Structure And Catalytic Performance Of Submonolayer Vanadium Oxide Supported On Anatase Titanium Dioxide (101) Surface

As a common semiconductor material,TiO₂ plays a very important role in the field of new energy materials due to its superior photocatalytic performance.However,its wide band gap and low surface activity greatly limit its application in the field of photocatalysis.At present,there are many methods to improve the photocatalytic activity of titanium dioxide materials.One of the most important methods is to load metal oxides on titanium dioxide surface.In this dissertation,based on first principles calculations,we studied the structure and catalytic performance of vanadium oxide supported on anatase TiO₂（101）.In this paper,we employed unbiased structural search based on the evolutionary algorithm（EA）combined with first principles calculations to determine the thermodynamically stable structures of V_mO_n clusters supported on anatase TiO₂（101）（V_mO_n/A（101）,m=1-3,n=3m）and their electronic structures.Our results indicate that monomeric cluster structures,such as VO,VO₂ and VO₃,are popular at low coverages（<0.25 ML）,while dimeric clusters（V₂O₅,V₂O₆）and trimeric clusters（V₃O₆）become more dominant at the coverage above 0.25 ML.The results of electronic structures and Bader charges show that the V atoms in the most stable structures of VO₂/A（101）,VO₃/A（101）and V₂O₅/A（101）are all in the oxidation states of V⁵⁺,while in the most stable structure of V₃O₆/A（101）one V atom is in V⁵⁺and the other two V atoms are in V⁴⁺oxidation states.Except structures with less oxygen,surface Ti atoms of most structures are in the Ti⁴⁺oxidation state.Based on the energetically stable structures,the structure-activity relationship were further investigated.The surface catalytic activity of V_mO_n/A（101）was characterized by calculating the surface hydrogen adsorption energy and oxygen vacancy formation energy,and the mechanism of selective catalytic reduction（SCR）of NO_x supported by vanadium oxide was analyzed by adsorption and dissociation of NH₃ molecule.The results show that VO₃/A（101）has very low hydrogen adsorption energy and oxygen vacancy formation energy,so it has very high catalytic activity for ODH and MVK type reactions.Compared with the pure A（101）surface,the photocatalytic activity of V₂O₅ and V₃O₆ surface was significantly improved.It is found that the values of hydrogen adsorption energy and oxygen vacancy formation energy are positively correlated with the amount of negative charges transfering from the load clusters to the substrate.According to the results of adsorption and dissociation of ammonia molecules,NH₃ is only prone to dissociation at VO₃/A（101）,which means that the one-site mechanism of SCR reaction of NO_x with NH₃can only act under the condition of low coverage of vadanium oxide（<0.25 ML）.We also studied the stable structures and catalytic activities of periodic vanadium oxide supported on anatase TiO₂（101）.The periodic structures with V coverage of 0.5 ML（V₂O_n,n=2-5）and 1 ML（V₄O_n,n=4-10）were studied.The results show that p-V₂O₅/A（101）is the main product when the coverage rate of V is 0.5 ML,and p-V₄O₈/A（101）is the main product when the concentration of V is 1 ML.In the most stable structure of p-V₂O₅/A（101）,all the V atoms are in their highest oxidation states V⁵⁺,while in the most stable structure of p-V₄O₈/A（101）,all the V atoms are in V⁴⁺oxidation states.The calculated hydrogen adsorption energies indicate that the oxidation states of V have strong influence on the catalytic reactivity of the system,and high oxidation states of V benefit for the improvement of catalytic reactivity.