Molybdenum carbide and oxycarbide from carbon-supported MoO3 nanosheets: phase evolution and DRM catalytic activity assessed by TEM and in situ XANES/XRD methods

Alkali cation stabilization of defects in 2D MXenes at ambient and elevated temperatures

Transition metal carbides have been adopted in energy storage, conversion, and extreme environment applications. Advancements in their 2D counterparts, known as MXenes, enable the design of unique structures at the ~1 nm thickness scale. Alkali cations have been essential in MXenes manufacturing processing, storage, and applications, however, exact interactions of these cations with MXenes are not fully understood. In this study, using Ti3C2Tx, Mo2TiC2Tx, and Mo2Ti2C3Tx MXenes, we present how transition metal vacancy sites are occupied by alkali cations, and their effect on MXene structure stabilization to control MXene's phase transition. We examine this behavior using in situ high-temperature x-ray diffraction and scanning transmission electron microscopy, ex situ techniques such as atomic-layer resolution secondary ion mass spectrometry, and density functional theory simulations. In MXenes, this represents an advance in fundamentals of cation interactions on their 2D basal planes for MXenes stabilization and applications. Broadly, this study demonstrates a potential new tool for ideal phase-property relationships of ceramics at the atomic scale.

The Impact of Oxygen Surface Coverage and Carbidic Carbon on the Activity and Selectivity of Two-Dimensional Molybdenum Carbide (2D-Mo2C) in Fischer-Tropsch Synthesis

Transformations of oxygenates (CO2, CO, H2O, etc.) via Mo2C-based catalysts are facilitated by the high oxophilicity of the material; however, this can lead to the formation of oxycarbides and complicate the identification of the (most) active catalyst state and active sites. In this context, the two-dimensional (2D) MXene molybdenum carbide Mo2CTx (Tx are passivating surface groups) contains only surface Mo sites and is therefore a highly suitable model catalyst for structure-activity studies. Here, we report that the catalytic activity of Mo2CTx in Fischer-Tropsch (FT) synthesis increases with a decreasing coverage of surface passivating groups (mostly O*). The in situ removal of Tx species and its consequence on CO conversion is highlighted by the observation of a very pronounced activation of Mo2CTx (pretreated in H2 at 400 °C) under FT conditions. This activation process is ascribed to the in situ reductive defunctionalization of Tx groups reaching a catalyst state that is close to 2D-Mo2C (i.e., a material containing no passivating surface groups). Under steady-state FT conditions, 2D-Mo2C yields higher hydrocarbons (C5+ alkanes) with 55% selectivity. Alkanes up to the kerosine range form, with value of α = 0.87, which is ca. twice higher than the α value reported for 3D-Mo2C catalysts. The steady-state productivity of 2D-Mo2C to C5+ hydrocarbons is ca. 2 orders of magnitude higher relative to a reference β-Μo2C catalyst that shows no in situ activation under identical FT conditions. The passivating Tx groups of Mo2CTx can be reductively defunctionalized also by using a higher H2 pretreatment temperature of 500 °C. Yet, this approach leads to a removal of carbidic carbon (as methane), resulting in a 2D-Mo2C1-x catalyst that converts CO to CH4 with 61% selectivity in preference to C5+ hydrocarbons that are formed with only 2% selectivity. Density functional theory (DFT) results attribute the observed selectivity of 2D-Mo2C to C5+ alkanes to a higher energy barrier for the hydrogenation of surface alkyl species relative to the energy barriers for C-C coupling. The removal of O* is the rate-determining step in the FT reaction over 2D-Mo2C, and O* is favorably removed in the form of CO2 relative to H2O, consistent with the observation of a high CO2 selectivity (ca. 50%). The absence of other carbon oxygenates is explained by the energetic favoring of the direct over the hydrogen-assisted dissociative adsorption of CO.

Evaluating metal oxide support effects on the RWGS activity of Mo2C catalysts

Mo2C supported on nonreducible metal oxides shows increased activity for the reverse water gas shift reaction compared to reducible oxides.

