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

Abstract

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 57Fe Mössbauer spectroscopy. In as prepared state, Fe is present as NiFe2O4 at the surface and as MgFe3+xAl2−xO4 within the support, whereas Ni is mainly present as NiO. During H2-TPR, NiFe2O4 and NiO form an alloy from 500 °C on and MgFe3+xAl2−xO4 is partially reduced to MgFe2+xAl2−xO4, such that Ni-Fe alloy, MgFe2+xAl2−xO4 and MgFe3+xAl2−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, Mössbauer analysis detects continued extraction of Fe from the support, sustaining ongoing Ni-Fe alloying.