Dramatic interference of Au TEM grid with model and supported MoNi/MgO catalysts during high temperature reduction and methane decomposition

The decomposition of methane in an inert atmosphere, in other words, methane pyrolysis results in CO 2 -free “turquoise” hydrogen and solid carbon. MgO-supported bimetallic molybdenum-nickel (MoNi) catalyst shows a promising synergistic effect in the reaction. Our objective was to efficiently investigate how changes in the Mo/Ni ratio influence the catalytic performance, with a particular focus on carbon deposition. A novel micro-combinatorial TEM technique has been successfully applied to generate, by magnetron sputtering, a continuous composition gradient of Mo-Ni nanoparticles on a SiO x -coated Au TEM grid, which is believed to be stable at high temperatures. The post-treatment of this model catalyst sample at 800 °C in diluted hydrogen yielded particles of uniform but variable composition. Attempts to induce carbon growth through methane exposure revealed a significant limitation: the Au and SiO x components of the grid interfered at high temperatures, causing Mo and Ni partial segregation and oxidation, which inhibited carbon formation. The model MoNi sample sputtered on an Au grid with a MgO-coated C membrane showed similar gold-induced effects, forming mixed metal oxides and no carbon after methane feed exposure. Interestingly, when a MoNi/MgO powder catalyst was deposited onto the Au grid, no carbon was formed either, while in the absence of the Au grid, carbon nanotubes were obtained, as expected. These results highlight the critical influence of the grid material on catalytic behavior and suggest that gold can actively inhibit coking, which may prove valuable in reactions where suppression of coking is desirable. • The variation of Mo/Ni ratio in catalytic methane decomposition was to be studied. • Micro-combinatorial TEM method resulted in MoNi nanoparticles over an Au TEM grid. • Change of Mo/Ni ratio on a full range was achieved on the grid (model catalyst). • Carbon deposition was investigated by TEM after ex situ CH 4 exposure at 800 °C. • Immobilization of Au from the grid caused alloy segregation and hindered coking.