Copper-based Catalysts Research Articles

Fe–Ni–Ce–Zr-modified CuO–ZnO catalyst for methanol steam reforming

Four copper-based catalysts were prepared by co-precipitation method. The catalysts were characterized by scanning electron microscopy, N2 adsorption-desorption, temperature programmed reduction process (H2-TPR), X-ray diffraction (XRD) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) to explore the effects of different components on catalyst activity. The performance of different catalysts for hydrogen production via methanol steam reforming was evaluated on a fixed-bed reactor. The effects of different components, reaction temperature and gas hourly space velocity (GHSV) on the catalytic performance were investigated. The results exhibited that among the investigated catalysts, 1# and 2# catalysts had the higher catalytic activity and stability. Under the conditions of 280 °C, a molar ratio of methanol to water of 1:1.3, and a GHSV of 13.3 L/(g·h), the methanol conversion rate reached 99.9%, the CO concentration was as low as 0.29%, and the selectivity was only 1.16%. The H2 production flow rate per unit mass of catalyst was 270 mL/(min·g). After 60 h of start-stop experiments, the methanol conversion rate remained above 98.4%, the CO concentration was always lower than 0.5%, and the start-up speed was significantly faster than that of the commercial catalyst M650. This study provided an effective method for the preparation of catalysts for methanol steam reforming.

Nanograin-Boundary-Abundant Cu2O-Cu Nanocubes with High C2+ Selectivity and Good Stability during Electrochemical CO2 Reduction at a Current Density of 500 mA/cm2.

Surface and interface engineering, especially the creation of abundant Cu0/Cu+ interfaces and nanograin boundaries, is known to facilitate C2+ production during electrochemical CO2 reductions over copper-based catalysts. However, precisely controlling the favorable nanograin boundaries with surface structures (e.g., Cu(100) facets and Cu[n(100)×(110)] step sites) and simultaneously stabilizing Cu0/Cu+ interfaces is challenging, since Cu+ species are highly susceptible to be reduced into bulk metallic Cu at high current densities. Thus, an in-depth understanding of the structure evolution of the Cu-based catalysts under realistic CO2RR conditions is imperative, including the formation and stabilization of nanograin boundaries and Cu0/Cu+ interfaces. Herein we demonstrate that the well-controlled thermal reduction of Cu2O nanocubes under a CO atmosphere yields a remarkably stable Cu2O-Cu nanocube hybrid catalyst (Cu2O(CO)) possessing a high density of Cu0/Cu+ interfaces, abundant nanograin boundaries with Cu(100) facets, and Cu[n(100)×(110)] step sites. The Cu2O(CO) electrocatalyst delivered a high C2+ Faradaic efficiency of 77.4% (56.6% for ethylene) during the CO2RR under an industrial current density of 500 mA/cm2. Spectroscopic characterizations and morphological evolution studies, together with in situ time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies, established that the morphology and Cu0/Cu+ interfacial sites in the as-prepared Cu2O(CO) catalyst were preserved under high polarization and high current densities due to the nanograin-boundary-abundant structure. Furthermore, the abundant Cu0/Cu+ interfacial sites on the Cu2O(CO) catalyst acted to increase the *CO adsorption density, thereby increasing the opportunity for C-C coupling reactions, leading to a high C2+ selectivity.

Manipulating local coordination of copper single atom catalyst enables efficient CO2-to-CH4 conversion

