J. Phys. Chem. 1985,89, 1593-1598
1593
Photoinduced Electron Transfer between Zn(TPPS),S- and Viologens Shigetoshi Aono, Ichiro Okura,* Department of Chemical Engineering, Tokyo Institute of Technology, Meguro- ku, Tokyo 152, Japan
and Akira Yamada The Institute of Physical and Chemical Research, Hirosawa, Wako-shi, Saitama 351, Japan (Received: May 21, 1984)
The photoreduction of viologens, such as methylviologen (MV2+) and zwitterionic viologenpropanesulfonate (PVS), with ZII(TPPS)~*were investigated. By laser flash photolysis, the quenching rate constant, the back-electron-transfer rate constant, and the charge separation efficiencieswere determined. Complex formation between ZII(TPPS)~~and viologen was observed, and the association constants were determined by laser flash photolysis.
Introduction Recently, much attention has been given to the photoinduced hydrogen evolution system for storage of solar energy. The four-component system containing an electron donor, a photosensitizer, an electron carrier, and a catalyst has been proposed for photoinduced hydrogen In our previous work, the enzyme hydrogenase instead of the colloidal platinum has been found to be a more favorable catalyst for hydrogen e v ~ l u t i o n . ~ The hydrogenase from Desulfovibrio vulgaris (Miyazaki type) is about 500 times more active than colloidal platinum? and the unfavorable side reaction, hydrogenation of methylvi~logen,~ does not occur. If the photoreduction of an electron carrier which is able to act as the substrate of hydrogenase praceeds, then hydrogen evolution can be achieved by adding hydrogenase. In this work, the photoreduction of viologens, which are well-known as an artificial substrate of hydrogenase, was investigated as that reaction is the important step of photoinduced hydrogen evolution. Zinc(I1) meso-tetraphenylporphinetrisulfonate (ZII(TPPS)~+)was used as a photosensitizer. It is highly active for photoreduction of Viologed and has the thermodynamic advantage for the quenching by viologen compared with the cationic porphyrins such as zinc(I1) meso-tetrakis(N-methylpyridini~m-4-yl)porphine.~*'As an electron carrier, two kinds of viologens were used; one is MV2+~ which is positively charged, and the other is zwitterionic viologenpropanesulfonate (PVS) which is neutral in the oxidized form and is negatively charged in the reduced form. It is expected that the photoreduction rate will depend on the difference of viologen's charge. The reaction mechanism of the photoreduction of viologens is proposed by kinetic studies under steady-state irradiation and laser flash photolysis. Experimental Section ZII(TPPS)~~and PVS were synthesized according to literature p r o c e d ~ r e s other ; ~ ~ ~ reagents were of the highest grade commercially available. (1) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, H.-C Coord. Chem. Rev. 1982, 44, 83 and references therein. (2) Kiwi, J.; Kalyanasundaram, K.;Gritzel, M.Srrucr. Bonding 1982,49, 37 and references therein. (3) Okura, I.; Kunsunoki, S.; Kim-Thuan, N.; Kobayasi, M.J. Chem. Soc., Chem. Commun. 1981, 56. (4) Okura, I.; Kusunoki, S. Znorg. Chim. Acra 1981, 54, 249. ( 5 ) Johansen, 0.;Launikonis, A.; Mau, A. W. H.; Sasse, W. H. F. Ausr. J. Chem. 1980, 33, 1643. (6) Okura, I.; Takeuchi, M.;Kim-Thuan, N. Phorochem. Phorobiol. 1981, 33, 413. (7) Kalyanasundaram, K.; Nerman-Spallart, M.J . Phys. Chem. 1982,86, 5163. ( 8 ) Fleischer, E. B.; Cheung, S. K. J . Am. Chem. Soc. 1976, 98, 3162. (9) Willner, I.; Otvos, J. W.; Calvin, M.J . Am. Chem. SOC.1981, 203, 3203.
