Abstract
The nature of the promotional effect of Fe addition to Rh/TiO2 and Rh/CeO2 catalysts for CO hydrogenation was investigated using FT-IR spectroscopy in an ultrahigh vacuum compatible transmission IR cell. CO adsorption experiments on Rh and FeRh showed vibrational signatures characteristic of linear and bridge bound CO on Rh0 as well as geminal-dicarbonyl species associated with Rh+. Compared to TiO2, the CeO2-supported catalysts show increased dispersion, reflected by decreased particle size, and a lower signal for linear versus geminal-dicarbonyl bonded CO. The absorption frequencies for CO on Rh/CeO2 are also redshifted relative to Rh/TiO2, which results from a weaker Rh–CO interaction, likely due to the increased reducibility of the CeO2 support. Upon addition of Fe, a new spectral feature is observed and attributed to CO bound to Rh in close contact with Fe, likely as a surface alloy. CO hydrogenation on (Fe)Rh catalysts on both supports was also studied. Compared to bare Rh, Fe containing catalysts promote formate and methoxy species on the surface at lower temperature (180 °C), which suggests an enhancement in methanol selectivity by Fe addition. At higher temperatures (220 °C), the spectral features appear similar, further confirming the role of Fe as a disrupter of large Rh0 crystallites and regulator of CO dissociation and CH4 formation.
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