Highly adsorbed systems

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

It should be mentioned that one can detect two types of equilibrium in the model of charge transfer in the absorbate - adsorbent system (i) complete transition of chemisorbed particles into the charged form and (ii) flattening of Fermi level of adsorbent and energy level of chemisorbed particles. The former type takes place in the case of substantially low concentration of adsorbed particles characterized by high affinity to electron compared to the work function of semiconductor (for acceptor adsorbates) or small value of ionization potential (for donor adsorbates). The latter type can take place for sufficiently large concentration of chemisorbed particles. 

Figure 8.13 Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system. Dipole scattering excites only the vibration perpendicular to the surface (v,) in which a dipole moment normal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering also excites the bending mode (v ) in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the highly intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

It was evident that a new-generation, high-capacity, high-recovery system was needed. The problem became one of making more efficient use of the adsorbents used in these systems and, concurrently, making a major increase in unit capacity. 

Since the previously stated limitations on vessel size were still in effect, it was decided to pursue a roate using multiple adsorber beds in a highly integrated system. It was necessary to define integrated high-capacity systems with at least 80% product recovery and to devise control schemes for such systems. 

Additional studies by Menon (32) have indicated the p can occur at lower pressures then those predicted by Equation 1 depending on the pore structure associated with the adsorbent. Empirically, adsorbents possessing microporosity exhibit a p that is 0.6-0.8 of the value predicted by Equation 1. This observation is attributed to the overlapping of potential fields in the adsorbent pores, thereby enhancing sorption of the gas at lower pressures. Experimental studies by Ozawa (33) have verified this trend as shown in Figure 4 for the C02/activated carbon system. Here the adsorption maxima for the gas occurs at a lower pressure than the critical pressure of carbon dioxide. It should also be noted that the amount of gas adsorbed is decreased at higher reduced temperatures and that additional compression is required to reach a defined adsorption maxima (i.e., at very high values of T it is sometimes difficult to discern a well-defined adsorption maximum). The above trend has also been found for other adsorbent/adsorbate systems, such as silica gel/C02. 

The Ag/Ru(001) series (Fig. 2.9, left panel) corresponds to zeroth-order desorption [20], To appreciate what this means, we need to explain this system in more detail. Adsorbate systems of low-melting metals such as Cu, Ag and Au, adsorbed on high-melting metals such as Ru and W, are highly suited to fundamental desorption studies, for two reasons (1) readsorption on the sample surface does not occur and (2) the rate of desorption is measured without any systematic error due to background pressures, as would be the case with gases such as H2, CO,... 

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