Shigeharu Kittaka

Effect of water on structure of hydrophilic imidazolium-based ionic liquid.

The state of water in room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF(4)(-)), has been investigated by measurements of absorption and desorption isotherms, attenuated total reflectance infrared (ATR-IR) spectroscopy, and (2)H NMR relaxation method. The absorption enthalpies of water for the ionic liquid were estimated from the absorption isotherms. The enthalpies in the water mole fraction range of x(w) <or= approximately 0.5 are lower than the enthalpy of vaporization for bulk water, but become larger than the value for bulk with increasing mole fraction of absorbed water. The ATR-IR spectra for EMI(+)BF(4)(-)-water solutions in the range of 0.09 <or= x(w) <or= 0.34 have revealed that individual water molecules hydrogen-bonded to the anions predominate in the solutions at x(w) <or= approximately 0.2, while approximately 30% of water molecules are hydrogen-bonded among them in the solutions at x(w) > approximately 0.3. In addition, the activation energies for the rotational motion of a water molecule estimated from the (2)H NMR relaxation rates have indicated that the motion of water molecules in EMI(+)BF(4)(-)-D(2)O solutions gradually becomes freer with increasing water content from x(w) = 0.10 to 0.30, but is retarded again at x(w) = 0.33. Therefore, all the present findings have suggested that the state of water molecules in EMI(+)BF(4)(-) significantly changes at x(w) approximately 0.3. On the other hand, to directly observe the effect of water on structure of EMI(+)BF(4)(-), LAXS experiments have been made on EMI(+)BF(4)(-)-water solutions. It has been suggested that the interactions between the C(2) atom within the imidazolium ring of EMI(+) and BF(4)(-) are strengthened with increasing water content, while those at the C(4) and C(5) atoms weaken. Thus, the present LAXS experiments have clarified the beginning of formation of ion pair in EMI(+)BF(4)(-) by adding water at the molecular level.

Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water and Maxwell-Wagner processes

The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 A, respectively, is examined by broadband dielectric spectroscopy (10(-2)-10(9) Hz) and differential scanning calorimetry for a wide temperature interval (110-340 K). The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.

Thermodynamic, structural, and dynamic properties of supercooled water confined in mesoporous MCM-41 studied with calorimetric, neutron diffraction, and neutron spin echo measurements.

Thermodynamic, structural, and dynamic properties of heavy water (D(2)O) confined in mesoporous silica glass MCM-41 C10, C12, and C14 were investigated by differential scanning calorimetry, neutron diffraction, and neutron spin echo (NSE) measurements, respectively. The DSC data showed that no crystallization of D(2)O confined in C10 occurs in a temperature range between 298 and 180 K, and that crystalline ice is formed at 204 and 221 K for C12 and C14, respectively. For C10, the neutron radial distribution functions of confined D(2)O suggested a structural change in the supercooled state between 223 and 173 K. For C10 sample, it has been found that the tetrahedral-like water structure is partially enhanced in the central part of pores at 173 K. For all the samples, the intermediate scattering functions from the NSE measurements are fitted by the Kohlrausch-Williams-Watts stretched exponential function which implies that confined supercooled D(2)O exhibits a wide distribution of relaxation times. For C10, C12, and C14 samples, between 298 and 240 K, the relaxation times of supercooled D(2)O follow remarkably well the Vogel-Fulcher-Tamman equation; for C10 sample, below 240 K, the relaxation times of nonfreezing D(2)O show an Arrhenius type behavior. From the present experimental results on calorimetric, structural, and dynamic properties, it has been concluded that supercooled D(2)O confined in MCM-41 C10 experiences a transition from high-density to low-density hydrogen-bonded structure at around 229 K.

Enthalpy and interfacial free energy changes of water capillary condensed in mesoporous silica, MCM-41 and SBA-15

The effect of confinement on the solid-liquid phase transitions of water was studied by using DSC and FT-IR measurements. Enthalpy changes upon melting of frozen water in MCM-41 and SBA-15 were determined as a function of pore size and found to decrease with decreasing pore size. The melting point also decreased almost monotonically with a decrease in pore size. Analysis of the Gibbs-Thomson relation on the basis of the thermodynamic data showed that there were two stages of interfacial free energy change after the constant region, i.e., below a pore size of 6.0 nm: a gradual decrease down to 3.4 nm and another decrease after a small jump upward. This fact demonstrates that the simple Gibbs-Thomson relation, i.e., a linear relation between the melting point change and the inverse pore size, is limited to the range not far from the melting point of bulk water. FT-IR measurements suggest that the decrease in enthalpy change and interfacial free energy change with decreasing pore size reflect the similarity of the structures of both liquid and solid phases of water in smaller pores at lower temperatures.

