*Article* **Fabrication of Poly(vinyl alcohol)/Chitosan Composite Films Strengthened with Titanium Dioxide and Polyphosphonate Additives for Packaging Applications**

**Tăchit,ă Vlad-Bubulac 1,\*, Corneliu Hamciuc <sup>1</sup> , Cristina Mihaela Rîmbu <sup>2</sup> , Magdalena Aflori <sup>1</sup> , Maria Butnaru <sup>3</sup> , Alin Alexandru Enache <sup>4</sup> and Diana Serbezeanu <sup>1</sup>**


**Abstract:** Eco-innovation through the development of intelligent materials for food packaging is evolving, and it still has huge potential to improve food product safety, quality, and control. The design of such materials by the combination of biodegradable semi-synthetic polymers with natural ones and with some additives, which may improve certain functionalities in the targeted material, is continuing to attract attention of researchers. To fabricate composite films via casting from solution, followed by drying in atmospheric conditions, certain mass ratios of poly(vinyl alcohol) and chitosan were used as polymeric matrix, whereas TiO<sup>2</sup> nanoparticles and a polyphosphonate were used as reinforcing additives. The structural confirmation, surface properties, swelling behavior, and morphology of the xerogel composite films have been studied. The results confirmed the presence of all ingredients in the prepared fabrics, the contact angle of the formulation containing poly(vinyl alcohol), chitosan, and titanium dioxide in its composition exhibited the smallest value (87.67◦ ), whereas the profilometry and scanning electron microscopy enlightened the good dispersion of the ingredients and the quality of all the composite films. Antimicrobial assay established successful antimicrobial potential of the poly(vinyl alcoohol)/chitosan-reinforced composites films against *Staphylococcus aureus*, *Methicillin-resistant Staphylococcus aureus* (MRSA), *Escherichia coli*, *Pseudomonas aeruginosa*, and *Candida albicans*. Cytotoxicity tests have revealed that the studied films are non-toxic, presented good compatibility, and they are attractive candidates for packaging applications.

**Keywords:** poly(vinyl alcohol); chitosan; titanium dioxide; polyphosphonate; casting from solution; xerogel composite film

### **1. Introduction**

Plastic packaging continues, in recent years, to be the most significant industrial use in the world (representing, on average, 30% of the total) [1–3]. Particularly, polymers, like polyethylene terephthalate, low-density polyethylene, polystyrene, polypropylene, and high-density polyethylene, were widely used as single-use packaging materials in the food and beverage industry because of their mechanical properties that could provide effective barriers to oxygen and carbon dioxide as well as their relative affordability and ease of availability [4–8]. Nevertheless, when plastic packaging based on the abovementioned polymers reaches the end life of its use, a significant amount frequently eludes formal

**Citation:** Vlad-Bubulac, T.; Hamciuc, C.; Rîmbu, C.M.; Aflori, M.; Butnaru, M.; Enache, A.A.; Serbezeanu, D. Fabrication of Poly(vinyl alcohol)/Chitosan Composite Films Strengthened with Titanium Dioxide and Polyphosphonate Additives for Packaging Applications. *Gels* **2022**, *8*, 474. https://doi.org/10.3390/ gels8080474

Academic Editors: Francesco Caridi, Giuseppe Paladini and Andrea Fiorati

Received: 5 July 2022 Accepted: 25 July 2022 Published: 28 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

collection and recycling processes and eventually leaks away, damaging the worldwide environment [9–11].

According to estimates, the increase in population growth will demand a 50% increase in global food supplies by the year 2050 [12]. Therefore, it is necessary to incorporate eco-innovations in primary packaging that can lessen the impact of packages on the environment and simultaneously maintain food quality and safety [13]. Additionally, more and more efforts are being put into creating bio-products for eco-innovations in food packaging, like biodegradable and compostable polymers [14,15].

The primary choice for packaging is represented by natural polymers, which should be researched and used extensively in the near future. The second-most prevalent polysaccharide in the world, chitosan, CS, is a biocompatible, biodegradable, and nontoxic substance that has proven to be a good candidate for use in packaging films. Structural versatility, attractive barrier properties, excellent film-forming and coating capabilities, as well as an innate antibacterial capability, have been proven for this biopolymer. As a result, several films made from CS have been produced and used in the food packaging sector [16–19]. Pure CS, however, is only soluble in acidic environments and has poor mechanical and thermal properties [20]. A unique bio-composite material with brand-new or improved properties could be produced by mixing biopolymers with existing polymeric matrices, natural or semi-synthetic, to meet specific needs. Such combinations of CS with starch, pectin, alginate, poly lactic acid (PLA), poly(vinyl alcohol) (PVA), gelatin, etc. have been reported to date [21–25].

Blends of PVA with CS, which combine a natural biomacromolecule and an easily biodegradable synthetic polymer, are among the most attractive bicomponent systems [26]. PVA is a thermoplastic synthetic polymer created from the hydrolysis of poly(vinyl acetate), unlike many other synthetic polymers. Due to PVA's distinct characteristics, which include high mechanical properties, chemical and thermal stability, non-toxicity, film-forming skills, and low manufacturing costs, its uses have increased over the past ten years. In addition to biodegradable goods like backing rolls, adhesives, coatings, and surfactants, PVA is utilized in a number of different industries, including those that deal with textiles, paper, and food packaging [27]. PVA, like other synthetic polymers, has applications in biology and medicine in addition to technical ones, and this has led to it being one of the main research areas for polymer scientists. Additionally, in recent years PVA have been studied for various smart applications such as shape memory hydrogels with improved viscoelasticity for printable applications [28], bio-based sensors and antimicrobial films [29], nanofibrous metallochromic sensors for colorimetric selective detection of ferric ions [30], etc.

It has also been shown that adding fillers to this PVA/CS bicomponent systems can be supplementary reinforcement, creating new composites with enhanced physical features, like water resistance, without sacrificing biodegradability. Because of their simplicity of processing, low cost, and superior synergistic characteristics, nanomaterials have been produced and employed in a wide variety of fields, including foods, medicines, and cosmetics. Incorporating nanosized compounds like nano-ZnO, nano-TiO2, and nano-silica into the polymers has been proven in some studies to help improve the characteristics of the polymers [31–33]. In our recent paper, synergistic effects of the presence of silica nanoparticles and a polyphosphonate mixed into PVA matrix have been discussed [34].

TiO<sup>2</sup> is a versatile and chemically inert mineral with various pertinent uses (food, pharmaceutical, biomedical, antibacterial agent, environmental, and clean energy) [34]. Despite the fact that its use as a food additive has recently been questioned and even banned in the European Union, TiO<sup>2</sup> <sup>0</sup> s physicochemical, mechanical, and photocatalytic qualities, as well as its reactivity and thermal stability, low cost, secure manufacturing, and biocompatibility, all contribute to TiO<sup>2</sup> <sup>0</sup> s widespread application [35]. However, the capacity of nano-TiO<sup>2</sup> to aggregate is one of its main drawbacks. According to certain reports, the interaction of nano-TiO<sup>2</sup> with biopolymers including starch, gums, and chitosan can aid to lessen the spontaneous agglomeration of TiO2, improving the functional aspects of the composite [36]. The addition of nanoparticles to the polymer matrix has two benefits:

first, it strengthens and functionalizes the matrix by improving its dispersibility, and second, it gives the mixture antibacterial capabilities [37]. In composite materials, PVA is frequently utilized as a polymer matrix, and CS is frequently employed as a reinforcing agent in an effort to broaden the application of PVA

can aid to lessen the spontaneous agglomeration of TiO2, improving the functional aspects of the composite [36]. The addition of nanoparticles to the polymer matrix has two benefits: first, it strengthens and functionalizes the matrix by improving its dispersibility, and

*Gels* **2022**, *8*, x FOR PEER REVIEW 3 of 17

second, it gives the mixture antibacterial capabilities [37].

In composite materials, PVA is frequently utilized as a polymer matrix, and CS is frequently employed as a reinforcing agent in an effort to broaden the application of PVA in the field of high-strength materials [26,38]. Thus, the co-existence of the hydroxyl groups in both PVA and CS, doubled by the presence of amino groups in CS, can produce intermolecular interactions, which may result in an improvement of performances of PVA composites. The use of polyphosphonate containing sulfur and phosphorus in the main chain and in the side chain in phosphaphenanthrene-type heterocycles is expected to induce thermal stability in the composite, then, to synergistically bring its own antimicrobial contribution to the composite, as was described in few studies in the recent past [39]. in the field of high-strength materials [26,38]. Thus, the co-existence of the hydroxyl groups in both PVA and CS, doubled by the presence of amino groups in CS, can produce intermolecular interactions, which may result in an improvement of performances of PVA composites. The use of polyphosphonate containing sulfur and phosphorus in the main chain and in the side chain in phosphaphenanthrene-type heterocycles is expected to induce thermal stability in the composite, then, to synergistically bring its own antimicrobial contribution to the composite, as was described in few studies in the recent past [39]. In the present study, optimized solutions of PVA/CS, PVA/CS/TiO2, or

In the present study, optimized solutions of PVA/CS, PVA/CS/TiO2, or PVA/CS/ TiO2/polyphosphonate have been used to obtain xerogels composite films via casting from solution method, followed by drying in ambient conditions. Schematic representation of the composites is presented in Figure 1. PVA/CS/TiO2/polyphosphonate have been used to obtain xerogels composite films via casting from solution method, followed by drying in ambient conditions. Schematic representation of the composites is presented in Figure 1.

**Figure 1.** Schematic representation of PVA/CS/TiO2/polyphosphonate composites. **Figure 1.** Schematic representation of PVA/CS/TiO2/polyphosphonate composites.

The effect of all the ingredients on the structure-properties relation of the developed

xerogel composite films was determined and discussed. Physical, chemical, morphologi-

The effect of all the ingredients on the structure-properties relation of the developed xerogel composite films was determined and discussed. Physical, chemical, morphological properties, surface properties, antimicrobial activity, and cytocompatibility of the matrices were studied by means of FTIR, SEM, swelling behavior, profilometry, contact angle measurements, antimicrobial, and cytotoxicity assays. matrices were studied by means of FTIR, SEM, swelling behavior, profilometry, contact angle measurements, antimicrobial, and cytotoxicity assays. **2. Results and Discussion** 

### **2. Results and Discussion** *2.1. Preparation and Structural Characterization*

#### *2.1. Preparation and Structural Characterization* Binary PVA/CS films and the composite PVA/CS films containing TiO2 or TiO2 with

Binary PVA/CS films and the composite PVA/CS films containing TiO<sup>2</sup> or TiO<sup>2</sup> with polyphosphonate, PFR-3, have been obtained by the casting from solution procedure. The details of this procedure are presented in the Materials and Methods section. The composition of the composite polymer matrices expressed in mass ratio for all the ingredients utilized in the preparation and the codes of the as-prepared composites are listed in Table 1. polyphosphonate, PFR-3, have been obtained by the casting from solution procedure. The details of this procedure are presented in the Materials and Methods section. The composition of the composite polymer matrices expressed in mass ratio for all the ingredients utilized in the preparation and the codes of the as-prepared composites are listed in Table 1. **Table 1.** Preparation details of the PVA/CS composite films.


