Impact of Hydrogen Concentration for CO2 reduction on PdHx

Pd hydride has shown much better electrochemical CO2 reduction reaction (CO2RR) performance compared to metal Pd implying that H in the PdHx surface plays a vital role in affecting the performance. Some experiments show phase transformation from metal Pd to PdHx via in-situ X-ray diffraction and electrochemical method. However, studying the stable atomic structures and reaction mechanisms at different H concentrations is experimentally very difficult. Most previous experimental and com- putational studies only focus on the comparison of CO2RR performance of pure metal Pd (0% H concentration) and Pure PdH (100% H concentration). However, there is also no systematical theoretical study about the H concentration effect of PdHx (from 0% to 100%) on CO2RR and the hydrogen evolution reaction (HER), and the atomic structure of the active substoichiometric PdHx catalyst have not been identi- fied. Based on density functional theory (DFT) calculations, active learning cluster expansion equipped with Monte Carlo simulated annealing is carried out to search for stable PdHx(111) surface configurations in this work. We identify 12 stable PdHx(111) configurations from the converged DFT convex hull and theoretically investigate the evolution of the H concentration of the stable structures as a function of applied potential. After automatically finding unique adsorption sites and calculating the binding energies of adsorbates, we analyze their free energy diagram, activity volcano and selectivity toward CO and H2. Finally, we identify the atomic structure of the PdHx phase most likely to produce syngas. It’s activity can be attributed to the fact that H segregation breaks the linear relation between COOH and CO adsorbates.