J . Phys. Chem. 1986, 90, 2388-2396
2388
showing the enhancement factor for the adsorbed H20was small. The present results make it clear that strong SERS is caused by the selectively chemisorbed species and the chemical mecha~~-~’ nism, such as C T effect or active site e f f e ~ t , ~ , contributes importantly to the appearance of SERS even on the Ag sols. This finding, however, does not rule out the EM effect caused by aggregation of Ag sols. Both effects might be cooperative each other. The cyclohexene-Ag sol system seems a good example which clearly exhibits the chemical effect. Since the importance of the EM effect has now been recognized, we have to emphasize the significance of the chemical effect. The E M effect28caused by the Ag sol aggregation might also contribute to the SERS of cyclohexene-Ag sol system to some extent. Registry No. Ag, 7440-22-4;cyclohexene, 110-83-8;pyridine, 11086-1.
together. The Raman intensity increases as the time increases, while the absorbance shows a maximum. In Figure 9b plots of the Raman intensities and the absorbances at 514.5 nm vs. time are given for the system of pyridine-Ag sol added with HNO,, as in the case of the cyclohexene-Ag sol system. The addition of HNO, quickens the decrease of SERS, because of the quickened aggregation of Ag sols. Further, we proceeded with the measurements with 600.0-nm excitation so that the similar curves to those in Figure 9b were obtained, where the time for the maximum Raman intensity did not coincide with that for the maximum absorbance. Namely, the time dependence of the Raman intensity differs from that of the absorbance. This result rather indicates that the S E R S intensity does not correlate directly with the absorption maximum in the visible region, even for the pyridine-Ag sol system. It is thus recognized that the SERS of pyridine adsorbed on Ag sols is also caused by some other important mechanism, such as chemical effect, in addition to the EM mechanism. Incidentally the H 2 0bands observed at -3400 cm-’ did not show any appreciable increase in apparent intensity on the Ag sols, which were overlapped with the SERS band of C H stretching at 3060 cm-’. The H 2 0bands were at almost constant intensity before and after pyridine, HNO,, or pyridine + HNO, was added,
(25) Ueba, H.; Ichimura, S.;Yamada, H. Surf. Sci. 1982, 119, 433. (26) Ueba, H. Surf. Sci. 1983, 129, L267. (27) Pettinger, B.; Gerolymatou, A. Ber. Bunsenges. Phys. Chem. 1984, 88, 359. (28) More recent calculations showed that the EM field for two single colloids can be very much larger than the surface field of one colloid, as pointed out by 0. Siiman at the 5th International Conference on Surface and Colloid Science, at Potsdam, NY, June 24-28, 1985.
Kinetics of Hydrogen Production from Illuminated CdS/Pt/Na,S Dispersions D. Neil Furlong, *+ Franz Grieser,t David Hayes,*Robert Hayes,* Wolfgang Same,+and Darrell Wellst CSIRO Division of Applied Organic Chemistry, Melbourne, Victoria, Australia and Department of Physical Chemistry, University of Melbourne, Parkville, Victoria, Australia (Received: October 1, 1985)
The production of hydrogen (H,) upon illumination of transparent CdS/Pt/Na2S dispersions has been studied over the complete life of the catalyst and as a function of the initial concentration of Na2S. Our results indicate that the observed decline in, and eventual cessation of, H2production results from the depletion of the electron donor.(Na2S) and the deactivation of the catalyst (poisoning). Donor depletion dominates when low initial concentrations of donor are used (less than ca. 4 X lo-, mol dm-3), whereas with higher concentrations catalyst poisoning is the,main deactivation pathway. Evidence is presented which suggests that polysulfides are responsible for the deactivation of the catalyst. A model reaction scheme is developed which accounts for the experimental observations and provides a detailed description of the “Lebenslauf‘ of the catalytic system.
Introduction
TABLE I: Preparation of CdS Sols“
Cadmium sulfide (CdS) particles absorb in the visible spectrum and are of potential use in solar energy conversion technology. It is not surprising, therefore, that dispersions of CdS particles have been used in numerous studies1-I7 to catalyze the photoproduction of hydrogen from aqueous solutions. The photophysical and photochemical characteristics of colloidal CdS particles have and it is well estabbeen investigated in considerable lished that the particles rapidly photodissolve unless electron donors or edta7-” are added (and consumed!). such as S2-,’-3S032-,1-4-6 While it is generally believed that CdS particles are relatively photostable in the presence of such additives and that photolysis can proceed for relatively long periods, detailed kinetic studies have not been undertaken. In many photolysis s t ~ d i e s ’ , ~ .the ”~ kinetics are not dealt with at all. Short periods of illumination are used with low conversions of available donor and only initial rates of hydrogen formation are reported. A few reports1*2,5-8,’7*29 have shown that hydrogen production slows with illumination time and that ultimate yields fall short of the theoretical limits, but kinetic analyses were not attempted. Instead a variety of speculative suggestions, such as consumption of d ~ n o r , ~changes . ’ ~ in P H , ~decomposition of s e m i c o n d u ~ t o r ,light ~ ~ ~ absorption by
sol no.
* Author to whom correspondence should be addressed.
’CSIRO Division of Applied Organic Chemistry. *Department of Physical Chemistry.
