J. Phys. Chem. C 2010, 114, 16401–16407
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CO Oxidation on Unsupported Dendrimer-Encapsulated Gold Nanoparticles Peter Kracke,† Terry Haas,‡ Howard Saltsburg,† and Maria Flytzani-Stephanopoulos*,† Department of Chemical and Biological Engineering and Department of Chemistry, Tufts UniVersity, Medford, Massachusetts 02155 ReceiVed: February 15, 2010; ReVised Manuscript ReceiVed: June 26, 2010
Unsupported gold nanoparticles in solution are reported here for the first time to catalyze the oxidation of CO at ambient conditions. Gold was stabilized in solution by various polyamidoamine dendrimers. Dendrimerencapsulated gold nanoparticles (DENs) 0.5-2.5 nm in diameter have low initial activity. With storage time, however, the activity of the aged DENs increased and became comparable to a reference Au-TiO2 catalyst with the same gold loading and average gold particle size, which was tested under the same reaction conditions. The activation is attributed to partial hydrolysis of gold as followed by UV-vis spectroscopy. Introduction The use of nanoscale gold in catalytic systems has attracted great interest ever since Haruta’s pioneering publications in the late 1980s.1,2 The progression of research on gold-based catalysts is demonstrated through the study of a few excellent review papers that have been published in the intervening years.3-5 Supported nanogold catalysts have been shown to be active for gas-phase reactions, including the water gas shift (WGS),6,7 CO oxidation,8,9 NOx reduction by hydrocarbons,10 and hydrocarbon hydrogenation.11 These catalysts have also been shown to catalyze various liquid phase selective oxidation reactions.12,13 Low-Temperature Oxidation of Carbon Monoxide. The oxidation of carbon monoxide has been extensively studied as it is of practical interest as well as being a good probe reaction for evaluating catalyst adsorption characteristics. It is generally agreed that nanoscale gold (12 h with a Spectra/Por Dialysis Membrane (MWCO, 3500) to remove residual Cl- ions. To better understand the effect of aging on DEN activity, samples were stored (aged) for various times before use. Each CO oxidation reaction experiment is labeled with the time lapsed between synthesis and reaction in weeks. Thus, Au 1.3-1 would indicate data from a CO oxidation experiment run with G5.OH(Au55) catalyst prepared within the past week, containing 1.3 mg of Au. The catalysts used in this work are listed in Table 1, along with their defining characteristics and initial reaction rates. Apparatus and Procedures for the CO Oxidation Experiments. Because all DEN catalysts were left in the aqueous phase, a novel thin film reactor system was designed to reduce mass transfer resistance during operation. This design most closely resembles a small flange. The body was constructed out of aluminum, which was capped with a polycarbonate lid. Several pieces of Whatman #41 filter paper (20 µm pores), supported on top of a Whatman 47 Anodisc (0.2 µm pores), were placed inside the reactor. The average thickness of the filter paper was 220 µm. Loading the reactor was accomplished by dropping 3-6 mL of DEN catalyst solution onto the filter paper. For the World Gold Council (WGC) supplied Au-TiO2 catalyst used here for comparison, loading involved agitation of the catalyst powder in DI water with the resulting slurry being deposited on the filter paper. This reactor was accompanied by the requisite gas management equipment to control gas composition and collect batch data by mass spectrometry. The relevant CO oxidation reaction conditions are shown in Table 2. Due to the aqueous nature of the DEN catalysts, Table 2 includes the amount of reactant gas expected to dissolve based on the solubility constants. All reaction rates were calculated from the amount of CO2 produced as measured by mass spectrometry and normalized by the number of moles of gold in the catalyst to give the reported rate. Batch reactor data were fitted to a first-order rate law, ln(C0/C) ) kt. An MKS (model PPT-2A-200EM) partial pressure transducer (PPT) series quadrupole residual gas analyzer (RGA) was used to measure gas concentrations present in the batch reactor during CO oxidation experiments. UV/vis spectra were collected on a Hewlett-Packard 8452A diode array spectrometer using quartz cuvettes with a path length of 1.00 cm and a spectroscopic resolution of 2 nm at ambient
