Catalytic performance of texturally improved Al–Mg mixed oxides derived from emulsion-synthesized hydrotalcites

Mixed oxides of aluminum and magnesium derived from hydrotalcites were prepared by means of a sol–gel method mediated by an emulsified sol as pore template. The emulsion consisted of ethanol as the continuous phase and n-dodecane droplets as the dispersed phase, which was stabilized by the presence of the surfactant Pluronic P123. The use of such an emulsion was essential for obtaining materials with a porous structure that were assessed by mercury intrusion porosimetry and nitrogen physisorption. Additional characterization by NH₃ and CO₂ temperature programmed desorption confirmed that despite the enhancement of their textural properties, the number of acid and base sites was reduced in comparison to a reference and conventionally prepared Al–Mg mixed oxide, as a consequence of the depletion of surface hydroxyls during condensation of the precursors around the nonpolar droplets of the emulsion. Catalytic conversion of 2-propanol under conditions of controlled mass and heat diffusion on the texturally improved Al–Mg mixed oxides evidenced the preparation of a more effective catalyst than the poorly porous reference.

Mixed oxides of aluminum and magnesium derived from hydrotalcites that were prepared by means of a sol–gel method mediated by an emulsified sol as pore template.     

Ethanol dehydrogenative reactions catalyzed by copper supported on porous Al–Mg mixed oxides

Mixed aluminum and magnesium oxides (AlMgO) prepared by means of an emulsion-mediated sol–gel method was impregnated with copper nitrate solution and used in the ethanol dehydrogenative reactions to produce acetaldehyde and ethyl acetate. The emulsified system allowed to obtain a macro–mesoporous support that resulted in an outstanding dispersion of copper. The porous catalyst was about 3 times more active than the non-porous counterpart, due to the formation on the support''s surface of Cu⁰ together with the more active Cu⁺ species. In fact, the simultaneous presence of Cu⁺ and Cu⁰ were advantageous for the catalytic performance, as the turnover frequencies, were 122 and 166 h⁻¹ for the non-porous reference catalyst and for the porous one, respectively. Both catalysts deactivated due to copper particles sintering, however the porous one deactivated less, as a consequence of the better dispersion of the Cu species on the macro and mesoporous support. Acetaldehyde was the main product, however by increasing the contact time by 6.6 times, the conversion of ethanol on the non-porous catalyst reached about 90% with a selectivity to ethyl acetate of 20% by means of the coupling reaction of ethanol and acetaldehyde. The selectivity to ethyl acetate was favoured on an increased support/copper interface that is given by larger copper particles.

Mixed aluminum and magnesium oxides (AlMgO) prepared by means of an emulsion-mediated sol–gel method was impregnated with copper species and used in the ethanol dehydrogenative reactions to produce acetaldehyde and ethyl acetate.     

An efficient removal mechanism for different hydrophilic antibiotics from aquatic environments by Cu–Al–Fe–Cr quasicrystals

The work studied the adsorption properties and mechanism of Cu–Al–Fe–Cr quasicrystals (QCs) for the adsorption of ibuprofen (IBU), tedizolid phosphate (TZD), and sulbactam sodium (SAM) for the first time. The experimental results showed that quasicrystals were good adsorbents with great potential. The structure, surface morphology, and elemental composition of QCs were investigated by XPS, XRD, SEM, EDX, particle size, DSC-TG, and FTIR. The adsorption pH, kinetics, thermodynamics, and isotherms of IBU, TZD, and SAM in QCs were systematically studied. QCs had good adsorption performance for antibiotics, and the adsorption capacities of IBU, TZD, and SAM were 46.964, 49.206, and 35.292 mg g⁻¹ at the concentration of 25 mg L⁻¹, respectively. The surface charge and hydrophobicity of QCs were affected by changing pH, thereby affecting the adsorption performance of QCs. The main driving forces of adsorption included electrostatic force and hydrophobicity.

Adsorption of three antibiotic drugs (ibuprofen, sulbactamsodium, and terdiazole phosphate) with different hydrophobicity by using Cu–Al–Fe–Cr quasicrystals with multilayer structure as the adsorbent was investigated.     

Acid–base sites synergistic catalysis over Mg–Zr–Al mixed metal oxide toward synthesis of diethyl carbonate

In heterogeneous catalysis processes, development of high-performance acid–base sites synergistic catalysis has drawn increasing attention. In this work, we prepared Mg/Zr/Al mixed metal oxides (denoted as Mg₂Zr_xAl_1−x–MMO) derived from Mg–Zr–Al layered double hydroxides (LDHs) precursors. Their catalytic performance toward the synthesis of diethyl carbonate (DEC) from urea and ethanol was studied in detail, and the highest catalytic activity was obtained over the Mg₂Zr_0.53Al_0.47MMO catalyst (DEC yield: 37.6%). By establishing correlation between the catalytic performance and Lewis acid–base sites measured by NH₃-TPD and CO₂-TPD, it is found that both weak acid site and medium strength base site contribute to the overall yield of DEC, which demonstrates an acid–base synergistic catalysis in this reaction. In addition, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements reveal that the Lewis base site activates ethanol to give ethoxide species; while Lewis acid site facilitates the activated adsorption of urea and the intermediate ethyl carbamate (EC). Therefore, this work provides an effective method for the preparation of tunable acid–base catalysts based on LDHs precursor approach, which can be potentially used in cooperative acid–base catalysis reaction.

Mg/Zr/Al mixed metal oxides were prepared via a facile phase transformation process of hydrotalcite precursors, which showed acid–base sites synergistic catalytic performance toward the synthesis of diethyl carbonate from ethanol and urea.     

