Mechanistic Features of the TiO2 Heterogeneous Photocatalysis of Arsenic and Uranyl Nitrate in Aqueous Suspensions Studied by the Stopped‐Flow Technique

The dynamics of the transfer of electrons stored in TiO₂ nanoparticles to As^III, As^V, and uranyl nitrate in water was investigated by using the stopped‐flow technique. Suspensions of TiO₂ nanoparticles with stored trapped electrons (e_trap^?) were mixed with solutions of acceptor species to evaluate the reactivity by following the temporal evolution of e_trap^? by the decrease in the absorbance at λ=600 nm. The results indicate that As^V and As^III cannot be reduced by e_trap^? under the reaction conditions. In addition, it was observed that the presence of As^V and As^III strongly modified the reaction rate between O₂ and e_trap^?: an increase in the rate was observed if As^V was present and a decrease in the rate was observed in the presence of As^III. In contrast with the As system, U^VI was observed to react easily with e_trap^? and U^IV formation was observed spectroscopically at λ=650 nm. The possible competence of U^VI and NO₃^? for their reduction by e_trap^? was analyzed. The inhibition of the U^VI photocatalytic reduction by O₂ could be attributed to the fast oxidation of U^V and/or U^IV.     

Aligning Electronic and Protonic Energy Levels of Proton‐Coupled Electron Transfer in Water Oxidation on Aqueous TiO2

The high overpotential in water oxidation on anodes is a limiting factor for the large‐scale application of photoelectrochemical cells. To overcome this limitation, it is essential to understand the four proton‐coupled electron transfer (PCET) steps in the reaction mechanism and their implications to the overpotential. Herein, a simple scheme to compute the energies of the PCET steps in water oxidation on the aqueous TiO₂ surface using a hybrid density functional is described. An energy level diagram for fully decoupled electron‐ and proton‐transfer reactions in which both electronic and protonic levels are placed on the same potential scale is also described. The level diagram helps to visualize the electronic and protonic components of the overpotential, and points out what are needed to improve. For TiO₂, it is found that its catalytic activity is due to aligning the protonic energy levels in the PCET steps, while improving the activity requires also aligning the electronic levels.     

Continuous Heterogeneous Photocatalysis in Serial Micro‐Batch Reactors

Solid reagents, leaching catalysts, and heterogeneous photocatalysts are commonly employed in batch processes but are ill‐suited for continuous‐flow chemistry. Heterogeneous catalysts for thermal reactions are typically used in packed‐bed reactors, which cannot be penetrated by light and thus are not suitable for photocatalytic reactions involving solids. We demonstrate that serial micro‐batch reactors (SMBRs) allow for the continuous utilization of solid materials together with liquids and gases in flow. This technology was utilized to develop selective and efficient fluorination reactions using a modified graphitic carbon nitride heterogeneous catalyst instead of costly homogeneous metal polypyridyl complexes. The merger of this inexpensive, recyclable catalyst and the SMBR approach enables sustainable and scalable photocatalysis.     

Facile Synthesis of Thermal‐ and Photostable Titania with Paramagnetic Oxygen Vacancies for Visible‐Light Photocatalysis

A novel dopant‐free TiO₂ photocatalyst (V_o^.‐TiO₂), which is self‐modified by a large number of paramagnetic (single‐electron‐trapped) oxygen vacancies, was prepared by calcining a mixture of a porous amorphous TiO₂ precursor, imidazole, and hydrochloric acid at elevated temperature (450 °C) in air. Control experiments demonstrate that the porous TiO₂ precursor, imidazole, and hydrochloric acid are all necessary for the formation of V_o^.‐TiO₂. Although the synthesis of V_o^.‐TiO₂ originates from such a multicomponent system, this synthetic approach is facile, controllable, and reproducible. X‐ray diffraction, XPS, and EPR spectroscopy reveal that the V_o^.‐TiO₂ material with a high crystallinity embodies a mass of paramagnetic oxygen vacancies, and is free of other dopant species such as nitrogen and carbon. UV/Vis diffuse‐reflectance spectroscopy and photoelectrochemical measurement demonstrate that V_o^.‐TiO₂ is a stable visible‐light‐responsive material with photogenerated charge separation efficiency higher than N‐TiO₂ and P25 under visible‐light irradiation. The V_o^.‐TiO₂ material exhibits not only satisfactory thermal‐ and photostability, but also superior photocatalytic activity for H₂ evolution (115 μmol h^?1 g^?1) from water with methanol as sacrificial reagent under visible light (λ>400 nm) irradiation. Furthermore, the effects of reaction temperature, ratio of starting materials (imidazole:TiO₂ precursor) and calcination time on the photocatalytic activity and the microstructure of V_o^.‐TiO₂ were elucidated.     

