3766
J . Phys. Chem. 1985, 89, 3766-3771
previously condensed. (Sample GI outgassed at only 600 ‘C for 24 h also showed complete wetting.) Significant surface area differences between the isotherms of Figure 6 exist. Taking the sample F isotherm as fixed, a normalization factor of 1.16 is needed for overlap of the sample H isotherm with F. For the 800 O C baked sample G2 the normalization is 2.07, confirming this loss of adsorption microvoids. The height of our second layer step is only about of monolayer capacity while the second layer step for the smokes is almost as large as for the monolayer. A glance at Figure 1, parts d and e, shows that the cubic habit grains comprise only a modest fraction of the total sample. There are studies of krypton wetting on other substrates. Partial wetting of kryton layers adsorbed on gold has been reported recently by Krim, Dash, and Suzanne.,’ Using temperature as a variable they find that partial wetting becomes complete as the bulk triple point temperature is approached from below. In another study, Bienfait et aLZ9showed that while krypton completely wets graphite substrates at low T, other adsorbates with only slightly different interaction parameters (i.e., N e and 0,) do not wet graphite. Huse30argues that equilibrum films strained relative to the bulk lattice spacing along directions parallel to the adsorption plane do not in general completely wet the surface. (Neon at 15 K is strained by a 3% misfit but krypton at 47 K has an almost perfect match to the underlying graphite mosaic.30) Gittes and Schick31 calculate that substrates which attract a solid adsorbate too strongly and also too weakly prevent that adsorbate from wetting. For krypton on MgO we have shown that increasing the chemical potential (and therefore presumably weakening the adsorption potential) actually causes nonwetting behavior. While gas selection provides a considerable range of lattice strains, our finding of Kr wetting/nonwetting on MgO shows that the outgassing treatment provides a continuous surface hydroxylation variable for exploration of the krypton wetting transition! The accompanying adsorption potential and lattice strain changes are (29) Bienfait, M.;Seguin, J. L.; Suzanne, J.; Lerner, E.; Krim, J.; Dash, J. S . Phys. Reo. E 1984, 29, 983. (30) Huse, D. A. Phys. Reo. B 1984, 29, 6985. (31) Gittes, F. T.; Schick, M. Phys. Reo. E 1984, 30, 209.
evidently sufficient to cause our observed alteration in wetting properties. We are presently exploring the Kr on MgO wetting transition in detail since it is of fundamental interest and is a potentially sensitive probe of surface hydroxyl concentration. A transition from capillary condensation to nonwetting was observed for N2 adsorption on microporous silica as a function of increasing powder c~mpaction.~’The authors interpreted their results in terms of changes in capillary diameter. However, hydroxyl groups are known to exist on silica surfaces33and could well contribute to changes in the adsorption characteristics. Conclusions
The history of the MgO samples, in particular their preparation, thermal treatment, and purity, appears to exert a strong influence on the morphology and microstructure of MgO. Even treatment at relatively high temperatures does not completely erase differences between MgO samples derived from various sources or subjected to different thermal treatment conditions. MgO seems to be very sensitive to surface rehydroxylation when exposed to ambient air. High surface hydroxyl concentrations shift A b / k B to higher values, indicating a greater affinity of Kr to clean rather than to hydroxylated MgO. The transition from wetting to nonwetting behavior depends on the nature of the starting material, the thermal treatment, and the outgassing conditions. The wetting observations made in the present investigation are to our knowledge unreported elsewhere. Detailed studies of substrate wetting and impurity effects are in progress. Acknowledgment. H.B.C. and M.B. gratefully acknowledge support of this work through the Institute of Science and Technology, University of Michigan. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Mr. A. Sachdev, Mr. J. J. Mulholland, and Mr. S. A. Shaheen for experimental assistance. Registry No. MgO, 1309-48-4; Kr, 7439-90-9. (32) Ramsay, J. D.F.; Avery, R. G. J . Colloid Interface Sci. 1975, 51, 205. (33) Boehm, H. P. Adu. Cutal. 1966, 16, 179.
Rate Enhancement of Photooxidation of CN- with TiO, Particles T. L. Rose* and C. Nanjundiah EIC Laboratories, Inc.. Norwood, Massachusetts 02062 (Received: January 22, 1985)
The rate of photooxidation of cyanide anion by TiOz particles of the anatase modification was investigated as a function of particle size, platinization, and pH. The rates were measured in 1 mM CN- solutions; the change in cyanide concentration was followed with an ion-sensitive electrode. The reaction rate constants were increased by increasing the surface area of the particles, platinizing the TiO,, and lowering the pH from 14 to 11. The fastest rate constant was over 500 times faster than had been reported previously with anatase particles. While the quantum efficiency for electrochemical processes appears to approach one, the competing process of photooxidation of water still accounted for 75% of the reaction in 1 mM cyanide solutions at pH 11.
