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

Authors: 
Bravo-Suarez, J. J., The University of Kansas
Subramaniam, B., University of Kansas
Chaudhari, R. V., The University of Kansas


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 CuMgAlOx 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 CuMgAlOx 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 CuMgAlOx 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 CuMgAlOx 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 CuMgAlOx MMOs could be correlated with the corresponding CuO domain sizes. An example will be presented in which well-dispersed Cu species in reduced CuMgAlOx 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.

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