J . Phys. Chem. 1986, 90, 6028-6034
6028
-
followed by agglomeration of the M n 0 2 particles formed n M n 0 2 (Mn02)" (5) or, via
-
2[Mn(HzO)4(OH)21' [Mn(H20)4(OH)OMn(H20)4(OH)]2+ + H 2 0 (6) followed by [Mn( H 2 0 ) 4 ( 0 H ) O M n (H 2 0 ) 4 (OH)] 2+
-
[ M I I ( H ~ O ) ~ ]+~ M + n 0 2 + 3 H 2 0 (7)
and reaction 5 . Equations 4 and 5 describe a process where the disproportionation occurs between two primary product molecules and is then followed by the agglomeration of MnOz formed. On the other hand, as described by eq 6 and 7, disproportionation may take place after condensation. It is even conceivable that condensation of more than two Mn(II1) species takes place before disproportionation. The induction period observed in the pulse experiments (Figure 7) may be caused by condensation to a certain size until disproportionation takes place. On the other hand, the mechanism of eq 4 and 5 also could explain the induction period, assuming that M n 0 2 particles of smaller size do not absorb. These reasons may also be given to explain the nonlinear behavior of the curves in Figure 2. Manganese(II1) oxide would according to these mechanisms be formed if condensation occurs much more rapidly than disproportionation. At a high degree of condensation, i.e., when colloidal manganese(II1) oxide is formed, there is no dispropor-
tionation. (On the contrary, in MnOl colloid containing Mn(I1) ions, even conproportionation takes place.6) According to the mechanism of eq 6 and 7, condensation would be a process of an order higher than one, while disproportionation would be a first-order-like reaction as the rate would probably not depend on particle size. High intensity of radiation should lead to a higher ratio of rates of condensation to disproportionation, Le., to a colloid of higher Mn(II1) content. This may explain why the colloid formed in neutral solution by pulse radiolysis (Figure 7, inset) contains Mn(III), while that produced by y-irradiation (Figure 1) consists of pure Mn02. Furthermore, the presence of OH- ions (pH 8.8 in Figure 9) seems to promote efficiently the condensation process as practically pure manganese(II1) oxide is produced in pulse radiolysis. This effect may be explained by OH- addition to the [Mn(H20)4(OH)OMn(H20)4(OH)]2+species (eq 6) or/and of higher condensation products. This also explains the decrease in conductivity during the formation of manganese(II1) oxide. An even better promotion of condensation or inhibition of disproportionation is caused by hexametaphosphate as manganese(II1) oxide is formed also in neutral solution (Figure 3).
Concluding Remarks In this work, the nature of the first product of oxidation of manganese(I1) by hydroxyl radicals was investigated in more detail than in the previous studies. The nature of the final colloidal products was also recognized. It was not possible to formulate in detail the mechanism for colloid formation, although we think that our more general outline has led to some understanding of the processes involved.
Photoassisted Platinum Deposition on TIO, Powder Using Various Platinum Complexes Jean-Marie Herrmann,* Jean Disdier, and Pierre Pichat Ecole Centrale de Lyon, Equipe CNRS Photocatalyse,t BP 163, 691 31 Ecully Cedex, France (Received: December 26, 1985)
The photocatalytic deposition of metallic platinum has been carried out with powder titania in aqueous suspension containing different complex solutions (chloroplatinic acid, sodium chloroplatinate, hexahydroxyplatinic acid, and platinum-dinitrodiammine). The deposition rate was found identical for the three first complexes and much more lower for the last one which is not ionic. In all cases the removal of the Pt ions from the solutions was achieved to the detection limit (1 ppm). Analyses of the solid, liquid, and gas phases indicated that the reduction of a (Pt"') complex ion induced, as expected, the release of n protons in the solution, whereas oxygen evolution from water oxidation was completely blurred by an initial photodesorption of preexisting ionosorbed oxygen species, followed by the dissociative chemisorption of O2on the Pt crystallites formed in the process. A mechanism is proposed from the effects of various parameters (initial concentration, light flux, temperature). An initial quantum yield of about 0.05 was calculated. Transmission electron microscopy showed that platinum deposits initially as small crystallitesof 1 nm diameter distributed on all the particles of titania. For higher loadings and longer illumination times, large agglomerates form; as a result, most of semiconductor surface remains accessibleto the photons so that the activity for the deposition does not decline. Pd, Ag, Rh, Au, and Ir have also been deposited on TiO, whereas Ni and Cu were not. Preliminary experiments indicated that ZnO, Nb205,and Tho2, for instance, can be used instead of Ti02.
