Adatoms catalysis

For Se and Te, oxidation of the adatom takes place at potentials higher than that of CO oxidation. The adatom is always in its reduced state, and no bifunctional catalysis through the transfer of oxygen from the adatom to the CO molecule can take place. 

Watanabe M, Motoo S. 1975. Electro catalysis by ad-atoms. Part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium adatoms. J Electroanal Chem 60 275. 

EXAFS data indicated that tin was only surrounded by four platinum atoms at the same distance of 0.276 nm (Scheme 2.40). This result clearly indicates that tin is located on the metal surface and not in the bulk. For example, in a bulk Pt3Sn alloy tin is surrounded by 12 platinum atoms while in a surface alloy on a platinum bulk it is surrounded by six platinum atoms only. Note that such tin adatoms are always obtained when low amounts of tin are deposited on the metal. This is probably because tetrabutyl tin coordinates first on the metal atoms of the faces, which are the most hydrogenolyzing, rather than corner or edges for which the alkyl ligands remain coordinated to the tin. This fact will be very important in catalysis since it explains selective poisoning of metal particles (see below). 

Using Phi as the substrate, the reaction mixture turned from a light yellow solution to a dark brown suspension after 20 min. However, no conversion was observed by GC analysis. We assumed that Pd ions, oxidised from the anode, were in turn reduced to adatoms at the Pt cathode and formed Pd° nanoparticles, ca. 11 nm in diameter (10). After 8 h, the Phi was totally consumed, giving 80% biphenyl and 20% benzene. Weighing the electrodes before and after the reaction showed difference of 2.5 mg in the Pd anode, equivalent to 0.1 mol% of the aryl halide substrate. This corresponds to a TON of 1000 at least (assuming that all the missing Pd participates in the catalysis). 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

Active O stoichiometry G(T,P) analysis. Figure 14 plots the free energy of adsorption as a function of temperature for the most stable Ag oxide overlayer at each O coverage.14 The pressure is 15 atmospheres, which is characteristic of the pressure used in industrial epoxidation catalysis [10]. We find that at zero Kelvin the most stable overlayer is the Agt 22O overlayer, i.e. the Ag oxide with an additional O adatom inside each oxide ring... 

Such fundamental knowledge about the microscopic processes on the gas-solid interface is also necessary for optimization of many catalytic processes. A statistical mechanical approach, which enables the solution of the many-body problem constituted by the adsorbate layer on the catalytic surface, is essential in the case when lateral interactions between adatoms and molecules are significant. In such cases, non-ideal surface adlayer mixing is often important and the adsorbates form islands on the surface. Hence, microscopic simulations of catalytic processes are necessary to develop an ab-initio approach to kinetics in catalysis. 

Aluminas are extensively used as catalysis and supports for catalytic materials. It is well known that the surface chemistry of adatoms may be influenced profoundly by addition on the alumina of minute amounts of electropositive or electronegative ions like alkalies or halogens (ref 1), Asa result the catalytic action of the active surface phase appears modified (refs 2-5), Such modiftcation may be due to (1) modified symmetry of the active sites, (2) different degree of dispersion of sites, or (3) altered surface coverage (refs. 6-9). Additives absorbed on the active sites of alumina may act as poisons. Such a poisoning action cun be expressed in formal terms, as a function of the concentration of added modifier. 

As in heterogeneous catalysis [44], small coverages of electropositive or electronegative adatoms, acting as promoters or poisons, can have very significant effects in the performance of electrocatalysts, as they can significantly modify the metal work function [10, 23, 44] and, thus, its chemisorptive propensity [10, 23, 44]. 

The overall conclusion reached, which is of importance to catalysis, is the possibility that the strong adatom-adatom interactions may alter electronic structure for the catalyst system and that this may be more important than a simple site-blocking mechanism for poisoning. 

In addition to the enantioselective effect, cinchona alkaloids also produce a rate acceleration, i.e. this is an example of ligand accelerated catalysis [14]. The model of a non-closepacked ordered array of cinchonidine molecules adsorbed on platinum, proposed by Wells and co-workers, was abandoned in their later study [15]. Augustine [16] deduced from the behaviour of this system at low modifier concentrations that the chiral sites are formed at the edge and comer platinum atoms, which involve the adsorbed cinchonidine and a metal adatom. The different authors agreed that the quinoline ring of the modifier is responsible for the adsorption on platinum, the quinuclidine part, through the nitrogen atom, interacts with... 

Calorimetric data for dissociative adsorption have highlighted the importance of lateral interactions between adatoms, particularly for catalysis. For example, the differential adsorption heat for O2 dissociative adsorption on Ni 100) falls from 570 kJ mol i at zero coverage to 170 kJ mol" at 0.5 ML L9 which indicates a next-nearest-neighbour 0-0 pairwise repulsive interaction energy as high as 40 kJ mokL These strong adatom-adatom repulsive... 

Only noble metal electrodes (mainly Pt electrodes) and some of their modified versions (electrodes modified by adatoms) will be considered in this survey. The vast majority of electrosoiption investigations have been performed at Pt-electrodes owing to the central role of Pt in catalysis and electrocatalysis. 

It is well known from catalysis that electropositive (e.g. Na, Cs, K) and electronegative (e.g. S, O, C, Cl) adatoms decrease or increase the reaction rate and thus poison or promote the reaction, respectively [153-155]. Alkali-metal influenced adsorption on transition metals was reviewed by Bonzel [154]. Coadsorption of alkali metals and H, or D, on Al(lOO) revealed that the sticking coefficient and dissociation rate are extremely weak ( 10 at all alkali coverages [156]). Upon exposing alkali-covered metal substrates to a beam of atomic H or D, alkali hydride formation was observed. 

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