Activation rate, maximum

Poliak. E. Quantum theory of activated rate processes a maximum free energy approach,... 

The third is the effect of temperature as typically the inactivation rate increases/decreases with temperature for gram-positive/gram-negative bacteria. The exception to this rule are coliforms. All microorganisms display, in any case, very narrow temperature ranges where photocatalytic disinfection activity reaches maximum values. ... 

Converging lines of evidence have led to a general acceptance of the monomeric metaphosphate mechanism for the hydrolysis of phosphate monoester monoanions. The pH rate profile for aryl and alkyl phosphate monoester hydrolyses commonly exhibits a rate maximum near pH 4. where the concentration of the monoanion is at a maximum. The proposed mechanism is based on these principal points of evidence (a) a general observation of P-O bond cleavage (b) the entropies of activation for a series of monoester monoanions are all close to zero, which is consistent with a unimolecular rather than a bi-molecular solvolysis where entropies of activation are usually more negative by 20 eu7 (c) the molar product composition (methyl phosphate inorganic phosphate) arising from the solvolysis of the monoester monoanion in a mixed methanol-water solvent usually approximates the molar ratio of methanol ... 

These can be explained if they involve a third reaction of the protonated ester, i.e. with a molecule or molecules of water, a reaction not, of course, observed in media such as SbF5-FS03H. The activity of water falls rather sharply as the sulphuric acid concentration increases from 60-100%, and we know that most esters become essentially completely protonated in this region. Thus the situation can arise where the increase in the concentration of protonated ester produced by a given increase in acid concentration is proportionately smaller than the concomitant decrease in the activity of water, so that bimolecular (or higher molecularity) hydrolysis goes more slowly as the acidity is increased. Similar behaviour is observed when amides are hydrolyzed in strong acid solutions, but the rate maximum occurs at lower acid concentration, since amides are more basic than esters, and protonation is complete in solutions of lower acidity. 

The efficiency of the catalyst in the archetypal system, EDTA/[Ru(bipy)3]2+/MV2+/Pt is dependent upon its mode of preparation, its concentration, possibly the particle size, and the nature of the support.219,223 226 Thus, unprotected colloids give lower rates of hydrogen production than those protected with PVA and most catalysts reach their maximum efficiency at [Pt] = 1-5 x 10s mol dm-3,129,224,226,227 although for particles with diameters of ca. 30 A (probably the most active particles), maximum rates are achieved at 2 x 1CT6 mol dm-3.138 Under these conditions... 

Reports on steady increases of polymerization rates with increasing polymerization temperatures usually refer to an upper limit of polymerization temperature of around 60 °C. At temperatures > 60 °C catalyst deactivation becomes more prominent and overall catalyst activities decrease. There are two reports which point in this direction. A decrease of catalyst activities at elevated temperatures was observed for NdV/DIBAH/fBuCl [455] and for NdN/TIBA/EASC [388]. Pires et al. studied the solution polymerization of BD whereas Ni et al. studied the polymerization of BD in the gas phase. The rate maximum observed by Pires et al. was at 80 °C whereas the reaction maximum in the gas-phase polymerization was at 50 °C. The reduction of polymerization rates at elevated temperatures can be explained by the decay of the number of active species. In gas-phase polymerization deactivation becomes evident at lower temperatures (50 °C) compared to the solution pro-... 

Table 13.2 Water activity for maximum rate of browning...

Soluble cobalt salts (acetate or naphthenate) are used as catalysts, most often together with manganese and bromide ions. Particularly in the presence of bromide source (as HBr, sodium bromide, or even organic bromides), the rate of the oxidation of methylbenzenes increases by up to 400 x. None of the other halogens approaches bromide in its promoting activity. The maximum effect is achieved with a 1 1 cobaltrbromine atomic ratio. 

A shift of rate control with temperature may occur from a mechanism with positive to one with negative apparent activation energy or vice versa. If so, the rate has a maximum or minimum in the temperature range of the shift. A rate maximum is sometimes seen, for example, in heterogeneous catalysis [12,15]. 

