The oxidation reaction was run until the inhibition
state was reached. After 3 h, the catalyst
was reactivated by adding a solution of KOH to
the reaction mixture until pH 9 was attained. The
pH was further kept constant in the usual way after
reactivation. The reaction restarted immediately
and the remaining glucose was further oxidized
(Fig. 3). This result is in agreement with the second
hypothesis. Since the selectivity towards Dgluconic
acid is very high at the beginning of the
reaction, we assume that this compound constitutes
the main poisoning species. Earlier we
reported on a similar poisoning effect on Pd/C as
catalyst during the oxidation of D-glucose in acidic
medium (reaction carried out without pH control)
[ 181. The ‘free’ carboxyl group of D-gluconic
acid, the only reaction product formed according
to HPLC, was considered to be responsible of the
poisoning of the active sites of the noble metal
catalyst. Also, electrocatalytic oxidation of D-gIu
case on a palladium electrode was found to suffer
from the poisoning by ‘free’ gluconic acid [ 181.
However in the electrocatalytic literature, poisoning
of platinum electrodes is generally attributed
to adsorbed CO,+ Most experimental evidence is
based on the oxidation of methanol [ 19,201. The
initial poisoning species in the case of the oxidation
of methanol were adsorbed CHOad which was
supposed to be transformed to adsorbed COad
[ 201. In the case of the oxidation of glucose on a
platinum electrode in acidic medium, COad was
detected as the major poisoning species [ 21-241.
However, the observation of COad by spectroscopic
methods is much easier than unambiguous
identification of other complicated species such as
D-gluconic acid. The adsorbed COad observed is
resulting from C-C bond breaking due to degradation
of D-gluconic acid to a lower aldonic acid.
Indeed, some arabinonic acid ( < 1%) was formed
(Table 1).
With the aim to check any poisoning of the
catalyst (5% Pt/C) by adsorbed COad, we have
carried out the following experiment. The catalyst
was reduced by hydrogen at room temperature in
the usual way ( see Experimental - Oxidation procedure)
and the reactor was flushed with nitrogen,then CO was conducted through the reactor for 30
min. Finally, the reactor was flushed with nitrogen
and the substrate (glucose) was added to the COtreated
catalyst. The system was equilibrated at
the pre-set temperature (50°C) and the gas phase
was replaced by oxygen to start the oxidation. The
reaction was performed without pH control
because under these conditions the poisoning of
the catalyst was observed.
不要翻译器翻译的，谢谢