Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single-Atom Catalytic Sites.

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) is an attractive approach for achieving enhanced photocatalytic performance. However, there is limited knowledge of the mechanism of photocatalytic enhancement in SAC-modified photocatalysts, which makes the rational design of high-performance SAC-based photocatalysts challenging. Herein, a series of photocatalysts for the aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu, and Fe ions) is reported. The most active SAC-modified photocatalysts outperform TiO2 modified with the corresponding metal oxide nanoparticles and state-of-the-art benchmark photocatalysts such as platinized TiO2 and commercial P25 powders. A combination of in situ electron paramagnetic resonance spectroscopy and theoretical calculations reveal that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, particularly with respect to the rate of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both the effective extraction of photogenerated electrons and fast catalytic turnover in the reduction of dioxygen, which translates directly into diminished recombination. These results provide essential guidelines for developing efficient SAC-based photocatalysts.

Kinetics and Optimization of the Photocatalytic Reduction of Nitrobenzene.

The photocatalytic reduction of nitrobenzene to aniline in alcoholic solutions appears as an interesting alternative to the classical hydration. However, little is known about the influence of reaction parameters on the kinetics of the reaction which were therefore studied herein. The effects of light intensity, catalyst concentration, initial concentration, and temperature were systematically investigated under more than 50 different conditions and accurately described with an appropriate kinetic model. The results show that the efficiency of the reaction is extremely high and apparent quantum yields of up to 142 % were observed under optimized conditions. Particularly interesting is the fact high efficiencies were also obtained at high reaction rates of up to 74.3 mM h-1. Overall these results demonstrate that heterogeneous photocatalytic reactions can be very efficient and productive at the same time and may therefore present a powerful tool in synthetic organic chemistry.

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NO x abatement.

Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NO x ) based air pollution through mineralisation of NO x to nitrate. However, most of the typically TiO2-based catalysts employed show a much higher reactivity towards NO than NO2, leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(iii) ions, the reactivity towards NO2 is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior.

On the underlying mechanisms of the low observed nitrate selectivity in photocatalytic NOx abatement and the importance of the oxygen reduction reaction.

Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate. However, the reaction typically shows poor selectivity, releasing a number of unwanted and possibly toxic intermediates such as nitrogen dioxide. Up to now, the underlying principles that lead to this poor selectivity were not understood so a knowledge-based catalyst design for more selective materials was impossible. Herein, we present strong evidence for the slow oxygen reduction being one the causes, as the competing back-reduction of nitrate leads to the release of nitrogen dioxide. Consequently, engineering the photocatalyst for a better oxygen reduction efficiency should also increase the nitrate selectivity.

Hierarchically structured nanoporous carbon tubes for high pressure carbon dioxide adsorption.

Mesoscopic, nanoporous carbon tubes were synthesized by a combination of the Stoeber process and the use of electrospun macrosized polystyrene fibres as structure directing templates. The obtained carbon tubes have a macroporous nature characterized by a thick wall structure and a high specific surface area of approximately 500 m²/g resulting from their micro- and mesopores. The micropore regime of the carbon tubes is composed of turbostratic graphitic areas observed in the microstructure. The employed templating process was also used for the synthesis of silicon carbide tubes. The characterization of all porous materials was performed by nitrogen adsorption at 77 K, Raman spectroscopy, infrared spectroscopy, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The adsorption of carbon dioxide on the carbon tubes at 25 °C at pressures of up to 30 bar was studied using a volumetric method. At 26 bar, an adsorption capacity of 4.9 mmol/g was observed. This is comparable to the adsorption capacity of molecular sieves and vertically aligned carbon nanotubes. The high pressure adsorption process of CO2 was found to irreversibly change the microporous structure of the carbon tubes.

Pseudomorphic transformation of amorphous silica microtubes into mesoporous MCM-41 type silica tubes. Synthesis, characterization and surface functionalization with titania, vanadia and zirconia.

Silica tubes with MCM-41 type mesostructures were successfully synthesized by a combination of the Stoeber process and a pseudomorphic transformation using electrospun macrosized polystyrene fibres as structure directing templates. Two different morphologies of mesoporous silica tubes are accessible with this method: a hollow morphology with tunable silica wall thickness and with a mesoporous silica shell structure and a core containing amorphous silica. All one dimensional tube like porous silica materials have a high specific surface area of approximately 1000 m(2) g(-1) with well-ordered hexagonal mesopores. Grafting of Ti, V and Zr metallocene dichloride molecular complexes has been employed resulting in the deposition of titanium-, vanadium-, zirconium-oxide in the interior of the silica tubes after ceramisation of the green body composites. The respective oxides were coated on top of the inner mesoporous silica surface of the tubes. Such silica based hybrids might be potential support materials in heterogeneous catalysis (e.g. vanadia) as well as interesting catalysts for photocatalysis (for TiO(2), ZrO(2)). All materials were characterised by X-ray diffraction (SAXS and XRD), nitrogen adsorption at 77 K, UV/VIS diffuse reflectance spectroscopy (UV-DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).