E.B.M. Doesburg

The influence of lanthanum oxide on the thermal stability of gamma alumina catalyst supports

Abstract The influence of 0–5 mol % lanthanum oxide on the thermal stability of gamma alumina catalyst supports was investigated. Sinter tests in air between 600°C and 1100°C show that sintering of gamma alumina proceeds via surface diffusion. Lanthanum oxide decreases the rate of sintering by the formation of a lanthanum aluminate surface layer. The phase transformation to alpha alumina was investigated by differential thermal analysis. A nucleation and growth mechanism was observed. Addition of lanthanum oxide increases the transformation temperature by 100°C. The results indicate that the phase transformation does not contribute significantly to the loss of surface area at high temperatures.

Thermal stabilization of high surface area alumina

Abstract The surface area loss of gamma and theta alumina at high temperatures (800–1100°C) was investigated. The cause is shown to be sintering, neck formation between crystallites via surface diffusion. Addition of lanthanum oxide decreases the rate of surface diffusion by the formation of lanthanum aluminate surface layers. In this way the thermal stability of these high surface area aluminas is increased. The phase transformation to alpha alumina, which also occurs in this temperature region, was investigated separately. Kinetic data from DTA experiments demonstrate that alpha alumina formation does not contribute significantly to the surface area loss. A new model for the nucleation of alpha alumina is proposed.

Preparation and characterization of stable copper/zinc oxide/alumina catalysts for methanol systhesis

A series of Cu/ZnO/Al2O3 catalysts for the low pressure methanol synthesis has been prepared by coprecipitation with a sodium carbonate solution from solutions of a mixture of the corresponding metal nitrates, followed by drying, calcination and reduction. The catalysts and their precursors were analyzed by techniques like X-ray diffraction (XRD), X-ray line broadening (XLB), differential thermal analysis (DTA), chemical analysis, adsorptive decomposition of N2O and B.E.T.-measurements. The catalytic activity for the methanol synthesis was determined in a flow reactor under industrial conditions. Depending on the metal ion ratio in the initial metal nitrate solutions different compounds were formed during coprecipitation, like rosasite (Cu, Zn)2(OH)2CO3, malachite, Cu2(OH)2CO3, Cu, Zn-hydrotalcite, (Cu, Zn)6Al2CO3(OH)16.4H2O and a ternary compound which we called roderite. Its structure is unknown and it contains, besides Zn2+, up to 28 at % Cu2+ and up to 17 at % Al3+. Addition of 7 at % Mg2+ stabilizes the Cu, Zn-hydrotalcite structure but leads to a drastic decrease in catalytic activity. The rate of methanol production depends on the phase composition of the precursors. Rosasite containing precursors give the highest activity; hydrotalcite proves to be an excellent catalyst stabilizer which evokes the formation of small Cu and ZnO particles. Mg2+ inhibits methanol production.

The sintering of coprecipitated nickel alumina catalysts

Abstract The sintering of a coprecipitated nickel alumina catalyst, calcined at 450, 600 and 900°C, was studied in a 70hydrogen/30% steam gas flow at a temperature of 700°C and a pressure of 1 bar. The phase composition of the fresh and sintered catalysts was determined by X-ray diffraction. Hydrogen chemisorption, X-ray line broadening, methanation activity determinations and BET surface area measurements were used to monitor the sintering of the catalysts. The results indicate a rather high sinter stability of the catalyst. During the initial sintering stage there is a parallel decrease of nickel surface area and total surface area. After 60 hours of sintering the nickel crystallite size reaches a constant level, while the total surface area decreases still further. There is no influence of the calcination temperature up to 900°C on the equilibrium nickel crystallite sizes of the sintered catalysts. To explain these results a new model of coprecipitated nickel alumina catalysts is proposed.

The effect of preparation conditions on the activity and stability of copreciptitated Ni/Al2O3 catalysts for the methanation of carbon monoxide

The activities and stabilities of coprecipitated Ni/Al2O3 methanation catalysts depend markedly on their preparation and pretreatment. The results are discussed in relation to the structure of the precipitate and to changes which occur during calcination and reduction.AbstractАктивности и стабильности совместно высажденных катализаторов метанирования Ni/Al2O3 сильно зависят от способа их приготовления и от предварительной обработки. Результаты обсуждаются в зависимости от структуры осадка и от изменений, происходящих при каляцинировании и восстановлении.

The morphology of coprecipitated nickel-alumina catalysts

Abstract The morphology of nickel-alumina catalysts prepared by coprecipitation was studied by scanning electron microscopy and X-ray diffraction. A very porous spongelike structure, consisting of rod-shaped particles, was observed for precipitated, calcined and reduced samples. Removal of nickel from the reduced sample did not effect the morphology

Characterization of the interaction between nickel oxide and aluminium oxide in coprecipitated catalysts

Abstract Nickel rich nickel/alumina catalysts prepared by coprecipitation show a remarkable thermal stability. Apparently a very effective interaction is present between the support and the active phase. The influence of the nickel content and the calcination temperature on this interaction is examined by X - ray diffraction and by measuring the reduction behaviour of the calcined samples in a thermo balance. It is found that the thermal decomposition of the coprecipitated catalyst precursor in air at 600 °C leads to the formation of two intermediate oxides of which the concentrations depend on the nickel content of the sample. One of these metastable oxides consists of a distorted nickel oxide lattice that contains a considerable amount of aluminium ions. For nickel/aluminium ratios higher than 0.5 calcination of the coprecipitate at 900 °C results in formation of three separate oxide phases viz. nickel aluminate and two nickel oxide phases. One of the two latter phases still contains aluminium ions in its crystallites. We suggest that after reduction a part of these aluminium ions remain in the metal lattice and provide the nickel crystallites with a tremendous thermal stability.

Kinetics of carbon monoxide methanation over lanthanum oxide doped nickel-alumina catalysts

Differential Scanning Calorimetry measurements were used to show that the addition of 5 mol % lanthanum oxide to nickel/alumina catalysts increases the methanation activity and changes the kinetics. The heats of CO and H2 chemisorption decrease; the temperature-independent factors increase. It is suggested that these changes are due to the basic properties of lanthanum oxide.

Effect of temperature of reduction on the activity and selectivity of a coprecipitated Ni–Al2O3 catalyst for the Fischer–Tropsch and methanation reactions

The reaction of CO with H2 at 473 K and at low pressures over a coprecipitated Ni–Al2O3 catalyst has been examined; it has been found that both methane and higher hydrocarbons are formed and that the activity and selectivity of the catalyst depends on the temperature (and hence, the degree) of reduction, low temperatures favouring the production of higher hydrocarbons.

Investigation of Alkali Carbonate Transport Toward the Catalyst in Internal Reforming MCFCs

A nickel catalyst to be used for internal steam reforming in a molten carbonate fuel cell (MCFC) must be resistant to the alkali components (Li and K species) of the electrolyte; these components can reach the catalyst from the anode by either transport via the vapor phase or by means of surface creep along the walls. In a series of experiments for determining the rates of transport, it was found that the amount of alkali transported by creep along a metallic wall (Au or Ni) was much smaller than that transported via the vapor phase. The vapor transport occurred by the formation of the alkali hydroxides. The vapor pressure of LiOH was found to be eight times larger than that calculated from thermodynamic data. All the Al-containing materials tested strongly took up alkali from the gas phase. The catalysts Ni/MgO and Ni/SiO2 sintered strongly during exposure to gaseous LiOH and KOH.