Catalytic Combustion Of Methane On Ce-based Monolith Catalysts Under Oxygen Lean Conditions

As a new,clean and efficient energy resource with high thermal value,coalbed methane is an important supplement for natural gas.But the mixing of oxygen during mining causes potential dangerous,which restricts the recycle of coalbed methane.Deoxygenation of coalbed methane is crucial before multiple utilization.Catalytic combustion is one of the most effective methods for deoxygenation,and the key point is to develop catalysts with high activity and stability under oxygen lean condition.Since Ce02 has good oxygen storage capacity and redox ability,and can promote activity through interaction with noble metals,and monolith catalysts has low pressure drop in high space velocity,and can promote mass and heat conductivity during reaction,many types of Ce-based oxides coatings monolith catalysts were prepared to investigate the reaction characteristics of catalytic combustion during oxygen-lean condition,and the activity,stability and reaction mechanism were deeply discussed.Composite oxides Ce0.8M0.2 coatings were prepared by citrate acid method,and modification of Ce02 was conducted by the doping of Cr,Mn,Fe,Co,Cu,Zr,La.Due to the different valences and radius between dopants and CeO2,the doping can cause oxygen vacancies and defects,and modify the surface area and reducibility of the oxides characterized by XRD and H2-TPR,thus can affect the activity of ignition and total conversion.The doping of some La improved concentration of oxygen species and oxygen vacancies,and the high temperature reducibility of CeO2,which reduced the total conversion temperature to 620 ?,80 ? lower than CeO2.Besides,the addition of noble metals as Pd and Pt marginally reduced the total conversion temperature.Pd/CexZry monolith catalysts were prepared by impregnation method,and support effects on the activity were investigated by adjusting Ce/Zr ratios.Oscillation occurred since catalysts cannot sustain its value with high activity,and the ability of oxygen exchange decreased.Catalysts supported by CexZry oxides can reach 100%02 conversion below 350 ?.The addition of CeO2 may maintain the value of Pd and its particle size,which benefits the stability of catalysts.Pd/Ce1Zr2/cordierite kept its activity after cyclic ignition and extinction tests,with only 5 ? up of the total oxygen conversion temperature,and can keep the O2 conversion up to 98%during 1000 h long-time stability test.To stabilize the valence and particle of Pd,solid state method was applied to prepare Pd-Ce02,and the interaction between Pd and Ce02 was modified by calcination.Though having low ignition activity,the activity drastically improved after cyclic reaction.By using XPS and TEM,the great change in activity is due to structural reorganization and redistribution of Pd on catalyst surface caused by reactants.Metallic Pd on catalyst surface is active for methane activation at ignition stage,and Pd2+ having interaction with CeO2 is benefit for total conversion of oxygen.Mechanism and kinetics studies on Pd-CeO2 calcined at 500 ?(PdCe-500)showed that activity is proportional to oxygen concentration.The mechanism matches well with Mars-Van Krevelen mechanism,which reaction follows redox route and the rate-limiting step is considered to be the reoxidization of oxygen vacancies.Pt-CeO2 coatings were prepared by citrate acid and impregnation methods to investigate the effects of interaction between Pt and CeO2 on oxygen conversion activity.Metallic Pt is considered as active sites for methane activation,and the activity may be inhibited through oxygen poison during programed heating.Pt-CeO2 by citrate acid method showed good activity and stability after calcination.The interaction between Pt and CeO2 inhibits the over-oxidation of Pt,and the segregation and sintering of Pt is avoided by the anchoring of CeO2.For catalysts by impregnation,Pt is dispersed on surface of support,and Pt supported by smaller CeO2 is more prone to be poisoned by oxygen due to the good oxygen exchange ability of CeO2.The pre-reduction by hydrogen promotes the interaction between Pt and CeO2,which benefits the activity.