J . Phys. Chem. 1990, 94, 8435-8439
8435
Radiation- Induced Dissolution of Colloidal PbO, Paul Mulvaney, Franz Grieser,* Department of Physical Chemistry, University of Melbourne, Parkville. Victoria 3052, Australia
and Dan Meisel* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: November 29, 1989; In Final Form: May 10, 1990)
The reduction of Ludox-stabilized colloidal Pb02 using radiolytically generated viologen radicals (ZV-) has been studied by using both steady-state y-radiolysis and pulse radiolysis techniques. The steady-state radiolysis results indicate that reduction of Pb02 is consistent with the overall reaction Pb02 + 4H++ 2ZV- Pb2+ 2ZV 2H20. Temporal examination of the reduction process, monitored conductometrically and spectroscopically, reveals that two main steps are involved in the overall reduction process. The first step occurs rapidly and involves a one-electron reduction of a PbO, colloid with virtually the simultaneous adsorption of a proton. Once the colloid possesses one electron, the second electron is more difficult to transfer and a slower electron-transfer step occurs consuming, again simultaneously, three protons to produce Pb2+.
-
+
+
Ultraflow apparatus using 4000-6000 molecular weight cutoff membranes. Matheson research grade N 2 0 was used to deoxygenate solutions prior to radiolysis, using the syringe technique! The colloidal solutions generally contained 2 X lo4 M 4,4-bis(sulfonatopropy1)bipyridinium-viologen (ZV) and 0.1 M propan-2-01 as a scavenger for O H radicals and H atoms produced radiolytically. The propyl-2-01 radical so formed then produced viologen radicals (ZV-) through reactions 1-3.'
Introduction Lead dioxide is a strongly conducting, powerful oxidant, which undergoes two electron reduction processes to the divalent metal ion in aqueous solution under reducing conditions. The reduction of lead oxides is of immense technological importance because it is the cathodic process in lead acid batteries.' Despite extensive investigation, few mechanistic details are available on the primary steps in the dissolution.2 Much of the electrochemical literature has concentrated on the morphology of the redox processes in batteries.) Little work has been done on the reduction of lead oxides in particulate form. In recent pulse radiolysis and flash photolysis experiments, quantum size effects were observed upon electron transfer to the particles. The cathodic dissolution of these radiolytically synthesized PbOz particles resulted from accumulation of a large number of electrons on the particle^.^ I n this report, we examine the reduction of colloidal P b 0 2by viologen radicals. Of particular interest is the observed relative rates of the electron- and proton-transfer processes and the ratio of proton-to-electron consumption during the reduction, which directly reflects the true stoichiometry of the dissolution process and also indicates whether any polarization of the oxide is possible under reducing conditions. As will be shown, no polarization occurs and the rate of dissolution is completely controlled by electron-transfer rates rather than protonation, disproportionation, or desorption.
eaq- + N 2 0 + H+ O H (H)
+ (CH3)ZCHOH
(CH3)ZCOH
+ ZV
-+
4
-
+ N2
+ H2O (H2) ZV- + (CH,)2CO + H+
(CH3)ZCOH
(1) (2) (3)
y-radiolysis was conducted using a 6oCo source at a dose rate of 40-100 krad/h. Time-resolved studies were conducted using the previously described Argonne pulse radiolysis system.' Analysis of Pb(I1) was performed polarographically using a Princeton Applied Research PA 174 polarographic analyzer. For analysis of Pb(IV), the sol was first reduced to Pb(I1) with concentrated hydrogen peroxide in acidic solution and then analyzed polarographically.
