J. Phys. Chem. 1982, 86, 242-247
242
with reasonable accuracy. Computer fitting gives a more accurate value. Since the value reported for k , is considered to have an uncertainty of about 10%,15 the absolute value of k, reported here has an uncertainty of about 20%.
Acknowledgment. This research was sponsored by the Division of Chemical Sciences/Office of Basic Energy Sciences, U.S.Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation.
Mechanism of the Uncataiyzed Formation of Dihydrogen In the Radiolytically Induced Reduction of Trls( 2,2’-bipyrldine)rhodium( III)Ion in Aqueous Soiutlonl Qulnto 0. Mulazzanl, Istltuto dl Fotochlmlca e Radlazbni D’Alta Energla, Conslgl& Nazbnak, Delk, Rlcercb, 40126 Bologna, Italy
MargherHa Venturl, Istltuto dl Scbnze Chlmlche, F a & % dl Farmacla, Universlts dl Bologna, 40126 Bdogna, Italy
and Morton 2. Hoffman’ Department of Chemkfry, Boston UnherNv, Boston, Massachusetts 022 15 (Recehed: April 20, 198 I; I n Final Form: September 28, 1981)
The reaction of Rh(bpy)$+ with radiation-generated eaq-and (CH3)2COHin deaerated aqueous solutions containing 0.1 M 2-propanol generates H2in the absence of any catalyst. The yield of H,, above that produced as “background”from the radiolytic act, is greatest at pH 4.2; the yield of extra Hz diminishes to zero at pH 2 and 7. The G value of extra Hz rises from zero in the limit of zero radiation dose to a plateau at -0.25 Mrd; G(H2)is also a function of the initial concentration of R h ( b p ~ ) ~approaching ~+, a maximum value at infinite substrate dilution. These results, coupled with the measured yield of Rh(I), give rise to a proposed mechanism for the uncatalyzed formation of H2 in which the direct precursor to Hzis RhH(bpy),+. This latter species is seen to arise from the reduction of RhH(bpy),,’, the form of Rh(1) stable in acidic solution, by Rh(bpy)?. The maximum turnover number for the generation of Hz from this system upon exposure to large radiation doses is -12. The data presented account for the origin of Hz in uncatalyzed photochemical systems.
Introduction The reduction of R h ( b p ~ ) (bpy ~ ~ + = 2,2’-bipyridine), photosensitized by R ~ ( b p y ) ~leads ~ + , to the generation of H2 from aqueous solution in the presence or absence of catalytic As a result, this system, in which R h ( b ~ y ) , ~serves + as a relay species to mediate the reduction of water, is viewed6 as having potentiality toward the development of solar energy conversien schemes. Indeed, in comparison with other R~(bpy)~~+-based systems which effect the photoreduction of water,’-’l the Rh(1) Resenrch supported in part by Consiglio Nazionale delle Ricerche and in part by the U.S. Department of Energy through Contract No. DEAC02-81ER10971. Presented at the 182nd National Meeting of the American Chemical Society, New York, Aug 2343,1981; Abstract MOR 154. (2) Lehn, J.-M.; Seuvage, J.-P. Nouo. J. Chem. 1977,1, 449. (3) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Helu. Chim. Acta 1979,62, 1345. (4) Brown, G. M.; Chan, S.-F.; Creutz, C.; Schwarz, H. A.; Sutin, N. J . Am. Chem. SOC.1979,101, 7638. (5) Chan, S.-F.;Chou, M.; Creutz, C.; Matsubara, T.;Sutin, N. J.Am. Chem. SOC. 1981,103,369. (6) Sutin, N. J. Photochem. 1979, 10, 19. (7) Kalyanasundaram, K.; Gritzel, M. Angew. Chem., Int. Ed. Engl. 1979. 18. 701. (si Moradpour, A,; Amouyal, E.; Keller, P.; Kagan, H. Nouo. J.Chim. 1978,2, 547. (9) DeLaive, P. J.; Whitten, D. G.; Giannotti, C. Ado. Chem. Ser. 1979, No. 173, 234. 0022-3654/82/2086-0242$01.25/0
