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Highly dispersed iron catalysts, consisting of 5 wt% Fe on high-purity carbon black (Carbolac-1), have been studied by in situ Moessbauer spectroscopy at 295, 77, and 4 K. About 75-80% of the iron in the Fe/C-1 catalyst is reduced to ..cap alpha..-Fe in H/sub 2/ at 740 K; the remaining iron is present as Fe/sup 2 +/. The ..cap alpha..-Fe particles in reduced Fe/C-1 exhibit behavior characteristic of very small (2 nm) particles. First, substantial contributions of superparamagnetic ..cap alpha..-Fe occur in the Moessbauer spectra at 295 and 77 K. Second, the magnetic hyperfine fields of the ..cap alpha..-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 Fe/sup 3 +/. Carburization in syngas (H/sub 2//CO = 3) converts all ..cap alpha..-Fe in the reduced catalyst to iron carbides; the Fe/sup 3 +/ remains unaffected. Conversion of the Fe/sup 2 +/ spectrum from a singlet at room temperature to a doublet at 77 K showsmore » that the unusual ferrous singlet is caused by cancellation of electron and lattice field gradients of opposite sign. 22 references, 5 figures, 2 tables.« less

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