Work Function Activators

Equations for the Work Function, Activity Coefficient, and Osmotic Coefficient for Particles Having Ion-Dipole Characteristics... 

Sodium was found to decrease and acids to increase the electron work function. Activation energies for the formation of acrolein, acetaldehyde, acid, and carbon dioxide change with A. 

It has been consistently found that small amounts of Pb in Sn + Pb alloys cause an appreciable decrease in the electron work function of Sn, which is in good agreement with data for liquid Sn + Pb alloys.816-818 The surface activity of Pb has been found to increase as the temperature decreases.817,818... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

There is an important point to be made regarding UWr vs t transients such as the ones shown in Fig. 4.15 when using Na+ conductors as the promoter donor. As will be discussed in the next section (4.4) there is in solid state electrochemistry an one-to-one correspondence between potential of the working electrode (UWr) and work function (O) of the gas exposed (catalytically active) surface of the working electrode (eAUwR=AO, eq. 4.30). Consequently the UWr vs t transients are also AO vs t transients. 

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

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.

Activation Energy and Preexponential Factor Dependence on Work Function... 

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. 

Such linear or near-linear variations in activation energy E with work function as the one shown in Fig. 4.28 but also in Figures 4.35 to 4.37 are quite common in electrochemical promotion studies and are usually accompanied by a concomitant linear variation in the logarithm of the preexponential factor, r°, defined from ... 

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 4.36. 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.

D. Tsiplakides, J. Nicole, C.G. Vayenas, and C. Comninellis, Work function and catalytic activity measurements of an Ir02 film deposited on YSZ subjected to in situ electrochemical promotion,/. Electrochem. Soc. 145(3), 905-908 (1998). 

Solid electrolyte cells can be used to alter significantly the work function catalytically active, catalyst electrode surface by polarizing the catalyst-solid electrolyte interface. 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd>Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as ... 

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... 

The variation in quasireference electrode in presence of reactive gas mixtures. This is due to its high catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

Figure 8.3. Effect of catalyst overpotential AUWR and work function

Figure 8.18. Effect of catalyst potential and work function on the apparent activation energy, E, and on the logarithm of the preexponential factor r° rfi is the open-circuit preexponential factor and T0, T are the two isokinetic points of C2H6 oxidation on Pt/YSZ for positive and negative potentials respectively.27 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 8.41 shows the effect of positive overpotential, i.e. increasing work function, on the apparent activation energies E, and preexponential factors kf of the epoxidation (i=l) and deep oxidation (i=2) reactions. After... 

The enhancement in the catalytic activity is due to the electrochemical supply of H+to the catalyst which decreases the catalyst work function and thus strengthens the chemisorptive bond of electron acceptor N while at the same time weakening the bonds of electron donor H and NH3. 

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