Theoretical Insight into the Decomposition Mechanism of Formic Acid over Diatomic Pd2/NC Catalysts

Pd-based catalysts exhibit good catalytic performance for the decomposition of formic acid (HCOOH). In this work, three diatomic catalyst models, i.e., Pd–Pd@N6V4 (A), PdPd@N6V4 (B), and PdPd@N7V4 (C), were rationally constructed. Over them, the reaction mechanisms of HCOOH decomposition have been theoretically investigated at the GGA-PBE/DNP level. For the decomposition of HCOOH, there are two competing pathways: one leads to the formation of CO2 and H2 (R1), while the other results in the formation of CO and H2O (R2). Both A and C selectively favor R1 because of the N-site’s Lewis basicity promoting R1 and the Pd-site’s Lewis acidity impeding the competing R2, whereas B selectively promotes R2, owing to the Pd-site’s Lewis acidity inhibiting R1 and facilitating R2. For R1, the catalytic activity decreases as A > B > C, with the formation of the H–H bond as the rate-determining step, where the higher activity originates from a synergistic interaction between the weaker Lewis acidity of Pd-sites and the weaker Lewis basicity of N-sites. Alternatively, for R2, the catalytic activity decreases as B > A > C, with the C–OH bond cleavage as the rate-determining step, where the higher catalytic activity stems from the stronger Lewis basicity at the second site (N/Pd), alongside the Pd1-site’s contribution. Overall, A is predicted to be a promising catalyst for the decomposition of HCOOH to CO2 and H2 (R1), owing to the synergistic effect of its two adjacent Pd–Pd sites. This work provides some theoretical guidance for the design of highly efficient and selective catalysts for the production of hydrogen.