Mahiko Nagao

Metal-insulator transition and thermoelectric properties in the system (R1−xCax)MnO3−δ (R: Tb, Ho, Y)

(R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}}(R: Tb, Ho, Y) solid solutions with the orthorhombic perovskite-type structure were synthesized and their electrical resistivity and thermoelectric power were measured in the temperature range 80 to 1000 K. At low temperature, the electrical properties of (R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}} are explained by the variable range hopping of electrons model. At high temperature, hopping conduction occurs in composition range 0.4{le}x{le}0.6, while metal-insulator transition occurs in the range 0.7{le}x{le}0.9. Transition temperature increases with increasing average Mn-O distance. It is concluded that the increase in Mn-O distance enhances the magnitude of the electrostatic field, and consequently the transition temperature is raised. (R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}}(R: Tb, Ho, Y, 0.7 {le}x{le}0.9) are candidates as high temperature thermoelectric conversion materials.

The state of excessively lon-exchanged copper in mordenite: formation of tetragonal hydroxy-bridged copper ion

Copper ions are exchanged in mordenite in amounts in excess of the value expected from stoichiometric considerations in the process of repeated ion-exchange of sodium ions in mordenite with copper ions. This is not the case for nickel and calcium ions used as exchanger ions. A series of experiments was designed to elucidate the state of excessively ion-exchanged copper in mordenite. DRS and EPR analyses together with XANES spectra revealed that the oxidation state of copper ion in mordenite is divalent in each exchange stage. IR, EPR, XANES and EXAFS studies provided evidence for the existence of a tetragonal hydroxy-bridged polymer of copper ions in excessively ion-exchanged mordenite; the Cu—O bond length is 1.97 A and the Cu–Cu distance is 3.05 A. This polymer species exhibited IR absorption bands near 3350 and 930 cm–1 for OH stretching and MOH bending vibrations of bridged species, respectively. The difference in exchange behaviour of divalent metal ions is interpreted in terms of the difference in the magnitude of their hydrolysis constants.

Surface characterization of LaMnO3+δ powder annealed in air

Abstract Perovskite-type LaMnO3+δ, synthesized using poly(acrylic acid) (PAA), was annealed at 400 and 700°C in air for 6–54 hr. The crystallite size, the specific surface area, the La/Mn ratio, and the catalytic activity of LaMnO3+δ were measured to characterize the surface. The specific surface area decreased slightly with increasing annealing time, while the La/Mn ratio of the surface, calculated from the XPS measurements, was independent of annealing time. The catalytic activity for the oxidation of CO increased with annealing. These results suggest that annealing improved the crystallinity (regularity of the ions) of the LaMnO3+δ surface.

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

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 N2 species that is one of the products of the decomposition reaction of NO. Two dominant types of exchangeable sites in the CuZSM5 sample were identified 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 different 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 different 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 oxidation–reduction process of copper ion species was also examined during NO adsorption and subsequent heat treatment invacuo. 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.

High-temperature phase transition of CaMnO3−δ

Abstract High-temperature phase transition of perovskite-type CaMnO3−δ was examined by both thermal analysis (DTA) and high-temperature X-ray diffractometry. The crystal structure is orthorhombic below ca. 896°C, tetragonal between ca. 896 and 913°C, and cubic above ca. 913°C.

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr1−xMnx)O3+δ (0.0⩽x⩽1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr1−xMnx)O3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn4+ ion content increases monotonically in the range 0.2⩽x⩽1.0. The magnetic measurement of La(Cr1−xMnx)O3+δ indicates that a Mn3+ ion is a high-spin state with (de)3(dγ)1. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr3+, the Mn3+, the Mn4+ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr1−xMnx)O3+δ is a p-type semiconductor and exhibits the hopping conductivity.

