(135c) First Principles Analysis of the Aqueous Phase Selective Oxidation of Ethanol Over Model Pd, Au and Pt Surfaces
The increasing demands for oil along with its limited supply will likely force much of the current petroleum-based chemical industry to transition to renewable feedstocks and their conversion into value added chemicals. The selective oxidation of complex biomass derivatives, such as hydroxymethylfurfural (HMF), glycerol and other polyols offers one such attractive route to selective production of organic acids used in polymerization processes. In order to understand the governing elementary steps involved in the catalytic oxidation of polyols we examined in detail the selective oxidation of ethanol over different metal surfaces. Ab initio density functional theoretical calculations were carried out to analyze the oxidation of ethanol to acetic acid over model Pd and Au (111) surfaces in both the vapor and aqueous phases. All of the reported calculations were performed using Vienna Ab Initio software program (VASP).
The results reveal that ethanol adsorbs and preferentially dehydrogenates to form the acetoxy intermediate [CH3CH(OH)] which is followed by the subsequent deprotonation to form acetaldehyde. Acetaldehyde is then oxidized via an -OH addition to the ethoxy-1,1-ethane-diol intermediate [CH3CH(OH)O] before ultimately dehydrogenating to form acetic acid.
The role of oxygen and hydroxide on the calculated reaction steps revealed that the presence of either oxidant on the surface (at low coverages) appears to enhance O-H activation while suppressing the activation of C-H during hydrogenation steps over the model Pd surface. Their presence can affect the activation barrier as much as 0.6 eV. In addition to the initial studies carried out in the vapor phase over Pd(111), both Pt(111) and Au(111) surfaces were also examined. The calculated reaction energies and activation barriers directly correlate with calculated changes in the oxygen binding energy over each of these metals and the predicted d-band center for each metal surface. Pt binds stronger than Pd by an average of 0.24 eV; Pd binds more strongly than Au by an average of 0.68 eV. Gold is known to be very noble in a closed packed crystal formation.
The aqueous solution phase was found to significantly influence surface reactivity. An aqueous bi-layer consisting of 23 water molecules was used in the calculations over all three metal surfaces to examine the influence of an aqueous media. The results clearly reveal the predominant influence of water is the result of changes in the hydrogen bonding between the key intermediates in the reactant and transition states. On Pd(111), the binding energy is reduced on an average by 0.33 eV, whereas on Au(111) and Pt(111), the binding energy actually increased. The reaction energies on Pd is reduced via the presence of water which suggests that water acts to lower the activation barrier. The results over different metals follow the classic Evans-Polanyi relationship. The resulting kinetics are compared with experimental results for the oxidation of ethanol over supported Pd and Au catalysts.