626
J . Phys. Chem. 1985,89, 626-632
Colloidal Semiconductors In Systems for the Sacrificial Photolysis of Water. 1. Preparation of a Pt/TiO, Catalyst by Heterocoagulatioh and Its Physical Characterization D. Neil Furlong,* Darrell Wells, and Wolfgang H. F. Sasse Division of Applied Organic Chemistry, CSIRO, G.P.O. Box 4331, Melbourne, Australia (Received: September 12, 1984)
Pt/Ti02 particles were prepared from dispersions of clean colloidal Pt and TiO, particles. The binding of Pt to TiOZwas found to be mainly electrcstatic in origin and was only slowly reversed by reversal of charge on the Ti02particles. Maximum coverage of Pt on Ti02 was attained at pH 3 and corresponded to only about 20% of geometric close-packing due to lateral repulsions between 'oound Pt particles. EDTA adsorbed on both Pt and TiOz particle surfaces. It was found that EDTA could pack more densely on the Pt particles than on the TiOZparticles. EDTA adsorption suppressed the binding of the Pt to TiOZ. The binding of Pt to Ti02 was found to cause the Ti02 particles to coagulate, particularly when Pt coverage on TiOz approached its maximum. EDTA and H2 bubbling caused coagulation of Pt/TiOz particles. PVA was found to retard coagulation induced by EDTA but not that induced by H2.
Introduction In recent years light-induced redox reactions involving particulate semiconductors have been widely studied.l Interest in the use of semiconductor particles for the light-induced production of hydrogen from water (so-called solar photolysis of water2-") has prompted the work reported in this and the following paper. However, the chemistry of photolysis systems is also relevant to processes such as the water-gas shift the oxidation of a l i p h a t i ~ ' ~and J ~ aromati@ compounds and of carbon itself," and the production of sulfuric acid from elemental sulfur.18 These and other processes have been discussed as useful ways of converting solar energy into chemical feedstocks or fuels. Efficient systems for the nonsacrificial cleavage of water using visible light are at a very preliminary stage of development. In such systems protons are reduced to give molecular hydrogen concurrently with the oxidation of hydroxyl ions to give molecular oxygen. Much progress has been made with so-called sacrifical systems in which an electron donor other than OH- is oxidized to provide electrons for the reduction of protons. Electron donors investigated include ethylenediaminetetraaceticacid (EDTA),19sm
(1) See, for example, chapters by: Kalyanasundaram, K., Sakata, T., Kawai, T.; Watanabe, T., Fujishima, A., Honda, K.; Hodes, G., Frank, A. J. In "Energy Resources Through Photochemistry and Catalysis"; GrBtzel, M., Ed.; Academic Press: New York; 1983. (2) Wagner, F. T.; Somorjai, G. A. Nature (London) 1980, 285, 559. (3) Darwent, J. R. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1703. (4) Lehn, J. M.; Sauvage, J. P.; Ziessel, R.Noun J . Chim. 1981,5, 291. (5) Borgarello, E.; Kiwi, J.; Pellizetti, E.; Visca, M.; GrBtzel, M. Nature (London) 1981, 289, 158. (6) Mills, A,; Porter, G. J. Chem. Soc., Faraday Trans. 1 1982. 78,3659. (7) Domen, K.; Nalto, S.; Onishi, T.; Tamura, K.; Soma, M. J. Phys. Chem. 1982,86, 3659. (8) Gutierrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1983, 87,
474. (9) Hashimoto, K.; Kawai, T.; Sakata, T. Nouv. J. Chim. 1983, 7, 249. (10) Houlding, M.; GrBtzel, M. J . Am. Chem. Soc. 1983, 105, 5695. (1 1) Matsumura, M.; Mitsuda, K.; Tsubomura, H.J . Phys. Chem. 1983, 87, 3807. (12) Sato, S.;White, J. M. J. Am. Chem. SOC.1980, 102, 7206. (13) Fang, S. M.; Chen, B. H.; White, J. M. J . Phys. Chem. 1982, 86, 3126. (14) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 4664. (15) Pichat, P.; Mozzanega, M. N.; Disdier, J.; Hermann, J. M. Nouv. J. Chrm. 1982, 6, 559. (16) Fujihara, M.; Satoh, Y.; Osa, T. Nature (London) 1981, 293, 206. (17) Kawai, T.; Sakata, T. J . Chem. SOC.,Chem. Commun. 1979, 1047. (18) Matsumoto, Y.; Nagai, H.; Sato, E. J. Phys. Chem. 1982, 86, 4664.
