Surface and bulk changes in iron nitride catalysts in H2/CO mixtures

Preparation of the ζ-, ε-, and γ′-iron nitride phases was confirmed by Mössbauer spectroscopy, X-ray diffraction, and quantitative mass spectrometry of NH3 evolved during decomposition. Computer fitting of the ε-nitride Mössbauer spectra with a distribution of hyperfine fields shows the conversion of iron with two nitrogen nearest neighbors (Fe 2nn) to Fe 3nn as the nitrogen content increases. The dynamic response of the nitrides to H2CO mixtures at reaction temperatures was followed by constant-velocity Mössbauer spectroscopy and transient mass spectrometry. The rapid decomposition of the iron nitrides in H2 at 523 K occurs with surface reaction as the rate-limiting step, initially. At lower temperatures or after significant nitride decomposition, the data are best fit with a shrinking core model. For reaction at 473 K, the Mössbauer effect identified an α-Fe shell, a ζ-Fe2N core, and an ε-FexN transition region. Surprisingly, loss of the pure nitride phase is barely retarded for H2CO mixtures compared to H2 alone at 523 K. Mass spectrometric studies show that the freshly prepared nitride has a substantial hydrogen inventory, equivalent to a monolayer of NH3 for ζ-Fe2N. On exposure to synthesis gas, the nitride catalysts produce no methane until one to two monolayers of N have been removed, but carbon is deposited on the catalyst by the Boudouard reaction. Mass spectral measurements show no evidence for active nitrogen on the surface after the synthesis reaction has been established. Both Mössbauer spectroscopy and mass spectral measurements confirm, however, that following the initial loss of nitrogen, bulk carbonitrides form which lose their nitrogen very slowly as the reaction proceeds. These data suggest that differences in the performance of iron and iron nitride catalysts may be strongly influenced by the way surface carbon is deposited during reactor startup.