Catalytic reaction cycle

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

If an adsorbed species, e.g. an intermediate in a catalytic reaction cycle, decomposes into products that desorb instantaneously, TPD can be used to monitor the reaction step. 

CATALYTIC REACTION CYCLES FOR THE NICKEL-CATALYZED CYCLO-OLIGOMERIZATION OF 1,3-BUTADIENE... 

Bis(p -octadienediyl-Ni11 species are shown (i) to be thermodynamically highly unfavorable, thus indicating them to be sparsely populated, and (ii) not to be involved as reactive intermediates along any viable path either for allylic isomerization or for reductive elimination. This leads to the conclusion, that bis(p ) species play no role within the catalytic reaction cycle. 

On the basis of the free-energy profiles presented so far for the refined catalytic reaction cycles for the C8- and Ci2-cyclo-oligomer production cycles, we now rationalize the critical factors that are decisive for the regulation of the product selectivity for the two reaction channels. 

II. Catalytic Reaction Cycles for the Nickel-Catalyzed Cyclo-Oligomerization... 

Figure 2. The catalytic reaction cycle of the Tetrahymena ribozyme (E), showing the binding and docking reactions (leading to the formation of E-S complex), followed by a bond cleavage breaking step (the rate constant for which is kchem) and release of the 5 -fragment in the multiturnover steps (rate constant equals k t).

CATALYSIS. Any condition promoting formation will tend to speed up the reaction rate, and catalysts are thought to accomplish rate enhancement chiefly by stabilizing the transition state. Shown in Fig. 8 is an enzyme-catalyzed process in which reactant S (more commonly called substrate in enzymology) combines with enzyme to form an enzyme-substrate complex. This complex leads to formation of the transition state complex EX which may proceed to form enzyme-product complex. The catalytic reaction cycle is then completed by the release of product P, whereupon the uncombined enzyme returns to its original state. 

STEP ONE Using just the different enzyme forms, write out the reaction scheme, such that a closed geometric pattern is generated. In this case (E) occurs at the start and end of the catalytic reaction cycle, and we get ... 

By replacing the natural substrate homoprotocatechuate (3,4-dihydroxyphenyl acetate) with the slower reacting substrate analog 4-nitrocatechol, six intermediates of the catalytic reaction cycle of 2,3-HPCD could be observed experimentally, four of which could even be characterized by X-ray crystal structure analysis [22], Scheme 2.4 gives an overview of the proposed reaction mechanism. 

These relationships, when incorporated into microkinetics models of catalytic reaction cycles, enable remarkable new predictive insights into the control of heterogeneously catalyzed reactions. Predictive models of catalytic activity as a function of catalyst composition as well as reaction conditiorvs have been constructed (22-24). The resultant volcano curves can be considered to be an application of the Sabatier principle (25,26). 

Because at steady state all the rates that constitute the catalytic reaction cycle are equal, there are several expressions that one can use for fhe rate of CO consumption. We start the derivation with the rate of CO consumption as given by the following expression ... 

FIGURE 6. Schematic representation of the catalytic reaction cycle in flavocytochrome b2. Five redox intermediates of FCB2 during the oxidation of one molecule of lactate at a steady-state turnover rate of 100 sec and the reduction of two molecules of cytochrome c at the rate of 200sec° are shown. Step 4 is the rate limiting step in the steady state and the maximal rates of some of the other electron transfer steps are indicated. Reproduced from Daff et al., 1996 with permission. 

Chain and catalytic reaction cycles provide energetically favorable pathways for reactant molecules to proceed to product molecules. This point is illustrated below for both types of cycles. Consider the reaction between dihydrogen and dichlorine to produce HCl that can be brought about in the gas phase by irradiating the reactants with light. It is known that over 10 molecules of HCl can be formed per absorbed photon. The reaction proceeds as follows ... 

Now consider what happens in a catalytic reaction cycle. For illustrative purposes, the decomposition of ozone is described. In the presence of oxygen atoms, ozone decomposes via the elementary reaction ... 

Within a few years from the discovery of the ODEs, the BrO - - BrO catalytic reaction cycle was confirmed as the main sink for ozone. The main process leading to the release of reactive bromine to the atmosphere was more difficult to identify. It now appears that the main source for the reactive gaseous bromine compounds are chemical reactions in the snowpack and on top of fresh (one-year-old) sea ice. 

Figure 4-1 Proposed catalytic reaction cycle involving cytochrome P-450 in the oxidation ol xenobiotics.

Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data. The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (see text) to the experimental data. (C) Five optimized structures (a)-(e) ofAu2C03, with bond lengths in A. The relative stability of these structures is... 

The first problem in Section II. A was the appearance of parasitic molecules to destroy the hypercycle—that is, the mutually catalytic reaction cycle. If only the replication process of molecules is concerned, it is not so easy to resolve the problem. Here we consider the dual level of replication—that is, molecular and cellular replication. 

FIGURE 16.7 Catalytic reaction cycle for manganese catalase turnover. (Adapted from Whittaker, Baryrun, Igarashi, Whittaker, 2003.)... 

Formation of the carbonylated products can be rationalized as in the following scheme. Two fused catalytic reaction cycles, A and B, proceed simultaneously. In cycle A, coordination... 

Fig. 1.34. Catalytic CO oxidation by N2O in an ICR mass spectrometer [24]. The ICR signal shown obtained for Fe+ while scanning the double-resonance oscillator to eject ions of a given mass from the ceU. Trace B is obtained with only N2O present in the trap. Trace A results when CO is added in addition to N2O to the ion trap. The increase in Fe+ signal after adding CO is caused by the regeneration of Fe in the catalytic reaction cycle indicated in the inset. This is evidenced by the double resonance at 72amu (FeO ), which indicates that FeO+ is reacting to Fe ...

Fig. 1.64. Production concentrations as a function of the reaction time for three different reaction temperatures and different reactant gas concentrations, (a) T = 300 K p(02) = 0.12Pa p(CO) = 0.12 Pa p(He) = 1.2Pa. (b) T = 150K p(02) = 0.04 Pa p(CO) = 0.04 Pa p(He) = 1.0 Pa. (c) T = 100K (Oa) = 0.02 Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data (squares), Aua (circles), AuaOa (triangles), Au2(C0)02 ). The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (1.65) to the experimental data...

The detailed catalytic reaction cycle emerging from experimental and theoretical evidence for the CO oxidation by gas-phase Au2 clusters is depicted in Fig. 1.67. Also included are the calculated energy barriers for the different reaction steps and the simulated intermediate structures. In addition to a comprehensive molecular mechanistic understanding based on experiment... 

The first catalytic cluster ion reaction cycle under thermal conditions was observed by Ervin and coworkers. This group demonstrated that the gas-phase cluster anions Pt (n = 3-7) efficiently catalyze the oxidation of CO to CO2 by N2O or O2 near room temperature in a combined flow tube and gas cell reactor instrument under multicollision conditions [7]. At the end of the process, the intact ciuster is regenerated and each step is exothermic and occurs rapidiy at thermai energies. The observed catalytic reaction cycles are summarized in Fig. 1.81. 

---