NEMCA Effect on Platinum Metals

Although the NEMCA effect is not limited to a particular metal catalyst or solid electrolyte (1), in most of the reactions examined so far platinum metals, and especially platinum, have been used as catalysts (1, 3-6). This choice has been dictated both by the importance of platinum metals as oxidation catalysts, and also by their particular suitability for NEMCA studies, as they exhibit in general high turnover frequencies and low values of I₀, compared to other metals, such as silver (Equation (iv)). Some examples are reviewed here and some new results are also presented.

Ethylene Oxidation on Pt/ZrO₂ (8 per cent Y₂O₃)

The kinetics of this reaction have been described in detail recently (6). Under fuel-lean conditions, P^O2⋙P_C2H4, where the catalytic rate is first order in ethylene pressure P_C2H4, and zeroth order in oxygen pressure P_O2, that is r = KP_C2H4, rate enhancement ratios q up to 55 and enhancement factor values up to 3×10⁵ were measured (6). Both the open-circuit and the NEMCA induced kinetic behaviours were modelled quantitatively (6). It was found that the kinetic constant K of the reaction between adsorbed oxygen and gas phase ethylene depends exponentially on V_WR and work function eΦ see Figure 2, with values of α = 0.5-1.0 (Equations (ii) and (iii)). The activation energy was found to decrease linearly with eΦ. This decrease counterbalances the compensating decrease in the pre-exponential factor K⁰, K = K⁰exp(-E/RT) (6), yielding to the experimentally observed exponential increase of k with v_wr and eΦ, also shown in Figure 2. Figure 3 shows a typical transient response of the catalytic rate and catalyst potential upon imposition of a constant current (galvanostatic transient). It can be seen that the effect is reversible and that the steady-state rate increase Ar is a factor of 26 higher than r₀, and a factor of 74,000 higher than the rate I/₂F of O^{2 _} supply. Also the relaxation time constant τ is of the order of ₂FN/I, where N is the total catalyst film surface area.

Ethylene Oxidation on Pt/β ”-Al₂O₃

This behaviour is qualitatively similar to that observed when using stabilised-zirconia as the solid electrolyte. As shown in Figure 4, the catalytic rate constant K decreases exponentially up to a certain point with decreasing work function eΦ caused by Na⁺ supply to the catalyst. The maximum rate constant decrease is over 90 per cent and corresponds to a sodium coverage of only 0.015 (9)

Measured Λ values were of the order of 10³ to 5 × 10⁴. The open circuit kinetic model developed in (6) was found to describe adequately the NEMCA catalytic behaviour (indicated by the dotted line on Figure 4) with the kinetic constant K (see above) depending exponentially on eΦ with slope 0.28/k_bT. The observed catalytic rate relaxation time constants τ during galvanostatic transients were approximated well by:

(v)

which is very similar to τ = ₂fn/i, used for τ estimation for O^{2 -} conducting solid electrolytes.

Methanol Oxidation on Pt/ZrO₂ (8 per cent Y₂O₃)

This system is quite interesting as it exhibits both positive (I>o,Λ>o) and negative (I<o, Λ<o) NEMCA behaviour at temperatures of 350 to 650°C (10). As in other NEMCA studies the kinetic constants Ki for the formation of the main products, which are formaldehyde and carbon dioxide, depend exponentially on catalyst work function:

(vi)

with K_i/K_{i o} ratios up to io for negative currents (removal of O^2- from the catalyst) and up to 3 for positive currents. Measured Λ values were of the order of 10³ to 10⁴ (10). Selectivity to formaldehyde can be varied deliberately between 35 and 60 per cent by controlling the catalyst potential. Unfortunately selectivity goes through a maximum near the open circuit potential (10).

