Theoretical Study On Hydrogenation And Deoxygenation Of Phenol On Pt(111) And Pd(111) Surfaces

With the developing of the society and economy, the energy demand will increase rapidly in the future. However, traditional energy resources cannot meet the demand of modern society, because fossil energy will be exhausted soon. It is a very severe situation for our human beings. Biomass has been paid more and more attention because it is a new energy resource with many advantages, such as renewable, environment friendly and low region restriction etc. Hydrogenation and deoxygenation of lignin derived phenolics are the key steps for conversion of lignin to fuels and chemicals. The adsorption, dissociation, hydrogenation and deoxygenation of phenol on the Pt(111) and Pd(111) surfaces have been studied, using density functional theory slab calculations. The results indicate that phenol favors adsorption through a mixed ?-? interaction on Pt and Pd surfaces through its phenyl ring, with the hydrogen atoms and hydroxyl group tilted away from the surface. The dissociation of phenol to phenoxy is both thermodynamically and kinetically favored on Pd but not on Pt. The phenoxy adsorbs on Pd through both the phenyl ring and the oxygen atom whereas the O atom points away from the surface on Pt. On Pt, the barrier for adding one hydrogen atom to the adsorbed phenol is 0.49 eV lower than the overall barrier for phenol dissociation to phenoxy followed by adding the hydrogen atom to its phenyl ring, resulting in direct hydrogenation of the adsorbed phenol to cyclohexanol as the dominant reaction pathway. In contrast, on Pd, the barrier for direct hydrogenation(1.22 eV) is higher than the overall barrier of dissociation followed by hydrogenation process(0.85 eV), resulting in hydrogenation of the adsorbed phenoxy to cyclohexanone as the major reaction pathway. Microkinetics analysis confirms that hydrogenation of the adsorbed phenol is the dominant pathway on Pt whereas phenoxy hydrogenation drives the turn-over on Pd. These results are consistent with the experimentally observed selectivity of phenol hydrogenation on Pd and Pt catalysts.The deoxygenation of phenol on Pt(111) has also been studied using density functional theory. It is found that the electron delocalization effect will strongly affect C-O bonding strength. When one ortho-C was hydrogenated, the electron delocalization would be weakened. The length of C-O increases from 1.36 ? to 1.37-1.39 ?, and the activation energy of deoxygenation reaction decreases from 2.61 eV to 2.04-2.14 eV. If both of ortho-C were hydrogenated, the electron would be localized around ?-C. The length of C-O increases further to 1.40 ?, and activation energy of deoxygenation reaction decreases to 1.70 eV. When the phenyl ring was saturated hydrogenated, all electrons would be localized. Even though the length of C-O bond is still 1.40 ?, the activation energy of C-O bond breaking decreases to 0.76 eV. Overall, these results indicate that the electron delocalization effect is an important factor that affects phenol deoxygenation reaction.