oxalate,21a ~ e t a t esulfide,24 ,~~~~~ and a l c o h o l ~ . ~In~ J ~ some cases the product of donor oxidation is also of interest.25 Although it has been claimedz6that the current state of development of homogeneous sacrificial photolysis systems warrants little further work, this certainly is not so for heterogeneous systems incorporating colloidal semicond~ctors.~~ Reported work6 involving a variety of semiconductors has shown that physical properties such as crystal morphology, particle size, and state of dispersion often exert a great influence on photolysis efficiency. Moreover many studies have been carried out with semiconductor particles that have not been fully characterized, so that experimental conditions are often ill-defined and the results are nonreproducible. The aim of the present study was to prepare a Pt/TiO, catalyst, that is, a dispersion of T i 0 2 particles carrying Pt particles, by controlled hetermgulation of well-characterized colloidal Pt and Ti02 particles. This simple method of preparation should produce well-dispersed Pt particles supported on TiOz particles and avoids the need for the salt addition,28i r r a d i a t i ~ n ,i ~m~p r e g n a t i ~ n , ~ ~ hydrogen reduction,30 or drying28*30procedures found in other catalyst preparations. The Pt and Ti02 particles used in our study were the smallest available-thus ensuring a catalyst of high specific surface area and hopefully maximum physical stability. In this paper we describe the loading of Pt onto Ti02 and the stability of the Pt/TiOz particles. The effects of pH, ionic strength, electron donor (EDTA), and polymer (poly(viny1 alcohol), PVA) stabilization on the Pt loading were investigated. In part 2 we will describe the performance of the Pt/TiOz catalyst in sacrificial photolysis systems. Experimental Section Materials. Details of all the colloids used are given in Table (19) Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H. B. Nouu. J. Chim. 1980, 4, 377. (20) Miller, D.; McLendon, G. Inorg. Chem. 1981, 20, 950. (21) Bliese, M.; Launikonis, A.; Loder, J. W.; Mau, A. W. H.; Sasse, W. H. F.Aust. J . Chem. 1983,36, 1873. (22) Koudelka, M.; Sanchez, J.; Augustynski, J. J. Phys. Chem. 1982,86, 277. (23) Ward, M. D.; Bard, A. J. J. Phys. Chem. 1982, 86, 3599. (24) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 301. (25) Kitamura, N.; Tazuke, S . Chem. Lett. 1983, 1109. (26) Harriman, A. Phozochemistry 1983, 14, Part V. (27) Aspnes, D. E.; Heller, A. J . Phys. Chem. 1983, 87, 4919. (28) Mills, A. J . Chem. Soc., Chem. Commun. 1982, 367. (29) Duonghong, D.; Borgarello, E.; Gratzel, M. J. Am. Chem. SOC.1981, 103, 4685. (30) Kiwi, J.; Gratzel, M. J. Phys. Chem. 1984, 88, 1302.
0022-3654/85/2089-0626$01.50/00 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No, 4, 1985 621
Preparation of a Pt/TiO, Catalyst
TABLE I: Colloids designation B1 F H1
P25 CLD640 BDH Linde A
material platinum platinum titania
titania titania
silica alumina
av particle radius, nm 1.oQ
sp surface area, m2 g-' 140b
2.0"
70b
4.4a.d 14d 41d 22d
16lC
5 3d
preparation/source preparation: citrate reduction of H2PtC1,' preparation: citrate reduction of H2PtC1,' preparation: hydrolysis of titanium isopropoxidd (anatase)
source:g Degussa Chemicals (anatase) source:g Tioxide International Ltd. (rutile) source:g BDH Chemicals source:g Union Carbide
5lC 17.4c 62.OC
15.OC
a Average particle size estimated from electron micrographs (Jeol CXlOO Microscope). *Surface area calculated from average particle size assuming spherical particles. cSee ref 34. dAverage particle size calculated from experimentally measured surface area. CSurfacearea determined from BET analysis of nitrogen adsorption isotherm at 77K. Samples outgassed at room temperature. 'Method of Hengleini4(pH 1.5). 'Materials used as received.
I. The TiO, sol H1 combined with the Pt sol B1 was chosen at the outset to form the catalyst for photolysis experiments (see part 2). With these two component sols a catalyst with high specific surface area can be obtained and scattering of light in a photolysis run can be reduced. Diffraction patterns obtained by electron microscopy confirmed31that TiO, sol H1 particles had the anatase structure. TiO, sol H1 and Pt sol B1 particles were approximately spherical (electron microscopy) but were too small to be observed in our microelectrophoresisapparatus or separated from each other by using our centrifuge. Therefore, the larger particle size Ti02 sols P25 and CLD640 and Pt sol F were used in some comparative characterization experiments. Silica and alumina colloids were used for comparative purposes in some Pt-binding experiments. All electrolyte solutions (NaCl, NaNO,, Na2EDTA) and acid (HCl) and base (NaOH) solutions were prepared with analytical reagent chemicals and triply distilled water (water conductivity less than 0.85 pS cm-' when equilibrated with air). Poly(viny1 alcohol) (PVA) solutions were prepared by using BDH Laboratory Reagent (average molecular weight 85 000). Methods. Electrophoretic mobilities were determined by using a Rank Bros. Mark I1 apparatus in conjunction with a flat ce11.32933 In all mobility/pH runs the pH was reduced from an initial value of around 9. All sols were maintained at essentially constant ionic strength at pH above 4 by the addition of lo-, mol dm-, of NaNO,. As the pH was reduced to below 3, however, the ionic strength increased significantly-electrophoretic mobilities would therefore decrease slightly with this decreasing pH due to ionic strength effects alone. Therefore, electrophoretic mobilities obtained at these low pH values provide only a semiquantitative guide to the pH dependence of particle charge. Turbidity and absorbance measurements were performed by using a Cary 219 UV/vis spectrophotometer. Ultrafiltration of T i 0 2 sol H1 was performed by using an Amicon stirred cell (65 cm3)with an XMlOOA (nominal molecular weight cutoff 100000) membrane. At least 20 cell volumes of wash liquor were used in each ultrafiltration run. Spectrophotometric investigation of filtrates revealed that Ti02 particles did not pass through the membrane. Coagulation experiments with TiO, sol H1 were performed by using a stirring/centrifugation procedure described elsewhere.34 The procedure identifies the formation of aggregates with diameter greater than approximately 250 nm. The uptake of colloidal Pt particles on colloidal oxide particles was determined by monitoring the Pt concentration (absorbance at 450 nm34)before and after equilibration with oxide. The Ti02 sol H1 particles were removed by ultrafiltration through a XMlOOA membrane which does not remove B1 Pt particles. The other oxide particles were removed from the Pt sol by centrifugation. EDTA adsorption onto TiO, and Pt particles was also determined by the equilibration/filtration technique described above (3 1) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 241. (32) Smith, A. L. In "Dispersions of Powders in Liquids", 2nd ed.;Parfitt, G. D., Ed.; Applied Science: London, 1973; p 86. (33) Furlong, D. N.; Parfitt, G. D. J. Colloid Interface Sci. 1978,65, 548. (34) Furlong, D. N.; Launikonis, A,; Sasse, W. H. F.; Sanders, J. V. J . Chem. Soc., Faraday Trans. 1 1984, 80, 571.
