MXenes, a recently discovered family of two-dimensional (2D) materials, are promising catalysts and supports for applications in heterogeneous catalysis; however, the thermal stability of MXenes and their surface chemistry are not fully explored. Here, we report that 2D molybdenum carbide Mo2CT x remains stable and shows no appreciable sintering up to ca. 550–600 °C in a reducing environment, as assessed by a combined in situ X-ray absorption near-edge spectroscopy (XANES) and powder X-ray diffraction (XRD) study during a temperature-programmed reduction (TPR) experiment. At higher temperatures, the passivating oxo, hydroxy, and fluoro groups defunctionalize the molybdenum-terminated surface, inducing a transformation to bulk β-Mo2C that is complete at ca. 730 °C. We demonstrate that Mo2CT x is a highly stable and active catalyst for the water–gas shift reaction with a selectivity >99% toward CO2 and H2 at 500 °C. The conversion of carbon monoxide on Mo2CT x starts to decline at temperatures that are associated with the decrease of the interlayer distance between the carbide sheets, as determined by the XRD-probed TPR, indicative of increasing mass transfer limitations at these conditions. Our results provide an insight into the thermal stability and reducibility of Mo2CT x and serve as a guideline for its future catalytic applications.

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.

Nanosheets of molybdenum(vi) oxide supported on carbon spheres were carburized and utilized for the dry reforming of methane (DRM). A molybdenum oxycarbide phase was identified as active for DRM and characterised by XANES and TEM methods.

Calcium looping, a CO 2 capture technique based on the cyclic carbonation and calcination of CaO, is a promising short-to midterm solution to reduce CO 2 emissions. However, CaO suffers from sintering under industrially relevant operating conditions, which reduces rapidly its cyclic CO 2 uptake capacity. Here, we report the design and manufacture of a hierarchical, porous (HP) CaO-based CO 2 sorbent. The hierarchically porous sorbent is created through the assembly of calcium carbonate nanoparticles and monodisperse oil droplets generated via microfluidic emulsification. The structure of the sorbent is stabilized by an Al 2 O 3 coating via atomic layer deposition (ALD). The sorbent outperformed the CO 2 uptake of the reference limestone by ca. 140%. The improved CO 2 uptake capacity is attributed to (i) the stabilization of the microand mesoporous structure of the material by the formation of Ca−Al mixed oxides, that is, Ca 12 Al 14 O 33 and Ca 3 Al 2 O 6 , and (ii) an improved mass transport within the sorbent particles owing to the HP structure of the material.

Carbon dioxide capture and storage (CCS) is a promising approach to reduce anthropogenic CO2 emissions and mitigate climate change. However, the costs associated with the capture of CO2 using the currently available technology, that is, amine scrubbing, are considered prohibitive. In this context, the so-called calcium looping process, which relies on the reversible carbonation of CaO, is an attractive alternative. The main disadvantage of naturally occurring CaO-based CO2 sorbents, such as limestone, is their rapid deactivation caused by thermal sintering. Here, we report a scalable route based on wet mechanochemical activation to prepare MgO-stabilized, CaO-based CO2 sorbents. We optimized the synthesis conditions through a fundamental understanding of the underlying stabilization mechanism, and the quantity of MgO required to stabilize CaO could be reduced to as little as 15 wt %. This allowed the preparation of CO2 sorbents that exceed the CO2 uptake of the reference limestone by 200 %.

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are emerging thermochemical CO2 capture cycles that allow the capture of CO2 with a small energy penalty. Here, the development of suitable oxygen carrier materials is a key aspect to transfer these promising concepts to practical installations. CuO is an attractive material for CLC and CLOU because of its high oxygen-storage capacity (20 wt %), fast reaction kinetics, and high equilibrium partial pressure of oxygen at typical operating temperatures (850-1000 °C). However, despite its promising characteristics, its low Tammann temperature requires the development of new strategies to phase-stabilize CuO-based oxygen carriers. In this work, we report a strategy based on stabilization by co-precipitated ceria (CeO2-x ), which allowed us to increase the oxygen capacity, coke resistance, and redox stability of CuO-based oxygen carriers substantially. The performance of the new oxygen carriers was evaluated in detail and compared to the current state-of-the-art materials, that is, Al2 O3 -stabilized CuO with similar CuO loadings. We also demonstrate that the higher intrinsic oxygen uptake, release, and mobility in CeO2-x -stabilized CuO leads to a three times higher carbon deposition resistance compared to that of Al2 O3 -stabilized CuO. Moreover, we report a high cyclic stability without phase intermixing for CeO2-x -supported CuO. This was accompanied by a lower reduction temperature compared to state-of-the-art Al2 O3 -supported CuO. As a result of its high resistance towards carbon deposition and fast oxygen uncoupling kinetics, CeO2-x -stabilized CuO is identified as a very promising material for CLC- and CLOU-based CO2 capture architectures.

Chemical looping is a promising process to produce high purity H 2 while simultaneously capturing CO 2 . The key requirement for this process is the availability of oxygen carriers that possess a high cyclic redox stability, resistance to carbon deposition, and thermal sintering. In this study, ZrO 2 -supported Fe 2 O 3based oxygen carriers were developed using a coprecipitation technique. We assess in detail the influence of the key synthesis parameter, i.e., the pH value at which the precipitation was performed, on the morphological properties, chemical composition, local structure, and cyclic redox stability. The performance of the new oxygen carriers was compared to unsupported Fe 2 O 3 and Al 2 O 3 -supported Fe 2 O 3 . A higher degree of disorder in the local structure of oxygen carriers precipitated at low pH values was confirmed by X-ray absorption spectroscopy (XAS) measurements. Electrical conductivity measurements showed that supporting Fe 2 O 3 on ZrO 2 lowered significantly the activation energy for charge transport when compared to pure Fe 2 O 3 . In line with this observation, ZrO 2supported oxygen carriers displayed a very high and stable H 2 yield over 15 redox cycles when precipitation was performed at pH > 5.

Calcium looping (CaL) is a CO2 capture technique based on the reversible carbonation/calcination of CaO that is considered promising to reduce anthropogenic CO2 emissions. However, the rapid decay of the...