J. Phys. Chem. C 2007, 111, 10461-10466
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Effect of Silica-Supported Silver Nanoparticles on the Dihydrogen Yields from Irradiated Aqueous Solutions Tomer Zidki,† Haim Cohen,*,†,‡ Dan Meyerstein,†,‡ and Dan Meisel§ Department of Chemistry, Ben-Gurion UniVersity, P.O. Box 653, Beer SheVa 84105, Israel, Department of Biological Chemistry, College of Judea and Samaria, Ariel, Israel 40700, and Radiation Laboratory and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: February 5, 2007; In Final Form: May 14, 2007
Metal nanoparticles (NPs) are used to catalyze the formation of molecular multielectron transfer products, for example, H2, from single-electron reductants, such as radicals. Noble metals, like silver and gold, were very instrumental in unraveling the mechanism of this interfacial process. In this study, we explore the effect of the support, silica nanoparticles, on the catalytic production of H2 on silver from radiolytically produced •C(CH ) OH radicals and water. We obtain very high concentrations of stable silica-supported silver 3 2 nanoparticles that remain suspended in solution for long periods of time. The presence of metallic silver particles on the silica surface further induces a very effective deposition of silver particles on the same silica particle leading to cooperative deposition of the silver. The silica support changes appreciably the reactivity of the silver NPs, reducing the yield of the molecular hydrogen produced at the high concentrations of the supported NPs to that of the primary molecular hydrogen G(H2) ) 0.45 molecules/100 eV from water radiolysis indicating that H2 production at the surface of the silver is inhibited. A possible explanation is that the catalyst (Ag on SiO2) catalyzes the disproportionation of the reducing radicals or the reduction of acetone at the expense of the H2 evolution catalysis.
Introduction Metal nanoparticles (NPs) have been used for nearly three decades as catalysts to induce the formation of molecular multielectron transfer products, for example, H2, from singleelectron reductants such as reducing radicals.1-5 In many studies, one electron reducing radicals (e.g., •C(CH3)2OH radicals) were used in aqueous solutions to reduce the NP generating negatively charged NPs, which can reduce water to yield H2. This solidaqueous redox catalysis is crucial in water splitting solar energy conversion schemes.6 The mechanism for this conversion, proposed by Henglein and co-workers,7 invokes electron transfer from the radical to the NP, creating a pool of stored electrons that can reduce water to H2, once the over-potential for hydrogen evolution is achieved. Low over-potential metals for this process, for example, Pt or Pd, are of course most efficient for H2 evolution, but noble metals like silver and gold were very instrumental in unraveling the mechanism of this interfacial process.7,8 Recently,9 it was found that methyl radicals, •CH3, react very fast with silver and gold NPs forming NP-C σ bonded unstable intermediates, which decompose via the formation of C-C covalent bonds forming ethane. Rarely were these particles utilized in conjunction with a support material, a common practice in solid-gas interfacial catalysis. Nonetheless, metal deposition on titania particles was utilized extensively in conjunction with the photocatalytic activity of TiO2.10-16 In this study, we explore the effect of the support, silica particles, on the catalytic production of H2 using radiolytically produced •C(CH ) OH radicals as precursors. 3 2 * Corresponding author. E-mail:
[email protected]. Fax: 972-86472943. Phone: 972-8-6472737. † Ben-Gurion University. ‡ College of Judea and Samaria. § University of Notre Dame.
