Joop C. Slaa

Catalytic and structural properties of ruthenium bimetallic catalysts: Preparation and characterization

In the framework of an EC Stimulation Action Programme, alumina-supported mono- and bimetallic Ru, RuGe, RuSn and RuPb catalysts have been prepared from both organic and inorganic precursors. Herein are presented some basic properties of these materials obtained by TPR and TPO experiments, H2 and CO chemisorption, TEM examination, XPS and EXAFS studies. The use of organic precursors led to very small Ru particles, with a coordination number between Ru atoms of 4.35 (Ru0-Ru0+Ru0-Ruδ+). The size of the metallic particles was not modified by adding Ge, Sn or Pb by the controlled surface reaction technique. In very small aggregates, some Ru atoms (10–15%) remained partially oxidised even after reduction at 623 K in flowing hydrogen. During the coimpregnation of alumina with inorganic precursors, the modifier played the role of a nucleation center for Ru, thus allowing small particles to be formed. We believe that CO could modify the surface composition of small bimetallic RuGe particles during the chemisorption.

Partial oxidation of methane to synthesis gas over Rh/alpha-Al2O3 at high temperatures

The partial oxidation of methane to synthesis gas has been studied in a continuous flow reactor using a Rh/α-Al2O3 catalyst under conditions as close as possible to those industrially relevant: pressures up to 800 kPa and temperatures higher than 1274 K in order to avoid the formation of carbon and to obtain high equilibrium selectivities to CO and H2. Intrinsic kinetic data were obtained when the feed was diluted with helium. Gas-phase reactions were found to occur at 500 kPa when the feed was not diluted. A reaction network has been derived from experimental results in which oxygen conversions range from 0 to 1. CO2, C2H6 and H2O are the primary products. C2H4 is formed by oxidative dehydrogenation of C2H6. CO and H2 are formed by reforming of CH4 by CO2 and H2O; an additional direct route to CO and H2 at low oxygen conversions cannot be excluded. The catalyst appears to be present in two states, the transition being at an oxygen conversion of 0.4 under the conditions used. The support probably enhances oxidation reactions by reverse spillover of oxygen or hydroxyl species onto rhodium. The support as such behaves similarly to the catalyst at low oxygen conversions, but shows no reforming activity.

Studies on ruthenium catalysts. Part 4: Hydrogenolysis of n-butane on Ru/Al2O3 catalysts-dependence of reaction kinetics on particle size and on pretreatment

Abstract Turnover frequencies (TOF) for hydrogenolysis of n-butane on Ru/Al2O3 catalysts, measured with a 10-fold excess of H2, depend on dispersion and on pretreatment. After a first high-temperature reduction at 753 K (HTR1), 1% Ru/Al2O3 showed high dispersion and moderate activity; oxidation at 623 K followed by reduction at 433 K caused dispersion to decrease, and rates and TOFs to increase. However, the magnitude and even the sign of the effects depended on the H2 pressure used, because the form of the rate dependence on H2 pressure changed significantly. Many of the discrepancies in the literature concerning particle size effects may originate in the use of different conditions for measuring rates and of different pretreatments. Alterations in the form of the dependence of rates and of product selectivities on H2 pressure suggest that “structure sensitivity” in these systems may in fact be due to variations in the strength of H2 chemisorption.

Catalytic and structural properties of ruthenium bimetallic catalysts: kinetics of hydrogenolysis of lower alkanes on variously pretreated RuAl2O3 catalysts

The H2 pressure dependence of rates of hydrogenolysis of ethane, propane and n-butane on RuAl2O3 catalysts differing in dispersion and type of pretreatment has been measured at a number of temperatures, and the results interpreted in terms of a mechanism involving adsorbed partially dehydrogenated intermediates CnHx, and modelled by the derived rate expression. The rate-limiting step is taken as the reaction of CnHx with an adsorbed H atom. We thus obtain best-fit values of the rate constant k1, the H2 adsorption equilibrium constant KH, an equilibrium constant for the dehydrogenation of the alkane KA, and of x, for each set of results. The shapes of the kinetic curves, and the constants that describe them, change markedly with dispersion, and with pretreatment: oxidation and low-temperature reduction (OLTR), as well as causing some loss of dispersion, gives rise to other effects, ascribed to ‘morphological’ factors, not seen when catalysts are reduced at high temperature (753 K). What is most striking is that differences in activity seem to be determined much more by the constants KA and KH than by the rate constant k1, which when expressed per Ru surface atom varies at most three-fold. In particular KH is much larger after the first high-temperature reduction (HTR1) than after OLTR. The true activation energy derived from the temperature-dependence of k1 is about the same for each alkane (≈ 60 kJ mol−1), the enthalpy changes for H2 chemisorption are small and for alkane dehydrogenation they lie between 50 and 130 kJ mol−1. The manner in which product selectivities vary with H2 pressure also depends on dispersion and pretreatment, the dominant factor being the strength of H2 chemisorption. Thus on a very highly dispersed catalyst for which KH is large, intermediate product selectivities are high because the high concentration of H atoms facilitates desorption of adsorbed species, and for this reason also selectivities scarcely respond to changes in H2 pressure. With the same catalyst after OLTR, however, when KH is much decreased, selectivities respond sensitively to H2 pressure because the adsorption is weaker. It is then deduced that approximately two more H atoms are required to effect desorption of intermediates as a product alkane than to cause further CC bond breaking. Our results strongly suggest that structure-sensitivity in alkane hydrogenolysis is more the result of variations in chemisorption energetics, and their consequential effects on surface coverage, than of kinetic effects; this concept also accounts for dispersion-dependent differences in the temperature-dependence of product selectivities previously reported.