**Table 1.** Preparation details of the PVA/CS composite films.

The chemical structure of the products was introspected by FTIR spectroscopy. Figure 2 presents the FTIR spectra of the binary PVA/CS-0 sample and of the PVA/CS composite films. In the spectrum of the PVA/CS-0 sample, the characteristic strong and wide band appeared at approximately 3320 cm−<sup>1</sup> due to hydroxyl stretching vibration. The chemical structure of the products was introspected by FTIR spectroscopy. Figure 2 presents the FTIR spectra of the binary PVA/CS-0 sample and of the PVA/CS composite films. In the spectrum of the PVA/CS-0 sample, the characteristic strong and wide band appeared at approximately 3320 cm−1 due to hydroxyl stretching vibration.

**Figure 2.** Fourier-transform infrared (FTIR) spectra for PVA/CS composites. **Figure 2.** Fourier-transform infrared (FTIR) spectra for PVA/CS composites.

Common characteristic absorption bands were found also at about 2930 cm−1 with a shoulder at 2860 cm−1 due to asymmetric and symmetric stretching vibrations of C–H, at 1414 cm−1 (δC–H), 1378 cm−1 (ωC–H), 1243 cm−1 (ωC–H), 1074 cm−1, 1023 cm−1 (νC–O), and 830 cm−1 (ρCH2) [40]. Due to C=O and C–O–C units, respectively, characteristic absorption bands for the non-hydrolyzed ester groups−O–CO–CH3 were observed at 1735 cm−1 and Common characteristic absorption bands were found also at about 2930 cm−<sup>1</sup> with a shoulder at 2860 cm−<sup>1</sup> due to asymmetric and symmetric stretching vibrations of C–H, at 1414 cm−<sup>1</sup> (δC–H), 1378 cm−<sup>1</sup> (ωC–H), 1243 cm−<sup>1</sup> (ωC–H), 1074 cm−<sup>1</sup> , 1023 cm−<sup>1</sup> (νC–O), and 830 cm−<sup>1</sup> (ρCH2) [40]. Due to C=O and C–O–C units, respectively, characteristic absorption bands for the non-hydrolyzed ester groups−O–CO–CH<sup>3</sup> were observed

1245 cm−1. The characteristic bands for CS could be also assigned in the FTIR spectra of the samples, at 1556 cm−1 (band characteristic for amide II, νN–H), 2920 cm−1 (νC–H), and

at 1735 cm−<sup>1</sup> and 1245 cm−<sup>1</sup> . The characteristic bands for CS could be also assigned in the FTIR spectra of the samples, at 1556 cm−<sup>1</sup> (band characteristic for amide II, νN–H), 2920 cm−<sup>1</sup> (νC–H), and approximately 3300 cm−<sup>1</sup> (wide band with νNH vibration overlapping with νO–H of polyvinyl alcohol). The characteristic band appearing at 1557 cm−<sup>1</sup> assigned for δNH (amide II) vibration of the NH<sup>2</sup> group, and the band observable at 1654 cm−<sup>1</sup> assigned for amide I (νC=O) of O=C–NHR groups, were revealed to be diminished and slightly shifted in the FTIR spectra of the ternary and quaternary composite films (samples PVA/CS-2 and PVA/CS-3), indicating some interactions between CS and PVA that could be catalyzed by the presence of the additives in the samples. Additionally, the band appearing enlarged at 846 cm−<sup>1</sup> in the multi-component composites (samples PVA/CS-1, PVA/CS-2, and PVA/CS-3) is the indicative of occurrence of hydrogen bonds formed between OH groups of the ingredients present in the composites.

From the FTIR spectra of the PVA/CS-2 and PVA/CS-3 samples, characteristic absorption bands at 1470 cm−<sup>1</sup> , 1210 cm−<sup>1</sup> (appearing as small shoulders), and 1140 cm−<sup>1</sup> (appearing as a distinctive peak), were assigned to stretching vibration of the P–Ar, P–O and P–O–C, respectively. Another distinctive band confirming the presence of the PFR-3 additive into the composition of the PVA/CS-2 and PVA/CS-3 samples could be observed as a sharp peak near 755 cm−<sup>1</sup> , which is attributed to the P–O–Ph group.

### *2.2. Surface Characteristics of the PVA/CS Composite Films*

Due to their outstanding biocompatibility, resistance to bodily fluids, mechanical qualities, anticorrosive capability, and flexibility, titanium-based composites are preferred for use in biomedical applications [41,42]. However, their properties depend on the surface, which is related with the mixing capacity of nanomaterials with titanium oxide [43]. Accordingly, it has been suggested that the interaction between titanium oxide nanoparticles (TiO2) and biopolymers (starch, gums, and chitosan) can aid in reducing the spontaneous agglomeration of nanoparticles, thereby enhancing the functional qualities of the composite [36].

PVA/CS composite films were analyzed using a profilometer. Microscopic images of the composite films were taken (histograms generated by the profilometer) and are presented in Figure 3, and the average roughness values were calculated (Table 2). According to the histograms and the microscopic images, it has been revealed that the addition of titanium dioxide to the polymeric matrix based on chitosan and PVA resulted in the production of composite films with a homogeneous composition and a surface devoid of cracks. The average roughness, Ra, representing the arithmetic mean of the highs and lows profiles, was in the interval of 65.5–570.3 nm. The surface roughness parameters decreased when introducing the PFR-3 in the PVA/CS matrix, according to the Ra value (Table 2), suggesting its contribution to the homogeneity and quality of the multi-component composites.


**Table 2.** Surface tension parameters of the test liquids used in contact angle measurements performed on PVA/CS composite films.

**Figure 3.** Microscopic images of the composite films and histograms generated by Alpha-Step D-500 Stilus profilometer (**a**) PVA/CS-0; (**b**) PVA/CS-1; (**c**) PVA/CS-2; (**d**) PVA/CS-3. **Figure 3.** Microscopic images of the composite films and histograms generated by Alpha-Step D-500 Stilus profilometer (**a**) PVA/CS-0; (**b**) PVA/CS-1; (**c**) PVA/CS-2; (**d**) PVA/CS-3.

The wetting characteristics, such as work of adhesion (Wa), the total solid surface free energy (γSV), solid-liquid interfacial tension (γSL), etc., of the PVA/CS samples with respect to the W and EG were investigated. A summary of all contact angle study parameters is provided. Table 2 provides a summary of all the contact angle investigations' parameters. W and EG were used as the test liquids, whereas PVA samples were examined using contact angle measurements at room temperature. Figure 4 displays the results of The wetting characteristics, such as work of adhesion (Wa), the total solid surface free energy (γSV), solid-liquid interfacial tension (γSL), etc., of the PVA/CS samples with respect to the W and EG were investigated. A summary of all contact angle study parameters is provided. Table 2 provides a summary of all the contact angle investigations' parameters. W and EG were used as the test liquids, whereas PVA samples were examined using contact angle measurements at room temperature. Figure 4 displays the results of measuring the contact angle for the PVA sample in W. **Figure 3.** Microscopic images of the composite films and histograms generated by Alpha-Step D-500 Stilus profilometer (**a**) PVA/CS-0; (**b**) PVA/CS-1; (**c**) PVA/CS-2; (**d**) PVA/CS-3. The wetting characteristics, such as work of adhesion (Wa), the total solid surface free energy (γSV), solid-liquid interfacial tension (γSL), etc., of the PVA/CS samples with respect to the W and EG were investigated. A summary of all contact angle study parameters is provided. Table 2 provides a summary of all the contact angle investigations' parameters. W and EG were used as the test liquids, whereas PVA samples were examined using contact angle measurements at room temperature. Figure 4 displays the results of

measuring the contact angle for the PVA sample in W.

measuring the contact angle for the PVA sample in W.

**Figure 4.** Contact angle of water on PVA/CS samples.

For water, the PVA/CS-0 sample exhibited a contact angle of 95.38◦ while by adding 0.16 g TiO<sup>2</sup> a slight decrease of the contact angle to 94.05◦ was observed in the case of the sample PVA/CS-1; a decrease was also observed in the roughness of the respective sample (Table 2). The same behavior, in terms of contact angle and roughness decreases, has been noticed when adding 0.2172 g of polyphosphonate PFR-3, which resulted in a further decrease of the contact angle as a result of the presence of phenyl-phosphonate groups within the macromolecule of the polymeric additive that could influence the molecular hydrogen bonding; these results are consistent with the FTIR findings. Thus, in the case of PVA/CS-2, the contact angle value decreased to 87.67◦ due to the orientation of macromolecules and the reorganization of the polar groups from the surface of the composite sample. After continuing to add the PFR-3 additive (0.4344), the hydrophobic character was preserved as a result of the reduced concentration of OH groups, whereas the phenyl-phosphonate group's impact was disrupted by the presence of bulky, rigid phosphaphenanthrene groups.

When polyphosphonate PFR-3 was added into the PVA matrix, the work of adhesion (Wa) increased (PVA/CS-2 sample). Therefore, an increase in the work of adhesion may have resulted from the polyphosphonate's effective dispersion in the PVA/CS matrix. For the PVA/CS-0 sample, γSV was 22.54 mN/m and increased up to 38.92 mN/m in the PVA/CS-2 sample, whereas the sample containing TiO<sup>2</sup> nanoparticles presented the highest value of 49.84 mN/m, suggesting the different nature of the forces interacting on the surface of the different compositions. Table 2 revealed that samples PVA/CS-1 and PVA/CS-2 show lower polar surface parameters γ <sup>P</sup> SV due to the presence of nano powders in the polymeric matrices. After continuing to add PFR-3 additive, a decrease of the Wa value and an increase of the polar surface parameters γ <sup>P</sup> SV were achieved, probably due to the increase in the hydrophobicity of the surface of the sample PVA/CS-3. Additionally, from Table 2 it can be observed that the dispersive component values for the PVA/CS samples are greater than the polar component values, which leads to the conclusion that there are more hydrophobic groups than hydrophilic ones on the surface of the PVA/CS films.