The primary yields of the products of the radiolysis of dilute neutral aqueous solutions by ionizing radiation (β or γ radiation) are given in eq 117 γ,β
H2O 98 e-aq (2.65), •OH (2.65), H (0.60), H2O2 (0.75), H2 (0.45) (1) where the numbers in parentheses are the yields of products in molecules (radicals)/100 eV of absorbed radiation energy in the solutions, often labeled G values. In concentrated solutions, like those used in this study, the yield of the radical species, e-aq and •OH, is somewhat larger and that of the molecular species is somewhat smaller because of the scavenging of radicals in the spurs.17 Acetone and 2-propanol in de-aerated solutions can act as scavengers for the e-aq, •H atoms and •OH radicals, via reactions 2-4:18
(CH3)2CO + e-aq (+H+) f •C(CH
3)2OH
k ) 6.5 × 109 M-1 s-1 (2)
(CH3)2CHOH + •OH f •C(CH
3)2OH
+ H2O
k ) 1.9 × 109 M-1 s-1 (3)
(CH3)2CHOH + •H f •C(CH
3)2OH
+ H2
k ) 7.4 × 107 M-1 s-1 (4)
Under these conditions, the yield of H2 increases from 0.45 to G(H2)tot ) 1.05 molecules/100 eV. The sources of H2 that constitute this yield are molecular hydrogen (G(H2) ) 0.45) and H atoms that react via reaction 4. When precious metal
10.1021/jp070984f CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007
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NPs are added to the solution containing the same scavengers, the yield of H2 can increase up to G(H2)tot ) G(H2) + G(H) + [G(e-aq) + G(OH) + G(H)]/2 - G(H2O2) ) 3.25 molecules/ 100 eV7,8 because of the reaction of the R radicals, •C(CH3)2OH, with the NPs (reactions 5-8). However, ∼15% of the H abstraction from the (CH3)2CH(OH) (by •H and •OH radicals) produces the β radicals (•CH2CH(CH3)OH), which are ineffective in producing H2, and does not produce the R radicals, •C(CH ) OH.19 Thus, the yield of H is reduced to 2.9 molecules/ 3 2 2 100 eV. This additional yield is due to the redox catalysis resulting from the electron pool on the metallic particle, (M)p; see reactions 5-7:
n•C(CH3)2OH + (M)p f (M)pn- + n(CH3)2CO + nH+ (5) (M)pn- + mH2O f (M)p(n-m)-sHm + mOH-
(6)
(M)p(n-m)-sHm f (m/2)H2 + (M)p(n-m)-
(7)
H2O2 + (M)pn- f 2OH- + (M)p(n-2)-
(8)
where the subscripted letter p represents particle state. The presence of high concentrations of SiO2 particles in aqueous solutions increases the yield of hydrated electrons in the water.20 This was attributed to escape of radiolytically generated charge carriers, electrons and excitons,21 from the solid particle to the aqueous phase. It may, therefore, be expected to increase the yield of H2 because most of the hydrated electrons are expected to be converted to molecular hydrogen. Indeed, even in the absence of metallic redox catalysts, silica and many other oxides have been shown to increase the yield of molecular hydrogen.22-25 However, since no oxidizing charge carriers from the silica were found in the aqueous phase when a significant fraction of the energy is absorbed by the particles,26 the interaction between the charge carriers of the two solids in a supported metallic catalyst may adversely affect the catalytic activity of the metal. Empirical observations from Yamamoto et al. indicate that addition of noble metals to TiO2 or Al2O3 may increase or decrease the yield of H2.27 The results reported herein demonstrate that the radiolytic yield of dihydrogen in irradiated aqueous solutions is considerably smaller in the presence of silver NPs bound to silica NPs than that in the presence of silver NPs. Similar observations with other metallic particles will be reported elsewhere. Experimental Section Materials. Silver perchlorate (AgClO4), tetraethyl orthosilicate (TEOS), (3-aminopropyl) trimethoxysilane (APS), and all other chemicals used were of the highest purity commercially available and were used as received. Nanopure water (R > 18 MΩ cm) was used across this study. De-aeration was done by bubbling pure argon for 30 min prior to irradiation. Instrumentation. An inline gas chromatograph (Varian CP3800) was used to determine the amount of hydrogen produced upon irradiation. A well-regulated argon stream was continuously bubbled through a radiation cell made of a 1 cm quartz cuvette. A four-way valve allowed the cell to be isolated from the argon stream during the course of the radiolysis. Opening the valve directs the accumulated volatile products to a 3 m molecular sieve 5A column held at 50 °C. Products were measured with a thermal conductivity detector. A septum for
