Safron,
There are a number of Reactive Empirical Bond Order (REBO) potentials based on the first generation Brenner potential (1990) which have been modified to include interactions between Fe, Cr, Ni and C for the study of the catalytic growth of carbon nanotubes. To my knowledge, none of these potentials (not even 1st generation REBO) have been implemented in LAMMPS. It would require pretty significant coding to accomplish this.
Dave Schall