Cu0 Ratio Research Articles

Tuning the interaction between Cu and surface -OH in Cu/Ti3C2 MXene derived catalyst for enhanced semi-hydrogenation of dimethyl oxalate

Methyl glycolate (MG), a crucial chemical raw material and fine chemical intermediate, holds immense potential in the production of the next-generation biodegradable material, polyglycolic acid, via polycondensation reactions. In this study, layered Ti3C2 MXene was used as support precursor to construct Cu based catalysts for the semi-hydrogenation of dimethyl oxalate to MG. By fine-tuning the loading method of Cu species, the interaction between Cu species and surface hydroxyl groups (-OH) was modulated, and the effect on the semi-hydrogenation performance of Cu-based catalysts were thereby explored. These catalysts underwent rigorous characterization using BET, SEM-EDS, XRD, FT-IR, H2-TPR and XPS techniques. The results revealed that varying loading methods significantly influenced the interaction between Cu species and surface -OH on Ti3C2 MXene during preparation process, altering the content of Ti-O-Cu as well as the Cu0 ratio of Cu/Ti3C2-derived catalysts. The Cu/Ti3C2 MXene-I catalyst displayed remarkable performance, achieving a MG selectivity of 88.85 % and stability of up to 120 h in the DMO hydrogenation reaction due to its high Cu0 ratio and large Cu particles.

Boosting hydrogenation properties of supported Cu-based catalysts by replacing Cu0 active sites

Balancing the Cu+/Cu0 ratio is a common strategy to improve catalytic activity of Cu-based catalysts but is still constrained by low atomic utilization and the inherent nature of charge distribution. Herein, we reported a strategy of replacing the Cu0 active sites in Cu-based catalysts by constructing bimetallic sites on ceria, which consist of spatially separated trace amounts of palladium metal and plate-shaped Cu+ clusters with one-atom layers. The catalytic activity of the prepared Cu100Pd1/CeO2-FA catalyst (using formic acid) was 4 times that of conventional Cu100Pd1/CeO2-H catalyst in selective hydrogenation of 5-hydroxymethylfurfural to 2,5-bis(hydroxymethyl)furan, even outperforming some existing noble metal catalysts. Multiple characterizations and theoretical calculations demonstrated that the Pd atom is the heterolytic activation site for H2 molecules while plate-shaped Cu+ metal clusters act as effective hydrogenation places. This directional control involving both spatial relationship and electronic structure of the active site provides a new strategy for designing hydrogenated catalysts.

Tracking the critical roles of Cu+ and Cu0 sites and the optimal Cu+/Cu0 ratio for CH3OH steam reforming (MTSR) to manufacture H2

Understanding the nature of surface-active sites is of critical importance for catalyst design, yet remains great challenges because of the complexity of solid surfaces. To tracking the valence states and roles of various Cu sites played in CH3OH steam reforming (MTSR) on Cu-based catalysts and the factors to determine the reactivity, appropriate Cu/Ce1-xZrxO2 solid solution catalysts have been purposely designed. On a catalyst having a Ce/Zr ratio of 0.7/0.3 (Cu/Ce0.7Zr0.3O2), the optimal Cu+/Cu0 ratio around 1.00 can be obtained to accomplish the best performance, with a high H2 production rate but very low CO selectivity. With the combination of experiments and DFT calculation, it has been discovered that CH3OH molecules mainly be adsorbed/activated on the Cu+ sites, but H2O molecules could mainly be adsorbed/activated on the Cu0 sites. We have substantiated that to fabricate Cu/Ce1-xZrxO2 catalysts with high efficiency, nearly equal amount of surface Cu+ and Cu0 sites should be engineered.

Insight into the correlation between Cu species and methanol selectivity from CO2 hydrogenation over Cu-based catalyst

The pre-activation of Cu-based catalysts is an important step to fully develop their catalytic potential. The production of active species (Cu+ and Cu0) which play the active site role generally occurs in the activation stage by reducing from Cu2+. Herein, the Cu+/Cu0 ratio of CuZnOAl2O3 (CZA) catalysts was adjusted using different activation atmospheres and the relationship between Cu+/Cu0 ratio of the catalyst and methanol selectivity from CO2 hydrogenation was investigated in detail. Combined with X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) characterization, etc. and experimental results, it was observed that the selectivity of methanol was obviously relied on the Cu+/Cu0 ratio of CZA catalyst. This work provides theoretical guidance and feasible scheme to rationally design and optimize the required catalysts.

