Study On Pt/MoO_x/TiO₂ Catalyst In CO₂ Hydrogenation To Methanol

The continuous accumulation of carbon dioxide in the atmosphere exacerbates global warming.Controlling and reducing CO₂ emissions has always been a very challenging and urgent issue.As a basic chemical raw material,CH₃OH is not only used as a precursor for various chemical products,but also used as a liquid fuel in paint,pharmaceutical and pesticide industries.Methanol production through CO₂ hydrogenation is one of the most critical ways to relieve global greenhouse gas emissions and drive sustainable economic trends,in line with the concept of"methanol economy".The highly efficient catalyst developing is the key skill for CO₂ hydrogenation to methanol to be applied towards industrialization.This work is mainly aimed at Pt/MoO_x/TiO₂ composite catalyst.Firstly,the impacts of TiO₂ cell phase and crystal plane variation on the physical characteristics and catalytic functions of Pt/MoO _x/TiO₂catalyst were discussed.On this foundation,the catalytic energy of the catalyst was boosted much further by pretreatment with a carbon-containing reducing atmosphere.Finally,in order to optimize the doses of the noble metal Pt,the law of the effect of the modification of the active metal Pt content on the catalyst efficiency was considered.The principal contents and conclusions of this work are described as given below:（1）Anatase TiO₂（a-TiO₂）and rutile TiO₂（r-TiO₂）were synthesized by hydrothermal method.At the same time,mixed phase commercial P₂₅ was selected to prepare three kinds of Pt/MoO_x/TiO₂ catalysts were obtained by means of sequential dipping method.The effect of TiO₂ crystal phase type on the performance of Pt/MoO_x/TiO₂ catalyst CO₂ hydrogenation to methanol was studied.The results show that the Pt/MoO_x/TiO₂catalyst with a-TiO₂ as the support has the best reaction performance.The catalyst structure-activity relationship showed that a-TiO₂ had the highest concentration of surface oxygen vacancy,which could effectively disperse MoO₃ and promote its reduction,resulting in the strongest interaction between the support and Pt,so that Pt could maintain a high hydrogen activation capacity.At the same time,Pt/MoO_x/a-TiO₂ catalyst also has a significant advantage of medium basic site,which is conducive to the adsorption of CO₂ and promote its conversion.（2）a-TiO₂ exposed{001}and{101}main crystal planes were synthesized,and two kinds of catalysts were obtained by sequential impregnation method.The effects of anatase TiO₂ crystal planes on the catalytic performance of Pt/MoO_x/TiO₂ hydrogenation to methanol were studied.The results show that the Pt/MoOx/a-TiO₂{101}catalyst supported by Anatase TiO₂{101}with Wulff stable structure has better activity.The characterization results showed that the high oxygen vacancy concentration on a-TiO₂{101}surface could not only stabilize Pt better,but also facilitate the activation of CO₂.At the same time,Pt/MoOx/a-TiO₂{101}catalyst has more moderate alkaline sites,which is conducive to the adsorption of CO₂molecules and promote the formation of methanol.（3）The dominant Pt/MoO_x/a-TiO₂{101}catalyst with the best activity was selected as the model catalyst,and further pretreatment was carried out through carbon-containing reduction atmosphere.The influence of pretreatment temperature and pretreatment time on the performance of CO₂ hydrogenation to methanol on Pt/MoO_x/a-TiO₂{101}catalyst was explored,and the reasons for the performance improvement were preliminarily explored.The results show that the optimum pretreatment temperature is 220℃and the pretreatment time is36h.Under these conditions,the activity of the catalyst can keep stable basically.After pretreatment,CO₂ conversion elevated from 867.60 mol·Pt^-1·h^-1 to 4246.61 mol·Pt^-1·h^-1,and methanol selectivity elevated from 49.22%to 93.66%.methanol yield elevated from 413.79mol·Pt^-1·h^-1 to 3977.25 mol·Pt^-1·h^-1,which was 9.6 times that of the unpretreated catalyst.The activity of catalyst was significantly improved.The results showed that the pretreated catalyst had little change in phase composition,reduction performance and Pt morphology.However,carbon accumulation was shaped on the catalyst surface after treatment,and it was confirmed by TPH that it was a composite carbon of atomic carbon and polymeric carbon.（4）The influence of different Pt loadings the performance of Pt/MoOx/a-TiO₂{101}catalysts for CO₂ hydrogenation to methanol was considered.The analysis revealed that the best loading amount of Pt was 1%.When the Pt level was 1%,the catalyst has larger pores,which is more conducive to the mass transfer of gas molecules.At the same time,Pt/MoOx/a-TiO₂{101}catalyst also has a high number of oxygen vacancies,which is conducive to the activation of CO₂ molecules and the generation of carbon-containing intermediates,thus affecting the catalytic behavior of the catalyst.