J . Phys. Chem. 1990, 94, 8594-8598
8594
Sodium Lauryl Sulfate-Ruthenium( I I)Interactions: Photogalvanic and Photophysical Behavior of Ru( II)-Mimine Complexes Tala1 S. Akasheh* and Nathir A. F. AI-Rawashdeh Chemistry Department, Yarmouk University, Irbid, Jordan (Received: November 20, 1989: In Final Form: March 27, 1990)
The photogalvanic and photophysical behavior of a number of mixed-ligand ruthenium(I1) complexes of 2,2’-bipyridine (bpy), 3,3’-bipyridazine (bpd), 24 2’-pyridyl)quinoline (pyq), 4,4’-dimethyl-2,2’-bipyridine(dmbpy), and 2,3-bis(2’-pyridyl)pyrazine (dpp) is reported both in water and in sodium lauryl sulfate (SDS) solutions. The effect of SDS on the photogalvanic experiment is predominantly an enhancement and/or modification of the photochemical and electrochemical processes. Luminescence, lifetime measurements, and photochemical behavior are affected by SDS and are used to predict possible modes of micelle-complex interactions.
Introduction
As the search for new ruthenium( II)-diimine complexes that are capable of solar energy conversion and H2 f~rmationl-~ continues, attempts are made to modify the behavior of such complexes by the use of substitution on the diimine ligand* or through ligand variation.”’ External modification of the photophysical behavior of Ru(l1) as well as Os(1I) photosensitizers can be accomplished by the use of surfactant^.'^-'^ For some time now, we have been involved in the synthesis of Ru(II)-diimine complexes in the hope of discovering new efficient photosensitisers for solar energy conversion. Several new complexes were prepared and s t ~ d i e d . ~ qAmong ~ - ~ ~ other experiments we attempted, the photogalvanic experiment was most attractive because of its simplicity and because it presents the ultimate test for the success or failure of a particular complex. This experiment utilizes electron-transfer quenching of the 3MLCT (metal-to-ligand charge transfer) excited state to set up a potential difference between an electrode kept in the dark and one exposed to light. With Fe3+ as the quencher, the following m e c h a n i s ~ n ’ ~has ~ ~been - ~ ~suggested:
- + -
R ~ ( I I LL ) * R ~ ( I I ) excitation Ru(I1) radiative and radiationless decay *Ru(II) d-d state thermal activation *Ru(ll) Fe(ll1) Ru(ll1) + Fe(l1) oxidative quenching *Ru(lI)
Ru(II1)
+ le-
--
Ru(I1)
electrode reaction
(1) (2) (3)
(4) (5)
Fe(II1) + leFe(l1) electrode reaction (6) From this scheme it is obvious that the success of the experiment requires the optimization of the electron-transfer process (4) in competition with decay (2), quenching by other scavengers (e.g., H 2 0 ) , and the thermal population to the d-d state (3), usually held responsible for degradative photosubstitution reactions. Thus, a long-lived excited state with a large 3MLCT to d-d energy gap (to minimize photochemistry) and favorable redox potentials largely contribute to successful solar energy conversion. Diffusion coefficients and electrode kinetics naturally enter into the picture, and theoretical considerations also require a large difference in the reversibility of the Ru(lIl)/Ru(lI) and Fe(llI)/Fe(II) redox couples as a precondition for a higher p h o t o p ~ t e n t i a l . ~ In ~-~~ addition, it is detrimental that back electron transfer between the photoproduced Ru(Il1) and Fe(l1) species is slow so that the stored energy would be used in steps 5 and 6. This largely depends on how fast these photoproducts are separated from each other.28 The main purpose of this work is to compare the photogalvanic behavior of Ru(l1) complexes and study the effect of photochemistry on such behavior. *To whom correspondence should be addressed.