Introduction
The photocatalytic deposition of noble metals on photosensitive semiconductors under the shape of single crystals, films, electrodes, or powders is a well-known phenomenon. As examples of metals deposited on metal oxide semiconductors, one can cite the following couples: Pd/Ti02,'-5 Pt/Ti02,3p6-9Ag/Zn0'&I4 or Ti02,12J5-18 Cu/Ti02 or wO3,l9Hg/ZnO" or Ti02.20 As our study was in progress,21a paper reported the deposition of gold on TiO, and W03.22 This method has been employed as a means of preparing metal catalysts3s5or photocatalyst~~~'' and suggested as a potential way of metal recovery from aqueous effluents. Despite these studies, certain points of the reaction mechanism deserve to be elucidated. J.E.CNRS 4594. 0022-3654/86/2090-6028$01 S O / O
Since ZnO samples are generally unstable in illuminated aqueous solutions and since W 0 3 specimens are generally much (1) Mollers, F.; Tolle, H. J.; Memming, R.J. Electrochem. SOC.1974, 121, 1160 and references therein. (2) Kelly, J. J.; Vondeling, J. K. J. Electrochem. SOC.1975, 122, 1103. (3) Dunn, W. W.; Bard, A. J. N o w . J . Chim. 1981, 5 , 651. (4) Yoneyama, H.; Nishimura, N.; Tamura, H. J. Phys. Chem. 1981,85, 268. (5) Stadler, K. H.; Boehm, H. P. Proceedings of the 8th International Congress on Catalysis, Berlin; Verlag Chemie: Weinheim, FRG, 1984; Vol. IV, p 803. (6) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 4317. (7) Koudelka, M.; Sanchez, J.; Augustynski, J. J . Phys. Chem. 1982,86, 4277. (8) Curran, J.; Domenech, J.; Jaffrezic-Renault, N.; Philippe, R. J . Phys. Chem. 1985, 89, 957. (9) Sato, S. J . Catal. 1985, 92, 11.
0 1986 American Chemical Society
Pt Deposition on T i 0 2 Powder
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 6029
less photocatalytically active than titania, we have chosen a TiO, sample for this study. The photoassisted deposition on TiOz of platinum from four complexes has been investigated through the effects of various parameters (initial concentration of the solution; duration and intensity of the illumination; presence or absence of air; temperature). A careful examination of the material balance has been carried out by analyzing the solid, liquid, and gas phases. In particular, the oxygen evolution, which was not that expected, is interpreted by desorptionadsorption phenomena. Transmission electron microscopy has been used to determine the texture of the platinum deposits. A mechanism is proposed. Experimental Section Materials. Ti02was nonporous Degussa P-25, mainly anatase, with a surface area of ca. 50 m2 g-'. Hexachloroplatinic acid (Merck), sodium hexachloroplatinate (Johnson-Matthey), and platinum(II)-dinitrodiammine and hexahydroxyplatinic acid (Comptoir Lyon-Allemand, Louyot) were reagents of high purity. Photoreactor. The deposition experiments were carried out in a Pyrex flask of ca. 100 cm3 with an optical window base receiving a radiant intensity of ca. 60 mW cm-2 from a Philip HPK 125-W mercury lamp. This photoreactor could be connected by stopcocks and metallic valves to a rotary pump or to a gas chromatograph. The slurry was agitated by a magnetic stirrer. For experiments performed at various controlled temperatures (between 277 and 351 K), a jacketed, but otherwise identical, photoreactor associated with a Huber HS40 thermostat was employed. Procedure. Ti02 (50 mg) was suspended in 10 cm3 of solution which enabled a complete absorption of the light entering the reactor. The gas phase was evacuated. The suspension was then stirred under static vacuum for 15 min in order to allow physisorbed or dissolved gases to be evolved in the gas phase which was subsequently evacuated. No oxygen was usually detected by gas chromatography after this procedure. During these operations the photoreactor was wrapped in an aluminum foil to avoid any illumination by ambient light. Analyses. The basic reaction for the deposition of a n-valent cation M"+ being theoretically given by
M"'
+ (n/2)H20
