Kunimitsu Morishige

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.

Two-dimensional condensation of water on the surface of Cr2O3

Abstract Physical adsorption of water was investigated on the surface of Cr3O3 (Cr3O3-I, Cr3O3-II and Cr3O3-III) prepared by the pyrolysis of (NH4)2Cr3Oτ, Cr2O3- xH2O and Cr2(C2O4)3, respectively. Two lumps were found in the adsorption isotherm at the relative pressure of 0.035 (jump-I) and of ∼0.17 (jump-II). The former was most clearly demonstrated as a vertical lump, which proved the occurrence of two-dimensional condensation of adsorbed water, on the samples 1373 K-treated Cr2O3-I 973 K and 1173 K-treated Cr2O3-III, while the latter was distinct on the 973 ∼ 1373 K-treated Cr2O3-II. The isosteric heat of adsorption of water substantiated the fact that the lateral interaction plays a significant role in adsorption of water molecules on the Cr2O3 surface and in appearance of lumps-I and -II.

Adsorption of Kr on ZnO, SnO2, MgO, and CdO: Surface homogeneity and the effect of surface hydroxyls

Abstract The change in surface structure arising from the formation and removal of surface hydroxyls on the homogeneous surface of ZnO, SnO2, MgO, and CdO was examined by the measurement of the Kr adsorption isotherm at the temperature of liquid N2. The samples dehydroxylated by outgassing at high temperatures gave stepwise isotherms of Kr, which are indicative of the existence of homogeneous surfaces on the samples. Furthermore, it was found that the surface hydroxyls on ZnO, SnO2, and MgO form an ordered monolayer, but those on CdO do not, and that every kind of metal oxides examined in the present study differs from each other in the desorption mode of surface hydroxyls when outgassed at high temperatures.

Adsorption of water on the surfaces of α-HCrO2 and Cr2O3

Abstract Physical adsorption of water was investigated on the surface of hydrothermally prepared α-HCrO 2 and its decomposition derivative Cr 2 O 3 . The (001) plane of α-HCrO 2 homogeneous for Kr adsorption gave a steep rise in water adsorption isotherm near zero pressure due to strong adsorption through H bonding. A step adsorption isotherm was observed for water on the (012) plane, suggesting a lateral interaction between admolecules on the homogeneous surface. The crystal plane of Cr 2 O 3 on which the two dimensionally condensed and solid film of water can be formed was assigned to the (001) plane. The structure of the water film was suggested to be a net constructed from hexagonal rings which is formed by H bondings between adsorbed water molecules on the sixfold symmetrical surface. During the decomposition of thick α-HCrO 2 in air, a transition material of CrO 2 was newly found.

Thermal desorption study of surface hydroxyls on ZnO

The desorbability of surface hydroxyls on ZnO was investigated by three different techniques: thermal desorption, infrared absorption spectroscopy and adsorption of water on hydroxylated ZnO. It was found that the thermal desorption spectrum of the surface hydroxyls has two distinct peaks at 493 and 543 K. The surface hydroxyls on ZnO which were considered to behave as an inert surface for water physisorption and which as a result play an important role in the appearance of a discontinuity in the water adsorption isotherm were identified to be those giving a peak at 543 K in the thermal desorption spectra. Furthermore, they also give rise to a broad band with a peak at 3540 cm–1 in the infrared absorption spectra.

The effect of surface hydroxyls of Cr2O3 on the adsorption of N2, Ar, Kr, and H2O in connection with the two-dimensional condensation

Abstract The adsorption properties of the Cr 2 O 3 surface for N 2 , Ar, Kr, and H 2 O were investigated by varying the extent of surface hydroxylation. The dehydroxylation made the Cr 2 O 3 surface active, which led to the chemical adsorption of the N 2 molecule at the temperature of liquid N 2 . Adsorption isotherms of Ar, Kr, and H 2 O on Cr 2 O 3 revealed a step, but the pressure at which the step appears changed drastically by evacuating the sample at 773°K. This suggests that the treatment modifies the adsorption force of the surface, keeping a surface homogeneity. Once the surface was dehydroxylated, the rehydroxylation did not reproduce the surface with the original homogeneity, on which the two-dimensional (2D) condensation of Ar and Kr had occurred. A crystal plane on which the 2D condensation of H 2 O occurs could be considered to be common to that at which such inert gases as Ar and Kr are adsorbed stepwise from the first to the higher layers. The ratio of the number of physisorbed H 2 O molecules to that of surface hydroxyls on the Cr 2 O 3 was found to be 0.40–0.67, which suggests that each physisorbed H 2 O molecule is bound to two surface hydroxyls except on the surfaces treated at higher temperatures.

