J . Phys. Chem. 1986,90,6025-6028
6025
Chemistry of Colloidal Manganese Oxides. 3. Formation in the Reaction of Hydroxyl Radicals with Mn2+ Ions S. Baral, C. Lume-Pereira, E. Janata, and A. Henglein* Hahn-Meitner-Institut fur Kernforschung Berlin, Bereich Strahlenchemie, D-1000 Berlin 39, Federal Republic of Germany (Received: December 9, 1985)
Aqueous solutions of Mn(C10&, which were saturated with N 2 0 , were y-irradiated and the colloidal manganese oxides formed investigated. In solutions of pH 3.5-9, colloidal MnO, was formed. At pH 11, the colloid consisted mainly of manganese(II1) oxide. A pure manganese(II1) oxide was produced in neutral solution in the presence of sodium hexametaphosphate. Pulse radiolysis experiments showed that the primary oxidation product of Mn(H20):' was [Mn(Hz0)50H]Z+, which dissociates protolytically with pK, = 5.0. Depending on pH, colloids of more or less Mn(II1) content were formed as the final products of pulse radiolysis. The results are explained in terms of condensation and disproportionation reactions of the primary oxidation products.
Introduction Aqueous solutions of Mn(C104), contain MnZ+ions predominantly as a hydrated species, [Mn(H20),$+, the pK for the formation of [Mn(HzO)50H]+ being 10.6 at 25 OC. The oxidation of this ion by the OH radical is known to generate a Mn(II1) species.' This as well as the oxidation of Mn(I1) by 0, radical^^-^ has been studied by various authors using the method of pulse radiolysis. However, only optical monitoring was applied, and the studies were essentially limited to the characterization of the immediate product of oxidation. In the present study, we have used simultaneous optical and conductometric monitoring techniques to obtain additional information regarding the nature of the immediate product of oxidation by OH and also have extended such study over a considerably longer time period to examine the evolution of further products. In addition, a steady-state irradiation study has also been carried out to provide complementary and substantiative information. The final products of oxidation of Mn(I1) often are colloids. This has already been recognized in one of the previous studies,, although the nature of the colloid and the mechanism of its formation were not investigated in detail. In the two preceding parts of this the absorption spectra and some chemical properties of the colloidal oxides of tetra- and trivalent manganese have been described. Knowing these properties, it was hoped that the colloids produced by oxidation of Mn(I1) could also be identified.
Experimental Section Crystalline Mn(C104),.6H,0 was obtained from Fluka and used as received. Fresh solutions of Mn(C104), were prepared just before every experiment. While adjusting the pH of these solutions, we took care to remove all dissolved oxygen by bubbling N 2 0or argon before adding any alkali to the solution to prevent oxidation of Mn2+ ions. Pulse radiolysis studies were carried out both under flowing and flow stopped conditions depending on the time scale of observation. For a time window of greater than 0.1 s, flow was stopped just before the electron pulse was applied. After the time of observation had lapsed the flow was resumed (1) Brown, D. M.; Dainton, F. S.; Walker, C. D.; Keen, J. P. In Pulse Rudiolysfs;Ebert,M.; Keene,J. P.; Swallow, A. J.; Baxendale, J. H., Eds.; Academic: New York, 1965; p 221. (2) Pick-Kaplan, M.; Rabani, J. J . Phys. Chem. 1976, 80, 1840. (3) Bielski, B. H. J.; Chan, P. C. J . Am. Chem. SOC.1978, 100, 1920. (4) Weinstein, J.; Bielski, B. H. J. J . Am. Chem. SOC.1979, 101, 58. (5) Cabclli, D. E.; Bielski, B. H. J. J . Phys. Chem. 1984,88, 3111. ( 6 ) Lume-Pereira, C.; Baral, S.; Henglein, A.f Janata, E. J . Phys. Chem.
once again to purge the cell of the reacted solution. Normally a flow rate of -20 mL/min was used. Detailed information about the instrumentation and data collection has been reported elsewhere.* The colloidal oxides of manganese(1V) and manganese(II1) oxidize Fez+ ions. This reaction was used to determine the oxidation state of manganese in the colloids. The reaction was carried out in 0.4 M H2S04and the concentration of Fe3+ions produced determined spectrophotometrically (e304nm = 2201 M-' cm-l). Irradiation of NzO-saturated solutions produces hydroxyl radicals with a yield of 5.4 radicals per 100 eV absorbed radiation energy. Hydrated electrons react with N 2 0 according to N 2 0 e, - H 2 0 N z + OH- OH. At the same time, H atoms are Iormed in the low yield of 0.5 atoms/100 eV.
