Unveiling the role of Ni nanometric particles in ultra-stable hierarchically porous Y zeolite to drive methane steam reforming and CO2 hydrogenation

Three catalysts containing 5, 10 and 20 % wt. Ni trapped in the porous zeolite framework (Ni5, Ni10 and Ni20, respectively), were prepared. An ultra-stable zeolite Y, with a Si/Al molar ratio of 385, selected for its high thermal stability, was impregnated under vacuum with an aqueous solution of Ni2+ ions and thermally treated at 600 °C in a reducing atmosphere. The Ni nominal contents were compliant with the nominal value as confirmed by X-ray fluorescence spectroscopy. The catalysts were tested in the steam reforming of methane and in the hydrogenation of carbon dioxide. Catalytic activity is significantly structure-dependant. Several physical-chemical characterizations were carried out. X-ray powder diffraction with synchrotron source, followed by Rietveld analysis, and H2-TPR analysis indicate that all the loaded nickel is in the metallic form. High-resolution transmission electron microscopy showed that the particle size distribution is centred around 30 nm for Ni5 and Ni 10, while particles of around 150 nm are obtained with Ni20. A thorough NLDFT analysis of the pore size distribution shows a certain fraction of large mesoporosity in addition to the microporosity typical of zeolites. This hierarchical structure, also evidenced by the TEM micrographs, has a profound impact on the catalytic response since the nickel particles are located in the mesoporosity, whereas the reaction appears limited by Knudsen diffusion through zeolite micropores. O2-TPOs carried out on used samples showed no coke deposition suggesting a good resistance to fouling, that can be related to a very low acidity, as measured by NH3-TPD measurements.