Methane Oxidation on Pt/ZrO₂ (8 per cent Y₂O₃)

This reaction also exhibits both positive and negative NEMCA behaviour, at temperatures 650 to 750°C and for a 1:1 ratio of oxygen to methane. Equation (iii) is again valid with α values +0.8 and -0.3 for increasing and decreasing work function, respectively. This is shown, for the latter case (I<o), in Figure 5. Again the activation energy changes linearly with work function. The slope is 1.05 ±0.1. Enhancement ratios ɐup to 70 and Λ values up to 5 have been measured

Carbon Monoxide Oxidation on Platinum and Palladium

These are well known oscillatory reactions and both were found to exhibit the NEMCA effect. In the case of the palladium catalysed carbon monoxide oxidation Λ values of the order of 10³ were measured at T = 290°C, P_co = 3×10^-4 bar and P_O2 = 0.15 bar. For the oxidation of carbon monoxide on platinum both negative and positive NEMCA behaviour has been observed, with rate enhancement ratios up to 6 and |Λ| values up to 500 (4). As shown in Figure 6 one can use NEMCA to create “volcano” type plots. These are, apparently, the first volcano plots generated for a single metal by changing its work function. Oscillatory states can also be induced, stopped or modified by controlling the catalyst potential (3, 5).

The Origin of NEMCA

The experimental features of all previous NEMCA studies can be adequately explained by a semiquantitative model, which is based on a uniform change of the catalyst work function induced by an electrochemically controlled spillover of ions onto the catalyst surface, and on the concomitant controlled change in the strength of chemisorptive bonds (1, 3, 6, 7, 11). The induced work function changes require only a small coverage of ions (typically <5 per cent of a monolayer, as calculated using the Helmholtz equation (1, 9) which does not significantly affect the coverages of covalently bonded species. The creation of such spillover ions depends solely on the presence of significant catalyst-electrolyte overpotential, so that the NEMCA effect seems to be of general interest and not restricted to any particular catalytic system. The surprisingly simple relationship (Equation (iii)) between catalytic activity and catalyst work function, measured in situ and for the first time varied at constant reactant composition, quantifies the importance of the electronic factor in heterogeneous catalysis, offering some new insights into the action of promoters.

As a result of NEMCA studies it was found that the e.m.f. of solid electrolyte cells with electrodes made of the same material provides a direct measure of the work function difference between the catalyst (working electrode) and the gas-exposed surfaces of the reference electrode (1). This observation is of particular importance for a deeper understanding of solid electrolyte potentiometric measurements, which have been used in recent years in catalyst research (8, 12).

The catalytic reactions for which the NEMCA effect has been studied so far are given in the Table. They fall into two groups: those exhibiting “positive” NEMCA behaviour, that is Λ>o, or equivalently, rate acceleration with increasing catalyst work function, and those exhibiting “negative” NEMCA behaviour, that is Λ<o or equivalently, rate acceleration with decreasing catalyst work function. The terms “electrophobic” and “electrophilic” have been proposed, respectively, for these two groups of reactions (1, 7).

The rate limiting step in electrophobic reactions, such as ethylene oxidation on platinum, involves cleavage of a metal/electron-acceptor adsorbate bond (6). In this case it is expected, provided that the nature of the activating complex is unchanging, that the rate of the reaction will increase exponentially with decreasing heat of adsorption of the adsorbate, for example atomic oxygen, (- ΔH_ad) and consequently with eΔΦ if one takes into account the semi-theoretical linear relation between Δ (-ΔH_ad) and eΔΦ, proposed by Boudart (II). This explains both Equation (iii) and the observed quasilinear decrease in activation energy with eV_WR and eΦ with slope of the order of -1.

If, on the other hand, the rate limiting step of a catalytic reaction involves cleavage of an intra-adsorbate bond in an electron-acceptor adsorbate, then it is expected from classical bond order conservation considerations that the strengthening of the metal-adsorbate bond with decreasing work function will cause a weakening of the intra-adsorbate bonds, thus producing an exponential catalytic rate increase (7). This is the case with methanol oxidation on platinum, where lowering the catalyst work function facilitates the abstraction of hydrogen from adsorbed methanol intermediates. In this way the rates of formation of formaldehyde and chemisorbed carbon monoxide are accelerated, and the latter can be rapidly oxidised by chemisorbed oxygen, as experimentally observed (7, 10).

