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The isothermal isotopic exchange reaction of 18 O 2 with 16 O of chromium(VI) oxide supported on zirconia, alumina, and titania has been investigated with in situ laser Raman spectroscopy. The isotopic exchange reaction is dependent on the support type, the Cr loading, and the reaction temperature. Complete isotopic exchange of chromium(VI) oxide with 18 O 2 is difficult to achieve and requires several successive butane reduction-18 O 2 oxidation cycles at relatively high temperatures. The efficiency of the isothermal isotopic exchange reaction increases from alumina over titania to zirconia and with increasing Cr loading and reduction temperature. The observed Raman shifts upon isotopic labeling are consistent with a mono-oxo surface chromium oxide(VI) species.

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The literature on methane dehydroaromatization (MDA) to benzene using ZSM-5 supported, group V–VIII transition metal-based catalysts (MOx/ZSM-5) is critically reviewed with a focus on in situ and operando molecular insights.

The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.

The involvement of lattice oxygen species is important toward oxidative coupling of the methane reaction (OCM) over supported Mn-Na 2 WO 4 /SiO 2 catalysts, but there is no consensus regarding the types, role, and origin of lattice oxygen species present in supported Mn-Na 2 WO 4 /SiO 2 catalysts, which hinders the understanding of the OCM reaction network. In the present study, by utilizing the temporal analysis of products technique, we show that supported Na 2 WO 4 /SiO 2 catalysts possess two different types of oxygen species, dissolved O 2 and atomic O, at an OCM-relevant temperature. The addition of Mn-oxide to this catalyst increases the total amount and release rate of dissolved O 2 species and improves C 2 selectivity of both dissolved O 2 and atomic lattice O species. KEYWORDS: Mn-Na 2 WO 4 /SiO 2 catalyst, oxidative coupling of methane (OCM), lattice oxygen, dissolved oxygen, molten salt, temporal analysis of products (TAP)

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