Reduction Of CuO Research Articles

Highly Dispersed Cu/Graphene Nanocatalyst Guided by MOF Structure: Application to Methanol Synthesis from CO2 Hydrogenation

AbstractThe dispersion of heterogeneous catalyst is directly related to the performance. In this work, highly dispersed Cu/graphene catalyst was successfully prepared by adopting a “template genetic inheritance” (TGI). The metal‐organic frameworks (MOFs) were introduced between the graphene sheets. Due to the spatial structure of MOFs, the Cu atoms arranged in the expected direction after removing the MOFs skeleton, which lead to a highly orderly dispersion of the Cu particles with uniform sized. Meanwhile, graphene provided a large specific surface area and a carbon source for in‐situ reduction of CuO. Furthermore, the CO2 hydrogenation to methanol performance was analyzed and more than 22 % conversion of CO2 and 84 % selectivity of methanol was achieved. This work provides a simple and novel strategy for the preparation of highly dispersed metal particle supported catalysts, which may be used in catalysis and other fields.

Mn- or Cu- substituted LaFeO[formula omitted]-based three-way catalysts: Highlighting different catalytically operating modes of La[formula omitted]Fe[formula omitted]M[formula omitted]O[formula omitted] (M=Cu, Mn)

The present work aims at presenting our investigations on the redox behaviour of Cu- or Mn-doped LaFeO3-based perovskite powders under three-way catalysis (TWC) relevant conditions. Two distinct La-deficient catalysts of generic formula La0.67Fe0.8Mn0.2O3 and La0.67Fe0.8Cu0.2O3 denoted as Mn-dLFO and Cu-dLFO, respectively, were prepared based on the conventional citrate complexation route and systematically investigated using complementary characterisation techniques. This study has made it possible to highlight fundamentally different structures. In Cu-dLFO, most Cu2+ cations are expelled from the LaFeO3 perovskite lattice in the form of a segregated CuO phase. On the other hand, in the case of Mn-dLFO, majority of Mn3+ cations are stabilised within the perovskite solid solution, while substantial iron exsolution in the form of an additional α-Fe2O3 phase was evidenced. The evolution of both catalysts during CO-TPR using operando Raman revealed the formation of polycyclic aromatic hydrocarbons (PAHs) besides the relative structural stability of the LaFeO3 lattice. The reduction of Mn3+ to Mn2+, indirectly suggested by Raman analysis, is further supported by a quasi-in situ XPS study. The latter also evidenced the reduction of CuO to metal copper to a large extent in Cu-dLFO. In addition, a share of the α-Fe2O3 phase present in Mn-dLFO is reduced to metal Fe0 during CO oxidation, and is fully re-oxidised upon NO reduction. Our investigation thus evidences that both copper and manganese sites in Cu-dLFO and Mn-dLFO, respectively, are redox-active centres upon CO oxidation/NO reduction with, however, varying operating modes underpinned by their fundamentally different structures.

Pure Cu particle obtained by ammonia reduction reaction: A new class of electrodes for hybrid supercapacitors

The metallic Cu material is synthesized via pyrogenic decomposition of Cu2(OH)2CO3 and ammonia reduction of CuO. The ammonia-induced solid phase transition process converts the material from semiconductor to conductor, and the internal chemical bond from ionic bond and covalent bond to metal bond, which makes it have high conductivity and more active sites as well as a rapid Faraday electrode reaction kinetics. Besides, the synthesized metallic Cu material can be used as positive materials for supercapacitors, which has high conductivity and displays a great specific capacity of 15 mAh g−1 (98.5 F g−1) at 0.5 A g−1. The hybrid supercapacitor device is further fabricated with activated carbon as cathode materials and it exhibits a good specific capacitance and excellent cycling performance (cost 16.7% of initial capacitance over 40,000 cycles). The electrochemical character of hybrid supercapacitor presents maximum specific energy of 6.5 Wh kg−1 at a specific power of 200.1 W kg−1. These results suggest the metallic material is a very promising electrode material for supercapacitors with superior electrochemical properties.

