Ningdong Feng

Understanding the High Photocatalytic Activity of (B, Ag)-Codoped TiO2 under Solar-Light Irradiation with XPS, Solid-State NMR, and DFT Calculations

The origin of the exceptionally high activity of (B, Ag)-codoped TiO(2) catalysts under solar-light irradiation has been investigated by XPS and (11)B solid-state NMR spectroscopy in conjunction with density functional theory (DFT) calculations. XPS experimental results demonstrated that a portion of the dopant Ag (Ag(3+)) ions were implanted into the crystalline lattice of (B, Ag)-codoped TiO(2) and were in close proximity to the interstitial B (B(int.)) sites, forming [B(int.)-O-Ag] structural units. In situ XPS experiments were employed to follow the evolution of the chemical states of the B and Ag dopants during UV-vis irradiation. It was found that the [B(int.)-O-Ag] units could trap the photoinduced electron to form a unique intermediate structure in the (B, Ag)-codoped TiO(2) during the irradiation, which is responsible for the photoinduced shifts of the B 1s and Ag 3d peaks observed in the in situ XPS spectra. Solid-state NMR experiments including (11)B triple-quantum and double-quantum magic angle spinning (MAS) NMR revealed that up to six different boron species were present in the catalysts and only the tricoordinated interstitial boron (T*) species was in close proximity to the substitutional Ag species, leading to formation of [T*-O-Ag] structural units. Furthermore, as demonstrated by DFT calculations, the [T*-O-Ag] structural units were responsible for trapping the photoinduced electrons, which prolongs the life of the photoinduced charge carriers and eventually leads to a remarkable enhancement in the photocatalytic activity. All these unprecedented findings are expected to be crucial for understanding the roles of B and Ag dopants and their synergistic effect in numerous titania-mediated photocatalytic reactions.

Direct visualization of a guest-triggered crystal deformation based on a flexible ultramicroporous framework

Host-guest composites may exhibit abnormal and/or controllable physical properties that are unavailable for traditional solids. However, it is still very difficult to control or visualize the occupancy and motion of the guest. Here we report a flexible ultramicroporous coordination polymer showing exceptional guest-responsive thermal-expansion properties. The vacant crystal exhibits constant and huge thermal expansion over a wide temperature range not only in vacuum but also in air, as its ultramicroporous channel excludes air adsorption even at 77 K. More interestingly, as demonstrated by single-crystal X-ray crystallography, molecular dynamic simulations and solid-state nuclear magnetic resonance, it selectively responds to the molecular rearrangement of N,N-dimethylformamide, leading to conformation reversion of the flexible ligand, which transfers these actions to deform the whole crystal lattice. These results illustrate that combination of ultramicroporous channel and flexible pore surface could be an effective strategy for the utilization of external physical and chemical stimuli.

New insights into Keggin-type 12-tungstophosphoric acid from 31P MAS NMR analysis of absorbed trimethylphosphine oxide and DFT calculations.

The acid and transport properties of the anhydrous Keggin-type 12-tungstophosphoric acid (H(3)PW(12)O(40); HPW) have been studied by solid-state (31)P magic-angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, (31)P NMR resonances arising from various protonated complexes, such as TMPOH(+) and (TMPO)(2)H(+) adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)(2)H(+) adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to (31)P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH(+) complexes are associated with protons located at three different terminal oxygen (O(d)) sites of the PW(12)O(40)(3-) polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.

Direct Detection of Supramolecular Reaction Centers in the Methanolto- Olefins Conversion over Zeolite H-ZSM-5 by C-13-Al-27 Solid-State NMR Spectroscopy

Hydrocarbon-pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon-pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H-ZSM-5 zeolite by advanced (13)C-(27)Al double-resonance solid-state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the (13)C nuclei (associated with HP species) and the (27) Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.

Signal enhancement of J-HMQC experiments in solid-state NMR involving half-integer quadrupolar nuclei

We show that for half-integer quadrupolar nuclei, the manipulation of the satellite transitions can accelerate and enhance coherence transfer to other isotopes. This novel strategy is demonstrated to improve the sensitivity of (31)P-{(27)Al} J-HMQC experiments for a layered aluminophosphate Mu-4.

