Elsevier

Thermochimica Acta

Volume 329, Issue 1, 6 April 1999, Pages 39-46
Thermochimica Acta

Au/iron oxide catalysts: temperature programmed reduction and X-ray diffraction characterization

https://doi.org/10.1016/S0040-6031(98)00664-9Get rights and content

Abstract

Gold on iron oxides catalysts have been characterized by temperature programmed reduction (TPR) and X-ray diffraction spectroscopy (XRD). The influence of preparation method, gold loading and pretreatment conditions on the reducibility of iron oxides have been investigated. On the impregnated Au/iron oxide catalysts as well as on the support alone the partial reduction of Fe(III) oxy(hydroxides) to Fe3O4 starts in the 550 and 700 K temperature range. On the coprecipitated samples, the temperature of formation of Fe3O4 is strongly dependent on the presence of gold. The reduction temperature is lowered as the gold loading is increased. The reduction of Fe3O4 to FeO occurs at about 900 K and is not dependent on the presence of gold and the preparation method. It is suggested that the effect of gold on the reducibility of the iron oxides is related to an increase of the structural defects and/or of the surface hydroxyl groups.

Introduction

Gold catalysts supported over metal oxides of the first transition series have been the subject of many recent investigations. These catalysts show an unusual high catalytic activity in the oxidation of CO and H2 at low temperature 1, 2, 3and in the water gas shift (WGS) reaction [4]. The catalytic performance of Au/iron oxides catalysts in the low temperature CO oxidation has been reported previously [5]. The remarkable enhancement of catalytic activity obtained from the combined effect of gold and the transition metal oxide strictly depends on the method of preparation and on the pretreatment conditions. Au/Fe2O3 catalysts prepared by coprecipitation were more active than the impregnated samples [5]. Moreover uncalcined samples have shown a higher initial activity than calcined catalysts.

It seems likely that a metal–support interaction, which is stronger when gold is coprecipitated together with the iron oxide support, is responsible for the enhanced catalytic activity. It is well known that the presence of Au(III) ions influences the nature and the growth of the iron oxides crystallites [6]which in turn can influence the metal–support interaction.

In order to characterize the Au/iron oxides system, a Mössbauer investigation has been carried out and the results are reported elsewhere [7]. Here a temperature programmed reduction (TPR) study on a series of iron oxy(hydroxides) supports and gold/iron oxides catalysts is presented. Samples obtained with different preparation methods, metal loadings and pretreatments conditions have been investigated.

Section snippets

Materials

Reagents used were analytical grade. The Au/Fe2O3 catalysts were prepared by using HAuCl4 and Fe(NO3)3·9H2O (Fluka). The gas mixture for TPR experiments was an ultra high purity H2/Ar (5 vol% of H2) mixture, purified with molecular sieves and oxygen absorbent traps.

Sample preparation

Iron oxide samples were prepared by addition of a solution of Fe(NO3)3·9H2O to an aqueous solution of Na2CO3 1 M (pH 11.9) or NaOH 1 M (pH 14) under vigorous stirring (500 rpm) at 7.5 ml/min rate and at temperature between 273 and 348 K.

Surface area and XRD

Structural characteristics of the samples under study have been investigated by BET surface area (SA) measurements and X-ray diffraction (XRD). The surface areas of the iron oxide samples and of the gold catalysts are reported in Table 1, Table 2, respectively. The iron oxides and the impregnated sample show SA in the range 1–195 m2/g. The SA of coprecipitated catalysts is however much larger; it increases (from 242 to 344 m2/g) with increasing the gold loading. The sample AF576/b, obtained by

Discussion

TPR experiments reported in the previous section have shown that the reduction of the iron oxy(hydroxy) phases of the support in the gold catalysts depends strongly on the preparation method, gold content and pretreatment conditions. It is well known that the reduction of bulk iron oxide by hydrogen proceeds through the following steps 12, 13:Fe2O3→Fe3O4→FeO→Fe

Our results agree with this picture and also show that the partial reduction to Fe3O4 is favoured on the samples with larger surface

Acknowledgements

This work has been carried out with the financial support of MURST.

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