Stable Cuδ+ species - Catalyzed CO₂ hydrogenation to methanol in silanol nests on Cu/S-1 catalyst

Fine-regulating the electronic state of metal species to enhance the catalytic activity has demonstrated to be an effective yet persistent challenge. The study introduces an innovative method for enhancing catalytic performance in hydrogenating CO2 to methanol via modulating copper-support interactions through the adjustment of surface silanol groups. The developed Cu/S-1 catalyst exhibits outstanding results: 85% methanol selectivity and 8.0% CO2 conversion under 240 °C and 3 MPa, while maintaining stability for over 200 h. This performance surpasses that of the Cu/SiO2 catalyst. Analyses indicate that the higher presence of Cuδ+ species in Cu/S-1 is attributed to the silanol nests in S-1 zeolite, which can stabilize these species and prevent the further reduction. The study identifies Cuδ+ and Cu0 as active species for CH3OH and CO production, respectively. A higher Cuδ+/Cu0 ratio eventuates better CO2 conversion and CH3OH selectivity. Copper-support interaction is a key to the exceptional stability of the catalyst.

Boosting 2,3-butanediol selective dehydrogenation over hierarchical Cu/silicalite-1 via tailoring metal-support interactions

Herein, Silicalite-1 supported Cu catalysts were synthesized by coupling the alkaline treatment and ammonia evaporation methods to synchronously introduce the hierarchical pores and strong metal-support interaction for anaerobic 2,3-butanediol dehydrogenation to acetoin. Characterization results demonstrated that mesoporous structures of resultant Cu/S-x-T can be enhanced by tuning the ammonia dosage in the synthetic solution to boost the formation of -Cu-O-Si- structures and improve the dispersion of copper species simultaneously. The increase in the reduction temperature would induce the reduction of Cu2+ existing in the form of -Si-O-Cu-O-Si-. Highly dispersed Cu0 and abundant Cu2+ sites with an appropriate Cu2+/Cu0 ratio brought about the superior activity of Cu/S-25-300 with a conversion of 78% and selectivity of 92%. A correlation analysis on the catalytic activity and Cu0/Cu2+ site densities as well as IR spectra of adsorbed 2,3-butanediol demonstrated that the synergic catalysis of Cu0 and Cu2+ was primarily responsible for the 2,3-butanediol dehydrogenation. Distinctively, DFT calculations revealed that Cu subnanoclusters was capable of catalyzing the 2,3-butanediol dehydrogenation to acetoin, following a similar reaction mechanism to the dehydrogenation of monohydric alcohols on Cu0 sites due to the size mismatch of reactive sites in reactants and catalytic structures.

Uniformly Dispersed Cu Nanoparticles over Mesoporous Silica as a Highly Selective and Recyclable Ethanol Dehydrogenation Catalyst

Selective dehydrogenation of ethanol to acetaldehyde has been considered as an important pathway to produce acetaldehyde due to the atom economy and easy separation of acetaldehyde and hydrogen. Copper catalysts have attracted much attention due to the high activity of Cu species in O-H and C-H bonds oxidative cleavage, and low process cost; however, the size of the Cu nanoparticle is difficult to control since it is easily suffers from metal sintering at high temperatures. In this work, the Cu/KIT-6 catalyst exhibited an ultra-high metal dispersion of 62.3% prepared by an electrostatic adsorption method, due to the advantages of the confinement effect of mesoporous nanostructures and the protective effect of ammonia water on Cu nanoparticles. The existence of an oxidation atmosphere had a significant effect on the valence state of copper species and enhancing moderate acid sites. The catalyst treated by reduction and then oxidation possessed a moderate/weak acid site ratio of ~0.42 and a suitable proportion of Cu+/Cu0 ratio of ~0.53, which conceivably rendered its superior ethanol conversion of 96.8% and full acetaldehyde selectivity at 250 °C. The catalyst also maintained a high selectivity of >99% to acetaldehyde upon time-on-stream of 288 h.

Converting waste PET plastics into automobile fuels and antifreeze components

With the aim to solve the serious problem of white plastic pollution, we report herein a low-cost process to quantitatively convert polyethylene terephthalate (PET) into p-xylene (PX) and ethylene glycol (EG) over modified Cu/SiO2 catalyst using methanol as both solvent and hydrogen donor. Kinetic and in-situ Fourier-transform infrared spectroscopy (FTIR) studies demonstrate that the degradation of PET into PX involves tandem PET methanolysis and dimethyl terephthalate (DMT) selective hydro-deoxygenation (HDO) steps with the in-situ produced H2 from methanol decomposition at 210 °C. The overall high activities are attributed to the high Cu+/Cu0 ratio derived from the dense and granular copper silicate precursor, as formed by the induction of proper NaCl addition during the hydrothermal synthesis. This hydrogen-free one-pot approach allows to directly produce gasoline fuels and antifreeze components from waste poly-ester plastic, providing a feasible solution to the plastic problem in islands.