0022-3654/86/2090-2388$01.50/0
1
2 3 4
-- -- - -
---
mixing seauence.6 DH Ludox, pH 10.0 7.0/Na2S pH 9.2/CdS04 Ludox, pH 10.0 9.2/CdS04 pH 7.6/Na2S CdS04, pH 5.5 3.6/Ludox pH 7.5/Na2S Ludox, pH I0.1/Na2S pH 10.2/CdS04 pH
pH 7.1 pH 6.8 pH 7.1 7.3
”Final concentrations: 6 X low3mol dm-3 of Si02 (Ludox);2 X IO4 mol dm-’ of CdS04;2 X lo4 mol dm-3 of Na2S. Ratio of CdS particles to SiO, particles ca. 2. *The shorthand used in the table can be understood from the following description of the sequence used in the preparation of sol 1: (i) Ludox sol, initially at pH 10.0, was adjusted to pH 7.0 with H,SO,. (ii) Na2S solution was then added and consequently the pH increased to 9.2. (iii) CdS04 solution was then added and consequently the pH decreased to 7.1. products,l back-reaction of products,2 production of surface deposits of metal,6 and establishment of ohmic contacts between (1) Serpone, N.; Borgarello, E.; Gratzel, M. J . Chem. SOC.,Chem. Commun. 1984, 342. (2) Biihler, N.; Meier, K.; Reber, J. F. J . Phys. Chem. 1984, 88, 3261. (3) Mau, A. W.-H.; Huang, C. B.; Kakuta, B.; Bard, A. J.; Campion, A. C.; Fox, M. A,; White, M.; Webber, S. E. J. Am. Chem. Sot. 1984,106,6537. (4) Aruga, T.; Domen, K.; Naito, S.; Onishi, T.; Tamura, K. Chem. Letf. 1983, 1037.
(5) Matsumura, M.; Saho, Y . ;Tsubomura, H. J . Phys. Chem. 1983,87, 3807.
0 1986 American Chemical Society
H2 Production from CdS/Pt/Na2S Dispersions semiconductor and noble metal catalyst,30have been proferred to explain certain kinetic features. The recent work of White and Bard3' does incorporate a kinetic analysis of electron-transfer processes at CdS/aqueous solution interfaces. However, the experimental basis for this study was electrochemical measurement and not the photoproduction of hydrogen. Clearly any lack of sustained photocatalytic activity with CdS suspensions must be overcome if CdS is to be a viable photocatalyst in solar energy conversion systems. The aim of the present study was to experimentally examine the kinetics of H2production from Pt/CdS/Na,S dispersions during prolonged illumination and to formulate a reaction scheme that is consistent with experimental findings. In particular we hoped to identify the steps responsible for the decrease in the rate of H2 production with time and those which control the ultimate yield of H2 from a given dispersion.
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2389 lo1 25
2
15 m
-2 F S
I
0
Experimental Section Materials. Silica-stabilized CdS sols were prepared by mixing aqueous solutions of C d S 0 4 and Na2S in the presence of Ludox-SM silica.'* The surface properties of sulfides are known to be sensitive to their conditions of preparation. Therefore, we have used four variations of mixing sequence and pH to produce sols 1-4 (Table I). The sols were yellow. Electron microscopy showed that while some of the CdS particles (average size in all sols ca. 4 nm) were uniformly distributed on Ludox particles (average nm) a large proportion were not attached to Ludox size ca' particles. X-ray diffraction showed the CdS particles to be mainly revealed that the cubic (8) form. An electrokinetic investi~ation~~ the amount of surface Cd(OH)2coformed wzh CdS was minimal. Sols were stored in the dark to prevent ambient photodegradation prior to use. The ZnS sol was prepared from ZnSO, following the procedure for CdS sol 1 (Table I). The Pt sol (average particle size ca. 2 nm) was prepared by the reduction of H2PtC16at 90 OC with aqueous sodium citrate. Excess citrate was removed by ion exchange.33
(6) Gutierrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1983, 87, 474. (7) Darwent, J. J . Chem. SOC.,Faraday Trans. 2, 1981, 77, 1703. (8) Darwent, J.; Porter, G. J. Chem. Soc., Chem. Commun. 1981, 145. (9) Harbour, J. R.; Wolkow, R.; Hair, M. L. J. Phys. Chem. 1981, 85, 4026. (10) Kalyanasundaram, K.; Borgarello, E.; Gratzel, M. Helu. Chim. Acta 1981, 64, 362. (1 1) Mills, A,; Porter, G. J . Chem. Soc., Faraday Trans. 1 1982,78,3659. (12) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H. J. Phys. Chem. 1984,88, 248. (13) Tricot, Y.-M.; Fendler, J. J. A m . Chem. SOC.1984, 106, 2475. (14) Kalyanasundaram, K.; Borgarello, E.; Duonghong, D.; Gratzel, M. Angew. Chem., I n f . E d i f . Engl. 1981, 20, 987. (1 5) Borgarello, E.; Kalyanasundaram, K.; Gratzel, M. Helv. Chim. Acta 1982, 65, 243. (16) Thewissen, D. H. M. W.; Thinnemans, A. H. A.; Eeuwhorst-Reinten, M.; Timmer, K.; Mackor, A. Nouu. J. Chim. 1983, 7, 191. (17) Borgarello, E.; Erbs, W.; Gratzel, M.; Pelizetti, E. Nouu. J . Chim. 1983, 7, 195. (18) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 301. (19) Rossetti, R.; Brus, L. J . Phys. Chem. 1982, 86, 4470. (20) Alfassi, 2.;Bahnemann, D.; Henglein, A. J. Phys. Chem. 1982,86, 4656. (21) Kuczynski, J.; Thomas, J. K. J. Phys. Chem. 1983.87, 5498. (22) Henglein, A,; Gutierrez, M. Ber. Bunsenges. Phys. Chem. 1983,87, 852. (23) Fojtik, A,; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1984,88, 969. (24) Lal, P.;Ganguly, P. B. J. Indian Chem. SOC.1929, 6, 547. (25) Williams, R. J. J . Chem. Phys. 1969, 32, 1505. (26) Gerischer, H.; Meyer, E. Z . Phys. Chem. (Wiesbaden) 1971,74,302. (27) Gerischer, H. J. Electroanal. Chem. Interfacial Electrochem. 1975, 58, 263. (28) Wilson, J. R.;Park, S.-M. J. Electrochem. SOC.1982, 129, 149. (29) Gerischer, H. Top. Appl. Phys. 1979, 32, 115. (30) Aspnes, D. E.; Heller, A. J. Phys. Chem. 1983, 87, 4919. (31) White, J. R.; Bard, A. J. J . Phys. Chem. 1985, 89, 1947. (32) Hayes, R.; Freeman, P.; Mulvaney, P.; Grieser, F.; Furlong, D. N.; Sasse, W. H. F., manuscript in preparation. (33) Furlong, D. N.; Launikonis, A.; Sasse, W. H. F.; Sanders, J. J . Chem. SOC.,Faraday Trans. 1 1984,80, 571.
