Study On The Performance Of Ni/ZrO₂-based Catalysts For CO₂ Methanation

Since the industrial era,the large-scale use of fossil fuels has directly led to a large amount of carbon dioxide emissions,and the climate change-greenhouse effect-caused by it has become more and more serious.Among the various concepts or technical methods proposed in recent years to solve the problem of carbon dioxide release,CO₂methanation is considered to be one of the most effective methods.At present,there are two challenges in the application of nickel-based catalysts to this reaction:poor activity at low temperature and easy sintering of metallic nickel nanoparticles at high temperature.In order to effectively improve the low-temperature activity and anti-sintering ability of the catalyst,the structure of the catalyst was optimized in terms of carrier composition,particle size and dispersity of active metals,respectively.In this paper,a series of Ce_xZr_1-xO₂solid solutions and Y₂O₃-ZrO₂composite oxides were prepared by coprecipitation method as catalyst supports.Then,Ni was loaded on the prepared carrier by citric acid method.And after reduction,a series of highly dispersed supported Ni-based catalysts with small-size Ni nanoparticles supported on carriers with large specific surface area are obtained.The catalysts were characterized by X-ray diffraction（XRD）,N₂adsorption-desorption（N₂-BET）,temperature programmed adsorption（TPD）,temperature programmed reduction（TPR）,etc.The methanation performance of the catalyst CO₂was tested on a micro fixed bed reactor at atmospheric pressure,to investigate the effects of carrier and nickel content on the catalytic performance of the catalyst.The effects of Ni loading and Ce/Zr ratio on the structure of Ni/Ce_xZr_1-xO₂catalyst and the catalytic performance of CO₂methanation were investigated.When the Ni loading is 12 wt.%,the CO₂conversion and CH₄selectivity of the12%Ni/Ce_0.25Zr_0.75O₂catalyst are 74%and 100%respectively at 200℃,atmospheric pressure and space velocity of 15,000 mL·g^-1·h^-1.The stability test results for 300 h showed that 12%Ni/Ce_0.25Zr_0.75O₂catalyst had better sintering resistance than Ni/ZrO₂catalyst.The excellent activity of the catalyst was attributed to the adoption of a new loading method-citric acid complexation method to load the active component nickel,which realized high dispersion of nickel.The experimental results show that as Ce⁴⁺partially replaces Zr⁴⁺to form Ce_xZr_1-xO₂solid solution,the crystal form of ZrO₂changed from monoclinic phase to tetragonal phase,and the ceria-zirconia solid solution carrier formed is conducive to enhancing the interaction between active component Ni and the carrier.Therefore,the sintering resistance of the Ni-based catalyst was remarkably improved.The doping of Ce in the carrier improved the oxygen storage,release capacity of the catalyst and the dispersion degree of Ni.Therefore,the 12%Ni/Ce_0.25Zr_0.75O₂catalyst showd excellent catalytic performance,as well as good carbon deposition resistance and sintering resistance.The effects of Y/Zr ratio on the structure of Ni/Y₂O₃-ZrO₂catalyst and the catalytic performance of CO₂methanation were investigated.The 12%Ni/Y₂O₃-ZrO₂catalyst with 3%Y doping has good catalytic performance at low temperature.Under the conditions of 200℃,atmospheric pressure and space velocity of 15,000 mL·g^-1·h^-1,the conversion rate of CO₂is 60%and the selectivity of CH₄is 100%.The experimental results show that with the increase of Y₂O₃doping amount in ZrO₂,cubic phase or tetragonal phase ZrO₂gradually increases,while monoclinic phase decreases with the increase of Y₂O₃doping amount.The formation of Y₂O₃-ZrO₂composite oxide significantly enhances the interaction between Ni nanoparticles and carriers in the catalyst,and significantly improves the dissociation adsorption capacity of CO₂.Thus the low-temperature catalytic performance of the series of catalysts was improved.