Combinatorial doping of TiO_2 with platinum (Pt), chromium (Cr), vanadium (V), and nickel (Ni) to achieve enhanced photocatalytic activity with visible light irradiation

Titanium dioxide (TiO_2) was doped with the combination of several metal ions including platinum (Pt), chromium (Cr), vanadium (V), and nickel (Ni). The doped TiO_2 materials were synthesized by standard sol-gel methods with doping levels of 0.1 to 0.5 at.%. The resulting materials were characterized by x-ray diffraction (XRD), BET surface-area measurement, scanning electron microscopy (SEM), and UV-vis diffuse reflectance spectroscopy (DRS). The visible light photocatalytic activity of the codoped samples was quantified by measuring the rate of the oxidation of iodide, the rate of degradation of methylene blue (MB), and the rate of oxidation of phenol in aqueous solutions at λ > 400 nm. 0.3 at.% Pt-Cr-TiO_2 and 0.3 at.% Cr-V-TiO_2 showed the highest visible light photocatalytic activity with respect to MB degradation and iodide oxidation, respectively. However, none of the codoped TiO_2 samples were found to have enhanced photocatalytic activity for phenol degradation when compared to their single-doped TiO_2 counterparts

Effects of the preparation method of the ternary CdS/TiO_2/Pt hybrid photocatalysts on visible light-induced hydrogen production

A variety of combinations of CdS, TiO2, and Pt in preparing the hybrid catalysts were studied for hydrogen production under visible light ( > 420 nm) irradiation. The preparation method sensitively influenced the activity of the ternary hybrid catalysts. The formation of the potential gradient at the interface between CdS and TiO2 is necessary in achieving the efficient charge separation and transfer and how the platinum as a cocatalyst is loaded onto the CdS/TiO2 hybrid catalysts determines the overall hydrogen production efficiency. The common method of photoplatinization of CdS/TiO2 hybrid [Pt-(CdS/TiO2)] was much less efficient than the present method in which Pt was photodeposited on bare TiO2, which was followed by the deposition of CdS [CdS/(Pt-TiO2)]. The CdS/(Pt-TiO2) has the hydrogen production rate ranging (6–9) × 10-3 mol h-1 g-1, which is higher by a factor of 3–30 than that of Pt-(CdS/TiO2). The photocatalytic activity of the ternary hybrid catalysts was extremely sensitive to where the platinum is loaded. The photoactivity of the hybrid catalyst was also assessed in terms of the photocurrent collected by the methyl viologen electron shuttle in the catalyst suspension. CdS/(Pt-TiO2) generated higher photocurrents than Pt-(CdS/TiO2) by a factor of 2–7. The extreme sensitivity of the preparation method to the hydrogen production activity should be taken into account when hybrid photocatalysts are designed and prepared

Effects of Single Metal-Ion Doping on the Visible-Light Photoreactivity of TiO_2

Titanium dioxide (M-TiO_2), which was doped with 13 different metal ions (i.e., silver (Ag^+), rubidium (Rb^+), nickel (Ni^(2+)), cobalt (Co^(2+)), copper (Cu^(2+)), vanadium (V^(3+)), ruthenium (Ru^(3+)), iron (Fe^(3+)), osmium (Os^(3+)), yttrium (Y^(3+)), lanthanum (La^(3+)), platinum (Pt^(4+), Pt^(2+)), and chromium (Cr3+, Cr6+)) at doping levels ranging from 0.1 to 1.0 at. %, was synthesized by standard sol−gel methods and characterized by X-ray diffraction, BET surface area measurement, SEM, and UV−vis diffuse reflectance spectroscopy. Doping with Pt(IV/II), Cr(III), V(III), and Fe(III) resulted in a lower anatase to rutile phase transformation (A−R phase transformation) temperature for the resultant TiO_2 particles, while doping with Ru(III) inhibited the A−R phase transformation. Metal-ion doping also resulted in a red shift of the photophysical response of TiO_2 that was reflected in an extended absorption in the visible region between 400 and 700 nm. In contrast, doping with Ag(I), Rb(I), Y(III), and La(III) did not result in a red shift of the absorption spectrum of TiO_2. As confirmed by elemental composition analysis by energy dispersive X-ray spectroscopy, the latter group of ions was unable to be substituted for Ti(IV) in the crystalline matrix due to their incompatible ionic radii. The photocatalytic activities of doped TiO_2 samples were quantified in terms of the photobleaching of methylene blue, the oxidation of iodide (I^(−)), and the oxidative degradation of phenol in aqueous solution both under visible-light irradiation (λ > 400 nm) and under broader-band UV−vis irradiation (λ > 320 nm). Pt- and Cr-doped TiO_2, which had relatively high percentages of rutile in the particle phase, showed significantly enhanced visible-light photocatalytic activity for all three reaction classes