Results and Discussion ( A ) Absorption Spectrum of Colloidal Pb02/Si02. In Figure la, the spectrum of a typical P b 0 2 sol is shown. The absorption coefficient, a (cm-I), was calculated from c, the extinction coefficient, via eq 4. The values obtained are in good agreement
Experimental Section Typically, the lead dioxide sol was prepared at about 0.1 mM concentration as follows: Lead(1V) tetraacetate was dissolved in glacial acetic acid (0.030 g mL-I). The acidic lead solution should be colorless. A yellow tinge indicates partial hydrolysis of the lead(1V). Four milliliters of this solution was immediately added dropwise to 1 L of vigorously stirred 0.2%v/v Ludox HS-40. The brown sol of hydrous PbO, formed readily. The sol was clear to transmitted light but showed slight turbidity to reflected light, in agreement with the original observations of Bellucci and ParravanoUs It was then dialyzed to pH 4.5 with an Amicon
cy
= 2.303 X 103ep/Mw
(4)
with the data of M i ~ ~ d t The . ~ ~slightly higher value for the colloidal P b 0 2 could reflect the use of the bulk density of 9.38 g cm-3 in eq 4, which may be substantially higher than the density of the hydrous precipitate formed by hydrolysis. Indeed, Stramel and Thomasiofound that the density of iron oxide sols was almost half that of the bulk oxide. vs hv were found to be linear over the range Plots of 2-4 eV, as is evident from Figure lb, suggesting an indirect
( I ) Bode, H. b o d Acid Batteries (Engl. Transl.); Wiley-Interscience: New York. ....., 1971. . (2) (a) Carr, P. J.; Hampson, N. A. Chem. Rev. 1972, 72, 679. (b) Hampson,N. A.; Jones, P.C.; Phillips, R. F. Con.J . Chem. 1967, 45, 2039. (c) Fleischmann, M.; Liler, M.Trons. Forodoy Soc. 1958,54, 1370. (d) Bard, A. J., Ed. Encyclopedio of Electrochemistry . of . the Elements; Marcel Dekker: New York, I973;Vol. 1; (3) (a) Pavlov, D.; Balkanov, 1.; Rachev, P. J. Electrochem. Soc. 1987, 134, 2390. (b) Ellis, S.R.: Hamwon. N. A,: Ball. M. C.; Wilkinson, F. J . A..d . Electrochem. 1986, 16, 159: (4) Kumar, A.; Henglein, A.; Weller, H. J . Phys. Chem. 1989, 93, 2262.
( 5 ) Bellucci, 1.; Parravano, N. 2.Anorg. Chem. 1906, 50, 107.
(6) Hart, E. J.; Anbar, M. The Hydroted Electron; Wiley-Interscience: New York, 1970; p 200. (7) Mulvaney, P.; Cooper, R.; Grieser, F.; Meisel, D. Longmuir 1988, 4, 1206. ( 8 ) Meisel, D.; Mulac, W. A,; Matheson, M. S. J . Phys. Chem. 1981.85, 179. (9) (a) Mindt, W. J . Electrochem. SOC.1969, 116, 1076. (b) Pohl, J. 0.; Schlechtriemen, G. L. J . Appl. Electrochem. 1984, 14, 521. ( 1 0) Stramel, R.; Thomas, J. K. J . Colloid Interfoce Sci. 1986, I I O , I2 1.
0022-3654/90/2094-8435~02.50/0 0 1990 American Chemical Societv , , I
OH
-
Mulvaney et al.
8436 The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 7
0l
I
4,
7
i
200
400 600 Wavelength (nm)
800
2
8
6
4
10
PH b
./ 200
n 1
2 3 Energy (eV)
4
Figure 1. (a) Absorption coefficient of colloidal Pb02/Si02sols derived from 9.9 X M sol at pH 4.5, using p = 9.38 g cme3and assuming Mw = 239.21 g mol-’.(b) Plot of the normalized absorption coefficient vs excitation energy for an indirect optical transition. The intercept yields an optical band edge of 1.7 eV.
