L.L. van Reijen

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.

Measurement of the thermal conductivity of solid substances by DSC

Abstract The thermal conductivity of solid samples can be measured with DSC by melting experiments with a pure metal (indium or gallium) on top of cylindrical pellets. Particularly samples with a low thermal conductivity, like plastics or porous substances, can be determined

A quantitative investigation of the phase transformation of gamma to alpha alumina with high temperature DTA

The phase transformation of gamma to alpha alumina was studied with high temperature DTA. The results were analysed with the methods of Borchardt and Daniels, Freeman and Carroll, Coats and Redfern and Kissinger. The order of the reaction is one, indicating a nucleation and growth mechanism with one nucleus being formed per crystallite. The activation energy is 0.60 MJ mol−1.

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 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

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.

Fine-structure EPR spectra of clusters of strongly exchange-coupled paramagnetic ions

Abstract At concentrations in the range of 1 to 10%, fine-structure EPR spectra of exchange-coupled paramagnetic ions, substituted in a host lattice, turn into a single broad cluster resonance. The linewidth of such a resonance does not obey the predictions about the exchange narrowing given in the classical theories of Van Vleck (moment method) or Anderson (random frequency modulation) derived for oncentrated paramagnetic compounds. A theoretical approach based on the calculation of EPR spectra of small clusters is presented, leading to a better description of the phenomenon.

Synthesis of thermostable nickel-alumina catalysts by deposition-precipitation

Abstract Nickel-alumina catalysts containing about 18 wt % nickel can be produced by deposition-precipitation from ammoniacal nickel nitrate solutions. Activation of the preformed gamma alumina support at high pH results in the formation of nickel aluminium hydroxycarbonate supported on gamma alumina. Catalysts made in this way are very active and much more thermostable than conventional impregnated and coprecipitated catalysts. The thermostability can be increased even further by incorporation of the lanthanum oxide. The nickel content of the catalysts can be increased by successive deposition-precipitation, but the performance is not much improved. The use of other alumina modifications as catalyst supports for this preparation procedure is not recommended. Scaling-up is possible.

Magnetic interactions between Fe(III) ions in the solid solutions CuM1 − xFexS2 (M = Al, Ga, In) and AgM1 − xFexO2 (M = Al, Ga).

Abstract Magnetic properties of CuM1 − xFexS2 (M = Al, Ga, In) and β-AgM1 − x, FexO2 (M = Al, Ga) solid solutions have been studied by Mossbauer spectroscopy (only on CuGa 1 − xFexS2), magnetization measurements and EPR spectroscopy. In the structures studied all ions have tetrahedral co-ordination. The magnetic interactions between Fe(III) ions are shown to be very strong for nn sites ( J k ∼100 K). More distant exchange interactions are found to fall off rather slowly as the distance between Fe(III) ions increases and are suggested to depend essentially on the number of bonds traversed.

DSC calibration for the methanation reaction, catalyzed by Ni/Al2o3 pellets

The nickel catalyzed methanation of CO is followed by DSC. This paper deals with the results, obtained by using Ni/Al2O3 pellets. Particularly the calibration of the DSC signal and the temperature increase of the pellet, caused by the exothermic methanation reaction, are determined.