Variation of catalytic activity

S. M. Augustine and W. M. H. Sachtler, Variation of catalytic activity over PtRe/AEOs, 7. Phys. Chem. 91, 5935-5956 (1987). 

Figure 3. Variation of catalytic activity of Na-Y zeolite with H+ content...

Fig. 13. Variation of catalytic activity after quinoline introduction at 430° 1,0.03 2,0.05 3, 0.07 mmoles of quinoline/g of catalyst (52). (Reprinted with permission of Plenum Press.)...

To analyse the variation of catalytic activities of the used catalysts, carbon contents measured on the used samples removed after tests using model molecule have been used. As can be noted in Table 1, the cyclohexane isomerization activity at 380°C or at 400°C is not measurable compared to the activities of the fresh catalyst. This indicates that the isomerization sites are strongly poisoned by the deposits. It is dear in Table 1 that even the samples containing less than 200 ppm V are strongly poisoned indicating that the catalyst acidity has been considerably neutralized by the carbon deposit. 

Table I. Variation of Catalytic Activity with Ca " Content of Na-Y Zeolite"...

The variations of catalytic activity which are observed in the first case have a quasi-permanent character they result from structural and electronic imperfections of long lifetime. Preliminary irradiation thus produces an activation of the catalyst. By this process new catalytic properties are imparted to the irradiated solid this addition, however, does not entail any modification of the thermodynamic laws applicable to the reacting system. The new catalysts created in this way, may modify the reaction rate and the reaction mechanism, or even orient the reaction system towards the formation of new products. 

There are important points to note about these variables. First, a small impurity composition can cause a big change in the figure of merit, as seen by the rapid variation of catalytic activity in the Cu/Rh oxidation catalyst (Cong et al., 1999). Second, the phases in thin film are not necessarily the same as those in bulk, as seen in the case of the thin-film dielectric, where the optimal material was found outside the region where the bulk phase forms (van Dover et al., 1998). Finally, the crystallinity of the material... 

A very frequent feature of electrochemical promotion studies is the observed linear variation of catalytic activation energies with varying catalyst work function [9,14,15]. It had been proposed that this is due to a linear variation in chemisorptive bond strengths with catalyst work function [9,14], a proposition recently supported by TPD studies for oxygen chemisorption on IVVSZ [26]. 

Nickel-copper alloys provide a good example of a bimetallic catalyst system in which the variation of catalytic activity with composition depends markedly on the type of reaction, thus leading to substantial selectivity effects. The catalysts to be considered here are alloy powders with a surface area of approximately 1 m2/g (6). Approximately one atom out of a thousand is a surface atom in such catalysts. 

The major products obtained in the alkylation of toluene on these catalysts are xylenes, tri- and tetra-methylbenzenes. The variation of catalytic activity and surface area of the system CsxHs. XPW12O40 as a function of x is shown in Fig.2. It is seen that the catalytic activity reaches a maximum at x = 2.5 and the surface area increases with increase in the extent of substitution. Similar results were obtained in the case of other salts. It has been reported, in the case of cesium salt, that the activity maximum occured at x = 2.5 for the alkylation of 1,2,3 trimethylbenzene with cyclohexene [ 16] and the high activity has been attributed to the high surface acidity of this catalyst. 

These considerations are strikingly demonstrated by the volcano-shaped pattern of variation of catalytic activity as shown schematically in Figure 7.3. While the heat of adsorption is steadily decreasing from left to right, the catalytic reaction rates peak at the group VIII metals in the periodic table. Figure 7.3 shows the pattern of variation of catalytic reaction rates across the series of transition metals Re, Os, Ir, Pt, and Au for the hydrogenolysis of the C—C bond in ethane, the C —N bond in methylamine, and the C —Cl bond in methyl chloride. 

However, the order of decreasing work function from crystal face to crystal face does not correlate with variations of catalytic activity. 

In cyclic voltammetry a sigmoidal variation of catalytic activity with electrochemical potential. Fig. 4, is expected from consideration of the Nemst equation as the active site is swept between its oxidized and reduced states. More complex variations of activity with electrochemical potential are frequently observed [1, 2]. These may be independent of the direction of potential change and scan rate. Alternatively they may display a hysteresis that is repeated over successive cycles or seen only on the first scan. Such behaviors arise from the chemical events, reversible or irreversible, that are coupled... 

---