Theoretical Investigations Of The Photo（electro）catalytic Reactions On Semiconductors:CO₂ Reduction And Lignin Conversion

Energy and environmental crisis are two major global problems we are facing today due to the heavy dependency on and overuse of the fossil fuels.Solar energy driven clean and efficient utilization and conversion of carbon-based fuels provide a promising way to address the energy and environmental issues.Herein,we mainly focus on the photo（electro）catalytic CO2 reduction and lignin conversion.The burning of fossil fuel is believed responsible for the increasing atmospheric CO2 concentrations.CO2 is one kind of greenhouse gases that is associated with the global warming problems.The conversion of CO2 into liquid fuel methanol driven by solar energy is an ideal strategy to reduce the concentration of CO2 in atmosphere.This process can address the energy and environmental problems at the same time.As is reported by Bocarsly and coworkers,when using p-GaP as the photoelectrode,pyridine as the cocatalyst,the CO2 can be reduced to methanol selectively at underpotential.However,the overall yield of methanol is quite low.The understanding of the role of GaP and pyridine in CO2 reduction is necessary for developing new catalysts with high efficiency.Lignin is the largest resource of aromatics.The conversion of lignin at mild conditions is of high economic value.Experimental studies show that the CdS quantum dots are able to catalyze the selective lignin conversion at room temperature under visible light.However,the mechanism is not clear.Photoelectrocatalytic reactions mainly occur at the solid/water interface.However,the simulation of proton coupled electron transfer reactions（PCET）at the solid/water interface is a great challenge.The method combining density functional theory based molecular dynamics（DFTMD）and free energy perturbation（FEP）methods performed well in the simulation of the solid/water interface.In this paper,mechanisms of photo（electro）catalytic CO2 reduction and lignin conversion are investigated with the combined DFTMD and FEP method:1.The photocatalytic activity of semiconductor depends on its band edge positons.The electronic structure of the GaP（110）/H₂O interface is investigated using the combined DFTMD and FEP method.Results show that the unsaturated dangling bond on the GaP（110）surface will cause the presence of the surface states near the band edges.These states can be removed by the water adsorption.The adsorption of 1ML water will shift up the VBM and CBM of the GaP（110）surface and the fully solvation effect will shift up the VBM and CBM further.The distinct difference between the band edge positions of the 1 ML adsorbed GaP（110）surface and the GaP（110）/H₂O interface is mainly due to the orientation of adsorption water and near-surface water.2.The acidity of the GaP（110）/H₂O interface is studied thermodynamically and kinetically.The proton affinity of the surface sites calculated are 11.7 and 13.7 respectively.This lead to a point of zero charge（PZC）of 12.7.The dissociation free energy of the surface water is estimated from the pKas’ difference,ΔAdiss=-0.1 eV.The negative value indicate that the dissociation of surface water is thermodynamically favorable.Kinetic studies about mechanism of surface water dissociation show that the proton relay mechanism is favored.The dissociation degree of adsorbed water is estimated to build reasonable GaP（110）/H₂O interface.3.The mechanism of pyridine catalyzed CO2 reduction is explored.Mainly,the possible first electron transfers steps are analyzed to find the active intermediate.First,the redox potentials of the related species are calculated with the effects of the solvent and the GaP（110）surface taken into account using combined DFTMD and FEP method.It’s found that both the solvent and the GaP（110）surface may have influence on the structures and redox potentials of molecules.Then,the band alignment of GaP（110）/H₂O interface is calculated with the effect of pyridine adsorption considered to determine the photocatalytic activity.Results show that pyridine adsorption will shift up the band edge positions of the GaP.Thus,in order to obtain the accurate band alignment of GaP and determine the photocatalytic activity,both the effect of water and pyridine adsorption should be taken into account.Finally,we compare the redox potentials with the CBM of the pyridine adsorbed GaP（110）/H₂O interface,and find that the PyCO2*will be reduced first by the photo generated electron and the PyCO2-*may be a potential active intermediate for CO2 reduction.4.Density functional theory（DFT）calculations are performed to understand the mechanism of selective conversion of lignin on CdS.Studies reveal that the lignin conversion proceeds through a EHCO（electron-hole coupled）mechanism.The Ca radical intermediate will be obtained first by oxidation dehydrogenation reaction of lignin.The formation of the Ca radical intermediate will markedly decrease the BDE of the β-O-4 bond.This is believed the key reason or the selective cleavage of β-O-4 linkages in this system.This study provides a promising tool to cleave selectively theβ-O-4 bond.