Characterization of active sites on copper ion-exchanged ZSM-5-type zeolite for NO decomposition reaction

Abstract

The necessary condition of the active sites for the NO decomposition reaction on copper ion-exchanged ZSM-5-type zeolite (CuZSM5) has been investigated by using an adsorbed species that is one of the N 2 products of the decomposition reaction of NO. Two dominant types of exchangeable sites in the CuZSM5 sample were identiÐed by means of IR spectra using CO as a probe molecule ; these sites are responsible for giving a 2159 and 2151 cm~1 band due to the chemisorbed CO species. From exploration of the decomposition reaction of NO on the samples having di †erent amounts of preadsorbed CO molecules, it was found that the NO decomposition reaction occurs only under the condition that both types of sites coexist. The quantitative relationship between the number of these sites and the ion-exchange capacity of the sample was also evaluated from the IR spectra for CO adsorption. Combination with a similar relationship between the NO decomposition activity and the copper ion-exchange capacity found in the reference convinces us that the presence of both types of sites located closely to each other is a necessary condition for the NO decomposition reaction. The structure of the copper ion in CuZSM5 under di †erent exchange levels was also studied by Cu-K-edge X-ray absorption spectroscopy, from which, evidence supporting an existence of dimer species of copper ions was obtained for samples that have excessive ion-exchanged copper ion exceeding the stoichiometric amount. In addition, the oxidationÈreduction process of copper ion species was also examined during NO adsorption and subsequent heat treatment in vacuo. It is concluded that zeolite having an appropriate Si/Al ratio, in which it is possible for the copper ion to exist as dimer species, may provide the key to the redox cycle of copper ion as well as catalysis in NO decomposition.

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

“…These findings together with the corresponding EXAFS data [49], showing a Cu· · ·Cu contribution at 2.84 Å with coordination number of 0.5, indicates the presence of a mixture of Cu + monomers and Cu + Cu + dimers after He pretreatment. This conclusion is shared by previous XAFS studies on pretreated Cu-ZSM-5 samples [62,63]. The presence of a major amount of Cu 2 O nanoparticles is not plausible since (1) their characteristic bandgap transition (at energy 16,200 cm −1 ) cannot be discerned in our UV-vis spectra and (2) an EXAFS Cu· · ·Cu separation of 2.84 Å is observed rather than the characteristic Cu· · ·Cu separation of 3.01 Å of Cu 2 O.…”

Adsorption of Nitrogen Monoxide and Carbon Monoxide on Copper-Exchanged ZSM-5: A Cluster and Embedded Cluster Study

“…At least two types of Cu(I) species had been identified from previous IR experiments. 16,17,24 The first Cu(I) species bonds symmetrically to two framework oxygen atoms, and the other bonds asymmetrically to three framework oxygen atoms. Those results agree with XAFS, IR, and UV-vis spectroscopy studies, which showed that the average coordination number of Cu(I) is 2.5 ( 0.3.…”

“…The first two bands occur due to spontaneous decomposition of NO, and the last band was attributed to NO adsorbing on the Cu + site. 8,9,16,41,42 The experimental binding energies of CO and NO largely depend on many factors, e.g., the Si/Al ratio, the Cu exchanged rate, temperature, and pressure. The relative binding energy between CO and NO is still questionable.…”

“…10,[13][14][15] CO is known to be adsorbed on Cu-ZSM-5, even under mild conditions such as at low pressure and room temperature. [16][17][18][19][20][21][22][23] The frequency shift of CO in the adsorption complex has been used to depict the characteristic of the active site and its bonding nature. At least two types of Cu(I) species had been identified from previous IR experiments.…”

Theoretical Studies of the Coordination and Stability of Divalent Cations in ZSM-5

“…There is also a growing body of evidence suggesting the presence of dimer cations in which two M 2+ cations are bridged by an O atom or two M 3+ cations are bridged by two O atoms. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Such structures can be represented by Z -[M(O)M] 2+ Z -and Z -[M-(O) 2 M] 2+ Z -. The experimental evidence also indicates that M 2+ cations can be reduced to form small metal particles or can react with oxygen and water to form metal oxide particles.…”