Catalytic Membranes Prepared Using Layer-by-Layer Adsorption of

Metal nanoparticles are attractive catalysts because their large surface area-to-volume ratio allows efficient use of expensive metals.1 Moreover, in ...
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NANO LETTERS

Catalytic Membranes Prepared Using Layer-by-Layer Adsorption of Polyelectrolyte/Metal Nanoparticle Films in Porous Supports

2006 Vol. 6, No. 10 2268-2272

David M. Dotzauer, Jinhua Dai, Lei Sun, and Merlin L. Bruening* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 Received July 21, 2006; Revised Manuscript Received August 24, 2006

ABSTRACT Layer-by-layer adsorption of polyelectrolytes and gold nanoparticles within porous supports provides a convenient method for forming catalytic membranes. The polyelectrolyte film effectively immobilizes the gold nanoparticles without inhibiting access to catalytic sites, as shown by the similar rate constants for nanoparticle-catalyzed 4-nitrophenol reduction in solution and in membranes. Modified alumina membranes reduce >99% of 0.4 mM 4-nitrophenol at linear flow rates of 0.98 cm/s, and the modification process is also applicable to track-etched polycarbonate supports.

Metal nanoparticles are attractive catalysts because their large surface area-to-volume ratio allows efficient use of expensive metals.1 Moreover, in some cases the electronic properties of nanoparticles are sufficiently different from those of the corresponding bulk metal to allow large enhancements in catalytic activities. Gold colloids, for example, demonstrate high catalytic activity even though bulk gold is typically an ineffective catalyst.2 However, aggregation of colloids frequently yields bulklike materials and can greatly reduce catalytic activity.3 To prevent aggregation, catalytic nanoparticles are usually immobilized on supports such as alumina,4 metal oxides,5 or zeolites6,7 or in polymeric materials including dendrimers,8,9 polyelectrolyte brushes,10,11 polystyrene microspheres,12 and polyelectrolyte multilayers.13-15 Porous membranes provide an alternative support for nanoparticle immobilization and are very attractive for catalysis because the membrane geometry allows for flowthrough reactions and avoids the need to disperse the catalyst and subsequently separate it from the reaction mixture. Importantly, flow through micrometer-diameter membrane pores results in rapid convective mass transport of reactants to immobilized metal nanoparticles, yielding conversions that often depend on kinetics or mass flow, rather than diffusion. In fluidized bed reactors or homogeneous solutions of catalytic beads, conversion can be limited by diffusion into * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (517) 353-1793. Phone: (517) 3559715 ext. 237. 10.1021/nl061700q CCC: $33.50 Published on Web 09/14/2006

© 2006 American Chemical Society

the pores of the support, but with membrane supported catalysts, convection through pores may overcome this problem.16 This Letter describes an especially convenient method for modifying alumina and polymeric membranes with metal nanoparticles; layer-by-layer adsorption of polycations and citrate-stabilized gold colloids (Figure 1). This technique, which has been developed by a number of groups for modification of flat surfaces,17-20 provides a simple way to immobilize highly accessible, well-separated nanoparticles in porous membranes and also affords control over the amount of colloid deposited.21-23 The resulting membranes can catalyze >99% reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) at solution fluxes of 0.29 mL/(cm2 s) (linear velocities of 0.98 cm/s assuming a 30% porosity). Our method for membrane modification is somewhat similar to that employed by Sehayek et al.,24,25 where porous alumina was modified with (aminopropyl)trimethoxysilane and subsequently exposed to citrate-stabilized gold colloids. Nevertheless, the Sehayek procedure produces gold nanotube structures due to aggregation and coalescence of the colloids, whereas the layer-by-layer procedure gives well-separated colloids. Chaudret and Schmid also exposed alkoxysilanized alumina membranes to metal nanoparticles and demonstrated catalytic hydrogenation and CO oxidation by these modified substrates.26-28 Their procedure was only utilized for gasphase catalysis, however, so it is not known whether this method yields membranes that are effective in aqueous catalysis. Bhattacharyya and co-workers employed adsorption of polyelectrolytes to modify membranes with catalytic Fe-

Figure 1. Schematic diagram showing the modification of a pore of an alumina membrane by adsorption of two polyelectrolytes and gold nanoparticles. Additional polycation/nanoparticle bilayers can be added by alternating steps 2 and 3.

