Surface structure and composition

Table 5. Diffraction and Scattering Methods for Surface Structure and Composition ...

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Kuntze J, Speller S, Heiland W, Atrei A, Rovida G, Bardi U. 1999. Surface structure and composition of the alloy Au3Pd(100) determined by LEED and ion scattering spectroscopy. Phys Rev B 60 1535-1538. 

The samples were collected from the cathodes 2.5 cm away from the current collector tab, washed in pure dimethyl carbonate (DMC), and soaked in DMC for 30 minutes after removal from Li-ion cells inside an argon-filled glove box. This procedure removed electrolyte salt from the electrode to prevent its reaction with air and moisture. An integrated Raman microscope system Labram made by ISA Groupe Horiba was used to analyze and map the cathode surface structure and composition. The excitation source was an internal He-Ne (632 nm) 10 mW laser. The power of the laser beam was adjusted to 0.1 mW with neutral filters of various optical densities. The size of the laser beam at the sample was 1.2 pm. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

The second most apparent limitation on studies of surface reactivity, at least as they relate to catalysis, is the pressure range in which such studies are conducted. The 10 to 10 Torr pressure region commonly used is imposed by the need to prevent the adsorption of undesired molecules onto the surface and by the techniques employed to determine surface structure and composition, which require relatively long mean free paths for electrons in the vacuum. For reasons that are detailed later, however, this so-called pressure gap may not be as severe a problem as it first appears. There are many reaction systems for which the surface concentration of reactants and intermediates found on catalysts can be duplicated in surface reactivity studies by adjusting the reaction temperature. For such reactions the mechanism can be quite pressure insensitive, and surface reactivity studies will prove very useful for greater understanding of the catalytic process. 

Microbeam Analysis Providing Microdomain, Surface Structure, and Composition... 

In surface studies, one is confronted with the difficulty of detecting a small number of surface atoms in the presence of a large number of bulk atoms a typical solid surface has 10 atoms/cm as compared with 10 atoms/cm in the bulk. In order to be able to probe the properties of solid surfaces using conventional methods, one needs the use of powders with very high surface-to-volume ratio so that surface effects become dominant. However, this technique suffers from the distinct disadvantage of an entirely uncontrolled surface structure and composition which are known to play an important role in surface chemical reactions. It is thus desirable to use specimens with well-defined surfaces which generally means small surface area, of the order of 1 cm, and examine them with tools that are surface sensitive. 

We shall prepare the various building blocks of the catalyst surface and study them separately. Then we put the parts together and the resultant structure should have all of the properties of the working catalyst particle. Just as in the case of synthetic insulin or the B12 molecule, the proof that the synthesis was successful is in the identical performance of the synthesized and natural products. Our building blocks are crystal surfaces with well-characterized atomic surface structure and composition. Cutting these crystals in various directions permits us to vary their surface structure systematically and to study the chemical reactivity associated with each surface structure. If we do it properly, all of the surface sites and microstructures with unique chemical activity can be identified this way. Then, by preparing a surface where all of these sites are simultaneously present in the correct configurations and concentrations the chemical behavior of the catalyst particle can be reproduced. The real value of this synthetic approach is that ultimately one should be able to synthesize a catalyst that is much more selective since we build into it only the desirable active sites in a controlled manner. 

First we study the surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. Then a well-chosen chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. Below 10 4 Torr the surface can be monitored continuously during the reaction with various electron spectroscopy techniques. Then the same catalytic reaction is studied at high pressures (1-100 atm) and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. Finally, the rates and product distributions that were determined at... 

Studies to correlate the reactivity and the surface structure and composition of platinum surfaces indicate that the active platinum crystal surface must be heterogeneous. The heterogeneity involves the presence of various atomic sites that are distinguishable by their number of nearest neighbors (atoms in terraces, steps, and kinks), and also variation in surface chemical composition. A model that depicts the active platinum surface is shown schematically in Fig. 28. Part of the surface is covered with a partially de-... 

The bulk crystal structure of the samples was determined by XRD (Rigaku D-max B) using a filtered Cu Ka radiation. Surface structure and composition were monitored by XPS (Perkin-Elmer PHI 5400). The N2 BET surface area and CO chemisorption were measured in a constant-volume adsorption apparatus. For the latter, two successive isotherms separated by evacuation were obtained at RT for a sample and the difference extrapolated to zero pressure was taken as chemisorbed CO. 

