In situ characterisation of supported iron-iridium catalysts by iron-57 and iridium-193 Mössbauer spectroscopy

Abstract

Some silica-supported iron-iridium catalysts with different iron to iridium ratios and formed by the incipient wetness technique have been examined in situ by⁵⁷Fe and¹⁹³Ir Mössbauer spectroscopy following pretreatment in hydrogen. The results show that the reduction of the iron component is enhanced by the presence of iridium metal.

The pretreated catalysts were evaluated for the hydrogenation of carbon monoxide at 270°C and 50 atmospheres pressure. The presence of iridium, which was shown by Mössbauer spectroscopy to result in the formation of reduced iron, iridium, and iron iridium alloy in the pretreated catalyst, was found to increase the catalytic activity and also influence selectivity. However, the systematic variation of the iridium content which was shown by Mössbauer spectroscopy to determine the exact phase composition of the pretreated catalysts, appeared to have little effect on catalytic performance.

The⁵⁷Fe Mössbauer spectra recorded from all the used catalysts showed the formation of large and small particle ε′-Fe_2.2C under the high pressure of the catalytic reaction. The results suggest that the formation of iron carbides on reduced iron, perhaps at the surface, and the adsorption of hydrogen on reduced iridium are important features of this catalytic system.