CO2 reduction via oxidative dehydrogenation and dry reforming of ethane over Fe3Ni1 nanoparticles: The influence of the oxide support

Abstract

The effect of the CO2:C2H6 feed ratio, the relative Lewis acidity of reducible and unreducible catalytically active metal oxide supports with and without Fe3Ni1 alloy nanoparticles on the activity, selectivity and stability for the CO2-mediated oxidative dehydrogenation of ethane (CO2-ODHE) is investigated. To circumvent the influence of the typically dissimilar textural properties of the supports in bulk form, overlayer oxide supports of V, Cr, Ga, Ti or Sm coated on a common γ-Al2O3 carrier were employed. Separately, (Ni0.75Fe0.25)Fe2O4 precursor nanoparticles were synthesized via a nonaqueous surfactant-free method, sonication-deposited onto supports and reduced in situ into an Fe3Ni1 alloy microstructure of bcc and fcc mixed phases captured with in situ XRD. When exposed to carbon dioxide at 255 °C, a selective re-oxidation of the bcc phase via CO2 dissociation is observed, while the fcc phase stays stable and only partially re-oxidizes above 525 °C. Upon exposure to CO2-ODHE conditions, the initial activity of the bare supports increases with increasing acid site strength, but this activity is rapidly lost in case of the strongly acidic supports. Comparison of the C2H4 and CO selectivity indicate direct dehydrogenation is preferred over the oxidative dehydrogenation pathway and is initially occurring in combination with some CO-forming routes, possibly the dry reforming of C2H6. This CO forming route is significant over the most acidic and reducible VOx@Al2O3 support in the early stages of operation. The addition of the Fe3Ni alloy increases the conversions of both C2H6 and CO2 across all supports, with a notably stronger effect observed on CO2 conversion especially over the highly acidic and reducible VOx@Al2O3 and CrOx@Al2O3. As a result, the CO selectivity is increased due to ethane dry reforming activity over the latter supports while CO2-ODHE activity is observed over the supports with intermediate and weak acid sites.