Effect of copper source on the structure–activity of CuAl2O4 spinel catalysts for CO hydrogenation

Due to the complexity of the structure–activity relationship of the CuAl₂O₄ spinel catalyst, optimization of the catalyst structure is a great challenge. In this paper, three different CuAl₂O₄ spinel catalysts were prepared by the solid-phase method using copper hydroxide, copper nitrate, and copper oxide as the copper source, respectively, to study the difference in the structure of CuAl₂O₄ spinel catalysts induced by the raw materials and the catalytic behavior for CO hydrogenation. The structure of CuAl₂O₄ spinel catalyst was characterized by XRD, BET, SEM, TEM, H₂-TPR and XPS. The activity of CO hydrogenation over the CuAl₂O₄ spinel catalyst without pre-reduction was evaluated in the slurry reactor. The results demonstrated that different copper sources had obvious influence on the CuAl₂O₄ spinel texture properties, surface enrichment degree, as well as decomposition and reduction ability, which further regulated the ratio of Cu⁺/Cu⁰ and thus affected the catalytic performance, especially the alcohol distribution. The CuAl₂O₄ spinel, employing copper hydroxide as the copper source, showed better selectivity of C₂₊OH, which was assigned to a higher ratio of Cu⁺/Cu⁰, along with larger pore size and pore volume. Moreover, the synergistic effect between Cu⁰ and γ-Al₂O₃ improved the selectivity of dimethyl ether.     

CuAl2O4 powder synthesis by sol-gel method and its photodegradation property under visible light irradiation

Nanocrystalline spinel CuAl₂O₄ powders were prepared by sol-gel method from nitrate Cu(NO₃)₂·3H₂O, Al(NO₃)₃·9H₂O and complex C₆H₈O₇·H₂O. Sintering was carried out at 400, 500, 600, 700, 800°C respectively for 2 h in air. The XRD patterns started to appear CuAl₂O₄ peaks after sintering of 500°C and consist of only CuAl₂O₄ peaks as spinel crystal after sintering of 700°C. The powders were analyzed by TEM and UV-vis diffuse reflectance spectrum to be round, about 10–30 nm in size and Eg=1.77 eV. Photodegradation property of nanocrystalline CuAl₂O₄ powders was investigated by using methyl orange as model pollutant and mercury lamp (λ>400 nm) as energy source. The results indicated that CuAl₂O₄ powders sintered at 700°C had the excellent visible photocatalytic property. Under the irradiation of visible light, methyl orange could be degraded 97% in 120 min.     

连续在线原位ATR-FTIR技术测定介孔CuAl2O4对黄药的吸附

以丁胺和正十二醇为混合模板剂, 采用共沉淀法制备了介孔纳米CuAl₂O₄. 用X射线粉末衍射(XRD)、傅里叶变换红外(FTIR)光谱、N₂吸附-脱附对产物的结构进行了表征. 采用连续在线原位衰减全反射傅里叶变换红外(ATR-FTIR)光谱技术研究了水溶液中丁基和辛基黄药在介孔CuAl₂O₄表面的吸附. 随着吸附时间的延长,1200 和1040 cm^-1两处黄药特征峰的高度逐渐增加, 根据1200 cm^-1处C-O-C伸缩振动峰的变化来评价黄药在CuAl₂O₄表面的吸附动力学过程. 结果表明, 介孔纳米CuAl₂O₄对黄药有很强的吸附能力, 在100 min 的时间内, CuAl₂O₄样品对丁基和辛基黄药的吸附量分别达到了236 和300 mg·g^-1, 且属于化学吸附. 对实验数据进行理论模拟, 发现吸附过程更接近于拟二级吸附动力学方程.     

Noble‐Metal‐Free Single‐Atom Catalysts CuAl4O7–9− for CO Oxidation by O2

The single copper atom doped clusters CuAl₄O_7–9^? can catalyze CO oxidation by O₂. The CuAl₄O_7–9^? clusters are the first group of experimentally identified noble‐metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl₄O₉CO^? is much more reactive than CuAl₄O₉^? in the reaction with CO to generate CO₂. One adsorbed CO is crucial to stabilize Cu of CuAl₄O₉^? around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble‐metal catalysis was discussed.     

Synthesis and characterization of spinel-type CuAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystalline by modified sol–gel method

Nanocrystalline Copper aluminate (CuAl₂O₄) was prepared by sol–gel technique using aluminum nitrate, copper nitrate, diethylene glycol monoethyl ether and citric acid were used as precursor materials. This method starts from of the precursor complex, and involves formation of homogeneous solid intermediates, reducing atomic diffusion processes during thermal treatment. The formation of pure crystallized CuAl₂O₄ nanocrystals occurred when the precursor was heat-treated at 600 °C in air for 2 h. The stages of the formation of CuAl₂O₄, as well as the characterization of the resulting compounds were done using thermo–gravimetric analysis, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The products were analyzed by transmission electron microscopy and ultraviolet–visible (UV–Vis) spectroscopy to be round, about 17–26 nm in size and E _g = 2.10 eV.     

