Effect of Cu content on the catalytic activity of CuSiBEA zeolite in the SCR of NO by ethanol: Nature of the copper species (original) (raw)
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Catalysis Today, 1999
Copper-exchanged zeolites with different structures (CuMFI, CuMOR and CuY) used as catalysts on the selective catalytic reduction (SCR) of NO by propene have been studied. Different types of Cu species were identified (Cu 2+ , Cu + , and CuO) by H 2 -TPR and NO TPD. The structure of each zeolite determines the nature and concentration of those species and the catalytic behavior for SCR of NO by propene in the presence of oxygen. A correlation was observed between the catalytic activity, and the presence of isolated Cu 2+ species, which is enhanced by MFI structure.
Role of the different copper species on the activity of Cu/zeolite catalysts for SCR of NOx with NH3
Applied Catalysis B: Environmental, 2014
The SCR of NO x with NH 3 has been studied by using different Cu zeolite catalysts, prepared both with ZSM5 and BETA zeolite supports by ionic exchange or by impregnation. The catalysts were characterized by ICP-AES, N 2 adsorption at-196 ºC, XRD, TEM, XPS and H 2-TPR. The catalysts characterization confirmed the presence of different Cu(II) species on all catalyst (CuO and Cu(II) exchanged on tetrahedral and octahedral positions of the zeolites framework). Clear evidences of Cu(I) or Cu(0) species were not obtained. CuO was more abundant in high copper-content catalysts and in ZSM5 catalysts, due to its lower ionic exchange capacity, while isolated Cu(II) ions are more abundant in low copper-content catalysts and in BETA catalysts. It was concluded that CuO catalyzes the oxidation of NO to NO 2 , and this favors the reduction of NO x at lower temperature (the NH 3-NO 2 reaction is faster than the NH 3-NO reaction because NO 2 is much more oxidizing than NO), whereas isolated Cu(II) ions maintain high NO x conversion at high temperatures.
Journal of Catalysis, 2015
A comparative study was carried out on a small-pore Cu-CHA and a large-pore Cu-BEA zeolite catalyst to understand the lower N 2 O formation on small-pore zeolite supported Cu catalysts in the selective catalytic reduction (SCR) of NOx with NH 3. On both catalysts, the N 2 O yield increases with an increase in the NO 2 /NOx ratios of the feed gas, suggesting N 2 O formation via the decomposition of NH 4 NO 3. Temperature-programmed desorption experiments reveal that NH 4 NO 3 is more stable on Cu-CHA than on Cu-BEA. In situ FTIR spectra following stepwise (NO 2 + O 2) and (15 NO + NH 3 + O 2) adsorption and reaction, and product distribution analysis using isotope-labelled reactants, unambiguously prove that surface nitrate groups are essential for the formation of NH 4 NO 3. Furthermore, Cu-CHA is shown to be considerably less active than Cu-BEA in catalyzing NO oxidation and the subsequent formation of surface nitrate groups. Both factors, i.e., (1) the higher thermal stability of NH 4 NO 3 on Cu-CHA, and (2) the lower activity for this catalyst to catalyze NO oxidation and the subsequent formation of surface nitrates, likely contribute to the higher SCR selectivity with less N 2 O formation on this catalyst as compared to Cu-BEA. The latter is determined as the primary reason since surface nitrates are the source that leads to the formation of NH 4 NO 3 on the catalysts.
Journal of Catalysis, 2008
The catalytic decomposition of N 2 O was studied over Cu-containing zeolites with different Cu loadings and framework topologies (MFI, MOR, FER, BEA, and FAU). The influence of NO, O 2 , and H 2 O on the rate of N 2 O decomposition was investigated in detail. A kinetic model was developed based on the relevant elementary reaction steps in the mechanism of N 2 O decomposition. The recombination of oxygen atoms into molecular oxygen is recognized as the rate-limiting step in N 2 O decomposition. The rate of oxygen desorption depends strongly on the Cu loading. At low Cu loadings, migration of oxygen atoms is required for recombinative desorption. NO accelerates oxygen recombination, because it provides an alternative route for oxygen migration via gas-phase NO 2 . The effect of water differs for Cu-containing zeolites with high and low Cu loadings. At high Cu loading, the rate is suppressed by competitive adsorption of water on the active sites, resulting in an increase in apparent activation energy. The rate of N 2 O decomposition is increased substantially for catalysts with a low Cu loading. This is tentatively attributed to waterinduced mobility of Cu ions, which facilitates oxygen migration. The effect of water addition is fully reversible. .be (R.A. Schoonheydt).
