Kinetics Consequences And Mechanisms Of Methane Catalytic Combustion On Pd-Pt Catalysts In Micro-scale Channels

The catalyst in the micro combustor not only solves the problem of radical quenching but also increases methane catalytic combustion rate in order to decrease the ignition temperature and stablize the combustion.Palladium and platinum catalysts are the excellent catalysts are widely used for methane combustion.By adding small amounts of platinum into the palladium catalyst,improved stability were obtained in comparison with the monometallic palladium catalyst.But kinetics and reaction mechanism of methane catalytic combustion have not be completed,especially for the methane catalytic combustion at low O2 concentrations.Hence,it is necessary to investigate kinetics and reaction mechanism of methane catalytic combustion at different oxygen concentration.Kinetic and mechanism of methane catalytic combustion carried out after elimination of transport corruptions.Besides,the physical and chemical properties of the catalysts,the kinetic consequence at different kinetic regime,reaction mechanisms and kinetic parameters were established.This research is beneficial to the design of micro combustor based on methane catalytic combustion.The efficient and stable combustion can be performed under different working conditions.Research on kinetics and reaction mechanism of methane combustion lay the theoretical foundation for the design of micro combustor.The main innovative achievements obtained in this paper are shown as follows:Addition of Pt to Pd catalyst can significantly change the chemical properties and thermodynamic stability of the active particles,and the Pt-rich catalysts in catalytic combustion own the advantages of activity and stability.Addition of Pt to Pd catalysts increase metal phase and decrease oxide phase,as well as the lattice constant of active phase increased.Among these different metal oxides,thermodynamic stability of the PdO is the best.Besides,addition of platinum reduced the thermodynamic stability of the oxides,transformation of oxide into metal in terms of transition temperature and oxygen release decreased with the increase of platinum content.Furthermore,oxygen release in heating and oxygen adsorption in cooling of the Pd-rich catalysts have temperature hysteresis effect.Unlike the core metal valence,surface Pd atoms valence of monometallic palladium catalyst are mainly +2,and addition of Pt to Pd lead to surface Pd valence increases.Pd based catalyst has the best activity but the initial stability was the worst,adding a small amount of Pt greatly improved the stability of the Pd catalysts.Methane catalytic combustion on Pd-Pt catalysts proceeds via five distinctly different kinetic regimes defined by operating oxygen to methane mole ratio,oxygen pressure and oxidation degree of crystal.These five kinetic regimes,each with unique rate dependencies for methane combustion.Besides,surface oxide morphology changes with the increasing O2 pressures,which lead to the different kinetics of the reaction processes.First-order combustion rate constants with the variation of oxygen to methane mole ratio on Pt catalysts divided into three kinetic regimes.When O2 pressure is low,Pt surface has enough active vacancies for dissociation of oxygen and methane,resulting in reaction rate was controlled by oxygen pressure(kinetic regime I).When the oxygen pressure continued to increase,oxygen atoms occupied most of metallic active sites.C-H bond activation on paired O*-* sites rather than on paired *-* sites,and metallic active sites decrease with increasing O2 pressure,which is not conducive to the methane combustion(kinetic regime II).As O2 pressure was further increased,metallic active sites were completely covered by O atoms,resulting in methane can only been dissociated on adsorbed oxygens.Hence,Pt activity is poor at low temperature,and methane reaction rate was independent on oxygen pressure(kinetic regime III).Pd catalyst has the better performance of oxytropism,and the surface was completely covered by O atoms in the low O2 pressure(kinetic regime III).With the increase of oxygen pressure,resulting in surface was been oxidized.Oxidation started from the surface to the core,small clusters to the larger,resulting in methane catalytic combustion rate increased with increasing oxygen pressure(kinetic regime IV).When the active clusters completely oxidation,methane catalytic combustion rate did not change with the oxygen pressure(kinetic regime V).The chemical components of bimetallic catalysts were more complex and Pt atoms are easy segregation to the surface of clusters at low oxygen pressure,resulting in the high methane conversion rate at low oxygen pressure as same as Pt catalyst.Furthermore,Pd atoms are easy segregation to the surface of clusters at high oxygen pressure,so the methane conversion rate is high at high oxygen pressure as same as Pd catalyst.Pd-rich bimetallic Pd-Pt catalysts own high catalytic methane combustion activity at different oxygen pressure.The key reaction steps in the process of methane combustion in a micro channel and the reaction mechanisms in different reaction regimes were studied in this study.It was found that methane and oxygen dissociation on the metallic active sites and the oxygen dissociation is irreversible at the clean metallic surfaces.Besides,the O=O bond activation was much easier than that of C-H bond,therefore methane combustion rate was determined by O2 activation.C-H bond and O-O bond activation on paired O*-* sites for the intermediate O* coverage.Besides,methane and oxygen become more difficult to be dissociated with the increasing O* coverage.In the case of surfaces saturated with chemisorbed oxygen,C-H bond activation on O*-O*,resulting in the low activation at low temperature.In the case of oxide surface,C-H bond activation on paired Pd atom and lattice oxygen,resulting in the high activation at low temperature.H2O and CO2 removal lead to the most recovery of initial turnover rates on Pd contained catalysts,indicating the inhibition processes are caused by the competitive adsorption rather than deactivation.H2 O strongly inhibits methane combustion rates,but inhibition effect of CO2 is much greater than H2 O.Beside,CO2 has no inhibition on methane combustion unless the CO2 pressure is larger than 3-5 kPa.Furthermore,CO2 and H2 O has no inhibition on methane combustion for Pt catalyst.The effects of H2 O on kinetic inhibition processes caused by the competitive adsorption between OH* and CH4.While the CO2* on kinetic inhibition processes caused by the competitive adsorption between CO2*(and CO3*)and CH4.