Adatoms hydrogenation

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

From chemisorption theory we know that adatom adsorption energies wiU decrease in a row of the periodic system of the group VIII metals when the position of the element moves to the right. The rate of hydrogenation of Cads vviU decrease with increasing adsorption energy of Cads and hence wiU decrease in the same order with element position in the periodic system. 

Addition of very small amounts of Bu4Sn can completely transform the performance of these catalysts by poisoning the hydrogenation sites. For example, when a Ni°/Si02 catalyst is used, the best result corresponds to a 2-carene yield of 30%, with at least 30% of the carenes transformed into by-products. Addition of 0.04 mole of Bu4Sn/Nis results in an increase of the yield of 2-carene, up to 37%, and a decrease of the amount of by-products to less than 10%. In this case, tin is present as adatoms on the most hydrogenating sites (very hkely those situated on the faces rather than on corners and edges). 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

The process for this irreversible adsorption has not been investigated in detail. The mechanism by which the metal is deposited has not been unambiguously elucidated, and several possibilities have been proposed. One possibility is the formation of local cells, with the ion of the adatom being reduced and either hydrogen [Szabo and Nagy, 1978] or platinum [Clavilier et al., 1988] being oxidized ... 

Then, the maximum adatom charge density corresponding to full blockage of hydrogen adsorption is... 

In this equation, it has been assumed that the adatom exerts the same blockage on hydrogen and anion adsorption (i.e., the stoichiometric number m is the same for... 

According to (7.8) and (7.12), the stoichiometry (m/n) can be extracted from the slope of the plots of adatom charge density versus hydrogen (or hydrogen plus anion) charge density. Some representative plots are shown in Fig. 7.3. The conclusions extracted from this kind of analysis are summarized in Tables 7.1 and 7.2 for Pt(l 11) and Pt(lOO) modified surfaces, respectively. The extension of this analysis... 

Figure 7.3 Plot of the platinum (hydrogen plus anion) charge density versus the charge density associated with the adatom redox process (Bi or Te, as indicated) on a Pt(l 11) electrode in 0.5 M H2SO4 solution. Straight lines represent the expected behavior for the stoichiometry indicated in the figure.

Completely different behavior is observed with S and Se, as shown in Fig. 7.8. With these adatoms, deposition on the terrace starts from the very beginning and no selectivity towards the step is observed. Additionally, deposition of the adatom changes the hydrogen adsorption energy on the (110) step sites, as reflected by the progressive shift of the peak at 0.12 V towards higher potential values. 

By varying the temperature at which the experiments were conducted and the distance between the activator and the sensor, the data were obtained (Fig. 4.17) which allowed us to calculate the activation energy of migration of hydrogen adatoms (protium and deuterium) along the carrier surface and coefficients of lateral diffusion of hydrogen atoms appearing due to the spillover effect (see Table 4.2). 

Atoms of metals are more interesting tiian hydrogen atoms, because they can form not only dimers Ag2, but also particles with larger number of atoms. What are the electric properties of these particles on surfaces of solids The answer to this question can be most easily obtained by using a semiconductor sensor which plays simultaneously the role of a sorbent target and is used as a detector of silver adatoms. The initial concentration of silver adatoms must be sufficiently small, so that growth of multiatomic aggregates of silver particles (clusters) could be traced by variation of an electric conductivity in time (after atomic beam was terminated), provided the assumption of small electric activity of clusters on a semiconductor surface [42] compared to that of atomic particles is true. 

Figure 8.7 Frames (23 by 35 A) of an STM movie taken at 65 K at close to a complete monolayer of hydrogen adatoms at Pd(l 11) showing vacancy diffusion. The images (b) and (c) show the aggregation of two nearest neighbour vacancies, which has the appearance of a three lobed object due to the rapid diffusion of one H atom next to the vacancy dimer. (Reproduced from Ref. 24).

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