1328 J. Phys. Chem. A, Vol. 101, No. 7, 1997
Jacobson et al.
equilibria (1,-1) and (6,-6) together with the cross reaction
14.
MnO2+ + HO2 + H+ f Mn2+ + H2O2 + O2 (14)
At even lower pH’s (pH < 2) the decay kinetics turned into
first-order as the excess of HO2 radicals over MnO2+ increases
with decreasing pH due to equilibrium (6,-6). Computer
+
modeling of the MnO2 decay yields the rate constant k14
)
(1.0 ( 0.3) × 107 M-1 s-1
.
+
In all cases studied, the end product of the MnO2 decay
was H2O2 and Mn2+. Neither with the aquo-ion nor with the
sulfate complex could indications of the formation of binuclear
species be observed. No formation of Mn(III) species could
+
be detected during decay of MnO2 neither in perchlorate nor
in sulfate medium. This is reasonable having in mind that the
+
MnO2 is formed in reaction 8 with both the aquo-ion and
sulfate complex and the difference of ca. 1.8 V in standard
-
reduction potential between Mn3+/Mn2+ and O2/O2 pairs. It
-
seems that formation of Mn(III) in Mn(II)/O2 requires the
presence of ligands that can radically decrease the reduction
potential of the Mn(III)/Mn(II) pair.
Figure 9. Spectrum of Mn+. Conditions: [Mn2+] ) 1.0 × 10-3 M,
air-free solution of 0.1 M HClO4 with 140 atm H2 at 22 °C.
Conclusion
In an air-saturated 0.1 M Mn2+ solution at pH 3 equilibrated
with 140 atm H2 about two-thirds (G ≈ 4.2) of the initial radicals
form Mn+, the rest being divided between Mn(III) (G ≈ 1.0)
and HO2 (G ≈ 1.3). A first-order decay observed at 350 nm
with an apparent rate constant proportional to the oxygen
concentration is attributed to reaction 12:
The MnO2+ complex forms in the reaction of Mn(I), Mn(II),
and Mn(III) with O2, O2-/HO2, and H2O2, respectively. Interac-
tions of O2-/HO2 radicals with Mn2+ in the absence of
complexing ligands or as the sulfate complex can be adequately
described by the equilibria (1,-1), (5,-5), and (6,-6).
Acknowledgment. The authors wish to thank Torben
Johansen for skillful operation of the linear accelerator. Fi-
nancial support from the Commission of the European Com-
munities within the Environment research program (Contract
RINOXA EV5V-ct93-0317) is gratefully acknowledged.
+
Mn+ + O2 f MnO2
(12)
Varying O2 concentration within the range (0.25-1.2) × 10-3
M, a rate constant k12 ) (6.0 ( 1.0) × 106 M-1 s-1 was
+
determined. Formation of the MnO2 species is confirmed at
References and Notes
higher pH by a concomitant buildup of the absorption spectrum
with λmax ) 270 nm, while at pH < 1 only the HO2 spectrum
(a result of equilibrium (6,-6) with a small amount of the Mn-
(III) absorption (formed in reaction 7) is observed.
(1) Cabelli, D. E.; Bielski B. H. J. J. Phys. Chem. 1984, 88, 3111.
(2) Cabelli, D. E.; Bielski B. H. J. J. Phys. Chem. 1984, 88, 6291.
(3) Jacobsen F.; Holcman J.; Sehested K. To be published.
(4) Sehested, K.; Holcman J. J. Phys. Chem. 1975, 82, 651.
(5) Christensen, H.; Sehested, K. Radiat. Phys. Chem. 1980, 16, 183.
(6) Sehested, K.; Rasmussen O. L.; Fricke H. J. Phys. Chem. 1968,
72, 626.
(7) Bjergbakke, E.; Rasmussen, O. L.; Sehested, K.; Christensen, H.
RISØ-M-2430, Risø National Laboratory, Roskilde, Denmark, 1984.
(8) Rasmussen, O. L.; Bjergbakke, E. RISØ-R-395, Risø National
Laboratory, Roskilde, Denmark 1984.
Decay of the MnO2+ Complex. In solutions close to neutral
pH with 140 atm H2 and 3 atm O2 and high concentrations of
Mn2+ and at fairly high doses (5-8 krad/pulse) MnO2+ decayed
in second-order kinetics according to eq 13. A second-order
rate constant k13 ) (6.0 ( 1.0) × 106 M-1 s-1 was determined.
2MnO2+ f 2Mn2+ + H2O2
(13)
(9) Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. J. Phys.
Chem. Ref. Data 1985, 14, 1044.
(10) Pick-Kaplan, M.; Rabani, J. J. Phys. Chem. 1976, 80, 1840.
(11) Baral, S.; Lume-Pereira, C.; Janata, E.; Henglein, A. J. Phys. Chem.
1986, 90, 6025.
(12) Davies, G.; Kirschenbaum, L. J.; Kustin, K. Inorg. Chem. 1968, 7,
146.
(13) Davies, G. Coord. Chem. ReV. 1969, 4, 199.
(14) Bidermann, G.; Palombari, R. Acta Chem. Scand. A 1978, 32, 381.
(15) Rabani, J.; Mulac, W. A.; Matheson, M. S. J. Phys. Chem. 1977,
81, 104.
(16) Dainton, F. S.; Phillipson, N. A.; Pilling, M. J. J. Chem. Soc.,
Faraday Trans. 1 1975, 71, 2377.
Determination of H2O2 formed by single-pulse radiolysis
showed stoichiometrical amounts of H2O2, according to eq 13.
On the basis of this result and experiments with H2O2 added,
we can estimate the upper limit for the reaction MnO2+ + H2O2
to k e 103 M-1 s-1
.
On lowering the pH the decay became faster but still
confirmed pretty well second-order kinetics. However, the
observed decay is now governed by a cooperative action of the