UHV experiments

In principle, a measurement of upon water adsorption gives the value of the electrode potential in the UHV scale. In practice, the interfacial structure in the UHV configuration may differ from that at an electrode interface. Thus, instead of deriving the components of the electrode potential from UHV experiments to discuss the electrochemical situation, it is possible to proceed the other way round, i.e., to examine the actual UHV situation starting from electrochemical data. The problem is that only relative quantities are measured in electrochemistry, so that a comparison with UHV data requires that independent data for at least one metal be available. Hg is usually chosen as the reference (model) metal for the reasons described earlier. 

The main problem in the analysis of E vs. 0 plots is that the two quantities are usually measured independently on different samples. It may happen that the surface structure differs somewhat so that for the sample on which E is measured is different from that of the sample used in UHV experiments. This is especially the case with polycrystalline surfaces, whose structural reproducibility is occasional, but it is also the case with well-defined crystal faces if reconstruction phenomena are possible.60 The problem persists also in the absence ofreconstruction since the concentration and/or distribution of surface defects may be differ-... 

In the case of ionic adsorbates, the variation in WS50is normally unable to provide a clue to the molecular structure of the solvent since free charge contributions outweigh dipolar effects. In this case UHV experiments are able to give a much better resolved molecular picture of the situation. The interface is synthesized by adsorbing ions first and solvent molecules afterward. The variation of work function thus provides evidence for the effect of the two components separately and it is possible to see the different orientation of water molecules around an adsorbed ion.58,86,87 Examples are provided in Fig. 6. 

As discussed in Section I.3(i), AX indicates the variation in the work function of a metal as an interface is created by bringing a solid and a liquid in contact. In principle, it should be possible to compare AX values with A values measured directly in gas phase experiments. This is the aim of UHV synthesis of the electrochemical double layer868 in which the electrode interface is created molecule by molecule, starting with the bare metal surface. It is thus possible to obtain evidence of ion-water interactions that can be envisaged from electrochemical measurements but that are not directly demonstrable. Wagner55 has given a recent comprehensive review of electrochemical UHV experiments. 

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

From the analysis described above, we now know that a very important molecule that may be adsorbed together with water is OH. Also, this system has been studied quite extensively within surface science [Thiel and Madey, 1987 Bedurftig et al., 1999 Clay et al., 2004 Karlberg and Wahnstrom, 2005]. It appears that a mixed water—OH system forms a hexagonal structure much like the water stmcture discussed above (see Fig. 3.13c, d). Both from DFT calculations and UHV experiments, the most stable stmcture appears to be that where every other molecule is water and every other OH. This is interesting, since it coincides with the electrochemical observation, discussed above, where the maximum OH coverage was measured to be about one-third of a monolayer [Stamenkovic et al., 2007a]. 

This would be consistent with a transformation of a COad-rich (2 x 2)(2CO + O) adsorbate phase [Schiffer et al., 1997] into an oxygen-rich, but stiU COad-containing phase, for example the (2 x 2)(CO + 20) phase known from UHV experiments [Narloch et al., 1994], CO adsorption on the Ru(OOOl) surface at 0.7 V is essentially inhibited [Wang et al., 2001], most likely as a result of surface blocking by OHad/ Oad species. 

Comparing Figure 1d with Figure le, it is evident that there are two important features of the electrochemical interface which cannot be reproduced yet by the simulation the above mentioned ionic excess charge in the diffuse layer and the bulk electrolyte ions with their screening properties. Fortunately, the condition of zero diffuse layer charge can often be extracted from electrochemical data such that the absence of the diffuse layer does not seriously depreciate the purpose of the UHV experiment. Similarly, it may be expected that the structural properties of the inner layer, tor a certain composition, do not depend on the electrolyte concentration in the bulk solution phase. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

One way to illustrate the effect of the EDL is to compare in situ electrochemical reactions with their equivalent UHV counterparts. Due to their roles in fuel cells, the methanol oxidation reaction and the oxygen reduction reaction are two such reactions for which numerous in situ and UHV experiments have been performed. 

During the last decade STM has proven to be a unique tool for the synthesis of novel structures. Perhaps the most elegant demonstration of this was the positioning of individual Xe atoms on Ni(l 10) with atomic precision in a low-temperature UHV experiment [516]. A variety of structures that exhibit the physics of quanmm confinement have been produced in this manner [517], and more recently, the manipulation of individual molecules at room temperature has been demonstrated [518,519]. It is now clear that there are several possible mechanisms for atomic and/or molecular manipulation [520]. Similarly, two reviews of various related schemes for sub-[im surface modification are also available [521,522]. In addition to published... 

Fig. 5. Left AG/Area plot of trial oxide and O adlayers as a function of temperature T at PO2 = 10—12 atm a pressure characteristic of UHV experiments. Right an ab initio phase diagram displaying stability regions (minimum values of AG/Area) as a function of T and PO2. Characteristic Industry and UHV pressures are indicated.

Realistic operating pressures in catalytic reactions are orders of magnitude higher than those used in most surface-science experiments, and the chemical potential of the gas, usually neglected in the UHV experiments, becomes a significant contribution to the free energy of the surface layer. This pressure difference implies that the structures monitored under... 

Vacuum valves for UHV experiments are bellows-actuated. Where samples and other objects must be transferred through an opening, gate valves are preferred. Metal sealed valves are employed otherwise. 

One would therefore expect that vibrational spectra of CO adsorbed on the platinum particle array should resemble spectra of CO on stepped high Miller index platinum single crystals. However, in preparation for UHV experiments, the... 

In summary, although there are open questions, the possible enhancement of SFG signals of CO on metal nanoparticles on dielectric substrates deserves further attention, and UHV experiments carried out with better controlled samples (particle morphology, cleanliness) are recommended. 

Comparatively more features related to adsorbed hydrogen were observed in UHV experiments for example, on Al203-supported Ir, bands in the region... 

A lack of CO desorption observed for surfaces with variable structures is difficult to reconcile with work on dispersed ceria powders which are known from FTIR to adsorb CO and retain it in vacuo. It would seem that CO adsorption must occur at many structurally distinct sites on the ceria surface. Appearance of surface species on reduced ceria, may indicate that in fact some CO does adsorb, but is not seen in TPD because CO dissociates and the C and O diffuse into the bulk, rather than desorbing. A lack of CO adsorption also implies inability of CO to reduce ceria surfaces, although CO readily reduces ceria powder. It is possible that the low pressure conditions of UHV experiments is partly responsible for the general lack of CO adsorption and surface reduction. Li et al have shown that CO species linearly bound to Ce are unstable in vacuum and these may be partly responsible for the reduction of CCO2/ ... 

Superlattice structure transformations occur usually as order-disorder phase transitions in UHV experiments [3,254, 3.273, 3.274],... 

---