Ag-Based Catalysts in Different Supports: Activity for Formaldehyde Oxidation

Rached Ousji¹, Zouhaier Ksibi¹, Abdelhamid Ghorbel¹, Céline Fontaine²

¹Laboratory of Materials Chemistry and Catalysis, Faculty of Sciences of Tunis, University of Tunis El Manar, Tunis, Tunisia.
²Institute of Chemistry of Poitiers: Materials and Natural Resources (IC2MP), University of Poitiers, Poitiers, France.

DOI: 10.4236/ampc.2022.128012                

Abstract

Through the impregnation method, Ag catalysts with different support (such as TiO₂ and γ-Al₂O₃) were prepared and then tested for catalytic oxidation of formaldehyde (HCHO) at low temperatures. The Ag/TiO₂ catalyst exhibited strong catalytic performance, converting HCHO to CO₂ and H₂O at around 95°C. However, the Ag/Al₂O₃ catalysts showed much lower activity and reached 100% conversion at 125°C. The Ag-based catalysts were next characterized by several methods (XRD, TEM, FT-R, BET and H₂-TPR). Results of characterization revealed that support dramatically impacts the size and dispersion of Ag particles. The XRD analysis showed the existence of different peaks of the silver on the surface of Al₂O₃ in the contrast with TiO₂ no specific peaks exist. Therefore, the size of the Ag particles and their dispersion are the most important factors that affect their catalytic performance for formaldehyde oxidation. In terms of catalytic performance for HCHO oxidation, the Ag/TiO₂ catalyst possesses the best Ag dispersion, as well as the smallest Ag particle size.

Keywords

Formaldehyde Oxidation, TiO₂, Al₂O₃, Silver (Ag), Impregnation, Sol-Gel

1. Introduction

In the indoor environment, formaldehyde (HCHO) is a major source of indoor air pollution, which is harmful to human health. Exposure to HCHO over a long period of time can cause irritation in the eyes, deficits in spatial memory, allergic reactions and even cancer. In response to the increasing concern about this hazard, huge efforts have been made to eliminate indoor HCHO. The pollution caused by HCHO can be eliminated via a variety of methods, but catalytic oxidation gas has proven to be the most promising [1] .

There have been certain types of catalysts that showed higher catalytic activity for oxidizing HCHO. It has been shown that noble metals supported catalysts, such Pt/TiO₂ [2] , Pt–CeO₂/NAC [3] , Pt/MnO₂ [4] , Pt/AC [5] , Pt/Ni_x/a-AlOOH [6] , Pd/CeO₂ [7] , Pd/SBA-15 [8] , Pd/TiO₂ [9] , Au/SiO₂ [10] , Au/CeO₂ [11] _, Au/Fe₂O₃ [12] , Au/Co-Cе/Al [13] are active at room temperature for complete oxidation of HCHO to CO₂ and H₂O. Then, the high price of Pt, Au and Pd prevents their wide application and encourages studies of supported Ag-based catalysts, although it has a much lower price, it still shows considerable efficiency for oxidizing HCHO at low temperature. Lei et al. [14] prepared Ag/CeO₂ nanosphere catalysts by a one-step hydrothermal method and it displayed 110˚C as the complete oxidation temperature. According to a recent study, Ag/TiO₂ [15] could catalyze the 100% conversion of HCHO at 95˚C. Using Ag/MnO_x-CeO₂ catalysts (Tang et al., 2006 [16] [17] ), it was found that HCHO could be transformed into harmless H₂O and CO₂ at a temperature below 100˚C. In order to promote the catalytic activity of Ag-based catalysts, some factors have been investigated, including support, additives, and active oxygen species. For the oxidation of HCHO, Chen et al., (2011) [18] compared the catalytic activity of Ag-based catalysts with different support. In their study, they found that Ag/MCM-41 displayed the best properties of all the catalysts, while the support played a significant role in catalytic activity. It was found that adding additives to Ag-based catalysts could enhance their catalytic activity. Bai and Li (2014) [19] found that the HCHO (100 ppm) removal efficiency increased to 55% at room temperature and 100% at 70˚C over a 1.7 %K-Ag/Co₃O₄ catalyst. In a recent study, Chen et al. (2017 [20] ) found that single Ag could facilitate the activation of gaseous oxygen, thereby increasing the amount of active oxygen species that are beneficial to catalysis. The effect of pretreated condition investigated by Qin et al. (2017 [21] ) is shown that the pretreatment condition of O₂ at 500˚C followed by H₂ at 300˚C has more effective catalytic activity on the oxidation of toluene over Ag/SBA-15 due to the small and highly dispersed Ag nanoparticles. However, Titanium oxide and alumina, used as a catalyst and support, have been of wide interest to scientists. Both supports present a very developed surface having acid-base sites and high mechanical and thermal stability, enabling a significant dispersion of active phases, which gives it exceptionally fascinating properties in heterogeneous catalysis. TiO₂ and Al₂O₃ can be synthesized by a variety of methods, including the sol-gel method. Sol-gel has the potential to produce different structures of materials such as bulk, fibers, sheets, films, and particles at low temperatures [22] . It makes conceivable both the creation of materials with high purity and homogeneity and additionally the control of particle-sized distributions at a nano-scale level [22] . In this paper, we prepared the Ag-based catalysts with supports of TiO₂ and Al₂O₃ by sol-gel and impregnation method, the silver content in the catalyst was 5% by weight, and then compared their performance for the catalytic oxidation of HCHO at low temperature. The catalysts are characterized by temperature-programmed reduction (TPR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscope (TEM), fourier-transform infrared spectroscopy (FTIR) and their catalytic performance for HCHO oxidation was evaluated. The results were used to discuss and explain the activity of Ag-based catalysts.

