Diniz da Silva Fernandes, Jorge
(2022)
Vibrational spectroscopy investigations of confined phenomena in mesopores.
PhD thesis, University of Nottingham.
Abstract
Chapter 1
An introduction to this Thesis is provided in this Chapter. It contains descriptions of silica-based mesoporous materials that were utilised in the investigations, and it gives details on their properties and applications, focusing on the water-silica interface and surface functionalisation. The work detailed in this Thesis has utilised vibrational spectroscopy to probe the confinement phenomena in mesopores; as such, this Chapter also provides brief descriptions of Raman and Fourier Transform Infrared (FTIR) spectroscopies, including explanations of the applied spectral analytical tools, such as band fitting, Multivariate Curve Resolution (MCR) and Two-Dimensional Correlation Spectroscopy (2DCOS) methods. Finally, the aims of the Thesis are outlined.
Chapter 2
The investigation of the phase properties of confined carbon dioxide, difluoromethane and ethane in grafted mesoporous silicas was carried out utilising Raman spectroscopy. The Chapter introduces the concepts regarding supercritical fluids and supercritical fluids electrodeposition (SCFED).
An understanding of the phase behaviour and the effects of confinement on these fluids during SCFED will help further understand this process and allow for fine tuning of the deposition product. Previous studies showed a fluid dependant decrease in the critical temperature (Tc) relative to bulk, suggesting the fluid wall interaction is an important factor. However, the effect of the surface chemistry of the templates on this parameter has not yet been fully investigated.
With regards to this, the present work has focused on the application of Raman spectroscopy to assess the critical temperature of carbon dioxide, difluoromethane and ethane confined in porous silica templates with different pore sizes (6-9.8 nm) grafted with trimethylsilane (TMS) functional groups. These results were compared with previous near-infrared (NIR) studies of analogous conditions and Raman findings in unmodified silica materials. The effect of pore surface modification on the fluid-wall interaction potential is evident and is reflected in the change of the pore critical temperature of the fluids when compared to their parameters in unmodified templates. The results are also in good agreement with previous NIR studies.
Chapter 3
In Chapter 3 FTIR spectroscopy has been utilised to probe the structure and vibrational properties of confined water at a wide range of concentrations in mesoporous silicas with different pore sizes and surface properties at low temperatures (290-185 K). A novel transmission FTIR spectroscopic method, combined with spectral analytical tools (band fitting and 2DCOS) have provided a simple yet powerful approach to acquire the quantitative and qualitative information of these confined systems.
The analysis of peaks sensitive to the different H-bonding arrangement of water in the OH stretch region, showed that pore width has a significant impact on the initial confined water structures, with smaller pores (3.6 nm) exhibiting increase percentages of water at the interface, and therefore an increase in low density water species. However, surface hydrophobicity also plays an important role in the distribution of the water arrangements and the respective average coordination number between water molecules. In modified pores, the Cl and TMS functional groups disturb the long-range, fully coordinated network water and the corresponding area percentage decreases in favour of weaker H-bonding species. This suggests that water does not completely freeze under hydrophobic environments within temperatures tested, since the formation of water network can be hindered due to broken H-bonds and agrees with the Gibbs-Thomson relationship.
No clear indication of crystallisation occurring inside the pores was detected and this investigation supports the idea of the coexistence of phases at low temperature by the fact that both characteristic H-bonding and weak/non-H-bonding vibrations were observed at temperatures well below bulk phase transitions.
Chapter 4
Chapter 4 builds on the knowledge of the previous Chapters concerning the surface properties of pores in modified and pristine silicas, and their implications on the confined species structure and adsorption phenomena. This Chapter will focus on the sorption of water from dichloromethane, one of the most widely used haloalkane solvents, into mesopores with varying pore size and chemistry, by utilising transmission spectroscopy in the MIR region for the understanding of the water penetration in these silica materials. The time-dependent spectra revealed that the water-silica interactions are dominant during the sorption processes and are strongly dependent on the quantity of water in the haloalkane but also on the hydrophilic/hydrophobic properties of the silica surfaces and pore width. The spectral intensity changes were further analysed by MCR-ALS and 2DCOS techniques, which indicated that possibly more water molecules are present inside the pores of the non-modified silicas due to enhanced fluid-pore interactions.
By utilising MCR-ALS strictly in the ѵB(H2O) region it was possible to extract sorption kinetic parameters from the estimated concentration profiles. These results indicated that the relaxation times for the two diffusion processes (fast and slow) describing the water sorption from CH2Cl2 are strongly dependent on pore size. Surface properties are significantly limiting during the initial process and imply that favourable water-silica interactions are responsible for the faster rates of this process. The investigation showed that the sorption kinetics are faster in the non-grafted MCM-41 with 3.6 nm pores, suggesting faster water uptakes in comparison with wider and grafted pores.
Chapter 5
This Chapter describes the general experimental procedures, novel methods and materials used in this Thesis. In addition, the apparatus and instrumentation, including the cryostat coupled with the low-temperature high-pressure Raman cell, the variable temperature liquid and low temperature IR transmission cells are described. The analytical methods utilising MATLAB are also provided.
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