Tailored Supported Nickel Nanoparticle Catalysts for Dry Reforming of Methane via a Molecular Approach

Dry reforming of methane (DRM) has become a very appealing reaction in the last decades because it allows a valorization of un-wanted greenhouse gases (CO2, CH4) toward syn-gas production. It is often use in industry as complementary process of steam reforming of methane (SRM) process in order to tune effectively the product composition (H2/CO), which ease the syn-gas conversion to synthetic fuel. Supported nickel nanoparticles (Ni NPs) catalysts show high DRM activity in comparison to most metals and due to the very high abundance of Ni are cost-effectively the best system for DRM. However, they suffer from severe deactivation upon reaction, caused by the dramatic loss of active sites through substantial formation of carbon on the NP surface as well as by sintering. Thus, this PhD thesis has aimed at improving and understanding supported Ni NP catalysts through Surface Organometallic Chemistry and inorganic colloidal NP synthesis. A central role of these different works is placed on structure evolution upon reaction using operando XAS techniques. Control reduction of Ni(II) formate precursor specifically adsorbed at alumina surface yields to 2 nm Ni(0) NP, which have the capability to efficiently reduce the rate of coke formation while doubling the initial activity compared to catalysts prepared by conventional methods. However, such catalyst still dramatically deactivates via migration of Ni in the alumina lattice as evidenced by Operando XANES. However, the stabilization of the 2 nm NP can be achieved by suppressing the formation of a solid solution between Ni and Al2O3 through the use of (Mg,Al,O) binary oxide supports. In addition, the stability of these small size Ni NP supported on a periclase material could be further improved by doping the nickel centre with iron (Fe) metal using a colloidal NiFe bimetallic synthesis. Tuning the Ni/Fe ratio has allowed to maximize the specific activity of the Ni(0) active site while retaining the iron capability to nearly supress coke formation. Operando XAS show the segregated structure of the two metal during DRM, in which FeO oxidizes carbon deposit. The decrease of Ni(0) ensembles was also investigating by using a phosphorus as promoter. NixP were first synthesized via a colloidal approach and then deposited on silica to provide 1-2 nm nickel phosphide NP with a Ni/P ratio of 3. Despite a low activity compared to its pure Ni(0) counterpart, the nickel phosphide nanoparticle show enhanced stability with neither coke formation nor sintering. Moreover, these Ni-based catalysts show an altered selectivity by supressing WGS thus favouring H2/CO ratio, suggesting the presence of an alternative active site. Thus, the combination of advanced spectroscopy techniques with a molecular approach towards the synthesis of Ni-based NPs permit to find general trends to form high performing DRM catalysts: 1) small size Ni(0) NPs (< 4 nm) provides catalysts with superior activity and stability due to the increase of Ni surface area and a lower rate of carbon formation, 2) designing the support is necessary to stabilize the small size NP from sintering while mitigating the migration of Ni inside the support matrix. The use of (Mg,Al,O) binary oxide material is a good alternative because of their thermal stability and high surface area, 3) the incorporation of adequate quantity of oxiphilic Fe promotor in proximity of small size Ni(0) NPs allow to nearly supress the formation of carbon, 4) the site isolation of the Ni center using phosphorus dopant permit both enhanced stability and selectivity