The role of sodium surface species on oxygen charge transfer in the Pt/YSZ system

Abstract

The role of sodium surface species in the modification of a platinum (Pt) catalyst film supported on 8 mol% yttria-stabilised-zirconia (YSZ) was investigated under a flow of 20 kPa oxygen at 400 o C. Cyclic and linear sweep voltammetry were used to investigate the kinetics of the oxygen charge transfer reaction. The Pt/YSZ systems of both 'clean' and variable-loading sodium-modified catalyst surfaces were also characterised using SEM, XPS and work function measurements using the Kelvin probe technique.Samples with sodium coverage from 0.5 -100% were used. It was found that sodium addition modifies the binding energy of oxygen onto the catalyst surface. Cyclic voltammetry experiments showed that higher overpotentials were required for oxygen reduction with increasing sodium coverage. In addition, sodium was found to modify oxygen storage and/or adsorption and diffusion increasing current densities at higher cathodic overpotential. Ex situ XPS measurements showed the presence of sodium hydroxide, carbonate and/or oxide species on the catalyst surface, while the Kelvin probe technique showed a decrease of approximately 250 meV in the work function of samples with more than 50% sodium coverage (compared to a nominally 'clean' sample).

Electrochemical Promotion of CO Oxidation on Na-Promoted Pt/YSZ: Interaction Between Multiple Promoting Species

“…This observation indicates that for this level of sodium coverage, sodium species may exist in the form of oxides or even superoxides form since a high oxygen/sodium mass ratio was recorded. This comes in agreement with the findings of Ibrahim et al [ 27 ], who studied through XPS the same case, i.e. the addition of sodium on Pt films supported on a YSZ pellet.…”

“…It was shown by in-situ (cyclic voltammetry, FTIR) and ex-situ techniques (SEM–EDX) that potassium oxides were formed and stabilised on Pt, after electrochemically pumped potassium ions reacted with adsorbed oxygen. The presence of sodium carbonates and oxides on the catalyst surface has also been confirmed by XPS analysis conducted by Ibrahim et al [ 27 ] in a previous study.…”

“…The explanation of this behaviour may be two-fold: (i) the permanent promotional effect has been attributed to oxygen ions storage at the catalyst–electrolyte interface and (ii) sodium at high coverages obtains a metallic character and bounds strongly with oxygen. Regarding to the latter, a previous study of Ibrahim et al suggested that oxygen storage is enhanced by high sodium coverages on Pt/YSZ, after application of positive potential [ 15 , 27 ]. In addition, the SEM–EDX analysis showed that sodium at high coverages, accumulated, possibly in the form of sodium oxides, inside the catalyst film pores.…”

“…Surface characterisation techniques (i.e. SEM, XPS) conducted by Ibrahim et al [15,27] showed that sodium species existed in a form of carbonates, hydroxides or oxides at coverage lower than 1% on platinum which blocked part of the catalyst active sites. These sodium species were described to be unstable and decomposed upon polarisation, generating new sodium species that induced further enhancement after EPOC activation.…”

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

“…An experimental rig was constructed using stainless steel Swagelok compression fittings and Perfluoroalkoxy (PFA) tubing as described in [36]. The flow of gas to the electrochemical reactor was controlled by electronic mass flow controllers (MFCs) provided by Chell Hastings.…”