-
Mo + nH+ + (n/4)O2
(1)
the reaction rate r is formally equal to r = -d[M"+]/dt = d[Mo]/dt = ( l / n ) d[H+]/dt = ( 4 / 4 d[O,l/dt
(2)
Consequently the kinetics was followed by measuring (i) the quantity of dissolved cations, (ii) the amount of deposited metal atoms, (iii) the pH of the solution, or (iv) the pressure of oxygen evolved in the gas phase. We did not succeed in developing an (IO) Korsunovski, G. A. Russ. J. Phys. Chem. 1965, 39, 1139. (11) Oster, G.; Yamamoto, M. J . Phys. Chem. 1966, 70, 3033. (12) Fleischauer, P. D.; Kan, H. K. A,; Shepherd, J. R. J . Am. Chem. Soc. 1972, 94, 283. (1 3) Hada, H.; Tanemura, H.; Yonezawa, Y. Bull. Chem. SOC.Jpn. 1978, 51, 3154. (14) Gonzalez-Elipe, A. R.; Soria, J.; Munuera, G. J. Caral. 1982, 76, 254. (15) Clark, W. C.; Vondjidis, A. G. J . Catal. 1965, 4, 691. (16) Hada, H.; Yonezawa, Y.; Saikowa, M. Bull. Chem. SOC.Jpn. 1982, 55, 2010. (17) Hada, H.; Yonezawa, Y.; Ishino, M.; Tanemura, H. J . Chem. SOC., Faraday Trans. 1 1982, 78, 2677. (18) Nishimoto, S.; Ohtani, B.; Kajiwara, H.; Kagiya, T. J . Chem. SOC., Faraday Trans. 1 1983, 79, 2685. (19) Reiche, H.; Dum, W. W.; Bard, A. J. J . Phys. Chem. 1979,83,2248. (20) C k h e t , P.; Martelet, C.; Martin, J. R.; Olier, R. C.R.Acad. Sci. 1978, 287, 405. (21) Herrmann, J. M.; Disdier, J.; Pichat, P.; Jaffrezic-Renault, N. J . Electrochim. Florence (Iraly) May 1985. (22) Borgarello, E.; Harris, R.; Serpone, N. Nouv. J . Chim. 1985, 9, 743. (23) (a) Lehn, J. ML.; Sauvage, J. P.; Ziessel, R. Nouv. J . Chim. 1980, 4, 623. (b) Duonghong, D.;Borgarello, E.; Grltzel, M. J. Am. Chem. SOC. 1981, 103, 4685. (c) Mills, A,; Porter, G. J. Chem. SOC.,Faraday Trans. I 1982, 78, 3659. (d) Biihler, N.; Meier, K.; Reber, J. F. J . Phys. Chem. 1984, 88, 3261, 5903.
0
2
4 Cx103 m ~ l . d m - ~
Figure 1. Adsorption isotherm in the dark of F'tC12- from solutions of Na,PtCl, (open circles) and H,PtCl, (black circles).
in situ analytical method. The submicronic Ti02grains strongly perturb the electrochemical response of specific electrodes or of a conductivity cell in a way which differs in the presence or absence of UV light. Spectrophotometric methods were also inoperative because of the presence of some colloidal titania. To determine the amount of Pt deposited on Ti02,the suspension was centrifuged and the solid, once washed three times in distilled water, was dissolved in a mixture of aqua regia and HF. The metal ions in the supernatant solution or those resulting from the dissolution of the solid were analyzed by atomic absorption spectroscopy using a nitrous oxide-acetylene flame.24
Results and Discussion I . Preliminary Remarks. i. Light Absorption by the P t Complex. A standard concentration of metal of M (=200 ppm) was selected to avoid too strong a light absorption by the M H2PtC16 solution exhibited an reactant. For instance a intense H absorption peak at 420 nm mentioned in ref 25 and no transmittance below a wavelength of 380 nm which just corresponds to titania's absorption threshold. However, when the concentration was decreased from to M, a substantial transmittance for h > 310 nm was observed. With an optical pathway of 1 cm of solution, the transmittance is equal to 40% at 385 nm. Experimentally, since the photodeposition reaction occurs on the Ti02 grains when they are close to the bottom optical window, the light absorption by the reactant is much smaller than that of the powder. Chromatographic analysis of the gas phase indicated that the homogeneous photochemical reaction corresponded to a conversion smaller than 1% after illuminating for several hours. ii. Adsorption in the Dark. Adsorption isotherms for H2PtC16 and Na2PtC1, have been determined for concentrations ranging from lo4 to 5 X lC3M. A suspension of 50 mg of TiOz in 20-cm3 solutions of various concentrations was stirred in the dark for 15 min. It was then centrifuged and the concentration of the remaining complex molecules was measured by atomic