J. Phys. Chem. 1984, 88, 912-918
912
1s photoelectron band between bridging and nonbridging oxygens in silicate glasses were observed: 7 1 the absolute shift of the 0 1s band of the bridging oxygen was greater than for the nonbridging oxygen in the Si02-Na20 system. The sensitivity of the physicochemical properties toward composition being different for different "coordinations", we may conclude that bridging hydroxyls will be more readily influenced by the average electronegativity than the terminal hydroxyls. The observations in the literature confirm this statement. Not only are the physical properties as "such" composition dependent, but also the sensitivity for external perturbations depends on the composition of the matrix. For terminal hydroxyls in mixed oxides S i 0 2 / M g 0 , SiO2/Al2O3,and AI2O3/Mg0 and their pure components, Lercher and N01ler~~ demonstrated that the shift of the hydroxyl bands after adsorption of acetone increases with increasing intermediate electronegativity of the oxide (electronegativity range in the Sanderson scale: 2.8-4.2; Av range: 260-340 cm-I). For bridged hydroxyls in H zeolites (electronegativity range: 3.8-4.2; Au range: 313-325 cm-'), similar effects were observed by Jacobs73for the adsorption of benzene. Because of the use of different donor molecules, it cannot be decided which type of hydroxyl is more compositional dependent. (71) Briickner, R.; Chun, H.-U.; Goretzki, H.; Sammet, M. J. Non-Crys?. Solids 1980,42, 49. (72) Lercher, J. A,; Noller, H. J . Catal. 1982,77, 152. (73) Jacobs, P. A. Catal. Rev.-Sci. Eng. 1982,24, 415.
Acknowledgment. W.J.M. thanks the "Belgisch Nationaal Fonds voor Wetenschappelijk Onderzoek" for a research position as "Onderzoeksleider", the Belgian Ministry of "Nationale Opvoeding and Nederlandse Cultuur" together with the T.U. Wien for financing a stay at the "Institut fur Physikalische Chemie", and Dr. D. Freude, Dr. A. Leonard, and Prof. H. Pfeiffer for stimulating discussions. The force constants were kindly calculated by K.-P. Schriider, J.S. thanks Prof. W. Schirmer for his promoting interest in these studies. Note Added in Proof: We are grateful to Prof. H. Pfeifer for careful reviewing of the manuscript and for drawing our attention to two recent in which evidence is produced for the presence of strongly acidic, bridging hydroxyls in amorphous aluminosilicates. In a more recent note76it was shown that this type of hydroxyl is thermally unstable and can be easily removed from the surface of amorphous aluminosilicates by outgassing at 670 K for 15 min. Registry No. la, 14475-38-8; 2a, 88337-13-7; 2b, 88337-14-8; 3a, 10193-36-9. (74) Kazansky, V. B. Kine?. Karol. 1982,23, 1334. (75) Hunger, M.; Freude, D.; Pfeifer, H.; Bremer, H.; Jank, M.; Wendlandt, K.-P. Chem. Phys. Let?., in press. (76) Borovkov, V. Yu.; Alexeev, A. A.; Kazansky, V. B. J . Catal. 1983, 80, 462.
