Theoretical Study On Several Important Reactions Catalyzed By Gold-based And Platinum-based Catalysts

The problem of energy and resources challenges human in the21th century. In order to relieve energy crisis and reduce environment pollution, many countries in the world devote to exploring efficient and clean energy. Fuel cell is the green and clean energy, which is the energy star of the21th century. Time in America treat it as the most important technology. In recent years, the low carbon economy promotes development and commercialization of fuel cell.Direct methanol fuel cells (DMFCs) and direct formic acid fuel cells (DFAFCs), as a viable power sources with potential applications in many systems, have attracted tremendous attention because of their excellent performance in all aspects. Pt and Pt-group metals are known as the most excellent electrocatalysts for methanol and formic acid oxidation, so they are extensively used as electrode materials of both the anode and cathode of DMFCs and DFAFCs. However, it is well known that pure Pt catalysts usually suffer from two major disadvantages,(ⅰ) high cost and limited resources, and (ⅱ) poor performance durability resulted from the poisoning of CO and CO-like species produced during methanol and formic acid oxidation at low temperatures. These disadvantages are the important bottlenecks of DMFC/DFAFC's scale applications. Therefore, searching for the catalysts that are of excellent catalytic activity, good stability and improved tolerance towards CO-poisoning is becoming a challenging issue in the study of DFAFCs.The recent studies show that alloying Pt with another transition metal or noble metal that is generally less expensive than Pt could address these two challenging problems due to their significantly improved tolerance towards CO-poisoning and enhanced electrocatalytic activity for methanol oxidation as compared to pure Pt catalysts. This provides new thought for designing efficient catalysts used in DFAFCs, which is using the synergism among the metals and exploring the Pt-and Pd-based catalysts. However, the relevant mechanism behind the phenomenon remains unclear, and some experimental observations are still not well understood.The dissertation traces international research frontier, and aims at the challenging issue in the study of DMFCs/DFAFCs. This proposal aims to elucidate the molecular mechanism of formic acid electro-oxidation promoted by a series of Pt-based catalysts based on the results of the density functional theory (DFT) calculations. Our studies will pay substantial attentions on the following several aspects:(ⅰ) Au, Pt mono-metal and bimetallic clusters and surfaces'structure and performance,(ⅱ) microscopic mechanism of catalysts and CH3OH, HCOOH'interactions,(ⅲ) the mechanism details of formic acid oxidation along various possible pathways,(ⅳ) the thermodynamic and dynamical properties of the formic acid oxidation,(ⅴ) the relative stability of various activate intermediates and transition states involved along each pathway,(ⅵ) key factors controlling the catalytic activity of Pt-based catalysts, and the new and reasonable theoretical model describing the electro-oxidation of formic acid. We believe that the theoretical results would provide valuable guidance and assist for the rational design of efficient Pt-based electrocatalysts of DMFCs/DFAFCs.The valuable results in this dissertation can be summarized as follows:1. The density functional theory (DFT) calculations are carried out to study the mechanism details and the ensemble effect of methanol dehydrogenation over Pt3and PtAu2clusters, which present the smallest models of pure Pt clusters and bimetallic PtAu clusters. The energy diagrams are drawn out along both the initial O-H and C-H bond scission pathways via the four sequential dehydrogenation processes, respectively, i.e. CH3OH→CH2OH→CH2O→CHO→CO and CH3OH→CH3O→CH2O→CHO→CO, respectively. It is revealed that the reaction kinetics over PtAu2is significantly different from that over Pt3. For the Pt3-mediated reaction, the C-H bond scission pathway, where an ensemble composed of two Pt atoms is required to complete methanol dehydrogenation, is energetically more favorable than the O-H bond scission pathway, and the maximum barrier along this pathway is calculated to be12.99kcal/mol. In contrast, PtAu2cluster facilitates the reaction starting from the O-H bond scission, where the Pt atom acts as the active center throughout each elementary step of methanol dehydrogenation, and the initial O-H bond scission with a barrier of21.42kcal/mol is the bottom-neck step of methanol decomposition. Importantly, it is shown that the complete dehydrogenation product of methanol, CO, can more easily dissociate from PtAu2cluster than from Pt3cluster. The calculated results over the model clusters provide assistance to some extent for understanding the improved catalytic activity of bimetal PtAu catalysts towards methanol oxidation in comparison with pure Pt catalysts. 