Isolated Copper Ions Research Articles

Evidence for clustering in Cu2+-doped CeO2

Abstract Electron spin resonance spectra of Cu 2+ in diluted solid solutions of Cu in CeO 2 have been studied at room temperature for Cu concentrations between 0.20 and 0.80 mol%. Isolated Cu 2+ ions were observed, as well as Cu 2+ ions coupled by the exchange interaction. The linewidth and intensity of the line associated with isolated copper ions have a maximum between 0.60 and 0.80 mol%. The results suggest that the solid solution of Cu in CeO 2 exhibits clustering effects, as was also observed in Mn:CeO 2 solid solutions. A theoretical calculation based on this model yields an effective range of the exchange interaction between Cu 2+ ions of 0.38 nm, smaller than that of Mn 2+ ions in the same host lattice.

Comparing CuAPO-5 with Cu:ZSM-5 in the Selective Catalytic Reduction of NOx: An in situ Study

The CuAPO-5 and Cu:ZSM-5 systems are compared in the selective catalytic reduction (SCR) of NOx. Within the Cu:ZSM-5 system materials with high and low metal contents were prepared by hydrothermal and conventional ion exchange, respectively. X-ray absorption spectroscopy (XAS) was used to follow changes in the valence state and the local environment of copper as the different materials reacted with the individual components of this particular deNOx process. This was effected by measuring the XAS spectra after each consecutive stage of the treatment with propene and NOx. In the CuAPO-5 system the reduction of copper(II) by propene to copper(I) is only partial and metallic copper or copper oxides are formed during the NOx and propene treatments. This can explain the relatively low activity (16-18%) of both the low and high copper content CuAPO-5 samples toward NOx reduction. By contrast, in the Cu:ZSM-5 system, copper(II) is reduced to copper(I) by propene and subsequently reoxidised by NOx. Copper is only partially reoxidised in Cu: ZSM-5 prepared by hydrothermal ion exchange (45% deNO(x) conversion), but the amount of copper exchanged by this method (133%) is significantly higher than in the conventional ion exchanged preparations (35%). The final propene treatment establishes that the Cu(I)/Cu(II) redox pair is formed in both Cu:ZSM-5 samples when reducing NOx. Previous studies suggest that dimeric copper species, in addition to isolated copper ions, are responsible for the activity of high copper content Cu:ZSM-5. We find no evidence of copper clustering in this system with the protocol used here.

Investigation of the structure of impurity defects in alkali halide crystals by the method of fluorescence EXAFS

It has been shown by experiment that the structure of impurity centers in NaCl–Sr crystals depends on impurity concentration and storage time of crystals. At low concentration and in quenched crystals the impurity is in site of the lattice. At concentrations higher than 0.15 wt% Sr, the impurity forms SrCl 2 phase. For the crystals containing Cu + impurity, it has been shown that an increase in the impurity concentration leads to the formation of clusters with CuCl structure. At the same time the existence of isolated copper ions shifted from the sites of the lattice is obvious. This leads to asymmetry of the environment of copper, which is manifested both in the absorption spectra in the ultraviolet region and in the EXAFS spectra. Heating of the crystals leads to destruction of complex clusters and formation exclusively of centers containing isolated copper ions.

Dependence of Copper Species on the Nature of the Support for Dispersed CuO Catalysts

Copper catalysts prepared by chemisorption-hydrolysis technique over silica (Cu/Si) and silica-alumina (Cu/SiAl) supports were studied to understand the role of the support on the nature and surface properties of the copper species stabilized on their surfaces. The morphological and surface properties of the copper phases have been characterized by complementary techniques, such as HRTEM, EXAFS-XANES, EPR, XPS, and FTIR. For the FTIR investigation, molecular probes (CO and NO) were also adsorbed on the surfaces to test the reactivity of the copper species. Moreover, the catalytic performances of the two catalysts have been compared in the HC-SCR reaction (NO reduction by C(2)H(4)) performed in highly oxidant conditions. The superior activity and selectivity of the supported silica-alumina catalyst with respect to that supported on silica could be related with the different nature of the copper species stabilized on the two supports, as emerged from the results obtained from the spectroscopic investigations. Small and well-dispersed CuO particles were present on silica, whereas isolated copper ions predominated on silica-alumina, likely in regions rich in alumina that made some exchangeable sites available, as indicated by FTIR spectra of adsorbed CO. The less positive global charge of copper species on Cu/SiAl than in Cu/Si has been confirmed by EPR, XPS, and EXAFS-XANES analyses.

