*Article* **Iron-Modified Titanate Nanorods for Oxidation of Aqueous Ammonia Using Combined Treatment with Ozone and Solar Light Irradiation**

**Silviu Preda <sup>1</sup> , Polona Umek <sup>2</sup> , Maria Zaharescu <sup>1</sup> , Crina Anastasescu 1,\*, Simona Viorica Petrescu <sup>1</sup> , Cătălina Gîfu <sup>3</sup> , Diana-Ioana Eftemie <sup>1</sup> , Razvan State <sup>1</sup> , Florica Papa 1,\* and Ioan Balint 1,\***


**Abstract:** Sodium titanate nanorods were synthesized by a hydrothermal method and subsequently modified with an iron precursor. For comparison, Fe2O<sup>3</sup> nanocubes were also obtained through a similar hydrothermal treatment. Pristine, Fe-modified nanorods and Fe2O<sup>3</sup> nanocubes were suspended in diluted ammonia solutions (20 ppm) and exposed to ozone and simulated light irradiation. Ammonia abatement, together with the resulting nitrogen-containing products (NO<sup>3</sup> −), was monitored by ion chromatography measurements. The generation of reactive oxygen species (·OH and O<sup>2</sup> −) in the investigated materials and their photoelectrochemical behaviour were also investigated. Morphological and structural characterizations (SEM, XRD, XRF, UV–Vis, H<sup>2</sup> -TPR, NH<sup>3</sup> -TPD, PL, PZC) of the studied catalysts were correlated with their activity for ammonia degradation with ozoneand photo-assisted oxidation. An increase in ammonia conversion and a decreasing amount of NO<sup>3</sup> − were achieved by combining the above-mentioned processes.

**Keywords:** titanate nanorods; Fe-modified titanate; Fe2O<sup>3</sup> nanocubes; ammonia catalytic ozonation assisted by solar light

#### **1. Introduction**

The decontamination of waste waters, including the removal of nitrogen-containing pollutants (NH3), is mandatory in order to sustain the increased global demand of drinking water since natural resources are limited. Therefore, biological water treatments, in addition to advanced catalytic oxidation processes (AOPs) are pathways that are nowadays largely explored by research studies and already validated depollution technologies [1]. High-performance water treatment processes with a view to removing ammonia play a beneficial role in the environment because they contribute to reducing water acidification and eutrophication, contributing to the sustainable use of water resources. In the last decade, many ammonia removal processes have been studied: biological treatment [2], bio filtration treatment [3], air/steam stripping [4] break-point chlorination [5], chemical precipitation [6], ion exchange [7], photo and catalytic ozone oxidation [8,9].

Catalytic ozonation is an advanced oxidation process and has become greatly significant in recent years. During the ozonation process, catalysts favour the decomposition of O3, generating reactive oxygen species [10]. Many studies [8] indicated the efficient removal of NH<sup>4</sup> <sup>+</sup> by oxidation with ozone using oxide-based catalytic systems of transition metals MO<sup>x</sup> (M = Co, Ni, Mn, Sn, Cu, Mg and Al). MgO has a high catalytic activity but low selectivity to N<sup>2</sup> gas, while Co3O<sup>4</sup> has good selectivity to N<sup>2</sup> gas but a lower activity.

**Citation:** Preda, S.; Umek, P.; Zaharescu, M.; Anastasescu, C.; Petrescu, S.V.; Gîfu, C.; Eftemie, D.-I.; State, R.; Papa, F.; Balint, I. Iron-Modified Titanate Nanorods for Oxidation of Aqueous Ammonia Using Combined Treatment with Ozone and Solar Light Irradiation. *Catalysts* **2022**, *12*, 666. https:// doi.org/10.3390/catal12060666

Academic Editor: Fernando J. Beltrán Novillo

Received: 1 June 2022 Accepted: 15 June 2022 Published: 17 June 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/).

A promising, effective catalyst for NH<sup>4</sup> + removal, which is selective to nitrogen gas, was reported by Chen et al. [11] using a MgO/Co3O<sup>4</sup> (molar ratio 2:8) composite catalyst. Noble metals supported on metal oxides are also widely investigated and used for aqueous ammonia oxidation into safe gaseous compounds [12–15].

Clearly, many efforts have been made to sustain the implementation of depollution technologies that generate non harmful degradation end-products, but recently, many of these efforts also focused on the usage of green and regenerable energies, such as solar light. For instance, an increased number of photocatalysts were developed and tested in order to achieve the selective degradation of aqueous NH<sup>3</sup> into N<sup>2</sup> under UV and solar light irradiation [16–18]. Most of these are materials based on TiO2, displaying a large scale of morphologies and modifiers.

Since titanates with a 1D morphology were extensively and successfully investigated, both for their electronic and optical properties [19–23] it is reasonable to assume that ammonia degradation on these materials can bring significant advancements for environmental preservation. In order to favour the ammonia photodegradation under solar light irradiation, pristine titanate nanorods were modified with iron for the improvement of light absorption and separation of photogenerated charges. These were also compared with bare Fe2O3. Additionally, iron-based compounds are largely used for water depollution processes because they are non-toxic and cheap.

The aim of this work was to optimize the aqueous ammonia oxidation process in order to obtain a high ammonia conversion but also increase selectivity to gaseous nitrogencontaining products. This was successfully achieved by combining the ozone oxidation of aqueous ammonia with its photodegradative abatement under solar irradiation.

#### **2. Results**

#### *2.1. Catalysts Synthesis*

Sodium titanate nanorods were prepared starting from commercial TiO2, anatase). Then, 8 g of TiO<sup>2</sup> was homogenized for 30 min at 3000 rpm in 80 mL 10 M NaOH aqueous solution. The mixture was ultrasonicated for 40 min, and then a definite amount (18 mL) was transferred to the PTFE-lined pressure vessels (Parr Instruments, Moline, IL, USA), with a filling degree of 80%. The pressure vessel was kept at 175 ◦C for 72 h. The as-obtained mixture was dispersed in distilled water, ultrasonicated for 5 min, filtered to remove excess solution, then dried overnight at 100 ◦C.

The as-prepared nanorods (denoted TiR) were washed several times using ultrapure water slightly acidified with HCl (pH 6) and further modified with Fe, according to the following procedure: 0.1 g of titanate nanorods were suspended in 3 <sup>×</sup> <sup>10</sup>−<sup>2</sup> M FeCl3·6H2<sup>O</sup> solution and gently shaken for 24 h. The filtered powder was subjected to hydrothermal treatment in 1 M NaOH solution at 160 ◦C for 3 h. The sample is denoted as FeTiR.

