Photocatalytic properties of CuO/(001)‐TiO2 composites synthesized by the vapor–thermal method

Composites of (001)‐face‐exposed TiO₂ ((001)‐TiO₂) and CuO were synthesized in water vapor environment at 250°C with various Cu/Ti molar ratios (R_Cu/Ti). The resulting CuO/(001)‐TiO₂ composites were characterized using a variety of techniques. The synthesis under high‐temperature vapor allows close contact between CuO and (001)‐TiO₂, which results in the formation of heterojunctions, as evidenced by the shift of valence band maximum towards the forbidden band of TiO₂. An appropriate ratio of CuO can enhance the absorption of visible light and promote the separation of photogenerated carriers, which improve the photocatalytic performance. The degradation rate constant K_app increased from 5.5 × 10^?2 to 8.1 × 10^?2 min^?1 for R_Cu/Ti = 0.5. Additionally, the results showed that superoxide radicals (^?O₂^?) play a major role in the photocatalytic degradation of methylene blue.     

Dye degradation studies of Mo‐doped TiO2 thin films developed by reactive sputtering

TiO₂ thin films with various Mo concentrations have been deposited on glass and n‐type silicon (100) substrates by this radio‐frequency (RF) reactive magnetron sputtering at 400°C substrate temperature. The crystal structure, surface morphology, composition, and elemental oxidation states of the films have been analyzed by using X‐ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy, respectively. Ultraviolet‐visible spectroscopy has been used to investigate the degradation, transmittance, and absorption properties of doped and undoped TiO₂ films. The photocatalytic degradation activity of the films was evaluated by using methylene blue under a light intensity of 100 mW cm⁻². The X‐ray diffraction patterns show the presence of anatase phase of TiO₂ in the developed films. X‐ray photoelectron spectroscopy studies have confirmed that Mo is present only as Mo⁶⁺ ions in all films. The Mo/TiO₂ band gap decreases from ~3.3 to 3.1 eV with increasing Mo dopant concentrations. Dye degradation of ~60% is observed in Mo/TiO₂ samples, which is much higher than that of pure TiO₂.     

Sol–gel preparation and properties of hydroxypropylcellulose–titania hybrid thin films

Hydroxypropylcellulose (HPC)–titania hybrid thin films were prepared by sol–gel method where titanium tetraisopropoxide Ti(OC₃H₇ ⁱ )₄ was hydrolyzed under acidic conditions in the presence of HPC, followed by dip-coating and drying at 120 °C for 24 h. The viscosity average molecular weight of HPC was 55,000–70,000 or 110,000–150,000, and the TiO₂/(HPC + TiO₂) mass ratio ranged from 0 to 1, which was calculated on the assumption that all Ti(OC₃H₇ ⁱ )₄ is converted into TiO₂. The films were 0.35–1.0 μm thick, transparent in visible region and opaque in ultraviolet (UV) region, where the optical absorption coefficient in UV region increased with increasing titania content. The refractive index increased with increasing titania content, ranging from 1.6 to 1.8 for the hybrid thin films. The pencil hardness increased from 6B to 5H, the durability in hot water significantly increased and the contact angle of water on films increased from 35° to 89° with increasing titania content. Crack-free films could be deposited on organic polymer substrates irrespective of titania or HPC contents, where cracking did not occur at higher HPC contents even when the substrate was bent.     

The Dependence on H2O and on NH3 of the Kinetics of the self-reaction of HO2 in the gas-phase formation of HO2·H2O and HO2·NH3 complexes

Electron pulse radiolysis at ?298°K of 2 atm H₂ containing 5 torr O₂ produces HO₂ free radical whose disappearance by reaction (1), HO₂ + HO₂ →H₂O₂ + O₂, is monitored by kinetic spectrophotometry at 230.5 nm. Using a literature value for the HO₂ absorption cross section, the values k₁ = 2.5×10^?12 cm³/molec·sec, which is in reasonable agreement with two earlier studies, and G(H) G(HO₂) ?13 are obtained. In the presence of small amounts of added H₂O or NH₃, the observed second-order decay rate of the HO₂ signal is found to increase by up to a factor of ?2.5. A proposed kinetic model quantitatively explains these data in terms of the formation of previously unpostulated 1:1 complexes, HO₂ + H₂O ? HO₂·H₂O (4a) and HO₂ + NH₃? HO₂·NH₃ (4b), which are more reactive than uncomplexed HO₂ toward a second uncomplexed HO₂ radical. The following equilibrium constants, which agree with independent theoretical calculations on these complexes, are derived from the data: 2×10^?20?K_4a?6.3 × 10^?19 cm³/molec at 295°K and K_4b = 3.4 × 10^?18 cm³/molec at 298°K. Several deuterium isotope effects are also reported, including k_H/k_D = 2.8 for reaction (1). The atmospheric significance of these results is pointed out.     