absorption spectroscopy. The adsorption isotherms for both reactants (Figure 1) present a niaximum for a final steady-state concentration close M. This maximum of adsorption is equal to 1.6 X to 6 X lo4 mol g-' which corresponds to a coverage of 2 X 10l8molecules m-2. This value is close to half the maximum coverage of titania by O H groups (5 X lo1*m-2) which could be indicative of the necessity of two hydroxyl groups for the adsorption of one chloroplatinic complex. At present we have no explanation for the decrease in the coverage for higher concentrations. Maybe the increase in concentration modifies the ionic equilibria of adsorption and thence the adsorption site density at the surface of titania. This chemisorption, which is a prerequisite of the photodeposition, is of an ionic type in relation with the OH surface groups, since (24) Urbain, H.; Cattenot, M. Analusis 1979, 7, 196. (25) Borgarello,E.; Pelizetti, E.; Mulas, W. A.; Meisel, D. J. Chem. SOC., Faraday Trans. 1 1985, 81, 143.
6030 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
r
Herrmann et al.
H*
60
10
In
. C
-
-
j I/--
J
-
0
0
E
E &
.
r. 20
5
20
t / h
0
Figure 3. Kinetics of photodeposition of Pt from M: sodium chlo02, and Pt as indicated. roplatinate. Variations of Ht,
6
Figure 2. Kinetics of photodeposition of Pt from chloroplatinic acid (C, = 10” M, mTiOl= 50 mg): variations vs. time of the amount of Ht, 02, and Pto produced as indicated (amount of photodeposited Pt determined by analysis of the supernatant solution (black circles) and by analysis of the solids (black squares)).
a decrease of the pH of the solution is observed when adding titania. 2. Photocatalytic Deposition of Platinum from Various Complexes. 2.1. PtCI6’- Complexes. Chloroplatinic Acid. As shown in Figure 2, most of the platinum is removed from the solution after illuminating for 1 h. There is a small discrepancy of unknown origin between the analyses of the solution and of the solid. The variations of the proton concentration parallel those of removed platinum with a ratio [H+]/[Pto] close to 6, in agreement with the stoichiometry of the equation H2PtC16 4- 2 H z 0
-
Pto + 6H+
+ 6C1- + 0 2 ( g )
(3)
Sodium Chloroplutinute. Similar kinetics curves (Figure 3) were obtained. Analyses of removed and deposited platinum are in close agreement. The ratio [H+]/[Pto] remains equal to 4 which is consistent with the expected dissociated state of sodium chloroplatinate Pt Cld-
+ 2H20
-
Pto + 6C1-
+ O,(gj + 4Hf
(4)
The similarity of the kinetics of photodeposition from H2PtCl, or from NaZPtCl6seems to indicate that the reaction mechanism basically depends on the nature of the complex. Oxygen Evolution. Gaseous oxygen production during platinum photodeposition was followed by gas chromatography. A sharp initial increase was observed during ca. 15 min up to a maximum corresponding to roughly 5 times the initial number of moles of Pt introduced in the reactor. Subsequently, the oxygen pressure decreased during 2 3 h down to zero. This important discrepancy between the stoichiometries of deposited platinum and of evolved oxygen expected from eq 3 and 4 was repeatedly observed in several tests. Such discrepancies in metal/oxygen stoichiometries had been already observed in previous studies concerning photodeposition of Ag on Ti02,18,26but with an excess of Ag. The first initial increase of Po may be. ascribed to the photodesorption of molecular oxygenl~7,*
0,- + pf
-
(x/2)0,(g)
x = 1, 2
(5)
The maximum of the oxygen evolved corresponds to about one monolayer. (26) Oosawa, Y.;GrBtzel, M. J. Chem. Soc., Chem. Commun. 1984, 1630. (27) Herrmann, J. M.; Disdier, J.; Pichat, P. Proceedings of the 7th International Vacuum Congress, Vienna 1977; Dobrozemski, R., et. al., Eds.; Berger F. and SBhne: Horn, Austria 1977; Vol. 2, p 9.51. (28) Pichat, P. Heterogeneous and Homogeneous Photocatalysis; Pelizetti, E . , Serpone, N., Eds.; Reidel: Dordrecht, The Netherlands, 1986; p 533.