0022-3654/85/2089-1593$01 .50/0
The sample solution was deaerated by repeated freeze-pump thaw cycles to remove the dissolved oxygen throughout this work. The ionic strength was kept constant by using NaCl aqueous solution if necessary. In the photolysis under the steady-state irradiation, the sample, in a Pyrex cell with a magnetic stirrer, was irradiated with light from a 200-W tungsten lamp at 30 O C . The light of wavelength less than 390 nm was cut off by using a Toshiba L-39 filter. The pH of sample solution was adjusted to 7.0 by Tris-HC1 buffer (0.2 mol dm-3). The absorption spectra were recorded with a Shimadzu Model MPS-5000 spectrometer. The concentration of the reduced viologen was determined by the absorbance at 605 nm (eMV+ = 1.1 X lo4 and epvs- = 1.3 X lo4 mol-' dm3 cm-I). Laser flash photolysis was carried out by using a Nd:YAG laser, Model HY-500, from JK Laser Ltd. The second harmonic (532 nm), ca. 100 mJ cm-2 and a flash duration of 20 ns, was used for the excitation of the sample solutions throughout this study. Analyzing light beams from a xenon lamp (Ushio UXL- 150 D 150 W) were intensified by a factor of ca. 20 during the detection of the transient spectra. Transient spectra having lifetimes longer than 200 ps were measured without intensification of the analyzing light beams. The light beam, after passage through a sample cell (1 X 1 cm, quartz cell), came into the entrance slit of a monochromator (Model MC-20 N from Ritsu Appl. Opt. Co.). The output from a Hamamatsu photomultiplier (R 758) attached to the exit slit of the monochromator was displayed on a Tektronix oscilloscope, Model 7904. The oscillogram was photographed with a Polaroid camera and analyzed. Results and Discussion Photoreduction of Viologen under Steady-State Irradiation. When the aqueous solution containing 2-mercaptoethanol (RSH), ZII(TPPS)~~-, and MVZ+was irradiated, the growth of MV+ was observed as shown in Figure 1. The concentration of MV+ increases with the irradiation time and tends to reach a constant value. When PVS was used instead of MV2+,the photoreduction of PVS also proceeds as shown in Figure 1. The initial reduction rates of viologens may depend on the concentration of ZII(TPPS)~~-, RSH, and viologens. The rates were proportional to the concentration of Z~I(TPPS),~and were independent of the concentration of R S H in the experimental conditions in which the experiments were carried out. However, as shown in Figure 2, abnormal phenomena were observed for the rate dependence on the viologen concentration. The reduction rate increased gradually with the increase of viologen concentration in both cases of MV2+and PVS, and then decreased beyond the maximum value. The degree of the depression of the reduction rate was smaller in the system containing PVS than in MV2+;i.e., the rate is about half of the maximum mol dmw3),while value at the concentration of MV2+ (1.0 X 0 1985 American Chemical Society
1594 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
Aono et al.
n
0.6
’E
-
n
E 5 7
4
. - 3 $2
0.4
t
-8 0
2
d ci
5
”
1
0.2
0
0
20
40
60
Time I min
Figure 1. Time dependence of (0) [MV’] and ( 0 )[PVS-1. Reaction mol dm-’; conditions: RSH, 0.21 mol dm-’; Zn(TPPS)’’-, 1.1 X MVZt or PVS. 6.07 X mol dm-’.
