The formation of semiconductor composites comprising multicomponent or multiphase heterojunctions is a very effective strategy to design highly active photocatalyst systems. This review summarizes the recent strategies to develop such composites, and highlights the most recent developments in the fi eld. After a general introduction into the different strategies to improve photocatalytic activity through formation of heterojunctions, the three different types of heterojunctions are introduced in detail, followed by a historical introduction to semiconductor heterojunction systems and a thorough literature overview. Special chapters describe the highly-investigated carbon nitride heterojunctions as well as very recent developments in terms of multiphase heterojunction formation, including the latest insights into the anatase-rutile system. When carefully designed, semiconductor composites comprising two or three different materials or phases very effectively facilitate charge separation and charge carrier transfer, substantially improving photocatalytic and photoelectrochemical effi ciency.

https://faculty.missouri.edu/~glaserr/3700s15/Marschall_Strategies_adfm201303214.pdf

Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity

The constructing of direct solid-state Z-scheme heterojunction photocatalytic system has received much attention in environmental purification and hydrogen generation from water. In this study, a novel direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions were synthesized via a facile in-situ growth strategy for the first time. The photocatalytic performance was evaluated by the degradation of rhodamine B (RhB) and tetracycline (TC) under visible light irradiation (λ > 420 nm). Results show that the as-synthesized heterojunctions can significantly enhance photocatalytic activity in comparison with pure g-C3N4 and V2O5. The optimum photocatalytic efficiency of VC1.0% sample for the degradation RhB was about 7.3 and 13.0 times higher than that of individual g-C3N4 and V2O5, respectively. In addition, the VC1.0% sample as well as can efficiently degrade methyl orange (MO) and methylene blue (MB) under visible light. By further experimental study, the possible for the enhancing photocatalytic mechanism was found to be a direct solid-state Z-scheme heterojunction system based on the active species trapping and electron spin resonance (ESR) experiments, which not only can improve the photogenerated electron–hole pair’s separation but also exhibit a strong oxidation and reduction ability for efficiency degradation of organic pollutants. This work will be useful for the design of other direct solid-state Z-scheme photocatalytic systems for application in energy conversion and environmental remediation.

In-situ synthesis of direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions with enhanced visible light efficiency in photocatalytic degradation of pollutants

The high photocatalytic activity of mixed phase (80% anatase and 20% rutile) titanium dioxide (Degussa P25) has attracted a great deal of interest in recent years. However, its low efficiency in visible light and nonporous nature limits the potential use and capabilities. Here, we report a novel preparation method for crystalline, thermally stable (up to 800 °C) TiO2 materials with tunable anatase/rutile phase compositions (0–100%) and monomodal mesoporosity. The control of the phase compositions was achieved by framework vanadium doping and various applied heat treatments. Vanadium (0% to 10% doping) decreased the anatase–rutile transformation temperature (from 1000 to 600 °C) and shifted the absorption band to the visible light region (narrowed the band gap). The mesopore structure was preserved in mixed phase TiO2. These materials are members of the recently discovered University of Connecticut (UCT) mesoporous materials family. The UCT materials are randomly packed nanoparticle aggregates and mesopore...

Crystalline Mixed Phase (Anatase/Rutile) Mesoporous Titanium Dioxides for Visible Light Photocatalytic Activity

MoS2 and Ni-promoted MoS2 catalysts supported on γ-Al2O3, siliceous SBA-15, and Zr- and Ti-modified SBA-15 were explored for the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrodenitrogenation (HDN) of o-propylaniline (OPA). In all cases, OPA reacted preferentially via initial hydrogenation, and DBT was converted through direct sulfur removal. HDN and HDS activities of MoS2 catalysts are determined by the dispersion of the sulfide phase. Ni promotion increased its dispersion and activity for DBT HDS and also increased the rate of HDN via enhancing the rate of hydrogenation. On nonpromoted MoS2 catalysts, HDS was strongly inhibited by NH3, and the addition of Ni dramatically reduced this inhibiting effect. The conclusion is that HDS is proportional to the concentration of Mo and Ni on the edges of sulfide particles. In contrast, the direct hydrodenitrogenation of OPA occurs only on accessible Mo cations and, hence, decreases with increasing Ni substitution. The nature of the supp...

Effects of the Support on the Performance and Promotion of (Ni)MoS2 Catalysts for Simultaneous Hydrodenitrogenation and Hydrodesulfurization

CO2 reforming of methane over highly active La-promoted Ni supported on SBA-15 catalysts: mechanism and kinetic modelling

Diverse approaches are being made in search of a visible-light active TiO2 photocatalysts. In this study, S-TiO2(x) nanocatalysts with x = 2, 4 and 6 (x = S/Ti molar ratio) were synthesized by sol–gel method and V2O5/S-TiO2(x) nanocomposites by wet impregnation method using ammonium metavanadate as a source of vanadium. The synthesized catalysts were characterized by diffuse reflectance UV–vis spectroscopy, X-ray diffraction, Raman spectroscopy, N2 adsorption desorption studies, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and high resolution transmission electron microscopy techniques. The photogenerated reactive species OH radical estimated from photoluminescence spectra was directly correlated to the visible-light absorption of the catalyst. The photocatalytic activities of all the synthesized catalysts were evaluated for the degradation of MB under visible light irradiation. The excellent stability of V2O5/S-TiO2(x) nanocomposites has been revealed from the linear relationship between the fluorescence intensity and the time. The addition of sulfur and V2O5 to TiO2 has significantly enhanced the visible-light photocatalytic activity by modifying the optical and electronic properties of TiO2. The enhanced visible-light photocatalytic activity has been attributed to the efficient visible-light absorption of V2O5/S-TiO2(x) nanocomposites and efficient charge separation due to migration of charge carriers between S-TiO2 and V2O5.

