Effects of Co-adsorbed Water on Different Bond Cleavages of Oxygenates on Pd (111)

Experimental studies have shown that co-adsorbates and solvents affect both the activity and selectivity of heterogeneous catalysts, but how they affect different bond cleavages and a network of parallel reactions is not well understood. Here we present a density functional theory (DFT) calculation of how co-adsorbed water affects different bond cleavages of oxygenates on metal surface, using decomposition of acetic acid over Pd (111) as a model system for oxygenates, with application in biomass conversion. The presence of co-adsorbed water generally enhances O-H bond cleavage while generally inhibiting the OC-O, C-C, and OC-OH bond cleavage. The influence of co-adsorbed water on C-H bond cleavage varies the most and depends on the nature of the transition state and how co-adsorbed water stabilizes the initial and final state. Although these trends are useful as general guidance, they are not sufficient to predict the effect on a complex reaction network such as acetic acid decomposition that has several parallel reaction paths. In the absence of co-adsorbed water, the two lowest energy pathways are decarboxylation and decarbonylation pathways through a common CH2COO intermediate. But through an inhibition of OC-O bond cleavage but enhancement of C-C bond cleavage of CH2COO, one of three exceptions to the general trend of CC inhibition in the presence of water, the two lowest free energy pathways are decarboxylation forming CO2 in the presence of water. This illustrates how a single reaction step can affect a complex reaction network with many parallel, energetically similar paths. This suggests that the presence of co-adsorbed water makes acetic acid decarboxylation (formation of carbon dioxide) more favorable than acetic acid decarbonylation (formation of carbon monoxide) over Pd (111).