Thermal desorption and infrared studies of ammonia, amines and pyridines chemisorbed on chromic oxide

The thermal desorption spectra of ammonia, amines and pyridines chemisorbed on Cr2O3 as well as the infrared absorption spectra have been measured in order to elucidate the acidic properties of the surface. The adsorption of ammonia on the bare Cr2O3 surface at room temperature leads to the formation of two kinds of coordination species giving rise to desorption peaks at 443 and 543 K, and the bonding of the latter species to the surface is strengthened to cause a shift in the peak to 593 K when the adsorption temperature is raised to 473 K. Dimethylamine bonded to Lewis-acid sites on Cr2O3 causes the N—H bond-cleavage reaction when the sample is heated at 473 K. Pyridine is strongly held on the surface: half the pyridine molecules absorbed on Cr2O3 at room temperature remain on the surface even after evacuation at 773 K. The infrared study of these molecules suggests the formation of an α-pyridone-like complex.

Isoelectric point of metal oxides and binary metal oxides having spinel structure

Abstract Isoelectric point (IEP) was measured on simple metal oxides (Co 3 O 4 , NiO, CuO, ZnO) as a function of treatment temperature and as a function of the composition of the binary metal oxide systems (CoOAl 2 O 3 , ZnOAl 2 O 3 , CoOFe 2 O 3 , NiOFe 2 O 3 , CuOFe 2 O 3 , ZnOFe 2 O 3 ). Most experimental IEP values for simple metal oxides showed good agreement with those calculated by the Parks equation, but some of them were smaller than the calculated ones, because of surface oxidation of heated metal oxides in air. Binary metal oxides having inverse spinel structure revealed characteristically small values of IEP. This can be explained in terms of the presence of tetrahedrally coordinated trivalent metal ions (Fe 3+ ), which causes the polarization of surface hydroxyls to release H + ions easily from the surface.

Thermodynamic and FTIR studies of supercooled water confined to exterior and interior of mesoporous MCM-41

The thermal properties of water confined to both exterior and interior of cylindrical mesoporous MCM-41 (pore diameter d = 1.8-3.6 nm) were analysed by differential scanning calorimetry and FTIR spectroscopy. A three-step freezing of the exterior water was observed just above 233 K, the homogeneous nucleation temperature of bulk water, before the interior water was frozen. The first freezing of water was ascribed to the outermost bulk water, the second one to water between bulk and water bound to the exterior wall, and the third one to the bound exterior water. With decreasing pore size, the second freezing water decreased in magnitude. This stepwise freezing of the exterior water has been found in porous zeolite materials. The exothermic peak of the interior water confined in MCM-41 was observed at 227.5 K before freezing, ascribed probably to a high-density liquid-low-density liquid phase change. FTIR data of the interior water confirmed this finding. The present results substantiate the static and dynamic crossover of supercooled water in MCM-41 reported from previous neutron scattering and NMR data.

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.

Neutron spin echo measurements of monolayer and capillary condensed water in MCM-41 at low temperatures

Neutron spin echo measurements of monolayer and capillary condensed heavy water (D(2)O) confined in MCM-41 C10 (pore diameter 2.10 nm) were performed in a temperature range of 190-298 K. The intermediate scattering functions were analyzed by the Kohlrausch-Williams-Watts stretched exponential function. The relaxation times of confined D(2)O in the capillary condensed state follow remarkably well the Vogel-Fulcher-Tammann equation between 298 and 220 K, whereas below 220 K they show an Arrhenius type behavior. That is, the fragile-to-strong (FTS) dynamic crossover occurs, which has never been seen in experiments on bulk water. On the other hand, for monolayer D(2)O, the FTS dynamic crossover was not observed in the temperature range measured. The FTS dynamic crossover observed in capillary condensed water would take place in the central region of the pore, not near the pore surface. Because the tetrahedral-like water structure in the central region of the pore is more preserved than that near the pore surface, the FTS dynamic crossover would be concerned with the tetrahedral-like water structure.

Mechanism of freezing of water in contact with mesoporous silicas MCM-41, SBA-15 and SBA-16: role of boundary water of pore outlets in freezing

The freezing mechanism of water contacted with mesoporous silicas with uniform pore shapes, both cylindrical and cagelike, was studied by thermodynamic and structural analyses with differential scanning calorimetry (DSC) and X-ray diffraction (XRD) together with adsorption measurements. In the DSC data extra exothermic peaks were found at around 230 K for water confined in SBA-15, in addition to that due to the freezing of pore water. These peaks are most likely to be ascribed to the freezing of water present over the micropore and/or mesopore outlets of coronas in SBA-15. Freezing of water confined in SBA-16 was systematically analysed by DSC with changing the pore size. The freezing temperature was found to be around 232 K, close to the homogeneous nucleation temperature of bulk water, independent of the pore size when the pore diameter (d) < 7.0 nm. Water confined in the cagelike pores of SBA-16 is probably surrounded by a water layer (boundary water) at the outlets of channels to interconnect the pores and of fine corona-like pores, which is similar to that present at the outlet of cylindrical pores in MCM-41 and of cylindrical channels in SBA-15. The presence of the boundary water would be a key for water in SBA-16 to freeze at the homogeneous nucleation temperature. This phenomenon is similar to those well known for water droplets in oil and water droplets of clouds in the sky. The XRD data showed that the cubic ice I(c) was formed in SBA-16 as previously found in SBA-15 when d < 8.0 nm.