Thermal analysis and temperature-programmed reduction studies of copper–zirconium and copper–zirconium–yttrium compounds

Differential thermal analysis evidenced that introduction of copper or yttrium into zirconia matrix delays its crystallization temperature. This result was connected to the stabilization of the zirconia tetragonal phase observed by X-ray diffraction. In addition, it was observed a decomposition delay of supported copper nitrate with yttrium content in a zirconia support. Temperature-programmed reduction revealed the existence of different copper species: isolated copper ions, copper clusters, small particles of copper oxide and bulk copper oxide. The reduction temperature of these species was strongly influenced by the preparation method of Cu–Zr systems and by the presence of yttrium in ZrO 2. But in all the samples, isolated copper ions and clusters were postulated to be reduced at the same temperature since these species have the same electron paramagnetic resonance parameters and then occupy the same sites. A correlation was done between the reducibility of these copper species and their activity in propene and toluene oxidation reactions. In the absence of yttrium, the best activity measured was correlated to an easier reduction but also to a higher quantity of isolated copper ions and clusters. Addition of yttrium to the support modified the copper interaction. At low copper/zirconium atomic ratio (0.01), the reduction temperature of small CuO particles increased with the yttrium content in the support. At high copper content (Cu/Zr = 0.1), the easier copper species reduction was obtained for Y 2O 3 content of 5 wt.%, the corresponding solid providing the best activity in the propene oxidation.

Comparative study of Cu(II) catalytic sites immobilized onto different polymeric supports

AbstractThe catalysts with copper(II) ions stabilized onto different polymeric matrixes are prepared on either bulk (Cu/chitosan, Cu/polyethyleneimine‐polyacrylic acid (PPA), and Cu‐diiminate‐impregnated polystyrene, polyarylate, or polymethylmethacrylate) or composite supports (egg‐shell type Cu/chitosan/SiO2 and Cu/PPA/SiO2). The morphology of the samples and peculiarities of Cu(II) cationic sites are studied by SEM and ESR methods, and the catalyst activities are compared in oxidation of o‐ and p‐dihydroxybenzenes by air in water. The catalytic activity of Cu(II) centers is governed by the coordination of isolated copper ions: for the most active catalysts, i.e., Cu/chitosan and Cu/PPA, the symmetry of isolated Cu2+‐sites approximates a coordinatively unsaturated square‐planar structure. At the same time, accessibility of active sites to water differs for different polymers, so the contribution of hydrophilicity to the reaction pattern cannot be excluded. Redox transformations of the active sites in the course of catalytic tests do not cause copper leaching from the polymer matrix. The binary composite systems with a film of low‐loaded hydrofilic Cu‐polymer supported on macroporous SiO2 demonstrate substantially higher activity in oxidation of hydroquinone and 3,4‐dihydroxyphenylalanine, as compared with the bulk Cu/polymer samples. In turn, the specific activity of Cu/chitosan/SiO2 exceeds significantly that of Cu/PPA/SiO2 due to stabilization of a thinner and more uniform film of chitosan at the surface of silica.

Influence of the preparation method on the activity and stability of copper–zirconium catalysts for propene deep oxidation reaction

Copper was introduced to zirconium-based catalysts using three methods before calcination at 600 °C: (i) coprecipitation of copper nitrate and zirconium oxychloride, (ii) wet impregnation of copper nitrate on zirconium oxyhydroxide (ZrO(OH)2), (iii) or on zirconium oxyhydroxide previously calcined at 600 °C (ZrO2). Two atomic ratios (Cu/Zr=0.01 and 0.1) were prepared. The crystallization phase of the catalysts was evidenced by X-ray diffraction (XRD). Copper stabilizes the metastable tetragonal phase of zirconia for the samples prepared by coprecipitation and by impregnation over oxyhydroxide. Electron paramagnetic resonance (EPR) study of Fe3+ isolated ions allowed also to evidence the stabilization of this phase. Moreover, this technique evidenced the presence of two types of paramagnetic copper species that occupy the same sites: the isolated copper ions in octahedral environment, which is lacunal in the case of the coprecipitates and the clusters. The activity of these catalysts for the complete oxidation of propene was investigated. The impregnated samples have shown better catalytic activities than the coprecipitated ones. EPR study and surface area measurements by the BET method, after the catalytic test, were done in order to detect an eventual deactivation of the catalysts. A decrease in the surface area due to water formation during oxidation reaction, accompanied by a structural change on the surface of the catalyst from tetragonal to monoclinic phase, was detected for the coprecipitates and that impregnated over oxyhydroxide samples. The catalysts copper impregnated over ZrO(OH)2 previously calcined at 600 °C have combined the best activity and the best resistance towards deactivation.

