Looking inside a Ni-Fe/MgAl2O4 catalyst for methane dry reforming via Mössbauer 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.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.A