Ultrahigh vacuum modeling

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. 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

The ultrahigh vacuum STM was used to investigate the addition of the 2,2,6,6-tetramethyI-l-piperidinyloxy (TEMPO) radical to the dangling bond of Si(l 0 0)-2 X 1 surface. ° ° The TEMPO can bond with a single dangling bond to form stable Si-O coupling products, in contrast to the thermal decomposition of TEMPO-silicon compounds. Semiempiiical and DFT calculations of TEMPO bound to a three-dimer silicon cluster model yielded... 

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. 

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. ... 

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 ... 

Modeling the Aqueous—Metal Interface in Ultrahigh Vacuum via Cryogenic Coadsorption... 

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). 

We will only briefly discuss the surface enrichment model proposed for the Ni-Cu alloy, since it has been reviewed several times elsewhere (38, 39). The phase diagram of this alloy has a miscibility gap. Figures 1 and 2 show the results of two experiments, which demonstrate composition differences between bulk and surface in Ni-Cu alloys (4a, 4b). The alloys are films deposited under ultrahigh vacuum. After sintering the binary systems all have nearly the same work function, despite the fact that the overall Cu Ni ratio in the copper-rich system is about four times that of the metal-rich system. 

Ultrahigh-vacuum (UHV) surface spectroscopy has been used with molecular beams of SiH4 and mass spectroscopy to elucidate the Si growth mechanism (67, 143). Joyce et al. (67) found that Si growth is preceded by an induction period when surface oxide was removed as SiO. The subsequent film growth proceeds by growth and coalescence of adjacent nuclei with no apparent formation of defects. Henderson and Helm (144) proposed a step-flow model in which adatoms from SiH4 surface reactions difluse to kink sites. 

In typical surface science experiments as presented previously, oxide-supported metal nanoparticles are deposited onto a clean oxide surface by physical vapor deposition. The precursor in this process is metal atoms in the gas phase, which impinge on the surface, diffuse until they eventually get trapped (either at surface defects or by dimer formation), and then act as nuclei for the growth of larger particles. These processes are well understood for ideal model systems under ultrahigh vacuum (UHV) conditions [56, 57]. In contrast, most realistic supported metal catalyst... 

Wang, H-F, Kaden, WE, Dowler, R, Sterrer, M, and Freund, H-J. Model oxide-supported metal catalysts - comparison of ultrahigh vacuum and solution based preparation of Pd nanoparticles on a single-crystalline oxide substrate. Phys Chem Chem Phys. 2012 14 11525-11533. 

In this study, we have used a set of polyimide model molecules to obtain information about possible interactions at the polyimide copper interface. Benzene, phthalimide (PIM), benzene-phthalimide (BPIM), methyl-phthalimide (MPIM), and malonamid (MAM) were deposited in ultrahigh vacuum (UHV) onto clean polycrystalline copper substrates, and the measurements were performed by means of XPS and UPS. 

In the present study we have used a thin, well-ordered atomically flat alumina film grown on a NiAl(llO) single crystal surface as a model support [27]. The atomic arrangement within line defects of this film have recently been investigated [28]. In addilion, Ihere exists a proposed structure of the film based on X-ray diffraction data which is controversially debated at the moment [29]. The structure and size of the oxide-supported metal particles were controlled utilizing nucleation and growth of vapor deposited metal atoms under ultrahigh vacuum conditions. 

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