Nobutsune Takezawa

New Supported Pd and Pt Alloy Catalysts for Steam Reforming and Dehydrogenation of Methanol

The catalytic performances of supported Group 810 metal (Co, Ni, Ru, Pd, Ir and Pt) catalysts for steam reforming of methanol, CH3OH + H2O → CO2 + 3H2, and dehydrogenation of methanol to methyl formate, 2CH3 OH → HCOOCH3 + 2H2, are markedly affected by the kinds of supports as well as the metals used. The selectivity for steam reforming and the formation of methyl formate was markedly improved when Pd or Pt were supported on ZnO, In2O3 and Ga2O3. The combined results of temperature-programmed reduction, XRD, XPS and AES revealed that Pd-Zn, Pd-In, Pd-Ga, Pt-Zn, Pt-In and Pt-Ga alloys were formed upon reduction. Over the catalysts having an alloy phase, the reactions proceeded selectively, whereas over the catalysts having a metallic phase, methanol was decomposed to carbon monoxide and hydrogen predominantly. It was shown that the reactivity of formaldehyde intermediate over the Pd and Pt alloys was markedly different from that over metallic Pd and Pt. Over Pd and Pt alloys, aldehyde species were stabilized and transformed into carbon dioxide and hydrogen or methyl formate by nucleophilic addition of water or methanol, respectively. By contrast, over metallic Pd and Pt, aldehyde species were rapidly decarbonylated to carbon monoxide and hydrogen.

New catalytic functions of Pd-Zn, Pd-Ga, Pd-In, Pt-Zn, Pt-Ga and Pt-In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga2O3 and In2O3 exhibit high catalytic performance for the steam reforming of methanol, CH3OH+H2O→CO2+3HH2, and the dehydrogenation of methanol to HCOOCH3, 2CH3OH→HCOOCH3+2HH2. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.

Highly selective supported Pd catalysts for steam reforming of methanol

Steam reforming of methanol, CH3OH + H2O → 3H2 + CO2, was carried out over various Pd catalysts (Pd/SiO2, Pd/Al2O3, Pd/La2O3, Pd/Nb2O5, Pd/Nd2O3, Pd/ZrO2, Pd/ZnO and unsupported Pd). The reaction was greatly affected by the kind of support. The selectivity for the steam reforming was anomalously high over Pd/ZnO catalysts.

Characterization of copper/zirconia catalysts prepared by an impregnation method

Abstract Copper/zirconia catalysts were prepared by an impregnation method and were characterized by UV—visible—near-infrared (UV—VIS—NIR) spectroscopy, X-ray diffraction (XRD), differential thermal analysis (DTA) and temperature-programmed reduction (TPR). It was concluded that copper-ammine complexes held on the surface of the zirconia support exist in three different states, viz., isolated and clustered copper(II) ions and tetraamminecopper (II) nitrate. These precursor species were transformed into bulky copper (II) oxide through highly dispersed copper (II) oxide at various temperatures, which depend on the calcination temperatures of the catalysts and the zirconia supports. In the TPR experiments, two TPR peaks were observed at 493 K (peak I) and 515–613 K (peak II). In conjunction with the observations by UV—VIS—NIR and XRD, it was concluded that highly dispersed copper(II) oxide and bulky copper (II) oxide were reduced to metallic copper, giving peaks I and II, respectively. The relative amounts of these precursor species were found to depend strongly on the conditions of preparation of both catalysts and supports.

Methanol-reforming reaction over copper-containing catalysts—The effects of anions and copper loading in the preparation of the catalysts by kneading method

Methanol-reforming reaction CH3OH + H2O = 3H2 + CO2 was carried out over copper-containing catalysts which were prepared from hydroxides of copper or from the hydroxide kneaded with various metal oxides. The specific activity (activity per weight of copper used) either of supported or support-free catalyst was markedly increased when the hydroxide was prepared from alkali solution with addition of copper salt solution at higher pH or when the weight percentage of copper on the support was decreased. However, other kinetic parameters such as activation energy and selectivity of the reaction were unaffected by the preparation of the catalyst unless copper chloride was employed as a starting material of the hydroxide preparation at lower pH. DTA, ir, XPS, AES and other chemical analyses of the catalysts revealed that hydroxide ion in the hydroxide precipitate prepared at lower pH exchanged in part with the anionic group of its starting material during the course of the preparation. The anion or its fragment was found to be strongly held on the surface and inhibited the reaction to a great extent. On the other hand, the anion held was markedly decreased when the catalyst was prepared at higher pH. This catalyst was found to be highly active for the title reaction. The surface area of metallic copper was considerably increased when copper was kneaded with the support. This gave rise to the increase in the specific activity of the catalyst. The effect of the support upon the reaction was examined for the catalyst kneaded with silica, manganese dioxide, ferric oxide, titanium oxide, calcium oxide, and nickel oxide. However, the supports other than nickel oxide exerted no chemical effect upon the reaction. Nickel oxide was suggested to be reduced during the course of the reaction and metallic nickel thus formed catalyzed the methanol decomposition reaction CH3OH = CO + 2H2.

