Tin oxide-based methane gas sensor promoted by alumina-supported Pd catalyst

In an attempt to promote the sensitivity of tin oxide-based sensors to methane gas, the parent tin oxide powder, pure or loaded with Ca and/or Pt (0.1 wt.%), was mixed with a fixed amount (5 wt.%) of alumina-supported Pd catalyst (net Pd loading 0.25 wt.%). The resulting sensor was found to exhibit excellent sensing properties to methane in the concentration range of 500-10 000 ppm at 658 K regardless of the difference in starting tin oxide powder. It gave higher sensitivity to methane than any other sensors for which the tin oxide powder was either mixed similarly with supported Pt, Rh or Ni catalyst or loaded with the same amount of Pd by conventional methods. The high dispersion of Pd (or PdO) particles appears to be responsible for the excellent promoting action of the supported Pd catalyst. At lower temperature of 573 K, however, the use of the Ca and/or Pt loaded powder of tin oxide gave higher sensitivity to methane than that of the unloaded powder. It is suggested that the mechanism of methane sensing consists of two steps, i.e. activation of methane molecules on the supported Pd catalyst and surface reaction of the activated species on the tin oxide particles. The first step is rate determining at 658 K, while the second step becomes also important kinetically at 573 K, allowing the promoting action of Pt to take place. (C) 1998 Elsevier Science S.A. All rights reserved.