8
5
100
200 300 Illumination Time (minsl
Figure 1. Yield and rate of hydrogen production with CdS sols I b and 4, sol designation as used in Table 11: C d S at 1.82 x 10-4 mol dm-3; Pt at 1.5 10-5 mol dm-3; initialNa2Sat 10-3mol dm4; initial pH
Solutions of CdSO,, ZnSO,, Na2S, and Na2S03,and of acid and base for control of pH, were prepared from analytical grade reagents and triply distilled water ( 4 at wavelengths below 420 nm). Blank experiments without CdS gave no production of H2. In a series of experiments using in turn a 300-, a 420-, a 480-, and a 520-nm cutoff filter the rate of H 2 mol dm-) Na2S/ production from our CdS (sol 1)/Pt/5 X initial pH 10 system decreased to zero-confirming that band-gap excitation of the CdS particles (2.42 eV, X < 5 15 nm) is central to the production of Hz The rate of H2 production was determined by gas ~hromatography~~ with a precision of mol min-' cmw3. The photolysis mixture was continuously purged with argon for 30 min prior to, and during, illumination, and the gas stream analyzed for H, via an automatic sampling value as often as required. Check injection experiments showed that the lapse time was less than 3 min between H2 production in the photolysis mixture and accurate determination of H, by the gas chromatograph. Pt Binding. Photolysis mixtures were prepared by mixing the CdS/Si02 and Pt sols at pH 10 for 30 min and then mixing with the Na2S solution for a futher 15 min. Pt and SiO, particles are negatively charged above pH -2,35 our CdS particles above pH ~ 4 Previous . ~ heterocoagulation ~ studies35show that Pt particles do not bind to silica particles at pH 10. Some photolyses were performed following redosing of an irradiated mixture. Redosing with Na2S was achieved by adding 0.05 cm3 of concentrated stock solution. Redosing with CdS/Pt was achieved by halving the irradiated mixture and adding 2.5 cm3 of freshly prepared CdS/SiO,/Pt sol. This latter redosing procedure results in the replenishment of half of the CdS/Pt catalyst and a twofold reduction in the concentration of Na2S. Binding experiments in the present study, using 1.8 X lod mol dm-3 of CdS/Si02 (sol 2) and 1.5 X mol dme3 of Pt, showed ~~
(34) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Phys. Chem. 1985.89, 1922. (35) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985,89, 626.
2390 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Furlong et al.
TABLE 11: H, Production from CdS/Pt/Na,S Dispersions" 108R,,,,mol 106(yieldof H, after sol min-' 200 min). mol cm-) I 1
1.86
repeat 1 repeat 1 1 (aged 6 months) 1 (aged 6 months) 7
I
2 3 4
2.1' 1.3' 1.6' 5.36 4.6' 2.9b 3.0' 1.8
5.3
2.1 2.5 2.2 2.4 2.2 2.1 2.3 2.1 2.5
mol dm-); [Pt] = 1.5 X mol dm-3; [Na2S] [CdS] = 1.8 X = 5.0 X IO-' mol dm3; initial pH 10.0; 420-nm filter. b'CRepeatruns using the same CdS dispersion.
that 66% of the Pt was bound to CdS/Si02 aggregates at pH 10-or ca. 1 Pt particle per 5 CdS particles. The rate of H2 production from this catalyst decreased by ca. 10 when the F 't was omitted confirming clearly that Pt bound to CdS enhances catalyst activity. The extent of binding was determined by membrane filtration of CdS/SiO,/Pt aggregates and measurement of residual concentration of Pt by absorbance at 450 311-n.~~The level of bound Pt increased to 100% at pH 5, suggesting that binding is in part due to local electrostatic attractions even though the surfaces of Pt and CdS particles carry a net negative charge. Saturation coverge of Pt onto CdS at pH 5, achieved by increasing the Pt/CdS ratio, corresponded to 17 nm2 of CdS surface per bound Pt. This area corresponds to an effective radius of ca. 2.3 nm per bound Pt which is in close agreement with the close-packed radius found for Pt bound to Ti025and represents the balance between Pt-CdS affinity and lateral Pt-Pt repulsions. At pH 10 no binding of Pt to ZnS was found. Binding did occur at pH 5 but the uptake was only about half that found for CdS.
