The key reaction for the reduction of metal ions in the photo-
catalytic cycle is the thermal reaction [eqn. (2)].
recovery of silver is 100% after 50 min of photolysis, whereas
Pd2` is recovered at about 83% after ca. 150 min. Under
these conditions, copper and nickel do not precipitate out.
The reduction of Ag` [E0(Ag`@0) \ 0.799 V vs. NHE] and
Thus, photolysis of the PW
ceeds to the steady-state formation of a one-equivalent
O
3~/S/Mn` system pro-
1
2 40
reduced tungstate, PW
on the rates of eqn. (2) (see Fig. 2). When this reaction is very
O
4~, whose concentration depends
Pd2` [E0(Pd2`@0) \ 0.987 V vs. NHE] by PW
O
4~ is
1
2 40
12 40
thermodynamically permitted, whereas that of Ni2`
[E0(Ni2`@0) \ [0.250 V vs. NHE] is not.
fast (as in the case of Ag`, Pd2` or dioxygen, for that matter),
no blue PW
O
4~ is obtained. In contrast, in the presence
The case of Cu2` requires special attention. PW
O
4~ is
12 40
12 40
of Ni2`, which does not reoxidize PW
O
4~, the blue color
a one-electron donor that reacts rapidly with Cu2` to
1
2 40
of PW
O
4~ develops to a steady-state value that is the
produce Cu`, as mentioned earlier. The reaction of Cu`
1
2 40
same as that observed in the absence of metal ions.
with PW
O
4~, although thermodynamically favored
1
2 40
In the case of Cu2`, the reduced catalyst attains a steady-
state concentration of a lower value than in the absence of
metals or presence of Ni2`. Several thermal experiments
[E0(Cu`@0) \ 0.521 V vs. NHE] seems to be slow, requiring
higher concentrations of reduced catalyst to proceed at a
reasonable rate. Thus, in the photolysis system
between PW
O
4~ (3È5 ] 10~4 M) and Cu2` (5È
PW
O
3~/S/Cu2`, an induction period is observed for
1
2 40
12 40
2
PW
5 ] 10~4 M) at pH 1 (4 ml) indicate rapid reoxidation of
copper precipitation; this period is shortened when the con-
centration of S is increased to enhance the rate of formation of
O
4~ within a subsecond time frame. This reaction did
1
2 40
not lead to metallic copper formation, whereas ca. 50% of the
PW
O
4~ [eqn. (1)]. Thus, when the concentration of sub-
1
2 40
PW
O
4~ reacted with Cu2` ions. This is also observed in
strate (2,4-DCP, or propa-2-ol) was 1 mM, about 5% Cu0 was
recovered after 1 h of photolysis, whereas about 36% copper
was recovered under otherwise identical conditions (1 mM
Cu2`, j [ 320 nm, pH 1, T \ 18.3 ¡C) when the substrate
concentration was increased 10-fold.
1
2 40
the corresponding photocatalytic experiments (Fig. 2), where
the concentration of reduced blue PW
steady-state value that is ca. 50% that observed in the absence
O
4~ reaches a
1
2 40
of Cu2` ions. This suggests that an equilibrium
Similar results have been obtained with SiW
12 40
except that the overall process was slower. Despite this fact,
O 4~,
PW12O40
4~ ] Cu2` EF PW
is established, in accordance with the closeness of the one-
O
3~ ] Cu`
(3)
12 40
SiW
O
4~ has the advantage of being stable in the pH
1
2 40
electron reduction potentials of PW
O
3~@4~ and Cu2`@`
range ca. 0.5 to 5.5.
1
2 40
(
0.221 and 0.153 V vs. NHE, respectively).
It should be noted that no reduction of Pd2`, Cu2` and
Ni2` is obtained, upon 1 h photolysis, in the absence of either
POM or the organic substrate (other conditions as in Fig. 3).
In the case of copper, an aqueous solution of 2 mM Cu2` and
2 M propan-2-ol was used). In contrast, illumination of an
aqueous solution of Ag` and organic substrate precipitates
out silver with a rate that is about half of that obtained in the
presence of POM.
We now show that, using a combination of thermodynamic
and kinetic e†ects, selective precipitation of metal ions can be
obtained. Fig. 3 demonstrates the selectivity in the recovery of
the metal ions Ag`, Pd2`, Cu2` and Ni2` when aqueous
solutions are subject to photolysis (j [ 345 nm) in the pres-
ence of PW12O40
3~ and 2,4-DCP. It can be seen that
As far as the 18-molybdodiphosphate is concerned, the two-
electron reduced species P Mo
O
8~ is not reoxidized by
2
18 62
dioxygen,12 so that it was easy to study its reaction with Mn`
[
eqn. (2)] independently. It was shown that P Mo
O
8~,
2
18 62
formed photochemically or otherwise, precipitates Pd2` in a
composite reaction process that is at least two orders of mag-
nitude faster than with Ag`, whereas no reaction takes place
with Ni2` and Cu2`, as the reactions with the latter ions are
thermodynamically forbidden [E0(P Mo
V vs. NHE].
O
6~@8~) \ 0.664
2
18 62
Thus, by a suitable choice of POM and, to a lesser extent,
organic substrate, selective photocatalytic reduction of several
metal ions in aqueous solutions can be achieved. Work is in
progress to study the photocatalytic mode of reduction of
several other metal ions through a combination of various
POMs and organic substrates and determine the feasibility of
this process in: (i) recovering metals and (ii) possible decon-
tamination of organic pollutants and metal ions from aquatic
systems.
Fig. 2 Steady-state formation of
a one-equivalent reduced 12-
tungstophosphate, PW 4~, showing the rate of its reoxidation by
O
12
40
Mn` [eqn. (2)] upon photolysis of a deaerated aqueous solution of
PW
O 3~ (0.7 mM), 2,4-DCP (1 mM) and Mn` (1.2 mM)
1
2 40
[conditions: pH 1 (HClO ), j [ 345 nm, T \ 20 ¡C].
4
Experimental
Deaerated aqueous solutions of the POM (PW
O
3~ or
1
2 40
SiW
O
4~; 0.7 mM), substrate (2,4-DCP or propan-2-ol;
1
2 40
0.5, 1.0 and 10 mM) and 1.2 mM metal ions (as the CuSO ,
4
NiSO , PdCl and AgNO salts) were photolyzed with a 1000
4
2
3
W Xe arc lamp (light intensity reduced mechanically by ca.
40%) using cuto† Ðlters at 320 and 345 nm to avoid direct
photolysis of the substrates. The photolytic process was fol-
lowed by: (i) recording the decomposition of the organic sub-
strate (2,4-DCP) by HPLC, equipped with a C-18 analytical
column and a UV-Vis detector adjusted to 290 nm; the
mobile phase consisted of CH CNÈH O, 50 : 50 v/v, in iso-
cratic mode, at a Ñow rate of 1 ml min~1; (ii) the development
of the characteristic blue colors of reduced POM15 and (iii)
Fig. 3 Recovery of metals through photolysis of a solution contain-
ing PW
3
2
O
3~ (0.7 mM), 2,4-DCP (1 mM) and Mn` (1.2 mM)
12
40
[
conditions: pH 1 (HClO ), j [ 345 nm, T \ 20 ¡C].
4
362
New J. Chem., 2001, 25, 361È363