Radiolytically Induced One-Electron Reduction of Methylvioiogen in Aqueous Solution. Platinum-Catalyzed Formation of Dihydrogen' Margherita Venturi, Istituto di Scienze Chimiche, Facoltci di Farmacia, Universitci di Bologna, 401 26 Bologna, Italy
Quinto G. Mulazzani,*2 Istituto di Fotochimica e Radiazioni D'Alta Energia, Consiglio Nazionale delle Ricerche, 401 26 Bologna, Italy
and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received: May 2, 1983)
The reaction of the methylviologen dication (MV2') with radiolytically generated (CH,),COH radicals in deaerated aqueous solution is rapid and quantitative, producing the methylviologen cation radical (MV'.). At pH 1, MV'. decays via [ MV2+]-dependentsecond-order kinetics in the course of minutes according to an H+-assisted disproportionation reaction that yields hydrogenated methylviologen as a final product; at pH 4.2, MV'. is more stable but does decay in the course of a day. Dihydrogen is a product of the radiolysis with G(H,) equaling the "background" yield of H2 from the primary radiolytic act and the scavenging of H atoms by 2-propanol. In the presence of poly(viny1 alcohol)-stabilized Pt sols (total [Pt] = 50 pM), MV+. decays in less than 1 s. G(H2) is at or near zero during the initial phases of the reaction when the Pt is adsorbing the radiolytically generated H2; at the same time, hydrogenated product is formed efficiently by Pt-catalyzed disporportionation. Continued exposure causes C(H2) to rise above "background" level and reach a plateau; the yield of hydrogenation product decreases concomitantly and reaches a minimum level that is not negligible. The maximum efficiency of H2production above "background", relative to that of the same system in the absence of MV'., is -79% at pH 1 and -43% at pH 4.2; the minimum efficiency of hydrogenation, relative to the zero-dose limit, is -10% at pH 1 and -25% at pH 4.2. The implications of these results to the photochemical generation of H2 in the presence of Ru(bpy)32', MV2*, and Pt are discussed.
Introduction The Pt-catalyzed formation of H2 in aqueous solutions in which a one-electron-reduced viologen radical has been photochemically generated serves as the basis of many model solar energy conversion schemes. In the most highly studied system, the me-
thylviologen dication (l,l'-dimethyl-4,4'-bipyridinium ion, MV2+) acts as the electron-transfer relay between the electronically excited state of R ~ ( b p y ) ~ (bpy ~ + = 2,2'-bipyridine) and the reduction of H 2 0 on the surface of the colloidal platinum aggregate^.^ The reactive species at the Pt is the methylviologen radical cation
(1) Research supported in part by Consiglio Nazionale delle Richerche and in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy.
(2) Visiting scholar, Boston University, Fall 1981. (3) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982,22, 117 and references therein.
0022-3654/84/2088-0912$01.50/0
0 1984 American Chemical Society
Platinum-Catalyzed Formation of Dihydrogen (MV'.), which transfers an electron to the metal particle; subsequent rapid proton uptake by the charged particle results in the storage of a pool of hydrogen atoms.4 Desorption of H2 from the platinum is competitive with hydrogenation of MV2+,which ultimately causes the failure of the systems by the removal of the electron-transfer relay.5 MV+. can also be generated conveniently by radiation chemical techniques, which permit an alternative examination of the interaction of MV+. with Pt to be made. In this study, we focus on the stoichiometry of H, formation and MV+- decay in the presence of Pt using the techniques of continuous and pulsed radiolysis.
Experimental Section Materials. Colloidal platinum was prepared via the reduction of H2PtCl, by sodium citrate according to literature procedures.6 Methylviologen dichloride (Aldrich) was recrystallized from water by the addition of acetone, collected on a sintered-glass filter, and dried over CaC12 in a vacuum desiccator. The purification of (CH3)2CHOH, (CH3),C0, and HzO has been d e ~ c r i b e d . ~ Poly(viny1 alcohol) (PVA; Fluka; average molecular weight 15OOO; viscosity of 4% solution: 3 CP at 20 OC; degree of hydrolysis: 86-89 mol %) was used as received. The pH of the solutions was adjusted with H2SO4 or NaOH (Merck, Suprapur). Merck Titrisol buffer (10% by volume) was