Electrochemical CO2 conversion to methane, powered by intermittent renewable electricity, provides an entrancing opportunity to both store renewable electric energy and utilize emitted CO2. Copper-based single atom catalysts are promising candidates to restrain C-C coupling, suggesting feasibility in further protonation of CO* to CHO* for methane production. In theoretical studies herein, we find that introducing boron atoms into the first coordination layer of Cu-N4 motif facilitates the binding of CO* and CHO* intermediates, which favors the generation of methane. Accordingly, we employ a co-doping strategy to fabricate B-doped Cu-Nx atomic configuration (Cu-NxBy), where Cu-N2B2 is resolved to be the dominant site. Compared with Cu-N4 motifs, as-synthesized B-doped Cu-Nx structure exhibits a superior performance towards methane production, showing a peak methane Faradaic efficiency of 73% at −1.46 V vs. RHE and a maximum methane partial current density of −462 mA cm−2 at −1.94 V vs. RHE. Extensional calculations utilizing two-dimensional reaction phase diagram analysis together with barrier calculation help to gain more insights into the reaction mechanism of Cu-N2B2 coordination structure.

Low-temperature NH3 abatement via selective oxidation over a supported copper catalyst with high Cu+ abundance

Selective catalytic NH3-to-N2 oxidation (NH3-SCO) is highly promising for abating NH3 emissions slipped from stationary flue gas after-treatment devices. Its practical application, however, is limited by the non-availability of low-cost catalysts with high activity and N2 selectivity. Here, using defect-rich nitrogen-doped carbon nanotubes (NCNT-AW) as the support, we developed a highly active and durable copper-based NH3-SCO catalyst with a high abundance of cuprous (Cu+) sites. The obtained Cu/NCNT-AW catalyst demonstrated outstanding activity with a T50 (i.e. the temperature to reach 50% NH3 conversion) of 174°C in the NH3-SCO reaction, which outperformed not only the Cu catalyst supported on N-free O-functionalized CNTs (OCNTs) or NCNT with less surface defects, but also those most active Cu catalysts in open literature. Reaction kinetics measurements and temperature-programmed surface reactions using NH3 as a probe molecule revealed that the NH3-SCO reaction on Cu/NCNT-AW follows an internal selective catalytic reaction (i-SCR) route involving nitric oxide (NO) as a key intermediate. According to mechanistic investigations by X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray absorption spectroscopy, the superior NH3-SCO performance of Cu/NCNT-AW originated from a synergy of surface defects and N-dopants. Specifically, surface defects promoted the anchoring of CuO nanoparticles on N-containing sites and, thereby, enabled efficient electron transfer from N to CuO, increasing significantly the fraction of SCR-active Cu+ sites in the catalyst. This study puts forward a new idea for manipulating and utilizing the interplay of defects and N-dopants on carbon surfaces to fabricate Cu+-rich Cu catalysts for efficient abatement of slip NH3 emissions via selective oxidation.

Synergistic Roles of Hexagonal Boron Nitride-Supported Cu0 and Cu+ Species in Dehydrogenation of Ethanol to Acetaldehyde: A Computational Mechanistic Study

Copper-based catalysts have been widely used in the dehydrogenation of ethanol to acetaldehyde due to their high activity. However, the inhibition of sintering and the identification of active sites remain challenging. In this work, by means of periodic density functional theory calculations, the catalytic mechanism is reported at the molecular level. The strong metal–support interaction between Cu 3d orbitals and O 2p orbitals and the electron transfer determine the high stability of copper clusters supported on h-BN. The enhancement of the interfacial interaction (binding energies of −11.0 and −12.5 eV on Cu+/BN and Cu0/BN, respectively) resists the sintering of copper clusters and benefits the retention of their active states. The free energy barriers to the C–H bond cleavage are 1.20, 0.90, and 0.53 eV on Cu(111), Cu0/BN, and Cu+/BN, respectively, and the barriers to the dihydrogen formation are 0.42, 1.51, and 2.36 eV, respectively. The results show the promotion of the d-band in the supported Cu+ and Cu0 species and introduce a synergistic effect of the two species, with Cu+ participating in the dehydrogenation of ethanol to acetaldehyde and Cu0 in the formation of H2. This work explains the origin of the resistance of h-BN-supported copper clusters against their sintering and identifies the synergistic roles of the Cu+ and Cu0 species in ethanol dehydrogenation This knowledge is expected to contribute to the rational design of heterogeneous copper-based catalysts for efficient ethanol dehydrogenation.