Synthesis of core–shell Ce-modified mixed metal oxides derived from P123-templated layered double hydroxides

Layered double hydroxides are a promising platform material which can be combined with a variety of active species based on their characteristic features. Silicon@P123-templated Ce-doped layered double hydroxide (SiO₂@CeMgAl-LDH(P123)) composites were synthesized via a facile in situ co-precipitation method, and characterized by TEM, X-ray diffraction, FTIR, XPS, CO₂-, etc. in detail. Meanwhile, the calcined powder (SiO₂@CeMgAl-LDO(P123)) possessed an excellent core–shell structure and a high surface area inherited from the LDH structure, which led to an outstanding catalytic activity (99.7% conversion of propylene oxide, 92.4% selectivity of propylene glycol methyl ether) under mild reaction conditions (120 °C). Cerium oxide provides a large number of oxygen vacancies and significantly improves the medium basic strength of the material, which facilitates the selective ring-opening of PO. Furthermore, the introduction and removal of P123 make the cerium oxide uniformly dispersed on the LDH layers, providing more reaction sites for the reaction of methanol and propylene oxide. The core–shell structure prepared by the in situ co-precipitation method could solve the shortcomings of agglomeration of layered double hydroxides and prolong the catalytic life evidently.

A shell of P123-templated CeMgAl-LDO was distributed in transverse and longitudinal directions on spheres of SiO₂. The composites displayed high catalytic activity in the synthesis of propylene glycol methyl ether.     

Mesoporous Cu–Cu2O@TiO2 heterojunction photocatalysts derived from metal–organic frameworks

This study reveals a unique Cu–Cu₂O@TiO₂ heterojunction photocatalyst obtained with metal–organic framework as the precursor, which can be utilized in dye photodegradation under visible light irradiation. The composition, structure, morphology, porosity, optical properties and photocatalytic performance of the obtained catalysts were all investigated in detail. The Cu–Cu₂O@TiO₂ nanocomposite is composed of lamellar Cu–Cu₂O microspheres embedded by numerous TiO₂ nanoparticles. Methylene blue, methyl orange and 4-nitrophenol were used as model pollutants to evaluate the photocatalytic activity of the Cu–Cu₂O@TiO₂ nanocomposite for dye degradation under visible light irradiation. Nearly 95% decolourisation efficiency of Methylene blue was achieved by the Cu–Cu₂O@TiO₂ photocatalyst within 3 h, which is much higher than that of TiO₂ or Cu₂O catalysts. The excellent photocatalytic activity was primarily attributed to the unique MOF-based mesoporous structure, the enlarged photo-adsorption range and the efficient separation of the charge carriers in the Cu–Cu₂O@TiO₂ heterojunction.

Cu–Cu₂O@TiO₂ heterojunction photocatalyst derived from a metal–organic framework shows high photocatalytic activity for dye degradation under visible light irradiation.     

Improvement of low-temperature NH3-SCR catalytic performance over nitrogen-doped MOx–Cr2O3–La2O3/TiO2–N (M = Cu,Fe, Ce) catalysts

A series of MO_x–Cr₂O₃–La₂O₃/TiO₂–N (M = Cu, Fe, Ce) catalysts with nitrogen doping were prepared via the impregnation method. Comparing the low-temperature NH₃-SCR activity of the catalysts, CeCrLa/Ti–N (xCeO₂–yCr₂O₃–zLa₂O₃/TiO₂–N) exhibited the best catalytic performance (NO conversion approaching 100% at 220–460 °C). The physico-chemical properties of the catalysts were characterized by XRD, BET, SEM, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. From the XRD and SEM results, N doping affects the crystalline growth of anatase TiO₂ and MO_x (M = Cu, Fe, Ce, Cr, La) which were well dispersed over the support. Moreover, the doping of N promotes the increase of the Cr⁶⁺/Cr ratio and Ce³⁺/Ce ratio, and the surface chemical adsorption oxygen content, which suggested the improvement of the redox properties of the catalyst. And the surface acid content of the catalyst increased with the doping of N, which is related to CeCrLa/TiO₂–N having the best catalytic activity at high temperature. Therefore, the CeCrLa/TiO₂–N catalyst exhibited the best NH₃-SCR performance and the redox performance of the catalysts is the main factor affecting their activity. Furthermore, in situ DRIFTS analysis indicates that Lewis-acid sites are the main adsorption sites for ammonia onto CeCrLa/TiO₂–N and the catalyst mainly follows the L–H mechanism.

A series of MO_x–Cr₂O₃–La₂O₃/TiO₂–N (M = Cu, Fe, Ce) catalysts with nitrogen doping were prepared via the impregnation method.     

Enhanced plasma-catalytic decomposition of toluene over Co–Ce binary metal oxide catalysts with high energy efficiency

In-plasma catalysis has been considered as a promising technology to degrade volatile organic compounds. Heterogeneous catalysts, especially binary metal oxide catalysts, play an important role in further advancing the catalytic performance of in-plasma catalysis. This work investigates the toluene decomposition performance over Co–Ce binary metal oxide catalysts within the in-plasma catalysis. Co–Ce catalysts with different Co/Ce molar ratios are synthesized by a citric acid method. Results show that the catalytic activity of Co–Ce catalysts is obviously superior to those of monometallic counterparts. Especially, Co_0.75Ce_0.25O_x catalyst simultaneously realizes highly efficient toluene conversion (with a decomposition efficiency of 98.5% and a carbon balance of 97.8%) and a large energy efficiency of 7.12 g kW h⁻¹, among the best performance in the state-of-art literature (0.42 to 6.11 g kW h⁻¹). The superior catalytic performance is further interpreted by the synergistic effect between Co and Ce species and the significant plasma–catalyst interaction. Specifically, the synergistic effect can decrease the catalyst crystallite size, enlarge the specific surface area and improve the amount of oxygen vacancies/mobility, providing more active sites for the adsorption of surface active oxygen species. Meanwhile, the plasma–catalyst interaction is able to generate the surface discharge and reinforce the electric field strength, thereby accelerating the plasma-catalytic reactions. In the end, the plasma-catalytic reaction mechanism and pathways of toluene conversion are demonstrated.

