W. Henry Weinberg

Theoretical studies of the dissociative adsorption of H2 on Ni(001) using ab initio parameterized LEPS calculations

Abstract Semi-empirical London-Eyring-Polanyi-Sato (LEPS) calculations are reported, comparing the energetics of H2 dissociation at linear, twofold, and fourfold sites on Ni(001). Parameters for the LEPS method were obtained from the results of first principles calculations of both atomic and molecular hydrogen adsorption on model Ni(001) surfaces. Several pathways are found to require no activation energy for dissociation, of which the most favorable is dissociation across a twofold site with subsequent atomic adsorption at fourfold sites. The experimentally observed β1 (high coverage) state is found to be consistent with a geometry in which two hydrogen atoms are adsorbed at a single fourfold site.

The geometrical and vibrational properties of the Rh(111) surface

Abstract Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.

Determining the angles of incidence in a LEED experiment

A new method for determining the angles of incidence in a back-reflection, post-acceleration, fluorescent display LEED apparatus is presented which uses the angles between the diffraction spots on a photograph of the LEED pattern. Absolute accuracies better than 0.1 degrees for both incidence angles should be routinely available.

2. Vibrations in Overlayers

Publisher Summary This chapter describes research into the static and dynamic properties of the surfaces of heterogeneous catalysts. It discusses one of the most important areas of current scientific research that is unquestionably the general field of surface and interfacial science. The reason for this significance is that the applications of research in this field are both diverse and extremely important to modern technology. A very appealing aspect of research in surface chemistry is its application to a wide variety of important technological problems. Fundamental research, in heterogeneous catalysis, is similar in its intent to the early research in homogeneous chemistry. Its purpose is to enable the chemist to predict reaction pathways and relative rates for heterogeneously catalyzed surface reactions. Thus, much current research in this field is directed toward the identification of molecular structures present on solid surfaces during reaction and relating the observed species to reaction mechanisms. However, a heterogeneously catalyzed chemical conversion, occurring under realistic industrial conditions, is very complicated indeed. The research discussed in this chapter is primarily concerned with the adsorptive and catalytic properties of the surfaces of metallic single crystals that are well characterized chemically and structurally. An understanding of a well-defined, well-controlled catalytic surface is obviously a necessary fundamental step toward understanding the vastly more complex surface of a real operating catalyst.

Effect of Cooperative Behavior on Molecular Vibrational IETS Peak Intensities

Contrary to previous theories of the intensity of molecular vibrational energy loss peaks in IETS, we have shown that the second derivative of the tunneling current varies as n4/3 rather than linearly in n, where n is the concentration of molecules adsorbed on the insulator surface during the junction fabrication. This is a result of considering the proper form for the potential in the junction region from a layer of vibrating dipoles with all of its images. Even though obtained with a simple model for the junction, this dependence agrees with the experimental results of LANGAN and HANSMA.

Application of IETS to Surface Chemistry and Heterogeneous Catalysis

A brief review is given of Inelastic Electron Tunneling Spectroscopy (IETS) as it is applied to investigations both of surface chemical phenomena as well as heterogeneous catatlysis. The use of IETS to probe the vibrational structure of admolecules is compared with alternate electron scattering methodologies, namely, Field Emission Energy Distribution (FEED) measurements and Inelastic Electron Scattering Spectroscopy (IESS). Both the advantages and the disadvantages of each type of measurement are emphasized.

The Structure and Catalytic Reactivity of Supported Homogeneous Cluster Compounds

Although tunneling spectroscopy has been applied heretofore to only a limited extent in the investigation of homogeneous cluster compounds supported on oxide surfaces, it is quite possible that it will prove to be one of the more valuable techniques that can be used to elucidate the structure and catalytic reactivity of this important class of catalysts. The principles of tunneling spectroscopy have been described in Chapter 1 of this book as well as in the previous review articles.(1–5) Examples of the use of tunneling spectroscopy to investigate the catalytic properties of metal crystallites supported on oxide surfaces have been described in previous review articles,(1–5) and, in particular, in Chapter 13 of this text. This application is closely related to the study of supported homogeneous cluster compounds. In the former case, one is concerned with aggregates of reduced metallic atoms attached to an oxide “support,” while in the latter case, the topic of this chapter, one is concerned with the attachment and catalytic reactivity of cluster compounds (which may or may not have lost one or more of their ligands) “supported” on an oxide surface.

Unified approach to photographic methods for obtaining the angles of incidence in low‐energy electron diffraction

An equation is developed to describe the geometrical relationships between the electron gun, the crystal surface, and the phosphorescent display screen in back‐reflection, post‐acceleration LEED experiments. Photographic methods for determining the polar and azimuthal angles of incidence in LEED experiments can be derived starting from this equation. In particular, two published procedures appear here as special cases. New methods are described for cases where the existing techniques do not apply. It is shown that the alignment of the electron gun and the positioning of the crystal can be checked using a photographic technique. An example illustrates that the angles of incidence can be measured with precisions of ±0.2° by recording data on several photographs taken over a wide range in electron energy.

Two‐Dimensional Phase Separation: Co‐Adsorption of Hydrogen and Carbon Monoxide on the (111) Surface of Rhodium

The co‐adsorption of CO and H2 on Rh(111) at low temperature (∼ 100 K) has been studied using thermal desorption mass spectrometry (TDS) and Low‐Energy Electron Diffraction (LEED). The probability of adsorption of CO on rhodium pretreated with hydrogen has been found to decrease slowly with increasing amounts of hydrogen on the surface. In addition, the effect of surface hydrogen on the CO LEED patterns indicates segregation of hydrogen and CO. These results can be explained in terms of a strong repulsive CO–H interaction and a mobile precursor model of CO adsorption.