Desorption study of NO and O2 on Cu-ZSM-5

A detailed temperature-programmed desorption (TPD) study on NO and O₂ saturated Cu-ZSM-5 at different temperatures (300–723 K) has been performed. In the temperature range 373–723 K, the evolution of O₂ and NO₂ accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O₂ absorbed was very much lower than that of NO. The desorption profile of O₂ after contacting Cu-ZSM-5 with O₂ at 623 K showed a low temperature peak (369K) confirming the spontaneous ability of O₂ desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O₂ and vice versa have been performed at various temperatures (298–623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O₂ are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O₂ and other NO_x species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O₂ for the adsorption sites, therefore the adsorption of NO at 623 K on O₂ saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed     

XANES and EXAFS analysis of copper ion-exchanged ZSM-5 zeolite catalyst used for nitrogen monoxide decomposition

XANES and EXAFS analysis of Cu ion-exchanged ZSM-5 zeolite, a highly active catalyst for NO decomposition, is performed. The copper species in the zeolite are Cu(II) ions in the zeolite cages. The contribution of Cu-Cu local structure is suggested for high loading samples. The Cu atoms in the zeolite are more ionic than CuO. The analysis of the catalyst deactivated by SOx treatment suggests the presence of Cu atoms surrounded by SO₄ ions which blocks the adsorption of NO molecules.     

Conversion of nitric oxide and methane over Pd/ZSM-5 catalysts in the absence of oxygen

We have studied the conversion of nitric oxide and methane on several H- and Na-ZSM-5 zeolite catalysts in the absence of oxygen. Our results suggest that the NO-CH₄ reaction can be explained in terms of a mechanism that starts with a nitric oxide decomposition step followed by the surface reaction of methane with the product oxygen regenerating the active site. We have found that reduced Pd/ZSM-5 catalysts are active for the nitric oxide decomposition reaction but deactivate rapidly due to self-poisoning by product oxygen. By contrast, in the presence of methane these catalysts can exhibit high activity and stability under certain conditions. For instance, when the nitric oxide decomposition and the reaction of methane with the surface oxygen proceed at comparable rates the catalyst is stable but when the methane conversion is lower than that required to remove all the oxygen produced (stoichiometric methane conversion) the catalyst rapidly deactivates. Under some conditions the methane conversion may be higher than the stoichiometric requirement leading to the deposition of carbonaceous species. These carbonaceous deposits can promote the reaction by helping to remove the product oxygen.     

Catalytic reduction of NOx over Cu/ZSM-5 catalyst

Catalytic decomposition of NO on Cu/ZSM-5 catalyst was carried out in tubular fixed-bed reactor at atmospheric pressure. The influence of temperature and flow rate on the reaction rate was investigated. The kinetic model of reaction was proposed and compared with the literature results. The formation of NO₂ during the catalytic decomposition of NO was also monitored. Therefore, additional experiments were performed aimed at examining the kinetics of NO oxidation under the same reaction conditions. Both kinetic models were used to develop the reactor model and to describe quantitatively the behavior of the overall reaction system. Satisfactory degree of correlation between experimental data and values predicted by reactor model has been achieved.     

Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO₂, NH₃, H₂O). NO₂ adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu²⁺NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N₂O₃ → N₂O₄ → NO₂ → NO₃. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO₂, N₂, O₂, and are only slightly affected by H₂O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO_x into highly active surface nitrates.     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

Well defined CuI(NO), CuI(NO)2 and CuII(NO)X (X = O− and/or NO 2 − ) complexes in CuI-ZSMS prepared by interaction of H-ZSM5 with gaseous CuCl

In this note an exchange procedure of the acidic protons of H-ZSM5 by Cu^I ions through reaction with CuCl in the gas phase is described. In the so obtained Cu^I-ZSM5 exchanged zeolite the Cu^I ions are in well defined configuration and form with NO mono and di-nitrosyl complexes of high structural and spectroscopic quality. The Cu^I(NO)₂ species are transformed at RT into Cu^II(NO)X (X=O^– and/or NO ₂ ^– ) species which could represent an intermediate in NO decomposition.     

Activity of ruthenium hydrogenation catalysts in copper(II) formate decomposition

Charcoal-, silica-, alumina- and titanium(IV) oxide-supported ruthenium catalysts, prepared by conventional impregnation and incipient wetness methods from a ruthenium(III) oxide precursor were tested in copper(II) formate decomposition in aqueous solution. Such a reaction was found to be an efficient and simple activity test of charcoal-supported catalysts. The application of this reaction for a bimetallic ruthenium–copper catalyst preparation was also suggested. Experimental results were compared with those obtained using commercial catalysts and ruthenium black.     

