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Cu-exchanged zeolites are widely studied materials because of their importance in industrial energetic and environmental processes. Cu redox speciation lies at the center of many of these processes but is experimentally difficult to investigate in a quantitative manner with regular laboratory equipment. This work presents a novel technique for this purpose that exploits the selective adsorption of CO over accessible Cu(I) sites to quantify them. In particular, isothermal volumetric adsorption measurements are performed at 50 °C on a series of opportunely pre-reduced Cu-ZSM-5 to assess the relative fraction of Cu(I); the setup is fairly simple and only requires a regular volumetric adsorption apparatus to perform the actual measurement. Repeatability tests are carried out on the measurement and activation protocols to assess the precision of the technique, and the relative standard deviation (RSD) obtained is less than 5%. Based on the results obtained for these materials, the same CO adsorption protocol is studied for the sample using infrared spectroscopy, and a good correlation is found between the results of the volumetric measurements and the absorbance of the peak assigned to the Cu(I)–CO adducts. A linear model is built for this correlation, and the molar attenuation coefficient is obtained, allowing for spectrophotometric quantification. The good sensitivity of the spectrophotometric approach and the precision and simplicity of the volumetric approach form a complementary set of tools to quantitatively study Cu redox speciation in these materials at the laboratory scale, allowing for a wide range of Cu compositions to be accurately investigated.

A simple kinetic model for the standard NH3-SCR reaction by Cu-ZSM-5 catalysts has been developed by assuming three reaction steps: NH3 adsorption (desorption), reactions of adsorbed NH3 with O2 (NH3 oxidation) and NO (NH3-SCR). The model is based on Arrhenius parameters for NH3-SCR and NH3 oxidation by a powder catalyst, combined with models for heat and mass transport and parameters of monolithic catalysts. The model is validated with the experimental results, not included in the estimation of the model, for NH3-SCR by monolithic catalysts with different parameters (catalyst loading and cell density). NOx removal from diesel exhaust, generated by engine bench, was also carried out by using monolithic Cu-ZSM-5 and Cu-AFX catalysts. The results suggest the poisoning effect of hydrocarbons in the exhaust emissions on NOx conversion is more significant for Cu-ZSM-5 than Cu-AFX.

Nitrous oxide (N2O) is an industrial emission that causes the greenhouse effect and damages the ozone layer. Density functional theory study on the N2O direct catalytic decomposition over Cu&ndash, ZSM-5 has been performed in this paper. Two possible reaction mechanisms for N2O direct catalytic decomposition over Cu-ZSM-5 were proposed (O2 formation mechanism and Nitric oxide (NO) formation mechanism). The geometrical parameters, vibration frequency and thermodynamic data of the intermediate states in each step have been examined. The results indicate that N2O can be adsorbed on active site Cu in two ways (O-terminal or N-terminal), and N2O decomposition reactions can occur in both cases. The NO formation mechanism exhibits higher N2O dissociation reaction due to lower energy barrier.

The integration of NOx reduction and catalytic soot oxidation was investigated for the SCRoF (Selective Catalytic Reduction on Filter) applications. By physically mixing a commercial SCR catalyst (either Fe-ZSM-5 and Cu-ZSM-5) with a soot oxidation catalyst (K/CeO2-PrO2), it was possible to lower the soot oxidation temperature by more than 150 degrees and, by optimizing the catalysts mass ratio in the mixture, NOx conversion simultaneously increased, because NO oxidation induced a fast SCR reaction pathway, unlike during standard SCR. Such an improvement in NOx conversion was more pronounced with the Fe-ZSM-5 than with the Cu-ZSM-5 zeolite, as the latter was more sensitive to the NO2/NOx ratio. In order to make the soot oxidation catalyst inactive towards ammonia oxidation, poisoning of the surface acid sites with 3.0 wt.% K2CO3 (corresponding to only 1.0 wt.% K) was performed. In the soot oxidation and SCR catalysts physical mixture, the soot was oxidized mainly by O2 and the contribution of NO2 to oxidation was negligible, as NO2 itself was a key reactant in the (kinetically much faster) SCR reaction.

