The Development Of New Methods For Potential Energy Surface Exploration And Their Applications During The Investigation Of Complex Heterogeneous Catalytic Systems

With the development of computer and quantum theory in recent years, using theoretical method to simulate heterogeneous catalytic system is becoming more and more popular nowadays. With reasonable model, and efficient potential energy surface searching method, people can get much useful information of concerned reactions in the field of thermodynamics and kinetics, which can be used to understand the physical and chemical properties of the catalyst. One of the most interesting catalytic systems is the gold catalyst during the last decade. People found the nano gold catalyst can be used to catalyze many reactions such as CO oxidation and alcohol oxidation. Although there has been many theoretical works focusing on the mechanism of gold catalytic system, most works were dealing with simple reactions. For complex systems, the current potential energy surface searching method, especially the transition state searching method is neither efficient nor stable. For the purpose of investigating complex gold catalytic systems, we developed several potential energy surfaces searching method. By using these methods, we have investigated the effect of the reaction environment on the activity of the gold catalyst. Specifically, we have focused on the following problems:Transition state searching:To determine transition state (TS) and thus predict chemical activity has been a challenging topic in theoretical simulation of chemical reaction. In particular, with the difficulty to compute the second derivative of energy (Hessian) in modern quantum mechanics packages with non-Gaussian basis set, the location usually involves a high demanding in computational power and lacks of stability in algorithm, especially for complex reaction systems of many degrees of freedom. Here an efficient TS searching method is developed by combining the constrained-Broyden-minimization algorithm with the dimer method that was firstly proposed by Henkelman and Jonsson. Based on our results on Baker reaction system and a heterogeneous catalytic reaction, our method is shown to increase the efficiency significantly, and is also more stable in finding TSs. Although the CBD method is very efficient, however, it often requires a high demanding in computational power and a good chemical intuition on reaction. Following the CBD method, a new reaction path searching method is developed by combining our recently-developed transition state (TS) location method, namely, the constrained Broyden dimer method, with a basin-filling method via bias potentials, which allows the system to walk out from the energy traps at a given reaction direction. In the new method, the reaction path searching starts from an initial state without the need for pre-guessing TS-like or final state structure and can proceed iteratively to the final state by locating all related TSs and intermediates. The method is tested successfully on the Baker reaction system (50elementary reactions) with good efficiency and stability, and is also applied to the potential energy surface exploration of multistep reaction processes in the gas phase and on the surface.The global searching of the potential energy surface:The global searching of the potential energy surface is also a very important part in the potential energy surface searching technics. By using this kind of method, we can automatically get many thermodynamic information of a certain system. Based on the BP-CBD method, and Metropolis Monte Carlo method, we propose an unbiased general-purpose potential energy surface (PES) searching method for both the structure and the pathway prediction of complex system. A central feature of the method is able to perturb smoothly a structural configuration towards a new configuration and simultaneously has the ability to surmount the high barrier in the path. We apply the method for locating the global minimum (GM) of short-ranged Morse clusters up to103atoms starting from random structure without using extra information of the system. In addition to GM searching, the method can identify the pathways for chemical reactions with large dimensionality, as demonstrated in a nano-helix transformation containing222degrees of freedoms.Moisture-assisted CO oxidation on gold/y-alumina:Being a poor catalyst under dry conditions, γ-Al2O3-supported gold (Au/γ-Al2O3) turns out to be a superior CO oxidation catalyst under moisture conditions. In this work, extensive density functional theory (DFT) calculations have been carried out to investigate the physical origin of the moisture promotion effect. By supporting Au strips on two most stable γ-Al2O3surfaces, namely (110) and (100) faces, we show that the majority (110) surface is catalytically inert due to the saturation of Al cationic sites by dissociated H2O. On the other hand, the minority (100) surface in combination with Au is responsible for CO oxidation activity, where O2can adsorb at the Au/γ-Al2O3(100) interface with a tilted Au-O-O-Alsc configuration. In the presence of coadsorbed H2O and CO, the adsorption energy of O2reaches to0.7eV. We find that H2O enables the direct dissociation of the reaction intermediate cis-OCOO produced by the bimolecular coupling between CO and O2, whereas an extra cis-to-trans rotation of OCOO is required in the absence of H2O. In the H2O-assisted pathway, no atomic oxygen is produced and the overall barrier is only0.25eV, which is0.28eV lower than that without H2O. By electronic structure analyses, we suggest that a modest acidity of the y-Al2O3(100) surface contributes to the O2adsorption, although Al2O3lacks the d-states that were shown to be important for O2activation.Origin and activity of gold nanoparticle as aerobic oxidation catalyst in aqueous solution:Whether gold is catalytically active on its own has been hotly debated since the discovery of gold-based catalysis in1980s. One of the central controversies is on the O2activation mechanism. This work, by investigating aerobic phenylethanol oxidation on gold nanoparticles in aqueous solution, demonstrates that gold nanoparticles are capable to activate O2at the solid/liquid interface. Extensive density functional theory (DFT) calculations combined with periodic continuum solvation model have been utilized to provide a complete reaction network of aerobic alcohol oxidation. We show that the adsorption of O2is very sensitive to the environment:the presence of water can double the O2adsorption energy to-0.4eV at commonly available edge sites of nanoparticles (-4nm) due to its strong polarized nature in adsorption. In alcohol oxidation, the hydroxyl bond of alcohol can break only with the help of external base at ambient conditions, whilst the consequent a-C-H bond breaking occurs on pure Au, both on edges and terraces, with a reaction barrier of0.7eV, which is the rate-determining step. The surface H from the a-C-H bond cleavage can be facilely removed by O2and OOH via a H2O2pathway without involving atomic O. We find that Au particles become negatively charged at the steady state because of facile proton-shift equilibrium on surface, OOH+OH(?)O2+H2O. The theoretical results are utilized to rationalize experimental findings and provide a firm basis for utilizing nano gold as aerobic oxidation catalysts in aqueous surroundings.