The effect of C-OH functionality on the surface chemistry of biomass-derived molecules: ethanol chemistry on Rh(100)

The adsorption and decomposition of ethanol on Rh(100) was studied as a model reaction to understand the role of C-OH functionalities in the surface chemistry of biomass-derived molecules. A combination of experimental surface science and computational techniques was used: (i) temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), work function measurements (Kelvin Probe - KP), and density functional theory (DFT). Ethanol produces ethoxy (CH₃CH₂O) species via O-H bond breaking upon adsorption at 100 K. Ethoxy decomposition proceeds differently depending on the surface coverage. At low coverage, the decomposition of ethoxy species occurs viaβ-C-H cleavage, which leads to an oxometallacycle (OMC) intermediate. Decomposition of the OMC scissions (at 180-320 K) ultimately produces CO, H₂ and surface carbon. At high coverage, along with the pathway observed in the low coverage case, a second pathway occurs around 140-200 K, which produces an acetaldehyde intermediate viaα-C-H cleavage. Further decomposition of acetaldehyde produces CH₄, CO, H₂ and surface carbon. However, even at high coverage this is a minor pathway, and methane selectivity is 10% at saturation coverage. The results suggests that biomass-derived oxygenates, which contain an alkyl group, react on the Rh(100) surface to produce synthesis gas (CO and H₂), surface carbon and small hydrocarbons due to the high dehydrogenation and C-C bond scission activity of Rh(100).