Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide

doi:10.1016/S0926-3373(01)00221-1

Applied Catalysis B: Environmental

Volume 34, Issue 4, 28 November 2001, Pages 277-285

https://doi.org/10.1016/S0926-3373(01)00221-1 Get rights and content

Abstract

This paper reports a study on the influence of calcination pretreatments on the catalytic behaviour of the Au/iron oxide system towards the combustion of some representative volatile organic compounds (2-propanol, ethanol, methanol, acetone and toluene). The catalytic activity of Au/Fe₂O₃ samples towards the total oxidation to CO₂ has been found to be strongly dependent on the catalyst pretreatment, decreasing on increasing the calcination temperature. On the basis of characterisation data (XPS, FT-IR, XRD, BET surface area) it has been proposed that the catalytic behaviour is related to the gold state and/or the iron oxide phase. It appears plausible to suggest that the gold oxidation state and/or the particle size play a key role in the catalytic combustion of volatile organic compounds.

Introduction

Volatile organic compounds (VOCs) are an important class of air pollutants, emitted from many industrial processes and transportation activities [1]. Catalytic combustion is one of the most promising technology for the destruction of VOCs, due to its definitive character and save energy. VOCs can be, in fact, oxidised over a catalyst at temperatures much lower than those of thermal oxidation. Both supported noble metals (Pt, Pd, Rh) and metal oxides (Cu, Cr, Mn) have been used as catalysts for this reaction [2], [3], [4], [5], [6], [7].

Gold has been generally considered a poor active heterogeneous catalyst because of its chemically inert character due to the completely filled 5d shell and the relatively high value of its first ionisation potential [8]. However, in the last years it has been reported [9], [10], [11], [12] that gold particles smaller than 10 nm, highly dispersed on metal oxides (Fe₂O₃, Co₃O₄, TiO₂, Mn₂O₃) by suitable preparation methods (i.e. coprecipitation, deposition–precipitation), can present a remarkable activity towards several oxidation reactions [13]. In particular coprecipitated Au/Fe₂O₃ catalysts were found to be very active for CO oxidation [9], [10], [14], [15]. However, the catalytic activity of the system appeared to be strongly affected by the preparation method and the pretreatment conditions used [9], [10].

More recently we have shown that the Au/iron oxide system is particularly active towards the VOCs catalytic oxidation [16]. On the basis of these results, in this paper, we now report a study on the influence of calcination pretreatments on the catalytic activity of the Au/Fe₂O₃ system towards the combustion of some representative VOCs (methanol, 2-propanol, ethanol, acetone and toluene). The main purpose was to elucidate the influence that different calcination pretreatments can induce on gold and/or iron oxide. These findings can be useful in order to better understand the role of the metal and the support on the catalytic combustion over supported gold systems.

Section snippets

Experimental

Au/Fe₂O₃ catalyst was prepared by coprecipitation from HAuCl₄ (Fluka) and Fe(NO₃)₃·9H₂O (Fluka). An aqueous mixture of the precursors was poured at 7.5 ml/min rate into an aqueous solution of Na₂CO₃ (1 M and pH=11.9) under vigorous stirring (500 rpm); the precipitation temperature was maintained at 75°C. The coprecipitated sample obtained was kept digesting for about 24 h, washed several times (until disappearance of nitrate and chloride), then dried under vacuum at 70°C and finally ground before

Catalytic activity

Fig. 1 shows the conversion of 2-propanol and the yields to CO₂ and acetone, as a function of reaction temperature, on the Au/Fe₂O₃ catalyst calcined at different temperatures (200, 300 and 450°C). It must be reminded that CO₂, water and acetone were the only products formed under the experimental conditions used. From Fig. 1, it can be seen that on the 200°C calcined sample (Fig. 1A) 2-propanol conversion is detectable already at 40°C with formation of acetone only. The total oxidation to CO₂

Discussion

In a recent paper [16] we have shown that coprecipitated Au/Fe₂O₃ catalysts in the presence of excess of oxygen are very active in the oxidation of VOCs (methanol, ethanol, 2-propanol, acetone and toluene) with an effect strongly related to the gold content. The light-off temperature has been found, in fact, to decrease on increasing the amount of gold.

The results reported in the present paper shows that the catalytic behaviour of the Au/Fe₂O₃ system is remarkably influenced by the thermal

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Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide

Abstract

Introduction

Section snippets

Experimental

Catalytic activity

Discussion

Appl. Catal. B

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