for Pt uptake. It was observed that adsorption of EDTA onto B1 Pt particles induced them to ~oagulate.,~EDTA concentrations were measured spectrophotometrically.36
Results and Discussion Catalysts were prepared by mixing aqueous dispersions of colloidal Pt and TiO, particles and allowing Pt-TiO, binding to take place. Description of this binding interaction requires knowledge of the surface characteristics of the T i 0 2 and Pt particles. ( a ) Cleaning and Characterization of T i 0 2 Particles. Clean TiO, surfaces are known to be hydroxylated when contacted with water.,' The pH dependence of the protonation/deprotonation of these surface hydroxyls results in an isoelectric point (iep) around pH 6. Chloride ions were present during the preparation of T i 0 2 sols H1 and CLD 640 involving the hydrolysis of aqueous Ti4+ions. Anions such as chloride adsorb at TiOzelectrolyte interfaces and lower the i e ~ . , ~The iep for oxide surfaces defines the pH below which particles carry net positive charge and above which they carry net negative charge. As particulate surface charge often controls the stability of particles and adsorption at interfaces in colloidal dispersions, it is likely that the surface charge/pH behavior of TiOz particles may affect their efficiency in photolysis systems. Therefore, it is essential that this behavior be characterized for any TiO, catalyst support. Ti02 sol H1 particles were too small to be tracked in our conventional microelectrophoresis apparatus; direct determination of an electrophoretic mobility/pH curve was therefore not possible. It has often been shown, however, that colloidal particles coagulate if the magnitude of the "surface" potential is less than 14-25 mV. This potential can be viewed as the potential that controls the electrostatic repulsion between particles and may in fact not correspond exactly to the potential at particle surfaces. At pH values sufficiently below the iep to ensure a positive potential greater than 14-25 mV, and, similarly, at pH values sufficiently above the iep to ensure a negative potential greater than 14-25 mV, particle charge can prevent coagulation. Therefore, colloidal stability as a function of pH can be used to locate the iep. The data of Figure 1 describe the pH/stability behavior of TiOz sol H1 after ultrafiltration in 10-1,6-10-4mol dm-, HC1 solutions (pH 1.6-4). The T i 0 2 particles were prepared at pH 1.6 in the presence of HC1 and 2-propanol. Ultrafiltration at this pH removes 2-propanol (curve I, Figure 1). Separate lots of sol H1 were ultrafiltered at each pH value indicated in Figure 1. After each ultrafiltration the sol appeared transparent. For the stability experiments the pH was increased after ultrafiltration by addition of NaOH. The T i 0 2 particles coagulated at higher pH as the ultrafiltration pH was increased (Figure 1). The effect of chloride on coagulation is clearly seen and probably results from the ~~~~
(35) Furlong, D. N.; Sasse, W. H. F. Aust. J . Chem. 1983, 36, 2163. (36) Chang, H. C.; Healy, T. W.; Matijevic, E. J . Colloid Interface Sci. 1983, 92, 479. (37) Furlong, D. N.; Yates, D. E.; Healy, T. W. In "Electrodes of Conductive Metal Oxides"; Trasatti, s.,Ed.; Elsevier: Amsterdam, 1981; Part B, p 361. (38) Parfitt, G. D. Prog. Surf. Memb. Sci. 1976, 1 1 , 181. (39) Parfitt, G. D.; Wharton, D. G. J. Colloid Intetface Sci. 1972,38,431.
628
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
Furlong et al.
p H / Instability of colloidal TiO,
Platinum p a r t i c l e s binding to o x i d e s
0 0
Ti0,ultrofiltered Ti0,ullrofiltered Ti0,ultrafillered
a t pH 1.6 at pH 2.5 at pH 3.0
- curve I - curve II - curve m
0
Ti0,ultrofilIered
at pH 4.0 a t pH 4.0
- curve E=pH decreasing
0
-- EO, ultrafiltered
- curve Ip,
Ti
2
3
4
5
6
7
8
9
PH
Figure 1. Coagulation of TiOz sol-effects of ultrafiltration and pH: TiOzsol HI at 0.33 g dm--’. Coagulation determined from turbidity at 500 nm. All turbidities normalized to 100 at pH 9 (curves I to IV) or
pH 5 (curve IV*). downward shift in iep that results from the adsorption of chloride anions. The coagulation behavior of the Ti02 particles will also depend on ionic strength in that electrolytes can induce coagulation. The dispersion that was ultrafiltered at pH 1.6 was at an ionic strength of 0.025; the dispersion that was ultrafiltered at pH 4 was at an ionic strength of 0.001. The data of Figure 1 will include the effects of this ionic strength difference on the interaction potential between particles as well as the increase in iep with pH of ultrafiltration, Ultrafiltration a t pH 4.5 resulted in rapid coagulation during ultrafiltration; the effect of adsorbed chloride on the iep of the Ti02 was therefore minimal at less than l P mol dm-3 of total chloride. Curve IV in Figure 1 presumably corresponds to clean positively charged Ti02 particles. When the pH of sol IV was adjusted from 4 to 9 by the addition of concentrated NaOH during sonication, the sol remained stable for several hours. Curve IV* describes coagulation as the pH was subsequently returned to around 4 and presumably represents the behavior of clean negatively charged particles. The surface charge/pH profile for T i 0 2 is known to be approximately symmetrical about the iep@(see also Figure 3). Hence cuxves IV and IV* indicate an iep near 6.1 for TiO, sol H1 particles, in agreement with the established literature value for a n a t a ~ e(see ~ ~ again ,~~ Figure 3 for Ti02sol P25). The effectiveness of ultrafiltration of Ti02sol HI at p H 4 is thus confirmed. Therefore, we stress that unless care is taken to clean TiO, surfaces, impurities are likely to be present which significantly affect surface charge/pH behavior. In some reported photolysis ~ t u d i e s ~in~which * ~ l colloidal Ti02 particles were used, the iep values quoted for the TiO, indicate that surface anion contamination was present. The question arises whether these (and perhaps other) surface impurities affect photochemical performance in the systems concerned. In this respect it is interesting that when 2-propanol was not completely (40) B6rub6, Y.G.; de Bruyn, P. L. J. Colloid Interface Sci. 1968,28, 92. (41) Moser, J.; Gratzel, M. J . Am. Chem. SOC.1983, 105, 6547.
