Effect of Aldehyde and Carboxyl Functionalities on the Surface Chemistry of Biomass-Derived Molecules.

The adsorption and decomposition of acetaldehyde and acetic acid were studied on Rh(100) to gain insight into the interaction of aldehyde and carboxyl groups of biomass-derived molecules with the surface. Temperature-programmed reaction spectroscopy (TPRS) was used to monitor gaseous reaction products, whereas Reflection absorption infrared spectroscopy (RAIRS) was used to determine the nature of surface intermediates and reaction paths. The role of adsorbate interactions in oxygenate decomposition chemistry was also investigated by varying the surface coverage. Acetaldehyde adsorbs in an η(C, O) configuration for all coverages, where the carbonyl group binds to the surface via the C and O atoms. Decomposition occurs below room temperature (180-280 K) via C-H and C-C bond breaking, which releases CO, H, and CH species on the surface. At low coverage, CH dehydrogenation dominates and surface carbon is produced alongside H and CO. At high coverage, about 60% of the CH hydrogenates to form methane, whereas only 40% of the CH decomposes further to surface carbon. Acetic acid adsorbs dissociatively on the Rh(100) surface via O-H bond scission, forming a mixture of mono- and bidentate acetate. The decomposition of acetate proceeds via two different pathways: (i) deoxygenation via C-O and C-C bond scissions and (ii) decarboxylation via C-C bond scission. At low coverage, the decarboxylation pathway dominates, a process that occurs at slightly above room temperature (280-360 K) and produces CO and CH, where the latter decomposes further to surface carbon and H. At high coverage, both decarboxylation and deoxygenation occur, slightly, above room temperature (280-360 K). The resulting O adatoms produced in the deoxygenation path react with surface hydrogen or CO to form water and CO, respectively. The CH species dehydrogenate to surface carbon for all coverages. Our findings suggest that oxygenates with a C═O functionality and an alkyl end react on the Rh(100) surface to produce synthesis gas and small hydrocarbons whereas CO and synthesis gas are produced when oxygenates with a COOH functionality and an alkyl end react with the Rh(100) surface. For both cases, carbon accumulation occurs on the surface.