*Article* **Highly Efficient Self-Assembled Activated Carbon Cloth-Templated Photocatalyst for NADH Regeneration and Photocatalytic Reduction of 4-Nitro Benzyl Alcohol**

**Vaibhav Gupta 1, Rajesh K. Yadav 1,\*, Ahmad Umar 2,3,4,\*,†, Ahmed A. Ibrahim 2,3, Satyam Singh 1, Rehana Shahin 1, Ravindra K. Shukla 1, Dhanesh Tiwary 5, Dilip Kumar Dwivedi 6, Alok Kumar Singh 7, Atresh Kumar Singh <sup>7</sup> and Sotirios Baskoutas <sup>8</sup>**


**Abstract:** This manuscript emphasizes how structural assembling can facilitate the generation of solar chemicals and the synthesis of fine chemicals under solar light, which is a challenging task via a photocatalytic pathway. Solar energy utilization for pollution prevention through the reduction of organic chemicals is one of the most challenging tasks. In this field, a metal-based photocatalyst is an optional technique but has some drawbacks, such as low efficiency, a toxic nature, poor yield of photocatalytic products, and it is expensive. A metal-free activated carbon cloth (ACC)–templated photocatalyst is an alternative path to minimize these drawbacks. Herein, we design the synthesis and development of a metal-free self-assembled eriochrome cyanine R (EC-R) based ACC photocatalyst (EC-R@ACC), which has a higher molar extinction coefficient and an appropriate optical band gap in the visible region. The EC-R@ACC photocatalyst functions in a highly effective manner for the photocatalytic reduction of 4-nitro benzyl alcohol (4-NBA) into 4-amino benzyl alcohol (4-ABA) with a yield of 96% in 12 h. The synthesized EC-R@ACC photocatalyst also regenerates reduced forms of nicotinamide adenine dinucleotide (NADH) cofactor with a yield of 76.9% in 2 h. The calculated turnover number (TON) of the EC-R@ACC photocatalyst for the reduction of 4-nitrobenzyl alcohol is 1.769 <sup>×</sup> 1019 molecules. The present research sets a new benchmark example in the area of organic transformation and artificial photocatalysis.

**Keywords:** EC-R@ACC photocatalyst; NADH regeneration; 4-nitro benzyl alcohol; solar light; photocatalytic

#### **1. Introduction**

Solar light has emerged as a sustainable and greener energy source for various solar chemical synthesis reactions. In the past few years, solar light-induced chemical transformations have been extensively achieved by eco-friendly processes [1,2]. In this context, the enlargement of an artificial substitute for this smart system continues to be an extraordinary challenge in the chemical society [1–6]. The recent research, therefore, involves synthesizing

**Citation:** Gupta, V.; Yadav, R.K.; Umar, A.; Ibrahim, A.A.; Singh, S.; Shahin, R.; Shukla, R.K.; Tiwary, D.; Dwivedi, D.K.; Singh, A.K.; et al. Highly Efficient Self-Assembled Activated Carbon Cloth-Templated Photocatalyst for NADH Regeneration and Photocatalytic Reduction of 4-Nitro Benzyl Alcohol. *Catalysts* **2023**, *13*, 666. https:// doi.org/10.3390/catal13040666

Academic Editors: Indra Neel Pulidindi, Archana Deokar and Aharon Gedanken

Received: 10 January 2023 Revised: 19 March 2023 Accepted: 23 March 2023 Published: 29 March 2023

**Copyright:** © 2023 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/).

and designing a photoreactor system as a photocatalyst for the selective regeneration of fine chemicals and the reduction of aromatic compounds under solar light. As reported previously, we noted that about 4% and 46% of the overall solar light accessible on the planet falls in the UV and visible ranges, respectively [7,8]. Consequently, a solar light i.e., a visible light-responsive photocatalyst is significantly important for the synthesis of solar fine chemicals, such as nicotinamide adenine dinucleotide phosphate (NADPH) and nicotinamide adenine dinucleotide (NADH) cofactor and the photoreduction of organic compounds. Additionally, a significantly important feature to be noted is that in photocatalytic systems, NADH is important for the fixation of carbon dioxide (CO2). Therefore, the regeneration of the NADH cofactor and the reduction of the organic compound via a highly efficient pathway is the only way to make it economically and industrially feasible [9–12].

In this context, green chemistry and a related photoreactor, solar light, as one of the reactants of chemical synthesis, is a rising research area [13,14]. Solar light sponsored NADPH, NADH cofactor, and the photoreduction of organic compounds by various solar light harvestings materials, such as graphitic carbon nitride, graphene composites, and titanium oxide TiO2, were well discovered [15,16]. Besides the photoreduction and degradation in the presence of solar light, different chemical reactions, such as [2 + 2] cycloaddition [17,18], reductive dehalogenation [19], hydrogen formation from 1,4 asymmetric alkylation [20,21], and dihydropyridine [22], are also described in the reported literature where various solar light-responsive metal complexes are utilized as the solar light-assisted photocatalyst. In spite of the utmost expensive metal complexes, solar chemical conversion can also be attained by using energy harvesting materials as a solar light-responsive photocatalyst [23]. It is evident from the literature that photocatalytic NADH regeneration and organic transformation consuming solar light illumination are a rising research zone, which provides various possibilities for future work. In this addition, NAD+ is a bio-enzyme that needs a steady flux of solar light to photocatalyze a solar chemical synthesis [24]. Thus, solar light is essential in the chemical transformation reaction catalyzed by the enzyme. In the nonexistence of solar light and a photocatalyst, the NAD+ type enzyme remains completely inactive during the catalytic reaction. To date, various types of photo enzymes have been investigated, which are used as a photosystem for organic transformation and solar chemical regeneration [25]. It is supposed that most of the possible formerly existing solar light active enzyme derivatives were sorted out by progression and that nowadays, solar light active coenzymes are only the past survivors of this pathway [24]. The use of many metal-based compounds for solar chemical regeneration and environmental remediation has achieved the utmost attention in current times due to the active utilization of naturally existing solar energy and an effective solar light active system to terminate various types of unwanted materials [26,27]. Furthermore, photosynthetic pathways can increase the fast and complete conversion of solar energy into solar fine chemicals [28]. Expensive metals such as CdS, ZnO, and TiO2 are expensive materials utilized as a photocatalyst in various fields due to their good photocatalytic ability [29]. However, the key weakness of such types of photocatalysts is captured only in the ultraviolet (UV) quota of the solar light spectrum [30]. A diversity of semiconductor materials has been broadly utilized as light-harvesting photocatalysts by engineering or tuning the energy gap position and for effective use in the solar light spectrum [31]. Over the decades, expensive metal-based semiconductor materials have played an important role in solar light photocatalysis due to their narrow band gap and ionic conductivity, etc. [32,33]. The metal orbital along with a lone pair is combined with supporting materials, such as graphene, carbon activated cloth, and graphitic carbon nitride to generate a shift valence band (VB) and conduction band (CB) that tends to create the suitable band gap [34]. Among the metal-based photocatalysts, graphene has lately been utilized for the photocatalytic NADH/NADPH regeneration and conversion of organic substrates in polar and non-polar solvents under solar light illumination [35]. Expensive metals may exist as different crystalline phases along with scheelite tetragonal zircon tetragonal and scheelite-monoclinic [36]. It is a fact that the properties of the photocatalyst always depend on the nature of the crystal structure [37].

