Sonochemical Reactions Chemical changes/reactions induced by ultrasound No direct interaction of ultrasound field with molecules (in contrast to photochemistry, ...) •Liquid phase reactions - chemical reactions driven by cavitation effects • Solid state reactions - introduction of defects = speeding up diffusion Sound Sound = pressure waves = periodic compression/expansion cycles traveling through a medium possessing elastic properties (gas, liqud, solid) Liquids and gases - longitudinal pressure waves - compression/rarefaction Solids - longitudinal and transverse waves The energy is propagated as deformations (tensile/compressive stress) in the media The molecules oscillate about their original positions and are not propagated The propagation of a sound wave = the transfer of vibrations from one molecule to another Sound In a typical liquid, the speed of sound decreases as the temperature increases, at all temperatures. The speed of sound in water is almost five times greater than that in air (340 m s-1) Substance Speed of sound, u [m s-1] Air Helium Water Lead Steel Granite 343 965 1482 1960 5960 6000 Speed of Sound The speed of sound (u) u2 = 1/KSp =[dP/dp]S ~ 1/(<(V)2>) ks = adiabatic compressibility p = density P = pressure Sound Intensity Sound Intensity = Power / area = Watts/m2 Source of Sound Jet Airplane 30 m away Air-raid Siren, nearby Threshold of Pain Concert Riveter Busy Traffic Normal Conversations Whisper Threshold of Hearing 0 dB (10-12 W/m2) 10 dB = 10 as intense 20 dB = 102 as intense 30 dB = 103 as intense 120 dB = 1012 as intense Intensity (W/m2) Sound level (dB) 102 1 10-1 ~10-1 10-3 10-5 10-6 10-10 10-12 140 120 120 115 100 70 60 20 0 Acoustic Pressure Pa = PA sin 2n f t Pa acoustic pressure PA pressure amplitude f sound frequency c = X f (for 20 kHz, X = 7.5 cm) Ptotal = Pa + Ph Ph hydrostatic pressure Acoustic Pressure P A = Compression and rarefaction (expansion) regions PA = driving pressure amplitude [Pa] I = irradiation intensity [W m-2] (500 W system - 1.3 105 W m-2) p = liquid density [kg m-3] c = sound velocity in liquid [m s-1] (Water 1482 m s-1) PA = 620 700 Pa = 6.2 bar Ultrasound Utrasound frequencies from 20 kHz to 50 MHz 0 10 —L 10 10 Human hearing ► Conventional powsr ultrasound Extended range for sonochenistry Diagnostic ultrasound frequency, Hz 16Hz- 18kHz 20kHz-100kHz 20kHz - 2MHz 5MHz-10MHz Generation of Ultrasound Transducer - a device converting one type of energy into another gas driven liquid driven electromechanical whistle (F. Galton), liquid atomizer siren liquid whistle homogeniser, a jet of liquid passed through an orifice on a thin metal blade, vibrations, cavitation, mixing of immiscible liquids, ketchup, mayonnaise magnetostrictive, Ni, Co/Fe, Al/Fe, Tb/Dy/Fe alloys shrink when placed in mg. field, solenoid, pulses, upper limit 100 kHz, cooling piezoelectric, oposite charges applied on crystal sides, contraction/expansion, quartz, Pb(Zr/Ti)O3 ceramics (PZT), up to MHz Generation of Ultrasound Sonochemical Reactor Piezoelectric transducer Sonochemical Reactor Ultrasound Processor VCX 500 W Frequency 20 kHz 0 to 40 °C Argon (flow rate 62 cm3 min-1) TIME of ultrasound treatment PULSE irradiation and a dwell time 2:2 TEMP maximum temperature 50 °C AMPL amplitude 50 % Sonochemical Reactor Ti