LEED analysis

We have discussed here, very briefly, some recent observations of small particle surfaces and how these relate to geometrical catalytic effects. These demonstrate the general conclusion that high resolution imaging can provide a direct, structural link between bulk LEED analysis and small particle surfaces. Apart from applications to conventional surface science, where the sensitivity of the technique to surface inhomogenieties has already yielded results, there should be many useful applications in catalysis. A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why results comparable to those described cannot be obtained from commercial catalyst systems. 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

LEED analysis, 28 30, 34 low surface concentrations, 28 46, 47 on metals and supported metals, 38 227-229... 

A qualitative structural model of the reconstructed c(2 x 2) W(1(X)) surface was first proposed by Debe and King on the basis of symmetry arguments. Figure 39 shows this reconstruction model. The surface atoms exhibit only inplane displacements along diagonal directions. A subsequent LEED structure analysis of Barker et al. ° supported this picture. In a more recent quantitative LEED analysis, Walker et a/ deduced a lateral displacement of 0.16A at 200K. 

In the case of Fe(lOO) + c(2 X 2)CO, the LEED analysis finds that the C and 0 atoms individually and randomly occupy fourfold hollow sites in a c(2 X 2) array, i.e., a c(2 X 2) array of unoccupied sites exists, all other sites being occupied at random by either C or 0 atoms. The average Fe-C and Fe-0 bond length is 1.93 A (C and 0 usually have very similar radii), somewhat smaller than for Fe(lOO) + p(l X 1)0 (where it is about 2.08 A) however, an expansion of the topmost substrate interlayer spacing has not been considered in this dissociative case (the bulk spacing was assumed), resulting in some uncertainty in the Fe-adsorbate bond length as well. 

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

For some references to this story, see note 5 in Ref. 29. An important early LEED analysis... 

A very stable species found on several metal surfaces, including Pt(lll), is ethylidyne, CCH3. ASED-MO calculations agree that the CCH3 species does take on the structure deduced from LEED analysis, namely the C-C axis is perpendicular to the surface, with one carbon bonded equally to three metal atoms. The LEED estimates of a 2.0 A Pt-C distance and a 1.50 A C-C distance agree well with the calculated values of 2.0 A and 1.55 A (see Figure 5). Ethylidyne is produced either with acetylene co-adsorbed with H2 or with ethylene adsorption, both of which are unstable relative to ethylidyne. 

Figure 7 Surface structure of ethylidyne adsorbed on Rfi(lll) solved by LEED analysis. Arrows represent directions of surface reconstruction from bulk positions...

Reconstructions similar to the ones observed on the Pt-Sn system have been observed in some other cases of binary alloys. For instance for Cu-Al(lll) [74] the quantitative LEED analysis [75, 76] showed that the topmost layer is a mixed plane of the same structure of the reconstructed PtsSnClll) surface. Also a (v X /3) R30°reconstruction has been observed for the (111) surface of the random substitutional Al-6.5at% Li alloy, [77] (Quantitative crystallographic data not available). Nothing comparable to the pyramidal structures observed by STM on the PtsSnClOO) system has been reported so far for other alloy systems. 

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