the injection of calibration gases was placed upstream from the sample cell. Gas flow through the sample was held at 40 mL min-1. The retention time of H2 under these conditions was ∼3 min. UV-vis measurements were carried out using a Varian Cary 50 Bio spectrophotometer. The yields of molecular hydrogen produced in the low-dose irradiation unit (see below) were determined according to the procedure described elsewhere.9 Transmission electron microscopy (TEM) analyses were performed using a JEOL-TEM-100SX instrument. Specimens for TEM analysis were prepared on Lacey Formvar/carboncoated 300 mesh copper grid from Ted-Pella. The grid was dipped into the desired solution (sometimes after dilution) and put on filter paper to remove excessive solution. To determine the silver concentration in the SiO2-Ag solutions, the silver was oxidized and dissolved in concentrated nitric acid and was then measured by inductively-coupled plasma (ICP). Synthesis of SiO2 Nanoparticles and Attachment of Bridging Molecules. Silica NPs were prepared according to the method of Sto¨ber et al.28 Briefly, 3 mL of NH4OH (29%) were added to 45 mL of dry ethanol followed by the addition of 3.12 mL of tetraethyl orthosilicate (TEOS) to give 0.28 M TEOS in the solution. The solution was stirred overnight to ensure that the reaction was completed. This procedure is reproducible, and it produces silica NPs of approximately 45 nm in diameter. The pH of this silica solution is between 8 and 10. The stability of the silica particles is pH dependent,29 and therefore, throughout this study, all dilutions were done with pH 10 solutions (using NaOH). The SiO2 particles were functionalized with the bridging APS molecules, where the silanol end of the molecule condenses onto the silica surface leaving an amino end that can be attached to the silver surface.30-32 Attachment of the bifunctional APS was carried out using a variation of the method described by Halas et al.33 The silica suspension was mixed with a quantity of APS sufficient to cover the silica NPs with half a monolayer of APS (147 mM to 2.5 × 10-7 M of SiO2 NPs of 45 nm in diameter) and not more to avoid self-condensation. The area of an APS molecule on the NP surface was assumed to be 60 Å2.34 The solution was stirred for 2 h and then refluxed for an additional hour. The solution was centrifuged after cooling, and the solid precipitate was redispersed by sonication in water. This generated the stock solution for further experiments. Highly concentrated solutions of the functionalized silica were prepared by centrifugation of the stock solution followed by drying the centrifuged solid under a stream of air. The dry powder was weighted and re-dispersed by sonication in water to give 25 wt % silica NPs. Attachment of Silver NPs onto the Silica. A 1.0 M aqueous solution of AgClO4 was added to the functionalized silica particles solution to produce a 10 mM solution of Ag+. A reducing solution, sodium borohydride (10 mM in ice-cold water at pH 10) was prepared by dissolving the solid borohydride. The latter solution was added dropwise. The total volume of the reducing solution added was the same as the functionalizedsilica solution prior to the addition. The resulting nanocomposites solution was centrifuged. The centrifuged solid with a very small amount of the supernatant liquid was re-dispersed in water at pH 10 by sonication to produce a solution of approximately 20 wt % of silica NPs in the composite solution. If needed, this solution was diluted for other experiments. The silver concentration in the nominal 20 wt % SiO2 solution was 120 mM (1.2 wt %, determined by ICP). The exact silica concentration was calculated from the Ag/SiO2 ratio to be 21 wt %. The attachment of the silver onto the silica NPs is demonstrated in Scheme 1.
Effect of Silica-Supported Silver Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10463
SCHEME 1: Schematic Representation of Silver Attachment onto the Silica NPsa
a
The exact way and the number of APS linkers by which the silver NPs are bound to the SiO2 NPs is not defined.
Irradiation. The γ irradiations were carried out at Notre Dame University in a Shepherd 109 60Co gamma source and at Ben-Gurion University in a 60Co gamma source by Noratom Gammacell. The dose rates in aqueous solutions have been measured by the Fricke dosimeter to be 106 Gy/min and 3.0 Gy/min in the 60Co gamma sources at Notre Dame and at Ben-Gurion, respectively.35 Unless otherwise stated, all doses are given for the aqueous solution; at high silica and silver concentrations, the dose increases because of the absorption of the energy by the solids. In these cases, it was assumed that the absorption of energy is proportional to the electron density of the system; a dose was then calculated accordingly and the total dose is clearly indicated. Samples were irradiated at room temperature, 25 °C.
