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

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 O-2 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.