In spite of the overall success of this particular scheme, there are some issues which need to be clarified especially when it comes to details of the structure of the electrode/monolayer/electrode crossings on an atomistic level [4]. In a recent review on the vapor deposition of metal atoms on organic monolayers [65], the complexity of the process and the subtle effects of the surface structure and composition on the outcome are illustrated. A reflection-absorption infrared spectroscopy study [66] of a system, which is similar to the actual cross-junction, suggests undamaged organic mono-layers with Ti coatings, but further research is needed for the complete characterization of these complex structures. 

Microcarriers are small particles, made of materials such as cellulose, dextran, glass, collagen, or gelatin. Generally, they have a spherical shape and present a surface structure and composition that promotes cell adhesion and growth. 

Mechanistic studies with real catalysts near atmospheric pressure conditions are complicated by several factors the surface structure and composition will be inhomogeneous and hence also the reactivity may be spatially different. In addition, the heat released by the reaction may change the (local) temperature, and as a consequence, kinetic oscillations are frequently associated with strong nonisothermal effects. These prob-... 

In spite of the advances made by these researchers, it remains unclear how membrane surfaces undergo restructuring and how these changes influence the catalytic and transport properties of the material. Furthermore, there is a need to link surface structure and composition with long-term performance of palladium membranes under continuous reaction conditions. One... 

Impressive theoretical progress has been made in the prediction of fundamental modifications of catalyst surface structures and compositions as a function of the chemical potential of the environment in relatively simple cases (N0rskov et al., 2006 Reuter and Scheffler, 2002 Stampfl et al., 2002). This level of dynamic analysis with either single crystals or realistic polycrystalline catalyst materials has not yet been attained experimentally and certainly not in experiments with XRD under reaction conditions. There are no investigations that provide quantitative links between the phases and texture of a catalyst with its performance. All investigations discussed here can at best provide evidence relating the catalytic activity with a phase or a defect structure of a phase. 

Applications of IR and Raman spectroscopy to the study of clinkers and unhydrated cements have been reviewed (B39,B40). The laser Raman microprobe, with which regions of micrometre dimensions on a polished surface may be examined, has been used to investigate structure and crystallinity, especially of the alite and belite (Cl9). Spectroscopic methods for studying the surface structures and compositions of cements are considered in Section 5.6.2. 

In order to increase the reaction rate and capacity, some methods to modify the surface structure and composition were recently attempted [42-49], They are surface oxidation [42-45] and surface fluorination [46-48] of graphite and carbon, and metal coating to carbon surface [49], These show that surface modifications are useful techniques to improve the electrochemical characteristics of graphite and carbon. 

This final section includes a brief outline of suggested future research directions, aimed at applying spectroscopy of functioning catalysts to more complex catalysts and reactions, mimicking technological systems even more closely. It is emphasized that such model investigations will have to sacrifice part of the control of surface structure and composition and cope with problems similar to those occurring on real catalysts. 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Issue (1) includes aspects of surface structure and composition, (il) surface area, porosity and heterogeneity. Regarding (iii), there was a certain emphasis on monolayer adsorption (which prevails for adsorption from solution) filling of pores by molecules has its analogies with the stacking of colloidal particles under certain confinements. 

Many of the Cu 100 based surface alloys discussed in this chapter are relatively well characterised in terms of their layerwise compositional profile, geometric structure and thermal stability. However, it is clear that the majority of structural studies performed to date have made the (often necessary) assumption that a single homogeneous structural phase with a somewhat idealised compositional profile is present. In many cases, particularly for adsorbates which exhibit considerable bulk solubility in copper this may be a oversimplification. Future work to investigate the sensitivity of quantitative probes of surface structure and composition such as LEED, ion scattering spectroscopies and photo-electron diffraction to structural heterogeneity will be invaluable. 

The aim of this chapter is to review our understanding of the fundamental processes that yield improved electrocatalytic properties of bimetallic systems. Three classes of bimetallic systems will be discussed bulk alloys, surface alloys, and overlayer(s) of one metal deposited on the surface of another. First, we describe PtjM (M=Ni, Co, Fe, Cr, V, and Ti) bulk alloys, where a detailed and rather complete analysis of surface structure and composition has been determined by ex situ and in situ surface-sensitive probes. Central to our approach to establish chemisorption and electrocatalytic trends on well-characterized surfaces are concepts of surface segregation, relaxation, and reconstruction of near-surface atoms. For the discussion on surface alloys, the emphasis is on Pd-Au, a system that highlights the importance of surface segregation in controlling surface composition and surface activity. For exploring adsorption and catalytic properties of submonolayer and overlayer structures of one metal on the surface of another, we summarize the results for Pd thin metal films deposited on Pt single-crystal surfaces. For all three systems, we discuss electrocatalytic reactions related to the development of materials... 

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