Results and Discussion Characterization of the SiO2-Ag Nanocomposite Particles. Figure 1a shows a TEM micrograph of the silverdeposited silica particles. The silver NPs are clearly visible as islands on the silica. Careful examination of the TEM micrographs shows that only a fraction of the silica particles contains silver islands. Quantitative counting indicates that only 25% of the silica particles contain silver and the rest are essentially void of silver. Furthermore, those silica particles that contain silver are covered by a large number of silver particles while the others contain none. Thus, the presence of a silver particle on the silica, or a precursor of that silver particle (e.g., APS or Ag+ ions), increases the probability that another one will deposit on the same silica particle. The origin of that cooperativity in silver
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Figure 1. (a) TEM micrograph of the SiO2-Ag NPs and (b) the UV-vis spectrum of those particles. The absorbance was measured in 200 fold diluted solution in a 1 mm optical path cuvette, and the spectrum is normalized to 1 cm optical path and to the original concentration.
Figure 2. Yield of H2 from irradiation of an aqueous solution of APSfunctionalized silica NPs at various silica concentrations. De-aerated solutions contained also 0.1 M acetone and 0.1 M 2-propanol at pH 10. Dose shown is dose absorbed by water only.
deposition, which was not reported up to date to our knowledge, is not clear at this time. From the TEM measurements, the size of the silica particles is estimated to be d ) 45 ( 5 nm. The size of the silver islands on the silica is d ) 6.0 ( 0.5 nm. Figure 1b shows the UV-vis spectrum of the SiO2-Ag nanocomposites. The narrow plasmon band, peaking at 401 nm, reflects the narrow size distribution of the particles observed also in Figure 1a. Irradiation of APS-Functionalized Silica NPs. The APSfunctionalized silica NPs were irradiated at the high dose rate of 106 Gy/min under reducing conditions. The •C(CH3)2OH radicals were produced by the irradiation of Ar-saturated aqueous suspensions containing 0.1 M 2-propanol and 0.1 M acetone according to reactions 2-4. All of these reactions are very fast and produce the same radicals (except for the ∼15% yield of the β radicals). As mentioned above, only half of the surface of the silica NPs is coated by the APS. Different concentrations of the functionalized silica were irradiated in the γ source, and the concentration of molecular hydrogen produced by the radiation was determined by gas chromatography. Figure 2 shows plots of H2 concentrations produced during the irradiation of 0.1 M of acetone and 0.1 M 2-propanol Ar-saturated aqueous suspensions containing various concentrations of 45 nm APSfunctionalized silica NPs versus the irradiation dose. The slope of the straight lines obtained is the yield of molecular hydrogen produced.
The yield of H2 in Ar-saturated aqueous solutions at pH 10, containing 0.1 M acetone and 0.1 M 2-propanol is G(H2) ) 1.1 ( 0.1 molecules/100 eV. This yield does not change upon addition of low silica concentrations (0.25-2.5 wt %) as shown in Figure 2. However, increasing the APS-functionalized silica concentration to 25 wt % leads to an increase in H2 yield to 1.4 ( 0.1 molecules/100 eV. At the high concentrations of silica NPs, the radiation dose absorbed by the sample increases because of the increase in sample density. The fraction of the energy that is initially absorbed by the solid silica NPs, nonetheless, increases the yield of e-aq.36 It should be recognized, however, that under the experimental conditions of Figure 2 none of the hydrated electrons contribute directly to the production of molecular hydrogen; they all react with acetone (reaction 2). The increased yield might be attributed to an increase in the yield of H atoms, and perhaps increased production of primary H2. From the increase in sample density of the 25 wt % silica samples, one estimates a 16% increase in the absorbed dose by the sample relative to the dose in the absence of silica. If the charge carriers arrive at the aqueous interface as efficiently as the escape of carriers generated in water, this is also expected to increase the yield in these samples. The expected yield then would be 1.3 molecules/100 eV. The observed yield of 1.4 is somewhat higher. Moreover, at the size of the particles used, 45 nm, most of the charge carriers that are produced in the silica are expected to annihilate by recombination or to become trapped before they arrive at the interface.36 It was previously suggested that the enhanced yield beyond the increase in absorbed dose might be attributed