Use in surface science studies

A paper by Szanyi and Goodman [56] on the synthesis of methanol over a copper single crystal provides a good example of how AES is often used in surface science studies of catalytic reactions. These authors investigated the formation of methanol from a mixture of CO2, CO and H2 on Cu(100) at tempera-... 

Nowadays, due to the great sophistication achieved by many of the experimental techniques used in surface science studies, a great deal of information can be obtained, at the atomic scale, about the mechanism of heterogeneous catalytic processes under well controlled conditions. In spite of that, it is still difficult, and sometimes impossible, to obtain a precise picture of the mechanism of the reaction, at the molecular level, without any theoretical support. 

It is important to note that catalysts are generally high surface area, three dimensional surfaces, quite different in that respect from the macroscopic two dimensional single crystals most commonly used in surface science studies. The reason such high area materials are used can be seen from the consideration of the rate equation for the simplest form of catalytic reaction, an isomerisation under conditions of pre-equilibrium ... 

Figure 2. Small area (1 cm ) single crystal surface that is used in surface science studies as well as a model heterogeneous catalyst. (Reproduced with permission from Lawrence Berkeley Laboratory.)...

The hydi ometer method is simpler in that the density of the suspension, which is related to the concentration, is read direc tly from the stem of the Iwdrometer while the depth is determined by the distance of the hydrometer bulb from the surface (ASTM Spec. Pub. 234, 1959). The method has low resolution but is widely used in soil science studies. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

Models based on chemisorption and kinetic parameters determined in surface science studies have been successful at predicting most of the observed high pressure behavior. Recently Oh et al. have modeled CO oxidation by O2 or NO on Rh using mathematical models which correctly predict the absolute rates, activation energy, and partial pressure dependence. Similarly, studies by Schmidt and coworkers on CO + 62 on Rh(l 11) and CO + NO on polycrystalline Pt have demonstrated the applicability of steady-state measurements in UHV and relatively high (1 torr) pressures in determining reaction mechanisms and kinetic parameters. 

The reflection technique has not been used as extensively as transmission. Its slow development may be attributed to several factors. It is used primarily on highly polished metal surfaces including those involved in fundamental studies of single crystals. The theoretical framework for reflection IR spectra has been developed only recently. Ultrahigh-vacuum techniques are required and modifications are needed for standard IR spectrometers. Since the reflection technique can be used with single crystals of metals, it is a bridge between the more sophisticated surface techniques used in surface science and the IR studies of the more practical catalysts. 

In conclusion, XPS is among the most frequently used techniques in catalysis. The advantages of XPS are that it readily provides the composition of the surface region and that it can also distinguish between chemical states of one element. XPS is becoming an important tool for studying the dispersion of active phases over the support. The related techniques UPS and AES are very useful in surface science but, with a few exceptions for AES, less suitable for the characterization of catalysts. 

A wide range of techniques have been developed to study surfaces in vacuum, and many techniques are commonly referred to by acronyms. Table I lists acronyms and brief descriptions of most common techniques used in surface science. 

Figure 10 Typical ultrahigh vacuum system used for surface science studies of catalytic reactions on model systems. The combined development of new preparation methods for realistic catalytic samples and in situ spectroscopies for the molecular level characterization of surface species during catalysis promises to advance the basic understanding of catalytic processes...

A TPD run on a supported metal by using a carrier gas at atmospheric pressure furnishes data for the mixture of surface sites present. Usually, such studies give only a qualitative idea of the different species present on the various sites. However, simultaneous measurement of the IR spectra of the adsorbed species may furnish valuable information (88). In addition, with a carrier gas it is extremely difficult to get kinetic data that are not altered by transport effects. Furthermore, the adsorption-desorption step is not unidirectional the reverse reaction (adsorption) may occur during the TPD. This effect is called rcadsorption in surface science studies in which it is unusual, but it is usually present in a TPD obtained with a carrier gas. 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

As already pointed out, the preparation of clean metal surfaces is an indispensable prerequisite for the study of surface reactions. The term clean , however, needs further definition. As it is commonly used in surface science, it means that the concentration of any contaminants on a clean surface is below or near the limits of detectability in Auger spectroscopy. Of course, this... 

Scanning probe microscopy is widely applied in surface science. It is particularly useful in material science studies for controlling the nature of surfaces and to measure the roughness and hardness of surfaces. The methodology also permits the examination of magnetic structures at the atomic level. 

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. 

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