Features of the potential energy surface for the reaction of HO2 radical with acetone

The mechanism of the reaction of acetone with HO₂ has been studied by quantum chemical computations. Different stationary points on the potential energy surface (PES) of the reaction have been characterized. These stationary points are the reactants, products, molecular complexes, and transition states. Three pathways have been studied: two H‐abstraction channels and one HO₂‐addition channel. The MP2 level of theory with the 6‐311G(d,p) basis set was employed for geometry optimization. The electronic energies was obtained at the PMP2, PMP4, and CCSD(T) level of theory with the 6‐311G(d,p) basis set on the computed geometries. The addition pathway is clearly the more favorable, contrary to the acetone + OH system. The pre‐reactive hydrogen‐bonded complexes have been characterized and show a large red shift between the O? H stretching frequency in the HO₂ radical and the one in the HO₂ fragment of intermolecular complexes. Our addition rate constant k₊ at T = 298 K (3.49 × 10^?16 cm³ s^?1) is consistent with previous experimental results (giving an upper limit of the rate constant of 6 × 10^?16 cm³ s^?1 at 298 K). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Improved photocatalytic performance of anatase TiO2 synthesized through ethanol supercritical drying technique

Anatase titania (TiO₂) nanoparticles were synthesized via a self‐developed ethanol vapor‐thermal method at 240°C (T240) and 250°C (T250), i.e. at temperatures lower and higher, respectively, than the supercritical temperature (243.5°C, 7.0 MPa) of ethanol. Compared to T240, T250 exhibited a higher ratio of exposed (001) facets, oxygen vacancies, and concomitant TiO_x. The specific surface area of T250 was 119.0 m² g^?1, smaller than that of T240 (144.2 m² g^?1). During the degradation of methylene blue, T250 exhibited a high apparent rate constant (K_app) of 14.5 × 10^?2 min^?1, which was 6.3 times larger than that for T240. Furthermore, compared to T240, T250 exhibited better performance toward degradation of phenol. Results of electron spin resonance spectroscopy and photoluminescence indicated that the photogenerated electron–hole pairs possessed higher separation efficiency for T250 than for T240. In summary, the excellent photocatalytic performance of T250 originates from the higher ratios of exposed (001) facets, oxygen vacancies or TiO_x, C═O groups adsorbed at the surface of particles, and higher separation efficiency of photogenerated electron–hole pairs. By employing this self‐developed vapor‐thermal method, a variety of catalysts and their composites can be synthesized, which may exhibit novel morphological characteristics and properties as well as excellent photocatalytic performance.     

Kinetics and mechanism of the reaction of Cl atoms with HO2 radicals

The kinetic and mechanism of the reaction Cl + HO₂ → products (1) have been studied in the temperature range 230–360 K and at total pressure of 1 Torr of helium using the discharge‐flow mass spectrometric method. The following Arrhenius expression for the total rate constant was obtained either from the kinetics of HO₂ consumption in excess of Cl atoms or from the kinetics of Cl in excess of HO₂: k₁ = (3.8 ± 1.2) × 10^?11 exp[(40 ± 90)/T] cm³ molecule^?1 s^?1, where uncertainties are 95% confidence limits. The temperature‐independent value of k₁ = (4.4 ± 0.6) × 10^?11 cm³ molecule^?1 s^?1 at T = 230–360 K, which can be recommended from this study, agrees well with most recent studies and current recommendations. Both OH and ClO were detected as the products of reaction (1) and the rate constant for the channel forming these species, Cl + HO₂ → OH + ClO (1b), has been determined: k_1b = (8.6 ± 3.2) × 10^?11 exp[?(660 ± 100)/T] cm³ molecule^?1 s^?1 (with k_1b = (9.4 ± 1.9) × 10^?12 cm³ molecule^?1 s^?1 at T = 298 K), where uncertainties represent 95% confidence limits. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 317–327, 2001     

Effect of heat treatment on electrochromic properties of TiO<Subscript>2</Subscript> thin films

