Reducibility of Al3+-Modified Co3O4: Influence of Aluminum Distribution.

The reduction of Co-based oxides doped with Al3+ ions has been studied using in situ XRD and TPR techniques. Al3+-modified Co3O4 oxides with the Al mole fraction Al/(Co + Al) = 1/6; 1/7.5 were prepared via coprecipitation, with further calcination at 500 and 850 °C. Using XRD and HAADF-STEM combined with EDS element mapping, the Al3+ cations were dissolved in the Co3O4 lattice; however, the cation distribution differed and depended on the calcination temperature. Heating at 500 °C led to the formation of an inhomogeneous (Co,Al)3O4 solid solution; further treatment at 850 °C provoked the partial decomposition of mixed Co-Al oxides and the formation of particles with an Al-depleted interior and Al-enriched surface. It has been shown that the reduction of cobalt oxide by hydrogen occurs via the following transformations: (Co,Al)3O4 → (Co,Al)O → Co. Depending on the Al distribution, the course of reduction changes. In the case of the inhomogeneous (Co,Al)3O4 solid solution, Al stabilizes intermediate Co(II)-Al(III) oxides during reduction. When Al3+ ions are predominantly on the surface of the Co3O4 particles, the intermediate compound consists of Al-depleted and Al-enriched Co(II)-Al(III) oxides, which are reduced independently. Different distributions of elemental Co and Al in mixed oxides simulate different types of the interaction phase in Co3O4/γ-Al2O3-supported catalysts. These changes in the reduction properties can significantly affect the state of an active component of the Co-based catalysts.

Photocatalysts Based on Graphite-like Carbon Nitride with a Low Content of Rhodium and Palladium for Hydrogen Production under Visible Light.

In this study, we proposed photocatalysts based on graphite-like carbon nitride with a low content (0.01-0.5 wt.%) of noble metals (Pd, Rh) for hydrogen evolution under visible light irradiation. As precursors of rhodium and palladium, labile aqua and nitrato complexes [Rh2(H2O)8(µ-OH)2](NO3)4∙4H2O and (Et4N)2[Pd(NO3)4], respectively, were proposed. To obtain metallic particles, reduction was carried out in H2 at 400 °C. The synthesized photocatalysts were studied using X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectroscopy and high-resolution transmission electron microscopy. The activity of the photocatalysts was tested in the hydrogen evolution from aqueous and aqueous alkaline solutions of TEOA under visible light with a wavelength of 428 nm. It was shown that the activity for the 0.01-0.5% Rh/g-C3N4 series is higher than in the case of the 0.01-0.5% Pd/g-C3N4 photocatalysts. The 0.5% Rh/g-C3N4 sample showed the highest activity per gram of catalyst, equal to 3.9 mmol gcat-1 h-1, whereas the most efficient use of the metal particles was found over the 0.1% Rh/g-C3N4 photocatalyst, with the activity of 2.4 mol per gram of Rh per hour. The data obtained are of interest and can serve for further research in the field of photocatalytic hydrogen evolution using noble metals as cocatalysts.

Novel Biocompatible Magnetoelectric MnFe2 O4 Core@BCZT Shell Nano-Hetero-Structures with Efficient Catalytic Performance.

Magnetoelectric (ME) small-scale robotic devices attract great interest from the scientific community due to their unique properties for biomedical applications. Here, novel ME nano hetero-structures based on the biocompatible magnetostrictive MnFe2 O4 (MFO) and ferroelectric Ba0.85 Ca0.15 Zr0.1 Ti0.9 O3 (BCZT) are developed solely via the hydrothermal method for the first time. An increase in the temperature and duration of the hydrothermal synthesis results in increasing the size, improving the purity, and inducing morphology changes of MFO nanoparticles (NPs). A successful formation of a thin epitaxial BCZT-shell with a 2-5 nm thickness is confirmed on the MFO NPs (77 ± 14 nm) preliminarily treated with oleic acid (OA) or polyvinylpyrrolidone (PVP), whereas no shell is revealed on the surface of pristine MFO NPs. High magnetization is revealed for the developed ME NPs based on PVP- and OA-functionalized MFO NPs (18.68 ± 0.13 and 20.74 ± 0.22 emu g-1 , respectively). Moreover, ME NPs demonstrate 95% degradation of a model pollutant Rhodamine B within 2.5 h under an external AC magnetic field (150 mT, 100 Hz). Thus, the developed biocompatible core-shell ME NPs of MFO and BCZT can be considered as a promising tool for non-invasive biomedical applications, environmental remediation, and hydrogen generation for renewable energy sources.

Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2-δ Catalyst of CO Oxidation Reaction.

A Mn0.2Zr0.8O2-δ mixed oxide catalyst was synthesized via the co-precipitation method and studied in a CO oxidation reaction after different redox pretreatments. The surface and structural properties of the catalyst were studied before and after the pretreatment using XRD, XANES, XPS, and TEM techniques. Operando XRD was used to monitor the changes in the crystal structure under pretreatment and reaction conditions. The catalytic properties were found to depend on the activation procedure: reducing the CO atmosphere at 400-600 °C and the reaction mixture (O2 excess) or oxidative O2 atmosphere at 250-400 °C. A maximum catalytic effect characterized by decreasing T50 from 193 to 171 °C was observed after a reduction at 400 °C and further oxidation in the CO/O2 reaction mixture was observed at 250 °C. Operando XRD showed a reversible reduction-oxidation of Mn cations in the volume of Mn0.2Zr0.8O2-δ solid solution. XPS and TEM detected the segregation of manganese cations on the surface of the mixed oxide. TEM showed that Mn-rich regions have a structure of MnO2. The pretreatment caused the partial decomposition of the Mn0.2Zr0.8O2-δ solid solution and the formation of surface Mn-rich areas that are active in catalytic CO oxidation. In this work it was shown that the introduction of oxidation-reduction pretreatment cycles leads to an increase in catalytic activity due to changes in the origin of active states.

Synthesis of Boron-Doped Carbon Nanomaterial.

A new method for the synthesis of boron-doped carbon nanomaterial (B-carbon nanomaterial) has been developed. First, graphene was synthesized using the template method. Magnesium oxide was used as the template that was dissolved with hydrochloric acid after the graphene deposition on its surface. The specific surface area of the synthesized graphene was equal to 1300 m2/g. The suggested method includes the graphene synthesis via the template method, followed by the deposition of an additional graphene layer doped with boron in an autoclave at 650 °C, using a mixture of phenylboronic acid, acetone, and ethanol. After this carbonization procedure, the mass of the graphene sample increased by 70%. The properties of B-carbon nanomaterial were studied using X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and adsorption-desorption techniques. The deposition of an additional graphene layer doped with boron led to an increase of the graphene layer thickness from 2-4 to 3-8 monolayers, and a decrease of the specific surface area from 1300 to 800 m2/g. The boron concentration in B-carbon nanomaterial determined by different physical methods was about 4 wt.%.

Efficient Photocatalytic Hydrogen Production over NiS-Modified Cadmium and Manganese Sulfide Solid Solutions.

In this work, new photocatalysts based on Cd1-xMnxS sulfide solid solutions were synthesized by varying the fraction of MnS (x = 0.4, 0.6, and 0.8) and the hydrothermal treatment temperature (T = 100, 120, 140, and 160 °C). The active samples were modified with Pt and NiS co-catalysts. Characterization was performed using various methods, including XRD, XPS, HR TEM, and UV-vis spectroscopy. The photocatalytic activity was tested in hydrogen evolution from aqueous solutions of Na2S/Na2SO3 and glucose under visible light (425 nm). When studying the process of hydrogen evolution using an equimolar mixture of Na2S/Na2SO3 as a sacrificial agent, the photocatalysts Cd0.5Mn0.5S/Mn(OH)2 (T = 120 °C) and Cd0.4Mn0.6S (T = 160 °C) demonstrated the highest activity among the non-modified solid solutions. The deposition of NiS co-catalyst led to a significant increase in activity. The best activity in the case of the modified samples was shown by 0.5 wt.% NiS/Cd0.5Mn0.5S (T = 120 °C) at the extraordinary level of 34.2 mmol g-1 h-1 (AQE 14.4%) for the Na2S/Na2SO3 solution and 4.6 mmol g-1 h-1 (AQE 2.9%) for the glucose solution. The nickel-containing samples possessed a high stability in solutions of both sodium sulfide/sulfite and glucose. Thus, nickel sulfide is considered an alternative to depositing precious metals, which is attractive from an economic point of view. It worth noting that the process of photocatalytic hydrogen evolution from sugar solutions by adding samples based on Cd1-xMnxS has not been studied before.

