Dynamic Cu/Zn Interaction in SiO₂Supported Methanol Synthesis Catalysts Unraveled by in Situ XAFS

Abstract

In situ X-ray absorption spectroscopy XAFS at the Cu and Zn K-edge has been used to unravel the Cu/Zn interaction and identify the possible active site of Cu-based methanol synthesis catalysts in the Cu/ZnO/SiO 2 ternary system. These highly dispersed silica supported catalysts, whose activity increases sharply as a function of the reduction temperature, were studied calcined, reduced at 200, 300, and 400°C, and for each reduction temperature under passivation/rereduction and methanol synthesis conditions. Results showed that the calcined form consists mainly of a mixed Cu/Zn hydrosilicate that is progressively transformed as the reduction temperature increases into (i) Cu metal particles, (ii) increasingly dispersed ZnO species on SiO 2 , and (iii) finally a Zn metallic phase forming segregated bimetallic CuÀZn α-brass alloy particles. These different structures and Cu/Zn interfaces may correspond to different active phases and activities in methanol synthesis. After reduction at 200 and 300°C, Cu 0 is likely composing most of the active phase, whereas above 300°C, the sharp increase in the number Zn 0 -based sites formed as a function of the reduction temperature could explain the major role played by this parameter in controlling the activity of these catalysts. The dynamic Cu/Zn interaction as a function of the temperature and gas environment pointed out in this ternary system may be at the origin of the existence of different and sometimes contradictory models to account for the mechanisms of the methanol synthesis.

Synthesis of an unexpected [Zn2]2+ species utilizing an MFI-type zeolite as a nano-reaction pot and its manipulation with light and heat

“…The HMFI-Zn 0 -5 sample exhibited characteristic absorption bands at 9663.7 and 9669.7 eV, being attributable to the 1s → 4p transitions for monomeric Zn 2+ ions exchanged in the MFI zeolite. 12,33 The XANES spectrum of Sample HMFI-Zn 0 -5 is consistent with that of Sample Zn 2+ -MFI (Fig. S1 †), supporting that the zinc metal-atoms used in the preparation process of the HMFI-Zn 0 -5 sample were quantitatively oxidized to monomeric Zn 2+ ions.…”

Microwave-assisted Cu-catalyzed Ullmann ether synthesis in a continuous-flow milli-plant

“…Previously, it has been reported that the Cu catalyst could be protected against oxidation and that leaching could be minimized by adding 50 wt% of Zn to the catalyst as a sacrificial reducing agent [53,82,83]. Consistent with this view, Figure 16b shows that a considerable yield increase was achieved when a CuZn-based heterogeneous catalyst was used in the fixed- 24 bed reactor.…”

Copper Nanowires as Nanoscale Interconnects: Their Stability, Electrical Transport, and Mechanical Properties

“…The Fourier transforms (FT) of the EXAFS spectra of the Cu foil and Cu NWs shown in Figure 2d demonstrate a strong CuÀCu peak located at 2.52 Å. 42 While for the Cu NWs, the new peak at around 1.42 Å might be ascribed to the formation of CuÀN bonds at the Cu nanowire surface. 42,43 The presence of NÀCu bonds on the surface of Cu NWs would greatly restrain the oxidation process.…”