Identification of copper(II) and copper(I) and their interconversion in Cu/ZSM-5 De-NOx catalysts

Lei, G.D.; Adelman, Brad; Sárkány, János; Sachtler, Wolfgang M.H.

doi:10.1016/0926-3373(94)00043-3

Cited by 233 publications

(178 citation statements)

References 37 publications

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“…10,11 A mechanism involving NO coordination to Cu 2+ without the involvement of reduction-oxidation cycles has also been suggested 12 but it has not been supported by experimental evidence. N-N formation has been proposed to occur via adsorption of two NO molecules on a single Cu + site to form dinitrosyl complexes {Cu + (NO) 2 4,[18][19][20] This step resolves the charge balance concerns and it is consistent with the observed increase in NO decomposition turnover rates as the Cu/Al ratio increases and oxygen-bridged dimers become more prevalent. 21 Oxidized Cu dimers {Cu 2+ -O 2À -Cu 2+ } 2+ and their reduced forms {Cu + -K-Cu + } 2+ are not the only Cu species that can co-exist in Cu-ZSM5, as shown by temperature-programmed reduction studies using H 2 or CO as reductants.…”

Section: Introductionsupporting

confidence: 70%

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Costa

Modén

Meitzner

et al. 2002

Phys. Chem. Chem. Phys.

148

145

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The number and type of Cu 2+ species present on O 2 -treated Cu-ZSM5 catalysts (Si/Al ¼ 13.1-14.6) with varying Cu/Al ratios (0.12-0.60) were measured using temperature-programmed reduction in H 2 or CO and desorption of O 2 with He as the carrier. The effluent stream was monitored using mass spectrometry and the structure and oxidation state was determined in parallel by X-ray absorption spectroscopy. Isolated Cu 2+ monomers and oxygen-bridged Cu 2+ dimers interacting with Al-Al next nearest neighbor pairs were the predominant Cu species on these catalysts. The fraction of Cu present as dimers increased from 0.46 to 0.78 as Cu/Al ratios increased from 0.12 to 0.60, as expected from the decreasing average Cu-Cu distance with increasing Cu content. In contrast, monomers reached a plateau of $0.15 Cu 2+ /Al, suggesting that only some Al-Al pairs can interact with small Cu 2+ monomer structures, while a much larger fraction can bind with larger oxygen-bridged Cu 2+ dimers. The measured distribution of Cu dimers and monomers is consistent with the number and bond distances of Al-Al pairs for the Si/Al ratio in these ZSM5 samples. The distributions of Cu species obtained from the amount of CO 2 formed (from CO), the amount of H 2 O formed (from H 2 ), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O 2 was significantly smaller than that removed with H 2 or CO, suggesting that only proximate Cu dimers autoreduce via recombinative desorption steps. NO decomposition turnover rates (normalized per Cu dimer) were nearly independent of Cu content, except at the lowest Cu/Al ratio, consistent with the involvement of Cu dimers as the active Cu species in NO decomposition redox cycles on Cu-ZSM5. Multiple O 2 and CO 2 peaks during desorption and reduction in CO suggest the presence of Cu dimers with varying oxygen binding energy and reactivity. The Cu dimers initially formed at low Cu/Al contents during exchange are less reducible, consistent with their lower NO decomposition turnover rates.

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Section: Introductionsupporting

confidence: 70%

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Costa

Modén

Meitzner

et al. 2002

Phys. Chem. Chem. Phys.

148

145

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“…The steps involved are shown schematically in eqs 3-5, as previously suggested. 7 NO desorbs at 490 K without the concurrent evolution of any other species, indicating that NO does not react to form N-N bonds or NO 2 molecules via reactions with reduced or oxidized Cu dimers at these temperatures. The species responsible for this NO desorption peak appear to be spectators, distinct from those adsorbed on reduced Cu dimers.…”

Section: Resultsmentioning

confidence: 99%

“…Cu dimers have been proposed as the active species in NO decomposition. [5][6][7] In contrast with isolated Cu 2+ cations, these dimers contain a removable oxygen atom; as a result, they can undergo the redox cycles implicated in NO decomposition turnovers.…”

Section: Introductionmentioning

confidence: 99%

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

et al. 2002

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The identity of reactive intermediates and of active sites and the details of redox cycles and oxygen removal pathways during NO decomposition on well-characterized Cu-ZSM5 were examined by combining previous spectroscopic and steady-state kinetic studies with measurements of the rate of evolution of NO, N 2 O, NO 2 , N 2 , and O 2 during isothermal and nonisothermal kinetic transients. The oxygen coverages measured during reversible isothermal switches from He to NO/He mixtures showed that NO decomposition involves bimolecular reactions of two NO molecules adsorbed on vicinal Cu + species with the formation of N 2 O as the initial product near ambient temperature. These vicinal Cu + species form via oxygen removal from {Cu2+ using NO 2 as an oxygen carrier among distant oxidized dimers. Adsorbed nitrate (NO 3 *) is the kinetically relevant intermediate in the formation of O 2 during NO decomposition. This NO 3 * decomposition reaction is one of the steps involved in the equilibrated formation of NO 2 observed during NO decomposition. These NO-mediated oxygen removal pathways, in which NO acts both as a reductant and as an oxidant, are significantly more rapid than recombinative desorption steps. The desorption of products and of unreacted NO during reactions of preadsorbed NO with increasing catalyst temperature confirmed the bimolecular nature and the low activation energy for N 2 O formation from NO. The facile nature of this reaction and the unfavorable NO adsorption thermodynamics as temperature increases combine to give the observed decrease in NO decomposition rates at high temperatures. These findings are consistent with some reported mechanism-based steady-state rate expressions and with previous infrared detection of the reaction intermediates proposed here based on isothermal and nonisothermal transients. These pathways appear to be relevant also to N 2 O decomposition reactions, which occur after the initial formation of N 2 O from NO and involve reactions of N 2 O with {Cu2+ and removal of oxygen via NO-mediated desorption pathways. This study brings consensus and some clarification into the mechanistic details for NO and N 2 O decomposition reactions and resolves some remaining discrepancies and some contradictory conclusions in previous reports.

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“…Some studies [27][28][29][30] have suggested that the ELO is associated with mononuclear copper and is of the structure Cu 2+ -O − or Cu 2+ -O 2− . Alternatively, others [21,22,33,[39][40][41] proposed that a single-O-bridged copper pair, i.e., [CuOCu] 2+ , is essential in the NO decomposition cycle. Recent computational contributions of Goodman et al [38,42] stressed the eventual role of double-O-bridged copper pairs, i.e., [CuO 2 Cu] 2+ .…”

Section: Introductionmentioning

confidence: 99%

An operando optical fiber UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5

Groothaert

Lievens

Leeman

et al. 2003

Journal of Catalysis

137

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The role of the bis(µ-oxo)dicopper core, i.e., [Cu 2 (µ-O) 2 ] 2+ , in the decomposition of NO and N 2 O by the Cu-ZSM-5 zeolite has been studied with combined operando UV-vis monitoring of the catalyst and on-line GC analysis. An optical fiber was mounted on the outer surface of the quartz wall of the plug-flow reactor and collected the UV-vis diffuse reflectance spectra under true catalytic conditions.

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Identification of copper(II) and copper(I) and their interconversion in Cu/ZSM-5 De-NOx catalysts

Cited by 233 publications

References 37 publications

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

An operando optical fiber UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5

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