# Effect of the amount of Ag on the performance of Ag/Ce-ZrO2 catalyst for the total oxidation of toluene

Rimeh Ismail, Jihene Arfaoui, Zouhaier Ksibi, Abdelhamid Ghorbel, Gérard Delahay

► To cite this version:

Rimeh Ismail, Jihene Arfaoui, Zouhaier Ksibi, Abdelhamid Ghorbel, Gérard Delahay. Effect of the amount of Ag on the performance of Ag/Ce-ZrO2 catalyst for the total oxidation of toluene. Journal of Chemistry Letters, 2024, 5 (4), pp.186-191. 10.22034/jchemlett.2024.471282.1218. hal-05315449

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Research Article

Journal of Chemistry Letters

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ISSN (online) 2717-1892 (print) 2821-0123

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# Effect of the amount of Ag on the performance of Ag/Ce-ZrO₂ catalyst for the total oxidation of toluene

Rimeh Ismail <fnref n="1" />,<fnref n="2" />,<fnref n="3" />* . Jihène Arfaoui <fnref n="1" />. Zouhaier Ksibi <fnref n="1" />. Abdelhamid Ghorbel <fnref n="1" />. Gérard Delahay <fnref n="4" />

ARTICLE INFO

Article history:

Received 2 August 2024

Received in revised form 4 September 2024

Accepted 5 September 2024

Available online 30 September 2024

Keywords:

Ag amount

2Ag/Ce-ZrO₂

Catalytic oxidation of Toluene

ABSTRACT

The oxidation of toluene, a volatile organic compound (VOC), plays a crucial role in reducing air pollution and promoting environmental sustainability. Catalysts based on silver have attracted considerable interest due to their exceptional catalytic capabilities, especially in the oxidation process of toluene. This study focuses on the development and optimization of silver-based catalysts Ag/CeZrO₂ for the catalytic oxidation of toluene. We examine how different silver loadings (0.5 wt. %, 1 wt. %, 2 wt. %, and 3 wt. %) influence the effectiveness of the Ag/CeZrO₂ catalyst. The Ag/Ce-ZrO₂ catalyst containing 2% Ag demonstrates the best performance at low temperatures, achieving 100% toluene conversion, primarily into CO₂, within the 300-550 °C temperature range.

## 1. Introduction

Environmental pollution is one of the most important problems that human beings face and due to the increase in population and the expansion of industries, the importance of controlling environmental pollution and preventing its exponential growth, is felt more than ever. [1] Toluene, a volatile organic compound (VOC), emitted from industrial activities poses significant environmental and health risks due to its toxicity and its role in the formation of photochemical smog. [2,3] Catalytic oxidation is a highly effective method to reduce these emissions because toluene is converted into less harmful substances, such as carbon dioxide and water, at lower temperatures compared to thermal oxidation. [4-7] Among the various catalysts studied, noble metal-based catalysts, especially those containing platinum (Pt) and palladium (Pd), show excellent activity and stability in the complete oxidation of toluene. [8-11] In addition, metal oxides [12,13], such as manganese oxide (MnO₂) and cerium oxide (CeO₂), supported on high surface area

materials, have shown promising results in enhancing catalytic performance.

Building upon our previous research efforts [14], this study focuses on the development and optimization of these catalysts for efficient and sustainable toluene oxidation. We investigated the effect of varying amounts of silver on catalyst performance. Silver, is known for its catalytic properties and cost-effectiveness, could potentially enhance the oxidation process when used in conjunction with other active components. By systematically studying the effect of silver content, our goal is to identify the optimal composition that maximizes catalytic activity and durability, thereby contributing to the development of more effective VOC emission control solutions.

