Surfaces vacuum techniques

Surface defects (Section VII-4C) are also influenced by the history of the sample. Such imperfections may to some extent be reversibly affected by processes such as adsorption so that it is not safe to regard even a refractory solid as having fixed surface actions. Finally, solid surfaces are very easily contaminated detection of contamination is aided by ultra-high-vacuum techniques and associated cleaning protocols [24]. 

Ultrahigh vacuum techniques have become common, especially in connection with surface spectroscopic and diffraction studies, but also in adsorption on very clean surfaces. The techniques have become rather specialized and the reader is referred to Ref. 8 and citations therein. 

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction. 

Soriaga M P 1992 Ultra-high vacuum techniques in the study of single-crystal electrode surfaces Prog. Surf. Sc/. 39 325... 

The implementation of high-pressure reaction cells in conjunction with UFIV surface science techniques allowed the first tme in situ postmortem studies of a heterogeneous catalytic reaction. These cells penult exposure of a sample to ambient pressures without any significant contamination of the UFIV enviromnent. The first such cell was internal to the main vacuum chamber and consisted of a metal bellows attached to a reactor cup [34]- The cup could be translated using a hydraulic piston to envelop the sample, sealing it from... 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

The scientific study of surfaces, and the full recognition of how much a surface differs from a bulk structure, awaited a drastic improvement in vacuum technique. The next Section is devoted to a brief account of the history of vacuum. 

Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

XPS and SIMS are both surface sensitive and surface-specific techniques that may be used to probe surface oxidation/degradation of polymers. They use X-rays and ions, respectively, to characterise surface species. Both XPS and SIMS are operated under ultra-high vacuum, UHV, conditions. 

Whereas ESCA and other surface specific techniques are often the obvious method of choice, IR-ATR may be a suitable technique when the composite material is heavily filled, and the IR radiation does not penetrate to far into the sample material. An advantage of, e.g., IR is that the sample can be investigated under ambient conditions rather than under a high vacuum, as required with techniques such as ESCA. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

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