DFT Study Of Partial Oxidation Of Methane On Pd And Its Modified Cu(111)surface By Oxygen Atom

Coalbed methane（CBM）is a kind of clean energy,but it is often mixed with air in the process of exploitation.Therefore,the conversion of methane（CH₄）and oxygen（O₂）into syngas by using the technology of partial oxidation（POM）is an effective means to make it efficient and safe.At present,the difficulty of POM reaction is to reduce the reaction temperature and increase the selectivity of CO.The results show that Pd noble metal can effectively reduce the reaction temperature.The performance of POM reaction is related to the d-band center and differential charge density of the catalyst surface,which can be changed by metal doping or adsorption of oxygen atoms.Therefore,it is of great significance to further understand the influence of different doping forms and oxygen atom adsorption states on the catalytic performance from the molecular and atomic levels.In this paper,the reaction process of POM on Pd clusters and single atom alloy Pd/Cu（111）catalysts was studied by using the Dmol³ module based on density functional theory,and the effect of local state change caused by oxygen adsorption on the active sites and rate determing step（RDS）was also discussed,which provided a theoretical basis for the study of POM reaction in Pd system and provided a catalyst with high conversion and selectivity catalysts provide guidance.Specific research contents are as follows:（1）The promotion of oxygen atom to POM reaction on Pd₂ clusterSince the adsorption of CH_X（x=1-4）on the catalyst is of great significance for POM reaction,the binding energy,the density of states（PDOS）,the dissociation energy and activation energy of CH_X（x=1-4）are calculated on Pd₂and Pd₂O reapectively,and the formation process of H₂ and CO is studied.The results show that:1)The existence of oxygen atom will weaken the binding energy between CH_X（x=1-3）and Pd₂,and makes the peak of PDOS move to the left;2)The existence of oxygen atom reduces the reaction energy barrier of CH₄disscoation and CO formation,and increases the reaction energy barrier of H₂formation;3)In the overall POM reaction,the oxygen atom does not change the RDS,which is CH₂→CH+H,but the rate constant increases from 7.27×10^–13 s^-1to 4.03×10^–8 s^-1.In general,the presence of oxygen atom can promote the POM reaction on Pd₂ and improve the selectivity of CO.（2）The POM reaction on Cu（111）surface modified by Pd single atomWhen Pd single atom modified Cu（111）,there may be doping and adsorption（D-Pd/Cu（111）and A-Pd/Cu（111）,respectively）.The process of CO formation by reaction of POM on two surfaces is studied.The adsorption energy of all possible intermediates,the formation path of CO and the thermodynamic stability of the two alloy surfaces are calculated.1)The adsorption energy of all intermediates except CH₃O and CO on A-Pd/Cu（111）surface is higher than that on D-Pd/Cu（111）surface;2)Pd atoms on the surfaces of the two alloys can promote the formation of CO on the surface of Cu（111）,but the formation path is different.On the surface of D-Pd/Cu（111）:CH₄→CH₃→CH₃O→CH₂O→CHO→CO;on the surface ofA-Pd/Cu（111）:CH₄→CH₃→CH₂→CH→CHO→CO.The total activation energy barriers are 1.49 e V and 1.46 e V,indicating that CO is easier to form on the surface of A-Pd/Cu（111）;3)The results show that the surface thermodynamic stability of A-Pd/Cu（111）is poor.In conclusion,the surface of D-Pd/Cu（111）is more favorable for POM reaction.（3）The regulation of oxygen atom on the formation of CO on D-Pd/Cu（111）surfaceThe formation of CO in POM reaction on the surface of D-Pd/Cu（111）surface（O-D-Pd/Cu（111））in the presence of O atom is studied and compared with that on the surface of A-Pd/Cu（111）（O-A-Pd/Cu（111））in the presence of O atom.Through the analysis of differential charge density on metal surface and calculation of adsorption energy of the intermediate in the process of CO formation is calculated on the stable O-D-Pd/Cu（111）and O-A-Pd/Cu（111）surfaces,and the optimal path is determined.The results show that:1)The best adsorption sites of oxygen atom on both surfaces are hollow sites,and on the other A-Pd/Cu（111）surface,oxygen will automatically combine with the adsorbed Pd atom to form Pd-O bond;2)The calculation results of d-band center on the catalyst surface before and after oxygen atom adsorption indicates that the adsorption of oxygen atom may affect the reaction mechanism;3)The existence of oxygen atom is conducive to the formation of CO on the different surface of D-Pd/Cu（111）and A-Pd/Cu（111）,and the best path is CH₄→CH₃→CH₃O→CH₂O→CHO→CO.However,RSD is different:CH₃+O→CH₃O on the surface of O-D-Pd/Cu（111）;CH₄→CH₃+H on the surface of O-A-Pd/Cu（111）,and the total activation energy barriers are 1.04 e V and 1.15 e V.Therefore,the presence of oxygen atom is beneficial to POM reaction.（4）Regulation of adsorption and dissociation of H₂O on D-Pd/Cu（111）and A-Pd/Cu（111）surfaces by oxygen atomIn the POM reaction process,H₂O is one of the main by-products,and the dissociation of H₂O will form oxygen species.Different oxygen species have different effects on POM reaction.Therefore,it is important to study the dissociation of H₂O for further understanding of POM reaction.Four surface models of D-Pd/Cu（111）,O-D-Pd/Cu（111）,A-Pd/Cu（111）and O-A-Pd/Cu（111）was established to study the adsorption of possible products of H₂O and its possible dissociation products,and the dissociation energy of H₂O and OH was calculated.The results show that:1)On the D-Pd/Cu（111）and A-Pd/Cu（111）surface,the existence of oxygen atom will affect the optimal adsorption sites of H atom and OH;2)The existence of oxygen atom increase the adsorption energy of H₂O on the surface of will not change the best adsorption site of H₂O on the surface of D-Pd/Cu（111）and A-Pd/Cu（111）,and reduces the reaction energy barrier of its disscoation;3)On the surface of D-Pd/Cu（111）and A-Pd/Cu（111）,there is no dissociation reaction of H₂O but adsorption desorption.In the present of oxygen atom,the dissociation of H₂O is mainly occurred;4)On the surface of O-D-Pd/Cu（111）,the OH produced by the decompoaition of H₂O will further split into H and O atoms,and the activation energy barrier is 1.52 e V.In conclusion,the dissociation of H₂O is beneficial to the reaction of POM on D-Pd/Cu（111）surface.