### *2.3. Swelling Behavior and Morphology of PVA/CS Composite Films*

The swelling behavior of the samples PVA/CS revealed that the PVA/CS-0 matrix without TiO<sup>2</sup> and PFR-3 additive exhibited the highest maximum swelling degree achieved at equilibrium, 834% (Figure 5). For the sample containing TiO2, the maximum swelling degree achieved at equilibrium was 688%, whereas introducing the polyphosphonate additive into the systems further decreased the swelling behavior of the composite in the series, with the swelling degree achieved at equilibrium being 639% and 593% in the case of the PVA/CS-2 and PVA/CS-3 sample, respectively. Another observation relates to a higher initial water uptake in the first 10 min when the samples PVA/CS-0 and PVA/CS-1 reached the maximum swelling degree, whereas in the case of the samples PVA/CS-2 and PVA/CS-3, the maximum swelling degree was achieved after approximately 30 min. *Gels* **2022**, *8*, x FOR PEER REVIEW 8 of 17 in the case of the PVA/CS-2 and PVA/CS-3 sample, respectively. Another observation relates to a higher initial water uptake in the first 10 min when the samples PVA/CS-0 and PVA/CS-1 reached the maximum swelling degree, whereas in the case of the samples PVA/CS-2 and PVA/CS-3, the maximum swelling degree was achieved after approximately 30 min.

**Figure 5.** Swelling degree of PVA/CS composites. **Figure 5.** Swelling degree of PVA/CS composites.

The morphology of PVA/CS composite films was investigated by scanning electron microscopy. Microphotographs for PVA/CS-0 in comparison with the samples containing additive are presented in Figure 6. The morphology of the PVA/CS-0 sample appeared

or both TiO2 and PFR-3 additive in their structure. A slight increase in the porosity could be observed in the sample PVA/CS-3 as the content of polyphosphonate PFR-3 increased.

The morphology of PVA/CS composite films was investigated by scanning electron microscopy. Microphotographs for PVA/CS-0 in comparison with the samples containing additive are presented in Figure 6. The morphology of the PVA/CS-0 sample appeared uniform and smooth, presenting a porous behavior. The aspect of the main polymeric matrix was also preserved in the samples containing either titanium dioxide nanoparticles or both TiO<sup>2</sup> and PFR-3 additive in their structure. A slight increase in the porosity could be observed in the sample PVA/CS-3 as the content of polyphosphonate PFR-3 increased. *Gels* **2022**, *8*, x FOR PEER REVIEW 9 of 17

**Figure 6.** SEM images of PVA/CS-0 (**a**), PVA/CS-1 (**b**), PVA/CS-2 (**c**), and PVA/CS-3 (**d**) composite films. **Figure 6.** SEM images of PVA/CS-0 (**a**), PVA/CS-1 (**b**), PVA/CS-2 (**c**), and PVA/CS-3 (**d**) composite films.

#### *2.4. In Vitro Evaluation of Antimicrobial Potential of the PVA/CS Composite Films 2.4. In Vitro Evaluation of Antimicrobial Potential of the PVA/CS Composite Films*

Due to the chitosan and the specific conditions of the culture medium and incubation parameters (37 °C/24 h), the tested matrices changed their disc-like shape and/or lost contact with the microbial culture, leading to a limitation in the interpretation of the antimicrobial activity. Due to the chitosan and the specific conditions of the culture medium and incubation parameters (37 ◦C/24 h), the tested matrices changed their disc-like shape and/or lost contact with the microbial culture, leading to a limitation in the interpretation of the antimicrobial activity.

However, qualitative evaluation of the antimicrobial potential of the PVA/CS composites films against Staphylococcus aureus ATCC 25923, *Methicillin-resistant Staphylococcus aureus* (MRSA) ATCC 43300, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Candida albicans ATCC 90028 (Table 3) was successful. Samples PVA/CS-2 and PVA/CS-3 contain both TiO2 and PFR-3 additive in their structure. The presence of TiO2 at a similar concentration as in PVA/CS-1 reflects the antimicrobial nature of the matrix, which was highlighted in PVA/CS-2 against Candida albicans and in PVA/CS-3 against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. At the same time, inhibition of antimicrobial activity was observed against MRSA (matrices PVA/CS-2 and PVA/CS-3), Escherichia coli (matrices PVA/CS-2 and PVA/CS-3), Pseudomonas aeruginosa (PVA/CS-2), and Staphylococcus aureus (PVA/CS-2). Comparing the results obtained for the PVA/CS-0 film (the sample containing only However, qualitative evaluation of the antimicrobial potential of the PVA/CS composites films against Staphylococcus aureus ATCC 25923, *Methicillin-resistant Staphylococcus aureus* (MRSA) ATCC 43300, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Candida albicans ATCC 90028 (Table 3) was successful. Samples PVA/CS-2 and PVA/CS-3 contain both TiO<sup>2</sup> and PFR-3 additive in their structure. The presence of TiO<sup>2</sup> at a similar concentration as in PVA/CS-1 reflects the antimicrobial nature of the matrix, which was highlighted in PVA/CS-2 against Candida albicans and in PVA/CS-3 against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. At the same time, inhibition of antimicrobial activity was observed against MRSA (matrices PVA/CS-2 and PVA/CS-3), Escherichia coli (matrices PVA/CS-2 and PVA/CS-3), Pseudomonas aeruginosa (PVA/CS-2), and Staphylococcus aureus (PVA/CS-2).

> PVA and chitosan) and the functionalized composite films (PVA/CS-1, PVA/CS-2, PVA/CS-3), it can be seen that the PVA/CS-1 sample (containing PVA, CS, and TiO2 nanopowder) has the best antimicrobial activity against both Gram-positive bacteria (*Staphylococcus aureus*, MRSA) and Gram-negative bacteria (*Escherichia coli*, *Pseudomonas aeru-*


**Table 3.** Qualitative evaluation of the antimicrobial activity of APV-chitosan-PFR-TiO<sup>2</sup> matrices.

Legend: [+] = inhibition zone present; [++] = clear inhibition zone the size of the matrix disc, [+++] = clear zone of inhibition went beyond the imprint area of the matrix disc.

Comparing the results obtained for the PVA/CS-0 film (the sample containing only PVA and chitosan) and the functionalized composite films (PVA/CS-1, PVA/CS-2, PVA/CS-3), it can be seen that the PVA/CS-1 sample (containing PVA, CS, and TiO<sup>2</sup> nanopowder) has the best antimicrobial activity against both Gram-positive bacteria (*Staphylococcus aureus*, MRSA) and Gram-negative bacteria (*Escherichia coli*, *Pseudomonas aeruginosa*). The yeast *Candida albicans* also proved to be sensitive to the action of the compounds in the PVA/CS-3 sample.

In the case of the PVA/CS-2 and PVA/CS-3 composite films containing both TiO<sup>2</sup> nanoparticles and PFR-3 additive in their structure, the presence of TiO<sup>2</sup> at a similar concentration as in PVA/CS-1 highlights the antimicrobial effect of the sample on all strains tested. The inclusion of PRF in their composition did not significantly alter the antimicrobial potential of the tested materials.

The tests performed in our study confirmed the well-recognized antimicrobial capability of TiO2. The antimicrobial potential and mechanisms of action of TiO<sup>2</sup> on bacterial cells are complex and diverse, exhibiting a broad spectrum of antimicrobial activity against bacteria (Gram-positive and Gram-negative), yeasts, and implicitly antibiotic-resistant microorganisms [44]. The mechanism thought to be responsible for the antimicrobial effect of TiO<sup>2</sup> is commonly associated with reactive oxygen species (ROS), which, when irradiated with bandgaps, produce photoinduced charges in the presence of O<sup>2</sup> [45]. Thus, the oxidative effect alters several cellular structures, but the first to be affected are the cell wall and cell membranes, leading to cell lysis and loss of cell integrity [46]. Specific studies on antimicrobial mechanisms have shown that microorganisms exposed to photocatalytic TiO<sup>2</sup> NPs exhibited cell inactivation at the level of the regulatory network and signal transduction, a significant reduction in respiratory chain activity, and inhibition of the ability to assimilate and transport iron and phosphorus. These processes, with the extensive cell wall and membrane changes, were the main factors explaining the biocidal activity of TiO<sup>2</sup> NPs [44].

### *2.5. In Vitro Citotoxicity and Proliferation Assay*

The cytotoxicity tests undertaken in this step of the study (MTT and cell morphology analysis) have a screening value and are performed and analyzed in accordance with ISO-10993-5 standards recommendations for cytotoxicity as a first-intended and eliminatory requirement for biocompatibility.

PVA/CS composite films have been subjected to cytocompatibility tests that were performed by culturing MCF 7 in the physical presence of material samples (Figure 7). The MCF 7 cells are a standard line for cytocompatibility testing, as recommended by ASTM.

alcohol/chitosan matrices with TiO2 nanoparticles and/or polyphosphonate additive PFR-

3 express a cytocompatibility, according to ISO 10993-5 recommendations.

**Figure 7.** MTT test results for all PVA/CS composites. **Figure 7.** MTT test results for all PVA/CS composites.

*2.6. Analysis of Cell Morphology*  The effect of direct exposure of cultured cells to the sample material was assessed microscopically by phase contrast, overlapped to fluorescence images of DAPI-stained nuclear DNA. The images of the fixed and stained cells at 20× objective magnification are Figure 7 shows the results of the MTT test, in which cell viability was expressed as an average and standard variation, obtained from the processing of absorbance values. Cell viability in control cultures was considered 100% ± SD and experimental values percentage of control value. Figure 7 shows the cell viability after 48 and 72 h of direct cell interaction with the material samples.

shown in Figure 8. The test revealed that all analyzed material samples decrease the cell viability by about 20% compared to control cultures. On the other hand, cell viability in the experimental cultures is approximatively the same, regardless of the TiO<sup>2</sup> ratio, which means that TiO<sup>2</sup> does not influence the biocompatibility of the materials. The decreasing of the cell viability in experimental cultures with the same value could be explained through the mechanical action of the direct contact of the membranes on the cells. The in vitro experimental condition for cell viability testing leads to the conclusion that polyvinyl alcohol/chitosan matrices with TiO<sup>2</sup> nanoparticles and/or polyphosphonate additive PFR-3 express a cytocompatibility, according to ISO 10993-5 recommendations.

### *2.6. Analysis of Cell Morphology*

The effect of direct exposure of cultured cells to the sample material was assessed microscopically by phase contrast, overlapped to fluorescence images of DAPI-stained nuclear DNA. The images of the fixed and stained cells at 20× objective magnification are shown in Figure 8.

From the images showing the experimental cultures, no differences of cell morphology were observed between the different compositions of the samples. All analyzed cultures contain epithelial cells with a shape typical of the MCF 7 cell line. It is observed that at 72 h of culture, the cells grow in confluent monolayers, with a cell density without significant differences between the binary PVA/CS sample and multi-component PVA/CS films containing TiO<sup>2</sup> and/or PFR-3 additives.

