Insight into the effect of Pd and Pd3Pb surface structure on activation of reactant O2 and product H2O in direct oxidative esterification

Abstract

Density functional theory is performed to systematically study the effect of surface structure and facet of Pd and Pd3Pb catalysts on activity for O2, H2O activation, O diffusion and stability of resistance toward deactivation induced by H2O adsorption and aggregation. Our results show that flat (1 1 1) (Pd3Pb(1 1 1), Pd(1 1 1)) and (1 0 0) (Pd(1 0 0)) facets rather than stepped (2 1 1) facet (Pd(2 1 1), Pd3Pb(2 1 1)) are active surfaces, mathing well with our previous experimental result that Pd(1 1 1) and Pd3Pb(1 1 1) planes detected in Pd-Pb alloy, further confirm that main exposed Pd3Pb(1 1 1) surface of Pd3Pb promotes O2, H2O activation, O diffusion and inhibits H2O adsorption, aggregation. Moreover, preadsorbed O coverage and numbers of H2O on Pd3Pb have little effect on H2O adsorption and cluster forming. Therefore, Pd3Pb catalysts not only present better activity and stronger stability but also decrease cost than pure Pd. Firstly finding that surface average charge through Mulliken charge analysis serves as a indicator judging minimum active region. Higher activity and stability of Pd3Pb catalysts are attributed to exposed rhomboid active unit with Pb synergistic effect. Dependence of Pd and Pd3Pb catalyst’s activity and stability on surface structure and facet revealed may provide new way for designing better, stable Pd-Pb catalysts in direct oxidative esterification.