Catalyst overpotential

Figure 5.13. Effect of catalyst overpotential, AUWR, on catalytic rate and on catalyst work function changes, AO, during ethylene oxidation on Pt/YSZ at 400°C.34Reprinted with permission from Elsevier Science.

Figure 7.Effect of catalyst overpotential r on the rate and reaction order of C2H4 oxidation on two Pt catalyst films, labeled R1 and R2. For Rb p02=4.8 kPa and Pc2H4=0-4 kPa. For R2, Po2=6.4 kPa and Pc2H4=0 4 kPa.1...

Figure 8.3. Effect of catalyst overpotential AUWR and work function

The effect of catalyst overpotential and potential on the rates of these two reactions is shown in Figs. 8.45 and 8.46. They both exhibit electrophobic behaviour for Uwr>U r and electrophilic behaviour for UWR< U, i.e. the reaction exhibits pronounced inverted volcano behaviour. 

Figure 8.45. Effect of Pt catalyst overpotential on the kinetic constant of CH3OH oxidation to C02 on Pt/YSZ for positive (a) and negative (b) currents. Pch30h= 0.9 kPa, p02=19 kPa. T=, 698 K A, 650 K , 626 K Reprinted with permission from Academic Press.50...

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.

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Figure 9.27. Steady-state effect of catalyst overpotential AUwii( riie-U he) on current I. Conditions as in Fig. 9.26.35 Reproduced by permission of The Electrochemical Society.

The complexity of the reaction rate transients, which consist of one fast and one slow stage, is in agreement with the cyclic voltammetric evidence abont the existence of differently accessible regions for surface charging. The first rapid step (a) is believed to be dne to accumulation of promoting species over the gas-exposed catalyst surface by the mechanism of backspillover, while the second step (b) is due to current-assisted chemical surface modification. Since no correlation between potential transients and reaction rate transients was manifested, a dynamic approach is justified and the applied current —rather than the catalyst overpotential— may be an appropriate parameter to describe the transient behavior of ethylene combustion rate at electrochemically promoted Ir02AfSZ film catalysts. For the interpretation of the fast transient steps (a) and (c), a dynamic model of electrochemical promotion has been developed, as presented in detail in Section 11.3. 

This depends on the catalytic activity of the semiconductor stnface, which can be quite low. By attaching suitable co-catalysts, overpotentials as low as 0.3-0.4 V can be achieved. 

Figure 57. Effect of AgA SZ catalyst overpotential on the activation energy E and preexponential factor of ethylene epoxidation (open symbols) and oxidation to CO2 (closed symbols). PC2H4 = 2.48 kPa, P02 = 3.15 kPa. (Reprinted, with permission from Academic Press, from Ref. 43.)...

---