temperature. DI water or methanol was used as the background reference spectrum depending on the solvent of the sample being characterized. Transmission electron microscopy (TEM) images were taken using a JEOL 200CX instrument with a Tungsten electron source operating at 200 kV. The TEM samples were prepared by deposition of 15 µL of methanolic sample on top of a carboncoated 200 mesh copper grid. All images were taken at a magnification of 100 000×. Particle size distribution histograms were prepared using Photoshop and Igor Pro. Results and Discussion Typically, nanoparticles are separated from dendrimers by some means, then deposited on a support prior to being used catalytically. Here, however, so as to test the gold catalytically in an unsupported state, no effort was taken to remove the nanoparticles from the dendrimers; they were instead left as dendrimer-encapsulated nanoparticles (DENs). It has been found that DENs in the presence of a solvent allow access to the nanoparticle surface.45,46 With the solvent removed, dry DENs collapse upon the nanoparticle, making it no longer accessible, even to small molecules like CO.47,48 In place of using a support, the DENs used here were kept in solution to prevent the collapse of the dendrimers and any potential agglomeration of the gold. Analysis of the spectral features in the UV-visible electronic spectrum has always been a key characterization technique for studying metal nanoparticle-organic systems. The use of UV/ vis for the characterization of gold sols has a long history.49,50 Plasmon absorptions arise due to the ability of the metal nanoparticles to be polarized by the absorption of certain photons of light.51 The polarization is determined by a resonance frequency in the conduction band caused by the complex terms of the nanoparticle dielectric constant. This phenomenon is typically referred to as the surface plasmon resonance (SPR) of a nanoparticle or colloid. According to classical optical absorption theory pioneered by Mie,52 SPR peaks only arise from nanoparticles that are much smaller than the impinging wavelength of light. The peaks associated with the gold SPR, as well as the ligandto-metal charge transfer (LMCT), can be found in the gold characterization literature.13,49,50,53-55 The Cl-Au LMCT occurs at around 315 nm in the UV spectrum.53,54,56 Also, the gold SPR peak located at 520 nm is sensitive to the size, shape, and concentration of the nanoparticles. Especially noted is the damped nature of the SPR on particles less than 2 nm in diameter.44,51,55 Particles of this size exhibit extremely damped SPR absorption. It has been theorized that, for particles below 2 nm, the assumption of a delocalized conduction band may no longer be valid, and the particles are better treated as atomic clusters with discrete electronic states.51,57 This type of UV/vis analysis has been used in the literature and in this work in diagnostic characterization to verify that the nanoparticles produced are indeed in the desired 1-3 nm
CO Oxidation on Unsupported Au DENs
Figure 1. Effect of preparation and purification on UV/vis spectra of PAMAM G5.OH(Au55): (a) Au 0.65-17 aged 17 weeks, (b) Au 0.98-1 aged 1 week, (c) no NaBH4, (d) 5 µM as prepared, (e) 5 µM after dialysis.
range.40,43,44 It has also been used to track the changes in the electronic state of the precursors throughout the preparation steps.40 The effects of preparation and purification on the catalyst electronic properties of various PAMAM G5.OH(Au55) samples are illustrated in Figure 1. The gold SPR peak can very clearly be seen in trace C, 15 µM G5.OH(Au55) without NaBH4
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16403 reduction. From this, it is clear that the particles in this sample are much larger than the desired 1-4 nm. Indeed, TEM of this material (not shown) confirmed that the particles were in the 10-20 nm range. By comparison, the rest of the samples have a significantly damped and blue shifted absorption peak, suggesting that their nanoparticles are in the 2 nm range where the Mie theory begins to break down.51,58 All of the samples show an increase in absorbance as the wavelength of light decreases, characteristic of Rayleigh scattering.44 Upon dialysis, the Au-Cl LMCT absorbance around λ ) 300 nm disappears, indicating the successful removal of chlorine. The change in the LMCT is clearer in samples of lower concentration (Figure 1, traces d and e), where Rayleigh scattering is less predominant. The TEM image in Figure 2 shows a large number of gold nanoparticles from a 20 µM solution of G5.OH(Au55) DEN in methanol. It should be noted that the carbon and amine groups in the dendrimer are not electron dense enough to be detected by TEM. Therefore, only the gold nanoparticles are visible in the TEM images.59 A count of 462 nanoparticles from the image of Figure 2a found an average particle size of 2.2 ( 0.8 nm. At 20 µM, the preparation of G5.OH(Au55) results in the formation of Au nanoparticles tightly distributed around 2.2 nm in diameter. Additionally, very few particles were produced outside the desired size range, as also found in the literature.43,44,60 Only 1.5% of the counted particles were between 4 and 5 nm in diameter, and there were no particles larger than 5 nm. Although the size distribution in this work is slightly larger than that shown by the Lambert group of PPh3-ligated Au55,32 the difference in mode size is within the error given by the pixel length of TEM images acquired. Also, with the technique
Figure 2. TEM images of 20 µM G5.OH(Au55) prepared in methanol: (a) as prepared, (b) after aging 12 weeks in storage. (c) Particle size distribution of (a).
