Mechanistic Studies On The Selective Catalytic Reduction Of NO_x With Hydrocarbons Over Co-based Catalysts

The surface intermediates formed during the selective catalytic reduction of NO with hydrocarbons(HC-SCR) over Co-based catalysts were investigated systematically by in situ diffused reflectance infrared Fourier transformed spectroscopy(in situ DRIFTS) and possible reaction mechanisms were also proposed.The formation and dispersion of cobalt species were studied by H₂-TPR,UV-vis and XRD.By combining these characterization results with the DRIFTS results of ad-species and related experiments,a positive correlation between the catalytic activity and the active sites of catalysts was obtained and the main innovative results presented in the dissertation have been summarized as follows:1.The adsorption and reaction of different gas mixtures over Co-ZSM-5 catalyst were investigated at different temperatures by in situ DRIFTS.The results showed that NO existed as Co²⁺-(NO)₂ and Co³⁺-NO when adsorbed on Co-ZSM-5 and was oxidized to various nitrates in excess oxygen.The kind of nitrate species was significantly influenced by reaction temperatures.C₂H₄ was partially oxidized to formate species at above 250℃C over Co-ZSM-5. Introduction of oxygen promoted the partial oxidation of C_H4.In a flow of NO+O₂+C₂H₄/He, we detected the appearance of-CN,-NCO and HCONO₂ over Co-ZSM-5 in addition to nitrate and formate species.Formate species were reactive towards nitrates to yield formyl nitro compound,an intermediate critical for the formation of cyanide and isocyanate species by further reacting with nitrates.Kinetics analysis suggested that the reaction rates of cyanide and isocyanate at different sites towards NO+O₂ were different for the formation of N₂(g) and CO_x (g).The consumption rate constants followed an order of:κ(Co²⁺-CN)＞κ(Al³⁺-NCO)＞κ(-CN_L)＞κ(-CN_B).2.Co-ZSM-5 was characterized by different methods:H₂-TPR results demonstrated that at least three types of Co species were co-existed over Co-ZSM-5:dispersed Co₃O₄,CoO_x and ion exchanged Co²⁺ ions.The DRIFTS results showed that Co₃O₄ particles contributed to the oxidation of NO to bidentate nitrates,the latter species were crucial for the formation of-NCO. In addition,Co₃O₄ also facilitated the partial oxidation of C₂H₄ to formate species and a reduction of Co³⁺ to Co²⁺ occurred during this process;addition of oxygen promoted the re-oxidation of Co²⁺ to Co³⁺ to maintain a proper amount of Co₃O₄ particles.Co²⁺ ions were responsible for the formation of monodentate and bridged nitrates.Besides,Co²⁺ ions also acted as active sites for the further reaction between formate and various nitrates to generate crucial N,C-containing intermediates,as well as the conversion of-CN to N₂.3.Co/Al₂O₃ catalysts were prepared,by the incipient wetness impregnation method and exhibited good activity in C₂H₄-SCR of NO.Calcination temperature strongly affected the DeNO_x performances of Co/Al₂O₃ catalysts.Co/Al₂O₃ samples calcined at 350 and 800℃as well as Al₂O₃ support were characterized by XRD,UV-vis and FTIR spectra.The results revealed that surface aggregated Co₃O₄ particles were dominant species on Co/Al₂O₃ calcined at 350℃.Increasing the calcination temperature to 800℃promoted the diffusion of Co²⁺ ions into the Al₂O₃ lattice,leading to the formation of CoAl₂O₄ species.By combining the catalytic activity with DRIFTS results of ad-species,we clarified the catalytic functions of active cobalt species and support during the C₂H₄-SCR reaction.In the absence of O₂,Co₃O₄ species contributed to the oxidation of NO and C₂H₄ to various nitrates and formate species, respectively,whereas the side reaction of ethylene combustion occurred simultaneously when excess oxygen was present.Tetrahedral Co²⁺ ions in CoAl₂O₄,which acted as the active sites, were responsible for the reaction between formate and nitrate species to form organic nitro compound.These organic nitro compounds were considered as important intermediates for the C₂H₄-SCR reaction.4.Ag was selected as additive metal for Co-ZSM-5 and the effect of Ag on the selective catalytic reduction of NO with methane was also investigated.It was found that the addition of Ag enhanced the low-temperature activity and water-resistant ability of catalyst for the CH₄-SCR of NO.Combined the activity with H₂-TPR and UV-vis investigations,it demonstrated that the introduction of Ag onto Co-ZSM-5 changed the redox properties of Co₃O₄ particles and facilitated the activation of methane.Additionally,Ag species inhibited the combustion of methane and thus promoted the selective reduction of NO_x with methane. DRIFTS results showed that Ag contributed to the reaction between NO and O₂,leading to the formation of a great amount of highly active nitrates.This is the main reason why the Co, Ag-ZSM-5 exhibited excellent activity in the CH₄-SCR of NO.