The monoclinic structure of a few expensive metals exhibits stronger photocatalyst ability under ultraviolet light illumination due to a small energy gap compared to other phasic structures [38]. Additionally, pure expensive metal has some restrictions, such as the rate of low absorption capacity of incident light, the fast recombination ability of solar light-created electrons, and a lack of pores in the structure [39]. It is of utmost importance to strengthen its solar light or solar spectrum absorption range ability and confine the recombination of solar light-generated holes and electrons in order to improve the ability of the photocatalyst under solar light illumination [40]. In this regard, many approaches have been designed by different researchers [16], such as n and p-type doping, fabricating a heterostructure solar light active system, and combining bias energy [41]. The combination of a few expensive metals with different types of porous materials, such as silica, alumina, glass, zeolites, and activated carbon cloth (ACC) is conducted to enhance the performance of the solar light absorption ability and overwhelm the charge carrier's recombination rate [42,43]. Among these, activated carbon cloth (ACC) has excellent physical and chemical properties to construct solar light active materials. The porous properties, structural stability, strong solar spectrum adsorption efficiency, and larger surface area of ACC enable healthier adsorption of substrates, making it a promising material that supports a photocatalytic procedure [44,45]. Thus, it permits the photocatalyst to absorb a solar spectrum that further leads to solar fine chemical production and the conversion of organic substrates [46,47]. A number of alterations have been described by many researchers on the solar light active catalyst surface using different supporting materials by various pathways of synthesis, such as sol-gel, co-precipitation, hydrothermal, solvothermal, and microwave synthesis [48]. Among these, the hydrothermal pathway is very easy to use to prepare different types of light-harvesting composites for solar chemical regeneration and organic transformations.

The solar chemical regeneration and photoreduction of NADH and organic compounds using solar radiation are developing new disciplines for future research. For the synthesis of new organic chemicals, the mechanism for the reduction of organic functionality is critical. To reduce organic functional groups, numerous methods have been reported, including (i) metal/acid reduction, (ii) photocatalytic reduction, (iii) electrolytic reduction, and (iv) catalytic hydrogen transfer [49]. One of the most prominent pollution control and disposal processes is the photocatalytic reduction of aromatic nitro compounds. Nitrogencontaining compounds are commonly created as by-products in a variety of industries and factories, including agrichemicals and pharmaceuticals. Among the many nitrogencontaining compounds, 4-nitrobenzyl alcohol is one of the most common by-products that is harmful to the environment [50,51].

4-amino benzyl alcohol (4-ABA) is made in the pharmaceutical industry by reducing 4-nitro benzyl alcohol (4-NBA). 4-ABA is a necessary precursor for the manufacture of a variety of drugs, including paracetamol, phenacetin, and acetanilide [52]. Metal-based photocatalysts in acidic conditions are utilized to reduce 4-NBA to 4-ABA to the greatest extent possible. However, such a procedure produces toxic metal oxide sludge that is harmful to the environment [52].

To address the aforementioned difficulties, a self-assembled metal-free self-assembled eriochrome cyanine R (EC-R) based ACC photocatalyst (EC-R@ACC) for the regeneration of NADH cofactors and the conversion of 4-NBA to 4-ABA is created. The synthesized metal-free EC-r@ACC photocatalyst has received a lot of interest in photocatalytic reactions because of its outstanding physicochemical properties, such as a suitable band gap, high molar extinction coefficient, low rate of intersystem crossing, excellent photocatalytic ability, easier synthesis, and excellent chemical stability. When compared to the metalbased photocatalyst, the metal-free self-assembled EC-r@ACC composite demonstrated significantly higher efficiency for NADH cofactor regeneration and the production of 4-ABA via artificial photocatalysis. Due to the utilization of environmentally acceptable and sustainable solar energy, artificial photocatalysis has sparked great interest in the synthesis of solar chemicals (NADH) and the reduction of 4-NBA to 4-ABA. A schematic

representation of the photocatalytic reduction of 4-NBA to 4-ABA and NADH regeneration under a solar light spectrum is represented in Scheme 1.

**Scheme 1.** Schematic representation of NADH regeneration and photocatalytic reduction of 4-NBA under solar light.

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

We introduced our study utilizing 4-NBA (a) as the substrate in the open air. The optimization of the photocatalytic reaction was performed in different solvents (Table 1). For photocatalytic optimization under different solvents, a low-cost, environmentally friendly EC-R@ACC (0.010 g) and 4-NBA (0.045 g) were utilized as a solar light spectrum harvester photocatalyst and starting material model substrate, respectively. The intended 4-ABA product was achieved with 50% conversion when the reaction was optimized in C2H5OH (49% yield) in 12 h. We screened the same reaction in different solvents: C2H5OH, PEG (Polyethylene glycol), CH2Cl2, and DMF under the same reaction conditions. DMF has the highest polar nature among the solvents, so it provides an excellent yield in 12 h.

We found that when the reaction was carried out with 4-NBA (0.045 g), the EC-R@ACC photocatalyst (0.010 g), and DMF (30 mL) at room temperature under a solar light spectrum for 12 h, the highest conversion (97%) and yield (96%) of 4-ABA was achieved. In contrast, the conversion and yield were reduced in different organic solvents as the reaction time increased, indicating that DMF is the most efficient at promoting the organic transformation reaction [53].

#### *2.1. Mechanistic Pathway for the Photoreduction of 4-NBA*

Based on the outcomes and the fiction, it is clear that the current photocatalytic reduction pathway includes several critical steps, involving: (I) the adsorption of the reacting molecules on the catalyst's surface, (II) the excitation of the EC-R@ACC photocatalyst to its triplet state and the transfer of electrons easily from sodium borohydride to the newly designed EC-R@ACC photocatalyst, (III) electron transfer from the EC-R@ACC photocatalyst to 4-NBA, (IV) the transfer of hydrogen from the BH4/solvent to 4-NBA, and (V) the desorption of the products from the edges of the newly designed photocatalyst. Scheme 2

depicts the likely mechanistic routes of the current photocatalytic conversion of 4-NBA to 4-ABA.

#### *2.2. Presence of Conformational Isomers during 1,4-NADH Synthesis*

As shown in Scheme 3, NAD+ is reduced directly for radical coupling and unselective protonation reaction. During this phase, many NAD isomers are formed, which have a presence in both enzymatically inactive and active states. To avoid the generation of enzymatically inactive isomers, an electron mediator must be utilized. Only when exposed to sun rays, the Rh-complex electron mediator supports the creation of the enzymatically solar light spectrum active 1,4-NADH isomer [54].

The reaction buffer medium for NADH regeneration contained 248 μL NAD<sup>+</sup> solution, 124 μL Rh-complex, 310 μL ascorbic acid (AsA), 2387 μL phosphate buffer, and 15 mg of the photocatalyst EC-R@ACC photocatalyst. Under continuous solar light irradiation cut by a 420 nm band-pass filter, the reaction was performed in a quartz cuvette as a reactor with a magnetic stir.

**Table 1.** Optimization of photocatalytic reduction of 4-NBA.


Reaction conditions: EC-R@ACC photocatalyst (0.010 g), 'a' (0.045 g), and NaBH4 (5 mg/L, various solvents (30 mL)) illuminated under a solar light spectrum for 12 h in an inert atmosphere at room temperature. (a) and (b) represent the reactant and product.

**Scheme 2.** Mechanistic studies of the photocatalytic reduction of 4-NBA to 4-ABA.

The cofactor of 1, 4-NADH rejuvenation was carried out in an inert environment at ambient temperature under the effect of sunlight (>420 nm). An FG@ACC photocatalyst (31 μL), AsA (310 μL), electron mediator (124 μL), and β –NAD+ (248 μL) were dissolved in 2387 μL of sodium phosphate buffer at neutral pH 7.0. First, the process was run in the absence of a solar light spectrum for 30 min, and no cofactor of NADH regeneration was achieved. The cofactor of 1,4-NADH regeneration was achieved in presence of solar light spectrum illumination, as illustrated in Figure 1. It was discovered that as the reaction time increased, so did the yield of 1,4-NADH. The absorbance at 340 nm in the UV-visible spectrum was used to calculate the amount of 1,4-NADH produced. As per a previous report [54], the molar absorption/extinction coefficient of the cofactor of 1,4-NADH is 6.22 mM−<sup>1</sup> cm−<sup>1</sup> [54]. We achieved 76.9% catalytic efficiency of 1,4-NADH in two hours (2 h) utilizing a highly stable and solar light active newly designed EC-R@ACC photocatalyst (shown in Figure 1). Because of the π-π interaction, the photocatalytic ability of the solar light active newly designed EC-R@ACC photocatalyst is greater than that of its precursor EC-R [40,54].

