In situ TEM and EELS analysis of Ni catalytic nanoparticles for dry reforming of methane.

Dry reforming of methane (DRM, CH+CO→2CO+2H) involves production of CO and H using two kinds of greenhouse gases, CH and CO, without requiring an expensive and complicated gas separation process. Using a developed specimen holder, we observed the Ni nanoparticles on AlO supports during DRM catalysis through in situ transmission electron microscopy and electron energy loss spectroscopy by tracking each individual nanoparticle to elucidate the structural and chemical features of the working catalyst under practical conditions. The average value of the Ni L/L intensity ratio, which relates to the valence state (i.e., oxidation state), shows that the Ni nanoparticles were oxidized at 450 °C in the DRM gas and then slightly reduced at 650 °C as a whole. But the Ni nanoparticles actually exhibited a range of oxidation states under DRM conditions. In particular, some oxidized Ni nanoparticles were reduced at 550 °C and drastically changed in size, becoming much smaller. Compared with in situ observations under only CH gas, without CO, oxidation of the Ni surface was directly and locally observed, presumably due to the reaction with AlO and the decomposition of CO. Additionally, the reduction of oxidized Ni was mainly due to hydrogen-containing gases. Furthermore, carbon deposition was observed at 350 °C, containing both amorphous carbon and graphene layers.