-
3
t
iepSi0,
4
5
6
7
8
I
iepTiO,
9
t
iepAI,O,
PH
Figure 2. Uptake of colloidal Pt on colloidal oxides: Pt sol F at 5 X 10-5 mol dm-3. Open symbols: citrate removed from Pt sol. Closed symbols: citrate not removed from Pt sol. Oxides at 10 mz dm‘3 of surface.
removed from Ti02 sol H1 it functioned as an electron donor.42 We have also found42that methanol, used as a medium in some catalyst preparation technique^,^^ will function as an electron donor. In addition TiO2 sol H1 particles were activated to visible light at pH 3, by using R ~ ( b p y ) ~[tris(2,2’-bipyridine)ruthe~+ nium(I1) dication] derivatives, when the sensitizer was salted-out onto particle surfaces by adding appropriate anions.42 All experiments discussed henceforth in this paper and part 2 were therefore conducted with Ti02 sols that were thoroughly cleaned by ultrafiltration of Soxhlet washing at pH 4 to 6. ( b ) Cleaning and Characterization of Platinum Particles. Detailed studies of the preparation and cleaning of Pt sols B1 and F have been reported previ~usly.~~ In the present study all Pt sols were treated by ion exchange34to remove impurities, mainly citrate present in excess during preparation. The Pt particles dispersed in water behave as if negatively charged above pH approximately 2. The negative charge most probably arises from adsorption of platinum-chloro complexes although there is also the possibility that surface layers of oxide are present and contribute a pH dependence to the charge. Sols F and B1 have previously been found34to coagulate when the pH was reduced below 3.5 and 2.8, respectively. At pH 5 the addition of KCl in excess of and lo-’ mol dm-3 causes coagulation of sols F and B 1, respectively. The differences between the pHs and the salt concentrations necessary to coagulate sols F and B1 reflect the different size Pt particles in the (c) PtlTiO, Catalyst-Preparation by Heterocoagulation. Mills28has recently reported the “precipitation” of Pt particles onto TiO, particles using 2 mol dm-3 NaCl as a method of preparing superior R/Ti02 catalysts. No doubt the 2 mol dm-3 NaCl causes homocoagulation of the TiO, particles and possibly also of the Pt particles. The state of Pt dispersion in catalysts prepared by the Mills procedure is, therefore, unclear. Since Pt particles will bind to substrates such as TiO, when the solution conditions (42) Furlong, D. N.; Wells, D., unpublished results
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 629
Preparation of a Pt/TiOz Catalyst 0 0.1 0
q/dm3 TiO,
0.1 q/dm3 TiO,,
molldm3 Pt
0.01 g/dm3 TiOZl IO+ mol/dm3 P I 0 0.01 q/dm3 TiO, IOw4mol/dm3 Pt mol/dm3 Pt Lf 0 01 g/dm3 Ti0,: 3 x 0
Figure 3. Microelectrophoresis-binding of colloidal Pt to TiO,: TiO, sol P25; Pt sol B1. IO-, mol dm-’ NaNO,.
