Catalysis surface analysis

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

A unique pilot plant/minlreactor/surface analysis system has been designed and put Into operation. This system represents the closest encounter reported In the literature to date between "real world" catalysis and-surface analytical techniques. It allows In depth studies of reaction kinetics and reaction mechanisms and their correlation with catalyst surface properties. 

Overall, LEED in UHV provides the exclusive ability to study stmcture-function relationships in heterogeneous catalysis, and for that reason it has become a routine surface analysis tool. 

Barr, T.L. (1990) In Applications of electron spectroscopy to Heterogeneous Catalysis in Briggs, D. and Seah, M.P. (eds.) Practical Surface Analysis, 2nd edn., John Wiley Sons, Chichester, England. 

Other techniques such as low-energy electron diffraction (LEED) are also used for surface analysis, primarily for large single crystals. Single crystal metal surfaces have been used to study hydrocarbon catalysis on platinum (Anderson 1975). Techniques such as x-ray photoelectron spectroscopy (XPS) are also used for surface analysis but normally the reports describe mostly idealized single-crystal surfaces in high vacuum as opposed to using real-life (practical) catalyst systems under reaction environments. 

Figure 13. Schematic representation of the surface coverage of platinum crystallites as they are used in hydrogenation catalysis The surface analysis instrument detects only the light parts and is affected by shadowing (grey zones arrows indicate direction of illumination).

In catalysis it is important not only to be able to study the surface region of the catalyst itself, but also any adlayer that may be present. The latter may arise from adsorption or preferential segregation of one component from the bulk to the surface. The spatial distribution of the elements at the surface can be obtained from scanning AES. Experiments, in which the surface is progressively eroded, e.g., by ion-bombardment, with surface analysis by AES, XPS or ISS carried out after various times, may provide concentration-depth profiles of the chemical species. 

Bluhm H, Havecker M, Kleimenov E, Knop-Gericke A, Liskowski A, Schlogl R, Su DS. In situ surface analysis in selective oxidation catalysis n-butane conversion over VPP. Topics in Catalysis. 2003 23(1) 99—107. 

When a polymer is treated with enzymes for surface modification, some of the undesired protein tends to adsorb on the polymer surface, which subsequently creates problems in the surface analysis and causes a slow down in the rate of catalysis. Adsorbed proteins can be removed from the surfaces by washing with large volumes of 1.5% Na2C03 and water (Eischer-Colbrie et al., 2006) as part of a preparation for surface analysis. Protein-resistant molecules such as polyethylene glycol can be used to prevent the nonspecific protein adsorption. Surfaces can be precoated with an inert protein such as bovine serum albumin (Salisbury et al., 2002) for increasing the rate of catalysis. 

Attributed to the mean free path required for the involved electrons, ions, atoms, etc. to reach the detector, surface analysis is typically restricted to a UHV environment (< 10 mbar), whereas practical heterogeneous catalysis is carried out at pressures > 1 bar. 

Surface science offers many opportunities in catalysis research because a variety of techniques are available to characterize in detail the composition and structure of the catalyst surface and to identify the adsorbed species. A frequent criticism of the surface science approach is that it is far removed from real catalysis since most of the surface science techniques can only be applied at low pressures and with model catalysts, often single-crystal surfaces. The so-called pressure gap has been bridged by combining, in the same apparatus, the techniques needed for surface analysis and characterization with the ability to measure reaction rates at elevated pressures. In addition, many techniques can also be apphed in situ at elevated pressures. 

It is important to consider the connection between the two types of studies. One often refers to the "pressure gap" that separates vacuum studies of chemisorption and catalysis from commercial catalytic reactions, which generally run above —often well above — atmospheric pressure. There is simply no way to properly simulate high pressure conditions in a surface analysis system. Reactions can be run in an attached reaction chamber, which is then pumped out and the sample transferred, under vacuum, into an analysis system equipped for electron, ion and photon spectroscopies. However, except for some optical and x-ray methods that can be performed in situ, the surface analytical tools are not measuring the system under reaction conditions. This gap is well recognized, and both the low- and high-pressure communities keep it in mind when comparing their results. 

Among the various surface analysis techniques which are currently available to catalysis chemists. X-ray photoelectron spectroscopy is certainly the one having found the widest application in the study of zeolitic materials. Reflecting this significance, this text will mostly dwell upon XPS and its relevance to zeolites, with some mentions of the contributions of such other techniques as Auger Electron Spectroscopy (AES), Ion Scattering Spectroscopy (ISS) and Secondary Ion Mass Spectrometry (SIMS). 

The discussion of a number of topics in electrocatalysis, including adsorption phenomena, surface reaction mechanisms and investigation techniques, electrocatalytic activity and selectivity concepts, and reaction engineering factors, may seem at first too diverse. We believe, however, that fundamental principles cannot be divorced from their natural counterpart, praxis. Here, we attempt to establish ties between basic and applied electrocatalysis and with their conventional similes, catalysis, surface physics (and spectroscopy) and reaction engineering. By taking a vitae parallelae perspective, we hope that a synthetic analysis of the present state of the art emerges. 

It is clear that the best experimental designs for addressing structure-function relationships in catalysis are those that minimize exposure of the catalyst surface to undesired vapors between high-pressure kinetic measurements and surface analysis. Thus one aims for a system where the transfer to UHV is as rapid and as clean as possible, and where the sample can be cooled as rapidly as possible once inside UHV. 

Most of the surface spectroscopic techniques require a vacuum environment. High vacuum conditions ensure that the particles used have long mean free paths to interact with the surface of interest. The vacuum environment also keeps the surface free from adsorbed gases during the surface analysis experiment. The exceptions to the high vacuum requirement arc the photon-photon techniques given in the last three rows of Table 21-T These allow examination of surfaces under conditions more akin to those used in applications such as catalysis, sensing, and corrosion studies. 

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