**Scheme 3.** The enzymatically active 1,4 NADH cofactor regeneration after NAD+ reduction.

**Figure 1.** The photocatalytic activity of the EC-R@ACC photocatalyst and EC-R for NADH regeneration under solar light.

#### *2.3. Reaction Mechanism of Photocatalytic NADH Regeneration*

Scheme 4 demonstrates a possible approach for photocatalytic regeneration of NADH using the EC-R@ACC photocatalyst. During the photocatalytic activity, the cationic type of the electron mediator (A) hydrolyzes, yielding a water-coordinated complex (B), symbolized as [Cp\*Rh(bpy)(H2O)]2+. The complex (B) interacts with the formate (HCOO<sup>−</sup>) during the hydride removal procedure [54]. This reaction generates the Rh hydride complex (C), i.e., [Cp\*Rh(bpy)(H)] +, and the release of CO2. When the EC-R@ACC photocatalyst contributes electrons to the complex of Rh, the reduced intermediate complex (D) is generated

(C). NAD<sup>+</sup> interacts with complex D via the activity of amide and transfer of hydride, due to which the NADH cofactor's region-selectivity regenerates.

**Scheme 4.** Photocatalytic NADH regeneration utilizing an EC-R@ACC photocatalyst mechanism.

#### *2.4. Solar Light-Induced Catalytic 1,4 NADH Regeneration*

We exclusively focused on regenerating the enzymatically active form of 1,4-NADH from the oxidized form of NAD+. As shown in Scheme 5, an electron mediator was used to prevent the transformation of undesirable isomeric forms, resulting in the artificial photocatalytic transformation of 1,4 NADH under sunlight irradiation. A neutral solution (pH 7.0) of phosphate-buffered solution (NaH2PO4–Na2HPO4, 0.1 M) and an NAD+ cofactor along with scavenger agents were used to regenerate NADH. In addition, combined with the recently synthesized EC-R@ACC photocatalyst, [Cp\*Rh(bpy)Cl]Cl was introduced to the reaction media.

Recycling experiments for NADH synthesis in the presence of EC-R@ACC photocatalyst were conducted by recycling the newly designed same photocatalyst several times under identical conditions to investigate the utility and sustainability of the EC-R@ACC photocatalyst under the same experimental conditions. During the reusability test, a nearly constant conversion yield was observed with no appreciable decline in efficiency, suggesting that the EC-R@ACC photocatalyst has strong catalytic strength [55]. Furthermore, the extra experiments were carried out under sunlight in the absence of NAD+. No absorbance peak was observed at a wavelength of 340 nm in this experiment, which suggests that NADH cannot be produced in the absence of NAD+ (Figure 1). Generally, every component of the artificial photosynthetic machinery is important, including solar light, EC-R@ACC, and NAD+. It should be noted that to eliminate the photo-saturation during the measurement of UV-Visible spectra, the concentration of the reaction media, which included AsA, NAD+, EC-R@ACC, and Rhodium complex, was kept quite low [54,55].

**Scheme 5.** A simplified potential energy diagram showing carrier generation and its migration in the photocatalytic system.

Scheme 5 shows the energy-labeled diagram, which depicts the pathway of the induced charge carriers in the photocatalytic system. Initially, on the irradiation of solar light, ascorbic acid becomes oxidized, and the electron of the EC-R@ACC photocatalyst is transferred from the valence band/highest occupied molecular orbital (HOMO) to the conduction band. Subsequently, the electron jumps from the conduction band/lowest unoccupied molecular orbital (LUMO) of the EC-R@ACC photocatalyst to the conduction band Rh-complex due to the lower band gap of the Rh-complex. Thus, the Rh-complex acts as an electron mediator. After the Rh-complex electron, the electron follows the same manner and easily jumps to the conduction band of NAD+. Here, NAD+ is reduced in NADH (Nicotinamide Dinucleotide Adenine Hydrate). In this photocatalytic process, after the reduction of NADH, the photocatalytic system is able to follow the Calvin cycle and mimic the natural photosynthetic route [54].

#### *2.5. Study of UV-Visible Spectra of Newly Designed Solar Light Spectrum Responsive EC-R@ACC Photocatalyst*

UV-Visible spectroscopy (UV-Visible-1900i, Shimadzu, Japan) was utilized to study the absorption spectrum of the ACC and EC-R@ACC in DMF (Figure 2). The UV-Visible spectra of the ACC were observed at about 250 nm [56], whereas the absorption band of the EC-R@ACC was observed at 545 nm. We estimated the optical band gap using the Scherrer equation (1240/λ) and found it to be 4.96 eV and 2.29 eV, respectively, indicating that it can operate as an active catalyst. The predicted optical band gap (2.29 eV) validates redshift and boosts solar-driven activation. The results showed that the EC-R and ACC absorb a lot of visible light. The absorption spectra of the newly designed EC-R@ACC photocatalyst in the bathochromic shift are most significant and are responsible for cofactor 1,4-NADH regeneration and organic transformation, which improves its solar light harvesting abilities/capabilities in the solar light spectrum region.

**Figure 2.** UV-Visible absorption spectra of the photocatalyst.

The optical band gap of the EC-R@ACC photocatalyst was computed using the Scherrer equation (1240/λ) [57], and it is close to 2.28 eV at about 540 nm. The cyclic voltammetry (CV, K-lyte electrochemical station,) measurement supports the computed optical band gap by the Scherrer method with the value of 2.20 eV [57]. The reduction and oxidation energy potential values of the newly designed EC-R@ACC photocatalyst were achieved by the CV measurement technique (Figure 3). The EC-R@ACC photocatalyst oxidation and reduction potential values were +1.10 V and −1.10 V, respectively. The collected reduction and oxidation energy potential data were utilized to calculate the energy gap/band gap.

The CV experiment authorizes the energy gap/band gap calculation (see Figures 3 and 4) [57]. A CV experiment was used to measure the reduction and oxidation energy potential values of the EC-R@ACC photocatalyst. The reduction and oxidation energy potentials were measured as +1.10 V and −1.10 V, respectively. The reduction and oxidation values gathered can be utilized to compute the band gap using the Latimer diagram (Figure 4), which verifies the optical band gap. Bathochromic shifts in the absorption spectra of the EC-R@ACC photocatalyst were detected, which boosts its ability to harvest sunlight.

**Figure 3.** Cyclic voltammetry of the EC-R@ACC photocatalyst.

**Figure 4.** Latimer Diagram displaying the photo-redox property of the EC-R@ACC photocatalyst; (\* represent the excited state.)

#### *2.6. Study of Zeta Potential of ACC and EC-R@ACC Photocatalyst*

The zeta potential (Malvern Panalytical, Nano-zetasizer (NZS90), Malvern, UK) of the EC-R@ACC photocatalyst was observed and showed an additional negative value of −40.1 mV, while the ACC showed a value of −23.7 mV (Figure 5) [58]. It is illustrated that the synthesis of the EC-R@ACC composite provides the more negatively charged fractions as it has a high content of EC-R. Additionally, the more negative zeta potential value for the EC-R@ACC photocatalyst proves that the interaction between the ACC and EC-R is quite good [59].

**Figure 5.** Studies of the (**a**) ACC (−23.7 mV) and (**b**) EC-R@ACC solar light spectrum photocatalyst (−40.1 mV) by Zeta potential pathway.