Switching CO2 Electroreduction Selectivity Between C1 and C2 Hydrocarbons on Cu Gas‐Diffusion Electrodes

Regulating the selectivity toward a target hydrocarbon product is still the focus of CO2 electroreduction. Here, we discover that the original surface Cu species in Cu gas‐diffusion electrodes plays a more important role than the surface roughness, local pH, and facet in governing the selectivity toward C1 or C2 hydrocarbons. The selectivity toward C2H4 progressively increases, while CH4 decreases steadily upon lowering the Cu oxidation species fraction. At a relatively low electrodeposition voltage of 1.5 V, the Cu gas‐diffusion electrode with the highest Cuδ+/Cu0 ratio favors the pathways of hydrogenation to form CH4 with maximum Faradaic efficiency of 65.4% and partial current density of 228 mA cm−2 at −0.83 V vs RHE. At 2.0 V, the Cu gas‐diffusion electrode with the lowest Cuδ+/Cu0 ratio prefers C–C coupling to form C2+ products with Faradaic efficiency topping 80.1% at −0.75 V vs RHE, where the Faradaic efficiency of C2H4 accounts for 46.4% and the partial current density of C2H4 achieves 279 mA cm−2. This work demonstrates that the selectivity from CH4 to C2H4 is switchable by tuning surface Cu species composition of Cu gas‐diffusion electrodes.

Efficient Catalytic Transfer Hydrogenation of Acetophenone to 1-Phenylethanol over Cu–Zn–Al Catalysts

Cu–Zn–Al catalyst was prepared by the co-precipitation method, and the catalytic performance for transfer hydrogenation of acetophenone (AP) to 1-phenylethanol (1-PhE) was evaluated using isopropanol (IPA) as a hydrogen source. The addition of Zn changed the morphology of the catalyst and reduced aggregation of catalyst particles, resulting in highly dispersed Cu active sites. As the Zn content increased, the valence state of Cu species changed. When the Cu/Zn molar ratio was 2/3, the Cu+/Cu0 ratio in the catalyst was 0.81, resulting in good selectivity for 1-PhE. The reaction conditions were optimized over the Cu–Zn–Al catalyst with a Cu/Zn molar ratio of 2/3. When the reaction was performed at 180 °C for 2 h with an IPA/AP molar ratio of 15, the AP conversion was 89.4% and the 1-PhE selectivity was 93.2%. The Cu–Zn–Al catalyst showed good stability, and AP conversion and the 1-PhE selectivity were essentially unchanged after five cycles.

Enhanced CO2 hydrogenation to methanol over La oxide-modified Cu nanoparticles socketed on Cu phyllosilicate nanotubes

The use of Cu-based catalysts in CO2 hydrogenation to methanol (MeOH) has attracted considerable attention due to their low reaction temperature, low cost, and high activity. However, Cu nanoparticles are usually sintered under high-H2 atmosphere. The difficulty in keeping the Cu+/Cu0 ratio stable during the reaction leads to the instability of Cu-based catalysts. In this work, La oxide-doped Cu nanoparticles socketed into parent Cu phyllosilicate nanotubes (La/CuSi-NT) are synthesized by exsolution in situ under a reducing atmosphere at elevated temperatures. They are used to catalyze CO2 hydrogenation to MeOH. Catalytic results show that the La/CuSi-NT catalyst is more stable than intact CuSi-NT and Cu-based catalysts prepared via traditional methods. Among the prepared catalysts, the 0.2La/CuSi-NT catalyst with the La/Cu molar ratio of 0.2 exhibits the highest MeOH yield of up to 428 mg gcat−1 h−1 and performance that remains stable for at least 200 h. Characteristic studies indicate that introducing a La promoter helps increase Cu+/Cu0 ratios and confirm that LaOx species favor generating the formate species of CO2 hydrogenation. Therefore, MeOH synthesis through CO2 hydrogenation is intensified over the La/CuSi-NT catalyst.