(**a**), PVA/CS-1 (**b**), PVA/CS-2 (**c**), and PVA/CS-0 (**d**) at 20× objective magnification.

**Figure 8.** Microscopic images of DAPI-stained cells at 72 h of culturing in contact with PVA/CS-0

From the images showing the experimental cultures, no differences of cell morphology were observed between the different compositions of the samples. All analyzed cultures contain epithelial cells with a shape typical of the MCF 7 cell line. It is observed that

**Figure 7.** MTT test results for all PVA/CS composites.

*2.6. Analysis of Cell Morphology* 

shown in Figure 8.

**Figure 8.** Microscopic images of DAPI-stained cells at 72 h of culturing in contact with PVA/CS-0 (**a**), PVA/CS-1 (**b**), PVA/CS-2 (**c**), and PVA/CS-0 (**d**) at 20× objective magnification. **Figure 8.** Microscopic images of DAPI-stained cells at 72 h of culturing in contact with PVA/CS-0 (**a**), PVA/CS-1 (**b**), PVA/CS-2 (**c**), and PVA/CS-0 (**d**) at 20× objective magnification.

alcohol/chitosan matrices with TiO2 nanoparticles and/or polyphosphonate additive PFR-

The effect of direct exposure of cultured cells to the sample material was assessed microscopically by phase contrast, overlapped to fluorescence images of DAPI-stained nuclear DNA. The images of the fixed and stained cells at 20× objective magnification are

3 express a cytocompatibility, according to ISO 10993-5 recommendations.

#### From the images showing the experimental cultures, no differences of cell morphol-**3. Conclusions**

ogy were observed between the different compositions of the samples. All analyzed cultures contain epithelial cells with a shape typical of the MCF 7 cell line. It is observed that at 72 h of culture, the cells grow in confluent monolayers, with a cell density without PVA/CS composite films were obtained by the casting from solution technique, starting from appropriate amounts of PVA and CS, as polymer matrices, and TiO<sup>2</sup> and PFR-3 polyphosphonate were utilized as reinforcing additives. Infrared spectroscopy enabled structural validation and the identification of distinctive bands for certain functionalities in the prepared composite films. According to the Ra value, the surface roughness characteristics decreased when the PFR-3 was introduced into the PVA/CH matrix, indicating its contribution to the homogeneity and quality of the four-component composites. Scanning electron microscopy demonstrated a small increase in porosity in the PVA/CS-3 sample as the polyphosphonate PFR-3 level increased. In vitro evaluation of antimicrobial potential in the series confirmed the antimicrobial role of TiO<sup>2</sup> in the PVA/CS-TiO<sup>2</sup> matrices. Cell viability testing revealed that polyvinyl alcohol/chitosan matrices containing TiO<sup>2</sup> nanoparticles or TiO2/polyphosphonate additive PFR-3 exhibit cytocompatibility in accordance with ISO 10993-5 guidelines.

### **4. Materials and Methods**

### *4.1. Materials*

Partially hydrolyzed PVA (Mowiol 26–88 (KURARAY POVAL 26–88)) with a molecular weight of 160,000 g/mol and an 87.7 percent degree of hydrolysis was supplied by ZAUBA (Munchen, Germany) and used as received. Chitosan hydrochloride (Mw = 302.11 kDa, DAC degree = 82%), titanium (IV) oxide nanopowder (21 nm primary particle size), and glacial acetic acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). 3,30 - Diaminodiphenyl sulfone, 4-hydroxybenzaldehyde, tetrahydrofuran anhydrous (THF), *N*,*N*-dimethylformamide (DMF), and phenylphosphonic dichloride were purchased from Sigma-Aldrich (Taufkirchen, Germany).

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Chemos GmbH, Germany and dehydrated before use. Polyphosphonate additive (PFR-3) has been prepared in our laboratory according to an adopted procedure from a previous work [47], starting from 3,30 -diaminodiphenyl sulfone and 4-hydroxybenzaldehyde. All other reagents were used as received from commercial sources.

### *4.2. Preparation of PFR-3 Polyphophonate Additive*

In the first step, a DOPO-containing bisphenol was obtained via condensation reaction of 20 mmol 4-hydroxybenzaldehyde (2.44 g) with 10 mmol 3,30 -diaminodiphenyl sulfone, in 20 mL THF, followed by the in-situ addition of DOPO (40 mmol, 8.64 g solved in 15 mL THF) to the newly formed azomethine groups. PFR-3 have been prepared through polycondensation reaction of equimolecular amounts of phenylphosphonic dichloride and DOPO-containing bisphenol, in DMF.

PFR-3, Yield: 95%.

FTIR (KBr, cm−<sup>1</sup> ): 3063 (=C–H), 1589 (C–C Aromatic), 1475 (P–Ar), 1353 and 1149 (O=S=O), 1200 (–P=O), 924 (P–O–Ar).

<sup>1</sup>H NMR (400 MHz, DMSO-d6, δ, ppm): 8.18–8.10 (m, 4H), 7.49 (m, 2H), 7.47–7.39 (m, 5H), 7.33–7.05 (m, 20H), 6.95–6.66 (m, 6H), 6.65–6.62 (m, 3H), 5.60–5.50 (m, 1H), 5.30–5.00 (m, 1H).

### *4.3. Preparation of PVA/CS Films following the Casting Procedure*

The first step in preparing these materials as composite films involved the treating of polyvinyl alcohol with oligophosphonate, a reaction which takes place in polar organic solvent, in this case DMF, working with concentrations of 5% PVA, by heating the mixtures at 95 ◦C for about 4–5 h. After the dissolution of the components is complete, the mixture is allowed to cool down to about 40 ◦C; then, the solution is poured into crystallizers, and the solvent is allowed to evaporate. The second stage consists in transferring the film obtained after drying in the crystallizer over a previously obtained chitosan solution (1% solution in water treated with glacial acetic acid 2%). After complete dissolution of the reaction mixture, the mixture was treated with an aqueous suspension of a predetermined amount of titanium dioxide nanoparticles, homogenized, and previously dispersed by ultrasound for 15 min. The resulting homogeneous composite mixture was poured into 10 <sup>×</sup> 10 cm<sup>2</sup> Teflon plates to slowly evaporate the solvent in ambient conditions. Afterwards, they were placed in the oven for an advanced drying treatment by heating to approximately 40–50 ◦C, under a vacuum, for a period of 8 h. The obtained films are stored and used for the characterization and testing of new materials. The complete data on the synthesis of composites in this series are presented in Table 1, whereas the schematic diagram of the preparation is given in Figure 9. *Gels* **2022**, *8*, x FOR PEER REVIEW 13 of 17 mixture is allowed to cool down to about 40 °C; then, the solution is poured into crystallizers, and the solvent is allowed to evaporate. The second stage consists in transferring the film obtained after drying in the crystallizer over a previously obtained chitosan solution (1% solution in water treated with glacial acetic acid 2%). After complete dissolution of the reaction mixture, the mixture was treated with an aqueous suspension of a predetermined amount of titanium dioxide nanoparticles, homogenized, and previously dispersed by ultrasound for 15 min. The resulting homogeneous composite mixture was poured into 10 × 10 cm2 Teflon plates to slowly evaporate the solvent in ambient conditions. Afterwards, they were placed in the oven for an advanced drying treatment by heating to approximately 40–50 °C, under a vacuum, for a period of 8 h. The obtained films are stored and used for the characterization and testing of new materials. The complete data on the synthesis of composites in this series are presented in Table 1, whereas the schematic diagram of the preparation is given in Figure 9.

**Figure 9.** General scheme of the process for the preparation of films based on PVA and chitosan. **Figure 9.** General scheme of the process for the preparation of films based on PVA and chitosan.

*4.4. Composite Films Structure and Performances Characterization 4.4. Composite Films Structure and Performances Characterization*

4.4.1. Chemical Structure of PVA/CS/TiO2 Composite Films 4.4.1. Chemical Structure of PVA/CS/TiO<sup>2</sup> Composite Films

The chemical structure of the PVA/CS/TiO2 films was analyzed using Bio-Rad 'FTS 135′ FTIR spectrometer equipped with a Specac "Golden Gate" ATR accessory. To record The chemical structure of the PVA/CS/TiO<sup>2</sup> films was analyzed using Bio-Rad 'FTS 1350 FTIR spectrometer equipped with a Specac "Golden Gate" ATR accessory. To record

scans between 4000 and 500 cm−1 at a resolution of 4 cm−1, a LUMOS Microscope Fourier Transform Infrared (FTIR) spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany),

The roughness of the composite films was analyzed at a recording speed of 0.10 mm/s using the Tencor Alpha-Step D-500 High Sensitivity Stylus Profiler (KLA-Tencor Corporation, Milpitas, CA, USA). By applying a stylus force of 15 mg and a long-range cutoff filter of 60 μm, arithmetic mean roughness, Ra, was obtained, which is the average of the

Two test liquids—double-distilled water (W) and ethylene glycol (EG)—were used to test their static contact angles on the composite film surfaces. The measurements were performed in triplicate, at room temperature, on a CAM 101 system (KSV Instruments, Helsinki, Finland) equipped with a liquid dispenser, video camera, and drop shape analysis software. The geometric mean approach was employed to calculate the surface tension parameters. This measurement allows for the calculation of parameters that describe the composite film's surface and its capacity for absorption, such as free surface energy

4.4.2. Roughness and Contact Angle Measurements

absolute value along the sampling length.

scans between 4000 and 500 cm−<sup>1</sup> at a resolution of 4 cm−<sup>1</sup> , a LUMOS Microscope Fourier Transform Infrared (FTIR) spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany), equipped with an attenuated total reflection (ATR) device was used.

### 4.4.2. Roughness and Contact Angle Measurements

The roughness of the composite films was analyzed at a recording speed of 0.10 mm/s using the Tencor Alpha-Step D-500 High Sensitivity Stylus Profiler (KLA-Tencor Corporation, Milpitas, CA, USA). By applying a stylus force of 15 mg and a long-range cutoff filter of 60 µm, arithmetic mean roughness, Ra, was obtained, which is the average of the absolute value along the sampling length.

Two test liquids—double-distilled water (W) and ethylene glycol (EG)—were used to test their static contact angles on the composite film surfaces. The measurements were performed in triplicate, at room temperature, on a CAM 101 system (KSV Instruments, Helsinki, Finland) equipped with a liquid dispenser, video camera, and drop shape analysis software. The geometric mean approach was employed to calculate the surface tension parameters. This measurement allows for the calculation of parameters that describe the composite film's surface and its capacity for absorption, such as free surface energy (*SV*), solid-liquid interfacial tension (*SL*), and work of adhesion (*Wa*). Detailed experimental setup and calculations were described in a previous publication [34].

