First-principles periodic density functional study of the hydrogenation of maleic anhydride to succinic anhydride over palladium(111)

Gradient-corrected periodic density functional (DFT) calculations were carried out in order to examine the hydrogenation of maleic anhydride to succinic anhydride via a Horiuti-Polanyi like mechanism on the well-defined Pd(111) surface. Reaction path calculations were performed at a surface coverage of maleic anhydride corresponding to 0.11 monolayer. The hydrogenation of maleic anhydride to the maleic anhydryl surface intermediate was found to have the highest intrinsic barrier (+95 kJ/mol) on Pd(111), of all the elementary steps involved in the hydrogenation of the olefinic group of maleic anhydride. This step is endothermic by 17 kJ/mol. Calculations indicate that the second step in the mechanism, the hydrogenation of maleic anhydryl to succinic anhydride, has a slightly lower barrier (+89 kJ/mol) and is exothermic by 37 kJ/mol. In an effort to understand the effect of change in the electronic properties of the metal on the intrinsic barrier for maleic anhydride hydrogenation, the C-H bond formation reaction was reexamined on Re(0001), PdML/Re(0001), and Pt(111). Among the surfaces studied here, the hydrogenation reaction has the highest barrier on Re(0001) and the lowest barrier on Pt(111). The calculations suggest that the PdML/Re(0001) pseudomorphic overlayer has a lower intrinsic barrier than Pd(111) and Re(0001) for C-H bond formation during maleic anhydride hydrogenation. A model based on frontier orbital theory was developed in order to explain the trends in surface reactivity. As the average position of the d-band of the surface metal layer shifts away from the Fermi-energy, the intrinsic activation barrier for C-H bond formation is lowered. The converse is true for the microscopic reverse reaction of maleic anhydryl β C-H bond activation. Based on the model predictions, the intrinsic barrier for C-H bond formation is expected to be small on the noble metals. However, the group IB metals do not make effective hydrogenation catalysts, because the overall reaction rate is more likely limited by the dissociative chemisorption of dihydrogen, rather than by C-H bond formation.