Jean-Alain Dalmon

In situ magnetic characterisation of supported cobalt catalysts under steam-reforming of ethanol

Various supported cobalt catalysts (Co/MgO, Co/Al2O3, Co/SiO2, Co/TiO2, Co/V2O5, Co/ZnO, Co/La2O3, Co/CeO2 and Co/Sm2O3), were characterised by magnetic measurements under ethanol steam-reforming conditions, in situ diffuse reflectance infrared spectroscopy and UV-Vis-NIR diffuse reflectance spectroscopy. Their magnetic behaviour was analysed as a function of reaction temperature and hydrogen treatment and was related to the cobalt species in the catalysts and to their catalytic behaviour in the steam-reforming of ethanol. The catalysts were prepared from Co2(CO)8 and used without previous calcination or reduction. All samples showed a paramagnetic or diamagnetic behaviour before reaction. The catalysts that performed well in the steam-reforming of ethanol presented under reaction conditions, metallic (ferromagnetic) cobalt particles and oxidised cobalt species. An easy exchange between small metallic cobalt particles (produced under H2 or under ethanol steam-reforming conditions) and oxidised cobalt species (produced under ethanol steam-reforming conditions) was found in these catalysts.

Magnetic and infrared study of CO chemisorption on silica supported nickel-copper alloys

CO adsorption at room temperature on Ni-Cu alloys supported on SiO2 is studied by two complementary techniques, infra-red spectroscopy and magnetic methods (saturation magnetization). The bond number between CO and the metallic surface calculated from magnetic data decreases from 1.8 to 1 as the Cu content increases. Two bands attributed to CO bonded to Ni are observed (the A band in the 2000–2050 cm−1 region, and the B band in the 1950–1900 cm−1 region). A small band assigned to CO bonded to Cu is also detected. As Cu content increases, the intensity of the B band decreases, and a noticeable and continuous frequency shift of the three bands is observed. Experimental results are fully accounted for assuming that: (i) two adsorbed species of CO on Ni, a monodentate and a bridged species (with small amounts of other multicentered species) are formed, as suggested by Eischens and Pliskin; (ii) dilution of Ni by Cu decreases the relative abundance of the bridged (and multicentered) species for some geometric reasons previously invoked by Soma-Noto and Sachtler; (iii) surface complexes are formed between CO and Ni; however Ni remains in its metallic state. The surface complex is sensitive to the electronic environment of the metallic atom, with a frequency shift of the three infra-red bands upon alloying as a consequence.

Influence of the nature of the support on the reducibility and catalytic properties of nickel: evidence for a new type of metal support interaction

Abstract Various nickel catalysts deposited on classical supports (Si0 2 , A1 2 0 3 , MgO) and on less conventional materials (TiO 2 , ThO 2 , CeO 2 , ZrO 2 , Cr 2 0 3 ) were prepared under conditions in which the strong metal support interaction of the Pt/TiO 2 -type does not occur. Their catalytic properties for ethane hydrogenolysis and carbon monoxide hydrogenation and the reducibility of the nickel phase deduced from magnetic measurements were investigated. Activity toward ethane hydrogenolysis per unit area varies by two orders of magnitude when the nature of the support is changed. A correlation between activity and nickel reducibility is demonstrated and interpreted in terms of geometric effects of dilution due to the presenceof unreduced surface-nickel species. These dilution effects are shown to play an important role in the case of the CO + H 2 reaction. However, additional effects leading to changes in apparent activation energy (not yet well understood but possibly linked to the carbon monoxide coverage) result in a poorer correlation between activity, selectivity toward C 2+ and nickel reducibility. These correlations lead to an interest in designing further experiments to study what can be termed the redox metal support interaction, which should not be confused with the strong metal support interaction.

Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions

Abstract The dehydrogenation of isobutane has been studied in a membrane reactor using either a mesoporous alumina membrane or a microporous zeolite membrane. It has been shown that for both membranes an increase in the isobutene yield is observed when compared to a conventional reactor. However, these increases are related to two different phenomena: a complete mixing of reactants, products and sweep gas in the case of the mesoporous membrane and a continuous separation of hydrogen when the microporous zeolite membrane was used.