Fe2O<sup>3</sup> nanocube synthesis follows the above-mentioned hydrothermal procedure, starting from 0.25 g FeCl3·6H2O and filtering, washing and drying at 80 ◦C. The sample is denoted as Fe2O3.

#### *2.2. Catalysts Characterisation*

#### 2.2.1. Scanning Electron Microscopy (SEM)

In order to identify the morphological properties of the materials of interest, SEM images were recorded in addition to EDAX analysis.

Figure 1a,b reveals a morphological similarity between the as-prepared sodium titanate nanorods and Fe-modified nanorods, their lengths ranging from tens of nanometers to micrometers. A slight loss of transparency, together with an incipient surface roughness, could be perceived in the FeTiR sample, relative to the as-prepared TiR sample. Additionally, EDAX spectra confirmed the presence of iron in the modified nanorods (1.84 wt%) and a smaller sodium amount (7.14 wt%) than for pure nanorods (12.27 wt%).

sample. Additionally, EDAX spectra confirmed the presence of iron in the modified nanorods (1.84 wt%) and a smaller sodium amount (7.14 wt%) than for pure nanorods (12.27

ters.

gated media.

exchange behaviour.

wt%).

(**c**) Fe2O3, as prepared.

**Figure 1.** SEM images and EDAX spectra: (**a**) TiR as prepared, (**b**) FeTiR, (**c**) Fe2O3 nanocubes. **Figure 1.** SEM images and EDAX spectra: (**a**) TiR as prepared, (**b**) FeTiR, (**c**) Fe2O<sup>3</sup> nanocubes.

Figure 1c shows well-defined Fe2O3 nanocubes with a narrow size distribution around 700–800 nm and smooth surfaces. Ma et al. [24] reported the obtaining of Fe2O3 microcubes and small nanoparticles depending on the hydrothermal treatment parame-

The identification of crystalline phases and elemental characterization was performed in order to explain the catalytic performance of the target materials in the investi-

The XRD pattern of the sample TiR, presented in Figure 2a (upper-side), presents typical reflections, which indicates the formation of sodium titanate with nanorods morphology, as also noticed by other groups [25,26]. The phase composition of the sodium titanate nanorods was identified as NaTi3O6(OH)·2H2O, according to PDF file no. 00-210- 4964. The crystal structure of NaTi3O6(OH)·2H2O was described as a layered structure, similar to Na2Ti3O7, belonging to monoclinic space group C2/m. The water molecules of crystallization rendered the structure more open, which is essential for cation-exchange behaviour [25]. The XRF measurement detected the Na/Ti weight ratio of 15/85. The deviation against theoretical ratio was a consequence of the thorough washing procedure after hydrothermal synthesis was completed and the vulnerability of this structure to cation-

Figure 1c shows well-defined Fe2O<sup>3</sup> nanocubes with a narrow size distribution around 700–800 nm and smooth surfaces. Ma et al. [24] reported the obtaining of Fe2O<sup>3</sup> microcubes and small nanoparticles depending on the hydrothermal treatment parameters.

#### 2.2.2. X-ray Diffraction (XRD) and X-ray Fluorescence (XRF)

The identification of crystalline phases and elemental characterization was performed in order to explain the catalytic performance of the target materials in the investigated media.

The XRD pattern of the sample TiR, presented in Figure 2a (upper-side), presents typical reflections, which indicates the formation of sodium titanate with nanorods morphology, as also noticed by other groups [25,26]. The phase composition of the sodium titanate nanorods was identified as NaTi3O6(OH)·2H2O, according to PDF file no. 00-210-4964. The crystal structure of NaTi3O6(OH)·2H2O was described as a layered structure, similar to Na2Ti3O7, belonging to monoclinic space group C2/m. The water molecules of crystallization rendered the structure more open, which is essential for cation-exchange behaviour [25]. The XRF measurement detected the Na/Ti weight ratio of 15/85. The deviation against theoretical ratio was a consequence of the thorough washing procedure after hydrothermal synthesis was completed and the vulnerability of this structure to cation-exchange behaviour. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 5 of 20

**Figure 2.** XRD pattern of (**a**) titanate nanorods: pristine TiR and FeTiR (**b**) Fe2O3. **Figure 2.** XRD pattern of (**a**) titanate nanorods: pristine TiR and FeTiR (**b**) Fe2O<sup>3</sup> .

Figure 2b (bottom side) presents the sample FeTiR, which contains sodium titanate nanorods submitted for the cation-exchange procedure. Sodium was partially exchanged

%. These results are in agreement with the EDAX elemental analysis. Besides the sodium titanate nanorods, no iron-based compounds were detected by XRD. Accordingly, the reasonable assumptions are: the amorphous phase of iron compounds located on the nanorod surface, a partial replacement of sodium by iron cations during the ion-exchange procedure (supported by the XRF results). Furthermore, a shifting of the (001) reflection to smaller 2θ, (for TiR relative to FeTiR sample, respectively, to a larger interlayer spacing (d-value) supports the ion-exchange approach (including sodium–proton exchange).

The XRD pattern of the sample Fe2O3 is presented in Figure 2b. Single-phase α-Fe2O3 (hematite) was identified in the sample, according to PDF file no. 00-033-0664. Even single-

phase hematite crystallizes, a ~5% amorphous phase can still be detected.

Figure 2b (bottom side) presents the sample FeTiR, which contains sodium titanate nanorods submitted for the cation-exchange procedure. Sodium was partially exchanged by iron, as the XRF measurements detected. The weight ratio Fe/Na/Ti was 6/10/84. The percentage of Fe relative to the overall composition, measured by XRF, was 2.9611 mass %. These results are in agreement with the EDAX elemental analysis. Besides the sodium titanate nanorods, no iron-based compounds were detected by XRD. Accordingly, the reasonable assumptions are: the amorphous phase of iron compounds located on the nanorod surface, a partial replacement of sodium by iron cations during the ion-exchange procedure (supported by the XRF results). Furthermore, a shifting of the (001) reflection to smaller 2θ, (for TiR relative to FeTiR sample, respectively, to a larger interlayer spacing (d-value) supports the ion-exchange approach (including sodium–proton exchange). *Catalysts* **2022**, *12*, 666 6 of 20

The XRD pattern of the sample Fe2O<sup>3</sup> is presented in Figure 2b. Single-phase α-Fe2O<sup>3</sup> (hematite) was identified in the sample, according to PDF file no. 00-033-0664. Even single-phase hematite crystallizes, a ~5% amorphous phase can still be detected. The XRD pattern of the sample Fe2O3 is presented in Figure 2b. Single-phase α-Fe2O3 (hematite) was identified in the sample, according to PDF file no. 00-033-0664. Even singlephase hematite crystallizes, a ~5% amorphous phase can still be detected. The structure parameters of the three samples are listed in Table 1.