Figure 2. Cross-sectional FESEM images of (a) a bare alumina membrane with 0.2 µm diameter pores and (b) a similar alumina membrane coated with one PAA/PAH bilayer and gold nanoparticles.

Ni and Fe-Pd nanoparticles for groundwater remediation.29 In that case metal ions were adsorbed to poly(ethylene glycol)-containing films and subsequently reduced with NaBH4.30,31 Formation of gold nanoparticles prior to their immobilization allows for control over both particle size and colloid-capping ligands to yield remarkably active catalysts. Characterization of Nanoparticle Immobilization in Membranes. Figure 2 contains field-emission scanning electron microscopy (FESEM) images of alumina membranes before and after modification with gold colloid/polyelectrolyte films. Modification occurred through the sequential flow Nano Lett., Vol. 6, No. 10, 2006

of 0.02 M poly(acrylic acid) (PAA), water, 0.02 M protonated poly(allylamine) (PAH), water, 0.02 mg/mL gold colloids (diameter of 12 ( 1 nm),32 and water through the membrane (Figure 1). (Concentrations of polymers are given with respect to their repeat units.) The FESEM images indicate that the nanoparticles are dispersed evenly on the pore walls of the membrane with no observable aggregation. Flow rates and colloid concentrations used for membrane modification were chosen to avoid formation of a nanoparticle cake layer on the membrane surface and to minimize particle aggregation. Although alumina should be positively charged when exposed to the pH 6 Au colloid solution, no visible adsorption of colloids occurred on untreated alumina. In contrast, after deposition of one PAA/PAH bilayer, adsorption of gold colloids yielded a red membrane, and the first 50 mL of a red 0.1 mM colloid solution (concentration is given with respect to gold atoms) exited PAA/PAH-modified membranes as a colorless liquid. Apparently, the PAA/PAH layer provided a more highly charged surface for nanoparticle adsorption, and coordination of amines with gold nanoparticles may also enhance colloid deposition. Dissolution of the adsorbed nanoparticles in aqua regia (3 parts HCl, 1 part HNO3) and analysis of this solution using atomic absorption spectroscopy revealed that the amount of gold loading in porous alumina modified with one layer of PAA/PAH and gold colloids was (4.9 ( 0.2) × 10-6 mol of gold per membrane. (The exposed surface area of the top of the 60 µm thick membranes was 2.8 cm2.) Assuming a 30% porosity33 and a pore diameter of 0.23 µm,34 this gold loading corresponds to 5.5 × 10-9 mol of gold per cm2 of internal (pore) surface area. For a nanoparticle diameter of ∼12 nm, this value is equivalent to 620 nanoparticles/µm2, which is reasonably consistent with the colloid density of ∼500 nanoparticles/µm2 seen in the SEM images (see Supporting Information). For membranes modified with two and three total PAH/Au colloid bilayers, the gold loading was (8.3 ( 0.2) × 10-6 and (11.4 ( 0.6) × 10-6 mol of gold per membrane, respectively. Enhanced gold loading with adsorption of more layers confirms the layer-by-layer deposition. Porous alumina membranes can also be modified through direct adsorption of polyethyleneimine (PEI)/Au colloid 2269

Figure 3. UV-vis absorption spectra of a 0.1 mM 4-NP, 10 mM NaBH4 solution before (blue) and after (red) passing it through a membrane modified with a PAA/PAH/Au colloid film (flux of 0.29 mL/(cm2 s)). The spectrum of a solution containing 0.1 mM 4-AP and 10 mM NaBH4 is also shown as a reference.

