Atop site

Carbon monoxide chemisorbs in atop sites on the clean Nl(lOO) surface for coverages up to 0=0.50 where a well-ordered c(2x2) lattice is observed. A further Increase In CO coverage to the... 

Final detennination of the structure was made by proposing a structural model with Cu sitting in threefold hollow sites and O atoms on atop sites with respect to the Cu atoms (Fig. 27.16). A program, FEFFIT, was used to analyze the data (Stem et al., 1995). This calculates the phase and amplitude parameters for the various backscatters. The EXAFS for the parallel polarization could be fitted six Cu-Cu interactions at a bond distance of 2.67 A and three Cu-Pt interactions at 2.6 A. For the perpendicular polarization, the data could be fitted one Cu-0 interaction at 1.96 A and three Cu-Pt interactions at 2.6 A. The Cu-Pt bond length is shorter than the sum of the metallic radii of Cu and Pt, which is 2.66 A. This indicates a Cu oxidation state different from zero, which agrees with the XANES results. 

Figure 7. Total internal reflection sum frequency generation (TIR-SFG) vibrational spectroscopy of high-pressure room temperature adsorption of carbon monoxide on PVP-protected Pt cube monolayers and calcined (373 K, 3h) monolayers [27], The infrared spectra demonstrate CO is adsorbed at atop sites, but is considerably red-shifted on the PVP-protected Pt cubes. After calcination, the atop frequency blueshifts to 2085 cm in good agreement with CO adsorption on Pt(l 0 0) at high coverages [28], (Reprinted from Ref [27], 2006, with permission from American Chemical Society.)...

Figure 7.4 (A) STM image (240 x 125)2 of two rotational domains of the Moire pattern formed at Pt(l 11) by CO at 1 bar at room temperature. (B) High-resolution image of the CO overlayer at 1 bar. (C) The ( /l9 x /T9) R23.40—13CO structure the unit cell is shown the dark circles represent CO molecules adsorbed in nearly atop sites. (Reproduced from Ref. 9).

CO occupies the atop site. The authors argue that adsorption, even at 4 K, is in the chemisorbed state with the molecular axis oriented perpendicular to the surface. In a physisorbed state, variations in the orientation, including where the C-0 axis is parallel to the surface, would be expected to maximise the van der Waals interaction. The oxidation of CO at Cu(110) is discussed elsewhere (Chapter 5). 

Figure 8 Photoelectron diffraction data (normal emission) for the surface formate species on (a) Cu 100] and (b) Cu 110). Insets A) The aligned atop site and B) the aligned bridge site. After [51.

In order to elucidate the results of the CO TPD experiment, the detailed structure of the oxygen-modified Mo(l 12) surfaces and the adsorption sites of CO on these surfaces have been considered. Zaera et al. (14) investigated the CO adsorption on the Mo(l 10) surface by high-resolution electron-energy-loss spectroscopy (HREELS) and found vinfrared reflection-absorption spectroscopy that at low exposures CO is likely bound to the substrate with the C-0 axis tilted with respect to the surface normal. They, however, have also shown that CO molecules adsorbed on O-modified Mo(l 10) exhibi Vc-o 2062 and 1983 cm L characteristic to CO adsorbed on atop sites. Thus it is supposed that CO adsorbs on top of the first layer Mo atoms. 

Quantitative determination of the local adsorption structure of carbonate on Ag(llO) has been done by Kittel etal. [110]. They have found that the carbonate species is essentially planar and adsorbs almost parallel to the surface at the off-atop site with respect to the outermost layer Ag atom. The C—Ag layer spacing was 0.264 0.009 nm, with a well-defined azimuthal orientation. This geometry is understood best in terms of the added-row model proposed by Guo and Madix. This model assumes that additional Ag atoms lie adjacent to the carbonate, such that the... 

Data was taken in the electron energy range of 10-200 eV, but little sensitivity to the organic adsorbate is found above 100 eV. The observed diffraction pattern arises from three equivalent 120° — rotated domains of (2 X 2) unit cells. The optimum agree "ent between calculated and experimental intensity data for the metastable acetylene structure is achieved for an atop site coordination. The molecule is located at a z-distance of 2.5 A from the underlying surface platinum atom. However, the best agreement is obtained if the molecule is moved toward a triangular site, where there is a platinum atom in the second layer, by 0.25 A, as shown in Fig. 7.2. 

This local adsorption geometry for formate on Cu(100) and Cu(110), with the molecular plane perpendicular to the surface and bonding through the two carboxylate atoms in near-atop sites, is also seen in other simple carboxylate species adsorbed on Cu(110), notably acetate (CH3COO—) [103] and benzoate (CgFIjCOO—) [104] formed, respectively, by exposure to acetic acid and benzoic acid. Relatively recent X-ray spectroscopy measurements combined with theoretical calculations provide further information on the bonding of formate and acetate on Cu(110) [105]. 

Figure 1.17. Electron charge density difference contour map for CO on Ni(100) and CO on Ni(100)/H in atop sites, derived from DFT calculations.

By combining the present ELS studies with previous TDS (23, 26, 59) and LEED (23, 59) experiments we can now present a fairly complete picture of CO chemisorption on Rh(lll). At very low exposures a single species is present on the surface located in an atop site ( Rh-CO = 480 cm-l, Vq=q = 1990 cm l). As the coverage increases, the bonding to the surface becomes weaker (vRh c decreases, increases, TDS peak maximum shifts to... 

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