Effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on catalytic performance for direct oxidative esterification of methacrolein into methyl methacrylate

Abstract

• Bimetallic Pd-Au catalysts with different coverageof modulated Au(Pd)atomsare used to model surface composition. • The presence of pre-adsorbed O on Pd modulated Au catalysts exhibit best stability for resistance toward deactivation induced by H2O. • CH3O species are formed via CH3OH+O+*→CH3O+OH rather than CH3OH+*→CH3O+H. • Pd modulated Au catalysts with higher coverage of modulated Pd atoms show better catalytic performance for direct oxidation esterification. • The better catalytic performance for direct oxidation esterification is relatedto surface charge andFermi level. Aiming at examining the effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on the catalytic performance for direct oxidative esterification of methacrolein with methanol to methyl methacrylate. Here, two kinds of surface composition with different coverage of modulated Au(Pd) atoms on Au modulated Pd (AuPd) and Pd modulated Au (PdAu) catalysts have been designed and employed to investigate direct oxidative esterification using density functional theory calculation. The results showed that Pd modulated Au catalysts with higher coverage of modulated Pd atoms in the presence of pre-adsorbed O exhibited higher stability for resistance toward deactivation induced by H 2 O deposition than Au modulated Pd and pure Au(Pd) catalysts. The formation of CH 3 O species mainly came from pre-adsorbed O assisted CH 3 OH activation rather than the direct breakage of CH 3 OH via O-H bond, and it was more advantageous than the activation of CH 3 OH via C-H bond with and without pre-adsorbed O. Additionally, Pd modulated Au catalysts with higher coverage of modulated Pd atoms improved the activity for MMA formation via MAL+*→CH 2 C(CH 3 )CO+(H)+CH 3 O+*→MMA than Au modulated Pd and pure Au(Pd) catalysts. Moreover, the better catalytic performance (stability and activity) for direct oxidation esterification of Pd modulated Au catalysts with higher coverage of modulated Pd atoms were attributed to less average charge of surfaces atoms and the d -band center close to the Fermi level compared with pure Au(Pd) and Au modulated Pd catalysts. Therefore, it is proposed that Pd modulated Au catalysts are promising for direct oxidative esterification reaction of methacrolein with methanol to methyl methacrylate. This study not only helps understanding theory on direct oxidative esterification mechanism on bimetallic Pd-Au catalysts, but also provides clues for the design of high-effectively bimetallic Pd-Au catalysts for direct oxidative esterification.