VOLUME I: ANOSTRUCTURED NITROGEN-CONTAINING CARBON
4.2.6 Characterization of the Fe phase with Mửssbauer Spectroscopy
At this point characterization of the Fe phase of the samples has not been possible using the techniques discussed thus far. The Fe particles are too small and sparse for analysis with XRD, and XPS cannot be used to detect such a small amount of Fe, especially when the particles are covered with carbon. TEM could provide insights into the Fe phases, but would be an expensive and time consuming technique to get a wide distribution of Fe particles. However, Mửssbauer spectroscopy is a powerful technique for analyzing Fe, even if the Fe composition is low, since 57Fe enriched samples can be prepared to allow characterization of low Fe compositions. For this study, precursor materials were partially enriched with 57Fe to improve the response, and this variable did not effect sample properties.
Mửssbauer spectroscopy, like any spectroscopy, measures absorption of electromagnetic radiation in a sample. In the case of Mửssbauer spectroscopy, the radiation is gamma rays created from the decomposition of a radioactive source, typically
57Co. The energy of the source is varied by vibration via the Doppler effect. Therefore, rather than reporting the energy of the radiation on the x-axis (as is usually done for other spectroscopy techniques), the velocity of the source is given on the x-axis. Various coordination, oxidation states, and magnetic field strengths (affected by grain size in iron) have an effect on the absorption spectra of samples. A discussion of the theory behind the different absorption characteristics can be found elsewhere [214].
A typical Mửssbauer spectra for the materials discussed here is shown in Figure 48. The spectra had to be fit to 4 species. Table 11 shows the typical ranges for some
common species in similar materials (based on the knowledge of an experienced collaborator, Dr. Jean-Marc Millet of the Institut de Recherches sur la Catalyse, Lyon, France), coincidentally the 4 deconvoluted species fit well within the known ranges for γ- Fe, θ-Fe3C, Fe3+, and Fe2+. Table 12 reports the parameters for all the fits that are presented. Gamma Fe is a high temperature phase of metallic Fe, that is apparently stabilized, perhaps, because it is covered by carbon. Baker et al. observed γ-Fe using Mửssbauer in samples where fibers grew from graphite supported Fe [215]. The determined mechanism for fiber growth in these materials began with Fe spreading on the support surface, followed by adsorption of carbon, and then elemental carbon growth from the Fe, while the Fe remained on the surface. θ-Fe3C is another species that should not be stable at room temperature while open to the atmosphere, so this species is likely encased in carbon. This species has been observed by Audier [216] and Millet [217] in materials where carbon fibers grew from Fe particles. The mechanism in this case involved the adsorption of carbon onto the metal, diffusion of carbon through the particle, and deposition of carbon out the other end, resulting in the metal being lifted off the support surface during fiber growth [218]. The partially oxidized Fe species may be from the previous species which were not completely encased in carbon, and therefore reacted with oxygen in the atmosphere. It is possible the oxidation proceeds through Fe2+, explaining the presence of this species. However, since a carbide species is present, one may also expect a nitride species to be present, since acetonitrile was used to carry out the treatment, and both nitrogen and carbon were present in the fibers. This could be an alternative explanation for the Fe2+ species, although a literature search of Fe nitride species could not uncover a species that has a similar Mửssbauer spectra.
The Mửssbauer spectra contained the same 4 species regardless of treatment time, although the relative intensities of the species changed. Figure 49 clearly shows the change in the relative intensities of the sextet yielding carbide species for treatments of 20 minutes, 2 hours, and 12 hours. The deconvolution results of the spectra in Figure 49 are shown in terms of relative compositions in Figure 50. From this figure it can be seen that the carbide contribution grows with treatment time, while the gamma Fe phase decreases. The oxidized Fe phases as a whole remain relatively constant, although the Fe3+ decreases with time. There are two possible explanations for this. First, if Fe2+ is an oxide, then apparently the amount of exposed Fe remains constant with treatment time, but it gets harder to fully oxidize the Fe after longer treatment times. This could be caused by larger particle sizes after longer treatments, or more carbon coverage. It should be noted that each sample was exposed to atmospheric conditions several weeks before being analyzed. A second theory is that the Fe2+ is a nitride phase, forming in a similar manner to the carbide phase, and thus increasing for the same reason the carbide phase increases. It should be pointed out that we have yet to find a nitride phase reported in the literature that has a similar isonomer shift and quadrapole splitting as the observed Fe2+ phase. However, there are a large number of possible nitrides with a wide range of different Mửssbauer spectra [219, 220].
