Catalytic Role Of Noble Metal-Like Molybdenum Carbide In Hydrogenation Of CO₂ To Methane

During the last decades,the growth of greenhouse gas emissions to the atmosphere has motivated society in the development of solutions in terms of CO2 capture and utilization.Hydrogenation of carbon dioxide to methane is an effective way to reuse and capture carbon dioxide.Moreover,CO2 methanation technology can convert CO2 into important chemical raw materials and clean fuel CH4,which can realize the recovery of carbon resources and the storage of hydrogen resources.So,highly efficient catalysts in CO2 methanation have always been the focus of scientists' research.In this thesis,a molybdenum carbide catalyst with direct synthesis of different crystal phases by calcination with aniline as carbon source was designed for the hydrogenation of carbon dioxide to methane.The bimetallic Ni-Rh was simultaneously loaded on ??-MoxC catalyst for carbon dioxide hydrogenation.Combined with the phase,morphology,surface area and adsorption performance of the prepared samples,the effects of structures and ingredients of catalysts on the catalytic performance were analyzed.Firstly,the precursors using aniline as a carbon source were calcined at different temperatures to prepare molybdenum carbides with four crystal phases of a-MoC1-x,???-MoC1-x,??-MoxC and?-Mo2C.The molybdenum carbides were used in CO2 hydromethanation reaction.It was found that the??-MoxC with ? and ? coexisting phases had the best catalytic effect.At the reaction temperature of 500?,the CO2 conversion rate was 55.7%and the CH4 selectivity was 69.8%.It is proved that there is indeed a transition state between the ? and ? molybdenum carbides.Secondly,we combined different transition metals and noble metals with ??-MoxC,and studied the CO2 hydromethanation activity of metal/??-MoxC.It was found that 3.0%Ni/??-MoxC and 1.0%Rh/??-MoxC are effective.3.0%Ni/??-MoxC has a CO2 conversion rate of 59.1%and a CH4 selectivity of 79.4%at the reaction temperature of 500?,which is about 10%higher than ??-MoxC alone in CH4 selectivity.1.0%Rh/??-MoxC has a CO2 conversion rate of 58.6%and a methane selectivity of 77.9%at the reaction temperature of 500?,which is about 7%higher than ??-MoxC alone in CH4 selectivity.Finally,the two active metals were combined with ??-MoxC by five methods.It was found that the catalytic activity of Ni-Rh/??-MoxC is the best when 1.0%Rh and 3%Ni were simultaneously loaded on ??-MoxC by co-precipitation method.At the reaction temperature of 500?,the CO2 conversion rate is 61.0%,and the CH4 selectivity is 86.6%.The CH4 selectivity of the Ni-Rh/??-MoxC is 16.7%higher than that ??-MoxC alone,7.2%higher than that 3.0%Ni/??-MoxC,and 9.7%higher than that 1.0%Rh/??-MoxC.Through SEM,HRTEM,H2-TPR,CO2-TPD,it is found that the Rh can reduce the reduction temperature of the single metal Ni catalyst,promote the dispersion of the metal Ni and reduce the particle size of the metal Ni.The bimetallic Ni-Rh shows the synergistic effect on catalytic activity.