Theoretical Study On Structure Sensitivity Of Lanthanum Oxide Towards Dissociative Adsorption Of Molecule Such As Methane And Oxygen

Oxidative coupling of methane(OCM)is one of pathways for methane utilization,and the design of the catalysts for it is greatly challenging.The work of this paper is divided into two parts.The first part is that we study the properties of lanthanum oxide facets((001),(110)and(100))and the adsorption,activation and dissociation of methane and oxygen on them by means of periodical density functional theory(DFT),based on the previous experimental investigation upon structure sensitivity of lanthanum oxide in OCM.We find that surface free energy of lanthanum oxide facets decreases in the order of(110)>(100)>(001),and the energy gap between valence and conduction bands follows the order of(110)<(100)<(001),meaning that the facet(001)is most stable and(110)is most reactive among the three facets.The associative adsorption of methane on lanthanum oxide is very weak(-0.03 eV),and the barriers for H-CH3 on lanthanum oxide(001),(110)and(100)surfaces are 2.16,0.68 and 0.90 eV,respectively,showing the reactivity of the facets is matched with the dissociation of methane.On the other hand,the associative adsorption energy of oxygen on lanthanum oxide(001),(110)and(100)surfaces is-0.04,-0.31 and-0.12 eV,respectively,and the dissociation barrier is 1.22,0.53 and 1.52 eV,respectively.The oxygen adsorption is greatly influenced by the varied structures of lanthanum oxide.The interaction of oxygen with the facet(001)is the weakest,but the strongest with the facet(110)among the three facets of lanthanum oxide,which promotes O-O bond cleavage at lower barrier(0.53 eV)on the facet(110)to lead to the formation of electrophilic peroxide species.As oxygen adsorption on lanthanum oxide is stronger than methane,and the dissociation adsorption energy of methane on the surface of lanthanum oxide pre-adsorbed with oxygen is greatly lowed compared with that of lanthanum oxide without pre-adsorption of oxygen,the structure sensitivity of lanthanum oxide in OCM should be closely associated with oxygen adsorption and activation.In addition,the mechanism of methane and oxygen activation on lanthanum oxide can be ascribed to the transfer of electrons from the surface to their antibonding orbitals,which implies that varied structures of the lanthanum oxide surfaces can change the driving forces on the electron transfer.In the second part,we also investigated the adsorption behavior of ammonia,water molecules,chlorine molecules and hydrogen chloride molecules on these crystal faces of lanthanum oxide,in order to have a more macro understanding of the behavior of the activation of small molecule on lanthanum oxide solid surface.From the viewpoint of the adsorption strength of different molecules on the surface of lanthanum oxide,the dissociative adsorption energy of small molecules such as oxygen and methane on the surface of lanthanum oxide follow the order of HCl<H2O<Cl2<NH3<O2<CH4?Through further analysis,We found that the order of LUMO energy of small molecules such as methane is O2<Cl2<HCl<H2O<NH3<CH4,and the order of the LUMO energy of HCl,H2O,NH3 and CH4 is consistent with the order of corresponding dissociative adsorption energy on the surface of lanthanum oxide.While the LUMO energy of O2 and Cl2 is smaller than that of HCl and H2O,the dissociative adsorption energy on the surface of lanthanum oxide is more high.Further analysis shows there is an acid-base interaction between the fragments formed by dissociative adsorption of HCl H2O,NH3 and CH4 on the surface of lanthanum oxide,and there is no acid-base interaction between the fragments formed by the dissociative adsorption of O2 and Cl2 on the surface of lanthanum oxide.This may be why the LUMO energy of O2 and Cl2 is smaller than HCl,H2O,and the dissociative adsorption energy on the surface of lanthanum oxide is larger.From the viewpoint of the adsorption strength of molecules on the different surfaces of lanthanum oxide,the dissociative adsorption energy of methane,ammonia,water,hydrogen chloride and chlorine on the surface of lanthanum oxide follow the order of(001)>(110)>(100),and the dissociative adsorption energy of oxygen on the surface of lanthanum oxide follow the order of(001)>(100)>(110)Further analysis shows that the orbital shape corresponding to the Fermi level of the lanthanum oxide(001)and(100)surface is matched with the shape of the LUMO of methane,ammonia,water,hydrogen chloride and chlorine with ?-bond orbital,and the orbital shape corresponding to the Fermi level of the lanthanum oxide(110)is matched with the shape of the LUMO of oxygen with ?-bond orbital,which may be the reason why different molecules exhibit different crystal plane effects.