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The thermal decomposition of Fe3(CO)12 has been studied for the first time by dispersing this cluster on an oxygen-free carbon surface and monitoring its behavior by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The Fe3(CO)12 clusters on carbon decomposed to Fe(CO)5 in either He or H2; consequently, the decarbonylation of both clusters could be followed. First-order rate constants of decomposition in He or H2 were determined for each cluster and compared to literature kx values for nucleophilic substitution reactions in solution. The kx values for each reaction were very similar for Fe3(CO)12, but the kx value for decomposition was much higher for Fe(CO)s. The rate-determining step in either the substitution or the decomposition reaction appears to be the removal of the first CO ligand. No stable hydrido-iron clusters were formed. The activation energy of decomposition of Fe3(CO)12 was near 18 kcal/mol in He and 21 kcal/mol in H2 while that for Fe(CO)5 was near 15.5 kcal/mol in either gas. These activation energies are lower than those observed for substitution reactions in solution, but this can be explained by Fe-Fe bond formation during the decomposition process to create very small Fe crystallites. Large amounts of chemisorbed CO confirmed the formation of well-dispersed Fe on carbon following decomposition at 673 K under either He or H2; however, DRIFTS spectra of CO at 195 K detected no IR-active CO species and at 300 K only Fe(CO)s was present. The heat of adsorption of CO on these small Fe particles was measured calorimetrically at 300 K and found to be 15.0 ± 1.6 kcal/mol. This study represents a portion of the first successful application of an IR spectroscopic technique to characterize carbon-supported metal catalysts.

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