Interestingly, when the Fe was supported by carbon rather than alumina, the spectra contained the same species, but had significantly different contributions from these species. A comparisons of the spectra are shown in Figure 51. When Fe is supported by Vulcan Carbon, it appears that Fe carbide is less likely to form, and γ-Fe is
preferred. The Fe2+ is only slightly larger in the sample with more carbide, so little can be said of its nature based on these results.
The Mửssbauer spectra also changed after the alumina supported sample was washed with HF acid. The spectra after HF washing is shown in Figure 52. Again, the same species could be fit to the deconvolution, however, the ratio of the species changed.
Specifically, the oxidized phases, and the carbide phases decreased after the wash.
Apparently, the carbide phase is not protected as well as the γ-Fe phase. Again, nothing can be said of the origin of the Fe2+ phase based on the results. It should be noted that when these species contribute such a small amount to the spectra that deconvolution becomes difficult, especially with the noise present in the signal. The relative compositions for the samples are shown in pie graph form in Figure 53 for ease of comparison. It should also be noted that the ORR activities of the 57Fe enriched samples were the same as the un-enriched samples prepared by the same treatments.
Figure 48: Deconvolution of Mửssbauer spectra for 2-wt% Fe/Al2O3 treated for 2 hours at 900oC with acetonitrile.
Isonomer Shift Quadropole Splitting
Phase Splitting δ (mm/s) ∆ (mm/s) H (kOe)
α Fe sextet ~ 0 ~ 0 300-550
γ Fe singlet -0.1 0 0
θ Fe3C sextet 0.18-0.25 0.01-0.9 ~ 200 α Fe2O3 doublet 0.35-0.37 1.00-1.02 0 Fe3+ species doublet 0.2-0.7 0.0-1.3 0 Fe 2+ species doublet 0.9-1.9 1.0-2.5 0 Table 11: Parameter ranges for Mửssbauer spectra of common Fe species.
Sample Species % of Fe δ (mm/s) ∆ (mm/s) H (kO) Fe / Al2O3 - 2 hours θ Fe3C 46% 0.204 0.016 203.9
γ Fe 30% -0.089 0 0
Fe2+ 14% 1.006 2.192 0
Fe3+ 10% 0.702 0.574 0
Fe / Al2O3 - 20 min θ Fe3C 40% 0.209 -0.008 199.98
γ Fe 34% -0.093 0 0
Fe2+ 10% 1.006 1.929 0
Fe3+ 16% 0.694 0.478 0
Fe / Al2O3 - 12 hours θ Fe3C 55% 0.181 0.046 206.6
γ Fe 17% -0.148 0 0
Fe2+ 21% 1.036 2.192 0
Fe3+ 7% 0.700 0.574 0
θ Fe3C 29% 0.24 -0.176 205.3
Fe / VC - 2 hours γ Fe 54% -0.068 0 0
Fe2+ 12% 1.006 2.192 0
Fe3+ 5% 0.702 0.574 0
Fe / Al2O3 - 2 hours θ Fe3C 37% 0.2 0.032 204.334
(HF washed) γ Fe 50% -0.085 0 0
Fe2+ 6% 1.365 2.145 0
Fe3+ 6% 0.594 0.536 0
Table 12: Values for deconvoluted Mửssbauer spectra.
Figure 49: Changes in the Mửssbauer spectra with treatment time for 2-wt% Fe/Al2O3.
Figure 50: Fe phase composition plotted versus treatment time for 2-wt% Fe/Al2O3
treated at 900oC with acetonitrile, as determined by Mửssbauer spectroscopy.
Figure 51: Comparison of Mửssbauer spectra for VC and alumina-supported Fe samples treated 2 hours at 900oC with acetonitrile.
Figure 52: Comparison of Mửssbauer spectra for HF washed and unwashed alumina- supported Fe samples treated 2 hours at 900oC with acetonitrile.
(a)
(b)
(c)
Figure 53: Comparison of Fe phase compositions for (a) 2-wt% Fe/Al2O3 – 2 hours, (b) 2-wt% Fe/Al2O3 – 2 hours, HF washed, and (c) 2-wt% Fe/VC – 2 hours.