Efficient Electromethanation from CO2 on Monodisperse Cu-Based Catalysts

The electrochemical CO2 reduction reaction (CO2RR) is a promising approach to upgrading CO2 into methane (CH4) and other high-value-added chemicals, critical for lowering carbon emissions and mitigating the greenhouse effect. However, the selectivity of CO2RR to methane remains low at the current densities required for commercialization. Herein, we report a simple one-step synthesis of monodisperse copper-based catalysts for efficient electromethanation of CO2. Through rational regulation of active sites, the selectivity for methane production can reach as high as 57.02% under the current density of 300 mA cm–2 as a result of suppressed C–C coupling for improved *CO hydrogenation. This work offers an effective strategy to regulate Cu-based catalysts to manage the hydrogenation of *CO for CO2RR toward CH4 formation.

Synthesis and Applications of Copper-Based Catalysts

The increasing demand for fine and high-value-added chemicals has motivated the lasting exploration of high-performance catalysts with desirable activity, selectivity, and durability [...]

Modulable Cu(0)/Cu(I)/Cu(II) sites of Cu/ C catalysts derived from MOF for highly selective CO2 electroreduction to hydrocarbons

Electrocatalytic CO2 reduction has been perceived as sustainable to realize the closed-loop carbon cycle and obtain high-energy-dense fuels and chemicals. Nevertheless, the efficient CO2 reduction into specific hydrocarbons is still highly challenging. In particular, the effects of Cu(0)/Cu(I)/Cu(II) active sites in copper-based catalysts remain elusive. Hence, this work investigated the modulable valence states of Cu(0)/Cu(I)/Cu(II) active sites derived from MOF Cu(btc) under different calcination conditions to enhance CO2RR performance. As evidenced by characterizations, the Cu/C-T-air was more inclined to produce C2H4, which may be associated with Cu(I)/Cu(II) coexistence and abundant oxygen vacancy. Cu/C-450-air showed excellent CO2RR properties with 34.8% Faradaic efficiency of C2H4 at 1.1 V vs. RHE after a continuous measurement of 12 h. Meanwhile, the Cu/C-T-H2 was inclined to produce C2H4, while Cu/C-T-N2 tended to produce CH4, which may be associated with smaller grain size and thinner carbon layer than the latter to exposure to more active sites and rapid electron transport. Moreover, the mechanism of electrocatalytic CO2 reduction of Cu/C-T-gas was conjectured. We furnished rational thinking to design Cu/C catalysts with tunable active sites to bolster CO2RR.

Selective hydrogenation of levulinic acid to γ-valerolactone over copper based bimetallic catalysts derived from metal-organic frameworks

The transformation of biomass or feedstocks derived from biomass into fuels or chemicals with added value is an essential step in the process of transitioning to a low-carbon energy system. In this study, metal-organic framework (MOF)-derived non-noble bimetallic catalysts (copper-cobalt [CuCo], copper-nickel [CuNi] and copper-iron [CuFe]) were synthesized and used for selective levulinic acid (LA) hydrogenation to γ-valerolactone (GVL). The results demonstrated that adding the second metal could significantly improve the Cu@C catalytic activity. A 100% yield of γ-valerolactone was obtained by using the CuCo@C catalyst at 220 °C, 10 bar of H2 and a reaction time of 4 h. Advanced characterizations demonstrated that the excellent catalytic activity benefits from the even concentration of both metals on the surface, the abundance of active sites (Lewis and Bronsted) created by the even dispersion of both metals and the accessible surface area and pore volume originated from MOF precursor. The copper-based bimetallic catalysts also showed excellent catalytic durability with no significant decrease in GVL selectivity. Using MOF as a precursor for catalyst preparation can enable high dispersion of active metal sites and prevent their leaching, coking (carbon deposit) or aggregation in the carbon matrix during catalytic testing.