Copper-Modified Titania-Based Photocatalysts for the Efficient Hydrogen Production under UV and Visible Light from Aqueous Solutions of Glycerol.

In this study, we have proposed titania-based photocatalysts modified with copper compounds for hydrogen evolution. Thermal pre-treatment of commercial TiO2 Degussa P25 (DTiO2) and Hombifine N (HTiO2) in the range from 600 to 800 °C was carried out followed by the deposition of copper oxides (1-10 wt. % of Cu). The morphology and chemical state of synthesized photocatalysts were studied using X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and XANES/EXAFS X-ray absorption spectroscopy. Photocatalytic activity was tested in the hydrogen evolution from aqueous solutions of glycerol under ultraviolet (λ = 381 nm) and visible (λ = 427 nm) light. The photocatalysts 2% CuOx/DTiO2 T750 and 5% CuOx/DTiO2 T700 showed the highest activity under UV irradiation (λ = 380 nm), with the rate of H2 evolution at the level of 2.5 mmol (H2) g-1 h-1. Under the visible light irradiation (λ = 427 nm), the highest activity of 0.6 mmol (H2) g-1 h-1 was achieved with the 5% CuOx/DTiO2 T700 photocatalyst. The activity of these photocatalysts is 50% higher than that of the platinized 1% Pt/DTiO2 sample. Thus, it was shown for the first time that a simple heat treatment of a commercial titanium dioxide in combination with a deposition of non-noble metal particles led to a significant increase in the activity of photocatalysts and made it possible to obtain materials that were active in hydrogen production under visible light irradiation.

Broadening the Action Spectrum of TiO2-Based Photocatalysts to Visible Region by Substituting Platinum with Copper.

In this study, TiO2-based photocatalysts modified with Pt and Cu/CuOx were synthesized and studied in the photocatalytic reduction of CO2. The morphology and chemical states of synthesized photocatalysts were studied using UV-Vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. A series of light-emitting diodes (LEDs) with maximum intensity in the range of 365-450 nm was used to determine the action spectrum of photocatalysts. It is shown for, the first time, that the pre-calcination of TiO2 at 700 °C and the use of Cu/CuOx instead of Pt allow one to design a highly efficient photocatalyst for CO2 transformation shifting the working range to the visible light (425 nm). Cu/CuOx/TiO2 (calcined at 700 °C) shows a rate of CH4 formation of 1.2 ± 0.1 µmol h-1 g-1 and an overall CO2 reduction rate of 11 ± 1 µmol h-1 g-1 (at 425 nm).

Effect of Phosphorus Precursor, Reduction Temperature, and Support on the Catalytic Properties of Nickel Phosphide Catalysts in Continuous-Flow Reductive Amination of Ethyl Levulinate.

Levulinic acid and its esters (e.g., ethyl levulinate, EL) are platform chemicals derived from biomass feedstocks that can be converted to a variety of valuable compounds. Reductive amination of levulinates with primary amines and H2 over heterogeneous catalysts is an attractive method for the synthesis of N-alkyl-5-methyl-2-pyrrolidones, which are an environmentally friendly alternative to the common solvent N-methyl-2-pyrrolidone (NMP). In the present work, the catalytic properties of the different nickel phosphide catalysts supported on SiO2 and Al2O3 were studied in a reductive amination of EL with n-hexylamine to N-hexyl-5-methyl-2-pyrrolidone (HMP) in a flow reactor. The influence of the phosphorus precursor, reduction temperature, reactant ratio, and addition of acidic diluters on the catalyst performance was investigated. The Ni2P/SiO2 catalyst prepared using (NH4)2HPO4 and reduced at 600 °C provides the highest HMP yield, which reaches 98%. Although the presence of acid sites and a sufficient hydrogenating ability are important factors determining the pyrrolidone yield, the selectivity also depends on the specific features of EL adsorption on active catalytic sites.

Single-Walled Carbon Nanotubes with Red Phosphorus in Lithium-Ion Batteries: Effect of Surface and Encapsulated Phosphorus.

