Dependence of copper(I) stability on long-range electromagnetic effects of Au under reducing atmospheres: the size effect of Au cores.

It has been widely recognized that adjusting the size of Au particles has emerged as a significant approach in catalyst design, catalyst screening, and comprehension of reaction mechanisms. However, the essential factors of Au nanoparticles used only as an additive to enhance the activity of traditional multicomponent thermocatalysts have not been fully revealed. In this study, a series of Au@Cu2O core-shell nanocatalysts were synthesized through a controllable method, featuring core sizes ranging from 11 to 33 nm and an average shell thickness of approximately 55 nm. It was revealed that the size effect of Au cores plays a very vital role in the stability of the active Cu+ species under reducing atmospheres (H2, acetylene and formaldehyde) as well as the catalytic performance of the catalysts in the ethynylation of formaldehyde. The experimental findings revealed that Au@Cu2O core-shell catalysts with Au core sizes ranging from 11 to 16 nm exhibited a higher abundance of electron-deficient Cu+ species in the shell, which is attributed to the strong long-range electromagnetic effects of the Au core in the absence of photoexcitation or an applied electric field. Additionally, the active Cu+ species demonstrated remarkable stability under reducing atmospheres. Although the stability of Cu+ decreased slightly when the Au core size exceeded 16 nm, the Cu+ content remained above 80%. Notably, the Au@Cu2O catalysts with Au core sizes ranging from 11 to 16 nm exhibited excellent catalytic activity in the ethynylation of formaldehyde.

Enhancement of Cu+ stability under a reducing atmosphere by the long-range electromagnetic effect of Au.

In conventional thermocatalytic reactions under a reducing atmosphere, stabilization of the active Cu+ component and inhibition of over-reduction into metallic Cu0 are extremely challenging. In this study, Au@Cu2O core-shell nano-catalysts with different Cu2O shell thicknesses were synthesized, and the effect of the Au nano-core on Cu+ stability under a reducing atmosphere and the catalytic performance of Cu+ in the ethynylation of formaldehyde were investigated. The Au nano-core facilitates Cu2O dispersion and leads to an increase of 0.2-0.5 eV in electron binding energies of Cu2O and Cu2C2 in the range of 27-55 nm, attributed to the long-range electromagnetic effect of Au NPs. Specifically, active Cu+ centers exhibit high stability under a reducing atmosphere due to the long-range electromagnetic effect of the Au nano-core. In the ethynylation of formaldehyde as a probe reaction, Cu+/(Cu0 + Cu+) on Au@Cu2O catalysts remained at 88-91%. The catalytic performance in the ethynylation of formaldehyde revealed that the introduction of an Au nano-core into Cu-based catalysts increased the TOF from 0.37 to 0.7 h-1, and decreased the activation energy from 42.6 to 38.1 kJ mol-1. Additionally, the Cu+/(Cu0 + Cu+) ratios and the catalytic performance in the ethynylation of formaldehyde (BD yield = 65%, BD selectivity = 95%) on Au@Cu2O catalysts remained constant after nine cycles, while pure Cu2O readily deactivated due to the dramatically reduced Cu+/(Cu0 + Cu+) ratios and carbyne deposition. In summary, Cu+ in Cu-based catalysts showed high catalytic activity and stability during the ethynylation of formaldehyde due to the long-range electromagnetic effect of the Au nano-core.

Application of CuxO-FeyOz Nanocatalysts in Ethynylation of Formaldehyde.

Composite nanomaterials have been widely used in catalysis because of their attractive properties and various functions. Among them, the preparation of composite nanomaterials by redox has attracted much attention. In this work, pure Cu2O was prepared by liquid phase reduction with Cu(NO3)2 as the copper source, NaOH as a precipitator, and sodium ascorbate as the reductant. With Fe(NO3)3 as the iron source and solid-state phase reaction between Fe3+ and Cu2O, CuxO-FeyOz nanocatalysts with different Fe/Cu ratios were prepared. The effects of the Fe/Cu ratio on the structure of CuxO-FeyOz nanocatalysts were studied by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet confocal Raman (Raman), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS, XAES), and hydrogen temperature-programmed reduction (H2-TPR). Furthermore, the structure-activity relationship between the structure of CuxO-FeyOz nanocatalysts and the performance of formaldehyde ethynylation was discussed. The results show that Fe3+ deposited preferentially on the edges and corners of the Cu2O surface, and a redox reaction between Fe3+ and Cu+ occurred, forming CuxO-FeyOz nanoparticles containing Cu+, Cu2+, Fe2+, and Fe3+. With the increase of the Fe/Cu ratio, the content of CuxO-FeyOz increased. When the Fe/Cu ratio reached 0.8, a core-shell structure with Cu2O inside and a CuxO-FeyOz coating on the outside was formed. Because of the large physical surface area and the heterogeneous structure formed by CuxO-FeyOz, the formation of nonactive Cu metal is inhibited, and the most active species of Cu+ are exposed on the surface, showing the best formaldehyde ethynylation activity.