Powder materials containing copper ions supported on ZSM-5 (Cu-Zeolite Socony Mobil-5) and SSZ-13 (Cu-Standard Oil synthesised zeolite-13), and predominantly CuO nanoparticles on amorphous SiO 2 were synthesised, characterised, wash-coated onto ceramic monoliths and, for the first time, compared as catalysts for direct conversion of methane to methanol (DCMM) at ambient pressure (1 atm) using O 2 , N 2 O and NO as oxidants. Methanol production was monitored and quantified using Fourier transform infrared spectroscopy. Methanol is formed over all monolith samples, though the formation is considerably higher for the copper-exchanged zeolites. Hence, copper ions are the main active sites for DCMM. The minor amount of methanol produced over the Cu/SiO 2 sample, however, suggests that zeolites are not the sole substrate that can host those active copper sites but also silica. Further, we present the first ambient pressure in situ infrared spectroscopic measurements revealing the formation and consumption of surface methoxy species, which are considered to be key intermediates in the DCMM reaction.

International audience; Des catalyseurs Cu-ZSM-5 ont été préparés par échange ionique en phase solide à 500 °C, à partir d'un mélange de CuCl2.2H2O et NH4-ZSM-5, en présence (dynamique) ou en l'absence (statique) de flux d'azote. Les techniques DRX, BET et RMN ont montré que la structure zéolithique a été légèrement détruite pour des teneurs accrues en cuivre. La nature des espèces de cuivre suite à l'échange solide dépend de la teneur en cuivre et de la méthode d'échange. Le cuivre se trouve majoritairement sous forme de Cu2+ pour les catalyseurs ayant un taux d'échange inférieur à 100 %. Pour des teneurs accrues en cuivre, la formation de particules de CuO de grandes tailles a été détectée par DRX pour les catalyseurs préparés sans flux d'azote. Par contre, la présence d'un flux d'azote induit la formation de particules de CuO bien dispersées sur les surfaces et dans les canaux de la zéolithe, non détectables par DRX. Les catalyseurs préparés présentent une bonne activité dans la réduction catalytique sélective de NO par le n-décane en atmosphère oxydante indépendamment de la méthode d'échange.

A systematic analysis of the thermodynamic and spectroscopic features of a vast set of data on carbonyl-like complexes present in the literature, combined with some new experimental data here reported, allowed us to attempt to single out the electrostatic U- and ﷿-contributions in the formation of carbonyl bonding for the so-called “nonclassical carbonyls” hosted inside zeolitic nanocavities. In particular, the room-temperature adsorption of CO on well-defined Cu(I) and Ag(I)-ZSM-5 systems (as testified by X-ray absorption experiments) was studied by means of the joint use of IR spectroscopy and adsorption microcalorimetry. The formation in the zeolite pores of heterogeneous [Cu(CO)2] + and [Ag(CO)] + complexes, the stoichiometry of which is in good agreement with the homogeneous nonclassical carbonyls, was monitored as a function of increasing pCO. In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by O ˜ COads > O ˜ COgas are formed on Cu(I) and Ag(I) sites. Conversely, labile adducts (electrostatic in nature) are formed on Na + and K + sites hosted in the same zeolite pores. The zero-coverage enthalpy of CO adsorbed on Cu(I) and Ag(I) sites (-¢adsH 120 and 100 kJ/mol, respectively) is much larger than the -¢adsH values measured for the two alkaline-metal adducts (35 and 28 kJ/mol for Na + and K + , respectively), despite the closeness of the charge/radius ratios of the two sets of metal cations [Na + and Cu(I); K + and Ag(I)]. The high -¢adsH values and the irreversible nature of (a fraction) of the d-block metal carbonyls suggest the onset of a ﷿-back-donation reinforcing the carbonyl bond with respect to a plain U-coordination. A clear deviation from an empirical rule, which linearly correlate ¢O ˜ CO and -¢Hads quantities for a large set of non-d/d 0 /d 10 metal carbonyls, was observed in the case of copper-and silver-carbonyls, confirming the interplay of U- and ﷿-back-donation contributions for such species, otherwise defined nonclassical carbonyls. The ¢O ˜ CO versus -¢adsH empirical rule was found of general validity, in that it allows to infer the enthalpy values from the blue-shift of the C-O stretching frequency (and vice-versa) in the case of non-d/d 0 metal carbonyls, whereas it allows to roughly estimate by the deviation from the line the extent of the ﷿-back-donation in the case of d-block metal carbonyls. Further, the spectroscopic and thermodynamic features of carbonyl species formed on (partially) reduced copper sites have shown that in the absence of strong electrostatic plus U-coordinative components the carbonyl bond is surprisingly weak, despite the presence of ﷿-back-donation.