0
600
550
500
650
WAVELENGTH I nm
0.8
1a 1 E
I
8
I
8
I
8
0.6 1
0
2
4
6
8
10
mol d N 3 Figure 2. Dependence of reduction rate on viologen concentration. (0) RSH-Zn(TPPS)33-MV2t system; ( 0 )RSH-Zn(TPPS)33-PVS system. Reaction conditions: RSH,0.21 mol dm”; Zn(TPFS)33, 1.1 X lO-’ mol dm-’. Viologenl I
the rate is about two-thirds of the maximum value at the same concentration of PVS. To clarify this abnormal phenomenon the following experiments were applied. When MVZ+was added to the Zn(TPPS),* solution, the change of the absorption spectra of Zn(TPPS)33-was observed as shown in Figure 3A (top), in which the Q-band region is shown. The absorption spectra of Zn(TPPS)33- changed with the isosbestic points, suggesting complex formation between Zn(TPPS)33- and MV2+. The spectra of Z ~ I ( T P P S ) ~ in , - the Soret band also changed with isosbestic points by adding MV2+, in which the decrease of the intensity was observed at first, and addition of more MV2+shifted the absorption maximum to the red as shown in Figure 3B (bottom). A similar spectral change for Zn(TPPS),> was observed by adding PVS instead of MV2+,which also suggests complex formation between Zn(TPPS),3- and PVS. These results showing that a complex is formed between porphyrin and viologen are in agreement with literature result^.'^^^ To clarify the role of the complex in the photoreduction of viologens, more quantitative analyses including the determination of the association constants between Zn(TPPS),’- and viologens are required. This problem is discussed later. Determination of Electron-Transfer Rate Constants between Zn(TPPS)3’ and viologens by h e r Flash Photolysis. As shown in Figure 1 and 2, the reduction rate of PVS was larger than that (lO)Schmehl, R. H.; Whitten, D. G. J. Phys. Chem. 1981, 85, 3473. (1 1) Rougee, M.; Ebbesen, T.; Ghetti, F.; Bensasson, R. V. J. Phys. Chem. 1982,86,4404. (12) Shelnutt, J. A. J . Am. Chem. SOC.1981, 203, 4275. (13) Okura, I.; Aono, S.; Takeuchi, M.; Kusunoki, S. Buff. Chem. SOC. Jpn. 1982, 55, 3637.
d o 0.4
0.2
0
380 400 420
440
460
480
WAVELENGTH / nm Figure 3. Spectrum change of ZII(TPPS)’~-by adding MV2’. Reaction conditions: (A, top) Q-band region, (B, bottom) Soret-band region. (A): (a) Zn(TPPS)’)- (3.03 X mol dm-’); (b) (a) + MV2+ (5.53 X lod mol dm-’); (c) (a) + MV2’ (1.21 X mol dm-)); (d) (a) + MV2’ (4.85 X mol dm-’); (e) (a) MV2’ (1.21 X lo4 mol dnl-’); (f), (a) + MV2’ (5.01 X lo-’ mol dm-’). (B): (a) Z ~ I ( T P P S(1.0 ) ~ ~X lod mol dm-’); (b) (a) + MV2+ (4.04 X mol dm-)); (c) (a) + MV2’ (1.21 X lo4 mol dm-’); (d) (a) MVZt (3.29 X lo4 mol dm-’); (e) (a) + MV2’ (9.87 X lo4 mol dm-’); (f) (a) + MV2+ (8.71 X mol dm-)) in 1 .O X 1.O cm quartz cell.
+
+
of MV2+. This may be caused by the difference of charge between MVz+ and PVS. The experiments by laser flash photolysis as described below were carried out to clarify the reasons the reduction rate of PVS was larger than that of MV2+.
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
Photoreduction of Viologens
O4 03
1595
i
“ I
400
500
600
WAVELENGTH
700 /
800
nrn
Figure 4. Difference transient absorption spectra for (a) Zn(TPPS),’and (b) Zn(TPPS)?- + PVS solution. Reaction conditions: (a) Zn(TPPS)?-, 3.03 X 10” mol drn-’; (b) Zn(TPPS),&, 3.03 X lo-’ mol dm-); PVS, 1.46 X lo4 mol dm-). Observed 30 p s after excitation.
