Highly Mixed Phases in Ball-milled Cu/ZnO Catalysts:  An EXAFS and XANES Study

Abstract

New highly mixed phases have been identified in Cu/ZnO systems by EXAFS and XANES at both the Cu and Zn K-edge. The phases were generated by ball-milling Cu 2 O/ZnO mixtures under three different atmospheres of synthetic air (SA), SA + CO 2 and CO 2 . The system milled in CO 2 shows disproportionation of Cu 2 O into Cu 0 , Cu 1+ (cuprite Cu 2 O-type phase) and Cu 2+ (tenorite CuO-type phase), while most of the Zn 2+ is transformed into a nanocrystalline/amorphous ZnO-type zincite that forms a superficial mixture of oxide and carbonate phases. When synthetic air is added to the CO 2 atmosphere, ball milling results in the oxidation of nearly half the Cu 1+ into Cu 2+ with no Cu metal formed. The copper phase in this material is almost entirely amorphous. In SA, a significant amount of Cu 2+ -and Zn 2+ -based phases appears to react to form a nanocrystalline/amorphous Cu 1-x Zn x O (x ≈ 0.3) solid solution. This distorted rock saltlike solid solution, in which Zn and Cu feature different octahedral environments, was never reported before. It is thought to be formed by incorporation of Zn 2+ in the Cu fcc sublattice of the cuprite Cu 2 O matrix and the concomitant oxidation of Cu 1+ into Cu 2+ . The formation of such a highly mixed Cu 1-x Zn x O phase indicates strong Cu/Zn interaction in the Cu/ZnO system, which also suggests the presence of highly mixed phases in conventionally prepared activated catalysts.

A nanocomposite superstructure of metal oxides with effective charge transfer interfaces

“…This is mainly because of the larger difference of Zn 2 þ and Cu 2 þ coordination environment in ZnO and CuO. The Zn 2 þ ions in ZnO prefer tetrahedral coordination, while Cu 2 þ ions in CuO are in Jahn-Teller-distorted octahedral coordination 41,42 . These different coordination environments result in the low degree of solid-state solubility between CuO and ZnO.…”

“…These different coordination environments result in the low degree of solid-state solubility between CuO and ZnO. In contrast, the coordination environment of the Zn 2 þ sites in ZnO is suitable for Ni 2 þ (refs 42,43). This is the reason why CuO and ZnO are phase separate, while NiO and ZnO form a solid solution.…”

New Highly Mixed Phases in Ball-Milled Cu/ZnO Catalysts as Established by EXAFS and XANES

“…This assumption is confirmed by the presence of a second shell of 2.4 Zn/Cu atoms at 2.97 Å, which does not correspond to the first Zn-Zn bond distances (3.19 and 3.24 Å) in bulk ZnO either. A plausible interpretation of these elements combined with the XRD data [3] is the formation of a highly disordered Cu 1-x . In this rock salt-like structure that has never been reported before, Zn and Cu feature both an octahedral coordination but with different geometries [3].…”

“…A plausible interpretation of these elements combined with the XRD data [3] is the formation of a highly disordered Cu 1-x . In this rock salt-like structure that has never been reported before, Zn and Cu feature both an octahedral coordination but with different geometries [3].…”

Towards ‘greener’ catalyst manufacture: Reduction of wastewater from the preparation of Cu/ZnO/Al2O3 methanol synthesis catalysts

“…These include the combustion of organic metal salts [17,15,18,19], mechano-thermal [20], sol-gel [21,22] and colloidal synthesis [23]. In addition, low-metal-loading catalysts have been prepared by impregnation or chemical vapour deposition of Cu and Zn precursors on high surface area supports [24][25][26][27].…”