Kinetic evaluation of mechanistic models for O2 release from ZSM-5-supported [Cu2+ –O–Cu2+] ions by thermal reduction or chemical interaction with impinging N2O molecules

For a series of oxidized Cu-ZSM-5 catalysts which were characterized in the catalytic amounts of the oxygen-bridged Cu²⁺-dimers, [Cu²⁺–O–Cu²⁺], activation energies required for the reduction of the Cu²⁺-dimer species by O₂ release were determined using the temperature-programmed experiments of thermal O₂ desorption (TPD) and N₂O decomposition reaction. The activation energy for the thermal reduction of the Cu²⁺-dimers during the TPD decreased linearly with increasing molar number of the Cu²⁺-dimers available on the ZSM-5, suggesting that the energy barrier of the O₂ formation via a Langmuir-Hinshelwood (LH) mechanism increased in proportion to the distance between the two Cu²⁺-dimers in the nearest neighbor. Activation energies of thermal O₂ release were comparable to the literature-reported binding energies of the differently spaced Cu²⁺-dimers. It was also revealed that the activation energy of O₂ release during the temperature programmed N₂O decomposition reaction over an oxidized catalyst was generally low as compared to that in the TPD, and that the degree of reduction of the Cu²⁺-dimers was much greater in the N₂O decomposition reaction than in the TPD at the same temperatures. These beneficial effects N₂O decomposition on the reduction of the Cu²⁺-dimers were discussed in respect of the removal mechanism of the Cu²⁺-dimer bridged oxygen.     

The preparation of Fe<Superscript>3+</Superscript> ion-exchanged mesopore containing ZSM-5 molecular sieves and its high catalytic activity in the hydroxylation of phenol

A series of Fe³⁺ containing catalysts were synthesized using ion-exchange technique over hierarchically porous ZSM-5 (M-ZSM-5) and micro-mesoporous composite ZSM-5/MCM-41 (ZSM-5/MCM-41), respectively. The prepared catalysts were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, N₂ adsorption–desorption, UV–Vis spectroscopy, temperature programmed reduction and inductively coupled plasma-optical emission spectroscopy. The characterization results exhibit that the hierarchically porous ZSM-5 was synthesized with intracrystalline mesopores, while the micro-mesoporous composite ZSM-5/MCM-41 was prepared with the well-ordered mesopores. Furthermore, the results also prove that the existence of iron in the catalysts was mainly presented in the form of Fe³⁺ ions. Catalytic performances of the samples for phenol hydroxylation were compared by using H₂O₂ as oxidant. Under the optimized conditions, Fe³⁺ ion-exchanged M-ZSM-5 (Fe-M-ZSM-5) shows that a phenol conversion of 42.3% obtained with 92.5% selectivity to dihydroxybenzenes, whereas Fe³⁺ ion-exchanged ZSM-5/MCM-41 (Fe-ZSM-5/MCM-41) give 46.2% phenol conversion and 90.1% dihydroxybenzenes selectivity, which are all better than most reported results. The recyclability tests show that Fe-ZSM-5/MCM-41 with ordered mesoporous structure and bigger surface area has better anti-deactivation performance than Fe-M-ZSM-5. The excellent catalytic performances were due to the improved diffusion performance with newly created mesopores and the highly active Fe³⁺ species obtained by ion-exchange technique.     

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.     

Thermodynamic features of the Cu-ZSM-5 catalyzed NO decomposition reaction

Over a Cu-ZSM-5 catalyst with a quantified amount of the active Cu²⁺-dimers (Cu²⁺-O^2--Cu²⁺), the kinetics of the catalytic NO decomposition to N₂ and O₂ was derived on the basis of the proposed reaction mechanism, and such thermodynamic data as adsorption enthalpies of NO and O₂ onto the Cu ion dimer sites were evaluated. It was revealed that the enthalpy of the adsorption of NO (δH=-34.1 kcal/mol) onto a reduced Cu⁺-dimer, as the initiating step of NO decomposition catalysis, was higher than that (δH=-27.8 kcal/mol) onto an oxidized Cu²⁺-dimer, or that (δH=-27.4 kcal/mol) of the dissociative adsorption of O₂ onto the two reduced Cu⁺-dimers in neighbor. The strong inhibition effect of gas phase oxygen on the kinetic rate of NO decomposition at 400–600 ‡C could be explained by the thermodynamic predominance of the oxidized Cu²⁺-dimers against the active reduced Cu⁺-dimers on the catalyst even at high temperature and under the low partial pressure of oxygen. It was also found that the maximum catalytic activity at temperatures around 500 ‡C, which was commonly observed in the Cu-ZSM-5 catalyzed NO decomposition reaction, was attributed to the relatively large enthalpy of NO adsorption onto the reduced Cu⁺-dimers as compared to that of the reaction activation energy (=19.5 kcal/mol), resulting in less favored NO adsorption at the higher temperatures than 500 ‡C.     