Results Experimental Determination of the Production of H,. ( 1 ) Preliminary Data. We will first discuss effects of sol preparation, composition, and pH. This preliminary study provides a basis for the choice of appropriate sol conditions for our detailed experimental analysis of the kinetics of H 2 production. ( a ) Sol Preparation. The yield (Y) of H2 and the rate ( R )of H2 production are described in Figure 1, a and b, for CdS sols 1 and 4. For each experiment the starting pH was 10 and 5 X mol dm-3 of Na,S was present at the beginning of illumination. The form of the curves, typical of all runs in the present study, shows that an induction period occurs during which R increases to R,,,. R then decreases with continued illumination for the runs shown in Figure 1b to ca. 1% of R,,, after 200 min. Table 11 gives R,,, and Y,,, values for sols 1-4. The reproducibility of R,,, and YZmfor any sol is ca f10% and Y2mwas the same within this level for all four sols. The pH had increased from 10.0 to 1 1.2 during illumination. As more than 90% of the donor solution will consist of N a H S in this pH range the net chemical reaction leading to H2 formation is HS-
hu
+ H20 cds/p~rSo + H2 + OH-
The limiting yield of H2 for the sols in Table 11, viz. 2.3 f 0.2 pmol d ~ n - corresponds ~, to the conversion of only ca. 46% of the available HS- via reaction 1. Clearly, either other processes must also consume HS- or the activity of the catalyst is lost (see sections 2 and 3). After being stored for 6 months sol 1 had become turbid. This turbid sol gave an R,,, that was three- to fourfold greater than that of the sol when freshly prepared. CdS sol 1 prepared without Ludox stabilizer, and therefore turbid when freshly prepared, also produced H, at an increased rate. Therefore, the state of dispersion of CdS particles affects R,,,. To ensure our kinetic analysis was not affected by such changes in degree of dispersion
7
t --A 1 2 , 3 4 5 6
i
[email protected] l
3 Or
-
+
~
-
&
cN=
$- --+ w
-
1
6
8
10 12 10:lCdSI
li
16
18
22
Figure 2. Dependence of R,,, on concentration of Pt and CdS. Na2S and pH as for Figure 1. (a) CdS at 1.82 X mol dm-'. (b) Pt at 1.5 X mol dm-'. N is the number of bound Pt particles per CdS particle. The arrow indicates the decrease in R,,, when a 50% neutral density filter was placed in the light beam.
we performed all experiments described in following sections using transparent, nonturbid sols. As described earlier all four freshly prepared CdS sols appeared well dispersed and consisted of particles of ca. 4 nm. With the exception of the one run with aged sol 1, the runs in Table I1 were all performed within 1 week of sol preparation with nonturbid sols. Thus it seems that the differences in R, seen in Table I1 are due to variations in the surface chemistry of the CdS particles. The four sols were prepared by using different sequences and pH of reactant mixing. We can only speculate, as did Biihler and co-workers,* that the extent of Cd(OH),/CdS co-precipitation during sol preparation, although may vary sufficiently between sols 1 to 4. Only a few previous s t ~ d i e s ~have J * ~mentioned ~~ any effects of sample preparation on hydrogen production and no clear picture emerges from these reports. We conclude from our data that the rate of production of H2, but not necessarily the yield, is sensitive to the surface chemistry and physical form of the catalyst particles. Comparisons of rates obtained in the present study with those of other workers are of little value unless details of catalyst surface chemistry and form are also known for all sols. The evaluation of kinetic parameters in section 2 was conducted using only one sol and the performance of this sol in a given photolysis mixture was periodically checked to ensure that no changes had occurred in its state of dispersion. (6) Concentration of CdS and Pt. Figure 2a shows that R,,, increased with the concentration of Pt for all four CdS sols of mol dm-3) Table I. The maximum Pt concentration used (5 X corresponds to ca. 1 bound Pt particle per 2 CdS particles ( N = 0.5) if we assume that Pt and CdS particles are monodisperse of 1 .O- and 2.0-nm radius, respectively. It should be noted though that an uncertainty of only 10% in either of the estimates for individual particle size introduces an uncertainty of 30% in N. The N values in Figure 2 are then only an approximate guide to Pt loadings. The data in Figure 2a nonetheless reflect how the performance of the system improves as more particles of CdS are activated by Pt particles. Figure 2b shows that R,,, increased with increasing concentration of CdS up to ca mol dm-3 and was unchanged by further increases in concentration. Note that R,,, was halved when the light intensity was halved at the highest concentration of CdS, indicating that, at normal light intensity, the system was undersaturated with light. Figure 2b reflects two opposing controls on R,,,. On one hand, R,,, increases with increasing N , as discussed for Figure 2a, while, on the other hand, R,,, decreases with decreasing concentration of CdS due to the decrease in the number of photons being adsorbed. Clearly the control via photon absorption dominates at the lower end of the concentration range in Figure 2b. ( 3 6 ) Yanagida, S.; Azuma, T.;Sakurai, H . Chem. Lerr. 1982, 1069.
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2391
H2 Production from CdS/Pt/Na2S Dispersions TABLE IIP initial DH 109R...”^ mol m i d
,...
10.0
11.2
13
9.9
I
% as
HS-
99 85
[CdS] = 1.8 x mol d d ; [Pt] = 1.5 X = 5.0 X lo-’ mol dm-’. a
% as S2-
L5
1
15
3.9
’
t
t
3.3 -
mol dm-’; [Na2S]
- i m 2.7
c
‘C
E
21-
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1.5-
E
P 0.9-
mx
03
-46
- 4 2 -30
- 3 4 -30 -26 log IINqSl initial)
-22 -1 8 -1
1,
Figure 4. Dependence of R,,, on initial concentration of Na2S. Other conditions as for Figure 1: (-0-) experimental data for CdS sol 3; (-) data from theoretical model.
Illumination Time lmin)
Figure 3. Variation of the rate of hydrogen production during illumination. All conditions as for Figure 1: (-0-) experimental data for CdS sol 3; (-) data from theoretical model.