0022-3654/90/2094-8594%02.50/0
After several ill-fated experiments, we were able to identify only one successful complex, [R~(bpy)(dmbpy)~]~+, and this prompted us to try and modify the behavior of the complexes using the surfactant sodium lauryl sulfate. This also led to the investigation of surfactant effects on photophysical and photochemical behavior of the photosensitizers [Ru(bpy)(dmbpy),]*+, [R~(bpy)(pyq)~] 2+, [ R u ( d p ~ ) ~ ( b p y ) ]and ~ + , [R~(bpy)~(bpd)]*+ (see Figure 1). Experimental Section
All complexes were prepared and purified according to standard ( I ) Lin, C. T.; Sutin, N. J. Phys. Chem. 1976, 80, 97. (2) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (3) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.;Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988.84, 8 5 . (4) Sutin, N.; Creutz, C. In Properties and Reactivities of the luminescent Excited States of Polypyridine Complexes of Ruthenium(Il) and Osmium(10;American Chemical Society: Washington DC, 1979; Adv. Chem. Ser. No. 168. ( 5 ) Nagle, K. K.; Young, R. C.; Meyer, T. J. Inorg. Chem. 1977,16,3366. (6) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim. Acta 1978, 61, 2720. (7) Crutchley, R. J.; Lever, A. B. P. J . Am. Chem. Soc. 1980, 102,7128. (8) Barqawi, K. R.; Llobet, A.; Meyer, T. J. J. Am. Chem. Soc. 1988,110, 7751. (9) Akasheh, T. S.;El-Ahmed, 2.M.Chem. Phys. Lett. 1988, 152,414.
(IO) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem.
1983, 22, 1617. ( I 1) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J . Am. Chem. SOC.1984, 106, 2613. (12) Dressick, W. J.; Hauenstein, Jr., B. L.; Gilbert, T. B.; Demas, J. N. J . Phys. Chem. 1984.88, 3337. (13) Pramauro, E.; Pelizzetti, E.; Diekmann, S.;Frahm, J. Inorg. Chem. 1982, 21, 2432. (14) Miesel, D.; Matheson, M. S.;Rabani, J. J. Am. Chem. SOC.1978, 100, 117. (15) Miyashita, T.; Murakata, T.; Yamaguchi, Y.;Matsuda, M. J . Phys. Chem. 1985,89,497. (16) Dressick, W. J.; Cline, Ill, J.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. SOC.1986, 108, 7567. (17) Ouyang, J.; Bard, A. J. Bull. Chem. Soc. Jpn. 1988, 61, 17. (18) Baxendale, J. H.; Rodgers, M. A. J. J . Phys. Chem. 1982,86,4906. (19) Dressick, W. J.; Raney, K. W.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1984, 23, 875. (20) Jibril, I., Akasheh, T. S.;Shraim, A. M. Polyhedron 1989,8,2619. (21) Akasheh, T. S.; Marji, D.; El-Ahmed, Z. M. Inorg. Chim. Acta 1988, 141, 125. (22) Dad, C.; Haas, 0.;Von Zelewsky, A. J. Electroanal. Chem. 1980, 107, 49. (23) Dad, C.; Haas, 0.;Lottaz, A,; Von Zelewsky, A,; Zumbrunen, H. R.J . Electroanal. Chem. 1980, 112, 51. (24) Dad, C.; Von Zelewsky, A.; Zumbrunnen, H. R. J . Electroanal. Chem. 1981, 125, 307. (25) Albery, W. J.; Archer, M. D. J. Electrochim. Acta 1976, 21, 1155. (26) Albery, W. J.; Archer, M. D. J. Electrochem. Soc. Electrochem. Sci. Technol. 1977, 688. (27) Albery, W. J.; Foulds, A. W. J. Photochem. 1979, 10, 41. (28) Klassen, D. M. Chem. Phys. Lett. 1976, 15, 3166.
0 1990 American Chemical Societv
SDS-Ruthenium(I1) Interactions
w
Q-Q
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8595 I
MCH3 H3
/
Tmin
b
S DS Figure 3. Photopotentials produced by (a) [ R u ( b p ~ ) ( p y q ) ~ ](2 ~ ’X M) and Fe(ll1) ( 5 X M) in 1 N H2S04. (b) Same system but with 0.1 M SDS added. 0 = on; 1 = off. dPp
I
Figure 1. Structure of the ligands and H bonding to bpd with an SDS molecule bound to the complex (see text).