This work demonstrates highly efficient plasma-catalytic decomposition of toluene over Co–Ce binary metal oxide catalysts with superior energy efficiency.     

One-pot synthesis of Ag–Cu–Cu2O/C nanocomposites derived from a metal–organic framework as a photocatalyst for borylation of aryl halide

Mixed metal–metal oxide/C (Ag–Cu–Cu₂O/C) nanocomposites were synthesized by the heat treatment of a metal–organic framework under a N₂ flow using the one-pot synthesis method. The as-prepared nanocomposites were characterized using a range of techniques, such as TEM, elemental mapping, XRD, N₂ sorption, UV-Vis DRS, and XPS. The nanoparticles were successfully formed with high dispersion in porous carbon materials and high crystallinity based on the analysis results. The Ag–Cu–Cu₂O/C nanocomposites (35 nm) showed high photocatalytic activity and good recyclability toward the borylation of aryl halides under a xenon arc lamp. This result can enhance the interest in photocatalysis for various applications, particularly in organic reactions, using a simple and efficient synthesis method.

Ag–Cu–Cu₂O/C nanocomposites derived from metal–organic framework through one-pot thermal reduction method were synthesized. The material exhibits high catalytic activity in the borylation of aryl halide under xenon lamp condition across 7 cycles, with no yield decrease.     

Facile preparation of Cu–Fe oxide nanoplates for ammonia borane decomposition and tandem nitroarene hydrogenation

A facile substrate involved strategy was used to prepare Cu–Fe LDO (layered double oxide) nanoplates. The material exhibited good-efficiency for decomposition of ammonia borane (AB) in alkaline methanol solution. Significantly, the material also demonstrated excellent catalytic performance in the reduction of various nitroarenes by coupling with AB hydrolysis in a one pot tandem reaction, and gave excellent yields of the corresponding amine products.

CuFe oxide nanoplates were developed which displayed good activity and stability toward AB solvolysis. The material transformed to Cu/Fe₂O₃ while catalyzing AB reduction and in a tandem system reduced nitro compounds to amines in substantial yield.

