In situ transmission electron microscopy study of the growth of Ni nanoparticles on amorphous carbon and of the graphitization of the support in the presence of hydrogen

In a specially equipped transmission electron microscope (TEM), Ni particles were vapor deposited onto thin films of amorphous carbon (a-C), and subsequent reactions with the carbon support were observed at elevated temperatures. Particles deposited at temperatures around 370 degrees C developed a graphite shell at above 600 degrees C and subsequently spread and graphitized the substrate. This activity was enhanced by hydrogen. The speed of graphitization significantly increased during spreading of the metal, which is attributed to the concomitant increase of the length of the reaction front, as well as to a purifying effect of hydrogen. It is concluded that the driving force for spreading of the Ni is the interdiffusion and catalytic conversion of carbon to graphite at the reaction front. Particles deposited at 500 degrees C remained inactive at 670 degrees C. This is probably due to the formation of a rather stable carbidic or graphitic interlayer during deposition.