Ultraviolet–Visible Spectroscopy and Temperature-Programmed Techniques as Tools for Structural Characterization of Cu in CuMgAlO_xMixed Metal Oxides

Abstract

Ultraviolet–visible (UV–vis) spectroscopy was used in combination with temperature-programmed reduction (TPR) methods to provide information about the Cu structure in CuMgAlO x mixed oxides. UV–vis spectra revealed the presence of highly dispersed, isolated and oligomeric, CuO species in a distorted octahedral environment. From these spectra, optical absorption edge energies were determined and correlated with the number of nearest CuO neighbors (a measure of CuO domain size) and Cu content in the mixed oxides. Increasing the Cu content from 4 to 21 at. % increased the number of nearest CuO neighbors in oligomeric CuO from about 2 to 4 and produced materials that were more easily reducible, as inferred from TPR of untreated and N2O-passivated CuMgAlO x . A further increase in Cu content to 38 at. % increased the number of nearest CuO neighbors to 4.5 but resulted in a decrease of reducibility because of the evolution of an amorphous CuO phase in the bulk of the mixed oxides. This work represents the first demonstration of combining UV–vis spectroscopy with TPR of untreated and N2O-passivated CuMgAlO x as a relatively simple and inexpensive methodology for in-depth structural characterization of Cu in mixed metal oxides from which the composition, domain size, and relative fraction of total (surface + bulk) and surface oligomeric CuO species can be determined. A methodology that allows assessment of the extent of CuO bulk enrichment in CuMgAlO x materials is also presented.

Conversion of 2,3-butanediol to butenes over bifunctional catalysts in a single reactor

“…Copper surface area (SA Cu ) and dispersion (D Cu ) were calculated by the N 2 O decomposition method [43,44]. As is shown in Table 1, the catalyst 9.5%CuO/ZSM-5(23) has the worst copper dispersion (0.03) and lowest copper surface area (20.3 m 2 /g Cu ), which is in agreement with the results of H 2 -TPR, SEM (Fig.…”

“…Copper surface area (SA Cu ) and dispersion (D Cu ) were determined by dissociative N 2 O adsorption method at 90°C [43,44] using the same system as H 2 -TPR and NH 3 -TPD. Prior to N 2 O adsorption, the catalysts (0.1 g) were first treated at 550°C in Ar at a flow of 40 mL/min for an hour.…”

“…Copper dispersion (D Cu ) is calculated as D Cu = 2Y/X, which is defined as the ratio of the surface copper atoms to the total copper atoms present in the catalyst. Copper surface area SA Cu (m 2 /g Cu ) is calculated as described in literatures [43,44] by the following equation:…”

Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol

“…TheU V/Vis spectrum of 1-a 700 presented ab road band centered at 900 nm corresponding to the d-d transition of d 9 Cu II species,a nd as harp signal centered at 250 nm (Supporting Information, Figure S10) corresponding to the LMCT of oxygen to asingle Cu center (Cu 2+ ÀO 2À ); this is consistent with the presence of monomeric Cu II ions. [28] Thea bsence of absorption bands from 300-600 nm indicates that CuO clusters or peroxo-or m-oxo species, [29,30] if present at all, are only very minor species,d iffering from the significant amount of Cu sites reacting with CH 4 in this material. EPR spectroscopy revealed an anisotropic signal with an axial configuration, implying the localization of the unpaired electron in the d x 2 Ày 2 orbital (g k = 2.31 A k % 540 MHz;g ?…”

Sustainable Carbon Dioxide Photoreduction by a Cooperative Effect of Reactor Design and Titania Metal Promotion

“…In particular, a broad absorption centred at 18,250 cm −1 , and due to the plasmonic resonance of gold nanoparticles [83,84], was observed for the Au-TiO2 sample. Differently, a weak absorption centred at 12,000 cm −1 , assigned to d-d transition in Cu(II) species, was detected and ascribed to the presence of copper oxide nanoparticles [85,86] on the CuO-TiO2 photocatalyst. Even if the presence of the promoters did not affect the titania band gap value, the presence of such species is aimed at reducing the electron-hole recombination by modifying the interaction between the titania surface and light.…”