Catalyst work function

By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

C.G. Vayenas, S. Bebelis, and S. Ladas, Dependence of Catalytic Rates on Catalyst Work Function, Nature 343, 625-627 (1990). 

Why do negative potentials (UWr=-1 V) fail to further enhance to any significant extent catalyst performance of the promoted catalyst whereas the unpromoted Rh catalyst is electrochemically promoted with both positive and negative potentials (Fig. 2.3). The answer will become apparent in subsequent chapters In a broad sense negative potential application is equivalent to alkali supply on the catalyst surface. They both lead to a substantial decrease (up to 2-3 eV) in the catalyst work function, O, aquantity which as we will see, plays an important role in the description of promotion... 

CATALYST WORK FUNCTION VARIATION WITH POTENTIAL IN SOLID ELECTROLYTE CELLS... 

It also allows for the first time to perform catalytic experiments under conditions of independently controllable catalyst work function. 

A catalytic reaction is termed electrophobic1,19,54 with increasing catalyst work function O when its rate increases... 

A general observation which has emerged from electrochemical promotion studies is that over wide ranges of catalyst work function catalytic rates depend exponentially on catalyst work function O ... 

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 4.35. 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 4.49. 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 5.10. Transient response of catalyst work function O and potential Uwr upon imposition of constant currents I between the Pt catalyst (labeled26 C2) and the Pt counter electrode p"-A1203 solid electrolyte T = 240°C, p02 = 21 kPa Na ions are pumped to (I<0) or from (I>0) the catalyst surface at a rate I/F.26 Reprinted with permission from Elsevier Science.

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.

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 6.25, Experimental71 (left) and modelled simulated" (right) dependence of the rate of CO oxidation on Pt deposited on J3"-A1203 as a function of pco, catalyst potential UWR and dimensionless catalyst work function Il(=A

Figure 8.29. NEMCA-generated volcano plots obtained by increasing the catalyst work function above its open-circuit value during CO oxidation on Pt pCo=0.2 kPa, Po2=t 1 kPa, , T=560°C, r0= 1.5x1 O 9 mol O/s O, T=538°C ro=0.9xl0 9 mol O/s.36 Reprinted by permission of Platinum Metals Review.

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. 

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]. 

During investigation of the NEMCA effect the rates of catalytic reactions were found to depend on catalyst work function, , via the equation In(r/r0) = aty/k T where a is a reaction-specific constant and kB is the... 

Figure 5 Rh/Na-/i" alumina linear variation of catalyst work function with Ai/wR-Copyright 2003 by Taylor Francis Group, LLC...

The promoter must have a large absolute dipole moment value, Pj, so that large variations, A, in the catalyst work function, d>, can be induced by relatively small coverages, Oj, of the promoter [Eq. (21)]. The latter is... 

Alkali metals satisfy both of these criteria [131,132] as can be seen in Figures 10 and 11. Due to their absolutely large dipole moments, Paik 5-10D, alkali coverages, 0aik of the order of 0.1 suffice to decrease the catalyst work function by more than 2.5 eV. 

Which types of catalytic reactions can be promoted (accelerated) by such a pronounced decrease in catalyst work function They are called electrophilic, and we discuss them in the next section, together with their counterpart, electrophobic reactions, which are promoted by increasing catalyst work function. 

Increasing catalyst work function causes an increase in the heat of adsorption... 

Figure 35 Experimental (from Ref. 68) (top) and model-simulated (from Ref. 147) (bottom) dependence of the rate of CO oxidation on Pt deposited on /1"-Al203 as a function of p o, catalyst potential Cwr, and dimensionless catalyst work function II (= AC>/k T) atpo = 6 kPa (from Ref. 68). Parameters used in Eqs. (52) and (53) = 9.133, ko = 8.715, 1a = —0.08,...

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... 

From Eq. (86), one can study the dependence of catalytic reactions on catalyst work function. Four types of rate-work function dependence have been identified experimentally, that is, electrophobic (3r/3 > 0), electrophilic (3r/3 < 0), volcano (where the rate exhibits a maximum with varying ) and inverted volcano type, where the rate exhibits a minimum with varying <1> (Fig. 52). 

A very frequent feature of electrochemical promotion studies is the observed linear variation of catalytic activation energies with varying catalyst work function [9,14,15]. It had been proposed that this is due to a linear variation in chemisorptive bond strengths with catalyst work function [9,14], a proposition recently supported by TPD studies for oxygen chemisorption on IVVSZ [26]. 

Electrochemistry can be used to affect oxidation catalysis on metals and metallic oxides [13] in a very pronounced and reversible manner. The observed promotional phenomena are due to an electrochemically driven and controlled backspillover of ionic species on the catalyst surface. These species, which in some cases cannot form via gas phase adsorption, alter the catalyst work function and affect the binding strengths of chemisorbed reactants and intermediates in a pronounced and theoretically predictable manner. This electrochemically controlled variation in the binding strength of adsorbates causes the observed pronounced modification in catalytic activity and selectivity. The ability of solid electrolytes to act as reversible promoter donors to influence oxidation catalysis is of considerable theoretical and, potentially, practical interest. 

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]. 

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