In situ and ex situ Mössbauer spectroscopy studies of iron phthalocyanine adsorbed on high surface area carbon

The physical state of monomeric iron phthalocyanine adsorbed on high surface area carbons has been investigated by using ex situ and in situ Mössbauer spectroscopy. Ex situ measurements have indicated that at relatively high loadings (>15%) the spectrum is characterized by two doublets (δ₁ 0.39 ± 0.01 mm/s (vs. α-Fe), Δ₁ 2.60 ± 0.01 mm/s; δ₂ 0.35 ± 0.02 mm/s (vs. α-Fe), Δ₂ 0.95 ± 0.06 mm/s^-1). The parameters corresponding to doublet 1 compare favorably with bulk FePc probably present in the form of small crystallites dispersed in the support matrix. At low loadings (∼3.5%) only doublet 2 attributed to FePc chemically bound to the carbon surface was observed. The relative fraction of these two iron species has also been shown to be sensitive to the specific method of preparation of the sample. In situ Mössbauer experiments have indicated that the quadrupole splitting decreases about 15-20% upon immersion of the electrode in an alkaline solution, corroborating earlier results obtained by this group. Further evidence concerning the formation of an octahedral complex with OH^- bound in an axial position was obtained by in situ measurements at pH 0. In this case the magnitude of the quadrupole splitting was larger than that obtained in basic solution. Conventional oxygen reduction polarization curves with fuel cell type electrodes prepared from Mössbauer-characterized 15% w/w FePc on XC-72 carbon (Cabot Corp.) have indicated that the crystalline material plays an important role in the overall electrocatalytic process. A thorough discussion of these results as well as a critical review of earlier work have been included.