Raman Observation of the "volcano Curve" in the Formation of Carbonized Metal-Organic Frameworks

The carbonization of various types of metal-organic frameworks (MOFs) was carried out under N₂ gas flow and high temperature. The formation of carbonized MOFs (CMOFs) was monitored by Raman spectroscopy. In addition to the well-known D and G bands in Raman spectra, the salient G′ band feature was observed only in Mn-, Fe-, Co-, and Ni-containing CMOFs. On the other hand, CMOFs containing other metals (Al, Cr, V, Cu, and Zr) do not show the G′ band. Furthermore, the G′ band was also observed when we mixed the nitrate salts of Mn(II), Fe(III), and Co(II) with Al-containing MOFs using the same treatment conditions as in the formation of CMOFs. The G′ band is known to be related to the stacking order of graphitic layers. The presence of the Raman G′ band in CMOFs can be ascribed to the catalytic activity of Mn, Fe, Co, and Ni. The trend of the G′ band to G band intensity ratio resembles the "volcano curve" in the description of the behavior of catalytic activities of transition metals. The G′ bands in Mn-, Fe-, Co-, and Ni-containing CMOFs were well-fitted with two-component peaks which indicates that these CMOFs have well-stacked graphitic structures.