C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

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Fig. 82: a) Activity test of Fe-700(5h). Conversion (%) of C2H6, CO2 and selectivity (%) towards dehydrogenation (C2H4) and dry reforming reaction (syngas), as a function of time on stream (TOS). Reaction conditions: 650°C, CO2/C2H6 = 1, Wcat/F0 = 10.8 kgcat∙s∙mol-1, TOS = 6h, P = 101.3 kPa. b) Performance of Fe-700(5h) after one hour and 30 hours of the stability test and regeneration by CO2 and air. Reaction conditions: 650°C, CO2/C2H6 = 1, Wcat/F0 = 14.9 kgcat∙s∙mol-1, TOS = 30h, P = 101.3 kPa. Regeneration conditions: 700°C, Ftotal=50ml/min (8.65vol% CO2 or 100% air), Duration= 1h, P = 101.3 kPa.

CO2-assisted ethane dehydrogenation (CO2-EDH) is a promising carbon economy process for CO2 valorisation, converting ethane (C2H6) and CO2 to valuable ethylene (C2H4) and carbon monoxide (CO) respectively. Currently, C2H4 is mainly produced through the energy-intensive thermal steam-cracking of hydrocarbons (state-of-the- art, SOTA), which is usually performed at temperatures above 800oC [2]. CO2-EDH is a promising alternative, with lower energy demands. The main scientific challenge is to develop materials that can simultaneously activate CO2 and be selective towards ethylene production.

In this work, environmentally friendly iron oxide catalysts were employed to activate the CO2-EDH reaction. Iron oxide catalysts can successfully activate CO2, while being selective towards C-H bond cleavage, suggesting C2H4 and not syngas formation [3-4]. 5wt%Fe/10wt%NiO- MgO-ZrO2 catalysts were synthesised and calcined at 600oC for three hours and at 700oC for five hours, hereafter named as Fe-600(3h) and Fe-700(5h), respectively. Fe-700(5h) exhibited the best performance, reaching a maximum ethylene yield of 21% and more than 25% CO2 conversion (Figure 82a). The regeneration ability of Fe-700(5h), by applying both air and CO2, is highlighted in Figure 82b. More specifically, the catalyst can be regenerated after one hour of CO2 flow at 700oC, resulting in an additional amount of CO2 converted to CO.

In order to optimise the catalytic performance, structure- performance correlations are required. To investigate the nature of the active sites that led to the enhanced catalytic performance of Fe-700(5h), advanced characterisation techniques of X-ray absorption spectroscopy (XAS) at the Ni and Fe K-edges and X-ray Raman scattering (XRS) at the Mg L2,3- and O K-edges, were performed at beamlines BM23 and ID20, respectively. Structural modelling of XRS data using the screening parameter made it possible to tune the electronic structure modifications necessary to reproduce the experimental spectra. The structural modelling of XRS data for the fresh Fe-700(5h) revealed a modification of Mg atom geometry and the formation of a stable, spinel-like arrangement between Mg and Fe atoms (Figure 83). The latter had a crucial impact on the catalytic performance, resulting in high CO2 conversion

X-ray spectroscopy validates an alternative process for CO2 utilisation

Developing selective catalysts for CO2 utilisation in tandem with ethylene production via ethane dehydrogenation (CO2-EDH) is a promising avenue for valorising CO2. Advanced X-ray characterisation techniques were applied to elucidate the structural modifications that led to enhanced catalytic performance during CO2-EDH, attaining 90% ethylene selectivity at 650oC.

The European Union Green Deal aims to transform the EU into the first climate-neutral continent, with a target of net-zero carbon dioxide (CO2) emissions by 2050 [1].