Effect of the preparation method on Au/Ce-Ti-O catalysts activity for VOCs oxidation

doi:10.1016/j.cattod.2007.12.141

Catalysis Today

Volume 137, Issues 2–4, 30 September 2008, Pages 367-372

https://doi.org/10.1016/j.cattod.2007.12.141 Get rights and content

Abstract

Studies concerning the preparation of gold phases dispersed on binary Ce-Ti oxide (Ce_0.3Ti_0.7O₂) were performed in order to elaborate catalysts for total oxidation of VOCs. Solids containing gold, cerium and titanium were synthesized by impregnation and deposition precipitation (DP) method using NaOH, Na₂CO₃ or urea as precipitant agent. These catalysts have been characterized by means of total surface area (BET), X-ray diffraction (XRD), diffuse reflectance ultra-violet–visible spectroscopy (DR/UV–vis) and temperature programmed reduction (TPR) and their reactivity towards the oxidation of propene was studied. Thus, it was revealed that the gold-based material prepared by DP method using urea as precipitant agent was the most efficient catalyst towards the total oxidation of propene. Based on the characterisation data, it has been shown that the preparation method has an effect on the catalytic activity.

Introduction

The increasing environmental awareness in the last two decades has prompted the emergence of stricter regulations covering automobile and industrial activities. Among these, the reduction of volatile organic compounds (VOCs) is particularly important because VOCs represent a serious environmental problem. The deep catalytic oxidation of these pollutants to carbon dioxide and water has been identified as one of the most efficient ways to destroy VOCs at low concentrations and to meet the increasingly stringent environmental regulations. In practice, the catalytic oxidation process requires heating large amounts of gas containing low concentrations of VOCs to the oxidation temperature. Therefore, highly active catalysts which work at lower temperatures are required.

In the last decade, it has been widely proved that it is possible to prepare gold nanoparticles deposited on metal oxide supports [1], [2], which exhibit high catalytic activity towards oxidation reactions [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Supported gold catalyst on titania is often studied since its efficiency for the CO oxidation at room temperature are evidenced [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. This activity depends on different parameters related to their preparation method [7] and the nature of the support [8]. Moreover, gold supported on cerium oxide has been shown to possess high activity for VOCs oxidation [11], [12], [13], [14], [15]. Ceria has been widely used in catalysis to purify vehicle exhausts and becomes the most rare earth oxide for controlling pollutant emission. It is known that ceria CeO₂ increases the dispersion of active components and its most important property is to serve as an oxygen reservoir which stores and releases oxygen via the redox shift between Ce⁴⁺ and Ce³⁺ under oxidizing and reducing conditions. The Ce³⁺/Ce⁴⁺ redox cycle leads to high catalytic activity of CeO₂ [16], [17], [18]. However, since single CeO₂ would be sintered after calcination at 750 °C, some mixed oxides are prepared by adding anti sintered oxides like titanium [19], [20].

Thus, a previous work [21] has shown that it seems to be interesting to combine the physico-chemical properties of gold, cerium and titanium in order to obtain a suitable catalytic material for the total oxidation of VOCs.

Therefore, the aim of this work is to deposit gold particles on Ce_0.3Ti_0.7O₂ and to investigate the effect of the preparation method on the catalytic total oxidation of propene. Propene was chosen as probe molecule for the catalytic oxidation, because it is often found in industrial exhausts and presents high photochemical ozone creation potentials (POCP) [22], [23]. The gold-based catalysts have been characterised by various techniques, such as BET, XRD, DR-UV–vis and H₂-TPR, and attempts have been made to correlate the catalytic activity with the physico-chemical properties of the catalysts.

Section snippets

Catalyst preparation

Ce-Ti oxide was synthesized using sol–gel method [24]. An aqueous solution of cerium nitrate Ce(NO₃)₃·6H₂O and ethanol CH₃CH₂OH were added under stirring to another solution of ethanol CH₃CH₂OH and titanium(IV) isopropoxide Ti(OC₃H₇)₄ with molar ratio Ti(OC₃H₇)₄/CH₃CH₂OH = 1/2. The molar ratio between H₂O and titanium(IV) precursor is H₂O/Ti(OC₃H₇)₄ = 5. The solution was gelled after finishing the reaction between titanium(IV) isopropoxide Ti(OC₃H₇)₄ and water. The gel was dried at 80 °C during 24 h

Results and discussions

Fig. 1 represents the conversion of propene as a function of the temperature in the presence of the catalytic support (Ce_0.3Ti_0.7O₂) and in presence of different gold-based catalysts. Thus, adding gold on the oxide support improves the catalytic activity for the propene oxidation. The propene conversion is performed at lower temperature and present 100% of selectivity towards formation of CO₂. When gold is deposited on the oxide using the DP method, a better activity is observed. This result

Conclusion

Au/Ce_0.3Ti_0.7O₂ catalysts were synthesised by several methods (deposition precipitation and impregnation) using HAuCl₄ as precursor compounds and their activity in the propene oxidation was evaluated. On the basis of the results reported, these solid can be classified by decreasing activities as follows: Au/Ce_0.3Ti_0.7O₂ [DP urea] > Au/Ce_0.3Ti_0.7O₂ [DP NaOH] > Au/Ce_0.3Ti_0.7O₂ [DP Na₂CO₃] ≫ Au/Ce_0.3Ti_0.7O₂ [Impregnation] > Ce_0.3Ti_0.7O₂. It can be concluded that the preparation method or the choice of the

Acknowledgements

The authors would like to thank the Urban Community of Dunkerque for the financial assistance allocated for the purchases of the UV–vis spectrophotometer and the specific area measurement apparatus. The “Conseil General du Nord”, the “Region Nord-Pas de Calais” and the European Community (European Regional Development Fund) are gratefully acknowledged for financial supports.

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Effect of the preparation method on Au/Ce-Ti-O catalysts activity for VOCs oxidation

Abstract

Introduction

Section snippets

Catalyst preparation

Results and discussions

Conclusion

Acknowledgements

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