Influence of Si/Al ratio on the activity and durability of Pd-ZSM-5 catalysts for nitrogen oxide reduction by methane

The selective reduction of NO by methane on Pd-ZSM-5 catalysts with different Si/Al and Pd loadings was examined in the presence and absence of water vapor. At low Si/Al, NO reduction activity was high and stable, while at higher Si/Al, activity was lower and deactivation was significant. The deactivated samples showed PdO bands in the Raman spectra, which indicated that dispersed Pd cations play a key role in NO reduction. The influence of Si/Al on the stability of dispersed Pd cations suggested the importance of Al site pairs for the stable dispersion of Pd cations.     

Phosphorus poisoning of ZSM-5 and Pt/ZSM-5 zeolite catalysts in diesel exhaust gas conditions

Powdered ZSM-5 and Pt/ZSM-5 catalysts were studied as fresh and after two treatments: hydrothermal ageing and phosphorus poisoning in hydrothermal conditions. The results showed that the deactivating effect of phosphorus was stronger than the effect of the hydrothermal treatment alone. Phosphorus had accumulated in the samples during the ageing. The decrease in specific surface area and increase in Pt particle size indicated the possible reasons for deactivation.     

The photocatalytic reduction of nitrous oxide with propane on lead(II) ion-exchanged ZSM-5 catalysts

UV irradiation of the Pb²⁺/ZSM-5 catalyst prepared by an ion-exchange method in the presence of N₂O leads to the decomposition of N₂O into N₂. This reaction is found to be dramatically enhanced by the addition of propane to produce N₂ and oxygen-containing compounds such as ethanol or acetone. UV light effective for the reaction lies in wavelength regions shorter than 250 nm where the absorption band of the Pb²⁺ ion ([Xe] 4f¹⁴5d¹⁰6s² [Xe] 4f¹⁴5d¹⁰6s¹6p¹) exists, indicating that the excited state of the isolated Pb²⁺ ions plays a significant role in this decomposition of N₂O both in the absence and the presence of propane, and the role of propane is found to be a capture of oxygen atoms formed by the decomposition of N₂O.     

Why Cu in ZSM-5 framework is active in DeNOx reaction—quantum chemical calculations and IR studies

In this paper, we present quantum chemical calculations for copper sites in the extended model of ZSM-5 framework based on seven T sites in two fused 5T rings cut off the MFI structure. Geometrical and electronic properties of cationic copper centres and their adsorption complexes with NO molecule are analysed from the point of view of the activation mechanism. The calculations show that the NO molecule adsorbed on Cu⁺ site becomes significantly activated contrary to Cu²⁺ centre, in full accordance to IR measurements. Our results indicate clearly that the ability of Cu⁺ site to donate electrons to π-antibonding orbital of NO causes significant bond weakening and makes Cu⁺ZSM-5 active in decomposition of nitrogen oxides.     

Effect of metal dispersion on CO hydrogenation over Pd/HZSM-5 catalysts

Pd/HZSM-5 catalysts prepared by ion-exchange method using Pd(NH₃) ₄ ²⁺ were calcined and reduced at different temperatures to provide different metal dispersions. The effect of Pd dispersion on CO adsorption characteristics and acidity were observed through FT-IR study. Methanol and dimethyl ether were the main products in CO hydrogénation over Pd/HZSM-5 catalyst with small Pd particles on which CO was weakly adsorbed, while the selectivity to methane increased with metal sizes.     

The existence of dual Cu site involved in the selective catalytic reduction of NO with propene on Cu/ZSM-5

In order to establish the role of surface species in the selective catalytic reduction (SCR), in situ IR studies were carried out using a DRIFT (diffuse reflectance infrared Fourier transform) cell in gas mixtures of various C₃H₆/NO ratios containing excess oxygen. The location and mobility of Cu ions were investigated by recording the relevant bands of CO adsorbed on Cu/ZSM-5. The nitro species coordinated on Cu²⁺ and the -NCO surface complex as possible intermediates were observed in the reduction of NO with propene on Cu/ZSM-5 between 350 and 400°C. The reactivities of these species toward NO, O₂ and propene were examined. The nitro species can react with propene very rapidly to form N₂ without the formation of NCO species. NCO also reacts with NO₂ and/ or NO at 350°C. IR spectra of CO adsorbed on cuprous ions show that two kinds of Cu ions, which are responsible for the activation of NO and propene respectively, exist on Cu/ZSM-5. From these results, a dual site mechanism involving nitro species and -NCO species as intermediates is suggested.     

In situ high temperature FTIR studies of NOx reduction with propylene over Cu/ZSM-5 catalysts

High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO_x with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm^–1 on Cu during reaction conditions (400°C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu⁺-CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.     

Towards a protocol for solution structure determination of copper(II) proteins: the case of Cu(II)Zn(II) superoxide dismutase

We have developed an optimized protocol to solve the solution structure of copper(II) proteins. After assignment, proton-proton NOEs are used for the shell where 1H spectra are conveniently observed. In a shell closer to the metal ion, 13C NMR spectra with band-selective homonuclear decoupling provide the assignment of all nuclei except for those of the metal ligands. A convenient method for the measurement of 13C longitudinal-relaxation rates (R1) of carbonyls and carboxylate moieties is proposed. 1H NOEs and 1H and 13C R1 data are sufficient to produce a good/reasonable solution structure, as demonstrated for a monomeric species of superoxide dismutase, a 153-residue protein.     