+
+
-
+
Results Continuous ?-Irradiation Experiments. Figure 1 shows the absorption spectra after a 90 min-irradiation period of 1 X lo4 M Mn(C104), solutions at various pH values. The spectra at pH 5 , 7 , and 9 are similar to those obtained for the colloidal MnO, prepared by radiolytic reduction of permanganate,6 although the maximum appears at a somewhat longer wavelength and with a lower absorption coefficient. The spectra in Figure 1 are broader than those for the MnO, prepared from permanganate. The colloids produced at pH 5 , 7, and 9 oxidized two equivalents of Fez+ ions, which indicates that the colloids consisted of MnOz. The differences in shape and height of the absorption band indicate a larger particle size as compared to that of MnO, from permanganate. The colloid formed at pH 3.5 has a very broad absorption band with a maximum at 500 nm. This solution showed strong light scattering. During the irradiation the maximum shifted to longer wavelengths until it reached the position at 500 nm. The reaction with Fez+ ions led to the yield of Fe3+expected for MnO,. It is also concluded that the particles in this solution were much larger than those in solutions at pH 5, 7, and 9. The spectrum obtained at pH 11 is distinctly different from the spectra of the solutions of lower pH values. As in the case of the other solutions, it did not change upon irradiation periods longer than 90 min. It resembles the spectrum of colloidal manganese(II1) oxide.6 The low yield observed in the reaction of the colloid with Fez+ was also in agreement with a manganese oxidation state of three. A similar spectrum was obtained after addition of degassed 2 X loT3M HzOzsolution to the 1 X lo4 M Mn(C104), solution at pH 11 (without irradiation). Addition of acid to the manganese(II1) oxide sol to produce a pH of 3.0 resulted in the precipitation of Mn02. No oxygen evolution was
1985,89, 5772.
(I) Baral, S.;Lume-Pereira, C.; Janata, E.; Henglein, A. J . Phys. Chem. (8) Henglein, A.; Lilie, J. J . Am. Chem. Sor. 1981, 103, 1059.
1985, 89, 5119.
0022-3654/86/2090-6025$01.50/0
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 0.8 ,& .6
I
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-- ----
e*-* L
n 0 0 .2 n m
Baral et ai.
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0
200
400
300
500
600
700
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Figure 1. Final spectra obtained after y-irradiation of 1 X lo-" M
Mn(C104)2solutions saturated with N 2 0and at various initial pH values. (dose rate: 7.2 X lo4 rad/h; irradiation time: 90 min).
t lminl Figure 4. Increase in conductivity of a 1 X lo-" M Mn(C10& solution at pH 9 as a function of time (dose rate: 1.9 X lo5 rad/h) pH of the solution was 3.8.
100% W
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Figure 2. Increase in absorption (expressed in percent of the final absorption) as a function of time and at various pH values.
Figure 5. Changes in specific absorbance at 260 nm and molar conductivity as functions of time after a pulse of radiation. Solution: 1 X M Mn(C104)2,saturated with N20, pH 9.
obtained at pH 11 in the absence of hexametaphosphate (Figure 1). The product was identified as colloidal manganese(II1) oxide by using the Fez+ ogidation method. The conductivity of the irradiated M I I ( C ~ Osolution ~ ) ~ changed after an induction period. Figure 4 shows a typical example for pH 9. Similar changes were observed at pH 7 and pH 3.5. The overall process [ M I I ( H ~ O ) ~ + ] ~2' 0 H M n 0 2 + 2H+ + 6 H 2 0 explains these observations. However, no changes in conductivity took place in the solution at pH 11. In this alkaline solution, Mr?+ is present as Mn(OH)z. The solution was still transparent, which indicates that there was no significant precipitation of Mn(OH)2. OH MXI(OH)~ The oxidation reaction Mn(OH)z 0.5Mnz03 1.5H20 is not accompanied by consumption or formation of conducting species. Pulse Radiolysis Experiments. Pulse radiolysis of a 1 X 10-3 M Mn(C10& solution at pH 9 yielded an increase in optical absorbance at 250 nm and a concomitant decrease in conductivity as can be seen from Figure 5. The second-order rate constant derived from these pseudo-first-order kinetic traces for various Mn2+ concentrations was (2.6 f 0.1) X lo7 M-I s-l. This rate constant is attributed to the reaction of O H with the Mn2+ species present at pH 9. The spectrum of the product as scanned from 340 to 250 nm was similar to that reported by Bielski and C h a ~ ~ The observed final conductivity change of 200 Q-]cm2 suggests that 1 equiv of base was consumed from the alkaline solution. This agrees with the following reaction:
-
"200
300
LOO
500
600
X Inml Figure 3. Absorption spectrum at various periods of y-irradiation (dose rate: 9.1 X IO5 rad/h). Solution: 1 X lo4 M Mn(C104)2;2 X lo4 M Na,(PO&; N20, 2.5 X lo-' M; pH 7.