I
I
a
I
0.01r
2hl n
I
I
;.-1---.b
I b
(bin Figure 2. (a) Photopotentials produced by [Ru(bpy)(dmbpy),lZ+ (2 X IOmJM)and Fe(lIl) ( 5 X IO-’ M) in 1 N HzSO,. (b) Same system but with 0.1 M SDS added. 0 = on; I = off.
literature procedure^.^^^^^^**^^ The photogalvanic setup’ consists of a left compartment covered with black tape and a right compartment exposed to the light from an ordinary Phillips slide projector with a 150-W tungsten bulb. A platinum gauze electrode is placed in each compartment, and a small magnetic stirrer is used for effective mixing of each compartment. The output of the electrodes is connected to an Hitachi 056 stripchart recorder. Solutions were argon-degased for 20 min before use, and the cell was thermostated by running water. The solutions consisted of the complex (1 0-5 M) with ferric ammonium sulfate (- IO-’ M) in 1 N H2S04.The experiment was also performed in a specially designed 1-cm fluorescence cuvette, to enable detection of any chemical changes in the solutions by taking emission spectra before and after exposure to the tungsten light during a photogalvanic experiment. Fluorescence on a Perkin-Elmer MPF-54B spectrofluorometer, UV-vis on a Cary 2390 series spectrometer, and lifetime experiments on an Edinburgh Instruments Ltd. Model 199 photon-counting system were performed on the appropriate solutions. Fluorescence and absorption changes were followed after exposure t o the output of a 200-W xenon lamp with an Applied Photophysics quarter meter monochromator set at 450 nm. All our photogalvanic results were optimized and checked by using the well-known photogalvanic photosensitiser [Ru(bpy)’12+, which under conditions similar to ours is known to give a photopotential of about 0.14 V.’ (29) (a) Ernst, S. D.; Kaim, W.Inorg. Chem. 1989, 28, 1520. (b) Nakamuru, Bull Chem. Soc. Jpn. 1982.55, 2691. (30) Alper, H.; Wollowitz, S. J . Am. Chem. Soc. 1975, 97, 3543.
-
4min
Figure 4. Photopotentials produced by (a) [ R ~ ( d p p ) ~ ( b p y ) ](2 ~ ’X IO” M) and Fe(II1) ( 5 X lo-’ M) in 1 N HfiO,. (b) Same system but with 0.1 M SDS added. 0 = on; 1 = off. TABLE I: Luminescence Maxima Shifts in Aqueous SDS Solutions L/nm complex in H20 in SDS/HzO [ W b P Y ) ( P Y ~ ) Z I ( P F ~ ) Z * ~ H Z ~688 700
[ R ~ ( ~ ~ Y ) ( ~ ~ ~ P Y ) z I ( P F ~ ) , 628 .HzO [R~(~PY)(~PP)Z](PF~)~.~HZO 657 [Ru(bPY)i(bpd)l(PF6)2.HzO 691
644 666 107
Results The Photogalvanic Effect. Figures 2-4 show the photopotentials produced by [R~(bpy)(dmbpy)~]~+, [Ru(bp~)(pyq)~]~+, and [ R ~ ( b p y ) ( d p p ) ~ ]respectively. ~+, [ R ~ ( b p y ) ~ ( b p d ) gives ]~+ traces similar to those of the dpp complex. Each figure includes traces with and without SDS. The trace shows the photopotential I, obtained upon first time exposure of the solution to light, followed by the photopotential on-off cycles (denoted by 0 for on and 1 for off). Ideal behavior is observed for aqueous [Ru(bpy)(dmbpy),lZ+ in Figure 2a, where a photopotential of 0.19 V is observed with high reversibility (Le., the potential goes back to the initial value at the end of every cycle) and instantaneous response. SDS reduces the photopotential (=43 mV) as well as the response time. Initially aqueous [ R ~ ( b p y ) ( p y q ) ~ (Figure ]~+ 3) gives a reasonably high initial photopotential, but this decays and reverses in the opposite direction. On turning off the light a small change is observed, and the on-off cycles show less dramatic photopotentials. SDS, though giving less photopotential, leads to more reversible as well as more sluggish response. The dpp (Figure (4)) and bpd complexes differ from [ R u ( b p ~ ) ( p y q ) ~ mainly ]~+
