J. Phys. Chem. 1983,87,2629-2636
2629
Design, Preparation, and Characterization of Ru02/Ti0, Colloidal Catalytic Surfaces Active in Photooxidation of Water G. Blondeel, A. Harrlman, George Porter, D. Urwln,+ and J. Klwl' Davy Faraday Research Laboratory, The Royal Institution, London, W7X 485 England (Received: November 8, 1982; I n Flnal Form: January IO, 1983)
This paper describes the preparation and optimization of a catalyst consisting of Ru02.xH20 obtained by hydrolysis of RuC13on Ti02powder. The catalyst is active in dark and light-inducedsacrificial systems mediating O2 evolution in solution. The influence of pH, temperature, concentration, and other experimental variables affecting the preparation of the catalyst is described. The first characterization of Ru02-xHzO/Ti02powdered surfaces by electron microscopy (EM),surface area measurements (BET), diffuse reflectance spectroscopy (DRS), optical acoustic spectroscopy (OAS), and powder conductivity measurements is presented. By EM, it is shown that the active catalytic species consists of islands of Ru02-xH20,present as agglomerates of 10-20 nm interspersed with Ti02 particles of 200-nm diameter.
Introduction The problem of O2 evolution in dark as well as photoinduced processes in sacrificial systems is a topic of relevant research in several laboratories working in the area of solar energy conversion processes. I t is generally considered that, contrary to hydrogen production, the oxidation half-reaction poses a difficult problem since four electrons have to be transferred to generate one molecule of OF It has recently been established that, of all noble metal oxides, ruthenium oxide has the lowest overvoltage for O2evolution in the Exact values for the low overvoltages shown by RuOz in electrochemical processes can be fourid in ref 1-3 and references therein. This observation was later used to obtain light-induced 02-evolvingprocesses,4-6using the same type of catalysts. Through the use of electrostatic stabilization of ruthenium oxide,' a useful colloidal catalyst based on Ru02 has been develaped. Mechanical mixtures8 up to 20% of Ru02with Ti02have recently been used to catalyze similar processes. More recently: it has been suggested that ruthenium oxyhydrates with valences 6+ and 7+ are formed on the surface of Ru02.xH20 when water oxidation takes place in a sacrificial system. The complicating factor is that several oxides of ruthenium have been reported but only Ru02 and Ru04 have been well characterized. In the case of hydrated ruthenium oxides, the situation is even lessdefined. In the present study, chemical hydrolysis has been used to produce islands of Ru02.xH20on submicron-size TiOz powders. This procedure in general is unlikely to give smooth continuous coatings that would block the surface of Ti02. These ruthenium oxide deposits may then be characterized by electron microscopy. The strategy followed during this work involved taking a low surface area Ti02 with BET area of 12 m2/g (sample T-1) having low microporosity as base material to deposit ruthenium dioxide. These Ti02 characteristics are beneficial in achieving oxidation in photochemically induced processeslO since smaller BET area and fewer OH groups on the surface mean that less of the evolved O2 will be trapped on the surface of Ti02 where it will be reduced by conduction band (cb) electrons via process 1. Here the radical 'Permanent address: Tioxide International, Ltd., Stockton-onTees, TS18 2NQ England. * On leave of absence, Ecole Polytechnique Federale Lausanne, CH-1015Lausanne, Switzerland. 0022-3654/83/2087-2629$01.50/0
O2 + e-(cb)
-
Oz- + H+ (acid pH)
-
HO,.
(1)
H02. is prone to be trapped on the surface. It is also important to reexamine the intervention of RuO2~xHzO in dark experiments involving Ce4+since the catalytic nature of these experiments has been q ~ e s t i o n e d .Once ~ a favorable catalyst has been developed for dark processes, the material will be tested in light-induced sacrificial processes to assess its relative efficiency under such conditions. This consideration is important, since an improved catalyst obtained in this way may accelerate kinetically reaction 2, taking place in cyclic water decomposition.ll Several 2H20
-
4H+
+ O2+ 4e-
(2)
experimental techniques are used to determine the controlling physical properties which improve favorably the kinetics of O2 evolution. Experimental Seption The following procedure was used in the preparation of colloidal Ru02/Ti02: For the preparation of a 3% Ru02/Ti02sample, Degussa P-25 T i 0 2 (1g) was stirred at 60 "C with 30 mg of RuC1,-3HzO (Alfa 64103) in 100 mL of solution for 5 h. The pH was initially adjusted to 4.5 by means of a 0.1 N KOH solution. The sample was then dried overnight at 100 OC. Degussa P-25 Ti02has a BET area of 50 m2/g, particle size between 15 and 25 nm, and is primarily anatase. The same procedure was used for T-1 Ti02 (British Tioxide) as base material. Dialysis was carried out for 24 h, the water being changed 5-6 times until the final pH was 4.5. Afterwards, the sample was (1) J. Kiwi and M. Gritzel, Angew. Chem., Int. Ed. Engl., 17,860 (1978). (2)J. Kiwi and M. Gratzel, Chimia, 33, 289 (1979). (3)A Mills and M. Zeeman, J. Chem. Soc., Chem. Commun., 948 (1981). (4)J. M. Lehn, J. P. Sauvage, and R. Ziessel, Nouu. J. Chim., 3, 423 (1979). (5)E. Borgarello, K. Kalyanasundaram, Y. Okuno, and M. Gratzel, Helu. Chim. Acta., 64,1937 (1981). (6)D. Thewissen, M. E. Rheinten, K. Timmer, A. Tinnemans, and A. Mackor, J. R. Neth. Chem. SOC.,101,79 (1982). (7)J. Kiwi, J. Chem. SOC.,Faraday Tram. 2, 78,339 (1982). (8)T. Kawai and T. Sakata, Chem. Phys. Lett., 72,87 (1980). (9)V. Shafirovich and V. Streleta, Now. J. Chim., 6, 183 (1982). (10)C. Jaeger and A. J. Bard, J.Phys. Chem., 83, 3146 (1979). (11)J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca, and M. Gratzel, Angew. Chem., 92,663 (1980).
