John H. Sinfelt

Nature of ruthenium-copper catalysts

Abstract The chemisorption and catalytic properties of ruthenium-copper catalysts with a metal dispersion of the order of 1%, in which the dispersion refers to the fraction of metal atoms present in the surface, were investigated and compared with previously published data on supported ruthenium-copper clusters of much higher dispersion. Ruthenium and copper are essentially completely immiscible in the bulk, but there is evidence of definite interaction between the two in ruthenium-copper catalysts. The presence of copper decreases the capacity of ruthenium for hydrogen chemisorption and also suppresses markedly the catalytic activity for hydrogenolysis of ethane to methane. The ethane hydrogenolysis activity of a ruthenium-copper catalyst is strikingly related to its capacity for strong hydrogen chemisorption, the latter being defined as the amount of chemisorbed hydrogen retained by the catalyst after evacuation at room temperature. The interaction between copper and ruthenium occurs at the surface and is likened to that which would exist if copper were chemisorbed on ruthenium. The state of dispersion of a ruthenium-copper catalyst has a major influence on the effect of the copper. The atomic ratio of copper to ruthenium required for a given degree of coverage of the surface by copper increases with increasing dispersion, as is clearly reflected by the results on hydrogen chemisorption and ethane hydrogenolysis.

Heterogeneous catalysis: some recent developments.

In recent years major progress has been made in the area of heterogeneous catalysis by metals. Much has been learned about the nature of metal catalysts and of catalytic phenomena on metals. Characteristic patterns of catalytic behavior among the metallic elements have been established for certain classes of reactions, and these patterns provide a first step toward a more comprehensive understanding of catalytic specificity. Studies on metal alloys and related bimetallic catalysts have revived interest in a geometric factor in surface catalysis to complement the traditional electronic factor. Closely related to this geometric factor is the discovery that selectivity, rather than activity alone, is a major factor in reactions on bimetallic catalysts. Concurrent with progress in understanding how catalysts work, advances are also being made in the development of new catalyst systems, examples of which are the bimetallic (or polymetallic) cluster catalysts. Research in this area provides an example of how advances in catalyst technology can be realized within a framework of fundamental research on catalytic phenomena (38).

Dispersion and structure of platinum-iridium catalysts

Abstract A series of supported platinum-iridium catalysts was investigated by hydrogen chemisorption and X-ray diffraction. The catalysts all contained equal amounts of platinum and iridium by weight. Chemisorption measurements were made on both silica and alumina supported catalysts containing amounts of platinum plus iridium ranging from 0.6 to 20 wt%. For given contents of platinum and iridium, hydrogen chemisorption was higher on catalysts supported on alumina, indicating a higher degree of metal dispersion on alumina than on silica. X-ray diffraction measurements were limited to silica supported samples with platinum plus iridium contents ranging from 5 to 20 wt%. The presence of platinum-iridium bimetallic clusters is evident from the diffraction patterns, which exhibit diffraction lines at positions approximately midway between the positions of the corresponding lines for platinum and iridium metals. X-ray diffraction studies on alumina supported samples are complicated by interference from the diffraction pattern of the alumina.

Role of surface science in catalysis

Abstract Around the time of World War I, Langmuir advanced a simple theory of chemisorption and showed how it could be used to formulate rate laws for reactions occurring on surfaces. From that time on, surface science has played an important role in heterogeneous catalysis. Between the two world wars, simple studies of extents of adsorption by catalyst surfaces led to the concept of activated adsorption and to a universally used method for determining the high surface areas associated with the pore structures of catalytic materials. After World War II, the application of various spectroscopic and structural probes made it possible to investigate catalyst surfaces at a more microscopic level. Studies with idealized surfaces such as the faces of single crystals in ultra-high vacuum apparatus also made their appearance. By the end of the twentieth century, direct information was being obtained on the rates of elementary reactions of well-defined surface species. The results of such work are beginning to put “finishing touches” on the great insight of early pioneers in surface science and heterogeneous catalysis. Much has been accomplished, but exciting opportunities still remain.

Hydrogenolysis of methyl chloride on metals

Metal-catalyzed hydrogenolysis reactions of carbon-halogen bonds have received less attention than hydrogenolysis reactions of carbon-carbon bonds. In this note the authors report results of a study on the hydrogenolysis of methyl chloride over a number of metals supported on silica. The products of the reaction are methane and hydrogen chloride. Somewhat earlier, a similar study was completed for the hydrogenolysis of the carbon-carbon bond in ethane, and, more recently, for the hydrogenolysis of the carbon-nitrogen bond in methylamine. Establishing activity patterns for the hydrogenolysis of these different types of bonds provides useful information regarding the specificity of metal catalysts for this important class of reactions. 12 references.

Analysis of EXAFS data on bimetallic clusters

In previous studies of the structures of bimetallic clusters using extended X-ray absorption fine structure (EXAFS), structural parameters were determined by fitting the single scattering expression for EXAFS to experimental data associated with an absorption edge of each component. The data for the two edges were fitted separately. Recently, with an extensive modification of the computer program for the analysis of EXAFS data, it has been possible to fit the EXAFS data for the two edges simultaneously. This improved method of analysis has the feature thatall of the EXAFS data are used in the determination ofall of the structural parameters, and permits one to impose in a direct manner certain necessary physical conditions regarding the system. Consequently, one has greater confidence in the values obtained for the parameters. The present paper first summarizes published results obtained by the previous method of analysis on a number of systems. For comparison, a summary is then given of results obtained from the same data on these systems with the new method of analysis.

