The Adsorption Of CO On Transition Metal Oxide Catalysts:A DFT Research

As an important chemical raw material,CO oxidation reaction has important practical value in fuel cell,laser gas purification,gas sensor and automobile exhaust control.Due to the limited reserves and high prices of transition metal catalysts,it is necessary to develop a new CO oxidation catalyst.The transition metal oxides is an ideal CO catalyst selection that they have the catalytic properties of redox and acid and alkali,the functions of promoting heat resistance,strong toxicity,heat sensitivity,light sensitivity,impurity sensitivity etc.,and have low cost.Since the oxidation mechanism of CO on the surface of metal oxides is complicated,understanding the adsorption structure of molecules on the catalyst surface is the key to determine the relevant catalytic reaction mechanism.Therefore,it is of great significance to study the adsorption of CO molecules on the surface of transition metal oxide catalysts..The CASTEP module in the Materials Studio?MS?software package is used to calculate the CO adsorption on diffirent surfaces of rutile MnO₂and rutile TiO₂with the first-principles method based on density functional theory.Due to the strong hybridization of transition metal 3d orbital in transition metal oxides with the oxygen 2p orbital,density functional theory calclations correlated by on-site coulomb intereactions?DFT+U?is used to calculate the systems.The bulk phase calculation of rutile MnO₂is first carried out.Then,three surfaces of?110?,?100?and?211?are selected.After surface relaxations,six adsorption sites of CO molecules on these three surfaces are calculated.Adsorption structure,adsorption energy,band structure,density of states,Milliken population analysis and other parameters are calculated.The results shows that CO adsorbs on the five-coordinated manganese on the?110?and?100?surfaces.Chemical adsorption can occur at the two adsorption sites selected on the?211?surface,but CO is more likely to adsorb on the surface of a coordinated oxygen.Due to the hybridization of the C 2p orbital,a new impurity level occurs at the surface state density of the adsorbed state,the band gap value of the system becomes larger,and the band could move toward the lower energy level as a whole.At this time,electrons flow from the surface to the CO molecule.The research surfaces of the rutile TiO₂system are?110?,?100?,?210?,and the?210?surface is a high-index surface.The CO molecules can be chemisorbed at the six adsorption sites selected on the three surfaces.In view of the distance between the CO molecules and the adsorption sites before and after adsorption and the bond length of the CO molecules,the CO is more inclined to adsorbed on the surface of the five-coordinated titanium on the on the?110?and?100?surfaces.And on the?210?surface the CO is more inclined to adsorb on the surface of the three-coordinated titanium.At this time,the CO molecule is negatively charged,and the more stable adsorption structure is,the higher the charge number of the CO molecule has.The results of this paper have a certain guiding role in the design of CO oxidation catalyst,and it gives ideas for the further study in the design and improvement of transition metal oxides catalysts with DFT+U method.