Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature

Abstract

Carbonic acid (H2CO3) forms in small amounts when CO2 dissolves in H2O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H2CO3 plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO2/H2O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H2CO3 inside celestial bodies. We present a novel method to prepare solid H2CO3 by heating CO2/H2O mixtures at high pressure with a CO2 laser. Furthermore, we found that, contrary to present understanding, neutral H2CO3 is a significant component in aqueous CO2 solutions above 2.4 GPa and 110 °C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C-O-H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H2CO3, a role in subduction related phenomena is inferred.

Figure 1

(a) Images of the CO₂/H₂O mixture before and after laser heating to 1500 °C at 3.5 GPa. The whole sample area was scanned with a CO₂-laser. The laser was switched off at the light area at the bottom of the image. The black piece in the center of the light area is gasket material (platinum). Raman spectra (b) and IR absorption spectra (c) of the sample were recorded at 3.5 GPa after cooling down to room temperature from 1500 °C. Reference Raman and IR absorption spectra of crystalline and amorphous H₂CO₃ are shown in blue for comparison. The vertical dashed lines highlight the observed high pressure absorption bands of H₂CO₃ while the reference spectra were recorded in vacuo. The spectrum recorded at 1.2 GPa shows the decomposition of H₂CO₃ after pressure was reduced to below 2.4 GPa where upon pressure dropped by itself to 1.2 GPa.

Figure 2. The still image (a) and Raman spectra (b) during laser heating at 4.0 GPa.

The arrows indicate the place of measurement – the fluid zones in the inner, directly heated area (i), around the directly heated area (ii), and the solid outer zone (iii). The color of the spectra in (b) corresponds to spectra recorded during heating (red) and after heating (blue). The liquid state around the directly heated area has been confirmed by a video recorded during heating (Supplementary Video S1). Region (iii) is solid during the entire experiment.

Figure 3. Pressure shifts of Raman modes (black stars) and IR modes (black circles) for a 1:1 CO₂/H₂O mixture after preparation of crystalline H₂CO₃ by laser heating to 1500 °C obtained by successive compression.

Reference data for the positions of Raman modes (blue stars) and IR modes (red circles) for crystalline H₂CO₃ are shown at the pressure of the experiment. The weak skeletal Raman mode at 639 cm⁻¹ at 3.5 GPa was not detectable during some experiments and the peak position could not be unequivocally be determined in this experiment.

Figure 4

(a) In situ IR absorption spectra of a 1:1 CO₂/H₂O mixture while heating from 24 °C to 280 °C at 2.4 GPa. The displayed spectra have been divided by the initial spectrum recorded at 24 °C (Supplementary Fig. S1). (b) Reference spectra of amorphous H₂CO₃ (ref. 13) and aqueous solutions of K₂CO₃ and KHCO₃ (ref. 22). The vertical dashed lines show five absorption bands assigned to carbonic acid, all of which are evident in the 280 °C spectrum.