Ethanol Reactions over the Surfaces of Noble Metal/Cerium Oxide Catalysts

This review focuses on the reactions of ethanol on the surfaces of platinum, palladium, rhodium and gold supported on ceria (of size 10-20 nm). The bimetallic compounds: Pt-Rh, Rh-Au, Rh-Pd, and Pt-Pd were also investigated. Initially this work was aimed at understanding the roles of the different components of automobile catalytic converters on the reactions of ethanol, which is used as a fuel additive. Some of the catalysts that showed high activity for ethanol oxidation were also investigated for hydrogen production. The addition of any of the above metals to CeO2 was found to suppress the oxidation of ethanol to acetates at room temperature, as there are fewer surface oxygen atoms available to oxidise the ethanol (the remaining oxygen atoms did not produce efficient oxidation). Ethanol dehydrogenation to acetaldehyde was facilitated by the presence of Pt or Pd; at higher temperatures the acetaldehyde condensed to other organic compounds, such as crotonaldehyde. By contrast, in the presence of Rh only traces of acetaldehyde or other organic compounds were seen on the surface, and detectable amounts of CO were found upon ethanol adsorption at room temperature. This indicates the powerful nature of Rh in breaking the carbon-carbon bond in ethanol. The effects of prior reduction were also investigated and clear differences were seen: for example, a shift in reaction selectivity is observed for the bimetallic Rh-containing catalysts. Methane was the dominant hydrocarbon on the reduced catalysts while acetaldehyde was the main product for the non-reduced ones. Hydrogen formation was monitored during steady state ethanol oxidation and Pt-Rh and Rh-Au were found to be the most active catalysts.