Asymmetric Sites on the ZnZrOx Catalyst for Promoting Formate Formation and Transformation in CO2 Hydrogenation.

The role of formate species for CO2 hydrogenation is still under debate. Although formate has been frequently observed and commonly proposed as the possible intermediate, there is no definite evidence for the reaction of formate species for methanol production. Here, formate formation and conversion over the ZnZrOx solid solution catalyst are investigated by in situ/operando diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy (DRIFTS-MS) coupled with density functional theory (DFT) calculations. Spectroscopic results show that bidentate carbonate formed from CO2 adsorption is hydrogenated to formate on Zn-O-Zr sites (asymmetric sites), where the Zn site is responsible for H2 activation and the Zr site is beneficial for the stabilization of reaction intermediates. The asymmetric Zn-O-Zr sites with adjacent and inequivalent features on the ZnZrOx catalyst promote not only formate formation but also its transformation. Both theoretical and experimental results demonstrate that the origin of the excellent performance of the ZnZrOx catalyst for methanol formation is associated with the H2 heterolytic cleavage promoted by the asymmetric Zn and Zr sites.

Layered ß-ZrNBr Nitro-Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction.

Developing mixed-anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide-based photocatalyst is still in shortage. Here we report a layered nitro-halide ß-ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO2 reduction under visible-light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro-halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of ß-ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro-halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Water-Stable Nickel Metal-Organic Framework Nanobelts for Cocatalyst-Free Photocatalytic Water Splitting to Produce Hydrogen.

Development of water-stable metal-organic frameworks (MOFs) for promising visible-light-driven photocatalytic water splitting is highly desirable but still challenging. Here we report a novel p-type nickel-based MOF single crystal (Ni-TBAPy-SC) and its exfoliated nanobelts (Ni-TBAPy-NB) that can bear a wide range of pH environment in aqueous solution. Both experimental and theoretical results indicate a feasible electron transfer from the H4TBAPy ligand (light-harvesting center) to the Ni-O cluster node (catalytic center), on which water splitting to produce hydrogen can be efficiently driven free of cocatalyst. Compared to the single crystal, the exfoliated two-dimensional (2D) nanobelts show more efficient charge separation due to its shortened charge transfer distance and remarkably enhanced active surface areas, resulting in 164 times of promoted water reduction activity. The optimal H2 evolution rate on the nanobelt reaches 98 µmol h-1 (ca. 5 mmol h-1 g-1) showing benchmarked apparent quantum efficiency (AQE) of 8.0% at 420 nm among water-stable MOFs photocatalysts.

Synthesis of Aluminophosphate Molecular Sieves in Alkaline Media.

Unlike conventional aluminosilicate zeolites synthesized in alkaline media, aluminophosphate molecular sieves (AlPOs) have always been prepared under acidic conditions in the past three decades; this has been regarded as one of essential factors for synthesis, except for the case of silica-substituted analogues (SAPOs). For the first time, we demonstrate herein a simple and generalized route for synthesizing various types of aluminophosphate molecular sieves in alkaline media. A series of aluminophosphate sieves and their analogues have been prepared with different quaternary ammonium cations as structure-directing agents in this manner. The above successes have extended the systematic media from acidic or neutral to alkaline for the preparation of a series of aluminophosphate molecular sieves, which possibly open an alternative route for the synthesis of aluminophosphate molecular sieves.

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling.

Currently, the synthesis of pure silica zeolites always requires the presence of organic structure-directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

Directly probing redox-linked quinones in photosystem II membrane fragments via UV resonance Raman scattering.

In photosynthesis, photosystem II (PSII) harvests sunlight with bound pigments to oxidize water and reduce quinone to quinol, which serves as electron and proton mediators for solar-to-chemical energy conversion. At least two types of quinone cofactors in PSII are redox-linked: QA, and QB. Here, we for the first time apply 257-nm ultraviolet resonance Raman (UVRR) spectroscopy to acquire the molecular vibrations of plastoquinone (PQ) in PSII membranes. Owing to the resonance enhancement effect, the vibrational signal of PQ in PSII membranes is prominent. A strong band at 1661 cm(-1) is assigned to ring CC/CO symmetric stretch mode (ν8a mode) of PQ, and a weak band at 469 cm(-1) to ring stretch mode. By using a pump-probe difference UVRR method and a sample jet technique, the signals of QA and QB can be distinguished. A frequency difference of 1.4 cm(-1) in ν8a vibrational mode between QA and QB is observed, corresponding to ~86 mV redox potential difference imposed by their protein environment. In addition, there are other PQs in the PSII membranes. A negligible anharmonicity effect on their combination band at 2130 cm(-1) suggests that the 'other PQs' are situated in a hydrophobic environment. The detection of the 'other PQs' might be consistent with the view that another functional PQ cofactor (not QA or QB) exists in PSII. This UVRR approach will be useful to the study of quinone molecules in photosynthesis or other biological systems.

