A review on identification, quantification, and transformation of active species in SCR by EPR spectroscopy

Electron paramagnetic resonance (EPR) is the only technique that provides direct detection of free radicals and samples that contain unpaired electrons. Thus, EPR had an important potential application in the field of selective catalytic reduction of nitrogen oxide (SCR). For the first time, this work reviewed recent developments of EPR in charactering SCR. First, qualitative analysis focused on recognizing Cu, Fe, V, Ti, Mn, and free-radical (oxygen vacancy and superoxide radical) species. Second, quantification of the active species was obtained by a double-integral and calibration method. Third, the active species evolved because of different thermal treatments and redox-thermal processes under reductants (NH₃ and NO). The coordination information of the active species in catalysts and their effects on SCR performances were concluded from mechanism viewpoints. Finally, potential perspectives were put forward for EPR developments in characterizing the SCR processes in the future. After all, EPR characterization will help to have a deep understanding of structure-activity relationship in one catalyst.

MOF-Derived Defective Co3O4 Nanosheets in Carbon Nitride Nanocomposites for CO2 Photoreduction and H2 Production

In photocatalysis, especially in CO₂ reduction and H₂ production, the development of multicomponent nanomaterials provides great opportunities to tune many critical parameters toward increased activity. This work reports the development of tunable organic/inorganic heterojunctions comprised of cobalt oxides (Co₃O₄) of varying morphology and modified carbon nitride (CN), targeting on optimizing their response under UV-visible irradiation. MOF structures were used as precursors for the synthesis of Co₃O₄. A facile solvothermal approach allowed the development of ultrathin two-dimensional (2D) Co₃O₄ nanosheets (Co₃O₄-NS). The optimized CN and Co₃O₄ structures were coupled forming heterojunctions, and the content of each part was optimized. Activity was significantly improved in the nanocomposites bearing Co₃O₄-NS compared with the corresponding bulk Co₃O₄/CN composites. Transient absorption spectroscopy revealed a 100-fold increase in charge carrier lifetime on Co₃O₄-NS sites in the composite compared with the bare Co₃O₄-NS. The improved photocatalytic activity in H₂ production and CO₂ reduction is linked with (a) the larger interface imposed from the matching 2D structure of Co₃O₄-NS and the planar surface of CN, (b) improvements in charge carrier lifetime, and (c) the enhanced CO₂ adsorption. The study highlights the importance of MOF structures used as precursors in forming advanced materials and the stepwise functionalization of the individual parts in nanocomposites for the development of materials with superior activity.

Ionic liquid/TiO2 nanoparticles doped with non-expensive metals: new active catalyst for phenol photodegradation

TiO₂ nanoparticles were synthesized using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) ionic liquid and doped with non-expensive metals Cu²⁺ and Fe³⁺ by the sol–gel method. The new generated photocatalysts had their morphological, textural and structural characteristics analysed by scanning electron microscopy and dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), Brunauer–Emmett–Teller analysis (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and diffuse reflectance spectroscopy (DRS). The results showed two phases by XRD analysis, anatase (majority) and rutile (minority). The SEM micrographs exposed spherical TiO₂ NPs/BMI·BF₄ IL and compact layers for Cu²⁺ and Fe³⁺-doped TiO₂ NPs in BMI·BF₄ IL, the EDX confirmed only the presence of Ti, O, Fe and Cu. The BET and BJH analyses exhibited high porous TiO₂ NPs/BMI·BF₄ IL. The BET and BJH analyses confirmed that the pore diameter of mesoporous materials was between 12 and 16 nm with similar values for surface area (55–63 m² g⁻¹). The TEM images exhibited spherical shape nanoparticles with mean diameter of 20–22 nm. The DRS analysis and Tauc equation were applied to estimate the optical energy band gap of the photocatalysts. The energy band gap values of 3.1 eV, 3.32 eV, and 2.78 eV were obtained for TiO₂ NPs/BMI·BF₄ IL, 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL and 1% Cu²⁺-doped TiO₂ NPs/BMI·BF₄ IL, respectively. Phenol photodegradation was realized using Cu²⁺ and Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL under UV/visible irradiation and quantified by HPLC-FLD. The phenol photodegradation was investigated by different concentrations of metal-doped TiO₂ NPs/BMI·BF₄ IL. The new active photocatalysts 1% Cu²⁺-doped TiO₂ NPs and 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL exhibited high catalytic activity (99.9% and 96.8%, respectively). The photocatalysts 1% Cu²⁺ and 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL were also evaluated using industrial wastewater from the tobacco industry. The results showed 56.7% phenol photodegradation, due to the complexity of the tobacco matrix wastewater.

