RESUMEN
Carbon dioxide capture is essential to achieve net-zero emissions. A hurdle to the design of improved capture materials is the lack of adequate tools to characterise how CO2 adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a promising probe of CO2 capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that 17O NMR is a powerful probe of CO2 capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by 17O NMR experiments on a series of CO2-loaded frameworks that clearly identify ammonium carbamate chain formation and provide evidence for a mixed carbamic acid - ammonium carbamate adsorption mode. We further find that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Our work paves the way for investigations of carbon capture chemistry that can enable materials design.
RESUMEN
Methanol is an attractive energy vector in a closed loop including its synthesis from CO2 and H2 and on-demand reforming to the starting feedstocks. Catalytic materials for the two reactions were mostly studied separately, with very few works assessing the feasibility of the same system for both. Here, key kinetic drivers of methanol synthesis (MS) and methanol steam reforming (MSR) were identified for the main catalyst families, with special focus on Cu-ZnO-Al2 O3 , In2 O3 , and Pd/ZrO2 . It was shown that the relative activity level was preserved in either direction, whereas the distinctly favored (reverse) water-gas shift modulated selectivity differently. Low selectivity in kinetically controlled MS could be overcome in MSR by exploiting thermodynamics as the driving force, with many catalysts unfit for MS still comprising appealing candidates for MSR and only few being suited for MS as well as MSR. Overall, readily identifiable properties describing catalyst behavior in the forward and backward reactions were highlighted, effectively linking research in the two fields and setting a stronger basis for developing a methanol-based hydrogen storage unit with a single reactor.