0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
dried overnight at 100 OC. Dialyzed samples show better catalytic performance than the undialyzed catalysts. I t is probable that the removal of the C1- content of the catalyst is beneficial in dark processes where Ce4+is used as oxidant2 since C1-/C12 oxidation is decreased, allowing H 2 0 / 0 2oxidation to proceed more effectively. T-1 Ti02 (Tioxide International) has a surface area of 12 m2/g, was calcined at 750 "C, and shows 200-nm particle size. Debye-Sherrer powder diffraction photographs carried out on these Ti02powders showed the three main lines for the d spacings to be at 3.52, 1.89, and 2.38 indicating the anatase form of Ti02.12 Irradiations were performed with a 150-W Xe-Hg lamp and the volume of the irradiated solution was 30 mL thermostated at 25 "C. The oxygen evolved was measured by the polarographic method as reported earlier.13 The rate of oxygen evolution was determined by the value of the tangent (pmol of 0,) vs. time (minutes) in the Ce4+ system under study. Electron microscopy was carried out by transmission EM in a JEM 120 CX microscope. Ruthenium oxide loading of Ti02 on Degussa P-25 or T-1 Ti02has been assessed by diffuse reflectance spectroscopy (DRS) on a Hitachi EPS-3T instrument. Optical acoustic spectroscopy (OAS) on T-1 Ti02 samples loaded with ruthenium oxides was performed on an EDT Research photoacoustic spectrophotometer Model OAS 400. Thermal decomposition experiments were done with a Mettler Thermoanalyser I1 thermobalance which enables the mass of the catalyst, the rate of mass change (the differentiation being carried out directly by the equipment), and the temperature to be recorded simultaneously while the sample is being heated at constant rate. A Pt crucible was used for the thermogravimetric analysis (TGA). Differential thermal analysis (DTA) and differential thermogravimetric analysis (DTG) were carried out simultaneously. The conductivity measurements reported in Figure 8 were done on a conductivity rig as reported earlier.14 It is assumed in these experiments that powders under pressure approach a sintered state since mechanisms such as bridging and necking take place when particles cohere under pressure.15 Compression was carried out on a steel compression cage insulated by two Teflon disks. Resistance was measured by an ITT digital multivoltmeter. In the samples under study, the resistance attained a minimum with compression at values of 75% of the theoretical density. Stainless-steelelectrodes were immersed in a TiOz slurry and dried at 60 OC, attaining constant weight (10 mg/ (2 cm2)). Upon illumination, transient photovoltages were observed during the early stages16J7and upon cessation of illumination.
Results and Discussion ( i ) Optimization of Catalyst Composition Using Ce4+ as a Test S y s t e m for Oxygen-Evolving Systems. Figure l a shows the decrease in Ce4+concentration in 1N H2S04 in the presence of 12 mg of Ru02.xH207.5% hydrolyzed on T-1 Ti02. The kinetics were followed spectrophotometrically with a 1-mm cell. After 5 min there is practi~
~
~
~~~~
(12) Powder Diffraction File, published by International Center of Diffraction Data, 1601 Park Lane, Swarthmore, PA., 19081 (L978). (13) A. Mills, A. Harriman, and G. Porter, Anal. Chem., 53, 1254 (1981). (14) A. C. Tseung and H. Evan, J. Mater. Sci., 5, 604 (1976). (15) W.Gray, "The Packing of Solid Particles", Chapman and Hall, London, 1968, Chapter 9. (16) F. Dekker, J. Juliao, and M. Abramovich, AppE. Phys. Lett., 35, 397 (1979). (17) J. Vandermolen,W.Gomes, and F. Cardon, J.Electrochem. SOC., 127,324 (1980).
