Studies On Ni-Co Bimetallic Catalysts For CO₂ Reforming Of Methane To Syngas

CO₂ and CH₄,two major greenhouse gases,can be converted into syngas by CO₂ reforming of methane to syngas（CRM）,which is meaningful for reducing greenhouse gas emissions and developing alternative energy.The objective of the dissertation is to develop high performance Ni-Co bimetallic catalysts for CRM under both atmospheric and high pressures,and investigate the stability,carbon deposition behavious,synergestic effect of Ni and Co,kinetics study,and deactivation.According to our previous research,the addition of β-cyclodextrin（CD）was benefit for dispersing Ni particles.In this thesis,xNiyCo/SBA-15-CD（x+y=5%,x/y=0/5⁵/0,mass ratio）bimetallic catalysts,prepared by β-CD modified impregnation method,were thoroughly characterized by comprehensive characterization methods and employed in CRM.The detailed structural parameters and local coordination environments were related to the catalytic performance in CRM reaction.Co species dissolved into the NiO lattice during the preparation process,resulting in the formation of NixCoyO,which was the precursor of Ni-Co alloy.Ni-Co alloy was formed during the reduction process.The TOFs of bimetallic Ni-Co catalysts were greater than the monometallic Ni or Co catalysts,and the stabilities of bimetallic catalysts with Ni/Co=2.5/2.5⁴.5/0.5 were better than the monometallic catalysts.The synergistic effect of Ni and Co,the addition of β-CD and the confined effect of ordered mesoporous structure contributed to the better catalytic performance.Among the bimetallic catalysts,4.5Ni0.5Co and 3Ni2 Co showed the best stability in CRM at both atmospheric and high pressures.The better stability of 4.5Ni0.5Co can be attributed to a Co-enriched surface segregation structure,whereas the highest proportion of Ni-Co alloy（precursor）in 3Ni2 Co should be the primary reason to the better performance.The effect of enrichment of Co and the formation of Ni-Co alloy on the stability were further confirmed by design experiments.The accumulation of carbon deposition took place mainly at the beginning of the reaction at high pressure CRM reaction.The type of coke,instead of the amount of carbon deposition,was proven to be the main reason responsible for the deactivation of xNiyCo/SBA-15-CD catalysts.The tests of different time on stream（TOS）on4.5Ni0.5Co/SBA-15-CD suggested that the disorder degree of carbon deposition,the reactivity of β-carbon towards oxygen as well as the sintering were identified as the primary reasons for deactivation.The kinetic measurements suggested that the forward reaction rates were monotonically increasing with the CH₄ partial pressures（200-400kPa）,and independent of CO₂ partial pressures（100-300 kPa）on4.5Ni0.5Co/SBA-15-CD at 2.0 MPa total pressure and 620-660 oC.Apparent activation energy of CRM reaction at high pressure measured on 4.5Ni0.5Co/SBA-15-CD was106.5 kJ/mol and the mechanism of stability improvement of Co addition in Ni-based catalysts was discussed.