### 4.4.3. Degree of Swelling

Swelling behavior of the PVA/CS composite films prepared via the solution casting method was studied. Thus, dried samples of 0.5 <sup>×</sup> 0.5 cm<sup>2</sup> were weighted and submerged in 10 mL Millipore water in a closed bottle that was set in a thermostatic bath at 37 ◦C. The composite film samples were taken at predetermined intervals, and, after the extra water was removed with filter paper, the films weights were measured. The following equation was used to compute the swelling ratio, which represents the water absorption of each sample:

$$SD(\%) = \frac{\mathcal{W}\_t - \mathcal{W}\_d}{\mathcal{W}\_d} \times 100\tag{1}$$

where SD (%) represents the amount of absorbed Millipore water; Wd—weight of the dry composite film; Wt—weight of the hydrated sample.

### 4.4.4. Microscopic Morphology

Microscopic examinations have been performed on Environmental Scanning Electron Microscope Type Quanta 200, operating at 10 kV with secondary electrons in low vacuum mode (LFD detector). The composite samples were fractured, and their cross-section surfaces were analyzed using scanning electron microscopy (SEM). An Energy Dispersive X-ray (EDX) system is a feature of the Quanta 200 microscope that allows for qualitative, quantitative analysis, and elemental mapping.

### 4.4.5. Evaluation of Antimicrobial Activity

Diffusimetric determinations were performed to highlight the antimicrobial activity of the PVA/CS samples containing PFR-3 and/or TiO<sup>2</sup> additives. This technique is common for such tests and is based on the principle of contact of the test matrices with the surface of a culture medium inoculated with different microbial species. The antimicrobial tests were performed with standardized strains: *Staphylococcus aureus* ATCC 25923, *MRSA* ATCC 43300, *Escherichia coli* ATCC 25922, *Pseudomonas aeruginosa* ATCC 27853, and *Candida albicans* ATCC 90028. Standard microbial suspensions with a density of 0.5 McFarland, prepared with a spectrophotometer, were used. An aliquot (500 µL) of the bacterial suspension was applied to the surface of the Mueller-Hinton aggregate culture medium (BioRad, Taufkirchen, Germany) using an exudate swab. After drying for 10 min in a thermostat with the lid open, the test matrices were spread on the surface of the medium. The results were evaluated after incubation at 37 ◦C for 24 h. The antimicrobial activity was

highlighted by identifying the areas of microbial inhibition formed upon contact with the tested matrices.

### 4.4.6. Evaluation of Cytotoxicity and Cell Morphology

The PVA/CS composite films were cut into 4 mm Ø discs, decontaminated with 70% aqueous ethyl alcohol solution for 20 min and then repeatedly washed in sterile ultrapure water and HBSS (Hank's Balanced Salt Solution) to remove any remaining contaminants. Following washing, the samples were equilibrated for 24 h in culture media (DMEM Ham/F12, supplemented with 10% fetal bovine serum and 1% antibiotic mixture). For equilibration, 0.5 mL of culture media were utilized for each piece of 4 mm Ø material. Using the MTT assay, the cytotoxicity of the growth medium and the material sample were both evaluated. Additionally, phase-contrast microscopy and fluorescence microscopy were used to examine the direct effects of the material samples on cell morphology. For biocompatibility study, the human epithelial cell line MCF 7, at the density of 20 <sup>×</sup> <sup>10</sup>4/well cells, was plated in 48-well culture plates and DMEM Ham/F12 culture medium supplemented with 10% bovine fetal serum and 1% mixture of Penicillin-Streptomycin-Neomycin antibiotics (all for in vitro use). Thus, material samples or the media of their extraction were put over the cell monolayers in the 48-well culture plates in order to assess the viability, density, and morphology of the cells.

The MTT technique uses tetrazolium salts as an oxidized substrate for mitochondrial dehydrogenases to measure the activity of cellular metabolism. The basic idea behind the procedure is that the yellow MTT compound is reduced into an insoluble purple (formazan) product. The formazan salts are solubilized with isopropyl alcohol to obtain a blue-violet solution, the intensity of which is directly proportional to the number of living cells in the culture. Briefly, for MTT experiments, the culture medium of the cell cultures was replaced with work MTT solution of 0.25 mg/mL and incubated with cells for 3 h. The MTT solution was replaced with an equal volume of isopropyl alcohol to solubilize the formazan crystals. The absorbance of the formazan solution was measured spectrophotometrically at 570 nm, using a TECAN UV/VIS plate reader. An equivalent amount of isopropanol was used as the blank reference sample. The cell viability was calculate applying the equation:

$$\text{Cell viability (\%)} = \frac{A\_{Sample}}{A\_{control}} \times 100\tag{2}$$

Each membrane sample was evaluated in triplicate, at 48 and 72 h of cell incubation with materials. The cultures developed in the absence of the material were used as growth control.

The cells were fixed in 4% paraformaldehyde solution and permeabilized for 30 min at +40 ◦C, in 0.05% Triton X 100 in PBS. After 72 h of interaction between the cells and material, the DAPI (2-(4-amidinophenyl)-1H-indole-6-carboxamidines) staining protocol was carried out for the cell morphology examination. PBS was used to properly clean the permeabilized cells before they were incubated with DAPI 10 µg/mL work solution for 30 min at room temperature. After being washed in PBS, stained cells were examined using fluorescence microscopy at λex 340 nm and λem 488 nm. To emphasize the cell nucleus and form, phase contrast was overlapped with the fluorescence.

**Author Contributions:** Conceptualization, C.H., D.S. and T.V.-B.; methodology, C.H., T.V.-B., C.M.R. and M.B.; validation, D.S., C.H., T.V.-B., M.A., C.M.R. and A.A.E.; formal analysis, C.H. and T.V.-B.; investigation, C.M.R., D.S., M.B., T.V.-B. and C.H.; resources, M.A. and A.A.E.; writing—original draft preparation, T.V.-B., D.S., C.M.R. and C.H.; writing—review and editing, T.V.-B., D.S., C.M.R. and C.H.; project administration, A.A.E. and M.A.; funding acquisition, A.A.E. and M.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of the current study are listed within the article.

**Acknowledgments:** The authors acknowledge the financial support of this research through the project "Partnerships for knowledge transfer in the field of polymer materials used in biomedical engineering" ID P\_40\_443, Contract no. 86/8.09.2016, SMIS 105689, co-financed by the European Regional Development Fund by the Competitiveness Operational Program 2014–2020, Axis 1 Research, Technological Development and Innovation in support of economic competitiveness and business development, Action 1.2.3 Knowledge Transfer Partnerships. The authors Tăchit,ă Vlad-Bubulac and Diana Serbezeanu acknowledge the financial support through CNCSIS– UEFISCSU, Project Number PN-III-P1-1.1-TE2019-0639 nr. 89/03.09.2020.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


## *Article* **Metallic Strontium as a Precursor of the Al2O3/SrCO<sup>3</sup> Xerogels Obtained by the One-Pot Sol–Gel Method**

**Eliza Romanczuk-Ruszuk <sup>1</sup> , Bogna Sztorch <sup>2</sup> , Zbigniew Oksiuta <sup>1</sup> and Robert E. Przekop 2,\***


**Abstract:** Two series of binary xerogel systems of Sr/Al with molar ratios of 0.1, 0.25, 0.5, and 1.0 were synthesized by the sol–gel technique with metallic strontium component as a precursor. The influence of the metallic precursor on the properties of the final xerogel was determined. The properties of the gels were determined on the basis of X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), low temperature nitrogen adsorption, transmission, and scanning electron microscopy with Energy Dispersive X-ray Spectroscopy (TEM, SEM, and SEM/EDS). The Al2O3/SrCO<sup>3</sup> xerogels were tested as supports for platinum catalysts. Hydrogen chemisorption was used to determine the platinum dispersion of the Pt/Al2O<sup>3</sup> -SrCO<sup>3</sup> systems. The original method of synthesis allows to obtain highly dispersed and stable strontium carbonate phases that allow for obtaining a high (42–50%) dispersion of platinum nanoparticles.

**Keywords:** sol–gel; metallic precursor; SrCO<sup>3</sup> ; xerogels; alumina; binary gels; one-pot

### **1. Introduction**

Xerogels are described as porous systems obtained by drying wet gels and retaining their porous structure after drying [1]. The advantages of xerogels are thermal stability, large surface area, and porosity. Xerogels are biocompatible and nontoxic and can be easily modified [2]. Silica xerogels are the most popular xerogels and can be used as fillers for polymer composites. They are characterized by low density, high thermal stability, low thermal conductivity, and good hydrophobic properties [3].

It is well-known that metal oxides characterized by large specific surfaces and thermal stability, such as Al2O3, SiO2, TiO2, and CeO2, can be used as catalyst supports. In addition to the well-recognized oxides, oxides of the alkaline earth metal groups are also used for catalytic processes. Strontium oxide (SrO) is an example of an oxide of the alkaline earth metal group. It can catalyze numerous synthetic reactions, like nitroaldol reactions, selective oxidation of propane, and oxidative coupling of methane [4–6]. Strontium has a lower electronegativity among metals from Group II of the Periodic Table. Therefore, strontium oxide has a higher basic strength compared to other group II oxides. The electronegativity increases in the order MgO < CaO < SrO < BaO [4]. Nevertheless, there is limited research on using SrO as a catalyst. The problem is related to the preparation and use of SrO as the base catalyst, as there are difficulties in the preparation of SrO [7].

Strontium carbonate (SrCO3) takes advantage of the production of X-ray tubes, hard magnets, ceramics, and special glasses [8]. Before or during the high-temperature production processes, strontium carbonate (SrCO3) is decomposed into strontium oxide (SrO) and carbon dioxide (CO2). Strontium carbonate decomposes into strontium oxide and carbon dioxide during calcination at a temperature above 1000 ◦C (1273 K) in atmospheric conditions [9,10]. Recently, much work has focused on the use of carbonate as a catalyst support. It is worth noting that carbonate is not a typical catalyst support. In the work of

**Citation:** Romanczuk-Ruszuk, E.; Sztorch, B.; Oksiuta, Z.; Przekop, R.E. Metallic Strontium as a Precursor of the Al2O3/SrCO<sup>3</sup> Xerogels Obtained by the One-Pot Sol–Gel Method. *Gels* **2022**, *8*, 473. https://doi.org/ 10.3390/gels8080473

Academic Editors: Francesco Caridi, Giuseppe Paladini and Andrea Fiorati

Received: 29 June 2022 Accepted: 25 July 2022 Published: 27 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Omat et al. [11] it was found that cobalt deposited on SrCO<sup>3</sup> showed exceptional activity for the dry reforming of methane. Another study investigated the selectivity and reactivity of cobalt deposited on an alkaline earth metal carbonate for the catalytic preferential oxidation of CO [12,13]. The catalytic properties of Co/SrCO<sup>3</sup> were greater compared to cobalt catalysts supported on popular metal oxides. Moreover, Co/SrCO<sup>3</sup> showed the best productivity. In Iida et al. [14], the reforming of the catalytic activity of toluene of Ru/SrCO3-Al2O<sup>3</sup> and Ru/Al2O<sup>3</sup> catalysts in steam was compared. The research shows that the carbonate catalyst shows higher activity.

