Infrared reflection-absorption spectroscopy of adsorbed molecules

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.

http://science.sciencemag.org/content/sci/273/5282/1688.full.pdf

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

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 CO2 product angular distribution varies from cosγ to cos4γ depending sensitively upon the adsorbed reactant concentrations.

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

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.

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

High resolution vibrational spectroscopy of CO on Ru(001): The importance of lateral interactions

Vibrational spectra and LEED structures of CO on the Ru(001) surface were measured as a function of coverage at 100 K. The LEED patterns indicate that CO occupies a variety of asymmetric sites at high coverage, while the vibrational spectra show a single Ru–C stretching frequency (445 cm−1) and a single C–O stretching frequency (1980–2080 cm−1) at all coverages. This shows that on this surface the CO adsorption site at high coverages is determined by interadsorbate interactions, and that the carbon–oxygen stretching frequency is not site sensitive within the resolution of electron scattering measurements.

The vibrational spectrum and adsorption site of CO on the Ru(001) surface

The adsorption of CO and O2 on the Ru(001) surface and the interaction between CO and chemisorbed oxygen on this surface have been studied by measuring the vibrational energies of the adsorbed layer with electron energy loss spectroscopy. Adsorbed CO shows two vibrational energies that are assigned to the metal–carbon and carbon–oxygen stretching frequencies of an upright CO with a single metal–carbon bond. Oxygen adsorbed on Ru (001) has a single vibrational energy that can be attributed to a mode of atomic oxygen perpendicular to the surface. The variation of the frequency spectrum of CO with CO coverage and with the coverage of oxygen in coadsorption experiments is discussed in terms of direct interadsorbate interactions and indirect through‐bond interactions.

High resolution electron energy loss spectroscopy of chemisorbed carbon monoxide and oxygen on the ruthenium (001) surface

Adsorption and Dissociation of Nitric Oxide on the Ru(001) Surface

An electron spectrometer is described which is designed to measure a variety of electron spectra of solid surfaces in ultrahigh vacuum. The instrument is capable of the high‐energy resolution (10–15 meV) required for vibrational inelastic electron scattering from atoms and molecules on surfaces. It has also been designed to carry out angle‐resolved photoemission measurements, Auger electron spectroscopy, and energy‐loss measurements of electronic excitations. The performance of the instrument in these modes of operation is discussed.

Versatile electron spectrometer for surface studies.

Inelastic electron scattering spectroscopy has been used to measure energy-gain as well as energy-loss transitions of oxygen chemisorbed on the Ru(001) surface The relative intensity of the energy-gain to the energy-loss transition was found to follow the expected Boltzmann distribution The intensities of the inelastic peaks relative to the elastic peak were observed to increase with temperature The temperature dependence and sources of experimental error are explained within the context of an approximate theory for the scattering of electrons from adsorbates on metal surfaces

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G. E. Thomas

Papers

The vibrational spectrum and adsorption site of CO on the Ru(001) surface

High resolution electron energy loss spectroscopy of chemisorbed carbon monoxide and oxygen on the ruthenium (001) surface

Adsorption and Dissociation of Nitric Oxide on the Ru(001) Surface

Versatile electron spectrometer for surface studies.

High resolution electron scattering from chemisorbed oxygen on the ruthenium (001) surface