2. Experimental

2.1. Preparation of Catalysts

All chemical reagents used to be of pure analytical grade. The TiO₂ with anatase structure and γ-Al₂O₃ powder were prepared by sol-gel method. Titanium (IV) isopropoxide (Ti[OCH(CH₃)₂]₄) formed a solution after 30 min of stirring absolute ethanol. Ethylacetoacetate (Eacac/Ti = 1, molar ratio) was added in the solution to control hydrolysis and condensation reaction rates. After 30 min of stirring an aqueous solution of nitric acid (HNO₃, 0,1 M) was introduced to the mix (HNO₃/Ti = 10, molar ratio), and stirring continued until a yellow gel formed. Then, the sample was dried, and the excess solvent was removed in an autoclave under the supercritical conditions, and calcined at 500˚C with ramping rate of 1˚C∙min⁻¹ under a flow rate 30 mL/min of oxygen. The same steps were used to synthesize the Al₂O₃ with Aluminum-tri-sec-butoxide (C₁₂H₂₇AlO₃) and 1-butanol, the uncolored gel formed was dried in different condition and calcined at 600˚C. The supported catalysts Ag/Al₂O₃ and Ag/TiO₂ were prepared by impregnation with an aqueous solution of silver nitrate (AgNO₃).

2.2. Characterization of Catalyst

BET surface area, pore volume and pore diameter of all samples were obtained from N2 adsorption isotherms using an ASAP 2020 micromeritics analyzer.

X-ray diffraction (XRD) measurements of catalysts were measured by using PANalytical X’pert PRO MPD diffractometer with a CuKα1 radiation (λ = 1.54060 Ǻ) and operated at 40 kV and 40 mA. The patterns were taken in the 2θ range of 10˚ - 70˚.

The programmed H₂ temperature reduction profiles (H₂–TPR) were carried out on chemisorption analyzer Autochem 2920 equipped with a TCD detector The TPR technique is based on the determination of hydrogen consumption for the reduction of metal oxides as a function of temperature. The temperature was in the range of 40˚C to 550˚C using a 5% H₂ /Ar gas mixture with a ramp of 5˚C/min and a flow rate of 30 ml/min.

Fourier transform infrared spectroscopy (FTIR) was used to identify types of molecular bonds in samples using the Perkin Elmer spectrometer. The samples were analyzed in the frequency range 400 - 4000 cm⁻¹ with an irradiating source in the IR medium.

2.3. Measurement of Catalytic Activity

The source of formaldehyde used during the tests, therefore, comes from the dissolution of paraformaldehyde at 50˚C overnight with stirring. The activity tests for the catalytic oxidation of HCHO (100 ppm) over the Ag/Al₂O₃ and Ag/TiO₂ catalysts (50 mg) were performed with a fixed-bed quartz flow reactor by passing a mixture gas of 19.5% O₂, 78% N₂, and 2.5% H₂O balance at a total flow rate of 100 cm³∙min⁻¹, within the temperature range of 50˚C - 350˚C. After reaching the steady state, the effluent gas was analyzed (MKS Multigas 2030) for HCHO conversion and CO₂ selectivity. The HCHO conversion was calculated by this equation:

$\text{HCHOConversion}\left(\%\right)=\frac{{\left[\text{HCHO}\right]}_{in}-{\left[\text{HCHO}\right]}_{out}}{{\left[\text{HCHO}\right]}_{in}}*100$

where ${\left[\text{HCHO}\right]}_{in}$ and ${\left[\text{HCHO}\right]}_{out}$ are HCHO concentration at the inlet and outlet, respectively.