The FTIR spectra (Shimadzu, IRspirit FTIR-8000, Anan, Japan) of the EC-R@ACC photocatalyst, as well as the ACC and EC-R in Figure 6, demonstrated the occurrence of an interaction in the EC-R@ACC. Figure 6 indicates that the FTIR spectrum of the EC-R@ACC displays a stretching peak of approximately 3450 cm−1, confirming the existence of the -OH group [60]. The stretching peak of SO3 <sup>−</sup> is also found at about 1250 cm−<sup>1</sup> [61]. The stretching peak of –COONa is also found at about 950 cm−<sup>1</sup> [62]. The stretching peak of –CO is also found at about 1050 cm−<sup>1</sup> [40]. The stretching peak of –CH3 is also found at about 2850 cm−1; however, it is completely absent in the ACC FTIR spectra [61]. The results show that the interaction in the EC-R@ACC photocatalyst was formed successfully. Additionally, we recycled the EC-R@ACC photocatalyst for more than four consecutive runs (i.e., four reuses) under the same reaction circumstances (Figure 7). It was perceived that the photocatalytic cofactor 1,4-NADH regeneration is almost constant in all the recycles, confirming the highest solar light spectrum harvesting stability of the EC-R and EC-R@ACC photocatalyst, respectively. In addition, the observed results revealed that the EC-R@ACC photocatalyst possesses higher stability (Figure 7a) compared to the EC-R photocatalyst (Figure 7b), which clearly revealed that the EC-R@ACC is superior to the EC-R

photocatalyst. The turnover number (TON) of the EC-R photocatalyst for the reduction of 4-nitrobenzyl alcohol is calculated from the below-mentioned equation [63]:

TON = No. of substrate molecules converted into the product by 1 g of photocatalyst

**Figure 6.** Studies of the ACC, EC-R, and EC-R@ACC photocatalyst by FTIR technique.

**Figure 7.** The recycle stability for NADH regeneration by the (**a**) EC-R@ACC photocatalyst and (**b**) EC-R photocatalyst.

So, the calculated TON of the EC-R@ACC photocatalyst for the reduction of 4-nitrobenzyl alcohol is 1.769 × <sup>10</sup><sup>19</sup> molecules.

#### **3. Experimental Details**

#### *3.1. Materials and Chemicals*

The ACC, sodium borohydride (NaBH4), EC-R, N, N-dimethyl formamide (DMF), 4-nitro benzyl alcohol (4-NBA), sodium phosphate monobasic dihydrate (NaH2PO4·2H2O), sodium phosphate dibasic dihydrate (Na2HPO4·2H2O), nicotinamide adenine dinucleotide (NAD+), and 2,2 bipyridine (pentamethylcyclopentadienyl) rhodium (III) chloride dimer were purchased from Sigma Aldrich (Munich, Germany) and TCI (Portland, OR, USA).

#### *3.2. Synthesis of ACC*

Activated Carbon Cloth (ACC) was synthesized in the reported way. Carbon fabric (1 cm × 1 cm) was initially washed many times with acetone and distilled water. Following multiple washes, the carbon fabric was cured with conc. HNO3 at more than 90 ◦C for roughly 4 h. Following the acid treatment, the carbon fabric was thoroughly cleaned with distilled water and acetone. After washing, the freshly produced activated carbon cloth (ACC) was dried in a 70 ◦C oven [64].

#### *3.3. Synthesis of EC-R@ACC Photocatalyst*

Typically, 350 mg of carbon powder (graphene) and 150 mg of EC-R were mixed in 20 mL DMF and stirred for 2 h to ensure complete mixing. Then, the solution was autoclaved at 150 ◦C for 12 h (Figure 8). Furthermore, the solution was cooled to room temperature. Then, the solvent in the solution was evaporated at its boiling point. The obtained compound was thoroughly washed with distilled water 2–3 times. Finally, the newly designed EC-R@ACC photocatalyst was dried in the oven overnight at 100 ◦C. The amount of EC-R@ACC achieved was 203 mg [65].

#### *3.4. Synthesis of Rh-Complex*

The Rh-complex [Cp\*Rh(bpy)Cl]+ was prepared using a well-standard technique. In 5 mL distilled methanol, 0.025 g of rhodium compound ([Rh(C5Me5)Cl2]2) was dissolved in an N2-purged environment. The methanol solution was then mixed in a dark-incubated environment at room temperature with 0.013 g of 2,2 -bipyridyl (2 eq.) [15].

As soon as diethyl ether was added, a yellow precipitate formed. In an N2-purged environment, the complete product was received by the filtration method and dried at room temperature.

#### *3.5. Synthesis of 4-ABA*

The mixture of the EC-R@ACC photocatalyst (0.010 g), 4-NBA (0.045 g), and NaBH4 (5 mg/L) was prepared in 30 mL DMF in a glass vial and mixed with a magnetic stirrer. The reaction mixture was stirred at room temperature for 12 h in the presence of air under continuous high solar irradiation. The reaction mixture was then examined using TLC after it was completed. After filtering, the mixture was thoroughly washed with 50 mL of distilled water. The filtrate was concentrated using a rotary evaporator to abstract the final product. The compound's yield was 97.61% [52].

**Figure 8.** The schematic diagram for the synthesis of the EC-R@ACC photocatalyst.

#### **4. Conclusions**

Overall, with this support, we have explained that the newly designed photocatalyst is feasible for the regeneration of NADH and organic transformation under solar light. In this context, photochemically under solar light irradiation, the regeneration of NADH and the reduction of 4-nitrophenol with NaBH4 can be carried out using a metal-free ACC templated EC-R@ACC photocatalyst. The regeneration of NADH, as well as the reduction of 4-NBA into 4-ABA, was accomplished using an EC-R doped ACC photocatalyst (EC-R@ACC) in conjunction with artificial photosynthetic machinery. The EC-R@ACC photocatalyst demonstrated good maintenance of catalytic effectiveness during numerous cycles of photocatalytic reaction due to its great thermal and chemical stability. Most importantly, the EC-R@ACC, under continuous solar light irradiation, permits the effective regeneration of NADH cofactors with a yield of 76.9%. This research suggests that solar light could be used to produce more effective and cost-effective NADH regeneration along with photocatalytic reduction of 4-NBA and many more reductive processes.

**Author Contributions:** Conceptualization, V.G., R.K.Y., A.U. and R.K.S.; software, V.G., R.K.Y., A.U., A.A.I., S.S., R.S., R.K.S., D.T., D.K.D., A.K.S. (Alok Kumar Singh), A.K.S. (Atresh Kumar Singh) and S.B., validation, V.G., R.K.Y., A.U., A.A.I., S.S., R.S., R.K.S., D.T., D.K.D., A.K.S. (Alok Kumar Singh), A.K.S. (Atresh Kumar Singh) and S.B.; formal analysis and investigation, V.G., R.K.Y., A.U., A.A.I., S.S., R.S., R.K.S., D.T., D.K.D., A.K.S. (Alok Kumar Singh), A.K.S. (Atresh Kumar Singh) and S.B.; writing—original draft, V.G., R.K.Y., A.U. and R.K.S.; writing—review and editing, V.G., R.K.Y., A.U., A.A.I., S.S., R.S., R.K.S., D.T., D.K.D., A.K.S. (Alok Kumar Singh), A.K.S. (Atresh Kumar Singh) and S.B.; visualization, D.K.D. and S.B.; project administration, R.K.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are thankful to the Deanship of Scientific Research and supervision of the Centre for Scientific and Engineering Research at Najran University, Najran, Kingdom of Saudi Arabia for funding under the Research Centers funding program Grant No. NU/RCP/SERC/12/6.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are thankful to the Deanship of Scientific Research and supervision of the Centre for Scientific and Engineering Research at Najran University, Najran, Kingdom of Saudi Arabia for funding under the Research Centers funding program Grant No. NU/RCP/SERC/12/6.

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

#### **References**


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## *Article* **One-Pot Synthesis of Green-Emitting Nitrogen-Doped Carbon Dots from Xylose**

**Gabriela Rodríguez-Carballo 1,\*, Cristina García-Sancho 1, Manuel Algarra 2, Eulogio Castro <sup>3</sup> and Ramón Moreno-Tost 1,\***


**Abstract:** Carbon dots (CDs) are interesting carbon nanomaterials that exhibit great photoluminescent features, low cytotoxicity, and excellent water stability and solubility. For these reasons, many fields are starting to integrate their use for a variety of purposes. The catalytic performance of VOPO4 has been evaluated in the synthesis of nitrogen-doped carbon dots (N-CDs). The synthesis reaction was carried out at 180 ◦C using VOPO4 as a heterogeneous catalyst for 2 to 4 h of reaction time. After reaction, the N-CDs were purified using a novel method for the protection of the functional groups over the surfaces of the N-CDs. The morphological, superficial, and photoelectronic properties of the N-CDs were thoroughly studied by means of TEM, HRTEM, XPS, and photoluminescence measurements. The conversion of the carbon precursor was followed by HPLC. After three catalytic runs, the catalyst was still active while ensuring the quality of the N-CDs obtained. After the third cycle, the catalyst was regenerated, and it recovered its full activity. The obtained N-CDs showed a great degree of oxidized groups in their surfaces that translated into high photoluminescence when irradiated under different lasers. Due to the observed photoelectronic properties, they were then assayed in the photocatalytic degradation of methyl orange.