Enhanced C2 and C3 Product Selectivity in Electrochemical CO2 Reduction on Carbon-Doped Copper Oxide Catalysts Prepared by Deep Eutectic Solvent Calcination

Copper and its oxides are the main catalyst materials able to promote the formation of hydrocarbons from the electrocatalytic CO2 conversion. Herein, we describe a novel preparation method for carbon-doped copper oxide catalysts based on an oxidative thermal treatment of copper-containing deep eutectic solvents (DES). XRD and EDX analysis of the samples show that thermal treatment at 500 °C in air for a prolonged time (60 min) provides exclusively carbon-doped copper(II) oxide catalysts, whereas shorter calcination time leads to a mixture of less oxidized forms of copper (Cu2O and Cu0), CuO, and a higher carbon content from the DES. Chronoamperometry of the electrode containing the prepared materials in 0.5 M KHCO3 electrolyte show the reduction of CuO to less oxidized copper species. The materials prepared by the use of different DES, copper precursors and calcination times were used as electrocatalysts for the electrochemical CO2 reduction. Chemical analysis of the products reveals an enhanced selectivity toward C2 and C3 products for the catalyst prepared from the DES galactose-urea with copper nanoparticles and calcination for 60 min in air. The electrocatalytic activity of the prepared materials were compared to commercial CuO and showed a higher product concentration at −1.7 V vs. Ag/AgCl, with formation rates of 7.4, 6.0, and 10.4 µmol h−1 cm−2 for ethanol, n-propanol, and ethylene, respectively.

One-Pot Synthesis of CeO2 Modified SBA-15 With No Pore Clogging for NO Reduction by CO

CeO2 modified SBA-15 composites have been prepared by adding cerium precursor (Ce(NO3)3·6H2O) directly into the mixture of soft template (P123), silica precursor (TEOS) and urea aqueous solution but without mineral acid. The products were characterized by X-ray fluorescence (XRF), powder X-ray diffraction (XRD), N2 physisorption and transmission electron microscopy (TEM). Results indicated that ceria were successfully grafted onto mesoporous silica matrix and no pore clogging was observed. Both ceria content and mesoporous ordering of the final products were found to depend on urea amount. Compared to CeO2/SBA-15 from conventional impregnation method, the one-pot synthesis not only showed simple and green operation, but also superior catalytic performance in NO+CO reaction after loaded with CuO. It was revealed that both the presence and location of ceria had great influence on the reducibility of CuO, and the catalytic performances were intimately related to the redox properties of crystalline CuO. That is, higher NO conversion and N2 selectivity were achieved over catalyst with easier reduction of crystalline CuO.

Development of CuO-based oxygen carriers supported on diatomite and kaolin for chemical looping combustion

Chemical Looping Combustion (CLC) technology has emerged as a promising alternative capable of restricting the effects of global warming due to anthropogenic gas emissions, especially CO2, through its inherent capture. This study aims to synthesize and evaluate Cu-based oxygen carriers supported on natural materials such as diatomite and kaolin, through the incipient wet impregnation method for CLC process applications. Oxygen carriers were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy with surface energy dispersive x-ray spectroscopy (SEM-EDS). The mechanical strength of the two oxygen carrier particles was determined after the sintering procedure resulting in high crushing force. Reactivity of oxygen carriers was evaluated in a thermobalance with CH4 and H2 gases. Different reaction pathways were attempted when undergoing the redox cycles: total direct reduction of CuO to Cu0 for Cu-K and partial reduction of CuO to Cu2O and CuO to Cu-D. However, the highest reactivity and reaction rate was achieved in Cu-D due to the pore structure of diatomite, the chemical composition and the resulting interaction between CuO and the support. H2 gas reactivity tests showed a higher conversion rate and greater stability between cycles for both oxygen carriers. Thus, the reducible CuO content present in Cu-Diatomite during the reactivity test with H2 as the fuel gas was ideal for achieving high solids conversion, tendency for greater stability and a higher reaction rate.