(bpy):+ system in the presence of Pt is quite efficient; H, is generated with a maximum quantum yield of 0.11 at pH 7.0-8.1. In this system, as in others that involve the oxidative quenching of * R ~ ( b p y ) ~a ~“sacrificial” +, reagent must be present to effect the reduction of the redox-generated Ru(bpy),S+ at a rate significantly greater than the reaction of Ru(bpy),3+ with the reduction product of the quenching step. For this purpose, EDTA and triethanolamine (TEOA) are employed. There is, however, dispute as to the mechanism of the catalyzed formation of H2 Kirch et al., proposed that the direct precursor to the formation of H,in the presence of Pt is Rh(1) as Rh(bpy),+ or a hydride (RhH(bpy)?+). Chan et al.5do not believe that this is likely, citing thermodynamic and mechanistic arguments to support their contention that R h ( b ~ y ) , ~is+the responsible species. Interestingly, the R~(bpy)~~+-Rh(bpy):+ photochemical system also leads to the uncatalyzed formation of Hz at pH 5 and 7 in the presence of EDTA or TEOA?v6 It is not at all clear from the work what the experimental parameters are that regulate the yield of H2 or the mechanism of its formation in the absence of Pt. (10) DeLaive, P. J.; Sullivan, B. P.; Meyer, T. J.; Whitten, D. G. J.Am. Chem. SOC.1979,101,4007. (11) Brown, G. M.; Brunwhwig, B. S.;Creutz, C.; Endicott, J. F.; Sutin, N. J. Am. Chem. SOC.1979,101, 1298.
0 1982 American Chemical Society
The Journal of Physical Chemlshy, Vol. 86, No. 2, 1982 243
Formatlon of H, in Reduction of Rh(bpy):+
I
I
3
G 2
0 0
01
02
03
04
Dose Mrad 0
2
4
6
6
1
0
PH
Flgure 1. G(H,) (0)and G(Rh(1)) (A)as a functlon of H from the hadlatbn of deaerated solutkns of 5.0 X lo4 M Rh(bpy), contalnlng 0.1 M 2-propanol; radlatlon dose = 50 krd. Dashed line: G value of "background" H,. Insert: AG(H,) as a function of pH.
E
Recently, we published12 a report on the generation of Rh(II) and Rh(1) from the radiolytically induced oneelectron reduction of Rh@pA3+ in which we characterized the spectra of the various intermediate species and the kinetics of their formation and decay. We also showed that H2, in excess of that which results directly from the radiolytic decomposition of water or indirectly from H-abstraction reactions of radiation-generated H atoms, was formed a t "natural" pH. In this paper, we examine in detail the yields of H2 and Rh(1) as a function of pH, radiation dose, and [Rh(bpy),3+]. The results permit us to suggest a mechanism for the uncatalyzed formation of H2 that results from the reduction of Rh(bpy),,+ and to speculate on the steps that may be operative in the photochemical system.
Experimental Section Materials. Rh(bpy),Cl3.5H20 was prepared and purified according to literature pro~edures.'~The purification of (CHd,CHOH, (CHJ2C0, and H20 has been deacribed.l'17 The pH of the solutions was adjusted with NaOH or H8O4 (Merck, Suprapur). Commercial buffer (Merck, Titrisol) (510% by volume) was also used for buffering in the range pH 4-10. Procedures. Continuous radiolyses were carried out at room temperature on 10-25-mL samples of solution contained in silica vessels fitted with silica spectrophotometric cells on a side arm. The irradiation vessels were also fitted with a separate compartment through which it was possible to add the required amount of concentrated NaOH solutions to the irradiated solutions under oxygen-free con(12) Mulazzani, Q. G.; Emmi, s.; Hoffman,M. Z.; Venturi, M. J. Am. Chem. SOC.1981,103,3362. (13) Crosby, G. A.; Elfring,W.H., Jr. J. Phys. Chem. 1976,80,2206. (14) S i i c , M. G.; Hoffman,M. 2.;Cheney, R. P.;Mulazzani, Q.G. J. Phys. Chem. 1979,83,439. (15) Mulazzani, Q. G.; Emmi,S.;Fuochi,P. G.; Venturi, M.; Hoffman, M. 2.;S i c , M. G. J. Phys. Chem. 1979,83,1583. (16) Mulazzani, Q. G.; Emmi, 5.;Fuochi,P. G.;Hoffman,M. 2.;Venturi, M. J. Am. Chem. SOC.1978,100,981. (17) Venturi, M.; Emmi, 5.;Fuochi,P. G.; Mulazzani, Q.G. J. Phys. Chem. 1980,84,2160.