8596
Akasheh and AI-Rawashdeh
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
r
TABLE 111: Relative Emission ( [ / I o ) Intensity after Photochemical IrradiationD
'h
complex
''zM 1 medium
H20 H2S04
H 2 0 , Fe(ll1) H2S04, Fe(l1l) H20, SDS H2SO4, SDS H 2 0 , Fe(III), SDS H2S04,Fe(III),SDS
0.8
0.4
0.01
3
8
1
I
'
'
'
I
0.000 0.024 0.048 0.072 CS D SI Figure 5. Effect of added SDS on the luminescence intensity ( l n / l vs [sbsl for aqueous solutions Of [Ru(pyq)2(bpyji2+-i-); [ R ~ ( b p y ) ~ ( b p d ) (-0-); ] ~ + [ R ~ ( d m b p y ) ~ ( b p y ) ](-X-). ~+ Io is intensity without SDS; I is with added SDS. A,, = 468 nm. A,, = maximum emission wavelength for each case.
TABLE 11: Lifetimes in SDS (nsb [SDSl/M comDlex 0 0.001 0.002 0.005 0.007 0.01 0.05 [ R u ( ~ P Y ) ( P Y s ) ~ ~ ~I57 ' 1 15 187 203 [Ru(bPy)(dPP),2t 208 182 198 239 123 117 116 [ R U V J P Y ) ~ ( ~ @ ) I ~ ' 1 10 [Ru(bpy)(dmbpy),12+ 342 325 430 435
in the lack of reversal in the signal. Spectral Shifts. In Table I, a red shift in the emission wavelength maxima at room temperature is observed in SDS. The absorption spectra however, are only slightly (if any) shifted by SDS (usually less than 1 nm, rarely reaching 2 nm). Such observations are very common,'6,'8,19,31-33 and generally a stabilization of the 3MLCT state with respect to the ground state and an increase in the 3MLCT to d-d gap is concluded.I6 Emission Intensities and Lifetimes. The effect of SDS on emission intensities varies from one complex to the other. Figure 5 shows a plot of l o / I , the intensity ratio at A,, vs [SDS] (Io is without SDS). Only three complexes are shown since [Ru(bpy)(dpp)J2+ behaves similarly to [ R u ( b p ~ ) ( p y q ) ~ ]and ~+ [Ru(bpy)(dmbpy),I2+ in the sense that the emission intensity decreases suddenly to a minimum at very low SDS concentrations, starts to increase rapidly, reaches a transition point near the SDS critical micelle concentration (cmc = 0.008 M), and steadies off at a value higher than that of the SDS-free solution. The lifetimes (Table 11) vary in a manner consistent with this trend. [Ru( b p ~ ) ~ ( b p d )shows ] ~ + smaller intensity variation with SDS concentrations but follows an opposite trend in luminescence intensities and lifetimes. Photochemical Changes. The role of photochemistry was studied by investigating the photoactivity of the complexes. Irradiation of [ R u ( b ~ y ) ~ ( b p d ) and ] ~ + [ R u ( b p ~ ) ( d p p ) ~ for ] ~ +3 h leads to no photochemistry. In Table I11 the variation of maximum Io = intensity at 0 time) for the other emission intensity (l/lo; two complexes in different media is shown. [Ru(bpy)(dmbpy),lZf is appreciably photoactive only in aqueous SDS solutions. [Ru( b p ~ ) ( p y q ) ~shows ] ~ + the most drastic changes (in intensity but not in spectral shape) where H2S04speeds up the reaction, while Fe( 111) and SDS offer good protection against photochemistry (31) Kaizu, J.; Ohta, H.; Kobayashi, K.; Kobayashi, H.;Takum, K.; Matsuo, T. J. Photochem. 1985, 130, 93. (32) Mandal, K.: Hauenstein, Jr., B. L.; Demas, J. N.; DeGraff, B. A . J . Phys. Chem. 1983, 87, 328. (33) Dressick, W. J.: Demas, J. N.; DeGraff, B. A. J. Photochem. 1984, 24, 45.
