INDEX structure sensitivity

Structure Sensitivity over Pe. Table II presents the rates of ammonia synthesis over each of the low Miller index planes of Pe. 

Carbon deposition from CO on a cobalt catalyst at low pressures is known to be a structure-sensitive process. CO is adsorbed molecularly on the low index surfaces (Co (0001)), but its dissociation occurs on the Co (1012), Co (1120), and polycrystalline surfaces.5762 Deposition of carbon on Co (1012) and the probable formation of Co3C have been established by Auger emission spectroscopy (AES) and low-energy electron diffraction (LEED) techniques.66... 

Much experience concerning the hydrogenation of conjugated dienes was obtained with butadiene hydrogenation. On Pt single crystals the reaction was found to be structure sensitive the activity sequence of different planes (marked with Miller s index) is... 

In a series of studies, the dehydrogenation and hydrogenolysis of cyclohexane was studied on both the stepped and low Miller index (111) crystal faces of platinum at a surface temperature of 300°C and a hydrogen to cyclohexane ratio of 20 1. While the rates on the stepped and low Miller index surfaces were not very different for the formation of benzene and hexane, the formation of cyclohexene was very structure sensitive, its rate being 100 times greater on the stepped surface than on the (111) crystal face. In Table III mrnnare the initial turnover numbers for the various reactions at low... 

The rate of a structure sensitive reaction catalyzed by a metal single crystal is also a function of the exposed plane. As illustrated in Figure 5.1.3, the low-index planes of common crystal structures have different arratigctncnls of surface atoms. Thus, caution must be exercised when the rates of structure sensitive reactions measured on single crystals are compared to those reported for supported metal particles. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

Figures 2-4 show a comparison of the ring-disk electrode for the oxygen reduction kinetic data along with the base voltammetry in oxygen-free solutions for each Vi hkl) surface. Clearly, the kinetics of the ORR on VtQikl) surfaces vary with crystal face in a different manner depending on the solution. In perchloric acid solution. Figure 2, the variation in activity at 0.8-0.9 V is relatively small between the three low-index faces, with the activity increasing in the order (100) < (110) (lll). A similar structural sensitivity is observed in KOH, Figure 3, with the activity...

Re preferentially occupies sites on low index Pt, so reducing the contribution from structure-sensitive reactions.110... 

These three chi indexes are not highly intercorrelated. The x index encodes size and branching information. The index encodes even more specific information about skeletal branching, x increases with the increase in skeletal branching in the hexane series the x values increase in the order hexane < 3-methylpentane, < 2-methylpentane, < 2,3-dimethylbutane, < 2,2-dimethyl-butane. The Xpc index is sensitive to specific structural aspects, especially to gem and vicinal substitution patterns. The negative sign on Xpc reflects a... 

In a later paper Lee et al. have confirmed the antipathic structure sensitivity of methanation over Ni/Si02 (337). They associate this result with the previous finding (335, 336) that the high-index planes of Ni tend to accumulate carbon that can transform to graphite, a poison for the CO/H2 reaction. Similar results have been found by Doering et al. (338) for Pd/mica. On Pt/mica, there is no CO dissociation (339). 

A pronounced structural effect was found in studies of methanol oxidation on the low index and high index planes of platinum. As mentioned in connection with the chemisorption of formic acid, it was found that the poisoning of the surface that occurs in the course of the oxidation process is structure sensitive. 

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. 

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