transition,”*12and the intercept yielded 1.7 eV for the optical band edge, also in good agreement with the data from sputtered for /3-Pb02 in particular. The intrinsic bandgap for PbO, has been estimated to be ca. 0.3 eV below this value.9a Two explanations may be invoked to rationalize the higher bandgap measured for the lead oxide particles: a quantum size effect, similar to that observed recently for colloidal Pb02,4or a Burstein-Moss band filling effect (or variations of it) which results from a high density of electrons in the semiconductor particles. From the effective mass of electrons in PbO, (-O.8mo): a shift of the order of 0.3 eV can be approximately calculated for particles with a radius of -10 A (our estimate of the radius of the P b 0 2 formed; see later). However, since Pb02 is notorious for possessing a high level of impurities (commonly oxygen vacancies) and since large shifts in the bandgap have been observed even for bulk material,”*’38it is possible that the Burstein-Moss effect is present in the system examined here. No apparent increase in the size of the Ludox particles was found by electron microscopy after deposition of the PbO,, indicating only minimal volume change of the particles. On average there are about 100 lead atoms per silica particle, much less than a monolayer coverage. No electron diffraction pattern could be observed from these particles, suggesting the lead dioxide to be amorphous. Convergent beam elemental analysis of the sol particles consistently showed low Pb/Si ratios, which supports the idea that there is little clumping of the PbO2 during hydrolysis. In the absence of SiO, it was found that the hydrolysis of lead tetraacetate led to unstable sols, and the addition of either 0.1-1 .O% w/v poly(viny1 alcohol) or 0.2% w/v hexametaphosphate was unable to prevent coalescence of the particles in the sols in acidic solution. In contrast, silica-stabilized sols were stable for several weeks before discernible settling occurred. Although PbO, is thermodynamically unstable in acidic solution, water oxidation is Indeed, analysis of dissolved Pb(I1) showed that the rate of thermal dissolution of the sol at pH 3.0 and pH 4.5, due ( I I ) Gratzel, M.Heterogeneous Photochemical Eleclron Transfer: CRC Press: Boca Raton, FL, 1989; p 91. ( I 2) Dare-Edwards, M.; Goodenough, J. B.; Hammett, A,; Trevellick, P. J . Chem. Soc., Faraday Trans. I 1979, 79. 2021. ( I 3) (a) Lappe, F. J . Phys. Chem. Solids 1962, 23, 1563. (b) Raviendra, D. Phys. Reu. E 1986,33,2660. (c) Dohrman, J . K.;Sander, U. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 605.
3
4
5
6
7
8
9
PH
Figure 2. (a) G(Pb2+)as a function of the pH at which the solutions were irradiated. Solutions contained 1 X lo4 M PbO,, 2 X lo4 M ZV, 0.1 M propan-2-01, and 0.2% w/v Ludox HS-40. Sols were N,O saturated. Dose was 10 krad. (b) Adsorption of Pb(l1) onto 0.2% Ludox as a function of pH. Solutions initially contained 4.4 X M Pb(I1) (no PbOJ and were not deaerated. Equilibration time was 18 h. to water oxidation, was negligible. ( B ) Reduction of P b 0 2 / S i 0 2Sols Using ZV Radicals. The M P b 0 2 sols by viologen radicals at pH reduction of 1 .O X 3.0 was found to be linearly dependent on the dose up to 60% conversion of the sol into Pb(I1) ions. No viologen radicals remained after radiolysis. The slope of a plot of released Pb(I1) concentration versus dose yielded G(Pb2+) = 3.0 ions/100 eV. Since the yield of reducing radicals is G(ZV-) = 6.8 radicals/100 eV, the dissolution is almost stoichiometric according to eq 5. Pb02
+ 4H’ + 2ZV-
-
Pb2+
+ 2ZV + 2 H 2 0
(5)
