Development of innovative processes and materials for the CO2 conversion and separation from mixtures of combustion products

The present thesis work has then been addressed to the study and implementation of the CO2 conversion process activated via photochemical irradiation. Particular focus has been given to the investigation of the reactivity, as photoactivators, of different silicate-based materials characterized by low cost and abundance: olivine, a natural silicate-based mineral class, and industrial waste products such as the steel slags produced by electric arc furnaces. Indeed, olivine, structurally characterised by a solid solution of Fe2SiO4 (fayalite) and Mg2SiO4 (forsterite), are among the most widespread classes of minerals on Earth’s crust. On the other hand, steel slags are a by-product obtained in large amount every year as the result of the industrial processes for the steel production. The common point that relies on these two classes of materials that have been investigated in this thesis, is the ability to provide a basic reaction in water solution, which, in turn, may facilitate CO2 dissolution in H2O. Redox and acid base natural reactions activated on such silicates in presence of water may fix the CO2 in form of metal carbonates and may produce a reducing environmen. The possibility to resort to raw materials or to industrial slags to promote CO2 conversion processes is, of course, a subject of great interest, either for basic science investigation, or in view of potential application purposes. However, the available literature data, relevant to the investigation of such reactivity behaviour, highlight that the above processes are, nowadays, characterized by slow kinetics that prevent the application on large scale. The solar-driven CO2 conversion processes are considered nowadays, instead, a more promising route to achieve a large-scale application. While pristine olivine minerals and steel slags are good candidates as substrates and activators for CO2 conversion strategies for their abundancy, low cost and reactivity, their poor photochemical efficiency represent a drawback that limited the research aimed at their actual implementation. The present work deals with such issue, addressing the problems posed by solar-driven CO2 conversion processes on silicate-based materials. The project purposes were to achieve the photochemical activated CO2 conversion processes on the silicate-based materials through their eventual modification, to increase the efficiency of the CO2 conversion processes, to identify and quantify the products of conversion, and to possibly evaluate the kinetics of the conversion processes. Considering the needs for a closed-circle CO2 re-utilization, the possibilities of utilization of metal carbonates produced by CO2 fixation have been investigated. In particular, as a possible industrial application, the possibility to increase the permeation efficiency at low temperatures of carbonate-based dual phase membranes have been investigated.

The possibility to resort to raw materials or to industrial slags to promote CO2 conversion processes is a subject of great interest, either for basic science investigation, or in view of potential application purposes. The thesis work has been addressed to the study and implementation of the CO2 conversion process activated via photochemical irradiation. Particular focus has been given to the investigation of the reactivity, as photoactivators, of different silicate-based materials characterized by low cost and abundance: olivine, a natural silicate-based mineral class, and industrial waste products such as the steel slags produced by electric arc furnaces. Redox and acid base natural reactions activated on such silicates in presence of water may fix the CO2 in form of metal carbonates and may produce a reducing environment. While pristine olivine minerals and steel slags are good candidates as substrates and activators for CO2 conversion strategies for their abundancy, low cost and reactivity, their poor photochemical efficiency represent a drawback that limited the research aimed at their actual implementation. The present work deals with such issue, addressing the problems posed by solar-driven CO2 conversion processes on silicate-based materials. The photochemical activated CO2 conversion processes was achieved on the silicate-based materials through their eventual modification, to increase the efficiency of the CO2 conversion processes. GC, XRD and SEM analysis performed were able to identify and quantify the products of conversion, and lead to the evaluation of the kinetics of the conversion processes. The possibilities of utilization of metal carbonates produced by CO2 fixation have been investigated. In particular, as a possible industrial application, the possibility to increase the permeation efficiency at low temperatures of carbonate-based dual phase membranes have been investigated.