Ultra-high vacuum techniques surface characterization

We begin with the most routine characterization methods—electrochemical methods. We then discuss various instrumental methods of analysis. Such instrumental methods can be divided into two groups ex situ methods and in situ methods. In situ means that the film on the electrode surface can be analyzed while the film is emersed in an electrolyte solution and while electrochemical reactions are occurring on/in the film. Ex situ means that the film-coated electrode must be removed from the electrolyte solution before the analysis. This is because most ex situ methods are ultra-high-vacuum techniques. Examples include x-ray photoelectron spectroscopy [37], secondary-ion mass spectrometry [38,39], and scanning or transmission electron microscopies [40]. Because ex situ methods are now part of the classical electrochemical literature, we review only in situ methods here. 

Ultra-High Vacuum Techniques of Surface Characterization... 

The combination of ultra-high vacuum (UHV) surface science techniques with electrochemical methods of electrode surface characterization (voltammetry, chrono-coulometry) resulted in a spectacular progress in the investigation and molecular level understanding of some processes occurring at electrode/solution interfaces. Evidently the experimental approach strongly depends on the aims of the investigation and the systems to be studied. 

The methodology of surface electrochemistry is at present sufficiently broad to perform molecular-level research as required by the standards of modern surface science (1). While ultra-high vacuum electron, atom, and ion spectroscopies connect electrochemistry and the state-of-the-art gas-phase surface science most directly (1-11), their application is appropriate for systems which can be transferred from solution to the vacuum environment without desorption or rearrangement. That this usually occurs has been verified by several groups (see ref. 11 for the recent discussion of this issue). However, for the characterization of weakly interacting interfacial species, the vacuum methods may not be able to provide information directly relevant to the surface composition of electrodes in contact with the electrolyte phase. In such a case, in situ methods are preferred. Such techniques are also unique for the nonelectro-chemical characterization of interfacial kinetics and for the measurements of surface concentrations of reagents involved in... 

Recent developments in surface-characterization methods have been made possible to a great extent by technological advances in areas such as lasers, ultra-high vacuum, charged-particle optics, and computer science. The surface-analysis techniques are commonly used to probe the interface between two phases after one phase is removed, but there is now a growing demand for additional methods for in situ interface characterization. 

Although the state of adsorbed ethylene on the Pt(lll) single-crystal surface under ultra-high vacuum conditions is rather well characterized, it is not clear that the species found on single-crystal surfrices is necessarily that present under high pressures of ethylene on supported Pt crystallites. Because NMR spectroscopy is applicable to supported catalysts even under h%h pressures, we have chosen to study the formation of carbonaceous deposits on silica-supported Pt catalysts heavily dosed with ethylene. By monitoring the evolution of the surface species on Pt using the NMR technique after beat treatment of catalyst samples at various temperatures, we have been able to characterize the coke-precursors. 

Surface analysis refers to the characterization of the outermost layers of materials. A series of techniques have been undergoing continuous development since the late 1960s based on ultra-high vacuum (UHV) technology. The most useful of these methods provide information on surface chemical composition and four techniques satisfy this requirement X-ray photoelectron spectroscopy (XPS or ESCA), Auger electron spectroscopy... 

Literature reports on interfaces are mainly limited to metallic solids while little is known on ceramic materials, which are mainly ionic solids of nonstoichiometric compounds. The reason for the scarcity of literature reports on ceramic interfaces results from the substantial experimental difficulties in studies of these compounds. Even the most advanced surface-sensitive techniques have experimental limitations in the surface studies of materials. Most of these techniques are based on ion and electron spectroscopy, such as XPS, SIMS, LEED, AFM, and LETS, and are still not adequate to characterize the complex nature of compounds. Namely, these surface techniques require an ultra-high vacuum and therefore may not be applied to determine surface properties during the processing of materials which takes place at elevated temperatures and under controlled gas phase composition. Consequently, the resultant experimental data allow one to derive only an approximate picture of the interface layer of compounds. 

Since that time, the method is widely developed experimental setups are improved and adjusted to many different purposes (e.g. for the investigations of oxidation and reduction reactions). Today, two main types of equipment are available those operating under ultrahigh vacuum and so-called flow systems. WeU-defined surfaces of single-crystalline samples are investigated in a continuously pumped ultra-high vacuum (UHV) chamber (this technique is often referred to as thermal desorption spectroscopy—TDS [5]). The equipment that is constructed to allow adsorption-desorption in the gas flow are most often used for the investigation of porous materials (catalysts, for example). Vacuum setups are customarily used for surface science studies, but they can be also useful for the characterization of porous materials. 

Because both the coordination of the H atoms and the energy of the H state change drastically between surface and bulk, it is realistic to consider that the absorption step is not an elementary step but involves a H subsurface state (sorbed H), intermediate between the surface adsorbed state and the bulk absorbed state, and located in the interstihal sites in the first metal layers beneath the surface [14]. This state has been characterized by ultra-high-vacuum (UHV) techniques on Pd, Ni, Pt, and Cu [13,15-22], and is believed to play a role in ordering of the H surface phases [19]. The subsurface sites become accessible by relaxation or reconstruction of the top layer of the metal substrate [13]. Figure 2.1 shows the different H sites at the vicinity of a metal surface [23]. 

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