Effects Of Additives On Surface Structure And Catalytic Performance Of CoMo Sulfur-tolerant Shift Catalyst

Water-gas shift(WGS)reaction(CO+H2O=H2+CO2)is one of the most important methods of large-scale cheap hydrogen production and widely used in catalytic processes such as the production of hydrocarbons and alcohols from syngas as well as ammonia synthesis.WGS reaction is a reversible process and the WGS catalysts is necessary to ensure reaction to run smoothly and efficiently.Among all kinds of WGS catalysts,CoMo sulfur-tolerant shift catalyst has attracted extensive attention for its merits of flexible operability,high activity,sulfur-tolerance as well as resistance to high sulfur content.Especially in recent years,along with a variety of high pressure gasfication advancing and the extnesive use of inexpensive raw materials such as high-sulfur coal and residue,the research and industrial application of CoMo sulfur tolerant shift catalysts has become a hot topic again.In this paper,A series of CoMo sulfur tolerant shift catalysts were prepared from two aspects:the active precursor control and the catalyst carrier modification,and tested their catalytic performance for water gas shift reaction.Some characterization methods,such as XRD,BET,Raman,UV-vis,H2-TPR,H2S-TPS,SEM,HR-TEM were used to investigated the infulence of the precursor for the active phase and support modification on the active phase structure and morphology.At the same time,the structure active relationship between the Co-Mo-S active phase and catalytic activity was established,which can provide theoretical guidance for developing the new and high efficient sulfur-tolerant shift catalysts.The main research contents in this dissertation are as follows:Firstly,a series of CoMo/MgAl2O4 sulfur tolerant shift catalysts were prepared by changing the calcination temperature of CoMoCA/MgAl2O4 catalyst and controlling the decomposition degree of CoMo citrate complex.The effect of CoMo complex structure on the WGS activity was studied.The activity tests revealed that with the calcination temperature rising from 110 ? to 600 ?,the catalyst activity appeared in a trend of first increased then decreased.Among them CoMoCA(380)calcinated at 380 ? presented the best catalytic activity.The characterizations showed that the different states of cirtic acid decomposition were determined by the different calcination temperatures,which leaded to the different interactions between the Co,Mo metals and the support.The optimum calcination temperature contributed to decrease the interaction between the metals of Co,Mo and the support and meanwhile obtain a higher dispersion degree of active metals,which can favor the formation of Co-Mo-S active phase during sulfurization process and the improvement of the catalytic performance.Secondly,CoMo complex with different structure was obtained by adjusting the complexing liquid pH and changing the coordination mode of citric acid with Co and Mo ions.Then,with these CoMo citric acid complexes as raw materials,a series of CoMoCA/MgAl2O4 sulfur tolerant shift catalysts were prepared.In addition,the effects of the structure of CoMo complex on the catalytic activity of the catalysts were studied.It was found that the CoMoCA-5(c)sample possessed the highest catalytic activity for WGS.Various characterization results show that pH has an important effect on impregnated catalyst system Co,Mo complex form;suitable pH value is conducive to the formation of active metal Co-Mo double metal complex.Such compound can be easily transformed to Co-Mo-S active phase in the process of vulcanization,which is beneficial to improve the catalytic activity of the catalyst.Finally,in view of the unique electronic structure and excellent electron transfer ability of rare earth elements,a series of MgAlM(M = La,Ce and Y)-LDO supports were synthesized.Then,a series of CoMo/MgAlM-LDO catalysts were prepared by equal volume impregnation method.The effects of different rare earth ions on the catalytic activity and the activity of CoMo sulfur tolerant shift catalyst Co-Mo-S were investigated.The results showed that the catalytic performance of CoMo sulfur tolerant shift catalyst with MgAlM-LDO as carrier was obviously improved,and the performance of MgAlCe-LDO catalyst was improved remarkably.The results show that the addition of rare earth elements can not only increase the number of basic sites on the surface of the catalyst,but also increase the dispersion of Co-Mo-S activity.In addition,the rare earth elements can also change the stacking layers and length of MoS2,which affects the number of Co-Mo-S active phase surface and the catalytic performance of the catalyst.