The increased human exploitation of fossil fuels has caused serious environmental problems and depletion of resources, which makes it necessary to develop new types of green and sustainable alternative energy sources.^1,2 Hydrogen has attracted great attention and is likely to become the main source of energy in the future.^3,4 As chemical hydrogen storage materials, B–N compounds generally have high hydrogen densities.⁵ Among which, ammonia borane (AB) has been widely studied due to its excellent hydrogen storage and production capacity; it is non-toxic, harmless and can be dissolved in solvents such as methanol and so on.^6,7 Great efforts have been made to investigate the hydrolysis of AB using noble and transition metal based catalysts.⁸ Nevertheless, the continuous search for economical and easily obtainable catalysts has been never-ending.^9,10On the other hand, functionalized aromatic amines are important reaction intermediates and raw materials for production of value-added compounds, pharmaceuticals and agricultural chemicals.^11,12 They are generally synthesized by hydrogenation of the nitroarenes under pressurized H₂ with transition metal based catalysts. The exploitation of alternative hydrogen storage source with efficient and stable catalysts are still key to achieve high catalytic performance.¹³ Although some noble-metal based catalysts (such as Pt,¹⁴ Rh¹⁵ and Pd¹⁶) can achieve good selectivity by alloying with other metals or coupling with supporting materials, the high price and scarcity limit their applications. Therefore, the development of earth-abundant transition metals catalysts with high activity and chemical selectivity is still very necessary.¹⁷ Meanwhile, although AB as hydrogen reservoir is not strongly reducing enough to initiate the hydrogenation of nitroarenes, the utilization of transition metal based catalysts which expediently decompose AB (NH₃·BH₃ + 2H₂O → NH₄⁺ + BO₂⁻ + 3H₂↑)¹⁸ while catalysing hydrogenation of nitroarenes to target amines under in situ generated H₂ in a one-pot tandem system provides a promising strategy to realize the reduction processes.^19,20To our knowledge, few work have explored the application of transition metal based LDHs (and LDHs derivatives) for AB decomposition and nitroarenes hydrogenation.²¹ The layered double hydroxides (LDHs) is a type of material composed of layers of di- and trivalent metal cations coordinated to hydroxide anions, the layered structure allows exposing a large number of active sites.^22,23 It has demonstrated promising activity for oxygen evolution reaction (OER),²⁴ nitrogen reduction reaction (NRR).²⁵ Due to its adjustable multiple metal centres and large surface areas derived from its lamellar structure, LDHs makes ideal candidate to serve as either synergistic catalyst or as supporting catalyst precursor.²⁶ In general, upon heat treatment LDHs would transform into LDOs (layered double oxides),²⁷ which maintains the layered structure with exposed active sites that can serve as ideal candidate for transition metal oxides catalysed reactions. These intrinsic properties led us to conceive that transition metal based LDHs (and LDOs) could be potentially desirable cost-effective material for AB hydrolysis and nitroarenes reduction.Herein, as a proof of study we report a fast and substrate involved preparation of Cu–Fe LDOs nanoplates as high-efficiency catalyst for tandem decomposition of AB and hydrogenation of various nitroarenes. We use iron foam (IF) as the supporting substrate, and add H₂O₂ to accelerate the redox process²⁸ to quickly synthesis uniform CuFe oxide nanoplates tightly attached to the surface of iron foam at room temperature for the first time. The resultant CuFe LDOs can release H₂ efficiently from alkaline (OH⁻) AB methanol solution. Significantly, the material also showed excellent catalytic performance in tandem reduction of various nitroarenes through coupling with AB hydrolysis. It gave very handsome yield of corresponding amine products within short reaction time. Characterization were performed on CuFe LDOs before and after AB hydrolysis and revealed that the synthesized CuO/Fe₂O₃ (LDOs) were in situ converted to Cu/Fe₂O₃ after reaction, active hydrogen in situ absorbed on Cu(0) nanoparticles renders the high selective activity for amine formation. This work demonstrates the feasibility of using well-structured transition metal LDHs as raw material to effectively achieve new reduction reactions.The Cu–Fe LDOs was synthesised in a one pot reaction by using Fe foam as substrate introduced into a H₂O₂ solution containing Cu²⁺ (4 mmol) and Fe³⁺ (0.2 mmol) ions (ESI). Fe foam participated in and accelerated the reaction while H₂O₂ decomposed and precipitated the target material. A substantial amount of O₂ bubbling and heat release was observed during reaction, it took only a few minutes to successfully grow the product that tightly binds on the surface of Fe substrate, with an observable colour change from silver white to brown. The reaction mechanism was proposed as follow: initially Fe³⁺ reacts with Fe foam to generate Fe²⁺ on surface; Fe²⁺, Fe³⁺ and H₂O₂ make up the classic Fenton reaction system in which Fe²⁺ is oxidized by H₂O₂ to Fe³⁺; then Fe³⁺ and Cu²⁺ co-precipitates on iron foam to form Fe(OH)₃ and Cu(OH)₂ LDHs as OH⁻ increases;²⁹ LDHs was eventually converted to LDOs under the internal heat releasing condition. Control experiments revealed that in the absence of either Cu²⁺ or Fe³⁺, reaction proceeded very slowly.The material was subsequently characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) as well as Fourier transform infrared spectroscopy (FTIR). As shown in Fig. 1b, the iron foam after reaction was covered by a uniformly distributed layer of rugged materials. Magnified SEM images of the surface materials (Fig. 1c and d) revealed the formation of chrysanthemum shaped ball-flowers with diameters around 2 μm. The ball-flower is composed of bundles of numerous thin and long nano-leaves, and therefore confer the material with large surface area and abundant exposed active sites (Fig. 1d). Elemental mapping images of different ball-flowers clearly shows the presence of Fe and Cu elements in the sample, which are in an approximate concentration ratio 4 : 1 based on EDX analysis (Fig. 1e and S2 in ESI^†). XRD characterization was performed with the as prepared Cu–Fe sample self-grown on the iron foam for many times, however, distinct diffraction peaks were not obtained. A weak peak which can be ascribed to Cu(111) was observed in some tests, which was due to metal replacement reaction between Fe foam and added Cu²⁺ (Fig. 2a). Therefore, the material could be amorphous or that a too small thickness of the material was synthesized. To provide more evidence, we further performed TEM experiments (Fig. 3a), the result showed the formation of ball-flower material corresponding to the SEM images in Fig. 1, and the transparent thin-plates proved the small thickness of layers. In addition, TEM-mapping were also performed, the result demonstrated the uniform distribution of Fe, Cu and O elements on the material (Fig. 3b–f). HRTEM clearly showed the stacking of very thin layered plates usually typical of layered structure, and the various complex lattice fringes revealed the material as poly-crystalline (ESI, Fig. S3^†).Open in a separate windowFig. 1(a) Scheme for CuFe LDO synthesis. (b) SEM image of CuFe LDOs generated on iron foam; (c and d) magnified SEM images showing the formation of flower-clump like layered structure. (e) CuFe LDOs nanoplates bundle and corresponding elemental mapping of Cu, Fe and O.Open in a separate windowFig. 