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J. Phys. Chem. C 2007, 111, 4596-4605
Synthesis and Size-Selective Catalysis by Supported Gold Nanoparticles: Study on Heterogeneous and Homogeneous Catalytic Process Sudipa Panigrahi,† Soumen Basu,† Snigdhamayee Praharaj,† Surojit Pande,† Subhra Jana,† Anjali Pal,‡ Sujit Kumar Ghosh, and Tarasankar Pal*,† Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India, and Department of CiVil Engineering, Indian Institute of Technology, Kharagpur-721302, India ReceiVed: NoVember 14, 2006; In Final Form: December 20, 2006
Core-shell nanocomposites (R-Au) bearing well-defined gold nanoparticles as surface atoms of variable sizes (8-55 nm) have been synthesized exploiting polystyrene-based commercial anion exchangers. Immobilization of gold nanoparticles, prepared by the Frens method, onto the resin beads in the chloride form is possible by the ready exchange of the citrate-capped negatively charged gold particles. The difficulty of nanoparticle loading, avoiding aggregation, has been solved by stepwise operation. Analysis of the gold particles after immobilization and successive elution confirm the unaltered particle morphology while compared to those of the citrate-capped gold particles in colloidal dispersion. It was observed that the rate of the reaction increases with the increase in catalyst loading, which suggests the catalytic behavior of the gold nanoparticles for the reduction of the aromatic nitrocompounds. The rate constant, k, was found to be proportional to the total surface area of the nanoparticles in the system. Kinetic study for the reduction of a series of aromatic nitrocompounds reveals that the aromatic nitrocompound exclusively adsorbs to atop sites of gold particles and that the rate of the reduction reaction increases as the particle size decreases. Similar reaction kinetics was observed involving gold sol of variable size (homogeneous catalysis) as catalyst. The induction time and the activation energy of the reaction decreases with decrease in particle size indicating the decrease in activation energy for the smaller particles, which also speaks for the increase of surface roughness with decrease in particle size. The observed rate dependence, in relation to particle size, is attributed to a higher reactivity of the coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in larger particles.
1. Introduction Inexpensive transportation fuels, high-temperature lubricants, chlorine-free refrigerants, high-strength polymers, stain-resistant fibers, cancer treatment drugs, and many thousands of other products required by modern societies would not be possible without the existence of catalysts. For a long time very limited attention was paid to study the catalysis by gold because of its completely filled d-band, accompanied by very low activities.1 The situation has changed since Haruta et al. deposited very small gold particles on metal oxides,2 which exhibits surprisingly high catalytic activity for CO oxidation at a temperature as low as 200 K.3,4 Gold catalysts have recently been attracting rapidly growing interests due to their potential applications to many reactions of both industrial and environmental importance. Recent theoretical calculations have explained why the smooth surface of gold is noble5 and how gold become an active catalyst in the nanoscale.6 Moreover, gold nanoparticles have been found to be catalytically active,7 which has led to a great number of investigations recently.8 Unsupported gold nanoparticles also present great potentials as unique catalysts in liquid-phase reactions.9 Parameters to control in transition-metal heterogeneous catalysis include particle composition, size and shape, support * To whom correspondence should be addressed. E-mail: tpal@chem. iitkgp.ernet.in. † Deparment of Chemistry, Indian Institute of Technology. ‡ Deparment of Civil Engineering, Indian Institute of Technology.
composition, and the organizational structure of the porous network. This multidimensional problem is complex; thus, the synthetic control and design of catalysts is a serious technological challenge.10 A logical approach to the problem is to isolate one of the parameters and learn how to control it in a systematic way and then test its effect on catalytic performance. Steric and electronic effects, and morphology of metal particle surface are some of the most effective factors controlling intramolecular selectivity.11,12 One of the most interesting aspects of metal and semiconductor nanoparticles is their size-dependent properties, which has been observed to be equally important from a fundamental as well as an application point of view.13-15 Size-sensitive catalysis of chemical reactions by metallic nanoparticles is an important area of research, attracting a lot of attention.16 Goodman and co-workers have reported an inspiring result on size-selective catalysis obtained by using a model Au/TiO2 catalyst.17 For heterogeneous catalysis involving gold nanoparticles deposited on metal oxides, size-sensitive catalysis is reported for CO oxidation,10c,18 propylene epoxidation,11 and the selective hydrogenation.12,16,19,20 Very recently Tsukuda et al. have reported the size-selective catalysis by gold and palladium nanoparticles for the oxidation of benzylic alcohols.21 Size-regime-dependent catalytic activity of gold nanoparticles for the reduction of eosin is reported from our group.22 Nanoparticles require a suitable support to prevent aggregation during the reaction to be catalyzed. If the activity of the bare
10.1021/jp067554u CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