Hydrothermal preparation of CuO-ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> catalyst for phenol <Emphasis Type="Italic">ortho</Emphasis>-alkylation with methanol: effect of the calcination temperature on the catalytic performance

The effect of heat-treatment on 10 wt% CuO-ZnAl₂O₄ catalytic activity in methylation of phenol and the degree of interaction of CuO active phase with support spinel phase were investigated. The CuO-ZnAl₂O₄ sample was subjected to heat-treatment up to 1000°C. The thermal products were characterized by X-ray diffraction (XRD) analysis, nitrogen adsorption-desorption at -196°C and temperature-programmed desorption (TPD-MS) of CO₂. Additionally, the reducibility of copper phases was investigated by temperature-programmed reduction (TPR). XRD patterns of the fresh catalyst sample (calcined at 600°C) indicated the presence of a mixture of poorly crystallized CuO and ZnAl₂O₄ spinel phase. The presence of two reducible copper species has been found on fresh CuO-ZnAl₂O₄ catalyst by TPR analysis. After subsequent calcinations in air at elevated temperatures some CuO disappeared with appearance of CuAl₂O₄ phase. The catalytic results revealed that the CuO addition to ZnAl₂O₄ increases the activity in ortho-methylation of phenol. Subsequent heat-treatment up to 900°C causes partial deactivation of copper centers, which is the result of transformation of CuO to the inactive CuAl₂O₄ phase.     

Improved adhesion of copper on Al2O3

Summary Improved methods for Al₂O₃ metallization by Cu are described. Good adhesion between Cu and Al₂O₃ substrate depends on the formation of chemical bonds between the substrate and the metallic layer. The temperature needed for the formation of a CuAl₂O₄ spinel interface is reduced from 1050°C to 900°C by the addition of various oxides. The adhesion between the CuAl₂O₄ interface and deposited Cu is stronger then the tensile strength of pure Cu. Plasma techniques for the formation of a Cu containing interface are also described. Bombardment of a Cu film with Xe⁺ ions in a rf-glow discharge implants Cu atoms into the substrate to a depth of 5 nm, as determined by SIMS depth profiling. Methods for reduction of the CuAl₂O₄ surface for subsequent metallization are also presented.     

Preparation of <Emphasis Type="Italic">p</Emphasis>-type conducting transparent CuCrO<Subscript>2</Subscript> and CuAl<Subscript>0.5</Subscript>Cr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> thin films by sol–gel processing

CuCrO₂ and CuAl_0.5Cr_0.5O₂ thin films were prepared by sol–gel processing and subsequent two-step annealing in air and inert gas atmosphere. Phase pure films with delafossite structure were obtained by adjusting the respective temperatures. The related phase development strongly affects the optical and electrical performance, giving leeway for optimization. The resulting CuCrO₂ (16 Ωcm, transmittance 21%) and CuAl_0.5Cr_0.5O₂ (11 Ωcm, transmittance 49%) films showed p-type conductivity by their positive Seebeck coefficients. The microstructure of the systems was characterized by scanning and transmission electron microscopy and correlated to the growth of different crystalline phases during the annealing steps. Thereby, crystal thermodynamics also affects the respective film performance, alleviating delafossite formation from the amorphous phase.     

Cu–Al Spinel Oxide as an Efficient Catalyst for Methanol Steam Reforming

Cu–Al spinel oxide, which contains a small portion of the CuO phase, has been successfully used in methanol steam reforming (MSR) without prereduction. The omission of prereduction not only avoids the copper sintering prior to the catalytic reaction, but also slows down the copper‐sintering rate in MSR. During this process, the CuO phase can initiate MSR at a lower temperature, and CuAl₂O₄ releases active copper gradually. The catalyst CA2.5‐900, calcined at 900 °C with n(Al)/n(Cu)=2.5, has a higher CuAl₂O₄ content, higher BET surface area, and smaller CuAl₂O₄ crystal size. Its activity first increases and then decreases during MSR. Furthermore, both fresh and regenerated CA2.5‐900 showed better catalytic performance than the commercial Cu–Zn–Al catalyst.     

