Structure-sensitive sulfur

Nevertheless, two factors strongly influence the heat of sulfur chemisorption on metal surfaces relative coverage and crystallographic structure. Thus sulfur chemisorbs at high coordination sites and, as a result, a selective poisoning of structure-sensitive reactions, preferentially catalyzed by these sites, may occur. Such a simple geometrical model can be used to explain change in selectivities induced by sulfur adsorption. 

The effect of promoters, such as zirconia, on the surface species are also uncertain. There are indications that the composition of the OSC materials can lead to improved sulfur tolerance [50]. It is possible that some combinations of oxides might block sulfate formation or destabilize the sulfate, in the same way that practical OSC materials require the presence of zirconia to maintain reducibility in ceria. Finally, just as the reducibility of ceria is structure sensitive [37], there may be structure sensitivity to adsorption of sulfur compounds as well. 

Acidic forms of zeolites are well suited as supports for metal functions which are employed for hydrogenation, since they can also withstand the presence of traces of sulfur compounds frequently found in feedstocks of petrochemical industry. It should be noted, however, that hydrogenation is a structure insensitive reaction so it will primarily depend upon the concentration of the accessible metal particles and the adsorption constant of the unsaturated hydrocarbon. This may offer an explanation as to why Pt catalysts, for example, are still active for hydrogenation, when their activity for dehydrocyclization or hydrogenolysis (i.e., for structure sensitive reactions) is completely lost (e.g., by poisoning). 

Kinoshita has also shown that ORR data for supported catalysts in hot, concentrated H3PO4 (180 °C, 97-98% acid) reported in three different studies were also fit by this model. Since the physical basis for the crystallite size effect in sulfuric acid is anion adsorption, it would be a considerable reach to suggest that the same physical basis applies to this size effect, i.e., structure-sensitive anion adsorption. There are, nonetheless, indications that this is the case. Anion adsorption in dilute phosphoric [43] has a very similar structure sensitivity as sulfate adsorption, i.e., strongest adsorption on the (111) face, and on poly-Pt anion adsorption and/or neutral molecule adsorption in dilute phosphoric has a strongly inhibiting effect on the kinetics of the ORR [43]. Sattler and Ross [16] report a similar crystallite size dependence of the ORR on supported Pt in dilute phosphoric acid at ambient temperature as that found in hot, concentrated acid with the same catalysts. But it is unclear whether similar adsorption chemistry would exist in the extreme conditions of hot, concentrated phosphoric acid. 

The UPD of Ag on Au and Pt is also an interesting reaction to investigate with surface structure-sensitive techniques. It has clearly been demonstrated that the iodine adlayers on Pt(lll) and Au(lll) strongly affect the UPD of Ag [1, 8, 36]. For example, Fig. 3 illustrates a clear difference in the electrochemical response of the UPD of Ag on a well-defined Pt(l 11) in sulfuric acid (a) and on a Pt(l 11) with the (V X V7)R19.1° iodine adlayer (b), respectively. Two sets of well-defined UPD peaks in the cyclic voltammogram were observed on a well-ordered Pt(lll) in sulfuric acid... 

A voltammetric study of the behavior of glyoxylic acid on platinum single crystal electrodes in sulfuric acid medium was performed in order to get information on the effect of surface structure on the electrosorption and oxidation. The oxidation (taking place at high potentials 1.0 V RHE) has been found structure sensitive. At lower potentials the formation of poisoning intermediates is considered as the predominant process. Two kinds of stable residues were distinguished ... 

Markovic et al. [17] review the data on platinum particles and suggest that the data in dilute sulfuric acid is consistent with Kinoshita s model and further suggest that essentially all of the reactivity can be attributed to the (100) surface. They go on to suggest that this difference in reactivity between the crystal faces is due to structure sensitivity of anion adsorption that impedes the reaction. They point out that in PEM systems, where anion adsorption by the sulfonic acid groups is unlikely, there might be considerably less of a particle-size effect. Still, most PEM catalyst layers employ platinum particles on the order of 3nm, roughly the same size as the maximum in mass activity identified by Kinoshita. 

In contrast with the extensive studies of the ORR on Pt(hkl) surfaces, there has been no substantial fundamental study of the effects of anion adsorption on the kinetics of the ORR on Cu(hkl) surfaces. Very recently, by utilizing the RRDE technique, Brisard and coworkers [101] have shown that the ORR on Cu(lll) and Cu(OOl) surfaces in sulfuric acid solution is a structure-sensitive process, see Fig. 31. As for Pt(hkl), an interpretation of the variation in the activity of this process with the different low-index crystal surfaces of Cu can be presented on the basis of the premise of the structure-sensitive adsorption of sulfuric acid anions on Cu(hkl) surfaces, for example, as for Pt(hkl) with the (1 — ad) term. 

A comparison of the polarization curves (positive sweeps) of the ORR on Cu(lll) and Cu(OOl) at the same rotation rate (1600 rpm). Fig. 31, shows relatively small, for example, less than a factor of 2, differences in activity, but very different reaction pathways below —0.4 V. Since very little is known about the nature of adsorbed anions on Cu(hkl) in sulfuric acid solution, or how the adsorption sites might differ between the different surfaces, it is difEcult to rationalize the structural sensitivity of the ORR on Cu hkl). For the Cu(lll)-S04 ad interface, both Wilms and coworkers [102] and li and Nichols [103] found by STM that a complex incommensurate ordered structure of sulfate anions formed at the Cu(lll) surface throughout the —0.1 to —0.4 V potential region in H2SO4. This ordered structure does not appear to lie along any high symmetry directions of the Cu(lll) substrate but it... 

This is a not structure sensitive reaction (also known as nondemanding reaction) (17), which means that no special arrangement of metal surface atoms is required. Small amoimts of metal are enough to reach equilibrium conversion. Deactivation of the metal by coke or sulfur typically does not inhibit the paraffin... 

System Reactivity Thermal stability Structure sensitive Price Sulfur tolerant ... 

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