Preparation, Characterization And The Catalytic Performance Of Co₃O₄ Catalyst In Low-temperature CO Oxidation

The process of low-temperature CO catalytic oxidation has become animportant research topic due to its many potential areas of applications.Particularly, these applications include air-purification devices for respiratoryprotection, carbon monoxide gas sensors, closed-cycle carbon dioxide lasers,and removing trace quantities of CO from the ambient air in sealed cabins. Inaddition, in the aspect of academic study the CO oxidation has been regardedas a probe reaction to show the relationship between the structure and theperformance, and also to investigate the reaction mechanism.The research about the base metal oxide Co₃O₄ catalyst forlow-temperature CO catalytic oxidation has become a hotspot over the years.At present, the methods for preparing the Co₃O₄ catalyst with high activityexist many disadvantages such as the complicated process of operation andthe post treatment. According to the literatures, the pretreatment conditionshave an great effect on the catalytic activity of Co₃O₄ for low-temperatureCO oxidation. There are some researches about the reaction mechanism andthe original factors of deactivation of the Co₃O₄ catalyst, and a littleconsensus has been obtained, however, there are many problems awaitingfurther investigation, and there is a lack of perfect study about thecharacterization and catalytic performance of the Co₃O₄ catalyst. Without useof any surfactant or oxidant, Co₃O₄ catalyst with high activity has beenprepared by a simple liquid-precipitation method in the dissertation. Firstly,the effects of preparation conditions on the catalytic performance forlow-temperature CO oxidation have been studied; secondly, the effects of calcination temperatures and evaluation conditions on the catalyticperformance for low-temperature CO oxidation have been studied. Thestructures of the catalysts have been investigated by TG-DTG,XRD,TEM,N₂-adsorption,IR and XPS. The adsorption performance of CO and O₂ andthe reaction mechanism and the reason of deactivation of low-temperatureCO oxidation have been investigated via CO titration, O₂-TPD and in situDRIFT. The main results and conclusions are as follows:1. The study on the effect of preparation methods and preparationparameters on the catalytic performance for low-temperature CO oxidationover the Co₃O₄ catalysts.(1) The Co₃O₄ catalyst with high catalytic activity can be obtained by theliquid-precipitation and the well-distributed precipitation method, and canconvert CO completely at 25℃with the same stability. However, the catalystprepared by the direct calcination has a poor catalytic performance.(2) The Co₃O₄ catalysts obtained by the liquid-precipitation can convertCO completely at 25℃with the same stability using Co(N_O3)₂·6H₂O orCoSO₄·7H₂O as the cobalt salt and NH₄HCO₃ as the precipitator. However,the conversion of CO over the catalyst is only 80% obtained usingCoCl₂·6H₂O as the cobalt salt and the stability is poor.(3) The Co₃O₄ catalysts obtained by the liquid-precipitation can convertCO completely at 25℃with the same stability using NH₄HCO₃ or NH₃·6H₂Oas the precipitator and Co(NO₃)₂·6H₂O as the cobalt salt. However, theconversion of CO over the catalyst is only 80% obtained using KHCO₃ as theprecipitator and the stability is poor.(4) The drying methods including the common drying and the super criticaldrying have no obvious effects on the catalytic performance.(5) The Pd/Co₃O₄ catalyst does not show superiority compared to theCo₃O₄ catalyst for CO oxidation at 25℃.(6) The Co₃O₄/Bent catalysts have the same catalytic activity as the Co₃O₄catalyst at 25℃, and the stability decreases with the content of Co₃O₄ increasing.2. The study on the effect of calcination temperature on the structure,surface properties and the low-temperature CO oxidation performance overthe Co₃O₄ catalysts.(1) The calcination temperatures have no obvious effect on the structure ofthe Co₃O₄ catalyst, and all the catalysts exist as a pure Co₃O₄ phase with thespinel structure within the calcination temperature 250～600℃. However,the calcinations temperatures have a great effect on the crystal size and thegeometry of the catalysts. The catalyst calcined at 300℃has the highestdispersion level, and the particles are apt to aggregate together randomly withthe increase of calcination temperature.(2) The calcination temperatures also have an obvious effect on the BETspecific surface area and the specific pore volume. The BET specific surfacearea and the specific pore volume of the catalyst calcined at 300℃are 54.3m²/g and 0.20 cm³/g respectively, and they are 9 m²/g and 0.02 cm³/g whenthe calcination temperature is at 600℃, so the catalyst has transformed into amaterial of none pore.