Low Temperature Dry Reforming of Methane With Nickel Manganese Oxide Catalysts

This thesis concerns the development of a stable nickel catalyst for the dry reforming of methane (DRM). Traditional (supported and unsupported) nickel catalysts are found to be inadequate due to deactivation and associated problems from sintering and carbon deposition. Formation of small supported nickel nanoparticles by impregnation of SBA-15 with single-source Ni4 cluster precursors is shown to create catalysts with good activities and stabilities for DRM. Commercially available 50 nm nickel nanoparticles, thinly coated in alumina using ALD, form extremely active catalysts for DRM with very high resistance to sintering. Addition of MnOx to unsupported nickel in low Mn/Ni ratios significantly improves catalytic activity and stability in comparison to bulk nickel. Coprecipitation via NixMn1-xO (x ≤ 0.5) to form ex-solid solution nickel supported on MnO gives catalysts with high activities and very high resistance to carbon deposition. This material was further developed by lowering the amount of Ni in the NixMn1-xO solid solution to x = 0.05, calcining at higher temperature, and pre-sintering the catalyst in a reductive atmosphere, which not only results in stable activity at temperatures of up to 700°C but also excellent resistance to carbon deposition, attributable to small and stable nickel particles and SMSI effects. Addition of silica led to reaction of MnO with SiO2 to form manganese orthosilicate, which depletes the catalyst of MnO and thus removes its beneficial effects. An experimental and theoretical investigation into DRM kinetics is presented, over the Ni/MnO catalyst and also over simulated catalysts using microkinetic and formal models from the literature. The DRM reaction over Ni/MnO is essentially first order with respect to the methane pressure and zero with respect to that of carbon dioxide, and the overall activation energy is 109 kJ mol-1, in agreement with the literature. CH4 activation is therefore rate determining, agreeing with a theoretical microkinetic sensitivity analysis, and CO2 activation is fast with an abundance of oxidant species on the catalyst surface to avoid solid carbon deposition. The behaviour of the model catalyst can be adequately described using LHHW kinetics from the literature, in particular the Bradford and Vannice formal kinetic model, with the assumption that methane activation occurs through decomposition of CHx or CHxO as the rate determining step(s)