Atomic-scale study of in situ metal nanoparticle synthesis in a Ni/TiO2 system

The nucleation and evolution of Ni nanoparticles during reduction of a Ni(NO3)2.6H2O precursor supported on a commercial titania substrate have been studied in situ at atomic resolution using environmental transmission electron microscopy. An incipient wetness technique was used to prepare the starting unreduced material (10 wt % Ni precursor on titania). The Ni precursor, before reduction, shows a nonuniform distribution over the titania support. It is observed that upon reduction, the initial Ni "seed" crystal nucleates within the precursor patch. The distribution and size of the Ni nanoparticles thus generated are influenced by the distribution and size of the precursor patches. In this system, we see no evidence of preferential nucleation of Ni particles on anatase or rutile. At 350 degrees C with CO as the reducing agent, the {111} surface facets of the Ni nanoparticles are predominant during the initial stage of nucleation and growth. However, the {111} facets are partially consumed with time, indicating that they are not thermodynamically favored in the CO atmosphere. In CO and H2 atmospheres, Ni particles show a nonwetting morphology on titania, while in a mild oxidizing environment, a thin layer of NiOx is formed, thus giving rise to a morphology that is indicative of wetting of the support. This work provides fundamental information on understanding and controlling the important parameters involved in the preparation of a well-designed supported Ni catalyst using the incipient wetness technique.