Catalytic Decomposition Of Nitrous Oxide By Transition Metal Oxides Supported On Silica Aerogel

As one of the six greenhouse gases in the Kyoto Protocol,nitrous oxide seriously damages the natural environment,which will lead to a serious greenhouse effect and destroy the ozone layer.Therefore,emissions reduction of nitrous oxide is of great significance to protect the global environment.In this paper,a series of silica aerogel-supported transition metal oxide catalysts were prepared by equal volume impregnation method.The activity of the catalytic decomposition of nitrous oxide was evaluated with a gas-solid phase fixed bed reaction device.The physical properties of the catalyst were characterized by XRD,H₂-TPR,BET et al methods.Intrinsic kinetics study of the catalyst catalytic decomposition of nitrous oxide was carried.Finally,the conditions of extruded catalyst were explored.Based on the support of commercially available silica aerogel,the optimization of catalytic decomposition activity tests was conducted on for one-component catalyst M/SiO₂-ag（M representing metal oxides by element symbols）.The examined elements incuded most of the transition metals often used in the literatures.Six of the loaded transition metal oxides expressed perfect catalytic activity.The results showed that the catalytic activity sequence was Ni O>Co₃O₄>Cu O>Fe₂O₃>Mn₂O₃>Zn O,in which Ni/SiO₂-ag showed the best activity.The temperature of complete conversion of nitrous oxide was 451℃for sample with optimal Ni O load of 27%.Zinc oxide revealed poor catalytic activity,with only 16%conversion of nitrous oxide even up to 700℃.Comparative experiments show that the sample prepared by the self-made silica aerogel possessed relatively low catalysic activity for supported nickel oxide.The experiments of catalytic activity for two-component composite metal oxide catalyst were investigated based on nickel oxide as the main component.The experiment results showed that second oxide of Co would significantly increase the activity of the catalyst.The second component of Cu,Fe,Zn,Cd,Zr,V,La,Ce,Y,Gd,Nd,Mg,Ca,Sr,Ba,Cs,Bi,Pb oxides had a little inhibition effect on catalyst activity.However,oxides of Li,Na,K,Al,Cr,W,Mo,and Mn would greatly reduce the activity of nickle oxide composite catalyst.The ritio factor effecting Ni Co/SiO₂-ag catalysts activity was investigated and sample Ni₂₇Co_2/SiO₂-ag loaded amount of 2%cobalt oxide showed good performance with nickle oxide support of 27%.Based on the optimal Ni₂₇Co₂/SiO₂-ag catalyst,experiments were carried out for inlet feed concentration of nitrous oxide and oxygen effect on the catalytic decomposition rate of nitrous oxide.Increase of the concentration of nitrous oxide in the feed gas（between 10 and 40%）would lead to increase the temperature of complete decomposition of nitrous oxide,that is,the decomposition conversion rate is reduced at the same reaction temperature.However,the impact is slight.The increase in oxygen concentration has little effect on the conversion of decomposition.The thermal stableility experiment was conducted at 445℃with Ni₂₇Co₂/SiO₂-ag sample.The results showed that no obvious deterioration of the decomposition rate of nitrous oxide was observed for lasted 400h run time.An intrinsic kinetics study for decomposition of nitrous oxide were explored based on Ni₂₇Co₂/SiO₂-ag catalyst.With the elimination of internal and external diffusion effects,nitrous oxide decomposition expressed one order reaction on the Ni₂₇Co₂/SiO₂-ag catalyst.Its activation energy of the reaction was 64.52 k J·mol^-1.With the aim of industrial application,the molding conditions of the preferred catalyst Ni₂₇Co₂/SiO₂-ag were investigated.An orthogonal experiment plan was designed with three levels and four factors.Catalyst activity was selected as the main goal and the mechanical strength of the molding catalyst as the second goal.The results showed that the amount of binder had the greatest influence on the mechanical strength of the catalyst.Binders had no significant effect on mechanical strength of catalyst for pseudo boehmite and silica sol.The temperature of the complete catalytic decomposition of nitrous oxide ranged from 500℃to 540℃on the molding catalyst.Compared with powder catalysts,the temperature of complete catalytic catalytic decomposition of nitrous oxide reduced about an average of 80℃for the molded catalyst.