I
I A T = O %
A T = O %
II
AT
I
100 %
Figure 5. Decay of T-T absorption of 3Zn(TPPS)33-.Reaction condimol drn-’. Observed at 470 nm. tions: Zn(TPPS)t-, 3.03 X
The transient difference spectrum at 30 ps after the excitation of Z I I ( T P P S ) ~in~ deaerated aqueous solution, which is attributed to the T-T absorption of the excited triplet state of Zn(TPFS)33(3Zn(TPPS)33-),was observed as shown in Figure 4. The decay of the T-T absorption, shown in Figure 5, obeyed first-order kinetics. The lifetime of 3Zn(TPPS)33-was estimated to be 1.6 ms by the first-order plot. MV2+and PVS were able to quench 3Zn(TPPS)33-efficiently as shown in Figure 6A and 7A, respectively. Those decay C U N ~ S consisted of two components, one with shorter and the other with longer lifetime. The transient spectrum of the solution containing ZII(TPPS)~’ and PVS a t 30 ps after the excitation is shown in Figure 4. The species with the shorter lifetime disappeared completely at 30 ps after the excitation, and only the species with the longer lifetime remained. The transient spectrum in Figure 4 should be attributed to the one-electron oxidation product of Z ~ I ( T P P S ) ~(Zn~(TPPS)?-) and PVS-, which may be produced through reaction 1, since the spectrum is in good accordance with the mixture of 3Zn(TPPS)2-
+ PVS
-
Zn(TPPS)32-
+ PVS-
(1)
the spectrum of Zn(TPPS)4* reported previ~usly’~ (the spectrum of Zn(TPPS)?- will be similar to that of Zn(TPPS).& and that of PVS-. The decay of the transient absorption observed at 605 nm, a t which the reduced viologens have the characteristic absorption band, obeyed second-order kinetics. This result is consistent with eq 2.
+
-
ZII(TPPS)~~+ PVS
(2) When MV2+ was used instead of PVS, similar results were obtained. On the other hand, the decays of the species with shorter lifetime in Figures 6A and 7A obeyed first-order kinetics and simultaneously accompanied the growth of the transient absorption a t 605 nm as shown in Figures 6B and 7B, in which the absorption can be attributed to that of the reduced viologens. That result ZII(TPPS)~~- PVS-
l
100 %
AT
l
Figure 6. Typical oscillograms observed at (A) 470 and (B) 605 nm. Reaction conditions: Zn(TPPS)?-, 3.03 X mol dm-’; MVZ+4.85 X mol dm”.
TABLE I: Rate Constant of Quenching and Back-Electron-Transfer Reaction k,’’O/mol-l kb/relative value k,/mol-’ dm3 5-I dm3 5-I quencher MV2+ (1.5 i 0.3) X 1O’O 1.5 X loTo 100 2.6 x 109 14 PVS (2.3 i 0.3) x 109 (4.7 i 1.9) x 104 RSH
suggests that the transient absorption with shorter lifetime in Figures 6A and 7A is ascribed to the T-T absorption of 3Zn(TPPS)33-and that reaction 3 takes place. 3Zn(TPPS)33-
+ viologen
-
+
ZII(TPPS)~~- viologen-
The rate constant for the quenching of 3Zn(TPPS)33-(k, in Scheme I) was determined by Stern-Volmer plots, and the k, values are summarized in Table I. The lifetime of 3Zn(TPPS)33was determined by the decay curve of the T-T absorption at 470 nm. The k, values for MV2+ are about 7 times larger than those for PVS. The difference of k, values may be caused by the difference of the electrostatic interaction between 3Zn(TPPS)33and viologens. MV2+is positively charged and PVS is electrically neutral. Thus, the electrostatic interaction between 3Zn(TPPS)2and MV2+ is larger than that between 3Zn(TPPS)33-and PVS. The electrostatic attraction between P and Q is a major component of the energy stabilizing the encounter complex. Since the redox and potentials of MV2+/+and PVSo/- are -0.44 V (vs.
~
(14) Neta, P. J . Phys. Chem. 1981, 85, 3678.
(3)
(15) Amouyal,
E.;Zidlcr, B. Isr. J. Chem. 1982, 22, 117.
