Most active surface intermediates

Adsorption of carbon monoxide on a gallium-doped sample [NiO(10 Ga)(250°)] precovered by oxygen decreases the electrical conductivity of the solid, whose color changes from black to green. Carbon dioxide is therefore formed. It appears from cycles 1 and 2 (Table XIII) that the interaction product remains adsorbed on the most active surface sites (6 = 0) and is desorbed from less active sites (6 = J0m)-Carbon dioxide is indeed found in the cold trap (1 cm /gm). Since cycle 3 (Table XIV) is balanced neither for 0 = 0 nor for 0 = 0m, the intermediate formation of C03-(ads) is precluded. 

The formate, formed by oxidative dehydrogenation of the acid, is quite stable and doesn t decompose until 480 K. This decomposition is a classical first-order case with a decomposition activation energy of 130 kJ mol-1 and a normal value pre-exponential of 1013 s-1. The great ability of the TPD technique is the separation of the individual steps in the reaction in temperature. It is clear that the step proceeding over the highest barrier in this case is the formate decomposition, and that in a catalytic oxidation of formic acid the most abundant surface intermediate is likely to be the formate with its decomposition being rate determining. 

The first paper on methanol electrocatalysis under UHV conditions was published by Attard et al. [139] on the most active surface, Pt(110). Similar results to those on Pt(l 11) were found, that is, carbon monoxide and molecular hydrogen, but with a slightly larger methanol surface coverage of 9 = 0.10. It was the first time that methoxy species were proposed as intermediates and were different from the carbon monoxide or formyl species proposed earlier by Bagotskii et al. [140], However, traces of the formyl species were also detected on reconstructed Pt(l 10) using vibrational spectroscopy, which was able to co-adsorb this species with atomic oxygen [117]. 

IN SSITKA, Np and the mean surface residence time of these most active reaction intermediates (xp) are determined. After a step-change between two reactant streams containing different isotopes of a reactant without disturbing other reaction conditions or reaction (as long as an H2/D2 switch is not used), the distributions of isotopically labeled products are monitored using a mass spectrometer. Tp is first determined by integration of the normalized isotopic transient of a product relative to an inert tracer (usually Ar) that delineates gas phase hold-up (see Figure 1 for the case of methanation). Np is then calculated from... 

The Most Abundant Surface Intermediate (MASI) Approximation Catalytic transformations may include the formation of many intermediates on the catalyst surface, which are difficult to identify. In these cases, it is impossible to formulate a kinetic model based on all elementary steps. Often, one of the intermediates adsorbs much more strongly in comparison to the other surface species, thus occupying nearly all active sites. This intermediate is called the most abundant surface intermediate masi [24]. For a simple monomolecular reaction, Aj A2, the situation can be illustrated with the following scheme ... 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

A second mechanism by which impurities present in the solution interfere with measurements in electrode kinetics is associated less with their providing alternative paths to reactions and more with their adsorption on the electrode surface. The presence of adsorbed impurity material on the electrode surface may affect the rate of an intended reaction, particularly where there is an intermediate in the mechanism of the intended reaction.25 Thus, the adsorbed impurity may block reactive sites on the electrode. Since the intended reaction (say, the reduction of oxygen) cannot now use the blocked active sites on the electrode surface, its kinetics are slowed down, particularly when the impurity adsorbs on the most active catalytic sites on the electrode. Again,... 

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

The intermediate region becomes enormously spread out and encompasses an interval in which the reaction rate varies in proportion to the extent that the rate of diffusion or other transport of the substance to the most easily accessible parts of the active surface exceeds the rate of transport to the least accessible parts.1... 

Whereas it seems justified to assume that the most unsaturated parts of the surface of iron catalysts are the most active in the synthesis, it may well be that on Mo and W, which are supposed to bind nitrogen stronger, an intermediate part of the energy spectrum functions most actively in the catalytic reaction, and it may therefore be more nearly correct to say that in this case surface nitrides are the catalysts. 

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