The aim of this study was to determine the properties of Al2O3/SrCO<sup>3</sup> xerogels obtained from metallic strontium. Therefore, a new synthesis approach using an Al2O3/SrCO<sup>3</sup> using the sol–gel method is presented. In the proposed method, the precursors of one of the matrix components were used in metallic form. Our previous work has indicated that the introduction of a metallic precursor changes the properties of the final product [15].

The advantage of using the sol–gel synthesis is the purity of the materials obtained without inorganic admixtures and residual ion content [16].

### **2. Results and Discussion**

### *2.1. X-ray Powder Diffraction*

Figure 1 presents X-ray diffraction patterns of Al2O3/SrCO<sup>3</sup> systems annealed in 500 ◦C (773 K) with different strontium (Sr) content and Sr1.0 annealed in different temperatures. The XRD results of Al2O3/SrCO<sup>3</sup> samples with a different amount of strontium indicate only the presence of SrCO<sup>3</sup> in the structure. The appearance of SrCO<sup>3</sup> in the XRD results can be explained by the strontium acetate decomposition, which takes place at 400–480 ◦C (673–753 K) [17]. As the Sr content increases, there is a change in the broadening and intensity of the peaks (Figure 1a). As expected, the highest intensity of the peaks in the sample with an Al/Sr molar ratio of 1.0 was detected. The XRD plot of the Sr1.0 annealed at different temperatures varies with the temperature operated (Figure 1b). XRD analysis of Sr1.0 materials annealed at 1000 ◦C (1273 K), 1150 ◦C (1423 K), and 1300 ◦C (1573 K) shows the same phases, but the peaks differ in intensity. The difference in the intensity of the peaks after different annealing times may be due to the amount of SrCO<sup>3</sup> crystallized [18]. In each of these materials one or more of the following compounds was identified: SrCO3, Al2O3, SrO, or SrAl2O4. The appearance of SrAl2O<sup>4</sup> can be explained by the interfacial reaction between SrCO<sup>3</sup> and Al2O<sup>3</sup> at a temperature above 500 ◦C (773 K) and the diffusion of Al3+ ions in the SrCO<sup>3</sup> lattice, which causes the formation of SrAl2O<sup>4</sup> [19]. Moreover, the presence of SrAl2O<sup>4</sup> may be related to the decomposition of strontium carbonate to SrO at higher temperatures and to the reaction with Al2O3, which is represented as follows [20]:

$$\text{SrCO}\_3 \rightarrow \text{SrO} + \text{CO}\_2 \tag{1}$$

$$\text{SrO} + \text{Al}\_2\text{O}\_3 \rightarrow \text{SrAl}\_2\text{O}\_4 \tag{2}$$

The presence of SrO in samples at temperatures above 1000 ◦C (1273 K) can be explained by the fact that in the temperature range 900–1175 ◦C (1173–1448 K) the equilibrium state moves toward the carbonate decomposition [21]. The SrO-CO2-SrCO<sup>3</sup> equilibrium diagram by Rhodes et al. [22] shows that at temperatures below 900–1000 ◦C (1173–1272 K) the equilibrium moves toward the formation of SrCO3; furthermore, at higher temperatures the possibility of SrCO<sup>3</sup> decomposition increases [21–23].

### *2.2. SEM, TEM, and EDS Analysis*

Figure 2 shows the surface of the samples with different molar ratios of Sr before annealing. Note that the surfaces of the tested materials vary depending on the amount of Sr. The SEM image of the Sr0.1 sample surface presents a granular structure. The Sr1.0 sample has a completely different structure compared to the other tested materials. The SEM EDS analysis (Figure 2e) shows that the elements in the examined xerogels systems decompose regularly. Oxygen clusters and a higher strontium content are observed in the Sr1.0 sample compared to the other tested xerogels.

*Gels* **2022**, *8*, x FOR PEER REVIEW 3 of 12

**Figure 1.** XRD patterns of Al2O3/SrCO3 systems: (**a**) annealed at 500 °C (773 K) with Sr molar ratios of 0.1, 0.25, 0.5 and 1.0; (**b**) Sr 1.0 molar ratio at different temperature. **Figure 1.** XRD patterns of Al2O3/SrCO<sup>3</sup> systems: (**a**) annealed at 500 ◦C (773 K) with Sr molar ratios of 0.1, 0.25, 0.5 and 1.0; (**b**) Sr 1.0 molar ratio at different temperature.

Figure 2 shows the surface of the samples with different molar ratios of Sr before annealing. Note that the surfaces of the tested materials vary depending on the amount of Sr. The SEM image of the Sr0.1 sample surface presents a granular structure. The Sr1.0 sample has a completely different structure compared to the other tested materials. The

*2.2. SEM, TEM, and EDS Analysis* 

Sr1.0 sample compared to the other tested xerogels.

**Figure 2.** SEM micrographs of the Al2O3/SrCO3 xerogels with various molar ratios: (**a**) Sr0.1, (**b**) Sr0.25, (**c**) Sr0.5, (**d**) Sr1.0, and (**e**) SEM-EDS analysis. **Figure 2.** SEM micrographs of the Al2O3/SrCO<sup>3</sup> xerogels with various molar ratios: (**a**) Sr0.1, (**b**) Sr0.25, (**c**) Sr0.5, (**d**) Sr1.0, and (**e**) SEM-EDS analysis.

decompose regularly. Oxygen clusters and a higher strontium content are observed in the

Figure 3 presents a comparison of the SEM micrographs of the Al2O3/SrCO3 samples with Sr to an Al ratio of 1.0 annealed at different temperatures. The structure of the gel after annealing at 1000 °C (1273 K) (Figure 3a) is non-homogeneous. The surface of the gels after annealing at 1150 °C (1423 K) and 1300 °C (1573 K) is similar (Figure 3b,c, respectively). In the material after annealing at 1150 °C (1423 K) and 1300 °C (1573 K), a monolithic structure with a uniform composition distribution without any traces of crystallization of the strontium phases is observed. In the xerogel annealed at 1000 °C (1273 K), a hierarchical structure with the effect of phase aggregation can be observed. Figure 3 presents a comparison of the SEM micrographs of the Al2O3/SrCO<sup>3</sup> samples with Sr to an Al ratio of 1.0 annealed at different temperatures. The structure of the gel after annealing at 1000 ◦C (1273 K) (Figure 3a) is non-homogeneous. The surface of the gels after annealing at 1150 ◦C (1423 K) and 1300 ◦C (1573 K) is similar (Figure 3b,c, respectively). In the material after annealing at 1150 ◦C (1423 K) and 1300 ◦C (1573 K), a monolithic structure with a uniform composition distribution without any traces of crystallization of the strontium phases is observed. In the xerogel annealed at 1000 ◦C (1273 K), a hierarchical structure with the effect of phase aggregation can be observed.

**Figure 3.** SEM micrographs of surface Al2O3/SrCO3 xerogels with the 1.0 Sr molar ratio annealed at: (**a**) 1000 °C (1273K), (**b**) 1150 °C (1423K), and (**c**) 1300 °C (1573K). **Figure 3.** SEM micrographs of surface Al2O3/SrCO<sup>3</sup> xerogels with the 1.0 Sr molar ratio annealed at: (**a**) 1000 ◦C (1273 K), (**b**) 1150 ◦C (1423 K), and (**c**) 1300 ◦C (1573 K). **Figure 3.** SEM micrographs of surface Al2O3/SrCO3 xerogels with the 1.0 Sr molar ratio annealed at: (**a**) 1000 °C (1273K), (**b**) 1150 °C (1423K), and (**c**) 1300 °C (1573K).

Figure 4 presents a comparison of the TEM micrographs of the Al2O3/SrCO3 samples with Sr to an Al ratio of 1.0 annealed at different temperatures. The black areas represent a strontium-rich phase (strontium carbonate), as confirmed by the XRD data presented in Figure 1a. The structures shown are typical for sol–gel systems [15]. Figure 4 presents a comparison of the TEM micrographs of the Al2O3/SrCO<sup>3</sup> samples with Sr to an Al ratio of 1.0 annealed at different temperatures. The black areas represent a strontium-rich phase (strontium carbonate), as confirmed by the XRD data presented in Figure 1a. The structures shown are typical for sol–gel systems [15]. Figure 4 presents a comparison of the TEM micrographs of the Al2O3/SrCO3 samples with Sr to an Al ratio of 1.0 annealed at different temperatures. The black areas represent a strontium-rich phase (strontium carbonate), as confirmed by the XRD data presented in Figure 1a. The structures shown are typical for sol–gel systems [15].

**Figure 4.** TEM images of Al2O3/SrCO3 xerogels after annealing at 500 °C with various Sr/Al molar ratios: (**a**) 0.1, (**b**) 0.5, and (**c**) 1.0. **Figure 4.** TEM images of Al2O3/SrCO3 xerogels after annealing at 500 °C with various Sr/Al molar ratios: (**a**) 0.1, (**b**) 0.5, and (**c**) 1.0. **Figure 4.** TEM images of Al2O3/SrCO<sup>3</sup> xerogels after annealing at 500 ◦C with various Sr/Al molar ratios: (**a**) 0.1, (**b**) 0.5, and (**c**) 1.0.

Figure 5 shows TEM images of Al2O3/SrCO3 sol–gel with Pt after annealing at 500 °C of samples with a molar ratio of Al/Sr 0.1, 0.25, and 0.5. The presented structures differ depending on the Al/Sr molar ratio. The Sr0.5Pt sample contains rod-shaped crystals, which are not observed in the other tested materials. The amorphous alumina structure is visible. The large dark fields show agglomerated strontium carbonate, and the smallest fields show platinum areas (marked with a red arrow). Figure 5 shows TEM images of Al2O3/SrCO3 sol–gel with Pt after annealing at 500 °C of samples with a molar ratio of Al/Sr 0.1, 0.25, and 0.5. The presented structures differ depending on the Al/Sr molar ratio. The Sr0.5Pt sample contains rod-shaped crystals, which are not observed in the other tested materials. The amorphous alumina structure is visible. The large dark fields show agglomerated strontium carbonate, and the smallest fields show platinum areas (marked with a red arrow). Figure 5 shows TEM images of Al2O3/SrCO<sup>3</sup> sol–gel with Pt after annealing at 500 ◦C of samples with a molar ratio of Al/Sr 0.1, 0.25, and 0.5. The presented structures differ depending on the Al/Sr molar ratio. The Sr0.5Pt sample contains rod-shaped crystals, which are not observed in the other tested materials. The amorphous alumina structure is visible. The large dark fields show agglomerated strontium carbonate, and the smallest fields show platinum areas (marked with a red arrow).

