Eizo Miyazaki

Catalytic activities of TiC, WC, and TaC for hydrogenation of ethylene

Evacuation temperatures of 1000/sup 0/C activated titanium monocarbide, of 600/sup 0/-1000/sup 0/C activated tungsten monocarbide, and of > 1000/sup 0/C activated tantalum monocarbide for the hydrogenation of ethylene at O/sup 0/C. The tantalum carbide activity was about one order of magnitude lower than that of the other two carbides. Changes in surface area during activation corresponded to the activation temperatures but were small. The activation temperatures corresponded to the temperatures at which metal oxide oxygen is removed, but hafnium, zirconium, and niobium monocarbides were not activated for ethylene hydrogenation at temperatures to 1100/sup 0/C, at which surface oxygen was completely removed.

Chemisorption of diatomic molecules (H2, N2, CO) on transition d-metals

Abstract The interaction energies of diatomic molecules, H 2 , N 2 , and CO with transition d -metal surfaces are empirically calculated using a bond energy bond order (BEBO) approach developed in earlier papers. The two problems, i.e., (i) does a molecule chemisorb on a given surface and (ii) whether the adsorbed species is in a molecular form or dissociative form, are examined from the potential energy curves obtained. The metals are then classified approximately into four groups, A, B, C, and D. The A group metals, which consist mainly of IV B , V B , and VI B group metals in the periodic table of the elements, strongly chemisorb all the above gases including O 2 and NO molecules in a dissociative form at room temperature. The group D metals consisting of noble d -metals, on the other hand, chemisorb NO and CO in a molecular form and do not decompose CO and N 2 molecules. The group B (Fe, Re) and C (Co, Ni, Tc) metals which are known as main elements of catalysts for the reactions of Fischer-Tropsch and ammonia syntheses have intermediate adsorption properties between the A and D group metals. These qualitative and quantitative results of heats of dissociative and nondissociative adsorption are summarized in tables and compared with the experiments.

Catalysis by transition metal carbides: IV. Mechanism of ethylene hydrogenation and the nature of active sites on tantalum monocarbide

The hydrogenation of ethylene on thermally activated TaC were studied. Similar to the catalysis by transition metals, the hydrogenation on TaC exhibited a maximum rate at a definite reaction temperature; the activation energy of the reaction was 33.8 kJ mole−1 below 370 K, but −54.4 kJ mole−1 above 420 K. The reaction of ethylene with D2 at 273 K yielded ethane ranging from d0 to d5 and highly exchanged ethylene with increased conversions. The isotope composition of gaseous hydrogen reached nearly equilibrium when the degree of conversion exceeded 40%. The chemisorption of hydrogen on TaC obeyed the dissociative Langmuir isotherm. The application of strongly adsorbed carbon monoxide and acetylene as poison showed that about 40% of the total surface was catalytically effective and that the reaction rate diminished with increasing fraction of poisoned surface, θp, being proportional to (1 − θp)3. The accumulation of graphite-like carbon on the TaC surface occurred by heating the catalyst above 400 K in an C2H4 atmosphere, and was accompanied by a drastic decrease in the catalytic activity. X-Ray photoelectron study revealed that the characteristic features of electronic states in the activated TaC were a partial electron transfer from Ta to carbon atom. The associative mechanism of hydrogenation was proposed and the possible structure of active sites on TaC was discussed.

Infrared and X-ray photoelectron spectroscopy studies of carbon monoxide adsorbed on silica-supported cobalt catalysts

The adsorption of CO on silica-supported cobalt catalysts has been studied by means of i.r. and X.p. spectroscopies in order to reveal the nature of Co—CO bonding in connection with the catalytic hydrogenation of CO. For a 7.5 wt% catalyst, a single i.r. band for CO appeared first at ca. 2000 cm–1(band A). With increasing exposure to CO this band shifted to 2040 cm–1, accompanied by a sharp band B at 2180 cm–1. In contrast, a 3.35 wt% catalyst produced only band B. Upon evacuation at room temperature, band B was readily removed, whereas band A, shifting to a lower frequency, remained up to 473 K. X.p.s. has revealed that the surface of the former catalyst has exposed metallic cobalt, whereas the surface of the latter consisted mainly of Co2+ ions even after exposure to H2 at 723 K. Bands A and B were assigned to linearly bonded CO on metallic sites and COδ+ on cationic sites, respectively. An analysis of the variation in frequency of band A as a function of the amount of CO suggested the presence of indirect interaction via the metal electrons. The effect of hydrogen was a shift in band A to lower frequency. This led to the conclusion of weakening of the C—O bond in the presence of H2 and no contribution from COδ+ species to the hydrogenation.

