Catalyst work function changes

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 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 5.11. Effect of applied current on induced work function change on Pt/p"-Al203 26 dashed line catalyst labeled26 Cl, T = 291°C, pQ2 = 5 kPa, pc2H4 2.1xl0 2 kPa solid lines catalyst labeled26 C2, T = 240°C, p02 = 21 kPa, Inset Effect of applied current on computed initial dipole moment of Na on Pt ( ) I>0, (A) I<0.26 Reprinted with permission from Elsevier Science.

The previous sections of this chapter have established that NEMCA, or Electrochemical Promotion, is caused by the electrochemically controlled backspillover of ionic species onto the catalyst surface and by the concomitant change on catalyst work function and adsorption binding energies. Although the latter may be considered as a consequence of the former, experiment has shown some surprisingly simple relationships between change AO in catalyst... 

Figure 6.3. Examples for the four types of global electrochemical promotion behaviour (a) electrophobic, (b) electrophilic, (c) volcano-type, (d) inverted volcano-type, (a) Effect of catalyst potential and work function change (vs I = 0) for high (20 1) and (40 1) CH4 to 02 feed ratios, Pt/YSZH (b) Effect of catalyst potential on the rate enhancement ratio for the rate of NO reduction by C2H4 consumption on Pt/YSZ15 (c) NEMCA generated volcano plots during CO oxidation on Pt/YSZ16 (d) Effect of dimensionless catalyst potential on the rate constant of H2CO formation, Pt/YSZ.17 n=FUWR/RT (=A(D/kbT).

Figure 8.8. Effect of catalyst potential (Jwr and corresponding work-function change A on the activation energy of C2H4 oxidation on Rh.13 Po2=l -3 kPa, Pc 2H4=7-4 kPa. Reprinted with permission from Academic Press.

Figure 8.15. Time dependence of the work function change, AO, the reaction rate, r, and the catalyst potential, Uwr, following galvanostatic steps during C2H4 oxidation on RuCVYSZ.20,21 Catalyst Ru02 (m=0.4 mg A=0.5 cm2), 1=50 pA, Pc2H4=1 14 Pa, po2=17.7 kPa, Fy=175 cm3 STP/min, T = 380°C.25...

Figure 8.21. (a) Effect of the rate, I/2F, of electrochemical oxygen ion removal (I<0) on the induced increase in the rate of propylene oxidation on Pt/YSZ.28 (b) Effect of catalyst potential and work function change on the rate enhancement ratio p (=r/r0) at a fixed gaseous composition. 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 9.6. Effect of catalyst potential Uwr corresponding work-function change AO, and linearized Na coverage 0n3 on the rate of CO oxidation on Pt/p"-Al203. Conditions T=350°C, po2=6 kPa , pco=5.3 kPa, O, Pco=2.8 kPa. Reprinted with permission from Academic Press.11...

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

Electrochemical and surface spectroscopic techniques [iii, v] have shown that the NEMCA effect is due to electro chemically controlled (via the applied current or potential) migration of ionic species (e.g., Os, NalS+) from the support to the catalyst surface (catalyst-gas interface). These ionic species serve as promoters or poisons for the catalytic reaction by changing the catalyst work function O [ii, v] and directly or indirectly interacting with coadsorbed catalytic reactants and intermediates [iii—v]. 

Surface Coverage, Catalyst Potential, and Work Function Changes ... 

The molecular origin of electrochemical promotion is currently understood on the basis of the sacrificial promoter mechanism [23]. NEMCA results from the Faradaic (i.e., at a rate I jnF) introduction of promoting species (Os in the case of O2- conductors, H+ in the case of H+ conductors) on the catalyst surface. This electrochemically introduced O2- species acts as a promoter for the catalytic reaction (by changing the catalyst work function and affecting the chemisorptive bond strengths of coadsorbed reactants and intermediates) and is eventually consumed at a rate equal, at steady state, to its rate of supply (I/2F) which is A times... 

Electrochemical promotion can be used to modify significantly the product selectivity, of catalytic oxidation reactions. An example is presented in Fig. 5 which shows the effect of catalyst potential and corresponding work function change on the selectivity to ethylene oxide (Fig. 5a) and acetaldehyde (Fig. 5b) of ethylene oxidation on AgA SZ at various levels of gas phase chlorinated hydrocarbon moderators [31] (The third, unde.sirable, product is CO2). As shown in the Figure a 500 mV decrease in catalyst potential causes the Ag surface to change from selective (up to 70%) ethylene oxide production to selective (up to 55%) acetaldehyde production. The same study [31] has shown that the total rate of ethylene oxidation varies by a factor of 200 upon varying the catalyst potential. (Fig. 6)... 

The behaviour of these systems may be quantitatively rationalised in terms of changes in adsorption energies (and therefore reaction activation energies) caused by changes in catalyst work function which result from backspillover of electrochemically pumped ions from the solid electrolyte to the active metal component [8]. The Electrochemical Promotion literature has been renewed recently [8]. 

Figure 26 Effect of catalyst potential and work function change on the rate...

Figure 31 TPD under electrochemical promotion conditions Effect of catalyst potential and Induced work function change on the peak desorption temperature Tp and desorption activation energy E of oxygen dissociatively adsorbed on Pt supported on YSZ. ...

Figure 32 Effect of Ag/YSZ catalyst potential and work function change on the rates of formation of ethylene oxide, acetaldehyde and COj at low (a) and high (b) Pq /Pq ratios/ ...

Figure 26. Time dependence of the work function change (A), the reaction rate (B) and the catalyst potential (C) following galvanostatic steps. Catalyst Ru02 (m = 0.4 mg A = 0.5 cm ), /= 50 pA during 23 min, = 114 Pa, P02 17-7 kPa, flowrate 175 mLmin STP, 7 =380°C.

Gayko, G., Wolf, D., Kondratenko, E.V., and Baems, M. Interaction of oxygen with pure and SiO-doped Nd203 catalysts for the oxidative coupling of methane study of work function changes. J. Catal 1998,178, 441 449. 

Figure 16. (a) Transient response of catalyst work function 0 and potential upon imposition of constant currents I between the Pt catalyst (C2) and the Pt counter electrode. )5"-Al203 solid electrolyte T= 240°C, Pq = 21 kPa. Na ions are pumped to (/ < 0) or from (/ > 0) the catalyst surface at a rate I/F. (b) Effect of applied current on induced work-function change. —, catalyst Cl, T= 291 °C, = 5 kPa, Pom ... 

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