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Langmuir 2000, 16, 4896-4900
Catalysis by Palladium Nanoparticles in Microemulsions Dionı´sia M. de Jesus and Michael Spiro* Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K. Received December 2, 1999. In Final Form: February 16, 2000 The oxidation by Co(NH3)5Cl2+ of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPPD) catalyzed by nanoparticles of palladium in an aqueous buffer/AOT/n-heptane microemulsion has been studied. The initial rates of reaction varied proportionately with the Co(NH3)5Cl2+ concentration, increased slightly with increasing TMPPD concentration, and rose linearly with the palladium concentration up to ca. 6 µM. However, a much steeper increase was discovered at higher catalyst concentrations. Evidence that adsorption of TMPPD on the palladium surface was playing a major role in the rate-determining step was obtained both from the fact that the activation energy at 15 °C was more than twice as large as that at 40 °C and from electrochemical experiments with the two reactants. A microreactor model for catalyst particles in microemulsion water pools was set out, and the resulting water pool sizes and numbers have been evaluated.
Introduction The catalytic potential of microemulsions incorporating solid nanoparticles was predicted several years ago,1 but to date, very few kinetic studies have been carried out with these novel media.2-5 We have recently studied5 the kinetics of the reaction between N,N-dimethyl-pphenylenediamine (DMPPD) and Co(NH3)5Cl2+ catalyzed by palladium nanoparticles in buffered aqueous/AOT/nheptane microemulsions, and now report the catalytic kinetics in the same medium of the related reaction
TMPPD + Co(NH3)5Cl2+ f +
S + Co
microemulsion (containing 1 M water and 0.1 M AOT, keeping their ratio at 10) and the preparation of the colloidal palladium within it. The pH 5.6 buffer was made from potassium hydrogen phthalate and NaOH. The kinetics of the formation of the blue S+ were followed by spectrophotometry at the two absorbance peaks of 563 and 612 nm. The respective extinction coefficients were taken as 1.24 × 104 8 and 1.27 × 104 M-1 cm-1 (cf. 1.30 × 104 at 610 nm9). The colloid characterization (which gave the radius of the Pd particles as 2.5 ( 0.1 nm) and the electrochemical experiments were carried out as described previously.5
Results and Discussion 2+
-
+ 5NH3 + Cl
(1)
where TMPPD stands for N,N,N′,N′-tetramethyl-p-phenylenediamine and S+ stands for p-Me2N‚C6H4‚NMe2+‚. The reason for investigating the reaction of the tetramethyl compound was that its deamination rate is lower than for DMPPD,6 and the deamination side reaction of DMPPD was thought to be one of the factors that affected the latter stages of the main reaction.5 A reaction such as eq 1 is relevant in photography, where catalytic oxidation of a diamine developer to a semiquinonediimine S+ is followed by its further oxidation to the corresponding quinonediimine T2+. This then reacts with organic couplers incorporated in the photographic film to form the required colored dyes.7 Experimental Section N,N,N′,N′-tetramethyl-p-phenylenediamine in the dihydrochloride form was supplied by Aldrich. The origin of the other chemicals was the same as before,5 as was the composition of the (1) Nagy, J. B.; Derouane, E. G.; Gourgue, A.; Lufimpadio, N.; Ravet, I.; Verfaillie, J. P. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum: New York, 1989; Vol. 10, pp 1-43. (2) Boutonnet, M.; Andersson, C.; Larsson, R. Acta Chem. Scand. 1980, A34, 639. (3) Boutonnet, M.; Kizling, J.; Touroude, R.; Maire, G.; Stenius, P. Appl. Catal. 1986, 20, 163; Catal. Lett. 1991, 9, 347. (4) Robinson, B. H.; Khan-Lodhi, A. N.; Towey, T. In Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; pp 198-200. (5) Spiro, M.; de Jesus, D. M., Langmuir 2000, 16, 2464. (6) Nickel, U.; Kemnitz, K.; Jaenicke, W. J. Chem. Soc., Perkin Trans. II 1978, 1188. (7) Keller, K., Ed. Science and Technology of Photography; VCH Publishers: Weinheim, 1980; Section 5.
