Vibrational spectra of carbon dioxide adsorbed on nanopore walls at supermonolayer and submonolayer coverage

Abstract

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy is applied to probe the microstructure of CO2 adsorbed in nanoporous glass Vycor with a mean pore radius of 2 nm. CARS spectra of the Q‐branch (1388 cm‐1) were measured at 22 °C at pressures decreasing in a range providing the transition from the condensed phase in the whole volume of the pores to submonolayer coverage on their walls. The spectral behavior demonstrates sequential changes in the fluid state inside the nanopores. The analysis of the experimental results allowed us to characterize and compare the spectral properties of gas‐like, liquid‐like, and surface adsorbed carbon dioxide. The results provide clear evidence that as the surface coverage is decreased, the vibrational frequency shifts down. Copyright © 2014 John Wiley & Sons, Ltd.

Spectroscopic characterization of adsorbate confined in small mesopores: Distinction of first surface‐adsorbed layer, polymolecular layers, and liquid clusters

“…Both the Raman shift and the linewidth show a tendency to slightly increase with pressure. This behavior is in a quantitative agreement with that observed earlier for CO 2 in Vycor at 22°C [31] using collinear CARS geometry. At the same time, the present results have been performed with higher spectral resolution, allowing more detailed analysis of the line shape.…”

“…In particular, there is an easily distinguishable broad peak at~1380 cm −1 , which grows with pressure ( Figure 1a). The peak was observed previously and associated with the surface adsorbate [31] as it dominated at low pressures. Another evident observation is that starting from 21 atm, there appears a bright peak at~1385 cm −1 , which then becomes predominant and finally grows~50-fold in intensity as pressure approaches the bulk saturation pressure ( Figure 1c,d,e, respectively).…”

“…Another evident observation is that starting from 21 atm, there appears a bright peak at ~1385 cm −1 , which then becomes predominant and finally grows ~50‐fold in intensity as pressure approaches the bulk saturation pressure (Figure c,d,e, respectively). This peak, observed previously at different temperatures, is caused by the liquid phase appearing through condensation in the pores. The two considered peaks and their origins seem to be quite clear.…”

“…Fundamentally, the local environment of molecules affects their vibrational levels, leading to that each of the phases appearing in the pores can be identified by specific shift and linewidth of a chosen Raman band. A number of previous studies showed the capabilities of coherent anti‐Stokes Raman scattering (CARS) spectroscopy in characterization of fluid adsorption in mesoporous glass with pores of few nanometers in radius. In particular, initial spectroscopic observations of carbon dioxide surface adsorption and condensation were then supported by calculations based on the model of capillary condensation, leading to a good quantitative agreement for the pores of two sizes .…”

Advances in nonlinear optical spectroscopies: a historical perspective of developments and applications presented at ECONOS

“…Reducing the pore size of the glass enabled an improved investigation of the molecules close to the walls of the pores. With a pore size of 2 nm, it was possible to study the super‐monolayer and sub‐monolayer coverage . The experimental results facilitated the characterization and comparison of the spectral properties of gas‐like, liquid‐like, and surface‐adsorbed carbon dioxide.…”