Insights of the Crystallization Process of Molecular Sieve AlPO4-5 Prepared by Solvent-Free Synthesis.

Crystallization of AlPO4-5 with AFI structure under solvent-free conditions has been investigated. Attention was mainly focused on the characterization of the intermediate phases formed at the early stages during the crystallization. The development in the long-range ordering of the solid phases as a function of crystallization time was monitored by XRD, SEM, IR, UV-Raman, and MAS NMR techniques. Particularly, the UV-Raman spectroscopy was employed to obtain the information on the formation process of the framework. J-HMQC (27)Al/(31)P double-resonance NMR experiments were used to identify the P-O-Al bonded species in the intermediate phases. For the first time the P-O-Al bonded species in the intermediate phases can be correctly described through using this advanced NMR technique. The crystallization under solvent-free conditions appears to follow the pathway: The initial amorphous raw material is converted to an intermediate phase which has four-/six-membered ring species, then gradually transformed into crystalline AlPO4-5. This observation is not consistent with the common idea that the intermediate phase is the semicrystalline intermediates with a three-dimensional structure.

Roles of adsorption sites in electron transfer from CdS quantum dots to molecular catalyst cobaloxime studied by time-resolved spectroscopy.

Electron transfer from CdS quantum dots (QDs) to cobaloxime (Co(dmgH)2pyCl) is demonstrated by transient absorption spectroscopy (TAS), and further confirmed using photoluminescence (PL) techniques. The analysis of the PL quenching results offers a novel way to understand the roles of the surface adsorption sites of CdS QDs in the performance of charge transfer in the CdS QDs-cobaloxime hybrid system. Two types of quenching dynamics reveal that there are two different adsorption modes of cobaloxime on the CdS QD surface. When cobaloxime substitutes the surface capping ligands of CdS QDs under low cobaloxime concentrations, the transfer is nearly unfavorable for both the free and trapped electrons. When cobaloxime occupies the surface defect sites of the CdS QDs under high cobaloxime concentrations, the transfer of both the free and trapped electrons is very effective, with an extremely high quenching rate constant of ∼10(12) M(-1) s(-1). Therefore, controlling the molecular adsorption sites and adjusting the surface defect properties of semiconductor QDs provide a strategy to improve the electron transfer efficiency of the QDs-cobaloxime photocatalytic system.

Direct Imaging of Highly Anisotropic Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst.

Spatially resolved surface photovoltage spectroscopy (SRSPS) was employed to obtain direct evidence for highly anisotropic photogenerated charge separation on different facets of a single BiVO4 photocatalyst. Through the controlled synthesis of a single crystal with preferentially exposed {010} facets, highly anisotropic photogenerated hole transfer to the {011} facet of single BiVO4 crystals was observed. The surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. The influence of the built-in electric field in the space charge region of different facets on the anisotropic photoinduced charge transfer in a single semiconductor crystal is revealed.

Sustainable synthesis of zeolites without addition of both organotemplates and solvents.

The development of sustainable and environmentally friendly techniques for synthesizing zeolites has attracted much attention, as the use of organic templates and solvents in the hydrothermal synthesis of zeolites is a major obstacle for realizing green and sustainable synthesis ways. Recently, the introduction of the organotemplate-free synthesis method allowed avoiding the use of organic templates, but water as solvent was still required; solvent-free routes on the other hand beared the potential to significantly reduce the amount of polluted wastewater, but organic templates were still present. In this work, we have demonstrated a combined strategy of both organotemplate- and solvent-free conditions to synthesize aluminosilicate zeolites Beta and ZSM-5 (S-Beta and S-ZSM-5), two of the most important zeolites relevant for industry. The samples are thoroughly characterized by XRD patterns, SEM images, N2 sorption isotherms, UV-Raman spectra, and (29)Si and (27)Al MAS NMR spectra. The results demonstrate that S-Beta and S-ZSM-5 zeolites exhibit almost the same textural parameters (e.g., BET surface area and pore volume) and catalytic performance in cumene cracking and m-xylene isomerization as those of conventional Beta and ZSM-5 zeolites synthesized under hydrothermal conditions (C-Beta and C-ZSM-5). The organotemplate- and solvent-free syntheses of S-Beta and S-ZSM-5 take place at a low-pressure regime and are free of harmful gases as well as give high product yields together with highly efficient consumption of the starting raw materials. These advantages plus the very simple procedures opened the pathway to a highly sustainable zeolite synthesis protocol compared to conventional methods currently employed for C-Beta and C-ZSM-5. Very interestingly, this simple synthesis is a good model for understanding zeolite crystallization. The detail characterizations indicate that the S-Beta crystals are formed from the assembly of zeolite building units, mainly 4MRs, while the 5MRs in the framework are just formed in the crystallization of S-ZSM-5, rather than existence in the starting solid mixture. During the crystallization processes, small traces of water play an important role for the hydrolysis and condensation of silica and/or aluminosilicate species.