Supported Gold Nanoparticles nanoparticles is to be studied in a quantitative manner, then the support should be totally inert. However, the support should be sufficiently stable to ensure the reuse of the catalyst after the reaction. An additional problem is the formation of well-defined nanoparticles with a narrow size distribution that should take place directly on the support. Several methods have been developed thus far to afford the synthesis of metal nanoparticles on high surface area support materials.23,24 In recent years, synthesis of metal nanoparticles14d in a polymeric matrix such as dendrimers,25 latex particles,26 microgels,27 or other polymers28 has fascinated scientists, because of their important applications in nanotechnology. The polymers, due to their stereospecificity, can present selective catalytic properties.29 The most prominent method for the immobilization of metal particles is ion exchange, in which the ion-exchange resin support is impregnated with metal precursors (simple salts of metal) in solution phase, followed by thermal treatment and/or reduction with H2 to form metal nanoparticles.30 The process can be further refined by postsynthetic modification of the support surface functionalization which directs the interaction of precursor with the surface.31 Because of its simplicity this method is successful for the large-scale production of catalysts. However, metal nanoparticles generated on the support typically lack uniformity in size and shape. Therefore, direct immobilization of preformed nanoparticles of well-defined and narrow size onto the solid support is very important to study the size-effect on the rate of a reaction. The realization of catalytic properties of ultrasmall gold nanoparticles has led us to probe the size-dependent catalytic activity of gold nanoparticles in the reduction of aromatic nitrocompounds. Here we report the catalytic activity of polystyrene-gold nanocomposites as a selective catalyst for the reduction of 4-nitrophenol (4-NP) to the corresponding aminophenol near 100% yield of the product as a function of their size as a model reaction. By preparation of the gold nanocomposite (R-Au), we have succeeded in probing the factors that dictate the overall size-specific catalytic activity of the gold nanoparticles on the reduction of aromatic nitrocompounds in aqueous solution. Although the metal catalysts have been widely employed in the reduction of aromatic nitrocompounds, the influence of particle size on these reactions are not understood. Characterization of the structural and electronic properties of all the catalysts particles were done by high-resolution scanning electron microscopy (HR-SEM), X-ray diffraction pattern (XRD), X-ray photoelectron spectroscopy (XPS), and Fouriertransformed infrared spectroscopy (FTIR) aimed at defining the immobilization of the preformed gold nanoparticles on to the resin beads. Immobilized gold particles on the resin surface can be eluted with the cationic surfactants like cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB) with the unaltered particle morphology. The rate constant k was found to be proportional to the total surface area S of the nanoparticles in the system. Furthermore, we have also probed the effect of particle size of gold nanoparticles in homogeneously dispersed sol system to have a general conclusion on the dependence of particle size on the rate of reduction of aromatic nitrocompounds for a wide range of particle diameter. The amount of catalyst of a particular size was also varied for both homogeneous and heterogeneous catalysis to study the dependence of rate of the reaction on the amount of catalyst. Repeated characterization of the gold nanoparticles in successive stages (after evolution, immobilization, elution, recovery of the catalyst) in turn authenticates the size-specific catalytic properties of gold nanoparticles.
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4597 TABLE 1: Details for the Size-Selective Synthesis of Gold Nanoparticles by the Frens Method
set no.
total volume solution (mL)
total volume of HAuCl4 (10-2 M) (µL)
volume of citrate (1% by weight) (µL)
particle size (nm)
1 2 3 4 5 6 7 8 9
50 50 50 50 50 50 50 50 50
1250 1250 1250 1250 1250 1250 1250 1250 1250
2000 1600 1300 1000 875 750 625 500 400
8 10 13 16 20 25 32 41 55
2. Experimental Section 2.1. Reagents. All the reagents were of AR grade. HAuCl4, sodium citrate, NaBH4, and aromatic nitrocompounds (nitrophenols, nitro cresols, nitro anilines, and nitro benzoic acid) were purchased from Aldrich. Seralite, SRA-400 was obtained from Sisco Research Laboratory, India. Mili-Q water was used throughout the experiment. 2.2. Instruments. Absorption spectra were recorded in a Spectrascan UV 2600 spectrophotometer (Chemito, India), and the solutions were taken in a 1 cm well-stoppered quartz cuvette. Transmission electron microscopic (TEM) analysis was performed in a Hitachi H-9000 NAR instrument on samples prepared by placing a drop of fresh gold sol bearing different size on Cu grids precoated with carbon films, followed by solvent evaporation under vacuum. HR-SEM measurements were done in SEM: JSM-6700F, JEOL with a 5 kV electron beam. FTIR spectral characteristics of the samples were collected in reflectance mode with Perkin-Elmer Spectrum RX 1 FTIR instrument. The solid resin beads were grinded with the KBr. The XRD pattern was recorded in an X’pert pro diffractometer with Cu (KR ) 1.54056) radiation. XPS measurement was made using a Vacuum Generators ESCALAB 220iXL electron spectrometer. 2.3. Preparation of Gold Sol of Different Size by the Frens Method. Gold nanoparticles of variable size (8-55 nm) were prepared according to the Frens method.32 A 50 mL aqueous solution of HAuCl4 (2.5 × 10-3 M) was heated to boiling, then 2 mL of trisodium citrate solution (1% by wt) was added to it under continuous stirring. Within 20 s of boiling, the solution turned faint blue. Under continuous stirring condition, after 50 s, the blue color suddenly changes to red, indicating the formation of gold nanoparticles. The reaction mixture was boiled for another ∼30 min for complete reduction of Au(III) ions. By this procedure, gold nanoparticles of 8 nm size were obtained. By a similar procedure, particles of a different size (8-55 nm) were prepared by varying the amount of citrate solution. The details of the size-selective synthesis are given in Table 1. The particle size was determined from TEM studies. 2.4. Procedure for Immobilization. In a conical flask 0.5 g of resin beads (size ∼ 560 µm) were taken and soaked overnight in water. Any impurity in the resin particles could cause gold particle aggregation and poisoning of the catalyst. So the beads were washed several times with water before immobilization. Then 40 mL of citrate-capped gold sol of variable size was added in portions, i.e., in small aliquots (5 mL) to the aqueous suspension of resin beads in separate sets for complete immobilization of the gold nanoparticles in resin matrix. The difficulty of loading, avoiding aggregation, is solved by stepwise
