Insight To The Difference Caused By Acid And Alkaline In Pd-Catalyzed Methanol Oxidation In The Light Of Quantum Chemistry

The direct methanol fuel cell (DMFC) is a variant of the proton exchange membrane (PEM) fuel cell, which uses aqueous methanol directly without prior reforming and releases final products just CO₂ and H₂O, meeting human being's environmental friendly demand. DMFC is regarded as one of the most promising fuel cells, but the cost and poor catalysis of electrode materials in DMFC, together with other technical problems, block DMFC from commercialization. In recent years, intensified research focuses on intermediates to speculate methanol dehydrogenation steps. To further and systemically investigate the methanol oxidation model is the hard and hot point in electrochemistry research.Electrocatalytic reaction is not only affected by the nature of catalyst, but also the electric intensity across the interface of electrode/electrolyte as well as the property of electrolyte. Whereas, vacuum model as a simple one widely used to study the species' function of adsorption and metal's catalysis capacity does not accurately simulate the real situation. In this paper, density function theory (DFT) was used to study the adsorption structures and energy of methanol and the related intermediates adsorbed on Pd (111) in vacuum at first, and the TS method was used to calculate the energy barrier of the methanol reaction in the first step. These results were further served as the initial configurations for the subsequent study, that is, insight to the difference caused by acid and alkaline in Pd-catalyzed methanol oxidation in the light of quantum chemistry.The mechanism for the first step of methanol dehydrogenation on palladium was studied in terms of energy barrier. The electric effect on methanol oxidation can't be omitted in the electrochemical system. Thus using CH3 or OH approaching to Pd as the initial model, the relationship between the methanol configurations adsorbed on Pd (111) surface and the electric effect was studied.Practically, methanol oxidation always happens in acid or alkali solution. So the last but not the least, effect of the medium on methanol oxidation was studied. In order to construct an appropriate acid or alkali model solution, an HCl or NaOH was introduced independently to the system by substituting for a water molecule in methanol. Considering the interaction among the unit cell and its images in periodic system, the corresponding calculations were executed with the slab model and DFT-GGA method.The results showed that the isolated methanol molecule was physically adsorbed atop Pd (111) with adsorption energy of 38.4kJ/mol. The first-step reaction of methanol oxidation on Pd (111) is the rupture of O-H bond because of a lower energy barrier. The Pauling interaction between the intermediates coadsorbed on metal surface would affect the adsorbing structures, thus there were differences among the different adsorbing models. Having been affected by the electric field, C-O bond axis would deviate from normal of Pd (111) plane at the lower electric field, and had few changes with further increase of electric field intensity in the case of methanol adsorbed through OH to the Pd surface. Being different from that, in the case of methanol adsorbed through CH3 group on Pd (111), the adsorption configurations of methanol had fewer change at a lower electric field, however, the configurations changed a lot at a higher electric field. In this case, it is O rather than CH3 group that is inclined to approach to the Pd (111) surface.It was found that there was great difference in acid and alkaline for Pd-catalyzed methanol oxidation. Methanol is not activated in pure water. However, bonded through hydrogen bond leads to adsorption energy of methanol decreasing by -23.2kJ/mol relative to the methanol adsorbed on vacuum Pd (111). In acid solution, methanol activation and puckered hexagonal hydrogen bonding network was not found. However, hydronium ion and stronger hydrogen bonds were formed due to introduction of proton ions. The puckered hexagonal methanol-water didn't exist at all owing to interference of Cl- ions. In alkaline solution, the elongation and even the rupture of OH in methanol could occur. The overall energy also changes a lot in alkaline solution. Apart from that, a much more compact new puckered hexagonal and a decagonal hydrogen bonding networks formed. Methanol activation in alkaline is undoubted.