J . Phys. Chem. 1985, 89, 1922-1928
1922
Colloidal Semiconductors in Systems for the Sacrificial Photoiysls of Water. 2. Hydrogen Production with Pt/Ti02 Catalysts D. Neil Furlong,* Darrell Wells, and Wolfgang H. F. Sasse Division of Applied Organic Chemistry, CSIRO, Melbourne, Australia (Received: December 18, 1984)
Pt/TiOz aggregates prepared by heterocoagulation catalyze the oxidation of the donors EDTA and oxalate when irradiated with light of energy greater than the band-gap of Ti02 The rate of hydrogen production decreased with prolonged irradiation not only because of donor consumption but also because of loss of catalyst activity. This loss of activity seems to be induced by hydrogen produced during irradiation. The greatest yield of hydrogen from a Pt/TiO,/EDTA system corresponds to the donation of ca. 23 electrons from each molecule of EDTA. The yield of carbon dioxide confirmed that the oxidation of EDTA extended beyond decarboxylation. The maximum rate of hydrogen production (R,,) was directly related to the adsorption density of EDTA on TiO, surfaces. This reflects the ability of adsorbed EDTA to reduce photogenerated holes before their combination with photogenerated electrons. R , increased when the adsorption of EDTA on Pt surfaces was suppressed by an increase in the ionic strength. Stabilization of the Pt/TiOz catalysts against coagulation, using PVA, lowered the photolysis efficiency apparently because PVA retarded the exchange between EDTA in solution and oxidation products at TiO, surfaces. When the concentration of Pt was varied, an optimum R,,, was observed. R,, first increased with Pt concentration due to the increasing number of Ti02 particles carrying Pt. At the higher concentrations of Pt, R , decreased partly because of the decrease in surface coverage of EDTA on Ti02 which accompanied the increase in Pt concentration. An optimum R,, was also found when the concentration of Ti0, was varied. R,, at first increased due to the increasing number of absorbing Ti0, particles and then decreased, at higher Ti0, concentrations, due to the reducing surface coverage of EDTA.
Introduction
TABLE I
In part 1I the heterocoagulation of colloidal platinum (Pt) and colloidal titanium dioxide (Ti02) was investigated with a view to preparing catalysts consisting of Pt particles supported on TiOz particles. The levels of loading of Pt onto TiO,, the adsorption of the electron-donor ethylenediaminetetraacetic acid (EDTA) on aggregates, and the physical stability of the aggregates were studied.' The production of hydrogen (H2) resulting from band-gap2 illumination of aqueous dispersions of such P t / T i 0 2 aggregate catalysts, in the presence of sacrificial electron donors, is now described. Experimental Section
Materials. A Pt sol designated earlier' as B1 and a TiO, sol H l l were used to prepare Pt/TiO, catalysts. The sols consist of particles of average size 2 and 9 nm, respectively. Absorbance measurements on the T i 0 2 sol (Cary 219 UV/vis spectrophotometer) showed an adsorption edge at 370 nm which corresponds* to the band-gap energy for anatase (3.3 eV). All electrolyte solutions (NaCl, NaNO,, Na2C204,and Na2EDTA) and acid (HCl), base (NaOH), and buffer solutions were prepared with analytical reagent chemicals and triply distilled water (maximum conductivity 0.85 pS cm-I). Methylviologen (MV2+) and tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)?+) salts were prepared and purified as described e l ~ e w h e r e . ~The poly(viny1 alcohol) (PVA) was a BDH Laboratory chemical of molecular weight 125 000. Methods. Pt/TiO, catalysts were prepared by mixing Pt sol B1 with TiOz sol H1 at pH 3. The compositions of the various Pt/TiOz catalysts are given in Table IA. The concentrations refer to the final mixture of Pt and TiO,. All N values (see Table I) are much less than the saturation binding value of 18,' and, therefore, the catalysts consist of virtually all the Pt particles bound to TiO, particles. With catalysts A and D not all T i 0 2 particles will carry Pt. The electron donor (EDTA or oxalate) was always added after binding of Pt to T i 0 2 had occurred to avoid any donor-induced suppression' of this binding. Catalysts containing PVA were prepared either according to Nord4 (catalyst E) or by ~~
~~
~
~~
~~
~
~~~
(1) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 626.
(2) Gerisher, H.; Willig, F. Top. Curr. Chem. 1976, 61, 50. (3) Braddpck, J. N.; Meyer, T. J. J . Am. Chem. SOC.1973, 95, 3158. (4) Rampino, L. D.; Nord, F. F. J. Am. Chem. SOC.1941, 63, 2745.
catalyst A
B C D
A. Pt/Ti02 Catalysts concn, mol dm-) Ti02 Pt 1.9 x 10-3 5.0 x 10-5 5.4 x 10-3 1.0 x lo4 3.1 x 10-3 1.0 x lo4 5.4x 10-3 5.0 x 10-5
N O
0.3 0.9
1.4 0.4
B. Catalysts Containing PVA
catalyst
description
E F1
Nord catalyst (Pt/PVA)4
F2
F3 F4
F5
Pt/Ti02/PVAb Pt/Ti02/PVAb Pt/Ti02/PVAb Pt/Ti02/PVAb Pt/Ti02/PVAb
wt 7% PVA
0.04 0.10 0.20
0.50 1 .oo
a N values refer to the calculated ratio of Pt to TiO, particles. bTi02 and Pt concentrations as in catalyst D.
mixing the appropriate quantities of Pt/TiO, catalyst D and aqueous PVA solution (catalysts F). Dispersions (5 cm3) were illuminated (25 "C) with a xenon 150-W (Cermax LX150) lamp. In all experiments the light beam was filtered by 11 cm of water, and in some experiments a 420-nm filter (absorbance >5 at
