Various techniques for selectivity in semiconductor gas sensors, emphasizing powder-based sensors, are reviewed. The use of catalysts and promoters is emphasized, although other techniques such as the use of ‘filters’ are described.

Because our understanding of how catalysts and promoters provided selectivity in gas sensors is so poor, a superficial review of the models of catalysis is presented to act as a basis for the discussion of catalysts in sensing. We review the reasons why spillover or Fermi energy control must occur for catalysis to be effective on sensors.

We emphasize the different objectives for catalyst behavior in normal heterogeneous catalysis from those in gas sensing. It is concluded from these arguments that a high catalyst dispersion is required in gas sensors, that partial oxidation catalyst may be counter-productive for catalysts designed for gas sensing.