Growth mode of carbide from C₂H₄ or CO on Ni(1 1 1)

The growth of carbide on a Ni(1 1 1) surface by the decomposition of C₂H₄ and the Boudouard reaction (2CO_g → C_a+CO_2,g) was studied using scanning tunneling microscopy (STM), Auger electron spectroscopy and low energy electron diffraction. STM results showed that the carbide growth by the Boudouard reaction started at step edges on Ni(1 1 1), while for the C₂H₄ decomposition the carbide was formed preferentially at terrace sites with very low concentration of carbide at the step edge. The different behavior for the carbide growth was ascribed to the difference in the dissociation sites of CO and C₂H₄. As for the Boudouard reaction, CO was dissociated at the step edge and then carbon migrated into the bulk at a reaction temperature of 500 K. The carbon was then segregated at room temperature to the surface from the bulk to form a single domain of the (√39 × √39)R16.1° structure at the step edge. On the other hand, the C₂H₄ decomposition took place on the terrace leading to an isolated carbide unit or carbide short strings on the terrace.