Cu–MgO Samples Prepared by Mechanochemistry for Catalytic Application

Abstract

High-energy ball milling was applied to prepare powder samples by treating copper or copper oxides with magnesium or magnesium oxide. The five samples thus prepared were characterized by physical methods. According to X-ray diffraction the mechanical treatment resulted in the formation of nanostructured powders. Carbon and oxygen impurities and Mg (mainly as oxide) were detected by X-ray photoelectron spectroscopy of the as-milled samples. Four samples, namely, nanocomposite materials prepared by the self-sustaining reaction of copper oxides and magnesium applied in stoichiometric amounts [(Cu2O)Mg and (CuO)Mg] and samples produced by mechanical treatment of copper oxides and magnesium oxide [(Cu2O)7(MgO)93 and (CuO)13(MgO)87] also had copper on the surface. The latter specimens and 3% Cu+MgO were unique in their characteristics to have, after a short hydrogen treatment or after catalytic application, an unusually large concentration of Cu0 measured by the N2O titration method. Temperature-programmed reduction showed, in each case, the existence of reducible copper species on the surface. The quantity of reducible copper was quite low for (Cu2O)Mg, (CuO)Mg, and 3% Cu+MgO, whereas almost the total amount of copper of (Cu2O)7(MgO)93 and (CuO)13(MgO)87 was available for reduction. Two types of copper species have been detected: bulk CuO with a reduction temperature of about 260°C [characteristic of (CuO)Mg and also found in (Cu2O)Mg] and species strongly interacting with the support with a reduction temperature of about 350°C [3% Cu+MgO, (Cu2O)7(MgO)93, (CuO)13(MgO)87, and also detected in (Cu2O)Mg]. The 3% Cu+MgO, exhibited the highest basicity measured by the decomposition of 2-methyl-3-butyn-2-ol and the dimerization of acetone. The samples showed high activity in the dehydrogenation of 2-propanol, whereas their activity in the one-step synthesis of methyl isobutyl ketone from acetone was moderate. Methyl isobutyl ketone (MIBK) was formed in low selectivity over the samples prepared by a self-sustaining reaction and 3% Cu+MgO, whereas very high MIBK selectivities were observed over the mixed oxides. These features were correlated with the relative concentration of active sites capable of hydrogenating the carbon–oxygen and carbon–carbon double bond.