The structure parameters of the three samples are listed in Table 1.


**Table 1.** The structure parameters. **Table 1.** The structure parameters.

#### 2.2.3. UV–Vis Spectroscopy 2.2.3. UV–Vis Spectroscopy

The optical properties of the materials were revealed by the recorded diffuse reflectance (DR) spectra, the light absorption characteristics of the catalysts being correlated with their photoactivity. The optical properties of the materials were revealed by the recorded diffuse reflectance (DR) spectra, the light absorption characteristics of the catalysts being correlated with their photoactivity.

Figure 3a illustrates a maximum absorption peak located at 260 nm, smaller for TIR and much higher for Fe-modified TiR sample. By enlarging the representation for TiR sample in Figure 3b, a small peak centred at 350 nm can be observed. This appears to be shifted to 390 nm and increased for the Fe-modified nanorods, which unlike the unmodified nanorods, also have a strong absorption in the visible range. Figure 3a illustrates a maximum absorption peak located at 260 nm, smaller for TIR and much higher for Fe-modified TiR sample. By enlarging the representation for TiR sample in Figure 3b, a small peak centred at 350 nm can be observed. This appears to be shifted to 390 nm and increased for the Fe-modified nanorods, which unlike the unmodified nanorods, also have a strong absorption in the visible range.

**Figure 3.** Comparative UV–Vis spectra are as follows: (**a**) Absorbance of TiR, FeTiR and Fe2O3 registeredin 250–1100 nm range. (**b**) Absorbance of TiR (multiplied 30 times), FeTiR and Fe2O3 (decreased 8 times) recorded in 330–1100 nm range. **Figure 3.** Comparative UV–Vis spectra are as follows: (**a**) Absorbance of TiR, FeTiR and Fe2O<sup>3</sup> registered in 250–1100 nm range. (**b**) Absorbance of TiR (multiplied 30 times), FeTiR and Fe2O<sup>3</sup> (decreased 8 times) recorded in 330–1100 nm range.

On the other hand, Fe2O3 emphasizes a broad light absorption, spanning on the whole spectral range. In the 750–1100 nm domain a broad absorption band can be observed, a similar but discrete shape is perceived in the inset of Figure 3b for the FeTiR sample. On the other hand, Fe2O<sup>3</sup> emphasizes a broad light absorption, spanning on the whole spectral range. In the 750–1100 nm domain a broad absorption band can be observed, a similar but discrete shape is perceived in the inset of Figure 3b for the FeTiR sample.

In conclusion, Figure 3a,b clearly shows the improvement of light absorption in the

Generally, photoluminescence measurements carried out using semiconductors are meant to act as photocatalysts that assess the photogenerated electron–hole pairs recombination and high PL signal, which indicates a reduced photocatalytic activity [27]. By modifying the photocatalyst and its encountering media, different photoluminescence

2.2.4. Photoluminescence Measurements

signals can be obtained.

In conclusion, Figure 3a,b clearly shows the improvement of light absorption in the visible range for Fe TiR nanorods relative to pristine TiRs.

#### 2.2.4. Photoluminescence Measurements

Generally, photoluminescence measurements carried out using semiconductors are meant to act as photocatalysts that assess the photogenerated electron–hole pairs recombination and high PL signal, which indicates a reduced photocatalytic activity [27]. By modifying the photocatalyst and its encountering media, different photoluminescence signals can be obtained.

For the TiR and FeTiR samples, Figure 4 shows the same PL emission maxima (λem = 357 and 425 nm), which slightly decrease for the Fe-modified nanorods in diluted ammonia solution. This indicates a beneficial photo-mediated interaction between the catalyst surface and the adsorbed reactant (NH<sup>4</sup> + ), which consumes part of the photo-generated charges, lowering their recombination. Consequently, the ammonia photodecomposition is expected to take place over the FeTiR sample. No significant PL signals were obtained for Fe2O3. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 7 of 20 charges, lowering their recombination. Consequently, the ammonia photodecomposition is expected to take place over the FeTiR sample. No significant PL signals were obtained for Fe2O3.

**Figure 4.** Room-temperature photoluminescence spectra of the powders suspended in water and diluted ammonia solutions, collected for λexc = 260 nm: (**a**) TiR, (**b**) FeTiR, (**c**) Fe2O3. **Figure 4.** Room-temperature photoluminescence spectra of the powders suspended in water and diluted ammonia solutions, collected for λexc = 260 nm: (**a**) TiR, (**b**) FeTiR, (**c**) Fe2O<sup>3</sup> .

#### 2.2.5. Reactive Oxygen Species Generation: Hydroxyl radical (·OH) and Superoxide Anion (O2−) 2.2.5. Reactive Oxygen Species Generation: Hydroxyl Radical (·OH) and Superoxide Anion (O<sup>2</sup> −)

Hydroxyl Radical (·OH)

Fe2O3 (**c**)

400 450 500 550 600

TiR 0 min TiR 5 min TiR 10 min TiR 15 min

**Wavelength (nm)**

**Intensity (a.u)**

Superoxide Anion (O2−)

spectrophotometric monitoring.

Hydroxyl Radical (·OH) According to our previously reported data [28], the generation of ·OH radicals was evaluated, taking into account the presence of fluorescent coumarin derivative (namely umbelliferone), caused by its interaction with photo-generated **·**OH radicals over the in-According to our previously reported data [28], the generation of ·OH radicals was evaluated, taking into account the presence of fluorescent coumarin derivative (namely umbelliferone), caused by its interaction with photo-generated ·OH radicals over the investigated materials.

vestigated materials. The characteristic broad peak is located around 470 nm and is depicted in Figure 5a,b. If ammonia photo-oxidation is mediated by ·OH radicals, this is likely to be caused by The characteristic broad peak is located around 470 nm and is depicted in Figure 5a,b. If ammonia photo-oxidation is mediated by ·OH radicals, this is likely to be caused by both TiR and FeTiR samples since they generate this oxygen species.

