Landong Li

Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile

The sole effect of surface/bulk defects of TiO2 samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO2 were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400-700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption-desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO2 samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO2 samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO2 and rutile TiO2. The surface defects of TiO2, i.e. oxygen vacancy clusters, could promote the separation of electron-hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.

Sub-10 nm rutile titanium dioxide nanoparticles for efficient visible-light-driven photocatalytic hydrogen production

Titanium dioxide is a promising photocatalyst for water splitting, but it suffers from low visible light activity due to its wide band gap. Doping can narrow the band gap of titanium dioxide; however, new charge-carrier recombination centres may be introduced. Here we report the design of sub-10 nm rutile titanium dioxide nanoparticles, with an increased amount of surface/sub-surface defects to overcome the negative effects from bulk defects. Abundant defects can not only shift the top of the valence band of rutile titanium dioxide upwards for band-gap narrowing but also promote charge-carrier separation. The role of titanium(III) is to enhance, rather than initiate, the visible-light-driven water splitting. The sub-10 nm rutile nanoparticles exhibit the state-of-the-art activity among titanium dioxide-based semiconductors for visible-light-driven water splitting and the concept of ultra-small nanoparticles with abundant defects may be extended to the design of other robust semiconductor photocatalysts.

Tungsten oxide single crystal nanosheets for enhanced multichannel solar light harvesting.

Substoichiometric tungsten oxide single-crystal nanosheets are successfully prepared via the exfoliation of layered tungstic acid and subsequent introduction of oxygen vacancies. The combination of different strategies, i.e., 2D-structure construction, the introduction of surface oxygen vacancies, and the creation of localized surface plasmon resonance can promote the light-harvesting performance of tungsten oxide through accumulative and synergistic effects.

Mesoporous Zr-Beta zeolites prepared by a post-synthetic strategy as a robust Lewis acid catalyst for the ring-opening aminolysis of epoxides

Mesoporous zirconosilicate, stannosilicate, and titanosilicate with the BEA structure framework have been prepared from the commercially available Beta zeolite via acid–alkaline treatments and subsequent dry impregnation of appropriate organometallic precursors. N2 adsorption–desorption isotherms and TEM observations confirm that alkaline treatment can induce desilication to create intra-crystalline mesopores from the partially dealuminated Beta sample. The incorporation of Zr species into the zeolite framework at formed silanol defect sites is monitored by infrared and 1H MAS NMR spectroscopy. Characterization results from UV-vis and XPS reveal that the majority of the incorporated Zr species exist in the form of tetrahedral Zr(IV) in the zeolite framework. The creation of Lewis acid sites with moderate acid strength upon Zr incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The as-prepared mesoporous Zr-Beta exhibits a remarkable catalytic activity and regio-selectivity to β-amino alcohols in the ring-opening aminolysis of epoxides, and the presence of mesopores can promote the reaction to a great extent through enhanced mass transfer. The impacts of the Lewis acidity of the catalysts, the basicity of amines and adsorption of reactants on the ring-opening aminolysis of epoxides are discussed in detail.

Solvent-free selective photocatalytic oxidation of benzyl alcohol over modified TiO2

Heterogeneous photocatalysis offers a promising route to realize green oxidation processes in organic synthesis. In this research, the solvent-free selective photocatalytic oxidation of benzyl alcohol to benzaldehyde in the presence molecular oxygen was studied by using TiO2 and modified TiO2 as photocatalysts. The surface modification of TiO2 by transition metal clusters dramatically enhanced the photocatalytic oxidation activity. Ir/TiO2 prepared by photodeposition showed a remarkably high activity for the photocatalytic oxidation of benzyl alcohol, and an average reaction rate of 14538 μmol h−1 gcat−1 could be obtained. The effect of preparation method, iridium loading and reaction conditions on the catalytic performance of Ir/TiO2 is investigated in detail. Based on the catalytic and characterization results, the problem of product selectivity and the reaction mechanism of the photocatalytic oxidation of benzyl alcohol over Ir/TiO2 are discussed.

A study on N2O catalytic decomposition over Co/MgO catalysts.

Different oxide supported cobalt catalysts were prepared by co-precipitation method and tested for the decomposition of nitrous oxide. Co/MgO with cobalt loading of 15% showed the best activity and a 100% N(2)O conversion was obtained at temperatures higher than 700 K. The active phase of cobalt species in Co/MgO catalysts was Co(3)O(4) highly dispersed in the matrices of MgO, based on XRD and XPS results as well as the kinetic analysis. The existence of NO, O(2) and H(2)O in reaction system showed different negative effects on N(2)O decomposition. Nevertheless, a 100% N(2)O conversion could be achieved at 800 K under simulated conditions of tail gas from nitric acid plant. Moreover, Co/MgO catalyst exhibited quite good durability and no obvious activity loss was observed in the 100 h stability test.

Synergetic promotion of the photocatalytic activity of TiO2 by gold deposition under UV-visible light irradiation.

