Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

Influence of parameters on the heterogeneous photocatalytic degradation of pesticides and phenolic contaminants in wastewater: A short review

Photocatalysis has been attracting much research interest because of its wide applications in renewable energy and environmental remediation. There are many materials that are found to show good photocatalytic activity in the presence of ultraviolet (UV) and visible light. However, the applications of these materials are limited to the UV portion of sunlight. α-Fe 2 O 3 has an advantage over the other conventional materials like TiO 2 , ZnO, etc . in using solar energy for photocatalytic applications due to its lower band gap ∼2.2 eV value. As a result of which Fe 2 O 3 is capable of absorbing a large portion of the visible solar spectrum (absorbance edge ∼600 nm). Also its good chemical stability in aqueous medium, low cost, abundance and nontoxic nature makes it a promising material for photocatalytic water treatment and water splitting applications. Except these advantages the usage of Fe 2 O 3 has been restricted by many anomalies such as higher e–h recombination effect, low diffusion length and VB positioning (VB is positive with respect to H + /H 2 potential). This article reviews the research that has been carried out to overcome these basic limitations and to enhance the photocatalytic activity of α-Fe 2 O 3 .

α-Fe2O3 as a photocatalytic material: A review

Heterogeneous Fenton reactions have gained widespread attention in removing recalcitrant organic contaminants as the reaction between solid Fenton catalysts and H2O2 can generate highly reactive hydroxyl radicals (HO[rad]). However, several drawbacks, such as the low-speed generation of Fe(II), high consumption of H2O2, and acidic reaction conditions (generally at ˜ pH 3), are always the core issues that hamper the large-scale application of heterogeneous Fenton reactions in environmental remediation. Thus, a large number of studies have been devoted to tackling these drawbacks, and this paper intends to comprehensively review the developed strategies for enhancing heterogeneous Fenton reactivity, mainly over the last decade. Based on a comprehensive survey of previous studies, we categorize these strategies according to their reaction mechanisms. For example, introducing additional electrons (e.g., from external electric fields, electron-rich materials, semiconductors, plasmonic materials, or doped metals) to heterogeneous Fenton catalysts can accelerate the generation of Fe(II); the in situ generation of H2O2 can be achieved by combining ultrasound, electricity, semiconductors, and iron-based catalysts in the system; and controlling the specific morphologies and exposed facets of heterogeneous Fenton catalysts can greatly promote the decomposition of H2O2. In addition, we briefly introduce some recent novel heterogeneous Fenton-like reactions that are of particular interest, including constructing dual reaction centers (i.e., the electron-poor center and the electron-rich center) and synthesizing single-atom catalysis-based heterogeneous Fenton-like catalysts. Moreover, this review article analyzes and compares the merits of each strategy for enhancing heterogeneous Fenton/Fenton-like reactions. We believe this review can motivate the construction of novel and efficient heterogeneous Fenton/Fenton-like systems and help readers choose proper Fenton/Fenton-like reaction systems for industrial applications.

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Strategies for Enhancing the Heterogeneous Fenton Catalytic Reactivity: A review

Nanosized rod-like, wire-like, and tubular α-MnO(2) and flower-like spherical Mn(2)O(3) have been prepared via the hydrothermal method and the CCl(4) solution method, respectively. The physicochemical properties of the materials were characterized using numerous analytical techniques. The catalytic activities of the catalysts were evaluated for toluene oxidation. It is shown that α-MnO(2) nanorods, nanowires, and nanotubes with a surface area of 45-83 m(2)/g were tetragonal in crystal structure, whereas flower-like spherical Mn(2)O(3) with a surface area of 162 m(2)/g was of cubic crystal structure. There were the presence of surface Mn ions in multiple oxidation states (e.g., Mn(3+), Mn(4+), or even Mn(2+)) and the formation of surface oxygen vacancies. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of rod-like α-MnO(2) > tube-like α-MnO(2) > flower-like Mn(2)O(3) > wire-like α-MnO(2), in good agreement with the sequence of the catalytic performance of these samples. The best-performing rod-like α-MnO(2) catalyst could effectively catalyze the total oxidation of toluene at lower temperatures (T(50%) = 210 °C and T(90%) = 225 °C at space velocity = 20,000 mL/(g h)). It is concluded that the excellent catalytic performance of α-MnO(2) nanorods might be associated with the high oxygen adspecies concentration and good low-temperature reducibility. We are sure that such one-dimensional well-defined morphological manganese oxides are promising materials for the catalytic elimination of air pollutants.