to the capture of excitons at the silica-water interface. Alternatively, the porous nature of the silica particles37 obtained by the Sto¨ber method28 may allow escape of carriers into the aqueous phase within the silica pores. The effect of dose rate was studied by irradiations at the low dose rate unit of 3.0 Gy/ min. The results indicate that within experimental error there is no change in the yield of molecular hydrogen produced. Irradiation of the SiO2-Ag Nanocomposites. The SiO2Ag nanocomposites at various concentrations were irradiated at the high dose rate 106 Gy/min, under the same conditions as the APS-functionalized silica NPs. The results are shown in Figure 3 for various particle concentrations but at a constant [SiO2]/[Ag] ratio; the yield of H2 depends on the concentration of the particles and the dose. As outlined in Introduction, the expected maximum yield in the absence of silica is G(H2) ) 2.9 molecules/100 eV.7 None of the yields in Figure 3A reach this limiting value. Moreover, as the concentration of the particles increases, the yield decreases. This is in reverse order to the observations for
Effect of Silica-Supported Silver Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10465
Figure 3. (A) H2 yields from irradiated SiO2-Ag suspensions of various concentrations at a constant ratio [SiO2]/[Ag] ) 17 by weight. (B) H2 yields at high dose irradiation of the highest concentration of SiO2-Ag suspensions (see blue data points in A). Different symbols indicate repetitive irradiations. Dose shown is only for the fraction absorbed by water.
silica particles alone as Figure 2 demonstrates. Thus, increasing the silica particles concentration (in the absence of metallic particles) or increasing the silver particles concentration (in the absence of silica)7 increases the catalytic yields, until complete conversion of the reducing radicals to H2 is achieved. The effect of dose rate was studied by irradiations at the low dose rate of 3.0 Gy/min. The results indicate that within experimental error there is no change in the yield of molecular hydrogen produced. In the absence of silver, the organic radicals react with one another either via disproportionation or via dimerization reactions. The results of Figure 3A indicate that there is hardly any catalytic reduction of the water as compared with the unsupported silver NPs (G(H2) ) 0.8 molecules/100 eV at 22.2 wt % of nanocomposite vs G(H2) ) 3.0 molecules/100 eV).7 At the low-concentration range, the average negative charge per nanocomposite particle is much higher than that at high concentrations, leading to a higher overpotential on the particle or higher surface concentration of [•H]ads. Under these conditions, the reduction of water to yield molecular hydrogen is faster and results in higher yields (1.9 and 1.1 for 0.22 and 2.22 wt %, respectively). However, this inverse dependence on particle concentration is not observed for unsupported silver particles. Thus, the deposition of silver on the silica particles significantly affects the overpotential (or other interfacial properties, depending on the intricate details of the mechanism, see below) of the combined microelectrode particle. Similar effects have been recently described for the hydrogenation of nitro compounds catalyzed by gold-titania nanocomposite particles.38 The initial step in the catalytic sequence is the reaction of the •C(CH3)2OH radical with the nanocomposite (reaction 5). This reaction probably proceeds via the formation of a Ag-C σ bond (often denoted [•C(CH3)2OH]ads). It has been shown that when the β radical •CH2CH(CH3)OH reacts with silver NPs the product is propene indicating the formation of intermediates with Ag-C σ-bonds.39 It is reasonable to assume that reaction 5 proceeds via a similar mechanism, that is, via the formation of an intermediate with a Ag-C σ bond. This intermediate can react via one of three plausible routes: (a) the dimerization and/ or disproportionation of the bound [•C(CH3)2OH]ads radicals to produce pinacol or 2-propanol and acetone at the silver surface, Scheme 2; (b) reduction of [•C(CH3)2OH]ads by the negatively charged silica-silver nanocomposite to yield 2-propanol; and
SCHEME 2: Plausible Reaction between Two Adsorbed Ketyl Radicals