Structure sensitivity of the hyperfine coupling constants was investigated by means of DFT calculations for selected surface paramagnetic species. $A\cdot CH_{2}OH$ radical trapped on silica and intrazeolite copper nitrosyl adducts encaged in ZSM-5 were taken as the examples. The surface of amorphous silica was modeled with a $[Si_{5}O_{8}H_{10}]$ cluster, whereas the zeolite hosting sites were epitomized by $[Si_{4}S=AlO_{5}(OH)_{10}]^{-}$ cluster. Three different coordination modes of the $\cdot CH_{2}OH$ radical were considered and the isotropic $^{13}\textrm{C}$ and $^{1}\textrm{H}$ hyperfine constants of the resultant van der Waals complexes, calculated with B3LYP/6-311G(d), were discussed in terms of the angular deformations caused by hydrogen bonds with the cluster. The magnetic parameters of the $\eta^{1}-N(CuNO)^{11}$ and $\eta^{1}-O(CuNO)^{11}$ linkage isomers were calculated at the BPW91/LanL2DZ and 6-311G(df) level. For the most stable $\eta^{1}-N$ adduct a clear dependence of the spin density distribution within the Cu-NO moiety on changes in the Cu-N-O angle and the Cu-N bond distance was observed and accounted for by varying spin polarization and delocalization.

The coordination of Cu+ at the T1 and T7 positions of the M7 ring of Cu-ZSM-5, and the interaction of NO with coordinated Cu+ were investigated by means of DFT/ONIOM calculations. The B3LYP, BLYP, PBE1PBE, PBE, M06, and M062X functionals with the def2-TZVP (def2-QZVP for Cu) basis set were used in the high-level part of ONIOM calculations, with the HF/3-21G, B3LYP/LANL2DZ, M06/LANL2DZ, and M062X/LANL2DZ methods in the low-level part. The ability of suitable combinations of the above methods to reproduce (i) the crystallographic structure of purely siliceous ZSM-5, (ii) the tendency of Cu+ to be twofold or fourfold coordinated by framework oxygen atoms of Cu-ZSM-5, and (iii) the interaction energy and the NO stretching frequency of adsorbed nitrogen oxide are discussed, showing that different results are obtained depending on the adopted computational approach. With reference to above properties, some considerations about the employment of the ONIOM approximations are also included. © 2015 Wiley Periodicals, Inc.

