Catalytic effects equilibrium/kinetics

DSC tests show a substantial reduction of the hydrogen desorption onset (red circles) (T J and peak (T ) temperatures due to the catalytic effects of n-Ni as compared to the hydrogen desorption from pure MgH also milled for 15 min. (Fig. 2.57). It is interesting to note that there is no measurable difference between spherical (Fig. 2.57a) and fdamentary (Fig. 2.57b) n-Ni, although there seems to be some effect of SSA. We also conducted desorption tests in a Sieverts apparatus for each SSA and obtained kinetic curves (Fig. 2.58), from which the rate constant, k, in the JMAK equation was calculated. The enhancement of desorption rate by n-Ni is clearly seen. At the temperature of 275°C, which is close to the equilibrium at atmospheric pressure (0.1 MPa), all samples desorb from 4 to 5.5 wt.% within 2,000 s. 

Reaction rates typically are strongly affected by temperature (76,77), usually according to the Arrhenius exponential relationship. However, side reactions, catalytic or equilibrium effects, mass-transfer limitations in heterogeneous (multiphase) reactions, and formation of intermediates may produce unusual behavior (76,77). Proposed or existing reactions should be examined carefully for possible intermediate or side reactions, and the kinetics of these side reactions also should be observed and understood. 

Actually, each step in both schemes involves a slow step and one or more equilibrium steps which have been combined for simplicity. Both kinetic expressions have [HA] terms in the denominator which decrease the catalytic effectiveness of the general acid catalyst at high concentrations. Catalysis by imidazole buffers is also observed a similar lowering of the catalytic coefficient is observed as the buffer concentration increases. The hydroxylaminolysis of other amides exhibits behavior similar to that of Wmamide. A search for the build-up of the tetrahedral intermediate by spectrophotometric means gave negative results. 

Noncatalytic reactions are less frequently used in kinetic-based determinations than are those involving a catalytic effect. However, recent advances in instrumentation mean that noncatalytic kinetic methods are powerful alternatives to equilibrium (nonkinetic) methods. This type of reaction is of especial relevance to the analysis of mixtures of closely related compounds, for which a munber of differential reaction rate methods have been developed. Whether for individual or joint determinations of species, the main field of application of noncatalytic reactions is organic analysis, unlike catalytic reactions, where a metal ion usually acts as the catalyst this has also contributed to their current wide acceptance. 

Concentration effects of the reactants and components on the hydrocarbomethoxyla-tion of cyclohexene with COMeOH, catalysed by (AcO)2Pd in the presence of p-TsOH and fra s-2,3-bis(diphenylphosphinomethyl)-norbornane as ligand, revealed first-order rate dependences on cyclohexene and (AcO)2Pd, while non-monotonic rate dependences were found for the diphosphine and p-TsOH concentrations and the CO pressure. The reaction follows first-order kinetics for the MeOH concentration below 0.4 mol r but it decelerates upon a further increase in the MeOH concentration. The results were interpreted by considering a hydride mechanism supplemented with ligand exchange, resulting in decreased catalyst reactivity, and with hydride complex annihilations by p-TsOH, which leads to a complete loss of catalytic activity. A kinetic equation was derived for the quasi-equilibrium conditions, which is consistent with the experimental data. ... 

The catalytic effect of nickel(n) on hydrolysis rates of the salicyl and phenyl esters of pyridine-2,6-dicarboxylic acid is ascribed to the intermediacy of a 1 1 complex between the ester and the nickel(n). Rates of hydrolysis and of reaction with hydroxide are reported.Kinetic and equilibrium studies have... 

The kinetic factor is proportional to the energetic state of the system and (for heterogeneous catalytic systems) the number of active sites per unit volume (mass) of catalyst. The driving-force group includes the influence of concentration and distance from chemical equilibrium on the reaction rate, and the hindering group describes the hindering effect of components of the reaction mixture on the reaction rate. The kinetic factor is expressed as the rate constant, possibly multiplied by an equilibrium constant(s) as will be shown later. 

The TEAF system can be used to reduce ketones, certain alkenes and imines. With regard to the latter substrate, during our studies it was realized that 5 2 TEAF in some solvents was sufficiently acidic to protonate the imine (p K, ca. 6 in water). Iminium salts are much more reactive than imines due to inductive effects (cf. the Stacker reaction), and it was thus considered likely that an iminium salt was being reduced to an ammonium salt [54]. This explains why imines are not reduced in the IPA system which is neutral, and not acidic. When an iminium salt was pre-prepared by mixing equal amounts of an imine and acid, and used in the IPA system, the iminium was reduced, albeit with lower rate and moderate enantioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Nevertheless, as other kinetic studies have indicated a pre-equilibrium with imine, it is possible that the proton formally sits on the catalyst and the iminium is formed during the catalytic cycle. It is, of course, possible that the mechanism of imine transfer hydrogenation is different to that of ketone reduction, and a metal-coordinated imine may be involved [55]. 

An important property of the S-nitroso thiourea derivatives is the ability to effect electrophilic nitrosation of any of the conventional nucleophilic centres. This is manifest kinet-ically by the catalysis of nitrous acid nitrosation effected by added thiourea (equation 29). The situation is completely analogous to the catalysis of the same reactions by added halide ion or thiocyanate ion. The catalytic efficiency of thiourea depends on both the equilibrium constant Xxno for the formation of the intermediate and also its rate constant k with typically a secondary amine65. Since Xxno is known (5000 dm6 mol-2), it is easy to obtain... 

It is clear from both Figs. 8.20 and 8.21 that CTP acts as an inhibitor while ATP acts as an activator. It is now established that both CTP and ATP bind to the regulatory binding site, which differs from the catalytic sites. The remarkable similarity between the equilibrium Bis on the one hand and the kinetic data on the other hand confirms the assertion made at the beginning of this chapter, that allosteric effects can be studied in equilibrated systems. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

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