Photodiode particles have been suggested as catalysts for decontaminating environmental pollutants or other processes for waste cleanup.1-5 The photooxidation of cyanide in oxygenated
solutions at pH 13 was one of the earliest systems Frank and Bard’ have tested several semiconductor particles and c O ~ A ~ d ethat d while ZnO had a slightly higher quantum Yield, TiO, particles had the most overall favorable characteristics because of their high stability against photocorrosion. Oxidized
(1) Frank, S . N.; Bard, A. J. J . Am. Chem. SOC.1977, 99, 303. (2) Frank, S. N.; Bard, A. J. J. Phys. Chem. 1977, 81, 1484. (3) Kraeutler, 9.; Bard, A. J. J . Am. Chem. Soc. 1978, 100, 5985. (4) Yoneyama, H.; Yamashita, Y.; Tamura, H. Nature (London) 1979, 282, 817.
( 5 ) Chum, H. L.; Ratcliff, M.; Posey, F. L.; Turner, J. A.; Nozik, A. J . J . Phys. Chem. 1983, 87, 3089. ( 6 ) Frank, S. N.; Bard, A. J. J . Am. Chem. SOC.1977, 99, 4667. (7) Kogo, K.; Yoneyama, H.: Tamura, H. J . Phys. Chem. 1980,84, 1705.
Introduction
0022-3654/85/2089-3766$01.50/0
0 1985 American Chemical Society
The Journal of Physical Chemistry. Vol. 89, No. 17, 1985 3161
Photooxidation of CN- with Ti02 Particles
done with this material. The particles were used without reduction. Platinization of the T i 0 2 powders was carried out as follows:I2 2.2 g of T i 0 2 (Degussa) powder was suspended in 10 mL of platinizing solution (0.1 M hexachloroplatinic acid in 0.1 M HCI) which had been neutralized with Na2C03. The pH was adjusted to about 4 by adding acetic acid. The solution was heated to 55 f 3 OC. The reduction mixture was irradiated with a 150-W xenon lamp for 42 h. The yield of platinized powder was 1.8 g. The amount of Pt deposited on the particles was measured by atomic absorption. A sample of the powder was dissolved in a minimum amount of aqua regia, and the solution evaporated to dryness. The residue was then dissolved in concentrated H 2 S 0 4 and again evaporated to dryness. This residue was dissolved in Ti02 2hv = 2e- 2h' (1) a solution of concentrated HC1 and 0.01 g of NaCl which was then evaporated to dryness. The final residue was dissolved in CN- 2 0 H - 2h' = CNO- H 2 0 (2) a 1:l solution of concentrated HC1 and H 2 0 and the solution The CNO- was not oxidized further since the CNO- detected as volume increased to 10 mL. This procedure was used for both the reaction product accounted for 90% of the decomposed CN-.2 platinized and unplatinized Ti02 powder. Stock solutions conReduction of oxygen occurs on the dark surface, thus taining 1 mg of Pt/mL obtained from Alfa Ventron were also treated in the above manner to make the standard solutions. The O2 2 H 2 0 4e- = 4 0 H (3) atomic absorption measurements were made with a Perkin Elmer or instrument Model 47 using an acetylene-air flame and a platinum lamp. Absorption was measured at 266 nm. The Pt concentration O2 2 H 2 0 + 2e- = H202 2 0 H (4) on the Ti02 was determined by comparison of the absorption at Rotating ring-disk electrode measurements on Ti02 single 266 nm with that obtained from the calibration curve generated crystals with or without Pt coatings showed that the oxidation by using the standard solution. of water competes with reaction 2 for the photogenerated holes.' Rutile single crystals 0.5 X 0.5 cm and 1-mm thick were The reaction in alkaline solution is purchased from Commercial Crystal Laboratories. Before mounting they were reduced by heating under vacuum for 75 min 2 0 H - 2h' = ' / 2 0 2 + H20 (5) at 650 "C. The back contact to the crystal was made by attaching The photooxidation of CN- on Z n 0 9 and T i 0 2 single crystals a wire with Ag epoxy. A coating of 5-min epoxy is applied over gives limiting currents which are larger than for other reducing the contact. The crystal was then mounted to the end of a Pyrex agents. This observation has been termed "current d o ~ b l i n g " . ~ ~ ' ~ tube with silicone rubber sealant (Dow Corning). The active area The current doubling mechanism involves formation of an indelineated by the epoxy was 0.25 cm2. Before use the electrode termediate which is more easily oxidized than the original species was polished with 1-pm alumina powder and etched for 10 s in and can donate an electron to the conduction band of the semia 50:50 mixture of HN03:HF. conductor. Thus two electrons are obtained for each input photon Polycrystalline T i 0 2 electrodes were prepared from Ti metal and each CN- oxidized. The amount of current enhancement by electrochemical ~xidation.'