Ultrahigh vacuum studies

The dynamics of high-temperature CO adsorption and desorption over Pt-alumina was analyzed in detail using a transient mathematical model. The model combined the mechanism of CO adsorption and desorption (established from ultrahigh-vacuum studies over single-crystal or polycrystalline Pt surfaces) with extra- and intrapellet transport resistances. The numerical values of the parameters which characterize the surface processes were taken from the literature of clean surface studies ... 

Graham, J. D., and J. T. Roberts, Interaction of HCI with Crystalline and Amorphous Ice Implications for the Mechanisms of Ice-Catalyzed Reactions, Geophys. Res. Lett., 22, 251-254 (1995). Graham, J. D J. T. Roberts, L. A. Brown, and V. Vaida, Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces Ultrahigh-Vacuum Studies, J. Phys. Chem.., 100, 3115-3120 (1996a). 

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. 

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

Jarvis S P, Yamamoto S-l, Yamada H, Tokumoto H and Pethica J B 1997 Tip-surface interactions studied using a force controlled atomic force microscope in ultrahigh vacuum Appl. Phys. Lett. 70 2238... 

In practical applications, gas-surface etching reactions are carried out in plasma reactors over the approximate pressure range 10 -1 Torr, and deposition reactions are carried out by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV below 10 Torr) or by chemical vapour deposition (CVD) in the approximate range 10 -10 Torr. These applied processes can be quite complex, and key individual reaction rate constants are needed as input for modelling and simulation studies—and ultimately for optimization—of the overall processes. 

Since then, STM has been established as an insttument fot foteftont research in surface physics. Atomic resolution work in ultrahigh vacuum includes studies of metals, semimetals and semiconductors. In particular, ultrahigh-vacuum STM has been used to elucidate the reconstructions that Si, as well as other semiconducting and metallic surfaces undergo when a submonolayer to a few monolayers of metals are adsorbed on the otherwise pristine surface. ... 

My principal objective in Section 10.4 has been to underline the necessity for a drastic enhancement of a crucial experimental technology, the production of ultrahigh vacuum, as a precondition for the emergence of a new branch of science, and this enhancement was surveyed in the preceding Section. It would not be appropriate in this book to present a detailed account of surface science as it has developed, so 1 shall restrict myself to a few comments. The field has been neatly subdivided among chemists, physicists and materials scientists it is an ideal specimen of the kind of study which has flourished under the conditions of the interdisciplinary materials laboratories described in Chapter 1. 

Another exception to the rule of contaminated surfaces involves very small particles, generally referred to as nanoclusters. These are generally formed and the adhesion of the.se particles to substrates studied in situ, under ultrahigh vacuum conditions. Owing to the vacuum and the short existence of these particles prior to deposition, it is possible for chemistry to occur. 

Harrington, D. A. Ultrahigh-Vacuum Surface Analytical Methods in Electrochemical Studies of Single-Crystal Surfaces 28... 

An ultraclean environment is another major reason for generating high vacuum. At atmospheric pressure, every atom on a solid surface is bombarded with gas molecules at a rate of trillions per second. Even under a reasonably high vacuum, 10 atm, a gas molecule strikes every atom on a solid surface about once per second. If the surface is reactive, these collisions result in chemical reactions that contaminate the surface. The study of pure surfaces of metals or semiconductors requires ultrahigh vacuum, with pressures on the order of 10 atm. 

It is important to realize that the assumption of a rate-determining step limits the scope of our description. As with the steady state approximation, it is not possible to describe transients in the quasi-equilibrium model. In addition, the rate-determining step in the mechanism might shift to a different step if the reaction conditions change, e.g. if the partial pressure of a gas changes markedly. For a surface science study of the reaction A -i- B in an ultrahigh vacuum chamber with a single crystal as the catalyst, the partial pressures of A and B may be so small that the rates of adsorption become smaller than the rate of the surface reaction. 

We have already mentioned that fundamental studies in catalysis often require the use of single crystals or other model systems. As catalyst characterization in academic research aims to determine the surface composition on the molecular level under the conditions where the catalyst does its work, one can in principle adopt two approaches. The first is to model the catalytic surface, for example with that of a single crystal. By using the appropriate combination of surface science tools, the desired characterization on the atomic scale is certainly possible in favorable cases. However, although one may be able to study the catalytic properties of such samples under realistic conditions (pressures of 1 atm or higher), most of the characterization is necessarily carried out in ultrahigh vacuum, and not under reaction conditions. 

Figure 1. An ultrahigh vacuum apparatus used for the study of single crystal catalysis before and after operation at atmospheric pressure in a catalytic reactor.

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

Soriaga, M. P., D. A. Harrington, J. L. Stickney, and A. Wiekowski, Ultrahigh-vacuum surface analytical methods in electrochemical studies of single-crystal surfaces, in Modem Aspects of Electrochemistry, J. O M. Bockris et al., Eds., Vol. 28, Kluwer, New York, 1996, p. 1. 

Thomas S, Sung Y-E, Kim KS, Wieckowski A. 1996. Specific adsorption of abisulfate anion on a Pt(l 11) electrode. Ultrahigh vacuum spectroscopic and cyclic voltammetric study. J Phys Chem 100 11726-11735. 

---