Catalytic activity contact time effects

The comparison of catalytic properties was made under identical reaction conditions, among three important candidate catalysts, namely, the Pt/y-Al203, Au/a-Fe203, and Cu Ce, x02 y systems [50], The catalytic tests were performed in the reactant feed containing CO, H2, C02, and HzO — the so-called reformate fuel. The effects of the presence of both C02 and H20 in the reactant feed on the catalytic performance (activity and selectivity) of these catalysts as well as their stability with time under reaction conditions have been studied. The composition of the prepared samples and their BET specific surface areas are presented in Table 7.6. The results obtained with the three catalysts in the presence of 15 vol% COz and of both 15 vol% COz and 10 vol% H20 in the reactant feed (with contact time wcat/v = 0.144 g sec/cm3 and X = 2.5) are shown in Figure 7.12. For comparison, the corresponding curves obtained under the same conditions but without water vapor in the feed are also shown in Figure 7.12. 

Data obtained in fixed-bed reactors and in continuous high-velocity coil-t ype reactors (fluid catalyst) indicate that the catalytic cracking of gas oils is approximately a first-order reaction, but that the apparent order approaches two because of the effect of nonhomogeneity of the feed and because of the increasing dilution of reactant with cracked products as conversion increases at constant total pressure (73). The extent of reaction is determined by the intrinsic activity of the catalyst surface, reaction time at the surface, temperature, and susceptibility of the feed to cracking. Superficial contact time in the reactor is of little consequence. The effective time of reaction is the time spent by oil on the active surface of the catalyst. For a given extent of adsorption, the reaction time should be inversely proportional to weight space velocity and should also be a function of the reactant partial pressure. Results of experiments with... 

Clearly, the best catalyst for the reduction reactions may not be the best for the oxidation reactions, so two catalysts are combined. The noble metals, although expensive, are particularly useful. Typically, platinum and rhodium are deposited on a fine honeycomb mesh of alumina (AI2O3) to give a large surface area that increases the contact time of the exhaust gas with the catalysts. The platinum serves primarily as an oxidation catalyst and the rhodium as a reduction catalyst. Catalytic converters can be poisoned with certain metals that block their active sites and reduce their effectiveness. Because lead is one of the most serious such poisons, automobiles with catalytic converters must use unleaded fuel. 

A further support to the belief that positively charged mediators are superior to neutral ones with respect to reaction rate was shown in a work by Katz and coworkers [200]. PQQ (Fig. 10), was covalently immobilized onto a thiol derivative-modified gold electrode. The PQQ incorporates an o-quinone functionality and when immobilized, has an E° value at pH 7 of—0.125 V versus SCE. In the presence of NADH, immobilized PQQ shows some moderate catalytic activity for NADH oxidation. However, when Ca + was added to the contacting solution, a much higher (R 10 times) catalytic activity was revealed by CV, although the addition of Ca + had virtually no effect on the E° of PQQ thus, the thermodynamic driving force (A ° ) remained constant. Addition of Ca + has also a... 

Catalytic Effects Deposition is affected by the condition of the surface and the time that deposit particles are in contact with one another on the component surface. Whether the component surface provides catalytic activity may be questioned, but relevant surface preparation procedures must be used. 

Schmidt and coworkers [551] have reported the effect of organoaluminum compounds on the catalytic properties of complex catalysts used in the oligomerization of propylene. The selectivity for the process depends strongly on temperature, pressure, and contact time. In a continuous process of oligomerization of propylene at 200 atm with AlEts activation it is observed that the yield of oligomers increases with temperature [552]. Studies on the oligomerization kinetics [553,5541 have shown that the reaction order with respect to propylene is close to unity. Activation energies of 11.7 and 14 kcal mol have been reported [553,554]. 

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