Elsevier

Solid State Ionics

Volume 225, 4 October 2012, Pages 386-389
Solid State Ionics

The role of low coverage sodium surface species on electrochemical promotion in a Pt/YSZ system

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Abstract

The effect of sodium-modification on the catalyst and electrocatalytic properties of a platinum catalyst supported on a YSZ solid electrolyte was studied. Increasing the sodium coverage on the catalyst surface appears to block some of the three-phase boundary (tpb) sites and reduces the rate of the charge transfer reaction. The promotion of the platinum surface reaction (ethylene oxidation) seems to a first approximation to be a function of the rate of oxygen supply or removal to or from the surface irrespective of whether this is contaminated by sodium or not (samples with sodium contamination require a higher overpotential to achieve the same current density as a clean sample because of poisoning in the tpb). At high negative polarisations (oxygen removed from the surface) the sodium contaminated samples show a significant increase in rate, possibly due to the decomposition of e.g. sodium hydroxides and carbonates.

Highlights

► Addition of sodium surface species modifies oxygen charge transfer. ► Sodium addition does not affect catalyst under open circuit or dynamic polarisation. ► Sodium addition enhances ethylene-oxidation EPOC under constant negative polarisation.

Introduction

Electrochemical Promotion of Catalysis (EPOC) has been observed in a wide range of reactions performed on various metal catalysts supported on a number of different solid electrolytes [1], [2], [3], [4]. In EPOC, or NEMCA (non-faradaic electrochemical promotion of catalysis), a small current supplied through an external circuit results in promoting species being pumped towards the catalyst surface, thus modifying the catalytic activity and selectivity of a reaction [5], [6]. A recent study found that variations in the catalyst surface morphology and the presence of impurities on the catalysts may have a significant impact on the catalyst behaviour [7] and could play an important role in electrochemical promotion. Nevertheless, the role of impurities in EPOC has not been studied in detail except perhaps the influence of Si impurities and electrode morphology on the Pt(O2)/YSZ system using cyclic voltammetry [7]. In this work, in order to systematically study the role of impurities in EPOC, a known type and amount of impurity (in this case sodium, Na) is gradually deposited in increasing amounts onto a nominally ‘clean’ catalyst surface (in this case platinum, deposited on yttria stabilised zirconia, YSZ) and the catalyst used for EPOC studies of ethylene oxidation.

Section snippets

Experimental

The catalyst/electrode and electrolyte materials used in the study were metal resinates provided by Metalor, UK and 8 mol% YSZ provided by Pi-Kem Ltd., UK. The platinum resinate was painted on two YSZ dense pellets of 15 mm diameter and 2 mm thickness and sintered at 850 °C for 2 h. The resulting samples have a platinum film of geometric projected surface area, A = 0.88 cm2. One sample was used for the non-reactive study and the other was used under reactive conditions. A three electrode pellet system,

Non-reactive conditions (oxygen charge transfer reaction)

Fig. 1a shows cyclic voltammograms of the platinum catalyst exposed to an oxygen flow of 20 kPa at 400 °C, at a scan rate, υ = 20 mV s 1. The voltammograms were obtained using one sample to which was gradually added NaOH of varying concentration from 10 12 to 10 1 M as shown in Table 1. As discussed earlier the sodium coverage in these samples ranges from 5.4 × 10 9% ([NaOH] = 10 12 M) to full coverage ([NaOH] = 10 1 M). One of the main features on the voltammograms is the sharp cathodic peak that involves

Conclusions

From our study, we have shown that addition of sodium to a platinum catalyst surface can affect, to some extent, the characteristics of the oxygen charge transfer reaction and modify the activity of a catalytic reaction. Sodium surface species (which could be in the form of e.g. hydroxides or carbonates) block sites in the tpb region and slow down the charge transfer reaction. They may also slightly reduce the open circuit rate of a catalytic reaction (in this case ethylene oxidation) by a

Acknowledgements

NI would like to thank the Ministry of Higher Education Malaysia for funding. DP acknowledges the financial support of EPSRC through grant EP/G025649/1.

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Cited by (3)

  • The role of sodium surface species on electrochemical promotion of catalysis in a Pt/YSZ system: The case of ethylene oxidation

    2013, Journal of Catalysis
    Citation Excerpt :

    Sodium was deposited dropwise using 1 μL of sodium hydroxide (NaOH) solutions (Alfa Aesar) on the platinum surface using a fixed micropipette followed by drying in air at 400 °C for 1 h. Table 1 summarises the sodium loading (per platinum surface area) and the percentage of nominal sodium coverage. Details of the sodium deposition method and the calculation of sodium loading/coverage are as reported in our previous work [37]. Unless otherwise stated, the sodium loadings reported in Table 1 are cumulative.

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