The detailed kinetic experiments of sections 2 and 3 were performed with 1.8 X lo4 mol dm-3 CdS and 1.5 X lW5 mol dm-3 Pt, a dispersion that uses all available incident photons and with minimal shielding of CdS by bound Pt. (c) p H . For the runs of Figure 1 the pH increased from 10.0 to 11.2 during 240 min of illumination. The rate of H2production had decreased to less than 1% of R,,. Table 111shows that R,,, was reduced by ca. only 24% if the dispersion pH was initially 11-2. Note that at pH 11.2 the ratio HS-:S2-is also less than at pH 10. Clearly the change in pH during photolysis is not a major cause of the decline in the rate of H2production during photolysis. Therefore, in our subsequent runs we have not attempted to buffer donor solutions. Interestingly, previous reports with sulfide dispersions are somewhat ambiguous with respect to the effects of pH. It has been claimed, on one hand, that the rate of hydrogen production is independent of pH between 8 and 1115 or 1437and, on the other hand, that it increases with pH above pH 11.5.16 By contrast Biihler and cc-workers2 found no hydrogen at pH values below pH 11. (2) Characteristics of the Kinetics (CdS Sol 3). Figure 3 shows R vs. illumination time ( t ) for an experiment with an initial concentration of sulfide of 5 X mol dm-3. We will discuss the calculated curves in the following sections. Clearly any valid description of the overall reaction pathway for H2 production should consider the Occurrence of R,,, as well as its magnitude, the length of the induction period (t,), and the yield of H2(Y,,,) at which R,,, occurs. Figure 4 shows that R,,, is not monotonic in initial concentration of Na2S. While the increase in R,,, with concentration has been observed in some previous studie~,’’,~’ the decrease in R,, at higher concentrations of donor is somewhat unusual. An optimum R,,, has also been found in the present study for the ZnS/Pt/Na2S system but not for the CdS/Pt/S032- system-for the latter R , increased monotonically with concentration of donor report^",^^ and plateaued at higher c o n c e n t r a t i ~ n . ~Previous ~ of a monotonic dependence of R,, on the concentration of sulfide were for systems not containing Pt. In two other studies in which platinized CdS particles were used the dependence on sulfide (37) Reber, J.-F.; Meier, K.J. Phys. Chem. 1984, 88, 5903. (38) Furlong, D. N.; Grieser, F.; Hayes, D.; Hayes, R.; Sasse, W. H. F.; Wells, D., manuscript in preparation.
39 -
J
33 r 27 -
-L6
-L2
-30
-36
-30
-26
-22
-10
-1 L
log IINa,S 1 initlal1
Figure 5. Dependence of f,,, on initial concentration of Na2S. Other conditions as for Figure 1: (-e-) experimental data for CdS sol 3; (-) data from theoretical model.
concentration was either not u n d e r ~ t o o d ‘or~ depended on the method of platinization.’ In neither study were the results presented in detail. From our data it is clear that the reaction pathway proposed for CdS/Pt/Na2S (and ZnS/Pt/Na2S) should include recognition that the donor molecule HS-has a particular relationship with the sulfide semiconductor. Figure 5 shows the dependence of t,,, on initial concentration of Na2S. Again a curve of similar form was found for the ZnS/Pt/HS- system but for the CdS/Pt/S032- system t,,, was constant over a wide range in donor concentration. Figure 6, a and b, describes the dependence of Y,,, on donor concentration, where the yield is expressed as the absolute yield of H2and percent conversion of Na2S (based upon eq 1 with [HS-] = [Na2S]), respectively. Note that in all systems the rate began to decline a t 17% or less conversion of Na2S into H2. Figure 7, a and b, shows the yield of H2 after 200 min of illumination for a range of initial concentrations of Na2S, expressed in absolute terms and as percentage conversion of Na2S, respectively. As with R , the yield of H2is not monotonic in donor concentration. The maximum conversion to H2 was ca. 60%-a further indication that HS- is converted in pathways other than that represented by eq 1, or that catalyst deactivation has occurred. Figures 3-7 provide a detailed experimental description of the kinetics of H2 production from our CdS/Pt/Na,S sols and show how the kinetic parameters depend upon the concentration of electron donor. The aim of the kinetic model described later in
2392 The Journal of Physical Chemistry, Vol. 90, No. I I , I986
Furlong et al. 22.5.
-- 19.5 f
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(39) Giggenbach, W. Inorg. Chem. 1972, 1 1 , 1201. (40) Ellis, A. B.; Kaiser, S.W.; Wrighton, M. S.J . A m . Chem. SOC.1976, 99, 1635. (41) Hods, G.; Manassan, J.; Cahan, D. Narure (London) 1976,261,403, Tsuiki, T.; Oki, T. Ber. Bunsenges. Phys. Chem. 1977, (42) Minoura, H.; 81, 588. (43) Gerischer, H.; Gobrecht, J. Ber. Bunsenges. Phys. Chem. 1978, 82, 520.
in H2 production with HS- as donor. We conclude that the formation of polysulfide causes catalyst deactivation as well as consumption of donor. The poisoning of active noble metal catalyst surfaces by adsorption of sulfur-containing compounds has long been known.44 The role of polysulfides formed during illumination experiments has been discussed in several previous s t ~ d i e s . * ~ ’ In ~ JCdS/ ~,~~ R u 0 2 systems,’s~’6 that is, without Pt present, polysulfides were deemed to have no influence on the course of H2production. By contrast, in a Pt-containing system,2 SZ2-was claimed to short circuit the production of H2because it was readily reduced to S2by conduction band electrons. A similar short circuit was proposed for a zinc sulfide sol in the absence of Pt.37 Because the precise mechanism of “poisoning” by polysulfide, be it by adsorption on active sites or by reaction with conduction band electrons, is not known for our Pt/CdS sols we include a general poisoning step in our reaction pathway. Our experimental kinetic study has shown (1) the dependence of the kinetic parameters R,,,, t,,,, Y,,,,and Yon the initial concentration of donor (Figures 3-7); (2) that a relationship exists between HS- and CdS (or ZnS), but not between S032-and CdS, that is central to the kinetics of H2 production; (3) that H2 production is not the only process leading to the consumption of HS-; (4) that the other processes referred to in (3) produce a catalyst poison on CdS/Pt surfaces. In the following section we will describe a kinetic model for H2 production from CdS/Pt/HS- dispersions which addresses points (1) to (4) above.
