G. Meitzner

Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) studies of Rh–Cu clusters

An investigation of the structure of the bimetallic clusters present in rhodium–copper catalysts was conducted with the use of extended x‐ray absorption fine structure (EXAFS) measurements. Two catalysts were studied, both employing silica as a support for the clusters and both containing 1 wt. % rhodium. In one catalyst the Cu:Rh atomic ratio was 1:2 and in the other 1:1. Studies were made of the EXAFS associated with the K absorption edges of the rhodium and copper. The results of the EXAFS studies indicate that copper concentrates at the surface of the rhodium–copper clusters. In this regard the results are similar to our earlier reported results on ruthenium–copper clusters. However, the extent of surface segregation of the copper appears to be less pronounced for rhodium–copper clusters. This result is reasonable on the basis that rhodium and copper, unlike ruthenium and copper, exhibit at least some miscibility in the bulk.

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.

Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) of Pt–Re and Pd–Re clusters

Extended x‐ray absorption fine structure (EXAFS) studies were conducted on catalysts containing platinum and rhenium, or palladium and rhenium, on alumina. The atomic ratio of rhenium to either platinum or palladium was close to one in the catalysts investigated. The metallic entities in the catalysts were characterized by analyses of the EXAFS associated with the LI absorption edge of platinum, the LIII edge of rhenium, and the K absorption edge of palladium. It was concluded that Pt–Re and Pd–Re bimetallic clusters are present in the catalysts. However, the clusters have regions rich in rhenium and other regions which are rich in either platinum or palladium. Exposure of Pt–Re clusters to sulfur has little influence on their structure.

Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) studies of Ag–Cu and Au–Cu clusters

Extended x‐ray absorption fine structure (EXAFS) studies were conducted on silica supported silver–copper and gold–copper bimetallic clusters to obtain information on their structures. The atomic ratio of copper to either silver or gold was close to one in the materials investigated. The EXAFS results, which were obtained in the presence of hydrogen, indicate extensive segregation of the components in both the silver–copper and gold–copper clusters, although it is much more pronounced in the former. The greater segregation in the silver–copper clusters is readily understandable, since silver and copper are only slightly miscible in the bulk, whereas gold and copper are completely miscible. The EXAFS results on the silver–copper clusters suggest that the copper‐rich region is in the interior of the clusters, with the silver concentrating at the surface. The location of the gold‐rich region in the gold–copper clusters is less clear from the EXAFS data, but there is a slight indication that it is present at ...

Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Ir--Rh clusters

The extended x‐ray absorption fine structures (EXAFS) associated with the LIII absorption edge of iridium and the K absorption edge of rhodium were investigated for catalysts containing bimetallic clusters of these elements dispersed on silica or alumina. The catalysts contained 1 wt.% iridium and 0.5 wt.% rhodium, which corresponds to an Ir/Rh atomic ratio close to 1. From the EXAFS data, it is concluded that the rhodium concentration in the surface region of the iridium–rhodium clusters is greater than it is in the interior. The EXAFS data also indicate that the iridium–rhodium clusters are more highly dispersed on alumina than on silica.

Extended x‐ray absorption fine structure (EXAFS) of highly dispersed rhodium catalysts

An investigation was conducted of the extended x‐ray absorption fine structure (EXAFS) associated with the K‐absorption edge of the rhodium in two catalysts, Rh/SiO2 and Rh/Al2O3, which contained 1 and 0.5 wt. % rhodium, respectively. Hydrogen chemisorption data indicated that the rhodium atoms were predominantly surface atoms in both catalysts. However, the EXAFS data indicated that the average number of nearest neighbor rhodium atoms about a given rhodium atom was much lower for the Rh/Al2O3 catalyst than for the Rh/SiO2 catalyst, 1.5 compared to 9. These values compare with a value of 12 for metallic rhodium. Although the detection of at least some Rh–Rh coordination for the Rh/Al2O3 catalyst indicates that the rhodium is not present exclusively as single atoms bound to the alumina, the EXAFS data do not exclude the possibility that rhodium in this form could be present along with rhodium clusters.

X-ray absorption studies of the electronic structures of Pd-Ag and Pd-Au alloys

Information on the number of unoccupied d-states in the valence band of a metal can be obtained from a study of the intensities of the X-ray absorption threshold resonances associated with LIII and LN absorption edges. Such resonances, which are attributed to excitations of electrons from 2p core levels to the unoccupied d-states, have been investigated for both Pd-Ag and Pd-Au alloys over the same range of composition. The spectra were obtained from measurements of the intensity of emission of electrons (total electron yields) from powder samples. This method avoids experimental artifacts associated with sample thickness effects of the type encountered with X-ray transmission or fluorescence measurements. The attenuating effect of the Group IB metal on the resonances of palladium in the alloys was much smaller than the effect predicted by the “rigid-band” model of the electronic structure, but substantially greater than that previously observed for Ni-Cu alloys.

Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) studies of Re–Cu, Ir–Cu, and Pt–Cu clusters

The extended x‐ray absorption fine structure (EXAFS) associated with the LIII absorption edges of rhenium, iridium, and platinum, and the K absorption edge of copper, has been used to study the structures of silica supported Re–Cu, Ir–Cu, and Pt‐Cu, bimetallic clusters. The atomic ratio of copper to the other metal was approximately one in all cases. The systems were chosen to investigate the effect of varying miscibility of the components on the structural features of the bimetallic clusters. At one extreme, platinum and copper are totally miscible in the bulk. At the other extreme, rhenium and copper are completely immiscible. Iridium and copper are only slightly miscible in the bulk. In the bimetallic clusters, atoms of rhenium, iridium, or platinum are all coordinated to copper atoms about as extensively as they are to atoms of their own kind. This result is particularly interesting for the Re–Cu and Ir–Cu clusters, in view of the limited miscibility of either iridium or rhenium with copper in the bul...

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.

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.