Reaction pathways for butanoic acid decarboxylation on the (111) surface of a Pd nanoparticle

Many-electron theory is applied to investigate decarboxylation of butanoic acid on the (111) surface of a Pd42 nanoparticle. Butanoic acid adsorbs weakly as an intact molecule (ΔH = − 28 kJ/mol) with the COOH moiety nearly parallel to the surface. In comparison, dissociative adsorption of butanoic acid to form surface butanoate (PrCOO) species and adsorbed H is exothermic by only 5 kJ/mol. Pathways for propane and CO2 formation are presented starting from adsorbed butanoic acid, PrCOO (ads) and a RCHCOO (R = C2H5) surface intermediate. An intramolecular pathway involves rotating the H of COOH toward the target (alpha) carbon. Following a series of bond distortions, the H is close enough to the target C to form a stretched HC bond. After propane begins to form, CO2 linearizes and CC dissociation is energetically favorable and rapid. Reaction of PrCOO (ads) and H (ads) also results in direct formation of propane and CO2. The energy barriers for H insertion are ~ 100 kJ/mol for both pathways; however, the dissociative pathway involves a high-energy precursor state with H in a 3-fold site under the CC bond undergoing reaction. An alternative decarboxylation pathway starting from RCHCOO leads to gas-phase CO2 and propylidene (in the absence of adsorbed H) or propyl (via insertion of adsorbed H) with energy barriers of 75 and 32 kJ/mol, respectively. The energy of the RCHCOO intermediate relative to gas-phase butanoic acid increases from 43 to 109 kJ/mol in the presence of adsorbed H, compensating for the lower decomposition barrier.