ARTICLE pubs.acs.org/JPCC
Photocatalytic Generation of Hydrogen Using Sonoluminescence and Sonochemiluminescence Leena Dharmarathne, Muthupandian Ashokkumar, and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010 Australia ABSTRACT: Acoustically generated sonoluminescence (SL) and sonochemiluminescence (SCL) have been used to produce molecular hydrogen in a photocatalytic process using Ru(bipy)32+ as a photosensitizer, methyl viologen as an electron relay agent, and colloidal platinum as the catalyst. SL and SCL and various radicals were generated in water using 355 kHz ultrasound. The efficacy of the photocatalytic hydrogen production was to some extent limited by sonochemical side reactions, however, the in situ photon generation was sufficiently intense for pronounced H2 generation above the background sonochemical processes.
’ INTRODUCTION The inertial collapse of a bubble in a liquid can create quite remarkable, albeit short-lived, physicochemical conditions in a fluid system.1,2 Depending on a number of factors, such as the nature of the fluid, the type of solubilized gas in the liquid, the solutes in solution and the temperature of the liquid, bubble core temperatures of several thousand degrees in multibubble systems can be achieved in localized transient “hot spots”. The bubbles needed for this remarkable concentration of energy can be rapidly, and readily, generated on the exposure of a liquid to ultrasound, and this has been increasingly exploited in the field of sonochemistry.1,3 The collapse of “active” acoustic bubbles leads to the production of free radicals, the formation of excited state species, and broad-wavelength light emission in the form of sonoluminescence (SL)46 or chemical reaction-based, giving sonochemiluminescence (SCL).79 In addition, collapsing bubbles can produce shock waves, and asymmetrically collapsing bubbles may generate fluid jets and intense shear flows, all of which can have a severe and damaging impact on surfaces.1,2,10 Studies examining all of these aspects, and more, of acoustic bubbles have provided considerable detail for exploiting cavitation chemistry. Whereas radical production and pyrolysis of material within cavitation hot spots have been widely used for the chemical degradation of organic solutes (typically pollutants1114) and colloid formation and dissolution,1518 the exploitation of the light emitted from collapsing bubbles has had relatively scant attention. We have shown in previous studies1921 that SL can be used to vibronically excite both fluorescence- and phosphorescenceemitting solutes in a process we have named sonophotoluminescence. We have also suggested that, in principle, a practical technique could be developed whereby ultrasound is used to photosensitize a molecule under conditions where photoexcitation by external illumination of the molecule is not possible. In the present study, we have examined the use of sonoluminescence and sonochemiluminescence as in situ photon sources to photocatalytically generate molecular hydrogen in aqueous r 2011 American Chemical Society
solutions. SCL was produced by the reaction of sonochemically produced OH radicals with luminol. Under the conditions used, SCL was significantly more intense than SL. A simplified diagram of the reaction system that we have examined is given in Scheme 1. The diagram shows photons emanating from either SL or SCL, sensitizing Ru(bipy)32+, that in its electronically excited state transfers an electron to methyl viologen, MV2+, an electron relay agent. The reduced methyl viologen (the methyl viologen radical cation, MV+•) in turn transfers an electron to colloidal Pt, from which H2 is catalytically generated. The EDTA is used as a sacrificial electron donor to ensure Ru(bipy)32+ is regenerated, and the cyclical excitation and electron transfer processes continue. The reaction mechanism and others very similar to that of Scheme 1 have been extensively studied as part of solar energy research dealing with the conversion of sunlight into a chemical fuel.2224
’ EXPERIMENTAL SECTION Experimental Arrangement. Reaction solutions were contained in a sealable quartz cell (4 mL) fitted with a sparging attachment (Figure 1). This cell was mounted at a fixed position in a sonication bath containing 250 mL of water. The experimental samples were sonicated using an ELAC Nautik USW-51052 ultrasonic transducer (plate diameter of 54.5 cm) operating at 355 kHz, 60 W/cm2, and the experimental temperature was maintained at 20 °C using a thermostatted water bath. Chemicals. H2PtCl6, NaBH4, PVA (98% hydrolyzed; MW = 106110 kg/mol), Ru(bipy)3Cl2, 1,10 -dimethyl-4,40 -bipyridinium dichloride (methyl viologen (MV2+(2Cl))), and luminol were supplied by Sigma-Aldrich; isopropyl alcohol (IPA), by Acros-Organics; NaOH, by Chem-Supply; and EDTA, by Merck. The high-purity argon gas was supplied by BOC. Received: October 16, 2011 Revised: December 6, 2011 Published: December 10, 2011 1056
dx.doi.org/10.1021/jp209946s | J. Phys. Chem. C 2012, 116, 1056–1060
The Journal of Physical Chemistry C
ARTICLE
Scheme 1. Schematic Representation of the Photocatalytic Generation of Molecular Hydrogen from in Situ Sonoluminescence and Sonochemiluminescence Using a Photosensitiser (Ru(bipy)32+), an Electron Relay Agent (MV2+), a Sacrificial Electron Donor (EDTA), and a Colloidal Electrode (Pt)a
a
EDTAox represents all species that may be formed once EDTA is oxidized. The two stages delineate the photo production of a radical intermediate (MV+•) and the catalytic production of H2.