observed. This shows that a disproportionation of the manganese(II1) took place. In fact, one calculates from the known thermodynamic properties of the molecular species involved that the reaction MnzO3 + 2H+ Mn2+ M n 0 2 H20has a greater driving force of AG = 30 kJ/mol at pH 3 than the reaction Mn203 4H+ 1/202 + 2Mn2+ + 2 H 2 0 . The development of the absorption maximum with time of irradiation is shown by Figure 2. The absorption in percent of the final absorption is plotted here as a function of the time of irradiation. At pH 11, a linear relationship is observed. At the lower pH values, the curves are bent toward the ordinate axis which indicates an increasing efficiency for the formation of the colloid with increasing irradiation time. The radiation chemical yield, G(-Mn2+), as calculated from the slope of the line for pH 11, is 1.3 ions/100 eV. Slightly higher yields are calculated from the slopes of the other curves at conversions above 60%. The maximum yield which would be observed if each OH radical produced participated in the oxidation of Mn2+ is 4.9 in the case of manganese(II1) oxide formation, and 2.4 in the case of manganese(1V) oxide formation. (4.9 is the yield of OH radicals of 5.4/100 eV minus the yield of H atoms of 0.5/100 eV, assuming that the H atoms offset the action of an equal amount of radicals.) An experiment was also carried out in which 2 X 1 p M sodium M Mn(ClO,), hexametaphosphate was added to the 1 X solution before irradiation. The pH of the solution was 7 and the dose rate 1.9 X lo5 rad/h. Fifty percent of the Mn2+ ions were adsorbed at the poiyphosphate anions, as could be shown by a polarography. The absorption spectrum at various times of irradiation is shown in Figure 3. It is quite similar to the one
+
-
-
+
+
+
+
[MII(H,O)~]~+ + OH
+ OH-
-
-
-
[Mn(H20)4(OH)2I' + 2H20 (1) Pulse radiolysis of an acidic solution at pH 3 showed practically no conductivity change although the optical signal and its buildup kinetics remained the same as shown in Figure 5. This suggests that a monohydroxo species is formed through the reaction [Mn(H2O)J2+
+ OH
-
+
[ M I I ( H ~ O ) ~ O H ] ~H + 2 0 (2)
The products from reactions 1 and 2 are the conjugate acid-base pair, the protolytic equilibrium
+
[Mn(H20)50H]2+ H 2 0 F?: [Mn(H20)4(OH)2]++ H30+ (3)
being attained rapidly. By measuring the magnitude of the conductivity signal and hence the amount of H30+released over a pH range we can calculate the equilibrium concentrations of
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The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 6027
Chemistry of Colloidal Manganese Oxides
-2 8 0 60 -
h
L
I
5 40E.
200
20 -
d
0
5
6
7 PH
8
9
Figure 8. First half-life of the buildup of absorption in the second range M Mn(ClO,&, N20saturated. Meaas a function of pH. 1 X surement at 300 nm. 2.5 X 10" M radicals produced per pulse.
3
2000
--
5 1200 E 800 1600
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-
0 0.5
I
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Figure 7. Absorption changes of a pulsed Mn(C10& solution at a more compressed time scale; pH 6.5. Inset: Spectrum of the final product (after 2.5 s ) .
the conjugate acid-base pair. Figure 6 shows the results of such a study. The pK, of [Mn(Hz0)50H]z+ was obtained as 5.0 f 0.1, To our knowledge this is the first determination of the second pKa of hydrated Mn3+ ions. The first pKa has been reported to be close to When the optical changes were observed at a much more compressed time scale than in Figure 5 , the formation of a strongly absorbing product was recognized as can be seen from Figure I. An examination of the kinetics of this development at a wavelength where the initially formed Mn3+ species has very little absorption (340-400 nm) shows the presence of a pronounced induction period for about 0.1 s after the pulse. The initial oxidation of Mn2+ by OH is complete within about 100 ps. Hence the induction period is indicative of the fact that the [Mn(H20),(OH)2]+species undergoes transformation to an intermediate x which is the precursor of the strongly absorbing final product. The formation of x is not accompanied with a significant optical (250-500 nm) or conductivity change a t p H 6.5. The spectrum of the final product is shown by the inset of Figure 7. By comparing it to the spectra of colloidal M n 0 2 and Mn203 one arrives at the conclusion that the final product is neither pure manganese(II1) nor pure manganese(1V) oxide but possibly a mixture of the two. The buildup in absorption in Figure 5 can be closely fitted to a second-order rate law after the initial induction period. This seems to be indicative of an agglomeration process. The spectrum of the final product measured 3-4 s after the pulse remained unchanged as the p H was decreased down to 4. However, the agglomeration process is considerably slowed down as shown by Figure 8. The induction period also became more pronounced a t higher acidities. When the long time experiments were carried out in alkaline solutions above p H 8, the end product has a different spectrum as shown by Figure 9. The considerably narrower absorption band resembles the band observed for colloidal manganese(II1) oxide.6 The conductivity, which in the beginning has dropped by 200 0-l cm2, further decreased to -300 R-' cmz during the slow (9)
Davies, G. Coord. Chem. Rev. 1969, 4,
199.