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SCHEME 1: Schematic Representation for the Immobilization, Recovery, and Catalytic Process of the Gold Nanoparticles onto the Resin Beads
operation: introduction of colloidal gold in portion into the resin beads in water followed by washing out the expelled Cl-. Repetition of the stepwise process, i.e., loading and washing are done carefully in successive steps for complete saturation of the resin beads by gold nanoparticles. After each step of immobilization, the supernatant solution became colorless. The resin beads were then washed thoroughly with water to remove the expelled Cl- ions. These beads do not uptake gold sol any more that confirmed that the resin particles are saturated by gold sol. The yellow-colored resin beads turn shining dark brown with immobilized gold particles. Finally the resin beads were washed, dried, and used as catalyst for the reduction of a series of aromatic nitrocompounds. 2.5. Catalytic Reaction. The reduction of 4-NP by NaBH4 was studied as a model reaction to probe the size-specific catalytic activity of gold nanoparticles for both heterogeneous and homogeneous systems. Under the experimental condition (25 °C), the reduction does not progress at all simply with NaBH4. But in the presence of a catalyst it goes to completion to produce 4-aminophenol (4-AP). So, to study the reaction, in a quartz cuvette, 2.77 mL water was mixed with 30 µL (10-2 M) 4-NP solution, and finally 200 µL freshly prepared NaBH4 solution (10-1 M) was added. Thus the final concentration of 4-NP became 10-4 M, and that of NaBH4 became 6.67 × 10-3 M. After mixing these solutions, 6 mg of solid R-Au particles bearing variable size of gold nanoparticles as surface atoms were added in different sets to study the reduction reaction. Immediately after the addition of catalyst, UV-vis spectra of the sample were recorded in every 5 min interval in the range of 200-500 nm. The rate constant of the reduction process was determined by measuring the change in absorbance of the initially observed peak at 400 nm, for the nitrophenolate ion, as a function of time. To have a general conclusion on the effect of particle size, the catalytic reaction was also carried out with colloidal dispersion of gold nanoparticles of variable size under similar experimental condition (25 °C). For homogeneous catalysis, 20 µL of the gold sol (prepared by the Frens method) were added instead of the R-Au particles. Both the catalytic reactions (heterogeneous and homogeneous catalysis) were studied with varying amount of catalyst particles of a particular
size. Under the similar condition reduction of a series of aromatic nitrocompounds were then studied. Scheme 1 shows the whole procedure for immobilization and catalysis. 3. Results and Discussion 3.1. Immobilization of the Gold Nanoparticles onto the Anion Exchange Resin. Anion-exchange resin is a polymer, containing amine or quaternary ammonium groups as integral part of the polymer lattice with an equivalent amount of anions such as chloride, hydroxyl, or sulfate.33 Seralite SRA-400, a polystyrene quaternary ammonium resin obtained in chloride form, was thoroughly washed before immobilization. It has been reported that polar head groups, such as the hydroxyl, thiol, amine, nitrile, and chloride groups, on the surface of the polymer microspheres have high affinity for metal nanoparticles.28a,34 After the reduction of Au(III) ions with excess citrate ions under boiling conditions, unreacted citrate ions cap the gold particles and thereby impart negative charge and stability to the generated particles. Hence the gold particle surface became negatively charged.32 Now the immobilization of gold nanoparticles is understandable due to the ready exchange of the anions of the anion exchange resin with the negatively charged gold nanoparticles.35 Here, an equivalent amount of exchangeable Clions is expelled from the resin beads and was removed by washing with water after each step of immobilization. Otherwise the expelled Cl- ions would cause gold particle aggregation in solution because of the electrolytic effect. If the gold sol is introduced all (40 mL) at once for immobilization onto the resin surface then the liberated chloride (exchangeable Cl-) ions from the resin moiety act as electrolyte and would cause gold particle aggregation in solution. Because of this problem total volume of the sol solution was introduced into the conical flasks slowly and in portions, i.e., in small aliquots (5 mL) to avoid the possible particle aggregation. Gold nanoparticles, as they bear a negative surface charge due to the adsorbed citrate ions32,36 cannot be immobilized on a cation-exchange resin bead, whereas strongly basic anion exchanger serves the purpose of immobilization well. Volume of gold sol used for the immobilization purpose was kept fixed for all the particle size to keep the
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Figure 1. UV-vis spectra of 8 nm gold nanoparticles (a) before and (b) after immobilization on to the resin beads.