The role of copper species and Brønsted acidity in CuCl/ZSM-5 catalysts during the selective catalytic reduction of NO by propene

The role of copper species and Brønsted acidity in the selective catalytic reduction (SCR) of NO by propene over CuCl/ZSM-5 catalysts prepared by dispersion method has been investigated. The results of IR and H 2-TPR show that the main active species of CuCl/H-ZSM-5 catalyst are isolated copper ions (Cu 2+ and Cu +) located in the ion exchange sites of ZSM-5, which formed during the high-temperature solid-state exchange reaction occurred between CuCl and the Brønsted acid sites of H-ZSM-5. While the high dispersed CuCl on the surface of ZSM-5 is inactive for this reaction. Some direct evidences indicate that the initial Brønsted acidity of the starting zeolite is useful to produce active copper ions species during the preparation process of catalysts. However, it is not necessary for the SCR of NO over Cu-ZSM-5 catalysts.

Hydrocarbon radicals and nitroxy complexes on the surface of HZSM-5 and CuZSM-5 zeolites: An ESR study

The conditions favorable for the formation of oligomer and benzene cation radicals upon C2H4, C6H6, and C3H6 adsorption on thermally activated HZSM-5 and CuZSM-5 zeolites were studied. In the case of isolated copper ions, the radicals were shown to be formed from C3H6 but not from C2H4 and C6H6. Paramagnetic nitroxy complexes, which were formed as a result of the interaction of adsorbed hydrocarbons C2H4, C6H6, and C3H6 with NO and NO2, were revealed on HZSM-5 zeolite and Al2O3. Hydrocarbon radicals and nitroxy complexes are localized on different zeolite sites. Nitroxy complexes are formed on aluminum cations. The role of radicals and nitroxy complexes in the process of NO x selective catalytic reduction by hydrocarbons on the zeolites was discussed on the basis of the temperature dependences of the concentrations of these species.

Comparison of the C3H8Oxidation by NO or by O2on Copper-Based Catalysts

The selective reduction of NO by C3H8is performed on copper-based catalysts, fresh and hydrothermally Cu-MFI with various Si/Al ratios and copper contents, Cu on conventional nonzeolitic supports with various Cu loadings. In the literature it is already known that the C3H8oxidation by NO occurs mainly on isolated copper ions. However, the sites responsible for the C3H8oxidation by O2are less studied. This note shows that nonisolated copper ions are active for the C3H8oxidation by O2. With or without NO in the feed, bulk oxides are more active for the C3H8-O2reaction than for the C3H8-NO (or NO2) reaction. Furthermore, NO inhibits the hydrocarbon combustion by O2.

Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM5

Copper ion-exchanged zeolites ZSM5 with SiO 2:Al 2O 3 molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h −1). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO 2 formation in the gas phase. A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO 2:Al 2O 3 ratio, enhances the conversion of NO. A vibrational band at 1631 cm −1 is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu 2+–O–Cu 2+ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N 2O 3, also binds in a bridge configuration to the same Cu 2+–O–Cu 2+ dimers. A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm −1 probes isolated copper ions. A third infrared band at 2130 cm −1 confirms previous observations of NO + 2-ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for NO + 2 adsorption are blocked by Cu 2+ ion-exchange. The 2130 cm −1 species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice. Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers. A high SiO 2:Al 2O 3 ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.

TPR, ESR, and XPS Study of Cu2+Ions in Sol–Gel-Derived TiO2

Fine titanium(IV) oxide amorphous powders containing two different amounts of CuO were prepared by the sol–gel process. The samples were crystallized to anatase by heating. The reduction of CuO–TiO2samples was investigated by temperature-programmed reduction (TPR). Three types of signals were detected: Two of them are strong and were assigned to two different types of copper ions distributed on the surface. The third kind was a weak broad signal and was ascribed to the simultaneous reduction of titanium and a third type of copper ion incorporated into the TiO2network. This was confirmed by electron spin resonance measurements of the samples before and after programmed reductions at various temperatures selected from the TPR profiles. The results showed that the lowest reduction temperature is due to the existence of isolated copper ions distributed on the surface. The second TPR peak was due to a CuO phase with small particle size. These results were consistent with X-ray photoelectron spectroscopy studies, which showed two signals assigned to –Cu–O–Ti–O– and CuO species.

