Oxidation on Pt

Nettesheim S, von Oertzen A, Rotermund FI FI and ErtI G 1993 Reaction diffusion patterns in the catalytic CO-oxidation on Pt(110) front propagation and spiral waves J. Chem. Rhys. 98 9977-85... 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

S. Bebelis, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of Catalytic Activity 1. The case of Ethylene Oxidation on Pt, J. Catal. 118, 125-146 (1989). 

J. Poppe, S. Voelkening, A. Schaak, E. Schuetz, J. Janek, and R. Imbihl, Electrochemical promotion of catalytic CO oxidation on Pt/YSZ catalysts under low pressure conditions, Phys. Chem. Chem. Phys. 1,5241-5249 (1999). 

C.G. Vayenas, C.Georgakis, J. Michaels, and J. Tormo, The role of PtOx in the isothermal rate and oxygen activity oscillations of the Ethylene Oxidation on Pt, J. 

Figure 4.11. Typical Tafel plots for Pt catalyst-YSZ interfaces during C2H4 oxidation on Pt the large difference in I0 values between the two Pt films (labeled R1 and R2) is due to the higher calcination temperature of Pt film R2 vs Pt film Rl.4 Reprinted with permission from Academic Press.

Both C2H4 oxidation on Pt/YSZ (Fig. 4.13) and C2H4 oxidation on Rh/YSZ (Fig. 4.14) are electrophobic reactions, i.e. the rate r is an increasing function of catalyst potential UWr. They are therefore enhanced with positive currents (I>0) which leads to an increase in UWr- As we will see soon this is one of the four main types of experimentally observed r vs UWr behavior. 

Figure 4.16, Effect of catalyst potential, dimensionless catalyst potential n(=FUWR/RT), corresponding linearized51 Na coverage 0ns and pCo on the rate of CO oxidation on Pt/(T-A1203. T=350°C, po2=6 kPa.51 Reprinted with permission from Academic Press.

Figure 4.22. Effect of the rate of O2 supply to the catalyst electrode on the increase in the rate of C2H4 oxidation on Pt deposited on YSZ.1,4 Dashed lines are constant faradaic efficiency, A, lines. Reprinted from ref. 4 with permission from Academic Press.

Figure 4.22 shows the steady-state effect of current, or equivalently rate, I/2F, of O2 supply to the catalyst on the rate increase Ar during C2H4 oxidation on Pt/YSZ. According to the definition of A (Eq. 4.19), straight lines passing from the (0,0) point are constant faradaic efficiency A lines. 

Figure 4.24. Effect of gaseous composition on the regular (open-circuit) catalytic rate of C2H4 oxidation on Pt/YSZ and on the NEMCA-induced catalytic rate on the same Pt catalyst film maintained at UWr=1V, T=370°C, pc2h4 =0.65 kPa.4 Reprinted with permission from Academic Press.

Typical examples of electrophilic reactions are the reduction of NO by ethylene on Pt32 and the CO oxidation on Pt under fuel-rich conditions.51,62... 

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 4.31. Transition from volcano-type behaviour at low Po2 to electrophobic behaviour at high po2 during CO oxidation on Pt/j3"-A]203.51 Effect of UWr and linearized51 Na coverage 0Na on the rate of CO oxidation on Pt/p"-Al203 at varying po2- Other conditions pco=2 kPa, T=350°C. The top part of the figure shows the corresponding variation of the actual51 Na coverage, 0Na, with UWr- Reprinted with permission from Academic Press.

Figure 4.33. Inverted volcano behaviour. Effect of catalyst potential and work function on the rate of C2H6 oxidation on Pt/YSZ. po2=107 kPa, pc2H6 65 kPa T=500°C , T=460°C , T=420°C.24 Reprinted with permission from Academic Press.

Electrophilic reactions also frequently conform to Eq. (4.49) with a<0. An example is shown in Fig. 4.29 for the case of C2H4 oxidation on Pt supported on CaIno.iZro.903.a, a H+ conductor. At a first glance it is surprising that the... 

Actually, as shown in Fig. 4.30, C2H4 oxidation on Pt/Na3Zr2Si2POi2, where Na3Zr2Si2POi2 (NASICON) is a Na+ conductor, can exhibit volcano-type behaviour, i.e. electrophobic behaviour at low potentials followed by electrophilic behaviour at higher potentials. 

The transition from volcano-type behaviour to electrophobic behaviour upon changing p02 is shown in Fig. 4.31 for the case of CO oxidation on Pt/p -Al203. Another example of volcano-type behaviour is shown in Fig. 4.32 for the case of NO reduction by C3H6 on Pt/ 3"-Al203.98,99... 

A fourth important case of r vs O dependence is the inverted volcano behaviour depicted in Figure 4.33 for the case of C2H6 oxidation on Pt/YSZ.24 The rate is enhanced by a factor of 7 for negative potentials and by a factor of 20 for positive ones. 

It has been known since the early days of electrochemical promotion that upon varying Uwr and thus , not only the catalytic rates, r, are changing in a frequently dramatic manner, but also the activation energy of the catalytic reaction is also significantly affected. An example was already presented in Fig. 4.28 which shows that both C2H4 and CH4 oxidation on Pt/YSZ conform to equation (4.50) with an values of -1 and -3, respectively. 

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 4.37. Effect of catalyst potential Uwr and work function on the activation energy E (squares) and preexponential factor r° (circles) of C3H6 oxidation on Pt/YSZ. open symbols open-circuit conditions. T is the isokinetic temperature 398°C and r is the open-circuit preexponential factor. Conditions po2 =3 kPa, PC3H6 0-4 kPa.25 Repnnted with permission from Academic Press.

Figure 4.39. NEMCA-induced compensation effect in the isokinetic point for C3H6 oxidation on Pt/YSZ. Conditions po2=3 kPa, Pcjh O-4 kPa.25 Reprinted with permission from Academic Press.

P.D. Petrolekas, S. Balomenou, and C.G. Vayenas, Electrochemical promotion of Ethylene Oxidation on Pt Catalyst Films deposited on Ce02, J. Electrochem. Soc. 

P. Beatrice, C. Pliangos, W.L. Worrell, and C.G. Vayenas, The electrochemical promotion of ethylene and propylene oxidation on Pt deposited on Yttria-Titania-Zirconia, Solid State Ionics 136-137, 833-837 (2000). 

Figure 5.2. NEMCA and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate, r, of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient)3 (b) the 02 TPD spectrum on Pt/YSZ4,7 after current (1=15 pA) application for various times t. (c) the cyclic voltammogram of Pt/YSZ4,7 after holding the potential at UWR = 0.8 V for various times t.

Figure 5.5. (a) Dependence of the NEMCA relaxation time constant x on 2FNc/I for C2H4 epoxidation on Agu and (b) for CO, C2H4 and CH3OH oxidation on Pt and Ag.12 Adapted from ref. 11 and reprinted from ref. 12 with permission from the American Chemical Society and from Elsevier Science respectively. 

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 5.58. Schematic of the effective double layer during C2H4 oxidation on Pt/YSZ (top) and Pt/p"-Al203.

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