Flgure 2. G(H,) as a function of radlation dose from the irradiation of deaerated solutions of 5.0 X lo4 M Rh(bpy),,+ containing 0.1 M P-propanol(0)or 0.1 M 2-propand and 0.02 M acetone (0)at pH 4.2. The solid lines represent empirical equations A and B.
ditions. Spectra were recorded with a Perkin-Elmer Model 555 spectrophotometer. The samples were deaerated by standard vacuum-line techniques. The absorbed radiation dose from the 6oCoy source was determined by Fricke dosimetry and had a value of 1.49 X 1017eV g-l min-l. Analyses. Gas analyses were performed on 10- or 25-mL samples which had been degassed before irradiation. The gaseous products were removed from the irradiation samplea by an automatic Toepler pump, and their total volume was measured in a gas buret. A liquid-nitrogen trap was placed between the samples and the Toepler pump. The gas collected in the gas buret was then injected into a gas chromatograph equipped with 2 X 4 mm i.d. columns of molecular sieve using helium as the carrier gas. The product gas in all cases was 100% H2. Generation qf Reducing Radicals. The generation of e,- and (CH3)2COHas the principal reductants by the use of selective scavengers in the radiolysis of aqueous solutions has been described in detail bef~re.'~J""
Results M The irradiation of deaerated solutions of 5.0 X Rh(bpy),Cl, containing 0.1 M 2-propanol generates H,; Figure 1shows the G value (number of molecules formed per 100 eV of energy absorbed) of the formation of H2 as a function of pH for a total dose of 50 krd applied at a dose rate of 1.5 krd mi&. The value of G(H2)is a function of the absorbed dose and reaches a plateau value of 3.9 for a dose of -250 krd at pH 2. Figure 1also shows G(Rh(1)) under the same conditions. The concentration of Rh(1) present is obtained by bringing the irradiated solution to pH 11by the addition of NaOH under 02-freeconditions and taking12 e515 9500 M-' cm-' for the red-violet form of Rh(1) at pH 110. As will be noted further in the Discussion section, a pH-dependent "background" yield of H2 is generated in the radiolysis system. Subtraction of the background G value from G(H2)shows (Figure 1)that AG(H2) reaches a maximum at pH -4 and decreases in more acidic or alkaline solution. As a result, pH 4.2 was the medium of choice for the examination of the dependence of G values on other system parameters. Figure 2 shows the effect of radiation dose on G(H2)for deaerated solutions of 5.0 X lo4 M Rh(bpy),Cl, containing 0.1 M 2-propanol or 0.1 M N
244
Muiazzani et ai.
The Journal of phvsical Chemistry, Vol. 86, No. 2, 1982
t
i
Ii
2 .o
2
A
01 0
2
6
4
Io
Dose.Mrad
Flgwe 5. G(H,) (0)and [Rh(I)] (X) as a function of radiation dose from the irradiation of deaerated solutions of 1.5 X lo4 M Rh(bpyh5+ containing 0.1 M 2-propanol at pH 4.2. I
0
02
04 Dose Mrad
-
06
-L
-
0'6
Flgwe 3. Concentration of Rh(1) generated as a function of radiation dose from the irradiation of deaerated solutions of 5.0 X lo4 M Rh(b~y),~+ Containing 0.1 M 2-propand (A)or 0.1 M 2-propanol and 0.02 M acetone ( 0 )at pH 4.2 or 0.1 M 2-propanol at pH 10 (0).
i
we examined the stability of the system over the course of exposure to rather high radiation doses. Exposure to up to 5.62 Mrd corresponds to the production of over 200 equiv of reducing radicals per equivalent of Rh(bpyIg3+ initially present. Figure 5 shows that, for deaerated solutions of 1.5 X M Rh(bpy),Cl, containing 0.1 M 2propanol at pH 4.2, G(H2) initially increases and then decreases with increasing irradiation dose. The concentration of the generated Rh(1) holds at a plateau level up to -2 Mrd and then diminishes toward zero.