Oxygen‐Controlled Catalysis by Vitamin B12‐TiO2: Formation of Esters and Amides from Trichlorinated Organic Compounds by Photoirradiation

An oxygen switch in catalysis of the cobalamin derivative (B₁₂)‐TiO₂ hybrid catalyst for the dechlorination of trichlorinated organic compounds has been developed. The covalently bound B₁₂ on the TiO₂ surface transformed trichlorinated organic compounds into an ester and amide by UV light irradiation under mild conditions (in air at room temperature), while dichlorostilbenes (E and Z forms) were formed in nitrogen from benzotrichloride. A benzoyl chloride was formed as an intermediate of the ester and amide, which was detected by GC‐MS. The substrate scope of the synthetic strategy is demonstrated with a range of various trichlorinated organic compounds. A photo‐duet reaction utilizing the hole and conduction band electron of TiO₂ in B₁₂‐TiO₂ for the amide formation was also developed.     

Three‐Dimensional Ordered Assembly of Thin‐Shell Au/TiO2 Hollow Nanospheres for Enhanced Visible‐Light‐Driven Photocatalysis

An Au/TiO₂ nanostructure was constructed to obtain a highly efficient visible‐light‐driven photocatalyst. The design was based on a three‐dimensional ordered assembly of thin‐shell Au/TiO₂ hollow nanospheres (Au/TiO₂‐3 DHNSs). The designed photocatalysts exhibit not only a very high surface area but also photonic behavior and multiple light scattering, which significantly enhances visible‐light absorption. Thus Au/TiO₂‐3 DHNSs exhibit a visible‐light‐driven photocatalytic activity that is several times higher than conventional Au/TiO₂ nanopowders.     

Semi‐heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides

Cross‐coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non‐recyclable noble‐metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal‐free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C?O cross‐couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales.     

Nano‐ and Microstructure Fabrication by Using a Three‐Component System

Two‐component amphiphiles based on hydrogen‐bonded complexes between terephthaloylbisalanine (H₂TBA) and dodecylamine (DA) are able to self‐assemble into nano‐ and microsized superstructures in an aqueous solvent. It is possible to modulate the morphology of these self‐assembled superstructures by modifying the composition of the complexes, which can be achieved by changing the molar ratio of the two components or by changing the chirality of H₂TBA. For example, right‐handed microhelical ribbon structures were formed with L ‐TBA_1.0DA_2.0, whereas in the case of rac‐TBA_1.0DA_2.0, flat ribbonlike structures were observed. Although L ‐TBA_1.0DA_1.0 exhibited entangled fibrous structures, rac‐TBA_1.0DA_1.0 exhibited wire structures. Different ratios of H₂TBA and DA were self‐assembled into fiber‐, wire‐, and tubulelike superstructures, as well as monoclinic, columnar, and lamellar aggregation patterns. The self‐assembled superstructures of TBA_xDA_y were significantly changed by adding metal ions. Transition metal (Cd^II, Co^II, and Zn^II) complexes with L ‐TBA_xDA_y self‐assembled into rod‐, tubule‐, wire‐, and platelike superstructures. Metal‐ion complexes with rac‐TBA_xDA_y exhibited different superstructures. Our work suggests that it is possible to fabricate a wide variety of nano‐ and microsized superstructures by using two‐ and three‐component amphiphiles.     

Mapping of the Photoinduced Electron Traps in TiO2 by Picosecond X‐ray Absorption Spectroscopy

Titanium dioxide (TiO₂) is the most popular material for applications in solar‐energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X‐ray absorption spectroscopy at the Ti K‐edge and the Ru L₃‐edge to address this issue for photoexcited bare and N719‐dye‐sensitized anatase and amorphous TiO₂ nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect‐rich surface shell in the case of anatase TiO₂, whereas they are inside the bulk in the case of amorphous TiO₂. In the case of dye‐sensitized anatase or amorphous TiO₂, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds.     