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Figure 3. CO conversion over various G5.OH(Au55) catalysts at 25 °C: Au 0.65-17 (aged), Au 0.98-9 (aged), Au 0.98-4 (fresh), Au 1.3-1 (fresh).
employed, it was difficult to differentiate particles e 1 nm from the image background.61 CO Oxidation by PAMAM G5.OH(Au55). For each experimental run of a particular catalyst synthesis, multiple CO oxidation batch reactions were run sequentially. The reactor was purged with an inert gas between each of the sequential runs, while the reactor remained sealed. Consequently, the catalyst was not exposed to air. The entire gas volume of each batch was emptied and analyzed after a set time. As a consequence of this sampling method, the data points in the subsequent graphs each represent a batch run for the amount of time shown on the x axis, with the lines connecting the batches, showing the order in which the batches were performed. This information is necessary because a catalyst that has previously been exposed to reactant gases for 6 h cannot be thought of as the same material as prepared, due to the possibility of activation or deactivation of the catalyst during reaction. A comparison of the conversions reached by the different G5.OH(Au55) catalysts is seen in Figure 3. The most important finding here is that these Au DENs are indeed active at RT for CO oxidation catalysis. Examples in the literature show the synthesis of similar Au DENs43,44 and have used Pd DENs46 and PPh3 Au5532 to catalyze liquid-phase hydrogenation; however, this is the first report of the Au nanoparticles catalyzing RT-CO oxidation while still encapsulated by the dendrimer. This activity was measured with the CO concentration in the fuelrich regime, above the explosion limit, and at an overall pressure of 500 kPa. To determine if gold particle sintering occurred under reaction conditions, TEM images of the sample Au 0.65-34 were taken before and after CO oxidation, as shown in Figure 4. A
Kracke et al. comparison of these images and the histograms of their respective particle size distributions do not show major discernible size change of the particles after 27 h under reaction conditions. This is consistent with the relatively stable performance of this catalyst over the 27 h seen in Figure 5. The aged catalysts are far more active when comparing the G5.OH(Au55) catalysts, as can be seen in Figure 3. Multiple samples of synthesis batch Au 0.65 were run, giving similar activity as shown in Figure 3, in all cases, more than 8 weeks after the synthesis. The difference between the aged and fresh catalysts is better evidenced by rate data. When comparing the rate data of G5.OH(Au55) catalysts, the aged catalysts are more active, as shown in Figure 5, whereas all the other DEN catalysts are similar in activity to each other. When the aged catalysts are fitted for a first-order rate, a rate constant k ) 1.36 × 10-6 s-1 is found. Additionally, rate data allow for better comparison with the literature, which reports data mainly in terms of rates or turnover frequencies (TOFs), such as in Ketchie et al.,62,63 Sanchez-Castillo et al.,30 and review papers.17,25 When attempting to determine the cause of increased activity with aging, both TEM and UV/vis characterization data should be analyzed carefully. A comparison between the TEM images in Figure 2 shows no obvious growth for methanolic DENs over the course of 12 weeks of aging. This is paralleled by UV/vis characterization of catalysts Au 0.65-17 (aged) and Au 0.98-1 (fresh) (Figure 1), which shows little difference in their SPR peaks, an indication that they are almost identical in size. Although TEM shows no obvious change during aging, the DEN catalysts are not static over the time period. It appears that activity increases after at least 8 weeks in storage, with the highest initial rates shown by catalysts stored 8 and 16 weeks. However, we also observed that, after at least 24 weeks, the DENs begin to flocculate and settle out of solution. Even so, these DEN catalysts can be mostly returned to solution using ultrasonic agitation. Au 0.65-34 demonstrates that these redispersed catalysts are still more active than the fresh ones (Figure 5). Although it is convenient to have direct images of nanoparticles by way of TEM, the limitations and nuances of TEM analysis and characterization as put forth by Pyrz,61 especially for DENs, should be taken into account before making any assumptions purely on the basis of TEM data. The only detectable difference between the aged and the fresh DEN catalysts that could account for the difference in activity is an increased extinction in the spectral feature at 290 nm of the UV/vis of the aged catalysts, as demonstrated by Au 0.6517 (Figure 1, trace a). Because of the low signal throughput at the wavelength of interest in the spectra of the used catalysts, additional spectra (Figure 6) were taken of 3× dilutions in which the change in absorbance with aging is more apparent. Several potential causes of this spectral feature can be found in the DEN literature;40,64,65 other Au-dendrimer interactions have been debated,66,67 as has the hydrolysis of Au13,68-70 and the occurrence of the Au-Cl LMCT in this region.53,54,56 Although other Au-dendrimer interactions cannot be completely ruled out as the source of the increased catalytic activity seen with aging, we believe that further hydrolysis of the Au in the DENs over time is the best explanation for the difference between the UV/ vis spectra of Au 0.65-17 and Au 0.98-1. The Cl-Au LMCT is unlikely because these samples have been dialyzed and should contain no more than trace amounts of Cl. The Bard group has shown that the oxidation of the PAMAM-OH dendrimer in the absence of gold gives rise to a new absorbance around 380 nm.67 Our aged dendrimer shows a similar behavior at 380 nm, in addition to an increase in absorbance at 290 nm (Figure 6, spectrum d).
