2733
J. Phys. Chem. 1986, 90, 2733-2738
Effect of Deposited Pt Particles on the Surface Charge of T102 Aqueous Suspensions by Potentiometry, Electrophoresis, and Labeled Ion Adsorption Nicole Jaffrezic-Renault? Pierre Pichat,* Ecole Centrale de Lyon, BP 163, 69131 Ecully Cedex, France
Alain Foissy,s and Ren6 Merciers UniversitC de Franche- ComtP, 25030 Besancon Cedex, France (Received: December 17, 1985)
Potentiometry, electrophoresis, and radiotracer technique have been used to study the charge density in different parts of the double layer at the Ti02-electrolyte interface for powder samples supporting 0.5-10 wt % platinum as particles of homogeneous size (ca. 2 nm diameter). Less positive charge densities were measured by potentiometry for increasing Pt content; the value of point of zero charge decreases accordingly. More negative electrophoretic mobilities were found for the metal-loaded samples from pH 3 to pH -8; as a result the value of the point of zero {potential decreases with increasing Pt content. From a comparison between the surface charge density, the charge density of the diffuse part of the double layer, and the charge density of the counterions (determined from adsorption isotherms of labeled Na+ and C1- ions for TiOz and 10 wt 9% Pt/Ti02), it is deduced that a part of the C1- ions form neutral adsorbed species at the Pt surface. Therefore, the changes observed in potentiometric titrations and electrophoretic mobilities are attributed to an increase in Ti02 acidity induced by the Pt particles, which reflects the decreased TiOz electron density.
Introduction Powders or colloids constituted by a semiconductor and a deposited metal of group VI11 (groups 8-10, see ref 33), in particular the system Pt/Ti02, have recently received much attention as catalysts and photocatalysts. In catalysis, it has been discovered that so-called strong metal-support interactions deeply modify the chemisorptive and catalytic properties of the metal.' In photocatalysis, the presence of a group VI11 (groups 8-10) metal is required to render catalytic reactions involving hydrogenZas a reactant or as a product, and such solids have been tentatively used in research aiming a t the cleavage of ~ a t e r . ~ - ~ In catalysis, the studies have been directed to the understanding of the changes in the metal properties resulting from the semiconductor support. In the case of photocatalysis, it is essential to determine the effects of the deposited metal on the semiconductor surface properties, since the basic steps take place on the semiconductor.2 Measurements of the electrical conductivity6 and photoconductivity7 of powder titanium dioxide, performed under vacuum, have shown that the semiconductor electron density is decreased in the presence of deposited group VI11 (groups 8-10) metal crystallites. This decrease should affect the surface properties of Ti02 Such properties can be evaluated by model catalytic or photocatalytic reactions and by the use of probe molecules, for example, to assess the acidic and basic sites8 Generally, these methods involve solid-gas interactions. Methods used to investigate the solid-aqueous solution interactions are much less commonly employed in the field of catalysis where gas-phase reactions predominate in industry, but are also of great interest, particularly for the present solids which are utilized in aqueous solutions as photocatalysts. Among these methods, potentiometry is based on the reactions of the surface hydroxyl groups with proton^.^*'^ For example, in the case of T i 0 2
+ H+ F? TiOHz+ TiOH TiO- + H+
TiOH
F?
(1) (2)
or TiOH
+ OH- @ TiO- + H 2 0
(2')
In addition, equilibria involving counterions can be considered." For example, in the presence of NaCl Laboratoire de Physico-Chimie des Interfaces (U.A. CNRS 404). * E q u i p CNRS Photocatalyse (J. E. 45 94). 8 Laboratoire d'Electrochimie des Solides (U.A. CNRS 810).
