(98f) The Reactivity of Polyols On Pd(111)
The growing complications associated with widespread petroleum usage have led to an increased interest in development of products from renewable sources. One of the largest and most diverse renewable feedstocks is plant biomass, which can be processed into fuels and chemicals in a biorefinery. The ability to process diverse feedstocks increases the resilience of the refinery, but it also introduces technical challenges. The large number of functional groups in biomass-derived carbohydrates results in a variety of potential reactions, not all of which may be desirable. For this reason, a high level of catalytic selectivity for the conversion of biomass derived oxygenates, such as polyols, is extremely important. This work focuses on understanding the fundamental reaction mechanisms of various polyols on Pd(111), a transition metal that has shown high catalytic potential. The insight gained can then be used to develop highly selective palladium based catalysts through modification of their surfaces and electronic properties. The surface chemistry of 1,2-propanediol (HOCH2CH(CH3)OH), ethylene glycol (HOCH2CH2OH), and 1,4-butanediol (HOCH2CH2CH2CH2OH) have been examined using high resolution electron energy loss spectroscopy and temperature programmed desorption. These investigations suggest that all three molecules initially interact with the surface through their oxygen atoms. As the thermal energy is increased, competing reaction pathways lead to the formation of multiple products. The dominant pathway for each molecule involves C-C bond scission, carbonyl formation, decarbonylation, and dehydrogenation, which is consistent with previous studies of similar molecules and surfaces. However, this work suggests that minor pathways also occur. Based on the experimental insight gained through this work, a reaction mechanism has been developed for each molecule on Pd(111). Connections between the surface reaction pathways identified in this work and heterogeneous catalysis involving polyol reagents or products will be discussed.