A Ca-Cu Chemical Loop Process for CO2 Capture in Steel Mills: System Performance Analysis

The conceptual design and modelling of the Calcium Assisted Steel-mill Off-gas Hydrogen (CASOH) process for the conversion of blast furnace gas (BFG) into H2-rich stream and CO2-rich stream at a large scale is discussed in this work. High temperature reactors packed with CaO- and Cu-based materials are used to remove CO2 from the gaseous phase and simultaneously shifting the WGS equilibrium towards H2-rich products. The incorporation of a Cu/CuO chemical loop to such sorption process allows an efficient regeneration of the CO2 sorbent. In this case, the heat needed for the calcination of the CaCO3 is supplied in situ by the exothermic reduction of CuO to Cu with a gaseous fuel (e.g. CH4, CO or H2). The Cu-based solid is firstly converted to CuO(s) during the oxidation step with air and later reduced during the regeneration step, which involves the combination of the endothermic reaction of CaCO3(s) calcination and the exothermic gas-solid reduction of CuO(s) to Cu(s) using a gaseous fuel (typically BFG or natural gas producing a highly concentrated stream of CO2 and H2O(v)). In this paper, the three reaction stages of the CASOH process are modelled with a new 1-D reactor model that integrates state of the art kinetic information on the gas solid reactions, predicting the molar composition of the product gases (dry basis) at the outlet of the packed-bed reactor and maximum temperature achieved in each stage. The 1-D reactor modelling results confirm that process allows the conversion of up to 99% of the inlet CO to H2 at intermediate temperatures (about 650 °C), because of the efficient and continuous removal of CO2 from the gas phase. The high pressure (10 bar) during the Cu-oxidation step causes a very low leakage of CO2 (1.1 vol. %) due to the partial calcination of CaCO3, i.e. only 8 wt. % of the CaCO3 is calcined in this stage. Finally, the feed of BFG as reducing gas during the regeneration stage leads to a maximum temperature of 850 °C in the bed, which allows the complete calcination of the sorbent and gives as a result a CO2-rich stream ready for purification and subsequent use or storage.

EPR and CV studies cast further light on the origin of the enhanced hydrogen production through glycerol photoreforming on CuO:TiO2 physical mixtures

Different CuO nanoparticles were synthesized and tested in CuO:TiO2 physical mixtures for hydrogen production through glycerol photoreforming. CuO alone was inactive whereas its presence in CuO:TiO2 mixtures significantly improved the photoactivity of TiO2, with smaller CuO nanoparticles leading to higher hydrogen productions. Results obtained through combination of EPR and CV studies suggested the existence of a Cu(II)-Cu0 catalytic cycle. Thus, electrons promoted to the conduction band of titania could be used for CuO reduction to Cu which in turn would enable hydrogen production regenerating Cu2+ for a new catalytic cycle. All in all, a hydrogen production of 88 mmol·g−1 was obtained after 5 h on a CuO:TiO2 (10% w/w) physical mixture which is comparable to that achieved in a previous study under identical reaction conditions on 1.5%Pt/TiO2.

Low-Temperature Exothermic Reactions in Al/CuO Nanothermites Producing Copper Nanodots and Accelerating Combustion

Several Al/CuO nanocomposite reactive materials, prepared recently by arrested reactive milling, were found to exhibit a distinct low-temperature exothermic peak around 600 K seen by differential scanning calorimetry. This work is an experimental study aimed to establish whether this low-temperature reaction affects combustion and why it is observed in some but not all materials with identical compositions. The peak is only observed when aluminum is initially separately milled in acetonitrile. Electron microscopy showed resulting composite powders to be porous, unlike fully dense powders obtained without premilling aluminum. The as-prepared and partially reacted powders recovered after heating just past the peak to 650 K were examined using high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. Additionally, X-ray diffraction measurements were performed for all materials. Finally, the reactive powders were ignited using plasma and shock generated by an electrostatic discharge. Results show that the low-temperature exothermic peak is associated with a redox reaction pathway involving release of oxygen by CuO that is not in direct contact with Al and free-molecular transport of that oxygen to the nearby surface of Al. This reaction pathway inhibits the formation of Al/Cu intermetallic phases. Instead, nanometer-scale metallic Cu particles are formed due to CuO reduction. It is also found that this reaction pathway accelerates ignition of reactive powders, which in turn leads to a higher burn rate of aerosolized powder.

