New Insight into the Decomposition Mechanism of Formic Acid on Pd(111): Competing Formation of CO2 and CO

While the high performance for electrooxidation of formic acid (HCOOH) has been recognized, Pd-based catalysts still suffer from CO poisoning, even though they are much more tolerant than Pt-based catalysts. Existing theoretical studies on the decomposition of HCOOH on Pd(111) surface cannot rationalize the catalyst poisoning effect. By performing density functional theory calculations, the present work reexamined the decomposition of HCOOH on Pd(111) along with the dual-path mechanism consisting of indirect and direct pathways. Two new adsorption configurations of HCOOH on Pd(111) are presented, from which the formation of CO is found to be either the same or more favorable in comparison with the formation of CO2. The present results are in distinct contrast to previous calculations where the barrier for the formation of CO2 was much lower than that for the formation of CO. Furthermore, this work also discussed the formation of CO through the reduction of CO2 and the effects of coadsorbed HCOOH and H2O molecules on the reactivity. From calculated results, it seems that the newly formed CO2 on Pd(111) can return to the surface to interact with adsorbed H atoms, partly contributing to the formation of CO. Coadsorbed HCOOH and H2O molecules are found to importantly affect the initial adsorption configuration and the decomposition mechanism of HCOOH on Pd(111). These results provide new insight into the reactivity of HCOOH on the Pd(111) surface and rationalize CO poisoning of Pd-based catalysts.