bilayers. In this case, the pH of the PEI deposition solution was 8.5, which allowed for the amine to adsorb directly to the alumina surface without the need for adsorption of a precursor PAA layer. Gold coverages were (9.2 ( 0.9) × 10-6, (1.7 ( 0.2) × 10-5, and (2.4 ( 0.3) × 10-5 mol per membrane for alumina modified with one, two, and three PEI/Au colloid bilayers, respectively. Deposition of PEI at pH 8.5 on a PAA precursor layer resulted in a surface that allowed similarly high Au colloid adsorption. Catalysis by Nanoparticle-Containing Membranes. Reduction of nitroaromatic compounds to their corresponding amino derivatives in the presence of NaBH4 provides a rapid, easily characterized reaction for examining the catalytic activity of nanoparticles immobilized in membranes. The reduction does not occur in the absence of nanoparticles and is readily monitored with UV-vis absorption spectroscopy.9,10,35-37 Specifically, the 4-nitrophenolate ion shows a strong absorbance maximum at 400 nm, and the rate of reduction is monitored by the disappearance of this peak. The gradual appearance of a peak at 298 nm indicates the corresponding production of 4-aminophenol. Colloid-modified membranes show remarkable conversions of 4-NP to 4-AP at high flow rates. Passage of a solution containing 0.1 mM 4-NP and 10 mM NaBH4 through PAA/PAH/Au-colloid modified membranes at a rate of 0.29 mL/(cm2 s) resulted in reduction of greater than 99% of the 4-NP (Figure 3). Similarly high conversions occur with 4-NP concentrations as high as 2 mM, provided that NaBH4 is in >50-fold excess. At such high conversions, the reaction is essentially mass-flow limited even at these high fluxes. (The solution flux of 0.29 mL/(cm2 s) corresponds to a residence time within the membrane of only 6.1 ms and a linear velocity of 0.98 cm/s.) At higher fluxes, conversion declines slightly as would be expected. Still, as Figure 4 shows, conversion is greater than 95% at fluxes as high as 0.44 mL/(cm2 s) (residence time of 4.1 ms). The maximum flux in this system is practically limited to about 0.53 mL/(cm2 s) because higher applied pressures result in membrane fracture. The data in Figure 4 conform well to a simple first-order kinetic model of the reaction. For a solution flowing through 2270

Figure 4. Plot of 4-NP conversion percentage vs flow rate for membranes containing one bilayer of PAA/PAH and adsorbed gold colloids. The curve represents a first-order reaction model with a rate constant (k′) of 900 s-1. Feed conditions: [4-nitrophenol] ) 0.4 mM, [NaBH4] ) 20 mM.

a catalytic membrane, first-order kinetics are described by eq 1 -

dCx dt k′Cx dCx )) dx dt dx V

(1)

where Cx is the concentration of 4-NP at a distance x into the membrane, k′ (s-1) is the first-order rate constant, t is time, and V is the linear velocity of the solution in the membrane. Integration of eq 1 across the length, l, of the membrane followed by rearrangement yields eq 2 Cl ) C0 exp

(-k′l V )

(2)

where C0 and Cl are the concentrations of 4-NP entering and exiting the membrane, respectively. The fit of eq 2 to the data in Figure 4 is excellent and yields an apparent rate constant (k′) for this system of 900 ( 30 s-1. To the best of our knowledge, this is the only investigation of the reduction of 4-nitrophenol via flow through a catalytic membrane. Previous batch reactions that employed platinum nanoparticles dispersed in solution produced apparent rate constants, k′, ranging from 1.25 × 10-4 to 0.06 s-1 depending on particle size.9,10,35 However, to accurately compare heterogeneous catalysts, the rate constants should be modified according to eq 3 k)

k′V A

(3)

where k (cm/s) is the first-order rate constant normalized to the surface area, A, of nanoparticles per solution volume, V. With eq 3, the platinum nanoparticles discussed in previous papers give rate constants, k, ranging from 0.0054 to 0.055 cm/s, while gold colloids in solution have a rate constant of 0.014 ( 0.002 cm/s (see Supporting Information Figure 8). When eq 3 is applied to a colloid-modified membrane, the value of k is 0.018 ( 0.002 cm/s. This calculation assumed a nanoparticle surface area of 250 cm2/membrane and a pore Nano Lett., Vol. 6, No. 10, 2006