Adsorption anomaly of water on the surface of tin(IV) oxide: effects of crystal growth of the solid

Abstract Two SnO 2 samples prepared by oxidizing metallic tin (SnO 2 -I) and by hydrolyzing SnCl 4 (SnO 2 -II) were used for the investigation of the effect of their calcination on the changes in pore size and crystallinity as well as on the appearance of the adsorption anomaly of water on them. Pore size and crystallinity were increased with rising temperature of calcination, discontinuously upon calcination at 673°K, where micropores changed to mesopores and the crystal growth was marked. The jump of the adsorption isotherm of water also appeared on the samples treated at this temperature. The height of the jump increased upon calcination over 600°K, passed a maximum value and then decreased, being more distinguished with SnO 2 -I than with SnO 2 -II. Electron diffraction analysis of the SnO 2 particles gave the evidence for the prediction that the (100) plane of the rutile structure of SnO 2 was responsible for the occurrence of the adsorption anomaly of water.

Interaction of the surface of BeO with water: In connection with the two-dimensional condensation of water

Abstract The adsorption isotherm of water was measured on BeO, and it was found that BeO reveals a step in the adsorption isotherm when the sample is subjected to hydrothermal treatment. The q st curve shows a weak maximum over the coverage region from 0.5 to 1.0, which manifests a characteristic lateral interaction between adsorbed water molecules. These phenomena can be explained in terms of the two-dimensional condensation of water which occurs on the hydroxylated surface of the (1010) plane of BeO as in the case of ZnO on which the step has been reported to appear. The preparation of a homogeneous surface of BeO which exhibits this phenomenon is rather difficult compared to that of ZnO, and the surface once prepared is apt to be damaged by heat treatment. This fact was considered to be due to a larger surface-ionic polarization of BeO because of a very small radius of the Be 2+ ion compared to that of the Zn 2+ ion. The infrared spectra substantiated that the peak at 3518 cm −1 is due to the OH stretching vibration of surface hydroxyls responsible for the appearance of the step.

Thermal desorption and infrared studies of methylamines adsorbed on dehydrated alkaline-earth-metal X zeolites

The effect of the methyl substituent on the interaction between the exchangeable cations in alkaline-earth-metal X zeolites and methyl-substituted amines has been examined by measuring the thermal desorption spectra and the infrared absorption spectra of adsorbed amines. The methylamines adsorbed on NaX, SrX, CaX and MgX gave two types of desorption peaks (α and β). The temperature range of the α peak, which appears on the lower-temperature side, was constant independent of a combination of exchangeable cations and amines, while the β peak, on the higher-temperature side, was shifted towards higher temperatures in the order NaX < SrX < CaX < MgX for a given methylamine. Furthermore, with SrX and CaX the temperature range of the β peak increased in the order NH3 < MeNH2 < Me2NH < Me3N, while for NaX and MgX it was determined solely by the cation, regardless of the amine. These facts lead to the conclusion that the β peak on the dehydrated alkaline-earth-metal X zeolites originates from the desorption of molecules interacting with site II cations in the supercages. Me2NH adsorbed on site II cations of the alkaline-earth-metal X zeolites gave rise to a broad band centred at ca. 3500 cm–1 due to the NH stretching vibration, although their free NH stretching frequency is 3384 cm–1. NH3 adsorbed on site II cations of SrX and CaX gave a very broad band centred at ca. 1580 cm–1 due to the NH3 antisymmetric bending vibration, contrary to the relatively sharp bands at 1650 and 1600 cm–1 observed for NH3 on MgX. Such a difference is explained on the basis of the size effect of the bivalent cations.

Structure and melting of a bilayer oxygen film on graphite

X‐ray diffraction measurements of oxygen physisorbed on graphite have been taken over the temperature range 32–52 K and the coverage range 0.13–0.27 molecule/A2. The results strongly suggest the model that bilayer oxygen at low temperatures consists of two independent layers of O2 molecules, a top layer of a centered rectangular lattice above a bottom layer of a hexagonal lattice incommensurate with the substrate. On warming, the top layer first melts at ∼38 K abruptly and then the bottom layer melts immediately after that. The resulting fluid II, which is a composite phase of a liquid giving rise to a very broad peak centered at 2θ=28.5° and a phase giving rise to a broad peak around 2θ=32°, is stable over the wide temperature range 38–47 K and eventually changes to the pure liquid. The structure of this composite phase remains unresolved, although the diffraction pattern shows evidence of long‐range order retained in this phase.