TiO₂ nanoparticles were synthesized using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) ionic liquid and doped with non-expensive metals Cu²⁺ and Fe³⁺ by the sol–gel method.

Sunlight-Operated TiO2-Based Photocatalysts

Photo-catalysis is a research field with broad applications in terms of potential technological applications related to energy production and managing, environmental protection, and chemical synthesis fields. A global goal, common to all of these fields, is to generate photo-catalytic materials able to use a renewable energy source such as the sun. As most active photocatalysts such as titanium oxides are essentially UV absorbers, they need to be upgraded in order to achieve the fruitful use of the whole solar spectrum, from UV to infrared wavelengths. A lot of different strategies have been pursued to reach this goal. Here, we selected representative examples of the most successful ones. We mainly highlighted doping and composite systems as those with higher potential in this quest. For each of these two approaches, we highlight the different possibilities explored in the literature. For doping of the main photocatalysts, we consider the use of metal and non-metals oriented to modify the band gap energy as well as to create specific localized electronic states. We also described selected cases of using up-conversion doping cations. For composite systems, we described the use of binary and ternary systems. In addition to a main photo-catalyst, these systems contain low band gap, up-conversion or plasmonic semiconductors, plasmonic and non-plasmonic metals and polymers.

Enhancement of Photocatalytic Oxidation of Glucose to Value-Added Chemicals on TiO2 Photocatalysts by A Zeolite (Type Y) Support and Metal Loading

TiO2-based photocatalysts synthesized by the microwave-assisted sol-gel method was tested in the photocatalytic glucose conversion. Modifications of TiO2 with type-Y zeolite (ZeY) and metals (Ag, Cu, and Ag-Cu) were developed for increasing the dispersion of TiO2 nanoparticles and increasing the photocatalytic activity. Effects of the TiO2 dosage to zeolite ratio (i.e., TiO2/ZeY of 10, 20, 40, and 50 mol %) and the silica to alumina ratio in ZeY (i.e., SiO2:Al2O3 of 10, 100, and 500) were firstly studied. It was found that the specific surface area of TiO2/ZeY was 400–590 m2g−1, which was higher than that of pristine TiO2 (34.38 m2g−1). The good properties of 20%TiO2/ZeY photocatalyst, including smaller particles (13.27 nm) and high surface area, could achieve the highest photocatalytic glucose conversion (75%). Yields of gluconic acid, arabinose, xylitol, and formic acid obtained from 20%TiO2/ZeY were 9%, 26%, 4%, and 35%, respectively. For the effect of the silica to alumina ratio, the highest glucose conversion was obtained from SiO2:Al2O3 ratio of 100. Interestingly, it was found that the SiO2:Al2O3 ratio affected the selectivity of carboxylic products (gluconic acid and formic acid). At a low ratio of silica to alumina (SiO2:Al2O3 = 10), higher selectivity of the carboxylic products (gluconic acid = 29% and formic acid = 32%) was obtained (compared with other higher ratios). TiO2/ZeY was further loaded by metals using the microwave-assisted incipient wetness impregnation technique. The highest glucose conversion of 96.9 % was obtained from 1 wt. % Ag-TiO2 (40%)/ZeY. Furthermore, the bimetallic Ag-Cu-loaded TiO2/ZeY presented the highest xylitol yield of 12.93%.

Elucidating the Nature of the Cu(I) Active Site in CuO/TiO2 for Excellent Low-Temperature CO Oxidation

Stabilized Cu⁺ species have been widely considered as catalytic active sites in composite copper catalysts for catalytic reactions with industrial importance. However, few examples comprehensively explicated the origin of stabilized Cu⁺ in a low-cost and widely investigated CuO/TiO₂ system. In this study, mass producible CuO/TiO₂ catalysts with interface-stabilized Cu⁺ were prepared, which showed excellent low-temperature CO oxidation activity. A thorough characterization and theoretical calculations proved that the strong charge-transfer effect and Ti-O-Cu hybridization in Ti-doped CuO(111) at the CuO/TiO₂ interface contributed to the formation and stabilization of Cu⁺ species. The CO molecule adsorbed on Cu⁺ and reacted directly with Ti doping-promoted active lattice oxygen via a Mars-van Krevelen mechanism, leading to the enhanced low-temperature activity.