amount of gold same, so that the rate of the reaction is not affected by gold loading. The absorption spectra of gold sol (size 8 nm) prepared by the Frens method display a significant plasmon band at 514 nm, which is the characteristic of metallic gold colloids.14d It can be seen that the surface plasmon absorption band of the gold sol is very sharp, which indicates the formation of particles with a narrow size distribution. However the formation of the particle with narrower size distribution has been confirmed from the TEM studies. The different-sized gold nanoparticles prepared by the Frens method were immobilized on the resin beads following the above-mentioned procedure. Figure 1 shows representative UV-vis spectra of the aqueous solution of gold sol before (trace a) and after (trace b) the immobilization process. It can be seen that the peak for the gold nanoparticles at 514 nm completely dies down indicating the complete immobilization of the negatively charged gold particles onto the resin matrices. Moreover, it is worth noting that the R-Au particles are quite stable. Neither obvious sedimentation nor denaturation was observed for the particles on resin matrix even after storing for months. UV-vis, HR-SEM, and XRD studies unequivocally proved that the particle size does not change after immobilization. It should be mentioned that the resultant charge due to the capping agent citrate on each gold nanoparticle has negligible bearing on the exchange reaction. In the present study, the rate of exchange of gold nanoparticles increases with the decrease in size of gold particles. As we have taken the same amount of Au(III) ions in each set therefore, the number of gold particle increases as the particle size decreases. This would have bearing on the rate of exchange.33 Neither anionic nor nonionic surfactant is capable of extracting the gold particles quantitatively from the resin. The immobilized gold particles can be eluted using different cationic surfactants (CPC and CTAB) without the change in particle morphology. They not only elute the gold nanoparticles efficiently from the resin beads but also stabilize the regenerated sol.35 These cationic surfactants possess a surface-active positive charge that is strongly adsorbed to the solid surfaces of the nanoparticles, which are already negatively charged. The surfactants have a long hydrocarbon chain containing headgroup -N+ and Cl- ions. Presumably, due to the attachment of the positive end of the surfactant to the nanoparticles surface, immobilized nanoparticles are released and subsequently Cl- ions are attached to the resin beads.35 The unaltered surface plasmon absorption band authenticates the stability which was due to the effective protection of gold nanoparticles by citrate against aggregation. 3.2. Characterization of the Gold Nanocomposites. 3.2.1. XRD Pattern. The XRD pattern of the free resin beads is shown in Figure 2a. The resin particles do not show any peak, but a broad hump was observed, which indicates the amorphous nature of the polymer matrix. When these beads were covered
Figure 2. XRD pattern of the (a) resin particles and (b) 8 nm R-Au particles.
TABLE 2: Calculation of Particle Diameter from XRD Analysis Using the Debye-Scherer Formula particle size (nm) before immobilization (calculated from TEM images)
particle size (nm) after immobilization (calculated from XRD)
8 10 13 16 20 25 32 41 55
8.2 9.9 13.1 16.2 20.1 25.2 31.9 42 57
by gold particles to form the core-shell type nanocomposites (R-Au) no such hump but the peaks corresponding to gold nanoparticles were observed (Figure 2b) corroborating the immobilization of the gold particles onto the resin beads. Five characteristic peaks were observed for the R-Au particles in the XRD pattern (Figure 2b) at 2θ ) 38.1, 44.2, 64.6, 77.5, and 81.4° corresponding to the (111), (200), (220), (311), and (222) planes, respectively, for the fcc gold lattice. The XRD pattern (Figure 2b) shows broad bands for the (111) and (200) reflections for the gold nanoparticles, indicating that the size of the particles is in the nanometer range without any aggregation.37 Estimation of the particle size based on XRD from Debye-Scherer formula is shown in Table 2. From the XRD pattern it was concluded that the particle size remain same after immobilization. 3.2.2. SEM and EDS Analysis. Figure 3a shows a representative HR-SEM micrograph containing 8 nm gold particles immobilized on the resin matrix. The bright spherical images are the representative particles. The inset shows the HR-SEM image of a resin particle. Figure 3b shows the HR-SEM micrograph of a 55 nm gold particle immobilized on the resin matrix. From the HR-SEM image it is clear that the size and the morphology of the immobilized gold particles in the resin matrices does not change at all while compared to those of the citrate-capped gold particles in colloidal dispersion even for the
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Figure 4. FTIR spectra of the (a) free resin particles, (b) 8, (c) 20, (d) 32, and (e) 41 nm R-Au particles.
Figure 5. XPS spectra of the (a) free resin particles and (b) 32 nm R-Au particles.
Figure 3. HR-SEM micrographs of (a) 8 nm R-Au particles, (b) 55 nm R-Au particles, and (c) EDS pattern for the 41 nm R-Au particles.