High-performance Cu/ZnO/Al2O3 catalysts for methanol steam reforming with enhanced Cu-ZnO synergy effect via magnesium assisted strategy

Methanol steam reforming (MSR) is an attractive approach to produce hydrogen for fuel cells. Due to the limited catalyst loading volume and frequent start-ups and shut-downs on board, it is highly desired to develop an extremely active and robust catalyst. Herein, on the basis of industrial Cu/ZnO/Al2O3 catalysts, a series of CuZnAl-xMg catalysts with enhanced Cu-ZnO synergy were synthesized via magnesium assisted strategy. The incorporation of magnesium was found to be beneficial to the enhancement of catalytic activity and stability of catalyst. A combination of complementary characterizations (e.g. XRD, H2-TPR, N2O chemisorption, TEM, XPS analysis etc.) proves that isomorphous substitution of Cu2+ in malachite phase gives rise to more dispersive Cu and ZnO NPs, and the increased Cu+/Cu0 ratio indicates the strengthened Cu-ZnO synergy effect, which leads to the boosted stability during the thermal treatment.

Understanding Structure‐activity Relationship on Metal‐Organic‐Framework‐Derived Catalyst for CO2 Electroreduction to C2 Products

AbstractMetal‐organic frameworks (MOFs) have been widely studied for electrochemical CO2 reduction reaction (CO2RR). However, the application of negative potential tends to trigger its structural reconstruction and component alteration. It is of great significance to study the relationship between the structural evolution of MOF‐derived materials and product selectivity under the reaction conditions. Herein, we fabricate a HKUST‐1 thin film on carbon fiber paper by a facile electrodeposition approach to investigate the variation of CO2RR activity with HKUST‐1 structural reconstruction. During the CO2RR process, the HKUST‐1 reconstructs into two structures successively over time: 3D nanospheres composed of numerous small fragments and 3D nano‐network composed of cross‐linked nanobelts in different directions. The former shows a better catalytic activity due to more active sites, lower charge transfer resistance and higher Cu+/Cu0 ratio. The optimum Faradaic efficiency (FE) of C2 products (ethylene and ethanol) reaches 58.6 % at −0.98 V versus reversible hydrogen electrode (RHE).

One-pot hydrogenolysis of cellulose to bioethanol over Pd-Cu-WOx/SiO2 catalysts

The chemo-catalytic production of ethanol from nonedible lignocellulosic biomass is essential for mitigating energy shortage. In this work, we developed a multifunctional Pd-Cu-WOx/SiO2 catalyst for the one-pot aqueous-phase conversion of cornstalk-derived cellulose to ethanol with a yield of 42.5C% at 300 °C and 4 MPa H2. Kinetic studies demonstrated that the transformation of cellulose to ethanol followed a number of consecutive steps namely, cellulose hydrolysis to glucose with H+ in hot water (k1), glucose conversion to glycolaldehyde over W species (k2), glycolaldehyde hydrogenation to ethylene glycerol (EG) over Pd (k3), and EG hydrogenolysis to ethanol over Cu (k4, rate-determining step). The individual rates were well balanced on Pd, Cu, and WOx species to achieve an ethanol formation rate of 0.163 g·gCat.−1h−1. The addition of Au to Cu-WOx species led to a relatively low k3 and thus the formation of a substantial amount of humins, while the Ru introduction catalyzed the over-hydrogenolysis of ethanol to methane and ethane with a high rate, thereby lowering the selectivity to ethanol. The characterization results showed that both the electronic properties of Cu+ species and the Cu+/Cu0 ratio on the Pd-Cu-WOx/SiO2 catalyst contributed to a specific CO bond cleavage, especially for EG hydrogenolysis to ethanol during the cellulose conversion.

Highly efficient Ag-modified copper phyllosilicate nanotube: Preparation by co-ammonia evaporation hydrothermal method and application in the selective hydrogenation of carbonate

Rapidly deactivation of Cu/SiO2 catalysts at high liquid hour space velocity (LHSV) has been an important obstacle for scale-up application. Herein, silver modified copper phyllosilicate nanotubes were fabricated by different strategies, and implemented to the selective hydrogenation of ethylene carbonate (EC) to methanol and ethylene glycol (EG) as alternative route for the indirect utilization of CO2. The CuPs Ag–copre catalyst synthesized by the co–ammonia evaporation hydrothermal process achieved 79% methanol and 99% EG yield within various ranges of EC LHSV, which was attributed to the balanced Cu+/Cu0 ratio and the enhanced H2 dissociation ability. Inlaid silver species over copper phyllosilicate promoted the interaction between the metal and the support, which substantially regulated the reducibility and dispersion of copper species, meanwhile, increased the stability for long-term running of the catalyst.