Enhanced visible-light photocatalytic activity of V2O5/S-TiO2 nanocomposites

Mesoporous Al-SBA-15 with different Si/Al molar ratio (10, 20, 30, and 40) was synthesized hydrothermally, and Mo–Ni catalysts were prepared by incipient wet impregnation method and characterized by different techniques. An impressive catalytic performance was observed on Mo–Ni/Al-SBA-15(10) for Methylcyclohexylamine conversion to hydrocarbons. The observed trend of activity is correlated with the aluminum content, and it is concluded that the variation in Si/Al ratio remarkably affects the hydrodenitrogenation ability of Al-SBA-15 supported Mo–Ni catalysts.

Mo–Ni/Al-SBA-15 (Sulfide) Catalysts for Hydrodenitrogenation: Effect of Si/Al Ratio on Catalytic Activity

A facile hydrothermal method was adopted for the synthesis of bare TiO₂ and titania nanotubes (TNT). In an effort to increase the efficacy of the existing photocatalyst, different weight percentage (0.2, 0.4, 0.6, 0.8 and 1.0%) of praseodymium were deftly doped on to the synthesized titania scaffolds. The physicochemical characteristics of the architectured photocatalyst were thoroughly elucidated by various sophisticated techniques. The doping of Pr₂O₃ (Pr) on to titania nanotubes (TNT) resulted into a significant bathochromic/hyperchromic shift in the optical absorption edge (towards the visible region) as perceived from the DRS-UV spectra. The XRD and TEM analysis showed average crystallite size of the synthesised photocatalyst to be as small as 4-7 nm with well-formed nanotube framework. Photoluminescence spectra of Pr doped TNT catalyst clearly exhibited greater suppression of photogenerated electron-hole pair as compared to the undoped counterparts. The photocatalytic activity of the synthesized catalysts was evaluated towards the degradation of organic pollutants namely Rhodamine B (90%) and Crystal violet (93%) in the presence of solar light and its activity and durability was compared to that of commercial TiO₂ (Degussa P25). The observed enhanced photocatalytic activity of TNT and Pr-TNT can be unambiguously attributed to the inhibition of recombination of the electron-hole pairs due to doping of Pr into TNT. Among catalysts synthesized, 0.4 wt.% of Pr on to TNT yielded the highest photocatalytic activity under visible light irradiation.

Enhanced Visible Light Activity of Pr-TiO₂ Nanocatalyst in the Degradation of Dyes: Effect of Pr Doping and TiO₂ Morphology.

The influence of Si/V ratio of mesoporous V-SBA-15 on the structure and catalytic behavior has been investigated for the vapor phase oxidation of diphenylmethane (DPM) to benzophenone (BP) using CO2 free air as oxidant under atmospheric pressure. The reaction conditions were optimized for a maximum conversion of DPM and selectivity for BP on V-SBA-15 (25). These catalysts were synthesized with different Si/V ratios of 10, 25, 50, 75 and 100 by the direct hydrothermal method under mild acidic conditions and characterized by using X-ray diffraction, ICP-OES, EDAX, DRUV–Vis, photoluminescence and transmission electron microscopy techniques. It was found that a large amount of vanadium is incorporated mainly in isolated tetrahedral environment with a terminal V=O bond. Under the optimized conditions, the activity of all the catalysts were compared and found that V-SBA-15 with Si/V = 25 ratio, exhibited the maximum activity. The difference in the catalytic activity is related to their physico-chemical properties.

Structural and catalytic properties of V-SBA-15 for the vapor phase oxidation of diphenylmethane

The Ce1-xMnxO2 nanocatalysts (x = 0.25, 0.50 and 0.75 wt.%) were synthesized by sol-gel method. The catalysts were characterized using various techniques such as XRD, N2 sorption study, DRSUV-Vis, TPR, SEM and TEM. The incorporation of Mn ions into the ceria lattice was confirmed by XRD analysis. DRUV-Vis spectra confirm the presence of Ce3+ ions in the lattice of Ce1-xMnxO2. H2-TPR study revealed the oxygen storage capacity of the catalyst. The 3D flowerlike morphology of the nanocatalysts was confirmed from FESEM and HRTEM images. The catalytic activity was tested for the vapor phase oxidation of cyclohexane using air as an oxidant. The key reaction parameters were varied to study the stability, activity and selectivity of the catalysts. The study concluded that suitable amount of manganese content is essential for the selective oxidation of cyclohexane at low temperature and Ce0.25Mn0.75O2 is the most suitable catalyst for high conversion and selectivity under the given reaction conditions. The activity of the catalyst is correlated with the characterization results.

M. Gurulakshmi

Papers

Enhanced visible-light photocatalytic activity of V2O5/S-TiO2 nanocomposites

Mo–Ni/Al-SBA-15 (Sulfide) Catalysts for Hydrodenitrogenation: Effect of Si/Al Ratio on Catalytic Activity

Enhanced Visible Light Activity of Pr-TiO₂ Nanocatalyst in the Degradation of Dyes: Effect of Pr Doping and TiO₂ Morphology.

Structural and catalytic properties of V-SBA-15 for the vapor phase oxidation of diphenylmethane

Selective oxidation of cyclohexane using Ce1-xMnxO2 nanocatalysts.