The mechanism of steam reforming of methanol over a copper-silica catalyst

Methanol steam reforming CH3OH + H20 → C02 + 3H2 was carried out over a copper-silica catalyst. It was concluded that the reaction proceeded through the route (1) 2CH3OH → HCOOCH3 + 2H2 (2) HCOOCH3 + H2O → HCOOH + CH3OH (3) HCOOH → C02 + H2 in which no carbon monoxide took part. When ethanol was admitted with water, acetic acid and ethanol were formed through steps analogous to reactions (1) and (2) above.

Mechanism of the reverse water gas shift reaction over Cu/ZnO catalyst

The reverse water gas shift reaction (RWGS) and the reaction with CO{sub 2} alone were carried out over a Cu/ZnO catalyst. The surface of the catalyst was characterized by N{sub 2}O titration, XPS, and FT-IR spectroscopy. CO{sub 2}dissociated to give CO and the surface oxygen species. Surface Cu(I) oxide was formed by the reaction with CO{sub 2}. The oxygen species were hydrogenated to H{sub 2}O and the surface Cu(I) oxide was reduced to metallic Cu. It is suggested that the RWGS reaction proceeds through surface oxidation and reduction with CO{sub 2} and H{sub 2}, and the dissociation of CO{sub 2} is the rate determining step.

Formation and reactivity of isocyanate (NCO) species on Ag/Al2O3

The formation of the reactivity of isocyanate species have been studied over Ag/Al2O3 by IR spectroscopy and mass spectrometry. Adsorbed CxHyNOz and NO3− species are produced by reaction among NO, O2 and C3H6 at room temperature. Thermal decomposition of adsorbed CxHyNOz species leads to the formation of two types of NCO species (NCO on Ag and NCO on Al2O3) above 423 K. These NCO species are thermally stable in vacuum at 673 K, while adsorbed NO3− species decompose completely. The NCO species are highly reactive toward NO+O2 at room temperature, being converted into N2, CO2, CO and a small amount of N2O. The NCO species are less active in NO or O2 alone than in the mixture of NO and O2. Thus, excess oxygen added in the NO reduction by C3H6 plays an important role in the formation of adsorbed CxHyNOz species and in the reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed CxHyNOz and adsorbed NCO is essential for the progress of the NO reduction with C3H6 in the presence of O2 under the present experimental conditions.

Difference in the selectivity of CO and CO2 methanation reactions

CO and CO2 methanation at steady states and under transient states was conducted over Ni, Ni/Al2O3, Ni/SiO2 and Ru/SiO2. The CO2 methanation proceeded highly selectively as compared with the CO methanation. In the CO methanation, weakly adsorbed CO retarded the hydrogenation of surface carbon species. In the CO2 methanation, the retardation was absent. The selectivity for the CO2 methanation was estimated from the selectivity for the CO methanation, the extent of the retardation and the ratio of the steady state rates of these reactions. The selectivity estimated was in fair agreement with that experimentally obtained.

Mechanisms of methanation of carbon dioxide and carbon monoxide over nickel/alumina catalysts

Abstract By the use of diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and the temperature-programmed reaction (TPRx) method, it was shown that two types of adsorbed CO in bridged structures (referred to as weakly adsorbed bridged CO(a) and strongly adsorbed bridged CO (a) species) were predominantly present along with formate species in the methanation of CO 2 on Ni/Al 2 O 3 . The amount of carbidic carbon species (referred to as C (a) species) and adsorbed CO in linear structures (referred to as linear CO (a) species) were practically negligible. In the methanation of CO, a considerable amount of linear CO(a), weakly and strongly adsorbed bridged CO(a) and C(a) species was present along with formate, methozide and surface hydrocarbon species. C (a) species were produced by dissociation of strongly adsorbed bridged CO (a) species, being hydrogenated to methane. On the basis of the reactions under transient states and those of C (a) species with H 2 , CO 2 /H 2 and CO/H 2 , it was suggested that one of the steps involved in the hydrogenation of C (a ) species to methane was markedly retarded in the presence of linear CO (a) species.