Highly Porous Cu2O Photocathode via Electrochemical Reconstruction of Dense Thin Films

Nanostructured light absorbers in PEC devices offer enhanced charge carrier collection and increased active area compared to planar structures. This article describes a simple two-step electrochemical treatment to obtain such nanostructured films of Cu2O absorber layer. A dense Cu2O film is electrodeposited on FTO coated glass substrate at a potential of −0.35 V (vs Ag/AgCl). This precursor film is then subjected to voltage cycling in a potential range of −0.6 V to 0.6 V, resulting in reconstruction of the top 0.5 μm dense film to 10–30 nm thick nano-walled structure. The reason for this reconstruction is a concomitant etching of Cu2O as Cu2+ ions along with oxidation to CuO. A morphology preserving reduction of the porous CuO during the cathodic scan results in a nano-walled Cu2O. From linear sweep voltammetry of pristine Cu2O and CuO films, we find that reduction of Cu2O to Cu is a slow process in this potential range and hence, Cu-impurity is not observed up to 50 cycles. Such an approach offers production of phase pure nanostructured films comprising features as small as 10 nm without following any complex procedure. The reconstructed porous Cu2O shows > 50% enhancement in photocurrent over dense Cu2O.

Operando Investigation of Ag-Decorated Cu2 O Nanocube Catalysts with Enhanced CO2 Electroreduction toward Liquid Products.

Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO2 electrochemical reduction reaction (CO2RR) can contribute to the decarbonization of the global economy. Here, well‐defined Cu2O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2O NCs, the catalyst with 5 at % Ag on Cu2O NCs displayed a two‐fold increase in the Faradaic efficiency for C2+ liquid products (30 % at −1.0 VRHE), including ethanol, 1‐propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X‐ray absorption spectroscopy revealed the partial reduction of Cu2O during CO2RR, accompanied by a reaction‐driven redispersion of Ag on the CuOx NCs. Data from operando surface‐enhanced Raman spectroscopy further uncovered significant variations in the CO binding to Cu, which were assigned to Ag−Cu sites formed during CO2RR that appear crucial for the C−C coupling and the enhanced yield of liquid products.

Syngas Production from Electrochemical CO2 Reduction on Copper Oxide Electrodes in Aqueous Solution

AbstractElectrochemical CO2 reduction to value‐added chemicals and fuels using renewable energy represents a promising strategy for reducing CO2 emissions and achieving effective energy storage. In this work, nano‐sized CuO catalysts were prepared by using the homogeneous precipitation method for electrochemical CO2 reduction to CO. The CO2 reduction studies combined with the characterization results show that structure‐activity relationships of the CuO catalysts depend on the calcination temperature. The as‐synthesized CuO catalysts calcined at different temperatures are capable of reducing CO2 to generate syngas with tunable CO/H2 ratios of 1 : 2 to 2 : 1. The desired CuO‐400 catalyst exhibits good morphology, small particle size, and enriched oxygen‐vacancy defects and, therefore, shows good performance for electrochemical CO2 reduction to CO. Under the given potential of −0.93 V vs. RHE, CuO‐400 exhibits good reduction activity and selectivity with a great electrochemically active surface area normalized CO partial current density of 1.44 mA/cm2 and a high CO faradaic efficiency of 48.2 %. The deactivation of CuO‐400 in the long‐term test is associated with the increase in particle size, the reduction of CuO to Cu2O and metallic Cu, and the coverage of surface active sites by the formed carbonate species. Our findings indicate that CuO catalysts with tunable physicochemical properties are promising candidates for electrochemical CO2 reduction to produce syngas.

