(618f) Novel UV-Visible/Temperature-Programmed Reduction Methodology for in-Depth Structural Characterization of Cu in Mixed Metal Oxides

Novel UV-visible/Temperature-programmed Reduction Methodology for in-Depth Structural Characterization of Cu in Mixed Metal Oxides

Cu containing mixed metal oxides (MMOs) are an important class of catalytic materials. Examples of these materials include the well-known CuO/ZnO/Al₂O₃ industrial catalyst for the low-pressure conversion of syngas to methanol (CO+2H₂⇆CH₃OH) and the low-temperature water gas shift reaction (CO+H₂O⇆CO₂+H₂), Cu chromite, and Cu/ZnO for (de)hydrogenation and oxidation reactions. Many characterization techniques are available for the structural characterization of Cu in these catalysts such as X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and X-ray diffraction (XRD)/pair distribution function (PDF). In this presentation, we will describe a simple methodology for the structural characterization of Cu in CuMgAlO_x MMOs based on a combination of commonly available UV-visible spectroscopy and temperature programmed reduction methods of untreated and N₂O-passivated MMO. The use of CuMgAlO_x MMOs is particularly advantageous because: (1) highly dispersed Cu containing MMOs can be prepared over a wide range of Cu compositions from well-defined precursor structures, namely, CuMgAlCO₃ layered double hydroxides; (2) the CuO UV-visible absorption bands in CuMgAlO_x do not overlap with those of MgO or Al₂O₃; and (3) CuO is the only reducible species in the MMO. These conditions allowed the unambiguous determination of Cu composition (by H₂-TPR), domain size (from CuO optical absorption edge energies as measured by UV-visible spectroscopy), and relative fractions of oligomeric CuO species in the bulk and on the surface of the MMOs (by UV-visible+N₂O/H₂-TPR). As a result, by increasing the Cu content in the CuMgAlO_x MMOs from 4 to 21 at.%, it was possible to determine the relative concentrations of isolated and oligomeric CuO species, displaying average numbers of nearest CuO neighbors between about 2 to 4. Additionally, the distinct reducibility patterns of untreated and N₂O-passivated CuMgAlO_x MMOs could be correlated with the corresponding CuO domain sizes. An example will be presented in which well-dispersed Cu species in reduced CuMgAlO_x are found to be highly active for C-O coupling reactions of alcohols and aldehydes to the corresponding esters. The developed methodology could be extended to the study of other Cu-containing MMOs and supported catalysts and can provide useful structural information of heterogeneous catalysts for which highly isolated and/or oligomeric surface metallic or metal oxides may be required.