Catalyst potential

Undercharge of catalyst. Potential for accumulation of reactants and subsequent runaway reaction. Possibility of no reaction resulting in a waste disposal issue. 

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.

Figure 2.3. Catalysis (0), classical promotion ( ), electrochemical promotion ( , ) and electrochemical promotion of a classically promoted (sodium doped) ( , ) Rh catalyst deposited on YSZ during NO reduction by CO in presence of gaseous 02.14 The Figure shows the temperature dependence of the catalytic rates and turnover frequencies of C02 (a) and N2 (b) formation under open-circuit (o.c.) conditions and upon application (via a potentiostat) of catalyst potential values, UWr, of+1 and -IV. Reprinted with permission from Elsevier Science.

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. 

How can one explain such a huge Faradaic efficiency, A, value As we shall see there is one and only one viable explanation confirmed now by every surface science and electrochemical technique, which has been used to investigate this phenomenon. We will see this explanation immediately and then, in much more detail in Chapter 5, but first let us make a few more observations in Figure 4.13. It is worth noting that, at steady-state, the catalyst potential Uwr, has increased by 0.62 V. Second let us note that upon current interruption (Fig. 4.13), r and UWr return to their initial unpromoted values. This is due to the gradual consumption of Os by C2H4. 

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 4.15. Rate and catalyst potential response to application of negative currents (a,b), for the case of volcano-type behaviour, see text for discussion. Conditions pCo=2 kPa, p02=2 kPa, T=350°C. Catalyst Cl. 51 Reprinted with permission from Academic Press.

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.

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t < - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a... 

Figure 4.17. NO reduction by H2 on Pt/p"-AI203.52 Transient effect of applied constant negative current (Na supply to the catalyst) on catalyst potential (a) under reaction conditions (solid line) and in a He atmosphere (dashed line) and on the rates of formation of N2 and N20 (b). Potentiostatic restoration of the initial rates see text for discussion. Reprinted with permission from Academic Press.

Figure 4.18. NO reduction by H2 on Pt/p"-Al2Oj. Effect of catalyst potential on the rates of formation of N2 and N20 and on the selectivity to N2.52 Reprinted with permission from Academic Press.

Figure 4.25. Dependence of pco2 and pN2 on the catalyst potential and on the oxygen concentration during NO reduction by C3H6 in presence of 02 on Rh/YSZ.70 Reprinted with permission from Elsevier Science.

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

Depending on the rate behaviour upon variation of the catalyst potential UWr and, equivalently work function , a catalytic reaction can exhibit two types of behaviour, electrophobic or electrophilic. These terms, introduced since the early days of electrochemical promotion, are synonymous to the terms electron donor and electron acceptor reaction introduced by Wolkenstein113 in the fifties. Electrochemical promotion permits direct determination of the electrophobicity or electrophilicity of a catalytic reaction by just varying UWr and thus 0. 

Dependence of Catalytic Rates and Activation Energies on Catalyst Potential UWRand Work Function [Pg.152]

Figure 4.29. Electrophilic behaviour Effect of catalyst potential and work function change AO on the rate of C2H4 oxidation on a Pt film deposited on CaZr0 9Ino 03.a which is a H+ conductor.104 Reprinted with permission from the Institute for Ionics.

Figure 4.30. Volcano-type behaviour Effect of catalyst potential on the rate of ethylene oxidation on a Pt film deposited on NASICON (Na3Zr2Si2PO 2), a Na+ conductor T=430°C, P02 =7.2 kPa, Pc2H4= kPa.102 Reproduced by permission of The Electrochemical Society.

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.

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

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 4.41. Effect of Ag/YSZ catalyst potential, work function and feed partial pressure of dichloroethane on the selectivity to ethylene oxide (a) and to acetaldehyde (b). T=270°C, P=500 kPa, 8.5% 02,7.8% C2H4.77 Reprinted with permission from Academic Press.

Figure 4.42. Ethylene epoxidation on Ag/p"-Al203.101 Steady-state effect of catalyst potential on the selectivity to ethylene oxide at various levels of gas-phase dichloroethane (a) and 3-dimensional representation of the effect of dichloroethane concentration, catalyst potential and corresponding Na coverage on the selectivity to ethylene oxide (b).101 Reprinted with permission from Academic Press.

Increasing catalyst potential and work function leads to a pronounced increase in total oxygen coverage (which approaches unity even at elevated temperatures) and causes the appearance of new chemisorption states. At least two such states are created on Pt/YSZ (Fig. 4.43) A strongly bonded one which, as discussed in Chapter 5, acts as a sacrificial promoter during catalytic oxidations, and a weakly bonded one which is highly reactive and causes the observed dramatic increase in catalytic rate. 

Figure 4.45. Thermal desorption spectra (bottom) and corresponding catalyst potential variation (top) after electrochemical O2 supply to Ag/YSZ at 260-320°C at various initial potentials Uwr Each curve corresponds to different adsorption temperature and current, thus different values of Uwr, in order to achieve nearly constant initial oxygen coverage.31 Reprinted with permission from Academic Press.

Figure 4.49. Transient effect of constant applied current (I=+10 pA) on the rate (r) of C2H4 oxidation on Ir02/YSZ, on catalyst work function (AO) and on catalyst potential (Uwr)-Conditions T=380°C, pc>2 = f 5 kPa and PC2H4 =0.05 kPa.88 Reprinted with permission of The Electrochemical Society.

Figure 5.12. Time evolution of catalyst potential and work function for Pt/p"-Al203 during potentiostatic transients, T = 200°C, po2 = 10 10 Pa.25...

Figure 5,14. Dependence of Ow-Or on catalyst potential eUWR for the systems (a) Pt(W)-Au(R) and (b) Pt(W)-Ag(R) at T=400°C. Open symbols Open-circuit operation in 02-He mixtures. Filled symbols Closed circuit operation at po2=12 kPa.32 Reprinted by permission of The Electrochemical Society.

Figure 5.24. Effect of catalyst potential, Uwr, on oxygen peak desorption temperature, Tp during 02 TPD from Pt/YSZ.4,5 The exact definition of Uwr has been given in Figure 4.45. It is the UWr value at the beginning of the TPD run and differs little (<0.1 V) from the UWR value at Tp.4,7 Reprinted with permission from the American Chemical Society.

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.

The technique of AC Impedance Spectroscopy is one of the most commonly used techniques in electrochemistry, both aqueous and solid.49 A small amplitude AC voltage of frequency f is applied between the working and reference electrode, superimposed to the catalyst potential Uwr, and both the real (ZRe) and imaginary (Zim) part of the impedance Z (=dUwR/dI=ZRc+iZim)9 10 are obtained as a function of f (Bode plot, Fig. 5.29a). Upon crossplotting Z m vs ZRe, a Nyquist plot is obtained (Fig. 5.29b). One can also obtain Nyquist plots for various imposed Uwr values as shown in subsequent figures. 

F/gwre 5 JO, (a) Complex impedance spectra (Nyquist plots) of the CH4,02) Pd YSZ system at different Pd catalyst potentials. Open circuit potential U R =-0.13 V. Dependence on catalyst potential of the individual capacitances, C4i (b) and of the corresponding frequencies, fmii, at maximum absolute negative part of impedance (c).54 Reprinted with permission from Elsevier Science. 

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