Research On Gas Sensing And Catalytic Properties Of Transition Metal Composite Oxides

Several kinds of transition metal composite oxides were investigated for their sensitivity to low concentration of harmful gases and the catalytic oxidation performance of some pollutants(CO,NO,soot)in vehicle exhaust,respectively.Different metal elements were added in the transition metal oxides and their influence on the gas sensing and catalytic properties were studied.XRD,BET,SEM,TEM,FTIR,H2-TPR and other methods were used to characterize the crystal structure,morphology,surface properties and redox ability of the materials.Furthermore,gas sensing measurements and catalytic reaction test of the materials were conducted and their changed performance due to the introduction of other elements were discussed based on the results of characterization.The main contents of this paper are as follows:(1)The hydrotalcite Ni/Al composite oxides with different aluminum contents were synthesized by co-precipitation method.Their sensitivity to 2-chloroethanol and catalytic oxidation performance of CO were investigated,respectively.The appropriate proportions of Al in the composites were determined.It was found that the samples with Ni/Al ratio of 3/1 showed better performance than the others.At 160?,the maximum response of sample Ni3AI(O)to 20 ppm of 2-chloroethanol can reach about 6.8.Moreover,it was also found that proper ratio of Al2O3 added in NiO could improve the catalytic oxidation of NiO to CO and the samples with Ni/Al ratio 2/1 have the best catalytic performance among the samples with T50-190?.Moreover,the temperature of 100%CO2 conversion for Ni2Al(O)is only 340? which is 100 0C lower than that of pure NiO.Therefore,the proper amount of Al addition could increase the performance of NiO on gas sensing and catalytic oxidation which were ascribed to the good dispersion and Lewis acid characteristics of alumina.(2)Series of nano-powders La0.7Ba0.3Fe1-XMnxO3(x = 0.0?0.6)with orthorhombic perovskite structure were prepared by Sol-gel method.The influences of Mn doping on the gas-sensing performance and catalytic properties of the material were investigated.It was found that the samples with low content of manganese show better response to NH3 than the undoped one the samples with Mn(x = 0.1)showed the highest response to ammonia,its response to 500 ppm ammonia was near 5 at 140?.the samples shows.Furthermore,the results of catalytic oxidation test showed that the doping of manganese could promote the oxidation reaction of NO at lower temperature and decrease the NOx storage temperature from 500? to 400?.In the series of materials,sample Mn0.6 has the best performance for the catalytic oxidation of NO with the highest conversion rate of NO2 40.1%at 365? which is lower than that of Mn0.0 for about 20?.The results show that appropriate proportions of Mn doping samples have improved gas sensing performance and low temperature activity which were ascribed to their increased oxidation and surface area.(3)Series of transition metal doped(Co,Mn,Fe and Cu)La2Sn2O7 pyrochlore oxides were prepared by CTAB-assisted sol-gel method.The influences of transition metal doping on the catalytic activity of the samples for soot oxidation were investigated.It was found that transition metals incorporation enhanced the activity and selectivity of La2Sn2O7 for soot combustion.Under O2 atmosphere,The Co doped samples show good catalytic performance with ignition temperature(T10)379 ? and the selectivity of CO2 close to 100%;The presence of NO in the gas phase significantly enhanced the soot oxidation activity,which is due to the promotion effect of NO2.At last,the densities of active oxygen sites and turnover frequency(TOF)values of the catalysts were used to explain the different soot combustion behavior of the samples.Among the series of pyrochlore oxides,the Co-doped one displayed the highest TOF value and ignition activity.The oxidation properties of transition metal oxides and the interaction between transition metal and tin may contribute to the improvement of the redox properties of the catalysts.(4)Cesium-added LaCoO3 perovskite-type composite oxides were synthesized via two different ways:Cs doping and Cs loading by Sol-gel method and impregnation,respectively.The effects of Cs addition(Cs loading and Cs doping)of the composite oxides on the removal of soot and NOx storage were studied.The results show that the oxidation ability of the materials were improved after the introduction of Cs with improved catalytic oxidation ability of soot.As shown of the results of soot combustion in O2 atmosphere,Cs doped catalyst showed the best performance with the half transition temperature(T50)at 425 ?,while that of Cs load samples and samples without Cs at 472 ? and 436 ?,respectively.The presence of NOx atmosphere can further enhance the activity of catalyst oxidation to remove soot.In addition,the Cs doped sample also has the largest NOx storage capacity among the samples.,which can be attributed to the enhanced oxidation ability by Cs doping and the strong alkaline of Cs.