Mechanistic Investigation Of The Low-temperature SCR Over Vanadia-based Metal Oxides And Copper-exchanged Small Pore Zeolites

As one of the main air pollutants,nitrogen oxides(NOx)are harmful to the overall ecology in terms of human health,acid rain,photochemical smog,haze etc.In such regard,the tremendous NOx emission in China has adsorbed extensive concerns from the whole society.Being as state-of-the-art technology for NOx abatement,selective catalytic reduction(SCR)of NOx has now been widely applied across different industries such as coal-fired power plants and diesel vehicles.SCR catalysts,the activity of which determines the overall De NOx performance,are nowadays a spotlight in the field of environmental catalysis researches.Reaction mechanism holds as the underlying substrate of catalytic processes,and for this reason,a detailed elucidation of elementary/pseudo-elementary steps that are occurring along the reaction coordinates serves as a roadmap for the catalytic enhancement.In this work,two typical SCR catalysts,i.e.vanadia-based metal oxides(VOx/Ce O₂)and copper-exchanged small pore zeolites(Cu-SSZ-13),are selected as two representatives due to their different structural topology and respective commercialization in stationary and mobile sources.A special chemical trapping technique has been used to capture and identify the unstable reaction intermediates,and was integrated with in-situ/operando spectroscopy,gas-phase kinetic measurements in both steady-state and transient modes,and theoretical computations using density functional theory(DFT,DFT+U+D,hybrid-functional HSE06)to investigate the detailed reaction steps of low-temperature(LT)SCR.By using such a coordinated approach,a consistent reaction mechanism that accounts for the reduction half cycle of LT-SCR has been proposed,based on which a first-order transient kinetic model was established.It has been demonstrated in the present work that the proposed mechanism and kinetic model can be extrapolated to several different SCR catalysts that are beyond VOx/Ce O₂ and Cu-SSZ-13.At the final stage,new SCR catalysts have been developed,on the basis of the proposed reaction mechanism and kinetic model,to be of enhanced LT-SCR activity and resistance to the alkali and arsenic poisoning.Summarized below are the major outcomes that are obtained in the present work.1.A transient reaction platform for the research of SCR has been established.Also,a combined methodology of in-situ/operando spectroscopy,gas-phase kinetic measurements in both steady-state and transient modes,and theoretical computations using DFT has been applied in the mechanistic investigatin of LT-SCR.Such a coordinated approach can be applied to various heterogeneous catalytic processes(SCR in the present work),enabling to track simultaneously the dynamics of oxidation states of catalyst active sites,the evolution of formation/consumption of surface active species,and the kinetic rates of gas-phase reactants and products in response to different external stimuli.In doing so,a complete description of catalytic processes from electronic,kinetic and structural perspectives,and both qualitative and quantitative analysis of the detailed reaction steps at a molecular level are authorized.2.DFT+U+D computations based on a VO₃H/Ce O₂(1 1 1)structure that embodies consistent features with the oxidized state of experimental model VOx/Ce O₂ samples,coupled with chemical-trapping techniques,in-situ spectroscopies and gas-phase TRM experiments,converge at suggesting HONO,a notoriously unstable intermediate that has been identified through the present integrated approach,as the primary intermediate of LT-SCR.Formation of HONO implicates pairing of V⁵⁺-OH and proximal Ce⁴⁺-O as the active centers,in which a strong synergism between vanadia and ceria surface has been highlighted in terms of both electron transfer and surface-oxygen migration.The HONO path is both energetically and kinetically favorable,and compatible with mechanistic findings and proposals over other types of SCR catalytic systems,such as Fe-and Cu-exchanged zeolites.These results cast new light on the long debated LT-SCR chemistry over vanadia-based catalysts,underscore the well-known synergetic interactions between active sites and supports in this specific case of SCR reactions over VOx/Ce O₂ systems,and also demonstrate an effective approach to progress understanding of the underlying SCR mechanism across,in principle,different kinds of SCR catalysts.3.The integratd strategy of hybrid DFT computations(HSE06 fucntional),transient response methods(TRM),chemical trapping techniques and operando UV-Vis-NIR spectroscopy has provided original fundamental insight into the detailed catalytic chemistry of the LT Standard SCR over Cu-SSZ-13,enabling both qualitative identification of the unstable reaction intermediates,and quantification of the reduction half-cycle(RHC)kinetics.Based on consistent theoretical and experimental evidence,herein we:i)reveal the involvement of HONO in RHC as the product of NO oxidative activation and of the associated Cu^II reduction to Cu^I;ii)specify different configurations of Cu-NH₃ complexes(one/two/three NH₃-ligands)in response to different reaction environments,and iii)stoichiometrically and kinetically validate the RHC scheme proposed by theory.Noteworthy,the mechanism here exhibits fair consistency with the one proposed for VOx/Ce O₂,showing its applicability across different SCR catalysts.4.The proposed mechanism is also applicable to WO₃/Ce O₂,on which NO oxidative activation into HONO has been demonstrated.Furthermore,In the absence of gas-phase O₂,intra-crystalline diffusion of lattice oxygen within Ce O₂ is disclosed,which severly decreases the reaction rates and also reduces the apparent activation energy to half of its intrinsic value.While in the presence of O₂,facile O₂ dissociation on the surface of Ce O₂ can readily fill the oxygen vacancy,and thus liberates the kinetic limiting role of intra-crystalline diffusion of lattice oxygen.Based on these observations,a first-order transient kinetic model that can closely simulate the spectroscopic dynamics is derived.Finally,this HONO-based mechanism has been successfully extrapolated to 10 different SCR catalysts(including Fe-ZSM-5 reported in the literature),and the kinetic model has been demonstrated to be applicable to the presently reported bin-and trinary metal oxide SCR catalysts.5.Based on the above established reaction mechanism,the LT-SCR activity and the resistance to alkali(Na)and arsenic poisoning on the vanadia-based metal oxide catalysts have been enhanced by doping additional catalytic components.A novel V-0.5Ce(SO₄)₂/Ti catalyst is developed and shows higher LT-SCR activity and improved resistance to Na poisoning when compared to commercial VOx-WO₃/Ti O₂catalysts.Also,an excellent durability of V-0.5Ce(SO₄)₂/Ti in the presence of SO₂ and H₂O is shown,indicating its promising potential of industrial application.The mechanism of As poisoning and its resistance is unraveled,and addition of SO₄^2-to commercial VOx-WO₃/Ti O₂ has been demonstrated to increase the surface acidity and thus contributes to the better SCR performance,suggesting that SO₄^2-can be an appropriate additive for the anti-poisoning of arsenic.Furthermore,NH₃ oxidation over V-0.5Ce(SO₄)₂/Ti is investigated,and a relatively higher polymerization degree of surface vanadia is found to account for its increased NH₃ oxidation activity.The NH₃ oxidation can be described using a successive NH₃ dehydrogenation mechanism,which thus provides an underlying and theoretical perspective for the future regulation of this notorious side reaction.