G. Ertl

Low‐Energy Electrons and Surface Chemistry

Basic Concepts Auger Electron Spectroscopy X-Ray Photoelectron Spectroscopy (XPS) Ultraviolet Photoelectron Spectroscopy (UPS) Electron Spectroscopy with Noble Gas Ions and Metastable Atoms Appearance Potential Spectroscopy Inverse Photoemission (IPE, BIS) Electron Energy Loss Spectroscopy (ELS, EELS) Low Energy Electron Diffraction (LEED) X-Ray Absorption Fine Structure (EXAFS) Vibrational Spectroscopy (HREELS, EELS) Electron and Photon Stimulated Desorption Appendix: Fundamental Constants Properties of Selected Elements Line Positions in XPS Using Al-Kd Radiation XPS Atomic Sensitivity Factors Kinetic Energies of Auger Electrons Relative Auger Sensitivity Factors Character Tables Characteristic Group Frequencies Abbreviations and Acronyms.

Chemisorption of CO on the Pt(111) surface

The system CO/Pt(111) was studied by means of LEED, thermal desorption and work function measurements. At 170 K a √ 3 × √3/R30° structure at θ = 13 is continuously transformed with increasing coverage into a c4 × 2 structure at θ = 12 and finally into a hexagonal close-packed layer with saturation at about θ = 0.68. Due to a decrease of the adsorption energy by about 4 kcalmole at θ = 0.5 (=7.5 × 1014 moleculescm2) adsorption is completed at this coverage at room temperature. The initial adsorption energy is about 33 kcal/rnole. A strong tendency for disordering far below room temperature manifests itself with the LEED patterns and with the Δφ data. The work function at first decreases, exhibits a (temperature-dependent) minimum at θ = 13, attains nearly the value of the clean surface at θ = 12 and again exhibits a second (shallow) minimum around θ = 0.6. A detailed discussion reveals that the observed effects may be explained by assuming the occupation of two different adsorption sites at θ⩽ 0.5 with different dipole moments (presumably bridge (A) and threefold coordinated (B)) whose adsorption energy differs by only 0.5 kcal/mole. At low temperatures at θ = 12 sites A and at θ = 13 sites B are preferentially occupied whereas their small energy difference favours disordering at increasing temperature.

Adsorption of hydrogen on a Pt(111) surface

Abstract The H 2 /Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H 2 was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H 2 /D 2 exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I / V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at −15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β 2 -) state desorbing with second order kinetics below 400 K and a low temperature (β 2 -) state that fills up, in the main, after the first peak saturates. The β 2 -state is associated with an activation energy for desorption E ∗ of 9.5 kcal/mole. The decrease E ∗ with increasing coverage and the formation of the β 1 -state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δϑ was found to have a small positive maximum (∼ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of −230 mV at saturation. The variation of Δϑ with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δϑ compared well with the results of the analysis of the thermal desorption spectra.

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √pH2, indicating atomic adsorption. Initial heats of H2 adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases Ead being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D2 is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.

A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface

The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E*LH =24.1 kcal/mole and a pre‐exponential υ4 =0.11 cm2 particles−1 sec−1. At very high oxygen coverage, E*LH decreases to about 11.7 kcal/mole and υ4 to about 2×10−6 cm2 particles−1 sec−1. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO2 below 540 K. The C...

Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals

Publisher Summary Catalytic oxidation of carbon monoxide over catalysts from the platinum group metals has been investigated. Apart from its enormous practical importance, this reaction is considered to proceed through a relatively simple mechanism because only diatomic molecules are involved and product formation occurs presumably only over a very few steps. This chapter discusses the adsorptive properties of the reactants, their mutual interaction, and the mechanism and kinetics of product formation as well as the investigations with well defined single-crystal surfaces. The activity of a catalyst for a particular reaction is strongly dependent on the surface structure. Directive investigation of the influence of the surface structure on the catalytic activity was performed by using a platinum single crystal whose surface was curved in such a way that not only the plane but also vicinals with varying step density of two different crystallographic directions were present.

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

Abstract The adsorption and desorption of O 2 on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O 2 flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν d = 2.4 × 10 −2 cm 2 s −1 and activation energy E D which decreases from 51 to 42 kcal/mole −1 with increasing coverage. A forward peaking of the angular distribution of desorbing O 2 flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.

Adsorption of CO on Pd single crystal surfaces

Abstract Studies of CO adsorption on Pd(110), (210) and (311) surfaces as well as with a (111) plane with periodic step arrays were performed by means of LEED, contact potential and flash desorption measurements. Isosteric heats of adsorption were evaluated from adsorption isotherms. Earlier work with Pd(111) and Pd (100) surfaces is briefly reviewed, yielding the following general picture: The initial adsorption energies vary between 34 and 40 kcal mole and close similarities exist for the dipole moments, the maximum densities of adsorbed particles and for the adsorption kinetics. At low and medium coverage the adsorbed particles are located at highly symmetrical adsorption sites, whereas saturation is characterized by the tendency for formation of close-packed layers.

Chemisorption geometry of hydrogen on Ni/111/ - Order and disorder

The location of a half monolayer of ordered hydrogen adatoms on Ni(111) has been analyzed by Low‐Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS), and Work Function (Δφ) measurements. It is found that the hydrogen atoms are arranged in an overlayer of graphitic structure with a (2×2) unit cell with respect to the substrate unit cell. In the ordered regions, the hydrogen adatoms occupy both types of three fold hollow sites without a detectable difference in the Ni–H bond lengths between the two sites. The Ni–H bond length is found to be 1.84±0.06 A, corresponding to an overlayer‐substrate spacing of 1.15±0.1 A. The relation between this structure and its observed order–disorder phase diagram as a function of temperature and hydrogen coverage is discussed. The disorder is discussed in detail, and a novel ’’atomic band structure’’ interpretation is given.

Identification of the "Active Sites" of a Surface-Catalyzed Reaction

The dissociation of nitric oxide on a ruthenium(0001) surface was studied by scanning tunneling microscopy. The distribution of nitrogen atoms after the dissociation allowed the identification of the “active sites” for this reaction, which are formed by the low-coordinated, top metal atoms of atomic steps. It is proposed that their activity is caused by local changes in the electronic structure. The structure of the steps determines whether they remain active or become deactivated by oxygen atoms. The results demonstrate the complex manner in which the structure of a catalytic surface determines the reactivity ofthe catalyst and confirm the active sites concept.