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.