Development of a novel Ba2BiV3O11 photocatalyst with dual functions of both water oxidation and proton reduction performance.

To overcome the limitation of photocatalysts with dual functionality of water oxidation and proton reduction, we proposed a novel bismuth-based Ba2BiV3O11 (BBVO) photocatalyst, which can simultaneously drive the proton reduction reaction under UV light and water oxidation reaction under visible light. After doping with sulfur through an in situ vulcanization strategy, the light absorption and charge separation efficiencies of the sulfur-doped BBVO were significantly improved, thus boosting its oxygen evolution activity (64 µmol h-1) by more than 16 times compared with independent BBVO. The experimental results demonstrate that BBVO can be employed as a very promising bismuth-based photocatalyst for solar energy conversion.

Diastereo-divergent synthesis of chiral hindered ethers via a synergistic calcium(II)/gold(I) catalyzed cascade hydration/1,4-addition reaction.

Hindered ethers are ubiquitous in natural products and bioactive molecules. However, developing an efficient method for the stereocontrolled synthesis of all stereoisomers of chiral hindered ethers is highly desirable but challenging. Here we show a strategy that utilizes in situ-generated water as a nucleophile in an asymmetric cascade reaction involving two highly reactive intermediates, 3-furyl methyl cations and ortho-quinone methides (o-QMs), to synthesize chiral hindered ethers. The Ca(II)/Au(I) synergistic catalytic system enables the control of diastereoselectivity and enantioselectivity by selecting suitable chiral phosphine ligands in this cascade hydration/1,4-addition reaction, affording all four stereoisomers of a diverse range of chiral tetra-aryl substituted ethers with high diastereoselectivities (up to >20/1) and enantioselectivities (up to 95% ee). This work provides an example of chiral Ca(II)/Au(I) bimetallic catalytic system controlling two stereogenic centers via a cascade reaction in a single operation.

Synthesis and morphology control of AM-6 nanofibers with tailored -V-O-V- intermediates.

Microporous vanadosilicates with octahedral VO6 and tetrahedral SiO4 units, better known as AM-6, have been hydrothermally synthesized with different morphologies by controlling the Na/K molar ratio of the initial gel mixtures. The morphology of the AM-6 materials changed from bulky cube to nanofiber aggregates as the Na/K molar ratio decreased from 1.9 to 0.2. Raman spectroscopy revealed that the VO3 (-) intermediate species plays an important role in the formation of the nanofiber morphology. The orientation of -V-O-V- chains in nanofiber aggregates was examined by confocal polarized micro-Raman spectroscopy. It was found that these aggregates are assemblies of short -V-O-V- chains perpendicular to the axis of nanofibers. The obtained AM-6 nanofibers greatly increase the exposed proportion of VO terminals, and thus improve the catalytic performance.

Flux-assisted synthesis of tungsten-doped layered perovskite oxychloride with promoted visible-light-responsive O2 evolution performance.

Here, we successfully prepared Ba2Bi3Ta2O11Cl via a simple one-step molten salt method and adjusted its crystal morphology and structure, based on which the O2-evolving activity was significantly improved. In addition, W doping promotes the charge separation efficiency, lowers the energy barrier for water oxidation reaction, and thus improves the activity. Finally, the optimized W-doped sample after molten salt treatment shows the best O2 production activity (55 µmol h-1) without loading any cocatalyst, which is 6 times higher than that of pristine Ba2Bi3Ta2O11Cl and 2 times higher than that of the undoped Ba2Bi3Ta2O11Cl treated with molten salt, respectively.

Suppressing non-radiative recombination in metal halide perovskite solar cells by synergistic effect of ferroelasticity.