© 2014. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/, A key target to reduce current hydrocarbon emissions from vehicular exhaust is to improve their abatement under cold-start conditions. Herein, we demonstrate the potential of factorial analysis to design a highly efficient catalytic trap. The impact of the synthesis conditions on the preparation of copper-loaded ZSM-5 is clearly revealed by XRD, N2 sorption, FTIR, NH3-TPD, SEM and TEM. A high concentration of copper nitrate precursor in the synthesis improves the removal of hydrocarbons, providing both strong adsorption sites for hydrocarbon retention at low temperature and copper oxide nanoparticles for full hydrocarbon catalytic combustion at high temperature. The use of copper acetate precursor leads to a more homogeneous dispersion of copper oxide nanoparticles also providing enough catalytic sites for the total oxidation of hydrocarbons released from the adsorption sites, although lower copper loadings are achieved. Thus, synthesis conditions leading to high copper loadings jointly with highly dispersed copper oxide nanoparticles would result in an exceptional catalytic trap able to reach superior hydrocarbon abatement under highly demanding operational conditions., The authors would like to thank MINECO and FEDER (CTQ2012/31762 and CTQ2012-37984-C02-01) for financial support. B.P. thanks the Spanish Ministry of Education (FPU studenship AP2009-3544). M.N.G. thanks the University of Alicante for the PhD studentship.

CuI-exchanged ZSM-5 contacted with N2 at room temperature or at 77 K exhibits a narrow IR band at 2295 cm–1. On the basis of 14N2/14N2 and 14N2/14N15N/15N2 isotopic substitution experiments this peak is assigned to the N—N stretching vibration of CuI—N2 end-on dinitrogen complexes formed on isolated CuI sites inside the zeolite channels. The problem of the location of the CuI ions and the role of surface heterogeneity in determining the FWHM (ca. 9 cm–1 at N2 coverage θ= 1) of the 2295 cm–1 band are discussed. Some hypotheses concerning the bonding structure are made by comparison with the spectra of the purely electrostatic M+–N2 and M+–CO adducts (where M+ is an alkali-metal ion) and of the CuI—CO complex formed on the same support.

DFT calculations on the coordination of Cu+ to the framework oxygen atoms of Al-substituted ZSM-5 were performed by using combinations of different basis sets in order to investigate the dependence of the results on the adopted computational level. With low-end basis sets, a large basis set superposition error (BSSE) favors the coordination of Cu+ to three to four oxygen atoms of the framework, only two of which belong to the AlO4 tetrahedron corresponding to the investigated T-site. More extended basis sets considerably lower the BSSE and favor the coordination of Cu+ to only two oxygen atoms of the AlO4 tetrahedron. Upon interaction with NO, the Cu+ ion is always coordinated by two oxygen atoms of the AlO4 tetrahedron, independently of the basis set adopted and of the coordination number before NO adsorption. The shift from three- to twofold coordination caused by the Cu+–NO interaction requires a deformation energy that lowers the final adsorption energy. Such an effect is relevant with low-end basis sets, whereas it is substantially absent with more extended basis sets, which favor the twofold coordination of Cu+ even before NO adsorption. As a result, high-end basis sets increase the NO interaction energy with respect to that calculated by low-end basis sets, in agreement with experiments and suggesting a possible re-interpretation of the catalytic properties of the investigated sites. Provided suitable scale factors are employed, the N–O stretching frequencies of adsorbed nitrogen oxide calculated by sufficiently extended basis sets turned out in fair agreement with experimental findings.

The adsorption of NO has been investigated in the temperature range 50–150 °C on Cu-ZSM-5 and La-Cu-ZSM-5 also in the presence of water and O 2 in a continuous flow reactor. TPD experiments have been performed after each adsorption test monitoring all desorbed NO x species. Water has been found to displace NO from copper sites strongly reducing its adsorption except for nitrate-like species which are partially preserved also in the presence of water. O 2 addition, although decreasing NO adsorption, limits the negative effect of water. Lanthanum co-exchange results in improving the NO adsorption capacity of the zeolite both in the absence and in the presence of water.

