Study On The Preparation Of Cobalt-based Spinel Compounds For Catalytic Oxidation Of VOCs

Volatile organic compounds(VOCs)are considered as major contributors to air pollution,and catalytic oxidation is considered as one of the most efficient methods to remove VOCs.Compared with noble metal catalysts,non-precious metal oxide catalysts such as cobalt oxide has attracted much attention due to their low cost.However,the non-precious catalysts are less active at low temperature than the noble-metal catalysts.Therefore,development of non-precious metal oxide catalyst with good low-temperature activity is highly desirable.In this paper,layered double hydroxides(LDHs)have been used as precursors to prepare Co3O4 based mixed metal oxides with small size and high surface area.The synthesized samples were characterized by ICP,N2 physical adsorption,TG-DTA,XRD,SEM,TEM,XPS and H2-TPR techniques,and their activities were tested in benzene total oxidation.Based the results,the structure-performance relationship was discussed.The main results are as follows:(1)The influence of preparation methods of CoAl-LDHs on the structure and activity property of Co-Al mixed oxides for benzene total oxidation was investigated.The CoAl-LDHs were synthesized by varying pH precipitation(dp),constant pH precipitation(cp)and urea homogeneous precipitation(hp).The characterization results showed that calcination of CoAl-LDHs gave rise to Co-Al mixed oxides consisting of Co(Co,Al)2O4 spinel as the main phase,which showed relatively small crystallite size and high specific surface area,i.e.,?12 nm,90?100 m2 gcat-1.The highest catalytic activity was obtained on the dp-CoAl mixed oxide,followed by cp-CoAl,while hp-CoAl mixed oxide was much less active.The activity difference can be related to the different reducibility of Co3+ species in Co(Co,Al)2O4 spinel.(2)A series of Co-Al mixed oxides with different Co/Al molar ratio were prepared from CoAl-LDHs.The influence of Co/Al ratio and calcination temperature on the structure and catalytic performance was investigated.The characterization results showed that the crystallite size of Co(Co,Al)2O4 spinel decreased with decreasing the Co/Al molar ratio,suggesting the inhibition of crystal growth by the incorporation of Al3+ ions in the spinel phase.The activity of the Co-Al mixed oxides increased with increasing the Co/Al molar ratio up to 5,which was related to the enhanced reducibility of active Co3+ species in the Co(Co,Al)2O4 spinel.Nevertheless,further increase in the Co/Al molar ratio lead to significant decrease in the activity performance caused by the structural change.On the other hand,the long-term stability test showed that the Co5Al mixed oxide was stable for the total oxidation of benzene.Increasing calcination temperature(573?973 K)resulted in the increase of particle size and as a result decreased the catalytic activity.(3)Co5-xCuxAl(x = 0.1?0.8)mixed oxides were prepared from CoCuAl-LDHs to investigate the effect of Cu2+ doping.The characterization results showed that calcination of CoCuAl-LDHs at Cu content x?0.25 obtained(Co,Cu)(Co,Al)2O4 spinel as the main phase,whereas additional CuO phase was formed at Cu content x ?0.5.The Cu doping significantly improved the catalytic activity of Co-Al mixed oxide.The Co4.75Cu0.25Al catalyst showed the highest activity(T50=477 K,T90=519K)in benzene combustion.The improvement of catalytic activity can be related to the enhancement of Co3+ reducibility.Excessive addition of Cu decreased the activity due to the formation of CuO.