Theoretical investigation of solution effects on metal catalyzed hydrogenation and oxidation processes

Liquid phase metal catalyzed processes are important in the syntheses of various fine chemicals and pharmaceutical intermediates. Because of the complex nature of solution-metal interfaces, little is currently known about the molecular mechanisms governing these processes. Herein, we begin to extend the application of first principles based methods to model surface phenomena in the presence of solution. Periodic density functional theory and ab initio molecular dynamics are used to study the effects of solution phase on the mechanisms and energetics of chemisorption and reactivity at the solution/metal interface.; Three different interfacial processes are examined: (i) Dissociative adsorption of acetic acid over Pd, (ii) Hydrogenation of formaldehyde (a model oxygenate) over Pd and (iii) CO oxidation over Pt-Ru surfaces in aqueous solution (relevant to direct methanol fuel cells). In all cases the solution phase is found to affect the nature of the interface and the surface kinetics. The dissociation of acetic acid over Pd(111) occurs homolytically in the vapor phase but proceeds heterolytically in the presence of solution, leading to the formation of chemisorbed acetate and solvated protons, Water forms hydrogen bonds with the surface intermediates and weakens their interactions with the surface. The acetate ions are more stable in bulk solution than over the surface. There is, however, an activation barrier for their displacement from the surface into the solution.; Water solvates the intermediates involved in the hydrogenation reaction to different extents and changes the energetics of the process. The activation barrier for hydrogenation in solution is lower than in the vapor phase since water stabilizes residual charges on the activated complex. In addition, water provides an environment that facilitates the formation and stabilization of protons at the metal surface. The solution offers a novel route for hydrogenation whereby the reaction occurs by the transfer of protons through water. The solution phase plays a similar role in the oxidation of CO. The solvent acts as a dielectric medium that stabilizes the reaction intermediates, and hence affects the reaction energetics, as well as plays a direct catalytic role in mediating the oxidation chemistry.