Applied Catalysis A: General, Vol.474, 87-94, 2014
Cu/Mn-based mixed oxides derived from hydrotalcite-like precursors as catalysts for methane combustion
Hydrotalcite-like precursors containing Cu and Mn have been prepared by adding Al as an element promoting the layered structure formation. Materials with (Cu + Mn)/Al equal 3 or 8, and Cu/Mn equal 0.5 or 1, have been investigated and characterized with XRD, ICP OES, BET, SEM/EDS, TPR, FTIR, and XPS. Almost pure hydrotalcite-like phase has been obtained for Cu:Mn:Al ratio = In other precursors, next to the hydrotalcite-like phase, a certain amount of poorly crystalline rhodochrosite (MnCO3)-like component (containing also some Cu and Al) was observed. Calcined solids were composed of Al-rich quasi amorphous mixed oxide phase, derived from hydrotalcite-like part of the precursor, and crystalline Cuts Mn1.5O4 spinel phase originating from the rhodochrosite-like component. In Al-poor samples, manganese present in excess with respect to the Cu1.5Mn1.5O4 stoichiometry appeared as crystalline Mn2O3. Addition of Ce and/or Zr dopants hindered the evolution of spinel phase. All catalysts derived from hydrotalcite-containing precursors were more active in CH4 combustion than the reference Cu-Mn mixed oxide of hopcalite-like stoichiometry. Catalysts activity depended on the (Cu + Mn)/Al and Cu/Mn ratios, whose proper choice ensured the balance between enhancing the content of highly dispersed mixed oxides derived from the hydrotalcite-like phase, and maximizing the content of the catalytically active transition metals. The latter was ensured by the lower relative Al content, while the lower value of Cu/Mn minimized formation of well crystalline Cu1.5Mn1.5O4. Addition of Ce and Zr improved the dispersion of mixed oxides obtained upon calcination, and helped to maintain the spreading of the active phase in the conditions of catalysis and/or steam ageing. (c) 2013 Elsevier B.V. All rights reserved.