~A 0.67-mm-thick titanium sheet observed depends on the relative ease of "reducing" the semiof (99.99%, Alfa Ventron) 1.3 X 1.5 cm was polished with fine conductor compared to reduction of other species in the electrolyte. sandpaper and then with 0.1-pm alumina powder. Anodization Morrison and Freund showed that on ZnO the current doubling was done in a solution of ethylene glycol containing 100 g/L of was observed when oxygen was present but not when H202 was sodium borate. The counter electrode was a platinum foil with added, indicating that the intermediate reduced the H202more an area of 48 cm2. The solution was stirred with a magnetic stirrer easily than Zn0.9 While reaction 2 describes the overall reaction during the anodization. A constant current of 1.95 mA/cm2 is for the oxidation of CN-, no detailed description of the mechanism applied. After 20 min the current was changed to 3.9 mA/cm2. has been given which accounts for the current doubling that has The anodization was stopped when the potential of the anode reached 50 V. The electrode was etched in concentrated HNO, been reported. The present work was undertaken to find the conditions which for 30 s before use. would maximize the rate of cyanide photooxidation. The rate of All reagents used in this work were reagent grade quality and the reaction was investigated as a function of pH. Platinized used without further purification. Solutions were prepared with anatase particles were used for the first time for the photooxidation distilled water which had been passed through an ion-exchange of CN-. By combining the enhancing effects of small particles column to give an ion conductivity of less than low6mho. of the anatase form, platinization, and reaction at a lower pH, Procedures. For the studies with the Ti02 powders, the reagent the rate of CN- photooxidation was increased by over a factor solution was typically 10 mL of 1 mM KCN in an alkaline solution of 500 compared to previous results. containing from 0.005 to 0.1 g of Ti02. The cell was a threenecked flask fitted with a quartz window 2.5 cm in diameter. The Experimental Section flask was surrounded by a thermostated water bath held at 20 Materials. Preliminary experiments were carried out using Ti02 OC. The solution was irradiated through the window with a 150-W particles in the anatase form obtained from three commercial xenon lamp. The energy impinging on the window was 82 sources: Matheson Coleman and Bell (particle size 125-250 pm, mW/cm2 as measured with an EGG photometer, Model 450-1. grain size 0.2 pm, surface area 1-10 m2/g),3 Baker Co. (particle During the irradiations the solution was stirred with a magnetic size 20-25 pm), and Degussa P-25 (particle size 0.015-0).040 pm, stirring bar, and oxygen was bubbled through the solution. A surface area 50 f 15 m2/g). The Degussa material has been cyanide ion-selective electrode (Orion, Model 9406) was used to characterized by Kiwi and Graetzel" as consisting of fused monitor the change in the cyanide concentration during the irspherical particles with a diameter of 0.014 pm and a hydrodyradiation. The concentration was determined from the electrode namic radius of 0.6 pm. Because of the much higher efficiency potential by using a calibration curve of emf vs. In [CN-] which obtained with the Degussa powder, most of the experiments were had been obtained by using standard cyanide solutions at the pH values used for the experiments. Because the solubility product
anatase was found to be more efficient than either the reduced anatase form or T i 0 2 with the rutile structure. The quantum efficiency for CN- oxidation was still very low; Frank and Bard estimated a value of 0.06 for the most efficient anatase particles.' Kogo et ale7studied the effect of platinization on the photocatalytic oxidation of CN- with T i 0 2 single crystals and polycrystalline powders having the rutile modification. They confirmed the generally observed trend that platinization enhances the conversion efficiency by decreasing the overvoltage for the reduction taking place in the dark.8 The following process was proposed by Frank and Bard2 for oxidation of CN- on T i 0 2 semiconductor particles:
+
+
+
+
+
+
+
+
+
+
(8) GrHtzel, M. Acc. Chem. Res. 1981, 14, 376. (9) Morrison, S. R.; Freund, T. Electrochim. Acta 1967, 13, 1343. (10) Gomu, W. P.; Freund, T.; Morrison, S. R. J. Electrochem. SOC.1968, 115, 818.
(11) Kiwi, J.; Graetzel, M. J . Phys. Chem. 1984, 88, 1302.
(12) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 4317. (13) Tamura, H.; Yoneyama, N.; Iwakura, C.; Murai, T. Bull. Chem. SOC. Jpn. 1977, SO, 153.