Blondeel et al.
a
b
i
0
I
2.5
I
5.0 % RuO, in TiO,+
1
7.5
1c
Figure 1. (a) Decrease of the absorption of a solution of 3.3 X M Ce4+ in 1 N H,S04 in a 1-mm cell taken at A = 320 nm when 12 mg of catalyst Ru0,.xH201TI0, is added to 30 mL of solution. At point A catalyst is added. (b) Kinetics of O2 evolution in a solution as described in part a as function of the percent ruthenium dioxlde loading on TiO,.
cally no Ce4+left. Figure l b shows the concomitant O2 evolution taking place in this time period. The maximum rate of O2formation is attained at about 7.5% ruthenium oxide loading and this observation will be justified at the end of this paragraph. To explain the stoichiometry observed in O2evolution and reported on the right-hand side of Figure lb, other products must be formed since only 70% of Ce4+is shown to oxidize H20to OB Indeed, RuO, was formed and had a concentration in redox equivalents less than 30% of the initial Ce4+ (RuO, was detected spectrophotometrically at the end of each run from the peak at 310 nm with e = 2960 M-l cm-' and our spectra of RuO, correspond exactly to the species reported in Figure 1of ref 18). Formation of higher oxidation states of ruthenium dioxide due to Ce4+has been reported.ls The oxygenated species formed subsequently pass into solution, accounting for the formation of ruthenium(6+) and -(7+) oxides. It is knownlg that Ru(6+) in acidic solution dismutates to Ru(8+) and Ru(4+) species. During our experiments, RuO, was also added to 1N H2S04to test for O2evolution but no oxygen formation was observed. Since RuO, has 1.6-V oxidation potential,20it should be able to (18) R.Connik and R. Hurley, J. Am. Chem. SOC.,74,5012 (1952). (19) "Gmelin Handbuch der Anorganischen Chemie",8th ed., Verlag Chemie, West Berlin, 1938. (20) W. Latimer, "The Oxidation States of the Elements and Their Potentials in Aqueous Solutions", Prentice-Hall, New York, 1938.
The Journal of Physical Chemistty, Vol. 87, No. 14, 1983 2631
Ru02/Ti02 Catalytic Surfaces
lt a/ l
f'
-1 1000 \
6
12 18 24 mg catalyst / ~ O C C ..-.+
Flgurr 2. Amount of 0, evolved in the same chemical system as reported in Flgue 1, but varying the amount of 7.5 % Ru0,.xH20/Ti02 catalyst in the 30-mL solutlon used.
oxidize H 2 0 to Oz but this reaction is slow under the present conditions. Only ruthenium(6+) and -(7+) oxide hydrated species having 0.2-V lower oxidation potentials than Ru04 are expected to oxidize water to oxygen (once formed by the action of Ce4+ under our experimental conditions). At the same time, dismutation to ruthenium@+) and -(4+) oxides accounts for some of the observations reported in Figure 1. It is interesting to comment here on the shape for the observed rates of oxygen formation as a function of R u 0 2 a H z 0loading on TiO, (reported in Figure lb). As more ruthenium dioxide particles are present on the surface of the catalyst, there is a greater probability of their encountering a Ce4+ion within a certain time period. When the plateau in Figure l b is reached at 7.5% RuOzaHzOloading, it is reasonable to assume that diffusional encounter between the reactants is no longer the rate-limiting step. Alternatively, after 7.5% Ru02.xHzO loading, the effectiveness of dispersal may decrease due to the agglomeration of ruthenium oxide particles when going to a higher concentration range as has been observed for silver particles loaded on TiOz catalysts used in ethylene oxidation?1 Preliminary results obtained from magnetic susceptibility measurements via the SUS 10 systemz2show a shape for the curves of x (magnetic susceptibility) similar to that of Figure l b for the kinetics of O2evolution vs. percentage loading of ruthenium oxide on the surface of TiOz. Thus, the activity of the present catalyst is dependent upon the number of active sites where catalyst particles are held in exposed positions whereby they can interact with the components of the reaction to be catalyzed. Figure 2 shows the rate of O2evolution in the same test system described in Figure 1. Different amounts of 7.5% RuOZaH2O/TiO2catalyst were added to a Ce4+solution 3.3 X lo-' M in 1N H2S04. Since the plot shown in Figure 2 is linear,it suggests a process taking place at the surface of the catalyst under conditions where diffusional encounters between reagents determine the rate of reaction. It is interesting to note that at 3% Ru02-xH20loading we begin to notice a difference in the relative efficiency of oxygen formation for the T-1 Ti02(30 p L of 02/(min.L*mg of catalyst)) and Degussa P-25 (20 p L of 02/(min.L.mg of (21) L. Vasilevich, E. Boreskov, R. Guryanova, I. Rizhal, A. Filippova, and I. Frollrina, kinet. Katal., 7, 525 (1965). (22) We thank Dr.Treyvaud of the Department of Physics, Geneva University. A full account for these observations will be given later elsewhere.