Decomposition of nitrous oxide in excess oxygen over Co- and Cu-exchanged MFI zeolites

The decomposition of nitrous oxide on several Co- and Cu-ZSM-5 zeolite catalysts was studied in the absence and presence of excess oxygen. Also, the effect of methane addition, as well as catalyst steaming in dry and wet feeds is reported. N₂O decomposition with no oxygen in the feed was proportional to metal loading on both catalysts. Co-ZSM-5 was much more resistant than Cu-ZSM-5 in excess oxygen. The tolerance of Co-ZSM-5 catalysts to excessive amounts of oxygen is high when Co²⁺ is stabilized in the zeolite framework and depends on the catalyst method of preparation. The presence of methane with no oxygen in the feed enhanced N₂O decomposition while the addition of both methane and oxygen to the feed decreased N₂O conversion on all catalysts tested. Co²⁺ ions stabilized by ZSM-5 framework have high hydrothermal stability in comparison to Cu²⁺ -exchanged ZSM-5.     

On the mechanism of nitric oxide decomposition over Cu-ZSM-5

Cu-ZSM-5, a copper-containing zeolite, catalytically decomposes NO at temperatures below those of other catalysts. A mechanism is proposed which is based on active sites consisting of coordinatively unsaturated cupric (Cu²⁺) ions in a square planar configuration. These sites are posited to chemisorb NO molecules in the gem-dinitrosyl form. The pair of adsorbed NO molecules desorbs as N₂ and O₂. This mechanism accounts for the experimental behavior in chemisorption and decomposition without invoking a cyclical oxyreduction of the surface sites.     

Time-resolved DXAFS study on the reduction processes of Cu cations in ZSM-5

The time-resolved reduction process of copper cations in/on ZSM-5 during temperature-programmed reduction (300–700 K) was studied by energy-dispersive X-ray absorption fine structure (DXAFS) as well as transmission electron microscopy (TEM). Two Cu-ZSM-5 samples with different Cu loadings were prepared by an ion-exchange method. The Cu K-edge DXAFS spectra for isolated Cu₂₊ species in the channels of ZSM-5 and CuO particles on the outer surfaces of ZSM-5 were recorded at an interval of 1 s during the reduction. The curve fitting analysis of the EXAFS data and the XANES analysis revealed that the isolated Cu₂₊ species in the channels were reduced stepwise. They were reduced to isolated Cu₊ species at 400–450 K and the Cu₊ species to Cu₀ metallic clusters at 550–650 K. Small clusters like Cu₄ were initially formed, followed by particle growth. A small part of them went out to the outer surfaces of ZSM-5 during the reduction. In contrast, CuO particles on the outer surfaces were reduced directly to Cu₀ metallic particles around 450 K.     

Effect of sulfur dioxide on nitric oxide adsorption and decomposition on Cu-containing micro- and mesoporous molecular sieves

NO decomposition was studied at different temperatures on copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. Cu-ZSM-5 zeolites presented the highest activity. SO₂ poisoning was also performed and Cu–NbMCM-41 was found to be more resistant. IR results of NO and SO₂ coadsorption either at room temperature or at 573 K show evidence that sulfate formation occurred at 573 K and partially prevented NO adsorption on Cu²⁺ in square planar structure in Cu-ZSM-5. Sulfation of Cu–NbMCM-41 was quite low due to niobium incorporated into the lattice. By contrast, niobium present in the extra-lattice position in CuNb-ZSM-5 and CuNb–AlMCM-41 did not protect the catalyst from sulfation. H₂-TPR results suggested that sulfates were formed on copper sites. IR spectra after treatment under SO₂ + O₂ at 673 K indicated that sulfated species were covalently bonded, their structure varying according to the nature of the support. This revised version was published online in August 2006 with corrections to the Cover Date.     

Turnover frequency for NO decomposition over Cu-ZSM-5 catalysts: insight into the reaction mechanism

In this letter we present a simple model useful to understand the relationship between the turnover frequency for NO decomposition over Cu-ZSM-5 catalysts (N _Cu), the number of Al atoms per unit cell of the ZSM-5 zeolite (p),and the copper loading expressed as percent of exchange (E). Our simple model is able to explain the literature data. We show that: (1) on catalysts with the highest activity (Cu exchange levelsE>90%),N _Cu increases withp (i.e. decreasing the Si/A1 ratio) indicating that the most active sites may contain two close copper ions; (2) at low Cu exchange levels (E<80%) the catalysts have lower activity and, moreover,N _Cu decreases withP, according to previous results of Iwamoto et al. (1986). The present results are also in agreement with the evidence that the redox couple Cu^2–/Cu^– play a key role in the reaction mechanism.Some of the ideas discussed in this letter were presented at the 2nd Italian national meeting on Science and Technology of Zeolites held in Modena, Italy, 6–8 October 1993.     