2(a) XRD of CuFe LDOs after reaction; (b) Raman spectra of CuFe LDOs before and after catalysis; (c and d) magnified SEM images showing the production of nanoparticles on the flower-clump like nanoplate structure.Open in a separate windowFig. 3(a) TEM; (b–f) TEM-mapping images; (g and h) Cu 2p and Fe 2p XPS spectra; (i) FTIR spectra of Cu–Fe LDOs.Furthermore, Raman spectrum provided evidence for the formation of material (Fig. 2b), analysis of the material formed on Fe foam displayed the main peak of CuO at 616 cm⁻¹, along with three minor peaks at 222, 285, 336 cm⁻¹. The weak peaks appeared at 1080 and 525 cm⁻¹ are indication of residue Cu(OH)₂ according to literature, which further proves the transformation process of LDH to LDO as described in above material preparation part.³⁰ The Raman spectra showed mainly the formation of CuO oxide, the peaks of iron species were not obvious as also happened to Raman of iron foam (Fig. 2b). Therefore, XPS experiments were also performed to provide more information. The XPS spectra where peaks corresponding to C 1s, Cu 2p and Fe 2p were detected (Fig. 3g–h). For Cu 2p, two typical peaks were located at 952.18 and 932.28 eV, corresponding to Cu 2p_1/2 and Cu 2p_3/2. The other two peaks at 942.96 and 962.27 eV were the satellite peaks of Cu 2p. These results indicates that the copper in the material appeared as Cu²⁺ with the outermost electron configuration of 3d⁹.³¹ For Fe 2p, the XPS peak of Fe 2p_1/2 was located at 725.60 eV, and the peak of Fe 2p_3/2 was two splitting peaks at 711.42 and 714.25 eV, indicating that iron appeared as Fe³⁺ with the outermost electron configuration of 3d⁵. In addition, the position of the binding energy at 714.25 eV indicated the presence of Fe–O on the surface of the composite.³¹FTIR spectra were also recorded using KBr method (Fig. 3i), which coincides with that of Cu–Fe LDH as reported in literature. The peaks at 1017 and 490 cm⁻¹ were ascribed to Cu–O–Fe stretching vibration and metallic bond vibration (M–O), respectively. The results indicated that the as-prepared material possessed a hydrotalcite-like structure, and the copper and iron atoms in the metal layer were connected by an oxygen atom.³²From the above characterization results, we may conclude as to the formation of layered double oxide materials of Cu–Fe oxides.Subsequently, the material was used as a model catalyst and tested for AB decomposition for H₂ production at room temperature. The produced gas was identified using gas chromatography and collected volumetrically with self-built setup. A 0.5 mol L⁻¹ NaOH methanol solution was used based on control experiments (ESI, Fig. S5^†) and report that OH⁻ could activate the B–N bond.³³ The molar ratio of Cu(NO₃)₂ and Fe(NO₃)₃ precursors used in material preparation were varied to find an optimized proportion that gave best H₂ release activity.As can be seen from Fig. 4a, when Cu²⁺concentration was varied from 0 to 6 mmol with fixed Fe³⁺ (0.2 mmol), the system showed increased activity for H₂ generation from AB (0.06 g in 25 mL H₂O) decomposition. It should be noted the material formed very slowly with only Fe³⁺ precursor did not show any activity, suggesting the essential role of Cu for LDO generation as well as dehydrogenation of AB. However, it was found that a thick layer of material was formed at high precursor concentration (Cu²⁺/Fe³⁺: 6/0.2) which easily fell off from iron foam and were difficult to handle for subsequent tests. Therefore, the catalytic system (Cu²⁺/Fe³⁺: 4/0.2) which generates catalyst with a tighter binding was chosen as model for most tests as well as characterizations. The kinetics of AB dehydrogenation by the model CuFe LDO was further studied under varying substrate concentrations.Open in a separate windowFig. 4H₂ evolution curves of AB solvolysis catalysed by catalyst (a) formed at different substrate ratios at 300 K; (b) with different AB amount by model LDO (added Cu²⁺/Fe³⁺: 4/0.2); (c) as a function of temperature in solvolysis of AB (0.06 g) with CuFe LDO (added Cu²⁺/Fe³⁺: 4/0.2; inset illustrations is logarithmic plot of H₂ generation rate versus 1/T). (d) Recycling test of CuFe LDO catalyst. (All reactions were conducted in 5 mL of 0.5 mol L⁻¹ NaOH methanol solution.)As demonstrated in Fig. 4b, the H₂ generation rate increased with increasing AB amount in solution (0.03–0.07 g), a higher reaction rate resulted from intimate contacts between catalyst and substrate. The maximum reaction rate was observed with 0.06 g AB, as system with a higher amount of 0.07 g added AB did not exhibit an even higher reaction rate. The reactions were performed in a temperature range of 298–315 K (Fig. 4c) To get the activation energy (E_a) of AB dehydrogenation by CuFe LDO. It is obvious that the reaction rate is enhanced upon increasing temperature. The Arrhenius plot of ln rate vs. 1/T is plotted (Fig. 4c, inset). The E_a is calculated to be approximately 35.9 kJ mol⁻¹. The material after one reaction was subjected to XRD, Raman, SEM characterizations. XRD revealed the formation of Cu/Fe₂O₃ after reduction reaction, the peaks appeared at 43.7°, 50.9° and 74.6° can be attributed to the (111), (200) and (220) crystal planes of Cu (JCPDS no. 04-0836). A small peak observed at 36.2° was evidence of the (119) crystal plane of Fe₂O₃ (JCPDS no. 25-1402).^20,21,34Furthermore, Raman characterization revealed the appearance of a peak at 628 cm⁻¹, which was right shifted compared to CuO (616 cm⁻¹) and was ascribed to the reduced Cu(0) in reference to literature (Fig. 2b).^21,30,35 In addition, the morphology of LDO after reaction was characterized by SEM, it was discovered that after AB solvolysis there were spherical nanoparticles in situ formed and attached on the surface of nanosheets clumps. The change in nanostructure coordinates with the reduction transformation of Cu species, and the generated nanoparticles was related to the Cu catalyst. The BET analysis indicates the lowering in porosity due to formation of nanoparticle clumps (Fig. S6^†). Therefore, it was evident from these data that the CuFe LDO material acted as precursor in AB solvolysis reaction, and reduction of CuO by ammonia borane would send forth the Cu(0) nanoparticles in situ from the nanosheets.The generated Cu/Fe₂O₃ after one reaction was collected and gently washed with deionized water, it was then subjected to recycling tests for AB decomposition. As can be seen from Fig. 4d, the reclaimed catalyst showed good recyclability and reusability. There is no apparent decrease in AB catalytic activity even after 7 repeated tests. The SEM image also provide evidence that the material maintained morphology after the recycling tests (Fig. S7^†). Very importantly, when Cu/Fe₂O₃ generated from LDO after the initial AB solvolysis was used for the second tests, the Cu/Fe₂O₃ showed faster reaction rate than the LDO in the initial test (Fig. S8^†). This result is due to the transformation of CuO/Fe₂O₃ to Cu/Fe₂O₃ catalyst in initial test, which in the second test Cu readily exists to catalyse AB decomposition.Additionally, these interesting results led us to exploit the use of CuFe LDO as catalyst for the hydrogenation of common functional aryl nitro compounds using AB as hydrogen source in a tandem reaction system, by simply adding nitro compounds to the AB solution. The activity and selectivity of this catalyst is comparable to noble metal nanocatalysts, which has meaningful significance for application to the industrial scale preparation of aromatic amines.As can be seen from †).CuFe LDO-catalyzed nitroarene hydrogenation

