Ni-alumina dry reforming catalysts: Atomic layer deposition and the issue of Ni aluminate

A catalyst consisting of 2 wt% Ni supported on a commercially available transition alumina is modified using TMA-H₂O ALD cycles to deposit thin alumina overcoats on the catalyst, and it is then investigated for the catalytic dry reforming of methane (DRM) at 700 °C. Highly dispersed Ni rapidly sinters to form bulk Ni particles on the uncoated catalyst. In contrast, deposition of an alumina overcoat by ALD significantly lowers the rate of Ni sintering, and also lowers the propensity towards carbon deposition during DRM. Additionally, it is experimentally demonstrated that the Ni aluminate (NiAl₂O₄) spinel phase is unstable under the present DRM conditions and slowly undergoes reduction to metallic Ni and Al₂O₃. Slow reduction of the Ni²⁺ from NiAl₂O₄ is proposed as the origin of the large increase in DRM activity observed for the alumina-overcoated Ni catalysts.