**Figure 5.** OH radicals trapping by coumarin under solar irradiation over TiR (**a**), FeTiR (**b**) and

In order to evaluate superoxide anion (O2<sup>−</sup>) generation, the XTT Formazan complex formation (due to the reaction of XTT with the photogenerated O2−) is evaluated based on

> 400 450 500 550 600 **Wavelength (nm)**

Fe₂O₃ 0 min Fe₂O₃ 5 min Fe₂O₃ 10min Fe₂O₃ 15 min

400 450 500 550 600

both TiR and FeTiR samples since they generate this oxygen species.

Fe TiR 0 min Fe TiR 5 min Fe TiR 10 min Fe TiR 15 min

**Intensity (a.u)**

TiR

TiR NH₄OH aq

300 350 400 450

**Wavelength (nm)**

both TiR and FeTiR samples since they generate this oxygen species.

**Figure 5.** OH radicals trapping by coumarin under solar irradiation over TiR (**a**), FeTiR (**b**) and Fe2O3 (**c**) **Figure 5.** ·OH radicals trapping by coumarin under solar irradiation over TiR (**a**), FeTiR (**b**) and Fe2O<sup>3</sup> (**c**).

charges, lowering their recombination. Consequently, the ammonia photodecomposition is expected to take place over the FeTiR sample. No significant PL signals were obtained

**Figure 4.** Room-temperature photoluminescence spectra of the powders suspended in water and

> 300 350 400 450 **Wavelength (nm)**

Fe₂O₃

Fe₂O₃ NH₄OH aq

2.2.5. Reactive Oxygen Species Generation: Hydroxyl radical (·OH) and Superoxide An-

According to our previously reported data [28], the generation of ·OH radicals was evaluated, taking into account the presence of fluorescent coumarin derivative (namely umbelliferone), caused by its interaction with photo-generated **·**OH radicals over the in-

The characteristic broad peak is located around 470 nm and is depicted in Figure 5a,b. If ammonia photo-oxidation is mediated by ·OH radicals, this is likely to be caused by

diluted ammonia solutions, collected for λexc = 260 nm: (**a**) TiR, (**b**) FeTiR, (**c**) Fe2O3.

300 350 400 450 **Wavelength (nm)**

a b c

Fe TiR

Fe TiR NH₄OH aq

#### Superoxide Anion (O2−) Superoxide Anion (O<sup>2</sup> −)

for Fe2O3.

ion (O2−)

Hydroxyl Radical (·OH)

vestigated materials.

In order to evaluate superoxide anion (O2<sup>−</sup>) generation, the XTT Formazan complex formation (due to the reaction of XTT with the photogenerated O2−) is evaluated based on spectrophotometric monitoring. In order to evaluate superoxide anion (O<sup>2</sup> −) generation, the XTT Formazan complex formation (due to the reaction of XTT with the photogenerated O<sup>2</sup> <sup>−</sup>) is evaluated based onspectrophotometric monitoring. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 8 of 20

> According to Figure 6a–c, there is no characteristic peak with maxima located at 475 nm for any of the investigated samples, only traces produced by Fe2O<sup>3</sup> after 15 min of irradiation. According to Figure 6a–c, there is no characteristic peak with maxima located at 475 nm for any of the investigated samples, only traces produced by Fe2O3 after 15 min of irradiation.

**Figure 6.** Investigation of O2<sup>−</sup> generation over TiR (**a**), FeTiR (**b**), Fe2O3 cubes (**c**) under simulated solar light irradiation. **Figure 6.** Investigation of O<sup>2</sup> − generation over TiR (**a**), FeTiR (**b**), Fe2O<sup>3</sup> cubes (**c**) under simulated solar light irradiation.

Consequently, it is reasonable to assume that the photogenerated electrons are not captured by O2 but possibly involved in H<sup>2</sup> production. Consequently, it is reasonable to assume that the photogenerated electrons are not captured by O<sup>2</sup> but possibly involved in H<sup>2</sup> production.

#### 2.2.6. Electrokinetic Potential Measurements 2.2.6. Electrokinetic Potential Measurements

lyst surface for all samples.

ionised water and diluted ammonia solution.

TiR aqFe TiR aq





**Electrokinetic potential (mV)**



0

TiR NH₄OH aq

Electrokinetic potential measurements can reveal the surface properties of the interest powders suspended in water and diluted ammonia aqueous solution. Electrokinetic potential measurements can reveal the surface properties of the interest powders suspended in water and diluted ammonia aqueous solution.

Figure 7 reveals the negative electrokinetic charges of all investigated samples. Nonetheless, a small difference in electrokinetic potential is observed for aqueous suspensions of titanate nanorods −49.54 mV and −45.24 for TiR and FeTiR, respectively. This could indicate more positive charges provided by the Fe presence in the titanate. Additionally, Fe2O3 cubes appear to be less negatively charged (electrokinetic potential being −29.2 mV). These above-mentioned values are clearly shifted toward the positive scale in the presence of ammonia. The difference is strongly related to the NH4+ adsorption on the investigated Figure 7 reveals the negative electrokinetic charges of all investigated samples. Nonetheless, a small difference in electrokinetic potential is observed for aqueous suspensions of titanate nanorods −49.54 mV and −45.24 for TiR and FeTiR, respectively. This could indicate more positive charges provided by the Fe presence in the titanate. Additionally, Fe2O3cubes appear to be less negatively charged (electrokinetic potential being −29.2 mV). These above-mentioned values are clearly shifted toward the positive scale in the presence of ammonia. The difference is strongly related to the NH<sup>4</sup> <sup>+</sup> adsorption on the investigated

surfaces. These data confirm the electrostatically driven adsorption of NH4+ onto the cata-

**Figure 7.** The electrokinetic potential of TiRs, Fe-modified TiRs and Fe2O3 cubes suspended in de-

Fe TiR NH₄OH aq

**Catalysts**

Fe₂O₃aq Fe₂O₃ NH₄ OH aq 1.5

2

2.5

**Intensity (a.u**)

3

3.5

4

surfaces. These data confirm the electrostatically driven adsorption of NH<sup>4</sup> <sup>+</sup> onto the catalyst surface for all samples. surfaces. These data confirm the electrostatically driven adsorption of NH4+ onto the catalyst surface for all samples.