The slight discrepancy of about 10% between the results obtained and the theoretical value expected from reaction 5 is attributable to Pb(I1) adsorbed onto the Pb02 portion of the colloid. Adsorption of Pb(I1) onto the silica stabilizer (which is in colloidal form) can be dismissed as it was found that no Pb2+ adsorbs on Ludox HS-40 (0.2% w/v) at pH 3.0 even after 24 h of equilibration (see below). In Figure 2a, the dissolution of P b 0 2 by viologen radicals is presented as a function of the pH at which the solution was irradiated. The yield of Pb(I1) following filtration of the irradiated solutions decreased rapidly above pH 4 and dropped to zero by about pH 8. Nevertheless, even a t high pH (>lo) no viologen radicals were left in the solution following the irradiation. When Pb(I1) is produced at higher pH’s, the adsorption onto the silica support becomes more likely. To demonstrate that the yield obtained at more alkaline pH’s was due to Pb(I1) adsorption onto the colloidal SO,, the adsorption of Pb(I1) onto Ludox was examined as a function of pH. The results are shown in Figure 2b. The adsorption edge is located at pH 5 , well below the first hydrolysis point for Pb(I1) in bulk s ~ l u t i o n . ’This ~ is in agreement Pb2+
+ H 2 0 * PbOH+ + H+,
pK = 7.71
(6)
with the results of Consalves et aI.,l5 who found that Pb(I1) adsorption on silica reaches its plateau level at pH 7. It seems from these steady-state results that adsorption onto SiO, is re(14) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley-Interscience: New York, 1976. ( 1 5 ) Gonqalves, M . D. L. S . ; Sigg, L.; Stumm, W. Enuiron. Sei. Technol.
1985, 19, 141.
The Journal of Physical Chemistry, Vol. 94, No. 22, I990 8437
Radiation-Induced Dissolution of Colloidal P b 0 2 sponsible for the lower yields of dissolved Pb(I1) at higher pH's, yet the reduction proceeds with the same yield as at pH 3.0. The broad pH range covered by the curve in Figure 2a (e.g., compare with Figure 2b) indicates that two adsorption isotherms are responsible for the declining yield of dissolving Pb(II), i.e., adsorption on P b 0 2 at lower pH's and on S O 2 at the higher pH's. When solutions irradiated at pH 3.0 were allowed to equilibrate for 24 h prior to filtering, the yield of Pb(I1) increased. This was not due to thermal dissolution which is negligible on this time scale. This slow postirradiation dissolution of P b 0 2 is ascribed to the reduction of the oxide by the radiolytically produced hydrogen peroxide. The yield of dissolved Pb(I1) at pH 3 and long times is 3.7 ions/ 100 eV as compared to the theoretical yield of 0.5G(ZV-) G(H202) = 4.0. (0 The Proton-to-Electron Stoichiometry. The ratio of electrons transferred to protons consumed could be determined from the conductivity signals in pulse radiolytic experiments. The procedure has been outlined earlier and will not be reiterated here.16 The limiting proton-to-electron ratio, as measured at the end of the proton consumption reactions (typically 0.2 s after the pulse), did not reach the expected value of 2:l based on the stoichiometry of reaction 5, and the final ratio was dose dependent. For the sol containing 1.23 X IO4 M Pb02, typical H+/e- ratios at 0.2 s were 1.30 (for 0.78-krad pulse), 1.38 (1.8-krad pulse), 1.72 (3.5-krad pulse), and 1.80 (4.0-krad pulse). Little change was observed up to 20 s after the pulse. Since all of the viologen radicals have reacted with the sol at the time that the final H+/eratio was recorded, this ratio reflects the existence of a reduced but partially hydrolyzed lead( 11) species (e.g., PbOH+). The limiting H+/e ratio of 1.8:1 at high doses is in reasonable agreement with the G( Pb2+) values from the steady-state dissolution, and the lower than stoichiometric ratio is thus similarly attributed to the adsorption of the partially hydrolyzed lead species. More significant is the very low H+/e- ratio at low doses. Considering the high solubility of Pb2+ at pH 3, it is unlikely that most of the reducing equivalents remain as Pb(OH)2 at the particle surface. More probably, Pb(II1) (e.g., PbOOH) centers are relatively stable on the P b 0 2 particles surface for long periods of time. This is to be expected