Electrochromic titanium oxide (TiO₂) films were deposited on ITO/glass substrates by chemical solution deposition (CSD). The stock solutions were spin-coated onto substrates and then heated at various temperatures (200–500 °C) in various oxygen concentrations (0–80%) for 10 min. The effects of the processing parameters on the electrochromic properties of TiO₂ films were investigated. X-ray diffraction measurements demonstrated that the amorphous TiO₂ films were crystallized to form anatase films above 400 °C. The electrochromic properties and transmittance of TiO₂ films were measured in 1 M LiClO₄–propylene carbonate (PC) non-aqueous electrolyte. An amorphous 350 nm-thick TiO₂ film that was heated at 300°C in 60% ambient oxygen exhibited the maximum transmittance variation (ΔT%), 14.2%, between the bleached state and the colored state, with a ΔOD of 0.087, Q of 10.9 mC/cm², η of 7.98 cm²/C and x in Li _x ClO₄ of 0.076 at a wavelength (λ) of 550 nm.     

Preparation and Photocatalytic Activity of TiO2 Films on Silica Glass Substrates by Assembly Technique

Nanostructured TiO₂ coating films on silica glass substrates were prepared by the assembly technique. TiO₂ colloids were synthesized employing the sol‐gel method using TiCl₄ as a precursor. The effect on the surface structures which was caused by the polyethylene glycol (PEG) added to the precursor solution and the photocatalytic activity were studied. The experimental results showed that the cobble‐like TiO₂ coating films were synthesized at 500 °C. On the surface of the samples, TiO₂ films exhibited uniform shape and a narrow size distribution. The result of proper PEG added to the precursor solution led to the decreasing of the size of TiO₂ particles and the increasing of the surface area of the samples. The photocatalytic activity of TiO₂ films with PEG was higher than that of samples without PEG.     

Substrate temperature influenced physical properties of silicon MOS devices with TiO2 gate dielectric

DC reactive magnetron sputtering technique was employed for deposition of titanium dioxide (TiO₂) films. The films were formed on Corning glass and p‐Si (100) substrates by sputtering of titanium target in an oxygen partial pressure of 6×10^?2 Pa and at different substrate temperatures in the range 303 – 673 K. The films formed at 303 K were X‐ray amorphous whereas those deposited at substrate temperatures ≥ 473 K were transformed into polycrystalline nature with anatase phase of TiO₂. Fourier transform infrared spectroscopic studies confirmed the presence of characteristic bonding configuration of TiO₂. The surface morphology of the films was significantly influenced by the substrate temperature. MOS capacitor with Al/TiO₂/p‐Si sandwich structure was fabricated and performed current–voltage and capacitance–voltage characteristics. At an applied gate voltage of 1.5 V, the leakage current density of the device decreased from 1.8 × 10^?6 to 5.4 × 10^?8 A/cm² with the increase of substrate temperature from 303 to 673 K. The electrical conduction in the MOS structure was more predominant with Schottky emission and Fowler‐Nordheim conduction. The dielectric constant (at 1 MHz) of the films increased from 6 to 20 with increase of substrate temperature. The optical band gap of the films increased from 3.50 to 3.56 eV and refractive index from 2.20 to 2.37 with the increase of substrate temperature from 303 to 673 K. Copyright © 2012 John Wiley & Sons, Ltd.     

Removal of dithioterethiol (DTT) from water by membranes of cellulose acetate (AC) and AC doped ZnO and TiO2 nanoparticles

Dithioterethiol (DTT) is a typical example of substances that contain sulfur with adverse effects on human health. Membranes-based cellulose acetate is used for the separation processes of thiols after the addition of ZnO and TiO₂ nanoparticles. The measurement of permeability allows us to estimate the efficiency of membrane cleaning. The permeability increases from 8.82 L.h^?1.m^?2.bar^?1 for CA membrane to 20.77 L.h^?1.m^?2.bar^?1 for CA-TiO₂ and 21.96 L.h^?1.m^?2.bar^?1 for CA-ZnO membranes. For the permeability values of DTT, we noted that the CA-ZnO membrane has the highest permeability (50.66 L.h^?1.m^?2.bar^?1). The CA-ZnO membrane changes from nanofiltration to ultrafiltration membrane. On the other hand, for the CA-TiO₂ modified membrane, the permeability decreases to 6.00 L.h^?1.m^?2.bar^?1. The CA-TiO₂ membrane is in the category of reverse osmosis membranes. This variation is explained by the interaction between nanoparticles and DTT. The contact angles of the incorporated membranes decrease progressively with the addition of TiO₂ or ZnO-NPs. The low contact angle with water means high hydrophilicity, indicated that the addition of TiO₂ and ZnO improved the hydrophilicity of the membranes. The CA membrane had the highest contact angle with water of 92.64 ± 1.5°. After the addition of 0.1 g of TiO₂ or ZnO, the contact angle of CA-TiO₂ and CA-ZnO was reduced to 86.7 ± 0.2° and 70.51 ± 1.5°, respectively. Both TiO₂ and ZnO caused strong hydrophilicity of membranes. From the elimination rates of DTT, it is concluded that there are optimal conditions of (1) Pressure P = 2 bars, (2) pH = 10 and (3) DTT concentration = 2 mM.     