A Kinetic Model for the Production of H2 The reactions used in our model are presented in Table VI. C* is considered as a steady-state “reactive entity” which may be identified with the photogenerated electron-hole (e-h+) pair. C* refers to the state where the electron has migrated from the CdS particle to the bound Pt and the hole has migrated to the surface of the CdS particle. For convenience we have assumed that electron capture by the hole from an adsorbed donor molecule (HS-) and electron reduction of water (H+) on Pt occur simultaneously (reaction 3). Reaction 4 yields elemental sulfur which we believe from our experimental study is involved in subsequent “poisoning” of the catalyst. Reactions 3 and 4 could be written as one step 2C*
+ HS,,j[ + H20
-
So
+ OH- + 2H. (+2C*)
although in this form the nature of intermediate species is hidden. It should be noted that neither reaction 3 nor 4 lead to a depletion of the steady-state concentration of C*, as this concentration is largely governed by the incident photon flux and by nonreactive deactivation pathways of the electron-hole pair. In reactions 4 and 5 we have treated the products of the electron/hole reactions, S - and H‘, as being homogenously distributed even though they are formed on the CdS and Pt particles, respectively. Our justification for this simplifying assumption is the recognition that H2 formation occurs on the time scale of minutes, while particle-particle “collisions” occur on a millisecond time The poisoning steps 6-9 account for the decline in the rate of H2 production with illumination time and for the non-H2 producing disappearance of donor. Poisoning is treated as a depletion of the steady-state concentration of C*. Alternatively, the action of the poison can be viewed as enhancement of the electron-hole recombination rate. In the model we have identified the poison with HS2- and HS). It is possible that higher polysulfides may be formed in the concentration and pH ranges of the experiment^^^ but to add these would only serve to embellish the model. The side reactions 10 and 11 have been included in the reaction scheme because oxidation of the CdS catalyst was found to occur to a small extent32during illumination. Since the Cd2+produced (44) Maxted, E. B. J . Chem. SOC.1921, 119, 225. (45) Wiegner, G.; Tuorila, P. Kolloid-Z. 1926, 38, 3. (46) Matheson, M. S.; Lee, P. C.; Meisel, D.; Pelizzetti, E. J . Phys. Chem. 1983, 87, 394.
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The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Furlong et al.
TABLE V: Effects of Added Polysulfide on R,, (10" mol min-' cm-')' add polysulfide then add 5 X add polysulfide initial run solution A Na2S after 72 min solution B 4.0 0.22 1.2 0 "Polysulfide solution A is nominally 5
6 7 8 9
10 11'
He
+ He
Recombination/H, Production H2
+
-
so+ HSadi
+ P,
-
50
500
+
CP
50
Side Reactions Cd2t So 2H. C* Cd2' + SToT C* -+
E,, = V,
+ +
+ Vb, = constant - 0.06 log [HS-],,,,,
If we also assume that the HS- anion acceptor state has its energy level fixed at E', independent of the solution concentration of the sulfide species because of its chemisorption onto a surface state of the colloid, then the overpotential for this system is represented
40
500
PI
c*+ P, CP PI + HSadL P,
c*
where [HS-I,, refers to the total concentration of HS- in solution, we may write for the valence-band energy
7 = Evb - E' = constant'
Poisoning -+
then add 5 X NazS after 60 min 0.04
mol dm-3 HS2'-. Polysulfide solution B is > IO-* mol dm-3 HS22-.
X
TABLE VI: Reactions Used for the Simulation Calculations of H2 Production from Aqueous Dispersions of Colloidal CIS" rate constant, dm3 mol-l no. reaction min-l or min-' Initiation 60[HS-]4'44 36 c * + HS,d,- -+ sa- + H. ( + c * ) So + He (+C*) + OH1000 C* + S*-+ HZO 4 5
then add 5 X Na2S after 60 min 0.02
- 0.06 log [HS-],,t,,
The Butler-Volmer equation, for transfer of an anion to an electrode under an overpotential 7,states that the rate constant for transfer is
2 x 10-5
4
QOtherparameters: K L = 5000; Ns = 1 the quantity adsorbed onto CdS given by
X
IO4.
[HS]-,d, refers to
where KL is the Langmuir adsorption constant (mol-' dm3) and NS is the concentration of surface sites available for adsorption (mol d n ~ - ~ ) . cSToTis the total concentration of sulfide not in the form of polysulfide. Under our experimental conditions at pH IO, STOT = [Na2S] N [HS-],,,, [HS-la,. No rate constant is used in reaction 1 1 . C* produced from reaction 1 1 maintains a constant concentration of this entity until all the Na2S has been consumed.
+
will react with any excess sulfide there is no loss of catalytic material until all sulfide has been consumed, Le., once S T ~(T HS,; HSad;) is lost then a depletion of the steady-state concentration of C* occurs. Reactions 10 and 11 are significant with respect to the H2 production profile at low added sulfide concentrations. In our first attempts to fit the experimental trends using eq 3-1 1 we failed to predict the decrease in both the H2 yield and R,,, with an increase in the concentration of added sulfide. In order to match this behavior reactions 3, 4,or 5 (or all 3) must have a rate constant that decreases with increasing sulfide concentration. Our approach is to include the variation in the rate of interfacial electron transfer with the changing potential on CdS due to the added potential determining ion, HS-. This can be achieved by using an approach similar to that developed by Moser and Gratze14' and Gratzel and Frank.48 These workers treated the difference between the energy of the semiconductor species (electron or hole) and the acceptor state as an overpotential for reaction. In our case, for simplicity, we have treated only reaction 3 in this manner with the rate-determining step being the transfer of a hole from the valence band of the CdS particle to an adsorbed HS- anion. A number of assumptions are involved in this treatment. First, we assume that the energy of the valence band (Evb)differs from the flat-band potential of the semiconductor ( V f i )by a fixed energy (Vbg). Then, using the value for Vfi obtained from single-crystal studies49and which has been applied to colloids in previous s t ~ d i e s , ~viz. .'~~~~
+
Vfi = -1.16
- 0.06 log
[HS-]t,,I
(47) Moser, J.; Gratzel, M. J. Am. Chem. SOC.1983, 105, 6547. (48) Gratzel, M.; Frank, A. J. J . Phys. Chem. 1982,86, 2964. (49) Inoue, T.; Watanabe, T.; Fujishima, A,; Honda, K. Bull. Chem. SOC. Jpn. 1979, 52, 1243.
