Oxidation of Nanodispersed MoS₂ in Ambient Air: The Products and the Mechanistic Steps

Abstract

Oxidation of finely divided molybdenum sulfide in ambient air has been studied for air exposure times from 10 min to 1 year to clarify the nature of the reaction products and the mechanistic steps. At the initial steps, for air exposure times from several minutes to several hours, rapid oxidation of MoS 2 edges occurs with simultaneous formation of hydroxyl species and surface disulfide S 2 2− moieties as attested by 1 H NMR, X-ray photoelectron spectra, and temperature-programmed reduction. Prolonged air exposure of MoS 2 nanodispersions leads to deep oxidation. According to the results of X-ray absorption spectroscopy, UV−visible, and electron paramagnetic resonance spectroscopies, the main oxidation products are soluble paramagnetic molybdenum blue species and sulfuric acid. As shown by EXAS fitting, the major product is oxo-bridged dimolybdenyl Mo(V, VI) species. Ambient moisture plays an important role in the oxidation process as it contributes to the formation of sulfuric acid which leads to liquescence of the material and to deep oxidation without formation of a protective passivation layer.

Influence of MoS2 on Activity and Stability of Carbon Nitride in Photocatalytic Hydrogen Production

“…This is characteristic of surface sulfate species, likely formed during preparation of 2H MS-CN (TD), which included a thermal decomposition step (Figure 5c). Though this was performed in argon, traces of oxygen might have promoted formation of surface sulfate that is also described in the literature [37].…”

“…Moreover, the sulfate XPS peak is more pronounced after use in sample Pt/2H MS-CN (TD) and it is stronger compared to the more active sample Pt/2H MS-CN (PD), in which this peak appears for the first time after use (Figure 5d). The oxidation of sulfur species during photocatalysis is not uncommon and also exploited e.g., in the photocatalytic oxidative desulfurization [37]. The formed sulfate, however, could interact with the surface Pt species and reduce their activity for proton reduction.…”

Stability of 2H- and 1T-MoS2 in the presence of aqueous oxidants and its protection by a carbon shell

“…The observed dissolution of MoS 2 will also presumably cause structural changes to MoS 2 , as shown previously for water and air matrices. 54,55,65,69 To protect 2D MoS 2 from oxidation, we propose two potential strategies with respect to the electrocatalytic HER that evolved from this research: (1) use pretreated water (e.g., tap water) containing relatively low concentrations of oxidants that is saturated with N 2 to remove DO, or (2) add a thin shell of carbon to protect the active edge sites of MoS 2 , but this may also change reaction mechanisms and reduce catalytic activity. For treatment applications, where MoS 2 would be used to reduce or oxidize various contaminants, removing DO may be enough to protect MoS 2 , even in the presence of naturally occurring oxidants.…”

“…Aer the reaction was complete, the 2H-and 1T-MoS 2 samples exhibited a bluish color, indicating the formation of Mo(V) species. 65 MoS 2 could potentially be acting as a hydrogenation catalyst (i.e., H 2 dissociation) and DO may play an intermediate role in the reaction. Thus, to investigate these effects, experiments were repeated with H 2 saturated water with limited DO (Fig.…”

“…When DO is present, a more complex reaction pathway occurs that promotes further degradation of MoS 2 and the removal of NO 2 À . It is possible that this stepwise reaction occurs through the oxidative bridging of S 2À sites to S 2 2À with DO, 65 which may then react with NO 2 À to form SO 4 2À (e.g., eqn (3)). For applications that involve the reduction of a target species (e.g., NO 2 À ), and thus the potential oxidation of MoS 2 , we caution the use of the word "catalyst" unless the reaction can be veried to be occurring through a truly catalytic pathway and not a sacricial reduction pathway.…”