Kinetic Results at 25 °C. As in the analogous reaction with DMPPD, the concentration of the S+ produced by TMPPD rose to a maximum and then slowly declined. However, the reaction was more complete, with some 69% of the Co(III) having reacted at the maximum compared with 61% for DMPPD under the same standard conditions of 5.7 µM Co(NH3)5Cl2+, 57 µM diamine, and 3 µM Pd. This is consistent with the deamination side reaction being less significant for TMPPD. The initial rates of the reaction were determined by fitting the absorbances in the rising stages to cubic polynomials as a function of time and differentiating. Because some of the runs spanned a period of several weeks, two runs at the standard conditions were carried out at regular intervals. These showed a gradual decline in the initial rates after the first 11 days, attributed to changes in the diluted colloid solution. The initial rates of the runs were then corrected accordingly. Table 1 lists the mean values at both wavelengths of the initial slopes of the absorbance-time plots obtained at various reactant and palladium concentrations (expressed in moles per cubic decimeter of the overall microemulsion). All tabulated values (bar one) are the mean values of two or more runs which always showed good reproducibility. Closely agreeing rates of v (in moles per liter per second) were obtained for any given run (mean difference 1.8%) when the initial slopes at the two wavelengths were divided by the appropriate extinction coefficients. (8) Pettersson, G. Acta Chem. Scand. 1968, 22, 3063. (9) Nickel, U.; Borchardt, M.; Bapat, M. R.; Jaenicke, W. Ber. BunsenGes. 1979, 83, 877.
10.1021/la9915668 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/12/2000
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Langmuir, Vol. 16, No. 11, 2000 4897
sions). The explanations offered in these cases (desorption of mediating species from the surface,10 partial poisoning by adsorption of impurities,11 or chemical reaction with a surface oxide layer12) do not appear so appropriate here. Instead, the unusual catalytic behavior at high palladium concentrations is ascribed to the further fast catalyzed oxidation of S+ to the p-quinonediimine T2+ which then rapidly synproportionates13 to yield two S+ ions for every one oxidized: Pd
TMPPD + Co(III) 98 S+ + Co(II) Pd
S+ + Co(III) 98 T2++ Co(II) Figure 1. Variation of the initial rates at 25 °C with the palladium concentration for microemulsion mixtures containing 5.7 µM Co(NH3)5Cl2+ and 57 µM TMPPD prepared in a phthalate buffer at pH 5.6. Table 1. Mean Corrected Initial Absorbance-Time Slopes for Runs in pH 5.6 Phthalate Buffer under Various Conditions temp [Co(NH3)5Cl2+] [TMPPD] [Pd] (°C) (µM) (µM) (µM) 25 25 25 25 25 25 25 25 25 25 25 25 10 15 35 45
5.7 2.8 4.3 11.4 17.1 5.7 5.7 5.7 5.7 5.7 5.7 5.7 5.7 5.7 5.7 5.7
57 57 57 57 57 28 114 171 57 57 57 57 57 57 57 57
3 3 3 3 3 3 3 3 1.5 6 9 12 3 3 3 3
initial slopes (10-5 s-1) 563 nm 612 nm 5.35 2.4 4.4 10.3 14.0 4.5 6.2 6.65 1.6 12.3 32.1 ∼60 0.78 1.68 12.4 19
5.45 2.3 4.6 10.5 14.6 4.7 6.4 6.9 1.7 12.7 32.4 ∼60 0.80 1.69 12.6 20