New light on the state of active sites in CuZSM-5 for the NO decomposition reaction and N2 adsorption

Ion-exchange of ZSM-5-type zeolites with copper ion was carried out by three different methods to obtain information on the states and roles of the effective sites for NO decomposition and N2 adsorption activity of copper-ion-exchanged ZSM-5-type zeolites (CuZSM-5). The first method of preparation is chemical vapour deposition (CVD) using bis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II), [Cu(hfac)2], as a volatile complex, and the second one is to utilize CuCl as a vaporizing source. These were compared with the third method, that is, the ordinary ion exchange method using an aqueous solution of CuCl2. It is apparent that in the first method the Cu2+ species deposited on ZSM-5 as [Cu(hfac)2] (i.e., via the hydrogen bonding between a Bronsted acid site and a pseudo-aromatic ring of ligands) is reduced to the monovalent species (Cu+) by evacuation at 573 K, and also that the reducibility of Cu2+ is superior to that in the sample prepared by the conventional ion-exchange in an aqueous solution. As for the sample prepared by using CuCl, Cu+ deposited as CuCl was exchanged with H+ on a Bronsted acid site in the HZSM-5 sample through treatment at temperatures above 573 K with a release of HCl. The CuZSM-5 sample prepared by the CVD method gave a single IR band at 2159 cm−1 due to the adsorbed CO species, while the sample prepared by evaporation of CuCl at 573 K and also the sample ion-exchanged in an aqueous solution gave a broad band at around 2155 cm−1 (composed of two bands at 2159 and 2151 cm−1) for the adsorbed CO species, indicating the existence of at least two dominant types of exchangeable sites in these CuZSM-5 samples. Each sample reveals different features for NO decomposition reactivity and N2 adsorption, and such behaviours are explained by the difference in ratio of the respective sites occupied by copper ions. As a result, it was clearly demonstrated that the simultaneous existence of two types of sites lying close together has important implications for the catalytic activity for NO decomposition by CuZSM-5, and that the site giving the 2151 cm−1 band is the effective site for N2 adsorption.

A more efficient copper-ion-exchanged ZSM-5 zeolitefor N2 adsorption at room temperature: Ion-exchange in anaqueous solution of Cu(CH3COO)2

The copper-ion-exchanged ZSM-5 type zeolite, prepared by ion-exchange in an aqueous solution of Cu(CH3COO)2 and evacuation at 873 K, gives a distinctive IR band at 2151 cm−1 due to the adsorbed CO species. More efficient adsorption of N2 was exhibited by this sample, compared with samples prepared by other methods, implying site-selective ion-exchange in the preparation process. On the basis of X-ray absorption near-edge structure (XANES) spectra the exchanged copper ion was proved to be in a monovalent state; one of the splitting strong bands, due to the 1s–4pz transition of the monovalent copper ion, loses its intensity on N2 adsorption. The extended X-ray absorption fine structure (EXAFS) spectral pattern around the copper ion also changed on N2 adsorption and a shoulder appeared at around 1.5 A (no phase-shift correction), in addition to the strong band at around 1.65 A (no phase-shift correction). It was concluded that the monovalent copper-ion-exchanged site giving the 2151 cm−1 band due to the adsorbed CO species is the active site for specific N2 adsorption. A first principles calculation was carried out with the object of finding the most appropriate model for the CO species adsorbed on the exchanged copper ions in ZSM-5. The data obtained suggest that a three-coordinate copper ion bonded to three lattice oxygen atoms adsorbs CO to give the 2151 cm−1 band. A pseudo-planar structure including the monovalent copper ion bound to three oxygen atoms is assumed to change to a pseudo-tetrahedral arrangement on N2 adsorption. Such a site-selectively ion-exchanged substance has potential for the development of materials for N2 separation or fixation and activation catalysts, as well as for the analysis of NO-decomposition sites.

