Catalytic regime

Transient kinetic experiments have also been carried out to complement the information deduced from the steady-state measurements [33], Systematic variations were observed during the transition from the clean surface to the steady-state catalytic regime that correlate well with the overall reaction rates in the latter. Specifically, there is a time delay in the production of molecular nitrogen because of the need to buildup a threshold of atomic nitrogen coverage on the surface. This atomic nitrogen coverage, which could... 

Perhaps the most extensive computational study of the kinetics of NO reactions on Rh and Pd surfaces has been provided by the group of Zgrablich. Their initial simulations of the NO + CO reaction on Rh(lll) corroborated the fact that the formation of N-NO intermediate is necessary for molecular nitrogen production [83], They also concluded that an Eley-Rideal mechanism is necessary to sustain a steady-state catalytic regime. Further simulations based on a lattice-gas model tested the role of the formation of... 

Only photons with X smaller than or equal to the absorption edge of the catalyst are effective in reaction activation. In addition, it must be checked that the reactants do not absorb light in order that the catalyst works in a truly heterogeneous catalytic regime. Besides, there is a direct proportionality between the conversion level and the radiation intensity that confirms the participation of photoinduced electrical charges (holes and electrons) to the photocatalytic mechanism. 

FIGURE 29 Schematic representation of the dynamic processes that determine the formation of the active catalyst phase from vanadyl pyrophosphate. The dashed separation in solid-state and catalytic regimes is artificial and supports only the explanation of the complex interrelations. Reproduced with permission from Ref. (757). Copyright... 

Therefore, the classical rrani-dioxoRu(VI) - oxoRu(IV) catalytic cycle [2] (Fig. 1) can be ruled out as the primary reaction pathway in case of rapid catalytic oxygenation. The apparent zero-order kinetics observed are consistent with a steady-state catalytic regime accessible from different initial states of ruthenium metalloporphyrin. Indeed, common oxidants, other than aromatic iV-oxides, such as iodosylbenzene, magnesium monoperoxyphthalate, Oxone and tetrabutylammonium periodate produced the trans-dioxoRu(VI) species from Ru (TPFPP)(CO) under reaction conditions but were ineffective for the rapid catalysis. 

The Pd-catalyzed carbonylation proceeds through an oxidative addition-reductive elimination cycle in which the Pd oxidation state cycles between -I- 2 and 0. Thus, there is a tendency for Pd metal to drop out of the catalytic regime. Supported Pd catalysts would conceivably lock the metal in a fixed environment and minimize such losses. In fact, Pd/C and Pd/zeolites with HI or C3H7I as promoters have been employed to carbonylate ethylene and higher olefins in acetic acid. High conversions and yields are obtained at 10 MPa at a 2 3 ratio of CO propylene and a 1 1 ratio of H20 propylene at 100°C. 

Metastable Ru(III) and oxoRu(V) species have been proposed as key intermediates in the catalytic cycle of rapid oxygenation which can be viewed as a part of the general scheme of diverse oxidative chemistry of ruthenium porphyrins (Scheme 1.16). The aerobic oxygenation pathway, involving the even oxidation states, is shown in the left half of Scheme 1.16. The new fast catal)dic process on the right half of the figure depicts the chemical interconnectivity between the fast and slow catalytic regimes. 

These definitions allow the well-known compilation of the advantages/disadvantages of both catalytic regimes (Table 1). 

Owing to the fact that under the conditions of the catalytic regime diffusion is easy compared to the surface reaction, reactant molecules can penetrate far into pores and are able to react there. The result is that the coating on a profiled substrate is uniform and the deposit has an excellent step coverage. Complicated forms and tubes can be covered with a uniform film in this regime even internally in the pores. 

If information on the surface reaction is to be collected, for instance to study the reaction mechanism, the process parameters must be chosen in such a way that the deposition is in the catalytic regime. In this regime only the observed growth rate is determined by the surface reaction. In the other two regimes the growth rates do not supply information on chemical reactions since rates are determined by mass transport. 

The extent to which gas-phase diffusion can be prevented from controlling the deposition rate is of considerable importance for chemical vapor infiltration (CVI). Low pressures and low temperatures (conditions in the catalytic regime) favor penetration. Both factors slow the deposition rate, however, and very long reaction times would be necessary for this way of doing CVI. Consequently, thermal gradients and forced reactant gas flows are sometimes applied to increase deposition rates. 

Hydrolysis of phosphites can be discussed from several points of view. Thus, besides their applications as flame retardants and stabilizers for polymers discussed earlier, [130] related phosphoric acid esters are constituents of important biomolecules (nucleotides). The reaction of the latter with water is a central aspect of their transformation, for example, activation and generation of energy. In hydroformylation, knowledge about the pathways and the nature of decomposition products is necessary to maintain a stable catalytic regime. Moreover, the formation of such acidic compounds over time can lead to the precipitation of insoluble gelatinous byproducts, which may block the recycle lines of a continuous technical reactor. 

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