Infrared spectroscopic, thermal desorption and X-ray photoelectron spectroscopic studies of NO, NO + CO and NO + O2 adsorbed on palladium surfaces

The adsorbed state of NO on polycrystalline Pd metal surfaces, its variation with heat treatment and the effect of CO and O2 have been studied by infrared spectroscopy (silica-supported), thermal desorption (powder) and X-ray photoelectron spectroscopy (powder, foil). The i.r. spectra of NO adsorbed at 298 K showed that there exist mainly bent or bridged NO (1660–1650, 1580–1570 cm–1) for θNO 0.6. Upon heat treatment, these i.r. bands changed in different ways. X.p.s. showed a broad N 1s peak of molecular NO (398.7 eV) at 298 K and another peak due to NO2-like surface species (404.0–404.2 eV) at 453 K. For θNO 0.6 additional peaks of NO appeared at 373–473 K. It is proposed that in N2 formation the dissociation of NO is rate-determining. Changes in the adsorbed state of NO upon heat treatment are explained by the interaction of NO with O atoms remaining on the surface after part of the NO is decomposed. The effect of preadsorbed oxygen supports this view. Preadsorbed CO works as a scavenger for this O atom and accelerates the formation of N2.

Angle-resolved photoemission study of NO chemisorption on Pd(111)

Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the chemisorption of NO on Pd(111) at room temperature. The NO molecule is found to have a bent orientation on the surface, from the study of incidence angle dependent measurements and symmetry arguments. Furthermore, the tilt angle is determined to be 20-25° from the surface normal direction, using the results of a study of the shape resonance of the 5σ + 1π emission. This is the first determination of the orientation of an NO molecule on a metal surface using shape resonance.

Theory of Electronic Structure of the Polar ZnO Surface by the Cluster Models

The electronic structures of polar ZnO srufaces are investigated by the use of the DV-Xα cluster calculations. The compensating charge of about 18∼28% of inner ionic charge is found to be induced on the polar surface of the cluster. The electronic origin of the charge compensation is discussed in detail. Drastic reduction of the ionic charge is revealed on the corner of the clusters, which is caused by the crossing of the surface level with the Fermi energy. Chemisorptions of oxygen and hydrogen on the ZnO polar surface are also studied by the cluster model.

Angle-resolved photoemission study of the surface state on NbC(111)

Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the surface electronic structure of a NbC0.9(111) single crystal. A strong emission from a surface state is observed just below the Fermi level (EF) which is rapidly quenched by hydrogen adsorption. The dispersion relation of this state is mapped in a surface Brillouin zone along the Γ M and Γ K directions. It is found that the surface state disperses upwards and crosses the Fermi level for both Γ M and Γ K dir dispersion and symmetry of this state are in good agreement with theoretical results for the surface state on TiC(111). The photon energy dependence of the cross section of the surface state is also investigated and some resonance effects are found.

Surface induced state on ZrC(111): Angle-resolved photoemission study

Abstract Angle-resolved photoemission spectroscopy (ARPES) has been used to study the electronic structure of a ZrC 0.9 (1 1 1) surface. A sharp emission is observed at just below the Fermi level in normal emission spectra. This peak is ascribed to the emission from a surface induced state formed through charge redistribution around the surface. The two-dimensional dispersion of the state is found to be small.

Exact occupation statistics for two‐dimensional lattices of single particles

A general expression is developed which describes exactly the ensemble average number of one‐ or two‐dimensional structures per arrangement, created when indistinguishable single particles are arranged on a two‐dimensional lattice. The expression obtained is applied to the calculation of some physically important structures which appear on a rectangular and a closest‐packed hexagonal lattice. The problem of nearest‐neighbor pairs is then solved as a special case from the general expression.