Investigation On Preparation And Catalytic Mechanism Of Co-Mo Sulfur-tolerant Shift Catalyst

The characteristics of fossil energy in china are less oil,short of gas and relatively abundant coal.Recently,due to the limitation of energy supplyment and transformation conditions,the raw materials for base-chemical products(e.g.synthetic ammonia,methanol)and new coal chemical industrial products(e.g.coal to olefins,oil,MEG,nature gas)which are beneficial to the people's livelihood,will have to be changed to bituminous or brown coal,heavy oil and residual oil with poor quality and high sulfur.Water gas shift reaction(CO + H2O = H2 + CO2)is one of the important steps in the production of hydrogen,which is a negligible material for the above industrial products.Co-Mo sulfur-tolerant shift catalyst is widely used in industry at present because of its excellent sulfur tolerance and broad operating temperature range.The aim of the dissertation is to solve problems such as the poor catalytic activity at low temperature of high pressure Co-Mo/MgAl2O4 sulfur-tolerant shift catalyst and uncertain active phase,The study was carried out from three aspects,which contained the precursor of active phase,the calcination temperature and the modifications of the support,respectively.Meanwhile,the prepared catalysts were characterized by XRD,BET,H2-TPR,Raman,XPS,HRTEM,ESR and other techniques to reveal the active phase structure and catalytic mechanism of the Co-Mo sulfur-tolerant shift catalyst.Firstly,the active phase structure of Co-Mo sulfur-tolerant shift catalyst was investigated.Since species in the catalysts prepared by conventional co-impregnation were complex,it is not favor to determine the active phase of catalyst.Therefore,in this part of the thesis,Co-Mo sulfur-tolerant shift catalyst with relatively simple active specie was prepared by precursor of Co2Mo10Am which simultaneously contained cobalt and molybdenum components.The results indicated that the catalytic activity for WGS of Co2Mo10/Al2O3 sample was significantly higher than Mo/Al2O3.The characterization showed that Co2+ can inhibit excessive reduction of Mo6+ species,and then promote the formation of oxysulfo-Mo5+.The active phases of Co-Mo sulfur-tolerant shift catalyst are the Co-Mo-S phases that consist of well dispersed MoS2 nanocrystallites decorated with the Co atoms.Secondly,the effect of calcination temperature on the performance of Co-Mo/MgAl2O4 catalysts was investigated.It was found that the activity of the Co-Mo/MgAl2O4 catalysts increased at first but later decreased while increasing the calcination temperature from 500 ? to 800 ?,when calcinated at 700 ?,the catalytic activity was the highest.The characterizations showed.that the catalytic activity was related to the appearance of CoxMg1-xMoO4 on catalyst.The presence of this intermediate compound can favor the formation of Co-Mo-S active phase,further improve the catalytic activity.However,the overhigh calcination temperature would lead to the serious decrease of specific surface area,followed by the decrease of catalytic activity.In addition,to solve the problem of poor catalytic activity at low temperature for high pressure Co-Mo/MgAl2O4 sulfur tolerant shift catalyst,the MgAl2O4 support was modified by acidic oxide(e.g.Nb2O5,ZrO2).It was found that adding the appropriate amount of acidic oxide can significantly improve the catalytic activity at a low temperature.However,excess amount will decrease the catalytic activity.It was attributed to that appropriate amount of acidic oxide would enhance the surface acidity of catalysts and promote the dissociation reaction intermediates,further improving the activity of catalyst at a low temperature.However,the excess acidic oxide would cause over allowance limit of support surface acidity,then poor dispersion of molybdate active species in the support surface,which led to the decrease of catalyst activity.