Different Mechanisms for the Formation of Acetaldehyde and Ethanol on the Rh–Based Catalysts

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts were investigated by the TPR (temperature programmed reaction) method, and the active sites were studied by CO-TPD, TPSR (temperature programmed surface reaction of preadsorbed CO by H2) and XPS techniques. The TPR results indicated that ethanol and acetaldehyde might be formed through different intermediates, whereas ethanol and methanol might result from the same intermediate. Results of CO-TPD, TPSR, and XPS showed that on the Rh-based catalyst, the structure of the active sites for the formation of C2-oxygenates is (Rh0xRh+y)–O–Mn+ (M=Mn or Zr, x⪢y, 2≤n≤4). The tilt-adsorbed CO species is the main precursor for CO dissociation and the precursor for the formation of ethanol and methanol. Most of the linear and geminal adsorbed CO species desorbed below 500 K. Based on the suggested model of the active sites, detailed mechanisms for the formation of acetaldehyde and ethanol are proposed. Ethanol is formed by direct hydrogenation of the tilt-adsorbed CO molecules, followed by CH2 insertion into the surface CH2–O species and the succeeding hydrogenation step. Acetaldehyde is formed through CO insertion into the surface CH3–Rh species followed by hydrogenation, and the role of the promoters was to stabilize the intermediate of the surface acetyl species.