Direct Synthesis of Nano‐Ferrierite along the 10‐Ring‐Channel Direction Boosts Their Catalytic Behavior

Ferrierite zeolites with nanosized crystals and external surface areas higher than 250 m² g^?1 have been prepared at relatively low synthesis temperature (120 °C) by means of the collaborative effect of two organic structure directing agents (OSDA). In this way, hierarchical porosity is achieved without the use of post‐synthesis treatments that usually involve leaching of T atoms and solid loss. Adjusting the synthesis conditions it is possible to decrease the crystallite size in the directions of the 8‐ and 10‐ring channels, [010] and [001] respectively, reducing their average pore length to 10–30 nm and increasing the number of pores accessible. The small crystal size of the nano‐ferrierites results in an improved accessibility of reactants to the catalytic active centers and enhanced product diffusion, leading to higher conversion and selectivity with lower deactivation rates for the oligomerization of 1‐pentene into longer‐chain olefins.     

Efficient Coupling of Solar Energy to Catalytic Hydrogenation by Using Well‐Designed Palladium Nanostructures

A Ru³⁺‐mediated synthesis for the unique Pd concave nanostructures, which can directly harvest UV‐to‐visible light for styrene hydrogenation, is described. The catalytic efficiency under 100 mW cm^?2 full‐spectrum irradiation at room temperature turns out to be comparable to that of thermally (70 °C) driven reactions. The yields obtained with other Pd nanocrystals, such as nanocubes and octahedrons, are lower. The nanostructures reported here have sufficient plasmonic cross‐sections for light harvesting in a broad spectral range owing to the reduced shape symmetry, which increases the solution temperature for the reaction by the photothermal effect. They possess a large quantity of atoms at corners and edges where local heat is more efficiently generated, thus providing active sites for the reaction. Taken together, these factors drastically enhance the hydrogenation reaction by light illumination.     

CaO as Drop‐In Colloidal Catalysts for the Synthesis of Higher Polyglycerols

Glycerol is an attractive renewable building block for the synthesis of polyglycerols, which find application in the cosmetic and pharmaceutical industries. The selective etherification of glycerol to higher oligomers was studied in the presence of CaO colloids and the data are compared with those obtained from NaOH and CaO. The materials were prepared by dispersing CaO, CaCO₃, or Ca(OH)₂ onto a carbon nanofiber (CNF) support. Colloidal nanoparticles were subsequently dispensed from the CNF into the reaction mixture to give CaO colloids that have a higher activity than equimolar amounts of bulk CaO and NaOH. Optimization of the reaction conditions allowed us to obtain a product with Gardner color number <2, containing no acrolein and minimal cyclic byproducts. The differences in the CaO colloids originating from CNF and bulk CaO were probed using light scattering and conductivity measurements. The results confirmed that the higher activity of the colloids originating from CaO/CNF was due to their more rapid formation and smaller size compared with colloids from bulk CaO. We thus have developed a practical method for the synthesis of polyglycerols containing low amounts of Ca.     

The Origin of Higher Open‐Circuit Voltage in Zn‐Doped TiO2 Nanoparticle‐Based Dye‐Sensitized Solar Cells

Zn‐doped anatase TiO₂ nanoparticles are synthesized by a one‐step hydrothermal method. Detailed electrochemical measurements are undertaken to investigate the origin of the effect of Zn doping on the performance of dye‐sensitized solar cells (DSSCs). It is found that incorporation of Zn²⁺ into an anatase lattice elevates the edge of the conduction band (CB) of the photoanodes and the Fermi level is shifted toward the CB edge, which contributes to the improvement in open‐circuit voltage (V_OC). Charge‐density plots across the cell voltage further confirm the increase in the CB edge in DSSCs directly. Photocurrent and transient photovoltage measurements are employed to study transport and recombination dynamics. The electron recombination is accelerated at higher voltages close to the CB edge, thus leading to a negative effect on the V_OC.