Nanoscale characterisation of model Ni-Mo catalyst active phases

Sammanfattning: An important stepping stone in transitioning from a fossil-fuel based transportation system is the production of bio-fuels such as bio-oils. For bio-oils to be commercially viable, catalytic upgrading via hydrodeoxygenation is required, typically using Ni-Mo oxide or sulfide catalysts. To optimise such processes, a thorough understanding of the catalyst is required. Spark discharge generation (SDG) of nanoparticles allows for a potentially useful model system for such catalyst due to the possibility of creating particles with a known composition, size distribution, and surface concentration. This report aims to characterise the oxidation and reduction of SDG-generated Ni-Mo nanoparticles and determine their suitability as model systems for the active phase of commercial Ni-Mo catalysts. Ni-Mo nanoparticles were deposited onto SiOx wafers or SiN TEM grids using an in-house spark ablation device. Characterisation was carried out using X-ray diffraction, HRTEM imaging, and compositional analysis with STEM-XEDS. The as-deposited particles exhibit Janus morphologies with a pure Mo part and a Ni-Mo part with structure and composition not expected from the Ni-Mo equilibrium diagram. Oxidation of isolated particles generated particles consisting mainly of β-NiMoO4; in contrast, oxidation of bulk powders generated mainly α-NiMoO4, while reduction followed previously published results on the reduction of NiMoO4. Upon oxidation, the Mo percentage in the particles decreased, which can be attributed to evaporation of MoO3. Previous literature indicating higher activity of supported Ni-Mo catalysts with β-NiMoO4 compared to α-NiMoO4 may be attributed to such catalysts having smaller active phase domains and less sintering. SDG-generated Ni-Mo particles can be used as a model catalyst active phase for Ni-Mo catalysts with less than ca 60 at. % Mo.