1596 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
Aono et al. IO“
-i 0
E
D
T.
-
ldo
E
U .r
I
I
L
I-’
I
. I 2
1 T = 100 %
J
us
I
Figure 7. Typical oscillograms observed at (A)470 and (B) 605 nm. Reaction conditions: Zn(TPPS)’’-, 3.03 X lo-’ mol dm-3; PVS, 4.85 X mol dm-3.
-0.41 V (vs. NHE),16 respectively, the thermodynamic driving
force for the quenching reaction seems to be the same for both. Consequently the difference of the kq values is caused mainly by the electrostatic interaction. 3Zn(TPPS)?- was also quenched by RSH, which has been used as an electron donor in the photoreduction of viologens under steady-state irradiation.” However, the quenching rate constant by RSH was about 5 orders of magnitude smaller than that by viologens as listed in Table I. Therefore, it is concluded that the quenching of 3Zn(TPPS)d- by viologen is dominant, or the oxidative quenching of 3Zn(TPPS)33- is more favorable under steady-state irradiation. Because charge transfer is influenced by the encounter complex formation, increasing the ionic strength will weaken ionic forces and, in this way, affect rate constants. The k , values were determined at various ionic strengths, which can be explained by the Debye-Brernsted equation as shown in Figure SA (top) B (bottom). As expected from the theory, for the Zn(TPPS)33-MVZ+system the linear relation with negative slope was obtained, while for the Zn(TPPS):--PVS system the value of k, was independent of the ionic strength. The k, values at zero ionic strength obtained by extrapolation of the straight lines were in good agreement with the values without adjustment of the ionic strength. Therefore, the experiments by laser flash photolysis were carried out without adding NaCl unless specially noted. The rate constants of the back electron transfer (kbin Scheme I) were determined by analysis of the decay curves observed at 605 nm. The transient absorption at 605 nm contains not only the absorption of reduced viologen but also that of Zn(TPPS)?-. However, the absorption of ZII(TPPS)~~at 605 nm seems to be small, which is supported by the result for ZII(TPPS)~~reported previo~sly.’~ Thus, the transient absorption of Zn(TPPS)?- at 605 nm is negligible. (16) Willner, I.; Yang, Jer-Ming; Laane, C.; Otovos, J. W.: Calvin, M. J. Phys. Chem. 1981,85, 3217. (17) Okura, 1.; Kim-Thuan, N. J. Mol. Coral. 1979, 6, 227.
0
0.1
0.2
0.3
0.4
0,5
P1‘2 Figure 8. Dependence of quenching rate constant on ionic strength. Reaction conditions: (A,top) Zn(TPPS)’’- (3.03 X 10” mol dm-3), mol dm”); (B, bottom) Zn(TPPS),’- (3.03 X MV2+ (4.85X mol dm-9, PVS (4.85X lo4 mol dm-’).
0
0.2
0.6 08
0.4
A
1.0
o.D!~’O
Figure 9. Relation between AOD605/r605 and AOD4’0.
The relative rate constants of the back electron transfer estimated from the decay curves at 605 nm are listed in Table I. The kb value for MV2+ system was about 7 times greater than that for PVS system. That result may be explained also by the difference of charge between MV+ and PVS-. The unfavorable back reaction is suppressed by the electrostatic repulsion between Zn(TPPS)?- and PVS- in the Zn(TPPS)32--PVS system. Efficiency of Charge Separation of Encounter Complex. The quenching of 3Zn(TPPS)3* and the formation of reduced viologen may proceed as shown in Scheme 1. The efficiency of the charge separation, Le., the efficiency for the formation of reduced viologen against 3Zn(TPPS)33- formed (@),may have an influence on the efficiency of the photoreduction of viologens under steady-state irradiation. In Figure 9, AOD4’0 shows the difference in absorbance observed at 470 nm immediately after excitation, which corresponds
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1597
Photoreduction of Viologens
l-----7
AT=O%
AT=O%
I
(d)
, 40 us,
1 A T = 100 %
A T = O %
5
n
AT = 100 %
. * r
AT = 100 %
A T = O % A T = O %
( f)
-
us
,40
1 rns
,
AT
Figure
100 %
10. Typical oscillograms obtained from laser flash photolysis observed (a-c) at 470 nm and (d-f) at 605 nm. Reaction conditions: Zn(TPPS)33(a,d) (1.21 X mol dm-)); (b,e) (5.01 X mol dm-'); (c,f) (5.01 X mol dm-?.