(c) C-H bond formation via the reaction between [•C(CH3)2OH]ads and [•H]ads. All of these reactions reduce the yield of molecular hydrogen that is produced via reactions 1 and 5-7. Furthermore, the negative potential of the silica-silver nanocomposites is inversely dependent on the particle’s concentration. However, the rate of pathways a-c does not depend on the overpotential while that of the H2 evolution does. At high concentration, these possible routes (a-c) compete effectively with reactions 5-7 thus reducing the yield of H2 to that expected from reaction 1. These routs can also take place in the reaction of the radicals with silver NPs in the absence of the support, but in that case, the concentration of the NPs is limited and full conversion to H2 is observed; alternatively, the effect of the support on the potential of the silver particle affects these rates. Regardless of the exact mechanism, it is clear that the silica support changes appreciably the catalytic activity of the silver NPs reducing the yield of dihydrogen produced at the high concentrations of the supported NPs. This change is probably due to the fact that the number of radicals bound to each nanocomposite is larger as the concentration of particles decreases. As Figure 3B demonstrates, a pronounced dose effect on hydrogen yields is observed in these suspensions. The dose effect depends on the concentration of the particles, increasing with particle concentrations, Figure 3. At the highest particle concentration studied, hardly any additional hydrogen is released when the particles are irradiated beyond 2.00 kGy. Addition of 2.00 kGy of absorbed dose beyond the first 2.00 kGy does not increase the amount of H2 collected (see limiting amount in Figure 3B). Several processes may lead to this dose dependence. As already discussed, the energy absorbed in the particles generates solvated electrons in the aqueous phase. However,
10466 J. Phys. Chem. C, Vol. 111, No. 28, 2007 the fate of the holes in this case is not clear. Accumulation of holes in the silica support is expected to generate silver ions and thus deteriorate the metallic catalyst and the efficiency of the catalytic hydrogen evolution. Indeed, pronounced changes in the absorption spectrum in the range of the plasmon absorption band were observed following irradiation to these large doses. However, this enhanced recombination process cannot explain the reduction in yield as it is not observed at low particle concentrations. The lack of H2 generation below that expected from the radiolysis of the water phase (0.5 molecules/100 eV) requires another interfering process. Since a large part of that yield is generated in the subpicosecond time regime,40 it is unlikely that any chemical process could interfere with the production of that fraction of the hydrogen. Rather, a reaction of molecular hydrogen needs to be invoked in order to explain the lack of H2 production, not the purely radiolytic fraction. The concentration of H2 produced following 2.00 kGy of irradiation exceeds its solubility in water under standard temperature and pressure (STP) conditions, that is, 8.5 × 10-4 M. It is reasonable that the overpotential required to sustain H2 evolution, at this concentration, at a fast enough rate to compete with the reactions of Scheme 2 cannot be achieved. Plausible reaction pathways to account for loss of H2 include reduction of acetone to 2-propanol by molecular hydrogen at high concentrations. Indeed, preliminary results point out that H2 reduced acetone to 2-propanol in solutions of acetone and to 2-propanol in the presence of SiO2-Ag nanocomposites, that is, the particles catalyze this reaction.39 Reduction of holes that are generated and accumulate within the silica might provide a route for preventing an increase in H2 production but not a destruction of the primary yield from water radiolysis. Conclusions (i) The deposition of silver metal NPs at the surface of colloidal silica is cooperative. Only 25% of the silica particles bind significant amounts of silver under the experimental conditions. Apparently, the presence of a metallic silver particle induces effective further deposition of silver particles on the same silica particle. The origin of this cooperativity (involvement of APS, reducing agent, or other factors) requires a further detailed study. (ii) A modified procedure to synthesize very high concentrations of stable silver NPs has been developed. (iii) The silica support changes appreciably the chemical nature of the silver NPs. Thus, the yield of the dihydrogen produced at the high concentrations of the supported NPs is reduced to that of the primary dihydrogen G(H2) ) 0.45. This means that molecular hydrogen production at the surface of the silver is inhibited. A possible explanation is that the adsorbed silver with active participation of the silica support catalyzes disproportionation and/or dimerization of the radicals. Acknowledgment. Support by the U.S. Department of Energy, Office of Basic Energy Science is gratefully acknowl-
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