Cu–Na–ZSM-5 Catalysts Prepared by Chemical Transport: Investigations on the Role of Brønsted Acidity and of Excess Copper in the Selective Catalytic Reduction of NO by Propene

Cu–Na–ZSM-5 catalysts were prepared by a dry technique from physical mixtures of Na–ZSM-5 with copper compounds (CuCl, Cu(NO3)2or Cu(CH3COO)2) in order to obtain intrazeolite copper species coexisting with the full amount of Na+ions of the parent zeolite. For this purpose, the parent mixtures were heated in inert gas or vacuum to 823–873 K. The transport of copper into the near-surface regions of the zeolite crystals was monitored by surface-analytical techniques (XPS, X-ray induced AES), into the bulk of the crystals by infrared spectroscopy of adsorbed pyridine. It was found that the chemical transport is most intense for copper associated with chlorine, but intrazeolite copper can be stabilized also in chlorine-free systems. When H–ZSM-5 is available together with Na–ZSM-5, solid-state ion exchange into the former predominates over chemical transport into the latter. The activity of these samples for the selective catalytic reduction of NO by propene was investigated in a broad temperature range and compared with the activity of reference catalysts prepared by aqueous exchange, which exhibited Brønsted acidity. Reaction rates normalized to the copper content were in the same order of magnitude for both types of Cu–ZSM-5 samples, which demonstrates the irrelevance of Brønsted acidity for the SCR by propene over Cu–ZSM-5. On the basis of surface-analytical data and trends in the temperature dependence of the activity, it was suggested that both isolated copper ions and intrazeolite but extralattice copper oxide clusters provide active sites for the SCR reaction.

Stabilization of the ethane oxidation catalytic activity of Cu-ZSM-5

The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Treatment by steam at 630°C for 17 h or high-temperature dry calcination at 850°C induce an irreversible change in coordination for practically all square-planar Cu2+ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu2+ ions remain accessible to gas-phase molecules, but the catalytic reactivity of these altered copper sites decreases drastically. A stabilizing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part of the copper ions (20–30%) preserves the parent square-planar Cu2+ state even after calcination at 850°C for 0.5 h. The effect of ca. 1% La or Ce is much less pronounced. The catalytic activity in the complete oxidation of ethane correlates well with the number of square-planar cupric cations retained by the samples after different treatments. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500–650°C.

Rearrangement of Cationic Sites in Cuh-ZSM-5 and Reactivity Loss upon High-Temperature Calcination and Steam Aging

The state and reactivity of isolated copper ions in Cu-ZSM-5 treated at elevated temperatures were monitored in situ by ESR under flowthrough conditions. Steam aging (620-630°C, 17 h) or dry calcination (850-900°C, 30 min) induces a change in Cu2+ coordination without noticeable agglomeration or encapsulation of the isolated ions. All Cu2+ ions remain accessible to gas-phase molecules: O2 causes dipole-dipole signal broadening; H2O sorption leads to the increase of the local crystal field symmetry; and the admission of CCl4 results in a noticeable change in CU2+ local coordination. At the same time, the reactivity of these altered CU2+ sites and their ability to adsorb different molecules is substantially affected. No stabilization of nitrosyl complexes on specimens calcined at 900°C was observed, distinct from the case of Cu-ZSM-5500. The bond strength between an altered Cu2+ site and the strong ligand-NO2 is decreased. The sorption of C3H6 on Cu-ZSM-5900 at 20°C is not accompanied by noticeable reduction of CU2+ sites. Even at 500°C in a [C3H6 + He] flow a noticeable part of copper ions (20-30%) preserves the CU2+ state. A partial stabilizing effect of the Cu2+ cations on framework Al3+ in ZSM-5 is demonstrated: the steam aging of pure H-ZSM-5 at 650°C results in an almost total destruction of the sites where Cu2+ ions may be exchanged. The introduction of the Cu2+ before steaming preserves these sites from total destruction and only a transformation in local topography takes place. The surface moieties, containing both CU2+ and Al3+ ions, are a part of the zeolitic framework but must also be "flexible" enough to change symmetry upon additional ligand bonding. The high-temperature calcination or steaming at intermediate temperatures appears to affect this ability. The practical implications are discussed.

Role of the nature of copper sites in the activity of copper-based catalysts for no conversion

The activities of the copper-based catalysts, Cu2+ /SiO2,Cu2+ /Vycor and Cu2+/ZSM-5, and V2O5/TiO2 for NO conversion to N2 in the presence or absence of NH3 and/or O2 have been investigated. The Cu2+ /ZSM-5 catalyst exhibited the highest activity, even higher than that of V2O5/TiO2. Photoluminescence studies of the dehydrated copper-based catalysts have suggested that the copper ions anchored onto ZSM-5 locate as isolated copper species near Bronsted sites in the zeolite channels while the copper ions anchored onto Vycor and SiO2 locate mainly as copper dimer forms. These results suggest the role of copper ions which are stabilized with near-lying oxygen vacancies created by dehydroxylation of the zeolite, in NO conversion. As a result, it may be concluded that the isolated copper ions near Bronsted sites play a significant role in NO conversion but dimeric or polynuclear copper species are less effective for the reaction.