Enhanced selective catalytic reduction of NO with CO over Cu/C nanoparticles synthetized from a Cu-benzene-1,3,5-tricarboxylate metal organic framework by a continuous spray drying process

Cu-benzene-1,3,5-tricarboxylate (BTC), Cu-BTC (FNP), and Cu-BTC (SD) precursors were prepared by direct mixing (DM), flash nanoprecipitation (FNP) and spray drying (SD), respectively. The precursors were pyrolyzed under nitrogen to obtain the corresponding Cu/C-DM, Cu/C-FNP, and Cu/C-SD catalysts. The physicochemical and catalytic properties of these samples were characterized by scanning electron microscopy (SEM), N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, electronic paramagnetic resonance (EPR), temperature-programmed reduction with hydrogen (H2-TPR), temperature-programmed desorption of oxygen (O2-TPD), and CO + NO model reactions. Regarding the CO + NO model reactions, the Cu/C-SD catalyst exhibited the best denitrification performance, reaching complete NO conversion and N2 selectivity of 98.8% at 300 °C. The Cu/C-SD catalyst removed 88.1% of the initial NO at temperatures as low as 200 °C, being this value was significantly higher than those obtained by Cu/C-DM (2.6%) and Cu/C-FNP (10.2%) catalysts. The Cu/C-SD catalyst also showed excellent stability in the independent presence of O2 (5 vol%), H2O (5 vol%) or SO2 (100 ppm). The Cu1+/Cu0 ratio played a key role in the selective NO catalytic reduction process. The larger specific surface area of the Cu/C-SD catalyst and its ability for the reduction and desorption of oxygen chemically adsorbed has a positive impact on the catalytic performance of this material. These results were theoretically supported by a DFT analysis.

Identification of the dehydration active sites in glycerol hydrogenolysis to 1,2-propanediol over Cu/SiO2 catalysts

Metal–support interaction is a hot topic in catalysis, but attention has seldom been drawn to the promotional effect of metal–irreducible SiO2 interfaces. Here, Cu/SBA-15 catalysts with CuOSiO interface structures were prepared by simple grinding, and showed high activity and selectivity to 1,2-propanediol above 97% in liquid-phase glycerol hydrogenolysis. Glycerol conversion of ground Cu/SBA-15 with 10% Cu loading was about seven times that of its impregnation counterpart. The linear relationship between turnover frequency and (CuOSiO induced Lewis acid sites)/Cu0 ratio indicated that CuOSiO structures were crucial for achieving excellent hydrogenolysis performance. More importantly, in situ Fourier transform infrared spectroscopy of glycerol adsorption and density functional theory calculation results confirmed that CuOSiO species were the dominant active dehydration sites of glycerol hydrogenolysis, while the adjacent Cu sites were involved in subsequent hydrogenation of the generated hydroxyacetone.

A strategy to control the grain boundary density and Cu+/Cu0 ratio of Cu-based catalysts for efficient electroreduction of CO2 to C2 products

Cu(OH)2/CuO nanocomposite-derived Cu2O/Cu achieves a total faradaic efficiency for C2 products of 65% via tuning the grain boundary density and Cu+/Cu0 ratio.

Efficient synthesis of methanol and ethylene glycol via the hydrogenation of CO2-derived ethylene carbonate on Cu/SiO2 catalysts with balanced Cu+–Cu0 sites

The preparation method for Cu/SiO2 catalysts had a great impact on the Cu+/Cu0 ratio and catalytic performance.

The role of copper oxidation state in Cu/ZnO/Al2O3 catalysts in CO2 hydrogenation and methanol productivity

CO2 hydrogenation was carried out over a series of Cu/ZnO/Al2O3 catalysts, which were prepared by different synthesis methods (co-precipitation, ultrasound-assisted, sol-gel and solid-state). The applicative physicochemical properties of the materials and their catalytic performance were investigated. Subsequently, the preparation methodology influence on the average Cu particle size, the interactions of metals, the exposed copper phases surface area and the ratio of Cu0/Cu+, as well as its specific effect on reactions, were assessed in detail. The ultrasonic synthetic route provided increased basic active sites' number and significant methanol selectivity, compared to the conventional precipitation procedure, while it also improved the dispersion of separate Cu metallic particles, which ultimately changed the intrinsic reactive activity of CuO–ZnO. The trend of apparent CH3OH productivity rate was consistent with the Cu+/Cu0 ratio or content as well. The presence of Cu+ species at high concentration levels is thus crucial for high methanol selectivity and an extremely low selectivity towards CO at the conventional industrial operating conditions analysed. The dependence of the selective methanol production on the fraction of determined alkaline moieties was found analogous to all catalysts studied. Cu+/Cu0 oxidation state role and distribution may, therefore, aid in CO2 conversion catalyst design and optimisation.