Solution Combustion Synthesis of Nanoscale Materials

Synthesis of Highly Efficient Bifunctional Ag/Co3O4 Catalyst for Oxygen Reduction and Oxygen Evolution Reactions in Alkaline Medium

Ag/Co3O4 catalysts using three different modes of solution combustion synthesis were developed and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy to identify crystallite size, oxidation state, composition, and morphology. Cyclic voltammetry and linear sweep voltammetry measurements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) confirm the bifunctionality of the electrocatalysts. The electrochemical evaluation indicates that a synergic effect between Ag and Co enhances the activity through the fast breaking of O-O bond in the molecular oxygen to enhance the reduction mechanism. The high content of cobalt (Co) in the catalyst Ag/Co3O4-12, synthesized by second wave combustion, improves the activity for ORR, and the reaction mechanism follows a 3.9 number of electron transfer in overall reaction. The kinetic and limiting current densities of Ag/Co3O4-12 are maximum when compared to those of other Ag/Co3O4 catalysts and are very close to commercial Pt/C. Moreover, the maximum current density of OER for Ag/Co3O4-12 makes it a promising candidate for various bifunctional electrocatalytic applications such as fuel cells and metal-air batteries.

Pt-Co and Pt-Ni Catalysts of Low Metal Content for H2 Production by Reforming of Oxygenated Hydrocarbons and Comparison with Reported Pt-Based Catalysts

New catalysts of Pt, PtNi, PtCo, and NiCo supported on Al2O3 were developed for producing hydrogen by aqueous phase reforming (APR) of oxygenated hydrocarbons. The urea matrix combustion technique was used for loading the metal on the support in order to improve several aspects: increase both the metal-support interaction and the metal dispersion and decrease the metal load. The catalysts were characterized by MS/ICP, N2 adsorption, XRD, TPR, CO chemisorption, and the test of cyclohexane dehydrogenation (CHD). The APR of a solution of 10% mass ethylene glycol (EG), performed in a tubular fixed bed reactor at 498 K, 22 bar, WHSV = 2.3 h−1, was used as the main reaction test. After 10 h on-stream, the catalysts prepared by UMC had better hydrogen yield and catalytic stability than common catalysts prepared by IWI. The UMC/IWI H2 yield ratio was 23.5/15.2 for Pt, 24.0/17.0 for PtCo, 26.6/21.0 for PtNi, and 8.0/3.9 for NiCo. Ni or Co addition to Pt increased the carbon conversion while keeping the H2 turnover high. Cobalt also improves stability. Reports of several authors were revised for a comparison. The analysis indicated that the developed catalysts are a viable and cheaper alternative for H2 production from a renewable resource.

Effects of Oxidizing/Reducing Agent Ratio on Phase Purity, Crystallinity, and Magnetic Behavior of Solution-Combustion-Grown BiFeO3 Submicroparticles

Fabrication of phase-pure well-crystalline BiFeO₃ submicroparticles in large scale is of great importance for the utilization of this rhombohedrally distorted perovskite material in applications such as memory storage and spintronic devices and visible photocatalyst for the degradation of organic pollutants. In fact, because of the narrow temperature range of phase stabilization, the fabrication of phase-pure BiFeO₃ in large scale remained elusive. We present the synthesis of phase-pure BiFeO₃ particles of submicrometric dimensions (246-330 nm average size) through the adjustment of oxidizing/reducing agent ratio in solution combustion process utilizing glycine as reducing agent and nitrate precursors as oxidizing agent. Utilizing X-ray diffraction and Raman spectroscopy, we demonstrate that the BiFeO₃ submicroparticles synthesized at equivalence ratio (Φ_e) close to 0.5 do not contain undesired impurities such as Bi₂Fe₄O₉ and Bi₂₄Fe₂O₃₉. Moreover, the submicroparticles are highly crystalline, possessing high room temperature magnetic moment and stable antiferromagnetic behavior across a wide temperature range. The superparamagnetic behavior at low magnetic field manifested by impurities attached to the BiFeO₃ submicroparticles might lead to their use as effective magnetically separable photocatalysts.

A simple, rapid and efficient flame synthesis of ultrafine LiNi0.95−xCoxTi0.05O2 (x = 0.25 and 0.30): a versatile route to synthesize Ni-rich cathode materials for Li-ion batteries

A simple, rapid, and efficient flame synthesis method was developed to fabricate LiNi_0.95−xCo_xTi_0.05O₂ (x = 0.25 and 0.30) cathode materials for Li-ion batteries.

Enhanced oxygen storage capacity of cation-ordered cerium–zirconium oxide induced by titanium substitution

Cation-ordered Ce_0.5Zr_0.4Ti_0.1O₂ with a high oxygen storage capacity (1310 μmol-O per g at 200 °C) was synthesized via a solution combustion method.

