A promising family of mixed transition-metal oxides (MTMOs) (designated as Ax B3-x O4 ; A, B=Co, Ni, Zn, Mn, Fe, etc.) with stoichiometric or even non-stoichiometric compositions, typically in a spinel structure, has recently attracted increasing research interest worldwide. Benefiting from their remarkable electrochemical properties, these MTMOs will play significant roles for low-cost and environmentally friendly energy storage/conversion technologies. In this Review, we summarize recent research advances in the rational design and efficient synthesis of MTMOs with controlled shapes, sizes, compositions, and micro-/nanostructures, along with their applications as electrode materials for lithium-ion batteries and electrochemical capacitors, and efficient electrocatalysts for the oxygen reduction reaction in metal-air batteries and fuel cells. Some future trends and prospects to further develop advanced MTMOs for next-generation electrochemical energy storage/conversion systems are also presented.

Mixed transition-metal oxides: design, synthesis, and energy-related applications.

It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Spinels with the formula of AB2O4 (where A and B are metal ions) and the properties of magnetism, optics, electricity, and catalysis have taken significant roles in applications of data storage, biotechnology, electronics, laser, sensor, conversion reaction, and energy storage/conversion, which largely depend on their precise structures and compositions. In this review, various spinels with controlled preparations and their applications in oxygen reduction/evolution reaction (ORR/OER) and beyond are summarized. First, the composition and structure of spinels are introduced. Then, recent advances in the preparation of spinels with solid-, solution-, and vapor-phase methods are summarized, and new methods are particularly highlighted. The physicochemical characteristics of spinels such as their compositions, structures, morphologies, defects, and substrates have been rationally regulated through various approaches. This regulation can yield spinels with improved ORR/OER catalytic activities, which can furth...

Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond

Iron oxide surfaces

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH3, the catalytic combustion of volatile organic compounds, Hg0 oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the rol...

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review.

This work reports the synthesis and characterization of single-phase cobalt manganese oxide (CMO) spinels Co3−xMnxO4 (0 ≤ x ≤ 0.34) prepared by the pulsed-spray evaporation chemical vapor deposition (PSE–CVD) method. Structure and cationic distribution of the obtained films were characterized by XRD, FTIR, XPS and Raman spectroscopy. Temperature-programmed reduction/re-oxidation (TPR/TPO) was used to elucidate the redox properties of the deposited films. The electrical resistivity was measured in the temperature range of 27–450 °C. XRD, FTIR and Raman spectra reveal the formation of single-phase cubic spinel structures up to x = 0.34. With the substitution of cobalt cations with Mn3+ and Mn4+ ions, the unit cell of the cubic spinel shows a linear increase; the TPR results indicate a lower reducibility while the TPO results display no evident change; also, the ratio Co3+/Co2+ decreased and both electrical resistivity and thermal stability showed increasing trends. The observed behavior is attributed to the progressive incorporation of manganese, which induces structural defects favoring the formation of anionic vacancies and the restriction of the oxygen mobility. The catalytic activities of the doped spinels were investigated for the deep oxidation of unsaturated hydrocarbons (C2H2 and C3H6). The introduction of a slight amount of manganese shifted the light-off curves toward lower temperatures. Based on the XPS results, the enhanced catalytic activity is thought to benefit from the abundant presence of oxygen vacancies in the doped oxide.

Catalytic oxidation of VOCs over mixed Co-Mn oxides

Pulsed spray evaporation chemical vapor deposition (PSE-CVD) was employed for the synthesis of cobalt-based spinel oxide thin films, Co3−xFexO4 with x = 0–1.56. XRD, Raman scattering and FTIR emission spectroscopy show that the normal spinel structure was retained for 0 ≤x≤ 0.65 by the selective insertion of Fe3+ in the octahedral sites. The spinel inversion was noticed above this range, whereas the insertion of Fe2+ was first indicated with x≥ 1. The room-temperature electrical resistivity of the thin films was controlled between 9 and 0.007 Ω cm by the adjustment of iron doping concentration. Furthermore an improvement of the thermal stability of the spinel was noticed upon doping by iron. The reducibility of the spinel in the presence of molecular hydrogen was efficiently adjusted by a shift of the reduction temperature by up to 110 °C upon the controlled insertion of iron in the octahedral sites of the spinel. The investigation of the catalytic oxidation of CO and ethanol over Co3−xFexO4 films with controlled structural modification enabled the confirmation of the Mars–van Krevelen mechanism for the oxidation of CO and the tight correlation between the selectivity of the conversion of ethanol to acetaldehyde and the abundance of surface basic sites. The controlled iron doping was demonstrated to be an efficient strategy to tune the reactivity and the selectivity of the cobalt-based spinel oxide. The doping-induced transition from normal to inverse spinel was observed to induce a clear discontinuity in the trend of all investigated physicochemical properties.

Tailoring the properties and the reactivity of the spinel cobalt oxide

Single-phase cobalt–manganese spinel oxides (Co 3− n Mn n O 4 , CMO) were studied for the catalytic oxidation of propene in a systematic optimization strategy. CMO films were synthesized by pulsed-spray evaporation chemical vapor deposition (PSE–CVD) and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and Ultraviolet–Visible (UV–Vis) spectroscopy. The effect of Co/Mn ratio in the mixed oxide systems on their catalytic activity was investigated in a fixed-bed reactor at T  = 100–800 °C, with a space velocity of 90,000 mL/g cat  h and a feed of 2% C 3 H 6 /20% O 2 /78% Ar. XRD patterns, FTIR and Raman spectroscopy reveal that a cubic single-phase spinel structure is obtained for n  ⩽ 1.23, while a tetragonal spinel structure is observed for n  > 1.23. With increasing of the manganese content, the temperature–programmed analysis demonstrates a lower reducibility, a general decrease of the temperature required for the reduced samples to be re-oxidized and increasing thermal stability. The catalytic tests show that the involvement of cobalt–manganese oxides in propene oxidation suppresses the formation of reaction intermediates, favoring the selectivity toward CO 2 at low temperatures. Co 2.35 Mn 0.65 O 4 exhibits the best catalytic performance, which follows in line with its better reducibility compared with the other compositions in the series of CMO oxides. These results show the great potential of CMO for future industrial application as a low-temperature catalytic system which does not rely on precious metals.

Structure sensitivity of propene oxidation over Co-Mn spinels

Thin films of cobalt-based oxide spinel were prepared by pulsed spray evaporation chemical vapour deposition (PSE-CVD), and doping with chromium was systematically investigated up to a Cr/Co ratio of 0.096, corresponding to a stoichiometry of Co3−x
 Cr
 x
 O4 with x = 0.00–0.26. The effect of doping concentration on the structure, assessed by X-ray diffraction and Raman scattering, and on the optical and electrical properties of the oxide films was investigated. Single-phase spinel could be preserved for stoichiometries below x = 0.22. The influence of Cr-doping on the band gap energies and on the electrical conductivity was determined, and the obtained results were exploited to discuss the cationic site occupations. The influence of Cr doping was complemented by the investigation of the surface catalytic reactivity towards the oxidation of dichloromethane.

Vincent Vannier

Papers

Catalytic oxidation of VOCs over mixed Co-Mn oxides

Tailoring the properties and the reactivity of the spinel cobalt oxide

Structure sensitivity of propene oxidation over Co-Mn spinels

Preparation and characterisation of chromium-doped cobalt oxide spinel thin films