## 2. Results and Discussion

### 2.1. X-ray diffraction (XRD)

In order to investigate the structural characteristics of the catalysts, X-ray diffraction (XRD) analysis was performed. Figure 1 shows the XRD patterns of the

<figure><img src="image_4.png" /></figure>

<footnote n="1">¹ Laboratory of Materials Chemistry and Catalysis (LCMC), University of Tunis el Manar, Tunisia</footnote>

<footnote n="2">²Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China</footnote>

<footnote n="3">³ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China</footnote>

<footnote n="4">⁴ICGM, University of Montpellier, ENSCM (MACS), CNRS, Montpellier, France</footnote>

<footnote>* Corresponding author. Tel. (+86)13999251709; e-mail: rimeh.ismail@fst.utm.tn / rimeh.ismail@ms.xjb.ac.cn
https://doi.org/10.22034/jchemlett.2024.471282.1218</footnote>

<footnote>This work is licensed under Creative Commons license CC-BY 4.0</footnote>
samples, indicating that all catalysts have good crystallinity. For pure zirconia, monoclinic and tetragonal phases of ZrO₂ (m-ZrO₂ and t-ZrO₂) were observed at specific angles, including 2θ = 24.38° (110), 28.26° (-111), 31.56° (111), 41.07° (102), 45.21° (-202), 54.09° (300), 55.60° (130), and 65.68° (320) [PDF 89-9066] for m-ZrO₂ and 2θ = 30.33° (101), 34.67° (002), 50.37° (112), 60.09° (211), and 62.57° (202) [PDF 79-1769] for t-ZrO₂. However, the XRD pattern of Ce-ZrO₂ aerogel oxide only shows the characteristic peaks of t-ZrO₂ phase. This suggests that cerium species are well-dispersed on the zirconia surface, contributing to the stabilization of its tetragonal phase, which is known for its metastability at low temperatures. For Ce-ZrO₂ sample, a slight decrease in the intensity of the tetragonal peak was observed, which may be due to the increase in lattice defects caused by the substitution of Ce⁴⁺ for Zr⁴⁺. XRD pattern analysis of xAg/Ce-ZrO₂ catalysts shows that the presence of different amounts of Ag does not affect the crystal structure of Ce-ZrO₂ support. However, for the 3% Ag sample, a low-intensity peak was observed at about 2θ = 38.1° (111), indicating the presence of trace amounts of metallic silver (PDF 65-2871). The crystal size of the material was determined using the Scherrer formula. [15] The values obtained for ZrO₂, Ce-ZrO₂, 0.5Ag/Ce-ZrO₂, 1Ag/Ce-ZrO₂, 2Ag/Ce-ZrO₂, and 3Ag/Ce-ZrO₂ are 10.6, 10.6, 7.8, 8.8, 15.3, and 15.3 nm, respectively. This indicates that the size of all samples is in the nanometer range and that their size is strongly affected by the silver content. The addition of a small amount of silver (< 1 wt. %) results in a reduction in the crystallite size of zirconia, while higher quantities of silver (≥ 2 wt. %) contribute to the growth of ZrO₂ crystals.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** XRD patterns of (a) ZrO₂, (b) Ce-ZrO₂, (c) 0.5Ag/Ce-ZrO₂, (d) 1Ag/Ce-ZrO₂, (e) 2Ag/Ce-ZrO₂ and (f) 3Ag/Ce-ZrO₂ samples.</figcaption></figure>

## 2.2. N₂-Adsorption-desorption at 77 K

Figure 2 (2A and 2B) shows the N₂ adsorption-desorption isotherms and the corresponding BJH pore size distribution curves of the samples. The texture parameters of the solids, including BET surface areas,

total pore volumes, and pore diameters, are summarized in Table 1. In particular, all the solids in Figure 2A exhibit type IV adsorption-desorption isotherms, which are characteristic of mesoporous materials with H1 or H2 type hysteresis loops according to the IUPAC classification. The presence of H2 type hysteresis observed in the supports (ZrO₂ and Ce-ZrO₂) indicates the presence of ink-pool pores in these materials. In contrast, the H1 type hysteresis observed in the catalysts (0.5Ag/Ce-ZrO₂, 1Ag/Ce-ZrO₂, 2Ag/Ce-ZrO₂, and 3Ag/Ce-ZrO₂) indicates that the materials consist of agglomerates or compacts of approximately spherical particles with relatively uniform arrangement.

