There and Back Again: The Unique Nature of Copper in Ambient Pressure Dried-Silica Aerogels

Abstract

We have previously reported the formation of framework single-site copper in silica ambient-pressure-dried (APD) aerogels and xerogels, for which the metal (2−11 wt %) was added during the sol−gel stage (Kristiansen, T.; Einarsrud, M.-A.; Bjørgen, M.; Nicholson, D. G. J. Phys. Chem. C 2011, 115, 19260−19268). We here present a fundamental study on the formation of ultrasmall metal nanoclusters in hydrogen and complete restoration of the initial surroundings in nitric oxide/oxygen, using in situ X-ray absorption spectroscopy (XAS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We coupled density functional theory (DFT) with the Cu−Cu shell multiplicities and distances from extended X-ray absorption fine structure (EXAFS) to investigate the dimensions and structure dynamics of the copper nanoclusters. Their average cluster size did not exceed dimensions above 1.6 nm in either gel system up to 450°C in hydrogen; however, a significant drop in the first Cu−Cu shell multiplicity suggests a change of morphology and structure. A subsequent treatment in nitric oxide/oxygen resulted in reoxidation to single-site copper(II) species. We believe the driving force for complete redispersion is framework vacancies containing acidic silanol clusters, created during cation removal as confirmed by DRIFTS. This unique reversibility of the copper(II) single sites establishes the silica aerogel and xerogel systems as highly functional supports for ultrasmall clusters and in redox applications.

Tailoring the Surface Structure of Silicon Carbide Support for Copper Catalyzed Ethanol Dehydrogenation

“…However, Si‐OH can also catalyze secondary reactions, such as condensation, or ketonization reactions of acetaldehyde to produce C 3 and C 4 compounds, thus decreasing the selectivity to acetaldehyde (<80 %). Moreover, these cascade reactions also take place on undesired Cu + sites generated in the reduction process originating from well‐dispersed copper phyllosilicate ,. In order to improve acetaldehyde selectivity by cutting down the secondary reactions, carbon materials with relatively inert surfaces have been successfully applied as supports for DHEA process,, indicating excellent acetaldehyde selectivity of 94.1 % at 280 °C .…”

“…Moreover, these cascade reactions also take place on undesired Cu + sites generated in the reduction process originating from well-dispersed copper phyllosilicate. [10,11] In order to improve acetaldehyde selectivity by cutting down the secondary reactions, carbon materials with relatively inert surfaces have been successfully applied as supports for DHEA process, [4,12] indicating excellent acetaldehyde selectivity of 94.1 % at 280°C. [12] However, Cu nanoparticles on carbon supports tend to agglomerate and easily deactivate on account of the weak interaction between Cu and carbon.…”

Identification of synergistic Cu/V redox pair in VCu:AlPO-5; a comparison with VCu:ZSM-5

“…Studies also show that interaction between the catalytically active metal and surface groups of 3-D carriers can promote reversible redox behaviour for instance by preventing oxide formation and sintering, crucial to catalyst lifetime. [18][19][20] We wish to study the redox properties of copper and vanadium cohabitating in the two 3-D systems of the neutral AlPO-5 compared to the acidic H-ZSM-5, and investigate possible interactions or synergistic effects. In addition to different acidic properties these two 3-D supports also have different pore characteristics; the zeolite has a 3-D zigzag pore system with pore size of P d = 5.4-5.6 Å, whereas AlPO-5 consist of 1-D channels with pore size P d = 7.3 Å. Additionally AlPO-5 and H-ZSM-5 show high thermal and hydrothermal stability (above 650 1C), and are easily synthesised at a reasonable cost.…”

Copper Supported on Hybrid C@SiO2 Hollow Submicron Spheres as Active Ethanol Dehydrogenation Catalyst

“…[6,9] These undesired Cu d + sites are generated during reduction pretreatment, which are unavoidable for Cu/SiO 2 catalysts due to the strong interaction between Cu nanoparticles and SiO 2 . [10,11] Reducing the density of Si-OH over SiO 2 surface is an effective way of promoting selectivity, but simultaneously leading to instability. [12] Compared with SiO 2 , carbon materials possess relatively inert surface.…”

“…It has been an agreement that exposed Cu 0 can catalyze the dehydrogenation of ethanol, while Cu δ+ and Si‐OH often accelerates secondary reactions ,. These undesired Cu δ+ sites are generated during reduction pretreatment, which are unavoidable for Cu/SiO 2 catalysts due to the strong interaction between Cu nanoparticles and SiO 2 ,. Reducing the density of Si‐OH over SiO 2 surface is an effective way of promoting selectivity, but simultaneously leading to instability .…”