On the thermal stability of metal particles supported on a thin alumina film

The structure of metal (Co, Rh, Pd) particles deposited on a thin alumina film grown on a NiAl(1 1 0) substrate were studied as a function of annealing temperature. The results reveal at least three processes to occur upon heating: (a) re-arrangement of metal atoms on the particle surface, (b) sintering of the particles, and (c) metal migration into the substrate. The latter process is probably mediated by the defect structure of the oxide film and dominates at higher temperatures. The thermal stability is found to be better for Co, which strongly interacts with the alumina support, as compared to Pd and Rh. Hydroxylation of the alumina film increases Pd and Rh dispersion and shifts processes (b) and (c) to higher temperatures, likely due to a stronger interaction of the metals with the hydroxylated film.