larger particles. It can be seen from the above images that the particles are well separated and have narrow size distribution even after immobilization. Figure 3c shows the results of EDS analysis for 41 nm gold particles immobilized on the resin matrix. The result indicates the presence of gold particles in the composite (R-Au) materials. The mass percentage of gold was found to be 13.94% for 41 nm particles immobilized on the resin surface. 3.2.3. FTIR Spectroscopy. FTIR spectroscopy has been used to probe the local surface structure of the surface-bound species.38 It helps to study the presence of immobilized gold particles onto the resin surface and the results are shown in Figure 4. FTIR spectra of the pure resin particles shows the antisymmetric vibration for the N-H stretching at 3447 cm-1 (trace a) due to the charged C-NH3+ moiety, but the band weakened after immobilization of the negatively charged gold particles onto the resin beads because of the new Au-N bond formation by electrostatic force.39 This was reflected by the maximum blue shift (trace b) for the smallest particles under consideration (8 nm). As the particle size increases in stages, this blue-shifted peak gradually red shifted with the increase in size of the gold particles as shown in Figure 4. Very recently
Somorjai et al. have reported that the stretching frequency of CO red shifted as CO goes to cover smaller to larger Pt nanoparticles surface.18 Cant40 et al. have shown a minor influence of particle size on the position of the atop CO vibration. Bischoff et al.41 have studied the effect of particle size on CO adsorption for Pt dispersed within a neutralized faujasite zeolite and reported red shifting of the peak position. Figure 4 gives sound evidence that gold particles are strongly bound to the resin surface and a strong electrostatic force is a reasonable suggestion. 3.2.4. XPS Spectra. The oxidation state of gold-coated resin beads were investigated by XPS studies. The XPS spectrum of the free resin particles and the gold coated resin particles is shown in Figure 5. The XPS spectra of the free resin beads are completely different from the gold coated resin particles (Figure 5a). The XPS spectrum of the gold nanocomposite (32 nm) is shown in Figure 5b. The intense peaks at 84.3 and 87.9 eV correspond to the binding energies of Au4f7/2 and Au4f5/2 electrons, respectively.42 The valence band spectra of the sample in “as-deposited” state are identical with that measured for bulk gold.43 Furthermore, formation of the gold nanoparticles is evidenced by a shift of Au 4f core level binding energy (BE) toward higher values (from 83.8 to 84.3 eV).44 The BE values of variable-size metal clusters become more positive compared with the bulk as the particle size decreases.45 The shifting of the peaks as compared to the literature value may also be due to the deposition of the metals on the surface of charged resin
Supported Gold Nanoparticles
Figure 6. UV-vis spectra for the reduction of 4-NP measured at 5 min intervals using (a) 20 nm R-Au particles and (b) 20 nm gold sol.
beads. These data delineate the zero oxidation state of the metallic gold. 3.3. Catalytic Reactions. To this end, there is a drive toward developing new strategies to synthesize catalysts whose catalytic performance can be predicted in a defined and rational manner. A definite conclusion can be drawn about the performance of a catalyst only when the size and shape of the catalyst particles is known both before and after a chemical reaction. Again, to have a concrete knowledge about the size-specific catalysis, some chosen reactions are to be performed. Therefore, as a model reaction, we have selected the reduction of 4-NP by NaBH4 to 4-aminophenol (4-AP). Separation of the product from the catalyst particles could be done by simple filtration. Now we are aware of the fact that water pollution by phenol and phenolic compounds is of great public concern. Among them nitrophenols are some of the most refractory pollutants that can occur in industrial wastewaters.27b More particularly, nitrophenols and their derivatives result from the production processes of pesticides, herbicides, insecticides, and synthetic dyes increases the pollution. So, this study becomes much more fascinating from the point of pollution abatement also. As there is a great demand of the aromatic aminocompounds in industry, therefore, the reaction becomes academically as well as technologically important. Additionally, this study becomes interesting in the development of methodologies for catalyzing organic reactions in aqueous solutions.46 The kinetics of 4-NP reduction in presence of metal nanocomposite (R-Au) particles was studied by UV-vis spectroscopy. Since the concentration of R-Au particles in the system is very low, the measurement of the absorption spectra of 4-NP and the reduction product, 4-AP, was not disturbed by the light scattering due to the catalyst carrier particles in solution. Figure 6 shows the UV-vis spectra for the reduction of 4-NP measured at a different time during the progress of the reaction. For a typical measurement, a successive decrease of peak intensity at 400 nm with time can be taken into consideration to obtain the rate constant.29b,47 This peak is attributed to the presence of 4-nitrophenolate ions in the system. Aqueous solution of 4-NP has an absorption maxima at 317 nm (trace a).9a After the addition of freshly prepared NaBH4 solution to the system the peak shifted to 400 nm, indicating the formation of 4-nitrophe-
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4601
Figure 7. Plot of the absorbance (At/A0) vs time (t) for the reduction of 4-NP of (a) 20 nm R-Au particles and (b) 20 nm gold sol.
nolate ions. This peak remains unaltered with time, which suggests that the reduction does not proceed in absence of a catalyst as reported in the literature.9b,29b,47-49 After the addition of R-Au particles, it was found that the peak height at 400 nm gradually decreases with time. With the gradual decrease in peak height at 400 nm, a new peak appeared at 295 nm, indicating the formation of 4-AP. Moreover, two points could be observed in the UV-vis spectra, where all spectra intersect each other. This indicates that in the catalytic reduction the conversion of 4-NP gives only one product, 4-AP.9b The conversion process can be directly read off these curves as the ratio of the concentration Ct of the 4-nitrophenolate at time t to its value C0 at t ) 0 is directly given by the ratio of the respective absorbances At/A0. Since the concentration of sodium borohydride largely exceeds the concentration of 4-NP, the reduction rate can be assumed to be independent of borohydride concentration. So, in this case, a pseudo-first-order rate kinetics with regard to the 4-nitrophenolate concentration could be used to evaluate the catalytic rate.9b,25b The UV-vis spectra for the heterogeneous catalytic process using 20 nm R-Au particles is given in Figure 6a. The corresponding UV-vis spectra for the homogeneous catalysis using 20 nm gold sol is given in Figure 6b. The plot of At/A0 vs time (t) for the reduction of 4-NP using 20 nm R-Au particles is shown in Figure 7a (heterogeneous catalysis), and Figure 7b shows the corresponding plots for the homogeneous catalysis, catalyzed by 20 nm gold particles as a representative case. A linear relation between ln(Ct/C0) and time, t has been obtained for all the particle size under consideration. Figure 8a shows the ln(Ct/C0) vs time, t, plot for the 20 nm R-Au particles as a model. The corresponding plot for the homogeneous catalytic process is given in Figure 8b. The rate constant k of the reaction was calculated from the slope of this straight line. Here, attention must be paid to the fact that a delay time t0 was found for the catalytic reduction in all the cases, which may be due to an activation of the catalyst in the reaction mixtures. This is in accord with other studies of the catalysis of this reaction by metal nanoparticles.9c,25a,29b,35,47 Moreover, a delay time t0 was found for the catalytic reductions, which were made under air. A similar behavior has been observed by
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Figure 9. The plot of rate constant k vs total surface area S.