**Keywords:** carbon dots; doping; VOPO4; heterogeneous catalysis; xylose; acetic acid; hydrothermal method; photoluminescence; photocatalysis

#### **1. Introduction**

Carbon dots (CDs) are zero-dimensional particles normally smaller than 10 nm in diameter [1]. Their morphology is quasi-spherical and comprises a graphite-like core composed mainly of carbon, which can present inclusions of different adatoms such as N and S, and of a surface decorated by different organic functional groups. This superficial functionalization allows CDs to develop different surface trap states, lowering the energy band gap [2]. This is coupled with the fact that graphitic sp<sup>2</sup> structures favor the projection of the recombination of electrons and hole [3], thus promoting a possible transition HOMO-LUMO and explaining their outstanding electronic properties. As for their optical properties, these nanoparticles are widely recognized as highly photoluminescent systems, with tunable and up-conversion photoluminescence properties [3]. CDs' optical properties show enormous variation with size. An increment in size directly translates into a decrease in the energy gap between HOMO and LUMO. Therefore, a bathochromic shift to higher-wavelength emissions is observed. However, sp<sup>2</sup> clusters can act as auxochromes, decreasing the energy gaps and exhibiting a completely opposite effect to the formerly described size-gap relation [4]. As CDs are functionalized by a great variety

**Citation:** Rodríguez-Carballo, G.; García-Sancho, C.; Algarra, M.; Castro, E.; Moreno-Tost, R. One-Pot Synthesis of Green-Emitting Nitrogen-Doped Carbon Dots from Xylose. *Catalysts* **2023**, *13*, 1358. https://doi.org/10.3390/ catal13101358

Academic Editor: Ermete Antolini

Received: 5 July 2023 Revised: 29 September 2023 Accepted: 4 October 2023 Published: 10 October 2023

**Copyright:** © 2023 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/).

of groups, they exhibit surface state photoluminescence, considering that the functional groups on their surface have multiple energy levels that can interact and result in emissive traps, thus dominating the emissions. The surface state is a synergetic contribution of all groups conforming to the surface of the carbon dot that hybridize with the outer graphitic core [5]. The presence of different functional groups on the surfaces of the CDs not only control their electronic properties, but are the cause of their water stability and solubility. Since they are carbon-based materials, their cytotoxicity has been proven to be very low and more environmentally friendly than their analogues, metallic quantum dots. Due to this phenomenon, known as quantum confinement, along with their low cytotoxicity and excellent optical properties, CDs have numerous practical applications: bioimaging [6], light-harvesting [7], photocatalysts [8] and photosensitizers [9], bio- and chemosensors [10], and drug delivery [11].

Their outstanding optical features are very sensitive to minimal structural changes, including heteroatomic inclusions [12]. For that reason, one of the main methods of tuning the photoluminescent properties of CDs consists of doping the CDs with either one kind of element, such as N, P, or S, in different concentrations, or combining them to synthesize codoped CDs [13]. The nature and quantity of the dopant are key parameters for the improvement of the photophysical properties of CDs, as their electronic structure is modified by the presence of adatoms originating from *n* or *p* carriers [14]. Nitrogen is frequently used for doping CDs, as the variety of precursors that can be used is wide, from organic to inorganic compounds. These dopants can be added during synthesis or postsynthesis, and its presence can highly impact the quantum yield (QY) of CDs [15], increasing it to values as high as 26% when it acts as a codopant along with sulfur atoms [16] in water, and up to nearly 100% depending on the emission wavelength when using solvents different from water [17]. Doping [18], along with surface functionalization [19] and size [20,21], are the three main factors controlling the emission wavelengths of CDs. A high amount of N as the dopant [22] reduces the bandgap between valence and conduction bans, causing a bathochromic shift. The presence of several organic groups on the surfaces of CDs can be observed, as different contributions appear in the photoluminescence spectra. Each emission can be assigned to a different group [23,24]. CDs energy gaps are also strongly affected by size. As size increases, the bandgap decreases, red-shifting the emission [25].

Generally, for the synthesis of CDs, whether the carbon precursor is a green precursor [26] or a commercial one and whether the dopant is added during the synthesis or post-synthesis, the preferred method is hydrothermal synthesis. Although hydrothermal synthesis is, in general terms, a green synthetic process, many authors still rely on strong mineral acids for the production of CDs [27,28]. These mineral acids are corrosive, contaminant, and cannot be recovered or reused after reaction, generating streams that must be neutralized before disposal. This is why greener alternatives based on the use of solid catalysts are starting to be considered [26,29]. However, heterogeneous catalysis also faces a major issue. In terms of acidity, they present much lower values of Brønsted acidity, which is necessary for the major transformation of the precursors into CDs. In this sense, catalysts based on V, Nb, Sn, and Zr are widely used in processes that require strong Brønsted acidity [30–32].

The aim of this work is to synthesize CDs doped with N adatoms by means of heterogeneous catalysis. Thus, in this work, vanadyl phosphate (VOPO4) is proposed as a bifunctional catalyst that will promote the dehydration and aggregation of the carbon precursor while oxidizing the functional groups on the surfaces of N-CDs. With this approach to CD synthesis, we present a mineral acid-free hydrothermal alternative that replicates the conditions of real biomass-derived monosaccharides without compromising the purification, quality, or quantity of N-CDs, nor their photophysical or catalytic properties, and ensuring the recyclability of the catalyst.

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

*2.1. Characterization of the CDs*

2.1.1. Morphological and Superficial Study

The synthesis of CDs was carried out following a bottom-up approach, using xylose as a model biomass molecule, VOPO4 as a catalyst, and nitrogen as the doping adatom of the surface. Moreover, considering that the fractionation of lignocellulosic biomass yields a liquor of hemicellulosic sugars with an acidic pH, basically originating from acetic acid, this acid was added to the xylose solution.

It is important to note that the acetic acid was not sufficient to produce the CDs; therefore, the catalytic activity was only due to the presence of the solid catalyst. The morphology and size distribution (Figure S1) were studied by TEM (Figure 1) and HRTEM (Figure S2). The resulting images show that the N-CDs were effectively obtaining using VOPO4 as the catalyst. There was a substantial difference in the yield of CDs when reaction time was doubled, as in Figure 1a, the reaction was stopped after 2 h, and in Figure 1b, it was kept for 4 h. The reaction time did not affect the quality of the nanoparticles in any way, as the shape remained quasi-spherical after 4 h and the average size ranged between 3.5 and 4 nm in diameter for both reaction times (Figure 1), meaning that a great degree of size homogeneity was achieved. It was thus decided to carry out the synthesis of CDs for 4 h. For further confirmation of the presence of N-CDs in the solution after reaction, the graphite spacing was identified by means of HRTEM and is presented in the Supplementary Materials (Figure S2). When measured, the spacing value was 0.287 nm on average. The obtained XRD pattern is also presented as further confirmation of the graphitic nature of the cores of N-CDs (Figure S3), as a broad peak at 21◦ was detected, corresponding to the (111) plane of graphite [33].

**Figure 1.** TEM images of N-CDs solutions synthesized using 100 mg VOPO4 as the catalyst, 0.75 M of xylose as the C precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after (**a**) 2 h of reaction and (**b**) 4 h of reaction. Both samples were dialyzed prior to the analysis.