NO oxidation on zeolite supported Cu catalysts: Formation and reactivity of surface nitrates
Catalysis Today, 2016
The comparative activities of a small-pore Cu-CHA and a large-pore Cu-BEA catalyst for the selective catalytic reduction (SCR) of NOx with NH 3 , and for the oxidation of NO to NO 2 and the subsequent formation of surface nitrates were investigated. Although both catalysts are highly active in SCR reactions, they exhibit very low NO oxidation activity. Furthermore, Cu-CHA is even less active than Cu-BEA in catalyzing NO oxidation but is clearly more active for SCR reactions. Temperature-programed desorption (TPD) experiments following the adsorption of (NO 2 + NO + O 2) with different NO 2 :NO ratios reveal that the poor NO oxidation activity of the two catalysts is not due to the formation of stable surface nitrates. On the contrary, NO is found to reduce and decompose the surface nitrates on both catalysts. To monitor the reaction pathways, isotope exchange experiments were conducted by using 15 NO to react with 14 N-nitrate covered catalyst surfaces. The evolution of FTIR spectra during the isotope exchange process demonstrates that 14 N-nitrates are simply displaced with no formation of 15 N-nitrates on the Cu-CHA sample, which is clearly different from that observed on the Cu-BEA sample where formation of 15 N-nitrates is apparent. The results suggest that the formal oxidation state of N
The Journal of …, 1994
A number of copper/ZSM-5 catalysts have been prepared and characterized by electron spectroscopy, ESR, and X-ray absorption spectroscopy. Preparation methods used include ion exchange to give excess levels of copper, solid state exchange, physical mixing of components, and impregnation. The combination of electron spectroscopy and X-ray absorption spectroscopy is shown to be very powerful and has allowed identification of six different copper species, differing in their environment, degree of aggregation, or location. Both the Auger parameter and XANES structure show particular sensitivity. The external surfaces of the as-prepared zeolites are highly enriched in copper oxide species, which however become well dispersed on pretreatment. In the exchanged zeolites X-ray absorption (EXAFS and XANES), ESR, and the observation of high XPS binding energies and unexpectedly low Auger kinetic energies show that copper becomes dispersed in the zeolite channels, in the form of both isolated, 5-fold coordinate ions and small clusters containing extralattice oxygen. The catalysts show varying activity for the reduction of NO, by hydrocarbons in the presence of oxygen, which correlated with the dispersion of the copper within the zeolite. In the active catalysts both electron and X-ray spectroscopic measurements indicate that there is ready conversion between Cu(1) and Cu(II), and possibly between clusters and ions, depending on the atmosphere present. The Cu(I1) state predominates under conditions relevant to lean-burn exhaust gas purification. Reduction in hydrogen results in metallic particles whose size and location depends on the reduction temperature. At 493 K, EXAFS indicates that small metallic clusters are formed within the zeolite channels, which nevertheless show electron spectroscopy relaxation parameters similar to Cu(1) species. On increasing the reduction temperature to 5 13 K, large metal particles are formed at the external surface of the zeolite. These are inactive in the lean NO, reaction but are partially reactivated under lean-bum conditions.
Role of the nature of copper sites in the activity of copper-based catalysts for no conversion
Research on Chemical Intermediates
The Cu 2+/ZSM-5 catalyst exhibited the highest activity, even higher than that of V205/7"i02. Photoluminescence studies of the dehydrated copper-based catalysts have suggested that the copper ions anchored onto ZSM-5 locate as isolated copper species near BrOnsted sites in the zeolite channels while the copper ions anchored onto Vycor and SiO 2 locate mainly as copper dimer forms. These results suggest the role of copper ions which are stabilized with near@ing oxygen vacancies created by dehydroxylation of the zeolite, in NO conversion. As a result, it may be concluded that the isolated copper ions near BrOnsted sites play a significant role in NO
Journal of the Brazilian Chemical Society, 2005
Zeólitas ZSM-5 contendo cobre ou cobalto, ou simultaneamente cobre e cálcio, ou cobalto e cálcio foram preparadas por troca-iônica e testadas como catalisadores na redução de NO a N 2 na presença de metano ou propano. Nos catalisadores trocados com cobre, este se apresentou como Cu 2+ e [Cu-O-Cu] 2+ em sítios de compensação de carga. A quantidade do último cátion aumentou com o teor de cobre e na presença de Ca 2+ e foi mais ativo na redução de NO; no entanto, em temperaturas superiores a 400 ºC a sua atividade diminuiu acentuadamente. Para os catalisadores trocados com cobalto, este foi encontrado como Co 2+ em sítios de compensação ou Co 3+ fazendo parte de precipitados extra-estruturais, sendo que, a quantidade de Co 3+ diminuiu na presença de Ca 2+ . Nestes catalisadores a presença de cálcio não provocou diferenças significativas na atividade específica para a redução de NO ou para a oxidação do metano.