alloy horn, minimum lenght is a half-wavelength of sound in a material, 26 cm for 20 kHz in Ti, multiples of 13 cm vibration amplitude 5 - 50 |j,m Sonochemical Reactor PZT wafers Sandwich transducer operating at 1-200 kHz Hydrodynamic Cavitation the passage of liquid through an orifice plate the kinetic energy/velocity of the liquid increases at the expense of the pressure throttling causes the pressure to fall (Bernoulli) below the threshold pressure for cavitation (vapor pressure) cavities are generated the liquid jet expands, the pressure recovers energetic collapse of the cavities Hydrodynamic Cavitation Lord Rayleigh for the British Admiralty 1895 cavitation erosion of propeller blades Snapping Shrimp snaps a claw shut to create a water jet -speed of 30 m/s, or 100 km/h a drop of the pressure to below the vapor pressure of water - cavitation bubbles acoustic pressures of up to 80 kPa at a distance of 4 cm The pressure wave is strong enough to kill small fish M. Versluis, B. Schmitz, A. von der Heydt, D. Lohse, How Snapping Shrimp Snap: Through Cavitating Bubbles. Science 289, 2114-2117 (2000) Snapping Shrimp D. Lohse, B. Schmitz, M.Versluis, Nature 413, 477-478 (2001) Snapping Shrimp cavitation bubbles D. Lohse, B. Schmitz, M.Versluis, Nature 413, 477-478 (2001) Re I al IV ŕ aCOusliC press Ufa ŕj — n S S B ôí R/RO (bubble radius / inilial radius) Relativa acouslic pressure R/RO (bubble radiue/initial rádius) Acoustic Cavitation Cavitation effects = creation, growth, and implosive collapse of bubbles (1-2 jus) in a liquid = implosion HOT SPOT (1 ns) TRANSIENT CAVITATION: THE ORIGIN OF SONOCHEMISTRY stable cavitation - bubbles oscillate for many cycles transient cavitation - transient cavities expand rapidly collapse violently Acoustic Cavitation Cavitation effects = creation, growth, and implosive collapse of bubbles in a liquid Bubble formation = breakage of liquid during expansion, overcoming tensile strength (pure water 1500 bar, only 6.2 bar available) Weak spots needed = dissolved gas molecules, solid particles, trapped gases Bubble growth (300 jus), energy absorption, size oscillations critical size (170-300 jum) = most efficient energy absorption, rapid growth, inefficient energy absorption, collapse ccmpt&ssim compression compnsssicn compression rwsfactkm rarefaction rarefaction tat&acticn rar&aciior) o o O o 3°ooO O BUBBLE GROWS IN REACHES U™*^^^F SUCCESSIVE CYCLES UNSTABLE SZE Acoustic Cavitation Standing wave Low pressure '—\—1 High pressure I Bubble expansion Bubble collapse Light emisssion Acoustic Cavitation Bubbles collapse = spherically symmetrical implosion, shear forces, adiabatic compression, life time 1-2 jus Hot spot = end of the collapse temperature of the gas inside bubble 5 000 - 20 000 °C (for 1 ns) surrounding liquid layer 2000 °C pressure 500 - 1500 bar Extreme cooling rates 1010 Ks-1 red hot steel poured into water 2500 K s-1 Homogeneous Sonochemistry Two-Site Mechanism Cavity interior Filled with gases and vapors temperatures 5 000 - 20 000 °C pressure 500 - 1500 bar Surrounding liquid layer temperatures 2000 °C Bulk liquid Shock waves, shear forces Homogeneous Sonochemistry ..... JMechanism • • Bubble interior A-B A* B* radicals A* B* exited A-B diffusion of volatile reagents Surrounding interface layer Bulk