used for buffering at pH 4.2. Procedures. Continuous radiolyses were carried out at room temperature on 10-25-mL samples of degassed solution contained in Pyrex or silica cylindrical vessels (total volume 100 mL) fitted with Spectrosil optical cells on a side arm. Absorption spectra were recorded with a Perkin-Elmer 555 spectrophotometer. The absorbed radiation dose from the ,OCo Gammacell source was determined by the Fricke chemical dosimeter by taking G(Fe3+) = 15.5 (C(X) = number of molecules of species X formed or destroyed per 100 eV of energy absorbed by the solution) and had a value of 8.6 X 1019 eV L-l min-l (1.38 krd min-'). Pulse radiolyses with optical absorption detection were performed using the 12-MeV linear accelerator of the FRAE Institute, CNR, Bologna. The pulse irradiations were performed at room temperature on samples contained in Spectrosil cells of 2-cm optical path length. Exposure of the solutions to unnecessary UV light was avoided or minimized by means of cutoff filters and a shutter. Changes in absorption following the pulse were measured relative to the unirradiated solution. The radiation dose per pulse was monitored with a 0.1 M KSCN aqueous solution saturated with 0, by using Gc = 2.15 X lo4 at 500 nm. In certain experiments, the samples used in continuous irradiations were subjected to Linac pulses. In one type of experiment, the vessel was placed in front of the Linac exit window in such a way so that the analyzing light beam passed through the optical cell on the side arm. The minimum amount of solution (-2 mL) was placed in the side arm and irradiated. In another type of experiment, the whole solution was irradiated with a certain number of pulses; an aluminum scatter plate was placed on the exit window of the Linac in order to irradiate the solution uniformly. Under such conditions, the overall irradiation dose delivered to the solution was determined with the modified Fricke dosimeter taking G(Fe3+) = 15.0 as valid for a dose rate of 1Olo rd SKI.* Analyses. Gas analyses were performed on 10- or 25-mL samples of solution. The gaseous products were removed from the irradiation vessel through a 1 i q ~ i d - Ntrap ~ by an automatic Toepler pump and collected in a gas buret. The gas was then injected into a gas chromatograph equipped with a molecular sieve (4) Henglein, A,; Lindig, B.; Westerhausen, J. J. Phys. Chem. 1981, 85, 1627. (5) Johansen, 0.; Launikonis, A.; M e r , J. W.; Mau, A. W.-H.; Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J. Chem. 1981, 34, 981.
(6) Wilenzick, R. M.; Russell, D. C.; Morris, R. H.; Marshall, S. W. J . Chem. Phys. 1967, 47, 533. (7) Mulazzani, Q.G.; Emmi, S.; Fuochi, P. G.; Venturi, M.; Hoffman, M. Z.; Simic, M. G. J . Phys. Chem. 1979.83, 1582. (8) D:aganii., I. G.; DraganiE, Z. D. "The Radiation Chemistry of Water"; Academic Press: New York, 1971; p 156.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 913 column using He as the carrier gas. In all cases, the gas collected in this way was 100% H2. Generation of Reducing Radicals. The radiolysis of aqueous solutions generates ea¶-, O H radicals, H atoms, H2, and H 2 0 2 according to reaction 1 where the numbers in parentheses represent
HzO
--+
ea¶- (2.8), O H (2.8), H (0.6), H2(0.45), H 2 0 2 (0.8) (1) the G values for ,the individual species. In the presence of 2propanol, (CH3)2COHis generated according to reaction 2. The OH/H
+ (CH3)2CHOH
k2 = 1.3
X
109/5.0
X
-
(CH3)2COH + H2O/H2 (2)
lo7 M-'s-l
(ref 10, 11)
reaction of H is probably quantitative while that of O H is known9 to be only 85% efficient, the remaining product being the /3-radical, .CH2(CH3)CHOH,formed in an analogous hydrogen-abstraction reaction. Hydrated electrons are efficiently scavenged by H+ (reaction 3) or by acetone (reaction 4). eaq-
k3 = 2.2
X
+ H+
X
H
1O'O M-ls-l
eaq- + (CH3),C0
k4 = 5.9
-
(ref 12)
H+
(3)
(CH3)2COH
lo9 M-' s-l
(4)
(ref 12)
Thus, in neutral and alkaline solutions containing 2-propanol, the major reactive radicals are ea¶- (C = 2.8) and (CH3),COH (G = 3.0); in acidic solution or in the presence ofacetone in neutral solution, the predominant radical is (CH3),COH (G = 5.8). According to reactions 1-3, the G values of H, from the primary radiolytic act and the scavenging of H by 2-propanol would be 3.9 in acidic solution and 1.1 in neutral and alkaline solutions.
