Looking inside a Ni-Fe/MgAl2O4 catalyst for methane dry reforming via Mossbauer spectroscopy and in situ QXAS

The evolution of the constituents of an 8 wt%Ni-5 wt%Fe/MgAl2O4 catalyst for dry reforming of methane (DRM) is monitored by in situ quick X-ray absorption spectroscopy (QXAS) and Fe-57 Mossbauer spectroscopy. In as prepared state, Fe is present as NiFe2O4 at the surface and as MgFex3+Al2-xO4 within the support, whereas Ni is mainly present as NiO. During H-2-TPR, NiFe2O4 and NiO form an alloy from 500 degrees C on and (MgFex+Al2-xO4)-Al-3 is partially reduced to MgFex2+Al2-xO4, such that Ni-Fe alloy, MgFex2+Al2-xO4 and MgFex3+Al2-xO4 are the prevalent phases in the reduced catalyst. During DRM, dominantly oxidizing environments (CH4/CO2 = 1/2, 1/1.5) lead to formation of FeOx nanoparticles at the surface of the Ni-Fe alloy, thereby affecting the DRM activity, and possibly to some reincorporation of Fe into the support. For CH4/CO2 = 1/1, no significant changes occur in the catalyst's activated state, as a consequence of reduction by CH4 dissociation species counteracting oxidation by CO2. However, Mossbauer analysis detects continued extraction of Fe from the support, sustaining ongoing NiFe alloying.