Yasunobu Inoue

Support and promoter effect of ruthenium catalyst. II: Ruthenium/alkaline earth catalyst for activation of dinitrogen

The turnover frequency (TOP) of the ammonia synthesis on Ru was promoted by using alkaline earths as supports. The TOF of this reaction on RuMgO and RuCaO was as high as those on RuCsOHAl2O3 catalyst. The activity was shown to be correlated with the electronegativity of oxide support. XPS results seem to support the electron transfer from alkaline earth to Ru. Activity of RuMgO was further promoted with CsOH, which increased not only TOF but also Ru dispersion. On the other hand, Ru/alkaline earth with K, which may be called “superbase-supported Ru catalysts,” proved to have excellent activities for N2 activation. TOF of ammonia synthesis over those catalyst was as high as that on RuKAC (activated carbon) which has been known to be the best catalyst under these conditions. Furthermore, TOF of N2 isotopic equilibration reaction on these catalyst were several times as high as those on RuKAl2O3 or RuKAC which have been believed to be the most active catalysts for this reaction.

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.

Pt/Al2O3: I. Percentage exposed and its effect upon the reactivity of adsorbed oxygen

Abstract This paper reports the preparation of a set of platinum catalysts on wide-pore silica gel by impregnation with H 2 PtCl 6 aq and by ion exchange with Pt(NH 3 ) 4 2+ . Some catalysts were reduced directly and others after preliminary calcination. Percentages exposed (platinum dispersion) measured at 25 °C by hydrogen chemisorption, hydrogen desorption, and hydrogen-oxygen titration vary from 6.3 to 81%. Hydrogen-oxygen titration gives values which are about 88% of those given by hydrogen chemisorption. Upon storage in air at 25 °C, the content in surface oxygen increases substantially beyond that resulting from the short exposure to oxygen employed in the hydrogen-oxygen titration. Exposure to oxygen at 300 °C results in still a further increase in the content in surface oxygen. The surface oxide resulting from the short exposure to oxygen employed in hydrogen-oxygen titration is rapidly reduced by hydrogen at 25 °C but coverages by oxygen much in excess of those resulting from the short exposures result in much reduced rates of reaction with hydrogen. The effect is structure sensitive: The degree of reduction in rate is much larger on the catalysts with a higher percentage exposed. The percentage exposed of the catalysts with the lowest percentage exposed appears to be larger when stored catalyst exposed to oxygen at 300 °C is reduced at 25 or 100 °C than when it is reduced at 300 °C and swept with argon at 450 °C.

New aspects of heterogeneous photocatalysts for water decomposition

Several new photocatalysts for overall water splitting are described. Under UV light irradiation (270 nm), La-doped NaTaO3 modified with NiO decomposed water into H2 and O2 with extremely high quantum efficiency. Under an optimized condition, the apparent quantum efficiency, which was estimated with numbers of irradiated photons and evolved H2 molecules, reached 56%. New stable photocatalytic materials containing elements with d10 electronic configuration such as In3+ Sn4+ and Sb5+ were developed for overall water splitting. Some mesoporous oxides were proved to be effective photocatalysts. (Oxy)nitrides of some early transition metals, i.e., Ta, Nb and Ti, were found to be stable materials having potentials for H2 and O2 evolutions under visible light irradiation (⪯600 nm). The electronic structures of these photocatalysts are also discussed based on DFT calculation.

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.

Pt/Al2O3: II. Activity and selectivity patterns for methylcyclopropane hydrogenolysis and cyclopentane exchange with deuterium: Comparison with Pt/SiO2

The activity and selectivity of a series of Pt/Al2O3 catalysts for the hydrogenolysis of methylcyclopropane and exchange between deuterium and cyclopentane have been determined. The series investigated included the range from 4.1 to 106 percentage exposed of platinum (Dh). Turnover frequencies, Nt, for methylcyclopropane hydrogenolysis at 0 °C determined after high-temperature pretreatment (H2, Tp > 350 °, 1) were independent of Dh as were the activation energies for i-butane and n-butane formation, 36.4 ± 2 and 42.3 ± 4 kJ mol−1, respectively. For cyclopentane-deuterium exchange at 81 °C the isotopic distribution patterns were clearly dependent upon Dh, particularly noted as a decrease in d10(d2-d10) with increasing Dh. Activation energy for overall exchange was about 75 kJ mol−1 with some dependence on Dh for the individual distributions. Catalyst pretreatment conditions affect activity for both reactions. For methylcyclopropane a minimum is observed for Tp about 200 °C, with an inversion in ordering of Nt vs Dh at higher temperatures, as previously observed for Pt/SiO2. For cyclopentane, Nt decreases with increasing Dh at Tp = 450 °C. The method of catalyst preparation also showed some effect on Nt for methylcyclopropane, but not for cyclopentane. A summary discussion of the factors responsible for various pretreatment effects on Pt/SiO2 and Pt/Al2O3 is given for both reactions.

Ag-ZSM-5 as a Catalyst for Aromatization of Alkanes, Alkenes, and Methanol

Ag-HZSM-5 is an active catalyst for the aromatization of alkanes. In the case of isobutane, the selectivity for aromatic hydrocarbons was 60% at the isobutane conversion of 68% at 773 K. Ag-ZSM-5 is characterized by the low selectivity for methane and ethane compared with Ga-ZSM-5 or Zn-ZSM-5. The analysis of the primary products revealed that Ag-ZSM-5 sharply enhances the C-H bond cleavage of the starting alkanes. On the other hand, both C-H and C-C bond cleavage are enhanced over Ga-ZSM-5 and Zn-ZSM-5. Ag-ZSM-5 is also a selective catalyst for aromatization of alkenes and methanol.

Comparative Studies of Mixed Oxide Perovskite Catalysts, LaCoO3, LaFeO3 and LaAlO3 for Hydrogenation of Alkenes and Hydrogenolysis of Alkanes

Catalysis by perovskite type mixed oxides, LaAlO 3 , LaFeO 3 and LaCoO 3 was comparatively studied. The former two oxides were found to catalyze only the hydrogenation of alkenes in contrast with LaCoO 3 which accelerates not only the hydrogenation but also the hydrogenolysis of alkanes. Ethylene hydrogenation on these three perovskites showed common kinetic features and is explained in terms of the associative mechanism which involves the change in the slow step from hydrogen adsorption to the hydrogenation of the ethyl radical with increasing temperature. Thermal desorption from a LaCoO 3 surface exposed to C 2 H 4 or C 2 H 6 gave spectra of C 2 H 2 and CH 4 produced by C-H and C-C bond dissociation, whereas desorption from LaAlO 2 and LaFeO 3 provided no species other than the original admolecules. The X-ray photoelectron spectrum of LaCoO 3 was compared with those of other perovskites and its variation upon deactivation emphasized the contribution of the Co(III) ion to the C-C bond rupture. A CNDO/MO calculation for C 2 H 6 adsorption on O-M-O-M-0 clusters (M=Fe, Al and Co) confirmed this view. The mechanism of the hydrogenolysis and the structure of the active surface are discussed.