Table 1. Percent 4-NP Reduction, k, and k′ vs the Number of PAH/Au-Colloid Bilayersa Deposited in Alumina Membranes Employed for Flow-Through Reactions no. of bilayers 1 2 3

moles of gold

% of 4-NP reducedb

rate constant k′ (1/s)c

k (cm/s)c,d

(4.9 ( 0.2) × 10-6 88.8 ( 0.9 9.0 ( 0.3 0.018 ( 0.002 (8.3 ( 0.2) × 10-6 94.1 ( 0.3 1.5 ( 0.1 0.018 ( 0.002 (11.4 ( 0.6) × 10-6 98.3 ( 0.5

a One layer of PAA was deposited prior to PAH/Au-colloid deposition. Feed conditions: [4-NP] ) 0.8 mM, [NaBH4] ) 20 mM, flux ) 0.30 mL/(cm2 s). c Feed conditions: [4-NP] ) 0.4 mM, [NaBH4] ) 20 mM, flux varied between 0.20 mL/(cm2 s) and 0.50 mL/(cm2 s). d Normalized to the colloid surface area/membrane pore volume ratio.

b

(solution) volume of 0.0051 cm3/membrane. (The nanoparticle surface area was estimated by assuming 12 nm diameter spherical nanoparticles and a gold loading of 4.9 × 10-6 mol of gold per membrane). Remarkably, the activities of gold nanoparticles in the membrane and in solution are essentially the same, showing that immobilization of nanoparticles does not significantly restrict access to catalytic sites or alter catalytic activities. Catalysis as a Function of Film Composition. Increasing the number of gold nanoparticles adsorbed in the membrane, which can be accomplished by increasing the number of adsorbed layers, should raise the value of k′ and enhance conversion in the membrane reactor. However, since conversion of 4-NP to 4-AP is nearly 100% even with a single layer of adsorbed nanoparticles, we had to decrease reaction rates to study the effect of further adsorption steps on the reaction. To do this, we reduced the initial NaBH4 excess from 50-fold to 25-fold relative to 4-NP. Table 1 shows that under these conditions, conversion increased from 89 to 98% on going from one to three adsorbed bilayers of PAH/Au colloid in the membrane. Furthermore, measurement of conversion percentage vs flow rate using a 50-fold excess of NaBH4 showed that the apparent rate constant, k′, increased by ∼70% on going from one to two bilayers. The fact that the amount of gold deposited also increased by 70% on going from one to two bilayers suggests that k is not affected by the number of layers deposited and that interior nanoparticles are readily accessible to both 4-NP and BH4-. Because of very high conversions, we could not determine the value of k′ for membranes containing three PAH/Au colloid bilayers. Capping of adsorbed films with a PAH layer also had little impact on the reactivity of underlying nanoparticles. For reductions using 4-NP feed concentrations of 0.4 or 0.8 mM and a NaBH4 feed concentration of 20 mM, membranes terminated with PAH gave essentially the same percent reductions of 4-NP as membranes terminated with a gold colloid layer. (See Figure 5 of the Supporting Information.) Membranes modified with PEI/Au nanoparticle films showed catalytic activities similar to those of membranes containing PAA/PAH/Au nanoparticle coatings. The reduction of 4-NP was greater than 99% even at fluxes of 0.30 mL/(cm2 s), and the percent conversion increased as a function of the number of layers deposited for PEI/Au nanoparticle films when using a 25-fold excess of NaBH4. Nano Lett., Vol. 6, No. 10, 2006

Figure 5. Plot of percent reduction of 4-NP vs the volume of solution passed through an alumina membrane modified with a PAA/PAH/Au colloid film. Feed conditions: [4-NP] ) 2.0 mM, [NaBH4] ) 100 mM, flux ) 0.17 mL/(cm2 s). The plot contains a compilation of five sequential passages of 500 mL of the reaction solution.