Discussion The radiolysis of aqueous solutions generates the primary radicals and molecular produ& according to reaction 1, where the numbers in parentheses represent the G H2O eaq-(2.8)) OH (2.8), H (0.61, H2 (0.45))H202(0.8) (1) *M-
values of the species. In acidic solution, e,; is converted to H via reaction 2, where k 2 = 2.2 X 1O'O M-' s-'.18 In eaq- H+ H (2)
-
+
0 0
05
1.0 [Rh(bpy):IxlO3
1.5 M
20
Figure 4. G(H,) (0)and G(Rh(1)) ( 0 )as a function of the initial concentration of Rh(bpy):+ from the irradiation of deaerated solutions containing 0.1 M 2propanol at pH 4.2 radiation dose = 50 krd. G(H,) G(Rh(1)): A.
+
2-propanol and 0.02 M acetone at pH 4.2. Figure 3 shows the dependence of [Rh(I)] on radiation dose under the same conditions; data at pH 10 in 0.1 M 2-propanol are also given. G(H.J and G(Rh(1)) are also functions of the initial concentration of Rh(bpy),,+ in deaerated solutions containing 0.1 M 2-propanol at pH 4.2 irradiated with a constant dose of 50 krd (Figure 4). It can be observed that G(H2)decreases and G(Rh(1)) increases with increasing [Rh(b~y)~,+] although their sum remains relatively constant. Because of the importance of the Rh(b~y)~,+ system to the generation of H2 over very long periods of irradiation,
the presence of 2-propanol, OH and H are scavenged according to reactions 3 and 4, where k3 = 1.3 X lo9 M-'s-l OH + (CH3)&HOH (CH3)2COH+ HzO (3) H + (CH&CHOH -.+ (CH,)2COH + H2 (4) (ref 19) and k, = 5.0 X lo7 M-' s-' (ref 20); the reactions also produce a low yield (- 15%) of the weakly reducing .CH2C(CH3)HOHradical. In the presence of acetone, e, is scavenged and forms (CHJ2COH according to reaction 5 for which k6 = 5.9 X log M-' s-'; l8 this reaction does not
-
-
eaq-+ (CH3)2C0
H+
(CH3)2C0H
(5)
produce the .CH2C(CH3)HOHradical. Yield of H2. As can be seen from the above equations, the background yield of H2from the radiolysis of deaerated (18)Anbar, M.;Bambenek, M.;Ross, A. B. Natl. Stand. Ref. Data Ser., Nat. Bur. Stand. 1973, No. 43. (19)Farhataziz; ROES,A. B.Nat. Stand. Ref. Data Ser. (US., Natl. Bu. Stand.) 1977, No. 59. (20) Anbar, M.;Farhataziz; Ross, A. B. Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.) 1975, No. 51.
The Journal of Physlcal Chemistry, Vol. 86, No. 2, 1982
Formatlon of H, in Reduction of Rh(bpy):+
i
neutral aqueous solutions containing 0.1 M 2-propanol, arising from steps 1and 4, corresponds to G(H2)= 1.05. In more acidic solution, where reaction 2 is operative in competition with the reduction of the substrate (reaction 6; k6 = 8.1 X 1O'O M-l s-l),12the background yield of H2 e,,
+ Rh(bpy),3+
-
Rh(b~y)~~+
245
(6)
reflects this competition. The dashed line in Figure 1 represents the background G value of H2 formation as a function of pH for 5.0 X 10"' M Rh(bpy)gB+from which AG(H2) is calculated and displayed in the insert. The radiolysis system is capable of generating extra H2between pH 3 and 7. Generation of Rh(I). The reduction of R h ( b ~ y ) , ~in+ this system occurs via reactions 6 and 7 with k, = 1.8 X (CH3)2COH+ Rh(bpy)gB+ Rh(bPY):+ + (CH&CO + H+ (7)
-
log M-'
4
s-'.12 R h ( b ~ y ) ~then l + undergoes slow ligand labilization (reaction 8; ke = 0.45 s-l),12followed by the re-
-
R ~ ( ~ P Y ) , ~Rh(bpy)z2+ + + bpy (8) duction by R h ( b ~ y ) of ~ ~the + resulting bis(bpy) complex (reaction 9); Chan et have determined kg to be 3 X 108 Rh(bpy)?+ + R h ( b p ~ ) , ~ + Rh(bpy)z+ + Rh(bpy)S3+
-
(9)