**Figure 6.** Investigation of O2<sup>−</sup> generation over TiR (**a**), FeTiR (**b**), Fe2O3 cubes (**c**) under simulated

Consequently, it is reasonable to assume that the photogenerated electrons are not

0.5 0.7 0.9 1.1 1.3 1.5 1.7

> 400 450 500 550 **Wavelength (nm**)

Fe₂O₃ 0 min Fe₂O₃ 5 min Fe₂O₃ 15 min Fe₂O₃ 30 min

Electrokinetic potential measurements can reveal the surface properties of the inter-

Figure 7 reveals the negative electrokinetic charges of all investigated samples. Nonetheless, a small difference in electrokinetic potential is observed for aqueous suspensions of titanate nanorods −49.54 mV and −45.24 for TiR and FeTiR, respectively. This could indicate more positive charges provided by the Fe presence in the titanate. Additionally, Fe2O3 cubes appear to be less negatively charged (electrokinetic potential being −29.2 mV). These above-mentioned values are clearly shifted toward the positive scale in the presence of ammonia. The difference is strongly related to the NH4+ adsorption on the investigated

est powders suspended in water and diluted ammonia aqueous solution.

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 9 of 20

captured by O2 but possibly involved in H<sup>2</sup> production.

400 450 500 550 **Wavelength (nm)**

a b c

Fe TiR 0 min Fe TiR 5min Fe Tir 15 min Fe TiR 30 min

2.2.6. Electrokinetic Potential Measurements

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 8 of 20

irradiation.

TiR 0 min TiR 5 min TiR 15 min TiR 30 min

400 450 500 550

**Wavelength( nm)**

solar light irradiation.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

According to Figure 6a–c, there is no characteristic peak with maxima located at 475 nm for any of the investigated samples, only traces produced by Fe2O3 after 15 min of

**Figure 7.** The electrokinetic potential of TiRs, Fe-modified TiRs and Fe2O3 cubes suspended in deionised water and diluted ammonia solution. **Figure 7.** The electrokinetic potential of TiRs, Fe-modified TiRs and Fe2O<sup>3</sup> cubes suspended in deionised water and diluted ammonia solution.

#### 2.2.7. Photoelectrochemical Measurements 2.2.7. Photoelectrochemical Measurements

Current–potential dependence was registered for all samples under simulated solar light irradiation, before and after ammonia was added to the electrolyte. Current–potential dependence was registered for all samples under simulated solar light irradiation, before and after ammonia was added to the electrolyte.

The investigated powder suspensions were deposited by drop casting on transparent conductive oxide (TCO)-coated glass substrates. Figure 8 presents the plotted current density (µA/cm<sup>2</sup> ) versus potential (0–1 V) for the above-mentioned working electrodes under solar irradiation, before and after NH4OH was added to the electrolyte solution. Lower values of the registered photocurrent density and open circuit voltage (VOC) can be observed for the FeTiR relative to the TiR sample in both investigated media. Unlike the nanorods, the Fe2O<sup>3</sup> cubes (green curve in Figure 8a) generate a much higher photocurrent that is significantly lowered by the ammonia addition. The investigated powder suspensions were deposited by drop casting on transparent conductive oxide (TCO)-coated glass substrates. Figure 8 presents the plotted current density (μA/cm2) versus potential (0–1 V) for the above-mentioned working electrodes under solar irradiation, before and after NH4OH was added to the electrolyte solution. Lower values of the registered photocurrent density and open circuit voltage (VOC) can be observed for the FeTiR relative to the TiR sample in both investigated media. Unlike the nanorods, the Fe2O3 cubes (green curve in Figure 8a) generate a much higher photocurrent that is significantly lowered by the ammonia addition.

**Figure 8.** (**a**) J/V curves recorded for the investigated materials in pure electrolyte and diluted ammonia containing electrolyte under simulated solar irradiation; (**b**) Voc representation for the investigated samples. **Figure 8.** (**a**) J/V curves recorded for the investigated materials in pure electrolyte and diluted ammonia containing electrolyte under simulated solar irradiation; (**b**) Voc representation for the investigated samples.

PCE (power conversion efficiency), η, was determined using the following equation:

Figure 9 emphasizes the higher power conversion efficiency of the Fe2O3 sample in water that strongly decreases after the ammonia addition. On the contrary, the behaviour

of the nanorods is scarcely affected by ammonia addition.

JSCVOC FF Pin

PCE = η =

PCE (power conversion efficiency), η, was determined using the following equation:

$$\text{PCE} = \eta = \frac{\text{J}\_{\text{SC}} \text{V}\_{\text{OC}} \text{ FF}}{\text{P}\_{\text{in}}}$$

where: Jsc—short-circuit current density, Voc—open-circuit voltage, FF—fill factor and Pin—power of incident light on the working electrode.

Figure 9 emphasizes the higher power conversion efficiency of the Fe2O<sup>3</sup> sample in water that strongly decreases after the ammonia addition. On the contrary, the behaviour of the nanorods is scarcely affected by ammonia addition. Figure 9 emphasizes the higher power conversion efficiency of the Fe2O3 sample in water that strongly decreases after the ammonia addition. On the contrary, the behaviour of the nanorods is scarcely affected by ammonia addition.

**Figure 9.** Comparative power conversion efficiency of the investigated samples measured in pure and ammonia containing electrolyte. **Figure 9.** Comparative power conversion efficiency of the investigated samples measured in pure and ammonia containing electrolyte.

2.2.8. Temperature-Programmed Reduction Measurements (H2-TPR) 2.2.8. Temperature-Programmed Reduction Measurements (H2-TPR)

Hydrogen temperature-programmed reduction (H2-TPR) measurements were carried out to determine the redox behaviour of the involved chemical species of catalyst. As can be seen from Figure 10, the TiR sample is not significantly reduced in this temperature range. For the FeTiR and Fe2O3 samples, Figure 10 presents three similar but slightly shifted (to the higher temperature) peaks above 500 °C. These correspond to the successive reduction stages of Fe2O3 to Fe0. The shift of the maxima to the higher temperatures in the case of Fe-modified nanorods could be due to the different encountering of the Fe species in titanate and Fe2O3 network, respectively. Unlike the Fe-modified nanorods, Fe2O3 cubes generate a distinct (TPR) peak around 450 °C, which the previous literature assigns to the presence of iron hydroxide [29]. The next overlapped peaks from 500 to 600 °C can be ascribed to the process of Fe2O3→Fe3O4. The largest hydrogen consumption at a high tem-Hydrogen temperature-programmed reduction (H2-TPR) measurements were carried out to determine the redox behaviour of the involved chemical species of catalyst. As can be seen from Figure 10, the TiR sample is not significantly reduced in this temperature range. For the FeTiR and Fe2O<sup>3</sup> samples, Figure 10 presents three similar but slightly shifted (to the higher temperature) peaks above 500 ◦C. These correspond to the successive reduction stages of Fe2O<sup>3</sup> to Fe<sup>0</sup> . The shift of the maxima to the higher temperatures in the case of Fe-modified nanorods could be due to the different encountering of the Fe species in titanate and Fe2O<sup>3</sup> network, respectively. Unlike the Fe-modified nanorods, Fe2O<sup>3</sup> cubes generate a distinct (TPR) peak around 450 ◦C, which the previous literature assigns to the presence of iron hydroxide [29]. The next overlapped peaks from 500 to 600 ◦C can be ascribed to the process of Fe2O3→Fe3O4. The largest hydrogen consumption at a high temperature 600–800 ◦C can be assigned to the reduction of Fe3O<sup>4</sup> to metallic iron via wüstite (FeO) Fe3O4→Fe<sup>0</sup> [30].

perature 600–800 °C can be assigned to the reduction of Fe3O4 to metallic iron via wüstite

(FeO) Fe3O4→Fe0 [30].