catalyst)) supports in the Ce4+test system. (ii)Size Factors and Surface Receptivity of TiO, for the Ruthenium Dioxide Catalyst. Figure 3 presents transmission electron microscopy data for four samples ( X 200000,l cm = 100 nm in print). As shown in Figure 3a, T-1 Ti02 consists of particles 200 nm in diameter and showed a more defined contour and homogeneity in size dispersion than Degussa P-25 Ti02 (20-nm particle size). With 0.5% ruthenium dioxide loading on T-1 TiO,, no ruthenium oxide agglomerates are seen on the border of the Ti02 particles (Figure 3b) which may suggest that intimate phase formation between ruthenium oxide and Ti02 has taken place at low loadings. The system ruthenium dioxide/Ti02 produced by hydrolysis at 60 "C of low loadings (below 3%) in RuOz apparently forms a very fine phase dispersion of ruthenium dioxide on the substrate. The possibility of interaction between similar ionic radii materials Ru4+ (0.62 A) and Ti4+ 0.60 A23 could partly explain this observation. Only at loadings of 3% and above of RuOz do agglomerates form showing a nonhomogeneous deposition of this material on the substrate. At 3% ruthenium dioxide loading on TiO, the ruthenium dioxide is seen in Figure 3c as a loose particular material attached to the Ti02 surface with a diameter of about 15 nm. The electron density of the ruthenium dioxide appears to be similar to that of TiO,. This may be the reason for the apparent absence of Ru02 at lower loadings than 3%. Also, when 3% RuO, is adsorbed in the high-area Degussa P-25, no Ru02clusters are observed. In both cases, the particle size of the adsorbed species will be smaller than that obtained in the sample shown in Figure 3c, making it much more difficult to see because of the small difference in contrast between Ru02and Ti02 Electron microscopy showed that larger agglomerates of ruthenium dioxides were formed on the Degussa P-25 Ti02 than on T-1Ti02,indicating a lower receptivity of this type of Ti02 towards ruthenium dioxide. The surface of Degussa P-25 Ti02 has a structure that does allow incorporation with formation of bigger agglomerates. Figure 3d presents micrographs of 7.5% Ru02.xH20on Ti02. It is readily seen that much more particulate material is attached to the Ti02surface by comparing these deposits with the number of similar deposits formed on 3% Ru02xH20/Ti02 (Figure 3c). Previously, ruthenium dioxide islands on TiOz have been formed by plasma techniques" vaporizing Ti and the volatile RuC13in an O2 atmosphere at 750 "C. Figure 4a shows the diffuse reflectance spectra (DRS) for T-1 TiO, loaded with different percentages of Ru02.xH20on the Ti02particle. The DRS accessory consists of an integrating sphere, mounting bracket, standard white plate of MgC03, and line adjuster. Light is emitted alternatively to the sample and reference, and the light scattered or reflected from them is led to the detector after one or more reflections inside the sphere. The absorption increases when the loading is increased to 5% Ru02.xH20; absorption from 450 to 750 nm is due to the ruthenium oxide loading and at 360 nm the optical cutoff of Ti02 occurs. The homogeneous adsorption of ruthenium dioxide on the surface is reflected by the smooth traces observed from 450 to 750 nm. To further confirm the importance of the surface structure on the receptivity toward ruthenium dioxide, trace 7 shows a 3% Ru02.xH20loading of the same T-1 Ti02sample in which Ru02.xH20has been dried not at 100 "C, but at 200 "C. (23) R. Shanon, Acta Crystallogr., Sect. A , 32, 751 (1976). (24) F. Hine, M. Yasuda, T. Noda, T. Yoshida, and J. Okuda, J.
Electrochem. Soc., 126,1439 (1979), and references therein.
2632 The Journal of mysical Chemistry, Vol. 87, No. 14, 1983
Blondeel et al.
a
C
b
Flgure 3. Electron micrographs of four samples with magnification X 200000 in all micrographs (1 cm on the print = 100 nm): (a) T-1 TiO,, (b) T-1 Ti0,/0.5% ruthenium dioxide, (c) T-1 Ti02/3% ruthenium dioxide, (d) T-1 Ti0,/7.5% ruthenium dioxide.
The higher temperature allows the growth of bigger particles of ruthenium dioxide which, in turn, is reflected by higher absorbance values (OD 0.8) when comparing with trace 4 (OD 0.6). The traces for Degussa P-25 Ti02 loaded as described in the Experimental Section are shown in Figure 4b. The lower traces shows that at 0.5% ruthenium dioxide loading there is not enough ruthenium dioxide to cover all the Ti02grains so that below 500 nm the absorption of Ti02 predominates. Traces 2-4 show that, at 1%,3%,and 5% ruthenium dioxide loading, the absorbance is 2-3 times higher than for T-1 Ti02 loaded with the same relative amounts of Ru02 This is due to larger aggregates of ruthenium dioxide formed on Degussa P-25 Ti02,originating higher absorbances for these com-
-
-
pounds as shown in Figure 4b. Electron microscopy25also showed larger aggregates of ruthenium dioxide on Degussa P-25 than on T-1TiOP The shapes of traces 2-4 in Figure 4b show that DRS is a suitable and sensitive method to detect homogeneity of ruthenium dioxide on Ti02surfaces since the trace is not as smooth as the ones presented in Figure 4a, indicating less dispersion of ruthenium dioxide on Degussa P-25 Ti02. Figure 5 shows the photoacoustic spectra (OAS) of ruthenium dioxide on T-1 Ti02 at loadings up to 5%. A ~~
(25) No electron micrographs of loaded Degussa P-25 have been presented in the text for clarity of presentation since T-1Ti02/ruthenium oxide was a more efficient 02-evolving catalyst.
