Characterization of Bismuth Manganese Oxide Catalysts for Methane Oxidative Coupling.

This paper reports a finding, namely a cooperation between separate phases, which may help understand the mechanism of oxidative coupling of methane. The activity of co-precipitated bismuth-manganese oxides in the oxidative coupling of methane, measured in the continuous flow regime, is maximum for Bi:Mn = 1:2. The catalysts have been investigated by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and microanalysis; catalytic pulse experiments were performed. The Bi2Mn4O10 phase is present in these catalysts. But, surprisingly, although it corresponds to the Bi:Mn ratio of 1:2 giving maximum activity, it decomposes to alpha-Bi2O3 and alpha-Mn2O3 in a few hours during the catalytic test. The higher catalytic activity observed at Bi:Mn = 1:2 thus does not seem due to the Bi2Mn4O10 phase. The high activity catalyst is actually composed of Bi2O3 in contact with an other phase. This might be a Bi-depleted Bi2Mn4O10-like phase; it is speculated that this phase could donate oxygen to Bi2O3 and make it more active. This would suggest that oxygen mobility or spill-over between phases. observed to be beneficial in other reactions occurring at 350-450-degrees-C, also promotes the selective oxidative coupling of methane above 700-degrees-C.