Hydrogen storage behavior of nanocrystalline and amorphous Mg–Ni–Cu–La alloys

Alloying and structural modification are two effective ways to enhance the hydrogen storage kinetics and decrease the thermal stability of Mg and Mg-based alloys. In order to enhance the characteristics of Mg₂Ni-type alloys, Cu and La were added to an Mg₂Ni-type alloy, and the sample alloys (Mg₂₄Ni₁₀Cu₂)_100−xLa_x (x = 0, 5, 10, 15, 20) were prepared by melt spinning. The influences of La content and spinning rate on the gaseous and electrochemical hydrogen storage properties of the sample alloys were explored in detail. The structural identification carried out by XRD and TEM indicates that the main phase of the alloys is Mg₂Ni and the addition of La results in the formation of the secondary phases LaMg₃ and La₂Mg₁₇. The as-spun alloys have amorphous and nanocrystalline structures, and the addition of La promotes glass formation. The electrochemical properties examined by an automatic galvanostatic system show that the samples possess a good activation capability and achieve their maximal discharge capacities within three cycles. The discharge potential characteristics were vastly ameliorated by melt spinning and La addition. The discharge capacities of the samples achieve their maximal values as the La content changes, and the discharge capacities always increase with increasing spinning rate. The addition of La leads to a decline in hydrogen absorption capacity, but it can effectively enhance the rate of hydrogen absorption. The addition of La and melt spinning significantly increase the hydrogen desorption rate due to the reduced activation energy.

In order to enhance the characteristics of Mg₂Ni-type alloys, Cu and La were added to an Mg₂Ni-type alloy, and sample alloys were prepared by melt spinning. The effects of La content and spinning rate on the hydrogen storage properties were explored.     

Systematic study of TiO2/ZnO mixed metal oxides for CO2 photoreduction

A two component three degree simplex lattice experimental design was employed to evaluate the impact of different mixing fractions of TiO₂ and ZnO on an ordered mesoporous SBA-15 support for CO₂ photoreduction. It was anticipated that the combined advantages of TiO₂ and ZnO: low cost, non-toxicity and combined electronic properties would facilitate CO₂ photoreduction. The fraction of ZnO had a statistically dominant impact on maximum CO₂ adsorption (β₂ = 22.65, p-value = 1.39 × 10⁻⁴). The fraction of TiO₂ used had a statistically significant positive impact on CO (β₁ = 9.71, p-value = 2.93 × 10⁻⁴) and CH₄ (β₁ = 1.43, p-value = 1.35 × 10⁻³) cumulative production. A negative impact, from the interaction term between the fractions of TiO₂ and ZnO, was found for CH₄ cumulative production (β₃ = −2.64, p-value = 2.30 × 10⁻²). The systematic study provided evidence for the possible loss in CO₂ photoreduction activity from sulphate groups introduced during the synthesis of ZnO. The decrease in activity is attributed to the presence of sulphate species in the ZnO prepared, which may possibly act as charge carrier and/or radical intermediate scavengers.

A novel example using a systematic design of experiments mixture design for developing mixed metal oxide photocatalysts for CO₂ photoreduction.     

Cu–Ag alloy for engineering properties and applications based on the LSPR of metal nanoparticles

Efficient generation of high-energy hot carriers from the localized surface plasmon resonance (LSPR) of noble metal (Ag, Au and Cu) nanoparticles is fundamental to many applications based on LSPR, such as photovoltaics and photocatalysis. Theoretically, intra- and inter-band electron transitions in metal nanoparticles are two important channels for the non-radiative decay of LSPR, which determine the generation rate and energy of hot carriers. Therefore, on the basis of first-principles calculations and Drude theory, in this work we explore the potential role of alloying Ag with Cu in modulating the generation rate and energy of hot carriers by studying the intra- and inter-band electron transitions in Cu, Ag and Cu–Ag alloys. It is meaningful to find that the d-sp inter-band electron transition rates are notably increased in Cu–Ag alloys. In particular, the inter-band electron transition rates of Cu_0.5Ag_0.5 become larger than that of single Cu and Ag across the whole energy range between 1.5 and 3.2 eV. In contrast, intra-band electron transition rates of Cu–Ag alloys become smaller than that of single Cu and Ag. Because the intra-band electron transitions mainly contribute to the resistive loss in metals, which finally results in a thermal effect rather than high-energy hot carriers, the reduction of intra-band electron transitions in Cu–Ag alloy is beneficial for the transforming the energy absorbed by LSPR into high-energy hot carriers through other non-radiative channels. These results indicate that alloying of Ag and Cu can effectively improve the generation rates of high-energy hot carriers through the inter-band electron transition, but decrease the resistive loss through intra-band transition of electrons, which should be used as a guide in optimizing the non-radiative decay processes of LSPR.

