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Magnetic Properties of Reduced and Reoxidized Mn–Na2WO4/SiO2: A Catalyst for Oxidative Coupling of Methane (OCM)

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journal contribution
posted on 11.09.2018, 00:00 by Wiebke Riedel, Lukas Thum, Jannik Möser, Vinzenz Fleischer, Ulla Simon, Konrad Siemensmeyer, Alexander Schnegg, Reinhard Schomäcker, Thomas Risse, Klaus-Peter Dinse
The magnetic properties of Mn–Na2WO4/SiO2, a promising catalyst for the oxidative coupling of methane (OCM), were investigated in two states: reduced with CH4 until reactivity ceased and reoxidized with O2 to probe for state-specific magnetic species and their involvement in the oxygen storage of the catalyst. Employing temperature- and frequency-dependent continuous-wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy combined with SQUID (Superconducting Quantum Interference Device) magnetization measurements allowed us to identify a variety of Mn species in different oxidation states and their role in the oxygen storage capability of the catalyst. For the reoxidized catalyst, the formation of magnetically ordered Mn­(II)- and Mn­(III)-containing Mn3O4 as well as Mn2O3 or MnMn6SiO12 phases were detected. The reduced catalyst exhibits almost ideal paramagnetic behavior and a strong, broad cw EPR signal consistent with the formation of short-range ordered nanosized Mn­(II) oxide, demonstrating the involvement of these Mn species in the oxygen storage capability of the catalyst. In contrast, rather isolated, highly oxidized Mn­(IV) as well as different Mn­(III) species were observed by pulsed EPR, which are not affected by the oxidation state of the Mn–Na2WO4/SiO2 catalyst, suggesting an inaccessible, buried location, presumably in the SiO2 support. Furthermore, paramagnetic sites with an effective S = 1/2 spin are detected, whose intensity depends on the oxidation state of the sample and are thus involved in the oxygen storage capacity of the catalyst.