The acid properties of the catalyst were evaluated by means of the adsorption of pyridine coupled with FTIR spectroscopy (Figure S4). The TEM images confirm that the acidic properties of the catalyst are strong enough for the synthesis of the nanoparticles to be carried out. Due to the noise of the spectra and the fact that the catalyst was diluted in KBr, since its yellow coloration was blocking the IR beam, the concentrations of both the Brønsted and Lewis acid sites were not calculated. Thus, the spectra are provided as a qualitative approach to understand the nature of the acid sites of VOPO4, due to its importance regarding the acidity of the medium needed to carry out the synthesis of the CDs. Despite the noise that hindered the interpretation of the spectra of VOPO4, a band attributed to a strong Brønsted acidity site could be identified. A main band regarding the adsorption of pyridine on the catalyst was located at 1677 cm<sup>−</sup>1. This band was ascribed to the presence of strong Brønsted acid sites [34] and corresponded to the ν8a vibration mode of pyridinium species [35].

As the temperature rose, the band maintained a similar value of absorbance until, drastically, it almost disappeared at 125 ◦C. The Lewis acid sites were not detected, indicating that this catalyst can be considered as a Brønsted solid. Additional characterization for the VOPO4 catalyst (XRD pattern and Raman spectrum) is presented in the Supplementary Materials of this publication (Figures S5 and S6).

The surfaces of the N-CDs were analyzed by means of XPS analysis. We found that 45.6% of the surface was C (Table 1), as was expected due to the graphitic nature of the core. As can be observed in the deconvolution of the C1s core level (Figure 2a), there was a major contribution at 284.7 eV (Table 2) that corresponded to the C-C bond [36]. The existence of highly oxidized groups was doubly confirmed, on the one hand, by the high percentage of O present on the surface (47.7%), and on the other hand, by the next two bands in the deconvolution at 286.3 eV and 288.6 eV, which were associated with the C–N/C–O bond and the O=C–O type of bond, respectively [37]. The N1s core level was analyzed to determine the nature of the species included on the surfaces of the N-CDs (Figure 2b). The band at 401.3 eV (Table 2) made the greatest contribution. This band is usually attributed to graphitic N. The other band (399.4 eV) present in the deconvolution spectrum was related to a small amount of amine nitrogen [38,39].

**Table 1.** XPS mass concentration table of the surface of N-CDs synthesized using 100 mg VOPO4 as the catalyst, 0.75 M of xylose as the C precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction.

**N-CDs C (1s) O (1***s***) N (1s) Cl (2***p***)**

168


**Table 2.** XPS energy binding deconvoluted bands of the C1s core level and N1s core level of N-CDs synthesized using 100 mg VOPO4 as the catalyst, 0.75 M of xylose as the C precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction.

In order to support the XPS results and to confirm the presence of N inclusions and the carboxylic functionalization of the surface of CDs, the Fourier-Transform Infrared (FTIR) spectrum of the sample was recorded (Figure 3).

**Figure 3.** FTIR of dialyzed and lyophilized N-CDs synthesized using 100 mg VOPO4 as the catalyst, 0.75 M of xylose as the C precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction. Before measurement, the N-CDs were dialyzed, purified, and lyophilized.

The IR spectrum presented five main bands, located at 3118 cm<sup>−</sup>1, 3018 cm−1, 2802 cm<sup>−</sup>1, 1726 cm−1, and 1388 cm−1. The bands that appeared at 3118 cm−<sup>1</sup> and 3018 cm−<sup>1</sup> corresponded to the N–H vibration bands [40–42] of the two N species present on the CDs, amine/pyrrolic N and graphitic N, respectively. On the other hand, the 2800 cm−<sup>1</sup> band could be assigned to the vibration of the C–H bond and the 1388 cm−<sup>1</sup> band to its corresponding bending band. The signal at 1726 cm−<sup>1</sup> was associated with the C=O bond vibration band from carboxylic acids. Thus, this corroborated the XPS characterization data, as N was successfully included in the structure and functional groups, and the surfaces of the CDs were fully oxidized to carboxylic species.

#### 2.1.2. Optical Properties of the CDs

The photoluminescent properties were studied at two different wavelengths, 325 nm and 473 nm, in order to study the possibility of tuning their emission. There was a clear difference between the photoluminescence of the solution of N-CDs that was left for 2 h and that that was left for 4 h. The intensity of the spectrum doubled as time passed, indicating a higher concentration of species emissions [43]. Two maximums can be identified in the spectrum of N-CDs irradiated under 325 nm (Figure 4a) after 4 h at 484 and 563 nm; these can be associated with C–N-emitting species and O–C=O-emitting species, respectively [44].

**Figure 4.** Photoluminescence spectra under (**a**) 325 nm (**b**) 473 nm irradiation of dialyzed solutions of N−CDs synthesized from 100 mg VOPO4 as the catalyst, 0.75 M of xylose as the precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction. No dilution was performed prior to the analysis.

A third emission at 518 nm can be observed for the spectrum of the N-CDs after 2 h of reaction. This emission can be attributed to an intermediate species of C–O/C=O; after another 2 h, it fully oxidized into O–C=O.

When the N-CDs were irradiated under 473 nm (Figure 4b), the intensity of the spectra was lower, as a less energetic laser was used. two major contributions can be identified, at 566 nm and 625 nm, which can be associated with the formerly mentioned species. They suffered bathochromic shifts of 82 and 62 nm, respectively, proving the tunable photoluminescence properties of N-CDs [45–47].

The QY and fluorescence lifetime were also measured for both N-CDs, resulting in QYs of 2.3% when the synthesis lasted 2 h and 6.2% when it was left for 4 h. Fluorescence lifetimes of 2.59 ns for N-CDs and 3.04 ns for N-CDs were also observed when the reaction time was set at 2 h and when the reaction was maintained for 4 h.

#### 2.1.3. Photostability Tests

Since the designated application for CDs is to work as photocatalysts, it becomes clear that photostability is a key parameter that confirms whether N-CDs are suitable for a photocatalysis reaction. The photostability tests were carried out in the same conditions as the photoluminescence measurements, changing only the exposure time. Samples for photoluminescence typically undergo a 2 min exposure time in order for the spectra to be recorded. During photostability assays, the solutions were irradiated for 28 min, and spectra were recorded every 2 min. During this time, there was a slight variation in intensity (Figure 5) in the spectra of both lasers, but it was not significant enough to consider the samples unstable in the selected conditions for the photocatalytic tests.

#### *2.2. Catalyst Recovery, Recycling, and Regeneration*

After 4 h of reaction, the catalyst was recovered, calcined to eliminate the organic matter over its surface, and re-assayed for several catalytic runs. The catalyst also underwent a regeneration step, at which point it was decided that its catalytic performance had excessively decreased. The changes in the active surface of the catalyst were followed by XPS (Figures S7–S10, Tables S1 and S2).

**Figure 5.** Photostability tests of dialyzed solutions of N−CDs using 100 mg of VOPO4 as the catalyst, 0.75 M of xylose as the precursor, NH4Cl as the dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction under (**a**) a 325 nm irradiation laser and (**b**) a 473 nm irradiation laser. No dilution was performed prior to the analysis.

#### 2.2.1. Recycling Tests

After three reaction cycles, the conversion did not suffer an alarming decrease in the conversion of xylose (Figure 6a). However, the presence of particles per squared micrometer dropped substantially with every cycle, suggesting that the catalyst's selectivity to N-CDs diminished as it was used in the reaction (Figure 6b). A possible reason for the decrease in the number of CD particles could be attributed to the presence of carbonaceous species adsorbed on the catalyst surface (Table S1), as shown by the increase in the atomic concentration of C after the reaction. Since no reactivation procedure was conducted between each catalytic run, these species may have been blocking the acid sites necessary for the dehydration reactions. Additionally, a partial reduction in V(V) was observed after the reaction (Table S2), indicating a potential decrease in the acidity of the catalyst. This reduction in acidity can be attributed to the fact that V(V) is more acidic than V(IV), providing a clue to the mechanism behind the observed changes.