Thermogravimetric insight in the reduction of xCuO – (1-x)MoO3 oxide system (0.1 ≤ x ≤ 0.9) by hydrogen

The oxide mixtures xCuO-(1-x) MoO3 were synthesized by gel-combustion procedure. The existence of phase mixture CuO + Cu3Mo2O9 and MoO3 + CuMoO4 in CuO-rich and MoO3 -rich composition region, respectively, were evidenced. The constant heating rate thermogravimetry in hydrogen atmosphere revealed that the reduction reactions proceed within the two clearly separated temperature regions. On the basis of mass changes, the mechanism of reduction processes was discussed. The measurements revealed considerable inhibition of CuO reduction by MoO3, and huge acceleration of MoO3 → MoO2 reduction step by copper. The particularities found in this system were commented in relation to our similar studies in NiO-MoO3 and CuO-WO3 systems. For particular composition, x = 0.5, existing preferably in form of a-CuMoO4, kinetic parameters of reduction were determined. The composition of oxide mixture influenced the particle size and morphology of resulting metallic Cu-Mo composites.

Hierarchical Cu2O/CuO/TiO2 composite films with high superhydrophobicity and strong adhesion

ABSTRACT Hierarchical Cu2O/CuO/TiO2 hollow nanostructured films with petal effect (high superhydrophobicity and high adhesion to water droplets) have been synthesized through a facile solvothermal approach, and the wettability of the product films was studied. The protocol was carefully designed for the preparation of the multi-scaled nanostructures. Cu(OH)2 nanorod arrays were used as precursors, which decomposed to form CuO and H2O molecules at high temperature. The produced tiny amount of water molecules was beneficial for both the slow hydrolysis of Ti(OC4H9)4 to formTiO2 nanostructures and the reduction of CuO to form Cu2O. The formation of hollow nanostructures is suggested to be based on kirkendall effect. CuO diffuses outward to react with ethanol in the solution, producing Cu2O/CuO/TiO2 hollow nanostructured films with rough surfaces. The water contact angle of the product film was estimated to be around 152.8°, and the water droplet remained firmly stuck on the surface when the film was vertically tilted, indicating the superhydrophobicity and strong adhesion of the product films. The Cassie impregnating wetting state was suggested as the wetting state of the product films.

Thermodynamics and Synthesis of Cu Powder from CuO in Waste Tire-Derived Pyrolytic Gas Atmosphere

The present study aimed to investigate the reduction behavior of CuO particles under the gaseous atmosphere generated by waste tire pyrolysis. Thermodynamics of the reduction process indicated that CuO could be reduced to the metal via the tire (rubber) pyrolysis route in the temperature range 700–900 K. Oxide reduction experiments were conducted as a function of the reactant mass ratio (mtire/mCuO) and temperature (600–900 K). The extent of waste pyrolysis increased as the temperature was raised to 900 K. This was accompanied by an increase in the oxide reduction. A significant reduction was attained at mtire/mCuO = 1.28 when the reactants were heated to 800 K and 900 K. Adding a small amount of waste high-density polyethylene to the tire sufficed for full CuO reduction. CuO reduction reactions and morphological evolution of flower-type CuO particles to relatively equiaxed Cu particles were discussed in terms of experimental and theoretical findings.

Study on the formation of 2-pentanone from ethanol over K-CuZrO2 catalysts

AseriesofK-CuZrO2catalysts with different Cu contents were prepared. The catalytic performance and reaction mechanism of 2-pentanone from ethanol condensation were investigated. The structure and properties of the catalysts were studied by N2 sorption, XRD, H2-TPR, CO2-TPD, TEM and XPS. The results showed that when the content of Cu was 9%, the conversion of ethanol reached the maximum (99.5%) due to the strong interaction between CuO and ZrO2 whichpromoted the reduction of CuO and resulted in the largest specific surface area of Cu on the catalyst surface. The selectivity of 2-pentanone reached the maximum (35.0%) because the strongest basicity related to medium-strength basic sites of the catalyst surface were suitable for the condensation reaction. The formation of 2-pentanone on K-CuZrO2 catalyst was speculated on the basis of the analysis of intermediates: ethanol was dehydrogenated to form acetaldehyde. Then, the acetaldehyde was condensed and decomposed to acetone followed by reaction with acetaldehyde to form 2-pentanone.