Examining the pore size distribution curves of the samples (Figure 2B) shows that the addition of cerium and silver leads to an enlargement of ZrO₂ pores. As suggested by previous studies, this extension indicates a strong interaction between the support and the supported active material. [16-18] The results in Table 1 show that the surface area and total pore volume of the Ce-ZrO₂ support increase when small amounts of silver are added (0.5 and 1 wt% Ag), while those of the Ce-ZrO₂ support increase when silver content is higher (2 and 3 wt% Ag). The XRD results suggest that this change may be attributed to pore blocking in the 2Ag/Ce-ZrO₂ and 3Ag/Ce-ZrO₂ solids caused by Ag particles with larger crystallite sizes. In contrast, the improvement in the textural properties of the catalysts 0.5Ag/Ce-ZrO₂ and 1Ag/Ce-ZrO₂ is attributed to the deposition of Ag particles with smaller crystallite sizes on their surface. This observation is consistent with the results of Hoffmann M et al. [19], who reported that the effect of impregnation on surface area is influenced by two factors: an increase due to the formation of small CeO₂ particles inside the pores and a decrease due to the blocking of the pores by CeO₂.

## 2.3. DR UV-vis spectroscopy

The results obtained from DR-UV-Vis spectroscopy are shown in Figure 3. The spectrum of pure ZrO₂ has an absorption band in the range of 200-250 nm, which is attributed to the charge transfer of O²⁻ → Zr⁴⁺. [20,21] In the UV-Vis spectrum of Ce-ZrO₂, different absorption bands in the range of 230-260 nm and 280-350 nm correspond to the charge transfer transitions O²⁻ → Ce³⁺ and O²⁻ → Ce⁴⁺, respectively. Generally, the d-d transitions (4d10-4d95s1) of highly dispersed Ag⁺ ions occur around 212 and 226 nm, while the absorption of Agn⁶⁺ clusters usually occur at 290-380 nm. In our xAg/Ce-ZrO₂ samples, these bands overlap with the bands attributed to cerium species. Strong absorption bands characteristic of metallic silver clusters and aggregates appear at approximately 480 nm in the UV-Vis spectra of 2Ag/Ce-ZrO₂ and at approximately 520 nm in the UV-Vis spectra of 3Ag/Ce-ZrO₂. It is important to note that the largest quantity of Agn⁶⁺ clusters (290-380
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig.2.** (A) $N_2$ adsorption-desorption isotherms and (B) BJH pore size distribution curves of (a) $ZrO_2$, (b) Ce-ZrO$_2$, (c) 0.5Ag/Ce-ZrO$_2$, (d) 1Ag/Ce-ZrO$_2$, (e) 2Ag/Ce-ZrO$_2$ and (f) 3Ag/Ce-ZrO$_2$ samples.</figcaption></figure>

<table><caption>**Table 1.** Textural properties of the samples</caption><thead><tr><th>Sample</th><th>BET surface area (m<sup>2</sup> g<sup>-1</sup>)</th><th>Total pore Volume (cm<sup>3</sup>/g)</th><th>Average Pore diameter (Φpore, Å)</th></tr></thead><tbody><tr><td>ZrO<sub>2</sub></td><td>107</td><td>0.33</td><td>88</td></tr><tr><td>10Ce-ZrO<sub>2</sub></td><td>119</td><td>0.35</td><td>87</td></tr><tr><td>0.5Ag/Ce-ZrO<sub>2</sub></td><td>162</td><td>0.41</td><td>71</td></tr><tr><td>1Ag/Ce ZrO<sub>2</sub></td><td>140</td><td>0.39</td><td>121</td></tr><tr><td>2Ag/Ce-ZrO<sub>2</sub></td><td>73</td><td>0.31</td><td>127</td></tr><tr><td>3Ag/Ce-ZrO<sub>2</sub></td><td>55.8</td><td>0.25</td><td>177</td></tr></tbody></table>

nm) appear to be generated on the surfaces of solids with 2% Ag. These species are less numerous for samples containing 3% wt. Ag and probably absent on the surfaces of samples with lower Ag contents (0.5 and 1% wt Ag). This observation suggests a possible correlation between the amount of silver present and the type and abundance of silver species.

