(687c) The Effects of Temperature and Hydrogen on Glycerol Adsorption on Ruthenium Metal

In efforts to understand the mechanism of aqueous hydrogenolysis of glycerol over noble metals, we seek to probe interactions between substrate molecules and the metal catalyst surface. Previous isotopic labeling work shows that substrate dehydrogenation is the first step in hydrogenolysis, although the exact mechanism of adsorption and is not known. This paper describes efforts to quantify the effects of temperature and adsorbed hydrogen on glycerol (GO) adsorption on ruthenium metal, by using a novel recirculating loop microreactor system. Glycerol adsorbs irreversible on a hydrogen free surface. In the presence of hydrogen GO does not adsorb. This suggests exclusion of glycerol from the surface by adsorbed hydrogen. Attempts to recover adsorbed glycerol species via washing with water and/or heating have not been successful. But hydrogen at elevated temperatures will remove adsorbed species in the form of methane. At 80 °C the addition of hydrogen stops the adsorption of GO and all the adsorbed carbon is converted to methane, while at 200 °C all adsorbed and solution species are converted to methane with a carbon recovery >90%. The total amount of carbon equivalents adsorbed does not vary at with increases in temperature but the GO conversion to other products increases with increase in temperature. The quantities of polyols adsorbed are significantly less, on a molar basis, than those obtained from gas-phase chemisorption of H2 (3.2 ìmol/g-Ru) and CO (5.8 ìmol/g-Ru), suggesting multiple substrate-metal contact points.