Ru02/Ti02 Catalytic Surfaces
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2633 100 ,
a
I
500
300
1
I
'
I
I
'
t
I
\
I
DTA
a
o.2
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700
'
TG A
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- 7
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7001
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b 100 -
I
t n
l
l
l
,
l
/
I
DTG
C
0
1
100
4001
5001
6001
700,nm
Flgure 4. (a) Dmwe reflectance spectra (DRS): (1) T-1 TiO,: (2) 0.5% ruthenium dioxide loaded, 100 OC dried; (3) 1% ruthenium dioxide loaded, 100 OC dried; (4) 2 % ruthenium dioxide loaded, 100 OC dried; (5) 3% ruthenium dioxide loaded, 100 OC dried; (6) 5 % ruthenium dioxide loaded, 100 OC dried; (7) 3 % ruthenium dioxide loaded, 200 OC dried. (b) DRS spectra: (0) Degussa P-25; (1) 0.5% ruthenium dioxide loaded on Degussa P-25; (2-4) are 1% , 3%, and 5 % , ruthenium dioxide, respectively; ruthenium dioxide was loaded on the same support and dried at 100 OC. 1.01
f
\
4-
3 2
I
1
300
1
600
900
n m+ Figure 5. Photoacoustic spectra In the UV-visible region for T-1 TiO, and T-1 Tii,/ruthenium dioxide loaded materials: (1) T-1 TiO, (2) 1% RuO,~xH,O/TiOz, (3) 3% RuO,.xH,O/TiO,, (4) 5 % RuO,~xH,O/TiO,.
photoacoustic signal is generated when a light-absorbing substance is exposed to a modulated incident radiation from a 300-W lamp. The resulting signal from the sample is dependent upon the energy of the incident radiation. Since the incident radiation is modulated, then the gen-
300
500
700
1
I 900 q
Flgure 6. (a) Differential thermal analysis: (1) T-1 TiO,, (2) T-1 Ti02/3% ruthenium dioxide, (3) T-1 Ti02/5% ruthenium dioxide, (4) Ru0,.3.6H2O Alfa as a function of temperature heated in air. (b) Variation of weight loss with temperature: (1) 7.5% Ru0,-xH,O/TiO, heated in air, (2) RuO2.3.6H,O (Alfa) heated in air. (c) Rate of weight loss with temperature for Ru02-3.6H,0 (Alfa) heated in air.
eration of thermal energy within the sample will be periodic and a thermal wave will be produced having the same frequency as the modulation. Energy is transferred by this wave toward the sample boundary where a periodic temperature change is generated. This period temperature change resulta in the generation of an acoustic wave in the gas. This wave propagates through the volume of the gas to the microphone where a signal is produced. It takes 4 min to record a UV-visible spectrum. The photoacoustic signal is registered and other signals out of the sample are compensated. In figure 5 the modulation frequency was 80 Hz,the scan rate 120 nm/min, the band-pass 8 nm, the sensitivity 1.5 mV, the phase 310°, and the lamp current 19 A. This technique is insensitive to radiation scattered from the sample surface making it attractive for the study of opaque powders like Ti02-ruthenium oxide. In the DRS technique, one should, ideally, have available a nonscattering sample of the same material under study as reference. Our studies reveal that the band-gap (bg) of TiOp is not affected by the ruthenium oxide loading and the anatase used shows a smooth inflection at -400 nm as expected (3.2-eV bg).26 Figure 5 shows that ruthenium dioxide absorbs the modulated light applied between 20 and 240 Hz originating a flat band beyond 450 nm, the intensity of which is proportional to the loading of this material. The OAS technique has an advantage over DRS in that it is possible to observe the bg of Ti02 in the presence of surface oxide or metal and to assess whether changes have taken place in the bg. In our case, changes in the bg upon oxide loading did not take place. Degussa (26) A.
J. Bard, Tech. Zng., 1, 2889 (1981).
2034
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Blondeel et al.
