Adsorption on metal catalysts

Figure 5.22 reveals the ability of solid state electrochemistry to create new types of adsorption on metal catalyst electrodes. Here oxygen has been supplied not from the gas phase but electrochemically, as 02 via current application for a time, denoted tj, of 1=15 pA at 673 K, i.e. at the same temperature used for gaseous O2 adsorption (Fig. 5.21). Figure 5.23 shows the effect of mixed gaseous-electrochemical adsorption. The Pt surface has been initially exposed to po2 =4x1 O 6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (1=15 pA) for various time periods ti shown on the figure, in order to simulate NEMCA conditions. 

The osmium complex is a model for sideways ethyne adsorption on metal catalysts [69,70]. The pattern of peaks is similar to that of ethyne on platinum black (see Fig. 7.12). The frequencies were consistently higher than for adsorbed ethyne (see Table 7.7) implying stronger binding of ethyne to osmium in the complex than to platinum black. 

Catalysis and the Adsorption of Hydrogen on Metal Catalysts Otto Beeck... 

All of the Au/metal oxide catalysts deactivate quickly, under the conditions shown in Figure 4. In addition, the deactivation of the Au/metal oxide catalysts appears to be enhanced in the presence of COj. In support of the theory that increased basicity of the metal oxides leads to lower stability, we carried out COj temperature programmed desorption experiments on the various catalysts. The COj TPD data also confirmed that an increase in the basicity of the metal oxides leads to an increase in the amount of COj adsorption on the catalysts. 

The conclusions on the mechanism of the double bond hydrogenation on metallic catalysts can be summarized as follows (1) with respect to structure effects on rate, all transition metals behave similarly (2) the reactivity of the unsaturated compounds is governed mostly by the number and size of the substituents on the carbon atoms of the double bond through their influence on adsorptivity (3) the electronic nature of the substituents plays a minor if any role. 

If one could use heterogeneous catalysts such as dispersed metals to promote this type of reaction, product separation would be facilitated and the more efficient flow processes could be used instead of the commonly employed batch mode. There are, however, a number of problems which must be overcome before such systems can be used practically. In the first place it must be shown that dispersed metals can promote these reactions and, secondly, a more detailed knowledge must be acquired of substrate adsorption on the catalyst and the interaction of the adsorbed species to give the product. 

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