Mechanism Insight into Catalytic Performance of Ni₁₂P₅ over Ni₂P toward the Catalytic Deoxygenation of Butyric Acid

The Ni/P ratio of nickel phosphide has an important effect on the catalytic performance toward the deoxygenation of fatty acids to biofuel. The Ni₁₂P₅ cluster is preferred to model Ni₁₂P₅ catalyst with butyric acid as the reactant model of palmitic acid. The catalytic deoxygenation mechanism of butyric acid over Ni₁₂P₅ cluster has been theoretically investigated at GGA-PBE/DSPP, DNP level in dodecane solution. From butyric acid, the hydrodehydration is predominated to form n-butanal. Then, from n-butanal, low temperature benefits the hydroreduction to form butanol and then hydrodehydration to produce n-butane, whereas high temperature favors the direct decarbonylation to yield propane. n-Butane originates from n-butanol through hydrodehydration and not from n-butylene. Propane comes from n-butanal through decarbonylation and not from propanol and/or propylene. Additionally, CO stems from n-butanal through decarbonylation, whereas CO₂ is ruled out from butyric acid through decarboxylation. Compared with Ni₁₂P₆ cluster, Ni₁₂P₅ cluster exhibits higher catalytic activity for the formation of butanal, n-butanol, and n-butane, while it displays lower catalytic activity toward the direct decarbonylation and dehydration to yield propylene. These results can be attributed to less negative charges of Ni-sites over Ni₁₂P₅ cluster, compared with Ni₁₂P₆ cluster.