at the very low doses when the initial concentration of reducing radicals is much smaller than the concentration of particles. Under such conditions disproportionation of Pb(ll1) species can only occur either by an interparticle electron-transfer process or via a dissolution-readsorption mechanism; both are expected to be very slow processes. (D)Kinetics of the Reduction of Colloidal Pb02/Si02by Z Y . The linear dependence of dissolved Pb(I1) on dose found at pH 3.0 (section B) suggests that reduction of the radiolytically generated Pb(I1) ions does not occur under the experimental conditions used. Nonetheless, further reduction of Pb2+ by the viologen radical is thermodynamically feasible:
+
2ZV-
+ Pb2+
-
2ZV
+ Pbo
AEO
N
+0.28 V
(7)
In order to establish the significance of this reaction in the system examined, blank solutions containing 2 X IO4 M Pb2+ at pH 3.0 (no colloid) were pulse-irradiated. The rate of ZV- formation and its yield were unchanged. No loss of the viologen radical could be observed up to 0.2 s following the electron pulse. However, the viologen radical was consumed on a time scale of minutes, indicating that reaction 7 is very slow. As will be seen below, the reaction of the viologen radical with the sol particles is much faster and thus reaction 7 should only become important when the Pb02 sol concentration has been substantially depleted. The slow rate of reaction 7 is attributed to the high energy barrier required for the formation of Pb(1). Checks on the effects of the colloidal silica itself on the reactions observed were also undertaken by both conductivity and spectrophotometric detections. No decay of the ZV- absorption could be observed in solutions containing 0.2% w/v Si02on the time scale of interest. The rate of formation of the viologen radical (16) Mulvaney, P.; Swayambunatham, V.;Grieser. F.; Meisel, D. J. Phys. Chem. 1988, 92, 6732.
0.15 '1
a .0.05
0.0
0.01
Time (sees)
Figure 3. Effect of dose on the amount of viologen disappearing by the fast step in pulse-irradiated 1.23 X lo-' M PbOzsols at pH 3.0. Solutions contained 2 X IO4 M ZV and 0.1 M propan-2-01 and were N,O-saturated. Signal amplitude normalized to the 1 krad.
Time (secs)
2.m
-200
7
4 . . 0-
.
.
,
.
.
,
-
. , . . . . , . . . .
010--
20 Time (secs)
I
Figure 4. Effect of dose on the amount of protons disappearing by the fast step at pH 3.0: top curve, 1360 rad; bottom curve, 3460 rad. Solution composition as per Figure 3.
was the same in the presence and absence of the Ludox. Since the rate of formation of the radical depends on the viologen concentration, it can be concluded that the parent ZV molecules do not significantly adsorb onto the silica. Similarly, the yield of protons measured by conductivity was close to the expected value of G(H+) = 6.8, and there was no decay of the signal up to 0.2 s. Thus, buffering by the silica is minimal at this pH. In the presence of colloidal Pb02, two distinct charge-transfer steps were observed spectrophotometrically at 600 nm. The fraction of the radical reacting by the faster step decreased with increasing pulse size, as can be seen from the overlays in Figure 3. However, the absolute concentration of radicals consumed in the fast step appeared to be roughly constant at a constant Pb02 concentration. At pH 3.0 and 1.23 X 10-4 M Pb02, approximately (3.0 f 0.3) X low6M radicals reacted in the fast step. The rate constant for the fast step increased with the pulse dose, suggesting that the radical concentration is in excess over the concentration of particles. The exponential decay of the radical yielded a bimolecular rate constant of 2.5 X 1O'O M-I s-I. This fast process did not disappear on repetitive pulsing and is therefore not due to impurities such as oxygen. In Figure 4,conductivity changes observed during the reaction of viologen radicals with colloidal Pb02/Si02 at pH 3.0 are shown
8438
Mulvaney et al.