0 250
300 350 X [nm 1
400
Figure 9. Absorption spectrum of the final product (after 0.3 s) at pH 8.8.
buildup of the colloid. Both the optical and conductivity half-lives decreased proportionally to the concentration of initially formed Mn3+ species.
Discussion The first products of the reaction of OH with Mn2+ are [Mn(Hz0)50H]2+or [Mn(H20),(OH),]+ depending on the pH of the solution (pKa = 5.0). The final colloidal products are formed from the primary products either via condensation to form manganese(II1) oxide or via disproportionation and condensation (or condensation plus disproportionation) to form manganese(1V) oxide. This general concept explains why the nature of the final products depends on the p H of the solution. However, the nature of the final product is also influenced by the rate of reaction, Le., by the dose rate. For example, compare the spectra in Figure 1 (for pH 5 or 7) obtained in the y-irradiation experiments with the spectrum of Figure 7 (for p H 6.5) obtained under pulse radiolysis conditions. In the y-irradiation, radicals are generated at a low stationary concentration, while they are produced in a pulse of high-energy electrons at a concentration larger by several orders of magnitude. At low radiation intensities, the product is more like MnO,, while it has a higher Mn(II1) content when it is formed at high intensity. Finally, in the presence of hexametaphosphate, the disproportionation of Mn(II1) does not take place as manganese(II1) oxide is formed even in neutral solution. The increase in the rate of formation of MnOz colloid occurs in a pH range close to the pK, of the equilibrium of eq 3 (Figure 8). This indicates that the acid form of the equilibrium disproportionates less rapidly or not at all, compared to the base form. The disproportionation of the base form could be written in two ways 2[Mn(H20),(OH),]+ [Mn(H20)a]2++ MnOz + 4 H 2 0
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(4)
J . Phys. Chem. 1986, 90, 6028-6034
6028
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followed by agglomeration of the M n 0 2 particles formed n M n 0 2 (Mn02)" (5) or, via
-
2[Mn(HzO)4(OH)21' [Mn(H20)4(OH)OMn(H20)4(OH)]2+ + H 2 0 (6) followed by [Mn( H 2 0 ) 4 ( 0 H ) O M n (H 2 0 ) 4 (OH)] 2+
-
[ M I I ( H ~ O ) ~ ]+~ M + n 0 2 + 3 H 2 0 (7)
and reaction 5 . Equations 4 and 5 describe a process where the disproportionation occurs between two primary product molecules and is then followed by the agglomeration of MnOz formed. On the other hand, as described by eq 6 and 7, disproportionation may take place after condensation. It is even conceivable that condensation of more than two Mn(II1) species takes place before disproportionation. The induction period observed in the pulse experiments (Figure 7) may be caused by condensation to a certain size until disproportionation takes place. On the other hand, the mechanism of eq 4 and 5 also could explain the induction period, assuming that M n 0 2 particles of smaller size do not absorb. These reasons may also be given to explain the nonlinear behavior of the curves in Figure 2. Manganese(II1) oxide would according to these mechanisms be formed if condensation occurs much more rapidly than disproportionation. At a high degree of condensation, i.e., when colloidal manganese(II1) oxide is formed, there is no dispropor-
tionation. (On the contrary, in MnOl colloid containing Mn(I1) ions, even conproportionation takes place.6) According to the mechanism of eq 6 and 7, condensation would be a process of an order higher than one, while disproportionation would be a first-order-like reaction as the rate would probably not depend on particle size. High intensity of radiation should lead to a higher ratio of rates of condensation to disproportionation, Le., to a colloid of higher Mn(II1) content. This may explain why the colloid formed in neutral solution by pulse radiolysis (Figure 7, inset) contains Mn(III), while that produced by y-irradiation (Figure 1) consists of pure Mn02. Furthermore, the presence of OH- ions (pH 8.8 in Figure 9) seems to promote efficiently the condensation process as practically pure manganese(II1) oxide is produced in pulse radiolysis. This effect may be explained by OH- addition to the [Mn(H20)4(OH)OMn(H20)4(OH)]2+species (eq 6) or/and of higher condensation products. This also explains the decrease in conductivity during the formation of manganese(II1) oxide. An even better promotion of condensation or inhibition of disproportionation is caused by hexametaphosphate as manganese(II1) oxide is formed also in neutral solution (Figure 3).