Facile synthesis of multi-type carbon doped and modified nano-TiO2 for enhanced visible-light photocatalysis

Nano-TiO₂ is a type of environment-friendly and inexpensive substance that could be used for photocatalytic degradation processes. In this study, the multi-type carbon species doped and modified anatase nano-TiO₂ was innovatively synthesized and developed to overcome the deficiency of common nano-TiO₂ photocatalysts. The multi-type carbon species were derived from tetrabutyl titanate and ethanol as the internal and external carbon sources, respectively. Meanwhile, diverse characterization methods were applied to investigate the morphology and surface properties of the photocatalyst. Finally, the visible-light photocatalytic degradation activity of the collected samples was evaluated by using methyl orange as a model pollutant. The promotion mechanism of multi-type carbon species in the photocatalytic process was also discussed and reported. The results in this work show that the doping and modification of multi-type carbon species successfully narrows the bandgap of nano-TiO₂ to expand the light absorption range, reduces the valence band position to improve the oxidation ability of photogenerated holes, and promotes the separation of photogenerated charge carriers to improve quantum efficiency. In addition, the further modification of the external carbon source can promote the surface adsorption of MO and stabilize the multi-type carbon species on the surface of nano-TiO₂.

The synergistic modification of nano-TiO₂ by multi-type carbon species results in excellent and stable visible-light photocatalytic degradation activity.

Interfacial structure-governed SO2 resistance of Cu/TiO2 catalysts in the catalytic oxidation of CO

Different types of Cu–Ti interfacial structures determine different tolerance abilities of catalysts towards SO₂ poisoning during CO oxidation at 250 °C.

Electrospinning Fabricating Au/TiO2 Network-like Nanofibers as Visible Light Activated Photocatalyst

Exploiting photocatalysts with characteristics of low cost, high reactivity and easy recovery offer great potentials for complete elimination of toxic chemicals and environmental remediation. In this work, Au/TiO2 network-like nanofibers were fabricated using a facile electrospinning technique followed by calcinations in air. Photocatalytic tests indicate that the Au/TiO2 network-like nanofibers possess an excellent photodegradation rate of rhodamine B (RB) under UV, visible and natural light radiation. The enhanced photocatalytic activity can be attributed to the plasmonic resonance absorption of Au nanoparticles, and photogenerated electrons and holes are effectively separated by the Au/TiO2 heterojunction structures. Furthermore, the three-dimensional network structure can provide a large number of active sites for RB degradation.

The Effect of Materials Architecture in TiO2 /MOF Composites on CO2 Photoreduction and Charge Transfer

CO₂ photoreduction to C₁ /C₁₊ energized molecules is a key reaction of solar fuel technologies. Building heterojunctions can enhance photocatalysts performance, by facilitating charge transfer between two heterojunction phases. The material parameters that control this charge transfer remain unclear. Here, it is hypothesized that governing factors for CO₂ photoreduction in gas phase are: i) a large porosity to accumulate CO₂ molecules close to catalytic sites and ii) a high number of "points of contact" between the heterojunction components to enhance charge transfer. The former requirement can be met by using porous materials; the latter requirement by controlling the morphology of the heterojunction components. Hence, composites of titanium oxide or titanate and metal-organic framework (MOF), a highly porous material, are built. TiO₂ or titanate nanofibers are synthesized and MOF particles are grown on the fibers. All composites produce CO under UV-vis light, using H₂ as reducing agent. They are more active than their component materials, e.g., ≈9 times more active than titanate. The controlled composites morphology is confirmed and transient absorption spectroscopy highlights charge transfer between the composite components. It is demonstrated that electrons transfer from TiO₂ into the MOF, and holes from the MOF into TiO₂ , as the MOF induces band bending in TiO₂ .

Bicrystalline TiO2 heterojunction for enhanced organic photodegradation: engineering and exploring surface chemistry

Bicrystalline TiO₂ anatase/rutile (TiAR) and anatase/brookite (TiAB) have been studied for photocatalytic degrdation of organics, while H₂O₂ treatment induced opposite mechanisms on the surface.

New insights into Ag-doped BiVO4 microspheres as visible light photocatalysts

This study describes the synthesis of Ag–bismuth vanadate (Ag–BiVO₄) microspheres, a highly efficient visible light photocatalyst for the degradation of methylene blue, via a one-step hydrothermal method.

Solar and visible light photocatalytic enhancement of halloysite nanotubes/g-C3N4 heteroarchitectures

The charged surface of HNTs allows efficient charge separation and increased pollutant adsorption, enhancing the overall photocatalytic performance of the HNTs/g-C₃N₄ heteroarchitectures.