Figure 1. Experimental arrangement showing the reaction cell positioned in a water bath that is directly coupled to an ultrasonic generator. Circulating cooling water surrounding the bath maintains a constant temperature for the complete reaction system (cell and bath). The complete setup was housed in a black box to eliminate any room light during sonication.
Synthesis of Pt0 Nanoparticles (NPs). The Pt0 NPs were
synthesized by reducing H2PtCl6 using NaBH4. In a typical synthesis, 1.0 mM H2PtCl6 (10.0 mL) containing 40 mg of poly(vinyl alcohol) (PVA) was vigorously mixed with ice cold 2.0 mM NaBH4 (30 mL) in a 1:3 volume ratio, and the solution was then slowly heated to 80 °C. The solution color turned to dark black at 6080 °C, and then the solution was cooled to room temperature. The TEM-determined diameter of the Pt0 particles was in the range of 520 nm. Preparation of Samples for the SL and SCL Experiments. The reaction solution inside the quartz cell consisted of 0.1 mM Pt0 (stabilized with 0.1 wt % PVA), 0.1 mM Ru(bipy)32+, 15 mM MV2+, 30 mM EDTA, and 100 mM IPA. Solutions were sparged with high-purity argon gas for 15 min prior to sonication and then sealed via a stopcock. [Under these solution conditions, 80% of the fluorescence from electronically excited Ru(bipy)32+ is quenched by the 15 mM MV2+ in solution, in perfect agreement with the work in ref 25.]. The same procedure was followed in the sonochemical production of H2 in the absence of
Figure 2. MV+• formation in the reaction cell as a function of sonication time for various sample solutions and sonication bath conditions. The inset shows the relative emission intensity of the SL and SCL in the bath for the conditions used. Photometer measurement of the SCL was 0.5 μW, compared with ambient room light of 70100 μW. (The sonication of Ru(bipy)32+, MV2+ solutions on their own or in the presence of IPA or EDTA for 60 min resulted in an ∼5% loss of these compounds.) [All sample solutions were argon-sparged and contained, with the exception of system a, 0.1 mM Ru(bipy)32+/30 mM EDTA/15 mM MV2+/100 mM IPA]. Systems: (a) 0.1 mM Ru(bipy)32+/30 mM EDTA/15 mM MV2+ (note: no IPA in this sample) with air-saturated water bath; (b) air-saturated water bath containing 50 mM IPA; (c) airsaturated water bath; (d) argon-saturated water bath; (e) air-saturated luminol solution at pH = 11 in bath.
Figure 3. H2 formation in the reaction cell as a function of sonication time under various sonication bath conditions. The reaction cell sample solution was argon-saturated 0.1 mM Ru(bipy)32+/30 mM EDTA/ 15 mM MV2+/100 mM IPA/0.1 wt % PVA-stabilized 0.1 mM Pt0. Relative emission intensities are the same as in the inset of Figure 2. Systems: (a) air-saturated water bath containing 50 mM IPA; (b) airsaturated water bath; (c) argon-saturated water bath; (d) air-saturated luminol solution at pH = 11 in the bath. The error bar shown is typical of the experimental variation in the measurements on duplicate experiments. [Inset shows the comparative H2 yields at 60 min of sonication for each of the solution systems ad.]
Ru(bipy)32+ and EDTA. The solution pH of the reaction samples was 4.6. Preparation of Solutions for the Sonication Bath. A 250 mL portion of Milli-Q water (conductivity
