Density Functional Studies On The Mechanisms Of Reduction Of N₂O With CO And Water-Gas Shift Reaction Catalyzed By Some Gaseous Transition Metal Oxide?Cluster?Ions

Environmental pollution and energy crisis are two serious problems for human survival and sustainable development to face.First,air pollution can trigger a series of environmental and ecological problems,it will not only affect people's daily life,but also undermine the sustainable development of our society.Therefore,it has been widespread concerned in today's society.The Nitrous?N₂O?and carbon monoxide?CO?are two major sources of air pollution,which produced by automobile exhaust and industrial production.Among them,CO is not only a kind of harmful gases in the atmosphere,but also harmful to the human body.It can bind with the hemoglobin quickly,thus preventing the hemoglobin bind with the oxygen,leading to human hypoxia and even death.N₂O is a stubborn greenhouse gas;it has strong absorption of infrared radiation in the atmosphere.Although its content in the atmosphere is only about 9%of the CO₂,but the greenhouse effect which produced by N₂O is 310 times as much as it by CO₂.Therefore,how to reduce these harmful gases is a hot and difficult issue in the fields of science and technology.Oxidation of CO and reduction with N₂O is highly exothermic,but the reaction barrier is too high.In addition,H₂ is one of the very important clean energies.The Water-Gas Shift Reaction is not only a way for the producing hydrogen,but also a key step in the processing of carbon based fuels.Meanwhile,the reaction can remove CO,so it is one of the hottest reactions today.However,the reaction often occurs under catalysis and at a high temperature.Therefore,looking for efficiently catalysts to achieve these two reactions under mild conditions is a very important research topic.Studying on gas phase catalytic models not only avoids the impact of the complex condensed phase effects in the real catalytic system such as solvent effects and surface effects,but also show the intrinsic properties of the reaction,and can play a supporting role in looking for the real catalysts.In recent years,scientists have found that some transition metals oxide cluster ions have better catalytic performance on oxidation of CO and reduction with N₂O in gas phase reaction.In this paper,based on the previous experimental observation,the reaction mechanism of the reduction of N₂O with CO has been studied in details,and the specific research contents are as follows:Firstly,the mechanism of the first title reaction catalyzed by ReO₂+ was calculated using two DFT methods of UPBEPBE and UB3LYP,and the results by the two methods are compared with each other.Results obtained by the two methods are qualitatively similar,the ReO₂+ as a catalyst is more favorable thermodynamically and kinetically,and is confirmed by the ratio of TOF.There are crossovers between two potential energy surfaces with different spin states,namely,the reaction may occur nonadiabatically on the two potential energy surfaces with different spin states.As a result,the lowest energy reaction pathway calculated by UB3LYP is:3ReO₂++N₂O?19?1TS9/10?110?1ReO₃⁺+N₂?111?1TS11/12?112?3ReO₂++CO₂.From the calculations by UPBEPBE and UB3LYP,the overall reaction is exothermic by 333.9 and 358.5 kJ·mol-1,respectively,and the barrier of the rate-determining step is about 133.2 and 149.4 kJ·mol-1,respectively,which are lower than the direct reaction barrier of 160.9 and 201.3 kJ·mol-1,respectively,which shows the catalytic role of ReO₂+.Furthermore,all transition states in the cyclic reaction locate below the enterance channel of the reaction.So the reaction can occur under normal temperature and pressure,which is in consistent with the available experimental facts.Secondly,two kinds of density functional theory methods of UPBEPBE and UB3LYP has been used to study the catalytic mechanisms of Water-Gas Shift Reaction?H₂O+CO?H₂+CO₂?catalyzed by ReO₂+/ReO₃⁺ couple.The results show that the two methods have similarly qualitative results,the Re?O₂?+ as a catalyst is more favorable in thermodynamics and kinetics,and is confirmed by the ratio of TOF.There are spin crossings between three potential energy surfaces with different spin states,the reaction may occur non-adiabatically on three potential energy surfaces with different spin states,the lowest energy reaction path obtained by UB3LYP is 5Re?O₂?++H₂O?516?3TS16/17?317?1TS17/18?118?1Re?O₂?O++H₂?119?3TS19/20?520?5Re?O₂?++CO₂.From the calculations by UPBEPBE and UB3LYP,the overall reaction is exothermic by 113.0 and 83.6 kJ·mol-1,respectively,and the barrier of the rate-determining step is about 235.4 and 231.8 kJ·mol-1,respectively,which are lower than the direct reaction barrier the reaction of 328.5 and 403.2 kJ·mol-1,respectively;and all transition states in the cyclic reaction are lower in energy than the entrance channel of the reaction.So the reaction can take place under room-temperature and pressure,which is in accordance with the previous experimental studies.Thirdly,the detailed mechanism for the reaction of reduction of N₂O with CO catalyzed by Pt7+ has been investigated by the UPBEPBE method.We successfully optimized the geometries for plausible stationary points in the reaction paths and calculated the corresponding energies.Results showed that there are spin crossings between three potential energy surfaces with different spin states,and the reaction of reduction of N₂O with CO catalyzed by Pt7+/Pt7O+ couple may take place non-adiabatically on three potential energy surfaces with different spin states.However,the lowest energy path of the reaction only involves two spin states:4Pt7++N₂O?21?4TS1/2?22?4Pt7O++N₂?23?2TS3/4?24?4Pt7++CO₂.The overall reaction is exothermic by 333.9 kJ·mol-1,and the barrier for the rate-determining step is 56.3 kJ·mol-1.TOF calculations show that the TDI?TOF-determining intermediate?and the TDTS?TOF-determining transition state?are 22 and 4TS3/4,respectively,which is not adjoined each other.This means the reaction rate is not determined by only one step but by two steps:22?4Pt7O++N₂?23?4TS3/4,which is different from the traditional one-step-determining rate,and the corresponding energetic span of the TOF is 127.7 kJ.mol-1,which is less than the direct reaction energy barrier of 160.9 kJ·mol-1.And this indicates that Pt7+ has good catalytic performance for the reaction of the reduction of N₂O with CO,and the results could justify the conclusion of the previous experimental studies.