Alloying Ag with Cu can effectively improve the generation rates of high-energy hot carriers.     

Microstructure–mechanical properties of Ag0/Au0 doped K–Mg–Al–Si–O–F glass-ceramics

In understanding the catalytic efficacy of silver (Ag⁰) and gold (Au⁰) nanoparticles (NPs) on glass-ceramic (GC) crystallization, the microstructure–machinability correlation of a SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ system is studied. The thermal parameters viz., glass transition temperature (T_g) and crystallization temperature (T_c) were extensively changed by varying NPs (in situ or ex situ). Tc was found to be increased (T_c = 870–875 °C) by 90–110 °C when ex situ NPs were present in the glass system. Under controlled heat-treatment at 950 ± 10 °C, the glasses were converted into glass-ceramics with the predominant presence of crystalline phase (XRD) fluorophlogopite mica, [KMg₃(AlSi₃O₁₀)F₂]. Along with the secondary phase enstatite (MgSiO₃), the presence of Ag and Au particles (FCC system) were identified by XRD. A microstructure containing spherical crystallite precipitates (∼50–400 nm) has been observed through FESEM in in situ doped GCs. An ex situ Ag doped GC matrix composed of rock-like and plate-like crystallites mostly of size 1–3 μm ensured its superior machinability. Vicker''s and Knoop microhardness of in situ doped GCs were estimated within the range 4.45–4.61 GPa which is reduced to 4.21–4.34 GPa in the ex situ Ag system. Machinability of GCs was found to be in the order, ex situ Ag > ex situ Au ∼ in situ Ag > in situ Au. Thus, the ex situ Ag/Au doped SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ GC has potential for use as a machinable glass-ceramic.

In understanding the catalytic efficacy of silver (Ag⁰) and gold (Au⁰) nanoparticles (NPs) on glass-ceramic (GC) crystallization, the microstructure–machinability correlation of a SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ system is studied.     

Catalytic ketonization of palmitic acid over a series of transition metal oxides supported on zirconia oxide-based catalysts

Modification of a ZrO₂ based catalyst with selected transition metals dopants has shown promising improvement in the catalytic activity of palmitic acid ketonization. Small amounts of metal oxide deposition on the surface of the ZrO₂ catalyst enhances the yield of palmitone (16-hentriacontanone) as the major product with pentadecane as the largest side product. This investigation explores the effects of addition of carefully chosen metal oxides (Fe₂O₃, NiO, MnO₂, CeO₂, CuO, CoO, Cr₂O₃, La₂O₃ and ZnO) as dopants on bulk ZrO₂. The catalysts are prepared via a deposition–precipitation method followed by calcination at 550 °C and characterized by XRD, BET-surface area, TPD-CO₂, TPD-NH₃, FESEM, TEM and XPS. The screening of synthesized catalysts was carried out with 5% catalyst loading onto 15 g of pristine palmitic acid and the reaction carried out at 340 °C for 3 h. Preliminary studies show catalytic activity improvement with addition of dopants in the order of La₂O₃/ZrO₂ < CoO/ZrO₂ < MnO₂/ZrO₂ with the highest palmitic acid conversion of 92% and palmitone yield of 27.7% achieved using 5% MnO₂/ZrO₂ catalyst. Besides, NiO/ZrO₂ exhibits high selectivity exclusively for pentadecane compared to other catalysts with maximum yield of 24.9% and conversion of 64.9% is observed. Therefore, the changes in physicochemical properties of the dopant added ZrO₂ catalysts and their influence in palmitic acid ketonization reaction is discussed in detail.

Catalyst screening and optimization of a series of ZrO₂ supported metal oxides for ketonization of undiluted, neat palmitic acid.     

Effect of unmelted lime on the element distribution behavior on a CaO–SiO2–MgO–Al2O3–FeO–CaF2(–Cr2O3) slag

In this study, a CaO–SiO₂–Al₂O₃–MgO–FeO–CaF₂(–Cr₂O₃) slag was chosen according to the compositions of the stainless steel slag for industrial production, and a CaO block was added to the molten slag after the synthetic slag was fully melted. The influences of unmelted lime on the distribution of elements and the structure of product layers at the lime/slag boundary, particularly the existing state of chromium oxide in the chromium-bearing stainless steel slag, were deeply discussed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and FactSage 7.1. The experiment results indicated that when the unmelted lime existed in the CaO–SiO₂–Al₂O₃–MgO–FeO–CaF₂ slag system, two product layers of periclase (MgO) and dicalcium silicate (Ca₂SiO₄) at the boundary of the CaO block were formed. However, when the CaO block was added in the CaO–SiO₂–Al₂O₃–MgO–FeO–CaF₂–Cr₂O₃ stainless steel slag, besides MgO and Ca₂SiO₄ product layers, needle-shaped calcium chromite (CaCr₂O₄) was also precipitated around the CaO block. Moreover, a small amount of Cr dissolved in the periclase phase. Eh–pH diagrams showed that the CaCr₂O₄ and MgO phase unstably existed in a weak acid aqueous solution. Therefore, the existence of unmelted lime in the stainless steel slag could enhance the leachability of chromium.

