| With the advancement of science, technology and society, massive consumption of primary energy such as coal and oil have caused severe energy and environment crisis, so it is extremely urgent to seek clean and efficient energy at present. Natural gas as a kind of high calorific value and clean fuel plays a more and more important role in the world energy structure. Generally, syngas is produced mainly by gasification of coal and biomass, wihle according to our country's energy structure and the increasing demond of natural gas, it is of strategic significance for developing the coal-to-SNG technology. The main reaction for coal-to-SNG is synthesis gas methanation, especially the methanation of CO. But during the actual process, CO2 will be introduced through side reactions. As known, CO2 as a kind of greenhouse gas can cause global warming. Especially since the industrial revolution, the content of CO2 in atmospheric increased rapidly, leading to the ecological and environmental issues much more serious. Currently, CO2 methanation has become the hot spot of research around the world with the increasing demand of clean energy. The reaction of CO2 methanation become an effective way to reduce the emissions of CO2 and meanwhile meet the demand of natural gas.As to CO methanation, the catalysts are prone to carbon deposition under high temperature, so it is significant to investigate the effect of C on catalytic performance. Besides, investigation into the mechanisms of CO2 methanation not only can understand the reaction process, but can provide theoretical guidance for calatyst preparation. The mechanisms of CO2 methanation still remain controversial, and the controversy mainly fouces on whether CO intermediate generates during this process. Recently, the catalsysts for methanation are mainly based on Ni catalysts in the industry production proces, therefore, it has important practical significance to investigate the reaction of CO2 methanation on Ni catalyst.In this paper, we have investigated the CO methanation on Ni(111) and CNi(111) surface using the DFT method with the periodic slab model. These results sugget the most stable adsorption sites and configurations of all reaction species (H, O, C, CO, CHx, etc.) of CO methanation. The main process of CO methanation is that CO firstly decomposes into C and O, then the adsorbed C hydrogenates to CH4 through multiple steps. The highest barriers of this reaction on Ni(111) and CNi(111) surface are 237.4 kJ/mol and 187.6 kJ/mol, respectively. By comparison, CO methanation on CNi(111) surface has a lower activation enegy and the rate-determining step on CNi(111) has also altered from CO?C+O on Ni(111) surfae to CH3+H?CH4.Three different mechanisms have been investigated of total CO2 methanation on Ni(111) surface using the DFT method. The most stable adsorption sites and configurations of all reaction species (H, O, C, CO, OH, CHx, HCOO, CH2O, C(OH)2, etc.) in three paths of CO2 methanation adsorbed on the Ni(111) surface are obtained. The three mechanisms can be devided into two categories:with and without the formation of CO as an intermediate. The main process of the first mechanism is that CO2 reacts with H to produce HCOO species. HCOO then dissociates into CO+ OH and CO subsequently hydrogenates to CH4. The highest energy barrier of this path is 306.8 kJ/mol, and the rate-determining step is HCOO?CO+OH; For the second mechanism,CO2 firstly decomposes into CO and O on the Ni(111) surface, then CO decomposes into C+O and adsorbed C hydrogenates to CH4. The highest barrier of the rate-determining step (CO?C+O) in path two is 237.4 kJ/mol; In the third mechanism, CO2 reacts firstly with H to produce C(OH)2 species, which then dissociate into CH2O and OH species followed by the dissociation of CH2O into CH2 species. The highest barrier of this path is 292.3 kJ/mol, and the rate-determining step is CO2+2H?C(OH)2. Based on these analysis, path 2 is the relatively optimal mechanism. |
PDF Full Text Request