Applied Catalysis A, General (v.259, #1)


Deactivation of Pt·PdO/Al2O3 in catalytic combustion of methane by Yasushi Ozawa; Yoshihisa Tochihara; Ayako Watanabe; Masatoshi Nagai; Shinzo Omi (1-7).
Catalytic combustion is being widely studied as a means of controlling the NO x of combustors. However, the poor durability of palladium-based catalysts hinders the development of catalytic combustors for the methane combustion reaction. The effects of the addition of platinum to PdO/Al2O3 on catalyst deactivation at 1073 K were studied using a fixed-bed flow reactor at atmospheric pressure. The adaptability of various deactivation equations to the data of methane combustion was investigated. The deactivation behavior of the catalysts could be expressed by an equation based on two kinds of deactivation species. The addition of Pt prolongs the lifetime of the PdO/Al2O3 catalyst by decreasing the initial activity. This is due not to the transformation of PdO to Pd–Pt but to the prevention of the growth of PdO and Pd–Pt particles during the reaction.
Keywords: Palladium; Platinum; Additive effect; Catalytic combustion; Methane; Deactivation; TPD;

The hydroconversion of n-heptane on 1 wt.% palladium supported on sulfated zirconium pillared montmorillonite (PdZMS) under isothermal and isobar conditions (P HC=7.99 kPa and P Tot=101.3 kPa) is studied. The catalyst activity of PdZMS is compared to those obtained on 1 wt.% palladium supported on γ-alumine (PdAl) and on sulfated zirconia (PdZS).The PdZMS catalyst develops an intermediary catalytic behavior between that of PdAl and that of PdZS and shows a very important isomerization selectivity at 300–330 °C. The monobranched isomers are the major reaction products formed on this catalyst and suggest a classical bifunctional mechanism.On the PdAl catalyst, it is rather a metallic mechanism charaterized by hydrogenolysis on Pd particles (demethylation reaction) and by a bond shift isomerization occurring through a cyclic intermediate.On the PdZS catalyst, a high selectivity in cracking products (propane and butane) suggests a mechanism involving a cyclopropanic protonated intermediate, followed by a β-scission attributed to the strong acidity of the sulfated zirconia.
Keywords: Zr-Pillared montmorillonite; Acidity; n-Heptane isomerization;

The effect of the impregnation method (co-impregnation and sequential) and the incorporation order of the active phases, Ni and Mo, on the structure and catalytic activity of NiMo hydrotreating catalysts supported on an Al-pillared montmorillonite has been investigated. The catalysts were characterised by X-ray diffraction, N2 adsorption, thermogravimetric analysis, temperature-programmed reduction, UV-Vis diffuse reflectance, in situ laser Raman, and X-ray photoelectron spectroscopies, and their catalytic behaviour tested in the hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of gas oil. It was found that the catalyst prepared by co-impregnation of Ni and Mo was more active than those prepared by sequential impregnation, and of the latter ones, the most active was that in which Mo was impregnated prior to Ni. Characterisation results revealed that by co-impregnation, the dispersion of the Mo phase was significantly increased due to the presence of Ni and, on the other hand, more Ni stay together to Mo on the external surface of the clay, favouring thus the formation of Ni-Mo–O interaction species, precursors of the “Ni-Mo–S” active phase. The relatively low activity of the sequentially impregnated Mo-Ni/Al-PILC catalyst was due to the absence of synergism between Ni and Mo because most of Ni was as Ni2+ ions into the octahedral layers of the clay, separated from the Mo phase. All NiMo bimetallic catalysts exhibited initially a better selectivity towards HDN than towards HDS reaction, probably due to their acidity properties.
Keywords: Hydrodesulphurisation; Hydrodenitrogenation; Al-pillared clays; NiMo catalysts;

Photocatalytic property of bismuth titanate Bi2Ti2O7 by Wei F. Yao; Hong Wang; Xiao H. Xu; Jing T. Zhou; Xue N. Yang; Yin Zhang; Shu X. Shang (29-33).
Bi2Ti2O7 crystals with pyrochlore structure were prepared by the chemical solution decomposition (CSD) method. The band gap of Bi2Ti2O7 crystals was estimated to be about 2.95 eV from the onset of UV-Vis absorption spectra of the photocatalyst. The photocatalyst based on Bi2Ti2O7 crystals for photodecolorization of methyl orange was examined. The Bi2Ti2O7 photocatalyst exhibits higher photocatalytic activity than the Degussa P-25.
Keywords: Photocatalyst; CSD; Bi2Ti2O7;

