| The emission control of volatile organic compounds(VOCs)has become the focus of air pollution control in China.In recent years,catalytic oxidation technology has become one of the most widely used terminal treatment technologies in industries.The development of highly efficient catalysts is the core of catalytic oxidation technology.The commercial catalysts that have been commonly used at this stage are noble metal catalysts.The high cost and low stability have limited its further industrial applications.Transition metal oxide catalyst has low manufacturing cost,but its low activity and high operating cost make it less promising.Therefore,improving the low-temperature catalytic activity of transition metal oxides has become one of the key issues to promote the development of catalytic oxidation technology.In this study,three different Co3O4 catalysts with different exposed facets and CeO2 loadings were selected as the research objectives.Their catalytic performances for converting toluene and stabilities under different stimulated industrial conditions have been measured.The structure-activity relationship and reaction mechanisms were thoroughly analyzed through an array of analytical techniques and in-situ experiments.Firstly,Co3O4 catalysts mainly exposed to {110},{111} and {112} crystal planes were successfully synthesized by the hydrothermal method.The activity test showed that Co3O4-{110} and Co3O4-{111} catalysts mainly exposed to {110} and {111}crystal planes had high toluene oxidation activity:when the toluene concentration was 1000 ppm.T90%is 250℃ and 249℃ respectively.At this temperature,the mineralization rate reaches 87.5%and 82.1%respectively,which is obviously superior to Co3O4-{112}.The results of structure and physicochemical properties analysis show that compared with Co3O4-{112},Co3O4-{110} has the highest Co3+concentration on its surface,while Co3O4-{111} has the largest specific surface area,so it shows higher catalytic oxidation activity at low temperature.At the same time,Co3+,as the main active site,can induce more oxygen defects on the catalyst surface,and enhance the activation ability of Co3O4-{110} catalyst for O2,thus enhancing its catalytic oxidation activity.Secondly,the effect of CeO2 loading with different mass fraction(1～10 wt.%)on the catalytic oxidation of toluene by Co3O4 at low temperature was investigated.When 5 wt.%CeO2 was loaded on Co3O4-{111} surface,the catalyst showed the best reaction performance:T90%was reduced to 223℃,and the mineralization rate was 79.7%.Compared with Co3O4 exposed to {110} and {112} crystal planes,Co3O4-{111} loaded with 5wt.%CeO2 significantly increased the content of Co3+active sites on the surface,and there was a strong interaction between Co3O4 and CeO2,and the ability to adsorb and activate O2 was further enhanced.Therefore,5%CeO2/Co3O4-{111} showed the best catalytic capacity for toluene oxidation.In-situ DRIFTS results show that Co3O4-{111} and 5%CeO2/Co3O4-{111} follow the same reaction path:toluene→benzyl alcohol→benzaldehyde→benzoic acid→maleic anhydride→CO2 and H2O.However,CeO2 loading improves the oxidation rate of benzoic acid on Co3O4-{111} surface to maleic anhydride,and improves the catalytic oxidation activity.Finally,the water resistance,long-term running stability and cycle stability of Co3O4 and its corresponding CeO2/Co3O4 catalyst exposed to different crystal planes were further investigated.Compared with Co3O4,CeO2/Co3O4 has better water poisoning resistance:when 2 vol%water vapor is introduced at T90%temperature,the conversion rate of Co3O4 decreased by 33%～40%,while the activities of 5%CeO2/Co3O4-{11} and 5%CeO2/Co3O4-{112} decreased by 12%and 11%respectively.CeO2-loaded catalyst had better water resistance,probably because CeO2’s strong oxygen absorption and desorption ability alleviates the competitive adsoption effect of water vapor on oxidation reaction.In addition,in the long-term activity test,the activity of the catalyst remained basically unchanged after 48 hours of continuous operation.In repeated cycle tests,all the catalysts have no obvious activity changes,which indicates that the catalysts have good stability.When the space velocity is further increased to 60 000 ml g-1 h-1,all catalysts had similar catalytic performance with slight decrease,and the T90%of all catalysts only increases by 4～12 ℃,which indicates that the catalysts have good adaptability to high space velocity.Therefore,the catalysts have strong stability and adaptability,and have good industrial application potential. |
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