are adjusted so that the substrate is positively charged, there is no need for salt addition. Under these conditions the distribution of Pt will be more uniform because it results from interaction between dispersed Pt particles and substrate particles. ( i ) Pt Uptake us. p H . The uptake of Pt sol F particles on to T i 0 2 CLD640 particles is shown in Figure 2, which, for comparison, also includes results obtained with silica and alumina. Silica, titania, and alumina represent a series of oxides in which surface behavior varies from acidic to neutral to basic. The data show that Pt particles bind to positive oxide particles at pH values below the iep and that the amount of binding increased with increasing positive charge on the oxide. Note that Pt uptake was independent of the presence of residual citrate in the Pt sol. We found earlier34that the Pt sol was stable toward coagulation at pH above 3.5. Figure 2 shows that Pt was not removed from the Pt-SiO, sol by centrifugation at pH 3.0. It appears, therefore, that in the present study the Pt particles did not bind to SiO, particles nor did they homocoagulate at pH 3 in the presence of the S O 2particles. This lack of homocoagulation of the Pt may also hold for the Pt-TiO, dispersions at pH values in the range 3 to 3.5. The additional stability of the Pt at low p H compared is probably due to slight variation in the to our previous preparation of the Pt sols between the present and the previous study. Electrophoretic mobility/pH curves for TiO, particles in dispersions also containing Pt particles (Figure 3) indicate that the binding of Pt particles resulted in the Ti0, particles acquiring extra negative charge. Curves A and B, which correspond to dispersions where the number ratio of Pt to T i 0 2 particles was very high (see discussion below), suggest that the Pt particles are negatively charged at pH above 2 and that the magnitude of that charge is approximately constant from pH around 4 to pH 8. Therefore, the dominant influence on binding of Pt to Ti02 in this pH range was the extent of the positive charge on the TiOz particles. When binding experiments were performed by using TiO, to Pt ratios that were approximately an order of magnitude above that used in the experiment of Figure 2, small levels of Pt uptake onto TiO, were observed at pH values well above the iep of the Ti02. This binding may result either from the small number of positive surface sites that persist on Ti0, surfaces above the iep43
or from a small nonelectrostatic component in the interaction between Pt and TiO,. The electrophoretic mobility of the Ti0, particles in the presence of the Pt particles can be used to probe the onset and extent of Pt binding. Figure 3 shows particle electrophoretic mobility/pH curves for various TiOZ(P25)/Pt(Bl) sol mixtures. The curve for TiO, alone is typical of a wide variety of TiO, samples in dilute ( mol dm-3) e l e ~ t r o l y t e . ~The ~ divergence of the Pt/TiO, curves from that of Ti0, indicates the onset of Pt binding as the pH was reduced below 9, confirming the observations from the uptake measurements. The unbound Pt particles were too small to be seen in the microelectrophoresis experiment. The effect of Pt binding to Ti02 on the electrokinetic behavior of the TiO, was dramatic. The iep was reduced from 6.3 to values below 3.4. P25 particles and sol B1 particles have radii of approximately 14 and 1 nm, respectively. A close-packed layer of the Pt particles corresponds, therefore, to around 600 per Ti0, particle. Curves A, B, and C in Figure 3 correspond to around 1000,100, and 31 available Pt particles per TiO,, respectively, if it is assumed that each sol is monodisperse in spherical particles. The mobility curve A, at least at the lower pH values where significant Pt binding occurred to cover TiO, surfaces, presumably resembles, therefore, the mobility curve that would be obtained for Pt sol B1 particles if they were of size accessible to our microelectrophoresis apparatus. It shows that the magnitude of the mobility decreased rapidly as the pH was lowered below around 4 and suggests an iep around pH 1 for the Pt particles. An iep of 1 has recently been reported by Kiwi and Gratze130for Pt particles prepared by reduction of Pt(NH3)42+. Curves B and C represent the behavior of mixed sols (Pt and Ti02) where the possible coverage of Pt on TiOz is less than saturation-defined as that level of coverage above which the electrokinetic properties of the Pt/Ti02 aggregate do not change. Kiwi and Gratzel have reported30 an iep of 4.2 for Pt/TiO, particles in which approximately 3% of the TiO, surface was covered by Pt. Our curve C (iep 3.4, Figure 3) is for a system with a maximum possible coverage of around 5%. Our electrophoretic mobility data appear consistent, therefore, with those of Kiwi and Gratzel, indicating that the surface charge properties of the catalysts prepared by our heterocoagulation procedure are similar to those of the catalysts prepared by the impregnation/reduction procedure of Kiwi and Gratzel. Curves A and B in Figure 3 suggest that Pt binding occurred at pH values above the iep of Ti0,. It also appears that the upper pH limit at which binding occurred decreased as the ratio of TiO, to Pt was reduced. Regardless of the precise mechanism of charge development on colloidal Pt, it is clear from Figure 3 that the binding of Pt particles to Ti0, particles changes the surface chemistry of the TiO, particles profoundly. Such change must be considered in the evaluation of Pt/TiO, aggregates in photolysis systems. Figure 4 shows the desorption, as well as the adsorption, of Pt on TiO, as the pH of the dispersion was varied in the range 3 to 6. Curve I shows the uptake of Pt as the pH was lowered from 6 to 3 in two separate experiments in which the dispersions were equilibrated for ca. 15 min a t each value of pH. Curves I1 and I11 show the release of Pt that occurred when the pH was raised again to 6. In the experiments corresponding to curve I1 the pH was raised from 3 to 4.2 and then to 5.5 with 15 min at each pH. In the experiments corresponding to curve I11 the longer times at each pH were as indicated. Clearly “desorption” of Pt occurred when the pH was increased, but the rate was much less than that of the adsorption. Similar slow desorption was observed for Pt bound to alumina. Pt desorption presumably occurs because positive charge on the T i 0 2 surfaces is lost as the pH is raised from around 3 to 6 with consequent loss of binding energy. The origin of this positive charge has been ascribed)’ to the protonation of surface hydroxyl groups. In the absence of “adsorbates” the protonation/deprotonation of surface hydroxyl groups of TiO,, (43) Smith, A. L. J. Colloid Interface Sei. 1976,55, 525. (44) Hunter, R. J. “Zeta Potential in Colloid Science-Principles and Applications”;Academic Press: New York, 1981.
630 The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
Furlong et al.