In the bottom-up mechanism of CD synthesis, the role of the VOPO4 catalyst was firstly to promote the dehydration of the xylose to furfural, which afterwards aggregated into higher structures that carbonized into N-CDs [43,48]. Secondly, the catalyst was able to oxidize the surfaces of the N-CDs. However, both the xylose and acetic acid dissolved in the reaction medium can act as reducing agents, promoting the rapid reduction of V(V) to V(IV), as confirmed via XPS (Table S2, before reaction). This partial reduction of vanadium could lead to leaching of the vanadium species, as it is a much more soluble species, especially in acidic media. Therefore, in order to avoid any contamination of the N-CDs with vanadium species, a purification step of the N-CDs was carried out. Thus, the pH was lowered to 2 after a reaction using a citrate/citric acid to ensure the protonation of the acidic groups on the surfaces of the N-CDs so they would not interact in any way with vanadium species such as VO2 <sup>+</sup> or VO2+. After the purification of the N-CDs by means of dialysis, the presence of vanadium in the sample inside the dialysis tube was analyzed via ICP-MS (Table S3), confirming, along with the absence of vanadium in the XPS survey spectra of the N-CDs (Figure S11), that whether the reaction lasted 2 h or 4 h, the purification method was effective, as no significant quantities of V were detected on the samples of the N-CDs solutions.

**Figure 6.** (**a**) Conversion of xylose in N-CDs solution samples after three catalytic runs of the VOPO4 catalyst. (**b**) Particle density per squared micrometer in N-CDs dialyzed solutions after 4 h of reaction time. No dilution was performed prior to the analysis.

#### 2.2.2. Regeneration Tests

After heating the used catalyst at 500 ◦C for the regeneration procedure, it was assayed for another catalytic run (Figure 7). The conversion of xylose attained in this new cycle was nearly the same as that obtained during the first catalytic run of the catalyst (blue bar). Nevertheless, this did not translate exactly into the same catalytic activity, as the ratio of particles per micrometer (red bar) achieved after the regeneration was higher than that obtained after the first recycling, but lower than the ratio reached when the catalyst was added fresh (Figure 7b). This can be attributed, again, to a partial blockage of the acidic sites due to residual carbonaceous species. Since, after regeneration, there is a reoxidation of V(IV) to V(V) a change in conversion or activity would not be due to a lack of V(V) species.

**Figure 7.** Conversion of xylose after the regeneration of the VOPO4 catalyst and particle density per squared micrometer in N-CDs dialyzed solutions after 4 h of reaction time. No dilution was performed prior to the analysis.

#### *2.3. Photocatalytic Assay*

Since the photoluminescence measurements revealed that the synthesized N-CDs had good photoelectronic potential, meaning that their surface properties would be optimal for electronic transfer and movement, it was decided to test their activity as catalysts in the photodegradation of methyl orange (MO) (Figure 8). The assay was performed under visible light. During the first 20 min of the reaction, the degradation was moderate, but after 30 min, the absorbance observed in the UV-Vis had greatly diminished. By the time the hour of reaction was reached, nearly no absorbance was detected, meaning that the CDs had effectively promoted the degradation of the colorant. The UV-vis absorbance spectrum of N-CDs was measured in order to ensure that no secondary absorbance to MO absorbance was taking place (Figure S12).

**Figure 8.** Photodegradation of methyl orange over time using 50 mg of N-CDs as the catalyst on an aqueous solution of 5 ppm of methyl orange.

#### **3. Materials and Methods**

#### *3.1. Materials*

Vanadium (V) oxide (V2O5) (>99.6%) and NH4Cl (>99.5%) were purchased from Sigma-Aldrich. Orthophosphoric (H3PO4) (>85%) and nitric acid (HNO3) (>65%) were purchased from Panreac (Barcelone, Spain) and VWR (Radnor, PA, USA). Xylose (>98%) was purchased from Millipore (Burlington, MA, USA).

#### *3.2. VOPO4 Catalyst Synthesis*

Briefly, the VOPO4 preparation was based on a previous existing method [49], in which 1.93 g of V2O5 is magnetically stirred along with 10.5 mL of H3PO4, 22 mL of water, and 2 mL of concentrated HNO3. The resulting suspension is kept for 2 h at 105 ◦C in reflux until the yellow precipitate is completely formed. Then, the vibrant yellow solid is filtered and left to dry in the stove overnight at 60 ◦C.

#### *3.3. CDs Preparation*

CDs were prepared following a hydrothermal procedure (Figure 9) that consisted of the addition of 100 mg of VOPO4 as the catalyst, 1.62 mL of CH3COOH, and 25 mL of 0.75 M xylose solution into a Teflon-lined steel hydrothermal reactor (Parr, Moline, IL, USA). As the ultimate target was to optimize the production for large-scale biomass

transformation of olive pits, in order to replicate the conditions of acidity of a biomassderived xylose solution, the expected amount of acetic acid that would be produced as a by-product when treating the biomass was added into reaction media. With the aim of doping the surfaces of the CDs to enhance their photocatalytic and photoluminescent properties, 5 g of NH4Cl was added into the hydrothermal reactor along with the rest of the components for the synthesis. The reaction temperature was set at 180 ◦C inside a muffle furnace in all cases, while the reaction time varied between 2 h and 4 h. Since the obtained N-CDs showed a high photoluminescence yield, it was decided that the synthesized N-CDs would be assayed in the photocatalytic degradation of methyl orange.

**Figure 9.** Scheme of the hydrothermal one-pot method for the synthesis of N-CDs from xylose as the carbon precursor, VOPO4 as the catalyst, and NH4Cl as the doping agent.

#### *3.4. CDs Purification*

After the reaction, the purification method involved a centrifugation step to separate the remaining carbonization solids and the majority of the catalyst; then, there was a second centrifugation step, along with 10 mL pH = 2 citrate-citric acid buffer. After centrifugation, the suspended solid particles of the catalyst were separated by filtrating the solution over 0.45 μm syringe filters. Before continuing with the purification process, after this filtering step, the photoluminescent emission of every sample was checked under a UV lamp (Electro DH, Barcelone, Spain) as a rapid way to confirm the success of the synthesis. Filtered solutions were then poured into dialysis membranes (Pur-A-LyzerTM 1 kDa, Sigma-Aldrich (St. Luis, MO, USA)), along with citric/citrate buffer solution. After 48 h, the samples were removed from the dialysis tubes, while keeping the rinsing water. To obtain solid N-CDs for XPS analysis and for their use in the photodegradation of methyl orange as catalysts, the samples were lyophilized after dialysis using Scanvac® Coolsave™ (Bjarkesvej, Denmark) lyophilizer equipment. After lyophilization, 10 mg of solid N-CD was recovered each time the procedure was carried out, so the average N-CD mass yield of the synthetic process rose to 0.36%.

#### *3.5. Catalyst Recovering*

The separated carbonaceous solid underwent a thermal treatment in order to remove all the organic residues masking the catalyst. The calcination of this solid was performed at 550 ◦C for 6 h, at a heating rate of 5 ◦C/min.

#### *3.6. Catalyst Recycling*

The calcined solid was then reused into further catalytic cycles in the same reaction conditions. As some of the catalyst was inevitably lost during the process of recovery and calcination, the recovered catalyst from two identical catalytic runs was evenly mixed for reuse.

#### *3.7. Catalyst Regeneration*

After three catalytic runs, the catalyst followed a regeneration step based on a previously published procedure [50], which involved the addition of H3PO4 along with HNO3 to a suspension in the rinsing water used for the dialysis of the catalyst after the reaction.

#### *3.8. Photocatalytic Assay*

For the photocatalytic assay, 50 mg of CD was added into 0.5 L of an aqueous solution consisting of 5 ppm of methyl orange dye. A photoreactor (Luzchem (Gloucester, ON, Canada) Model CCP-4V 220 V 50 Hz 3 A) was used to irradiate the samples with visible light, employing the fourteen white visible lamps inside the reactor. After 5, 10, 20, 30, 60, and 80 min, an aliquot of the solution was taken to control methyl orange (MO) photodegradation by determining its remaining concentration. After 5, 10, 20, 30, 60, and 80 min, an aliquot of the solution was taken to control the methyl orange (MO) photodegradation by determining its remaining concentration. Prior to the measurement of the sample, a calibration curve was performed using five standards of methyl orange of 1, 2, 3, 4, and 5 ppm.