P-25 ruthenium dioxide samples could not be analyzed via OAS since these samples were too dark in color (almost black) so that all incoming signals were absorbed. (iii) Mode of Stabilization of Ruthenium Dioxide by the Host Ti02. In Figure 6a the data for differential thermal analysis (DTA) in an aerobic atmosphere are presented and all samples showed an exothermic change with increasing temperature. Peak 1 for T-1 Ti02 indicates a phase transformation from one hydroxide form of Ti02to another hydrated form at 350 oC.n In traces 2 and 3 with ruthenium dioxide loadings of 3% and 5%, the interphase transition for these double oxides moves towards 250 "C, the value for Ru02.xH20 (Alfa Ventron) (trace 4). For 7.5% Ru02.xH20/Ti02,the peak is observed at -280 "C but it is not shown in Figure 6a for clarity of presentation. Therefore, the temperature for phase transformation in loaded samples of Ti02 is proportional to the amount of ruthenium dioxide loading. A t 7.5% ruthenium dioxide loading, there is maximum dispersion of Ru02 on Ti02 (Figure lb), while at higher concentrations the contact between oxide particles is not as uniform. The contact between ruthenium dioxide and T i 0 2 would then be maximal at 7.5% ruthenium dioxide (peak 280 "C). This catalyst shows the closest peak for the position of a loaded ruthenium dioxide/Ti02 to the pure Ru02.xHz0 (Alfa). In Figure 6b, trace 1 shows the stabilizing action of the host matrix on the surface ruthenium dioxide. No weight loss was observed by thermogravimetric analysis (TGA) for ruthenium dioxide/TiOz loading from 0.5% to 10%. The stabilization action of the host matrix on ruthenium dioxide is apparent if one compares trace 1with trace 2. Trace 2 reports a 50% weight loss for RuOZaH20(Alfa) up to 325 "C. As reported earlier%water loss occurs only in ruthenium dioxide hydrate up to 400 "C and this loss is a multistage process which depends upon the preparation used for the hydrated ruthenium dioxide. According to the TGA data reported in Figure 6b, the dehydration occurs by the following steps: (a) 50-100 "C, 0.87-mg weight lost from 8.66 mg of initial Ru02.xH20, (b) 100-250 "C, 2.84 mg lost from the same initial weight, (c) 250-325 "C, 0.61 mg lost from the same initial weight. This simple determination allows a formula RuOz.3.6H20for hydrated Ru02 (Alfa) since 3.6 molecules of HzO is equivalent in weight to 50% of the weight of Ru02 (133.7). Ru02 anhydrous is then the sole species present above 325 "C. In the regions 50-100 and 100-250 "C, the hydrated species which would predominate are RuOZ.2.9H20and RuOz. 0.54H20, respectively. The structural interactions at the Ti02 interface responsible for stabilization of ruthenium dioxide in trace 1 are not well-known at present. Thermogravimetric analysis (DTG) of Ru02.3.6Hz0 is presented in Figure 6c. The maximal rate of water loss at the observed peaks for these processes is used to characterize the temperature at which HzO is lost (water is lost at 100, 150, and 275 "C and the observed rates are 0.12,0.25, and 0.14 mg/min, respectively). Figure 7 compares the O2 evolution kinetics when the catalyst 7.5% RuOz-xH20/Ti02has been dried at temperatures over 100 "C. When the RuOz~xHzO/TiO2 catalyst is dried at 400 "C, its activity is about 1/7 of its value when dried at 100 "C. Ru02.xH20 (Alfa) powder was dehydrated and its catalytic activity analyzed at the same temperatures and compared with experiments carried out with ruthenium dioxide loaded on Ti02. The relative
kinetics for O2 evolution here follow the same profile as the ruthenium dioxide/TiO, reported in Figure 7, indicating that the active catalytic form of ruthenium dioxide is the Ru02.3.6H20 hydrate. More important, the activity per milligram of ruthenium dioxide was similar when ruthenium dioxide hydrated (Alfa) and the one deposited on Ti02 were assessed showing that the dispersion of ruthenium dioxide on TiOz does not decrease its catalytic efficiency. The shape of Figure 7 reflects the observations reported in Figure 6c. In the regions of maximum water loss (between 100 and 300 "C) a larger reduction in catalytic activity is observed for the ruthenium dioxide/Ti02 (Figure 6c). This observation points to the existence of a hydrated species active in catalysis which is destroyed at higher temperatures and recent work using RuO, hydrates as catalysts in electrochemical processesmswreports similar results. Since no X-ray diffraction for an intermetallic phase of the type TiOz-RuOz could be found, this lends more support for the hydrated Ru02 acting as the catalytic species. And a final remark: the activity of insoluble Ru02 (Alfa 64104) used a few years ago2gave less
(27) J.Barksdale, "Titanium,Ita Chemistry and Applications", Ronald Press, New York, 1966. (28) C. Keattch and J. Redfer, J. Less-Common Met., 4,460 (1962).
(29) L. Burke and J. Healy, J. Electroanal. Chem., 124,327 (1981). (30)L. Burke, M. Lyons, and M. McCarthy in uProceedingsof the 4th World Hydrogen Conference",California, Verizoglu, Ed., 1982, p 267.
60
-
t
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m
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-
I
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-
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.-C
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> 200 -0
f I
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200
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300
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400
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Flgure 7. Klnetics of O2 evolution In a solution 3.3 X M Ce4+ in 1 N H2S0, loaded wtth 12 mg of 7.5% Ru02.xH20/T102in 30 mL, as a function of the drying temperature of the catalyst.
-
4
I-
E f 3 l
'\
1
2
3 tonnes-
Flgure 8. Variation of reslstance with applled weight for (1) T-1 T102, (2) 0.5% ruthenium dioxide on TiO,, (3) 1% ruthenium dioxide/TiO,, (4) 3 % ruthenium dloxIde/TlO,, (5) 5 % and 7.5 % ruthenium dioxideITiO,.