I00
80
s
.-c
-n
24 hrs
0
L
-
60
0 0 >
5
n
E
40
20
01
I
2
I 3
I 4
I 5
6
-
I-
I
I
7
8
9
I IO 0
I
I
Figure 4. Binding/removal of colloidal Pt to/from Ti0,: Ti02 sol CLD640 at 10 m2 dm-’ of surface: Pt sol F at 5 X mol dm-’.
as revealed by the mobility curve for Ti02 in Figure 3, is readily reversible with respect to pH. Therefore, the data in Figure 4 suggest that deprotonation of surface hydroxyl groups is retarded within the region of contact between Pt and Ti02. Figure 4 also suggests that the longer the Pt/TiO, aggregates are left intact the less likely they are to dissociate. It is possible that exclusion of interfacial water within contact regions retards desorption. When Pt/Ti02 aggregates formed at pH 3 were dried in air and then redispersed in water at pH 6, release of Pt was not observed after several days. In summary, the pH dependence of the binding of Pt particles to T i 0 2 results from the pH dependence of particulate surface charges and the largely electrostatic binding interaction. Increases in the pH of dispersions containing Pt/Ti02 particles result in a slow release of bound Pt particles. (ii) Limiting Coverage of Pt on T i 0 2 . At pH 3 the particles of Pt sol B1 and T i 0 2 sol H, are well dispersed and the negative particles of sol B1 attract the positive sol H 1 particles. Figure 5 describes the extent of uptake of Pt particles onto the T i 0 2 particles. The Pt concentration was maintained constant, and the Ti02 concentration was varied over two orders of magnitude; each point in Figure 5 represents a separate experiment. For the experiments corresponding to the segment BC in Figure 5, the area of Ti02 surface per bound Pt increased with increasing concentration of T i 0 2 and the equilibrium concentration of unbound Pt was less than 5% of the initial concentration in each case. The area was calculated from the uptake of Pt and from the known particle sizes, assuming the particles to be spherical. It appears that these dispersions contain an excess of TiOz with consequent low packing densities of Pt. By contrast, for all of the experiments corresponding to the segment AB in Figure 5 the average area of Ti02 surface per bound Pt particle was calculated to be 14 2 nm2. Therefore, we conclude that the segment AB represents mixed Pt plus Ti02 sols in which unbound Pt particles are in equilibrium with Pt/Ti02 aggregates whose surface density of Pt particles is the maximum possible at pH 3. The area of 14 2 nm2 probably corresponds to the minimum area per Pt particle attainable on T i 0 2 because the positive charge on the Ti02 is probably fully developed at pH 3 (Le., at 3.1 pH units below the iep) and the negative charge on the Pt particles decreases when the pH is reduced below 3. Binding of Pt at pH 3 to TiO, sols P25 and CLD640 and to Linde A alumina at pH 4 gave the same limiting area per Pt, suggesting that a factor independent of the type of oxide surface is decisive in determining the maximum packing density of Pt. Since an area of 14 nm2 is projected by
*
*
I
I
-3
-4
DH
I
I
-2
log [ T i 0 2 ]
1
I
I
I
10
100
in2 dm-3
o f Ti0, surface
Figure 5. Uptake of colloidal Pt on TiO, at pH 3.0: Ti02 sol H1;Pt sol B1 at 1.1 X mol dm-’.
a sphere of 2.1-nm radius, each Pt particle of 1.0-nm radius occupies an area at maximum packing density that is much larger than that of the Pt particle itself. The observation that the same maximum packing density was found for both Ti02 samples and the alumina seems to preclude any surface topographical restriction on packing. The inability of the Pt particles to pack into geometric close packing is probably caused by repulsions between adjacent Pt particles resulting from overlap of the electrical double layers at the Pt-electrolyte interface. The sols of Figure 5 contain H+ and C1- at an ionic strength of approximately mol dm-3. At this ionic strength diffuse electrical double layers extend approximately 10 nm. Hence Pt particles separated by 2.1 nm will interact strongly with each other via their electrical double layers. These Pt-Pt repulsions could be reduced by an increase in the ionic strength, say, by the addition of salt. However, this will also reduce the electrostatic attractions between Pt and Ti02 particles and could cause homocoagulation of the Ti02 and of the Pt.34 The overall result of salt addition might therefore be a coagulated Pt/Ti02 system in which the Pt particles are not well dispersed. Such a catalyst is not likely to function as well as one with well-dispersed Pt particles. (iii) Effects of EDTA. Most electron donors used in photolysis systems (e.g., EDTA and oxalate) are electrolytes and are used at concentrations above l e 2mol dm-3. EDTA anions have already been shown to interact strongly with Pt35and o ~ i d eparticles. ~ ~ , ~ ~ Figure 6 shows the binding of Pt (sol F) to Ti02 (sol CLD640) at pH 3.9 in the presence of EDTA. The data for lo-, mol d m 3 EDTA presented in Figure 6 have been corrected for the slight loss of Pt due to its homocoagulation induced by the EDTA. The extent of this homocoagulation was determined from a blank experiment in the absence of Ti02. At a concentration of mol dm-3 EDTA did not destabilize Pt sol F.35 Inspection of Figure 6 shows that EDTA suppresses the binding of Pt to TiO,. This effect was not seen when NaCl instead of EDTA was used. mol However, when both NaCl (lo-’ mol dm-3) and EDTA ( dm-3) were added to the Pt sol the level of Pt uptake on TiO, was the same as that when neither NaCl nor EDTA was present. We will discuss this effect later. The suppression of the Pt-TiO, (45) Rubio, J.; Matijevic, E. J . Colloid Interface Sci. 1979, 68, 408.
Preparation of a Pt/TiO, Catalyst
0 0 0
TiO, TiO, TiO,
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 631
7.5
+ 10‘3M
EDTA
+ IO-‘M
EDTA I *I )
E
0
0
5
E 0
>
0
E
e
s
-
.-
0
a 2.5
*, 0
-3
-4
-2
3.5
log [TiO,]
0.1
I IO
I
rn‘drr~-~ofTiO, s u r f o c e
Figure 6. Uptake of colloidal Pt on Ti02 at pH 3.9-effect of added EDTA: Ti0, sol CLD640;Pt sol F at 7.5 X lo-’ mol dm-).