+ H+ + C1- TiOHz+...C1TiOH + Na+ s TiO--Na+ + H+
TiOH
(3)
(4)
Potentiometric titrations' 1-13 allow one to determine the surface charge density uo which is given (in pC cm-2) by uo = 106F (TiOH2+.-C1-) - (TiO-) - (TiO-.-Na+)] -[(TiOH2+) mS (5)
+
where m is the concentration of the solid in the solution (in g dm-3), S is the surface area of Ti02 (in cmz g-l), the concentrations being expressed in mol dm-3. The charge density Ud of the diffuse part of the double layer can be calculated from electrokinetic experiments.'"16 Finally, the adsorption of counterions can be mea~
(1) See,for example: Tauster, S.J.; Fung, S.C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. Metal-support and Metal Additive Effects in Catalysis; Imelik, B. et al., Eds.; Elsevier: Amsterdam, 1982. Metalsupport Interactions; Tauster, S.J., Baker, R. T. K., Dumesic, J., Us.ACS ; Symp. Ser. 286; American Chemical Society: Washington, DC, 1986. (2) See, for example: Pichat, P. In Organic Phototransformations in Nonhomogeneous Media; Fox, M. A., Ed.; ACS Symp. Ser. 278; American Chemical Society: Washington, DC, 1985; p 20 and references therein. (3) See, for example: Harriman, A. Spec. Period. Rep.: Photochemistry 1984, 14, 513. . (4) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanega, M.-N. N o w . J . Chim. 1981, 5, 627 and references therein. ( 5 ) Mills, A.; Porter, G. J . Chem. Soc., Faraday Trans. I 1982, 78, 3659 and references therein. (6) Herrmann, J.-M.; Pichat, P. J . Catal. 1982, 78, 425. (7) Disdier. J.; Herrmann, J.-M.; Pichat, P. J . Chem. Soc., Faraday Trans. 1 1983, 79,651. (8) Tanabe, K. In Catalysis; Anderson, J. R., Boudart, M.; Eds.; Spinger-Verlag: New York, 1981; Vol. 2, Chapter 5. For TiO,, see for example: Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1221. Graham, J.; Rochester, C. H.; Rudham, R. J. Chem. Soc., Faraday Trans. 1, 1981, 77, 1973. Morterra, C.; Ghiotti, G.; Garrone, E.; Fisicaro, E. J . Chem. Soc., Faraday Trans. I 1980,76,2102. Busca. G.; Saussey, H.; Saur, 0.;Lavalley, J.-C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (9) See for example: Furlong, D. N.; Yates, D. E.; Healy, T. W. In Electrode of Conductive Metal Oxides; Trasatti, S.,Ed.; Elsevier: Amsterdam, 1981; Part B, pp 367-432. (10) Schindler, P. W.; Gamjager, H. Discuss. Faraday Soc. 1971,52,286. (1 1) Davis, J. A.; Leckie, J. 0. J. Colloid Interface Sci. 1978,67, 90; 1980, 74, 32. (12) Berube, Y. G.; De Bruyn, P. L. J. Colloid Interface Sri. 1968, 27, 305. (13) Yates, D. E.; Healy, T. W. J . Chem. SOC.,Faraday Trans. 1 1980, 76, 9. (14) Foissy, A.; M'Pandou, A.; Lamarche, J. M.; Jaffrezic-Renault, N. Colloid Surf.1982, 5, 363.
0022-3654/86/2090-2733$01.50/0 0 1986 American Chemical Society
2734
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
sured by a radiotracer technique.14v1’ Several potentiometric titrations of T i 0 2 samples have been performed:J0J3J4 but, to our knowledge, the effect of a supported metal has never been investigated by this method. Electrophoretic measurements have already been used to determine the point of zero {potential (PZZP) of rutile and it was inferred that impurity anions, namely chloride ions, have the effect of lowering the PZZP.’* This effect was also evoked to explain a PZZP at 4.7 for colloidal particles of anatase and amorphous Ti02 prepared from TiCl4.I9 A more negative electrophoretic mobility for IO wt % Pt/Ti02 was tentatively attributed to the adsorption of C1ions on the platinum depaskm A decreased PZZP was also found by electrophoresis for colloidal Ti02 loaded with Pt (8%) and R u 0 2 (18%).21 As our study was in progress,22more positive or more negative electrophoretic mobilities were reported for Li-doped Ti0223or for Pt/Ti02 (0.05-0.8 wt % Pt),24 respectively. Quite recently, a study of the coagulation of colloidal Pt particles to T i 0 2 grains showed that platinum renders the T i 0 2 mobility more negative and it was concluded that it changes the Ti02 surface chemistry p r o f o ~ n d l y . ~ ~ We report measurements performed by the three complementary methods (potentiometry, electrophoresis, adsorption of labeled ions) with T i 0 2 and Pt/Ti02 samples containing OS-10 wt % platinum deposited as particles of homogeneous size, and we discuss the origin of the differences found.
Experimental Section 1 . Pt/ T i 0 2 Catalysts. The P t / T i 0 2 samples were prepared by impregnation of nonporous titanium dioxide powder (Degussa P-25, 50 m2 g-I, mainly anatase) with a H2PtC16aqueous solution and by reduction in flowing H2at 753 K as previously detailed.4 The T i 0 2 sample, used as a reference, was similarly treated except that the platinum complex was replaced by HC1. The high-temperature treatments in N 2 / 0 2 and in H2 included in the preparation procedure of all the samples eliminated the chloride ionsls as indicated by chemical analysis. Transmission electron micrographs showed that the size of the Pt particles was nearly independent of the platinum loading, and the distribution, obtained by counting 1500 crystallites for each sample, was relatively narrow (1-4 nm diameter) around the surface weighted mean diameter of ca. 2.0 nm.26 As a result one should represent the samples studied as titania spheres and polyhedra of 20-30-nm diameter or width supporting ca. 1, 2, IO, or 20 Pt particles (-2 nm diameter) in the case of Pt contents 0.5, 1, 5, or 10 wt %, respectively. In other words, the coverage of the T i 0 2 surface by the Pt particles amounts to a maximum of 6% for the highest content. 2. Potentiometric Titrations. The principle of the measurement of the surface chargeI2 is based on the determination of the consumption of H+ (or OH-) ions by the solid surface as a function of p H (eq 1-4). This determination is deduced from the proton balance in the total system (cell, solution, electrodes, and powder). The measurements were performed in a tight electrochemical cell, under nitrogen pressure. Before its admission into the cell, nitrogen
-
(15) Shaw, D. J. Electrophoresis; Academic: New York, 1969. (16) Wiersema, P. H.; Loeb, A. L.; Overbeek, J. Th. G. J . Colloid Interface Sci. 1966, 22, 78. (17) Smit, W.; Holten, C. L. M.; Stein, H. N.; Goeij, J. J. M.; Theelen, H. M. J. J. Colloid Interface Sci. 1978, 63, 120. (18) Parfitt, G. D.; Ramsbotham, J.; Rochester, C. H. J . Colloid Interface Sci. 1972, 41, 437. (19) Moser, J.; Gratzel, M. J . Am. Chem. SOC.1983, 105, 6547. (20) Dunn, W. W.; Aikawa, Y . ;Bard, A. J. J. Am. Chem. SOC.1981,103, 3456. (21) Duonghong D.; Borgarello, E.; Gratzel, M. J . Am. Chem. SOC.1981, 103, 4685. (22) 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 Dehli, 1985; p 741. (23) Kiwi, J.; Morrison, C. J . Phys. Chem. 1984, 88, 6146. (24) Kiwi, J.; Grltzel, M. J . Phys. Chem. 1984.88, 1302. (25) Furlong, D. N.; Wells, D.; Sasse, W. H. F.J . Phys. Chem. 1985.89, 626. (26) Pichat, P.; Mozzanega, M.-N.; Disdier, J.; Herrmann, J.-M. Nouo. J . Chim. 1982, 6, 559.