Adsorbate interactions of Cu(II) in mesoporous Cu-GaMCM-41 studied by electron spin resonance and electron spin echo modulation

Gallium-containing mesoporous MCM-41 materials (GaMCM-41) were synthesized by adjusting the gel equilibrium between the source gallosilicate mixture and a GaMCM-41 phase using an organic acid. The location of Cu(II) exchanged into the mesoporous GaMCM-41 and its interactions with various adsorbate molecules were investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The results are compared with those of Cu-AlMCM-41. In a fresh, hydrated Cu-GaMCM-41, Cu(II) is octahedrally coordinated to six water molecules and is located in a cylindrical channel and rotates rapidly at room temperature. Evacuation removes some of these water molecules, leaving the Cu(II) coordinated to fewer water molecules and anchored to oxygens in the channel wall. Dehydration produces one Cu(II) species located on the internal wall of a channel which is accessible to oxygen as evidenced by broadening of its ESR lines by oxygen. Adsorption of water, methanol, ammonia, pyridine, aniline, acetonitrile, benzene and ethylene on dehydrated Cu-GaMCM-41 materials causes changes in the ESR spectra of Cu(II), indicating complex formation with these adsorbates. Cu(II) fully coordinates with a maximum coordination number of polar adsorbates in MCM-41 mesopores. Cu(II) forms a complex with six molecules of methanol as evidenced by an isotropic room temperature ESR signal and ESEM data. Cu(II) forms square planar complexes with four molecules of ammonia, pyridine and aniline based on resolved nitrogen superhyperfine interactions and other ESR parameters. However, Cu(II) forms a complex with six molecules of acetonitrile based on the ESR parameters and ESEM data. Interestingly, Cu(II) does not fully coordinate with nonpolar molecules. Cu(II) interacts directly with only one molecule of benzene, and only part of Cu(II) interacts indirectly with one molecule of ethylene based on ESR and ESEM analyses.     

Competitive adsorption of N2O and CO on CuZSM-5, FeZSM-5, CoZSM-5 and bimetallic forms of ZSM-5 zeolite

The behavior of copper-, iron- and cobalt-exchanged ZSM-5 (Si/Al = 20) and bimetallic forms of the same zeolite was investigated with reference to the adsorption of N₂O and CO at 303 K. The interactions of both gases with investigated zeolites have been studied using microcalorimetry and infrared spectroscopy. The values of differential heats of adsorption and the amounts of adsorbed gases were determined. Both gases react with the charge-balancing cations as active sites for adsorption. CuZSM-5 shows significantly better performances than FeZSM-5 and CoZSM-5, toward both gases; additionally, nitrous oxide can be chemisorbed on CuZSM-5. Adsorption capabilities of FeZSM-5 and CoZSM-5 are increased with the addition of copper ions. The heats of CO interaction with copper-containing samples have been found to be higher then the heats of N₂O interaction, while for FeZSM-5 and CoZSM-5 samples similar heats were detected for both gases. In the case of CuZSM-5, as a result of competitive adsorption between previously adsorbed N₂O and incoming CO from the gas phase, a surface reaction is found.     

Unusual Forms of Molecular Hydrogen Adsorption by Cu+1 Ions in the Copper-Modified ZSM-5 Zeolite

DRIFT and IR transmittance spectra of H₂ adsorbed at 77 K or at room temperature by the copper-modified ZSM-5 zeolite pre-evacuated or pre-reduced in CO at 873 K indicated several unusual forms of adsorbed hydrogen. H–H stretching frequencies of adsorbed species at 3075–3300 cm^–1 are by about 1000 cm<>^–1<> lower than in the free hydrogen molecules. This indicates unusually strong perturbation of adsorbed hydrogen by reduced Cu<>⁺¹<> ions that has been never before reported neither for hydrogen nor for adsorption of other molecules by any cationic form of zeolites or oxides.     

Encapsulation of bis(ethylenediamine) copper(II) complex in zeolite matrices

Bis(ethylenediamine) copper(II) complex was encapsulated in NaY, KL, Naβ and NaZSM-5 zeolite. The redox properties of metal complexes in neat and encapsulated state were studied by cyclic voltammetry. The redox potential of metal complex is altered towards negative value upon encapsulation in various zeolites. This may be due to axial interaction with various zeolite matrix. The redox properties of biological systems are also altered (cytochrome-C, and Vitamin-B₁₂) when they are immobilized on NaAlMCM-41 materials. The copper ethylenediamine complex in constrained environment shows higher activity for the oxidation of dimethyl sulfide compared to that of neat complex.