M-' s-'. Thus, from this mechanism, the yield of formation of R h ( b ~ y ) ~in+ the , absence of any secondary reactions, has a maximum value of G 3. In our earlier study,12we found that, at relatively low radiation dose (-5 krd), G(Rh(1)) has a virtually constant value of -2 across the entire pH range (2-14). We attributed the less-thanstoichiometric yield of Rh(1) to its reaction with radiolytically generated H202(G = 0.8). In the present study, we find that the yield of Rh(1) with increasing radiation dose is linear at pH 10 (Figure 3) with G = 2.15 f 0.05; at doses higher that 0.1 Mrd, the insoluble violet form of Rh(1) precipitates. However, as Figure 3 shows, the concentration of Rh(1) formed at pH 4.2 does not increase linearly with increasing radiation dose but ultimately reaches a plateau value at -0.3 Mrd. As a result, G(Rh(1)) decreases with increasing dose; Figure 1 shows G(Rh(1)) for a 50-krd dose. An apparent pK, of -6 is suggested by the variation of G(Rh(1))with pH. In this pH region, the absorption spectrum of Rh(1) dramatically changes, indicating the conversion of that species from red-violet Rh(bpyI2+to colorless RhH(bpy)?+ via reaction 10. As Rh(bpy)2++ H+ -w RhH(bpy)z2+ (10)
-
well the H2 yield exhibits its maximum value in that pH region. Effect of Radiation Dose. The variation of G(H2)with radiation dose a t pH 4.2 shown in Figure 2 is described by eq A for solutions containing 0.1 M 2-propanol and by G(H2) = 1.1+ 1.3(1 - e-1sD) (A) eq B for solutions containing 0.1 M 2-propanol and 0.02 G(H2) = 1.1 + 0.85(1 - e-11D) (B) M acetone, with D being the radiation dose expressed in Mrd. Both equations require G(H2)= 1.1at zero dose, the value representing the background H2 The preexponential terms represent the maximum AG(H2) obtainable from these systems and give a direct indication of their efficiencies. The presence of acetone, which converts e,, to (CH3)2COHvia reaction 5 in competition with reaction 6, reduces the ability of the system to generate extra H2 This
I 0
10
05
[Rh (bpy):
15
20
] x lo3 M
Figure 6. Reciprocal of G(H,) (0)and G(Rh(1)) (A)as a function of the i n b i concentration of Rh(bpyh3+ from the irradiation of deaerated solutions containing 0.1 M 2propanol at pH 4.2; radiation dose = 50 krd.
observation suggests that (CH3)2COHradicals, while serving to reduce R h ( b p ~ ) via ~ ~ reaction + 7, are also involved in secondary reactions which compete with the process leading to the generation of extra Ha. This phenomenon may be the reason that AG(H2)diminishes to zero in acidic solution where reaction 2 is operative. However, it must be noted that the functional dependence of the concentration of Rh(1) on radiation dose at pH 4.2 in solutions containing 2-propanol is independent of the presence or absence of acetone and that G(CH2C(CH3)HOH) is the same in these media. Effect of Substrate Concentration, The experimental values of G(H2)and G(Rh(1)) as a function of the initial concentration of Rh(bpy)gB+displayed in Figure 4 fit eq C and D if G(H2), and G(Rh(I)),, the G values at infinite
1
-
G(Rh(I)), - G(Rh(1)) 0.55 + (0.75 X 103)[Rh(bpy)33+](D) [Rh(bpy)l+],are taken as 1.0 and 2.2, respectively; the radiation dose is constant at 50 krd. The quality of eq C and D is demonstrated by Figure 6; rearrangement of eq C and D yields eq E and F, which reflect the dependence G(H2) = 1.0 + 1/(0.55 + (1.44 X lO3)[Rh(bpy)g3+]) (E) G(Rh(1)) = 2.2 - 1/(0.55 + (0.75