The apparent selectivity for carbon dioxide, are calculated as presented by the following equation:

$\left(\text{Pi}\right)\text{Selectivity}\left(\%\right)=\frac{\left[\text{Pi}\right]}{{{\displaystyle \sum }}^{\text{}}\left[\text{Pj}\right]}*100$

where $\left[\text{Pi}\right]$ is the product concentration and $\left[\text{Pj}\right]$ is the sum of all reaction product concentrations.

The previous selectivity values will make it possible to calculate the yields of these products as shown on the equation:

$\left(\text{Pi}\right)\text{Yield}\left(\%\right)=\text{HCHOConversion}*\text{S}\left(\text{Pi}\right)$

3. Results and Discussion

3.1. Catalytic Test

The catalytic activities and the selectivity of by-products, of pure supports (TiO₂ and Al₂O₃) and the catalysts (Ag/TiO₂ and Ag/Al₂O₃) were evaluated for the HCHO oxidation as shown in Figure 1. It is evident that the most efficient catalyst for HCHO oxidation was the Ag/TiO₂, while the worst active catalyst was pure Al₂O₃ the order of activity observed was Ag/TiO₂ >> Ag/Al₂O₃ > TiO₂ > Al₂O₃. To more understand the effect of support modification, the catalytic properties of TiO₂ and Al₂O₃ without silver were studied. The results obtained showed that the pure supports have an average activity, but cannot completely convert HCHO into CO₂ and H₂O. The main by-products found it in the outlet of gases is the CH₃OH and CO with a low selectivity of dioxide of carbon (CO₂).

Figure 1. HCHO conversions (a), CO₂ yield (b), CO yield (c), CH₃OH yield (d) on TiO₂, Al₂O₃, Ag/TiO₂ and Ag/Al₂O_3. Reaction conditions: HCHO 100 ppm, catalyst mass 50 mg, O₂ 19.5 vol%, total flow rate 100 cm³∙min⁻¹, GHSV 84,000 h⁻¹

Zhang and all [15] proved that TiO₂ and Al₂O₃ showed no activities for HCHO oxidation in the range of temperature 35˚C - 125˚C. The study of Hang and al [23] showed that TiO₂ and hydrogenated TiO₂ have a little activity after 200˚C. According to these data, the catalytic activity of the supports synthesized by sol-gel method is significantly important. Therefore, the supports modification by impregnation of silver on the surface, results in an increase of catalysts performance as well as the selectivity of formaldehyde oxidation to CO₂. The Ag/TiO₂ catalyst has better oxidation activity, the complete HCHO conversion to CO₂ is achieved at 95˚C. Total elimination of formaldehyde over Ag/Al₂O₃ catalyst is exhibited at 125˚C. We can conclude that the addition of Ag can provide sufficient active sites for the HCHO oxidation reaction. Note that the efficiency of the Ag/TiO₂ catalyst in the conversion of HCHO at low temperature (particularly T₅₀) is better than some literature data Table 1.

3.3. Characterization of the Catalysts

Figure 2 displays the XRD patterns of TiO₂ and Ag/TiO₂ catalysts. Diffraction peaks at 25.44˚, 38.0˚, 48.3˚, 54.43˚ and 63.4˚ correspond to (101), (004), (200), (105) and (204) and were well matched of tetragonal anatase TiO₂ (JCPDS 01-071-1166). The XRD pattern of Ag/TiO₂ showed an increase in peaks intensities with no additional peak related to the silver. These results showed that Ag

Table 1. Catalytic activity of silver-based catalysts in the HCHO oxidation.

With GHSV: gas hourly space velocity.

Figure 2. X-ray diffraction patterns of: TiO₂ and Ag/TiO₂.

species were widely dispersed on TiO₂ support that may have been caused by the strong interaction metal-support [25] . XRD patterns of Al₂O₃ and Ag/Al₂O₃ catalysts are shown in Figure 3. Only the phase γ-Al₂O₃ was detected Al₂O₃ [26] . As for the Ag/Al₂O₃ the diffraction peaks of Ag₂O at 38.2˚ and 64.5˚ were observed, which assigned as (111) and (220) crystal planes. In addition, a peak of Ag was detected at 44.3˚ corresponded to the (200) lattice plane, which could be indexed to face-centered cubic phase [27] . The crystallite sizes of the synthesized catalysts were calculated using the Debye-Scherrer equation and are presented in Table 2.

Figure 3. X-ray diffraction patterns of: Al₂O₃ and Ag/Al₂O₃.

Table 2. Material properties of synthesized catalysts.

^aDetermined by BET, ^bDetermined by XRD.