A plot of ln v against ln[Co(III)] for runs with 57 µM TMPPD and 3 µM Pd gave a good straight line with a least-square slope of 0.965, indicating a kinetic order in Co(NH3)5Cl2+ of almost unity. The data points of ln v plotted against ln[TMPPD] for runs with 5.7 µM Co(III) and 3 µM Pd lie on a straight line with a slope of 0.21, so giving a fractional kinetic order in TMPPD. For runs with the standard concentrations of 5.7 µM Co(III) and 57 µM TMPPD, the rate at first increased linearly with the concentration of the palladium catalyst according to the equation
d[S+] ) v ) v0 + kcat[Pd] dt
(2)
where kcat ) 1.9 × 10-3 s-1 and v0 ) -1.5 × 10-9 M s-1. The small negative intercept arose from a slow side reaction between S+ and AOT that had been observed in blank experiments without palladium. However, at palladium concentrations above 6 µM, the catalytic rate increased much more than proportionately, as shown in Figure 1. This was not caused by an optical effect since dilutions of the stock palladium solution obeyed the BeerLambert law at both wavelengths, and the reference cuvette always contained the same concentration of palladium microemulsion as the reaction cuvette. Kinetic orders in catalyst greater than unity have been reported in the literature for several other heterogeneously catalyzed reactions in solution (but not so far in microemul-
fast
(3) (4)
T2+ + TMPPD 98 S+ + S+
(5)
d[S+] ) v3 - v4 + 2v5 ) v3 + v4 dt
(6)
Hence
since v4 ) v5 by application of the stationary state assumption to T2+. The rate of the catalyzed eq 4 would be expected to depend on [S+], which is present at low concentration, as well as on [Co(III)] and [Pd]. On the assumption that these dependences are all first-order, we can write
d[S+] ) k′3[Co(III)][TMPPD]0.21[Pd] + dt k′4[S+][Co(III)][Pd] (7a) ) k3[Pd] + k4[S+][Pd]
(7b)
where k′n is the rate constant for reaction n while kn incorporates some of the concentration terms as shown. Integration gives
(
ln 1 +
)
k4[S+] ) k4[Pd]t k3
(8)
Expansion of the logarithmic term for low values of [S+] gives a series which, on differentiation, leads to
d[S+] 1 ) k3[Pd] + k3k4[Pd]2t + k3k42[Pd]3t2 + ... (9) dt 2 The initial slope from fitting a polynomial in t to the observed values of [S+] would normally give k3[Pd]. However, at high catalyst concentrations, the reactions are so fast that parts of the following terms become included even in the early experimental points on which the initial slope is based. This would account for the higher dependence on [Pd] shown in Figure 1. Moreover, as indicated in eq 7a, v4 and k4 are likely to include a term in [Co(III)]. According to the explanation above, the rates obtained from the initial slopes at high palladium concentrations would then show a kinetic order in [Co(III)] greater than 1. Additional experiments with 9 µM Pd led to an apparent order of 1.37 in Co(III), so confirming the (10) Mureinik, R. J. J. Catal. 1977, 50, 56. (11) Spiro, M.; Freund, P. L. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1649. (12) Albery, W. J.; Bartlett, P. N.; McMahon, A. J. J. Electroanal. Chem. 1985, 182, 7. (13) Michaelis, L.; Schubert, M. P.; Granick, S. J. Am. Chem. Soc. 1939, 61, 1981.
