First-principles investigations of the Ni3Sn alloy at steam reforming conditions

The structure and surface composition of a Ni₃Sn alloy at conditions relevant for the steam reforming reaction was investigated using density functional theory calculations. Both the flat Ni₃Sn(0 0 0 1) surface and a surface with steps in the closed packed direction [1 0  0]were considered. The adsorption geometries and energies of the species CO, C, OH and H were calculated. Chemical potentials were used to map out which adsorbates are on the surface under varying conditions. It was found that adsorbates preferably bind to Ni as nearest neighbor with Sn as second-nearest neighbor. The binding energy is slightly stronger than on pure Ni. Adsorbate binding to Sn was found to be very unfavorable. Binding free energies indicate that at high temperature the alloy surface will be predominantly covered by CO and C, and at low temperatures one may find H and almost no OH. Even though the nominal composition of the investigated alloy is Ni₃Sn, the surface composition may differ significantly depending on temperature and pressure of the gas phase. This effect was investigated by calculating segregation energies both in the absence and in the presence of adsorbates. For the flat surface, it was found that only the bulk termination is present under relevant conditions. In contrast, it was found that for steps preferential adsorption of CO and C on Ni sites may lead to adsorption-induced segregation at temperatures below 400 °C. When taking segregation into account, the most stable Ni₃Sn surfaces will not bind CO or C at the same condition that Ni does. This is in excellent agreement with the previously proven ability of Ni-Sn alloys to inhibit graphite formation.