Ultrahigh vacuum investigations experiments

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. 

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. 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

There are two main reasons why high vacuum (10 to 10 Torr range) must be maintained around the samples during some phase of the surface chemical experiment. First, it is often desirable to start our investigation with initially clean surfaces and ultrahigh vacuum (less than 10 Torr) is needed to achieve a surface that is free from... 

From previously stated, it follows that temperature-programmed experiments can be performed under ultrahigh vacuum or in the flow of gas. Still, whatever is the experimental design, three main parts of equipment are always necessary to perform this kind of investigations ... 

The success of surface research is based on the so-called surface science approach, that is, the investigation of well-defined and clean single crystal surfaces under ultrahigh vacuum (UHV) conditions using a seemingly unlimited arsenal of highly surface-sensitive techniques that were developed continuously over the past four decades. UHV environment is not only necessary to maintain the state of a surface - once cleaned - for the duration of the experiment but also a condition for the application of many of the surface analytical methods, which are often based on electron, ion, or atom beams. Colhsions between these beam particles and residual gas atoms or molecules must be minimized in order not to disturb the measurements. 

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