Metal Phosphides and Phosphates-based Electrodes for Electrochemical Supercapacitors

Phosphorus compounds, such as metal phosphides and phosphates have shown excellent performances and great potential in electrochemical energy storage, which are demonstrated by research works published in recent years. Some of these metal phosphides and phosphates and their hybrids compare favorably with transition metal oxides/hydroxides, which have been studied extensively as a class of electrode materials for supercapacitor applications, where they have limitations in terms of electrical and ion conductivity and device stability. To be specific, metal phosphides have both metalloid characteristics and good electric conductivity. For metal phosphates, the open-framework structures with large channels and cavities endow them with good ion conductivity and charge storage capacity. In this review, we present the recent progress on metal phosphides and phosphates, by focusing on their advantages/disadvantages and potential applications as a new class of electrode materials in supercapacitors. The synthesis methods to prepare these metal phosphides/phosphates are looked into, together with the scientific insights involved, as they strongly affect the electrochemical energy storage performance. Particular attentions are paid to those hybrid-type materials, where strong synergistic effects exist. In the summary, the future perspectives and challenges for the metal phosphides, phosphates and hybrid-types are proposed and discussed.

Catalytic Palladium Film Deposited by Scalable Low-Temperature Aqueous Combustion

This article describes a novel method for depositing a dense, high quality palladium thin film via a one-step aqueous combustion process which can be easily scaled up. Film deposition of Pd from aqueous solutions by conventional chemical or electrochemical methods is inhibited by hydrogen embrittlement, thus resulting in a brittle palladium film. The method outlined in this work allows a direct aqueous solution deposition of a mirror-bright, durable Pd film on substrates including glass and glassy carbon. This simple procedure has many advantages including a very high deposition rate (>10 cm² min^-1) and a relatively low deposition temperature (250 °C), which makes it suitable for large-scale industrial applications. Although preparation of various high-quality oxide films has been successfully accomplished via solution combustion synthesis (SCS) before, this article presents the first report on direct SCS production of a metallic film. The mechanism of Pd film formation is discussed with the identification of a complex formed between palladium nitrate and glycine at low temperature. The catalytic properties and stability of films are successfully tested in alcohol electrooxidation and electrochemical oxygen reduction reaction. It was observed that combustion deposited Pd film on a glassy carbon electrode showed excellent catalytic activity in ethanol oxidation without using any binder or additive. We also report for the first time the concept of a reusable "catalytic flask" as illustrated by the Suzuki-Miyaura cross-coupling reaction. The Pd film uniformly covers the inner walls of the flask and eliminates the catalyst separation step. We believe the innovative concept of a reusable catalytic flask is very promising and has the required features to become a commercial product in the future.

Structure sensitive photocatalytic reduction of nitroarenes over TiO 2

It is a subject of exploration whether the phase pure anatase or rutile TiO₂ or the band alignment due to the heterojunctions in the two polymorphs of TiO₂ plays the determining role in efficacy of a photocatalytic reaction. In this work, the phase pure anatase and rutile TiO₂ have been explored for photocatalytic nitroarenes reduction to understand the role of surface structures and band alignment towards the reduction mechanism. The conduction band of synthesized anatase TiO₂ has been found to be more populated with electrons of higher energy than that of synthesized rutile. This has given the anatase an edge towards photocatalytic reduction of nitroarenes over rutile TiO₂. The other factors like adsorption of the reactants and the proton generation did not play any decisive role in catalytic efficacy.

Structure sensitive photocatalytic reduction of nitroarenes over TiO 2

It is a subject of exploration whether the phase pure anatase or rutile TiO₂ or the band alignment due to the heterojunctions in the two polymorphs of TiO₂ plays the determining role in efficacy of a photocatalytic reaction. In this work, the phase pure anatase and rutile TiO₂ have been explored for photocatalytic nitroarenes reduction to understand the role of surface structures and band alignment towards the reduction mechanism. The conduction band of synthesized anatase TiO₂ has been found to be more populated with electrons of higher energy than that of synthesized rutile. This has given the anatase an edge towards photocatalytic reduction of nitroarenes over rutile TiO₂. The other factors like adsorption of the reactants and the proton generation did not play any decisive role in catalytic efficacy.

Role of silver doping on the defects related photoluminescence and antibacterial behaviour of zinc oxide nanoparticles

The Ag doped ZnO (ZnO:Ag) NPs with a hexagonal wurtzite structure were synthesized by a solution combustion method. X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) were used to study the defects, local electronic and atomic structures before and after Ag doping. XPS and XANES studies confirmed the deficiency of concentration of defects in ZnO after Ag doping. The photoluminescence study showed the deep level emission in the orange-red region in addition to the band to band emission. It was also found that the defect related emission of ZnO was decreased with an increasing in Ag concentration. The antibacterial behaviour of ZnO and ZnO:Ag NPs was studied against the gram positive and gram negative bacteria. The role of Ag doping and defects in the ZnO NPs were discussed for the observed antibacterial and photoluminescence behaviour.

A combustion synthesis route for magnetically separable graphene oxide–CuFe2O4–ZnO nanocomposites with enhanced solar light-mediated photocatalytic activity

A novel GO–CuFe₂O₄–ZnO ternary nanocomposite has been designed as an efficient photocatalyst for the degradation of four toxic organic pollutants.

Hydrothermally prepared nanosized and mesoporous Ce0.4Zr0.6O2 solid solutions with shape dependence in photocatalysis for the degradation of methylene blue

The shape dependence of photocatalysis is confirmed.

Hematite nanoparticle clusters with remarkably high magnetization synthesized from water-treatment waste by one-step “sharp high-temperature dehydration”

Hematite (α-Fe₂O₃) nanoparticle clusters with an exceptionally high magnetization of 51 emu g⁻¹ were synthesized for the first time. This material was prepared from water-treatment waste by a new “sharp high-temperature dehydration” process.