Surface step techniques

Kontturi et al. studied TEA ion transfer across water-1,2-DCE microinterfaces covered by different PCs using short potential step techniques [12]. The enhancement in the forward rate constant was observed for all lipids and increased with the surface coverage (Fig. 6). 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

The REM and SREM techniques have recently been shown to be very powerful for the study of flat surfaces of large crystals or bulk specimens (19,20). Single-atom surface steps may be seen clearly with a lateral resolution of 1 nm or better and the interactions of surface steps with bulk defects can be investigated. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

The relative simplicity of CO oxidation makes this reaction an ideal model system of a heterogeneous catalytic reaction. Each of the mechanistic steps (adsorption and desorption of the reactants, surface reaction, and desorption of products) has been probed extensively with surface science techniques, as has the interaction between O2 and CO " . These studies have provided essential information necessary for understanding the elementary processes which occur in CO oxidation. 

This very short treatment of reversal techniques has the following basis. There are certainly treatments in the literature of chronopotentiometiy dealing with current reversal, or reversed-step voltammetry. However, their validity has to be diligently examined in each application. For example, is an assumption of a first-order reaction tacitly involved, when the actual solution may correspond to a fractional reaction order Another reason for the limited treatment has an eye on the future. There are those who see in the rapid development of in situ spectroscopic techniques (see, e.g., Section 6.3), together with advances in STM and AFM, the future of surface analysis in electrochemistry. If these surface spectroscopic techniques continue to grow in power, and give information on surface radicals in time ranges as short as milliseconds, transient techniques to catch intermediate radicals adsorbed on surfaces may become less needed. 

The experiment can be done using a stationary technique, but the well-known large-amplitude step techniques (either galvanostatic or potentio-static) can also be applied. The latter have the advantage that the response vs. time function can be extrapolated to t = 0, thus approaching more closely the situation where the surface concentrations have not been changed (see Sects. 2.1.2 and 2.2.2). 

The four sections above followed the typical workflow of screening approaches of the early days. In recent years, it became evident that additional steps have to be added on top of the workflow. One such step is the analysis of the true kinetics and the interplay of its elemental reactions by modem surface-science techniques. 

Finally, Marks and Smith52 have shown that surface reconstruction as well as surface steps and facetting can be detected at the atomic level. They observe a (2 x 1) surface reconstruction of a Au surface leading to the loss of every second column of Au atoms at the surface. This work provides surface structural information at the atomic level which is comparable to the macroscopic surface structural information which has been available from LEED techniques for several years. 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

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