Preparation effects on the activity of Cu-ZSM-5 catalysts for NO decomposition

Effects of Cu-ZSM-5 catalyst preparation on the activity of over-exchanged copper for NO decomposition are reported. The Cu-ZSM-5 catalysts were prepared by incorporating Cu²⁺ cations into ZSM-5 zeolites from an aqueous cupric acetate solution adjusted to different pH values by adding either acetic anhydride or aqueous ammonia in the solution. The Cu²⁺ exchange levels increased with increasing pH level. STEM/EDX analysis identified CuO particles (5–6 nm) on the zeolite surface for the materials exchanged at pH>6. Conversion and kinetics measurements of NO decomposition to N₂ over these catalysts showed that the over-exchanged copper was not active. Short-time wash with aqueous ammonia removed this copper. The catalyst activity correlated very well with the amount of copper remaining in the ZSM-5 channels.     

Theoretical analysis of oxygen-bridged Cu pairs in Cu-exchanged zeolites

O- and O₂-bridged Cu pairs in zeolitic environments are examined using density functional theory with cluster models. Both types of oxocation ([CuOCu]²⁺ and [CuO₂Cu]²⁺) are found to be highly stable for conditions likely to exist in Cu-ZSM-5. A variety of geometric isomers with different electronic states and preferred Cu–Cu distances are described. Possible implications for “autoreduction”, NO decomposition, and other reactions involving Cu pairs in ZSM-5 are explored. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partial oxidation of butadiene to furan at bismuth molybdate catalysts

The adsorption of NO at 80°C on Cu-overexchanged ZSM-5 (Si/Al=80) has been investigated in a flow apparatus. The formation of N₂O during a transient phase was observed after He pretreatment of the zeolite at 550°C. A negligible amount of N₂O was detected, in contrast, from O₂ pretreated zeolite. The formation of N₂O has been associated to the reoxidation from Cu⁺ to Cu²⁺ of a fraction (about 40%) of copper in the zeolite. The redox chemistry in NO adsorption at low temperature can be related to the activity of Cu-ZSM-5 zeolite in NO decomposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox chemistry in excessively ion-exchanged Cu/Na-ZSM-5

TPR/TPD and FTIR are used to characterize excessively ion-exchanged Cu/Na-ZSM-5. After calcination in O₂ at 773 K at least two copper-oxygen species are present in addition to Cu²⁺ ions; these have been identified as CuO and [Cu-O-Cu]²⁺. Reduction in H₂ transforms all these into Cu⁰ below 773 K. [Cu-O-Cu]²⁺ is autoreduced to Cu+ during outgassing. Reoxidation of Cu⁰ by zeolite protons to Cu⁺ is observed above 723 K in He or Ar; in the presence of CO this process is considerably enhanced and observed at much lower temperature, because CO is strongly adsorbed on Cu⁺. At 293 K CO adsorption causes reversible changes in the FTIR spectra.On leave from: Center for Catalysis, Surface and Material Science, Department of Organic Chemistry, József Attila University, Dóm tér 8, Szeged, H-6720 Hungary.     

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO₂ intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO₂ itself is reduced only back to NO. Thus, for the selective reduction of NO₂ to N₂ (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O₂ reaction to form NO₂ parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al₂O₃ or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO₂ back to NO and O₂ while the other catalysts are much less active. The inhibiting effect of water on the NO + O₂ catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO₂ intermediate.     

Bifunctional mesoporous Cu–Al–MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs

This study explored the possibility of using waste organic solvent as the source of volatile organic compound (VOC) and it served as a reducing agent of selective catalytic reduction (SCR) deNO_x process, in which the VOC itself can be catalytically oxidized on the mesoporous Cu and/or Al substituted MCM-41 catalysts. The synthesized Cu–Al–MCM-41 catalysts were extensively characterized by powder low-angle X-ray diffraction (XRD), N₂ adsorption–desorption measurements, transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), ²⁷Al magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma–mass spectrometer (ICP–MS) analysis. The XRD, TEM and N₂ adsorption–desorption studies clearly demonstrated the presence of a well ordered long range hexagonal array with uniform mesostructures. The Cu–Al–MCM-41 materials showed a better long-term-stability than that of copper ion-exchanged H–ZSM-5 (Cu–ZSM-5) zeolite. The Cu–Al–MCM-41 material was found to be an efficient catalyst than that of Cu–MCM-41 without aluminum for the simultaneous catalytic abatement of NO_x and VOCs, which was attributed to the presence of well dispersed and isolated Cu²⁺ ions on the Cu–Al–MCM-41 catalyst as observed by UV–Vis DRS and EPR spectroscopic studies. And the presence of aluminum (Al³⁺ ions) within the framework of Cu–Al–MCM-41 stabilized the isolated Cu²⁺ ions thus it led to higher and stabilized activity in terms of NO_x reduction.     