How Phenol and α-Tocopherol React with Ambient Ozone at Gas/Liquid Interfaces

The exceptional ability of α-tocopherol (α-TOH) for scavenging free radicals is believed to also underlie its protective functions in respiratory epithelia. Phenols, however, can scavenge other reactive species. Herein, we report that α-TOH/α-TO^− reacts with closed-shell O_3(g) on the surface of inert solvent microdroplets in <1 ms to produce persistent α-TO−O_n^−(n = 1−4) adducts detectable by online thermospray ionization mass spectrometry. The prototype phenolate PhO^−, in contrast, undergoes electron transfer under identical conditions. These reactions are deemed to occur at the gas/liquid interface because their rates: (1) depend on pH, (2) are several orders of magnitude faster than within microdroplets saturated with O_3(g). They also fail to incorporate solvent into the products: the same α-TO−On^− species are formed on acetonitrile or nucleophilic methanol microdroplets. α-TO−O_n(=1−3)^− signals initially evolve with [O_3(g)] as expected from first-generation species, but α-TO−O^− reacts further with O_3(g) and undergoes collisionally induced dissociation into a C_(19)H_(40) fragment (vs C_(19)H_(38) from α-TO^−) carrying the phytyl side chain, whereas the higher α-TO−O_(n≥2)^− homologues are unreactive toward O_3(g) and split CO_2 instead. On this basis, α-TO−O^− is assigned to a chroman-6-ol (4a, 8a)-ene oxide, α-TO−O2^− to an endoperoxide, and α-TO−O3^− to a secondary ozonide. The atmospheric degradation of the substituted phenols detected in combustion emissions is therefore expected to produce related oxidants on the aerosol particles present in the air we breathe

Absorption of Inhaled NO_2

Nitrogen dioxide (NO_2), a sparingly water-soluble π-radical gas, is a criteria air pollutant that induces adverse health effects. How is inhaled NO_2(g) incorporated into the fluid microfilms lining respiratory airways remains an open issue because its exceedingly small uptake coefficient (γ 10^(−7)−10^(−8)) limits physical dissolution on neat water. Here, we investigate whether the biological antioxidants present in these fluids enhance NO_2(g) dissolution by monitoring the surface of aqueous ascorbate, urate, and glutathione microdroplets exposed to NO_2(g) for 1 ms via online thermospray ionization mass spectrometry. We found that antioxidants catalyze the hydrolytic disproportionation of NO_2(g), 2NO_2(g) + H_2O(l) = NO_3^−(aq) + H^+(aq) + HONO, but are not consumed in the process. Because this function will be largely performed by chloride, the major anion in airway lining fluids, we infer that inhaled NO_2(g) delivers H^+, HONO, and NO_3^− as primary transducers of toxic action without antioxidant participation

