Deep oxidation of pollutants using gold deposited on a high surface area cobalt oxide prepared by a nanocasting route

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Abstract

Gold deposited on a cobalt oxide with high surface area (138 m2 g−1), obtained through a nanocasting route using a siliceous KIT-6 mesoporous material as a hard template, has demonstrated high activity for the total oxidation of propane and toluene, and ambient temperature CO oxidation. The addition of gold promotes the activity when compared to a gold-free Co3O4 catalyst prepared using the same nanocasting technique. The enhanced catalytic activity when gold is present has been explained for the deep oxidation of propane and toluene in terms of the improved reducibility of cobalt oxide when gold is added, rather than to the intrinsic activity of metallic gold particles. The improved behaviour for CO oxidation has been linked to the simultaneous presence of Auδ+ and Au°.

Introduction

Amongst the number of methods employed to remove volatile organic compounds (VOCs), catalytic oxidation is considered as one of the most efficient and effective, since it eliminates pollutants yielding only carbon dioxide and water [1]. One additional advantage for catalytic oxidation is the particular ability to eliminate VOCs, both gases and condensables, especially if the concentration of the VOC in the effluent stream is low [2], [3], [4]. Amongst VOCs, linear short chain alkanes are some of the most difficult to destroy, and especially methane [5], [6] which is not strictly a VOC, presents a global warming potential of about 20 times greater than CO2. Another light paraffin, propane, is also released to the atmosphere in increasing amounts, since LPG, composed of primarily propane and butane, is increasingly used as a substitute for gasoline and diesel in transport vehicles. Currently around 12 million drivers worldwide are using LPG vehicles, since they produce less harmful emissions in terms of particulates, CO and CO2 than conventional fuels. However, during LPG combustion some unburnt hydrocarbons are emitted to the atmosphere. Therefore, a catalyst that completely removes propane, which is more difficult to eliminate than butane at low concentrations, is essential to reduce the impact on the environment [7]. Stationary power sources can also release propane to the atmosphere in relatively high concentrations, and hence the control of propane emissions is important for a range of applications.

Although noble metals are usually employed for the elimination of VOCs there are some oxides of non noble metals which can present comparative activity. Amongst metal oxides cobalt oxide, in the form of Co3O4, has been demonstrated to be one of the most active non noble metal oxides for VOC elimination [8], [9], [10], and in the case of propane it seems to be the most active [8], [9], [10]. It has been reported that cobalt oxides with high surface areas not only increase the catalytic activity but also the activity normalized per surface area [11], [12], [13] if compared to a conventional Co3O4. This enhanced catalytic activity has been related to an improved reducibility [11], a higher concentration of O-electrophilic species [12] and a higher amount of oxygen vacancies [13]. One way to synthesize cobalt oxides with high surface area is through a nanocasting route which in the case of cobalt oxide can exceed 170 m2 g−1 [14]. This method allows the synthesis of ordered cobalt oxide with a structure determined by the shape of the pores of the mesoporous silica used as a hard template. In recent work fully replicated Co3O4 and Au/Co3O4 catalysts have been found to be very active towards total oxidation of traces of ethylene [15]. This method makes it possible to prepare metal oxides with high areas using high heat treatment temperatures. These ordered materials have demonstrated high catalytic activity in the total oxidation of several volatile organic compounds, which is remarkably higher than that obtained by conventional Co3O4. However, both the catalytic activity and the specific activity are higher if the degree of replication is lower than for fully ordered replicas [13]. This fact has been related to the higher concentration of highly reactive oxygen defects in the non-fully replicated cobalt oxides. Additionally, this mainly disordered sample still presents a very high surface area. The degree of replication of the cobalt oxide can be controlled by adjusting the synthesis parameter, such as the temperatures of both the aging and the calcination temperature of the mesoporous silica used as a hard template [13], [16], [17].

In the present paper the total oxidation of propane, toluene and CO has been studied over optimized catalysts, which were prepared following the combination of two methodologies. The first is by the addition of gold to several metal oxides by coprecipitation or deposition–precipitation, as this has been shown previously to increase the catalytic activity during the oxidation of alkanes [8], [9], [10]. The second is to prepare non-fully ordered nanocast Co3O4, which presents a high surface area, and shows outstanding catalytic performance for the deep oxidation of VOCs [11], [12], [13]. Consequently, we have prepared a partly ordered cobalt oxide that presents a high surface area (138 m2 g−1) and to this gold has been added through a deposition–precipitation method.

Section snippets

Preparation of catalysts

High surface area cobalt oxide has been prepared using a mesoporous silica as a hard template. Below the details of the preparation of the silica template, the final cobalt oxide and the gold on cobalt oxide catalysts are described.

Catalyst characterization

Reproducibility of the preparation of the gold-free cobalt oxide was checked by repeating the synthesis procedure for 4 different batches, and surface areas between 132 and 146 m2 g−1 were obtained. All of these batches were mixed and the resulting catalyst presents a surface area of 138 m2 g−1. Some properties of the catalysts synthesized are shown in Table 1. By adding gold to the partly ordered cobalt oxide, the surface areas do not significantly vary except for the catalysts with the highest

Conclusions

It has been demonstrated that the addition of gold to a high surface area cobalt oxide produces optimal materials (very active and stable) as catalysts for the deep oxidation of propane and toluene to CO2. For these catalysts we have simultaneously used two approaches in order to improve the activity for total oxidation. These are (i) a cobalt oxide prepared through a preparation procedure that results in the desired crystalline phase (Co3O4) with a suitably high surface area, and (ii) the

Acknowledgements

The authors would like to thank Spanish Ministerio de Educación y Ciencia (Project CTQ2006- 02386 and Project CTQ2009-14495) for financial support.

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