Jaffrezic-Renault et al. 1
3
Figure 1. Schematic illustration of the device used to measure electrophoretic mobilities: ( 1 ) thermostat (298 K), (2) suspension reservoir, (3) N2 bubbling, (4) pH measurement, (5) ultrasonic source, (6) peristaltic pump, (7) stopcocks, (8) Pt electrodes, (9) observation tube, (10) halogen quartz lamp, ( 1 1 ) TV camera.
was washed successively in water, in a 3 N soda solution, in a 1 N nitric acid solution, and in three flasks of pure water. The titration was carried out with an automatic titrator (“T.T. processed’, TACUSSEL) fitted with a thin-wall glass electrode (T. B. 10, TACUSSEL) and a saturated calomel electrode. The suspensions were strongly agitated with a magnetic stirrer during the experiment. Experimentally, to 50 mL of lo-’ M NaCl solutions, 0.1 N soda was added up to pH 9.5. An aliquot of 1 g of a sample powder was then dispersed in this solution. After the solution was stirred for a few minutes, the pH value stabilized between 4 and 7, depending on the sample. The pH was again adjusted at 9.5 by a new addition of 0.1 N soda. The titration was then carried out: 0.1 N hydrochloric acid was added with 5-pL increments and pauses of 60-180 s before the pH was recorded. The titration was stopped at pH 4. Then the pH was once more adjusted at 9.5 by soda addition. 3. Electrophoretic Mobility Measurements. A Rank Brothers (Cambridge, U.K.) apparatus, Model MARK 11, was used. The mobilities of the particles were measured over 170 pm in a channel having a rectangular section (1 mm X IO mm). The conditions (pH, ionic strength, voltage) were chosen to avoid any gaseous evolution. Twice-distilled water and reagents of analytical grade were employed. The suspensions (20 mg of powder catalyst per liter of NaCl or N a N 0 3 solution) were continuously sonicated and submitted to N 2 bubbling in a 200-mL reservoir where the pH adjustments were also made by adding N a O H or HC1 or HNO, (Figure 1). They were circulated by a peristaltic pump through the electrophoresis cell which was isolated during the mobility measurements by stopcocks. The enlargement of the TV system was 145. The mobilities were measured for the particles situated in the “stationary” planes defined as the planes where the liquid was not moving. The locations of these planes were calculated by the formula s = 0.194d, where s is the distance between the stationary plane and the cell wall and d is the cell thicknes~.’~ For each plane the motions of at least 10 particles were observed, the polarity of the platinum electrodes being inverted for two successive determinations to minimize the polarization effects. The sizes of the particles chosen during the observation were about 0.1-1 pm. Large particles fell too rapidly, while very small particles were not clearly visible. Therefore, the observed particles were ag-
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2735
Charge Density at Pt/Ti02-Electrolyte Interface
0
- 0.2-
0.2
E r 0 >
\
I
>= I1
0.4
'0.4-
0.6-
0.6
4
i
;
I
Figure 2. Potentiometric titrations of a 10 wt % R/Ti02 sample in NaCl solutions: I, M; 11, M;111, lo-' M.
Figure 3. Variations of the surface charge of a 5 wt % F't/Ti02 sample against pH in NaCl solutions: A, lo-) M; B, M; C,lo-' M.
gregates containing between 10 and 1000 elementary grains. The { potentials relate to these large aggregates; presumably, they reflect an average of the { potentials inside the aggregates. The electrophoretic mobility uEwas calculated from uE = v / E , where u is the velocity of the particles in the applied electric field E. The accuracy varied between 0.1 and 0.3 pm s-'/(V cm-l); the most important relative errors concerned the lowest mobilities (