Moreover, these oxygen species are activated and, instead of desorbing, can oxidize some traces of organic contaminants preadsorbed on titania during storage, thus accounting for the very small amount of COz detected in the gas phase. Note that the C 0 2 traces disappear when titania was freshly calcined in O2 (or air) at T > 723 K. This phenomenon is quite independent of the photodeposition process. The subsequent decrease of oxygen pressure corresponds to a period of time for which most of Pt(1V) complex ions have been reduced to metal atoms, Le., when less and less photoelectrons are needed to deposit platinum. Consequently this decrease can be ascribed to the chemisorption of O2(i) on titania with the excess of photoelectrons now available (x/2)02(g)
+ e--
O;(ads)
x = 1, 2
(6)
and (ii) on surface platinum atoms according to the well-established reaction29 2Pt,
+ O,(g)
-
2Pt,-0
(7)
2.2. Hexahydroxyplatinic Acid, H2Pt(OH),. This complex was chosen in order to determine at constant Pt valency the possible influence of the nature of the ligand upon the kinetics of the photodeposition. Since it is almost insoluble in cold water, complete dissolution was achieved by progressively increasing the pH with N a O H up to a value of 8.8. Figure 4 represents the kinetics of the photodeposition of platinum followed by several means. Reaction completion is reached within slightly more than 2 h according to platinum analysis. The following overall equation can be proposed to account for the process:
The discrepancy in oxygen stoichiometry between eq 8 and experiment and the subsequent disappearance of O2 can be interpreted as in the case of PtC1,- photodeposition. Also, the quantity of OH- ions released in the solution is much smaller than expected from eq 8. This difference might be accounted for by a fixation of OH- ions at the surface of T i 0 2 as found by electrophoresis m e a s ~ r e m e n t s ~ ~ , ~ ~ TiOH
+ OH- F? TiO- + H 2 0
An alternative explanation might be the formation of OH‘ radicals (29) Benson, J. E.; Boudard, M. J . Catal. 1965, 4, 704. (30) Parfitt, G. D.; Ramsbotham, J.; Rochester, C. H. J. Colloid Interface Sri. 1972, 41, 437. (31) (a) Pichat, P.; Herrmann, J. M.; Jenny, B.; Disdier, J.; Courbon, H.; Mozzanega, M. N.; Jaffrezic, N . Advances in Catalysis Science and Technology;Prasada Rao, T. S. R., Ed.; Wiley Eastern: New York, 198.5;p 741. (b) Jaffrezic-Renault, N.; Pichat, P.; Foissy, A,; Mercier, R. J . Phys. Chem. 1986, 90, 2733.
Pt Deposition on T i 0 2 Powder
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 6031
*t 30
15
-
E"
: . g
.
t
c
I' 0
IO
5
1
I
I
@I@,,
1
Figure 6. Variation of the initial photodeposition rate (in pmol H+m i d ) as a function of the relative light flux. For conditions, see text.
0
Figure 4. Kinetics of photodeposition of Pt from Pt(OH)62-!10-3 M): variations vs. time of Pt, 02,and OH- as indicated (black circles, determination of deposited Pt by analysis of the solution; black squares, by analysis of the solid).
lot
1 0
L
C
I
I
I
2
4
6
I
'""'h
Figure 5. Kinetics of photodeposition of Pt from Pt(N02)2(NH3)2 M) (curve A) and, for comparison, photodeposition from H2PtC&(curve B) and Na2PtCI6 (curve c ) .