(3.03 x IOp5 mol dm-)); MV?
to the concentration of 3Zn(TPPS)33-formed initially, AOD605 shows the maximum difference absorbance observed at 605 nm, c605 is the molar absorption coefficient of reduced viologens at 605 nm, and the ratio AODa5/t6OScorresponds to the concentration of reduced viologens formed by charge separation. The ratio of the slopes of the straight lines in Figure 9 can give the relative value of 3. Compared with the slopes of the straight lines, the relative value of 3 for the Zn(TPPS)3*-PVS system was obtained system). to be 1.6 (when Q, = '1.0 for the ZII(TPPS)~~--MV~+ These results indicate that the recombination of Zn(TPPS)?and MV+ within the solvent cage ( k , in Scheme I) takes place more easily than that of Zn(TPPS)32- and PVS- because of etectrostatic interaction, i.e., in the ZII(TPPS),~--MV+ system electrostatic attraction, while in the ZII(TPPS),~--PVS- system electrostatic repulsion. The value of 3 for PVS, therefore, is larger than that for MV2+. This is one of the reasons why the reduction rate of PVS is larger than that of MV2+ as shown in Figure 2 . Complex Formation between Zn( TPPS)?- and Viologens. As described in the previous section complex formation takes place between ZII(TPPS)~* and viologens. In this section quantitative analysis was carried out by using laser flash photolysis. The association constants between Zn(TPPS),,- and viologens can be
derived from the measurements of the T-T absorption of ,Zn(TPPS)33-. The typical oscillograms monitored at 470 nm after a laser flash of the sample solution are shown in Figure 10a-c. The absorbance of the T-T absorption measured immediately after the laser flash decreased with the increase of MV2+concentration, and the absorption disappeared at a concentration higher than 5.01 X mol dm-3. Because of the decrease in absorbance at 470 nm, MV+ formed after the laser flash decreased as shown in Figure 10d,e, and the formation of MV' was not observed in the absence of the T-T absorption, as shown in Figure 1Of. These results are shown in Scheme 11, which 1:l complex (PQ) formation between Zn(TPPS)33- and viologen (Kis the association constant). SCHEME I1 K
P+QSrrPQ
+ P
,P*
hu
Q
PQ 2 PQ*
3P* P+ + Q-
2%
PQ
J. Phys. Chem. 1985,89, 1598-1601
1598
‘9 I
I
TABLE II: Efficiency of Charge Separation and the Association Constants between Zn(TPPS)3* and Viologens
quencher
0
1
CMV”1
2 mol dm-3
0
0
5 [PVSI /
10 mol dmV3
Figure 11. Relation between AOD,470/AOD470and [viologen].