where for 7 3 0.1 V the former term dominates. For a singly charged species (lzl = 1) we obtain
The constants in this last equation have their usual meaning and a is the transfer coefficient. Substituting for 7 we obtain the following expression for k ,
In calculations for our theoretical model we have used a = 0.44 and k3/ = 60 (see Table VI). Both numbers have been treated as adjustable parameters in order to model the trends in the experimental data. The above approach, which was used in our calculations, is not the only way of introducing a rate constant that decreases with increasing concentration of sulfide. It is possible to obtain such a trend if we assume that the transfer of an electron from the conduction band of CdS to water (or H') to form the H radical is the rate-limiting step. Since the flat-band potential of CdS (V,) is considerably more cathodic than that of the H+/H2couple ( E H ) at pH 10, and becomes even more so with increasing concentration of HS-, a Marcus fluctuating energy model may be applicable. The form of the rate constant would then bes1 k,, = ke: exp(-(V, - EH - X)'/4kT) where X is the rearrangement parameter.s1 The use of this equation for k3 predicts a somewhat sharper decrease in the rate of production of H2 with increasing concentration of Na2S than does the expression for k3 derived via the Butler-Volmer equation. At present we cannot say which form for k3 is the correct or more appropriate one. In this study we chose to use the Butler-Volmer approach (Table VI). We stress, however, that without the inclusion of some such rate expression the experimental trends over the range of sulfide concentrations studies could not be simulated. Note also that the inclusion of the potential-determining role of HS-, with respect to CdS, satisfies our experimental requirement that the kinetics include a contribution specific t o CdS/HS- which is not present for the system CdS/S032-. In (50) Atkins, P. W. Physical Chemistry Freeman: San Francisco, 1978;
p 966.
(51) Morrison, S. R. Electrochemistry at Semiconductor and Oxidised Metal Electrodes; Plenum Press: New York, 1980; Chapter 6.
H2 Production from CdS/Pt/Na,S Dispersions I
'
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2395 1
I
4.80
I
1
............____
4.16 m
' E 3.52 U
-2 E s
2.24
8
1.60
5
6
288
*x
g 0.96 0.32 12
36
60
81,
108
132
156
180
204
Illurnination Trne (mind Figure 10. Concentration of reaction species (calculated from theoretical model) vs. illumination time: initial NazS at 5 X lo4 mol dm-3. Note the change of scale for [HS,,-]on inset. Concentration of P2< lod mol dm-' at all times.
summary then, the form of the experimental R,, dependence on the concentration of HS- is brought about by a balance between, on one hand, an increase in adsorbed HS- (which we have taken to follow a Langmuir isotherm, see Table VI), which increases the rate of H production, and, on the other hand, a decrease in the rate constant with increasing [HS-] due to the changing overpotential for electron transfer. Using the rate constants given in Table VI in the kinetic model, we can predict all the experimental trends (see Figures 3-7), including the total consumption of sulfide donor with remaining catalyst activity, which occurs at initial sulfide concentrations less than mol dm-3, and the total deactivation of catalyst with sulfide donor remaining, which occurs at concentrations of Na2S mol dm-3. greater than about 4 X The model does not take into account the effect of changing pH on the flat-band potential of CdS. As H+ is a secondary potential determining ion for CdS it is expected to exert an inJ ~ examined ,~~*~~ fluence similar to HS-. Several ~ t u d i e s ~ J ~have the dependence of E f bon pH and the results are inconclusive, or 5 5 varying from no dependence up to pH 1 12J2 to a 40-'5*31 mV3' change per pH unit. Although our experiments did show the effect to be small, addition of the variation to the rate constant of reaction 3 would make for completeness. The trends, however, are not altered. In summary, the proposed kinetic model can account for the observed experimental trends of H2 production in the presence of varying amounts of sulfide. The most significant conclusion of the model calculations is the need to incorporate a step which decreases the rate of H2 production with an increase in the concentration of sulfide donor. We have been able to rationalize this requirement by identifying the decrease in rate with a variation in overpotential due to the potential determining HS-.
Discussion
In order to attain a better understanding of the reaction scheme for the production of H, it is instructive to consider some aspects of the kinetic model in more detail. The rate constants used to model the experimental data are treated as adjustable parameters and the values used may not be in close accord with the actual values. However, the good correlation obtained between the calculated and the experimental data suggests that the relative values used in the model are similar to the real relative values. The reaction scheme can be considered in two parts. The first part, represented by reactions 3-5, determines the duration of the induction period for the production of H2. A decrease in the rate constants for reactions 3-5, or alternatively a decrease in the
Illumination Time lmins)
Figure 11. Concentration of reaction species (calculatedfrom theoretical model) vs. illumination time: initial Na2S at mol dm-).
