Mössbauer characterisation of gold/iron oxide catalysts

Abstract

57Fe and 197Au Mo ssbauer data have been reported and discussed for a variety of catalysts of gold supported on iron oxide. The 57Fe Mo ssbauer spectra show that iron is present as ferrihydrite, haematite and goethite in amounts that depend on the conditions of preparation. The highest amount of ferrihydrite is observed in the samples prepared by coprecipitation. The 197Au Mo ssbauer spectra reveal the formation of two main gold species, i.e., metallic gold and oxidic trivalent gold. The presence of gold favours the formation of ferrihydrite over that of haematite. No direct correlation was found between catalytic activity towards CO oxidation and the Au species identiÐed by Mo ssbauer spectroscopy, but catalyst activity was found to increase with the relative amount of ferrihydrite. It is suggested that the ferrihydrite is important for activation of molecular oxygen.

Gold as a novel catalyst in the 21st century: Preparation, working mechanism and applications

“…It is unlikely that oxidic Au species are the major catalytically active phases because the most active supported Au catalysts are prepared by calcination in air at 573K, where Au precursors (hydroxides or organo complexes) are mostly transformed into metallic particles. A certain fraction of Au species remain as atomically dispersed species in the matrix of the support, as proved by EXAFS (8,50,53,54), XPS (51), Mössbauer (52,55), and IR of CO adsorbed (43,49).…”

“…The data for Pt group metals are taken from ref. [41] (51,52). It is unlikely that oxidic Au species are the major catalytically active phases because the most active supported Au catalysts are prepared by calcination in air at 573K, where Au precursors (hydroxides or organo complexes) are mostly transformed into metallic particles.…”

Gold supported on iron oxy-hydroxides: a versatile tool for the synthesis of fine chemicals

“…A detailed characterisation by 197 Au and 57 Fe Mossbauer spectroscopy and XPS spectroscopy of gold catalysts prepared by coprecipitation (entries 2-9) have revealed that gold is present as Au(III) and Au (metal) and the support is a mixture of ferrihydrite, and hematite [25,27], The amount of ferrihydrite on such catalysts has been found to increase with the gold loading. XRD analysis have shown that all the samples, but AF046/b, are amorphous [28].…”

“…However, gold in a higher oxidation state has been also shown to chemisorb carbon monoxide [37]. 197 Au Mossbauer spectroscopy [25] and XPS spectroscopy [26], have revealed that when the catalysts are not calcined at high temperature (as in the case of the catalysts reported in this paper) gold is mainly present as Au(III).…”

Structured Au/FeOx/C Catalysts for Low-Temperature CO Oxidation

“…An increase in the temperature to 600 K provides a higher O/Au ratio (up to 2). The presence of trivalent gold in the Au/MeO x catalysts was detected by 197 Au Mössbauer spectroscopy (45,46), XPS (37,47), and XANES (37). Different hydroxylated species of gold, such as Au(OH) 3 (37) and AuO(OH) · H 2 O (46), were also found.…”