The concentration of PQ increases with the increase of Q concentration. The absorbance of the T-T absorption at 470 nm is considered to be proportional to the concentration of uncomplexed Z ~ I ( T P P S ) ~on~ -the assumption that the photoexcited complex (PQ*)goes back to PQ very rapidly without the formation of the separated ion pairs. The concentration of the uncomplexed ZXI(TPPS)~~can be written as eq. 4 when the 1:l complex is
(4) [PI = [P10/(1 + H Q 1 ) formed, with [PI, and [Q] being the concentrations of Zn(TPPS)33-and viologen added, respectively. Therefore, the following relation can be derived AOD,470/AOD470= 1 + K[Q1
(5)
where AOD,470and AOD470are the difference absorbance at 470 nm measured immediately after the laser flash in the absence and in the presence of viologen, respectively. As shown in Figure 11, the fairly linear relationship between AOD,470/AOD470 and [Q] indicated that the scheme proposed with
@,,I
MV2+
1.O
PVS
1.6
Klmo1-I dm’ (1.7 f 0.2) x 104 (7.3 f 0.6) X IO2
some assumption described above is adequate. From the slope of the straight line in Figure 11, the value of K obtained was (1.7 f 0.2) X lo4 mol-’ dm3 in the case of MV2+ and (7.3 f 0.6) X lo2 mol-’ dm3 for PVS. The difference of K values between MV2+ and PVS is also explained by the difference of the electrostatic interaction between ZXI(TPPS)~~and viologens. The difference between the redox potential of MV2+/+and that of PVSo/- is so small that the large K value of MV2+ compared with that of PVS should be mainly due to an electrostatic effect. Since the positively charged MV2+ is more favorably associated with negatively charged ZII(TPPS)~~-, the large K value detected for MV2+ compared with PVS is plausible. As the complexes formed between ZII(TPPS)~~and viologens (PQ) did not produce separated ion pairs as described above, PQ was not favorable for the photoreduction of viologens. Therefore it may be concluded that only the uncomplexed ZXI(TPPS)~~acts as a photosensitizer for the photoreduction of viologens. The parameters obtained in this study are summarized in in Table 11. The fact that the reduction rate of PVS under steady-state irradiation was larger than that of MV2+ was explained by the difference of Or,, and K values; Le., the Orelvalue of PVS was larger than that of MV2+,and the K value of PVS was smaller than that of MV2+. As the larger K value means the high concentration of PQ which is inactive for the photoreduction of viologens, the concentration of uncomplexed ZII(TPPS)~~is greater in the Zn(TPPS)33--PVS system than that in the Zn(TPPS)33--MV2+ system at the same concentration of viologens. Therefore the reduction rate of PVS was larger than that of MV2+ under the steady-state irradiation.
Acknowledgment. We thank Dr. M. Hoshino of The Institute of Physical and Chemical Research for the measurement of the laser flash photolysis. Registry No. MV2*, 4685-14-7; ZII(TPPS),~-,95646-69-8; HS(CH&OH, 60-24-2; 1,l’-dipropylviologen sulfonate, 7795 1-49-6.
Potassium Coadsorption Induced Dissociation of CO on the Rh( 111) Crystal Surface: An Isotope Mixing Study J. E. Crowell, W. T. Tysoe, and G . A. Somorjai* Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720 (Received: August 1, 1984)
The formation of 13C1*0 from a mixture of I3Cl6Oand 12C180 proves unequivocally that molecular CO dissociateson Rh( 1 1 1) when potassium is coadsorbed. The presence of a surface complex between the alkali metal and the CO is confirmed by the simultaneous desorption of potassium and CO. A minimum of 0.08 potassium atoms per surface Rh atom is necessary to induce any CO dissociation. A maximum of three CO molecules are observed to dissociate per potassium atom.
Introduction The influence of alkali metals on the chemisorptive properties of co is dramatic. is, in fact, one of the most important and interesting of the ability of additives to modify the chemical properties of surfaces. Previous investigations on the Rh( 111) i and Pt( 111) surfaces2 using electron energy loss spec-
troscopy (EELS) have demonstrated that alkalis cause considerable -carbon-oxygen bond weakening and simultaneous strengthening of the M-CO surface bond. This was concluded from observed changes in the corresponding bond vibrational frequencies, namely, substantial decreases in the C-O stretching frequencies and slight increases in the M< vibrational frequencies. (2) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. Surf. Sci. 1982, 121,
(1) Crowell, J. E.; Somorjai, G. A. Appl. Sur/. Sci., in press.
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303.
0 1985 American Chemical Society