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de Jesus and Spiro
Figure 2. Arrhenius plot of ln(initial rate) against 1/T for runs carried out with microemulsions containing 5.7 µM Co(III), 57 µM TMPPD, and 3 µM Pd in pH 5.6 buffer. Table 2. Activation Energies Calculated from the Data in Figure 2 Fitted by Eq 10 temperature (°C) activation energy (kJ mol-1)
15 96.7
25 72.4
40 38.8
prediction. As a further check, some extra runs were also carried out with 9 µM Pd on the corresponding microemulsion reaction between Co(NH3)5Cl2+ and DMPPD (N,N-dimethyl-p-phenylenediamine) at 25 °C where the rate had previously been found5 to vary linearly with [Pd] at lower palladium concentrations. Here, too, a greater rate rise than that expected from the linear dependence was found with 9 µM palladium catalyst. Effect of Temperature on Catalytic Rates. Figure 2 shows the Arrhenius plot for the catalyzed reaction to be a pronounced curve, as in the case of the analogous DMPPD + Co(III) reaction. The curve can be well represented by the second-order polynomial equation
ln v ) -131.5 + 7.5664 × 104/T - 1.2577 × 107/T2 (10) where T is the temperature in Kelvin. Differentiation leads to the activation energies in Table 2. The marked rise in activation energy as the temperature falls points to a rate-determining process that is increasingly hindered by greater TMPPD adsorption on the surfaces of the palladium nanoparticles. More evidence for such adsorption is provided by the electrochemical experiments. Electrochemical Studies. Voltammograms were determined with a rotating palladium electrode of 1 mm diameter in phthalate-buffered aqueous solutions of Co(NH3)5Cl2+ and of TMPPD. Their concentrations were 55.6 times greater than those in the overall microemulsions so as to represent the concentrations in the water pools. The current-potential curve in Figure 3 shows the two oxidation steps of TMPPD to S+ and then to T2+. The currents for the reduction of Co(NH3)5Cl2+ in Figure 4 are much smaller and are obviously affected by the presence of even micromolar amounts of added TMPPD. When both TMPPD and Co(NH3)5Cl2+ are present together, as they are in the reaction system, they will impose a mixed (or mixture) potential Emix at the palladium surface.14,15 The net current is then zero because the anodic current of TMPPD oxidation balances the cathodic current of Co(NH3)5Cl2+ reduction. These mixture currents Imix are directly proportional to the catalytic rates according to (14) Wagner, C.; Traud, W. Z. Elektrochem. 1938, 44, 391. (15) Spiro, M. Chem. Soc. Rev. 1986, 15, 141.
Figure 3. Current-potential curves at a palladium electrode rotating at 9 Hz for the oxidation in pH 5.6 buffer of 1.6 mM TMPPD, at a sweep rate of 10 mV s-1.
Figure 4. Current-potential curves at a palladium electrode rotating at 9 Hz for the reduction in pH 5.6 buffer of 0.16 mM Co(NH3)5Cl2+: (1) without added TMPPD and (2) with the addition of 20 µM TMPPD, both at a sweep rate of 10 mV s-1.
Faraday’s law. It follows from the figures that Emix lies in the S+ region of TMPPD oxidation and close to the diffusion-controlled limiting current region of the cobalt curves. The catalytic rates would thus largely be determined by the rate of diffusion of Co(NH3)5Cl2+ ions to the surface of the palladium rod. However, the hydrodynamic conditions in the microemulsion water pools will be quite different. The thickness of the diffusion layer at the surface of the rotating palladium rod is ca. 2 × 10-5 m according to the Levich theory,5 whereas that at the surface of the Pd nanoparticles in the water pools5,11 is close to the radius of the particles themselves, and hence some 104 times smaller. Within the microemulsion, the limiting diffusion current for Co(NH3)5Cl2+ reduction would thus be greater by the same factor. In consequence, Emix would lie in surface-controlled rather than in diffusion-controlled regions of the curves, and the catalytic process would also be surface-controlled. Catalytic Mechanism. Adsorption of TMPPD and its oxidized forms has been detected on silver surfaces by Liu et al.16 using surface-enhanced Raman spectroscopy. Similar adsorption of TMPPD on the palladium surface explains why the Co(NH3)5Cl2+ currents in the surfacecontrolled region of Figure 4 are significantly decreased by the presence of quite small amounts of TMPPD. Such adsorption will likewise reduce the catalytic rate. This is supported by the fact that the activation energy of the catalyzed reaction is much greater at lower temperatures where TMPPD will be adsorbed more strongly. The small kinetic exponent of 0.21 found for TMPPD suggests that it is adsorbed by a Freundlich adsorption (16) Liu, C.-Y.; Zhang, Z.-Z.; Reu, X.-M. J. Imag. Sci. Technol. 1994, 38, 229.
