Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Abstract

Sodium‐based catalysts (such as Na2WO4) were proposed to selectively catalyze OH radical formation from H2O and O2 at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na2WO4, which can form OH radicals, using in situ techniques including X‐ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient‐pressure X‐ray photoelectron spectroscopy (AP‐XPS). As a result, Na2O2 species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas‐phase hydrocarbon reactions, including the selective transformation of methane to ethane.

The Influence of Reactor Parameters on the Boron Nitride-Catalyzed Oxidative Dehydrogenation of Propane

“…We emphasize that radical-mediated selective oxidation mechanisms have been previously proposed in the literature. The group of Marin extensively studied the reaction kinetics of oxidative methane coupling (OCM) in empty reactors and using Li/MgO catalysts, and they proposed an overall mechanism coupling surface and gas-phase reactions. , Takanabe and co-workers proposed that hydroxyl radicals play an important role during OCM using Mn/Na 2 WO 4 /SiO 2 catalysts as well as during ODH of ethane. − Leveles et al developed a reaction kinetics study of Li/Dy/MgO catalysts for ODH of propane and identified reaction regimes where homogeneous reactions play a comparable role to surface-catalyzed propane conversion. Furthermore, their observed partial pressure dependencies show trends similar to those observed on hBN catalysts, ,, albeit with a significantly higher selectivity toward CO 2 .…”

Fluctuating Storage of the Active Phase in a Mn‐Na2WO4/SiO2 Catalyst for the Oxidative Coupling of Methane

“…In addition to this, a significant broadening of the ν sym (W−O) stretching mode of Na 2 WO 4 at 923 cm −1 was observed in the Raman spectrum of the catalyst above the melting point of Na 2 WO 4 (Figure A, top). Both, Takanabe et al., and Yu. et al., reported similar observations, describing a reversible disappearance and intensity loss of the characteristic Raman bands for Na 2 WO 4 supported on TiO 2 and CeO 2 , respectively.…”

“…The source of oxygen has been attributed to manganese oxide, or any unspecified lattice oxygen . Potentially more decisive is the release of an active form of sodium oxide species that has been proposed to catalyse the oxidative coupling of methane by generation of OH radicals at high temperatures, presumably under involvement of homogeneous gas‐phase reactions due to the high volatility of sodium compounds under operation conditions …”

“…A new concept is proposed that involves the fluctuating storage and release of an active phase in heterogeneous catalysis. According to Equation (1), active sodium oxide species, which are responsible for high activity and selectivity in the oxidative coupling of methane, are generated in the catalytically relevant temperature regime in small amounts. The extent of this reaction is controlled by the oxygen partial pressure in the gas phase and the redox chemistry on the surface (Scheme ).…”

“…In the presence of gas‐phase oxygen, the phase transition of Mn 7 SiO 12 and Na 2 WO 4 into Na 2 O and MnWO 4 [Eq. (1)] is largely impeded and generates only transient amounts of active sodium oxide species. Apparently, only a small concentration of oxygen is necessary to keep the system in this highly active state (>88 % oxygen conversion in the steady state, see Supporting Information, Figure S21, t >8 h).…”