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We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr0.8Ca0.2FeO3−δ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N2 physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr K-edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite via X-ray photoelectron and in situ Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO2-free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO2, surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping. [ABSTRACT FROM AUTHOR]

Early transitional metal carbides are promising catalysts for hydrogenation of CO2. Here, a two-dimensional (2D) multilayered 2D-Mo2C material is prepared from Mo2CTx of the MXene family. Surface termination groups Tx (O, OH, and F) are reductively de-functionalized in Mo2CTx (500 °C, pure H2) avoiding the formation of a 3D carbide structure. CO2 hydrogenation studies show that the activity and product selectivity (CO, CH4, C2–C5 alkanes, methanol, and dimethyl ether) of Mo2CTx and 2D-Mo2C are controlled by the surface coverage of Tx groups that are tunable by the H2 pretreatment conditions. 2D-Mo2C contains no Tx groups and outperforms Mo2CTx, β-Mo2C, or the industrial Cu-ZnO-Al2O3 catalyst in CO2 hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO2 hydrogenation. The development of robust and efficient catalysts for CO2 hydrogenation to value-added chemicals is an urgent task. Here the authors report two-dimensional carbide catalyst based on earth-abundant molybdenum that hydrogenates CO2 with high activity, stable performance and tunable selectivity. [ABSTRACT FROM AUTHOR]

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh 0 ). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh 2 C catalyst and comparing the results with those for a Rh 0 /C catalyst obtained from Rh 2 C via H 2 pretreatment. While the catalysts Rh 2 C and Rh 0 /C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H 2 addition route on rhodium catalysts., (© 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Transformations of oxygenates (CO 2 , CO, H 2 O, etc.) via Mo 2 C-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 Mo 2 C T x ( T x 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 Mo 2 C T x in Fischer-Tropsch (FT) synthesis increases with a decreasing coverage of surface passivating groups (mostly O*). The in situ removal of T x species and its consequence on CO conversion is highlighted by the observation of a very pronounced activation of Mo 2 C T x (pretreated in H 2 at 400 °C) under FT conditions. This activation process is ascribed to the in situ reductive defunctionalization of T x groups reaching a catalyst state that is close to 2D-Mo 2 C (i.e., a material containing no passivating surface groups). Under steady-state FT conditions, 2D-Mo 2 C yields higher hydrocarbons (C 5+ 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-Mo 2 C catalysts. The steady-state productivity of 2D-Mo 2 C to C 5+ hydrocarbons is ca. 2 orders of magnitude higher relative to a reference β-Μo 2 C catalyst that shows no in situ activation under identical FT conditions. The passivating T x groups of Mo 2 C T x can be reductively defunctionalized also by using a higher H 2 pretreatment temperature of 500 °C. Yet, this approach leads to a removal of carbidic carbon (as methane), resulting in a 2D-Mo 2 C 1- x catalyst that converts CO to CH 4 with 61% selectivity in preference to C 5+ hydrocarbons that are formed with only 2% selectivity. Density functional theory (DFT) results attribute the observed selectivity of 2D-Mo 2 C to C 5+ 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-Mo 2 C, and O* is favorably removed in the form of CO 2 relative to H 2 O, consistent with the observation of a high CO 2 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., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr 0.8 Ca 0.2 FeO 3- δ ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N 2 physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr K -edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite via X-ray photoelectron and in situ Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO 2 -free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO 2 , surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The thermal carburization of MoO 3 nanobelts (nb) and SiO 2 -supported MoO 3 nanosheets under a 1 : 4 mixture of CH 4  : H 2 yields Mo 2 C-nb and Mo 2 C/SiO 2 . Following this process by in situ Mo K-edge X-ray absorption spectroscopy (XAS) reveals different carburization pathways for unsupported and supported MoO 3 . In particular, the carburization of α-MoO 3 -nb proceeds via MoO 2 , and that of MoO 3 /SiO 2 via the formation of highly dispersed MoO x species. Both Mo 2 C-nb and Mo 2 C/SiO 2 catalyze the dry reforming of methane (DRM, 800 °C, 8 bar) but their catalytic stability differs. Mo 2 C-nb shows a stable performance when using a CH 4 -rich feed (CH 4  : CO 2 = 4 : 2), however deactivation due to the formation of MoO 2 occurs for higher CO 2 concentrations (CH 4  : CO 2 = 4 : 3). In contrast, Mo 2 C/SiO 2 is notably more stable than Mo 2 C-nb under the CH 4  : CO 2 = 4 : 3 feed. The influence of the morphology of Mo 2 C 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 Mo 2 C/SiO 2 features a higher resistance to oxidation under DRM than the highly crystalline unsupported Mo 2 C-nb and this correlates with an improved catalytic stability. Lastly, the oxidation of Mo in both Mo 2 C-nb and Mo 2 C/SiO 2 under DRM conditions in the in situ XAS experiments leads to an increased activity of the competing reverse water gas shift reaction., Competing Interests: The authors declare that there is no conflict of interest regarding the publication of this article., (This journal is © The Royal Society of Chemistry.)