### *2.3. Porous Structure—Low Temperature Nitrogen Adsorption–Desorption*

Table 1 shows surface area, pore equivalent diameter, and volume. Figure 6 shows plots of isotherms, pore volume distribution, and pore area distribution. The adsorption isotherms for samples with different Sr/Al ratios are type IV with the hysteresis loop (IUPAC) present in the range of the relative pressure p/p<sup>0</sup> 0.5–0.8, which is characteristic of mesoporous structures [24]. Additionally, all the analyzed samples are characterized by

this isotherm. The shape of the hysteresis loops is similar for the samples with an Sr/Al ratio higher than 0.1. The adsorption isotherms for platinum samples differ from those with different Sr contents. Type III with the H<sup>2</sup> hysteresis loop (except for the Sr1.0Pt system) is in the relative pressure range p/p<sup>0</sup> 0.5–1. *Gels* **2022**, *8*, x FOR PEER REVIEW 6 of 12

**Figure 5.** TEM images of Al2O3/SrCO3 sol–gel with Pt after annealing at 500 °C with molar ratios of: (**a**) Sr0.1 Pt, (**b**) Sr0.25 Pt, and (**c**) Sr0.5 Pt. **Figure 5.** TEM images of Al2O3/SrCO<sup>3</sup> sol–gel with Pt after annealing at 500 ◦C with molar ratios of: (**a**) Sr0.1 Pt, (**b**) Sr0.25 Pt, and (**c**) Sr0.5 Pt.


*2.3. Porous Structure—Low Temperature Nitrogen Adsorption–Desorption*  **Table 1.** Textural properties of Sr systems.

Al203/SrCO3 xerogel samples exhibited specific surface areas above 100 m2/g (except for the Sr1.0 and Sr1.0 Pt). The surface area of the samples in which SrCO3 was used for the synthesis is greater compared to the samples with metal strontium. The average pore diameter is smaller for Al2O3/SrCO3 systems compared to the other samples. The average pore diameter and average pore volume are similar for the metallic strontium and platinum strontium systems. However, for systems with Sr/Al 0.5 and 1.0 molar ratios, a change in the geometry (a slight increase in the diameter) of the pores is visible, which may indicate the location of platinum crystals in the pores of the xerogel of a smaller size. For the systems with Pt, a slight increase in the specific surface area was also observed. This phenomenon may be related to the influence of platinum on the oxidative decomposition of carbon deposit residues that may occur in xerogel systems. **Table 1.** Textural properties of Sr systems. The pore distribution curves depend on the desorptive branch of the BJH isotherm. In catalytic systems, the presence of scattered platinum on the surface leads to a slight increase in surface area (Table 1). In every system tested, the surface area of the Sr0.1 sample is twice that of a sample with an Sr/Al molar ratio of 1.0. Thus, the prepared Al203/SrCO<sup>3</sup> xerogel samples exhibited specific surface areas above 100 m2/g (except for the Sr1.0 and Sr1.0 Pt). The surface area of the samples in which SrCO<sup>3</sup> was used for the synthesis is greater compared to the samples with metal strontium. The average pore diameter is smaller for Al2O3/SrCO<sup>3</sup> systems compared to the other samples. The average pore diameter and average pore volume are similar for the metallic strontium and platinum strontium systems. However, for systems with Sr/Al 0.5 and 1.0 molar ratios, a change in the geometry (a slight increase in the diameter) of the pores is visible, which may indicate the location of platinum crystals in the pores of the xerogel of a smaller size. For the systems with Pt, a slight increase in the specific surface area was also observed. This phenomenon may be related to the influence of platinum on the oxidative decomposition of carbon deposit residues that may occur in xerogel systems.

Sr0.1 135 7 0.30 Sr0.25 204 6 0.34

Sr1.0 69 6 0.13 Sr0.1 Pt 154 7 0.29 Sr0.25 Pt 209 6 0.31 Sr0.5 Pt 159 7 0.25 Sr1.0 Pt 75 6 0.12

**Average Pore Diameter DBJH [nm]** 

**Average Pore Volume DBJH [cm3/g]** 

**[m2/g]** 

**System Composition Surface Area SBET**

**Figure 6.** (**a**,**b**) Isotherm of Al2O3/SrCO3 and Pt/Al2O3-SrCO3, (**c**–**f**) pore volume distribution of Al2O3/SrCO3 and Pt/Al2O3-SrCO3. **Figure 6.** (**a**,**b**) Isotherm of Al2O3/SrCO<sup>3</sup> and Pt/Al2O<sup>3</sup> -SrCO<sup>3</sup> , (**c**–**f**) pore volume distribution of Al2O3/SrCO<sup>3</sup> and Pt/Al2O<sup>3</sup> -SrCO<sup>3</sup> .

#### *2.4. Chemisorption of Hydrogen on Pt-Al-Sr Catalysts 2.4. Chemisorption of Hydrogen on Pt-Al-Sr Catalysts*

Table 2 shows hydrogen chemisorption results on Pt/Al2O3-SrCO3 systems with 1% metal content loading. Based on the results of hydrogen chemisorption, the platinum dispersion, the metallic surface area values, and the volume of the adsorbed hydrogen were determined. The results of hydrogen chemisorption for samples with a metallic strontium precursor showed no significant differences. The lowest metal dispersion occurred in the sample with an Sr/Al molar ratio equal to 1. Note that the Al2O3/SrCO3 system is alkaline. In such system (alkaline), dispersion is significantly lower, because the chemical nature of the surface of materials with large pores and low surface area is important. In our study, the alkaline nature of the system reduces platinum dispersion, but the presence of nanopores stabilizes the platinum nanocrystallites and, as a result, dispersion is beneficial. Table 2 shows hydrogen chemisorption results on Pt/Al2O3-SrCO<sup>3</sup> systems with 1% metal content loading. Based on the results of hydrogen chemisorption, the platinum dispersion, the metallic surface area values, and the volume of the adsorbed hydrogen were determined. The results of hydrogen chemisorption for samples with a metallic strontium precursor showed no significant differences. The lowest metal dispersion occurred in the sample with an Sr/Al molar ratio equal to 1. Note that the Al2O3/SrCO<sup>3</sup> system is alkaline. In such system (alkaline), dispersion is significantly lower, because the chemical nature of the surface of materials with large pores and low surface area is important. In our study, the alkaline nature of the system reduces platinum dispersion, but the presence of nanopores stabilizes the platinum nanocrystallites and, as a result, dispersion is beneficial.


**Table 2.** Platinum dispersion, surface area, and volume of adsorbed hydrogen of the Pt/Al2O<sup>3</sup> - SrCO<sup>3</sup> systems. *2.5. Thermal Analysis*  For thermogravimetric testing, xerogel dried for one week at room temperature was

**Table 2.** Platinum dispersion, surface area, and volume of adsorbed hydrogen of the Pt/Al2O3-SrCO3

Sr0.1 Pt 47 116.94 0.27 ± 0.006 Sr0.25 Pt 49 122.55 0.29 ± 0.004 Sr0.5 Pt 48 119.15 0.28 ± 0.001 Sr1.0 Pt 42 102.94 0.24 ± 0.003

**Metallic Surface [m2/gmetal]** 

**Volume of Adsorbed Hydrogen [cm3/g]** 

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**[%]** 

**System Composition Metal (Pt) Dispersion** 

#### *2.5. Thermal Analysis* a fourth stage of weight loss above 900 °C (1173 K) with the decomposition of the stron-

systems*.* 

For thermogravimetric testing, xerogel dried for one week at room temperature was used. Thermograms of the analyzed systems are shown in Figure 7a and the DTG curves in Figure 7b. Four visible areas of thermal changes in the xerogel were observed. The first step at temperatures up to 100 ◦C (373 K) is to remove the water adsorbed by the system. The second step of mass loss between 180 and 260 ◦C (453 to 533 K) was attributed to a correspondence to the removal of internally absorbed and trapped solvent residues and to the water of hydration in the gel. The third step of mass loss between 350 and 480 ◦C (623 to 753 K) can be attributed to the decomposition of anhydrous strontium acetate to SrCO3, as confirmed by the XRD results (Figure 1). In the sample Sr0.5 and Sr1.0, there is a fourth stage of weight loss above 900 ◦C (1173 K) with the decomposition of the strontium carbonate. tium carbonate. Furthermore, the decomposition processes can be described in the following steps [17–25]: Sr(CH3COO)2 → SrCO3 (s) + CH3COCH3 (g) +CO2 (g) (3) SrCO3 (s) → SrO (s) + CO2 (g) (4) Al2O3 + SrO வଵଵହ ℃ ሱ⎯⎯⎯⎯ሮ SrAl2O4 (5) The DTG curves shown in Figure 7b indicate a multi-stage distribution. The decomposition of anhydrous strontium acetate to SrCO3 is shifted to the right as the strontium molar ratio increases.

(**a**) (**b**)

**Figure 7.** (**a**) TGA weight loss, and (**b**) DTG curves of Al2O3-SrCO3 xerogels obtained with a metallic strontium precursor. **Figure 7.** (**a**) TGA weight loss, and (**b**) DTG curves of Al2O<sup>3</sup> -SrCO<sup>3</sup> xerogels obtained with a metallic strontium precursor.

Furthermore, the decomposition processes can be described in the following steps [17–25]:

$$\text{Sr(CH}\_3\text{COO)}\_2 \rightarrow \text{SrCO}\_3\text{(s)} + \text{CH}\_3\text{COCH}\_3\text{(g)} \star \text{CO}\_2\text{(g)}\tag{3}$$

$$\text{SrCO}\_{\text{3\text{ (s)}}} \rightarrow \text{SrO}\_{\text{(s)}} + \text{CO}\_{\text{2}\text{ (g)}} \tag{4}$$

$$\text{Al}\_2\text{O}\_3 + \text{SrO} \xrightarrow{>1150\text{ °C}} \text{SrAl}\_2\text{O}\_4\tag{5}$$

The DTG curves shown in Figure 7b indicate a multi-stage distribution. The decomposition of anhydrous strontium acetate to SrCO<sup>3</sup> is shifted to the right as the strontium molar ratio increases.