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990
?OM
4000
( E ) The Slow Proton and Electron Uptake by Pb02 Colloids. The observed rates of slow electron and proton transfer to the colloid were found to be first order over several half-lives. The rate of both these processes linearly increased on increasing the P b 0 2 concentration, independent of radical concentration (see Figure 5 ) . Both rates were equal to one another within experimental error. The apparent second-order rate constant was (2.4 f 0.2) X lo5 M-I s-I. The true second-order rate constant is of course larger than this (approximately 1 X lo7 M-' s-') and expected to be slower than diffusion controlled. The only apparent effect of prepulsing the colloidal system was to reduce the rate of all processes as the Pb02 concentration decreased. No further slower steps were observed up to 20 s after the pulse. Considering the above observations, the overall reaction describing the slow step can be represented by
6000
Dose (rad)
[(PbOz),,]-H+
[PbOJ x IO sLI
Figure 5. Observed rate constant of the slow electron- and protontransfer steps to colloidal PbO,/SiO, sols as a function of (a) colloid concentration and (b) dose. Solutions were N20-saturated at pH 3; [PbOJ = 1.23 X IO" M, 0.2% w / v Ludox, 0.1 M propan-2-01, Circles are for spectrophotometric data: squares are for conductivity data.
for two different doses. Again, there are two distinct processes associated with the decay of the conductivity signal. The fraction of protons transferred decreased with increasing the dose in the pulse, as is clear from the traces in Figure 4, and the rate constants were similar to the rates of viologen radical decay. These observations indicate that one proton was taken up by toe colloid per each electron transferred during the fast process and at the same rate as electrons were transferred to the particles. Accordingly, the fast step may be interpreted in terms of the reaction (PbOJ,
+ ZV- + H+
-
(Pb02),,..lPb11'OOH
+ ZV
(sa)
where n is the number of P b 0 2 molecules per particle. The observation of a fast step which proceeds only to a limited extent is often observed on reduction of colloidal metal and is attributed to equilibration of the colloid's quasi-Fermi level with that of the electrolyte redox potential. The observations of Figures 3 and 4 are, however, distinctly different from earlier observations since the equilibrium level at the end of the fast process is independent of the reductant concentration. This would indicate that once an electron is injected into the particle its Fermi level is raised close to the V/V- level. Injection of a second electron to the same particle is then a much slower process. While earlier experiments were conducted with relatively large particles (aggregation number >lOOO), the particles in this study are much smaller and thus the effect of an excess electron on the Fermi level is more pronounced. If this hypothesis is correct, then the number of viologen radicals that reacted in the fast process is equal to the concentration of particles. Under the experimental conditions of Figure 3 this leads to an aggregation number of 40. Since the presence of an excess electron is felt across the particle, the question of whether the electron is localized in a particular site or delocalized over all of the particles constituents may be raised. Reaction 8a then may be better described by eq 8b, which em(PbOZ),, + ZV-
+ H+
-
[(PbO,),]-H+
+ ZV
(8b)
phasizes the delocalization of the charge over the particle. Nevertheless, further electron-transfer reactions do proceed. The ensuing slower reactions are discussed below. (17) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik. A . J . J . Phys Chem. 1984,88, 4278.
