Adsorption of CO on Pd(100)

Journal Article · Mon Sep 15 04:00:00 EDT 1980 · J. Chem. Phys.; (United States)

DOI:https://doi.org/10.1063/1.440430· OSTI ID:5076480

Behm, R J; Christmann, K; Ertl, G; Van Hove, M A

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2..sqrt..2 x ..sqrt..2) R 45/sup 0/ structure occurring at THETA=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd--C distances of 1.93 +- 0.07 A and a C--O bond length of 1.15 +- 0.1 A, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short-range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (E/sub ad/=38.5 kcal/mole) is constant up to a coverage of THETAapprox. =0.45. The photoelectron spectra exhibit two maxima at 7.9 (5sigma+1..pi.. level) and 10.8 eV (4sigma level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with coverage is described in terms of a second-order precursor state model with repulsive interactions. The pre-exponential factor for desorption (3 x 10/sup 16/ sec/sup -1/) varies only little with coverage. The dipole moment of the adsorbate remains constant up to THETAapprox. =0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near THETA=0.5. The maximum work function increase is ..delta..phi/sub max/=0.93 V at THETA=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to THETA=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier.

• Research Organization: Institut fuer Physikalische Chemie, Universitaet Muenchen, Muenchen, West Germany
• OSTI ID: 5076480
• Journal Information: J. Chem. Phys.; (United States), Journal Name: J. Chem. Phys.; (United States) Vol. 73:6; ISSN JCPSA
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360104* -- Metals & Alloys-- Physical Properties
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201 -- Chemical & Physicochemical Properties
ADSORPTION
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
COHERENT SCATTERING
DIFFRACTION
ELECTRON DIFFRACTION
ELECTRON SPECTROSCOPY
ELEMENTS
METALS
OXIDES
OXYGEN COMPOUNDS
PALLADIUM
PHOTOELECTRON SPECTROSCOPY
PLATINUM METALS
SCATTERING
SORPTION
SORPTIVE PROPERTIES
SPECTROSCOPY
SURFACE PROPERTIES
TRANSITION ELEMENTS