Research Output
The role of sodium surface species on oxygen charge transfer in the Pt/YSZ system
The role of sodium surface species on electrochemical promotion of catalysis in a Pt/YSZ system: The case of ethylene oxidation
1 - 2 of 2
Publication
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°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-coverage 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 to 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).
Publication
The role of sodium addition as foreign (impurity) species on the electrochemical promotion of ethylene oxidation in a Pt/YSZ system was investigated. It was found that the presence of sodium surface species on the catalyst surface can significantly affect its catalytic and electrocatalytic properties, but there is no clear evidence at this stage that such species are necessary for the observation of EPOC. Under negative polarisation, low coverage sodium was found to have a pronounced effect on the electrochemical promotion of ethylene oxidation as an electronic promoter. The reaction changed behaviour from electrophilic at low sodium coverage (0.11%) and low to intermediate oxygen partial pressure (pO2 ⩽ 3.0 kPa) to electrophobic at high sodium coverage (65%) and under high oxygen partial pressures (pO2 = 8.0 kPa). In between the two sets of conditions, the reaction showed volcano-type behaviour depending on the coverage of sodium and gas-phase oxygen partial pressure.