Adlayer evolution

The STM has also been used to follow the evolution of surface-confined reactions such as the oxidation of adsorbed sulfide to form adsorbed Sg and iodide to polyiodide [275,288,289]. The substrate exerts a strong influence on the dimensions and ordering of the adsorbed molecules, particularly the formation of the first monolayer. In a similar manner, studies of the impact of different adlayer structures on the electron transfer kinetics of various soluble redox species have been initiated [290]. 

Figure 3 Sketch of an example of the evolution of a system during a temperature-programmed desorption experiment in the system s phase diagram. The fat line indicates the change of the temperature and coverage during the experiment, and the thin lines indicate the phase transitions (see text). The snapshots below the order-disorder transition line are taken during a simulation of the experiment. The coverages are 0.3, 0.5, and 0.7 ML. The snapshots above the order-disorder transition line show adlayers of 0.3 and 0.7 ML at high temperatures...

We will skip reactions in which three or more atoms or molecules take place. The rate equations for these reactions can be derived in the same way as those for the bimolecular reactions. We want to address here the question of what to do when the mean-field approximation is too crude. It is possible the simulate the evolution of the adlayer taken into account the exact configuration using dynamic Monte Carlo. The way to do this is described later on. Here we want to show how the rate equations can be improved. Working with these improved equations is often preferable, because they require less (computer) work, and they are easier to interpret than the results of simulations. Even when one does perform simulations, it is useful to have rate equations, or their improved version, to interpret the results of the simulations. 

Cut 1001 In comparison to Cu( 111), it is clear from Fig. 1 that the desorption charge prior to the onset of hydrogen evolution amounts to far less than a monolayer equivalent charge. At potentials above - -0.300 V the surface is covered by a (V2 x x/2)R45° chloride adlayer as shown in Fig. 5, while at slightly more negative potentials an order-disorder transition occurs that is accompanied by the desorption of less than 0.006 mC/cm2 (i.e. the first desorption wave in Fig. 7). The (V2 x V2)R45° adlayer leads to step faceting in the <100> direction. This corresponds to the close packed direction of the adlayer which stabilizes the... 

Fig. 11. Potential modulation of the order-disorder transition of the chloride adlayer on Cu( 100) may be used to influence roughness evolution during film growth. Film deposited at -0.1 ML/s from 0.1 M HC104 + 0.001 M Cu(C104)2 + 0.00001 KC1.

The evolution of the adlayer and the substrate is described by the Master equation... 

The adsorption of cw-l,2-dichloroethene on Cu(llO) was followed as a function of exposure over the temperature range of 85 K to 450 K. RAIRS was used to obtain chemical and molecular information on the adlayer while TPD and MBARS were utilised to detect the evolution of products. From these data, three main reaction regimes can be identified, as shown in Figure 1 which displays the TPD trace of molecular desorptions following 14 L exposure of cj5-l,2-dichloroethene to Cu(l 10) at 83 K. Regime I, occurring between 85 and... 

In this review, after a brief overview of the structural and electronic properties of metal adlayers, there are six sections describing catalytic effects on redox couples, oxidation of organic molecules, carbon monoxide, organic electrosynthesis reactions, hydrogen evolution, oxygen reduction, and metal electrodeposition. Outside the scope of this review are other UPD processes that play a role in determining the catalytic properties of electrode surfaces such as the UPD of H and OH. 

Hydrogen evolution has been found without exception to be inhibited by adlayers of Bi, As, Cu, and Sn on Pt [141-143], Pb, Tl, and Cd on Pt [144, 145] and Au [144, 145], and by Pb and Tl on Ag [146] electrodes. All these metals exhibit a large overpotential for hydrogen evolution. Adlayers inhibiting H2 evolution are of interest for fundamental electrocatalysis and electrode kinetics, but they also have practical significance in promoting sorption of H into... 

The mechanism of significant H2 evolution on Pt with large coverage of inhibiting metal adatoms is not clear. For Bi on Pt(lOO), the existence of the c(2 x 2) Bi adlayer was assumed, and it was proposed that Fl2 evolution takes place through fourfold symmetry holes in the adlayer [150, 151]. This adlayer exists at Pt(lOO) [71, 72], but its structure still needs to be verified during H2 evolution. In situ verification of... 

Several approaches were used to quantify inhibition effects of metal adlayers. These involved calculations of currents of H2 evolution based on the order-disorder theory of alloys (141, 144, 145], and simulations based on geometric [150, 151, 153] and long-range electronic effects [150, 151], Verification of the models used in some simulations seems... 

Figure 2. A) Voltammetric desorption of bromide from an irreversibly adsorbed Pt(lll)-Br layer, in O.IM H2SO4. Arrows indicate the evolution of the voltammetric profile. Sweep rate 50 mV s . B) Constant current STM image of the close-packed bromine adlayer on Pt(l 11).

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