The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4− in Superoxide Dismutation

The reaction of OH^? with O₃ eventually leads to the formation of ^.OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982 , 16, 676–681), it was suggested that the first step occurred by O transfer: OH^?+O₃→HO₂^?+O₂ and that ^.OH was generated in the subsequent reaction(s) of HO₂^? with O₃ (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one‐step O transfer such as that mentioned above would require the release of O₂ in its excited singlet state (¹O₂, O₂(¹Δ_g)); this state lies 95.5 kJ mol^?1 above the triplet ground state (³O₂, O₂(³Σ_g^?)). The low experimental rate constant of 70 M ^?1 s^?1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH^? with O₃ to form an adduct (OH^?+O₃→HO₄^?; ΔG=3.5 kJ mol^?1) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (ΔG=26.7 kJ mol^?1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO₄^? have been probed: HO₄^?→HO₂^?+¹O₂ is spin allowed, but markedly endergonic (ΔG=23.2 kJ mol^?1). HO₄^?→HO₂^?+³O₂ is spin forbidden (ΔG=?73.3 kJ mol^?1). The decay into radicals, HO₄^?→HO₂^.+O₂^.?, is spin allowed and less endergonic (ΔG=14.8 kJ mol^?1) than HO₄^?→HO₂^?+¹O₂. It is thus HO₄^?→HO₂^.+O₂^.? by which HO₄^? decays. It is noted that a large contribution of the reverse of this reaction, HO₂^.+O₂^.?→HO₄^?, followed by HO₄^?→HO₂^?+³O₂, now explains why the measured rate of the bimolecular decay of HO₂^. and O₂^.? into HO₂^?+O₂ (k=1×10⁸ M ^?1 s^?1) is below diffusion controlled. Because k for the process HO₄^?→HO₂^.+O₂^.? is much larger than k for the reverse of OH^?+O₃→HO₄^?, the forward reaction OH^?+O₃→HO₄^? is practically irreversible.     

Does the HO2 radical react with H2S,CH3SH,and CH3SCH3?

The kinetics of the title reactions were investigated in a discharge flow tube by using laser magnetic resonance detection of HO₂. The upper limits for the bimolecular rate constants for the reactions of HO₂ with H₂S (k₁), CH₃SH (k₂), and CH₃SCH₃ (k₃) are <3 × 10^?15, <4 × 10^?15, and <5 × 10^?15 cm³ molecule^?1 s^?1, respectively, at 298 K. Our upper limit for k₁ is three orders of magnitude lower than the previously reported value. Measurements at higher temperatures also yield similar upper limits. Our results suggest that HO₂ is not an important oxidant for these reduced compounds in the atmosphere. © 1994 John Wiley & Sons, Inc.     

Electrochemical Fabrication and Properties of Highly Ordered Fe‐Doped TiO2 Nanotubes

Highly‐ordered Fe‐doped TiO₂ nanotubes (TiO₂nts) were fabricated by anodization of co‐sputtered Ti–Fe thin films in a glycerol electrolyte containing NH₄F. The as‐sputtered Ti–Fe thin films correspond to a solid solution of Ti and Fe according to X‐ray diffraction. The Fe‐doped TiO₂nts were studied in terms of composition, morphology and structure. The characterization included scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray diffraction, UV/Vis spectroscopy, X‐ray photoelectron spectroscopy and Mott–Schottky analysis. As a result of the Fe doping, an indirect bandgap of 3.0 eV was estimated using Tauc’s plot, and this substantial red‐shift extends its photoresponse to visible light. From the Mott–Schottky analysis, the flat‐band potential (E_fb) and the charge carrier concentration (N_D) were determined to be ?0.95 V vs Ag/AgCl and 5.0 ×10¹⁹ cm^?3 respectively for the Fe‐doped TiO₂nts, whilst for the undoped TiO₂nts, E_fb of ?0.85 V vs Ag/AgCl and N_D of 6.5×10¹⁹ cm^?3 were obtained.     

