| Adsorption and reaction of catalyst surface are considerable important due to its great potential in many industrial applications, environmental protection and in catalysis science study fields. The nature of the catalyst surface is closely related to its surface electron structure, adsorption species and its environment. Therefore, it is important to study the atomic arrangement, bond formation and chemical reaction on the surface of the catalyst at atomic level. The aim of controlling matter at the molecular scale by engineering the electronic structure is not restricted to catalytic materials, it is a general challenge in chemistry,physics and materials science.In this dissertation, we take methanol dehydrogenation as the model system, using the first-principles density functional theory (DFT) to study the reaction process on Pt-Ni alloy surface, comprehensively investigate the reaction potential energy surface and the complex dehydrogenation reaction networks. On the basis of the existing experimental research, the theoretical calculation results and the experimental phenomena are compared and analyzed to find out the key steps and control factors of the reaction, and provide theory clues to the designation of new efficient catalyst. The structural optimization, the transition state and the potential energy surface are determined by DFT calculation with the periodic slab model. The electronic structure of the adsorption structure and the reaction element process is realized by means of density of states, work function and difference charge density. The kinetic analysis of chemical reaction was further discussed to determine the reaction selectivity. On this basis,we put emphasis on the following contents which are related to the reaction microcosmic mechanism and reaction selective. The main results are as follows:1. The intrinsic facet-dependent adsorption properties of CH3OH on the surface of Pt3Ni catalyst was investigated quantitatively. I analyzed the adsorption characteristics of the CH3OH by optimizing the geometrical structures and calculating the adsorption energies at each site on the (111),(100),and (110) surfaces of Pt3Ni catalyst based on density functional theory (DFT) calculations with van der Waals (vdW) corrections to the DFT total energy. The results indicate that the adsorption strength of the CH3OH at the surface of Pt3Ni catalyst is facet-dependent,and follows the order of Pt3Ni( 110) >Pt3Ni(111) > Pt3Ni(100). The mechanism of the facet-dependent adsorption of CH3OH on the catalyst is further discussed and explained based on the shifts of the d-band center of the Ni component relative to the Fermi level, the density of states, and work function changes of each surface of Pt3Ni catalyst, and the polarization effects of the adsorbed CH3OH. It is believed that the results gained through this work will provide useful information that guides the rational design and construction of nanoarchitectured catalyst surfaces for the optimization of heterogeneous catalysis.2. The detailed mechanism of methanol decomposition on Pt3Ni(111) was studied based on self-consistent periodic density functional theory calculations. The geometries and energies of methanol and its intermediates are analyzed, and the decomposition network is mapped to illustrate the decomposition reaction mechanisms. On Pt3Ni(lll),the less electronegative Ni atoms are more favorable for adsorbing radical intermediates and intermediates with lone-pair electrons (such as O-containing species). The possible pathways through initial scission of the O-H, C-H, and C-O bonds in methanol are studied and discussed based on the steric effect and electronic structure of the related transition states and the Br(?)nsted-Evans-Polanyi (BEP) relationships. The initial scission of the O-H bond is the most favorable and bears the lowest energy barrier among the three decomposition modes (initial scission of O-H, C-H, and C-O bonds). The decomposition of the energy barrier analysis indicates that the high energy barrier for initial C-H and C-O bond scission is caused by the large structural deformation, strong repulsive interaction,and the low adsorption ability of the decomposed species in their transition states. Potential energy surface (PES) analysis confirmed that the favorable decomposition pathway for methanol on Pt3Ni(111) proceeds via CH3OH → CH3O → CH2O → CHO → CO, in which scission of the O-H bond is the rate-limiting step. The comparison between the current results and CH3OH decomposition on other systems shows that Pt3Ni(111) can efficiently promote methanol decomposition and alleviate the CO poisoning problem when it is used as an anode catalyst in direct methanol fuel cells (DMFCs).3. The detailed mechanism of methanol decomposition on Pt3Ni(100) was studied based on self-consistent periodic density functional theory calculations. The geometries and energies of methanol and its intermediates are analyzed, and the decomposition network is mapped to illustrate the decomposition reaction mechanisms. Similar to the result of Pt3Ni(111), On Pt3Ni(100), the less electronegative Ni atoms are also more favorable for adsorbing radical intermediates and intermediates with lone-pair electrons(such as O-containing species). The possible pathways through initial scission of the O-H,C-H, and C-O bonds in methanol are studied and discussed based on the steric effect and electronic structure of the related transition states and the Bronsted-Evans-Polanyi (BEP)relationships. The results indicate that the O-H bond scission of CH3OH is thermodynamically and kinetically favorable on Pt3Ni(100) while the C-O bond scissions are unlikely to occur at low temperature. Potential energy surface (PES) analysis confirmed that the favorable decomposition pathway for methanol on Pt3Ni(100) proceeds via CH3OH → CH3O → CH2O → CHO → CO, CHO is the major intermediate in the decomposition process. The C-H bond scission of CHO to CO is the rate-limiting step. It provides a theoretical guidance for better understanding of the catalytic performance of(100) in the PtNi alloy and to improving the performance of the DMFC. |
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