Single-crystal faces preparation

Simple experimental conditions for an investigation of the relation (5.28) can be obtained in the case of electrodeposition of silver on silver single crystal faces prepared by the capillary technique [5.29, 5.74, 5.75]. As already shown, such crystal faces are usually intersected by only a few single screw dislocations which produce growth pyramids or cones with an uniform slope (cf. Fig. 5.31). The parabolic dependence (5.28) has been experimentally found in the systems standard AgQtkl)/... 

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

Ni single-crystal faces in HzO + HCIO4 or H2SO4 solutions have been investigated by Arold and Tamm using impedance.743 Ni (100), (110), and (111) single-crystal faces were prepared by the method described by... 

Some measurements with Pt single-crystal faces have been published recently.140,210,773 Iwasita and Xia210 prepared platinum single crystals according to the method of Clavilier et a/.186 773 After flame annealing and cooling in an H2 + Ar mixture, the electrode was protected... 

Table 26 shows some steps in the chronological sequence of compilations, which are evidently related to improvements in the preparation and control of electrode surfaces. In second order, the control of the cleanliness of the electrolyte solution has to be taken into consideration since its effect becomes more and more remarkable with solid surfaces. A transfer of emphasis can in fact be recognized from Hg (late 1800s) to sp-metals, to sd-metals, to single-crystal faces, to d-metals, although a sharp chronological separation cannot be made. 

While the pzc of Hg in F solution has not changed by more than 1 mV for over 70 years, marginal variations are visible for Ga, Tl, In, Cd, Bi, Sn, and Sb that are related to electrolyte effects (weak specific adsorption or disturbance of the adsorbed water layer, as for Ga).847 Important variations can be seen, on the other hand, for polycrystalline Ag, Zn, Ni, Fe, and Cu. For all these metals a drop of the pzc to much more negative values has been recorded this is evidently related to an improvement in the preparation of the surface with more effective elimination of surface oxides. All these metals, with the exception of Ag, are naturally sensitive to atmospheric oxygen. Values of pzc for single-crystal faces first appeared in a 1974 compilation,23 in particular for the three main faces of Ag and for Au (110). Values for a number of other metals were reported in 1986.25 However, for sd-metals, an exhaustive, specific compilation of available experimental data was given by Hamelin etal. in 1983.24... 

The main problem in Eas0 vs. correlations is that the two experimental quantities are as a rule measured in different laboratories with different techniques. In view of the sensitivity of both parameters to the surface state of the metal, their uncertainties can in principle result of the same order of magnitude as AX between two metals. On the other hand, it is rare that the same laboratory is equipped for measuring both single-crystal face is not followed by a check of its perfection by means of appropriate spectroscopic techniques. In these cases we actually have nominal single-crystal faces. This is probably the reason for the observation of some discrepancies between differently prepared samples with the same nominal surface structure. Fortunately, there have been a few cases in which both Ea=0 and 0 have been measured in the same laboratory these will be examined later. Such measurements have enabled the resolution of controversies that have long persisted because of the basic criticism of Eazm0 vs. 0 plots. 

The existence of a supersaturation or overvoltage threshold is a characteristic feature of nucleation-induced processes as, e.g., electrochemical phase formation. Based on this phenomenon, several experimental techniques for electrociystallization studies have been developed (cf. Section 4.2). Before going into further details, however, let us discuss some technical skills that can lead to the preparation of well-developed low dislocation density single crystal faces. 

In addition to flame annealing methods for the preparation of single crystal faces of metals with extended atomically smooth terraces [5.10-5.13], the already traditional technique of electrolytic growth of single crystals [5.6-S.9] has remained unrivaled for the preparation of perfect, screw dislocation-free faces or faces with single, isolated screw dislocations. ... 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

In Chapter 1, Antoinette Hamelin deals with the topic of characterization of the double layer at well-defined single-crystal faces of solid metals. She also gives valuable instructions for the preparation and characterization of single-crystal faces of known orientation and index which will be very useful to many workers in this field, especially those who are just entering it as novices or to those whose crystals are not always too accurately characterized Most of the work reported is concerned with crystal faces of gold and silver which this author has specialized in studying. It is her work in recent years that has taken experimental studies of doublelayer behavior well beyond the conventional area of research at liquid mercury. 

There are, of course, some limitations. An obvious one is that this method cannot be applied yet to the preparation of industrial catalysts, another one is its cost because ultra-high-vacuum (UHV) equipment is required. This drawback explains why this method is usually coupled to surface techniques such as XPS, UPS, RHEED, and AES, which also require UHV. The last disadvantage is that the best suited supports are those that are flat, i.e., oxide single-crystal faces, oxides produced by oxidation of a metal single crystal, or compressed powder oxides. There have been several examples where the preparation chamber also serves as sample chamber for surface techniques and is coupled to a catalytic reactor. Whereas there are a number of works using this approach for bulk metals (80), there are, by contrast, few studies dealing with metals supported on either single crystals (81) or polycrystalline supports (78, 79,82, 83). The latter type of system appears to be the model catalysts closest to the real catalyst. 

When we recall that practically all adsorbing systems studied are polycrystalline it is no surprise that Rhodin s results are almost unique, being most nearly approximated by isotherms on salt crystals. Furthermore, the entirely different results for the same absorbate on two crystals of the same substance purified in the same way is a strong argument for heterogeneity operating in at least one case, presumably the polycrystalline preparation. We can only hope that studies will be made on other single crystal faces, to test this conclusion. 

It is to be noted that in many cases, the preparation of a single-crystal face is not followed by a check of its perfection by means of appropriate spectroscopic techniques, and therefore we actually have nominal single-crystal faces. The only values of AX of high reliability are, in... 

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