Catalysis by Palladium Nanoparticles
Langmuir, Vol. 16, No. 11, 2000 4899 Table 3. Variation of Size and Number of “Empty” Water Pools as a Function of the Size of the Catalyst-Containing Pools in a Microemulsion Containing 1 M Water, 0.1 M AOT, and 3 µM Pd as Particles of Radius 2.5 nm
Figure 5. Schematic model of a catalyst particle inside a microemulsion water pool surrounded by stabilizing surfactant species.
isotherm. The kinetic order of almost 1 for the other reactant, Co(NH3)5Cl2+, then points to a mechanism in which these ions diffuse slowly through the adsorbed TMPPD to reach the palladium surface where a relatively rapid electron transfer via the metal takes place between the reactants. Microreactor Model. The accepted model for a simple water-in-oil microemulsion, without any catalyst particles, assumes that the water is present inside monodisperse inverse micelles. Their number and size should permit all the surfactant species to fit in a monolayer around the total interfaces. This leads to a core radius of the pools of 4,17
r ) 3Vwcw/AAOTcAOTNAv
(11)
where cw is the molar concentration of water (here 1 M), cAOT is the concentration of AOT (here 0.1 M), Vw is the molar volume of water, NAv is Avogadro’s number, and AAOT is the cross-sectional area of the surfactant species at the pool interface (taken as18 0.50 nm2). Hence, r ) 1.800 nm. The number of water pools is given by
N ) 3Vwcw/4πr3
(12)
which equals 7.40 × 1020 dm-3. It is reasonable to assume that in microemulsions containing catalyst microparticles the latter are located inside the water pools. However, in the present catalytic system and in that used previously,5 the radius rPd of the palladium particles is 2.5 nm, i.e., larger than the 1.8 nm calculated above. A similar situation was encountered by Clint et al.19 with Pt particles larger than the initial water core radius in water/CTAB/chloroform + heptane microemulsions and by Pileni17 with bigger Cu and CdS particles in water/AOT/isooctane solutions. The catalyst-containing (or filled) pools must therefore possess a larger radius, as illustrated in Figure 5. The number of such enlarged water pools, if each contains one colloidal particle of palladium, is given by
Nf )
3cPdMPd 4πrPd3FPd
(13)
where MPd is the molar mass of the metal and FPd is its density. For our 3 µM Pd microemulsion, Nf equals (17) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (18) Caselli, M.; Luisi, P. L.; Maestro, M.; Roselli, R. J. Phys. Chem. 1988, 92, 3899. (19) Clint, J. H.; Collins, I. R.; Williams, J. A.; Robinson, B. H.; Towey, T. F.; Cajean, P.; Khan-Lodhi, A. Faraday Discuss. 1993, 95, 219.
a
rf (nm)
re (nm)
Ne (1020 dm-3)
0 5 20 50 100 150 200
1.800 1.800 (1.800)a 1.799 1.780 (1.595)a 1.633 1.233 0.448
7.40 7.40 (7.40)a 7.405 7.56 (9.38)a 8.97 15.70 118.4
For a similar system but containing 30 µM Pd.
