| Low-concentration methane gas is widely existed in coal-bed gas and industrial waste gas,and methane is around 20 times more potent as a greenhouse gas than carbon dioxide.Catalytic combustion is one of the efficient and practical methods to treat low concentration methane.For catalytic reaction of low-concentration mehane,it shows good catalytic performance of Pd and Pt catalyst.Bimetallic Pd-Pt sample becomes a research hot spot due to the excellent reactivity.At present,the reaction of the gas fuel represented by methane on bimetallic catalyst mainly focuses on the interaction between metals and the reaction kinetics.The structural characteristics of bimetallic catalysts and the quantitative differentiation of active sites need to be solved.This paper aims at the above problems,and have significances to elucidate the structure of bimetallic catalyst and the mechanism of catalytic combustion of gas fuel.This paper mainly investigates methane catalytic oxidation over bimetallic Pd-Pt catalyst,and from the aspect of methane reaction over monometallic Pt sample,it employes the method of differential reactor and chemisorption,to study the sensitivity or insensitivity of methane reaction rate on each site to Pt particle size.It employs chemisorption method at high temperature to inject oxygen pulse to bimetal surface,which makes use of the oxygen diffusion in bulk catalyst to synthesize Pt-core@PdO-shell catalyst.Catalytic consequence provides evidence for demonstrating the existence of single Pt particles.A bimetal catalyst model based on single Pt particle and Pd-Pt alloy core-shell structure was established.It utilizes the tecknique of HAADF?high angle annular dark field?–STEM and catalyst dispersion,to explore the particle type,cluster diameter,and particle distribution over supporter surface.Via DRIFTS and EDS method,it demonstrates the existence of Pt plane,and elucidates the variation of single Pt particle and core-shell structural alloy with Pt loading.By using differential reactor,it distinguishes Pd sites and Pt sites over alloy surface,calculates particle diameter of single Pt particle and core-shell alloy particle in bimetallic 1Pd0.5Pt,1Pd1Pt,and 1Pd2Pt catalysts,and demonstrates the improvement effect of Pt to Pd species for methane oxidation.The theory and innovation of this investigation is listed below:?1?It investigates reaction regions,active sites and rate-determining steps for methane reaction over monometallic Pt sample.Based on the Pt cluster size sensitivity or insensitivity to site reaction rate over methane catalytic oxidation,it establishes quantitative relation between rate constant and monometallic Pt particle.There are 3 reaction regions for methane catalytic reaction over monometallic Pt sample:Region A,active sites show as metal vacancy?*+*?;Region B,active sites show as the pair of chemisorbed oxygen and metal vacancy?*+*?;Region C,active sites show as the pair of chemisorbed oxygen?*+*?.In Region A,it shows that rate-dependent step determined by oxygen dissiociation is nothing to do with particle size,that is to say cluster size insensitivity.In Region B,it shows that reaction rate determined by both oxygen dissiociation and C-H bond dissiociation has a certain quantitative relation with catalyst particle size,that is to say cluster size sensitivity.Cluster size insensitivity is used to distinguish surface active sites,while cluster size sensitivity is used to calculate Pt particle size.?2?It provides a novelty synthesis method for prepareing Pd-Pt core-shell structural catalyst via using oxygen titration at high temperature,and a particle model of alloy catalyst is provided,that is single Pt particles and Pt-core@PdO-shell particles.For Pd-Pt alloy caystals,as oxygen pulse is injected into catalyst particle surface,oxygen atoms adsorb on crystal surface and diffuse into the inner part of alloy cluster.Pd species gradually migrate to crystal surface forming PdO shell duo to oxygen induced segregation,and Pt species migrate into the inner part forming Pt core.Core shell structure makes the alloy grain reach thermodynamic equilibrium.Single Pt particles also form on supporter surface,which is composed by Pt element precipitated from alloy,and show Pt property for methane catalytic reaction.Alloy cluster shows large particle size and all the Pd species concentrate on the shell,which is supported by Pt core.Therefore,core shell structural alloy displays Pd property for methane catalysis?particle surface is covered by Pd atoms,and Pd species mainly contributes to methane oxidation?,and there are interactions between Pd and Pt species.?3?It discusses the variation of cluster diameter of single Pt particle and core shell alloy particle with Pt/Pd ratio,differentiates Pd sites and Pt sites,and normalizes total catalytic activity to single catalytic site to show the real site reaction rate.By employing DRIFTS method,it demonstrates the existence of Pt plane via CO bridge adsorption at 1820 cm-1.Combined with catalytic performance,it considers that all the three bimetal Pd-Pt catalyst have single Pt particles,and the particle size increases with increasing Pt loading.HAADF-STEM tecknique proves the segregation of Pd-Pt elements that a large amount of particles display the Pt-core and Pd-shell structure.Doing the peak separation for CO adsorption curve,can distinguish the surface Pd site and Pt site when compared with CO adsorbing on monometallic Pt catalyst surface.Finally,normalizing total catalytic activity to single catalytic site,can obtain each catalytic consequence of Pd and Pt to catalytic reaction.?4?It provides a novelty method based on reaction kinetic method via the characteristics of methane catalytic reaction to differentiate surface Pd active sites and Pt active sites,and calculates the number of Pd and Pt sites.This method mainly proceeds in reaction Region A?Pd-Pt?,combined with the relation between the insensitivity of cluster size to methane reactivity.For three bimetallic catalysts?1Pd0.5Pt,1Pd1Pt,1Pd2Pt?,to use total reaction rate of bimetallic catalyst to divide by turnover rate of monometallic Pt sample,it can differentiates surface Pd sites and Pt sites,quantitatively calculated the number of surface active sites.The results of sites separation and quantitative calculation are in good agreement with that of IR spectra method. |
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