Small-particle effects in Moessbauer spectra of a carbon-supported iron catalyst

Highly dispersed iron catalysts, consisting of 5 wt % Fe on high-purity carbon black (Carbolac-1), have been studied by in situ Mössbauer spectroscopy at 295, 77, and 4 K. About 75-80% of the iron in the Fe/C-1 catalyst is reduced to α-Fe in H2 at 740 K, whereas the remaining iron is present as Fe2+. The α-Fe particles in reduced Fe/C-1 exhibit behavior characteristic of very small (2 nm) particles. First, the most interesting feature of this work is the occurrence of substantial contributions of superparamagnetic α-Fe in the Mössbauer spectra at 295 and 77 K. Second, the magnetic hyperfine fields of the α-Fe in the spectra at 4 K are enhanced by about 7 kOe, due to the influence of the demagnetizing field in the particle. Third, the effective Debye temperatures of the Fe/C-1 catalysts are much smaller than in iron bulk compounds. Exposure of reduced Fe/C-1 to air results in the oxidation of all iron to Fe3+. Carburization in syngas (H2/CO = 3) converts all α-Fe in the reduced catalyst to iron carbides, whereas the Fe2+ remains unaffected. Conversion of the Fe2+ spectrum from a singlet at room temperature to a doublet at 77 K shows that the unusual ferrous singlet is caused by cancellation of electron and lattice field gradients of opposite sign. The work illustrates the usefulness of in situ Mössbauer spectroscopy at cryogenic temperatures in the investigation of highly dispersed systems.