2. By performing density functional theory calculations, we studied the methanol decomposition promoted by neutral, anionic, and cationic Au trimers, which represent three simplest prototypes of Au-cluster-based catalysts with different charge states. The theoretical results show that the Au3-and Au3+-mediated reactions proceed via the four successive single dehydrogenation steps CH3OH→CH3O→CH2O→CHO→CO, while Au3--mediated reaction occurs through two double dehydrogenation steps CH3OH→CH2O→CO. The additional negative charge remarkably reduces the binding capability of CO (the completely dehydrogenated product of methanol) on the cluster, and thus is favorable for reducing the catalyst poisoning by CO. In contrast, the neutral and positively charged clusters present strong interaction with CO, making the catalyst is easily poisoned by CO. Furthermore, the reaction promoted by the cationic cluster shows a much higher energy barrier than those by the neutral and anionic clusters. So selecting suitable substrates that make Au nanoparticles negatively charged may be a promising strategy for promoting methanol oxidation.3. By performing density functional theory calculations, we have studied the CO pathway and non-CO pathway of the methanol oxidation on the PtAu(111) bimetallic surface. CO is shown to possess larger adsorption energy on the PtAu(111) surface than that on the pure Pt(111) surface, and the non-CO pathway on the bimetallic surface is found to be energetically more favorable than the CO pathway. These calculated results propose that the improved electrocatalytic activity of PtAu bimetallic catalysts for methanol oxidation should be attributed to the alternation in the major reaction pathway from the CO pathway on pure Pt surface to the non-CO pathway on the PtAu bimetallic surface rather than the easier removal of CO on PtAu catalysts than that on pure Pt catalysts.4. By performing density functional theory calculations, we have studied the dual-path mechanism of formic acid (HCOOH) oxidation on the PtAu(111) surface in the continuum water solution phase. The direct pathway involving the dehydrogenation of HCOOH to form CO2occurs with a barrier of15.5kcal/mol, which is in contrast to the much higher barrier of99.2kcal/mol in the indirect pathway involving the dehydration of HCOOH to form CO intermediate. In comparison, the calculated barriers on Pt(111) surface in direct and indirect pathways are5.8and32.9kcal/mol, respectively. The theoretical results emphasize that bimetallic PtAu(111) surface significantly increases the barrier difference between the two pathways to83.7kcal/mol from27.1kcal/mol on the Pt(111) surface, and thus can hinder remarkably the indirect pathway. The theoretical results rationalize well the experimental finding that bimetallic PtAu catalysts show higher catalytic activity towards HCOOH oxidation than pure Pt catalysts.5. We establish the new and reasonable theoretical model. HCOOH molecules can form stable dimers through a variety of hydrogen-bonded structures with a formation energy up to15kcal/mol, so we conjecture that HCOOH electro-oxidation is relative with dimers'structure. We design dimer model and water assistance proton bridge model of HCOOH electro-oxidation. The theoretical results show that (ⅰ) the HCOOH dimer which adsorbs on the Pt(111) surface forms CO intermediate via intermolecular dehydration and leads to catalysts'CO-poisoning,(ⅱ) the carbonyl and hydroxyl group of HCOOH can form hydrogen bonds with surrounding water molecules, which may play an important role to HCOOH electro-oxidation, and water assistance proton bridge can promote the reaction and reduce the energy barrier. The newly proposed mechanism improves our understanding for the mechanism of catalytic HCOOH oxidation and rationalizes the easy CO poisoning of Pt-based catalysts.