Catalytic properties of copper oxide supported on zinc aluminate in methane combustion

Copper oxide was deposited on a zinc aluminate prepared from high specific area alumina and zinc nitrate. The activity of the solid obtained has been measured in the catalytic combustion of methane before and after an aging test performed at 1340 K. The environment of the copper species was followed by UV–VIS reflectance spectroscopy whereas the state of copper was deduced from temperature-programmed reduction experiments. By comparison with a series of CuO/Al2O3 catalysts, the CuO/ZnAl2O4 sample is slightly less active in the complete oxidation of methane. This decrease in catalytic activity is connected with the presence of bulk CuO particles on the zinc aluminate support instead of isolated copper ions in the case of the alumina carrier.

EPR of layered magnetic metal-amino acid salts. II. Cu(L-Met) 2

Single crystals of bis(L-methioninato)copper(II), Cu(L-Met) 2, were studied by EPR at 9.7 and 33.6 GHz, at 300 K. The position and the peak-to-peak linewidth of the single observed EPR line were measured in three perpendicular planes of the samples. This single resonance is due to the collapse of the resonances of the two magnetically inequivalent copper ions in the lattice caused by the exchange interaction. The components of the molecular g tensor for isolated copper ions were obtained using a model which assumes axial symmetry. The results indicate that the unpaired electron occupies the d( x 2 − y 2) orbital, and the orientation of the molecule obtained from the EPR data agrees with the crystallographic result. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic lattice, an incomplete collapse of the hyperfine structure, and a frequency-dependent contribution in the planes where the g factors for the two sites of copper are different. An analysis of this latter contribution, allows to evaluate an exchange coupling constant | J′| = 0.10 K between inequivalent copper neighbors. Besides, the analysis of the hyperfine contribution to the linewidth gives | J| = 0.18 K for the average value of the exchange interaction of one copper ion with its six nearest neighbors within the layer. Concerning the possible superexchange paths between inequivalent copper ions, we suggest they can be of two types: the CuNH…OCu one, involving hydrogen bonds between equatorial nitrogens and equatorial oxygens, and the other consisting of CuOCOCu carboxylate bridges involving apical and equatorial oxygens. They are discussed in view of the experimental results.

EPR of layered magnetic metal—amino acid salts. I. Cu(L-PHE)2

EPR measurements in single crystals of Cu(L-PHE)2, the copper derivative of the amino acid L-phenylalanine, were performed at 300 K and 9.7 GHz. The gyromagnetic factor, g, and the linewidth of the single EPR line were measured in three perpendicular planes of the sample. The gyromagnetic tensor was obtained, and its principal values are g1 = 2.211, g2 = 2.134, and g3 = 2.075. The single resonance is explained by the collapse of the resonances of the two magnetically non-equivalent copper sites due to the exchange interaction. The molecular g-factor of isolated copper ions is obtained from a model assuming axial symmetry. The results are g| = 2.266 and g⊥ = 2.075, indicating a d(x2 - y2) ground orbital. The orientations of the molecules in the crystal obtained by this model are in agreement with the crystallographic values. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic structure, an incomplete collapse of the hyperfine structure, and contributions arising from non-isotropic exchange. A mean value ¦J¦=0.19 K is calculated for the isotropic exchange interactions between one copper and its six copper neighbors in Cu(L-PHE)2. Also, a lower limit ¦J′¦ > 0.02 K for the exchange coupling J′ between non-equivalent copper neighbors is obtained.

Etude des phases β et haute pression du phosphate tricalcique par la RPE de l'ion Cu 2+

Copper (II)-doped powdered samples of β-Ca 3(PO 4) 2 and of its high-pressure phase, which is an isotype of Ba 3(VO 4) 2, were studied by ESR. In each case the RT spectrum is due to a unique type of defect exhibiting orthorhombic and respectively axial symmetry. The spin-Hamiltonian parameters were determined using the method proposed by Rollmann and Chan for polycrystalline specimens. The contribution of each of both copper isotopes was resolved with stressed samples. Our results show that isolated copper ions substitute preferentially for calcium in Ca(III) sites of β-calcium orthophosphate, these sites corresponding to those of Ca(I′) in the high-pressure phase.