UHV results

A (111) face of a fee crystal exhibits two types of threefold adsorption sites as shown in Fig. 3.14a. Thus, the number of adsorption sites is twice the number of surface atoms. 

In the following, structural aspects of substrate surfaces and Me UPD overlayers obtained by in situ GIXS, STM and AFM are discussed. UPD systems without and with different crystallographic Me-S lattice misfit are presented. 

Indeed, in situ STM imaging with lateral atomic resolution of a flame-annealed Au(lOO) substrate in deaerated perchloric or sulphuric acid solutions fi ee of Ag ions shows the existence of a potential-induced surface reconstruction with an undulated quasi-hexagonal ( quasi-hex ) structure at Ew 240 mV (cf. Figs. 2.5 and 2.7). Reconstructed domains were not observed at higher hE, which indicates that the quasi-hex reconstruction of the Au(lOO) surface is lifted by the applied positive electrode potential. In deaerated perchloric or sulphuric acid solutions containing Ag ions, Ag UPD is clearly indicated in cyclic voltammograms (Fig. 3.20a) as well as in q E) or r E) isotherms at = constant in the underpotential range 0 mV A 600 mV (Fig. 3.20b). Electrosorption valency measurements under ITL 

Similar Ag UPD phenomena were found on the Au(lll) substrate [3.157, 3.159, 3.165]. In sulphuric acid solution containing Ag ions, an expanded Au(lll)- JZ X VS) R 30 overlayer structure (cf. Figs. 3.14a and 3.15a) was observed at medium underpotentials (A = 500 mV) as shown in Fig. 3.22 [3.165]. This overlayer structure was attributed to Ag UPD on the basis of q measurements. It was concluded that S04 ions are specifically )sorbed to the same extent on both the Au(lll) substrate and the expanded Ag overlayer [3.165, 3.167]. However, in situ AFM studies in the same system combined with ex situ UHV measurements indicated an expanded Au(lll)-(3 X 3) - 4Ag overlayer structure at A = 420 mV as illustrated in Fig. 3.23 [3.157]. The composition of the surface coverage was estimated from a comparison of 

Morphological in situ STM studies showed 2D Ag island formation and limited growth on AuQikl) with (hkt) = (100) and (111) in the UPD range 0 mV AE 250 mV depending on AE and the polarization time, as illustrated in Fig. 3.24, for Au(lOO) [3.196, 3.298]. The islands dissolve during subsequent anodic stripping. The analysis of DTS results obtained in the system Au (100)/Ag , CIO4 at AE = 100 mV revealed 

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A water-alone monolayer potential above the pzc is in accordance with an absolute work function measurement for the water monolayer on Pt(lll) of 4.8 eV (29). Comparing this to the hydrogen electrode (4.7 eV below vacuum (30) for the normal hydrogen electrode NHE) corrected by 7x0.059 V for a nominaI pH 7 yields a water-alone mono-layer potential of +0.5 V vs. RHE at pH 7. This lies 0.3 V above our proposed pzc of 0.2 V RHE. This relatively high apparent potential of the water monolayer has been noted previously (Sass, J.K., private communication), and has raised concern about the relevance of the UHV monolayer to real electrochemical conditions, since most electrochemical measurements of the pzc of polycrystalline Pt have been closer to 0.2 V than to 0.5 V (31). By showing that the water monolayer lies above, not at, the pzc, the present H.+H-O data remove part of the apparent discrepancy between the electrochemical and UHV results. If future UHV work function data show a large ( 0.3 V) decrease in the water monolayer work function upon addition of small (<20X saturation) amounts of hydrogen, all of the apparent discrepancy could be quantitatively accounted for. 

Rotating ring-disk electrodes were employed for an anion effect on the Cu underpotential deposition on Pt(lll) in fluoric and perchloric acid solutions, the results of which were discussed in comparison with UHV results of AES and LEED. ... 

SFG, HP-XPS, and PM-IRAS were employed to characterize elementary steps of these reactions. As shown below, processes that have a minor impact under UHV (such as C O bond scission on Pd(l 11)) may dominate a reaction at ambient pressure. Thus, once again, we have a demonstration that UHV results cannot generally be extrapolated to conditions of technological catalysis. 

In the region, where the reaction is first order with respect to CO and zeroth order with respect to O2 one can approximately calculate the intrinsic reaction rates. They are a linear extrapolation from the UHV-results impingement rates and sticking coefficients of the reactants are rate determining even at normal pressure. 

Our own results (foils) obtained at pqq < 5 10 2 mbar perfectly agree with the UHV results of Golchet and Wliite [l](pco < mbar) and results obtained by Coulston and Haller [7] for pco =5. 10-3 tnbar. For oxidising gas composition, approximately 10 percent of the impinging CO molecules react to form CO2... 

Recently the CO oxidation reaction on supported Pt particles (of different sizes) was studied under applied conditions (elevated pressures and temperatures, as well as steady-state conditions) and by means of different techniques. Monitoring changes in plasmon frequency (INPS, Sect. 5.2.2) the reaction as a function of the mole fraction (at ambient pressures) was measured on Pt catalysts (2-20 nm size) on Si02 and proved to be able to detect CO poisoning [28], comparable to UHV results. Similar sized catalysts were investigated under near atmospheric pressures (in a... 

Conversely, let us examine the situation from a different point of view. Let us suppose that the UHV value for SHE is about 4.8 V. In this case the UHV value for E0-o of Hg would work out to be 4.61 V. This would measure 0, the work function of Hg in contact with water. Vi0 for clean Hg is indeed 4.50 V as measured, the outcome is that the work function of Hg would increase by 0.11 eV upon contact with water. This result is highly improbable on the basis of common observations. 

The strength of the Cu-0 bond will be lower on the Cu(lll) face than on the Cu(100) and Cu(110).593 Indeed, the Cu-0 stretching frequency in UHV is lowest on the (111) face and only a disordered oxygen structure is observed.596 These results suggest that a specific Pourbaix pH -E phase diagram is needed to describe the behavior of each low-index face of Cu. 

Figure 27.42 is a derivative AES spectrum of Pt(lll) before and after UPD Sn deposition (TiUmann et al., 2003). These experiments were done in a UHV transfer system. The results indicate a very clean surface. The oxygen is probably associated with the adsorbed Sn. 

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