Rose and Nanjundiah
3768 The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
-O4I
.-
8
0 Pt-AnatasePawder(Left Scale) o Anatase Powder(Left Scale x 1/20) 0 Single Crystal Rutile (Right Scale)
'C
E
90 -2 5 0
20
40
60
80
I00
120
140
160
TlME(min)
0
Figure 1. Plot of change in [CN-] with time for oxidation of cyanide with Ti02 particles under different experimental conditions. All experiments done with 1 mM KCN, 0.1 g Ti02/10 mL solution. 0 , Platinized Ti02 (P-25), pH 11; 0,platinized T i 0 2 (P-25) pH 14; 0,Ti02 (P-25), no platinization, pH 1 1 ; A, T i 0 2 from MCB, no platinization, pH 1 1 . TABLE I: Effect of Particle Size and Platinization on Photochemical Oxidation Rate of CN-Solutions with TiOs Particles" koW, mid av particle wt of catalyst, size, r m g/lO mL unplatinized platinized 125-250 0.1 0.2 20-25 0.1 0.7 0.015-0.040 0.1 2.0 39.0 0.015-0.040 0.05 1.2 5.1 0.01 5-0.040 0.005 1 .o 0.9 "Solutions containing 1 mM cyanide at pH 1 1 were illuminated through a 2.5-cm-diameter quartz window with a 150-W xenon lamp. Oxygen was bubbled through the solution during the irradiation.
of AgCNO is much higher than AgCN, the presence of CNOdoes not interfere with measurements of [CN-1. Current-voltage curves for the single-crystal and polycrystalline electrodes were obtained with a cyclic voltammetric unit consisting of a Bioanalytical Instrument CV-1B cycler and a Houston Instruments Model 2000 x-y recorder. The T i 0 2 electrodes were illuminated with the xenon lamp as described above. Platinum and the saturated calomel electrode (SCE) served as the counter and reference electrodes, respectively. All solutions were degassed with nitrogen. The coulometric experiments were done on an illuminated electrode held at a potential of 2 V vs. SCE. Phosphate-buffered solutions were used for the measurements at pH 11 and 12. Nitrogen was bubbled through the solution during the irradiation to remove the oxygen formed. Current-time curves were recorded and integrated with a Bascom-Turner Model 21 10 recorder. The changes in the cyanide concentration were monitored with the cyanide ion-selective electrode.
Results The rate of the photooxidation of CN- with T i 0 2 particles of different size and methods of preparation were measured in solutions with pH l l . The initial concentration of cyanide was l mM for all the studies. Oxygen was bubbled through the solution during the irradiation. Control experiments run in the absence of either the T i 0 2 or light showed no decrease in the CN- concentration over a period of 24 h. If the rate-determining step for the loss of cyanide depends on the first order of cyanide concentration, the rate expression can be expressed as -d[CN-] /dt = k,bd[CN-]
(6)
where the rate constant koW may be a function of [OH-] or [h'] but will be constant for a given pH and irradiation condition. Integration of eq 6 between limits 0 and t gives In ([CN-lt/[CN-lO) = -kobsdt
(7)
where [CN-1, and [CN-1, are the cyanide concentrations initially and at time t , respectively. /cobs,, was determined from the slope
0
1
I
I
I
O
TABLE II: Effect of p H on Photooxidation Rate of CNon TiOl Particles" koMr mi& pH [OH-]/[CN-] unplatinized platinized 11 1 2 39 12 10 0.7 9.7 13 100 0.6 8.8 14 1000 0.2 6.3 "Ten milliliters of 1 mM CN- solution containing 0.1 g of T i 0 2 (Degussa) were illuminated with 150-W xenon lamp. Oxygen was bubbled through the solution during the irradiation.
of the plot of In [CN-],/[CN-], vs. time. Figure 1 shows some of the plots for different samples of Ti02. In most cases the plots were linear up to conversion values of 80% which supports the assumption of first-order dependence on [CN-1. The rate constants determined from the linear portion of the plot are given in Table I for a number of different reaction conditions. For T i 0 2 particles there is a general trend toward faster rate constants as the particle size decreases. The ratios of the rates constants from the MCB and Degussa particles is about the same as the ratio of their surface areas. The total amount of catalyst, Le., the loading, also had an effect on the rate constant. A decrease in the rate constant was observed with smaller amounts of catalyst. This result is different than reported by Frank and Bard,' who found the rate was independent of the amount of catalyst in the range of 0.05-0.2 g/ 10 mL solution. The platinization of T i 0 2 anatase powder increased the rate of photooxidation of cyanide. The weight percent of Pt on the T i 0 2 particles was determined from the atomic absorption measurements to be 1.5%. This loading is in the percentage range where efficient H2 production on Pt/Ti02 photoelectrodes was reported." In the present case, however, where the Pt is applied photoelectrochemically, the high-temperature reduction step used when Pt is applied by the impregnation or ion-exchange method is not required. The effect of Pt was most marked when the amount of Ti02 was large. In contrast to results obtained with the unplatinized particles, the reaction on platinized particles was so fast when the highest loadings of catalyst were used that room light was adequate to cause significant photooxidation of the cyanide. The rate of increase by a factor of 20 compared to the unplatinized particles was considerably higher than the factor of 2 reported on platinized rutile powders.' There was scarcely any rate enhancement at the lowest loadings of Ti02 that were investigated. The smaller effects observed with the low loadings may be due to poisoning of the Pt layer by CN- as was reported by Sawyer and Day.14 (14) Sawyer, D. T.; Day, R. J. J . Electroanal. Chem. 1963, 5, 195.