Effects of lanthanum addition on Ni-B/γ-Al2O3 amorphous alloy catalysts used in anthraquinone hydrogenation by Yongjiang Hou; Yaquan Wang; Fei He; Wanliang Mi; Zhenhua Li; Zhentao Mi; Wei Wu; Enze Min (35-40).
A series of amorphous Ni-B/γ-Al2O3 catalysts with different La loadings were prepared by KBH4 reduction, characterized by inductively coupled plasma (ICP), DSC, H2-TPD, X-ray photoelectron spectroscopy (XPS) and H2-chemisorption, and studied in the hydrogenation of anthraquinone (AQ). In comparison with the catalyst without La loadings, the presence of proper amounts of lanthanum in the supported amorphous alloy catalyst leads to more active centers, higher extents of H2-chemisorption, the shift of H2 desorption peaks to lower temperatures, the enhancement of thermal stability as well as higher reaction rates and higher TOF values. The effects of lanthanum were attributed to both its structure effect, dispersing Ni, leading to more active centers and enhancing the thermal stability, and its electronic effect, resulting in electron-rich Ni, weakening the adsorption of anthraquinone and the bond strength of NiH and therefore activating the adsorbed hydrogen. However, higher La loadings were harmful to the activity because too many surface Ni active centers might be covered by La3+ species.
Keywords: Ni-B/γ-Al2O3 amorphous catalyst; Anthraquinone; Hydrogenation; Lanthanum;

Methane catalytic combustion over Pd/Al2O3 in presence of sulphur dioxide: development of a deactivation model by Salvador Ordóñez; Paloma Hurtado; Herminio Sastre; Fernando V. Dı́ez (41-48).
The effect of the concentration of sulphur dioxide (in the range 15–65 ppm) and temperature (350–550 °C) on the deactivation of a commercial Pd/Al2O3 catalyst used in the oxidation of methane has been studied. It was observed that, although at a given temperature a common steady-state of constant methane conversion was attained for all sulphur dioxide concentrations, the deactivation rate increased as sulphur dioxide concentration in the feed increased. On the other hand, the deactivation rate decreased as temperature increased the activity at steady state increasing as temperature increased.The behaviour of the system has been modelled considering that the methane combustion follows a pseudo-first-order with respect to methane kinetics, whereas the deactivation is caused by the formation of surface Pd sulphate, which is less active than Pd oxide. The rate of formation of Pd sulphate is considered to be first-order for both sulphur concentration and fraction of unsulphated Pd. The proposed deactivation model, in addition to being in good agreement with the deactivation mechanisms proposed in the literature for this process, provides a good fit for the experimental results for which the deactivation may be assumed to be caused by sulphur poisoning only.
Keywords: Catalyst deactivation; Methane oxidation; Sulphur poisoning; Palladium catalyst; Deactivation modelling;

The catalytic activity and selectivity of a series of rhodium(I), (II) and (III) complexes have been examined in a model hydrosilylation reaction of 1-octene with triethoxysilane and in a more applied system composed of oligomeric allyl ethoxylate alcohol and poly(dimethylsiloxane-co-methylsiloxane). The results were compared with the catalytic activity of the widely used Karstedt’s system (Pt2[(ViSiMe2)2O]3 (1)). The tris(3-phenyl,5-methylpyrazol-1-yl)borate (TpPh,Me) dicarbonyl rhodium(I) compound TpPh,MeRh(CO)2 (2) was found to be more active in both reactions than (1), but in case of hydrosilylation of the polymeric species, (2) was less selective leading to a higher amount of the undesired Markovnikov addition. The catalytic activity of the other rhodium complexes Rh(Hdmg)2(PPh)3Cl (3), [Rh(Hdmg)2(PPh3)]2 (4), [Rh(Hdmg)2(PPh3)2]ClO4 (5), TpPh,MeRh(CO)(PPh3) (6), Rh(CH3COO)(PPh3)3 (7), [Rh(Hdmg)2(ClZndmg)(PPh3)]2 (8) (Hdmg = diemthylglyoximate) was similar to or somewhat lower than 1. No clear relationship was found between the formal oxidation state of rhodium and the catalytic activity of its complex, which is in contrary to theoretical suggestions.
Keywords: Hydrosilylation; Rhodium; Alkene; Triethoxysilane; Alkene; Polyether; Homogeneous catalysis;

Addition of La and Sn to alumina-supported Pd catalysts for methane combustion by M.A. Fraga; E. Soares de Souza; F. Villain; L.G. Appel (57-63).
The effects brought about by introducing both a stabiliser cation and a potentially promoting oxide phase, namely lanthanum and tin oxide, respectively, on a conventional support for Pd-based catalysts for methane combustion were evaluated. The characteristics of both supports and catalysts were analysed by XRD, LRS, XPS and PdO decomposition; their catalytic activity was evaluated in the combustion of methane under lean conditions (O2/CH4=4). The behaviour of these systems were also evaluated after high temperature treatment, carried out under conditions similar to those created during the combustion of methane. The results shown that the addition of tin and La on PdO/Al2O3 improves the thermal stability of Al2O3 and PdO but did not bring any benefit to activity in methane combustion.
Keywords: Methane; Catalytic combustion; Tin oxide; Lanthanum; Alumina stabilisation;