Reconstructing two-dimensional defects in CuO nanowires for efficient CO2 electroreduction to ethylene.

Here we report that in situ reconstructed Cu two-dimensional (2D) defects in CuO nanowires during CO2RR lead to significantly enhanced activity and selectivity of C2H4 compared to the CuO nanoplatelets. Specifically, the CuO nanowires achieve high faradaic efficiency of 62% for C2H4 and a partial current density of 324 mA cm-2 yet at a low potential of -0.56 V versus a reversible hydrogen electrode. Structural evolution characterization and in situ Raman spectra reveal that the high yield of C2H4 on CuO nanowires is attributed to the in situ reduction of CuO to Cu followed by structural reconstruction to form 2D defects, e.g., stacking faults and twin boundaries, which improve the CO production rate and *CO adsorption strength. This finding may provide a paradigm for the rational design of nanostructured catalysts for efficient CO2 electroreduction to C2H4.

Cu-Modified SrTiO3Perovskites Toward Enhanced Water–Gas Shift Catalysis: A Combined Experimental and Computational Study

The water gas shift reaction (WGS) is important and widely applied in the production of H2. Cu modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu modified SrTiO3 (nominally SrTi1-xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into a stable metallic phase under conditions of WGS reactions for all compositions. Among the characterized materials, the x = 0.20 composition showed the highest turnover frequency, lowest activation energy, and the highest WGS rate at 300C. According to density functional calculations, the formation of CuO is energetically less favorable compared with SrTiO3, explaining why the segregated CuO phase on the SrTiO3 surface is reduced to Cu during the catalytic reaction, while SrTiO3 remains. For x = 0.20, the size of the segregated CuO phase is optimum for facilitating the catalytic reaction. In contrast, a higher Cu content (x = 0.3) results in an aggregation of smaller CuO particles, resulting in fewer surface active sites and a net decrease in catalytic performance.

Aqueous phase catalytic hydrogenation of furfural to furfuryl alcohol over in-situ synthesized Cu–Zn/SiO2 catalysts

The aqueous phase catalytic hydrogenation of furfural to furfuryl alcohol is of environmental and technical advantages. In this study, a series of Cu-based catalysts supported on SiO2 were synthesized and used for furfural hydrogenation to furfuryl alcohol. Firstly, the effect of preparation methods of Cu/SiO2 catalysts on the structure and performance was investigated. The results indicate that, different from those by the impregnation and deposition-precipitation, the in-situ synthesized Cu/SiO2-IS catalyst leads to the formation of silicates, increasing the dispersion and surface area of Cu and thus improving the catalytic activity. Furthermore, the addition of Zn species not only promotes the reduction of CuO and improves the dispersion of metal Cu, but also effectively prevents decarbonylation, leading to higher furfuryl alcohol selectivity. Especially, due to its good reducibility and high metal Cu surface area, the Cu–Zn/SiO2-IS catalyst with the Cu/Zn ratio of 2/1 shows an excellent catalytic performance. So, the in-situ synthesized catalyst has a great potential as catalyst for catalytic hydrogenation of furfural to furfuryl alcohol in industry.

Copper-nickel mixed oxide catalysts from layered double hydroxides for the hydrogen-transfer valorisation of lignin in organosolv pulping

Copper and nickel mixed catalysts obtained by calcination of iron and aluminium hydrotalcites (layered double hydroxides, LDH) have been tested in the conversion of a lignin model dimer in subcritical methanol. Phase distribution and textural properties of the catalysts were characterized by X-ray diffraction Rietveld analysis and N2 physisorption. The presence of copper was critical for effective hydrogenation, both by direct hydrogen transfer from methanol to aldehyde groups and by reactivity of products from methanol reforming. TPR experiments showed that the hydrogenation activity was promoted by an enhanced reducibility of the Cu-catalysts, related to the presence of other oxide components.Characterisation of the catalysts after reaction indicated that metallic copper was formed by the reduction of CuO by methanol and that modifications of the oxide catalysts in the reaction medium played a major role in the formation of active sites.