Decrease site activity

In contrast, substitution of oxygen for —NH— in open-chain compounds causes a great decrease in activity. He states that the heterocyclic ring of D-24 locks the carbomethoxy group at the active site and thus accomplishes the same amount of restriction as does binding of both the aromatic and acylamido groups of the open-chain analogs (82). 

Two space velocities, i.e. 0.03 and 0.3 h l, have been used in the evaluation of catalytic activities of catalysts B and C at 823 K. Figure 6 shows a decrease in activity of the catalyst B when space velocity increases. The accessible sites are saturated at the lowest space velocity. This explains thus the lower conversion levels at a higher space velocity. However, for catalyst C, the evolution of the conversion, which is also depicted in Figure 6, is almost identical for both space velocities. This result could be explained by a better dispersion of the platinum due to the presence of tin. 

Intrinsic Activity Poisons. These poisons decrease the activity of the catalyst for the primary chemical reaction by virtue of their direct electronic or chemical influence on the catalyst surface or active sites. The mechanism appears to be one that involves coverage of the active sites by poison molecules, removing the possibility that these sites can subsequently adsorb reactant species. Common examples of this type of poisoning are the actions of compounds of elements of the groups Vb and VIb (N, P, As, Sb, O, S, Se, Te) on metallic catalysts. 

Degree of dealumination. Moderate dealumination generally increases the catalytic activity or leaves it unchanged, while advanced dealumination leads to a decrease in activity. Such a decrease is due to a loss of active sites with advanced framework dealumination. 

The effect of variation in the N-l side-chain of (26) on activity was also examined [52]. In the benzazepinone series (26, n = 2), no loss of activity occurred on extension of the side-chain from acetic acid (m = 1) to propanoic acid (m = 2). The butanoic acid homologue (m = 3) resulted in only a slight decrease in activity, whereas the next higher homologue (m = 4) became 1000-fold less active. These results imply unexpected degrees of freedom at the carboxyl binding site. 

The schematic below shows that in this dynamic system PAN could affect the synthetic process (site 1), the enzyme itself (site 2), or the degradation process (site 3). If the site of attack were site 2, the synthetic process might compensate for degradation of the enzyme by producing more. If the enzyme activity were measured as a function of time after exposure, there would be first a decrease and then recovery of activity. (Such a response has been observed for the effect of ozone on respiration.) Effects at site 3 would show first an increase in activity and en, if the system were regulated, a decline to normal, as the synthetic process slowed down. Effects at site 1 would cause a decrease in activity commensurate with the rate of enzyme degradation. 

---