Theoretical Investigation Of Mechanism Of De-NO_x On Metal-Exchanged Zeolite

Diesel engines are occupying a large share of transport market with human civilization,owning to their efficiency and durability.Among the product of combustion in the diesel engine,nitrogen oxides,NOx,are of primary concern for the responsibility of photochemical smog and acid rain.Typical diesel engine NOx exhaust contains predominately NO and much less NO2.Therefore,the catalytic decomposition or reduction of NO is a dominant target to be achieved.The reaction is slow without reducing agent as well as in the presence of catalysts.In rencent years,the catalytic decomposition of NO has been widely investigated on Cu-exchanged molecular sieves.Among literature,the 5T model has often been used.However,the error made by replace of real large system with a small cluster exists and is inevitable.Recent studies use hybrid methods,such as QM/MM(combined quantum mechanics/molecular mechanics)method,to study adsorption and mechanism of reaction on different types of zeolites showing results which are consistent with experiments.Application of ONIOM method for its minimal computation time and expense is common in fields of zeolite,among these scientific researches.At the same time,due to including the environmental effect,one can get an accurate conclusion with less expense.The present work is undertaken over 160 T cluster model with implemented ONIOM(DFT:UFF)calculation in Gaussian 09 software.The aim is to study the adsorption of gases and the mechanism of direct decomposition of NOx correctly.Then we use the ONIOM method in later investigation of selective catalytic reduction(SCR)of NOx.We obtain the differences of gas adsoption as well as the accruracy of ONIOM method.Besides,this work will fill the blank of SCR on Cu-ZSM-5 to make the study of De-NOx more complete.The results show that ONIOM is accurate.The difficulty order of the gases adsorption is as follows: NH3>NO?H2O>N2?O2.The results of this work also show that intermediate including copper and gas N2 O are generated in the direct decomposition of NO process.In SCR,fast-SCR(F-SCR)has good performance for the removal of NO.NO2-SCR will produce pollutant of N2 O.HONO-SCR is also a side reaction,which needs also be controlled.The clarification of reaction mechanism is of importance for the catalyst design and modification in the future.