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3769
Photooxidation of CN- with TiO, Particles I
TABLE 111: Efficiency of CN- Photooxidation on n-Ti02 Single-Crystal Electrode" decrease total CNin [CN-1, charge, charge," PH soh mM/L C C efficiency, % 11 PB' 0.2 1.58 0.33 24 l l d PBC 0.14 1.15 0.27 23 0.18 2.34 0.35 15 12 PBC 13 0.1 M K O H 0.16 3.00 0.30 10 14 1MKOH 0.32 7.20 0.62 8.6
1.0-
-
0.9
0.8
fa \
-
0.7-
E
I
t
06-
"Solutions irradiated with 150-W xenon lamp initially contained 1 m M CN-. The Ti02 electrode was held at 2 V vs. SCE and nitrogen was bubbled through the solution. bCalculated assuming there is no current doubling occurring. CThesolution was buffered with 0.05 M HP042-. dSolution illuminated with light of half the intensity used in the other irradiations.
v)
W
a a
3 0
04-
TABLE I V Comparison of Relative Rate Constants for Photooxidation of CN- with Different Types of TiO, Particles particle relative (sa." m2/a) olatinized DH rate ref
-10
-8
-6
-4
-2
0
2
4
6
POTENTIAL( V v s SCE)
Figure 3. Current-potential curves for T i 0 2 electrodes illuminated with 150-W xenon lamp at pH 13 in the absence (solid lines) and presence (dashed lines) of 1 M CN-. Curves a and b = single-crystal rutile; c and d = polycrystalline T i 0 2 electrode after heat treatment; e and f = Ti02 polycrystalline T i 0 2 before heat treatment.
The effect of pH on the rate constant for photooxidation of CNusing platinized and nonplatinized TiO, powder is given in Table I1 and Figure 2. The p H was varied from pH 11 to pH 14. Lowering the pH led to an increase in the rate of photooxidation of CN-. This trend is the same as reported for photooxidation of the halide ions on TiO2.I5J6 As seen from Table 11, the change in the rate constant is not directly proportional to the change in the [OH-]/[CN-] ratio. The greatest increase is observed in going from pH 12 to pH 11. While the absolute rates are higher for the platinized particles, the same trend with change in pH is observed with both platinized and unplatinized particles. The current efficiency of cyanide photooxidation on singlecrystal n-Ti02 was measured at different pH values. The current-voltage curve with the single-crystal electrode in l M KCN and 0.1 M N a O H was recorded as a preliminary test of the properties of the electrode. The current doubling phenomenon for cyanide photooxidation reported previouslp9 was not observed. Our polycrystalline TiOz electrode made by anodization of Ti, however, did show current doubling. On the other hand, when this electrode was heated at 400 OC for 1 h in argon, current doubling was not observed in the presence of 1.O M KCN although the limiting current increased by a factor of 20 compared to that obtained prior to the reduction of the electrode. Plots of the photocurrent as a function of voltage for the T i 0 2 electrodes are given in Figure 3. It is apparent that current doubling is very sensitive to the type and method of production of the TiO, electrode. Since we did not observe current doubling with the single-crystal electrode used in the efficiency measurements, we have calculated the efficiency assuming eq 2. According to this mechanism 2 C of charge are generated for each mole of CNthat is oxidized. The coulometric experiments were carried out with the photoanode held a t a potential of +2.0 V vs. SCE which was in the current-limited range. The efficiency of the reaction was determined by monitoring the current during the photooxidation and measuring the loss of CN- at the completion of the reaction. The (15) Tamura, H.; Aridado, T.; Yoneyama, H.; Matsuda, Y. Electrochim. Acta 1974, 19, 273. (16) Fujishima, A.; Inoue, T.; Honda, K. J . Am. Chem. SOC.1979, 202, 5582.