Metal-catalyzed reactions of methylamine in the presence of hydrogen

Abstract Rates of formation of various products obtained from methylamine in the presence of hydrogen have been determined over a number of metals supported on silica. The metals included ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, and gold. Ammonia and methane were observed as products on all of the metals investigated. Dimethylamine was formed in significant amounts over all the metals except rhodium. Various other products were also observed on certain metals. With regard to the kinds of products formed, the results of the present work are in excellent agreement with previously published results on catalysts in the form of metal films. In addition to extending the earlier work on metal films to supported metals, the present work includes data on several additional metals as well. When the present results are combined with our earlier reported results on ethane hydrogenolysis, we are able, for a common series of metals, to compare patterns of variation of catalytic activity for the hydrogenolysis of carbon-carbon and carbon-nitrogen bonds. Such a comparison is of interest in the general question of the catalytic specificity of metals.

Kinetics of hydrogenolysis of methylamine on a rhodium catalyst

The kinetics of hydrogenolysis of methylamine to methane and ammonia were investigated over a catalyst consisting of small clusters of rhodium dispersed on silica. Data obtained in the temperature range 353–408 K exhibit a characteristic pattern in which the rate passes through a maximum as the hydrogen partial pressure is increased by two orders of magnitude from 0.01 to 1.0 atm. At a given temperature, the position of the maximum shifts slightly in the direction of higher hydrogen partial pressure when the methylamine partial pressure increases by one to two orders of magnitude. Of particular interest is the finding that the rate increases with decreasing methylamine partial pressure over a broad range of hydrogen partial pressures covered in the investigation. As the hydrogen pressure increases, the inverse dependence of the rate on methylamine pressure becomes less pronounced and eventually disappears at a sufficiently high hydrogen pressure. At hydrogen partial pressures somewhat higher than those at which the rate maxima are observed, there is some indication that the inverse dependence changes to a positive dependence, especially at the lowest temperatures investigated. It seems likely that the rate limiting step of the reaction changes when the hydrogen pressure varies over a wide range. At the highest hydrogen pressures studied, it is suggested that the limiting step is one in which the scission of the carbon-nitrogen bond occurs in a hydrogen deficient surface intermediate formed in the chemisorption of methylamine, with no direct participation of hydrogen as a reactant in the step. On the other hand, at the lowest hydrogen pressures investigated, it is proposed that the rate is limited by a step in which chemisorbed hydrogen does participate directly as a reactant.

Laser Raman studies of the preparation of platinum and iridium catalyst systems

Abstract Steps in the preparation of Pt, Ir, and PtIr on alumina catalysts have been followed by laser Raman spectroscopy. The samples investigated were prepared by impregnation of alumina with the appropriate chloroplatinic acid and chloroiridic acid solutions. The resulting materials were then heated in air at varying temperatures. Laser Raman spectra indicated significant differences in the nature of the surface species remaining when Pt Al 2 O 3 and Ir Al 2 O 3 preparations were subjected to such thermal treatments. After a drying step at 110 °C, the Pt Al 2 O 3 preparation exhibits Raman bands at frequencies very close to those at which bands have been reported for the PtCl 2− 6 ion, whereas the Ir Al 2 O 3 preparation yields a spectrum significantly different from that reported for the IrCl 2− 6 ion. With increasing temperature, both the platinum and the iridium species decompose with loss of chlorine ligands. After being heated in air at 500 °C, the Ir Al 2 O 3 sample exhibits a Raman spectrum characteristic of crystalline IrO 2 . The presence of platinum in the bimetallic PtIr Al 2 O 3 sample inhibits the formation of crystalline IrO 2 to some degree.

Methylamine hydrogenolysis on a rhodium catalyst: kinetics at high hydrogen partial pressures

The kinetics of hydrogenolysis of methylamine to methane and ammonia on a rhodium catalyst were investigated at hydrogen partial pressures in the range of 2–10 atm at temperatures of 368, 383, and 408 K. At a fixed methylamine partial pressure, the rate decreased with increasing hydrogen partial pressure. When the hydrogen pressure was held constant, the rate increased with increasing methylamine pressure. Results of a previous investigation by our group at lower hydrogen partial pressures (0.01–1 atm) indicated that the hydrogenolysis rate passed through a maximum with increasing hydrogen pressure. Moreover, at the lower hydrogen pressures, there was an inverse rather than positive dependence of the rate on methylamine partial pressure. With the aid of the present results, there is a much clearer definition of the maximum in the experimental data relating the reaction rate to hydrogen partial pressure. The inversion of the effect of methylamine pressure on the rate as the hydrogen pressure is varied over a sufficiently wide range is also firmly established. With regard to the interpretation of the many interesting features of the kinetics, we retain the suggestion from our earlier work that the rate limiting step at the highest hydrogen pressures is the scission of the carbon-nitrogen bond in a partially dehydrogenated methylamine intermediate chemisorbed on the rhodium, with no direct participation of hydrogen as a reactant in this step. At the lowest hydrogen pressures, however, there is a different rate limiting step in which hydrogen does participate directly as a reactant.