An Extended In Situ Cu−K XAFS and XRD Study on the Site Preference and Valence of Copper Ions in (Mg_1-xCu_x)O

Abstract

The local environment and valence of copper ions in copper-doped magnesium oxide, (Mg1 - x Cu x )O, were investigated under in situ conditions as a function of temperature T, composition x, and oxygen activity a O 2 . By comparison of Cu−K extended X-ray absorption fine structure (EXAFS) data with corresponding multiple scattering EXAFS simulations, copper was proven to occupy preferentially magnesium lattice sites. The bivalency and the site preference of copper was inferred from a comparative X-ray absorption near edge structure (XANES) study using standards containing copper in various oxidation states and coordination geometries. Strong similarities between the Fe−K XANES of Fe2+ in (Mg1 - x Fe x )O and the discussed Cu−K XANES support these interpretations. Additionally, the Jahn−Teller distortion was estimated to be small at room temperature. As the site preference and valence of copper is not influenced by variation of the experimental conditions, the local thermal expansion around copper in (Mg0.948Cu0.052)O was characterized by quantitative EXAFS analysis. The thermal expansions extracted from the nearest and next-nearest radial distribution function coincide with those determined from in situ recorded X-ray diffraction data. Furthermore, the Cu−K step height Δμ d is found to correlate with the lattice parameter a of (Mg0.948Cu0.052)O, indicating the step height change to be also a measure of thermal expansion.

Synergistic Effects in Bimetallic Palladium–Copper Catalysts Improve Selectivity in Oxygenate Coupling Reactions

“…Based on the XAS data of PdCu/HT (Figure 1A,B) a significant fraction of Cu exists as Cu 2+ , forming a ternary Cu−Mg−Al oxide. 30 The shape of the white line, reminiscent of the octahedral hexaaquocopper complex ion, is consistent with the conclusion that in this material Cu 2+ ions are octahedrally coordinated, presumably replacing Mg 2+ and Al 3+ in the HT structure. Replacement of Mg and Al in HT structures by first-row 31 and second-row 31,32 transition metals has been widely reported and also accounts for the small amount of oxidation of Pd (Figure 1A).…”

“…The location of Cu atoms in the catalyst and their oxidation state give interesting insights to explain these observations. Based on the XAS data of PdCu/HT (Figure A,B) a significant fraction of Cu exists as Cu 2+ , forming a ternary Cu–Mg–Al oxide . The shape of the white line, reminiscent of the octahedral hexaaquocopper complex ion, is consistent with the conclusion that in this material Cu 2+ ions are octahedrally coordinated, presumably replacing Mg 2+ and Al 3+ in the HT structure.…”

The Jahn−Teller Effect in Cu(II) Doped MgO

“…Examining the parameters in Table 2 does not reveal a significant temperature dependence of the bond lengths or Debye-Waller factors. In agreement with the high temperature experiments, 8 the third order cumulant does increase with temperature (Supporting Information Figure S3), but the associated uncertainties are large.…”

“…33 The bond lengths are then expected to be temperature independent; the increase observed at temperatures 300-1376 K is attributed to the overall thermal expansion of the MgO host. 8 In the X-ray photoelectron spectrum of CuO, the splitting in the Cu 2p 3/2 shakeup satellites has been attributed to an elongated geometry (4 × 1.96 Å, 2 × 2.66 Å). 34 This splitting remains the same for Cu x Mg 1-x O (x ) 1f0.05) and it has been concluded 34 that Cu(II) is also tetragonally elongated in MgO in agreement with the present study.…”

A Genuine Jahn–Teller System with Compressed Geometry and Quantum Effects Originating from Zero‐Point Motion

“…[48,49] It can be noticed thatt he octahedral distances, R oc ,o btained for Ni + and Ni 3 + are higher and lower than those calculated in Cu 2 + systems embedded in thes amel attice,r espectively.These facts are thusqualitatively consistentwith the relationship between the ionic radii of the Ni + ,C u 2 + ,a nd Ni 3 + species, r(Ni + ) > r(Cu 2 + ) > r(Ni 3 + ). Although there are no extended X-ray absorption fine structure (EXAFS) measurements on irradiated CaO:Ni 2 + ,t he value R oc = 2.14 p reviously calculated forM gO:Cu 2 + [50] is coincident, within 1.4 %, with R oc = 2.11 measuredb yH ilbrandt and Martin [51] by means of that technique.…”