#### *3.9. Characterization Conditions*

Transmission electron microscopy (TEM) was carried out in a FEITalos F200X (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an FEG 200 kV electron gun, four STEM detectors, and four FEG detectors. A Thermo Scientific-FEI Tecnai G2 20 Twin Transmission Electron Microscope equipped with LaB6 filament, tomography software, a cryo-transmission system, and an EDS/EDX (energy-dispersive X-ray spectroscopy) elemental analyzer (Thermo Fisher Scientific, Waltham, MA, USA) was used to evaluate the characteristics of the surfaces of the CDs. TEM and HRTEM images were processed using ImageJ v. 1.53k software. The photophysical characteristics of the CDs were studied by photoluminescence measurements using a LabRAM Odyssey PL microscope (Horiba, Kyoto, Japan). Samples were irradiated under two lasers: (1) 325 nm, ×40 lens, 100-hole aperture, 5% ND filter, 28 mW power. (2) 473 nm, ×40 lens, 100-hole aperture, 5% ND filter, 100 mW power. Photostability was measured using the same equipment and irradiation conditions over time, following a previously published study [51]. QY and the fluorescence lifetime were measured using an Edinburgh Instruments FLS920 fluorimeter coupled with a 1-M-1 integrating sphere for calculating the QY, and using the ultra-rapid F-G05 detector. XPS analysis was utilized to characterize the superficial composition of the CDs and the surface of the catalyst, VOPO4. It was performed by means of a Physical Electronics PHI5700 spectrometer using monochromatic Al Kα of 15 kV and 1486.6 eV, with a dual charge beam and a hemispheric multichannel detector for the VOPO4 and N-CDs spectra. The analysis zone comprised an area 100 μm in diameter when AlKα radiation was used. The constant pass energy mode was set at 29.35 eV. The obtained spectrum was processed using MultiPak v.9.3 software. The XPS analysis was carried out for the lyophilized samples in the case of N-CDs. All values were referenced to adventitious carbon (C 1s at 284.8 eV). FTIR was performed for N-CDs in a Bruker Vertex70 coupled with a Golden Gate Single Reflection Diamond ATR System (Bruker, Billerica, MA, USA). The spectral resolution was set at 4 cm−<sup>1</sup> in a spectral range of 4000–500 cm<sup>−</sup>1. The Raman spectrum of VOPO4 was measured using a FT-Raman RFT-6000-JASCO spectrometer (JASCO, Tokyo, Japan) with a 1064 nm laser at 150 mW of power. The XRD patterns were collected on a PANanalytical EMPYREAN (Malvern Panalytical, Malvern, UK) automated diffractometer. The PIXcel 3D detector was set at a step size of 0.017◦ (2θ). The diffractograms were

recorded between 4 and 70 in 2θ. The remaining xylose in the reaction samples was controlled using an HPLC instrument (JASCO, Tokyo, Japan) equipped with an autoinjector (AS-2055), which injected 6 μL of the sample into a Phenomenex (Torrance, CA, USA) Rezex ROA-Organic Acid H+ (8%) (300 mm × 7.8 mm) column. The mobile phase (0.0025 M H2SO4) was pumped by a quaternary gradient pump (PU-2089) at a 0.35 mL/min−<sup>1</sup> flow rate to the column, heated at 40 ◦C. The possible remains of vanadium species after the reaction were studied by means of ICP-MS in a Nexon 300D at a flow rate of 0.8 L/min of nebulizer gas, 18 L/min of gas for the plasma, and 1.2 L/min of auxiliary gas at a potential RF of 1600 W. The Brönsted–Lewis acidic sites were determined via pyridine adsorption measurements. The catalyst was shaped into wafers along with KBr and saturated in pyridine for 10 min at room temperature; then, it was gradually heated in a tubular oven until 125 ◦C, recording the spectrum each 20 to 30 ◦C. The adsorption and desorption spectra, at different temperatures, were recorded by a SHIMADZU (Kyoto, Japan) FTIR-8300 infrared spectrophotometer at a fixed irradiation wavelength and power of 632.8 nm and 0.5 mW, respectively. The photodegradation of MO was followed by UVvis spectrophotometry (UV 1800 SHIMADZU (Kyoto, Japan) UV). The UV-vis spectrum of N-CDs was recorded using the same equipment in a 300–900 nm range.

#### **4. Conclusions**

Green-emitting N-CDs were successfully obtained using heterogeneous catalysis via a hydrothermal synthetic procedure. Nevertheless, there is still work to be done as this procedure could be improved in terms of production yield. VOPO4 played a bifunctional role in the synthesis, while Brønsted acidity promoted the dehydration and condensation of the carbon precursor. Its oxidizing properties provoked a complete oxidation of the organic groups functionalizing the surface of the N-CDs, as was deduced from the XPS results and confirmed by photoluminescent emissions. The addition of a certain amount of CH3COOH in order to replicate the composition of a real biomass liquor positively affected the synthesis, as it enhanced the oxidizing properties of the catalyst. The VOPO4 catalyst was successfully recovered, recycled, and regenerated with no further negative effect on its performance in terms of the dehydration, aggregation, and oxidation of the surfaces of the N-CDs. CDs were doped with N atoms using NH4Cl, and the doping was confirmed by XPS. The doping of CDs to N-CDs greatly improved the photophysical properties of pristine CDs, as was observed in the photoluminescent measurements that affected the photocatalytic performance of the CDs in a positive way, as methyl orange was easily degraded at a high rate after 30 min of reaction. After 1 h of reaction, the degradation was considered to have been completed.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal13101358/s1, Figure S1: Size histograms of N-CDs synthesized using 100 mg VOPO4 as catalyst, 0.75 M of xylose as C precursor, NH4Cl as dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction calculated from TEM images; Figure S2: Graphitic spacing of N-CDs dialyzed solution samples synthesized using 100 mg VOPO4 as catalyst, 0.75 M of xylose as C precursor, NH4Cl as dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction; Figure S3: XRD pattern of dialyzed and lyophilized N-CDs synthesized using 100 mg VOPO4 as catalyst, 0.75 M of xylose as C precursor, NH4Cl as dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction; Figure S4: Pyridine adsorption and desorption on VOPO4 followed by FTIR; Figure S5: XRD pattern of freshly synthesized VOPO4; Figure S6: Raman spectrum of freshly synthesized VOPO4; Figure S7: XPS spectrum of V 2*p* core level binding energy of freshly synthesized VOPO4; Figure S8: XPS spectra of V 2*p* core level binding energy of (a) VOPO4 before reaction\* and (b) VOPO4 after reaction; Figure S9: Survey XPS spectrum of freshly synthesized VOPO4; Figure S10: Survey spectra of (a) VOPO4 before reaction\* and (b) VOPO4 after reaction; Table S1: XPS atomic concentration table of VOPO4; Table S2: XPS energy binding deconvoluted bands of VOPO4; Table S3: ICP-MS measurements of vanadium species in N-CDs-dialyzed solutions; Figure S11: XPS survey spectrum of dialyzed and lyophilized N-CDs synthesized using 100 mg VOPO4 as catalyst, 0.75 M of xylose as C precursor, NH4Cl as dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction; Figure S12: UV-vis spectrum of N-CDs dialyzed synthesized using 100 mg VOPO4 as catalyst, 0.75 M of xylose as C precursor, NH4Cl as dopant, and 17 g/L of CH3COOH at 180 ◦C after 4 h of reaction solution from 300 nm to 900 nm. No dilution was performed prior to the analysis. References [52–57] are cited in the Supplementary Materials.

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

**Funding:** This research was funded by the Spanish Ministry of Science and Innovation (PID2021- 122736OB-C42, PID2021-122613OB-I00) and FEDER (European Union) funds (PID2021-122736OB-C42, P20-00375, UMA20-FEDERJA88).

**Data Availability Statement:** No data is available.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **References**


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