What are the important factors determining the state of copper ion on various supports? Analysis using spectroscopic methods and adsorption calorimetry

The important factors that determine the state of copper ion supported on the SiO2·Al2O3, SiO2 and ZSM-5 samples have been elucidated by using various spectroscopic techniques and adsorption calorimetry. When CO was adsorbed on the copper ion supported SiO2·Al2O3 (Cu/SiO2·Al2O3) sample or the copper ion exchanged ZSM-5 (CuZSM-5) sample which had been evacuated at 873 K in advance, a band was observed at around 2155 cm-1 which can be assigned to the CO species adsorbed onto the monovalent copper ion in these samples. In the case of CO adsorption on the copper ion deposited SiO2 (Cu/SiO2) sample, the band due to the adsorbed CO species appeared at 2132 cm-1. The differential heat of adsorption (Hd) of CO on Cu/SiO2·Al2O3 gave a value of ca. 100 kJ mol-1 at the initial adsorption stage and it gradually decreased with increasing amount adsorbed. The same relationship in the Hd–νCO (wavenumber of absorption band due to the C–O stretching vibration) plots was observed in the systems of Cu/SiO2·Al2O3–CO and CuZSM-5–CO, which indicates that the same σ bonding interaction is operative in these systems. In the case of CO adsorption on the Cu/SiO2 sample, the amount adsorbed is too small to get meaningful values of the adsorption heat for the discussion of the bonding nature between the copper ions and CO molecules, and we speculated that the same σ-bonding interaction is operative. The existence of the Bronsted acid sites on the original proton-type SiO2·Al2O3 and ZSM-5 samples was confirmed by the IR spectra using CO as a probe molecule and by the measurement of solid NMR spectra. These data provide an explanation for the appearance of an IR band (2155 cm-1) due to the CO species adsorbed on the Cu/SiO2·Al2O3 and CuZSM-5 samples. The existence of Bronsted acid sites, due to the existence of Al in the lattice, can be regarded as an important factor in their role as catalysts in the various reactions. The state of copper ions that act as the active sites in the catalytic reactions is different, depending on the Si:Al ratio of the sample; the Cu2+ species supported on the SiO2·Al2O3 sample having a lower Si:Al ratio resist reduction, because the exchanged divalent ions may occupy two exchangeable sites simultaneously. It seems that the higher Si:Al ratio is a necessary condition for keeping an amount of copper ion deposited on the support sufficient for redox reaction as well as for acting as a good NO-decomposition catalyst. From the spectroscopic observations such as IR, emission, X-ray absorption, and electron paramagnetic resonance spectra, it is also found that the copper ions on the SiO2 sample reduced in the evacuation process are dispersed appropriately in Cu2O-like sites.

On the possibility of AgZSM-5 zeolite being a partial oxidation catalyst for methane.

A silver-ion-exchanged HZSM-5 zeolite sample (Ag(H)ZSM-5) evacuated at 573 K exhibited prominent catalytic behavior in the partial oxidation of CH(4) at temperatures above 573 K, exceeding the performance of Ag/SiO(2)Al(2)O(3) and Ag/SiO(2) catalysts. From the infrared (IR) and X-ray absorption fine structure (XAFS) spectra, as well as the dioxygen adsorption measurement, it was concluded that the simultaneous existence of Ag(+) ions and small clusters of Ag particles leads to the partial oxidation of methane. Taking the magnitude of the formation enthalpy (per oxygen atom) of Ag(2)O (DeltaH=26 kJ/mol) into consideration, we propose the interpretation that the dioxygen activated on small Ag metal clusters formed in ZSM-5 elaborates a surface oxide layer on small Ag clusters and the thus-formed species is simultaneously and easily decomposed at 573 K or above, and the oxygen activated in this way on the Ag metal spills over and can react with methane that has been activated by the Ag(+) ions exchanged in ZSM-5, resulting in the high catalytic activity of the Ag(H)ZSM-5 sample in the partial oxidation of methane. This interpretation is also well evidenced by XAFS and IR data. It is anticipated that this material has the potential to be a promising catalyst in the conversion of natural gas into higher value-added chemicals and fuels.