liquid C-D nonvolatile reagents Shock waves, shear forces How to Measure the Temperature inside a Bubble ? Sonoluminescence - Light generated during the implosive collapse of bubbles in liquids irradiated with ultrasound Kenneth S. Suslick University of Illinois 95% H2SO4(aq.) under Ar 20 kHz (14 W/cm2) Ti horn directly immersed T = 298 K • Apparent blackbody temperature • Ar emission • SO and O2+ emission 8 000 - 15 000 K Temperature/Pressure inside a Bubble Neppiras Equation Tmax T0 Pa (y-1) Q P = Q max z~ V Q y y-i J Pa = acoustic pressure T0 = solution temperature y = Cp/Cv Q = gas pressure inside a bubble upon initiation of the collapse, at its maximum size Gas y = Cp/Cv Kr 1.66 Ar 1.66 He 1.63 O2 1.41 Fate of Bubbles under Ultrasonic Irradiation Ultrasound Bubble nuclei Dissolution \ Fragmentation Coalescence Recti fied diffusion Buo> ancy Resonance size Collapse SL Rectified diffusion - during expansion phase the bubble has larger surface area - more gas diffuses inside than during compression gets out Single Bubble Sonoluminescence SBSL D. F. Gaitan, L. A. Crum, 1990 a method to trap a single sonoluminescing bubble within an acoustic standing wave field Standing acoustic wave field One bubble trapped The bubble oscillates for many cycles Bubble sonoluminescence Single Bubble Sonoluminescence SBSL D. F. Gaitan, L. A. Crum, 1990 Standing acoustic wave field 1 bar One bubble levitates in the acustic field The bubble oscillates for many cycles Bubble sonoluminescence C. A. and V. Bjerknes The force on an object in a liquid depends on its volume and the pressure gradient, the time averaged force drives the bubble towards the antinode of sound pressure and keeps it there. Single Bubble Sonoluminescence SBSL Proper conditions for a single sonoluminescing bubble within an acoustic standing wave field Single Bubble Sonoluminescence SBSL Sonoluminescence Pulses 50 ps Single Bubble Sonoluminescence SBSL Red - MBSL in dodecane Blue - MBSL in water, 16 kHz Green - SBSL in water, 43 kHz Black - blackbody curve for 16200 K Single Bubble Sonoluminescence SBSL Red - bubble radius Green - bubble temperature Blue - acoustic pressure 1.3 bar/25 kHz Multi Bubble Sonoluminescence MBSL Multi-bubble sonoluminescence Spatial and temporal average 250 bar Sonoluminescence Light generated during the implosive collapse of bubbles in liquids irradiated with ultrasound Apparent blackbody temperature (all 4 spectra) 12500 ± 1500 K Sonoluminescence 95% H2SO4(aq.) blackbody temperature Ar emission an optically opaque plasma core Sonoluminescence 95% H2SO4(aq.) SO and O2+ emission with vibronic progression 1580 ±110 K at 3.3 bar 2470 ± 170 K at 4.2 bar 3480 ± 240 K at 5.1 bar Sonofusion Fraud Degassed deuterated acetone (CD3)2CO, 0 °C 4 105 neutrons s-1 Power Measurement in Sonochemistry Calorimetry P = power, W P el = input power to generator P hf = high-freq. power output P th = power input into liquid ± th dT dt cpm Power Measurement in Sonochemistry Calorimetry P = power, W T = temperature, K t = time, s cp = heat capacity, J g-1 K-1 m = mass, g Volume 50 cm3 Argon atmosphere Error 5% heat capacity, J g-1 K-1 Water Tetraglyme 4.2 2.08 P — cpm Power Measurement in Sonochemistry Chemical dosimetry The Weissler reaction Volume 50 cm3 KI 0.1 M CCl4 0.2 cm3 Time 30 min X......