by reaction between photoholes and OH- surface groups followed by production of H20zwhich would subsequently decompose into O2and H20. No attempt has been made to verify this hypothesis. 2.3. Platinum(Zl)-Dinitrodiammine,Pt"(NOz)z(NH3)2.This complex was selected because it is quite transparent in the near-UV region where TiOz absorbs and it does not photodecompose in the aqueous phase. In addition, since it contains divalent Pt, an approximately twice higher quantum yield was expected by comparison with PtCbZ-ions for equivalent reactivities. Moreover, it seemed interesting to observe how the ionicity of the preceding complexes affects the chemisorption and accordingly their reactivity. The kinetics of Pt photodeposition is shown in Figure 5, curve A, which plots the amount of the complex disappeared from the solution or that of the metal deposited on TiOz as a function of time, since both sets of values coincide. For comparison, the curves
obtained with PtC16'- ions under similar conditions are also presented. The slower kinetics found with the Pt" complex can be mainly ascribed either to a higher stability of this complex and/or to a smaller adsorption coefficient due to its nonionic character. Simultaneously N 2 0 , as the main product, and N 2 are evolved. The production of these gases is not unexpected since they are the only two products resulting from the photocatalytic oxidation of ammonia over TiOz.32 Their partial pressures increased linearly as a function of time within 5 h. However, it was not possible to determine, from the quantities produced, the stoichiometry of the decomposition reaction. As the solution becomes depleted of the platinum complex, the partial pressures progressively decreased to zero. This phenomenon implies a reaction of both gases with the illuminated titania surface once the photoelectrons are no more consumed by the platinum deposition. Note that the fixation of N2 on TiOz (probably as (Nz02)2-species) has been proposed.33 3. Kinetics Studies of the Effects of Various Parameters. 3.1. Limit of Exhaustion of Pt Solutions. In a solution of hexachloroplatinic acid, the initial concentration of PtN (10-j M) could be decreased to the detection limit, Le., below 5 X 10" M. This lowest detectable limit corresponds to =1 ppm, which underlines the possibility of photocatalytic deposition for the platinum recovery in highly diluted effluents. 3.2. Influence of the Light Flux. The influence of the photonic flux 3 upon the reaction rate has to be determined under initial conditions, since for larger conversions the decrease in concentration diminishes the adsorption coverage by the reactant and thence decreases the rate. A quasimonochromatic light was used with a Coming 7-60 UV filter, centered at the 365-nm line of the mercury lamp which transmitted a flux of 6.8 m W cm-2. This light flux was attenuated with calibrated grids. A suspension of 50 mg of TiOZ in 10 cm3 of a M Na2PtC16 solution was illuminated for a period of 5 min, as considered as the optimum reaction time compatible with the assimilation of the mean reaction rate to the initial one and with an acceptable conversion for analysis. For a better sensitivity, the determination of the initial reaction rate ro was based on the proton production since four H+ are released per Pt atom deposited (eq 4). Moreover, the measurement of ro from metal analysis would be largely erroneous, since, in initial conditions, the disappearance of metal complexes from the solution corresponds to a combination of photodeposition and of ion adsorption. The initial pH was measured in the dark after ~
(32) Mozzanega, H.; Herrmann, J. M.; Pichat, P. J . Phys. Chem. 1979, 83, 225 1. (33) Bickley, R. I.; Vishwanathan, V. Nature 1979, 280, 306.
Herrmann et al.
6032 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
0
v. 0.5
1
c o x l o3/ m o l
cim-3
Figure 7. Variation of the initial photodeposition rate as a function of the initial concentration. For conditions, see text.
stirring and pumping. Figure 6 shows that the initial reaction rate is proportional to the light flux as observed, for instance, for ~ means the photodeposition of Pd on a TiOz single ~ r y s t a l .This that electron-hole recombination is not preponderant as attested by the high rate of initial oxygen photodesorption (Figures 2, 3, 4) which results from photohole consumption. The extrapolated straight line does not pass through the origin (Figure 6). This can be related to the fact that the initial pH is not independent of the chemisorption of PtC16'- in the dark. 3.3. Influence of the Initial Concentration. Figure I shows that the conversion firstly increases almost linearly with Countil it reaches a plateau for Co > 2 X M. This isotherm has a Langmuir shape already found for the photodeposition of Ag on TiOz or ZnO single crystals14 and for other aqueous phase photocatalytic reactions, such as oxidation of halide34or oxalic35ions and alcohol dehydrogenation.36 For Co> 2 X 1W3M, the reaction rate will not be limited by the reactant adsorption. According to a Langmuir model, the reaction rate is given by
r = kept = k K C / ( l
+ KC)
where k is the rate constant (which depends on various factors, such as reactor geometry, light radiant flux, efficiency of photon absorption by the catalytic bed, etc.); Opt is the coverage by Pt complexes and K is the constant of adsorption. For low concentrations (KC
