Deuterium chemisorption

Stobinski, L. (1996) Molecular and atomic deuterium chemisorption on thin gold films at 78 K an isotope effect. Applied Surface Science, 103, 503-508. 

The specific metal surface area of our nickel samples was established by means of deuterium chemisorption, the amount of deuterium adsorbed being determined by exchange with a known quantity of hydrogen followed by mass speetrometric analysis. It was assumed in the calculation that 1 cm3 (NTP) of deuterium corresponds to 3.64 m2 of nickel surface area. 

Figure 6. Time-of-flight mass spectrum of deuterium chemisorption products of Vg, V, and V,o. Reactant mixture is 10% Dj in He.

Deuterium chemisorption on platinum clusters is difficult to observe by product analysis due to the high IP (> 6.42 eV) of the clusters, the high mass of the metal, and the large number of naturally occurring isotopes of Pt. The amount of deuterium uptake is estimated from the increase in the widths of the observed mass peaks. No reaction is observed on the atom through the tetramer either by depletion of the metal ion signal or by product formation. 

For CDIE, besides the inhibition of deuterium chemisorption necessary to regenerate OD photoactivable sites, the electron excess in the metal increases the recombination of holes which is detrimental for the basic activation step of the reaction,... 

The interaction of hydrogen (deuterium) molecules with a transition metal surface c an be conveniently described in terms of a Lennard--Jones potential energy diagram (Pig. 1). It cxxislsts of a shallcw molecular precursor well followed by a deep atomic chemisorption potential. Depending on their relative depths and positions the wells m or may not be separated by an activation energy barrier E as schematically Indicated by the dotted cur e in Fig. 1. 

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

Section II will summarize experimental work designed to determine the extent and manner of benzene chemisorption by the use of physical methods and the radioisotope carbon-14. In Section III, evidence obtained by use of deuterium as a tracer is examined. The relatively small amount of information concerning the mode of benzene chemisorption that is afforded by studies of its hydrogenation is presented in Section IV. 

To summarize, the use of heavy water as a deuterium source has provided a wealth of experimental information. Evidence for the associative ir-adsorption of benzene [species (I) J is secure (2). Evidence for hydrogen exchange in the benzene ring by an abstraction-addition mechanism is less well established, partly because of uncertainties that surround the mode of chemisorption and reaction of water at metal surfaces. Nevertheless, it would be wrong to deny that Scheme 6 is consistent with a large body of experimental work. 

A number of theoretical works have been devoted to the study of the hydrogen-deuterium exchange reaction. Hauffe (25) examined this reaction from the standpoint of the boundary layer theory of chemisorption. Dowden and co-workers (26) undertook a theoretical investigation of the hydrogen-deuterium exchange reaction from the viewpoint of the theory of crystalline fields. 

We shall consider the hydrogen-deuterium exchange reaction from the viewpoint of the electronic theory of chemisorption and catalysis (27),... 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

Infrared studies show that when water is adsorbed on the surface, the background intensity in the hydroxyl region increases new bands may appear but hydrogen-bonding effects make such conclusions uncertain. If such a catalyst is then exposed to hydrogen (or deuterium), no bands due to adsorbed hydrogen (or deuterium) are observed. Thus, adsorption of water apparently occurs on the active sites and blocks out type I chemisorption. 

The spectra of C3H6 and C3D6 show that chemisorption of propylene is dissociative, but they fail to identify which carbon-hydrogen bond is broken on adsorption. To this end the spectra of a number of deuterium-labeled propylenes were studied and compared. These results are summarized in abbreviated form in Table VI, which specifies the hydrogen fragment formed on adsorption the fragment was identified as an OH if a band appeared near 3593 cm-1 or as an OD if a band appeared near 2653 cm-1. In those cases where the spectrum changed with time the summary... 

The outcome of this research naturally raised the question whether the catalysts investigated differed also in chemisorptive and catalytic properties. We therefore examined how the adsorption of CO and CO2, and the reaction of benzene with deuterium proceed on these catalysts. This study forms the subject of the present article. 

However, it will at any rate be clear now that the palladium, nickel, and iridium catalysts used in our experiments differ widely in surface characteristics, as is evident from the variations in chemisorptive behavior. An obvious question that may be asked now is whether the catalysts differ also in catalytic behavior. This induced us to study the reaction of benzene with deuterium on the nickel and iridium catalysts. 

---