The gas-sensing properties of sintered bismuth iron molybdate catalyst

Sintered pellets of bismuth iron molybdate catalysts act as sensors of reducing vapors by bulk conductivity changes mediated by oxygen vacancy donors. These donors, originating from the direct reduction of the semi-conductor catalyst, are highly mobile and can quickly diffuse through the sample, thereby having a direct effect on the electronic carrier density throughout. In air the vacancies are reoxidized into lattice oxygen. The sensitivity and selectivity of pellets embedded with gold wire contacts are examined for a wide number of gases and vapors and the effect of high-temperature sintering (calcining) as well as the effects of platinum doping is examined. Mechanisms are proposed to explain the power law dependencies of the sensor conductivity versus concentration and the response time versus concentration. Direct measurements are made on the diffusion of oxygen vacancies under voltage and thermal-induced drift. Bismuth iron molybdate is shown to be almost completely immune to the effects of water vapor. The pellets show great long-term stability of sensor response under most operating conditions, as well as great sensitivity to very small amounts of organic vapor (less than 1 ppm).