Ab Initio Microkinetic Modelings of Molecular Reactions on Pd(100), Pd(110), and Pd(111) Surfaces: A CO2/CO Reduction Case Study

Palladium metal (Pd) and its alloys are favorable materials for hydrogen storage and purification but are facile to surface adsorption inhibition by impurities like CO2 and CO. In this work, density functional theory calculations coupled with microkinetic modelings were performed to analyze and compare reaction processes occurring on Pd(100), Pd(110), and Pd(111) surfaces when exposed to mixing gases of H-2 + CO2 and H-2 + CO. Simulation results indicate that perfect Pd surfaces are nonactive for catalyzing CO2-to-CO reactions and thus resistant to CO2 inhibition when acting as membranes for hydrogen permeation. Effects of CO on retarding hydrogen adsorption on perfect Pd surfaces are notable at temperatures below 600 K by preferentially occupying the active sites both thermodynamically and kinetically. Rates of CO2-to-CO reactions are mainly limited by the COOH formation and dissociation steps, and the direct CO2 dissociation step becomes influential on Pd(100) and Pd(110) at temperatures above 750 K. For H-2 + CO reactions, the rate-limiting steps on all of the Pd surfaces are the dissociation of COH species, and carbon deposition was only observed on the Pd(100) slab.