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Figure 4. Catalyst Au 0.65-34 (a) before reaction and (b) after reaction for 27 h TOS. (c, d) Particle size distribution of (a, b), respectively.
Figure 6. Effect of aging on UV/vis spectra of PAMAM G5.OH(Au55): (a) aged 125 weeks, (b) aged 104 weeks, (c) aged 1 week, (d) G5.OH aged 125 weeks, (e) G5.OH fresh. Samples (d) and (e) contain no gold. Samples were diluted to 5 µM concentration for improved UV/vis signal.
Figure 5. Comparison of reaction rates on various G5.OH(Au55) catalysts: Au 0.65-17, Au 0.98-9, Au 0.65-34, Au 0.98-4, Au 1.3-1, and TiO2 Au 0.75.
However, the increase in the absorbance at 290 nm is much greater in the DEN. We believe that partial oxidation/hydrolysis68-70
of gold surface atoms to provide Au-OH bonding sites may account for this change in absorbance. The concomitant increase in catalytic activity of the aged samples may then be attributed to the increase of the number of Au-OH sites, which have been implicated in the mechanism of the gold-catalyzed CO oxidation reaction.8,17 Comparing the reaction rates (expressed as TOFs) of the DENs with those of the Au-TiO2 catalyst, the latter is initially
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2-3 times more active than the aged DENs and about 10 times more active than the other DEN catalysts (Figure 5). In these tests, the variables of gold loading and nanoparticle size were held relatively constant between the gold DEN catalysts and the reference Au-TiO2 catalyst. Only a couple of the DEN catalysts had a greater gold loading, 1.3 and 0.98 mg, than the reference powder catalyst, 0.75 mg. By scaling the rates with the total gold loading for each catalyst, we have probably underestimated the TOF. At least for the DENs, it appears that the number of hydrolyzed gold species must be calculated for proper comparison. Although this work did not examine whether the dendrimer could have blocked catalytic sites on the gold, other work has shown that, as long as the dendrimer is carried in a liquid solvent, the encapsulated nanoparticle is accessible to CO.45 These TOFs may actually be further underestimated due to interactions with the dendrimer potentially blocking active sites. Therefore, activities reported here may represent a conservative baseline catalytic activity of gold, in the absence of any promotional effects due to interactions with a support. The important finding from this work, however, is that unsupported DEN gold has comparable activity to the well-studied, active Au-TiO2 (WGC sample 6A #02-04, 1.5 wt % Au/TiO2). Another corollary is that the catalytic activity is demonstrably higher when a mixture of Au0 and Auδ+ are present, the latter from the hydrolysis of gold. Conclusions This is the first report of Au nanoparticles catalyzing the CO oxidation reaction at ambient conditions, while still encapsulated by a dendrimer in solution. Activity increases over time while the DENs are stored in water. This increase in activity coincides with an increase in absorbance at λ ) 290 nm, indicating a change in the electronic nature of the Au nanoparticle, attributed to hydrolysis. The aged catalysts are comparable in activity to Au-TiO2 powders used under similar reaction conditions, whereas the activity of the as-prepared (fresh) DENs is within an order of magnitude. Only after catalysts have been aged on the order of years do they show the smallest indication of gold aggregation. Acknowledgment. The financial support of this work by the National Science Foundation/NIRT program, under Grant No. 0304515, is gratefully acknowledged. P.K. thanks Dr. R. Valluzzi, Dr. R. Si, the CMSE at MIT, Prof. B. D. Chandler of Trinity U. TX, E. Coombs, and C. Codington-Lacerte. References and Notes (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405–408. (2) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301–309. (3) Haruta, M. CATTECH 2002, 6, 102–115. (4) Bond, G. C.; Thompson, D. T. Appl. Catal., A 2006, 302, 1–4. (5) Thompson, D. T. Gold Bull. 1999, 32, 12–19. (6) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938. (7) Andreeva, D.; Idakiev, V.; Tabakova, T.; Andreev, A. J. Catal. 1996, 158, 354–355. (8) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41–51. (9) Fierro-Gonzalez, J. C.; Guzman, J.; Gates, B. C. Top. Catal. 2007, 44, 103–114. (10) Ilieva-Gencheva, L.; Pantaleo, G.; Mintcheva, N.; Ivanov, I.; Venezia, A. M.; Andreeva, D. J. Nanosci. Nanotechnol. 2008, 8, 867–873. (11) Haruta, M. Catal. Today 1997, 36, 153–166. (12) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed. 1998, 37, 1415–1418. (13) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552–560.
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