Tailoring *H Intermediate Coverage on the CuAl2O4/CuO Catalyst for Enhanced Electrocatalytic CO2 Reduction to Ethanol

The direct electrochemical conversion of CO₂ to multi-carbon products offers a promising pathway for producing value-added chemicals using renewable electricity. However, producing ethanol remains a challenge because of the competitive ethylene formation and hydrogen evolution reactions. Herein, we propose an active hydrogen (*H)-intermediate-mediating strategy for ethanol electroproduction on a layered precursor-derived CuAl₂O₄/CuO catalyst. The catalyst delivered a Faradaic efficiency of 70 % for multi-carbon products and 41 % for ethanol at current density of 200 mA cm⁻² and exhibited a continuous 150 h durability in a flow cell. The intensive spectroscopic studies combined with theoretical calculations revealed that the in situ generated CuAl₂O₄ could tailor *H intermediate coverage and the elevated *H coverage favors the hydrogenation of the *HCCOH intermediate, accounting for the increased yield of ethanol. This work directs a pathway for enhancing ethanol electroproduction from CO₂ reduction by tailoring *H intermediate coverage.     

Selective wet air oxidation of diluted aqueous ammonia solutions over co-precipitated transition metal-aluminium catalysts

Catalytic wet air oxidation of ammonia over a co-precipitated transition metal-aluminium catalyst was investigated. Copper-aluminium (Cu-Al-O) catalyst exhibited the highest activity and N₂ selectivity among those prepared from Co, Fe, Mn, and Ni. 50% of 1500 ppm of ammonia could be removed from wastewater of pH 12 at 503 K under 2.0 MPa of air by using 4.0 g of catalyst without formation of toxic nitrogen containing compounds. Cu and Al ions were not found in solution after the reaction. It has been found that the catalytic performance of Cu-Al-O catalyst was strongly dependent on the preparation methods. The co-precipitated Cu-Al-O catalysts showed high N₂ selectivity. The presence of CuO is concluded to promote the reaction and CuAl₂O₄ in bulk phase is needed to stabilize the catalyst.     

Synthesis of Single‐Crystalline Spinel LiMn2O4 Nanorods for Lithium‐Ion Batteries with High Rate Capability and Long Cycle Life

The long‐standing challenge associated with capacity fading of spinel LiMn₂O₄ cathode material for lithium‐ion batteries is investigated. Single‐crystalline spinel LiMn₂O₄ nanorods were successfully synthesized by a template‐engaged method. Porous Mn₃O₄ nanorods were used as self‐sacrificial templates, into which LiOH was infiltrated by a vacuum‐assisted impregnation route. When used as cathode materials for lithium‐ion batteries, the spinel LiMn₂O₄ nanorods exhibited superior long cycle life owing to the one‐dimensional nanorod structure, single‐crystallinity, and Li‐rich effect. LiMn₂O₄ nanorods retained 95.6 % of the initial capacity after 1000 cycles at 3C rate. In particular, the nanorod morphology of the spinel LiMn₂O₄ was well‐preserved after a long‐term cycling, suggesting the ultrahigh structural stability of the single crystalline spinel LiMn₂O₄ nanorods. This result shows the promising applications of single‐crystalline spinel LiMn₂O₄ nanorods as cathode materials for lithium‐ion batteries with high rate capability and long cycle life.     

A study of the formation of CuAl2O4 from CuO and Al2O3 by solid state reaction at 1000°C and 950°C

The formation of the spinel CuAl₂O₄ from the oxides CuO and Al₂O₃ has been studied at 1000 and 950°C in air by measuring the fraction reaction completed as a function of time. In the experiments the molar oxide ratios were CuO/Al₂O₃ = 0.5, 1.0 and 2.0 and the grain size for CuO was 1–3 μ throughout while for Al₂O₃ fractions of 40–60 μ, 71–100 μ and 100–125 μ, were used. The rate of reaction could be explained quite well assuming a three-dimensional diffusion mechanism.     

One pot facile hydrothermal synthesis of superparamagnetic ZnFe2O4 nanoparticles and their properties

Superparamagnetic ZnFe₂O₄ nanoparticles were synthesized by facile hydrothermal method using different amount of diethylamine (DEA) as a precipitating agent. The phase formation, morphological and magnetic properties were investigated using X-ray diffraction, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy and vibrating sample magnetometer. The amount of DEA required for forming single phase ZnFe₂O₄ was optimized. For 2 ml of DEA, a mixed phase of α-Fe₂O₃ and ZnFe₂O₄ was formed whereas single phase of ZnFe₂O₄ was formed with high crystalline quality for 4, 6, 8 and 10 ml of DEA. FE-SEM results indicated that all the synthesized products were in spherical shape with small aggregations. Particle size of 9 nm was obtained from TEM image for sample synthesized using 10 ml of DEA. Superparamagnetic property was observed for the samples synthesized using 4, 6, 8, and 10 ml of DEA.     

Structural study of copper-nickel aluminate (CuxNi1−xAl2O4) spinels

CuAl₂O₄, NiAl₂O₄, and three ternary spinels Cu_xNi_1?xAl₂O₃ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, NiO, and Al₂O₃ at 1223 K. X-Ray powder diffractometry, coupled with adequate computational methods, allowed determination of the unit-cell length, oxygen positional parameter, and cation distribution for each compound. Interdependence of these structural parameters is closely analyzed on the ternary oxide spinels. The one-electron difference between the Cu²⁺ and Ni²⁺ ions was found to be enough to render them distinguishable by X-ray powder diffraction.     