Figure 8. Plot of ln(Ct/C0) vs t of (a) 20 nm R-Au particles and (b) 20 nm gold sol.
other groups as well.9c,35,50 Ballauff et al. has proved that this delay time was due to the reduction of oxygen present in the system.28b In the present case the positive charge (R+) of the resin matrix invites the negatively charged nitrophenolate and borohydride ions for adsorption and in turn facilitates the electron transfer from BH4- (donor) ion to the nitrophenolate (acceptor) ion through gold surface. As soon as we add NaBH4, the metal particles start the catalytic reduction by relaying electrons from the donor BH4- to the acceptor 4-NP right after the adsorption of both onto the catalyst particle surface. The excess of NaBH4 used increases the pH of the reaction medium and thus retarded the degradation of the borohydride ions. Obviously, the reduction of oxygen proceeds much faster than the nitrophenols present in the system. The reduction reaction of 4-NP only starts after all the oxygen in the system has been reacted. Therefore, the aerial oxidation of the product, 4-AP was prevented. Again, evolution of small bubbles of hydrogen gas surrounding the catalyst particles helps to stir the solution. As a result, catalyst particles remain well distributed in the reaction mixture during the course of reaction and offer a favorable condition for a smooth reaction to occur. As NaBH4 was present in large excess, its consumption for the reduction of oxygen did not alter its concentration notably. Induction period observed at the initial stages of the reaction becomes shorter as the particle size decreases. 3.3.1. Effect of Surface Area. There is a strict relation between the rate constant k and the surface area of the gold nanoparticles. Moreover, k will certainly be proportional to the total surface area S of the metal nanoparticles in the system47b,49
dCt ) kCt ) k1SCt dt
(1)
where Ct is the concentration of 4-nitrophenolate at time t and k1 is the rate constant normalized to S, the surface area of gold nanoparticles normalized to the unit volume of the system. The plot of the k vs surface area S is shown in Figure 9. From Figure 9 it is observed that the rate constant k is proportional to the total surface area of the nanoparticles in the system. It is wellknown that the rate of the reaction increases with the available
surface area and the amount of the catalysts. As can be seen from the Table 1 (Experimental Section) that same amount of gold has been immobilized in all systems. Therefore, with decrease in particle size the number of particles increases and thereby the surface area also increases. However, a decrease in rate constant k with the increase in particle size has a bearing on the total surface area S of catalyst particles. Hence, Figure 9 validates the assumption given in eq 1. Therefore, it can be concluded that the catalysis takes place on the surface of the nanoparticles and the catalytic activity at a given temperature must, hence, depend on the total surface area S of the gold nanoparticles. The above discussion relates to the homogeneously dispersed gold sol system (homogeneous catalysis). Because of the inherent difficulty we could not calculate the surface area of the R-Au particles. However, a similar trend (linear increase in rate with surface area) is expected for the heterogeneous catalysis, as the particles size remains same even after immobilization. 3.3.2. Influence of Catalyst Dosage. The amount of catalyst was varied keeping other parameters constant to determine the effect of the amount of catalyst on the rate of the reaction. Rate values are plotted against varying amount of catalyst (for 20 nm R-Au particles), as shown in Figure 10a. As expected, with an increase in the amount of catalyst the reduction of nitrophenol occurs at a faster rate and the rate of the reaction increases linearly with the amount of catalyst dosage as shown in Figure 10a.51-53 Similar observation was found for the homogeneous catalysis by gold nanoparticles using 20 nm particles, and the result is shown in Figure 10b. It was found that, with an increase in amount of catalyst (for both homogeneous and heterogeneous catalysis), the rate increases linearly for a particular size of the particles while the other parameters are constant. It should be mentioned that the rate of reaction also increases with increase in the concentration of sodium borohydride. 3.3.3. Influence of Temperature. The activation energy of the reaction at different temperatures using the R-Au particles as catalyst was obtained from the Arrhenius equation
k ) Ae-Ea/RT
(2)
where A is a constant, k is the rate constant of the reaction at temperature T, Ea is the activation energy, and R is the universal gas constant. The catalytic reduction of 4-NP was studied at four different temperatures (15, 25, 45, and 60 °C) and the rate of the reaction was calculated for the different temperatures for the 20 nm R-Au particles. Table 3shows the dependence of the rate of reaction on temperature. From Table 3 it can be seen that the rate of the reaction increases with the increase in temperature. The activation energy for the catalytic reaction was found to be 31.01 kJ mol-1 for the 20 nm R-Au particles. It was observed that the activation energy of the reduction reaction decreases with decreasing particle size might be due to the
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TABLE 3: Rate of Reduction of 4-NP at Different Temperatures for 20 nm R-Au Particles temperature (°C)
rate of reduction (min-1)
15 25 45 60
0.632 × 10-2 0.976 × 10-2 2.144 × 10-2 3.637 × 10-2
TABLE 4: Rate Constant Values and the Corresponding TOF for the Reduction of 4-NP by 20 nm R-Au Particles particle size (nm)
rate of reaction (min-1)
TOF × 107 (g-1 s-1)
8 10 13 16 20 25 32 41 55