103)[Rh(bpy)33+]) (F) of G values on [ R h ( b ~ y ) ~ ~These + ] . quantitative relationships predict that, for infinite R h ( b p ~ ) dilution, ~~+ G(H2)a= 2.8 and G(Rh(I)), = 0.4; with increasing initial substrate concentration, G(H2)diminishes and G(Rh(1)) increases. The maximum difference in the G values between infinite dilution and infinite concentration is 1.8 in both cases. This value represents the maximum yield of H2 above background that can be obtained from the irradiation of this system with a dose of 50 krd. It should X
248
The Journal of Physical Chemistty, Vol. 86, No. 2, 1982
be compared with the maximum G value of 2.2 for the formation of Rh(1) at pH 10, a value that appears to be constant over the entire pH range. Relationship between H2and Rh(l). The data indicate that the extra H2 is produced in this system at the expense of the Rh(1) in virtually a 1:l ratio. However, as seen from the previous section, a residual G value of Rh(1) of 0.4 does not appear as AG(Hz). Furthermore, the presence of acetone in-the system at pH 4.2, while increasing the yield of (CH,),COH radicals, decreases G(H2)but not at the expense of G(Rh(1)). Again, there is a quantity of Rh(1) generated that does not appear as H2. These facts lead us to suggest the occurrence of a reaction involving Rh(1) in the form(s) in which it exists at pH 4.2 with (CH3)&OH that removes part of the reducing equivalents from the system during the course of the irradiation. Stoichiometrically, the most satisfying reaction, in the sense that no side reactions are indicated, is described by reaction 11, which would compete with reaction 7, especially at low
-
Rh(1) + 2(CH3)2C0H+ 2H+ Rh(II1) + 2(CHs)ZCHOH (11) [ R h ( b ~ y ) ~ ~A+t ]high . substrate concentrations, reaction 11would be negligible and the full yield of Rh(1) (G = 2.2) would be generated. The detailed mechanism of reaction 11 is not known at this time. However, it can be stated that the decrease in G(H2) in the presence of acetone cannot be attributed to the .CH2C(CH3)HOHradical. Mechanism of H2 Formation. We pointed out previously12 that Rh(1) in all its forms is perfectly stable in acidic, neutral, and alkaline 02-free aqueous solutions without showing any generation of H2 over the course of at least 3 months. Furthermore, under continuous irradiation conditions, Rh(I1) species, which are one-electron transport agents, are present only at very low steady-state ~ ~ ~neither J~ concentrations. It has been c ~ n c l u d e d that R h ( b ~ y ) ~its+ ,various aquated, hydroxylated, andlor dimeric forms, nor R h H ( b ~ y ) ~nor ~ +Rh(II) , is capable of generating H2 by itself at appreciable rates in the uncatalyzed system. We must rule out reaction 12 as the origin
Mulazzani et at.
RhH(bpy)2+ + HzO
+
H2 + R h ( b p ~ ) 2 + ~ +OH-
-
(15)
bimolecular interaction (reaction 16). One can see that, 2RhH(bpy)2+
H2 + 2Rh(bpy)z+
(16)
-
according to reactions 16 and 10, RhH(bpyI2+would act as a homogeneous catalyst for the reaction 2e- + 2H+ HP Note should be taken here that a binuclear homolytic reaction has been proposed as one of the pathways for H2 evolution from hydridocobaloxime.21 The decrease of G(H2) and the increase of G(Rh(1)) (Figure 4) as the initial concentration of Rh(bpy),3+ is increased can be accounted for by reaction 17 in compeRhH(bpy)2+ + Rh(bpy)S3+ RhH(bpy),2+ + Rh(bpy)gZ+ (17) tition with H2-generatingreactions such as reactions 14-16. Reaction 17, which is the reverse of reaction 13, would be driven by the relatively high concentration of substrate. As [ R h ( b ~ y ) ~is~ increased, +] the direct precursor of H2 (RhH(bp~)~+) would be increasingly scavenged but Rh(1) (as the hydride) would be regenerated. Because of reaction 17, no extra H2 would be produced when relatively concentrated solutions of Rh(bpy)gS+ are irradiated with relatively low doses, a situation that is described by eq A and E. Under these conditions, the effect of reaction 11 would be negligible and the maximum yield of Rh(1) would be obtained (eq F). Irradiation with Large Doses. The experimental values of G(Hz) for 1.5 X lo4 M Rh(bpy)gS+in 0.1 M 2-propanol at pH 4.2 shown in Figure 5 can be expressed by eq G, G(H2) = 1.05
