Activation energy catalyst effect

Activation energy—Continued effect on reaction rates, 261 hydrogen desorption, 183 use of differences for selectivity control, 260 Active metals content, effect on catalyst activity, 283,285/... 

The maximum catalytic activity for IVI-AA copolymer has been observed at a polymer copper(I I) ratio of 17 1. Activation energies and effective rate constants, Ar ff, have been determined from results of the measurement of the rate of quinone formation over a 288-313 K range of temperature. Thus, at pH 5.25 and [Cu " ] = 1.2-10 mole/1, these values are 111.4kJ/mole and 1.32-10 min , and 79.5 kJ/mole and 8.55 10 min for low-molecular weight and polymeric catalysts respectively. Because the limiting step of the process is the oxidation from Cu(I) to Cu(Il), the incorporation of polyampholytes which form more stable complexes with Cu(II) than with Cu(I) is expected to increase the catalytic activity of the Cu(II) ions. 

Diffusion Continued) effect of, on activation energy, 104 effect of, on reaction order, 105 effect of, oh reaction rate, 95 importance of, 115 Knudsen, 484 molecular, 484 on surface, 98, 208 Diffusivity. See Effective diffusivity Dispersion (of catalyst), 19 Dispersion coefficients (axial and radial), 287, 493, 497... 

Three commercial processes that use these various hot carbonate flow arrangements are the promoted Benfield process, the Catacarb process, and the Giammarco-Vetrocoke process (26—29). Each uses an additive described as a promoter, activator, or catalyst, which increases the rates of absorption and desorption, improves removal efficiency, and reduces the energy requirement. The processes also use corrosion inhibitors, which aHow use of carbon—steel equipment. The Benfield and Catacarb processes do not specify additives. Vetrocoke uses boric acid, glycine, or arsenic trioxide, which is the most effective. 

Conversion processes are either thermal, where only heat is used to effect the required change, or catalytic, where a catalyst lowers the reaction activation energy. The catalyst also directs the reaction toward a desired product or products (selective catalyst). 

Enzymes, like all other catalysts, lower the activation energy for reaction. They can be enormously effective it is not uncommon for the rate constant to increase by a factor of... 

Figure 8-8 shows the analogous situation for a chemical reaction. The solid curve shows the activation energy barrier which must be surmounted for reaction to take place. When a catalyst is added, a new reaction path is provided with a different activation energy barrier, as suggested by the dashed curve. This new reaction path corresponds to a new reaction mechanism that permits the reaction to occur via a different activated complex. Hence, more particles can get over the new, lower energy barrier and the rate of the reaction is increased. Note that the activation energy for the reverse reaction is lowered exactly the same amount as for the forward reaction. This accounts for the experimental fact that a catalyst for a reaction has an equal effect on the reverse reaction that is, both reactions are speeded up by the same factor. If a catalyst doubles the rate in one direction, it also doubles the rate in the reverse direction. 

Catalysts increase the rate of reactions. It is found experimentally that addition of a catalyst to a system at equilibrium does not alter the equilibrium state. Hence it must be true that any catalyst has the same effect on the rates of the forward and reverse reactions. You will recall that the effect of a catalyst on reaction rates can be discussed in terms of lowering the activation energy. This lowering is effective in increasing the rate in both directions, forward and reverse. Thus, a catalyst produces no net change in the equilibrium concentrations even though the system may reach equilibrium much more rapidly than it did without the catalyst. 

Findings with Bench-Scale Unit. We performed this type of process variable scan for several sets of catalyst-liquid pairs (e.g., Figure 2). In all cases, the data supported the proposed mechanism. Examination of the effect of temperature on the kinetic rate constant produced a typical Arrhenius plot (Figure 3). The activation energy calculated for all of the systems run in the bench-scale unit was 18,000-24,000 cal/g mole. 

Methyl- and 2,6-dimethylpyridine as catalysts with sterically hindered a-com-plexes give greater isotope effects (k2n/k2D up to 10.8). Such values are understandable qualitatively, since the basic center of these pyridine derivatives cannot easily approach the C-H group. The possibility of tunneling can be excluded for these reactions, as the ratio of the frequency factors 4h 4d and the difference in activation energies ED—EU (Arrhenius equation) do not have abnormal values. 

Catalysis by hydrogen chloride or iodine monochloride in chlorination in carbon tetrachloride has also been examined. For the chlorination of pentamethylbenzene, the reaction was first-order in both aromatic and chlorine and either three-halves, or mixed first- and second-order in hydrogen chloride, but iodine monochloride was more effective as a catalyst and the chlorination of mesitylene was first-order in iodine monochloride the activation energy for this latter reaction (determined from data at 1.2 and 25.0 °C) was only 0.4 273. 

Figure 4.28. Electrophobic behaviour Effect of catalyst work function on the activation energy E and catalytic rate enhancement ratio r/r0 for C2H4 oxidation on Pt p02 4.8 kPa, Pc2H4 0.4 kPa (a) and CH4 oxidation on Pt p02 =2.0 kPa, Pch4 =2.0 kPa (b)."4 Reprinted with permission from Elsevier Science.

Figure 4.35. Effect of catalyst work function on the activation energy EA, preexponential factor k° and catalytic rate enhancement ratio r/r0 for C2H4 oxidation on Pt/YSZ 4 p02=4.8 kPa, Pc2H4=0-4 kPa,4,54 kg is the open-circuit preexponential factor, T is the mean temperature of the kinetic investigation, 375°C.4 T0 is the (experimentally inaccessible) isokinetic temperature, 886°C.4 25,50...

Figure 4.36. Effect of catalyst potential UWR and work function on the activation energy E (squares) and preexponential factor r° (circles) of C2H4 oxidation on Rh/YSZ. open symbols open-circuit conditions. Te is the isokinetic temperature 372°C and r is the open-circuit preexponential factor. Conditions po2=l.3 kPa, pc2n =7.4 kPa.50 Reprinted with permission from Academic Press.

Figure 4.47. Effect of catalyst potential on the desorption activation energy of 02 from Pt/YSZ calculated from the modified Redhead analysis ( ) and from the initial slope of the TPD spectra (O).30 Reprinted with permission from Academic Press.

Figure 5.26. Effect of catalyst potential on the oxygen desorption activation energy, Ed, calculated from the modified Redhead analysis for Pt, Ag and Au electrodes deposited on YSZ.44,46 Reprinted from ref. 44 with permission from the Institute for Ionics.

Figure 8.3. Effect of catalyst overpotential AUWR and work function

Figure 8.18. Effect of catalyst potential and work function on the apparent activation energy, E, and on the logarithm of the preexponential factor r° rfi is the open-circuit preexponential factor and T0, T are the two isokinetic points of C2H6 oxidation on Pt/YSZ for positive and negative potentials respectively.27 Reprinted with permission from Academic Press.

Figure 8.24. Effect of catalyst potential Uwr and work function change (vs 1=0) on the activation energy E and preexponential factor K° of the kinetic constant K of CH4 oxidation to C02 an average T value of 948 K is used in the rhs ordinate p°H4 =p°2 =2kPa, 29Reprinted with permission from Academic Press.

Figure 8.53. Effect of catalyst overpotential on the apparent activation energies of formation of H2CO ( ), CO (A), and CH4 ( ) during CH3OH dehydrogenation and decomposition on Ag.S6 Reprinted with permission from Academic Press.

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