Effect of alkali addition on a Cu/SmCeO2 @TiO2 catalyst for NO reduction with CO under oxidizing conditions

The accumulation of alkali metals generated during biomass combustion onto the catalysts employed for off-gas treatment can have a detrimental effect on their performance. This work investigates the effect of potassium as a model alkali on an efficient copper-based catalyst supported on SmCeO2 @TiO2. The addition of potassium modifies the redox properties of the Cu species or, more specifically, the metal-ceria interface, providing better catalytic activity for NO reduction in oxidizing conditions. SmCeO2 @TiO2 stabilizes highly dispersed copper species, and the presence of potassium introduces surface distortions that modify the lability of oxygen atoms. The results demonstrate the key role of surface defects and labile oxygen species associated with the CeO2 support. It was found that both Cu/SmCeO2 @TiO2 and K/Cu/SmCeO2 @TiO2 catalysts fully oxidized CO below 350 °C and actively reduced NO with CO in the presence of excess oxygen, reaching a maximum NO conversion of 65% at 316 °C and 83% at 330 °C, respectively, while undesired NO2 release was minimized in the alkaline-loaded sample.

Dual-site catalysts featuring platinum-group-metal atoms on copper shapes boost hydrocarbon formations in electrocatalytic CO2 reduction

Copper-based catalyst is uniquely positioned to catalyze the hydrocarbon formations through electrochemical CO2 reduction. The catalyst design freedom is limited for alloying copper with H-affinitive elements represented by platinum group metals because the latter would easily drive the hydrogen evolution reaction to override CO2 reduction. We report an adept design of anchoring atomically dispersed platinum group metal species on both polycrystalline and shape-controlled Cu catalysts, which now promote targeted CO2 reduction reaction while frustrating the undesired hydrogen evolution reaction. Notably, alloys with similar metal formulations but comprising small platinum or palladium clusters would fail this objective. With an appreciable amount of CO-Pd1 moieties on copper surfaces, facile CO* hydrogenation to CHO* or CO-CHO* coupling is now viable as one of the main pathways on Cu(111) or Cu(100) to selectively produce CH4 or C2H4 through Pd-Cu dual-site pathways. The work broadens copper alloying choices for CO2 reduction in aqueous phases.

Low temperature direct growth of carbon nanostructures on basalt fibers

Among natural fibers, basalt fibers have recently emerged as the main candidate to replace glass fibers in composite materials. Since the fiber/matrix interface strongly influences the final mechanical properties in a fiber reinforced composite, the current aim is to optimize the efficiency of the load transfer between the two different phases in the composite by growing carbon nanostructures (CNS), both nanofibers and nanotubes, on the surface of unsized basalt fibers exploiting less common catalysts compared to transition metals. Carbon nanostructures were grown onto unsized basalt fibers by chemical vapor deposition under different growth temperatures, using three different catalysts: copper, sodium, and iron. Considering the negative influence that high temperature has on the mechanical properties of basalt fibers, by lowering the synthesis temperature, we preserved the intrinsic properties of the basalt fibers, losing only 5% of the tensile strength at 300 °C growth temperature with a copper-based catalyst, while obtaining dense and uniform CNS growth. The different morphologies of the CNS obtained on the surface of the fibers were investigated by morphological and spectroscopic analyzes, while fiber/matrix adhesion was characterized by single fiber pull out tests, showing increases of 30% and 60% in interfacial shear strength and total pull out work, respectively. These less common catalysts provide an expanded design strategy for obtaining composites with improved interfacial bonding and multifunctional properties.

Copper-Based Electrocatalysts for Nitrate Reduction to Ammonia.