Direct NO and N2O decomposition and NO-assisted N2O decomposition over Cu-zeolites: Elucidating the influence of the CuCu distance on oxygen migration

Several zeolites of varying topology and with different Cu/Al ratios were investigated in the catalytic decomposition of NO, N₂O and the NO-assisted N₂O decomposition. The highest activity in the direct NO and N₂O decomposition was found for bis(μ-oxo)dicopper cores in Cu-ZSM-5 followed by the EPR silent Cu in MOR, FER and BEA, while almost no activity was observed over the isolated, EPR detectable Cu sites. This sequence of decreasing activity follows the increasing average distance between Cu sites. An average volume of 35 ų per Cu atom (average CuCu distance 4.1 Å) appears to be a threshold value separating high and low activity. The high activity for catalysts with small CuCu distances is explained by facile oxygen migration over the Cu sites, enabling recombination into gaseous O₂. When the distance between the Cu centers is increased, oxygen migration is hampered. Adding NO results in the scavenging of the deposited O atoms, thereby transporting them into the gas phase. Hence an alternative oxygen migration pathway is created that has the greatest impact on activity of the isolated EPR-detectable Cu sites and a negative effect on the bis(μ-oxo)dicopper cores in Cu-ZSM-5.     

Iron exchanged ZSM-5 and Y zeolites calcined at different temperatures: activity in N2O decomposition

ZSM-5 and Y zeolites were modified with iron by an ion-exchange method and then calcined at 773, 873, 973 and 1,073 K. The obtained materials were characterized with respect to textural parameters (low-temperature N₂ sorption), structure (X-ray diffraction, UV–vis–DRS), redox properties (H₂-temperature programmed desorption, TPD) and surface acidity (NH₃-TPD). The obtained results have shown that the structure of zeolites influenced form, aggregation and content of the introduced iron species. In case of the FAU type structure characterized by wide pores (max. ring size, T-atoms—12) mainly iron in form of mononuclear Fe³⁺ cations and Fe _x ³⁺ O_y oligonuclear clustered species was found. On the other hand for the MFI type structure characterized by smaller pores (max. ring size, T-atoms—10) significant contribution of iron in the form of bulky Fe₂O₃ clusters located possibly on the outer surface of ZSM-5 was detected. Such significant differences in distribution of iron species is probably related to various mobility of iron species in the pore systems of both zeolites. The obtained materials were tested as catalysts in the process of N₂O decomposition. Calcination of zeolites at different temperatures influenced neither the properties nor the activity of the obtained catalysts.     

Hydrotalcite derived Cu-Zn-Cr catalysts admixed with γ-Al2O3 for single step dimethyl ether synthesis from syngas: Influence of hydrothermal treatment

The Cu-Zn-Cr catalysts derived from hydrotalcite (HT) structures were prepared by the hydrothermal and non-hydrothermal methods. The catalysts were admixed with Al₂O₃ to synthesize DME (dimethyl ether) from syngas. XRD analysis revealed the presence of hydrotalcite (HT)-like structures in the oven dried form was decomposed to disperse copper species in the calcined conditions. ESR spectra of the fresh calcined catalysts disclosed both the isolated and clustered copper species and bulk Cr³⁺ species are seen in used catalysts. The TPR analysis indicated that the Cu²⁺ ions are reduced in two stages. DME synthesis experiments showed that the CO conversion and DME yield were linearly correlated with Cu metal surface areas of Cu-Zn-Cr catalysts admixed with γ-Al₂O₃. Activity results indicated that hydrothermal treatments have pronounced influence on the dispersion of copper species and consequently on DME synthesis rates.     

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O₂, C₃H₈ + O₂, and NO + C₃H₈ + O₂ reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO₂ formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO₂ formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO₂ formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO₂ formation. Increase in NO₂ formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C₃H₈ conversion in the C₃H₈ + O₂ and C₃H₈ + O₂ + NO reactions provides evidence that NO₂ activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.On leave from N.D. Zelinskii Institute of Organic Chemistry, Leninskii Pr. 47, Moscow, Russia.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.