4.06 × 1014 dm-3. If all the 1 M water were inside these filled pools, the radii rf would be given by the equation
4 π(r 3 - rPd3)Nf ) Vwcw 3 f
(14)
which works out to 220 nm. The combined surface area of these filled pools would then be 4πr2fNf or 2.47 × 1020 nm2, providing room for only 0.82% of the AOT molecules present. It follows that at least two types of water pool must be present, larger ones containing catalyst particles (radius rf) and smaller “empty” ones without catalyst (radius re), to create enough surface area for all the AOT molecules. We therefore require balance equations for both water content and interfacial area:
4 4 Vwcw ) π(rf3 - rPd3)Nf + πre3Ne 3 3
(15)
AAOTcAOTNAv ) 4πrf2Nf + 4πre2Ne
(16)
These equations imply that the empty and filled water pools are each monodisperse, that no catalytic pool contains more than one catalyst particle, and that all the surfactant species sit in a monolayer at the water-oil interface. However, no assumption has been made about the location of the particle within the filled water pool as shown in Figure 5. Selection of a series of values of rf for our system leads to the values of re and Ne listed in Table 3. The calculations show that rf has to rise to around 40 nm before there is a decrease of at least 0.01 nm in the size of the empty water pools together with a 1% increase in their number. However, as the radius of the catalyst-containing water pools approaches its limit of 220 nm, the radius of the empty pools shrinks faster and faster and their number rises dramatically so as to provide sufficient interfacial area for the AOT species. It can also be seen that, at any given higher rf value, larger concentrations of catalyst lead to a greater number of empty water pools, which are also of smaller size. A similar but more detailed model has been proposed by Caselli et al.18 for the uptake of proteins by water-inoil microemulsions. By making further structural assumptions, these workers were able to estimate the conditions for the minimization of the Gibbs free energy of the system and then evaluated rf as well re for various proteins. In general, rf was found to be approximately 1 nm greater than re. If a similar result were to apply to the present 3 µM Pd catalyst system, rf would be ca. 2.8 nm, a realistic value in view of the 2.5 nm radius of the Pd particles. The values of re and Ne would be 1.800 nm and
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Langmuir, Vol. 16, No. 11, 2000
7.40 × 1020 dm-3, respectively, the same as in the absence of catalyst because of the small concentration of palladium used. Higher catalyst concentrations, as Table 3 shows, would produce a reduction in re and an increase in Ne. Conclusions (1) The oxidation of TMPPD by Co(NH3)5Cl2+ at pH 5.6 in buffered water/AOT/n-heptane microemulsion was catalyzed by the incorporation of colloidal particles of palladium of 2.5 nm radius. Under the standard conditions of 5.7 µM Co(III), 57 µM TMPPD, and 3 µM Pd at 25 °C, the initial catalytic rate was approximately twice as large as that for the analogous DMPPD reaction.5 (2) The initial rates of the catalyzed TMPPD oxidation were found to be first order in Co(NH3)5Cl2+, of order 0.21 in TMPPD, and first order in palladium at low Pd concentrations. At higher palladium concentrations, the rate rose more than proportionately, a phenomenon ascribed to further fast catalytic oxidation to the quinonediimine T2+ and its rapid synproportionation with the reduced form TMPPD. (3) The activation energy of the catalytic reaction decreased from 97 kJ mol-1 at 15 °C to 39 kJ mol-1 at 40 °C. The values are all larger than those for the corresponding DMPPD reaction.
de Jesus and Spiro
(4) Electrochemical studies showed that the Co(NH3)5Cl2+ reduction current at a rotating Pd electrode decreased in the presence of micromolar amounts of added TMPPD. This finding, together with the marked temperature variation of the activation energy, indicated that adsorption of TMPPD on the Pd particles affected the rate-determining step of the catalysis. Slow diffusion of Co(NH3)5Cl2+ through adsorbed TMPPD species is likely to be the slow step in the catalysis followed by electron transfer via the metal from the adsorbed diamine to the cobalt ion at the surface. (5) In the present work, as well as in earlier reports, the sizes of incorporated colloidal catalysts and other solids were found to be larger than those of the water pools of the microemulsion itself. A model of bigger pools which can contain the solid particles plus aqueous solution was therefore examined. Calculations showed that a much larger number of small catalytically empty water pools was needed in addition to provide sufficient interfacial area to accommodate all the surfactant species present. Acknowledgment. We thank Junta Nacional de Investigac¸ a˜o Cientı´fica e Tecnolo´gica for the award of an Overseas Scholarship to D.M.de J. and the Leverhulme Trust for the award of an Emeritus Fellowship to M.S. LA9915668