(34) Williams. R. J.: Phillips, J. N.: Mysels, K. 1. Trans. Faraday SOC. 1955, 51, 728.
[RU(bPY)(PYS)2I2+ [Ru(bPY)(dmbPY)212+ (Illnafter 3 h) ( I / l n after 100 min) 0.94 1 .o 1 .o 0.48 b b 0.91 0.82 0.55 0.66 0.98 0.52 1 .o 1 .o
1 .o
0.59
Io and I are emission intensity before and after irradiation, respectively (Ae,,, = maximum emission wavelength). Irradiation wavelength [SDS] = 0.05 M, [Fe(lll)] = 5 X = 450 nm. [Ru(Il)] = 2 X M, [H2S04]= 0.05 M. All solutions were argon bubbled for 20 min before use. bNot enough fluorescence to allow observation.
TABLE IV: Relative Emision Intensity ([/Io) after the Photogalvanic Experiment I / Io (SDS free)
complex''
[ W b P Y )(PYCl)212+ [ W b P Y NdmbPY )*I2+ [RU(~P~)(~PY)~I~+ [ W b P Y )(dPP),l 2+
1/10
(0.1 M SDS)
0.45
0.3 1 0.82
1.2c
0.85c
1 .o 1 .Ob
0.70b
"Ru(I1) = 2 X M, Fe(I1l) = 1 X M, [H2S04]= 0.05 M; I,, I = initial and final intensity at the maximum emission wavelength. A,, = 468 nm. Time of exposure to the photogalvanic experiment = 60 min except b was for 100 min, and c was for 80 min.
l m
~
560
~~~
640
720
800
hlnml Figure 6. Luminescence spectrum of 2 X M aqueous [ R ~ ( p y q ) ~ (bpy)I2+with Fe(ll1) (1 X IO-' M) and 1 N H 2 S 0 4(a) before and (b) after I-h exposure to white tungsten light during the photogalvanic experiment.
whenever added individually to any medium. Photoelectrochemistry. Any permanent changes that occur during the photogalvanic experiment were observed, and Table 1V shows the emission intensity ratio (I/Io;Io is before the experiment, and I is after a period of irradiation with the tungsten lamp while current is withdrawn by the strip-chart recorder). SDS enhances the reaction. No new spectral features are observed for all complexes in aqueous or SDS media except for [Ru(bpy)(pyq)J2', where a blue-shifted feature appears in the presence of SDS (Figures 6 and 7 ) .
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8597
SDS-Ruthenium(I1) Interactions
n?
L
3
I
640
I
1
720
I
80 0
A("
Figure 7. Same as Figure 6 but with 0.1 M SDS.
