| CH4/CO2reforming catalyzed by supported Ni-based catalysts is a key reaction in the process of coal chemical engineering. Because of its good initial activity and low cost, Ni-based catalysts have been widely applied and investigated. However, a fatal shortcoming exists for Ni catalyst, i.e., the serious carbon deposition. Therefore, to solve the problem of carbon deposition using supported Ni-based bimetal catalysts, it is necessary to systematically investigate the formation mechansim of carbon deposition. In this study, based on quantum chemical DFT calculations combined with experimental characterization methods, all possible mechanisms of carbon deposition formation are firstly proposed and disscussed in order to illustrate micro-dynamic of carbon deposition formation at the electronic level, the structure-activity relationship of the catalysts is elucidated, and the simplified electron-gas model of metal-support interaction is proposed to clarify the effect of support on the activity of catalyst. Focusing on the problem of carbon deposition on Ni-based catalysts, the methods of solving the carbon deposition are proposed, which can provide the basic theoretical clues for filtration, modification and design of the new supported catalysts. In this work, the ideas to solve carbon deposition from muti-path view, i.e., suppressing, eliminating and inhibiting carbon deposition, were proposed, and a series of models including of single metal(Fe,Co,Ni,Cu) and NiM(M=Fe,Co,Cu) bimetallic alloys were built. Based on the models, the carbon deposition were investigated in CH4/CO2reforming, and the results are in line with those from experimental observation. At the same time, the models of species migration on the surfaces also were built, on which the process of migration of C and O were investigated. The results show that elimination of carbon deposition by increasing the migration ability of O is found to be infeasible in theory. Whereas, that by decrease the migration ability of C is found to be feasible. Then, the dissociation pathway of CH4on catalysts are systematically investigated to illustrate the change rule of rate-determining and activation barrier, meanwhile, the mechanism of pyrolytic C elimination is analyzed to obtain the microscopic mechanism of C plus O reaction. Further, the mechanism of pyrolytic C accumulation are analyzed to clarify the reaction on different catalyst surfaces.On the basis of the metal-support interaction, the electron-gas model of supported catalyst are provided to adjust the metal-support interaction by making the electron transfer from metal to support, in which the basic principle of inhibiting the carbon deposition are obtained. The elementary reaction involving carbon deposition on different catalysts are synthetically analyzed, which show that when the activation barrier (1.36eV) of rate-determining step for CH4dissociation increase by15-50%in comparison with that on Ni surface, the formation of pyrolytic C can be inhibited in the expense of decreasing the rate of CH4dissociation to some extent, further the formation of carbon deposition is inhibited.Our results show that the carbon deposition is easily formed on Fe, Ni, NiFe, NiCo and uniform NiCu surfaces, whereas, the carbon deposition is hard to form on segregate NiCu surface. As a result, the theoretical clues for the design of anti-carbon deposition are provided, in which the d-band center away from Fermi level is the microscopic factor to inhibit the formation of pyrolytic C in reaction systems. Meanwhile, the electrons transfer from metal to support by adjusting the metal-support interaction can be used to realize the inhibitation of carbon deposition. Finally, adding different metal into Ni can inhibit the migration of pyrolytic C, further inhibit the accumulation of pyrolytic C to form carbon deposition. |
PDF Full Text Request