J. H. Sinfelt

Effect of chemical environment on magnitude of x‐ray absorption resonance at LIII edges. Studies on metallic elements, compounds, and catalysts

We report spectra isolating the LIII x‐ray absorption threshold resonance of the elements iridium, platinum, and gold in the pure metallic state and in a variety of compounds. When normalized spectra obtained on the metals are subtracted from those obtained on the compounds, the resulting difference spectra are related to differences in the electronic structure of the absorber atom in the two types of environment. The change in area of a threshold resonance line obtained from such a difference spectrum can be related to the ionicity of the bonds of the absorber atom in its compounds. Measurements on supported platinum and iridium catalysts provide information on electronic changes in the metal due to the small size of the metal clusters or to interaction with the support material. Information on electronic changes due to interaction of the catalysts with gas molecules may also be obtained.

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

Extended x‐ray absorption fine structure (EXAFS) studies were made in a series of highly dispersed metal catalysts to obtain structural information on the metal clusters present. Clusters of osmium, iridium, and platinum dispersed on silica or alumina were investigated. The metal clusters in the catalysts constituted 1.0 wt.% of the total mass. Chemisorption measurements indicated that the metal dispersions approached unity, where dispersion is defined as the ratio of surface metal atoms to total metal atoms in the clusters. When EXAFS data on the metal clusters are compared with data on the corresponding bulk metals, the lower average coordination number of the metal atoms in a cluster is evident. The decrease in coordination number is accompanied by an increase in the root mean square deviation of interatomic distance about the equilibrium value.

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

A study was made of the extended x‐ray absorption fine structure (EXAFS) associated with the K absorption edges of ruthenium and copper in a silica supported ruthenium–copper catalyst containing 1.0 wt.% ruthenium and 0.63 wt.% copper (1:1 atomic ratio of ruthenium to copper). Previous chemisorption and catalysis studies indicated a strong interaction between the ruthenium and copper in the catalyst, and led to the suggestion that the catalyst consisted of bimetallic clusters of ruthenium and copper dispersed on the silica. The EXAFS data of this investigation provide information on the structure of the ruthenium–copper clusters. The ruthenium in the bimetallic clusters appears to be very similar to the ruthenium in a silica supported ruthenium reference catalyst, in that the ruthenium atoms are coordinated predominantly to other ruthenium atoms and only to a minor extent to copper atoms. By contrast, the copper atoms in the clusters have nearest neighbors which are more nearly equally distributed among c...

Application of EXAFS in Catalysis. Structure of Bimetallic Cluster Catalysts

Abstract In recent years the utility of extended x-ray absorption fine structure (EXAFS) as a probe for the study of catalysts has been clearly demonstrated [1–13]. Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. With such catalysts it is difficult or impossible to obtain structural information by conventional x-ray diffraction methods [14].

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

The extended x‐ray absorption fine structures (EXAFS) associated with the LIII absorption edges of platinum and iridium were investigated for several catalysts consisting of metal clusters of these elements dispersed on silica or alumina. Previous x‐ray diffraction studies on silica supported platinum–iridium catalysts containing 5%–20% by weight of platinum plus iridium indicated that these elements were present in the form of bimetallic clusters with sizes in the approximate range of 25–50 A. The present EXAFS investigation has provided more detailed structural information on such clusters and has extended the scope of the research to include more highly dispersed platinum–iridium catalysts which cannot be investigated satisfactorily by x‐ray diffraction. In this way, it has been possible to extend the structural evidence for platinum–iridium bimetallic clusters to the case in which the ratio of surface atoms to total atoms approaches unity.

NMR studies of simple molecules on metal surfaces.

In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup.

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

An investigation was conducted of the extended x‐ray absorption fine structure (EXAFS) associated with the Liii absorption edge of osmium and the K absorption edge of copper in silica supported osmium–copper catalysts with a 1:1 atomic ratio of copper to osmium. The effect of the copper on the Liii x‐ray absorption threshold resonance of the osmium was also investigated. Chemisorption and catalysis studies had earlier indicated a strong interaction between osmium and copper in such catalysts, and led to the proposal that bimetallic clusters of osmium and copper were present on the silica. Information on the structure of the osmium–copper clusters has been obtained from the EXAFS data of the present investigation. The osmium atoms in the clusters are coordinated to a large extent to other osmium atoms, whereas the copper atoms are coordinated extensively to osmium atoms as well as to copper atoms. The results are consistent with a model in which the copper in the clusters is present at the surface and are ...

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.

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 ...