= 355 nm •max s = 26303 dm3 mol-1 cm-1 CO4 + H2O 2 KI + O2 I2 + 2 S2O32- Cl2 + CO + 2HCl| I2 + 2 KCl 2 I- + S4O62- Weissler Reaction CO4 + H2O —► Cl2 + CO + 2HCl 2 KI + Cl2 I2 + 2 KCl |l2 + 2 S2O32- —*~ 2 I- + S4O62- 020! 1 0,10. 0 20 40 60 Calorimetrically determined ultrasonic power (W) 80 Power Measurement in Sonochemistry Chemical dosimetry The Fricke reaction Volume 50 cm3 (NH4) Fe(SO4)2.6H2O 0.001 M H2O-► H. + OH. Fe2+ +OH. Fe3+ +OH- H2SO4 0.4 M NaCl 0.001 M Time 30 min Fe3+ A,mav = 304 nm III(I A s = 2197 dm3 mol-1 cm-1 Fricke Reaction H2O-► H* + OH* Fe2+ + OH* Fe3+ +OH-I 1.0x10"*- (a) e.OxlO"5- O". 0.0x10"5- o £ 4.0x10"5- 2.0x10 0.0 10 100 Frequency (kHz) n-1-1—1—i—ry _i_i_1_1_i_i_ 1000 Power Measurement in Sonochemistry Chemical dosimetry Porphyrin decomposition ratio CTPPS 1 - RTPPS = 1 - 0 CTPPS TPPS 3.3 10-6 M Volume 50 cm3 TPPS A,mav = 412 nm 111(1 A 8 = 500000 dm3 mol-1 cm-1 "O3S SO3" SO3" -SO3" Reactor Optimization cavitating bubbles in the optimised cell (water, 20 kHz, Pus = 10 W) and simulated intensity distribution for the same geometry Heterogeneous Sonochemistry Solid surfaces = implosion, microjets, shock waves 200 jum minimum particle size at 20 kHz for microjets surface erosion removal of unreactive coatings (oxides, nitrides, carbonaceous) fragmentation of brittle materials, increased surface area Heterogeneous Sonochemistry Heterogeneous Sonochemistry Solid particles in liquid = shock waves high speed interparticle collisions (500 km/s) surface smoothing, surface coating removal Ni catalytic activity in hydrogenation increased 105 fold by NiO removal localized melting of metal particles at the impact point fragmentation, increased surface area intercalation rates enhanced 200 fold in layered oxides and sulfides (V2O5, MoO3, MoS2, ZrS2, TaS2) Heterogeneous Sonochemistry Metal powders Cr (mp 2130 K) and Mo (mp 2890 K) agglomerate W (mp 3683 K) does not temperature at the point of impact ~ 3000 °C Control of Sonochemical Reactions sound intensity - minimum for cavitation threshold, depends on frequency, optimum intensity for given reaction conditions, at high powers great number of bubbles hinder sound transmission, decoupling of a liquid from the source, breakdown of transducer material, 10 - 100 W cm-2 sound frequency - 20 - 100 kHz, the higher the frequency, the higher power needed to actuate cavitation, stronger cavitation effects, rarefaction phase shortens at high frequency sound attenuation - proportional to the frequency, more power needed at high frequencies Effect of Frequency on Cavitation in Water The frequency dependence of the intensity required to produce cavitation for degassed water at room temperature. The intensity required to produce vaporous cavitation above the frequency of 100 kHz rises rapidly. Control of Sonochemical Reactions volatile reactants - primary reaction site inside the bubbles, diameter 200 jum, 5000 °C, easy bubble formation, more reactant vapors inside bubbles, but the cavitation is cushioned Fe(CO)5 Fe(acac)3 FeSO4 nonvolatile reactants - reaction in the thin layer (200 nm) surrounding the bubble, 2000 °C, less cushioning, more energetic cavitation (collapse) high boiling solvents - high vapor pressure inside the bubble cushions the implosion, nonvolatile solvents give less