The N₂ adsorption-desorption isotherms of Al₂O₃, Ag/Al₂O₃, TiO₂ and Ag/TiO₂ samples are presented in Figure 4(a) and Figure 4(b) show the isotherms of Al₂O₃ support and Ag/Al₂O₃ catalyst. Both solids belonged to type IV with a type H3 hysteresis, typically indicates the presence of closely packed slit pores [28] . As shown, in Figure 4(b) the isotherm of TiO₂ and Ag/TiO₂ samples resembled the type IV curves with H2 hysteresis loop, corresponding to mesoporous textures and in-bottle shaped. The Barrett-Joyner-Halenda (BJH) pore size distribution (Figure 5(a), Figure 5(b)) Al₂O₃, Ag/Al₂O₃, TiO₂ and Ag/TiO₂ catalysts indicating that all samples characterized by a monomodal size distribution. The Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore diameter obtained from N₂ adsorption-desorption measurement were showed in Table 2. The surface area of both TiO₂ and γ-Al₂O₃ supports gradually decreased after silver impregnation.

The optical analysis of FT-IR was used to determine the functional groups and vibration bonds in the samples in the range of 4000 - 400 cm⁻¹. Figure 6 shows the FT-IR of Al₂O₃ support and Ag/ Al₂O₃ catalyst. The absorption peak at 3480 cm⁻¹ is related to the vibration bonding of the hydroxyl groups OH. The peak of water molecules is formed at 1630 cm⁻¹. The vibrations peaks generated in the range of 700 - 500 cm⁻¹ correspond to Al-O-Al, which proves the existence of γ-form [29] . Figure 7 presents the FT-IR spectra of TiO₂ and Ag/TiO₂. The adsorption band in the range of 3800 - 3000 cm⁻¹ are assigned to the stretching vibrations of OH. The peak absorption at 1620 cm⁻¹ is ascribed to the adsorbed water molecules. The observed peaks at the range of 900 - 450 cm⁻¹ is associated with the vibration of Ti-O-Ti [30] [31] .

Figure 4. The curves of nitrogen adsorption-desorption isotherms of: Al₂O₃, Ag/Al₂O₃ (a) and TiO₂, Ag/TiO₂ (b).

Figure 5. Pore size distribution curves of: Al₂O₃, Ag/Al₂O₃ (a) and TiO₂, Ag/TiO₂ (b).

Figure 6. FT-IR spectrum of Al₂O₃ and Ag/Al₂O₃.

Figure 7. FT-IR spectrum of TiO₂ and Ag/TiO₂.

Transmission electron micrographs (TEM) of Ag/Al₂O₃ and Ag/TiO₂ catalysts are shown in Figure 8 and Figure 9. The image of Ag/Al₂O₃ showed a distribution of Ag particles with nonuniform shapes, the silver present in the form of huge agglomerates composed of large particle sizes (Table 1). In contrast, in Ag/TiO₂ it can be observed that the Ag particle sizes are much smaller, and the distribution was relatively uniform. The addition of silver in the surface of TiO₂ support caused some black spots with a hexagonal structure. The results of TEM indicate that the impregnation of silver is very affected by the support used.

H₂-TPR experiments were conducted to investigate the redox ability of Ag/Al₂O₃ and Ag/TiO₂ catalysts and the results are shown in Figure 10. No reduction peaks observed in the profiles reported for the pure supports TiO₂ and γ-Al₂O₃. Two peaks were detected for Ag/Al₂O₃, peak around 100˚C caused by large Ag₂O clusters and the peak around 350˚C is assigned to small Ag₂O clusters [32] . For Ag/TiO₂ the peak below 100˚C was ascribed to reduction of oxygen species absorbed on the dispersed Ag surface, and the second peak was due to the reduction of large Ag₂O groups [27] .

Figure 8. TEM images of Ag/Al₂O₃.

Figure 9. TEM images of Ag/TiO₂.

Figure 10. H₂-TPR profiles of Ag/Al₂O₃ and Ag/TiO₂ catalysts.

4. Conclusion

Ag/TiO₂ and Ag/Al₂O₃ were effective for the removal of formaldehyde (HCHO) leading to the selective production of carbon dioxide, unlike pure supports which release CO, and methanol as by-products. However, the activity of Ag/TiO₂ (100% conversion at 95˚C) was higher than Ag/Al₂O₃ (100% conversion at 125˚C). Characterization results indicated that Ag particle sizes and their dispersion in the surface of support are the important factors in the catalytic activity for formaldehyde oxidation. In summary, we have reported that the catalytic activity of supported Ag catalysts is dramatically influenced by the support morphology.

Author’s Contributions

All authors conceived and designed the study. Ousji Rached conducted the experiments, analyzed the data and wrote the paper. All authors contributed to manuscript revisions. All authors approved the final version of the manuscript and agree to be held accountable for the content therein.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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