Fig. 2.

IR spectra of mixed silicalite and CO₂ (thin sample, approximately 1 μm), showing the formation of silicon carbonate. The compound is identified by the A, B, and C peaks, assigned to unidentate, bidentate, and bridged silicon carbonate species, respectively. (Upper) IR spectra before and after the heating to 740 K. Heating lasted for about 1 h. Gray dashed line indicates spectrum of carbonia (13). The peak at 2,400 cm^-1 and the high-frequency shoulder at 2,450 cm^-1 are assigned to the ν₃ mode of confined and bulk CO₂, respectively. After heating, the confined CO₂ has almost vanished, and the ν₃ peak is dominated by the component of bulk CO₂, which did not react. (Left Inset) IR spectra of a thick (20 μm) sample, before and after temperature annealing, showing the formation of the strong A and B peaks. (Right Inset) Raman spectrum of the ν₊ line of CO₂ on the 1-μm-thick sample, before annealing. Peaks of confined (asterisk) and bulk CO₂ have an integrated intensity ratio of about 3. (Lower) IR spectra measured upon decreasing pressure, after annealing. The behavior of the ν₃ peak shows that confined CO₂ forms again (sharp component), starting at about 16 GPa; instead, bulk CO₂ (broad component) vanishes, indicating that this material progressively extrudes from the sample chamber. Vertical black sticks (Upper and Lower) indicate frequency of the IR stretching mode of the free CO₃ ion (24). Blue, green, and orange bars (Upper and Lower) indicate spectral ranges, at ambient pressure, of the split high- and low-frequency components of the IR stretching mode of the free CO₃ ion, as found in unidentate (blue), bidentate (green), and bridging, T-CO₃-T (orange) carbonates, obtained from CO₂ adsorption on metal oxides and dissolution in silicate melts (see text). The two components correspond to the antisymmetric and the symmetric C─O stretching modes, respectively, involving the two equivalent oxygen atoms. Pink bar (Lower) indicates spectral range, at ambient pressure, for the highest-frequency IR stretching mode of bridging carbonates also obtained from adsorption of CO₂ on some metal oxides (see text). Vertical, dashed lines (Upper and Lower) indicate room pressure extrapolated frequencies of the A and B peaks.