Ru02/Ti0, Catalytic Surfaces
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2635
1.0 2.5
5.0 ,ic,
7.5
10
RuO, in T O z - +
Rate of evolution of O2 induced by vislble light (A > 400 nm) In the system Ru(bpy)&I, (7 X M)/Na,S,O, (lo-’ M). Catalysts added in the amounts shown, pH 4.8 via acetate buffer (lo3 M): (a) 6 mg of catalyst/30 cm3of solution, (b) 12 mg of catalyst/30 cm3of solution, (c) 3 mg of catatyst/30 om3 solution. Flgure 9.
than 8% of the catalytic activity compared with the hydrated Ru02 (Alfa 88781) used. This coincides with the degree of reduction of catalytic activity for ruthenium dioxide on Ti02 at 400 “C. Resistivity measurements as a function of applied pressure were made on powder samples and are presented in Figure 8. The samples examined were T-1 Ti02 pure and loaded with ruthenium oxide up to 7.5%. To compute the resistivity, the resistance has been measured at the plateau for each particular material and the values give an indication of the number of charges in the materials under study. Taking p as resistivity, a as the area of the powder piston under compression (0.50 cm2),and length 1 (0.2 cm) the expression p = aR/1 (R resistance in ohms) gives a value of p for Ti02 of 5.0 X lo6 D cm and 0.12 X lo6 D cm for 7.5% ruthenium dioxide/TiOz. This shows that the conductivity of ruthenium dioxide/Ti02 at a 7.5% loading level is -40 times higher than in unloaded TiOz. This observation partially accounts for the beneficial effect of ruthenium dioxide on semiconductor Ti02powders. At higher conductivities the materials allow more efficient charge transfer, rendering the catalyst kinetically faster when intervening in redox processes. Since the materials have similar grain microstructure, grain-to-grain contact under pressure follows a similar pattern15making the trend presented in Figure 8 representative. (iv) Parameters Affecting O2 Evolution in Light-Induced Processes. Figure 9 presents the kinetics for O2 evolution in the photosystem R ~ ( b p y ) ~ C l ~ / N at a ~!pS ~ 0 ~ 4.8. Details of this system have been reported previously. Our aim was to explore the role of the catalyst in the photochemical oxidation of water induced by visible light using persulfate ions to oxidize the triplet excited state of Ru(bpy)l+ to Ru(bpy),3+. (Note, there is no dark reaction under the present experimental conditions). As seen from Figure 9, a 6 mg/30 mL solution catalyst concentration provides good kinetics for 0, evolution over the whole range of Ru02loading on Ti02 and affords enough surface to promote favorable catalytic activity and does not absorb a high amount of the incident light. At 3 mg of catalyst loading/30 mL of solution, not enough Ru02 surface is available for catalysis and at 12 mg of catalystj30 mL too much light is absorbed by the catalyst, hindering light absorption by the chemical system as shown by the DRS and OAS measurements. Ruthenium dioxide loadings on
3.0 5.0 photons 8 10”/9cm2/m i n -+
Figure 10. Effect of visible light intensky on the klnetics of oxygen M)/Na2S20, M) at evolution for a solution Ru(bpy),CI, (7 X pH 4.8. Catalyst 7.5% Ru02.xH20was used with 6 mg/30 cm3 of solution.
Ti02 higher than 2% led to a decrease in rate of O2 evolution. Figure 9 shows the maximum catalytic activity takes place at 2% loading of Ru02 on Ti02 since high amounts of incident light are not adsorbed at this Ru02 loading, but still enough surface is achieved to promote adequate catalytic activity for the process under study. In dark processes, as reported in Figure lb, a 6% Ru02-Ti02 shows the highest catalytic activity. This is not surprising since Ru02activates the process reported in section i. The amount of loading increases the observed effect until the limiting dispersion of Ru02 on Ti02 found at 6% Ti02 loading. The catalytic activity of the Ru02/Ti02system increases with Ru02 loading and is independent of the capacity of the system to absorb light in a dark oxidative process as previously reported. No Ru04 was produced in these light-induced experiments and Ru04 added prior to illumination was not stable at pH 4.8. The maximal evolution rate is 5% of the maximal values reported for O2 evolution (Figure lb) using the Ce4+test system. The stoichiometric yields have not been reported since these yields were imprecise at these low levels of conversion. Figure 10 shows that the rate of O2 evolution is proportional to the intensity of applied light. As shown in eq + 3, the O2generation goes through excited R ~ ( b p y ) , ~with 4R~(bpy)3~+ +0
2
+ 4H+ (3)
subsequent oxidation to R ~ ( b p y )by ~ ~S202+ present in the system.6 The R ~ ( b p y ) , ~oxidizes + water to O2in the presence of the catalyst. Figure 10 shows that O2generation is proportional to the first power of the light intensity. A few photoelectrochemical experiments were performed with Ti02 powders deposited upon stainless-steel electrodes in contact with an 02-saturated aqueous solution at pH 4. In the dark, the equilibrium potential (ca.30 mV) was determined by the Ti02 composition, the concentration of 02, the nature of the electrolyte used, impurities, and surface states. Upon illumination with UV and visible light the potential dropped immediately by ca. 27 mV, probably due to reaction with O2 which is adsorbed upon the TiOz surface in the form of 02--and 02H-. Within 1 min of illumination, the potential began to increase and reached a final value of ca. 90 mV after 20-min illumination, This large photopotential is most probably a reflection of the photoadsorption of O2 which was shown to
2636
J. Phys. Chem. 1983,87,2636
occur under similar experimental conditions by monitoring the 0, concentration in a closed vessel. In identical experiments but using TiO, samples loaded with 3% Ru02.xHz0the dark potential was ca.280 mV and this moved slowly (over 20 min or so) to ca. 230 mV upon illumination. Perhaps this reflects the trapping of positive holes by RuO, islands but further work is required before this finding can be satisfactorily explained. In conclusion, it has been shown that surface loading of small particles may introduce changes in the behavior of the whole particle whenever the ratio of surface to volume is large. The first direct observation of ruthenium dioxide islands prepared via a low-temperature method has been reported. Although the uniformity of the dispersion is low, the catalyst produced is kinetically fast in O2evolution. A possible model for a Ti02-Ru02 microelectrode would
be represented by a bubble of TiOp loaded with deposits of RuO,. The whole surface of TiOz is then available for charge injection from oxidants in the electrolyte solution but the charges can only go out of the particle at the RuO, islands on its surface. In summary, the catalyst structure has to create adequate conditions for the micro-macro transport process extending beyond the atomic level before efficient 0, evolution can occur.
