Study On The Structure-Function Relationship In Catalyzing CO Oxidation And Water-Gas Shift Reaction Over Copper-Ceria Catalysts

CO oxidation reaction and the water-gas shift reaction(WGS)have great value of application in eliminating residual CO and purifying H2 in industrial production.However,harsh reaction conditions,such as high temperature and moisture,may lead to the sintering and deactivation of catalysts easily.Therefore,the coexistence of high stability and high activity is the primary goal of catalyst synthesis.At the same time,it is expected that the limited volume in the use of catalysts can bring higher catalytic efficiency,but the high content tends to bring about the agglomeration and complexity of species.The above aspects pose great challenges to how to prepare more efficient and practical catalysts.The supported catalysts with rare earth oxide CeO2 as the support and cheap transition metal Cu as the active metal are potential materials for the application in CO oxidation and WGS.However,due to the lack of effective preparation and characterization methods,the CuO-CeO2 catalysts with high-activity,high-stability,high-content,and high-dispersion are rare.At the same time,the understanding of the basic catalytic problems in the reaction of related catalysts is unclear.Therefore,we choose CeO2 with specific morphologies as supports and obtain a high-performance catalyst by adjusting the catalyst calcination temperature and metal loading.By combining in situ and ex situ characterization methods,the phase composition and local coordination environment of the catalysts are analyzed detailedly.The evolution of the catalyst structure is tracked and the reaction mechanism of the catalyst is revealed thoroughly.Moreover,the activation effect between the metal and the support is explained definitely.The catalytic behavior on the reaction performance has been revealed,so as to guide the design of a more efficient catalyst.The main work of this thesis is as follows:(1)Construction of active site in a sintered copper-ceria nano rod catalyst Ususlly,the adsorption and activation of gaseous reactants on solid surfaces are alaways barriers for designing high performance nanocatalysts in practical heterogeneous catalysis.The key to solving this problem is constructing coordinatively unsaturated centers,which often act as sites for the activation of reactant molecules at the catalysts interface.However,the current active sites are apt to sinter on-stream.Sintering always leads to many disadvantages to catalysts,including the decrease in surface area,the loss of active site and so on.But it can induce surface reconstruction that may change the coordination structure of some atoms,through which the coordinatively unsaturated atoms may be created.Thus,sintering brings possibility to construct new active site,although seems difficult.Here,through air-calcination at 800?,atomically dispersed and coordination-unsaturated copper species were successfully constructed in a sintered copper-ceria catalyst,which showed a very high activity with the CO consumption rate of 6100 ?molCO·gCu-1·s-1 at 120 ?.It was at least 20 times that of other reported copper catalysts at same temperature.Besides,the long-term stability was unbroken even under harsh reaction conditions.Through comprehensive characterizations,the copper atoms in this sintered catalysts was identified in the form of Cu1O3,which was the sole active site for both CO and O2 molecules activation.All in all,the coordination-unsaturated Cu1O3 induced remarkable CO oxidation activity with a very low copper loading(1 wt%).(2)In situ generation of surface oxygen vacancies in the copper-ceria catalysts for the WGS reactionThe water-gas shift reaction(WGS)is a very important industrial reaction due to its production and purification of H2,which is very important for energy supply.When considered the reaction molecules,the adsorption of CO on active metals and the dissociation of H2O on oxygen defects are two important aspects of the reaction.For the catalyst that has highly dispersed Cu species produced by calcination at a high temperature of 800?,the adsorption and activation of CO is very effective,which is a strong guarantee for the efficient progress of the WGS reaction.However,the catalyst owned limited specific surface area and oxygen defects,which is not conducive to the progress of the WGS reaction.The adsorption of CO on the metal and the dissociation of H2O on the defects are a synergistic process.Therefore,in order to clarify the dynamic change of the catalyst with extremely strong CO adsorption capacity and scarce oxygen defects in the WGS reaction,a low content(2 wt%)of copper was loaded on CeO2-NR and subjected to high temperature.The catalyst calcined at 800? has better catalytic performance than when calcined at low temperature.With the help of in situ characterizations,it was found that the catalyst with a lack of oxygen defects can generate a large number of defects in situ during the reaction to ensure the dissociation of H2O molecules.The effective dissociation of H2O ensures the reaction with the CO adsorbed on the Cu atom,and jointly promotes the efficiency in the reaction.This discovery provides a new idea for the study of the WGS reaction,which is helpful to guide the construction of high-efficient catalysts.(3)Very high loading oxidized copper supported on ceria to catalyze the water-gas shift reactionActive metals in supported catalysts are expected to disperse finely on the surface of the support at nanoscale even atomically dispersed,through which the high atomic efficiency is created.While,the highly dispersed sites are very mobile and easy to sinter,leading to a very low metal loading.It limits their practical catalytic applications,so high metal loading is essentially expected.However,high metal loading often makes the catalysts unstable and structurally complicated.The water-gas shift(WGS)reaction is very crucial to energy providing by upgrading and producing H2 for practical use.However,the catalysts generally have poor overall activity and the active sites are unclear.To solve these problems,we report the fabrication of very reactive catalyst with finely dispersed copper supported on high-surface area CeO2 at high content(20 wt%)for the WGS.It shows superb activity with a CO consumption rate of 116.7 umolCO·gcat-1·s-1 at 300?,which was mor eactive than other copper-based catalysts.Based on the integrated characterizations,the oxidized Cu is proved to be active centers for CO adsorption.The CO molecules directly reacted with bridged-OH groups.The oxidized Cu sites are confirmed to be induced by the activation effect from Cu to CeO2.The activation effect promotes the O in bulk CeO2 diffuse to surface,ensuring Cu species oxidized even under reductive atmosphere.