Jerzy Zieliński

Morphology of nickel/alumina catalysts

Abstract The morphology of impregnated nickel/alumina catalysts before and after the reduction process was studied. The temperature-programmed reduction method was successfully utilized for the study of the catalyst prior to the reduction. Nickel oxide was found to appear in the catalysts in two forms, as “free” or “fixed” oxide. The occurrence of the fixed form of the oxide in the catalysts is connected with the formation of nickel aluminate. As a result of the reduction of free and fixed forms of nickel oxide, respectively, large and small nickel crystallites are produced and a bidispersed structure consequently develops in the catalysts. Small and large nickel oxide crystallites are found to undergo reduction with the same ease. The difficult reduction of supported catalysts which is observed is due to chemical interaction of nickel oxide with the support.

Characterization of supported palladium catalysts: III. PdAl2O3

In Part I of this series (W. Juszczyk et al., in Proceedings, 9th International Congress on Catalysis, Calgary, 1988 (M.J. Phillips and M. Ternan, Eds.), Vol. 3, p. 1238. The Chemical Institute of Canada, Ottawa, 1988) using the catalytic conversion of neopentane as a virtually noncoking probe, the authors found that after reduction at moderate temperatures (573-773 K), Pd/Al{sub 2}O{sub 3} is roughly two orders of magnitude more active than Pd/SiO{sub 2}. An even higher, though transient, catalytic activity is now reported after reducing Pd/Al{sub 2}O{sub 3} at 873 K and extensively purging it in He at this temperature. The IR spectra of adsorbed CO reveal the presence of Pd{sup n+} ions in superactive Pd/Al{sub 2}O{sub 3}; their concentration positively correlates with catalytic activity. Since other potential causes such as strong acid sites are excluded, it is proposed that Pd{sup n+} ions are sites of high catalytic activity. A possible mechanism of their formation and a model of neopentane hydrogenolysis are briefly discussed. Completely reduced Pd appears necessary for neopentane isomerization.

Effect of K, Cs and Ba on the kinetics of NH3 synthesis over carbon-based ruthenium catalysts

The kinetics of NH3 synthesis over carbon-based ruthenium catalysts promoted with barium or alkali was studied. Both the ammonia partial pressure dependencies of the reaction rates (T = 400°C, p = 63 bar, H2 : N2 = 3 : 1) and the pressure variations of the activity (T = 370°C, p= 4–63 bar, H2 : NN2 = 3 : 1) were found to be different for Ba and for the alkali (K, Cs). Ba–Ru/C proved to be more sensitive to the NH3 content and to the total pressure. The rate of synthesis over the alkali-promoted catalysts is, in turn, much stronger influenced by the ruthenium dispersion. TOFs of NH3 synthesis for the promoted samples at 370°C and 4 bar (Ba 0.085 1/s, Cs 0.05 1/s, K 0.035 1/s) are significantly higher than that for the Ru(0001) basal plane (0.0085 1/s results from the literature data at 370°C, 2 bar). The most active Ru/C samples (Ba or Cs) exceed significantly the fused iron catalyst, especially at high conversions.

Decomposition of ammonia over potassium promoted ruthenium catalyst supported on carbon

The rate of NH3 decomposition has been measured over the K-Ru/C catalyst, the studies being accompanied by the N2-TPD experiment. The catalyst proved to be extraordinarily active in ammonia decomposition. The TOF values over K-Ru/C were almost 103 times higher than those for the triply promoted magnetite, both referred to the nitrogen chemisorption. The rate of nitrogen desorption was found to be significantly lower than the rate of NH3 decomposition under the comparable conditions. It is suggested that the adsorbate–adsorbate interaction is responsible for the higher rate of the latter. The modelling based on the Langmuir–Hinshelwood kinetics shows the coverage of Nad in high pressure NH3 synthesis to be very small.

Morphology of coprecipitated nickel/alumina catalysts with low alumina content

Abstract A series of low-alumina Ni/Al 2 O 3 catalysts, designed for carbon monoxide hydrogenation, was examined by temperature-programmed reduction, the BET method, oxygen chemisorption, and X-ray diffraction. Pre-reduced catalysts consisted of nickel crystallites and amorphous non-stoichiometric NiO·Al 2 O 3 residues. According to the proposed model of the catalysts the residues appeared in two forms: as clusters situated between/on the nickel crystallites and as inclusions encapsulated within large nickel crystallites. The size of the nickel crystallites, the size and composition of the clusters, and the amount of inclusions were estimated. A decrease of alumina content in the catalysts resulted in an increase in the size of the nickel crystallites, and did not affect the size, composition, and abundance of the clusters.