Discussion Spectral Shifts. Electrostatic binding of the complex to the SDS micelle surface occurs with possible partial penetration of the hydrophobic ligands into the hydrophobic micellar core.1619 The excited electron in the )MLCT state is localized on one ligand,35 which may point toward the water pool, thus resulting in an excited state that is more polar than the ground state. The environmental reorganization that follows stabilizes the )MLCT in comparison with the ground state and the emission is red shifted (Table II).16 Absorption is too fast for the reorganization, and less dramatic shifts occur.I6 Emission Intensities and Lifetimes. The nature of the aggregates formed between SDS and the complex below and above the cmc determine the variation of emission intensities (Figure 5 ) and lifetimes (Table 11) with SDS concentrations. Furthermore, the fact that water is a good quencher of the excited )MLCT compared to organic solvents plays an important r ~ l e . ' ~ . ~ ~ At concentrations much below the cmc, SDS molecules bind to the complex to form small premicellar aggregates. The loose SDS tails add vibrational degrees of freedom, and radiationless decay is enhanced with reduction in intensities and lifetimes. Quenching by water is still as effective as in water. As the concentration increases, the aggregates (with more SDS molecules) are bulkier and the hydrophobic SDS tails may partially surround the complex. This protects from quenching by water so that both intensities and lifetimes start to increase; above the cmc each complex molecule is attached to a micelle surface. The complex, partially buried in the hydrophobic core, is better protected from water, and a maximum in intensity and lifetime is reached. Increasing SDS increases the micelles but has no further effect so that little variation (within experimental error) is observed. (35) (a) Baraqawi, K. R.; Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips. G. 0.J . Phys. Chem. 1988,82,291. (b) Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips, G. 0. J . Phys. Chem. 1986, 90, 5651. (c) Braterman, P. S.; Heath, G. A.; Yellowlees, L. J. J . Chem. Soc., Dalton Trans. 1985, 1081. (d) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S.J . Chem. Soc.,Chem. Commun. 1981. 287. (e) Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. Soc. 1979,101,4391. (0 Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981,81,42. (g) Myrick, M. L.; Blakely, R. L.; DeArmond, M. K.; Authur, M. L. J . Am. Chem. SOC.1988, 110, 1325. (h) Kumar, C. V.; Barton. J . K.; Could, 1. R.; Turro, N. J.; Van Houten, J. Inorg. Chem. 1988, 27, 648. (i) Tait, C. D.; Donohoe. R. J.; De Armond, M. K.; Wertz, D. W. Inorg. Chem. 1987, 26, 2754. (j) Danielson, E.; Lumpkin, R. S.; Meyer, T. J . J . Phys. Chem. 1987. 91, 1305.
While three of our complexes follow the above trend, [Ru( b p ~ ) ~ ( b p d )behaves ] ~ + in an opposite manner but with smaller variations. A possible explanation may be sought in the fact that the nonbonded nitrogens on bpd could be available for H bonding with water at sites very close to the metal center (see Figure 1). The clefts between the metal and the ligand are large enough to be penetrated by water molecules.12 However on the side of bpd, the H-bonded solvent might prevent such penetration. On the other sides of the metal, SDS tails in the premicelles (Figure 1) assist in this blockage. If one assumes penetration of the clefts by water is a precondition for quenching, both H bonding and aggregation to SDS protect from quenching compared to aqueous solutions and both intensities and lifetimes increase. Above the cmc the excited electron in the micelle-bound )MLCT is localized on bpd, which has lower T* orbitals than b ~ y . )Thus ~ the bpdmoiety protrudes into the water pool. The negatively charged micellar surface could compete with H bonding on bpd, thus allowing penetration of water into the clefts, enhancing water quenching, and decreasing the intensity. The increase in lifetimes above the cmc is within experimental error (&lo%) and is not in conflict with this argument. Although in [ R u ( b p ~ ) ( d p p ) ~ ] ~ + H-bonding is possible on dpp, the nonbonded nitrogens are far removed from the metal. Thus, this complex behaves similar to cases where no H bonding is possible. Photochemical Changes. The higher d-d to 3MLCT energy gap resulting from SDS-complex interactions above the cmc is expected to reduce ligand photosubstitution due to reduced thermal population of the photolabile d-d state ()MLCT d-d). Table 111 shows exactly the opposite effect. The longer lifetimes in SDS and the protection against water quenching actually favor thermal population to the d-d state, and both [ R u ( b p ~ ) ( p y q ) ~ and ]~+ [R~(bpy)(dmbpy)~]~+ are much more photoactive in SDS. Within experimental error [R~(bpy)(dmbpy)~]~+ remains photoinactive in all other media. The addition of H2S04 to the SDS solution reduces the photochemistry in [R~(bpy)(dmbpy)~]~+ since protons can bind to the micellar surface in competition with the complex, which becomes more exposed to water quenching and less liable to thermal population. By contrast, acidity inexplicably enhances photochemistry in [ R ~ ( b p y ) ( p y q ) ~(regardless ]~+ of other additives). A pH-dependent mechanism could be the cause. Effective quenching of the )MLCT by Fe(II1) competes very well with thermal population, and this reduces photosubstitution. This is particularly effective in the presence of SDS. Obviously binding of Fe(II1) in close proximity to the )MLCT on the micellar surface enhances quenching and [ R u ( b p ~ ) ( p y q ) ~becomes ]~+ completely photoinactive. Photogalvanic Effect and Photoelectrochemical Reactions. During the photogalvanic experiment redox and photochemical degradative reactions may occur. Table IV indicates that no changes occur to [Ru(bp~)(dmbpy)~]~+ in water. This is confirmed by the highly reversible on-off photopotential cycles (Figure 2a). A high difference in reversibility between the Ru(III)/Ru(II) and Fe(III)/Fe(II) couples is also implied.25-27The excited-state redox potentials for this complex are almost the same as for [Ru(bpy))12+, and hence the success encountered here is not surp r i ~ i n g .The ~ ~ addition of SDS induces a photoelectrochemical change (Table IV) in the complex that is also photoinactive (Table 111) in the same medium. This reaction may result from SDSenhanced quenching by Fe(III), changes in redox couple reversibilities, enhanced electrode reactions, and even a change in the reduction potential of the )MLCT state (reaction 4). The resulting photopotential (Figure 2b) is smaller and less reversible due to photoelectrochemistry and enhanced back electron transfer between Fe(I1) and Ru(II1) held together on the micellar surface.28 The redox potentials for the excited states of the other three complexes are less favorable than for [ R ~ ( b p y ) , ] ~ +but , ) ~ the behavior of each complex is best explained alone. The photogalvanic trace for [ R u ( b p y ) ( ~ y q ) ~in] ~water + (Figure 3) and the emission intensities following the photogalvanic experiment are consistent with each other, since large photochemical (Table 111:
-
(36) Akasheh, T. S.; Jibril, 1.; Shraim, A . M., unpublished results.
J . Phys. Chem. 1990, 94, 8598-8603
8598
H2S04, Fe(II1)) and photoelectrochemical changes are indicated. The resulting trace shows a large initial photopotential followed by a large reversal of the signal that becomes steady due to a new product. This product gives smaller, more reversible photopotentials during the later on-off cycles. SDS reduces the photopotential by causing an increase in product formation (Table IV), and better reversibility is obtained. The explanation for this behavior is similar to that for [R~(bpy)(dmbpy)~]~+. The different photogalvanic product implied in Figure 7 hints at some unknown modification of the electrochemical reactions in SDS. Permanent changes do not seem to occur for [R~(bpy)~(bpd)] in water. Yet the photogalvanic trace does indicate an initial chemical change, and a steady-state intermediate subsequently gives negligible photopotentials. However, on stopping the experiment, the reaction is reversed and the emission spectrum before and after the experiment is of the same shape and intensity. Again SDS enhances permanent photoelectrochemical change, and for this reason the photogalvanic trace exhibits a larger slowly reversible signal (than in water). [ R ~ ( b p y ) ( d p p ) ~ (Figure ]~+ 4) behaves similar to [ R ~ ( b p y ) ~ ( b p d )except ] ~ + that Table IV shows a permanent photoelectrochemical reaction with an increase in emission intensity in water, while SDS causes a decrease in intensity. This could be taken as an indication of different products being formed in the two media. Conclusion
In water, high reversible photopotentials were obtained for [Ru(bpy)(dmbpy)J2+. The other three complexes are plagued
by unfaborable ,MLCT redox potentials,36photochemical and/or photoelectrochemical changes, and possibly by a less reversible Ru(III)/Ru(II) redox couple than is known for [Ru(bpy)J2+. The previously expected SDS-induced protection against photochemistry, due to less thermal population of the photolabile d-d state, is overcome by protection against quenching of the 3MLCT state. The enhanced quenching by the micelle-bound Fe(II1) and the competition between protons and complex for adherence to the micelle play an important role as well. The same factors play an equally important role in shaping the outcome of the photogalvanic experiment in SDS. In addition to such factors photoelectrochemical changes and enhanced back electron transfer between micelle-bound Fe(I1) and Ru(II1) species contribute to the poor phototraces of [R~(bpy)(dmbpy)~]~+. In the three remaining complexes, SDS slightly enhances the photopotential traces due to enhanced formation of products, which seem to be responsible for the improvement.
