Adatom selectivity

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

Figure The effects of chlorine adatom coverage C upon the rates, selectivity, and 0q on Ag(llO) at h90 K and Pq2 150 torr and Pjjt torr.

These results are all consistent with a mechanism which involves the comhination of O2, a and Eta in the formation of an adsorbed intermediate, Ig, which can then branch out to evolve EtO (leaving an oxygen adatom behind on the surface) or to produce CO2 + H2O. The oxygen adatom thus inherent to EtO production must itself go on to make either CO2 or H2O, which sets a theoretical upper-limit on the selectivity of 6/7. This agrees within experimental error with the maximum selectivity observed on both high surface area catalysts (2,35,37) and single-crystals (26,27). 

As already mentioned, the poison formation reaction is potential-dependent, and the poisoning rate for the basal planes is Pt(llO) > Pt(lOO) > Pt(lll) [Sun et al., 1994 Iwasita et al., 1996]. The case of Pt(lll) is special, since the poisoning has been associated with the presence of defects on the surface. Selective covering of the defects on the Pt(l 11) electrode by some adatoms prevents the formation of CO on the electrode surface [Macia et al., 1999, 2001 Smith et al., 2000]. 

Interestingly, it has been shown that some adatoms can be selectively deposited on step sites, taking advantage of the enhanced reactivity of these sites. Figure 7.5 shows the voltammogram of a Pt(775) surface in 0.5 M H2SO4. The hard sphere model for... 

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. 

Ensemble or Third-Body Ejfects. These effects concern the selective blockage of a particular adsorption site by adatom deposition. This can be advantageous when the reaction mechanism contains parallel paths that can be affected differently by blocking particular sites. In some cases, the undesired reaction needs more than one free adjacent site (ensemble), and can be inhibited by blocking particular sites without decreasing the reactivity of the surface for the catalyzed reaction. 

In the isomerization of (+)-3-carene into (+)-2-carene or the dehydrogenation of 2-butanol into 2-butanone, the selectivity into the desired product is also increased by the introduction of small amounts of Sn, which will form adatoms poisoning unselective sites.324... 

With the BiPt/C catalyst with a Pt/Bi surface atom ratio of 3, in acidic medium (2 < pH < 4) glycerol could be oxidized with a selectivity of 80% into dihydroxyacetone. It is proposed that the Bi adatoms function as blockers of the Pt(lll) surface, controlling the glycerol surface orientation. 

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). 

Adatoms (a very small amount), a situation corresponding to tin complexes having reacted with the most hydrogenolyzing sites. Those sites are thus selectively poisoned (see below). 

Several examples showing the effects of adatoms on activity and selectivity of a given catalytic reaction were observed. In most cases, this effect can be rationalized as a selective poisoning of undesirable sites. Usually, the presence of adatoms leads to a simultaneous decrease of the global activity and to a significant increase of selectivities in favor of the desired products. We describe here two examples. 

All the studied catalysts, bimetaUics composed of aUoys of a given composition, catalysts in which adatoms of tin are located on the surface of transition metal particles or catalysts having organometaUic fragments attached on the surface, presented remarkable activity, stabiUty and selectivity. 

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