rutile (3.8) rutile (1-10) rutile (3.8) anatase (1-10) anatase (50) anatase (50) anatase (50)
no no yes no no yes yes
13.3 13.0 13.3 13.0 13.0 13.0 11.0
0.1 0.2 0.3 1 .O
9 125 560
7 1 7 this this this this
work, 2 work work work
" Surface area. efficiency is calculated as the fraction of the total current represented by the oxidized CN-. It is assumed that the other principal competing reaction is oxidation of the OH- ion by the photogenerated holes. Table I11 shows the change in the efficiency of CN- photooxidation on single-crystal T i 0 2 as a function of pH. The effect of light intensity on the efficiency was tested at pH 11. The efficiency was unchanged when the intensity of light was halved, indicating that our measured efficiency at the higher light intensity was not being limited by a mass-transfer effect of the CNreaching the electrode surface. Our value of 10% efficiency at pH 13 is in reasonable agreement with the 14% efficiency reported for measurements made on single-crystal rutile a t pH 13.3.' Figure 2 shows that the relative changes in the efficiency with changes in pH on single-crystal TiO, are comparable to the changes observed in the reaction rates with TiO, particles. The efficiency increases at lower pH, and the most marked effect is again observed going from pH 12 to pH 11. Discussion Rate of CN- Photooxidation on TiO,. The previous investigations of photooxidation of cyanide on semiconductors measured the amount of cyanide reacted for a given irradiation time and used the values as a reaction rate for cyanide loss. At high light intensities from a 2.5-kW Hg-Xe lamp, the rate of cyanide reacted was independent of the concentration of CN- for measurements of 90% conversion starting with concentrations of 1, 10, and 100 mM CN-.' In our studies, however, we have observed first-order dependence on [CN-] a t 1 m M CN- using irradiation from a 150-W Xe lamp. The results of Frank and Bard at comparable light intensities and 1 mM CN- also gave a linear plot of In [CN-],/ [CN-I,, indicating first-order dependence., Their values measured at 3 mM initial concentration, however, are a better fit to a zero-order than a first-order dependence on the cyanide concentration. A similar change in the order of cyanide dependence as its concentration is increased has been reported for the electrochemical oxidation of cyanide on Pt electrodes.I4J5 This change to first-order dependence at lower concentrations reduces the rate of cyanide oxidation as the concentration decreases, although the rate constant does not change. The results given in Tables I1 and I11 show that using small Ti02 particles with high surface area, lowering the pH, and
3110
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
platinizing the surface all enhanced the rate constant. It is difficult to compare rates of CN- photooxidation published by others with the values in this work because of differences in amount of TiO,, platinization, and intensity of absorbed light. Nevertheless, Table IV represents an attempt to compare the results for a number of different studies. The rate constants from the earlier studies have been calculated assuming first-order dependence on cyanide as discussed above. Since in some cases only a single data point was available to determine the rate constant, the value may not be precise, but its order of magnitude is adequate to draw some qualitative conclusions. The results are normalized to the values obtained with unplatinized TiO, particles obtained from MCB which were studied both in this work and by Frank and Bard'q2 under similar conditions of loading and pH. The results on unplatinized rutile powders under comparable conditions from two different lab~ratories'.~ allow comparison of the anatase and rutile results. This procedure should take into account differences in cell geometries, light intensities, and loadings for the three different investigations. Comparison of the increase in the rate of photooxidation of [CN-] with the addition of Pt in this work with that of Kogo et aL7 shows that the increase on anatase particles was 5 times that obtained on the rutile particles. While the results on rutile were done with Pt applied by an absorption technique, it was stated that similar results were obtained when the Pt was applied by the photoelectrochemical deposition method used here.I2 The main differences in their experimental conditions were use of 3 times higher concentration of cyanide, use of larger particles which had a smaller surface area per gram, and Ti02 with the rL tile rather than the anatase structure. Since relative measurements with and without Pt are being compared, however, it is difficult to understand how the particle type or size should have caused the difference. Thus, the smaller increase observed with rutile is probably due to the higher cyanide concentration. Measurement of the cathodic current in the dark in oxygenated solutions without cyanide showed that the onset potential for oxygen reduction shifted anodically with increasing amounts of Pt. They did not report, however, the effect of the addition of cyanide on the cathodic dark current. The higher concentration of cyanide used by Kogo et al. may have poisoned the Pt on the Ti02 and inhibited the reduction of oxygen as is observed on Pt metal e1ectr0des.I~ The reaction rate constant for cyanide oxidation increased as the pH of the solution was decreased. It was also observed that on single-crystal T i 0 2 the efficiency of cyanide photooxidation relative to competing processes, assumed to be primarily photooxidation of water, increased with decreasing pH. The pH dependence was comparable for the T i 0 2 particles with or without Pt and with the Ti02single-crystal electrode for which there was no oxygen in the system. These results indicate that changes in the pH are probably affecting the rate of the anodic reaction on the illuminated semiconductor which, in turn, is the rate-controlling step of the reaction. Figure 4 shows an energy diagram as a function of pH for the TiO, system along with the relevant redox potentials for reactions involved in this study. The Ti02 flat band potential decreases with pH by 59 mV/pH which is the same change as the redox potential for water oxidation and redu~ti0n.l~The redox potential calculated for CN- oxidation based on reaction 2 is -0.97 vs. N H E a t pH 14. With such a negative potential, it is energetically possible that CN- can be oxidized in the dark by donating its electrons to the conduction band of TiO,. There would obviously be severe kinetic and mechanistic barriers, however, since the reaction as written involves transfer of two electrons and participation of two OH- molecules. Reaction 2 also implies that the redox potential for CN- oxidation should depend on the pH. Several results, however, indicate that the potential does not change with pH. Polarization curves of CN- oxidation on graphite showed no dependence with changes in pH from 13.8 to 11.9.18 We have also measured the cur-
Rose and Nanjundiah
-
CNOXN-
-1 2
-0E -08 -04
-
I
,(CN)2/CN-04
C 0 04
04 W
W V
I 08
z
-
VI
z
cn 3 8 In
Br2/Br-
W
I 2
12
IE 16
2c 20
24 4 - 1
PH
Figure 4. Positions of energy levels of TiOz and reversible potentials of redox couples at a Pt electrode for the pH range of 10-14. TiOl (dec) is the decomposition potential of Ti0, and TiO, (ss) the proposed energy level of TiO, surface states.