Experimental Evidence on the Formation of Ethene through Carbocations in Methanol Conversion over H-ZSM-5 Zeolite

The methanol to olefins conversion over zeolite catalysts is a commercialized process to produce light olefins like ethene and propene but its mechanism is not well understood. We herein investigated the formation of ethene in the methanol to olefins reaction over the H-ZSM-5 zeolite. Three types of ethylcyclopentenyl carbocations, that is, the 1-methyl-3-ethylcyclopentenyl, the 1,4-dimethyl-3-ethylcyclopentenyl, and the 1,5-dimethyl-3-ethylcyclopentenyl cation were unambiguously identified under working conditions by both solid-state and liquid-state NMR spectroscopy as well as GC-MS analysis. These carbocations were found to be well correlated to ethene and lower methylbenzenes (xylene and trimethylbenzene). An aromatics-based paring route provides rationale for the transformation of lower methylbenzenes to ethene through ethylcyclopentenyl cations as the key hydrocarbon-pool intermediates.

Transfer Channel of Photoinduced Holes on a TiO2 Surface As Revealed by Solid-State Nuclear Magnetic Resonance and Electron Spin Resonance Spectroscopy

The detailed structure-activity relationship of surface hydroxyl groups (Ti-OH) and adsorbed water (H2O) on the TiO2 surface should be the key to clarifying the photogenerated hole (h+) transfer mechanism for photocatalytic water splitting, which however is still not well understood. Herein, one- and two-dimensional 1H solid-state NMR techniques were employed to identify surface hydroxyl groups and adsorbed water molecules as well as their spatial proximity/interaction in TiO2 photocatalysts. It was found that although the two different types of Ti-OH (bridging hydroxyl (OHB) and terminal hydroxyl (OHT) groups were present on the TiO2 surface, only the former is in close spatial proximity to adsorbed H2O, forming hydrated OHB. In situ 1H and 13C NMR studies of the photocatalytic reaction on TiO2 with different Ti-OH groups and different H2O loadings illustrated that the enhanced activity was closely correlated to the amount of hydrated OHB groups. To gain insight into the role of hydrated OHB groups in the h+ transfer process, in situ ESR experiments were performed on TiO2 with variable H2O loading, which revealed that the hydrated OHB groups offer a channel for the transfer of photogenerated holes in the photocatalytic reaction, and the adsorbed H2O could have a synergistic effect with the neighboring OHB group to facilitate the formation and evolution of active paramagnetic intermediates. On the basis of experimental observations, the detailed photocatalytic mechanism of water splitting on the surface of TiO2 was proposed.

Unravelling the Efficient Photocatalytic Activity of Boron-induced Ti3+ Species in the Surface Layer of TiO2

Ti3+ species are highly unstable in air owing to their facile oxidation into Ti4+ species, and thus they cannot concentrate in the surface layer of TiO2 but are mainly present in its bulk. We report generation of abundant and stable Ti3+ species in the surface layer of TiO2 by boron doping for efficient utilization of solar irradiation. The resultant photocatalysts (denoted as B-TiO2−x) exhibit extremely high and stable solar-driven photocatalytic activity toward hydrogen production. The origin of the solar-light activity enhancement in the B-TiO2−x photocatalysts has been thoroughly investigated by various experimental techniques and density functional theory (DFT) calculations. The unique structure invoked by presence of sufficient interstitial boron atoms can lead to substantial variations in density of states of B-TiO2−x, which not only significantly narrow the band gap of TiO2 to improve its visible-light absorption, but also promote the photogenerated electron mobility to enhance its solar-light photocatalytic activity.

Acidity of Solid and Liquid Acids Probed by P-31 NMR Chemical Shifts of Phosphine Oxides

A novel method for acidity characterization of solid and liquid acids is proposed. Surpassing conventional methods, this technique employs 31 P nuclear magnetic resonance (NMR) of adsorbed phosphine oxides is capable of

Heteronuclear correlation experiments of 23Na-27Al in rotating solids ☆

We demonstrated that the heteronuclear correlation experiments between two quadrupolar nuclei, 23Na and 27Al, with close Larmor frequencies can be achieved via D-HMQC and D-RINEPT approaches by using a diplexer connected to a conventional probe in magic-angle-spinning solid-state NMR. Low-power heteronuclear dipolar recoupling schemes can be applied on 23Na or 27Al to establish polarization transfers between the central transitions of 23Na and 27Al for a model compound, NaAlO2. Further, we showed a practical implementation of the two dimensional 23Na-27Al dipolar-based heteronuclear correlation experiment on a heterogeneous catalyst, Na2CO3/γ-Al2O3. This allows to determine spatial proximities between different 23Na and 27Al sites, thus the surface Na species adjacent to octahedral-coordination Al can be clearly discriminated.