Study On Pd/CeO₂ Catalyst For The Catalytic Lean Methane Oxidation

Methane is widely distributed in nature and it is a major component of natural gas,biogas,coal mine gas and combustible ice.It has the advantages of high calorific value,less pollutant emissions when completely burning,etc.However,methane is a typical greenhouse gas,and the greenhouse effect caused by direct discharge into the atmosphere is about 22 times that of equivalent CO2.For the last 20 years,the greenhouse effect caused by methane was about 70 times that of carbon dioxide.When the concentration of CH4 is less than 3%,conventional flame combustion will no longer be applicable.Therefore,the realization of CH4 catalytic oxidation reaction under low temperature conditions is of great significance for reducing the methane content in the exhaust gas.Catalytic combustion of methane is a technology with industrial application value.It promotes the oxidation of low concentrations of methane at low temperatures.The key to the development of this technology is to design a catalyst with high activity,chemical stability and economical applicability.When a single-material precious metal catalyst is used for low-concentration CH4 catalytic combustion,Pd has the least volatility at high temperatures and high catalytic activity at low temperatures.The performance of the catalyst depends not only on the type of active material being supported but also on the nature of the support material.Three different morphologies of CeO2 carrier were prepared by typical hydrothermal synthesis reaction.Mainly included rod type r-CeO2,cubic c-CeO2,and octahedral o-CeO2.A series of Pd/CeO2 catalysts with corresponding morphology were prepared by impregnation method using noble metal Pd as active ingredient.Catalyst activity test was carried out in a quartz tube microreactor with an internal diameter of 3 mm under standard atmospheric pressure.The effects of the characteristic crystal planes exposed by CeO2 with different morphologies on the activity of Pd/CeO2 catalysts were investigated.The anti-influenza ability of the catalysts was also investigated.In the low concentration methane catalytic oxidation reaction,Pd/octahedral-CeO2(Pd/o-CeO2)with(111)crystal faces had much higher catalytic activity comparing with Pd loaded on c-CeO2 with(100)crystal faces and r-CeO2 with(110)and(100)crystal faces.The catalytic activity of simple CeO2 carriers was ranked in the order ofrod-CeO2(r-CeO2)>cube-CeO2(c-CeO2)>octahedral-CeO2(o-CeO2).In this paper,BET specific surface test(BET),TEM or HRTEM,XPS,1CP-AES,X-ray diffraction(XRD),H2 temperature-programmed reduction(H2-TPR),EDS spectrum,In-situ XPS(Quasi In-situ XPS),SEM and elemental maps were used to characterize the catalyst.Based on the microscopic,this paper further explains the relationship between the structural properties of the catalyst and the surface electronic properties and its catalytic activity.The results of XRD,HRTEM,EDS,H2-TPR,XPS and elemental mapping showed that the Pd species in the form of Pd0 and PdO were highly dispersed on surface of all CeO2 carriers.Quasi In-situ XPS results convinced that Ce3+-Ce4+ ion pairs played an important role in the catalytic combustion of low-concentration methane by adsorbing or releasing oxygen.Compared with r,CeO2 and c-CeO2,the concentration of Ce3+on the surface of o-CeO2 carrier was greatly increased when the Pd species were present,which indirectly reflected that the concentration of oxygen vacancies on the carrier surface also increased.The presence of water vapor inhibited the activity of the Pd/CeO2 catalyst.In a word,the morphology of carrier material and the characteristic crystal surface exposed by carriers were important factors for affecting the activity of catalyst.