Hubert Nijs

METAL-PARTICLE SIZE DISTRIBUTIONS AND FISCHER-TROPSCH SELECTIVITY - AN EXTENDED SCHULZ-FLORY MODEL

A new model is presented which accounts for all possible Fischer-Tropsch product distributions. By proper selection of the critical parameters, namely, polymerization growth rate, average particle size of the metal particles which catalyze the reaction, and width of the size distribution, all published Fischer-Tropsch product distributions can be fitted very closely. The method correctly predicts the amount of these carbon numbers which usually deviate strongly from Schulz-Flory kinetics. The model is based on the assumption that a particle size effect exists. On a given size only molecules lower than a given carbon number can be synthesized. On each particle polymerization occurs according to Schulz-Flory kinetics.

New evidence for the mechanism of the fischer-tropsch synthesis of hydrocarbons

Abstract New mechanistic evidence is advanced for Fischer-Tropsch synthesis on Ru-loaded Y-type zeolites, Ru-on-silica and Co promoted on kieselguhr. The experiments were carried out in real Fischer-Tropsch conditions, i.e., at high growth rates. Experiments were done in pulse reactors and under transient reaction conditions. 13C-labeled carbon monoxide was used and addition of water and olefins to the synthesis gas was studied. From a series of well-chosen experiments it is deduced that carbon monoxide disproportionation initiates the reaction. Surface carbon is hydrogenated to methane and the reverse water-gas shift reaction accounts for water formation. Water reacts with surface carbon and forms hydroxy-species which initiate chain propagation. The mechanism accounts for a particle size effect and explains why polar conditions stimulate chain growth.

Redox behaviour of transition metal ions in zeolites. Part 5.—Method of quantitative determination of bidisperse distributions of metal particle sizes in zeolites

Temperature programmed reduction and oxidation techniques were used as a fast and quantitative method to determine the amount transition metal inside or outside the zeolite after a reductive treatment. The method has been applied to Ag, Cu and Ni and to different zeolites—X, Y, Z and L.For Ag and Cu zeolites it was found that small metal particles located in the zeolite cages show a low temperature maximum in the rate of oxidation. The maximum in the rate of reoxidation of the metal phase outside the zeolite is found at distinctly higher temperatures. For Ni zeolites, the determination of the bidisperse metal distribution requires first a complete reoxidation of the sample. During a subsequent reduction, two metal dispersions can be distinguished on the basis of maxima observed in the rate of reduction.

Selective Fischer–Tropsch synthesis of hydrocarbons: particle size effect of ruthenium metal in Faujasite-type zeolites

Selective synthesis of hydrocarbons from CO and H2 over RuY zeolites shows a drastic decline in chain growth probability after C5 or C10, depending upon the particle size of the Ru metal.

Chain limitation of Fischer–Tropsch products in zeolites

Fischer-Tropsch synthesis on RuY-molecular sieves shows a non-linear Schulz–Flory plot and a drastic decline in chain growth probability after C10; thus, pure dimensions in zeolite-based catalysts are capable of limiting reaction product chain growth.

Redox behaviour of transition metal ions in zeolites. Part 7.—Characterization of a nickel metal phase in zeolite NaY

NiY zeolite has been treated under steaming and non-steaming conditions and reduced in hydrogen at different temperatures. In the solids obtained this way, the Ni(O) metal phase was characterized by temperature programmed reduction and oxidation techniques and by ferromagnetic resonance spectroscopy.Both techniques show that a bidispersion of metal particle sizes exists. The size of the particles inside the zeolite is restricted to the dimensions of the supercage. For samples reduced at elevated temperatures, small particles are found in structural defects of the crystals. In each case, the Ni metal particles either strongly interact with the support or are incompletely reduced.Large Ni metal particles are formed outside the zeolite. They are completely reduced and do not interact with the support. They are ellipsoidal since they exhibit shape anisotropy.

Characterization of Reduced Natural Garnierite and its Catalytic Activity for Carbon-monoxide Hydrogenation

Natural garnierite, a nickel-containing mineral from New Caledonia, was reduced in hydrogen. The mineral and its reduced forms were characterized by x-ray diffraction electron microscopy, and microprobe analysis. The reduction was followed volumetrically and the Ni/sup 0/ phase characterized by H/sub 2/ chemisorption. The catalytic activity of this mineral was determined in the hydrogenation reaction of carbon monoxide. Experiments were done in typical methanation and so-called Fischer-Tropsch conditions. The mineral consists of a mixture of a 10-A phase (talc-like fluffy particles) and a 7-A serpentine-like phase (fibers). The major amount of nickel is associated with the talc phase. Minor amounts are in the serpentine fibers and possibly substituted in the lattice of the minerals. Qualitative evidence for a redispersion of Ni in the serpentine fibers is advanced. At the moment this occurs, the turnover numbers of CO disappearance are optimum. Compared to other supports, they are considerably lower. The product distribution is within C/sub 1/-C/sub 4/ and follows closely a Schulz-Flory distribution.

Photo-oxidation of water on the surface of hectorite using trans-diaquabis-(2,2′-bipyridine)ruthenium(2+) as catalyst

trans-Ru(bpy)2(H2O)22+(bpy = 2,2′-bipyridine) was shown to be an efficient catalyst for the oxidation of water and allowed the construction of a catalytic water photo-oxidation system within an adsorbed layer.