Concluding Remarks In this work, the nature of the first product of oxidation of manganese(I1) by hydroxyl radicals was investigated in more detail than in the previous studies. The nature of the final colloidal products was also recognized. It was not possible to formulate in detail the mechanism for colloid formation, although we think that our more general outline has led to some understanding of the processes involved.
Photoassisted Platinum Deposition on TIO, Powder Using Various Platinum Complexes Jean-Marie Herrmann,* Jean Disdier, and Pierre Pichat Ecole Centrale de Lyon, Equipe CNRS Photocatalyse,t BP 163, 691 31 Ecully Cedex, France (Received: December 26, 1985)
The photocatalytic deposition of metallic platinum has been carried out with powder titania in aqueous suspension containing different complex solutions (chloroplatinic acid, sodium chloroplatinate, hexahydroxyplatinic acid, and platinum-dinitrodiammine). The deposition rate was found identical for the three first complexes and much more lower for the last one which is not ionic. In all cases the removal of the Pt ions from the solutions was achieved to the detection limit (1 ppm). Analyses of the solid, liquid, and gas phases indicated that the reduction of a (Pt"') complex ion induced, as expected, the release of n protons in the solution, whereas oxygen evolution from water oxidation was completely blurred by an initial photodesorption of preexisting ionosorbed oxygen species, followed by the dissociative chemisorption of O2on the Pt crystallites formed in the process. A mechanism is proposed from the effects of various parameters (initial concentration, light flux, temperature). An initial quantum yield of about 0.05 was calculated. Transmission electron microscopy showed that platinum deposits initially as small crystallitesof 1 nm diameter distributed on all the particles of titania. For higher loadings and longer illumination times, large agglomerates form; as a result, most of semiconductor surface remains accessibleto the photons so that the activity for the deposition does not decline. Pd, Ag, Rh, Au, and Ir have also been deposited on TiO, whereas Ni and Cu were not. Preliminary experiments indicated that ZnO, Nb205,and Tho2, for instance, can be used instead of Ti02.
Introduction
The photocatalytic deposition of noble metals on photosensitive semiconductors under the shape of single crystals, films, electrodes, or powders is a well-known phenomenon. As examples of metals deposited on metal oxide semiconductors, one can cite the following couples: Pd/Ti02,'-5 Pt/Ti02,3p6-9Ag/Zn0'&I4 or Ti02,12J5-18 Cu/Ti02 or wO3,l9Hg/ZnO" or Ti02.20 As our study was in progress,21a paper reported the deposition of gold on TiO, and W03.22 This method has been employed as a means of preparing metal catalysts3s5or photocatalyst~~~'' and suggested as a potential way of metal recovery from aqueous effluents. Despite these studies, certain points of the reaction mechanism deserve to be elucidated. J.E.CNRS 4594. 0022-3654/86/2090-6028$01 S O / O
Since ZnO samples are generally unstable in illuminated aqueous solutions and since W 0 3 specimens are generally much (1) Mollers, F.; Tolle, H. J.; Memming, R.J. Electrochem. SOC.1974, 121, 1160 and references therein. (2) Kelly, J. J.; Vondeling, J. K. J. Electrochem. SOC.1975, 122, 1103. (3) Dunn, W. W.; Bard, A. J. N o w . J . Chim. 1981, 5 , 651. (4) Yoneyama, H.; Nishimura, N.; Tamura, H. J. Phys. Chem. 1981,85, 268. (5) Stadler, K. H.; Boehm, H. P. Proceedings of the 8th International Congress on Catalysis, Berlin; Verlag Chemie: Weinheim, FRG, 1984; Vol. IV, p 803. (6) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 4317. (7) Koudelka, M.; Sanchez, J.; Augustynski, J. J . Phys. Chem. 1982,86, 4277. (8) Curran, J.; Domenech, J.; Jaffrezic-Renault, N.; Philippe, R. J . Phys. Chem. 1985, 89, 957. (9) Sato, S. J . Catal. 1985, 92, 11.
0 1986 American Chemical Society