Catalytic and structural properties of ruthenium bimetallic catalysts: hydrogenolysis of propane and n-butane on RuAl2O3 catalysts modified by a Group 14 element

Abstract The addition of Ge as GenBu4 to H-covered Ru Al 2 O 3 (RuEC1) leads after in situ reduction at high temperature (HTR1; 758 K) to decreases in H2 chemisorption capacity and even more marked falls in activity for hydrogenolysis of propane and of n-butane. Changes in product selectivities are slight, and all Ge-containing samples exhibit similar characteristics to the parent RuEC1, which is very highly dispersed. Although loss of activity is chiefly due to encapsulation of the Ru0 particles by amorphous GeOx species, these affect neighbouring active sites by decreasing the equilibrium constant for alkane dehydrogenation to the reactive species. This conclusion follows from determinations of the rate-dependence on H2 pressure and mathematical modelling of the results. Similar behaviour is shown by the less well-dispersed RuEC3, where activity loss and selectivity changes are partly due to encapsulation by GeOx of smaller particles: some evidence for selective blocking of Ru atoms in low coordination number sites by Sn species formed analogously is however obtained. H2 pressure-dependence of the rate of n-butane hydrogenolysis was determined at four different temperatures with a GeOx-modified RuEC3, whence a true activation energy (50 kJ mol−1) and an enthalpy change for the alkane dehydrogenation step were obtained. Oxidation and low-temperature reduction ( O LTR ) of catalysts previously subjected to HTR1 led to very large increases in rate, and to changes in product selectivites, that suggested the creation of large, essentially pure, Ru particles; the modifier could not be wholly re-united with them by a second HTR. Ru Al 2 O 3 catalysts of the NI series, prepared from inorganic precursors, showed greater dispersion in the presence of the modifier, the catalytic behaviour being in harmony with this observation.

The origin of particle size effects in supported metal catalysts : propane hydrogenolysis on Ru/Al2O3 catalysts

The kinetics of the hydrogenolysis of propane on Ru/Al 2 O 3 catalysts differing in particle size have been analysed according to a classical rate equation which supposes the reactive species to be a dehydrogenated form of the alkane. With small ruthenium particles there is a sharp maximum in rate as hydrogen pressure is increased ; with large particles the peak is broader and at higher hydrogen pressures. Published results showing the latter to give higher turnover frequencies are only valid at high hydrogen : alkane ratios. Ethane selectivity (S 2 ) is almost independent of hydrogen pressure on small particles, but on large particles it increases markedly as the hydrogen pressure is raised. The results are interpreted by supposing hydrogen chemisorption to be strong on small ruthenium particles, and weaker on large ones ; effects of temperature on selectivities are also explained in this way. The geometric basis of structure-sensitivity is thereby called into question.

Catalytic and structural properties of ruthenium bimetallic catalysts: effects of pretreatment on the behaviour of various RuAl2O3 catalysts in alkane hydrogenolysis

Abstract Very highly dispersed 1% Ru Al 2 O 3 prepared from Ru(acac) 3 , after high-temperature reduction, catalyses the hydrogenolysis of n-butane with a high and almost temperature-independent selectivity to ethane. Catalysts having higher Ru contents (2.5 and 4%) are less well dispersed, but show higher turnover frequencies for this reaction, and selectivities which change with temperature, with methane the major product throughout. Oxidation and low-temperature reduction of these catalysts causes loss of metal area, and much enhanced turnover frequencies: product selectivity behaviour is then in each case that characteristic of large metal particles. A final high-temperature reduction leads to lower turnover frequencies, with little change in product selectivity. Results are also reported for catalysts prepared from RuCl 3 and from Ru(NO) (NO 3 ) 3 , and for Ru powder: propane hydrogenolysis has also been examined as an adjunct to this study.

What do we mean by “catalytic activity”?

The relative reactivities of the lower alkanes in hydrogenolysis on a Pt/Al2O3 catalyst depend on the H2 pressure used, as do those of a Ru/Al2O3 catalyst, pretreated in various ways, for propane hydrogenolysis. Apparent activation energies also vary with H2 pressure. No single rate measurement adequately represents “catalytic activity”, which is properly defined as the rateconstant for the slow step.