Investigations of mechanism, surface species and support effects in CO hydrogenation over Rh

Abstract

The Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates over Rh/ZrO2 and Rh/SiO2 was studied. Catalytic reaction tests show a support effect with a faster steady state reaction rate over Rh/ZrO2 compared to Rh/SiO2. Temperature programmed hydrogenation (TPH) experiments reveal that the CO dissociation on the metal surface is rate limiting, and the support effect thus accelerates the CO dissociation on the metal. Combined TPH and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies after low-temperature CO pre-adsorption reveal a H-assisted C-O bond breakage through CH3O species on the Rh surface. Transient measurements indicate that this mechanism is also likely to be in operation during the steady state reaction at higher temperatures, although here the methoxide is too short-lived to be detected at steady state. This hydrogen-assisted CO activation can help to explain that previous studies have observed an inverse H/D isotope effect for Rh despite CO dissociation being the rate limiting step. TPH studies show that both CO pre-adsorption at 30 ℃ and CO/H2 exposure at 250 ℃ lead to so high CO coverages that it restricts the CO activation, which only starts once part of the CO has desorbed. The near-complete CO coverage on the working Rh surface is restricting the rate, but is essential for the selectivity towards oxygenates. Studies of acetaldehyde conversion in various atmospheres over Rh/SiO2 and Rh/ZrO2 catalysts show that acetaldehyde decomposes over a bare Rh surface with a rate that greatly exceeds the oxygenate formation rate in CO hydrogenation. In the presence of CO the acetaldehyde decomposition is strongly inhibited. The high CO coverage on the surface of the working Rh catalysts thus prevents oxygenate decomposition and is therefore essential for the ability to produce oxygenated products. The reaction temperature is observed to play a role for the establishment of the high coverage. During exposure to CO/H2 at reaction temperatures (>200 ℃) an activated process occurs whereby both the stability and coverage of the CO adlayer increases. This activated stabilization of the CO adlayer shifts the CO activation up in temperature in a subsequent TPH. The results contribute to a fundamental understanding of the reaction mechanism and support effects in the Rh-catalyzed CO hydrogenation, which can assist the formulation of improved Rh-based catalysts. The results, such as the identification of a H-assisted mechanism of CO hydrogenenation via methoxide, could also be of general relevance for the understanding of CO hydrogenation over other metals.