Effect of the CuAl2O4 and CuAlO2 Phases in Catalytic Wet Air Oxidation of ETBE and TAME using CuO/γ-Al2O3 catalysts

This paper studies Cu/Al₂O₃ catalysts, synthesized in two ways: copper deposit in the synthesis of alumina (sol gel) and incipient impregnation stabilized at 400 °C. The materials were characterized by X-ray diffraction studies, nitrogen physisorption, temperature programmed reduction of H₂, dehydration of isopropanol, scanning electronic microscopy, transmission electronic microscopy, which were evaluated in the liquid phase oxidation reaction of ethyl tert-butyl ether and tert-amyl methyl ether. The formation of CuAl₂O₄ and CuAlO₂ in the samples synthesized by sol gel, led to a modification of the texture, thus resulting in an expansion of the specific area of the materials. CuAl₂O₄ and CuAlO₂ have been identified by DRX from a content of 10 % Copper, the first showed the highest intensity with this technique. In the same way, these species favor the presence of Lewis acid sites; this is reflected in the materials with (Di-isopropyl Ether) DIPE of 96.7 % and 91.1 % for the samples SAlCu5 and SAlCu15 respectively. The catalytic activity of the materials prepared by sol gel is in the function of the number of surface acid sites, the smaller particle size of the Cu and the surface of the contact, in the case of the ETBE meanwhile for TAME the activity was based mainly on the strength of the present acid sites. With impregnated materials, the activity is attributed to the smaller particle size of the Cu and the greater strength of the surface acid sites in the solid. The formation of spinel species inhibits the leaching phenomenon in the reaction milieu.     

Study of CuCl2 Supported on SiO2 and Al2O3

The nature and stability of surface species of CuCl₂ supported on α-Al₂O₃, γ-Al₂O₃, and SiO₂ were investigated by using X-ray diffraction techniques and reflectance spectroscopy. No specific chemical interaction of CuCl₂ is observed on an inert α-Al₂O₃ support, as opposed to hydrated carriers as SiO₂ and γ-Al₂O₃. On these supports the coordination sphere of Cu²⁺ consists of surface groups (OH^? or O^? at drying and activation, resp.), H₂O and Cl^?, with the H₂O ligands decreasing in concentration in the process of impregnation, drying and calcination. γ-Al₂O₃ samples, calcined at 400°C, show γ-Cu₂(OH)₃Cl as opposed to CuAl₂O₄ at higher temperatures. The absence of Cu₂(OH)₃Cl on SiO₂-supported samples is related to the acid-base characteristics of the carriers. The various supports can be arranged in the following order of stability of the complexes formed: γ-Al₂O₃ > SiO₂ ? -Al₂O₃.     

CuAl2 revisited: Composition, crystal structure, chemical bonding, compressibility and Raman spectroscopy

The structure of CuAl₂ is usually described as a framework of base condensed tetragonal antiprisms [CuAl_8/4]. The appropriate symmetry governed periodic nodal surface (PNS) divides the space of the structure into two labyrinths. All atoms are located in one labyrinth, whereas the second labyrinth seems to be ‘empty’. The bonding of the CuAl₂ structure was analyzed by the electron localization function (ELF), crystal orbital Hamiltonian population (COHP) analysis and Raman spectroscopy. From the ELF representation it is seen, that the ‘empty’ labyrinth is in fact the place of important covalent interactions. ELF, COHP in combination with high-pressure X-ray diffraction and Raman spectroscopy show that the CuAl₂ structure is described best as a network built of interpenetrating graphite-like nets of three-bonded aluminum atoms with the copper atoms inside the tetragonal-antiprismatic cavities.     

CTAB-Mn3O4 nanocomposites: Synthesis,NMR and low temperature EPR studies

We are reporting on the synthesis of Mn₃O₄ nanoparticles and CTAB-Mn₃O₄ nanocomposites via a sonochemical route using MnCl₂, ethanol, NaOH and CTAB. The crystalline phase was identified as Mn₃O₄. The crystallite size of the CTAB-Mn₃O₄ nanocomposite was identified as 13 ± 5 nm from X-ray line profile fitting and the particle size from TEM was 107.5 ± 1.4 nm. The interaction between CTAB and the Mn₃O₄ nanoparticles was investigated by FTIR and ¹H NMR spectroscopies. Two different magnetic phase transitions were observed for both samples below the Curie temperature (43 °C) by using a low temperature Electron Paramagnetic Resonance (EPR) technique. Also we determined the effect of the capping with CTAB on the reduction in absorbed power.