(3) Within the temperature 250～600℃, the calcination temperatures haveon obvious effect on the valence state of Co on the Co₃O₄ surface.(4) There are three kinds of oxygen species on the Co₃O₄ surface. Theamount of the adsorption oxygen species decrease a little while the hydroxideradical oxygen species decrease a lot, and the percent of the lattice oxygen atthe whole oxygen species increase.(5) The results of CO titration indicate the Co₃O₄ catalysts possess theactive oxygen species after pretreatment. The calcinations temperature havean obvious effect on the amount of active oxygen species, and the catalystcalcined at 300℃possesses the most amounts of active oxygen species,about 143.74μmol/g.(6) All the catalysts have good catalytic activity for CO oxidation in thetemperature range used. The catalyst calcined at 300℃shows the beststability. The calcination temperatures that higher or lower than 300℃lead to a decrease of the catalytic stability.(7) The results of investigation about the structure,surface properties andthe low-temperature CO oxidation performance indicate that the catalystcalcined at 300℃has the small particle size and the highest dispersion levelexisting as the spinel structure, and so possesses the most amounts of activeoxygen species and the best catalytic performance. Obviously, the smallparticle size,high dispersion level and many active oxygen species are theimportant factors for the high catalytic activity and stability oflow-temperature CO oxidation over the Co₃O₄ catalysts.3. The optimization of the technological condition of low-temperature COoxidation over the Co₃O₄ catalyst(1) The pre-oxidized Co₃O₄ catalyst shows very high CO oxidation activityand stability at 25℃. Treatment of catalyst with inert gas causes the catalyticstability lower, while the catalyst has bad catalytic activity without anypretreatment.(2) The stability decreases with the space velocity increasing within2500～20000 h^-1, and the stability decreases with the content of COincreasing within 0.5～2.0% at 25℃.(3) The pre-oxidized Co₃O₄ catalyst shows very high CO oxidation activitywithin -78～80℃. The stability increases with the reaction temperatureincreasing within -78～25℃, and the stability decreases with the reactiontemperature increasing within 25～80℃(4) The results of optimization of the technological condition indicate that:the pre-oxidized Co₃O₄ catalyst can be able to maintain its activity for COcomplete oxidation more than 630 min under the reaction conditions: CO0.5%,space velocity 2500 h^-1 and 25℃. The result is better than theliteratures.(5) The low-temperature activity of Co₃O₄ is inhibited by the presence ofmoisture in the feed gas, and the resistance to water is promoted greatly bythe addition of traces of Pd. (6) The regeneration of the deactivated catalyst could be achieved via theheat treatment at 200℃under the following drying air.(7) The catalyst not only has the high catalytic performance forlow-temperature CO oxidation in continuous flowing conditions, but also canbe used inconsecutively.4. The investigation of the reaction mechanism and the reason ofdeactivation for the low-temperature CO catalytic oxidation over the Co₃O₄catalysts.(1) The O₂-TPD analysis of the fresh and deactivated Co₃O₄ catalystsindicate the Co₃O₄ catalysts have several kinds of oxygen species, and theO₂^-,O₂^2- and O^- species responsible for the possible active oxygen species.The lattice oxygen has no effect on the catalytic performance of Co₃O₄ inlow-temperature CO oxidation.(2) The valence state of Co has no obvious difference between the freshand the deactivated Co₃O₄ catalysts, and it indicates that the deactivationdoes not originate in the change of the valence state of Co. There is water onthe surface of the deactivated, and the ratio of the adsorption oxygen to thelattic oxygen decreases after the deactivation.(3) The results of in situ DRIFT indicate that: there is a little wateradsorbing on the catalyst without pretreatment, and the water and CO arecompeting for the same site which leads to the weak adsorption of CO, andthe CO can react with the—OH to form the formate species. There is linearadsorbed CO on the surface of the pre-oxidized Co₃O₄ catalyst, and the COcan react with the active oxygen species to form CO₂, and the carbonates arethe possible intermediate product.(4) The presumable reaction mechanism of low temperature CO oxidationover the Co₃O₄ catalyst is that: there is adsorption of CO and O₂ on thesurface of the catalyst, and the CO can react with the active oxygen species toform CO₂, and the carbonates are the possible intermediate product. In thereaction process, the deactivation of the catalyst probably due to traces of moisture in the feed gas adsorption and accumulating on the surface leadingto the decrease of CO adsorption and the formation of the formate species.