In this study data are presented which support the view that a special configuration of dimeric Cu(I) species in Cu-ZSM-5 catalysts may be the active site for NO decomposition. Such dimeric Cu(I) species, produced by self-reduction of Cu(II) during the thermal activation of Cu-ZSM-5 catalysts in He or under vacuum, are the sites where N 2 is generated from two adsorbed NO molecules, and from which O 2 desorbs from the catalyst at the steady state. It was found that these dimeric Cu(I) species adsorb N 2 irreversibly at low temperature (273–325 K). DFT calculations for cluster models obtained from the crystallographic structure of orthorhombic ZSM-5 suggested that Cu(I)–N 2 –Cu(I) species may be IR-silent or have a very low extinction coefficient, as suggested by previous diffuse reflectance FT results. The isosteric heat of adsorption of N 2 on Cu-ZSM-5, obtained with the Clausius–Clapeyron equation from adsorption isotherms in the temperature range of 273–325 K, is 10.3 kcal mol −1 at a coverage of 0.4 mol N 2 /mol Cu. This value is much larger than the heat of condensation of N 2 at 77 K (1.33 kcal mol −1 ) and significantly higher than the heat of adsorption of N 2 on Li-ZSM-5 (8.68 kcal mol −1 at zero coverage) obtained with the Clausius–Clapeyron equation from adsorption isotherms in the temperature range of 196–273 K. The turnover frequency (TOF) for the NO decomposition, calculated assuming that the active sites are those titrated by the irreversible adsorbed N 2 at 273 K, is independent of the copper loading and of the ZSM-5 Si/Al atomic ratio. The present model is also supported by the evidence that Cu-Y, Cu-S-1, and Cu-[Al]-MCM-41 catalysts, containing Cu(I) species that do not adsorb irreversibly N 2 at 273–325 K, were found to be inactive or scarcely active (Cu-Y) for the NO decomposition reaction under the same experimental conditions as employed for Cu-ZSM-5.

Starting from (NH4)+–ZSM-5, we have prepared Ag(I)–ZSM-5 and Cu(I)–ZSM-5. In both cases a nearly total cation exchange has been achieved. The local environment of Ag(I) and Cu(I) cations, as determined by an EXAFS study, is very similar in both cases: Cu(I) is surrounded by 2.5±0.3 oxygen atoms at 2.00±0.02 A and Ag(I) is surrounded by 2.5±0.5 oxygen atoms at 2.30±0.03 A. Notwithstanding the same electronic configuration of the two cations (d10), nitrosylic complexes formed in the two systems exhibit quite consistent differences. At liquid-nitrogen temperature, in Cu(I)–ZSM-5, IR spectroscopy reveals the stepwise formation of [Cu(NO)]+ and [Cu(NO)2]+ complexes, whereas only mononitrosylic [Ag(NO)]+ has been observed in Ag(I)–ZSM-5; moreover, the stability of cuprous nitrosylic adducts upon decreasing the NO equilibrium pressure and/or increasing the sample temperature has been observed to be much greater than that of corresponding silver nitrosylic adducts. Our results indicate that, in contrast to Cu(I), the interaction between NO and Ag(I) cations hosted in zeolitic systems is mainly of an electrostatic nature.

N 2 O decomposition on over-exchanged Cu-ZSM-5 zeolite was investigated in the range of temperatures from 323 to 400°C. When only N 2 O in helium was fed, regular periodic oscillations of the outlet concentration of N 2 O were observed. Catalyst pretreatment affected the pattern of the oscillations, but not their periods or amplitudes. Also the average conversion of N 2 O to N 2 and O 2 was dependent on the initial oxidation state of the catalytic active sites, being lower when the catalyst was pretreated in oxygen. The addition of NO, O 2 or both in the feed resulted in disappearance of the oscillations and decrease of N 2 O conversion. The experimental observations are in agreement with a scheme of reactions in which N 2 O acts either as oxidizing or as reducing agent for the catalytic sites.

The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu2+ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called “self-reduction” of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu+. The onset of this latter phenomenon starts at 470 K and, in the range 470−670 K, involves the majority of copper ions present in the system. Rehydration of Cu+-containing samples does not cause direct...