**Figure 10.** Temperature-programmed reduction behaviour of the investigated samples.

**Figure 10.** Temperature-programmed reduction behaviour of the investigated samples.

The amount of H2 (µmol) consumed by each catalyst is listed in Table 2. It is clear The amount of H<sup>2</sup> (µmol) consumed by each catalyst is listed in Table 2. It is clear that TiR sample is not significantly reduced in this temperature range.


that TiR sample is not significantly reduced in this temperature range.


Fe2O3 1625 3092 4098 9352 18,167 In conclusion, in the FeTiR and Fe2O3 samples, the presence of Fe3+ as a single species can be identified using the TPR analysis. In the case of the Fe2O3 sample, besides hematite, the additional presence of an amorphous hydroxide phase is suggested. In fact, the XRD analysis indicates the presence of an amorphous phase (5%) for the Fe2O3 sample. The theoretical amount of H2 consumption needed for the complete reduction of Fe3+→Fe0 in pure Fe2O3 hematite is 18,750 μmol×gcat−<sup>1</sup>. The measured value was 18,167 μmol×gcat−1, which is close to the theoretical value. Concerning the synthesis procedure, the amorphous phase presence can be explained as follows. Despite the fact that the hydrothermal treatment applied for the obtaining of Fe2O3 cubes and FeTiR is similar, there is a significant difference regarding the overall synthesis procedure. The iron source of Fe2O3 nanocubes sample, obtained by hydrothermal synthesis, was FeCl3×6H2O in the aqueous solution. The sodium titanate nanorods were first subjected to an ion-exchange procedure In conclusion, in the FeTiR and Fe2O<sup>3</sup> samples, the presence of Fe3+ as a single species can be identified using the TPR analysis. In the case of the Fe2O<sup>3</sup> sample, besides hematite, the additional presence of an amorphous hydroxide phase is suggested. In fact, the XRD analysis indicates the presence of an amorphous phase (5%) for the Fe2O<sup>3</sup> sample. The theoretical amount of H<sup>2</sup> consumption needed for the complete reduction of Fe3+→Fe<sup>0</sup> in pure Fe2O<sup>3</sup> hematite is 18,750 µmol·gcat −1 . The measured value was 18,167 µmol·gcat −1 , which is close to the theoretical value. Concerning the synthesis procedure, the amorphous phase presence can be explained as follows. Despite the fact that the hydrothermal treatment applied for the obtaining of Fe2O<sup>3</sup> cubes and FeTiR is similar, there is a significant difference regarding the overall synthesis procedure. The iron source of Fe2O<sup>3</sup> nanocubes sample, obtained by hydrothermal synthesis, was FeCl3·6H2O in the aqueous solution. The sodium titanate nanorods were first subjected to an ion-exchange procedure in the presence of FeCl3·6H2O and were then hydrothermally treated.

> in the presence of FeCl3×6H2O and were then hydrothermally treated. 2.2.9. Temperature-Programmed Desorption Measurements (NH3-TPD)

2.2.9. Temperature-Programmed Desorption Measurements (NH3-TPD) The total acidity and distribution of the catalyst acid sites were calculated from the peak integration.

The total acidity and distribution of the catalyst acid sites were calculated from the peak integration. NH3-TPD were performed in order to study the surface acidity of the FeTiR catalyst by comparing with TiR and Fe2O3. According to Figure 11 and Table 3, NH3-TPD profiles of FeTiR, TiR and Fe2O<sup>3</sup> catalysts were mainly composed of three desorption zones at the

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 12 of 20

following temperatures: 50–300 ◦C (weak acid groups), 300–600 ◦C (moderate acid sites) and 600–800 ◦C (strong acid sites), respectively. following temperatures: 50–300 °C (weak acid groups), 300–600 °C (moderate acid sites) and 600–800 °C (strong acid sites), respectively.

NH3-TPD were performed in order to study the surface acidity of the FeTiR catalyst by comparing with TiR and Fe2O3. According to Figure 11 and Table 3, NH3-TPD profiles of FeTiR, TiR and Fe2O3 catalysts were mainly composed of three desorption zones at the

**Figure 11.** NH3-TPD profile. **Figure 11.** NH<sup>3</sup> -TPD profile.

**Table 3.** The total acidity and distribution of the acid sites of the TiR, Fe2O3 and FeTiR catalysts. **Table 3.** The total acidity and distribution of the acid sites of the TiR, Fe2O<sup>3</sup> and FeTiR catalysts.


The thermal stability of the nanorods reached up to 350 °C, and the comparative evaluation of all investigated sample was perfectly valid for this temperature, which was similar to the structure of catalysts in our catalytic media. High-temperature investigations are relevant for comparisons with the literature data. The thermal stability of the nanorods reached up to 350 ◦C, and the comparative evaluation of all investigated sample was perfectly valid for this temperature, which was similar to the structure of catalysts in our catalytic media. High-temperature investigations are relevant for comparisons with the literature data.

The total acidity of TiR sample increases after iron addition (FeTiR), while Fe2O3 does not absorb ammonia. Additionally, a shift of low-temperature desorption (from 349 to 253 °C) is observed after the Fe addition to TiR, indicating change in the surface acidity of the FeTiR catalyst. Wang et al. [31] showed that the NH3 species adsorbed at a low temperature T < 500 °C can be related to the presence of Brønsted acid sites, while adsorption at a high temperature (>600 °C) was correlated with the adsorption of NH3 species on Lewis acid sites. The total acidity of TiR sample increases after iron addition (FeTiR), while Fe2O<sup>3</sup> does not absorb ammonia. Additionally, a shift of low-temperature desorption (from 349 to 253 ◦C) is observed after the Fe addition to TiR, indicating change in the surface acidity of the FeTiR catalyst. Wang et al. [31] showed that the NH<sup>3</sup> species adsorbed at a low temperature T < 500 ◦C can be related to the presence of Brønsted acid sites, while adsorption at a high temperature (>600 ◦C) was correlated with the adsorption of NH<sup>3</sup> species on Lewis acid sites.

