Adatoms single-crystal surfaces

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

A block model of defects on a single-crystal surface is depicted in Figure 2.4.17 The surface itself in reality is a two-dimensional defect of the bulk material. In addition, one-dimensional defects in the form of steps which have zero-dimensional defects in the form of kink sites. Terraces, which are also shown in the figure, have a variety of surface sites and may also exhibit vacancies, adatoms, and point defects. Surface boundaries may be formed as a result of surface reconstruction of several equivalent orientations on terraces. 

The other study examined the influence of Sn adatoms on the dipole-dipole interactions at Pt single-crystal surfaces. On Pt(llO), CO was bonded predominantly on... 

FIGURE 1.11 The structure of the face of a defect-containing single crystal surface with a face-centered cubic system. The spheres indicate the ions in solution, the cubes the metallic adatoms, and the rest the metal adsorbed ensembles. 

Thiophene HDS Activities have been reported by Bussell et al. over three low Miller index single-crystal surfaces of Mo and four of Re (104-106). The reaction over Mo was insensitive to the surface structure. The Re(0001) surface showed about the same activity as the Mo surfaces, but Re( 1121) was twice as active, and the (1120) and (1010) surfaces were approximately sixfold more active (106). While Mo surfaces were covered with a near monolayer of partially hydrogenated carbon after reaction, the Re was not moderated by a carbon overlayer. The product distribution over Mo(100) was similar to that reported for powder MoS2 catalysts, although the single-crystal surface of pure Mo was much more active (104). Measurements of the rate of hydrogenation of 35S on Mo(100) suggest that sulfur adatoms are not intermediates in thiophene HDS (104). 

In the last three to fom years the study of the coadsorption of anions and other solution components with adatoms plays a central role in order to clarify the structure of adlayers formed on single-crystal surfaces. In most of these cases the layer of Cu adatom formed on Pt(lll) [43-51] and Au(lll) [50, 52-61] surfaces was studied. Several studies were devoted to UPD of Ag on Au(lll) surfaces [62-66]. 

Regarding surface spectroelectrochemistry, the intense interest in the electrochemistry of single crystal surfaces, adatoms, oxide layers, and monolayers in the last decade has seen not only the enhancement of traditional methods (e.g., infrared (IR) and Raman microspectroscopy) but also the development of relatively new surface specific methods such as second-harmonic generation, sum-frequency generation, surface plasmon resonance (SPR), and surface-enhanced resonance spectroscopy (SERS). The increased access to synchrotron radiation has led to X-ray absorption and X-ray diffraction methods becoming more feasible as in situ techniques for thin films or species generated close to the electrode. In this article, for... 

In a relatively simple model by Leiva et al., the impact of adatom coverage in terms of a nearest-neighbor electronic effect versus a third-body effect was developed and compared to experimental results for both bismuth (Bi)- and antimony (Sb)-modified Pt single-crystal surfaces, shown in Fig. 3.7 [56]. One of... 

The most commonly investigated substrates have been Pt and Pd, ranging from well-defined single-crystal surfaces to nanoparticles. Bismuth (Bi) has been the most extensively tested adatom [18-28]. Other adatoms that have also exhibited performance enhancements are lead (Pb) [29-31], antimony (Sb) [2,22,29,32,33], arsenic (As) [34, 35], gold (Au) [36], tellurium (Te) [37, 38], selenium (Se) [39], ruthenium (Ru) [40], and palladium (Pd) [5,40,41]. Researchers have seen that, for the various adatoms, higher coverages promote the direct reaction pathway. 

Molecules that dissociate upon adsorption can act as intermediates in surface-catalysed chemical reactions. A good example is the catalytic formation of biphenyl (CgHj-CgHj) from adsorption of iodobenzene on metal surfaces. The reaction has been studied on well-defmed single-crystal surfaces in vacuum. With the Cu(lll) surface, the reaction proceeds via a number of steps. First, iodobenzene adsorbs to the copper surface, dissociating above 180 K to form surface-bound iodine atoms (an adatom, i.e. adsorbed atom) and phenyl intermediates. Subsequently, between 180 and 300 K, phenyl moieties diffuse on the surface, while above 300 K they react to form biphenyl, which subsequently desorbs upon its... 

As the crystal temperature is raised starting from 0 K, the first process that is activated is surface diffusion. This is the random walk of adatoms, molecules, small clusters, or vacancies over atomically flat terraces or across atomic steps of a single crystal surface. Since this volume deals with clean surfaces, we focus on surface self-diffusion, or homodiffusion, in which the diffusing species is of the same chemical nature as the substrate. 

These results are all consistent with a mechanism which involves the comhination of O2, a and Eta in the formation of an adsorbed intermediate, Ig, which can then branch out to evolve EtO (leaving an oxygen adatom behind on the surface) or to produce CO2 + H2O. The oxygen adatom thus inherent to EtO production must itself go on to make either CO2 or H2O, which sets a theoretical upper-limit on the selectivity of 6/7. This agrees within experimental error with the maximum selectivity observed on both high surface area catalysts (2,35,37) and single-crystals (26,27). 

Clues for the Molecular-Level Understanding of Electrocatalysis on Single-Crystal Platinum Surfaces Modified by p-Block Adatoms... 

Most recently, we have been able to obtain the in situ surface EXAFS spectrum of a half-monolayer of underpotentially deposited copper on a bulk Pt(lll) single crystal pretreated with iodine. The spectrum shown in Fig. 23 is a bit noisy (due to limited number of scans) but at least five well-defined oscillations can be observed. Preliminary data analysis indicates that the copper adatoms sit on threefold hollow sites with copper neighbors at 2.80 0.03 A. This distance is very close to the Pt—Pt distance in the (111) direction and indicates the presence of a commensurate... 

Under conditions of step flow, the ability to grow good crystalline material is related to the mobility of the adatoms on the surface. These must be able to diffuse freely and find the proper crystal lattice sites for growth, wherever these are available. In this section, we discuss our calculations of the diffusion barriers on the Si (100) surface and the single-height steps. We shall restrict our discussion to the motion of adatoms even though there is considerable evidence that mass transport via dimer diffusion plays a role at high temperatures as well. ... 

Surface science studies of CO oxidation on Au(llO) single crystals have been made previously in which a Pt filament was used to adsorb oxygen adatoms ( o 0.25) on the Au(llO) surface and a CO titration was performed subsequently (7). CO did react to form CO2 with Eapp = 2 1 kcal/mole. Since CO was not observed to adsorb on Au(llO) at 125 K, it is only physisorbed (as on Au(lll)) and we can estimate that Elh = 7 kcal/mol on Au(llO). The difference from Au(lll) is probably due to a weaker Au-O bond on Au(lll) which leads to a lower barrier for reaction. No surface carbonate was formed from CO2 + Oa on Au(llO) either (7). This is in contrast to the behavior on Ag. Exposing oxygen covered Ag(llO) (16) or Ag(lll) (17) to CO2 produces carbonate species which are stable to 485 K on the surface. 

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