Isomerization of n-hexane on the Pt-promoted Keggin and Dawson tungstophosphoric heteropoly acids supported on zirconia by A.V. Ivanov; T.V. Vasina; V.D. Nissenbaum; L.M. Kustov; M.N. Timofeeva; J.I. Houzvicka (65-72).
The activity of the Pt-promoted systems prepared on the basis of Keggin H3PW12O40 (HPW) and H3PW11Zr (HPW11Zr), and Dawson H6P2W18O62 (HP2W18) and H6P2W21O71 (HP2W21) heteropoly acids supported on zirconia in the isomerization of n-hexane was studied. The Pt/HPW/ZrO2 catalytic system shows high activity in n-hexane isomerization. The yield of isohexanes was close to 80% and the selectivity was 96–98% at 190 °C. The DTA and FTIR spectroscopic studies allowed us that partially distorted grafted Keggin ions are responsible for the catalytic activity. However, these species cannot be formed by dispersion of HP2W n and HPW11Zr heteropoly acids because of their insufficient stability. The support of these systems on ZrO2 is accompanied by significant structural transformations. The Pt/HP2W n /ZrO2 or HPW11Zr/ZrO2 catalytic systems shows the best performance (yield is close to 80%) at higher temperatures (30–40 °C) than Pt/HPW/ZrO2. However, these systems show higher selectivity due to lower density of active sites and inhibition of bimolecular alkylation–cracking side reactions.
Keywords: Isomerization; Heteropolyacid; Zirconia; n-Hexane; Platinum;

Liquid-phase hydrogenation kinetics of isooctenes on Co/SiO2 by Mikko S Lylykangas; Petri A Rautanen; A.Outi I Krause (73-81).
The kinetics of the hydrogenation of isooctenes (2,4,4-trimethyl-1-pentene, TMP-1, and 2,4,4-trimethyl-2-pentene, TMP-2) to isooctane (2,2,4-trimethylpentane) was studied on a commercial Co/SiO2 catalyst in a three-phase laboratory-scale reactor. The terminal double bond in TMP-1 was substantially faster hydrogenated than the internal double bond in TMP-2. Double bond isomerization was found to be of no importance under the applied reaction conditions. Kinetic parameters were estimated for power-law rate equations as well as for equations based on the Horiuti–Polanyi mechanism assuming competitive and non-competitive adsorption of the alkenes and hydrogen. All models described the experimental data accurately with physically reasonable values of parameters (E app 34–35 kJ/mol for TMP-1 and 40–43 kJ/mol for TMP-2). The hydrogenation rate per catalyst mass was considerably lower on cobalt than on nickel, evidently mainly due to the smaller number of active sites.
Keywords: Hydrogenation; Kinetics; Isooctene; Isooctane; Cobalt; Liquid-phase;

CO formation/selectivity for steam reforming of methanol with a commercial CuO/ZnO/Al2O3 catalyst by H Purnama; T Ressler; R.E Jentoft; H Soerijanto; R Schlögl; R Schomäcker (83-94).
A study of CO formation for steam reforming of methanol on a commercial CuO/ZnO/Al2O3 catalyst has been performed in the temperature range 230–300 °C and at atmospheric pressure. The reaction schemes considered in this work are the methanol–steam reforming (SR) reaction and the reverse water gas-shift (rWGS) reaction. Power rate laws for the SR and reverse WGS reactions were used in a refinement of rate equations to the experiment data. For the temperature range studied the reaction order of methanol was determined under differential conversion (less than 10%) and was found to be 0.2. The integral method (partial pressure of the reactants and products measured as a function of contact time) was then applied to determine the reaction rate constants, activation energies, and pre-exponential factors for both reactions. The experimental results of CO partial pressure as a function of contact time at different reaction temperatures show very clearly that CO was formed as a consecutive product. The implications of the reaction scheme, in particular with respect to the production of CO as a secondary product, are discussed in the framework of on-board production of H2 for fuel cell applications in automobiles. Potential chemical engineering solutions for minimizing CO production are outlined.
Keywords: Methanol–steam reforming; Commercial catalyst CuO/ZnO/Al2O3; Reaction kinetics; CO formation; Intraparticle diffusion limitation; Dilution effect; Reverse water gas-shift reaction;

