Catalysis

Oxidation of Carbon Monoxide 195
experiments with 18 02 that lattice oxide ions can react with carbon monoxide
on Au/TiO2. 50 > 51 ' 59 ' 69 ' 71 - 149
The specific role played by surface anion vacancies is reinforced by the
observation, frequently made, that a common cause of deactivation is unreactive
'spectator' carbonate ions 3 > 5 > 38 > 58 > 69 > 92 that block these sites, thus
forbidding them to oxygen molecules. The carbonates are readily decomposed
on heating in air, and activity partially if not wholly restored. Other
by-products such as bicarbonate 33 and formate ions 59 ' 92 have been also
detected. The stability of the carbonate at low temperature, when the titania
surface becomes saturated with them, 58 ' 150 has led to the suggestion
that reaction then occurs uniquely on the gold 3,5 ' 151 (see also Section 6.2.5).
The toxicity of chloride ion can also be understood by its occupation of
anion vacancies.
A further mechanism 4 attributes importance to cationic gold at the
interface between the metal and the support, and appearing also at the particle
edge (Figure 6.10). It is worth explaining how this idea arose. At the
time it was advanced, there was much confusion in the literature about
the importance on even the necessity of calcination in preparing active catalysts,
an uncertainty that still exists, and it was felt that this might be
understood if the gold entities were responsive to the reaction environment.
If they started as metal, they might become somewhat oxidised, or if they
started as oxide, they might become partly reduced. The most likely location
for any cations was thought to be at the interface, since with supported
metals of Groups 8-10 a layer of cations forming a 'chemical glue' was wellestablished
as being responsible for the stability of small particles. Such
species, shown as Au 3+ ions in that publication, 4 but they could just as
well have been Au + , would have to bear an anion, such as the hydroxyl ion
(Figure 6.10), and the reaction might then proceed as shown, by oxidation
of an hydroxycarbonyl ion by O^. This mechanism therefore resembles that
discussed in the last section, but no support involvement was invoked then
(Figure 6.7).
Other supports are more easily reducible than titania, and with them a
role for surface anion vacancies is even more plausible. Based on a TAP
study, an elaborate mechanism has been proposed for the reaction on
Au/Fe203 33 (Figure 6.11); this appears to employ lattice oxide ions adjacent
to the gold particle, and O^ ions occupying vacancies created by their
use, the product being formed by decomposition of -CO3H. Extensive use
was made of hydroxyl groups on both the gold and the support, as well as
water molecules on the latter.