Insight into the effect of promoter Pb in Pb-Pd catalyst on methyl methacrylate formation via direct oxidative esterification: A DFT study

The effect of promoter Pb in Pb-Pd alloy on the direct oxidative esterification of methacrolein (MAL) and CH3OH to methyl Methacrylate (MMA) reaction has been systematically investigated using density functional theory for the first time. Formation energies results showed Pb-4-Pd(1 1 1) was the easiest formation surface in Pb-Pd alloy. Adsorption and co-adsorption energies results revealed the addition of Pb weakened CH3OH adsorption, but enhanced adsorption capacity of CH3O, MAL and MMA, and greatly increased co-adsorption energies of CH3O with H, MAL and carbonyl species, which were further demonstrated by charge density difference, PDOS and HOMO/LUMO orbitals analysis. Activation energies results indicated that MMA formation via MAL dehydrogenation following carbonyl coupling with CH3O was the favorable path on the Pb-4-Pd(1 1 1) surface. The effect of promoter Pb on adsorption, co-adsorption, and activation energies were attributed to strong electronic interactions between Pb and Pd atoms. Promoter Pb played an electronic donor role realizing bimetallic synergetic effect, and its introduction unchanged actual catalytic active site of Pd, which was confirmed by comparative results of combined electronic structure analysis and reaction mechanism on the pure Pd and Pb-4-Pd(1 1 1) surfaces. These results are beneficial for the design of Pb-Pd bimetallic catalyst implied in direct oxidative esterification reactions.