The structural evolution of Mo2C and Mo2C/SiO2 under dry reforming of methane conditions: morphology and support effects

The thermal carburization of MoO₃ nanobelts (nb) and SiO₂-supported MoO₃ nanosheets under a 1 : 4 mixture of CH₄ : H₂ yields Mo₂C-nb and Mo₂C/SiO₂. Following this process by in situ Mo K-edge X-ray absorption spectroscopy (XAS) reveals different carburization pathways for unsupported and supported MoO₃. In particular, the carburization of α-MoO₃-nb proceeds via MoO₂, and that of MoO₃/SiO₂via the formation of highly dispersed MoO_x species. Both Mo₂C-nb and Mo₂C/SiO₂ catalyze the dry reforming of methane (DRM, 800 °C, 8 bar) but their catalytic stability differs. Mo₂C-nb shows a stable performance when using a CH₄-rich feed (CH₄ : CO₂ = 4 : 2), however deactivation due to the formation of MoO₂ occurs for higher CO₂ concentrations (CH₄ : CO₂ = 4 : 3). In contrast, Mo₂C/SiO₂ is notably more stable than Mo₂C-nb under the CH₄ : CO₂ = 4 : 3 feed. The influence of the morphology of Mo₂C and its dispersion on silica on the structural evolution of the catalysts under DRM is further studied by in situ Mo K-edge XAS. It is found that Mo₂C/SiO₂ features a higher resistance to oxidation under DRM than the highly crystalline unsupported Mo₂C-nb and this correlates with an improved catalytic stability. Lastly, the oxidation of Mo in both Mo₂C-nb and Mo₂C/SiO₂ under DRM conditions in the in situ XAS experiments leads to an increased activity of the competing reverse water gas shift reaction.

In situ Mo K-edge X-ray absorption spectroscopy reveals different carburization pathways for unsupported and supported MoO₃ yielding Mo₂C and Mo₂C/SiO₂. Mo₂C/SiO₂ features higher resistance to oxidation under dry reforming of methane than Mo₂C.

Understanding the Preparation and Reactivity of Mo/ZSM-5 Methane Dehydroaromatization Catalysts

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X‐ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM‐5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mo‐species proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM‐5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

Probing Active Sites in CuxPdy Cluster Catalysts by Machine-Learning-Assisted X-ray Absorption Spectroscopy

Size-selected clusters are important model catalysts because of their narrow size and compositional distributions, as well as enhanced activity and selectivity in many reactions. Still, their structure-activity relationships are, in general, elusive. The main reason is the difficulty in identifying and quantitatively characterizing the catalytic active site in the clusters when it is confined within subnanometric dimensions and under the continuous structural changes the clusters can undergo in reaction conditions. Using machine learning approaches for analysis of the operando X-ray absorption near-edge structure spectra, we obtained accurate speciation of the Cu_xPd_y cluster types during the propane oxidation reaction and the structural information about each type. As a result, we elucidated the information about active species and relative roles of Cu and Pd in the clusters.

Metal (Mo, W, Ti) Carbide Catalysts: Synthesis and Application as Alternative Catalysts for Dry Reforming of Hydrocarbons-A Review

Dry reforming of hydrocarbons (DRH) is a pro-environmental method for syngas production. It owes its pro-environmental character to the use of carbon dioxide, which is one of the main greenhouse gases. Currently used nickel catalysts on oxide supports suffer from rapid deactivation due to sintering of active metal particles or the deposition of carbon deposits blocking the flow of gases through the reaction tube. In this view, new alternative catalysts are highly sought after. Transition metal carbides (TMCs) can potentially replace traditional nickel catalysts due to their stability and activity in DR processes. The catalytic activity of carbides results from the synthesis-dependent structural properties of carbides. In this respect, this review presents the most important methods of titanium, molybdenum, and tungsten carbide synthesis and the influence of their properties on activity in catalyzing the reaction of methane with carbon dioxide.

Exploiting two-dimensional morphology of molybdenum oxycarbide to enable efficient catalytic dry reforming of methane

The two-dimensional morphology of molybdenum oxycarbide (2D-Mo2COx) nanosheets dispersed on silica is found vital for imparting high stability and catalytic activity in the dry reforming of methane. Here we report that owing to the maximized metal utilization, the specific activity of 2D-Mo2COx/SiO2 exceeds that of other Mo2C catalysts by ca. 3 orders of magnitude. 2D-Mo2COx is activated by CO2, yielding a surface oxygen coverage that is optimal for its catalytic performance and a Mo oxidation state of ca. +4. According to ab initio calculations, the DRM proceeds on Mo sites of the oxycarbide nanosheet with an oxygen coverage of 0.67 monolayer. Methane activation is the rate-limiting step, while the activation of CO2 and the C-O coupling to form CO are low energy steps. The deactivation of 2D-Mo2COx/SiO2 under DRM conditions can be avoided by tuning the contact time, thereby preventing unfavourable oxygen surface coverages.