### **3. Conclusions**

In this study, a new method of obtaining binary Al2O3-SrCO<sup>3</sup> xerogels systems is presented. The obtained xerogels are characterized by the presence of a stable carbonate phase for the full range of concentrations, which is confirmed by the XRD results. The effectiveness of the synthesis method using a reactive metallic precursor (metallic strontium) was confirmed. The obtained Al2O3/SrCO<sup>3</sup> xerogels are characterized by a high dispersion of the carbonate phase and a large specific surface area for alkaline systems. Carbonate xerogels with an alkaline element are characterized by a similar dispersion of the metallic phase (42–50%) in all the tested systems, which is a very good result for alkaline systems. Changes in the nanoporosity system may confirm the theory of stabilization of platinum nanoclusters in the structure of nanopore carriers obtained by the sol–gel method. Xerogels obtained by the described method are also an attractive precursor for high-temperature ceramics with a strictly defined microstructure.

Carbonate xerogels with an element of basic nature are characterized by a similar dispersion of the metallic phase (42–50%) in all the tested systems, which is a very good result for alkaline systems.

Some strontium aluminates (such as SrAl2O4) are used as phosphors. Phosphors based on strontium aluminate are characterized by good luminescent properties such as long-lasting afterglow and high quantum efficiency in comparison to classic sulfide phosphors [26,27].

### **4. Materials and Methods**

### *4.1. Materials*

Strontium carbonate, aluminum isopropoxide, acetic acid, and toluene were purchased from Sigma-Aldrich (Saint Louis, MO, USA) and used as received.

### *4.2. Preparation*

Al2O3-SrCO<sup>3</sup> mixed systems with different molar ratios of Sr to Al: 0.1; 0.25; 0.5; and 1.0 were synthesized by aqueous sol–gel methods. Alumina gel was prepared according to our previous study [15]. Reactions were executed in a 1L glass reactor equipped with a mixer and a thermostat. Aluminum isopropoxide (500 g of fine powder) was added and hydrolyzed in 440mL of water at 75 ◦C. Then the obtained suspension was stirred for 2 h, and 175 g (167.5 cm<sup>3</sup> ) of 98% acetic acid was peptized. The sol was heated under reflux for 24 h at 95 ◦C (368 K), followed by metallic strontium addition in small portions as the second component. The next step was refluxing the resulting mixture for 72 h with vigorous stirring. The obtained product was a homogeneous liquid gel. To obtain a monolithic xerogel, part of the obtained gel was spilled into Petri dishes and dried for 5 days at room temperature. Next, the dry gel was annealed in a tube furnace at 500 ◦C (773 K) for 6 h under air flow. Part of the sample was air dried for 3 h, then annealed in 1000 ◦C (1273 K). Next, a portion of the sample was taken and air dried for 3 h and annealed in 1150 ◦C (1423 K). Finally, the sample batch was air dried for 3 h and annealed in 1300 ◦C (1573 K). The sample obtained after the annealing was crushed and sieved. Two grain fractions were collected: 0.1–0.2 mm and <0.1 mm. The particle size fraction with a diameter of 0.1 to 0.2 mm was used to determine the porous structure.

To prepare the Pt catalysts, 1.98 g of the support annealed at 500 ◦C (773 K) was weighed and placed in a 100 mL round-bottom flask. The samples were wetted with 2 mL of distilled water, then 1 mL of aqueous H2PtCl<sup>6</sup> solution (platinum content: 20 mg Pt/mL) was added. The round-bottom flask was placed on a vacuum evaporator, and the solvent was evaporated. The dried system was heat treated at a temperature of 500 degrees for 2 h in air atmosphere.

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Figure 8 shows the scheme of the synthesis of Al2O3/SrCO<sup>3</sup> xerogels.

**Figure 8.** Schematic procedure of Al2O3/SrCO3 xerogels synthesis. **Figure 8.** Schematic procedure of Al2O3/SrCO<sup>3</sup> xerogels synthesis.

#### *4.3. Characterization 4.3. Characterization*

The samples obtained in this work were characterized by the following techniques. The samples obtained in this work were characterized by the following techniques.

#### 4.3.1. X-ray Diffraction Analysis 4.3.1. X-ray Diffraction Analysis

The X-ray diffraction (XRD) analysis was performed using the Philips PW1050 diffractometer (Almelo, the Netherlands) working in the θ–2θ geometry with Cu-Kα (λ = 0.15406 nm) radiation of 35 kV and 20 mA. For all the samples, an angular range (2θ) of 10° to 100° with a step width of 0.01° and a step time of 3 s was used [28]. The X-ray diffraction (XRD) analysis was performed using the Philips PW1050 diffractometer (Almelo, the Netherlands) working in the θ–2θ geometry with Cu-K<sup>α</sup> (λ = 0.15406 nm) radiation of 35 kV and 20 mA. For all the samples, an angular range (2θ) of 10◦ to 100◦ with a step width of 0.01◦ and a step time of 3 s was used [28].

### 4.3.2. SEM, TEM, and EDS (Energy Dispersive X-ray Spectroscopy) Analysis

4.3.2. SEM, TEM, and EDS (Energy Dispersive X-ray Spectroscopy) Analysis The surface morphology of the oxide xerogels was depicted by a Scanning Electron Microscope (SEM, Hitachi 3000N, Tokyo, Japan), which was operated in high-vacuum conditions at 15 kV acceleration voltage and low-vacuum conditions at 20 kV acceleration voltage. Chemical composition was performed using the Energy Dispersive Spectroscopy (EDS). TEM observations were performed using a JEOL 200 CX (Tokyo, Japan) transmission electron microscope worked at 80 kV. The surface morphology of the oxide xerogels was depicted by a Scanning Electron Microscope (SEM, Hitachi 3000N, Tokyo, Japan), which was operated in high-vacuum conditions at 15 kV acceleration voltage and low-vacuum conditions at 20 kV acceleration voltage. Chemical composition was performed using the Energy Dispersive Spectroscopy (EDS). TEM observations were performed using a JEOL 200 CX (Tokyo, Japan) transmission electron microscope worked at 80 kV.

### 4.3.3. Porous Structure

4.3.3. Porous Structure In order to determine the porosity of the structure, measurements of low-temperature nitrogen adsorption were carried out using the Autosorb iQ Station 2 (Quantachrome Instruments, Boynton Beach, Florida, United States) in the standard analysis mode; 200– 300 mg of material with a particle size fraction from 0.1 to 0.2 mm were tested. Prior to testing, the samples were degassed for 10 h at 350 °C (623 K) and 0.4 Pa to constant weight. The adsorption and desorption isotherm branches were assumed in the p/p0 0–1 range. Quantachrome ASiQwin software (version 2.0) was used. The Boer t-method and the BJH method were used to determine the distribution of the pore surface and pore volume. The In order to determine the porosity of the structure, measurements of low-temperature nitrogen adsorption were carried out using the Autosorb iQ Station 2 (Quantachrome Instruments, Boynton Beach, Florida, United States) in the standard analysis mode; 200–300 mgof material with a particle size fraction from 0.1 to 0.2 mm were tested. Prior to testing, the samples were degassed for 10 h at 350 ◦C (623 K) and 0.4 Pa to constant weight. The adsorption and desorption isotherm branches were assumed in the p/p0 0–1 range. QuantachromeASiQwin software (version 2.0) was used. The Boer t-method and the BJH method were used to determine the distribution of the pore surface and pore volume. The volume and diameter of the pores were determined by the BJH method from the adsorption branch of the isotherm.

volume and diameter of the pores were determined by the BJH method from the adsorption branch of the isotherm. The BET multipoint linear regression method was used to The BET multipoint linear regression method was used to calculate the surface area using the p/p0 0.1–0.3 window and the available seven degrees of freedom (nine data points) [25].

### 4.3.4. Thermal Analysis

On the NETZSCH TG 209 F1 Libra thermogravimeter (Selb, Germany), the thermal conversion of unprocessed (air-dried) samples was carried out. Five mg of the sample was placed in an alumina vessel (volume 85 <sup>µ</sup>L) and heated from 20 ◦C·min−<sup>1</sup> to 1000 ◦C (1273 K). For analysis, the fraction with a grain size <0.1 mm was used. TG traces were recorded with air flow (20 cm<sup>3</sup> ·min−<sup>1</sup> ) with a resolution of 0.1 µg. Drying under vacuum or at elevated temperature was not applied [28].

### 4.3.5. Chemisorption of Hydrogen on Pt-Al-Sr Catalysts

Hydrogen chemisorption was carried out by means of an ASAP 2010C sorptometer (Micromeritics, Norcross, GA, USA). Samples had previously been reduced with H<sup>2</sup> at 400 ◦C (673 K) during 2 h. Then the samples were evacuated in a sorptometer at room temperature for 0.25 h, then at 350 ◦C (623 K) for 1 h. After 1 h, the samples were reduced with a hydrogen flow (2.4 L/h) at 350 ◦C (623 K) and degassed for 2 h at 350 ◦C (623 K). Hydrogen chemisorption studies were performed at 35 ◦C (308 K). The platinum dispersion was determined from the total amount of chemically adsorbed hydrogen. The following equation was used to calculate the metal surface area S [29]:

$$S = \frac{v\_m \cdot N\_A \cdot n \cdot a\_m \cdot 100}{22414 \cdot m \cdot wt} \left[ m^2 \cdot g\_{Pt}^{-1} \right] \tag{6}$$

where *vm*—volume of adsorbed hydrogen (cm<sup>3</sup> ), *<sup>N</sup>A*—Avogadro's number (6.022 <sup>×</sup> 1023 mol−<sup>1</sup> ), *n* = 1 is the chemisorption stoichiometry, *wt* (%)—the metal loading, *am*—the surface area (m<sup>2</sup> ), and *m*—the sample mass (g). The following formula was used to calculate the dispersion of the active phase:

$$D = \frac{\mathbb{S} \cdot \mathcal{M}}{a\_m \cdot \mathcal{N}\_A} \tag{7}$$

where *S* is the metal surface area, *M* is the platinum atomic weight, *N<sup>A</sup>* is Avogadro's number, and *a<sup>m</sup>* is the surface occupied by one platinum atom.

**Author Contributions:** Conceptualization, R.E.P. and E.R.-R.; methodology, R.E.P. and E.R.-R.; formal analysis, R.E.P. and E.R.-R.; investigation, R.E.P., E.R.-R. and B.S. data curation, B.S. and E.R.-R.; writing—original draft preparation, E.R.-R., R.E.P., B.S. and Z.O.; writing—review and editing, R.E.P., E.R.-R. and Z.O.; visualization, E.R.-R. and B.S.; supervision, R.E.P. and Z.O.; project administration, B.S.; funding acquisition, R.E.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** Research funded by the statutory funds of the Center for Advanced Technology AdamMickiewicz University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