Ammonia (NH3) is a highly important industrial chemical used as fuel and fertilizer. The industrial synthesis of NH3 relies heavily on the Haber-Bosch route, which accounts for roughly 1.2% of global annual CO2 emissions. As an alternative route, the electrosynthesis of NH3 from nitrate anion (NO3-) reduction (NO3-RR) has drawn increasing attention, since NO3-RR from wastewater to produce NH3 can not only recycle waste into treasure but also alleviate the adverse effects of excessive NO3- contamination in the environment. This review presents contemporary views on the state of the art in electrocatalytic NO3- reduction over Cu-based nanostructured materials, discusses the merits of electrocatalytic performance, and summarizes current advances in the exploration of this technology using different strategies for nanostructured-material modification. The electrocatalytic mechanism of nitrate reduction is also reviewed here, especially with regard to copper-based catalysts.

Cu2S Nanoparticle-Modified Copper Hydroxide Nanowire Arrays for Optimized Electrochemical CO2 Reduction Selectivity

A copper-based catalyst exhibits a wide distribution of catalytic products in electrochemical CO2 reduction. However, it faces a grand challenge in reaction selectivity. A deeper understanding is needed regarding the mechanism of its diversified product selectivity. Herein, we design a cuprous sulfide nanoparticle-modified copper hydroxide nanowire array precatalyst by a simple solvothermal method for revealing the catalytic mechanism of copper-based catalysts in electrochemical CO2 reduction. The role of cuprous sulfide nanoparticle modification in copper hydroxide nanowires on formate formation during CO2 reduction is investigated. The intermediate during CO2 reduction is altered by sulfur modification, resulting in improvement in formate selectivity. Density functional theory is used to investigate the effect of trace sulfur modification in copper on activity and selectivity toward formate formation. Compared with a pure copper surface, sulfur modification in copper can promote the formation of *OCHO, a key intermediate along the formate pathway.

Effect of the Preparation Method on Cu-MOR/g-C3N4 for Direct Methanol Synthesis from Methane Oxidation by Photothermal Catalysis

Copper-based zeolite catalysts are widely used in methanol synthesis from methane oxidation, but their photothermal catalytic properties have seldom been explored. This study examines the effect of the preparation method on Cu-based zeolite composite graphite-phase carbon nitride catalysts (Cu-MOR/g-C3N4) for direct methanol synthesis from methane oxidation by photothermal catalysis. Four different preparation methods are employed: liquid phase ion exchange (Cu-MOR/g-C3N4-IE), isovolumetric impregnation (Cu-MOR/g-C3N4-IM), solid-state ion exchange (Cu-MOR/g-C3N4-GR), and hydrothermal synthesis (Cu-MOR/g-C3N4-HT). Cu-MOR/g-C3N4-IE shows the highest methanol yield (3.09 μmol h−1 gcat−1) due to strong interactions between the CuxOy species and g-C3N4, as well as smaller interfacial charge transfer forces. This study provides a new method for the design and synthesis of catalysts for the conversion of methane.

Creating interfaces of Cu0/Cu+ in oxide-derived copper catalysts for electrochemical CO2 reduction to multi-carbon products

Electrochemical carbon dioxide reduction reaction (CO2RR) is an effective approach to capture CO2 and convert it into value-added chemicals and fuels, thereby reducing excess CO2 emissions. Recent reports have shown that copper-based catalysts exhibit excellent performance in converting CO2 into multi-carbon compounds and hydrocarbons. However, theselectivityto the couplingproductsispoor. Therefore, tuningCO2-reductionselectivitytoward C2+productsover Cu-based catalyst is one of the most important issues in CO2RR. Herein, we prepare a nanosheet catalyst with interfaces of Cu0/Cu+. The catalyst achieves Faraday efficiency (FE) of C2+ over 50% in a wide potential window between − 1.2 V to − 1.5 V versus reversible hydrogen electrode (vs. RHE). Moreover, the catalyst exhibits maximum FE of 44.5% and 58.9% towards C2H4 and C2+, with a partial current density of 10.5 mA cm−2 at − 1.4 V. Density functional theory (DFT) calculations show that the interface of Cu0/Cu+ facilitates CC coupling to form C2+ products, while inhibits CO2conversion toC1products.