+ (1.036 X lO-,)D (1- De-24LJ - e-D)
(G)
where D represents the radiation dose in Mrd. The first term in the equation (1.05) represents the background yield of Hz. The quantity 1.85 X represents the total number of moles of extra H2that can be obtained, per liter of solution, from the exhaustive irradiation of the system; the origin of this figure will be discussed below. The term 1.036 X contains various conversion factors relating H20 R h H ( b p ~ )+ ~ ~H30+ + H2 + Rh(bpy)z(HzO)z3+ dose, energy deposition, Avogadro's number, etc. The (12) De-24Dterm is only important during the low-dose part of the curve becoming negligible at -0.25 Mrd. Treatment of Hz which Kirch et al.3 suggested as a possible process of the data to express the gas yield in terms of the number in homogeneous solution. We do not believe that the of moles of H2 generated per liter of irradiated solution immediate precursor to H2 in the uncatalyzed system is leads to the plot in Figure 7. The background yield Rh(bpy),2+; this species is believed5to be responsible for corresponds to the straight line for G = 1.05 while the the reduction of water to H2 in the catalyzed system. experimental yield curves upward from the origin to Instead, we suggested earlier12that, under continuous parallel the background. The difference between the radiolysis in an uncatalyzed system, RhH(bpy)22+,which parallel lines (1.85 X mol) represents the total number is designated as a rhodium(II1)hydride, is capable of being of moles of extra H2 that can be generated from 1 L of reduced to R h H ( b ~ y ) ~this + ; latter species could be the solution by the application of a dose exceeding 6 Mrd. direct precursor of H2. The reducing agent suggested With respect to the initial concentration of Rh(bpy)p (1.5 there12 and in the uncatalyzed photolysis system5 was X M), the maximum turnover number for this system R h ( b p ~ ) ~ Rh(bpy),2+ ~+. is sufficientlylong-lived to engage is 12. Experiments conducted with a more concentrated in reaction 13, particularly if k13 were of the order of solution of R h ( b p ~ )have ~ ~ +indicated that this turnover number does not depend upon the initial concentration Rh(bpy)S2++ RhH(bpy)2'+ of Rh(b~y),~+. In other words, the total amount of extra Rh(bpy)g3++ RhH(bpy)z+ (13) H2 that can be obtained from the radiolytic reduction of 106-107M-' s-l. If that were the case, we would conclude R h ( b ~ y ) ~is, +limited only by the initial concentration of that the potential of the RhH(bpy),2+/+couple is similar the substrate. Naturally, the radiation dose that is reto or slightly less negative than that of the R h ( b ~ y ) , ~ + / ~ + quired to generate the total amount of extra H2 increases couple ( E O = -0.7 V).5 R h H ( b ~ y ) ~ could + generate H2 with increasing concentration of Rh(b~y),~+. through reaction with H30+in acidic solution (reaction 14), The irradiated solutions which give rise to the results shown in Figures 5 and 7 were also analyzed for Rh(1). A RhH(bpyIz++ H30+ H2 + R h ( b p ~ ) + ~ ~H+2 0 (14)
-
-
-
-
reaction with HzO in neutral solution (reaction 15), or
(21) Chao, T.-H.; Espenson, J. H. J. Am. Chem. SOC.1978, 100, 129.