1.648 × 1.490 × 10-2 1.184 × 10-2 1.056 × 10-2 0.976 × 10-2 0.790 × 10-2 0.554 × 10-2 0.351 × 10-2 0.201 × 10-2
9.956 8.99 7.141 6.371 5.885 4.765 3.343 2.117 1.212
10-2
increase in surface roughness with decrease in particle size.18 Hence, the above result indicate that smaller particles exhibits a considerably higher activity. 3.3.4. Turnover Frequency (TOF). Many studies have already focused on the use of catalytic processes for the reduction of nitrophenols.9c,53 Catalysis may be performed in homogeneous systems; however, in the case of expensive catalysts such as those involving precious and strategic metals,54 it is important to recover the metal catalysts at the end of the process. The R-Au particles are not only very effective for the catalytic reduction of aromatic nitrocompounds but also recoverable and can be recycled for a number of times. At the end of the reaction, the catalyst particles remain active and were separated by simple filtration from the product. After one course of reaction, the particles were washed thoroughly with water, dried at room temperature and recycled a number of times for the reduction reaction. The rate constant values and the corresponding TOF for the 4-NP reduction by the R-Au particles bearing variable gold particles are shown in Table 4. It was observed that the TOF decreases with an increase in particle size. This may be due to the fact that increase in particle size decreases the adsorbate mobility, which is critical for catalytic turnover55 and increased residence time of the adsorbate on the surface may lead to eventual poisoning of the catalyst surface and thereby decreases the value of TOF. Similar trends have been observed on supported Pt catalysts56 and might be a general phenomenon for catalysis involving noble metal nanoparticles. 3.3.4. Effect of Particle Size on the Rate of the Reaction. Currently, exhaustive experimental and theoretical studies are conducted to understand the cluster size effect in nanocatalysis.18,21,52,57 In spite of the wealth of information available for the reduction of aromatic nitrocompounds catalyzed by metal nanoparticles, the aspect of the effect of particle size has not been understood to date. Since the surface atoms of colloidal metal nanoparticles are very active, there is a great possibility that significant change in the nanoparticle size and shape could take place during the course of its catalytic function.58 For this reason, supported catalysis has been extensively investigated during the past few decades. Figure 11 shows the rate of reduction of 4-NP as a function of the size of the gold nanoparticles on resin matrix. From Figure 11 it was observed that the rate of the reaction decreases with increase in particle size. It is evident from Figure 11 that, as the particle size increases, a linear decrease in the rate of the reaction is found
Figure 10. Plot of the rate of reduction vs amount of catalyst of (a) 20 nm R-Au particles and (b) 20 nm gold sol.
up to a particle size of 32 nm, after that the decrease in rate with the increase in particle size is less significant for the larger particles, though the rate of the reaction decreases with increase in particles size. Smaller particles are more active because of the fact that they have a larger numbers of surface atoms available for catalysis, i.e., the higher rate of reduction involving smaller particles is due to the higher surface area. The observed rate dependence, in relation to particle size, could be attributed to a higher reactivity of the coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in larger particles. Furthermore, we have probed the effect of particle size of gold nanoparticles in homogeneously dispersed system to have a general conclusion on the dependence of particle size on the rate of reduction of aromatic nitrocompounds. The rate of the reaction against the particle size in solution phase has been shown in Figure 12. Similar observation was found for both the cases (homogeneous and heterogeneous catalysis). But for homogeneous catalysis it can be seen that the rate decreases linearly with increase in particle size up to 20 nm but after that the effect of particle size on the rate of the reduction is not that prominent. From the above two figures (Figures 11 and 12), it can be seen that for homogeneous catalysis the decrease in rate with increase in particle size is very sharp for the smaller particles while for the larger particles the rate of the reaction does not change to a noticeable extent while for the heterogeneous catalysis the rate of reaction is also influenced by the larger particles even. In this case due to the absence of support the particles possibly aggregate. Thus the larger particles do not affect the rate of the reduction to a meaningful extent in solution phase. In general, the faster rate of reduction of nitrocompounds with smaller particles might be attributed to the larger Fermi level shift (∝1/R) in presence of highly electron injecting species59 such as BH4- ions. On the other hand Fermi level shift for larger particles are negligible and thereby does not affect the rate of the reaction remarkably.22 Here, it is unequivocally proved that for both homogeneous and heterogeneous catalysis the rate of the reaction decreases with the increase in particle size. In the recent past, it has been reported from our group60 that, for some redox reactions, the rate of catalysis involving the
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Panigrahi et al. The catalytic reduction was also successful for a series of aromatic nitrocompounds like 2- and 3-NP, nitro cresols, nitro anilines, nitrobenzoic acid, etc. 4. Conclusions
Figure 11. Plot of rate of the reduction of the 4-NP as a function of the particle size of the R-Au particles (heterogeneous catalysis).