Here, M — iAl, is an aluminum-filled Si-O-Si-linked surface site, s equals the number of aluminum ions exchanged to create a surface precursor site, and A,- stands for the rth aqueous species involved in the formation of the precursor. The rate model described in Equation (21) predicts two types of behavior (Oelkers and Schott, 1995a) at low aluminum activity, rates are at a maximum and are independent of aqueous aluminum activity (or concentration), and at higher aluminum activity, fewer precursors are found on the mineral surface and the rate is slower and dependent on dissolved aluminum activity (or concentrations). As discussed later, these authors further argue that a region exists in which the rate is dependent also on an affinity term (Equation (66)). 

Assuming co-reaction, the cure reaction of a mixture of bis(4-maleimido phenyl) methane and BACY was followed by FTIR [221]. The reaction kinetics, studied by DSC, suggested dependency of cure mechanism on blend composition. The apparent activation energy computed by the Prime method increased with BMI content. The rate maximum at a fractional conversion range of 0.32-0.33 indicated an autocatalytic nature of the reaction. The different pattern of activation energy with fractional conversion for two different blend compositions indicated non-identical cure mechanisms for the two compositions. The cyclot-rimerization of BACY occurred during the cure of a 1 2 molar ratio of BMI and BACY. Since activation parameters derived from DSC method are generally not consistent, and since the cyanate cure can be catalyzed by impurities present in BMI, which was not taken into consideration, the authors conclusions on the cure mechanism based on DSC kinetics can be erroneous. 

Salt pair Equilibrium water activity Rate of water transfer Maximum temperature (°C) ... 

Carbon Deposited on Nickel via 02 Exposure TPSR with 1-atm H2 Carbon deposited on alumina-supported nickel (17 wt%) following ethylene exposure at various temperatures has a wide range of activity for reaction with H2 (Figure 2). Different states of carbon are identified by maxima in the rate of CH production. The temperature of the rate maximum (T ) for a particular carbon state was generally found to he independent of both the amount of carbon in that state and the temperature of deposition. Thus T will be taken as characteristic of the reactive state of... 

The esterification activity of acetic acid with primary and secondary alcohols with varying feed rate is also shown in Table 2. Esterification activity is maximum at high contact time and it is seen decreasing with increasing feed rate steadily with normal alcohols. The esterification activity of isopropanol and sec-butanol decreased considerably with a two fold increase of the feed. 

For the NO-CO reaction over Rh(111) at high pressures, we find that the reaction is positive order in NO and surprisingly is zero order in CO. As is shown in Fig. 2, from 500 K to 650 K the reaction has an activation energy close to 30 kcal/mole. After reaction the Rh(111) surface is nearly covered with nitrogen atoms. (The nitrogen atom coverage is also high near the rate maximum in low pressure studies [8].) The elementary steps which were used to model the NO-CO reaction shown below were also chosen based on the UHV measurements of the rate constants of each step. 

The dependence of DCB HDCl rates over Ni/Si02 on H2 and DCB pressure can be described in terms of the involvement of two types of active sites with a logarithmic dependence on H2 partial pressure. A rate maximum is observed with respect to DCB partial pressure where, under conditions of ascending rate, the reaction order with respect to DCB exceeds unity. The mechanism, which explains such kinetic regularities, assumes adsorption of two DCB molecules on adjacent sites and reaction with spillover hydrogen. Selectivity towards CIB was seen to be independent of H2 pressure but dependent on the nature of the isomer 1,2-DCB undergoes complete dechlorination to a greater extent and HDCl selectivity trends for 1,3-DCB and 1,4-DCB coincide. 

The pronounced rate maximum below R = 4.5 may arise from a strong adsorption of the active complex on precipitated cobaltous dicyanide, which greatly increases the amount of catalyst in the system. 

The kinetics of the CO oxidation reaction over all three noble metals exhibit a Langmuir-Hinshelwood type behavior, due to competitive adsorption of CO and oxygen, characterized by the appearance of rate maximum with increasing CO partial pressure. This is a typical behavior which has been described by many investigators for this reaction system [19,20], The catalytic activity, presented as turnover number of CO2 production, was measured by varying either the partial pressures of CO or O2, or temperature, keeping the other parameters constant. 

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