Acknowledgment. We thank the SERC, the EEC, NATO, and GE (Schenectady). We thank the Chemistry Department of the City University of London for use of their optical-acoustic as well as their thermal-analysis facilities. Registry No. RuOz, 12036-10-1; TiOz,13463-67-7; H,O, 7732-18-5; RuCl,, 10049-08-8.
COMMENTS Vlbratlonal Numbering of Bands In the Spectra of Polyatomlc Molecules
Sir: In discussions of the vibrational assignments of bands of polyatomic molecules, extensive use has been made of the nomenclature introduced by Brand, Callomon, and Watson,’ e.g., 2:4& 6k16:. The principal numbers refer to the vibrations excited and the superscripts and subscripts refer to the number of quanta involved in the upper and lower states, respectively. No confusion has arisen when these symbols are written but there has been some confusion when these symbols are spoken. This unfortunate situation may have been caused by the need in some journals to stagger the superscripts and subscripts. We strongly recommend that, when spoken, the superscript should precede the subscript in conformity with the recognized convention in molecular spectroscopy that the upper state precedes the lower We also recommend that, when staggering is necessary in print, the same order be followed, e.g., 410. Reading from left to right, the assignment is then read as four-one-zero, in conformity with standard spectroscopic usage for molecules. (1)Private communication by J. C. D. Brand, J. H. Callomon, and J. K. G. Watson circulated to participants at the 35th Discussion of the Faraday Society on “The Structure of Electronically Excited Species in the Gas-Phase”, held at Queen’s College, University of St. Andrews, Dundee, Scotland on April 2-3,1963. The notation first appeared in the Ph.D. thesis of J. K. G. Watson, University of Glasgow, Scotland, 1962 and in the paper by J. H. Callomon and K. K. Innes, J.Mol. Spectrosc., 10, 166-81 (1963). (2) G. Herzberg, “Molecular Spectra and Molecular Structure”, Van Noatrand, Princeton, N J Vol. I, 1950; Vol. 11, 1945; Vol. 111, 1967. (3) R. S. Mulliken, J. Chem. Phys.,23, 1997-2011 (1955). (4) Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7.
0022-3654/03/2007-2636$01.50/0
( 5 ) Department of Chemistry, University College London, 20 Gordon Street, London, England WClE 6BT. (6) Department of Chemistry, State University of New York, Binghamton, NY 13901. (7) Department of Chemistry, Tel-Aviv University, Tel-Aviv, Israel. (8)Laboratoire de PhotoDhvsiaue MoMculaire, Universit6 de ParisSud, Bitiment 213,91405-0r&y, France. (9) James Franck Institute, University of Chicago, Chicago, IL 60637. (10) Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1Y6. (11)Department of Chemistry, University of Reading, Whiteknighta, Reading RG6 ZAD, England. (12) Department of Chemistry, University of California, Berkeley, CA 94720. (13) Department of Chemistry, Indiana University, Bloomington, IN 47405. (14) Henberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6. (15) Department of Physics, The Ohio State University, Columbus, OH 43210. (16) Institut fiir Physikaliache Chemie und Theoretische Chemie der Technischen Universitit Miinchen, D-8046 Garching, West Germany. (17) Department of chemistry, Stanford University, Stanford, CA 94305.
J. C. D. Brand‘ J. H. Callomon5 K. K. Inner6 J. Jortner’ S. Leach* D. H. Levy’ A. J. Yerer‘’ I.M. Mlllc” C. B. C. Pannenterla D. A. Ramsay”‘ K. Naraharl Reo“ E. W. ScMag16 J. K. 0. Watson“ R. N. Zare”
s.
Received: April 11, 1983
0 1903 American Chemical Society
