The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation. Here, the photocatalytic oxidation of CH4 into CH3OH by O2 is carried out on Ag-decorated facet-dominated TiO2. The {001}-dominated TiO2 shows a durable CH3OH yield of 4.8 mmol g−1 h−1 and a selectivity of approximately 80%, which represent much higher values than those reported in recent studies and are better than those obtained for {101}-dominated TiO2. Operando Fourier transform infrared spectroscopy, electron spin resonance, and nuclear magnetic resonance techniques are used to comprehensively clarify the underlying mechanism. The straightforward generation of oxygen vacancies on {001} by photoinduced holes plays a key role in avoiding the formation of •CH3 and •OH, which are the main factors leading to overoxidation and are generally formed on the {101} facet. The generation of oxygen vacancies on {001} results in distinct intermediates and reaction pathways (oxygen vacancy → Ti–O2• → Ti–OO–Ti and Ti–(OO) → Ti–O• pairs), thus achieving high selectivity and yield for CH4 photooxidation into CH3OH.

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.

The structures and local environments of boron species in B-doped and (B, N)-codoped TiO2 photocatalysts have been investigated by solid-state 11B NMR spectroscopy in conjunction with density functional theory (DFT) calculations. Up to seven different boron sites were identified in the B-doped anatase TiO2, which may be classified into three categories, including interstitial, bulk BO3/2 polymer, and surface boron species, and has been supported by results obtained from FT-IR and XPS spectroscopy as well as from DFT calculations. Two types of interstitial borons, namely the tricoordinated (T*)- and pseudotetrahedral-coordinated (Q*) borons, were observed in addition to the two types of bulk BO3/2 polymer and three types of surface B, in good agreement with experimental data. Further density of state analyses revealed that, compared to undoped TiO2, the T* species in boron-doped TiO2 are solely responsible for the observed increase in energy band gap, whereas the presence of Q* species tend to lead to a decrease in band gap and hence are more favorable for the absorption in the visible-light region. In comparison with B- and N-doped TiO2, (B, N)-codoped TiO2 tends to exhibit a much higher visible-light photocatalytic activity for the oxidation of rhodamine B. Accordingly, a photochemical mechanism of the (B, N)-codoped TiO2 under visible-light irradiation is proposed.

A unique insight into the acidic nature of the tri-coordinated framework aluminum (AlFR) in H-ZSM-5 zeolite catalysts has been provided using multi-nuclear and multi-dimensional solid-state NMR spectroscopy in conjunction with TMPO probe molecules.

1H–71Ga internuclear spatial proximity/interaction between Brønsted acid site (BAS) and cationic Ga species (Lewis acid sites) in Ga-modified ZSM-5 zeolites, which leads to a synergic effect in the methanol-to-aromatics (MTA) conversion, was identified with solid-state NMR spectroscopy. The internuclear distance between BAS and Ga species was measured, which is similar to that of a neighboring BAS pair located in the six-membered rings of ZSM-5. The Brønsted acidity of the Ga-modified zeolite was considerably enhanced due to the synergic effect, and the synergic active sites were quantified by 1H–71Ga double-resonance solid-state NMR, which shows a correlation with the aromatics selectivity in the MTA reaction.

The methanol-to-aromatics (MTA) reaction was investigated on Ga-modified ZSM-5 zeolites (Ga/ZSM-5). As revealed by 71Ga and 1H solid-state NMR and FT-IR of pyridine adsorption measurements, cationic Ga species are formed as Lewis acid sites by substitution of Brønsted acid sites on H-ZSM-5. Further experimental studies show that C5- and C6-cycloalkenes are generated during the MTA reaction, which lead to the formation of cyclic carbocations as precursors to aromatics. Isotope exchange experiments demonstrate that the reactivity of the cyclic carbocations on Ga/ZSM-5 is much higher than that on H-ZSM-5 and they play an intermediate role in the formation of aromatics. In addition to the traditional bimolecular hydrogen transfer (HT) reaction, the dehydrogenation of alkenes with the release of H2 (DeaH2 process) was identified to significantly contribute to the formation of aromatics. The transformation of cycloalkenes to aromatics is favored by promotion of the DeaH2 process and competes with the HT route on Ga/ZSM-5, while these cycloalkenes tend to crack back to alkenes, and the dominating HT route results in lower aromatic selectivity on H-ZSM-5. A DeaH2-aromatization route mediated by the cooperation of cationic Ga species and Brønsted acid sites was proposed for the enhanced formation of aromatics on Ga/ZSM-5 zeolite.

The detailed structure-activity relationship of surface hydroxyl groups (Ti-OH) and adsorbed water (HO) on the TiO 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 H solid-state NMR techniques were employed to identify surface hydroxyl groups and adsorbed water molecules as well as their spatial proximity/interaction in TiO photocatalysts. It was found that although the two different types of Ti-OH (bridging hydroxyl (OH) and terminal hydroxyl (OH) groups were present on the TiO surface, only the former is in close spatial proximity to adsorbed HO, forming hydrated OH. In situ H andC NMR studies of the photocatalytic reaction on TiO with different Ti-OH groups and different HO loadings illustrated that the enhanced activity was closely correlated to the amount of hydrated OH groups. To gain insight into the role of hydrated OH groups in the h transfer process, in situ ESR experiments were performed on TiO with variable HO loading, which revealed that the hydrated OH groups offer a channel for the transfer of photogenerated holes in the photocatalytic reaction, and the adsorbed HO could have a synergistic effect with the neighboring OH 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 TiO was proposed.

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