Langmuir adsorption constant, leads to an increase in the induction period. The second part of the reaction scheme, encompassing reactions 6-1 1, controls the ultimate yield of H2. With an initial concentration of Na2S less than 4 X lo-) mol dm-3 the death of the system, that is cessation of production of H,, is due to the total consumption of Na2S. In this concentration range of Na,S a small portion of the H, is produced via reaction 10, and the model predicts that H2 is still being produced after 200 min but at rates below experimental sensitivity. Figure 10 shows the (calculated) concentration of each major intermediate species and products, as a function of illumination time, when the initial concentration mol dm-3. It can be seen that (1) active of Na2S was 5 X catalyst (C*) remains after the Na2S is completely consumed 0) and ( 2 ) the production of H2 (via reaction 5 ) (HSad; continues long after the time when all the HSad; is consumed. Figure 10 shows, however, that although the HSad; is consumed after about 10 rnin the concentration of CP continues to increase for much longer times (up to about 84 min), indicating that H' produced via reactions 3 and 4 is reacting to give H2 subsequent to the consumption of HSa,-. Catalyst deactivation (reaction lo), while a potential source of H from the start of illumination, will be the main pathway for H2 production when illumination is continued for longer than about 84 min. Indeed, measurements using a Cd2+ selective electrode (in a pH range where all free cadmium is as Cd2+) showed that the level of Cd2+ in solution had increased during illumination. Our calculations also reveal that at any concentration of Na2S mol dm-3 the production of H, ceases greater than ca. 4 X before the Na2S is totally consumed because of poisoning. For example, C* decreased to zero during ca. 200 min of illumination when the initial concentration of Na2S was lo-, mol dm-3 (Figure 11). HS,d; had not decreased significantly during the same period. Calculations based on the reaction scheme of Table VI indicate that almost complete consumption of HSad[ and catalyst deactivation should w u r during the first 100 min or so of illumination mol dm-3. when the starting concentration of Na2S is ca. 4 X The experiments of Figures 8 and 9 confirm this prediction. Figures 10 and 11 show that the generation of polysulfide becomes considerably more important at the higher concentrations of Na2S. Increased formation of polysulfide represents a loss in performance in terms of the conversion of donor to make H1. Thus the maximum use of Na2S is obtained by working at low concentrations of donor mol dm-3). The most crucial equation in the reaction scheme is that giving the rate constant for step 3. A change in the valence band energy of CdS with the concentration of HS- is consistent with the potential-determining role of HS-. As indicated earlier, without such a dependence in the rate constant k 3 , we could not model the maximum in the R , vs. concentration curve found experimently.
-
2396
J . Phys. Chem. 1986, 90, 2396-2407
In summary, it is clear that for sustained production of H2 from CdS/Pt/Na,S (or CdS/Pt/H2S) systems two conditions must be achieved: (1) the polysulfide poison must be continually removed and (2) decomposition of the CdS lattice must be prevented. The second condition can be met by constant readjustment of the solution concentration of Na2S (or H2S). The first condition may prove more troublesome. It may be possible to wash the catalyst but this would probably require that the CdS be suspended in a support matrix. Bard and c o - w o r k e r ~and ~ * ~Thomas ~ and cow o r k e r ~have ~ ~ demonstrated the use of a Nafion membrane to (52) Krishnan, M.; White, J . R.; Fox, M. A.; Bard, A. J J. Am. Chem. SOC.1983, 105, 7002. (53) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J . Phys. Chem. 1984, 88. 980.
support CdS. Alternatively,z additives could be used which react with So provided that they do not adversely affect the surface energy levels. Catalyst poisoning may be partly retarded if an electron donor other than Na2S (or H2S) is used to produce H2. However, CdS decomposition (reaction 10) may still proceed and a catalystcleaning method would still be required.
Acknowledgment. The research reported was supported in part by a CSIRO/University of Melbourne Collaborative Research Grant and by the University of Melbourne Special Studies Program. Registry No. H 2 0 , 7732-18-5; CdS, 1306-23-6; Na,S, 1313-82-2; H2S, 7783-06-4; HS-, 15035-72-0; H2, 1333-74-0; Pt, 7440-06-4; S, 7704-34-9.
H2S Adsorption on A1203, Modified A1203, and Mo03/A1203 Yasuaki Okamoto,* Minoru Oh-Hara, Akinori Maezawa, Toshinobu Imanaka, and Shiichiro Teranishi Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560. Japan (Received: October 2, 1985)
A systematic investigation of H2S adsorption on A1203,modified AI@,, and MoO,/A1,O3 catalysts was carried out by using temperature-programmed desorption techniques, coupled with IR and XPS. The modified aluminas involve Na’, Mg2+, Zn2+,F, or S042-/A1@, catalysts with various doping levels. SiO2-AI2O3and AIPOl were also studied for comparison. It was found that A120, showed two distinctly different chemisorption modes of HIS, associative (a)and dissociative ( p ) H2S, whereas SiO2-AI20, and AIPOI provided predominantly a-H2S. The heats of adsorption were 1 3 and 3 4 kcal mol-’ for a-and P-H2S, respectively, and the corresponding site densities were 4.8 and 5.5 X IO” c d . The modification of AI2O3 with cations was found to induce a significant decrease in the desorption temperature of @-H2Swith a concomitant increase in the total amount of H2S adsorption. On the contrary, the adsorption amounts of a-and, in particular, @H2Swere decreased by modifying with anions. The adsorption sites are concluded to be cus (coordinatively unsaturated) Altet3+cations for a-H2S and cus A1,,3+-strong base pair sites for @-H2S.The effects of calcination and evacuation temperatures on the H,S adsorption behavior of A 1 2 0 3 are also interpreted in terms of the above site models and a surface reconstruction of yA1203. The isomerization of 1-butene on A1203 was found to be poisoned by @-H2S. On the basis of IR studies of C02-H2S coadsorption, cus A1,e,3+-strongly basic oxide anion pair sites are proposed to be responsible for the reaction. When the loading level of MOO, was low (