Copper-based catalysts synthesized during hydrogenation

Impregnation is a traditional method for preparing heterogeneous catalysts, which typically includes impregnation, calcination and reduction steps. In this study, catalyst preparation was combined with its use in catalytic hydrogenation for avoiding ex-situ catalyst-synthesis. Specifically, Cu/MgO were prepared by simply mixing CuSO4, MgO and the reactant to be hydrogenated such as levulinic acid (LA) or ethyl levulinate (EL) in ethanol with hydrogen. Heating of the mixture to 160 °C could obtain both synthesized Cu/MgO and hydrogenation product of gamma-valerolactone (GVL) (highest yield: 99.9%). The formation of Cu/MgO catalyst proceeded via the steps of fixing Cu2+ by reacting with the basic sites of MgO to form Cu(OH)2. The metastable Cu(OH)2 decomposed to form CuO that was reduced to Cu for catalyzing conversion of LA/EL to GVL. Such a method is very simple and applicable to prepare varied types of Cu-based catalyst by changing copper salts and supports of varied alkalinity.

Effect of Cu and Zn on the performance of Cu-Mn-Zn/ZrO2 catalysts for CO2 hydrogenation to methanol

CO2 hydrogenation to methanol is an important technology for carbon utilization which not only provides a solution to the greenhouse gas mitigation but also produces value-added products. Copper-based catalyst has always been the research focus of the process. Cu1.5Mn1.5O4 spinel has become an interesting alternative because it contains more oxygen defects and highly dispersed copper species that promote the CO2 adsorption and conversion. In this paper, the effects of Cu and Zn on the performance of Cu-Mn-Zn/ZrO2 catalysts for hydrogenation of CO2 to methanol were studied. It was found that after Zn modification, the catalytic performance of the catalyst was greatly improved. Among all catalysts, Cu3MnZn0.5Zr0.5 has the best CO2 conversion (7.14%) and methanol selectivity (69.74%) at 260 °C and 5 MPa. XPS analysis showed that doped Zn replaced the position of Cu in Cu1.5Mn1.5O4 spinel, forming the ZnOx and then increasing the content of oxygen defects, which resulted in the higher methanol selectivity. The increased Cu content promoted the activation of H2 and the rate of *CO3 hydrogenation to *HCOO, which in turn increases the conversion of CO2.

Structured nanoporous copper catalysts prepared by laser powder bed fusion and dealloying for on-board methanol steam reforming

The use of online methanol steam reforming (MSR) as an efficient hydrogen production approach is considered an attractive route for supplying hydrogen to onboard fuel cell vehicles (FCVs). Current catalysts face challenges, such as a large pressure drop, easy pulverization, and coating peeling. To address these challenges, in this study, a structured Cu-based catalyst with a body-centered cubic skeleton was prepared using laser powder bed fusion (LPBF) combined with a dealloying method. The three-dimensional (3D) hierarchical nanoporous copper structures with high specific surface areas were observed. The catalytic performance of the 3D structured Cu-based catalyst is superior to those of reported catalysts with 93.6% methanol conversion and a CO concentration of less than 0.3% in dry gas under the reaction conditions of a mass space velocity of 34.6 h−1, a temperature of 280 °C, and a H2O/CH3OH molar ratio of 1.3. In addition, computational fluid dynamics (CFD) simulations confirmed the excellent mass and heat transfer performance and low pressure drop of the 3D-structured copper-based catalyst. Structured Cu-based catalysts have great potential for applications in FCVs.

Improving the Performance of Heavy Oil Oxidation Using Oil-Soluble Copper-Based Catalysts in In Situ Combustion for the In Situ Upgrading Process

Improving the Performance of Heavy Oil Oxidation Using Oil-Soluble Copper-Based Catalysts in In Situ Combustion for the In Situ Upgrading Process