Acknowledgment. Financial support by the Ministry of Planning, Jordan, the Kuwait Development Fund, and Yarmouk University Research Fund is deeply appreciated. This work is part of N.A.R.'s Master Thesis. Registry No. [R~(bpy)(pyq)~](PF~)~, 129448-55-1; [Ru(bpy)124272-94-2; (dn~bpy)~](PF,),,99666-66-7; [R~(bpy)(dpp)~](PF~)~, [ Ru(bpy)i( bpd)](PF6)2, 126290-34-4; [Ru(bpy)(pyq)2]2t, 74 1 7 1-83-8 ; [Ru( bpy)(dmb~y)~] 2t, 97 135-47-2; [ Ru(bpy)(dpp)2]2t, 124272-93- 1; [R~(bpy)2(pyq)2]~*, 119743-37-2; SDS, 151-21-3.
Valence States of Vanadia-on-l?tania/SHka and Molybdena-on-Silica Catalysts after Reduction and Oxidation J. J. P. Biermann, F. J. J. G. Janssen,* Joint Laboratories and Other Services of the Dutch Electricity Supply Companies, R& D Division, Chemical Research Department, P.O. Box 9035, 6800 ET Arnhem, The Netherlands
and J. R. H. Ross Department of Chemical Technology, University of Twente, P.O. Box 21 7 , 7500 AE Enschede, The Netherlands (Received: January 16, 1990; In Final Form: June 14, 1990)
The behavior of vanadia-on-titania/silica and molybdena-on-silica catalysts during reduction by ammonia and oxidation by nitric oxide or oxygen at elevated temperatures was studied. The heat evolved during adsorption, reduction, and oxidation was measured by means of a differential scanning calorimeter. Ammonia reduces the catalysts to a certain extent under the formation of N2 and HzO.On the basis of the heat measured, the amount of reduced species was calculated. The reduced producing an amount of heat that corresponds to the respective reactions. The valence catalysts were oxidized by NO and 02, changes of V(V) to V(1V) and Mo(V1) to Mo(1V) were confirmed by means of X-ray photoelectron spectroscopy. The results of the differental scanning calorimetry measurements were used to calculate initial apparent rates of reduction and of oxidation. It was found that the rate of oxidation by O2was about 5 times higher than that by NO. Moreover, the initial apparent rate of reduction by NH, of the molybdena catalyst was significantly higher than that of the vanadia catalyst. These results were used to explain the differences in reactivity of the gas mixture of NO, NH,, and O2over the catalysts; namely, NO does not participate in the reaction over the molybdena catalyst, while it does over the vanadia catalyst. 4NH3 + 4 N 0
Introduction
The selective catalytic reduction (SCR) of NO, is one of the options for postcombustion control of NO, emissions. Of the possible reducing agents such as NH3, H2, CO, and CH4, only ammonia was found to be selective for the reduction of NO, in atmospheres containing excess oxygen.' The mechanism of the overall reaction of NO, NH,, and O2 (reaction 1) over vanadia catalysts has been studied by using la be led molecules .293
*To whom correspondence should be addressed. 0022-3654/90/2094-8598$02.50/0
+ O2
-
4N2 + 6 H 2 0
(1)
Two types of active sites were present on the surface of the catalyst. The sites were assigned to vanadium species with valences 4+ and 5+. Two different reactions occurred on these sites. Ammonia reduces the oxidized sites, while reoxidation was brought (1) Bosch, H.; Janssen, F. J. J. G . Coral. Today 1988, 2 (4). 369.
(2) Janssen, F. J. J. G.; van den Kerkhof, F. M. G.; Bosch,
R. H. J . Phys. Chem. 1987, 91. 5921.
(3) Janssen, F. J. J. G.; van den Kerkhof, F. M. R . H. J . Phys. Chem. 1987, 91, 6633.
0 1990 American Chemical Society
H.;Ross,J.
G.;Bosch, H.; Ross, J.
