Carbon monoxide interaction does not produce any effect in the absence of
moisture. Under moist air, a number of vacancies were produced following the
reactions
H 2 O þ Sn Sn þ O 0 þ CO $ HCO-O 0 þ HO-Sn sn
Sn Sn þ HCO-O 0 $ Sn Sn À HCO 2 þ V 0
V 0 $ V 0
þ
þ e
À
Figure 5.20 describes the ESR spectrum of the oxygen vacancies V 0
þ and of the
superoxo derivative Sn
4+ -O 2
− .
The resonance lines at g = 1.89 are attributable to the oxygen vacancies, under
reducing CO/argon atmosphere (Fig. 5.20), while the spectrum of Sn
4+ -O 2
−
g 1 = 2.024, g 2 = 2.009, g 3 = 2.004 becomes evident when O 2 was introduced. This
assesses the possibility of understanding the structural variation at the SnO 2 surface
that parallels the variations in conductivity. If toxic gases are mixed to air, being
these mainly reducing gases, the conductivity variations are indicative of their
Fig. 5.20 ESR spectra of the
paramagnetic species under
a (stronger line) reducing and
b oxidizing atmosphere [17]
5.29 Discussion of the Case
113