+ ZV- + 3H+
----L
Pb2+ + 2H20
+ (Pb02)w1 + ZV
(9)
As written, the second electron is transferred to the conduction band of the singly reduced colloidal particle. Proton uptake by the doubly reduced oxide essentially takes place instantaneously following the second electron-transfer step. Since at this stage all of the particles in the ensemble already contain one excess electron, the dependence of the rate on the P b 0 2 concentration is expected. Since no dependence of the rate on dose was observed, the disproportionation of Pb(1II) species is not the rate-determining step in the reduction to Pb2+. The proton-to-electron ratio at the end of this step leads to the values quoted in section C. Thus, three protons must be consumed during this slow step. Interestingly, none of the various primary steps which lead to net reaction of eq 9 is rate determining, and the electron transfer controls the dissolution rate. (F)Reduction of PbO, Suspensions and pH Effects. Similar signals to those observed for the Ludox-stabilized P b 0 2 solutions were obtained from 0.26 g L-I P b 0 2 suspensions following irradiation at pH 3.0. The oxide used for these experiments was an electrochemically prepared sample with a surface area of only 8 m2 g-l. Because of relatively rapid settling of this suspension, rate constants were not reproducible but the decay profiles were. Again, a fast and slow proton-transfer step were seen similar to the observations for the transparent sols. The final proton-toelectron ratio again reached a value of 1.8:1 at long times. Thus, the colloidal material behaved very similarly to the bulk lead dioxide powder. The effect of pH on the rate of slow electron and proton transfer was also briefly examined. The pH appeared to have a small effect on the rate of electron transfer. The observed rate constants at M P b 0 2 were about 14 f 2 s-l at pH 3.0-3.9 and 4 s-I 9X at pH 9.3. The decrease at higher pH may reflect some partial aggregation as the sol passed through the pH of minimum stability for the Ludox. It may also reflect the interaction of the negatively charged radical with the electrostatic field of the S i 0 2which may largely determine the double-layer effectsI6 on the rates of charge transfer. I n fact, it would be expected that the rate of electron transfer of ZV- would decrease with increasing pH as the Si02 colloid is more negatively charged at high pH. Conclusions
The P b 0 2 particles on the Si02colloid seem to be extremely small (50-100 molecules per particle). The onset of light absorption of these particles is shifted to higher energy relative to the estimated intrinsic bandgap of the bulk material. This shift is consistent with quantization effects often associated with such small particles; however, it is also possible that it is the result of nonstoichiometry existing in the particles, which is commonly found in Pb02. The P b 0 2 sols undergo close to stoichiometric dissolution with reducing radicals formed radiolytically in acidic media. The primary step involves simultaneous transfer of one electron and proton to the colloid, yet no more than one electron per particle is injected in this step. Excess electrons in Pb02 have previously been shown to lead to bleaching at the adsorption edge of the
J . Phys. Chem. 1990, 94, 8439-8450
8439
Acknowledgment. Work at Argonne National Laboratory is performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under Contract No. W-31-109-ENG-38. The dedicated Linac operation by D. Ficht and G. Cox is much appreciated. P.M. acknowledges the receipt of a Commonwealth Postgraduate Research Award. Electrochemically prepared PbOp was a gift from M. Horne, CSIRO, Division of Mineral Products.
~ o l l o i d .Both ~ effects, the adsorption bleaching and the barrier to the addition of a second electron, may result either from band tilling by the excess electron or from the local electric field created by the presence of the electron, even when it is localized. Following the fast process, a second slower electron-transfer process is observed accompanied by a second proton-transfer process. The final conductivities from the pulse radiolysis are in good agreement with the observed G(Pb(I1)) values from the steady-state experiments, both indicating some adsorption of lead(l1) species onto the particle surface.
Registry No. ZV-, 1 13 1 1 1-40-3; Pb02, 1309-60-0.
Surface Catalytic Sites Prepared from [HRe(CO),] and [H,Re3(C0),2]: Mononuclear, Trinuclear, and Metallic Rhenium Catalysts Supported on MgO P. S. Kirlin,+ F. B. M. van Zon,t D. C. Koningsberger,t and B. C. Gates*.' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 1971 6, and Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 M B Eindhouen, The Netherlands (Received: January 23, 1990: In Final Form: June 6, 1990)
MgO-supported catalysts were prepared from [HRe(CO)5]and [H,Re3(CO),,] and characterized by extraction of surface organometallics, infrared and ultraviolet/visible spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS analysis and other data show that [H,Re,(CO),,] was initially deprotonated on the MgO surface, giving a surface-bound anion with a structure comparable to that of the salt [Ph,As] [H2Re,(C0)12]and having a Re-Mg distance of 2.39 8,. Heating of the supported cluster anion in helium to 225 OC led to oxidation and breakup of the cluster framework, giving a mononuclear complex formulated as [Re(C0)3(0MgJ3] (where the braces refer to groups terminating the bulk oxide). The distances characterizing the bonding of the Re to the support are Re-O = 2.15 8, and Re-Mg = 2.80 8,. A Re-Re distance of 3.94 8, was observed, consistent with the decomposition of the cluster on the support to form ensembles consisting of three of the Re subcarbonyls, for which a structural model is presented. Treatment of this sample in hydrogen at 350 "C gave a Re species with oxygen neighbors at average distances of 1.94 and 2.45 8,. Heating of this sample to 500 OC in hydrogen led to reduction and conversion of most of the Re into metal crystallites. The several samples were tested as catalysts, as described in a companion paper.
