Dual-Element Modulation Excitation XAS to Probe the Redox Surface Chemistry of a Ni–Fe Dry Reforming Catalyst
Valentijn De Coster, Nadadur Veeraraghavan Srinath, Parviz Yazdani, Hilde Poelman, Vladimir Galvita
Abstract
Fe and Ni K edge CO 2 /H 2 modulation excitation X-ray absorption spectroscopy (MEXAS) was performed to gain insights into the CO 2 /H 2 redox behavior of Ni and Fe species within activated, i.e., reduced, Ni/MgFeAlO 4 . These results are compared with those of “monometallic” Ni/MgAl 2 O 4 and MgFeAlO 4 . Concurrent phase-sensitive detection (PSD) analysis and kinetic differentiation of demodulated Fe and Ni K edge MEXAS data, obtained after the exposure of the activated catalyst to CO 2, evidence oxidation of Fe before Ni. Fe oxidizes into FeO x, which interacts with Mg from the support to engage in MgFeO x formation and further re-incorporates into the MgFeAlO 4 support lattice. Metal oxide–metal boundaries, present between FeO x /MgFeO x and Ni, as well as metal–support interfaces provide active sites for CO 2 activation and channel the formed oxygen into metallic Ni, leading to NiO formation. Subsequent exposure to H 2 first forms Ni 0, then Fe 0 . Once Ni 0 is present, H 2 is activated on the metal and spills over toward oxidic Fe within the support, causing MgFeO x segregation and, later, Fe 0 formation, which eventually engages with Ni 0 in Ni–Fe alloying. This study reveals the highly synergistic Ni–Fe redox dynamics of restructuring in Ni/MgFeAlO 4, in response to methane dry reforming (DRM)-related alternating CO 2 /H 2 exposure, and highlights the metal oxide–metal interface as an exploitable pathway for improved DRM catalyst design. Moreover, the bi-element MEXAS–PSD kinetic differentiation approach in this work is extendable to other material science studies in view of gaining information on the sequence of element-specific electronic and structural transformations.