Porphyrin As Sustainable Catalyst For Water Dissociation:DFT Study

In the field of producing clean and renewable energy,splitting water to produce hydrogen is currently the most efficient method.The key point is to find a kind of catalyst which can reduce the energy barrier and enhance the reaction efficiency.Using first-principles calculations,we firstly study water splitting catalyzed by alkaline-earth or transition metal porphyrin?MPP??M=Mg,Ba,Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,and Zn?at charge neutrality state?H₂O+MPP?or at the state releasing electron?H₂O+MPP⁺?.The results show that hydrogen proton will split from the water molecule whether in the H₂O+MPP system or in the H₂O+MPP⁺system.In the H₂O+MPP system,the dissociation energy is positive value which means it is an energy absorption process.However,in the H₂O+MPP⁺system,the dissociation energy is negative value representing an exothermic reaction process which means the splitting of the hydrogen proton is extremely easy.Through the Bader charge analysis we conclude that for the same porphyrin system if there are more total electron transfer quantities from the H₂O molecule and the center metal atom to the outer-ring of the porphyrin substrate,the dissociation energy will be lower.For all kind of porphyrin system,the titanium porphyrin?TiPP?catalyzes the water splitting with the minimum dissociation energy.Next,in order to analyze the catalysis of TiPP in detail,we explore the water splitting adsorbed on the top of TiPP at neutral state?H₂O+TiPP?,TiPP at injecting electron state?H₂O+TiPP^-?and TiPP at the state releasing electron?H₂O+TiPP⁺?.For the H₂O+TiPP and H₂O+TiPP⁺system the H₂O tend to split hydrogen proton,while for the H₂O+TiPP^-system the H₂O tend to split H atom.The dissociation energy of the first H are in the order of-2.71 eV?H₂O+TiPP⁺?<1.23 eV?H₂O+TiPP?<1.31 eV?H₂O+TiPP^-?.In order to investigate the influence factors for the water dissociation energy,we plot the optimized geometry structures,Bader charge analysis image and the density of state for the three systems.Through contrastive analysis,when there are more total electron transfer quantities from the H₂O molecule and Ti atom to the outer-ring of the TiPP substrate,the polarity of O-Ti bond and the adsorption between Ti and O atom will be stronger.The electron transfer also will lead to the less electrons in H atoms which results in the strong repulsive interactions between TiPP and H atom,and these make the water tend to split hydrogen proton with very low energy barrier.The dissociation process of the second hydrogen proton is an exothermic reaction process too,and the relationship between the charge distribution and the dissociation energy is consistent with the above conclusion.After the splitting of two hydrogen protons,the O atom adsorb on the TiPP substrate.The energy barrier is 7.32 eV,if we want to desorb the O atom,so the oxygen decorated titanium porphyrin?OTiPP?is a stable structure.In the final part,we study the water splitting catalyzed by OTiPP at the releasing electron state.The dissociation process of the two hydrogen protons will release 2.38 eV in all.The desorption of the O atom only needs 0.72 eV,which is rather small and can be readily realized.All in all,the OTiPP can catalyze H₂O splitting into hydrogen protons and O atom,and then return to original state.In these processes there is no byproduct,so all the catalyzed processes are pure clean.In other word OTiPP is the clean high efficiency and recycled catalyst for water splitting.The composition of the paper is as follows.In chapter 1,we introduce the research background and the latest research progress of water dissociation.Moreover,we make a brief introduction about the porphyrin and give the research process of this paper.In chapter 2,we briefly introduce the development process of the first principles and the density functional theory.In chapter 3,we firstly investigate the water dissociation catalyzed by alkaline-earth or transition metal porphyrin and then study the catalytic efficiency of TiPP and OTiPP for the water splitting.Additionally,we have concluded the influence factors for the water dissociation energy.In chapter 4,we summarize the entire research work.OTiPP is the clean high efficiency and recycled catalyst for water splitting.