Introduction Understanding of the catalytic activities of transition metals for rupture of C-H and C-C bonds in alkanes, important in the conversion of petroleum and petrochemicals, has been a goal of researchers for decades.l,, Progress has been hampered by the seeming impossibility of determining the contribution of a single, isolated active site of a structurally nonuniform metal c a t a l y ~ t ~ . ~ and by the lack of molecular models that catalyze the intermolecular rupture of C-C bonds ( h y d r o g e n o l y ~ i s ) . ~The ~ ~ results of kinetics investigations with supported metal catalysts indicate that the C-C bond breaking in alkane hydrogenolysis is preceded by multiple dehydrogenation steps and that the active site is comprised of more than one surface metal Multiple dehydrogenation (C-H insertion) of cycloalkanes has been observed with soluble mononuclear Re complexes; the extent of dehydrogenation increases with decreasing ring size from eight to five carbon atoms.*-9 These results suggest that an ensemble of metal atoms with labile ligands may catalyze dehydrogenation of alkanes through C-H insertion, leading to the rupture of the C-C bond on adjoining metal centers.I0 The goal of the present research was to prepare metal oxide supported Re catalysts consisting of ensembles of varying nuclearities (numbers of Re atoms) and to determine how the nuclearity influences the catalytic properties for activation of the C-C bond. A mononuclear precursor, [HRe(CO)5], was used to prepare surface structures designed to incorporate isolated 'University of Delaware. f Eindhoven University of Technology.
0022-36S4/90/2094-8439$02.50/0
surface-bound Re carbonyl complexes. A trinuclear precursor, [H3Re3(C0)12], was used to prepare ensembles consisting of three such complexes. The surface chemistry has been characterized by extraction of surface anions and infrared and other spectroscopies. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the catalyst prepared from [H,Re,(CO),,I. Precise structural characterization of surface species with EXAFS requires the use of reference materials in the experimentation and data analysis." Since EXAFS spectroscopy can lead to precise characterization of the metal-support bonds in supported organometallics, these samples are valuable models for the metal-support interface', in supported metal catalysts. EX(1) Boudart, M. Ado. Catal. 1969, 20, 153. (2) Somorjai, G.A.; Carrazza, J. Ind. Eng. Chem. Fundam. 1986,25,63. (3) Gault, F. G. Adu. C a r d 1981, 30, I , and references therein. (4) Van Broekhoven, E. H.; Schoonhoven, J. W. F. M.; Ponec, V. Sur/. Sci. 1985, 156, 899. ( 5 ) Crabtree, R. H. Chem. Reo. 1985, 85, 245, and references therein. (6) Bergman, R. G. Science 1984, 223, 902. (7) Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts, and Applicafions;Wiley: New York, 1983. (8) Bandy, J. A.; Cloke, G. N.; Green, M. L. H.; O'Hara, D.; Prout, K. J. Chem. SOC.,Chem. Commun. 1984, 240. (9) Baudry, D.; Ephritikhine, M.; Felkin, H. J. Chem. Soc., Chem. Commun. 1982, 606. (IO) Crabtree, R. H.; Dion, R. P. J . Chem. SOC.,Chem. Commun. 1984, 1260. ( 1 1) Koningsberger, D. C., Prins, R., Eds.; X-ray Absorption: Principles,
Applications, Techniques of EXAFS, SEXAFS, XANES Wiley: New York, 1988.
0 1990 American Chemical Society