Ethynylation of Formaldehyde over CuO/SiO2 Catalysts Modified by Mg Species: Effects of the Existential States of Mg Species.

The highly effective catalytic synthesis of 1,4-butynediol (BD) from the Reppe process is a fascinating technology in modern chemical industry. In this work, we reported the effects of the existential states of Mg species in the CuO/silica-magnesia catalysts for the ethynylation of formaldehyde in a simulative slurry reactor. The physichemical properties of the supports and the corresponding catalysts were extensively characterized by various techniques. The experimental results indicated that the introduced Mg species in the form of MgO particles, MgO microcrystals, or Si-O-Mg structures effectively resulted in an abundance of medium-strong basic sites, which can synergize with the active Cu+ species, facilitate the activation of acetylene, and improve the ethynylation activity. For the CuO/MgO-SiO2 catalyst, the existence of Si-O-Mg structures strengthened the Cu-support interaction, which were beneficial to improving the dispersion and the valence stability of the active Cu+ species. The highly dispersed Cu+ species, its stable valence state, and the abundant medium-strong basic sites enhanced the synergistic effect significantly, leading to the superior activity and stability of CuO/MgO-SiO2. The insights into the role of the existential states of Mg species and the revelation of the synergistic effect between active Cu+ species and basic sites can provide theoretic guidance for future rational design of catalysts for the ethynylation reation.

Ethynylation of Formaldehyde over Binary Cu-Based Catalysts: Study on Synergistic Effect between Cu+ Species and Acid/Base Sites.

Most studies on the Cu-based catalysts in the ethynylation of formaldehyde are merely focused on the tuning of electronic configuration and dispersion of the Cu+ species. So far, little attention has been paid to the synergy between Cu species and promoters. Herein, binary nano-CuO-MOx catalysts (M = Si, Al, and Mg) were synthesized and the effects of the promoter on the surface basicity/acidity were systematically studied as well as the ethynylation performance of the nano-CuO-based catalysts. The results show that the introduction of MgO provided a large number of basic sites, which could coordinate with the active Cu+ species and facilitate the dissociation of acetylene as HC ≡ Cδ- and Hδ+. The strongly nucleophilic acetylenic carbon (HC≡Cδ-) is favorable to the attack at the electropositive carbonyl Cδ+ of formaldehyde. The MgO-promoted CuO catalyst showed the highest yield of BD (94%) and the highest stability (the BD yield decreased only from 94% to 82% after eight reaction cycles). SiO2 effectively dispersed Cu species, which improved catalytic activity and stability. However, the introduction of Al2O3 resulted in a large number of acidic sites on the catalyst's surface. This led to the polymerization of acetylene, which covered the active sites and decreased the catalyst's activity.

Regulation of Cu Species in CuO/SiO2 and Its Structural Evolution in Ethynylation Reaction.

A Cu-based nano-catalyst has been widely used in the ethynylation of formaldehyde; however, the effects of the presence of Cu on the reaction have not yet been reported. CuO/SiO2 catalysts with different Cu species were prepared by impregnation (IM), deposition-precipitation (DP), and ammonia evaporation (AE). The structural evolution of the Cu species in different states of the ethynylation reaction and the structure-activity relationship between the existence state of the Cu species and the catalytic properties of the ethynylation reaction were studied. The results show that the Cu species in the CuO/SiO2 (IM), prepared using the impregnation method, are in the form of bulk CuO, with large particles and no interactions with the support. The bulk CuO species are transformed into Cu+ with a low exposure surface at the beginning of the reaction, which is easily lost. Thus, this approach shows the lowest initial activity and poor cycle stability. A high dispersion of CuO and copper phyllosilicate exists in CuO/SiO2 (DP). The former makes the catalyst have the best initial activity, while the latter slows release, maintaining the stability of the catalyst. There is mainly copper phyllosilicate in CuO/SiO2 (AE), which is slowly transformed into a highly dispersed and stable Cu+ center in the in situ reaction. Although the initial activity of the catalyst is not optimal, it has the optimal service stability.