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982
Formation of H2 in Reduction of R h ( b p ~ ) ~ ~ + I
I
/ /
Dose,M rad
Figure 7. Number of millimoles of H, generated per liter of irradiated solution as a functlon of radlatlon dose. Initial concentration of Rh(bpyb3+ = 1.5 X lo4 M; sokrtkns contain 0.1 M 2propand at pH 4.2.
concentration of Rh(1) of -4 X lod5M was obtained with a radiation dose of 0.05 Mrd and remained practically constant up to a dose of 0.8 Mrd. The visible absorption spectrum of the solution irradiated with a dose of 2.13 Mrd, made alkaline under 02-free conditions, showed the presence of a shoulder in the 470-nm region superimposed on the spectrum of the red-violet form of R h ( b ~ y ) ab~+ sorbing a t 515 nm. The latter species was still, however, the major visible absorbing component of the irradiated system. When a larger radiation dose (5.62 Mrd) was delivered to the system, only the spectrum of a species absorbing at 470 nm was obtained when the irradiated solution was made alkaline under 02-freeconditions. The major part of this visible absorption disappeared when the solution was exposed to air, suggesting that the species absorbing at 470 nm is a Rh complex with the metal center in a low oxidation state. No attempt was made in this study to characterize this species. It can be observed that the rate of formation of extra H2,which is proportional to the tangent to the c w e shown in Figure 7, presents ita maximum value at zero dose and decreases exponentially with increasing radiation dose becoming zero at -6 Mrd. It is clear that some process consuming R h H ( b ~ y ) ~(and/or ~+ its precursors or successors) takes place during the long-term irradiation of this system which ultimately destroys the ability of the system to generate extra H2. We suggest that this degradative process is the dearomatization of the ligand, specifically via hydrogenation and/or radical addition to the ligand system. It has been demonstrated, in fact, that the hydrogenation of the methyl viologen radical cation is the main factor limiting the long-term efficiency of generating H2from the photochemical system containing Ru(bpy)32+, methyl viologen, EDTA, and Pt.22*23 (22) Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H. B. Now.J. Chim. 1980,4, 377.
247
Scheme I
In.
e-t
Conclusions The mechanism of the formation of H2 from the radiolysis-induced one-electron reduction of R h ( b ~ y ) by ~~+ eaq-and (CH3)2COHin deaerated, mildly acidic solution containing 2-propanol is shown in Scheme I. Because the substrate completely scavenges the reducing radicals which are generated with a total G value of -6, Rh(bpy)gP+is generated with a G value of -6. Because the generation of Rh(1) involves a disproportionation process with 2 equiv of.Rh(II), the maximum yield of formation of RhH(bpy)?+ at pH 4.2 is G 3. The data that we have presented account for the origin of the H2 in the photochemical systems containing Ru(bpy)l+,Rh(bpy),3+, and EDTA (or TEOA) in the absence of Pt.395For those systems, one merely replaces reactions 1-7 with reaction 18; EDTA (or TEOA) reduces Ru*Ru(bpy)S2++ R h ( b ~ y ) 3 ~ +Ru(bpy)?+ + Rh(bpy)gz+ (18) ( b ~ y ) ~In ~+ the . case where TEOA is used as the sacrificial reagent, the oxidized form is converted rapidly into a reducing radical via H abstraction from TEOA.5 This reducing radical reacta with R h ( b p ~ )to ~ ~generate + another equivalent of Rh(bpy)gP+. Chan et al.5 report a quantum yield of 0.02 for the uncatalyzed formation of H2at pH 5.0 in the system containing TEOA; (PHP= 0.04 in the presence of EDTA. Inasmuch as the quantum yield of formation of Rh(bpy)gP+ (ref 5 ) is 0.15 from reaction 18, the lower observed quantum yields of H2 are easily rationalized in terms of the stoichiometry, the pH, and the substrate concentration. From the data, it appears that TEOA reducing. radicals may interact with Rh(1) as does (CH3)2COHto cause a further lowering of the H2yield. It appears that, with the judicious choice of experimental parameters, the uncatalyzed photochemical generation of Hz mediated by R h ( b p ~ ) could ~ ~ + approach an efficiency of unity relative to the yield of R h ( b ~ y ) , ~and + the stoichiometry of intermediate reactions.
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-
Acknowledgment. We thank L. Minghetti for assistance with the vacuum line and L. Ventura for the drawing of the figures. (23) Johansen, 0.;Lane, J. E.; Launikonis, A,; Mau, A. W.-H., Sasse, W. H. F.; Swift, J. D. "Abstracts of Third International Conference on Photochemical Conversion and Storage of Solar Energy"; Boulder, CO, 1980,p 145.