Single-walled carbon nanotubes (SWCNTs) with their high surface area, electrical conductivity, mechanical strength and elasticity are an ideal component for the development of composite electrode materials for batteries. Red phosphorus has a very high theoretical capacity with respect to lithium, but has poor conductivity and expends considerably as a result of the reaction with lithium ions. In this work, we compare the electrochemical performance of commercial SWCNTs with red phosphorus deposited on the outer surface of nanotubes and/or encapsulated in internal channels of nanotubes in lithium-ion batteries. External phosphorus, condensed from vapors, is easily oxidized upon contact with the environment and only the un-oxidized phosphorus cores participate in electrochemical reactions. The support of the SWCNT network ensures a stable long-term cycling for these phosphorus particles. The tubular space inside the SWCNTs stimulate the formation of chain phosphorus structures. The chains reversibly interact with lithium ions and provide a specific capacity of 1545 mAh·g-1 (calculated on the mass of phosphorus in the sample) at a current density of 0.1 A·g-1. As compared to the sample containing external phosphorus, SWCNTs with encapsulated phosphorus demonstrate higher reaction rates and a slight loss of initial capacity (~7%) on the 1000th cycle at 5 A·g-1.

The Formation of Mn-Ce Oxide Catalysts for CO Oxidation by Oxalate Route: The Role of Manganese Content.

The Mn-Ce oxide catalysts active in the oxidation of CO were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transition electron microscopy (TEM), energy dispersive X-Ray (EDX), and a differential dissolution technique. The Mn-Ce catalysts were prepared by thermal decomposition of oxalates by varying the Mn:Ce ratio. The nanocrystalline oxides with a fluorite structure and particle sizes of 4-6 nm were formed. The introduction of manganese led to a reduction of the oxide particle size, a decrease in the surface area, and the formation of a MnyCe1-yO2-δ solid solution. An increase in the manganese content resulted in the formation of manganese oxides such as Mn2O3, Mn3O4, and Mn5O8. The catalytic activity as a function of the manganese content had a volcano-like shape. The best catalytic performance was exhibited by the catalyst containing ca. 50 at.% Mn due to the high specific surface area, the formation of the solid solution, and the maximum content of the solid solution.

Comparative Study of the Photocatalytic Hydrogen Evolution over Cd1-xMnxS and CdS-ß-Mn3O4-MnOOH Photocatalysts under Visible Light.

A series of solid solutions of cadmium and manganese sulfides, Cd1-xMnxS (x = 0-0.35), and composite photocatalysts, CdS-ß-Mn3O4-MnOOH, were synthesized by precipitation with sodium sulfide from soluble cadmium and manganese salts with further hydrothermal treatment at 120 °C. The obtained photocatalysts were studied by the X-ray diffraction method (XRD), UV-vis diffuse reflectance spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 low temperature adsorption. The photocatalysts were tested in hydrogen production using a Na2S/Na2SO3 aqueous solution under visible light (λ = 450 nm). It was shown for the first time that both kinds of photocatalysts possess high activity in hydrogen evolution under visible light. The solid solution Cd0.65Mn0.35S has an enhanced photocatalytic activity due to its valence and conduction band position tuning, whereas the CdS-ß-Mn3O4-MnOOH (40-60 at% Mn) samples were active due to ternary heterojunction formation. Further, the composite CdS-ß-Mn3O4-MnOOH photocatalyst had much higher stability in comparison to the Cd0.65Mn0.35S solid solution. The highest activity was 600 mmol g-1 h-1, and apparent quantum efficiency of 2.9% (λ = 450 nm) was possessed by the sample of CdS-ß-Mn3O4-MnOOH (40 at% Mn).

Composite photocatalysts based on Cd1-x Zn x S and TiO2 for hydrogen production under visible light: effect of platinum co-catalyst location.

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd1-x Zn x S/TiO2 were compared. In the first case, platinum was deposited on the surface of synthesized Cd1-x Zn x S (x = 0.2-0.3)/TiO2 P25; in the second one, Cd1-x Zn x S (x = 0.2-0.3) was deposited on the surface of Pt/TiO2 P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd1-x Zn x S (10-50 wt%; x = 0.2-0.3)/Pt (1 wt%)/TiO2 photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd1-x Zn x S (10-50 wt%; x = 0.2-0.3)/TiO2 ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd0.8Zn0.2S/1% Pt/TiO2 in the amount of 26 mmol g-1 h-1 (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

The influence of the sacrificial agent nature on transformations of the Zn(OH)2/Cd0.3Zn0.7S photocatalyst during hydrogen production under visible light.