Reductibility of silica supported nickel oxide

The reduction of nickel/silica catalysts was studied in parallel by temperature programmed reduction (TPR) and X-ray diffraction (XRD) methods. The calcined materials contain nickel in the form of nickel oxide and nickel hydrosilicates and a part of the oxide particles is covered with the hydrosilicate species. This oxide is reduced in the same way as unsupported nickel oxide in the absence of water, and in a similar way to nickel hydrosilicates in the presence of water.

Effect of alumina on the reduction of surface nickel oxide; morphology of the surfaces of the surfaces of Ni/Al2O3 catalysts

Abstract Hydrogen reduction of surface nickel oxide, formed during the low temperature oxidation of nickel, was studied for a series of Ni/Al 2 O 3 catalysts. The examinations were carried out mainly by the temperature-programmed reduction (TPR) method. The results obtained show that alumina considerably inhibits the reduction, especially when the reaction proceeds in the presence of water. Analysis of the results lead to conclusion that nonstoichiometric nickel aluminate decorates corners and edges of Ni crystallites. It is supposed that this decoration is responsible for the unique properties of Ni/Al 2 O 3 catalysts, e.g. low ability of the catalysts to form Ni (CO) 4 during the interaction with CO, and high thermal stability of nickel in the specimens with low alumina content.

The effect of water on the reduction of nickel/alumina catalysts

The crucial role of water in the reduction of calcined nickel/alumina catalysts is demonstrated. A fraction of nickel oxide in the catalysts is reduced as NiO powder in the absence of water vapour, and as a nickel aluminate in the presence of water. A much higher dispersion of nickel is attained when the reduction is carried out at low concentration of water vapour.

Interaction of carbon monoxide with supported nickel catalysts

Abstract The interaction of carbon monoxide with Ni/SiO 2 , Ni/Al 2 O 3 and Ni/TiO 2 catalysts was studied by temperature-programmed desorption (TPD) of pre-adsorbed carbon monoxide. The TPD spectra exhibit a number of CO and CO 2 features reflecting various processes occurring during the interaction. Silica and alumina have a moderate effect on the spectra for supported nickel. In contrast, titania significantly affects the spectra, and the degree depends on the extent of reduction of titania and thus its ability to accept/donate oxygen. Analysis of the spectra obtained indicates that silica, alumina and non-reduced titania lower equally, by about 40°C, the temperature of dissociation of CO pre-adsorbed on supported nickel; on the other hand partially reduced titania lowers the dissociation temperature by ca . 100°C, by accepting the oxygen that is released. The influence of silica, alumina and non-reduced titania on the rate of CO dissociation is ascribed to the formation of a perturbed nickel surface at the nickel—support boundary; at the same time the effect of partially reduced titania on the dissociation is ascribed to joint action of perturbed nickel surface and the oxygen accepting TiO x species at the boundary.

Potassium-Promoted Carbon-Based Iron Catalyst for Ammonia Synthesis. Effect of Fe Dispersion

Potassium-promoted iron catalysts supported on thermally modified, partly graphitized carbon were studied in the ammonia synthesis reaction. Iron nitrate was used as a precursor of the active phase and KOH or KNO3 were used as promoters. The kinetic studies of NH3 synthesis were carried out in a differential reactor under 63 bar and 90 bar pressure. Hydrogen chemisorption, X-ray diffraction and transmission electron microscopy experiments were performed to determine the dispersion of iron in the specimens. All the K+–Fe/C catalysts proved to be sensitive to ammonia, the NH3 partial pressure dependencies of their reaction rates being close to that of the commercial magnetite catalyst (KM I, H. Topsoe). The catalytic properties of the promoted Fe particles on carbon were shown to be dependent upon the iron dispersion, i.e. smaller particles exhibited higher turnover frequency in NH3 synthesis. It is suggested that either small Fe crystallites expose more highly active sites, e.g. C-7 (B-5) or the promotion of small crystallites by the alkali is more efficient.