Local coordination configuration of Ni and Co in MgAl2O4 spinel structure and the performance of NiCo/MgO-Al2O3 catalyst for dry reforming of methane

MgAl2O4 spinel exhibits high flexibility in accommodating doped metal atoms and other solid defects, providing a powerful tool for tailoring the metal-support interaction (MSI) and developing high-performance catalysts for dry reforming of methane (DRM). We prepared a series of Ni and/or Co catalysts supported on MgO-Al2O3 (MAO) with different MgO/Al2O3 ratios (x = 0-4). We found that the coordination configuration of Ni and Co in the MgAl2O4 spinel structure depends on the Mg content: Ni atoms are pushed from the tetrahedral (Td) site to the octahedral (Oh) site as the Mg content increases (x >= 1.2), while Co tends to occupy both the Td and Oh sites. A proper MSI strength achieved by tuning the metal coordination configuration not only increases the thermal stability of the catalysts but also reduces the crystallinity of the deposited carbon species, making them more amorphous and prone to be removed during DRM. We proposed that driven by high temperature and the fluctuations in the local chemical environment, Ni and Co atoms may undergo reversible redox cycles and shuttle between the metal and support across the metal-support interface. This self-regeneration process helps retards the sintering of metal nanoparticles and suppresses the formation and crystallization of filamentous carbon. The CH4 conversion over bimetallic NiCo/MAO-2 achieves 84.4 % at 750 degrees C, remaining stable over a 110-h long-term test. This study provides an atomic-scale understanding of the catalysts, enabling the full utilization of the MgAl2O4 spinel structure for the rational design of high-performance DRM catalysts.