4 2t/ 0
0
4.5
PH
I
I
4.0
I I
I
I
2
3
IO’ x EDTA] 1 mol/dm3 )
Figure 7. Adsorption isotherm of EDTA on Ti02at pH 3: Ti02sol P25.
binding induced by EDTA was limited to the “adsorption” of Pt since addition of EDTA ( mol dm-3) after the uptake of Pt did not cause desorption of Pt (up to 5 h). Thus while the addition of EDTA to a preformed Pt/Ti02 catalyst can induce coagulation of Pt/TiOz particles, it seems not to significantly change the state of dispersion of Pt particles on each Ti0, particle. The ability of EDTA to suppress Pt binding to TiO, undoubtedly results from its adsorption at interfaces present. The extent of this adsorption may well also affect the efficiency of EDTA as an electron donor in photolysis systems where the donation of electrons is from EDTA to the TiOz particle. Figure 7 shows the adsorption isotherm for EDTA on TiO, a t pH 3. The isotherm is very similar to those reported for iron o ~ i d e sand ~ ~reveals .~~ that near-saturation coverage of EDTA is obtained only when the equilibrium concentration of EDTA exceeds ca. mol dm-3. The isotherm for P25 TiO, shows an area per adsorbed EDTA species, at saturation coverage, of 0.40 nm2; TiOz sol H1 gave a value of 0.34 nm2. These areas are similar to that reported4’ for saturation adsorption of EDTA on positive goethite (FeOOH) surfaces. Interestingly, studies36on another oxide of iron, hematite (Fe203),revealed an area of 0.96 nm2 per adsorbed EDTA under the surface charge conditions which gave rise to 0.34 nm2 for
Figure 8. Coagulation of Pt/Ti02 dispersions-effect of Pt:Ti02 ratio and pH: Ti02 sol H1, ultrafiltered at pH 3.0, 0.43 g dm-’; Pt sol B1. N = calculated ratio of total Pt to Ti02 particles. Coagulation determined from turbidity at 500 nm. All turbidities normalized to 100 at pH 5.0.
adsorption on goethite. Clearly, the mode of adsorption of EDTA on oxides depends on the chemical nature of the oxide surface as well as on surface charge. We found that the adsorption of EDTA on TiO, was not significantly changed by the addition of lo-’ or 1 mol dm-3 NaCl. An area of 0.074 nm2 was found for saturation adsorption of EDTA on Pt particles. Molecular models show that this area is about equal to the smallest cross-sectional area of an EDTA molecule, and it would seem that EDTA anions are stacked with their N-N axis directed out from the Pt surfaces. Such stacking would be facilitated by the high relative curvature of the Ptelectrolyte interface for these 2-nm Pt particles. This curvature would allow more room for unbound segments of EDTA out from the surface even though the packing at the site of attachment was very dense. That the mechanism of EDTA adsorption on Pt is different from that on TiO, is further suggested by the observation that the addition of lo-’ mol dm-3 NaCl completely suppressed the adsorption of EDTA on Pt; no such suppression was found for the adsorption of EDTA on TiO,. The total adsorption of EDTA on each of several Pt/TiO, catalysts of different Pt:TiO, ratios was found in each case to be approximately equal to the sum of adsorption on the separated Pt and TiO, particles. This result is consistent with the Pt being well dispersed on the TiO, surfaces at its maximum packing density in a manner allowing access of EDTA to both TiO,-electrolyte and Pt-electrolyte interfaces. The addition of lo-’ mol dm-3 NaCl suppressed EDTA adsorption on the Pt component of the Pt/TiOz catalyst but not on the TiO, component. Noting again that NaCl inhibited the suppression effect of EDTA on Pt-TiO, binding, it seems than that this suppression is due to EDTA binding to Pt and not to its binding to TiO,. This may be of relevance to the performance of Pt/Ti02 catalysts in photolysis as presumably EDTA adsorption on the TiO, surface facilitates the scavenging of photogenerated holes while EDTA adsorption at the Pt-electrolyte interface may hinder desired electron transfer at this interface. This will be discussed further in part 2. ( c ) Stability of P t / T i 0 2 Catalyst. ( i ) Effects of p H and Pt Loading. Figure 8 shows the coagulation of mixed (Pt plus TiO,) dispersions as a function of pH for various values of N, where N is the calculated ratio of the number of Pt to Ti0, particles. For N < 18 nearly all Pt particles will be bound to TiO, at pH 3.
Furlong et al.