Isobutane dehydrogenation in a membrane reactor influence of the operating conditions on the performance

Abstract Isobutane dehydrogenation has been investigated in a membrane reactor combining a bimetallic PtIn/zeolite fixed-bed catalyst and a microporous MFI-alumina tubular membrane. The membrane reactor performance has been studied as a function of the feed and sweep flow rates and of the sweep (co- or counter-current sweep modes). Isobutene yields up to four times higher than that observed in a conventional reactor have been obtained. Depending on the conditions, it is shown that the performance of the membrane reactor is controlled either by the membrane or by the catalyst.

Zeolite membrane reactor for isobutane dehydrogenation: Experimental results and theoretical modelling

Dehydrogenation of isobutane has been studied in a packed-bed zeolite membrane reactor using Pt–In catalyst. Enhanced dehydrogenation yields were obtained in the membrane reactor due to the separation of hydrogen from the reaction medium. Two sweeping modes were studied, namely the co-current and the countercurrent modes. Although the separation factor is higher in countercurrent than in co-current, the yield of reaction in these two sweeping modes is quite the same. The dehydrogenation reaction was limited by transport properties of the membrane in co-current mode while in countercurrent mode the limitation by kinetics was predominant. Reactor operation was described through theoretical modeling. A good agreement between the model and the experimental values was obtained with a co-current sweep gas. In countercurrent mode, the values predicted by the model for the yield of isobutene were slightly overestimated, due to hydrogen removal which induced catalyst local deactivation, or because the kinetic rate is inadequate when we are working far from the neighborhood of the equilibrium.

Characterization of a zeolite membrane for catalytic membrane reactor application

Abstract This paper describes the morphological and transport properties of a composite zeolite (silicalite)-alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported.

Characterization of nickel catalysts by chemisorption techniques, x-ray diffraction and magnetic measurements: Effects of support, precursor and hydrogen pretreatment

Two series of supported nickel catalysts were prepared by using alumina, silica and titania as supports and nickel nitrate or nickel chloride as impregnating salts. The catalysts, prereduced with hydrogen in the range 300–700°C, were characterized by adsorption of hydrogen and oxygen, X-ray diffraction (XRD) and magnetic methods. Strong effects of the nature of the support, of the nickel salt precursor and, in a few instances, of the reduction temperature on the adsorptive and textural properties of nickel catalysts were observed. For the series prepared from nickel nitrate, alumina support gave the highest dispersions of nickel, which varied only slightly with the reduction temperature, whereas the dispersion of titania-supported catalysts decreased significantly when the reduction temperature was increased. In contrast, the series prepared with nickel chloride always exhibited low metal dispersions which were nearly independent of the nature of the support and the reduction temperature. A strong decrease in hydrogen adsorption was observed on all samples prepared from nickel chloride. This decrease was recorded, for the nitrate preparation series, only on Ni/TiO2 reduced at 500 and 700°C and on Ni/SiO2 reduced at 700°C, which, in this instance, may be related to a strong metal-support interaction. On the other hand, oxygen chemisorption took place on all catalysts, allowing the determination of their metallic dispersion. Nickel crystallite sizes calculated from oxygen chemisorption were in good agreement with those determined from XRD and magnetic measurements, provided that the adsorption stoichiometry O/Nis=2 is assumed for typical catalysts whereas O/Nis = 1 should be applied to Ni/TiO2 under the strong metal-support interaction state.

Nitrobenzene liquid-phase hydrogenation in a membrane reactor

Abstract This work presents the potentialities of a catalytic membrane reactor in a gas-liquid-solid reaction. A catalytic membrane has been prepared via well-controlled platinum deposition within the porous framework of a γ-Al 2 O 3 mesoporous membrane. The hydrogenation of nitrobenzene to aniline has been performed using the catalytic membrane as an active gas-liquid contactor. Some operating parameters controlling the membrane reactor performance have been explored and compared with the behaviour of conventional reactors.

Membrane reactor for selective oxidation of butane to maleic anhydride

A simulation of a packed-bed membrane reactor acting as an oxygen distributor for the selective oxidation of n-butane to maleic anhydride (MA) has been performed by recreating specific reactive atmospheres in a microreactor. In the membrane reactor, the oxidation state of the catalyst depends on its position in the bed, leading to an important change in the MA yield. However, this heterogeneity can be turned to an advantage using a reverse of n-butane flow. Co-promoted catalysts have also been developed to enhance the global performance of the membrane reactor.