Experimental determination of the rate constants of the reactions of HO2 + DO2 and DO2 + DO2

The rate constants of the reactions of DO₂ + HO₂ (R1) and DO₂ + DO₂ (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO₂ and DO₂ by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO₂ was generated by photolyzing Cl₂ in the presence of CH₃OH and O₂. Low concentrations of DO₂ were generated simultaneously by adding low concentrations of D₂O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH₃OD and thus formation of DO₂. Excess DO₂ was generated by photolyzing Cl₂ in the presence of CD₃OD and O₂, small concentrations of HO₂ were always generated simultaneously by isotopic exchange between CD₃OD and residual H₂O. The rate constant k₁ at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D₂O concentration k₁ = (1.67 ± 0.03) × 10⁻¹² × (1 + (8.2 ± 1.6) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The rate constant for the DO₂ self-reaction k₂ has been measured under excess DO₂ concentration, and the DO₂ concentration has been determined by fitting the HO₂ decays, now governed by their reaction with DO₂, to the rate constant k₁. A rate constant with insignificant pressure dependence was found: k₂ = (4.1 ± 0.6) × 10⁻¹³ (1 + (2 ± 2) × 10⁻²⁰ cm³ × [He] cm⁻³) cm³ s⁻¹ as well as an increase of k₂ with increasing D₂O concentration was observed: k₂ = (4.14 ± 0.02) × 10⁻¹³ × (1 + (6.5 ± 1.3) × 10⁻¹⁸ cm³ × [D₂O] cm⁻³) cm³ s⁻¹. The result for k₂ is in excellent agreement with literature values, whereas this is the first determination of k₁.     

Single-step sol-gel deposition and dielectric properties of 0.4 μm thick, (001) oriented Pb(Zr,Ti)O<Subscript>3</Subscript> thin films

Single-step sol–gel deposition was attempted for realizing submicron thick, (001) oriented Pb(Zr_0.53Ti_0.47)O₃ (PZT) thin films, using an alkoxide solution containing polyvinylpyrrolidone (PVP). A solution of molar composition, Pb(NO₃)₂:Zr(OC₃H₇ ⁿ)₄:Ti(OC₃H₇ ⁱ)₄:PVP:H₂O:CH₃COCH₂COCH₃:CH₃OC₂H₄OH:C₃H₇ ⁿOH = 1.1:0.53:0.47:0.5:5:0.5:22:0.98, was prepared as a coating solution. Gel films were prepared on Pt(111)/TiO₂/SiO₂/Si(100) substrates by spin-coating, and calcined at 350 °C and annealed at 650 °C either in an electric furnace or in a near-infrared (IR) furnace. When calcined in the near-IR furnace, the films became (001) oriented on annealing. When calcined in the electric furnace, on the other hand, the films became randomly oriented on annealing. These observations indicate that the heating the gel films from the substrate side in the calcination step at 350 °C induces crystallographic orientation in the annealing step at 650 °C. The effects of the heating methods on the thermal decomposition of the gel films, and the microstructure and dielectric properties of the fired films were studied. Finally 0.4 μm thick, (001) oriented PZT films could be successfully prepared by non-repetitive, single-step deposition. The oriented film thus obtained had the remnant polarization 2P _r of 39 μC/cm² and the dielectric constant ε′ of 960 ± 169.     

Yields of O2(b1Σ) from reactions of HO2

A discharge-flow apparatus with resonance fluorescence and chemiluminescence detection has been used to monitor O₂(b ¹σ) production from several reactions of the HO₂ radical at 300 K and 1-torr total pressure. O₂(b), HO₂, and OH were observed when F atoms were added to H₂O₂ in the gas phase. Signal strengths of O₂(b) were proportional to initial concentrations of H₂O₂ and HO₂. These observations were analyzed by using a simple three step mechanism and a more complete computer simulation with 22 reaction steps. The results indicate that the F + HO₂ reaction yields O₂(b) with an efficiency of (3.6 ± 1.4) × 10^?3. By monitoring [O₂(b)] and [HO₂] upon addition of an excess second reactant to HO₂, O₂(b) yields from the reactions of HO₂ with O, Cl, D, H, and OH were found to be <1 × 10^?2, <5 × 10^?4, <2 × 10^?3, <8 × 10^?3, and <1 × 10^?3, respectively. Yields of O₂(b) from the HO₂ ± HO₂ reaction were found to be less than 3 × 10^?2.     

