Theoretical Studies Of Methanol Oxidation On CdS?100? And Rutile TiO₂?110? Surfaces

As a classical model reaction,photocatalytic methanol oxidation attracts much attention.There were divergent behaviors in methanol oxidation on CdS(100)and TiO2(110)surfaces.On CdS(100)surface,(CH2OH)2 is formed with high selectivity,whereas on rutile TiO2(110)surface,HCHO is formed with high selectivity.But the reaction mechanism is still unclear.To solve this problem,we firstly investigated the basic properties of materials on a micro level with theoretical calculation.Herein,solid-liquid interface is an important reaction zone,but its simulation is rather difficult.Fortunately,Cheng and Michiel had developed a method named Density Functional Theory Based Molecular Dynamics(DFTMD)in a few years ago,which performed well in the calculation of band alignment and pKa at the solid-liquid interface.In this paper,methanol oxidation on CdS(100)and TiO2(110)has been investigated in the following aspects with the DFTMD method:1.The band positions of materials affect the reaction activity in the methanol oxidation.In this work,we calculated the band alignment at CdS(100)/H2O interface using PBE and HSE functional,which are relative to the standard hydrogen electrode.Compared with the energy levels in clean surface,it is found that the VBM and CBM will upshift at solid-water interface due to the water orientation.2.The acidity of CdS(100)/H2O surface is studied using DFTMD method and free energy perturbation(FEP)theory.The pKa values we computed for the two active surface species on CdS(100),the terminal H2O adsorbed on a 3-coordinated Cd site(CdOH2)and SH(SH+)are 14.9 and-10.8,respectively.They lead to a point of zero proton charge(PZC)of 2.05,which agrees well with experimental value of 2,within the computational error margin of 2 pKa units.From the two pKas' difference,we can estimate the dissociation free energy of adsorbed water,1.52 eV.Thus,the water is not easy to dissociate on perfect aqueous CdS(100)surface.Compared with rutile TiO2(110)surface,it is found there is lower dissociation free energy(0.23 eV)on TiO2(110).3.Thermodynamically,we calculated the energies of PCET steps in methanol oxidation on CdS(100)and TiO2(110).Kinetically,we explored the reaction pathways and calculated the energy barriers in methanol oxidation on ground state.It is found the C-H activation is easier than O-H activation with a photogenerated hole and C-H activation via a concerted PCET pathway on CdS(100)and TiO2(110)surfaces.Adsorption energy plays a determinant role in selectivity of the reactions.In addition,their reactivity is inconsistent in thermodynamics and kinetics.I think it is caused by the different oxidation ablitity of generated-holes on different surfaces and is still need to be understood.