Stepped single crystal surface

Hydrogen Electrosorption and Oxidation of Formic Acid on Platinum Single-Crystal Stepped Surfaces... 

Identification of Peaks for Hydrogen Adsorption on the Disordered Low Index Planes. Besides the major objective for studying electrocatalysis on single crystal stepped surfaces mentioned above, these studies offer a wealth of information on the behaviour of polycrystalline surfaces, of preferentially oriented surfaces and, as we suggested recently (12), of disordered low-index surface. 

Figure 7. Oxidation of HCOOH on the Pt(ll,l,l) and Pt(332) single crystal stepped surfaces in 0.5 M H S0.. Sweep rate 50...

Hamelin A (1979) Lead adsorption on gold single crystal stepped surfaces. J Electroanal Chem 101(2) 285-290... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

In the following section, we focus on imaging single-crystal electrode surfaces that are of relevance to electrocatalysis. We will first deal with flat, defect-free terraces as well as with more real surfaces with monoatomic high steps as the most common active sites. We will then explore various strategies for nano structuring surfaces, for example, by repetitive oxidation-reduction cycles (ORCs). 

Oxidation of formic acid shows a pronounced structural dependence which is well illustrated by data on the low index planes (8-10) and preliminary data on stepped single crystal surfaces (11). In this work further investigations of hydrogen adsorption and oxidation of formic acid on single crystal Pt surfaces with 15 orientations are reported. 

Fig. 4.39. Single crystal metal surface with cut forming steps. After adsorption of a hydrocarbon, islands are formed (preferentially on terraces) of hydrogen-lean-carbon-rich species.

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

Earlier we described the catalytic reaction as a series of consecutive steps at the surface, in which adsorbate and adsorbate-surface bonds are formed and/or broken on the reaction path towards the product molecule. The forces between surface atoms and adsorbate atoms responsible for rearrangement of the chemical bond are similar to those responsible for strong adsorption (E > 10 kcal/nx)l). The adsorption process dominated by such interaction is called chemisorption. Even on a single crystal metal surface, several adsorption modes are conceivable and for dissociation of a diatomic molecule many different reaction paths can be envisioned. However, usually only one particular surface atom configuration is preferred to lead to the idea of catalytic active site. If catalysis of a molecule is studied that has several reaction possibilities, some desirable and others not, a selective reaction usually requires a particular surface atom composition and rearrangement. 

The influence of the catalyst morphology on electrocatalytic activity becomes manifest in strong experimental [50-52] and theoretical [53,54] evidence that the recombination step, Eq. (2.25), proceeds preferentially at defect, edge, or step sites. Such sites may therefore act as active sites for the reaction. On well-defined stepped single crystal Pt surfaces, Koper and coworkers have related the active site fraction to the step site density [52,55]. 

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

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