Study of the kinetics of the reactions of NO3 with HO2 and OH

The Absolute rate constants for the gas-phase reactions of NO₃ with HO₂ and OH have been determined using the discharge flow laser magnetic resonance method (DF-LMR). Since OH was found to be produced in the reaction of HO₂ with NO₃, C₂F₃Cl was used to scavenge it. The overall rate constant, k₁, for the reaction, HO₂ + NO₃ → products, was measured to be k₁=(3.0 ± 0.7)×10^?12 cm³ molecule^?1 s^?1 at (297 ± 2) K and P=(1.4 – 1.9) torr. This result is in reasonable agreement with the previous studies. Direct detection of HO₂ and OH radicals and the use of three sources of NO₃ enabled us to confirm the existence of the channel producing OH:HO₂+NO₃→OH+NO₂+O₂ (1a); the other possible channel is HO₂+NO₃→HNO₃+O₂ (1b). From our measurements and the computer simulations, the branching ratio, k_1a/(k_1a + k_1b), was estimated to be (1.0). The rate coefficient for the reaction of OH with NO₃ was determined to be (2.1 ± 1.0) × 10^?11 cm³ molecule^?1 s^?1. © 1993 John Wiley & Sons, Inc.     

Is the HO4− Anion a Key Species in the Aqueous‐Phase Decomposition of Ozone?

The role of the HO₄^? anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O₃ + HO^? or HO₂ + O₂^?. The determination of the stability of HO₄^? in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short‐lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug‐cc‐pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO₄^? anion has a planar structure in the gas phase, with a very large HOO? OO bond length (1.823 Å). In contrast, HO₄^? adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 Å for the HOO? OO bond length) with a very small energy cost. The formation of the HO₄^? anion is predicted to be endergonic by 5.53±1.44 and 2.14±0.37 kcal mol^?1 with respect to the O₃ + HO^? and HO₂ + O₂^? channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O₃ + OH^? → HO₂ + O₂^?) is endergonic by 3.39±1.80 kcal mol^?1, the electron transfer process (O₃ + O₂^? → O₃^? + O₂) is exergonic by 31.53±1.05 kcal mol^?1, supporting the chain‐carrier role of the superoxide ion, and the reaction O₃ + HO₂^? → OH + O₂^? + O₂ is exergonic by 12.78±1.15 kcal mol^?1, which is consistent with the fact that the addition of small amounts of HO₂^? (through H₂O₂) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO₄^? anion as a key reaction intermediate.     

Effect of inorganic oxide supports on the activity of chromium‐based catalysts in ethylene trimerization

Novel heterogeneous catalysts were prepared using immobilization of bis(2‐decylsulfanylethyl)amine–CrCl₃ (Cr‐SNS‐D) on various supports, namely commercial TiO₂, Al₂O₃ and magnetic Fe₃O₄@SiO₂ nanoparticles, to yield solid catalysts denoted as support@Cr‐SNS‐D. The structure of the catalysts was confirmed on the basis of spectroscopic analyses, N₂ adsorption–desorption and inductively coupled plasma (ICP) analysis. The surface areas of Al₂O₃@Cr‐SNS‐D, Fe₃O₄@SiO₂@Cr‐SNS‐D and TiO₂@Cr‐SNS‐D catalysts were determined to be 70, 23 and 41 m² g^?1, respectively. A decrease in surface area from that of the supports clearly establishes accurate immobilization of Cr‐SNS‐D catalyst on the surface of the parent carriers. The loading of Cr was determined to be 0.02, 0.16 and 0.11 mmol g^?1 for Cr‐SNS‐D supported on TiO₂, Al₂O₃ and Fe₃O₄@SiO₂, respectively, using ICP analysis. After preparation and full characterization of the catalysts, ethylene trimerization reaction was accomplished in 40 ml of dry toluene, at 80°C and 25 bar ethylene pressure and in the presence of methylaluminoxane (Al/Cr = 700) within 30 min. The supported chromium catalysts were found to be efficient and selective for the ethylene trimerization reaction. The highest activity (74 650 g_1‐hexene g_Cr^?1 h^?1), as well as no polyethylene formation during reaction processes, was observed when TiO₂ was used as the catalyst support.