Reduced TiO2with prolonged electron lifetime for improving photocatalytic water reduction activity.

The reduction of anatase TiO2with NaBH4under argon atmosphere at a high temperature resulted in a longer electron lifetime and a larger electron population. The reduced gray anatase sample with disorder layer showed a higher evolution rate of H2(130.2µmol h-1g-1) compared to pristine TiO2(24.1µmol h-1g-1) in the presence of Pt co-catalyst in an aqueous glucose solution under exposure to ultraviolet light (λ⩽ 400 nm). Ti3+and oxygen vacancy defects were proposed to exist in the reduced TiO2. A continuum tail forms above the valence band edge top as a result of these two defects, which contribute to the lattice disorder. This is presumably also the case with the conduction band, which has a continuum tail composed of mid-gap states as a result of the defects. The Ti3+and oxygen vacancy defects operate as shallow traps for photoexcited electrons, thereby preventing recombination. Since the defects are primarily located at the surface, i.e. in the disorder layer, the photoexcited electrons in shallow traps hence become readily available for the reduction of H3O+into H2. The prolonged electron lifetime increases the photoexcited electron population in the reduced TiO2, resulting in enhanced water reduction activity.

Efficient electron extraction by CoS2loaded onto anatase TiO2for improved photocatalytic hydrogen evolution.

Titanium dioxide (TiO2) as a benchmark photocatalyst has been attracting attention due to its photocatalytic activity combined with photochemical stability. In particular, TiO2with anatase polymorph holds promise for driving reduction reactions, such as proton reduction to evolve H2via photocatalysis. In this study, anatase TiO2is loaded with CoS2through the hydrothermal route to form a CoS2@TiO2photocatalyst system. X-ray absorption near edge structure confirms the +2-oxidation state of the Co cation, while extended x-ray absorption fine structure shows that each Co2+cation is primarily coordinated to six S-anions forming a CoS2-like species. A small fraction of the Co2+species is also coordinated to O2-anions forming CoxOyspecies and substitutionally resides at the Ti4+-sites. Further investigations with steady-state IR absorption induced by UV-light and time-resolved microwave conductivity suggest an efficient electron transfer from the conduction band of TiO2to the surface-loaded CoS2which acts as a metallic material with no bandgap. The CoS2shallowly traps electrons at the host surface and facilitates proton reduction. An appreciably enhanced H2evolution rate (8 times) is recognised upon the CoS2loading. The CoS2is here proposed to function as a proton reduction cocatalyst, which can potentially be an alternative to noble metals.

Mechanistic understanding of the increased photoactivity of TiO2 nanosheets upon tantalum doping.

Anatase TiO2 is doped with Ta cations through a hydrothermal route. Based on X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy, the Ta dopants exist in the 5+-oxidation state. The oxidation state is insensitive to the Ta loading amount. Extended X-ray absorption fine structure spectroscopy confirms that the local structure around Ta cations is not identical between the Ta-doped samples. The Ta-O distance monotonically increases with the Ta loading amount due to a gradually expanding lattice. The Ta-doped samples show higher activity than pristine TiO2 for photomineralizing recalcitrant organics. The enhanced photocatalytic activity is proposed to be due to an enhanced population of photoexcited electrons, as probed using light-induced IR absorption spectroscopy, and an extended electron lifetime, as probed using time-resolved microwave conductivity, which are associated with the formation of Ti3+ defect states acting as shallow electron traps. The maximum photocatalytic activity is observed for TiO2 doped with 2 mol% of Ta, which shows enhancement of mineralization efficiency (about 3 times) and enhancement of electron population (up to 20 times), as compared to those of pristine TiO2. The fundamental question of why a proper metal doping into TiO2 increases photocatalytic activity is discussed in this study.

Mechanistic insights into the adsorption of methylene blue by particulate durian peel waste in water.

This study aims to investigate the adsorption of methylene blue (MB) over particulate durian peel waste, which is chemically activated with hydrogen peroxide. The equilibrium data are well described by the Freundlich isotherm model, which indicates that the MB adsorption takes place predominantly on multilayers and heterogeneous surfaces of the biosorbent. The Freundlich adsorption constants, KF and n, are 11.06 L/g and 2.94, respectively. Thermodynamic data suggest that the MB adsorption occurs spontaneously and endothermically. The enthalpy and entropy for the MB adsorption are obtained as 10.26 kJ/mol and 0.058 kJ/mol K, respectively, in the temperature range of 303-323 K. Based on the stepwise desorption method, the adsorption of MB is dominated by physical interactions, particularly hydrogen bonding.

Structural properties and catalytic activity of a novel ternary CuO/gC3N4/Bi2O3 photocatalyst.

In the present study, CuO/gC3N4/Bi2O3 composite is constructed as a ternary visible light active photocatalyst. Since CuO plays a critical role in enhancing the photocatalytic activity of the formed composite, its structural properties are particularly studied using synchrotron X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES confirms the presence of Cu species with +2 oxidation state in the form of CuO. EXAFS furthermore confirms that each Cu cation is coordinated to four O anions in an approximately square planar configuration. The length of the Cu-O coordination is estimated to be 1.92 Å, slightly shorter than that of bulk CuO (1.95 Å). The CuO/gC3N4/Bi2O3 composite exhibits highly enhanced photocatalytic activity in the 2,4-dichlorophenol decomposition under visible light. The enhanced photocatalytic activity is due to the increased population of electrons and the successful consumption of the photoproduced electrons by the dissolved oxygen through the one-electron transfer reaction.

Direct quantitative analysis of arsenic in coal fly ash.

A rapid, simple method based on graphite furnace atomic absorption spectrometry is described for the direct determination of arsenic in coal fly ash. Solid samples were directly introduced into the atomizer without preliminary treatment. The direct analysis method was not always free of spectral matrix interference, but the stabilization of arsenic by adding palladium nitrate (chemical modifier) and the optimization of the parameters in the furnace program (temperature, rate of temperature increase, hold time, and argon gas flow) gave good results for the total arsenic determination. The optimal furnace program was determined by analyzing different concentrations of a reference material (NIST1633b), which showed the best linearity for calibration. The optimized parameters for the furnace programs for the ashing and atomization steps were as follows: temperatures of 500-1200 and 2150°C, heating rates of 100 and 500°C s(-1), hold times of 90 and 7 s, and medium then maximum and medium argon gas flows, respectively. The calibration plots were linear with a correlation coefficient of 0.9699. This method was validated using arsenic-containing raw coal samples in accordance with the requirements of the mass balance calculation; the distribution rate of As in the fly ashes ranged from 101 to 119%.