Atomic-level Mechanism Of Energy Minor Molecules Conversion Catalyzed By Two-dimensional Materials

The energy crisis and environmental problems caused by the over-exploitation and use of fossil energy have posed a great challenge to the sustainable development of human society.Therefore,the development of clean and renewable alternative energy is a global issue that needs to be addressed urgently.Hydrogen energy is an important clean energy,and the technology of producing hydrogen energy by water electrolysis can effectively utilize renewable solar and wind energy and other natural energy,thus solving the problems of energy shortage and environmental pollution,which is a technology with great development potential.The electrolysis of water molecules（H₂O）consists of the hydrogen evolution reaction（HER）,which produces hydrogen molecules（H₂）at the cathode,and the oxygen evolution reaction（OER）,which produces oxygen molecules（O₂）at the anode.The electrocatalytic HER only requires the adsorption of H⁺on the catalyst surface and the transfer of two electrons to form an H₂ molecule,while the electrocatalytic OER is a four-electron process that also involves the breakage of O-H bonds and generation of O-O bond,which has a very high energy barrier to electrocatalytic OER.Therefore,the activity of the electrocatalytic OER is the key to the activity of the overall electrolytic H₂O reaction,which directly affects the activity of the electrocatalytic HER for H₂ production.Methanol（CH₃OH）is rich in hydrogen and could be the engine of a new generation of fuel cells,which could provide significant environmental benefits.If natural gas,which is primarily methane（CH₄）,could be economically converted to methanol,the generated liquid fuel,methanol,would be easier to store and transport than natural gas and hydrogen.The process of CH₄ oxidation to CH₃OH involves the breaking of CH₃-H bonds with high energy barriers and the bonding of CH₃ radicals with OH,thus,it is necessary to search for efficient catalysts to lower the energy barriers of the reaction.Therefore,in this paper,based on first-principles calculations,we systematically investigate the activity of two-dimensional dual atom nanofilm and Mo Si Ge N₄ monolayer materials catalyzing the conversion of energy minor molecules（H₂O,CH₄）and summarize their intrinsic mechanisms.This research will promote the development and utilization of clean energy such as hydrogen energy,and provide a theoretical basis for solving the problems of carbon-based fuel shortage and environmental pollution.The main contents of this paper and its conclusions are as follows:（1）We have calculated the reaction process and activities of dual transition metal N-coordinated graphene(2TM-N_PG,2TM-N_PAG)nanofilms for the electrolysis of H₂O to produce O₂（OER）in alkaline electrolyte and summarized the intrinsic mechanism.The mechanism of electrocatalytic OER by dual atom catalysts is 2TM-N_PG as a pre-catalyst,while the real catalysts are 2TM-N_PAG and 2TM-N_PG-O.The Co Ni-NG has excellent electrocatalytic OER activity,where Co Ni-N_PAG has an overpotentialη=0.31e V and Co Ni-N_PG-O has an overpotentialη=0.38 e V.The CI-NEB kinetic pathway analysis reveals that the potential-determining step is also the rate-determining step,that is,the thermodynamic rate-determining step is also the kinetic rate-determining step.The electronic properties of the catalytic sites were calculated,and it was further found that the overpotential of Co Ni-NG has a volcano-like relationship with the atomic charge and the d-band center of Co atoms,that is,the best electrocatalytic OER activity is achieved when the electronic properties of the catalytic site are located near the peak of the volcano.It is suggested that the electronic properties of the catalytic sites can be adjusted to regulate the catalytic activity,and this research insight can be extended to other dual atom catalyst systems.（2）We have calculated the thermodynamic pathway and kinetic pathway for the electrolysis of H₂O to produce O₂（OER）by two-dimensional Mo Si Ge N₄ monolayer in unbuffered neutral electrolytes and summarized the intrinsic mechanism.By analyzing the energy band structure and density-of-states of the Mo Si Ge N₄ monolayer,the Ge-N layer is predicted to be more susceptible to redox reactions than the Si-N layer.In unbuffered neutral electrolytes,the highest catalytic activity with an overpotentialη=0.64 e V was found at 4%biaxial strain of Mo Si Ge N₄ monolayer by analyzing the thermodynamic pathway of electrocatalytic OER.Therefore,the kinetic pathway of the Mo Si Ge N₄ monolayer at 4%biaxial strain for electrocatalytic OER in unbuffered neutral electrolytes was next analyzed by CI-NEB method,the OH*-Chain Mechanism（OHCM）was concluded,since OH*is involved in each step of the OER process like a chain.The atomic charges of the Mo Si Ge N₄ monolayer were further analyzed at 0%～8%biaxial strains,and it was found that the largest number of electrons were lost at the Ge sites and the highest electrocatalytic OER activity was observed at 4%biaxial strain.This study fills a blank in the mechanistic study of electrocatalytic OER in unbuffered neutral electrolytes and provides a theoretical basis for the study of the electrolysis of pure water and even seawater.（3）With H₂O₂ as the oxidant,we have calculated the reaction process and activities of the CuZn N-coordinated graphene(CuZn-N_PG,CuZn-N_PAG)nanofilms for the CH₄oxidation to CH₃OH and summarized the intrinsic mechanism of its rate-determining step.The Cu 3d_z² orbital in CuZn-N_PAG and the Cu 3d_yz orbital in CuZn-N_PG were predicted to play major roles in the adsorption reaction by analyzing the valence band maximum and spin polarization density-of-states of CuZn-N_PAG.For the non-radical pathway of the CH₃-H bond activation,we found that additional adsorption is possible on the Cu 3d_yz orbital of CuZn-N_PG-O,but the energy barrier is high,while the Cu 3d_z²orbital of CuZn-N_PAG-O already interacts with O atom and no additional adsorption exists,that is,there is no non-radical pathway.Therefore,CuZn-NG catalyzes the CH₄oxidation to CH₃OH in a direct way without side reactions.The CH₃-H bond activation was found to be a spontaneous exothermic reaction with CuZn-N_PAG as the catalyst,that is,there is no energy barrier in this process.By analyzing the atomic charges of the Cu center and O atom,we defined the special electronic mechanism for CuZn-N_PAG to promote CH₃-H activation as the“bow-release effect”.Finally,the activity of CuZn-N_PAG catalyzing methane oxidation to methanol was calculated based on an implicit solvation model,and the electrolyte environment had no significant effect on the catalytic process and activity.This study provides an effective way to tune the catalytic activity of the dual atom nanofilm catalysts to facilitate the"Dream reaction"that catalyzes the oxidation of gaseous methane molecule to liquid methanol molecule.