Microstructural comparison of calcined and uncalcined gold/iron-oxide catalysts for low-temperature CO oxidation

Abstract

A series of gold/iron-oxide catalysts has been prepared by an inverse co-precipitation method from a mixture of HAuCl 4·3H 2O and Fe(NO 3) 3·9H 2O. Samples calcined at 400 °C for 3 h exhibited poor activity towards CO oxidation, whereas uncalcined materials that had only been dried at 120 °C for 16 h exhibited a far superior catalytic activity. The most active material of this latter type is shown to have 100% CO conversion at 20 °C for at least 10 h. Detailed characterisation of the dried and the calcined materials using ICP and BET analysis, XRD, HREM, STEM-EDX and Mössbauer spectroscopy has shown significant differences in their microstructure. The dried materials consist of micron scale agglomerates of 4–8 nm disordered Fe 5HO 8·4H 2O particles on which the Au is uniformly dispersed in the form of a mixture AuOOH· xH 2O and Au 0. By comparison, the calcined materials are comprised solely of 3–5 nm cuboctahedral metallic Au particles supported on 20 nm diameter well-crystalline α-Fe 2O 3 particles. Our microstructural observations and catalytic measurements are discussed in the context of the Bond–Thompson mixed Au x+ /Au 0 model for the low-temperature CO oxidation catalyst.