632 The Journal of Physical Chemistry, Vol. 89, No. 4 , 1985
E s t
P I ~ T ~ O ~~ d PVA d
O'
A
pldd
EDTA
Bubble w t h H2
11, o1:****,Io,,0:2 Minuler
Hrl
HIS
HIS
HW
Time
Figure 9. Coagulation of a Pt/Ti02 dispersion at pH 3.0-stabilization using PVA: Ti02 sol H1, ultrafiltered at pH 3.0, 0.43 g dm-'; Pt sol Bl-1O4 mol dm-). EDTA--IO-) mol dm-'. Inset: Stability of the Pt/Ti02 dispersion without added PVA while under illumination or H2
treatment. Therefore, all dispersions represented in Figure 8 consisted mostly of Pt/Ti02 particles with very few free Pt particles. The dispersions were equilibrated for approximately 2 h at pH 3, and then the pH was increased, with intervals of approximately 15 min between each pH change. Experiments of the type described in Figure 4 indicated that no significant "desorption" of Pt would have occurred during the experiments described in Figure 8. At pH 3 the Pt/TiOz particles were less stable with higher ratios of Pt to Ti02, i.e., at higher N values. The sol with N = 5 was found to be almost totally unstable using the centrifugation procedure. A sol with N = 15 (Le, at near the saturation value of 18) could be seen by eye to have coagulated within seconds of mixing of the Pt and Ti02 sols. Coagulation would appear to be the result of overall charge reduction due to binding of Pt to Ti02 particles. When the pH was increased above 3 all the sols of Figure 8 became less stable. As discussed (Figure l), the TiO, particles will homocoagulate when the pH is increased above 4 because of deprotonation of surface hydroxyl groups with subsequent reduction in surface charge. Figure 8 indicates that this reduction in the surface charge of uncovered Ti0, surfaces also occurred with the Pt/TiO, particles. (ii) Effects of EDTA, Light, and H2. With respect to Pt/Ti02 catalysts in photolysis mixtures the data of Figure 8 show clearly that the extent of Pt loading onto T i 0 2 particles significantly affects their stability. In an operating photolysis mixture, the presence of electron donor particularly, but also illumination and the generation of hydrogen, may affect Pt/Ti02 stability. Figure 9 describes the stability of a Pt/TiO, catalyst at pH 3, with respect to time, when EDTA (curves I and 11) and hydrogen and illumination (inset) are present. This catalyst ( N = 1) was stable when dispersed in water at pH 3, as expected from the above discussion, and this is shown by the relatively constant turbidity over the initial 180 min in curves I and I1 in Figure 9. Addition of mol dm-3 EDTA caused very rapid coagulation of the Pt/TiO, particles (curves I and 11, Figure 9). These coagulated
dispersions were stirred for several hours and the aggregates removed by centrifugation. Analysis of the supernatant confirmed that Pt had not been desorbed from TiO, as a result of coagulation. Illumination had no apparent effect on dispersion stability; after a few hours' bubbling with H2, however, dispersions became much more turbid. Prolonged bubbling (24 h) resulted in aggregate sedimentation. It has been showd4 that Pt sol B particles will coalesce under an H2 atmosphere. It is possible that coalescence of Pt particles bound to different Ti0, particles may play a role in the coagulation induced by H2 bubbling. (iii)Stabilization with PVA. It is clear then that addition of a dispersed Pt/Ti02 catalyst to a photolysis mixture containing EDTA can cause coagulation of that catalyst. Often41in photolysis research polymers are used to stabilize catalysts. Figure 9 shows that PVA, at a concentration above 0.5 wt % for the catalyst concerned, effectively retarded the coagulation of the Pt/Ti02 dispersion induced by mol EDTA (curves IV and V). The results suggest that between 0.5 and 1.0 wt % PVA was required to achieve sufficient surface coverage of PVA to provide stabilization. The adsorption of all of the PVA from a 1 wt % solution corresponds approximately to 1.4 nm2 per adsorbed PVA molecule. The process of adsorption of PVA at particle-electrolyte interfaces has been shown to be complex46- particularly as the PVA is polydisperse in molecular weight. Previous studies46with dispersed solids indicate that the area of 1.4 nm2 per adsorbed PVA on Ti02 is consistent with a saturated adsorbed layer of PVA at Pt/Ti02 particle-electrolyte interfaces. Curves IV and V in Figure 9 show that the PVA did not render the system Pt/ Ti02/EDTA resistant to coagulation by hydrogen bubbling.
Conclusions (1) Extensive ultrafiltration at pH not less than 4 was found to give clean Ti02 H1 particles. These particles coagulated when the pH was raised above 4. (2) Negatively charged Pt particles (pH above 2) bind strongly to positively charged oxide particles (Le., below the iep), indicating a mainly electrostatic interaction. Bound Pt particles desorbed very slowly from oxide particles when the pH was raised to the iep of the oxide. (3) Saturation coverage of Pt on TiO, lowered the iep of the TiO, from 6.3 to below 3. Such saturation coverage corresponded to a Pt-Pt separation equal to one Pt particle diameter. This separation is thought to be due to lateral repulsions between Pt particles resulting from overlap of electrical double layers on the Pt particles. mol dm-3 (4) Pt did not bind to Ti02 in the presence of EDTA. EDTA did not induce desorption of bound Pt. EDTA was found to adsorb on both Pt and Ti02 but apparently more densely on Pt. NaCl suppressed the adsorption of EDTA on Pt but not the adsorption on Ti02. (5) Pt binding to T i 0 2 (at pH 3) induced coagulation when Pt coverages became significant. Coagulation in all Pt/Ti02 systems increased when the pH was raised above 3. EDTA caused rapid coagulation of Pt/TiO, sols without causing desorption of Pt from Ti02 particles. ( 6 ) Pt/Ti02 particles coagulated when treated by H2 bubbling. (7) PVA was found to stabilize the Pt/Ti02 particles towards coagulation by EDTA but was not effective against coagulation induced by H,. Acknowledgment. We are grateful to Dr. J. Sanders (CSIRO Division of Materials Science) for obtaining electron micrographs and to P. Freeman (University of Melbourne) for performing the microelectrophoresis experiments. Registry No. Pt, 7440-06-4; Ti02, 13463-67-7;EDTA, 60-00-4; PVA
(homopolymer),9002-89-5. (46) Koopal, L. K. J . Colloid Interface Sci. 1981, 83, 116.