#### **3. Catalytic Assays 3. Catalytic Assays**

Prior to ozone exposure and light irradiation, the steady state of the catalytical system is achieved (the suspended catalyst powder in aqueous ammonia solution is stirred for 30 min). It is expected that the reactant adsorption on the catalyst surface (certified by the electrokinetic potential measurements) and the working pH of the diluted ammonia Prior to ozone exposure and light irradiation, the steady state of the catalytical system is achieved (the suspended catalyst powder in aqueous ammonia solution is stirred for 30 min). It is expected that the reactant adsorption on the catalyst surface (certified by the electrokinetic potential measurements) and the working pH of the diluted ammonia solution are around 10. For these parameters, equilibrium occurs between free ammonia (NH3) and ammonium ions (NH<sup>4</sup> + ).

#### *3.1. Ammonia Oxidation with Ozone 3.1. Ammonia Oxidation with Ozone*

(NH3) and ammonium ions (NH4+).

The following steps are presumably involved in aqueous ammonia catalytic oxidation with ozone: The following steps are presumably involved in aqueous ammonia catalytic oxidation with ozone:

solution are around 10. For these parameters, equilibrium occurs between free ammonia

− An equilibrium reaction (Equation (1)): − An equilibrium reaction (Equation (1):

$$\mathrm{NH\_4}^+ + \mathrm{OH}^- \rightleftharpoons \mathrm{NH\_3} + \mathrm{H\_2O} \tag{1}$$

− Direct oxidation with ozone, especially for low pH (pH < Pka): − Direct oxidation with ozone, especially for low pH (pH < Pka):

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$$\rm NH\_4^+ + \rm 3O\_3 \rightarrow NO\_2^- + H\_2O + \rm 3O\_2 + 2H^+ \tag{2}$$

− The reaction of ·OH radicals resulting from the decomposition of O<sup>3</sup> (Equation (3)) (pH > Pka): − The reaction of **·**OH radicals resulting from the decomposition of O3 (Equation (3)) (pH > Pka):

$$\text{6-OH} + \text{NH}\_3 \rightarrow \text{NO}\_2^- + \text{H}^+ + 4\text{H}\_2\text{O} \tag{3}$$

$$\text{NO}\_2\text{}^- + \text{O}\_3 \rightarrow \text{NO}\_3\text{}^- + \text{O}\_2 \tag{4}$$

$$2\text{ NO}\_2^- + 2\cdot\text{OH} \rightarrow \text{NO}\_3^- + \text{H}\_2\text{O} \tag{5}$$

In both cases (Equations (4) and (5)), the oxidation of NO<sup>2</sup> <sup>−</sup> to NO<sup>3</sup> − occurs rapidly due to the strong oxidizing agents. In both cases (Equations (4) and (5)), the oxidation of NO2<sup>−</sup> to NO3- occurs rapidly due to the strong oxidizing agents.

Figures 12 and 13 present the recorded catalytic results for the following working parameters: NH<sup>4</sup> + initial concentration: 20.00 ppm, starting pH: 10.2, catalyst amount: 0.15 g/L, reaction temperature: 25 ◦C, and reaction time: 3 h. Figures 12 and 13 present the recorded catalytic results for the following working parameters: NH4+ initial concentration: 20.00 ppm, starting pH: 10.2, catalyst amount: 0.15 g/L, reaction temperature: 25 °C, and reaction time: 3 h.

**Figure 12.** Reaction products distribution for aqueous ammonia oxidation with ozone and a photoassisted process (carried out for FeTiR catalysts). **Figure 12.** Reaction products distribution for aqueous ammonia oxidation with ozone and a photoassisted process (carried out for FeTiR catalysts).

The amount of converted NH<sup>4</sup> + in the absence of the catalyst (denoted as blank test in Figure 13) is low; a higher NH<sup>4</sup> + conversion for the catalytic oxidation of ammonia with ozone is recorded over the FeTiR catalyst. The main degradation product in the solution is nitrate, and the nitrite ions are not present. A rapid oxidation of NO<sup>2</sup> <sup>−</sup> to NO<sup>3</sup> − in the presence of ozone occurs.

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 14 of 20

**Figure 13.** NH4+ conversion for catalytic oxidation with ozone and combined photo-assisted degradation pathway (for FeTiR catalyst). **Figure 13.** NH<sup>4</sup> + conversion for catalytic oxidation with ozone and combined photo-assisted degradation pathway (for FeTiR catalyst). Taking these data into account, the FeTiR sample was tested in a combined degradation pathway: ozone and solar light irradiation. An increase in conversion was recorded

**blank test FeTiR TiR**

The amount of converted NH4 <sup>+</sup> in the absence of the catalyst (denoted as blank test in Figure 13) is low; a higher NH4+ conversion for the catalytic oxidation of ammonia with ozone is recorded over the FeTiR catalyst. The main degradation product in the solution is nitrate, and the nitrite ions are not present. A rapid oxidation of NO2<sup>−</sup> to NO3<sup>−</sup> in the Taking these data into account, the FeTiR sample was tested in a combined degradation pathway: ozone and solar light irradiation. An increase in conversion was recorded (Figure 13), together with a lower amount of NO<sup>3</sup> −. Based on these results, the increased selectivity of ammonia degradation to gaseous-nitrogen-containing end products is expected. (Figure 13), together with a lower amount of NO3−. Based on these results, the increased selectivity of ammonia degradation to gaseous-nitrogen-containing end products is expected.

#### presence of ozone occurs. *3.2. Photocatalytic Assays 3.2. Photocatalytic Assays*

presence of ozone occurs.

**+**

**conversion** 

0

40

**+**

Taking these data into account, the FeTiR sample was tested in a combined degradation pathway: ozone and solar light irradiation. An increase in conversion was recorded (Figure 13), together with a lower amount of NO3−. Based on these results, the increased selectivity of ammonia degradation to gaseous-nitrogen-containing end products is ex-The above-mentioned working parameters were preserved for the photocatalytic tests of the TiR, FeTiR, Fe2O<sup>3</sup> samples. Similarly, the comparative evaluation of the combined photocatalytic ammonia oxidation with ozone was performed on the FeTiR sample and is presented in Figures 14 and 15. The above-mentioned working parameters were preserved for the photocatalytic tests of the TiR, FeTiR, Fe2O3 samples. Similarly, the comparative evaluation of the combined photocatalytic ammonia oxidation with ozone was performed on the FeTiR sample and is presented in Figures 14 and 15.