Figure 12. Plot of rate of the reduction of the 4-NP as a function of the particle size of the gold sol (homogeneous catalysis).
growing metal nanoparticles was higher than the corresponding rate involving fully grown nanoparticles. Size-regime-dependent catalysis by gold nanoparticles for the reduction of eosin suggests that there are two size regimes that show completely different size effects.22 Work on the single crystals61 and supported catalyst40,41 suggest that the position of the atop CO vibration red shifts with increasing surface roughness and decreasing particle size. The work presented here is in agreement with previous studies and suggests that the rate of reaction is faster for smaller particles with a larger fraction of lowcoordination metal sites relative to that of a larger gold particle with a large fraction of high-coordination surface sites.62 It has been reported that on the larger particles there is less surface heterogeneity as the surface is terminated by large terraces of identical surface atoms. As the nanoparticle size decreases, the surface heterogeneity increases due to a change in surface atom statistics63 and in turn increases the surface roughness.18 Smaller particles are composed of higher fraction of coordinatively unsaturated surface atoms18 analogous to a stepped single crystal, which increases the surface roughness and promotes the chemisorption of the nitrophenolate ions and thereby facilitates the reaction, while the surfaces of the larger gold particles are terminated primarily by low-index, high-coordination surfaces related with the lower surface roughness. Theoretical calculations on Pt slabs and clusters have shown that barriers to bond activation are lower on a stepped Pt(211) surface relative to those on a flat (111) surface.64 Comparison of ethane hydrogenolysis rates on Ni(111) and (100) has led Goodman to suggest that the spatial coordination of surface metal atoms is a decisive factor in determining reactivity; differences in spacing between surface atoms on the two surfaces could account for the differences in activity.65 Recent work has shown that methane activation rate increases with Pt dispersion suggesting that the reaction is taking place on the coordinatively unsaturated atoms.66 Particle size dependent catalytic behavior of the 4-NP reduction suggest that surface roughness is a primary component of the active site. In this elegant example, the morphology and surface roughness of the surface not only influence activity but also play an important role in determining reaction selectivity.
The effect of size of gold nanoparticles on the rate of reduction of aromatic nitrocompounds has been studied for a wide variation of particle size. The citrate-capped particles were immobilized onto the polystyrene-based commercial anion exchanger taken in the chloride form. The immobilization process proceeds through electrostatic interactions, which follows a simple ion exchange mechanism. The regenaration of the gold sol using different cationic surfactants is an important aspect for the storing of the particles. The resulted material was used as the size-selective solid-phase catalyst for the reduction of an important model compound, 4-NP. Conversion of aromatic nitrocompounds to the corresponding aminocompounds confirms that the catalyst is very active for the reduction reaction. Smaller particles were found to be more active for the reduction reaction suggesting that coordinatively unsaturated surface atoms prevalent in small crystallites were more reactive for the reduction. Characterization of catalysts particles before and after immobilization confirm that immobilization does not alter the particle morphology. The most significant impact on heterogeneous catalysis is in catalyst synthesis: the ability to design and control catalytic structures during synthesis which enables production of catalysts capable of specific (i.e., selective) function. In the present example, the ability to synthesize monodispersed particles by solution-phase reduction enabled the study of intrinsic activity and selectivity of particle size rather than ensemble (particle size) averaged values of these kinetic phenomena. To have a general conclusion the study was also extended toward homogeneous catalysis using gold sol of variable sizes. All data demonstrate that spherical particles immobilized on to the resin beads serve as good substrates to probe the size-specific catalysis by gold nanoparticles for a series of aromatic nitrocompounds. This is the first fruitful exploitation of its kind where the rate kinetics was studied for both homogeneous and heterogeneous system on the reduction of aromatic nitrocompounds to have a general overview on the cluster size effect for the reduction of aromatic nitrocompounds. Further study on the size-selective catalysis by gold nanoparticles for other reactions is currently under progress. Acknowledgment. The authors are thankful to the CSIR, DST, and UGC New Delhi and IIT Kharagpur for financial assistance. References and Notes (1) Bond, G. C.; Thompson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319. (2) Haruta, M. Catal. SurV. Jpn. 1997, 1, 61. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (4) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (5) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238. (6) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Ha1kkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (7) Haruta, M. Catal. Today 1997, 36, 153. (8) (a) Haruta, M. Chem. Rec. 2003, 3, 75. (b) Mohr, C. H.; Claus, P. Sci. Prog. 2001, 84, 311. (c) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P., Jr.; Goodmann, D. W. J. Am. Chem. Soc. 2005, 127, 1604. (9) (a) Pradhan, N.; Pal, A.; Pal, T. Langmuir 2001, 17, 1800. (b) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf. A 2002, 196, 247. (c) Pasquato, L.; Rancan, F.; Scrimin, P.; Mancin, F.; Frigeri, C. J. Chem. Soc., Chem. Commun. 2000, 2253.
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