Proton Availability at the Air/Water Interface

The acidity of the water surface sensed by a colliding gas is determined in experiments in which the protonation of gaseous trimethylamine (TMA) on aqueous microjets is monitored by online electrospray mass spectrometry as a function of the pH of the bulk liquid (pH_(BLK)). TMAH^+ signal intensities describe a titration curve whose equivalence point at pH_(BLK) 3.8 is dramatically smaller than the acidity constant of trimethylammonium in bulk solution, pK_A(TMAH^+) = 9.8. Notably, the degree of TMA protonation above pH_(BLK) 4 is enhanced hundred-fold by submillimolar LiCl or NaCl and weakly inhibited at larger concentrations. Protonation enhancements are associated with the onset of significant direct kinetic solvent hydrogen isotope effects. Since TMA(g) can be protonated by H_2O itself only upon extensive solvent participation, we infer that H3O^+ emerges at the surface of neat water below pH_(BLK) 4

Time-resolved Microwave Conductivity. Part 2.-Quantum-sized TiO_2 and the Effect of Adsorbates and Light Intensity on Charge-carrier Dynamics

Charge-carrier recombination dynamics after a pulsed laser excitation are investigated by time-resolved microwave conductivity (TRMC) for quantum-sized (Q-) TiO_2 and P25, a bulk-phase TiO_2. Adsorbed scavengers such as HNO_3, HC, HCIO_4, isopropyl alcohol, trans-decalin, tetranitromethane, and methyl viologen dichloride result in different charge-carrier recombination dynamics for Q-TiO_2 and P25. The differences include a current doubling with isopropyl alcohol for which electron injection into Q-TiO_2 is much slower than into P25 and relaxation of the selection rules of an indirect-bandgap semiconductor due to size quantization. However, the faster interfacial charge transfer predicted for Q-TiO_2 due to a 0. 2 eV gain in redox overpotentials is not observed. The effect of light intensity is also investigated. Above a critical injection level, fast recombination channels are opened, which may be a major factor resulting in the dependence of the steady-state photolysis quantum yields on l^(–1/2). The fast recombination channels are opened at lower injection levels for P25 than for Q-TiO_2, and a model incorporating the heterogeneity of surface-hole traps is presented

Prompt Formation of Organic Acids in Pulse Ozonation of Terpenes on Aqueous Surfaces

A major atmospheric process, the gas-phase ozonation of terpenes yields suites of products via a cascade of chemically activated intermediates that ranges from primary ozonides to dioxiranes. If a similar mechanism operated in water, as it is generally assumed, such intermediates would be deactivated within picoseconds and, henceforth, be unable to produce carboxylic acids in microseconds. Herein, we report the online electrospray mass spectrometric detection of (M + 2O – H^+) and (M + 3O – H^+) carboxylates on the surface of aqueous β-caryophyllene (C_(15)H_(24), M = 204 Da) microjets exposed to a few ppmv of O_3(g) for < 10 μs. Since neither species is formed on dry solvent microjets and both incorporate deuterium from D_2O, we infer that carboxylates ensue from the interaction of nascent intermediates with interfacial water via heretofore unreported processes. These interfacial events proceed much faster than those in bulk liquids saturated with ozone

Gas-Phase Photodegradation of Decane and Methanol on TiO_2: Dynamic Surface Chemistry Characterized by Diffuse Reflectance FTIR

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminated TiO2 surfaces under both vacuum conditions, and in the presence of organic molecules (decane and methanol). In the presence of hole scavengers, electrons are trapped at Ti(III)–OH sites, and free electrons are generated. These free electrons are seen to decay by exposure either to oxygen or to heat; in the case of heating, reinjection of holes into the lattice by loss of sorbed hole scavenger leads to a decrease in Ti(III)–OH centers. Decane adsorption experiments lend support to the theory that removal of surficial hydrocarbon contaminants is responsible for superhydrophilic TiO2 surfaces. Oxidation of decane led to a mixture of surface-bound organics, while oxidation of methanol leads to the formation of surface-bound formic acid

R. M. Harrison and S. J. De Mora: Introductory Chemistry for the Environmental Sciences [book review]

Harrison and De Mora have revised the first edition of their book, Introductory Chemistry for the Environmental Sciences, which has an intended reading audience of college or university undergraduates who are studying or majoring in the environmental sciences, environmental chemistry, or ecology. Their intent is to present the basic concepts of chemistry within the context of the thermodynamic universe known at ‘the environment'