30 Fe₂O₃ PHOTO TiR PHOTO **Figure 14.** Comparative NH4+ photodegradation for the investigated samples. **Figure 14.** Comparative NH<sup>4</sup> <sup>+</sup> photodegradation for the investigated samples.

10 20 **% NH4**FeTiR PHOTO After solar light irradiation of the system, the following steps are taken:

$$\text{Catalyst} + \text{hv} \rightarrow \text{e}^- \text{CB} + \text{h}^+ \text{VB}$$

$$\text{e}^-\text{C}\_\text{CB} + \text{O}\_2 \rightarrow \text{O}\_2\text{}^-\tag{6}$$

FeTiR PHOTO+O₃

$$\cdot \text{h}^{+} \text{v}\_{\text{VB}} + \text{H}\_{2}\text{O} \rightarrow \cdot \text{OH} \tag{7}$$

$$\text{h}^+\text{VB} + \text{OH}^-\text{surf} \rightarrow \text{OH} \tag{8}$$

0 50 100 150 200 250

**Time (min)**

After solar light irradiation of the system, the following steps are taken: Catalyst + hν→e−CB + h+VB According to the reported data [32], in the presence of oxygen, the following reactions are possible:

$$\text{4NH}\_3 + \text{3O}\_2 \rightarrow \text{2N}\_2 + \text{6H}\_2\text{O} \tag{9}$$

$$\text{2NH}\_3 + \text{2O}\_2 \rightarrow \text{N}\_2\text{O} + \text{3H}\_2\text{O} \tag{10}$$

$$2\text{NH}\_3 + 3\text{O}\_2 \rightarrow 2\text{NO}\_2^- + 2\text{H}^+ + 2\text{H}\_2\text{O} \tag{11}$$

$$2\text{NO}^-\text{}\_2 + \text{O}\_2 \rightarrow 2\text{NO}\_3^-\tag{12}$$

According to the reported data [32], in the presence of oxygen, the following reactions are possible: 4NH3 + 3O2→ 2N2 + 6H2O (9) 2NH3 + 2O2→ N2O + 3H2O (10) 2NH3 + 3O2→ 2NO2<sup>−</sup> + 2H+ + 2H2O (11) Figure 14 shows the highest NH<sup>4</sup> + conversion over the course of 180 min under solar irradiation and bubbled O<sup>2</sup> for the Fe-modified nanorods. The selectivity to gaseous nitrogen compounds is high (Figure 15) since the measured NO<sup>3</sup> − amount is very low and nitrite is also missing. By adding ozone to the photocatalytic system, an increased catalytic activity of this sample can be obtained. Figure 15 also emphasizes the improvement of NH<sup>4</sup> + conversion induced by ozone, adding to the photo-driven degradative process and reducing NO<sup>3</sup> − formation (1.72–0.93%).

2NO<sup>−</sup><sup>2</sup> + O2→2NO3- (12) Figure 14 shows the highest NH4+ conversion over the course of 180 min under solar irradiation and bubbled O2 for the Fe-modified nanorods. The selectivity to gaseous nitro-Figure 16 shows the nitrate yield versus time of the ozonation process and photooxidation process. Due to the fact that the gaseous reaction products cannot be separately quantified, the yield of NO<sup>3</sup> − formation was measured. The lowest value was registered for the ozone photo-assisted degradation of ammonia for the FeTiR sample. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 16 of 20

**Figure 16.** Yield of NO3- vs. time for ammonia ozonation in addition to photo-oxidation. **Figure 16.** Yield of NO<sup>3</sup> − vs. time for ammonia ozonation in addition to photo-oxidation.

Taking into account the previously mentioned structural and functional characterization of the investigated catalysts, the following can be concluded for the photodegrada-

− The photogenerated charges on the catalyst surface play a major role in ammonia

− The highest photocatalytic activity of the Fe-modified titanate nanorods can be re-

The sodium titanate nanorods (TiR), Fe-modified titanate nanorods (FeTiR) and Fe2O3 were prepared, starting from commercial TiO2, anatase (Aldrich), FeCl3×6H2O

In order to identify the morphological properties of the materials of interest, scanning electron microscopy (SEM) was used with a high-resolution microscope, FEI Quanta3 DFEG model, Brno, Czech Republic, working at 5 kV voltage, in high-vacuum mode with Everhart–Thornley secondary electron (SE) detector, coupled with energy-dispersive X-

Elemental analysis of the samples was carried out using a Rigaku ZSX Primus II spectrometer, Rigaku Corp., Tokyo, Japan with wavelength dispersion in vacuum atmosphere. The spectrometer was equipped with 4.0 kW X-ray Rh tube. The XRF results were analysed using EZ-scan combined with Rigaku SQX fundamental parameters software (standard less), which is capable of automatically correcting all matrix effects, including

Powder X-ray diffraction patterns were recorded using Rigaku's Ultima IV multipurpose diffraction system, Rigaku Corp., Tokyo, Japan, a Cu target tube (λ = 1.54060 Å) and a graphite (002) monochromator, with working conditions of 30 mA and 40 kV. The data were collected at room temperature between 3 and 75° in 2θ, with a 0.02° step size and a scanning rate of 1°/min. Phase identification was performed using Rigaku's PDXL

lated to the increased light absorption relative to the bare samples.

(Fluka), NaOH 97% (Alpha Aesar), and HCl 37.5% (Alfa Aesar).

4.2.2. X-ray Diffraction (XRD) and X-ray Fluorescence (XRF)

4.2.1. SEM—Scanning Electron Microscopy

*4.2. Structural and Functional Characterization of the Obtained Materials*

tive oxidation of the ammonia:

oxidation.

**4. Materials and Methods** *4.1. Synthesis of Materials* Synthesis of Materials

ray (EDAX) spectrometer.

line overlaps.

Taking into account the previously mentioned structural and functional characterization of the investigated catalysts, the following can be concluded for the photodegradative oxidation of the ammonia:


## **4. Materials and Methods**

#### *4.1. Synthesis of Materials*

The sodium titanate nanorods (TiR), Fe-modified titanate nanorods (FeTiR) and Fe2O<sup>3</sup> were prepared, starting from commercial TiO2, anatase (Aldrich), FeCl3·6H2O (Fluka), NaOH 97% (Alpha Aesar), and HCl 37.5% (Alfa Aesar).