cushioning, more energetic cavitation less cavitation in viscous liquids, viscosity resists shear forces low surface tension facilitates cavitation, in water add surfactants Control of Sonochemical Reactions temperature - higher temperature increases vapor pressure of a medium, lowers viscosity and surface tension, many bubbles formed at temperatures close to solvent boiling point, a barrier to sound transmission, reaction rates decrease with increasing temperature, more vapors in bubbles ambient gas energy developed on bubble collapse: monoatomic (Ar) > diatomic (N2) > triatomic (CO2) Xe: low thermal conductivity, heat of the collapsing cavity retained He: high thermal conductivity, heat of the collapsing cavity dissipitated, no reaction external pressure - higher pressure suppresses bubble formation but makes cavitation more energetic, optimum pressure for a given frequency Effect of Temperature on Cavitation in Water The effect of temperature on cavitation and its associated hysteresis effect for tap water. The increase in intensity as the temperature is increased can be observed before it falls away at the boiling point. When the temperature is allowed to fall an increase in intensity is found in the region of 50-60 °C. This is quite a significant effect and appears to occur in all liquids. Sonochemical Reactions Solid surfaces = implosion, microjets, shock waves 200 jum minimum particle size at 20 kHz for microjets surface erosion removal of unreactive coatings (oxides, nitrides, carbonaceous) fragmentation of brittle materials, increased surface area Li, Mg, Zn, Al, Cu react at room temperature MCl5 + Na + CO M(CO)5- (M = V, Nb, Ta) Mo + 6 CO — Mo(CO)6 r. t., 1 bar, normally needs 300 bar, 300 °C R2SiCl2 + Li — [-SiR2-SiR2-]n + LiCl monomodal MW distribution Homogeneous Sonochemical Reactions Liquids = heating/cooling by cavity implosions H2O H. + OH. H2 + H2O2 precursor decomposition: metals Fe(CO)5 — Fe + 5 CO oxides Ga3+ + H2O — Ga(O)(OH), diaspore nitrides, carbides, sulfides alkane cracking polymer degradation, lower MW, surface modification emulsification of immiscible liquids (oil-water, Hg-organics, polymer-inorganics) M(acac)n as Precursors Me .o——M—0v O Me Me Me M(acac)3 • Well studied class of compounds •Many elements form acac complexes • Metal complexes - precursors in CVD, sol-gel, thermolysis routes to oxides • Easily chemically modified • Volatile, organics soluble • Nontoxic Chemistry of M(acac)n Precursors Thermal decompositon pathway H H3C CH3 200 °C O C3H4 OO OO M 300 °C CH3COCH3 765 °C MCO3 -► MxOy CO2 Ismail, H. M. J. Ana* Appl. Pyrolysis 1991, 21, 315-326. Ligand Removal by Water 18o H \l/ « M » OO OO CVD OH M18o + 2 O Pinkas, J.; Huffman, J. C.; Baxter, D. V.; Chisholm, M. H.; Caulton, K. G. Chem. Mater. 1995, 7, 1589-1596. Sonochemical Synthesis of Iron Oxide Nanoparticles Fe(CO)s ))))) decaline Cao, X.; Prozorov, R.; Koltypin, Y.; Kataby, G.; Feiner, I.; Gedanken, A. J. Mater. Res. 1997, 12, 402-406. Cao, X.; Koltypin, Yu.; Prozorov, R.; Katabya, G.; Gedanken, A. J. Mater. Chem. 1997, 7, 2447-2451. hexadecane ))))) Amorphous product, by heating to 7CC °C converted to a-Fe2O3 2C-4C nm Nikitenko, S. I.; Moisy, Ph.; Seliverstov, A. F.; Blanc, P.; Madic, C. Ultrasonics Sonochem. 