rent-potential curves on Pt for 1 mM solutions of NaCN at pH's from 10 to 14. At pH 10 a peak due to cyanide is obtained at 1.1 V vs. SCE in agreement with that reported by Sawyer and Day.14 As the pH is increased, the CN- oxidation peak becomes a shoulder and then is obscured by the increasing background from oxidation of water whose redox potential is moving in the cathodic direction at 59 mV/pH unit. If the CN- oxidation was dependent on pH, its potential should also move cathodic and remain separated from the background due to water oxidation. As the pH is reduced, therefore, the increasing efficiency of oxidation of CN- can be explained on the basis of the change in the relative positions of the redox potentials. Cyanide has been described as a pseudohalogen with an electron affinity between that of C1-and F.I9 Our measured potential of +1.1 V vs. SCE, which lies between the oxidation potentials of C1- and F,seems reasonable for oxidation of CN- to the cyanide radical. CN- = CN- e(8)
+
Reaction 8 would then be the first and rate-determining step in the photooxidation of CN- on Ti02 surfaces. Our studies, which measured only the decrease of CN- with time, do not provide any information on the subsequent steps leading to CNO-. Several possible detailed mechanisms of cyanide oxidation have been discussed by Arikado et a1.18 One of them involved formation of cyanogen, (CN),, as an intermediate after reaction 8; thus 2CN. (CN), (9) (CN), 2 0 H CN- CNO- H20 (10) -+
+
--L
+
+
The rate-determining step, however, was still deemed to be reaction 8. The direct conversion of CN- to (CN),, though energetically more favorable than the stepwise mechanism through CN. (see (18) Arikado, T.; Iwakura, C.; Yoneyama, H.; Tamura, H. Electrochim. Acfa 1976, 21, 1021.
(17) (a) Bolts, J. M.; Wrighton, M. S. J. Phys. Chem. 1976,80, 2641. (b) Tomkiewicz, M.: Fay, H. Appl. Phys. 1979, 18, 1.
(19) Douglas, B. E.; McDaniel, D. H. "Concepts and Models in Inorganic Chemistry"; Blaisdell: Waltham, MA, 1965; p 248.
J . Phys. Chem. 1985,89, 3771-3774 Figure 4), would be second order in CN- which was not observed either in this work or in the electrochemical studies on Pti4 or carboni8 electrodes. The oxidation potential of CN- becomes more favorable compared to water oxidation as the pH is lowered. The situation is comparable to that of the oxidation of halogen anions on Ti02 reported by Fujishima et a1.I6 They observed that for the competitive oxidation between Br- and water in acid or neutral solutions, the efficiency of halogen ion oxidation increased as the pH decreased. The effect at the low concentrations of Br-, however, was much less marked than for CN- results reported here. While the Br- oxidation potential actually becomes negative of the water oxidation potential in acidic pH, the redox potential for water oxidation in basic solutions always lies negative of that for CN-. This relationship may explain why the oxidation of water is still 3 times more efficient than cyanide oxidation even at pH 11. The rapid rate of cyanide photooxidation corresponds to the loss of 8 pmol of cyanide in 5 min for the 10-mL solution of 1 mM [CN-1. A rough estimate of the quantum efficiency for the reaction was made using the following assumptions: 10% of the energy of the xenon lamp is at wavelengths shorter than 380 nm, the wavelength at which T i 0 2 begins to absorb light. All the ultra-band-gap light impinging on the cell is absorbed by the Ti02 in the slurry. The absorbed photon flux is then 2.7 X lOI9 photons during the 5-min irradiation based on the intensity of the xenon lamp measured by the radiometer, and using an average energy of the photons of 3.5 eV. Since two photons are used in the oxidation of each cyanide molecule, this flux could oxidize a
3771
maximum of 22 pmol of cyanide. If the ratio of rates of cyanide to water oxidation is the same on the particles as on the singlecrystal electrode at pH 11, namely 1:3, then 5.5 pmol of CNwould be oxidized if the quantum yield for electrochemical reactions, both water and cyanide oxidation, was unity. Despite the many approximations, the calculated and observed moles of photooxidized CN- are fairly close and indicate that very few of the electron-hole pairs formed by the light are lost by recombination. The optimization of the experimental conditions, therefore, has led to a significant improvement in the quantum efficiency for cyanide oxidation compared to the value of 0.06 reported in Frank and Bard’s initial studies.2 To determine if the rapid rates measured in the laboratory obtained for more real-life situations, a few tests were run in sunlight. One-tenth gram of platinized anatase from Degussa dispersed in 10 mL of a 1 mM CN- solution at pH 11 was exposed to the afternoon sunlight. The solution was contained in a 2cm-diameter quartz tube. Oxygen was bubbled through the solution before the irradiation, and the slurry was stirred during the exposure. In a 5-min irradiation, 85% of the CN- was removed. When the experiment was carried out at pH 12, 57% of the CN- was photooxidized. These results show that Ti02 particles could indeed be an efficient method for cleaning cyanide from wastes streams as proposed earlier.2 Acknowledgment. This work was supported by the Office of Naval Research. The many helpful discussions with Dr. R. David Rauh are gratefully acknowledged. Registry No. CN-, 57-12-5; anatase, 1317-70-0.