Under UV-visible light irradiation, multiple synergetic promotion effects on the photocatalytic activity of TiO2 by gold deposition can be observed, and plasmonic Au/TiO2 shows great potential for photocatalytic solar conversion.

A procedure for the preparation of Ti-Beta zeolites for catalytic epoxidation with hydrogen peroxide

Ti-Beta zeolite has been successfully prepared via a reproducible and scalable two-step post-synthesis strategy, which consists of creating vacant T sites with associated silanol groups by dealumination of H-Beta and subsequent dry impregnation of the resulting Si-Beta with titanocene dichloride. The mechanism of Ti incorporation into the framework of Beta is investigated by diffuse reflectance infrared Fourier transform (DRIFT) and multinuclear solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Characterization results obtained from diffuse reflectance ultraviolet-visible (UV-vis) and X-ray photoelectron spectroscopy (XPS) reveal that the majority of incorporated Ti species exist in the form of isolated tetrahedrally coordinated Ti(IV) in the zeolite framework while a minority exists in the form of isolated octahedrally coordinated Ti(VI) at framework or extra-framework positions. The obtained Ti-Beta zeolites are highly active and selective catalysts for the epoxidation of unsaturated ketones, e.g. 2-cyclohexen1-one, with hydrogen peroxide as an oxidant. A quasilinear correlation between the epoxidation rate and the number of framework Ti(IV) species could be drawn evidencing that these Ti(IV) species are responsible for the epoxidation activity of the Ti-Beta zeolites under study. The impact of preparation parameters and reaction conditions on the catalytic performances of the Ti-Beta zeolites in the epoxidation of unsaturated organic compounds with hydrogen peroxide is discussed in detail.

Low temperature CO oxidation on Cu–Cu2O/TiO2 catalyst prepared by photodeposition

A series of Me/TiO2 samples (Me = V, Cr, Mn, Fe, Co, Ni, Cu and Zn) with designed Me:TiO2 ratio of 1:10 were prepared by a photodeposition method and studied for the oxidation of CO. The Cu/TiO2 catalyst exhibited remarkably high activity and an overall CO oxidation could be achieved at <100 °C. The effects of activation temperatures, GHSV and initial CO concentration on CO oxidation over Cu/TiO2 were further investigated. XRD, TEM, XPS-AES, H2-TPR and FTIR of CO adsorption were employed to characterize Cu/TiO2 samples and the exact composition of Cu/TiO2 prepared by photo-deposition was determined to be Cu–Cu2O/TiO2. Based on the catalytic and characterization results, the possible mechanism for CO oxidation over Cu–Cu2O/TiO2 was discussed. Finally, the durability and deactivation of Cu–Cu2O/TiO2 during CO oxidation was investigated in time-on-stream reaction.

Fate of Brønsted Acid Sites and Benzene-Based Carbenium Ions During Methanol-to-Olefin Conversion on SAPO-34

The silicoaluminophosphate SAPO-34 with chabazite-cages and 8-ring windows is the most interesting candidate for application as shape-selective acidic catalyst in the conversion of methanol to olefins (MTO). As an important advantage, SAPO-34 has a high selectivity to light olefins, such as ethene and propene. On the other hand, this catalyst shows a rapid deactivation depending on the weight hourly space velocity of the reactant methanol. A number of studies focused on the mechanism of the MTO reaction on SAPO-34, which indicated that a hydrocarbon pool consisting mainly of polyalkylaromatics formed during an induction period. In the steady-state of the MTO reaction, carbenium ions, formed from polyalkylaromatics on Brønsted acidic SAPO-34 catalysts, were suggested to act as catalytically active compounds, which reacted with methanol to form larger hydrocarbons and split-off olefins. Polycyclic aromatics, which formed upon long times-on-stream by the above-mentioned aromatics, were discussed as being responsible for the catalyst deactivation as demonstrated, for example, by analyzing the GC-MS data of coke compounds upon dissolution of the catalyst framework. 2] The work presented here focused on the investigation of the number of accessible Brønsted acid sites and benzenebased carbenium ions on working SAPO-34 catalysts after different MTO reaction times by applying a novel method consisting of ammonia adsorption and quantitative solid-state NMR spectroscopy of the different types of formed ammonium ions. Catalyst samples were taken from the fixed-bed reactor for H and C solid-state NMR studies after various MTO conversion times until the catalyst was deactivated. For this purpose, the MTO reaction was quenched after different times-on-stream, and then the catalyst samples were transferred from the fixedbed reactor to the MAS NMR rotors, without contact to air, and loaded with ammonia. An important prerequisite for this study was the application of H high-speed MAS NMR spectroscopy, which led to a significant improvement of the resolution of H MAS NMR spectra and a high sensitivity for the different surface species under study. Silicoaluminophophate SAPO-34, which had an nSi/ (nAl+nSi+nP) ratio of 0.11, was used. [3] Upon preparation and calcination of this catalyst (see the Supporting Information), the methanol conversion was performed in a standard fixedbed reactor connected to a glass fiber UV/Vis spectrometer. Figure 1 shows the conversion of methanol and selectivities to