Manganese Oxides with Rod-, Wire-, Tube-, and Flower-Like Morphologies: Highly Effective Catalysts for the Removal of Toluene

Synthesis and characterization of nano-sized porous gamma-alumina by control precipitation method

Rare earth metal oxides doped TiO2 were prepared by an incipient wetness impregnation method by varying different mol% of La, Nd and Pr and characterized by various advanced techniques such as powder X-ray diffraction (PXRD), BET-surface area, N2 adsorption–desorption measurements, UV–vis DRS, FTIR and scanning electron microscope (SEM). Analytical results demonstrated that the TiO2 nanoparticles are mesoporous in nature and doping of lanthanides could inhibit the phase transformation, increases the surface area and decreases the crystallite size of mesoporous structures of TiO2. We investigated the visible light induced photocatalytic activities of these materials towards reduction of hexavalent chromium and degradation of methylene blue for the first time. La3+-TiO2 containing 0.4 mol% lanthanum, activated at 673 K showed highest surface area (124.8 m2/g), lowest crystallite size (8 nm) and exhibits highest photocatalytic activity. The reduction of hexavalent chromium and methylene blue degradation rate followed first-order kinetics.

Visible light induced photocatalytic activity of rare earth titania nanocomposites

Hydrated titania was prepared by a sol–gel method, taking tetraisopropyl orthotitanate as starting material, and then promoted with different weight percentages of sulfate by an incipient wetness impregnation method The materials were characterized by various advanced techniques such as PXRD, BET surface area, N2 adsorption–desorption measurements, FTIR, and SEM Analytical results demonstrated that TiO2 is mesoporous in nature, and sulfate modification could inhibit the phase transformation and enhance the thermal stability of TiO2 It was also found that sulfate modification could reduce the crystallite size and increase the specific surface area of the catalysts The degradation of methyl orange under solar radiation was investigated to evaluate the photocatalytic activity of these materials Effects of different parameters such as pH of the solution, amount of catalyst, additives, and kinetics were investigated At 25 wt% sulfate loading, the average percentage of degradation of methyl orange was nearly two times than that of neat TiO2 The photocatalytic degradation followed first-order kinetics

Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light.

Mesoporous S,N−TiO2 nanocomposite was prepared by a one-pot template free homogeneous coprecipitation technique using titanium oxysulfate sulfuric acid complex hydrate, thiourea, ethanol, and water. Nano gold deposition on mesoporous S,N−TiO2 was preformed by a borohydrate reduction method. To evaluate the structural and electronic properties, these catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), UV−vis DRS, photoluminescent (PL) spectra, Fourier transform infrared (FTIR), and TPO/TPD. CO adsorption and CO + O2 interaction over these catalysts were investigated by in situ FTIR. Sulfur and nitrogen doping enhances the catalytic activity of Au/TiO2. Higher catalytic activity of Au/S,N−TiO2 compared to Au/TiO2 was attributed to the presence of oxygen vacancy and creation of new adsorption sites at Au/TiO2 interfaces for the adsorption and activation of O2 molecules.

Gold Promoted S,N−Doped TiO2: An Efficient Catalyst for CO Adsorption and Oxidation

A highly efficient S, N co-doped α-Fe2O3 has been synthesized by a simple co-precipitation technique using thiourea as the precipitating agent, as well as the source for sulphur and nitrogen. Detailed characterizations are performed for establishing the structural, morphological, optical and electronic property of the synthesized materials. Sulphur doping induces a strong diffraction along the [110] plane which is a determining factor for the enhancement in photocatalytic activity. The presence of both sulphur and nitrogen has remarkable enhancement of the degradation of Rhodamine B (RB) compared to the bulk material, as well as the single non-metal doped hematite. A comparison has been made between the adsorption, Fenton, photo-Fenton and photocatalytic degradation of RB. A maximum degradation of 95% was obtained in just 4 h under natural sunlight illumination. The difference in catalytic activity has been discussed in terms of their (1) particle size, (2) surface area, (3) [110] plane in the sulphur doped material, (4) formation of OH radicals and (5) co-doping of sulphur and nitrogen creating the trap state, etc.

Fabrication of S, N co-doped α-Fe2O3 nanostructures: effect of doping, OH radical formation, surface area, [110] plane and particle size on the photocatalytic activity

Nruparaj Sahu

Papers

Synthesis and characterization of nano-sized porous gamma-alumina by control precipitation method

Visible light induced photocatalytic activity of rare earth titania nanocomposites

Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light.

Gold Promoted S,N−Doped TiO2: An Efficient Catalyst for CO Adsorption and Oxidation

Fabrication of S, N co-doped α-Fe2O3 nanostructures: effect of doping, OH radical formation, surface area, [110] plane and particle size on the photocatalytic activity