Theoretical Insight Into The Surface Structure And Surface Reactions Of PdZn And PdCu Alloy

Along with economic development and social progress,the consumption of fossil and pollution from human activity also increases.Clean energy attracts more and more attention.Hydrogen energy has many merits such as high-efficiency,condensed-density and zero-emission etc.methanol is an ideal energy cattier which can produce hydrogen in situ for on-board fuel cells.One proposed method is the methanol stream reforming(MSR,CH3OH+H2O?CO2+H2).The Cu and PdZn alloy can be use as catalysts for MSR.Although the Cu catalyst has high performance,it can be easily sintered at high temperature and limited the scope of applications.Iwasa et al discovered that Pd/ZnO catalyst displays good stability and exhibits high activity and selectivity towards MSR reaction.Quite interesting,on pure Pd metal,MSR produces exclusively CO.Obviously,the addition of Zn atoms leads to a high selectivity of PdZn catalyst.Rameshan et al founded that the catalytic properties of two PdZn model catalysts made by depositing 2?3 monolayers of Zn on Pd(111)and then annealing between 300K and 700K respectively for 10 minutes.The catalyst obtained between 430K?570K(low temperature)exhibit?100%CO2 selectivity whereas poisonous CO is produced exclusively on the one at high than 630K(high temperature).The low-energy ion scattering(LEIS)indicates that the low temperature is a 1:1 multilayer PdZn surface alloy while the high temperature is interpreted to be a 1:1 monolayer PdZn supported on Pd(111).However,the selectivity strongly depends on the experimental parameters of catalyst synthesis and other reaction conditions.In theoretical,a lot of work has been done on MSR by DFT method.As participators among the MSR process,a thorough understanding of the behavior of water and methanol on PdZn alloy is a prerequisite for obtaining comprehensive knowledge of MSR.The PdZn alloy catalysts reported in previous theoretical study were almost artificial and cannot fully reflect the actual catalysts.Pd-Cu alloy is an important material that can be used to catalyze various reactions such as water-gas shift reaction,nitrate reduction,molecular sensors,oxidation reaction,hydrogen purification and alkynes semi-hydrogenation.The surface composition and atomic arrangement of a heterogeneous catalyst is a key factor controlling the surface physical and chemical processes.However,for alloys possessing surface segregation their surface composition and structures can differ significantly from the bulk ones.Existence of surface segregation in Pd-Cu alloy has been well confirmed by Miller and co-workers.Surface segregation is a complicated phenomenon and is affected by several factors including bulk composition,crystal facet,temperature and pressure etc.However,the formation and segregation of alloy form as a result of long timescale diffusion processes,which are impractical to simulate with molecular dynamics(MD).In contrast,the kinetic Monte Carlo method is capable of modeling such long timescale diffusion processes,because in kMC only the relevant atomistic configuration changes are explicitly simulated,while the detailed atomic vibrations incorporated in MD simulations are treated implicitly.In order to further understand the formation and segregation processes of binary alloys and find the active sites,we use kinetic Monte Carlo method combined density functional theory to investigate the formation process of binary alloy,to evaluate the activity on PdZn alloy towards MSR reaction and finally to get a closer link between the theory and experiment.The primary investigation contents are briefly introduced as follows.Firstly,the annealing process is modeled with a rectangular grid 50×50×21(PdZn)and 20×20×31(PdCu)with periodic boundary condition considered.Every atom,except the bottom layer,can direct exchanges with the nearest atoms.Obviously,the system changes according to the atoms exchanges.The most important stage of the kMC simulation is establishing the rate equations and the critical step is accurately calculating the energy barriers.Here the Marcus theory and Bugakov-van-Limpt rule were adopted to obtain the energy barrier and the intrinsic activation energy,respectively.we propose the concept of microenvironment to calculate the intrinsic energy.The melting point of the alloy and the bong energy among the environment is concentration dependent on the alloy phase diagram.In the experiment,the composition can be measured at a certain time.However,in the theoretical simulation,except for some very simple systems,the theoretical results are difficult to correspond to the experimental results.In order to make the results of theoretical and experimental consistent,we introduces the parameters ?,?as a function of concentrations and temperatures.The results shown that,whether in terms of low temperature range of 430K-570K or high temperature above 630K,it is difficult to form a multilayer or monolayer PdZn 1:1 alloy.It is just gradually decreasing from top to bottom layer.The results are determined by the nature of Pd and Zn element,the sum of EPdPd and EZnZn is far less than twice EPdZn.On the contrary,because of the less surface free energy,Cu atoms are easily segregated to alloy surface.We also firstly reported the phenomenon of bulk-inside blocking.To further examine the results of kMC,we carried out DFT study of methanol and water dissociation on the low-temperature,high-temperature,multilayer PdZn(111)and monolayer PdZn(111)and zigzag model.On the multilayer model surface,water adsorption energy is 0.27eV,on the high-temperature model water adsorption energy is 0.21eV only.Zigzag model show similar adsorption energy,0.24eV.On this surface it is easy for water to desorb into the gas phase,on the low-temperature model,we calculated large adsorption energy of 0.49eV.Obviously,large adsorption energy would help water dissociate on the alloy surface.On the low-temperature model surface,our calculated barrier,0.72(0.55)eV,is quite low and comparable to the water adsorption energy,0.49 eV,implying that water dissociation is likely on it.Water can be dissociated on the low-temperature model alloy whereas it remains intact on the high-temperature model alloy.Here our calculated results on the two models present a strong support to this point of view.It is worthy to point out that the low-temperature model could basically be regarded as a 1:1 multilayer PdZn alloy,because it has the same surface composition as the ideal 1:1 multilayer surface alloy.The main difference is the surface atomic arrangement.For the methanol,we calculated large adsorption energy of 0.57eV.In comparison,the series decomposition reaction of methanol(CH3OH?CH3O?CH2O?CHO?CO),the low temperature model are favorable for the decomposition of methanol.Based on the experiment,we used the kMC method combined DFT study the annealing process of binary alloy and obtained the expected results in methanol and water decomposition.These results suggest model about atoms exchange in binary alloy had a certain reference value to explain experimental phenomenon and catalyst design.To recap,this dissertation elucidates how the morphology of binary alloy changed and the decomposition of water and methanol and demonstrates in different PdZn models.The low temperature PdZn alloy model contributes the MSR reaction.Finally,we proposed a new calculation method for energy barrier of metal atom exchanges.