However, the higher gold loading and subsequently higher conversions with PEI/Au nanoparticle coatings made it difficult to determine rate constants for membranes modified with these films. Stability and Turnover Numbers. In addition to high activity, stability is also vital for developing useful catalytic membranes. Dry membranes modified with gold colloids could be stored in the laboratory environment for several months without negative effects on their activity. Figure 5 demonstrates that membranes are also reasonably stable under flow of a solution containing 4-NP and NaBH4. The percent reduction remains essentially constant, even though over 140 000 membrane volumes have passed through the system. Although we do not yet know the limits of membrane stability, the data in Figure 5 show a turnover number >1000 mol of 4-NP per mol of gold contained within the membrane. After attainment of this high turnover number, 4-NP conversion at a flux of 0.17 mL/(cm2 s) decreased only marginally, from 99.8% to 99.7%. These results indicate that in a 50fold excess of NaBH4, minimal fouling occurs on the surface of the gold colloids and little or no gold is leached from the membrane. Larger turnover numbers may be more easily achieved with higher concentrations of 4-NP and NaBH4, but the high pH of such solutions results in slow dissolution of the alumina support. Formation of Catalytic Polymer Membranes. To demonstrate the versatility of the layer-by-layer technique for membrane modification, we deposited polyelectrolyte/Au colloid films in polycarbonate (PC) track etched membranes containing nominal pore sizes of 0.2 µm. The PC membranes are approximately 20 µm thick and are significantly less porous than the alumina (PC porosity is 10-15%).38 As a result, less gold can be immobilized within the track etched polymers. To modify PC membranes, we used polystyrene sulfonate (PSS) in the formation of the precursor polyanionic layer because PSS adsorbs well to polymer films.39 At a solution flux of 0.03 mL/(cm2 s), PSS/PAH/Au-colloid2271

modified membranes can reduce more than 99% of the 4-NP in a solution containing 0.4 mM 4-NP and 20 mM NaBH4 (see Figure 6 of the Supporting Information). However, it is more difficult to perform reductions at high flow rates with PC membranes than with alumina due to the lower burst strength (0.7 atm) of the track etched PC.38 The rate constant, k, for PC membranes modified with PSS/PAH/Au-colloid films ((6.2 ( 0.2) × 10-7 mol of gold per membrane) is approximately 0.016 ( 0.002 cm/s (see Figure 7 of the Supporting Information), which is consistent with the k values for gold colloids immobilized in alumina membranes or suspended in solution. Overall, membranes modified with gold colloids in multilayer polyelectrolyte films present a stable system that has great potential for catalyzing fast reactions. The polyelectrolyte multilayer provides a convenient platform for immobilizing active, accessible catalytic nanoparticles in a variety of porous supports including alumina and polycarbonate. The ability to vary both the polyelectrolyte and the catalytic material should make this method even more versatile. Acknowledgment. We are grateful for financial support from the Department of Energy Office of Basic Energy Sciences. Supporting Information Available: Procedures for nanoparticle synthesis, membrane preparation, and catalytic reduction reactions, as well as TEM images of nanoparticles and additional figures mentioned in this Letter. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Fendler, J. H. Nanoparticles and Nanostructured Films: Preparation, Characterization, and Applications; Wiley-VCH: Weinheim, Germany, 1998. (2) Haruta, M. Chem. Rec. 2003, 3, 75-87. (3) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877-4878. (4) Ivanova, S.; Pitchon, V.; Zimmermann, Y.; Petit, C. Appl. Catal., A 2006, 298, 57-64. (5) Mallick, K.; Scurrell, M. S. Appl. Catal., A 2003, 253, 527-536. (6) Mukherjee, P.; Patra, C. R.; Kumar, R.; Sastry, M. Phys. Chem. Commun. 2001, 5, 1-2. (7) Chen, J.; Lin, J.; Kang, Y.; Yu, W.; Kuo, C.; Wan, B. Appl. Catal., A 2005, 291, 162-169.

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NL061700Q

Nano Lett., Vol. 6, No. 10, 2006