Platinum Metals Rev., 1988, 32, (4), 199
Lean-Burn Oxygen Sensor Material
Platinum Catalyst Improves Response Time
Oxygen sensors are widely used as automobile engine control devices in order to obtain an optimised balance of exhaust emissions, fuel economy and vehicle drivability, and generally this is achieved by controlling the air to fuel ratio at the stoichiometric mix of 14.7:1. Now there is increasing interest in controlling the air to fuel ratio away from the stoichiometric point, in the lean-burn region, with the aim of increasing engine efficiency and decreasing nitrogen oxides emissions.
Lean-burn oxygen sensors are generally classified as either semiconducting or electrochemical pumping. The former are small, simple, low cost devices which are based upon the resistivity changes that take place in oxide semiconductors as the partial pressure of oxygen in the surrounding atmosphere varies.
A recent paper by C. Yu, Y. Shimizu and H. Arai of Kyushu University, Fukuoka, Japan, reports on their investigation of several species of magnesium-doped SrTiO3 in the exhaust gas resulting from air-propane combustion containing water vapour (“Mg-Doped SrTiO3 as a Lean-Burn Oxygen Sensor”, Sens. Actuators, 1988, 14, (4), 309–318). At temperatures between 600 and 800°C, the highest sensitivity to oxygen in the lean-burn region and the lowest sensitivity in the rich-burn region was shown by SrTi0.6Mg0.4O3−δ , and therefore it was considered to be a suitable material for a lean-burn oxygen sensor. However the response time was about 1.5 seconds, which is too long for use in an automobile engine system. When 1 weight per cent platinum was added as a catalyst the response time was reduced significantly and in addition the sensitivity was increased in the lean-burn region.