Blankenship, L. Scott
(2023)
Understanding the relationship between synthetic conditions, porosity and CO2 uptake capacity of turbostratic carbons.
PhD thesis, University of Nottingham.
Abstract
As of 2021, the atmospheric concentration of CO2 is 412 ppm and continues to rise. It is well established that these concentrations are resulting in rising temperatures, an increased incidence of extreme weather, and thus unspeakably disastrous consequences for all life. It follows then that CO2 capture and removal technologies must be rapidly developed to ensure the longevity of human society. One area of research is CO2 capture via physisorption onto porous materials and in particular on the easily synthesised turbostratic carbon. This application requires fine control over porosity (surface area, pore volume, pore size) according to conditions of sorption, and it therefore follows that both the ability to precisely measure pore sizes, as well as a definitive knowledge of the relationship between CO2 uptake capacity and pore size is needed. This thesis attempts to address all three of these issues.
In terms of routes to activated carbons, this work investigates two principal synthetic methods. Firstly in chapter 4 - developing on the author's previous work - a simplification of the production of turbostratic carbons from unwrapped used cigarette filters (UCFs) was attempted by activation of whole used cigarette butts (UCBs) with KOH. The simplified method resulted in much less porosity as compared to the previous work (maximal A_{BET} of 4300 compared with 1960 m^2 g^{-1} and the samples derived by this method possess a hierarchical - as opposed to narrow, microporous - pore size distribution (PSD). Nevertheless, these new materials may perform well for CO2 capture in Pressure Swing Adsorption (PSA) applications. Pyrolysis of UCB in the absence of a porogen created minimal porosity, perhaps as a result of contaminants present from the ucb wrapping paper.
The other approach to activation used (chapter 5) is a set of methods which have been collectively coined as impregnation routes, i.e. methods which attempt to achieve close contact between precursor and the activating species whilst maintaining a homogeneous distribution of the latter throughout the former. Impregnation was achieved through (i) the hydrothermal carbonisation of sawdust (SD) with KOH prior to pyrolysis, as well as (ii) direct activation of a polymeric sodium salt, sodium carboxymethyl cellulose (NC). In both cases, PSDs achieved were generally narrow and situated principally in the small micropore region, making the products potential candidates for low pressure CO2 capture. Both sets of materials also showed unexpected features, SD-derived samples having extremely low bulk density, and those obtained from NC exhibiting reduction in porosity at surprisingly low porogen:precursor ratios. The latter of these suggests pore formation effects outside of the caustic nature of porogenesis with Na compounds, and is a potential route for further investigation of activation mechanisms.
For materials derived through the synthetic routes mentioned above, reliable, accurate, and efficient isothermal porosimetry proved difficult due to poor diffusion of N2 into the materials' pores. As such, alternative porosimetric techniques were investigated in chapter 6. It was found that dual isotherm porosimetry using O2 and H2 isotherms measured at -196 C results not only in more expedient equilibration of the sorptive-sorbent system, but allows the measurement of sub-angstrom level developments in porosity associated with changes in quantity of porogen used. These subtle developments in porosity are not measurable through traditional porosimetry using N2.
As for improving the understanding of the relationship between CO2 uptake as a function of pressure and pore size, chapter 7 details the development and deployment of the pyPUC which, using experimental PSDs and gravimetric gas uptake isotherms applies a brute force approach to determine the correlation between porosity within some pore width range and CO2 uptake at a given pressure. This is performed for all user-defined pore width ranges and pressures, and correlation coefficients are compared to give optimum pore size ranges, Ω at each pressure. When applied to CO2 uptake on turbostratic carbons, it was confirmed that Ω broadens with increasing pressure. In addition, the relationship between Ω and pressure-dependent CO2 uptake was calculated to a more granular level of detail than has been previously reported. Furthermore, following on from findings in chapter 6 it was found that these relationships at low pressures are best described using dual isotherm O2/H2 porosimetry.
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