Crossover temperature hydrogen transfer

The temperature dependence of this rate constant was measured by Al-Soufi et al. [1991], and is shown in Figure 6.17. It exhibits a low-temperature limit of rate constant kc = 8x 105 s 1 and a crossover temperature 7 C = 80K. In accordance with the discussion in Section 2.5, the crossover temperature is approximately the same for hydrogen and deuterium transfer, showing that the low-temperature limit appears when the low-frequency vibrations, whose masses are independent of tunneling mass, become quantal at Tisotope effect increases with decreasing temperature in the Arrhenius region by about two orders of magnitude and approaches a constant value kH/kD = 1.5 x 103 at T[Pg.174]

The rate constant for hydrogen atom transfer (conversion II into III) spans six orders of magnitude in the range 290-80 K. The quantum limit of the rate constant and crossover temperature are 5xl0 3s 1 and 100 K, respectively. The ratio kH/ku increases from 10 to 5 x 103 as the temperature falls from 290 to 100 K. It is the H atom in position a that is transferred, since the substitution of deuterium atom at position b (R = H) does not change the rate constant. 

Kensy et al. [1993] showed that the zwitterion formation and subsequent cyclization due to hydrogen transfer take place in the lowest triplet excited state of diphenylamine and its methyl substituents. The crossover temperature is about 100 K, and the values of C(H) are 10 2-10 4 s 1 for various substituents. The H/D kinetic isotope effect at T [Pg.177]

As discussed above, the temperature can influence the OCV of a PEM fuel cell and thermodynamics, electrode kinetics, membrane conductivity, hydrogen crossover, and mass transfer process, and this influence will be reflected in the overall cell performance. However, the dependence of performance on temperature can be complicated by the fact that other conditions, such as RH, backpressure, gas stochiometry, flow field design, and electrode structure, also affect performance. 

Example 4.12 Calculating Crossover Losses In ref. [9], the authors noted a hydrogen crossover loss of 3.3 mA/cm for their automotive H2 PEFC applications. Calculate the mass crossover rate of hydrogen through the membrane. Also, calculate and plot the cathode activation overpotential loss at open circuit and 1 A/cm as a function of cathodic exchange current density. Assume the cathodic charge transfer coefficient at the cathode is 1.5 at a temperature of 353 K, and the fuel cell has a 50 cm geometric area. 

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