## 2.4. Catalytic tests

Figure 4 illustrates the oxidation of toluene $C_7H_8$ in the presence of water vapor (3.4 vol %) as a function of the

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** DRUV-Vis spectra of (a) $ZrO_2$, (b) Ce-ZrO$_2$, (c) 0.5Ag/Ce-ZrO$_2$, (d) 1Ag/Ce-ZrO$_2$, (e) 2Ag/Ce-ZrO$_2$ and (f) 3Ag/Ce-ZrO$_2$ samples.</figcaption></figure>

reaction temperature over the catalytic support and xAg/Ce-ZrO$_2$ catalyst. The temperatures corresponding to 10%, 50% and 90% toluene conversion (denoted as T10, T50 and T90) were used to compare the catalytic behavior of the materials. $ZrO_2$ and Ce-ZrO$_2$ materials previously reported [13] were shown in this paper only to be considered as reference materials. As shown in Figure 4, the reactivity of xAg/Ce-ZrO$_2$ catalysts is significantly higher than that of the $ZrO_2$ and Ce-ZrO$_2$ aerogel supports, increasing in the 300 °C to 500 °C temperature range as the silver content is increased up to 2 wt%. Specifically, 100% conversion of toluene to $CO_2$ is achieved at temperatures ≥ 300 °C and ≥ 400 °C for the 2Ag/Ce-ZrO$_2$ and 3Ag/Ce-ZrO$_2$ catalysts, respectively. This result demonstrates that 2 wt% Ag is the optimal silver content, leading to the creation of more active sites for the total oxidation of toluene. Additionally, the Ag-Ce interactions likely play a crucial role, promoting the dispersion of Ag and Ce active species, increasing oxygen availability, facilitating the mobility and activation of oxygen species, and enhancing the reactivity of the active sites in this reaction. A previous study by Guo Hao et al. [22] showed that the strong interaction between Ag and $Co_3O_4$ produces a synergistic effect, promoting the reducibility of the Ag/$Co_3O_4$ catalyst and the formation of abundant active lattice oxygen, thereby improving its reactivity in benzene oxidation. The results shown in Table 2 confirm the excellent reactivity of the 2Ag/Ce-ZrO$_2$ system in the complete oxidation of toluene to $CO_2$. Specifically, the temperature required to achieve 90% toluene conversion (T$_{90}$) follows this order: 330 °C for 2Ag/Ce-ZrO$_2$ < 373 °C for 3Ag/Ce-ZrO$_2$ < 475 °C for 1Ag/Ce-ZrO$_2$ < 494 °C for 0.5Ag/Ce-ZrO$_2$ < 511 °C for Ce-ZrO$_2$. The effect of silver content on the catalytic performance of the catalyst has been previously studied. [23-25] They demonstrated that the addition of Ag increased the number of oxygen species on the lattice surface, improved the conversion and migration of lattice oxygen, and effectively promoted the low-temperature activity of toluene oxidation. Compared with some literature reports [26,27] the 2Ag/Ce-ZrO$_2$ catalyst
exhibits high catalytic reactivity in the total oxidation of toluene, especially under challenging conditions: the presence of water vapor (3.4 Vol %), a high gas hourly space velocity (GHSV = 60,000 h⁻¹), and a low catalyst amount (m = 20 mg). This highlights the key role of the co-presence of silver and cerium species in improving the acidity, reducibility, oxygen mobility, and consequently, the catalytic behavior of the Ag/Ce-ZrO₂ system in the total oxidation reaction.