Dual catalyst bed concept for catalytic partial oxidation of methane to synthesis gas by J. Zhu; M.S.M.Mujeebur Rahuman; J.G. van Ommen; L. Lefferts (95-100).
A system with two catalyst beds instead of one single metal catalyst bed is proposed for catalytic partial oxidation of methane (CPOM) to synthesis gas. In this dual catalyst bed system, an irreducible stable oxide, such as yttrium-stabilized zirconia (YSZ), is used in the first catalyst bed to obtain selective oxidation to syngas with significant highly exothermic deep oxidation occurring as well. This feature results in milder temperature profiles in the reactor because less heat is liberated compared to initial deep oxidation as, e.g. on metal catalysts. More importantly, all oxygen is completely consumed in the oxide catalyst bed. The second bed comprises a metal catalyst, e.g. Co-based, for reforming methane with H2O and CO2 exclusively. In this way the catalysts are exposed to less extreme temperatures and, exposure of metallic catalysts to oxygen at high temperatures is prevented. Therefore catalyst deactivation via evaporation of precious metal oxides is circumvented. Finally, synthesis gas with an equilibrium composition (almost 100% CO and H2 yields) is produced.
Keywords: Partial oxidation of methane; Yttrium-stabilized zirconia; LaCoO3; Dual catalyst beds;

Role of additives in cobalt-mediated oxidative crosslinking of alkyd resins by Stefania Tanase; Elisabeth Bouwman; Jan Reedijk (101-107).
Spectroscopic and chromatographic studies have been carried out to investigate the interaction between bis(acetylacetonato)cobalt(II) and pyrazole ligands as possible anti-skinning additives, and their role in the oxidative crosslinking reactions has been tested. From experimental results (UV-Vis, TR-FTIR, SEC) it has been concluded that Co(acac)2 can be used as drier promoter for alkyd paints and its catalytic activity is similar to that of the actual commercial drier (Co Nuodex). The presence of pyrazole ligands as anti-skinning additives precludes association of Co(acac)2 that usually occurs in nonpolar media and it facilitates the formation of stable coordinated ROO• radicals, that inhibit the radical chain reaction. From EPR results it has been established that the stability of the [CoIII(acac)2(ROO•)L]+ complex radicals is highly dependent of the molar ratio between the cobalt catalyst and the organic additive L. The dependence of anti-skinning activity on the concentration of the pyrazole additives has been studied and a mechanism of inhibiting properties of azoles on the oxidative crosslinking reactions has been proposed.
Keywords: Cobalt; Drier; Ethyl linoleate; Alkyd paint; Pyrazole; Anti-skinning; Coating;

Propyne hydrogenation over alumina-supported palladium and platinum catalysts by D.R Kennedy; G Webb; S.D Jackson; D Lennon (109-120).
Propyne hydrogenation over alumina-supported palladium and platinum catalysts at 293 K has been investigated under pulse-flow conditions. Catalytic activity and selectivity is shown to be critically dependent on hydrogen concentration. An equimolar C3H4:H2 mixture was 100% selective to propene for the Pt/Al2O3 catalyst, whereas the Pd/Al2O3 catalyst produced both propene and propane in comparable amounts. Correlations in activity, carbon mass balance and variations in the product distribution for excess hydrogen mixtures indicate distinct regions of the catalyst conditioning process, as the catalyst approaches steady state operation. The active site for propene formation is attributed to a hydrocarbonaceous overlayer, formed during the early stages of the reaction sequence. Comparisons with comparable studies on silica-supported palladium and platinum catalysts suggest that the metal–support interaction plays a role in defining the hydrogen transfer characteristics of the overlayer. Furthermore, it is tentatively suggested that, for supported Pd catalysts, the degree of carbon retention under reaction conditions is structure sensitive, with larger metal particles exhibiting a greater degree of carbon laydown.
Keywords: Propyne hydrogenation; Palladium; Platinum;

This paper reports a study on the influence of calcination temperature on the catalytic behavior of the Au/iron oxide systems in the water–gas shift reaction. The catalytic activity has been found to be strongly dependent on the catalyst pretreatment, decreasing on increasing the calcination temperature. Detailed characterization of the uncalcined and the calcined sample at different temperatures, employing BET analysis, XRD, H2-TPR, HRTEM and XPS, has shown that the catalytic behavior is related to the gold state and/or the particle size and the properties and structure of iron oxide support. Both the oxidation state and dispersion of gold and the phase transition and its crystallinity and reductive property of the support exert great influence on the catalytic performance of the gold/metal oxide. The wave of the activity within the middle reaction temperature range (i.e. 523–623 K) could be attributed to the difference of the operating temperatures belonging to the gold species and the active magnetite phase.
Keywords: Water–gas shift; Calcination; Supported gold catalyst; Iron oxide;