The Effect Of Metal And Support Particle Size In CO₂ Methanation Over Ni/CeO₂

At a time when environmental problems are becoming increasingly serious,it has been a consensus among governments to reduce the energy consumption and CO₂emissions.Owning to mild reaction conditions,high product selectivity and facile separation,CO₂ methanation exhibits broad application prospects for realizing the resource utilization of CO₂.Development of catalysts with high activity and stability at low temperature for CO₂ methanation has been the reseach focus of methanation process.Focusing on the size effect in methanation,the structure-activity relationships were studied from multiple perspectives,including oxygen vacancy concentration,metal-support interaction,Ni-CeO₂ electronic effect and reaction mechanism.Regarding the particle size effect of Ni active component in Ni/CeO₂,the oxygen vacancy concentration and Ni particle size were controlled by tuning the Ni loading on the same support.Through comparison with Ni/Si O₂ and CeO₂/Ni catalysts,the promotional effect of metal-support interaction and oxygen vacancies on the catalyst activity was demonstrated.The Ni size sensitivity of Ni/CeO₂-catalyzed CO₂methanation was revealed,where the turnover frequency（TOF）of CO₂ decreased with increasing the Ni particle size.Furthermore,Operando FTIR spectrascopy confirmed that formate is a key intermediate in the surface reaction and the rise in CO selectivity of Ni/CeO₂ with small particle size was due to the occurence of the surface carboxylate pathway.The stability evaluation and thermogravimetric analysis showed that the catalyst deactivation originated from the agglomerative reconstruction of Ni particles and the carbon deposition on the surface of Ni/CeO₂ with small Ni particles.The size effect of catalyst support was investigated by changing the particle size of CeO₂ supports.The catalyst with samll CeO₂ particle size（mean=5 nm）exhibited much larger specific surface areas than other catalysts,which promoted the dispersion of Ni active components on the catalyst surface in a cluster structure.Ni-CeO₂ electron interaction was formed between Ni clusters and support,producing more oxygen vacancies and unique reactant molecule（CO₂,H₂）and CO adsorption properties.The results of Operando FTIR spectroscopy for CO₂ hydrogenation revealled that more carbonate and formate species generated on the catalyst surface during the reaction.Compared with the highly active non-noblemetal-based catalysts reported in CO₂methanation,Ni/CeO₂ catalyst developed here is more advantageous in CO₂ conversion rate and is comparable to noble metal-based catalysts,while it possesses high CH₄selectivity.These results provide an experimental basis and theoretical guidance for the design of high-performance catalysts for CO₂ methanation.