Photocatalysts based on zinc hydroxide and a solid solution of CdS and ZnS were prepared via the precipitation method and used for photocatalytic hydrogen production from aqueous solutions of inorganic (Na2S/Na2SO3) and organic (ethanol) sacrificial agents. The photocatalysts were tested in cyclic experiments for hydrogen evolution and studied using X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) techniques. Different transformations of the ß-Zn(OH)2 co-catalyst were observed in the presence of inorganic and organic sacrificial agents; namely, ZnS was formed in Na2S/Na2SO3 solution, whereas the formation of ε-Zn(OH)2 was detected in solution with ethanol. The composite Zn(OH)2/Cd1-x Zn x S photocatalysts have great potential in various photocatalysis processes (e.g., hydrogen production, CO2 reduction, and the oxidation of organic contaminants) under visible light.

Effects of high mechanical treatment and long-term annealing on crystal structure and thermal stability of Ti2O3 nanocrystals.

The effect of high-energy milling and long-term annealing on the stability of Ti2O3 nanocrystals was studied using a magnetic susceptibility method. In situ temperature dependences revealed that the crystal size greatly affects the magnetic susceptibility value. According to XRD, SEM and TEM data, Magnéli phases Ti9O10, Ti4O7, Ti7O19 and Ti3O5 are formed.

Study of heteroaggregation and properties of sol-gel AlOOH-Fe3O4 composites.

In this work, AlOOH-Fe3O4 powder nanocomposites for Cr(VI) adsorption were obtained for the first time using oppositely charged boehmite and citric acid modified magnetite sols. The process of heteroaggregation of oppositely charged AlOOH and Fe3O4 nanoparticles was also studied as one of the stages in the preparation of adsorption active material. Сomposition, surface area, porous structure, thermal and surface properties, adsorption efficiency, and regenerability of nanocomposites were investigated using a wide range of analytical methods. It is noted that a low content of magnetite (2 wt.%) in the AlOOH-Fe3O4 composite promotes an increase in the surface area, weakly affects the Cr(VI) adsorption capacity, and imparts magnetic properties to the composite. Low cost, simplicity of preparation, high Cr(VI) adsorption capacity (up to 21 mg/g), and stability in cyclic use are the advantages of the obtained nanocomposites in comparison with similar systems. They can easily be separated from the purified liquid using a permanent magnet due to their magnetic properties.

The Influence of Cu and Al Additives on Reduction of Iron(III) Oxide: In Situ XRD and XANES Study.

The reduction of Fe-based nanocomposite catalysts doped with Al and Cu has been studied using in situ X-ray diffraction (XRD), in situ X-ray absorption near-edge structure (XANES), and temperature-programmed reduction (TPR) techniques. The catalysts have been synthesized by melting of iron, aluminum, and copper salts. According to XRD, the catalysts consist mainly of Fe2O3 and Al2O3 phases. Alumina is in an amorphous state, whereas iron oxide forms nanoparticles with the protohematite structure. The Al3+ cations are partially dissolved in the Fe2O3 lattice. Due to strong alumina-iron oxide interaction, the specific surface area of the catalysts increases significantly. TPR and XANES data indicate that copper forms highly dispersed surface CuO nanoparticles and partially dissolves in iron oxide. It has been shown that the reduction of iron(III) oxide by CO proceeds via two routes: a direct two-stage reduction of iron(III) oxide to metal (Fe2O3 → Fe3O4 → Fe) or an indirect three-stage reduction with the formation of FeO intermediate phases (Fe2O3 → Fe3O4 → FeO → Fe). The introduction of Al into Fe2O3 leads to a decrease in the rate for all reduction steps. In addition, the introduction of Al stabilizes small Fe3O4 particles and prevents further sintering of the iron oxide. The mechanism of stabilization is associated with the formation of Fe3- xAl xO4 solid solution. The addition of copper to the Fe-Al catalyst leads to the formation of highly dispersed CuO particles on the catalyst surface and a mixed oxide with a spinel-type crystalline structure similar to that of CuFe2O4. The low-temperature reduction of Cu2+ to Cu0 accelerates the Fe2O3 → Fe3O4 and FeO → Fe transformations but does not affect the Fe3O4 → FeO/Fe stages. These changes in the reduction properties significantly affect the catalytic performance of the Fe-based nanocomposite catalysts in the low-temperature oxidation of CO.