2003, 10, 95-102. Sonochemical Synthesis of Iron Oxide Nanoparticles Amorphous sono-Fe2O3 ))))) TG Fe2O3 maghemite 340 oC dynam/isothermal Composite particles (20-30 nm) Amorphous Fe2O3 particles (2 to 3 nm) Embedded in organic matrix (acetate) Fe203 hematite Defect spinel J. Pinkas, V. Reichlova, R. Zboril, Z. Moravec, P. Bezdicka, J. Matejkova: Sonochemical synthesis of amorphous nanoscopic iron(III) oxide from Fe(acac) Ultrasonic Sonochem. 2008, 15, 256-264 Corundum SEM of Nanoscopic Fe2O3 5.0kV X150,000 100nm WD 3.0mm IR Spectrum of Sono-Fe2O3 1.0 0.9 0.8 0.7 0.6 E 0.5 : 0.4 E 0.3 0.2 0. 1 0. 0 as-synthesized Fe2O3 (red) after calcination to 500 °C (blue) 4000 30 00 2000 Wavenumber (cm-1) 1000 IR Spectrum of Sono-Fe203 Acetate stretching Diketo nate vibr. absent 0.65 = 0.60 = 0.55É 0.50: 0.45: c 0.40: ° 0.35: 0.30: 0.25: 0.20 = 0.15 = 0.10 = 0.05 0.00 = ,(C00) 1566 cm1 A = vas(C00) -vs(C00) = 134 cm-1 T-r -1—I—I—I—I—I—I— 2000 Wavenumber (cm-1) vs(C00) 1432 cm-1 4000 3000 1000 Decomposition of Acac Ligands Speculation about the nature of residual organic groups H3C H2O CH3 H3C C O CH3 Acetone H3C O Fe O Acetate O o / \ Oo \ / O Fe \/ Fe Fe Fe Binding modes of acetate groups H3C-C C- ■H Deacon-Phillips Rules A = vas(COO) -vs(COO) A CH3COO- = 164 cm-1 A larger than ionic form = unidentate A smaller than ionic form = bidentate A comparable to ionic form = bridging CH3 CH3 CH3 O O o' nxo O' x o Fe Fe Fe Fe )eacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 3, 227-250. proves amorphous character Crystallization of Amorphous Fe2O3 under TEM Beam Amorphous Fe2O3 * * ^ M'.' HR-TEM Fe2O3 calcined at 300 °C 5 nm Crystallization induced by heating (300 °C) E nm 20 nm Smaller particle size on calcination - why? 20 nm Specific Surface Area Surface area 48 to 260 m2 g-1 (BET) depending on H2O content BET surface area of the Fe2O3 heated to different temperatures during 12h outgassing periods 220 200 - 180 - < 160 140 - 120 0 50 100 150 200 250 300 350 400 Temperature, °C The oxide surface area increases as the acetate groups are removed, then the particle size increases because ofsintering Composite Particles of Sono-Fe2O3 HR-TEM (5 nm bar) 5 nm nm bar) after heating to 250 °Cv./ Organic matrix >/■ partially removedv.. . ■ Composite Particles of Fe2O3 TEM (10 nm bar) Iron oxide particle size 2 to 3 n Embedded in organic matrix XRD of amorphous Fe2O3 heated dynamically in air 24000 16000 8000 up to 250, 300, and 360 °C 1 4000 3000 Maghemite Y -Fe2O3 2000 6000 4000 2000 20 1 1 I 1 I 1 ^ ^ ^ maghemite 40 360 °C 300 °C 250 °C 60 80 100 29 [deg] TEM of Fe203 Calcined at 600 °C EH Iron oxide particle size 10 to 20 nm 50 nm HT-XRD of Sono-Fe2O3 280 - 390 0C 1OOOO 9000 8000 7000 Hematite 6000 ^^^^ 5000 ■ 4OOO 3000 Pseudo-isothermal 390 oC 380 oC 370 oC 360 oC j*. *4 350 oC ^ 340 oC 2OOO ^^^^ Calcination to 1000 °C 330 oC 320 oC 310 oC 300 oC 290 oC 280 oC 1000 .ptwiJ^-v^^ (pseudo-isothermal heating) provides a different polymorph - Hematite 20.0 30.0 70.0 2Theta Ramp 1 °C min-1, 1 min equilb., 30 min data collect., 10 °C steps Hematite Particle Size coherence length D (nm) 31,0 29,0 27,0 25,0 23,0 21,0 19,0 17,0 15,0 330 380 430 480 T, oC Dependence of the coherence length, D (nm) of a-Fe2O3 on the crystallization temperature under dynamic-isothermal conditions of the HT-XRD measurement