Results 1 . Solutions Containing M p + in the Absence of Pt. The pulse radiolysis of Ar-purged solutions of 0.1 mM MVZ+in 0.1 M 2-propanol at pH 1 and 10 yielded the characteristic absorption spectrum of MV+..I3 At pH 1, the formation of MV+. occurred via clean pseudo-first-order kinetics ( [MV2+] = 0.05-0.5 mM) according to reaction 5 for which k5 = (2.9 f 0.2) X lo9 M-' s-I, comparing well with the value of (3.5 h 0.2) X lo9 M-' s-l previously reported.I4 At pH 10, two reactions forming MV+were clearly distinguishable: a faster process due to reaction 6 (CH,)2COH
+ MV2+
-+
+ (CHJ2CO + H+
MV+*
(5)
for which a value of k, = 8.4 X lolo M-' s-I has been reportedI5 and a slower process corresponding to reaction 5. The absorption ea¶-
+ MV2+
+
MV+-
(6)
of MV+. generated at pH 1 and 10 was stable in the longest time frame of pulse radiolysis (seconds). It should be noted that in 1 M HzSO4, MV+. disappeared much more rapidly via well-defined second-order kinetics with a rate constant that is an inverse function of [MV2+]. The details of this behavior have been published separately.16 (9) Asmus, K.-D.; MGckel, H.; Henglein, A. J . Phys. Chem. 1973, 77, 1218. (10) Farhataziz; Ross, A. B. N a f l .Stand. ReJ Data Ser. US.,(Narl. Bur. Stand.) 1977, No. 59. (1 1) Anbar, M.; Farhataziz; Ross,A. B. N a f l .Sfand. ReJ Data Ser. (US'., N a f l . Bur. Stand.) 1975, N . 51. (12) Anbar, M.; Bambenek, M.; Ross, A. B. Natl. Stand. Ref Data. Ser. (Natl. Bur. Stand.) 1973, No. 43. (13) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524. (14) Meisel, D.; Mulac, W. A,; Matheson, M. S . J . Phys. Chem. 1981, 85, 179.
(15) Farrington, J. A,; Ebert, M.; Land, E. J.; Fletcher, K. Biochim. Biophys. Acta 1973, 314, 372. (16) Venturi, M.; Mulazzani, Q.G.; Hoffman, M. Z. Radiaf. Phys. Chem. 1984, 23, 229.
914
Venturi et al.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
h
N
3 u
P 0.2
0.4
0.6
0.8
1.0
Dose, Mrad
Figure 2. G(H2)as a function of irradiation dose for the continuous radiolysis of deaerated aqueous solutions at pH 1. Contents of solution: 0.1 M 2-propanol (0); 0.1 M 2-propanol and Pt/PVA ( A ) ; 0.1 M 2propanol, Pt/PVA, and 0.5 mM MV2+(0). represents two independently made samples. O0
0.2
0.4
0.6 L
Dose, Mrad
Figure 1. Dependence of the absorbance of MV2+ at 255 nm and the hydrogenated methylviologen product at 220 nm on the irradiation dose delivered to deaerated aqueous solutions of 0.5 mM MV2+in 0.1 M (absorbance at 255 2-propanol; optical path length 2 mm. Curve I (0) nm) and curve I1 ( 0 ) (absorbance at 220 nm) refer to pH 1 in the absence of Pt/PVA catalyst. Curves I11 (0)and IV (A)(absorbance at 255 nm) refer to pH 1 and 4.2, respectively, in the presence of Pt/PVA.
The continuous radiolysis of deaerated solutions of MV2+ containing 0.1 M 2-propanol at pH 11.7 generated MV+., which was infinitely stable in the absence of air; MV+. reacts rapidly with O2 according to reaction 7 for which k7 = 7.7 X lo8 M-' MV+.
+0 2
-
MV2+
+ 0,-
(7)
€605 = 1.37 X lo4 M-l cm-l for MV+.,I7 we evaluated a value of G(MV+.) = 5.8 from the generation of MV+. as a function of irradiation time extrapolated to zero dose. In fact, G(MV+-) decreases slightly with increasing dose due to the further reduction of MV'. to MVo by ea
