UHV studies

Single erystals are traditionally used in UHV studies beeause they provide an opportunity to well eharaeterize a surfaee. However, as diseussed above, single erystals are quite different from industrial eatalysts. Typieally, sueh eatalysts eonsist of supported partieles that ean have multiple erystal orientations exposed at the surfaee. Therefore, an obstaele in attempting surfaee seienee studies of eatalysis is the preparation of a surfaee in sueh a way that it mimies a real-world eatalyst. 

While the measurement of the work function is losing importance in UHV studies (because other more specific techniques have been developed), such a quantity retains its role in electrochemistry because it is intimately related to the electrode potential. A major problem is thus the dichotomy between samples for which is known but not and vice versa. This is one of the major obstacles to the unambiguous interpretation of Eam0- plots. However, this point has been recently addressed in a few cases and the outcome has allowed us to clarify some debated aspects. It is now well established that within a major group of sp- and sd-metals AX (the decrease in 4> as the metal comes in contact with the solution) increases as

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Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

Related UHV Studies The interaction of molecules relevant for the present purpose, such as H2, O2, and H2O, with Ru(OOOl) surfaces under UHV conditions has been studied in much detail over the last decades [Diemant et al., 2003 Liu... 

Electrochemical cells, particularly those with aqueous electrolytes, operate well above the freezing temperature of water. Since water desorbs from Pt below its freezing point ca. 180 K), there is a very low temperature ceiling for which reactions may take place. While the low temperature of water desorption precludes UHV study of many, if not most, electrocatalytic reactions, the surface reaction... 

Recent reviews by Ertl and Engel have summarized most of the chemisorption and low pressure catalytic findings . In general, the reaction proceeds through a Langmuir-Hinshelwood mechanism involving adsorbed CO and O atoms. Under reaction conditions typical in most high pressure, supported catalyst studies, and most low pressure UHV studies, the surface is almost entirely covered by CO, and the reaction rate is determined by the rate of... 

The Langmuir-Hinshelwood reaction between adsorbed CO and O atoms is well established as the dominant reaction mechanism for conditions where CO is the primary surface species . This mechanism has been confirmed by numerous UHV studies of the coadsorption of the transient kinetic... 

UPD of As on Au single crystals resulted in well-ordered atomic layers, as observed with TEED. In addition, subsequent UPD of Ga results in formation of an ordered GaAs monolayer. The sUiicture of the monolayer is similar to that for CdTe, in that it consists of a 1/2 ML each of Ga and As on Au(lOO). The observed LEED pattern, however, was not a c(2 X 2), but a p(2 X 2) structure (Fig. 43). This larger unit cell has been proposed to result from dimer formation in the top layer (Fig. 44), which is consistent with previous UHV studies of GaAs single crystal surfaces [335-339]. Those results concerned mostly the formation of a single monolayer of... 

The majority of studies aimed at understanding the mechanisms of ethylene epoxidation rely on either experimental physical chemistry studies of single crystal-silver surfaces or computational studies looking at enthalpies of different reaction states. One unfortunate caveat of the experimental studies lies in the fact that the binding of ethylene to the silver surface is too weak for ultrahigh vacuum (UHV) studies to be of any use. Therefore, the majority of silver surface epoxidation reactions use olefins that stick to the surface better both before and after epoxidation (e.g., butadiene or styrene). 

As indicated above, the Ap technique has been applied to several other phenomena involving Pt-based electrocatalysts. The first report of Ap applied to operating Pt electrocatalysts was based on Hads at anodic potentials. The nature of Ha on Pt, and its contribution to the effective double layer, had long been a matter of debate. " Ap analysis of Pt Lmn XANES showed the H to be highly delocalized, and hopping between one-fold and three-fold (fee) sites on the Pt surface. While prior research had pointed to such activity, the realistic extent in respect to potential was murky due to the nature of the analytical techniques (e.g., IR spectroscopy, UHV studies, etc.) employed. The study by Teliska et al., ... 

Reaction rate oscillations may be accompanied by temperature oscillations [temperature fluctuations of up to 500 K have been reported (24)] or they may be isothermal. Isothermality occurs either because the catalyst can conduct heat away much faster than the rate at which it is produced by the reaction, as is the case in UHV studies, or because isothermal conditions are forced on the system by anemometry, as described in the work of Luss and co-workers (757). Oscillation frequencies can range from more than 10 Hz (24) up to periods of several hours (217,219). Often there is evidence for several time scales in a single oscillating stem. Relatively regular high-frequency oscillations may be superimposed over relaxation oscillations (93,98), with the two types of oscillations caused by different changes on the catalyst surface. 

Schultze et al. [3.115-3.120], and Kolb et al. [3.121-3.128]. The experimental results gave evidence of the formation of well-ordered 2D Meads overlayers in the underpotential range depending on the crystallographic orientation and the crystal imperfection density of S. Electrochemical results were first hypothetically interpreted in terms of 2D Meads superlattice structures" [3.87-3.89, 3.93, 3.98], which were also observed in comparative [3.121, 3.122, 3.129-3.133] and ex situ [3,123-3.128] UHV studies. 

The studies reviewed here focus on Sn/Pt because of the opportunity afforded by the ordered alloys formed in this system for improving our basic understanding, as well as the commercial importance of Pt-Sn catalysts in naphtha reforming and their potential for other selective hydrogenation and dehydrogenation reactions. These studies combined detailed structural characterization of the alloy surfaces with UHV studies of adsorption and reaction of hydrocarbons and other small molecules, and measurements of the rate and selectivity of catalytic reactions at atmospheric pressure over these model catalysts. 

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