The effect of unmelted lime on the distribution of elements and structure of product layers in CaO–SiO₂–MgO–Al₂O₃–FeO–CaF₂(–Cr₂O₂) stainless steel slag and the action of unmelted lime phase mechanism in experimental slags was conducted.     

Formation kinetics of Sr0.25Ba0.75Nb2O6 and Li2B4O7 crystals from 0.25SrO–0.75BaO–Nb2O5–Li2O–2B2O3 glass

We have investigated the transition kinetics of Sr_0.25Ba_0.75Nb₂O₆ (SBN) and Li₂B₄O₇ (LBO) crystals from 0.25SrO–0.75BaO–Nb₂O₅–Li₂O–2B₂O₃ (SBNLBO) glass under isothermal and non-isothermal processes. With increasing temperature, there are two consecutive steps of crystallization of SBN and LBO from the glass. The Johnson–Mehl–Avrami function indicates that the crystallization mechanism of SBN belongs to an increasing nucleation rate with diffusion-controlled growth. The crystallite size of SBN ranges from 40 to 140 nm but it is confined to within 30–45 nm for LBO during the whole crystallization process. The relationship between the nano size and strain of SBN based on the Williamson–Hall method, and the change of activation energies of SBN and LBO crystallization analyzed by using the isoconversional model are discussed. A comparison of phonon modes between as-quenched glass and fully transformed crystals clearly shows that the low dimensional vibration modes in the structurally disordered glass change to highly dimensional network units with the formation of crystals.

We have investigated the transition kinetics of Sr_0.25Ba_0.75Nb₂O₆ (SBN) and Li₂B₄O₇ (LBO) crystals from 0.25SrO–0.75BaO–Nb₂O₅–Li₂O–2B₂O₃ (SBNLBO) glass under isothermal and non-isothermal processes.     

CeO2-modified P2–Na–Co–Mn–O cathode with enhanced sodium storage characteristics

To improve the cycling stability and dynamic properties of layered oxide cathodes for sodium-ion batteries, surface modified P2–Na_0.67Co_0.25Mn_0.75O₂ with different levels of CeO₂ was successfully synthesized by the solid-state method. X-ray photoelectron spectra, X-ray diffraction and Raman spectra show that the P2-structure and the oxidation state of cobalt and manganese of the pristine oxide are not affected by CeO₂ surface modification, and a small amount of Ce⁴⁺ ions have been reduced to Ce³⁺ ions, and a few Ce ions have entered the crystal lattice of the P2-oxide surface during modification with CeO₂. In a voltage range of 2.0–4.0 V at a current density of 20 mA g⁻¹, 2.00 wt% CeO₂-modified Na_0.67Co_0.25Mn_0.75O₂ delivers a maximum discharge capacity of 135.93 mA h g⁻¹, and the capacity retentions are 91.96% and 83.38% after 50 and 100 cycles, respectively. However, the pristine oxide presents a low discharge capacity of 116.14 mA h g⁻¹, and very low retentions of 39.83% and 25.96% after 50 and 100 cycles, respectively. It is suggested that the CeO₂ modification enhances not only the maximum discharge capacity, but also the electric conductivity and the sodium ion diffusivity, resulting in a significant enhancement of the cycling stability and the kinetic characteristics of the P2-type oxide cathode.

The CeO₂ modification significantly enhances the maximum discharge capacity and cycling stability of a P2–Na_0.67Co_0.25Mn_0.75O₂ cathode.     

Nonstoichiometric oxygen in Mn–Ga–O spinels: reduction features of the oxides and their catalytic activity

The subject of this study was the content of oxygen in mixed oxides with the spinel structure Mn_1.7Ga_1.3O₄ that were synthesized by coprecipitation and thermal treatment in argon at 600–1200 °C. The study revealed the presence of excess oxygen in “low-temperature” oxides synthesized at 600–800 °C. The occurrence of superstoichiometric oxygen in the structure of Mn_1.7Ga_1.3O_4+δ oxide indicates the formation of cationic vacancies, which shows up as a decreased lattice parameter in comparison with “high-temperature” oxides synthesized at 1000–1200 °C; the additional negative charge is compensated by an increased content of Mn³⁺ cations according to XPS. The low-temperature oxides containing excess oxygen show a higher catalytic activity in CO oxidation as compared to the high-temperature oxides, the reaction temperature was 275 °C. For oxides prepared at 600 and 800 °C, catalytic activity was 0.0278 and 0.0048 cm³ (CO) per g per s, and further increase in synthesis temperature leads to a drop in activity to zero. The process of oxygen loss by Mn_1.7Ga_1.3O_4+δ was studied in detail by TPR, in situ XRD and XPS. It was found that the hydrogen reduction of Mn_1.7Ga_1.3O_4+δ proceeds in two steps. In the first step, excess oxygen is removed, Mn_1.7Ga_1.3O_4+δ → Mn_1.7Ga_1.3O₄. In the second step, Mn³⁺ cations are reduced to Mn²⁺ in the spinel structure with a release of manganese oxide as a single crystal phase, Mn_1.7Ga_1.3O₄ → Mn₂Ga₁O₄ + MnO.

The hydrogen reduction of Mn_1.7Ga_1.3O_4+δ proceeds in two steps. In the first step, excess oxygen is removed, Mn_1.7Ga_1.3O_4+δ → Mn_1.7Ga_1.3O₄. In the second step, Mn³⁺ cations are reduced to Mn²⁺ in the spinel structure and formation of MnO, Mn_1.7Ga_1.3O₄ → Mn₂Ga₁O₄ + MnO.