The low fraction of non-radiative recombination established the foundation of metal halide perovskite solar cells. However, the origin of low non-radiative recombination in metal halide perovskite materials is still not well-understood. Herein, we find that the non-radiative recombination in twinning-tetragonal phase methylammonium lead halide (MAPbIxCl3-x) is apparently suppressed by applying an electric field, which leads to a remarkable increase of the open-circuit voltage from 1.12 V to 1.26 V. Possible effects of ionic migration and light soaking on the open-circuit voltage enhancement are excluded experimentally by control experiments. Microscopic and macroscopic characterizations reveal an excellent correlation between the ferroelastic lattice deformation and the suppression of non-radiative recombination. The calculation result suggests the existence of lattice polarization in self-stabilizable deformed domain walls, indicating the charge separation that facilitated by lattice polarization is accountable for the suppressed non-radiative recombination. This work provides an understanding of the excellent performance of metal halide perovskite solar cells.

A thorough investigation of the active titanium species in TS-1 zeolite by in situ UV resonance raman spectroscopy.

A thorough investigation of the active titanium species in TS-1 zeolite was conducted by in situ UV resonance Raman spectroscopy combined with UV/Vis diffuse reflectance spectroscopy, DFT calculations, and epoxidation experiments. A new titanium species was identified with a characteristic Raman band at 695 cm(-1) when excited at the 266 nm laser line. It is shown that the newly found titanium species is active in the epoxidation reactions in addition to the tetrahedrally coordinated titanium species. However, the acidity of the new titanium species could catalyze the ring-opening reactions of the epoxy products. It results in a lower selectivity toward the epoxy products relative to that of the tetrahedrally coordinated titanium species. The side reaction can be suppressed by the addition of a weak basic reagent.

"Extracting" the key fragment in ETS-10 crystallization and its application in AM-6 assembly.

The mechanism of crystallization of microporous titanosilicate ETS-10 was investigated by Raman spectroscopy combined with (29)Si magic-angle spinning (MAS) NMR spectroscopy, DFT calculations, and SEM imaging. The formation of three-membered ring species is shown to be the key step in the hydrothermal synthesis of ETS-10. They are formed by means of a complex process that involves the interaction of silicate species in the reaction mixture, which promotes the dissolution of TiO(2) particles. These insights into the mechanism of ETS-10 growth led to the successful development of a new synthesis route to the vanadosilicate AM-6 that involves the use of intermediates that contain three-membered ring species as an initiator.

Chiral sulfur compounds studied by Raman optical activity: tert-butanesulfinamide and its precursor tert-butyl tert-butanethiosulfinate.

Two chiral sulfur compounds, tert-butyl tert-butanethiosulfinate (1) and tert-butanesulfinamide (2), with inversion of configuration, have been studied by Raman optical activity (ROA) and electronic circular dichroism combined with density functional theory calculation. With the S-S linkage in 1, the couplings between the two tertiary carbon atoms often generate large ROA signals, whereas the tertiary carbon atom itself generally makes a large contribution to ROA signals in 2 for similar vibrational modes. The conformational dependence of ROA parameters provides probing conformation around the S-S bond from a new perspective. The simultaneous use of electronic circular dichroism and ROA is warranted to extract reliable conformational information. ROA provides a suitable candidate for the stereochemical study of chiral sulfur compounds, especially its capability of sensing the conformation around the S-S bond.

Photocatalytic overall water splitting promoted by an α-ß phase junction on Ga2O3.

When Alpha met Beta: a tuneable α-ß surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or ß-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-ß phase junction.

Switching between classical/nonclassical crystallization pathways of TS-1 zeolite: implication on titanium distribution and catalysis.

In the MFI zeolite crystallization process, the classical crystallization mechanism based upon the addition of silica species is often concomitant with the nonclassical route that is characteristic of the attachment of silica nanoparticle precursors. However, the factors that govern the preferences for each mechanism remain unclear. In this work, we present the impact of switching between these two crystallization pathways on the active sites and the resulting catalytic performance of the titanosilicate TS-1 zeolite. By controlling the self-assembled precursor structures in the early crystallization stage which are mediated by the Ti and H2O in the reaction system, we could achieve the preferred modes of crystal growth of the TS-1 zeolite. We indicate that by directing the predominant crystallization path from the classical to the nonclassical route, it is possible to generate more stable bridging peroxo species upon reaction with hydrogen peroxide, as confirmed by 17O solid-state nuclear magnetic resonance spectroscopy, thus substantially increasing the catalytic performance of the resulting TS-1 for olefin epoxidation.