G.A. Martin

Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide

Abstract Several supported nickel catalysts were tested for the methane reforming reaction at 700°C. The initial activity is found to depend essentially on the state of the nickel phase (reduction and dispersion) and little on its environment (support, additive). The product distribution is controlled by the WGS equilibrium. The zero order aging process is mostly due to carbon deposition, though slight Ni sintering also occurs. Among the deposited carbon, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles. Another form, less stable and arising from methane activation, is rapidly accumulated on the catalyst, but at a low level and limited extent.

Adsorption of CO on well-defined Ni/SiO2 catalysts in the 195–373 K range studied by infrared spectroscopy and magnetic methods

CO adsorption on NiSiO2 catalysts has been studied by two complementary techniques, infrared spectroscopy and saturation magnetization. Special care was taken to measure the degree of reduction and the metallic particle size. Two main bands attributed to CO bonded to Ni are observed: the A band at 2070-2040 cm−1, and the B band at 1935 cm−1 with a shoulder at 1800 cm−1. For completely reduced samples, the ratio r = Aa(Aa + ab) of the integrated intensities of A and B bands, as well as the bond number n calculated from magnetic measurements (n = 1.85) are particle size independent in the 2.5–9.5 nm range, temperature independent in the 20–100 °C range, and coverage independent. On partially reduced samples, n is smaller and r higher. Experimental results are fully accounted for by assuming that the A band corresponds to a linear form and that the B band is a bridged species, the shoulder at 1800 cm−1 being attributed to multicentered species. Ni2+ or NiO present on partially reduced samples acts as a diluent of metallic nickel atoms, similar to Cu atoms in NiCu alloys; the stability of the concentration ratio of linear and bridged species suggests an analogy with coordination complexes. The presence of two bands in the region corresponding to linear species (2070 and 2040 cm−1) with intensities ratios varying with the degree of reduction, Ni particles size, coverage, and temperature is also discussed.

Adsorption of hydrocarbons and various gases on Ni-SiO2 catalysts studied by high field magnetic methods

Abstract The adsorption of various gases (H 2 , H 2 S, alcanes, alcenes and alcynes containing up to five carbons, benzene, cyclohexane, cyclopentane, and some mixtures) on Ni-SiO 2 catalysts has been studied by measuring variations of saturation magnetization. The adsorption has been performed at −78 °C and the system heated in steps. When coverage is small intermediate adsorbed compounds thermally stable have been detected. The observed bond number is even (2, 4, 6, 8) and it corresponds to strongly dehydrogenated states for alcanes, alcenes, cyclanes and benzene, and to an associative model for acetylene and propyne. Complete cracking occurs at relatively low temperatures (50 °C to 200 °C) with formation of Ni 3 C and NiH: the greater the saturation of the hydrocarbon, the smaller its carbon number, and the lower is its cracking temperature. Sulfur of H 2 S is rapidly and irreversibly dissolved into the bulk of nickel. These results which are not always in good agreement with those obtained by low field techniques, strongly supports the following view: nickel atoms, on which a molecule is adsorbed cease to participate to the collective magnetism.

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.

Reactions of CO and CO2 on NiSiO2 above 373 K as studied by infrared spectroscopic and magnetic methods

Reactions of CO and CO2 on NiSiO2 catalysts have been studied in the 373–700 K range by ir, magnetic (saturation magnetization), and volumetric measurements. At room temperature, CO is adsorbed as a mixture of the linear form, NiCOads, and the bridged form, Ni2COads. Upon heating the system beyond 400 K, these forms are irreversibly transformed into new CO adspecies bonded to four Ni atoms, Ni4COads, with a vco frequency at about 1830 cm−1. This multibonded species is in equilibrium with gaseous CO2 according to the reaction 2 CO CO2 + C; the remaining carbon atom is reversibly dissolved into the bulk in an interstitial position. This CO disproportionation is assumed to occur via the CO rupture of Ni4COads with formation of a surface carbon atom bonded to three nickel atoms Ni3Csurf and an oxygen atom bonded to one nickel atom. CO2 is dissociated on pure nickel into CO, adsorbed as Ni4COads, and adsorbed oxygen atoms.

