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Carbon dioxide (CO2) is one of the most harmful greenhouse gases, and it is the main contributor to climate change. Its emissions have been constantly increasing over the years due to anthropogenic...

MAX phases are layered ternary carbides or nitrides that are attractive for catalysis applications due to their unusual set of properties. They show high thermal stability like ceramics, but they are also tough, ductile, and good conductors of heat and electricity like metals. Here, we study the potential of the Ti3AlC2 MAX phase as a support for molybdenum oxide for the reverse water–gas shift (RWGS) reaction, comparing this new catalyst to more traditional materials. The catalyst showed higher turnover frequency values than MoO3/TiO2 and MoO3/Al2O3 catalysts, due to the outstanding electronic properties of the Ti3AlC2 support. We observed a charge transfer effect from the electronically rich Ti3AlC2 MAX phase to the catalyst surface, which in turn enhances the reducibility of MoO3 species during reaction. The redox properties of the MoO3/Ti3AlC2 catalyst improve its RWGS intrinsic activity compared to TiO2- and Al2O3-based catalysts. We thank the Netherlands Organisation for Scientific Research (NWO) for the grant “Developing novel catalytic materials for converting CO2, methane and ethane to high-value chemicals in a hybrid plasma-catalytic reactor” (China.15.119). We also acknowledge financial support by MINECO (Spain) through projects MAT2017-86992-R and MAT2016-80285-P. Z.S. was supported by the project LTAUSA19034, from the Ministry of Education Youth and Sports (MEYS). M.H. and M.T. gratefully acknowledge NWO under LIFT, Launchpad for Innovative Future Technology, PreCiOuS, 731.015.407. We also thank the staff of the Swiss Light Source (SLS) synchrotron (SuperXAS beamline and proposal number 20190956), Maarten Nachtegaal and Adam Clark, for support during synchrotron measurements.

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO2 associated with Mg2+-O2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO2 molecular path in MgO-based CO2 adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO2 on the surface of MgO exhibited the highest CO2 uptake (15.7 mmol g-1) at 350 °C compared to less than 0.1 mmol g-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO2 at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO3 formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO2 partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO2-capturing performance gained and its considerable significance for future practical designs of precombustion CO2 capture.

Selective conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is an attractive CO2 conversion process, which may be integrated with many industrial catalytic processes such as Fischer−Tropsch synthesis to generate added value products. The development of active and cost friendly catalysts is of paramount importance. Among the available catalyst materials, transition metal phosphides (TMPs) such as MoP and Ni2P have remained unexplored in the context of the RWGS reaction. In the present work, we have employed density functional theory (DFT) to first investigate the stability and geometries of selected RWGS intermediates on the MoP (0001) surface, in comparison to the Ni2P (0001) surface. Higher adsorption energies and Bader charges are observed on MoP (0001), indicating better stability of intermediates on the MoP (0001) surface. Furthermore, mechanistic investigation using potential energy surface (PES) profiles showcased that both MoP and Ni2P were active toward RWGS reaction with the direct path (CO2* → CO* + O*) favorable on MoP (0001), whereas the COOH-mediated path (CO2* + H* → COOH*) favors Ni2P (0001) for product (CO and H2O) gas generation. Additionally, PES profiles of initial steps to CO activation revealed that direct CO decomposition to C* and O* is favored only on MoP (0001), while H-assisted CO activation is more favorable on Ni2P (0001) but could also occur on MoP (0001). Furthermore, our DFT calculations also ascertained the possibility of methane formation on Ni2P (0001) during the RWGS process, while MoP (0001) remained more selective toward CO generation.