The almost linear correlation of work function change with activation energy, found in NEMCA studies, seems to imply that NEMCA is actually a “long-range” electronic effect mediated through the electronic band structure of the catalyst. Long-range effects have been predicted theoretically for some adsorbates on metal surfaces (13) and have been invoked to explain the dependence of the sticking coefficient of oxygen on alkali-doped germanium; this was found to depend exponentially on the alkali induced e$ change, regardless of alkali type (14).

Nevertheless short-range effects involving sites adjacent to the spillover ions cannot be entirely ruled out as one could also observe, macroscopically in this case, a “long-range” effect if the spillover ion is fairly mobile on the catalyst surface. There is also convincing experimental and theoretical evidence for short-range effects in explaining the promoting and poisoning action of co-adsorbed electronegative and electropositive species (for example 2, 15). It is very likely that there is no universal answer and that the long- or short-range electronic nature of NEMCA is catalyst- and spillover-ion-specific. The use of TPD and of in situ surface science techniques, such as XPS, UPS and STM (16) under conditions of NEMCA behaviour will definitely help the exact characterisation of the electrochemically created spillover species and the elucidation of the nature of their interaction with the catalyst surface.

Practical Applications of NEMCA

Apart from the interesting theoretical consideration briefly outlined above, it is clear that the NEMCA effect can be utilised in at least two different and practically meaningful ways:

First, as a means of selecting promoters for conventional supported catalysts. NEMCA can be used to determine if a given reaction exhibits electrophobic (Λ>o, α >o), or electrophilic (Λ<o, α <o) behaviour. In the former case, promoters based on fluorine, chlorine, bromine and other elements causing an increase in catalyst work function are expected to have a beneficial effect on catalyst performance. In the latter case alkali based promoters are the best choice for catalyst performance enhancement.

Second, for use in solid oxide fuel cell (SOFC) type reactors (12) to carry out reactions where NEMCA can give high selectivity to the desired valuable products. In this application, the reacting mixture should be fed to one side of the solid electrolyte, which is covered by the porous catalyst, while the other side, covered by a suitable electrocatalyst (12), is exposed to a suitable gaseous or liquid (9) environment, for the oxidation or reduction of the ions, which are pumped through the solid electrolyte to or from the catalyst. A suitable voltage is constantly applied between the catalyst and the electrocatalyst. Since current densities of a few µ A/cm² suffice to induce significant overpotential and catalyst work function changes, provided the catalyst-solid electrolyte interface is polarisable (1, 3-10), the SOFC reactor can operate at low operating temperatures (200-650°C), which cover the vast majority of industrial chemical reactions.

In the latter application platinum group metals, of use in many catalytic reactions of technological interest, could naturally play a leading role.

There are sound theoretical reasons to expect that the NEMCA effect is more pronounced (higher α values) on metals with high local density of states (LDOS) near the Fermi level, as is the case for most of the platinum group metals.

However there is already some experimental evidence supporting this idea. The LDOS at the Fermi level is 2.2 and 2.3 states/eV atom, for platinum and palladium, respectively, while it is only 0.27 states/eV atom for silver. In comparison the absolute value of α is 0.3-1.0 for platinum-catalysed reactions and 0.1-0.2 for reactions which are catalysed by silver.

Conclusions

Solid electrolytes can be used as ion donors or acceptors to control the work function of metal catalysts and to induce dramatic and reversible changes in catalytic activity and selectivity. The effect is very pronounced for platinum, but is not restricted to a particular metal or solid electrolyte. The use of NEMCA, that is of in situ electrochemical promotion of catalyst surfaces, allows for new areas of surface catalytic chemistry to be explored, and could lead to technological applications by influencing catalyst performance in desirable directions.