<table><caption>**Table 2.** Conversions of toluene at different temperatures over the catalysts</caption><thead><tr><th>Catalyst</th><th>T 10 (°C)</th><th>T 50 (°C)</th><th>T 90 (°C)</th></tr></thead><tbody><tr><td>ZrO2</td><td>469</td><td>550</td><td>-</td></tr><tr><td>Ce-ZrO2</td><td>327</td><td>436</td><td>511</td></tr><tr><td>0.5Ag/Ce-ZrO2</td><td>267</td><td>400</td><td>494</td></tr><tr><td>1Ag/Ce-ZrO2</td><td>242</td><td>330</td><td>475</td></tr><tr><td>2Ag/Ce-ZrO2</td><td>312</td><td>320</td><td>331</td></tr><tr><td>3Ag/Ce-ZrO2</td><td>200</td><td>320</td><td>373</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig.4.** (A) Toluene conversion and (B) CO₂ formation (ppm) over (a) ZrO₂, (b) Ce-ZrO₂, (c) 0.5Ag/Ce-ZrO₂, (d) 1Ag/Ce-ZrO₂, (e) 2Ag/Ce-ZrO₂ and (f) 3Ag/Ce-ZrO₂ samples.</figcaption></figure>

## 3. Experimental

### 3.1. Materials and Methods

Zirconium (IV) butoxide (Zr(OC₄H₉)₄, Sigma-Aldrich, 80%), anhydrous ethanol (C₂H₆O, Aldrich, 99.8%), ethyl acetoacetate (C₆H₁₀O₃, Fluka, >99.5%), Ce(NO₃)₃,6 H₂O

(Sigma-Aldrich, ≥98.5%), HNO₃ and AgNO₃ aqueous solutions were used in the experiment for the preparation of materials. Sol-gel and impregnation methods are used to prepare the materials, using similar procedure to that reported in our previous work [13,28-30]. We used the following techniques to characterize the samples. N2 adsorption-desorption at 77 K, X-ray diffraction (XRD) and UV-vis spectroscopy (UV-Vis).

### 3.2. Catalyst characterization

The textural properties of catalysts were examined by N2-Physisorption at 77 K, using a Micromeritics Asap 2000 Analyser. Before analysis, the sample was pretreated at 250 °C for 5 h. The phase composition of solids was identified by XRD technique on a Brüker AXS D8 diffractometer with CuKα radiation source. The XRD patterns were collected in 2θ range of 20°-70°, at a scanning rate of 5° min⁻¹ with a step size of 0.02°, using Cu Kα radiation (λ = 1.54 Å). The average crystallites size of solids was estimated using the Scherer equation:

$$D = \frac{0.90\lambda}{\beta \cos \theta}$$

Where D is the crystallites size (nm), λ is the wavelength (nm), β is the corrected full width at half maximum (radian) and Θ is the Bragg angle (radian). Diffuse reflectance UV-Vis spectra were obtained, in the 800-200 nm range, with a Lambda 40, Perkin Elmer spectrophotometer operating at room temperature.

### 3.3. Catalytic activity measurements

Toluene oxidation measurement was done, using similar experimental conditions to that reported in ref [13-15].

## 4. Conclusion

In this work, a new series of mesoporous xAg/10Ce-ZrO₂ catalysts, containing various amounts of silver (x = 0, 0.5, 1, 2, and 3 wt. %), were successfully prepared for the total oxidation of toluene. The results demonstrate that increasing the amount of Ag influences the crystallite size of the catalyst and the nature of the silver species. The addition of 2 wt. % Ag resulted in the most reactive catalyst, achieving 100% toluene conversion, predominantly into CO₂, within the 300-550 °C temperature range.

## Acknowledgements

This work was supported by the Ministry of Higher Education and Scientific Research (Tunisia) and the Laboratory of Materials Chemistry and Catalysis (LCMC), University of Tunis el Manar, Tunisia. All experimental procedures were performed at the Laboratory of Materials Chemistry and Catalysis
(LCMC). The catalytic test was performed at ICGM,
University of Montpellier, ENSCM (MACS), CNRS,
Montpellier, France.

Declaration of Competing Interest

The authors confirm that they have no known financial
or interpersonal conflicts that would have appeared to
impact the research presented in this study.

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