Conformational Dependence of the Pyramidallzation of Alkyl Radicals Michael N. Paddon-Row*+and K. N. Houk* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania I5260 (Received: March 25, 1985)
Ab initio MO calculations reveal that the presence and degree of pyramidalization of the ethyl, isopropyl, and tert-butyl radicals are dependent upon the conformation(s) of methyls attached to the radical center. In conformations lacking a plane of symmetry, pyramidalization toward a partially staggered conformation occurs. In the resulting structures, the CH bond which is coplanar with the radical orbital lengthens and has a lowered vibrational frequency, indicating enhanced hyperconjugation with the radical center. The pyramidalization of radicals is explained in terms of torsional and hyperconjugative effects.
Introduction Ab initio calculations predict that normally planar trigonal centers (alkenes,’ carbonyls,2 and radical^^-^) are pyramidalized when substituted by alkyl groups which are fied in conformations that are asymmetric with respect to the plane of the trigonal center and the three attached ligands. The direction of pyramidalization is predicted to be that which results in partial staggering about the bond to the substituent. In the case of alkenes and carbonyls, experimental evidence in support of these predictions has been For radicals, experimental evidence is consistent with this prediction, if not overwhelming.6 We have proposed that the pyramidalization is a manifestation of closed-shell repulsion and hyperconjugative interactions, which cause ethane, propene, etc., to prefer staggered conformation^.^ However, in a recent publication, it was proposed that ‘‘u conjugation” of C C bonds causes pyramidalization of alkyl radicals.’ The u conjugation model implies that the conformation of alkyl groups will not influence the pyramidalization of alkyl radicals. However, Visiting Research Professor, University of Pittsburgh, 1984-1985. Permanent address: University of New South Wales, New South Wales, Australia.
0022-3654/85/2089-3771$01.50/0
ab initio M O calculations (vide infra) reveal that pyramidalization is strongly dependent on conformational effects. In this paper, a model which does provide a satisfactory rationalization of the relationship between alkyl substituent conformations and radical (1) Rondan, N. G.; Paddon-Row, M. N.; Caramella, P.; Houk, K. N. J . Am. Chem. SOC.1981,103,2436. Houk, K. N.; Rondan, N. G.; Brown, F. K.; Jorgensen, W. L.; Madura, J. D.; Spellmeyer, D. C. Ibid. 1983,105,5980. (2) Jeffrey, G. A.; Houk, K. N.; Paddon-Row, M. N.; Rondan, N. G.; Mitra, J. J . Am. Chem. SOC.1985, 107, 321. (3) (a) Pacansky, J.; Dupuis, M. J . Chem. Phys. 1978, 68, 4276. (b) Pacansky, J.; Dupuis, M. Ibid. 1979, 71,2095. (c) Pacansky, J.; Dupuis, M. Ibid. 1980, 73, 1867. (d) Yoshimine, M.; Pacansky, J. Ibid. 1981, 74, 5168. (4) Paddon-Row, M. N.; Houk, K. N. J . Am. Chem. Soc. 1981,103, 5046. (5) (a) Frisch, M. J., unpublished data. (b) Binkley, J. S., unpublished data. These results are included in: Whiteside, R. A,; Frisch, M. J.; Binkley, J. S . ; DeFrees, D. J.; Schlegel, H. B.; Krishnan, R.; Pople, J. A. ‘CarnegieMellon Quantum Chemistry Archive”, 3rd ed.; Department of Chemistry, Carnegie-Mellon University: Pittsburgh, PA 15213. (6) (a) Pacansky, J.; Coufal, H. J. Chem. Phys. 1980, 72, 5285. (b) Pacansky, J.; Coufal, H. Ibid. 1980, 72, 3298. (c) Wood, D. E.; Williams, L. F.; Sprecher, R. F.; Lathan, W. A. J. Am. Chem. SOC.1972,94,6241. (d) Griller, D.; Ingold, K. U.; Drusic, P. J.; Fischer, H. Ibid. 1978,100, 6750. (e) Pacansky, J.; Chang, J. S. J. Chem. Phys. 1981, 74, 5539. (7) Dewar, M. J. S. J . Am. Chem. SOC.1984, 106, 669.
0 1985 American Chemical Society