Low temperature oxidative dehydrogenation of ethane over catalysts based on group VIII metals

Abstract Unsupported Fe, Co and Ni catalysts are active in the oxidative dehydrogenation of ethane (ODHE) at low temperature. A conversion of 1% is achieved at 585, 438 and 487 K, respectively. The selectivity towards ethylene is found to be ca. 60% at low conversion. As the reaction temperature increases, the selectivity remains nearly constant for Ni whilst it decreases for Fe and Co. This behaviour has been explained by comparing the catalytic properties of Co and Ni towards ethane and ethylene oxidation. The ODHE intrinsic activity sequence (Co>Ni>Fe) is similar to that observed for the homomolecular exchange of oxygen, confirming that these catalysts are in an oxidized state in the course of the ODHE reaction. The study of the reduction and oxidation of unsupported and silica-supported nickel catalysts using magnetic methods has shown that the catalytic activity is not related to the ease of the Ni 2+ Ni 0 transition. TAP experiments carried out over Ni have revealed that the oxygen species involved in the reaction are irreversibly held by the catalyst at 573 K (possibly O − ) from which a reaction mechanism is proposed. Furthermore, this series of experiments have shown that ethane is irreversibly adsorbed and that CO 2 originates from a parallel-consecutive scheme.

Oxidative Conversion of Methane and C2 Hydrocarbons on Oxides: Homogeneous versus Heterogeneous Processes

Abstract A study of the reactions of methane, ethane and ethylene with oxygen in a quartz reactor, either empty of containing a lithium-magnesium oxide catalyst, was carried out to assess the relative importance of heterogeneous versus homogeneous processes in the oxidative convertion of these hydrocarbons. At low temperatures the oxidative dimerization of methane does not occur; only the catalytic total oxidation of hydrocarbons to carbon dioxide, which is probably a primary product , proceeds at a measurable rate. At high temperatures the dimerization proceeds catalytically into ethane, which is dehydrogenated to ethelyne. Ethylene is oxidized in the gas phase to carbon monoxide at 600 °C. The view according to which the surface is a source of radicals, the other steps occurring exclusively in the gas phase, is criticized.

Activation of hydrogen on Fe/MgO catalysts studied by magnetic methods and mössbauer spectroscopy

Fe/MgO catalysts with large metallic particles were prepared by coprecipitation, and studied with the help of Mossbauer spectroscopy and magnetic techniques. Two activation processes of H 2 were observed. At room temperature, H 2 was adsorbed on metallic particles with a small decrease in saturation magnetization, as previously reported in the literature. Above 470 K, adsorbed hydrogen was activated according the reversible redox scheme Fe n + + n H ads Fe + n H + , with n probably equal to 2. Protons are trapped on basic sites of the catalyst giving OH groups.

Physico-chemical properties of potassium-promoted Ni/SiO2 catalysts

Abstract K-promoted Ni/SiO 2 catalysts have been prepared by adding KNO 3 to the Ni/SiO 2 precursor and subsequent hydrogen reduction. They were studied by XPS, infrared spectroscopy and magnetic methods. Chemisorbed hydrogen is strongly interacting with Ni surface atoms of K-promoted nickel catalysts, whereas oxygen penetrates deeply into the nickel particles with the formation of bulk NiO. Evidence of an increase of the electron density of the metallic nickel due to the presence of potassium is given. CO is adsorbed on nickel in a bridge bonded form. K is mainly present in the K + form and a Ni-O-K surface complex is probably formed. This complex is concentrated on the nickel surface into patches or islands. Ni interacting with the K-containing phase is accessible to hydrogen adsorption and not to CO adsorption. The hypothesis of Ni(111) arrangements of the free nickel atoms has been considered and no experimental evidence of a direct interaction between the promoter and the adsorbed CO was found.