Density Functional Theory Study of Decarboxylation and Decarbonylation of Acetic Acid on Pd (111)

Acetic acid (CH3COOH) decomposition over the Pd(111) surface is a good model system for decomposition of acids and small oxygenates common in biomass conversion. Here we present density functional theory calculations of the energy landscape of CH3COOH decomposition on Pd(111), focusing on the reaction mechanism for acetic acid decarboxylation (DCX) and decarbonylation (DCN). Our results suggest that the most favorable DCX pathway proceeds through dehydrogenation of CH3COOH to acetate (CH3COO), followed by dehydrogenation of CH3COO to carboxylmethylidene (CH2COO), C-H bond cleavage of CH2COO to carboxylmethylidyne (CHCOO), and finally C-C bond cleavage to form CH and CO2. The DCN pathway proceeds via the same initial dehydrogenation steps to CH2COO, followed by deoxygenation of the CH2COO to ketene (CH2CO). This follows dehydrogenation of CH2CO to ketyne (CHCO) and finally C-C bond cleavage to yield CH and CO. Carboxylmethylidene (CH2COO), which is formed in both mechanisms, is a key reaction intermediate determining the branching to DCN or DCX, but the free energy barrier difference between the DCN and the DCX route is Delta G(a) = 0.13 eV, making the DCX more favorable than DCN.