The combination of techniques including switching experiments, temperature programed reduction and in
situ neutron scattering-conductivity are used to investigate the sensing mechanism of 1% Pd/SnO2 toward
hydrogen-containing gas mixtures. In particular, the use of the in situ neutron scattering-conductivity for the
first time allows the simultaneous monitoring of electrical conductivity and inelastic neutron scattering spectra
of the sensor material. Direct evidence is obtained on a reversible migration of hydrogenic species from and
to the metal and the underlying tin oxide surface, i.e., reversible hydrogen spillover. As a result of this spillover,
a dramatic change in electrical conductivity of the Pd doped tin oxide material is observed. We confirm, in
accordance with the known mechanism, that the change of conductivity is based upon the creation or destruction
of negatively charged adsorbed oxygen species on the sensor surface. In addition, we report a new but important
sensing mechanism, the spillover hydrogen species behaving like a shallow donor to the semiconductor oxide
as the direct source of conductivity occurs concurrently with the known mechanism.