Research On CO₂ Hydrogenation And And Its Selectivity On Ni/TiO₂ Catalyst

Substantial CO₂ emission is one of main causes for the increasingly serious greenhouse effect in the world,which negatively affects the living environment of human beings.CO₂ hydrogenation to high value-added chemicals or fuels is an effective way to solve the carbon emission and has economic and environmental benefits.CO₂ methanation is a basic reaction of CO₂ hydrogenation with vital research value.Researches around these reactions help to deepen the understanding of CO₂hydrogenation and to develop new industrial catalysts with low expense,high activity,and strong stability.In this work,we focued on Ni/TiO₂ catalysts,the performance of catalyst was comprehensively study,structure-activity relationship of catalyst was systematically discussed,the intrinsic kinetics of catalyst was accurately investigated,and the key intermediate of the catalytic reaction was deeply revealed.First of all,3%-15%Ni/TiO₂ catalysts were synthesized by impregnation method and then used for performance testing and structural characterization.The results showed that the metal size was closely related to the catalytic performance.Catalysts with different metal loadings had similar specific surface area,pore structure,morphology,but different metal sizes in structure.In terms of performance,3%Ni/TiO₂showed a CO selectivity of 99%,while 15%Ni/TiO₂ exhibited a CH₄ selectivity over95%,and the main product turned from carbon monoxide to methane with the increase of nickel loading.Secondly,based on TEM,TPR,XPS,chemisorption and other characterizations,further research showed that the strong metal-support interaction played a more essential role.The different degrees of encapsulation of Ni by TiO_x on Ni/TiO₂catalysts led to different amount of Ni⁰ on catalyst surface,which changed the capacity to supply hydrogen for catalysts and affected the conversion of reactive intermediate.By tuning strong metal-support interaction on 15%Ni/TiO₂,the reduction-oxidation-reduction?ROR?treatment nearly achieved the reversible adjustment of catalytic performance,which experimentally verified the effect of strong metal-support interaction on performance.After high-temperature reduction,the 15%Ni/TiO₂ showed the characteristic of strengthened strong metal-support interaction:weaker capacity for hydrogen absorption,which made the conversion of CO₂ drop to 2%and carbon monoxide selectivity exceed 99%.After an additional high-temperature oxidation,the catalyst exhibited the characteristic of eliminated encapsulation:the catalyst almost recovered its performance,which expressed a CO₂ conversion of 7.4%and a methane selectivity of 95%.Subsequently,kinetic experiments were implemented to study the kinetic behavior of 3%Ni/TiO₂ and 15%Ni/TiO₂,and found that reverse water gas shift reaction was mainly performed on 3%Ni/TiO₂,while CO₂ methanation was mainly performed on15%Ni/TiO₂.In addition,the results suggested that the partial pressure of hydrogen was closely related to methane formation rate on 15%Ni/TiO₂ catalyst.Finally,in-situ DRIFT was applied to study the dynamic conversion of carbon dioxide.The results showed that the TiO₂ played a role in CO₂ activation,while the Ni was responsible for hydrogenation of intermediates,and that formate was the key intermediate for generating both CO and CH₄.On 3%Ni/TiO₂catalyst,formate tended to decompose to produce carbon monoxide due to weak hydrogen donating ability,while the formate mainly hydrogenated to methane due to strengthened hydrogen donating ability on 15%Ni/TiO₂.