Climate change and global warming caused by the increasing greenhouse gases emissions (such as CO2) in the atmosphere recently attract the attention of the scientific community. These large emissions have been correlated to the Global Warming effect which has many consequences across the globe, including glacial retraction, ocean acidification and increased severity of weather events. With green technologies still in the infancy stage, it can be expected that CO2 emissions will stay this way for a long time to come. It is necessary to find an alternative way to get rid of the resulting environmentally harmful emissions. A promising solution is the use of CO2-free electrical energy produced, for example, by renewable or nuclear sources, for dissociation of CO2 or other greenhouse gases, followed by their conversion into easily storable fuels. In this context, the CO2 re-utilization to synthesize syngas, fuels or chemical compounds as well as pure CO2 dissociation into CO and O2, is of a special interest. Among the different methods to convert CO2 into added-value products (thermolysis, thermochemical cycles, electrolysis, photocatalysis, etc), the discharges sustained by microwave radiation combining high electron and low gas temperature have already demonstrated huge potential for plasma-assisted CO2 conversion. The present research work is targeted to the systematic investigation of the microwave-assisted conversion of various CO2-based gas mixtures being especially focused on plasma catalysis. The different physical effects affecting the efficiency of plasma catalysis are considered, for a better understanding of the synergistic effects between plasma and catalyst. The characterization of microwave discharges is performed by various plasma diagnostics methods, including optical spectroscopy and gas chromatography. In addition, the catalysts have been characterized by the state of art material characterization techniques, such as Transmission electron microscopy (TEM), Raman spectroscopy, etc. Such a combined characterization of both plasma and catalysts is performed for the sake of better understanding of the plasma-catalytic processes.In the first part of this study, the different dissociation pathways of the studied molecule as a function of different plasma parameters are considered by evaluating the composition with different plasma diagnostic techniques. A simple increase of Specific Energy Input (SEI) is not a promising solution since in this case the energy efficiency drops. The beneficial effects of lowering the pulse frequency for increasing CO2 conversion efficiency are observed and discussed. The obtained results are explained by the relation between the plasma pulse parameters and the rates of the relevant energy transfer mechanisms in the discharge. Simultaneous dissociation of CO2 and H2O has been investigated as well. It was clearly demonstrated that both H2 and CO productions are strongly affected by the different plasma parameters. The second part of this study deals with the effects of catalyst preparation method, nature of plasma activation gas, gas admixture, as well as NiO content and their influences on the CO2 conversion and energy efficiencies in microwave plasma. It was found throughout this work that the catalyst preparation method has a significant effect on the chemical and physical properties of the catalysts, which in turn strongly influences CO2 conversion and energy efficiencies of this process. In particular Ar plasma treatment results in a higher density of oxygen vacancies and a very favorable distribution of nickel oxide on the TiO2 surface. It is concluded that, the oxygen vacancies are the key factor explaining high catalytic activity in CO2 decomposition. The dissociative electron attachment of CO2 at the catalyst surface enhanced by the oxygen vacancies and plasma electrons can explain the observed increase of CO2 conversion efficiency as well as the energy efficiency. A mechanism explaining the observed plasma–catalyst synergy is proposed. The overall aim is to establish a model describing the interaction between highly reactive species produced in plasma discharge and supported catalyst for the conversion of CO2 into useful compounds. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Photocatalysis is an advanced oxidation process, which has shown to possess an enhanced capability to remove a wide range of contaminants. It involves the use of a semiconductor photocatalyst and a photon source. Photocatalysis has several advantages such as mild reaction conditions like ambient temperature and pressure, good control over the reaction and faster reaction kinetics. Semiconductor photocatalysts such as TiO2, ZnO, Fe2O3, CdS, ZnS, etc. absorbs light of energy greater than or equal to its band gap and the electron in the valence band gets excited to conduction band leaving behind the hole in valence band. These charge carrier pair results in the formation of various reactive oxygen species such as hydroxyl and superoxide radicals which results in the degradation of chemical contaminants and inactivation of microorganisms. TiO2 is the most widely used catalyst in photocatalytic studies because of its high photocatalytic activity, non-toxicity and wide availability. Anatase phase TiO2 has been reported to possess higher photocatalytic activity than the rutile phase. Although there are several methods to synthesize TiO2, solution combustion synthesis is a single step process to produce pure anatase phase TiO2. The catalyst produced by this method has been shown to be superior to the commercially available Degussa P-25 catalyst for the degradation of various chemical contaminants. The present investigation focuses on the use of combustion synthesized catalyst for the inactivation of microorganisms. The photocatalytic activity was compared with commercial Degussa P-25 catalyst. The various aspects of photocatalytic inactivation reactions studied in this dissertation are: i) photocatalytic inactivation of microorganisms in presence of UV light, ii) effect of various parameters on the inactivation, iii) photocatalytic inactivation in presence of visible light, iv) use of immobilized catalyst for the photocatalytic inactivation, v) understanding of mechanism and kinetics of inactivation. Combustion synthesized TiO2 (CS-TiO2), combustion synthesized 1% Ag substituted TiO2 (Ag/TiO2 (Sub)) and 1% Ag impregnated CS-TiO2 (Ag/TiO2 (Imp)) were used as photocatalysts. The catalysts were characterized by powder XRD, TEM, BET surface area, UV-Vis spectroscopy, TGA and photoluminescence spectroscopy. The photocatalytic inactivation experiments were carried out using E. coli (K-12 MG 1655), a bacterial strain and P. pastoris (X-33), a yeast strain, as model microorganisms. The results demonstrate higher photocatalytic activity of all the combustion synthesized catalysts than commercial Degussa P-25 catalyst. The optimum catalyst concentration was 0.25 g/L and the maximum inactivation was observed in the presence of Ag/TiO2 (Imp) catalyst. Rapid and complete inactivation of the microorganisms was observed at lower initial cell concentrations. A reduced photocatalytic inactivation was observed in presence of various anions (HCO3¯ , SO4 2¯ , Cl¯ and NO3¯ ) and cations (Na, K, Caand Mg). Even a small addition of H2O2 was observed to improve the photocatalytic inactivation. At higher dosage of H2O2, a 2 min exposure was sufficient to result in a complete inactivation. Changing the initial pH of the solution was observed to have no significant effect on the photocatalytic inactivation. All the combustion synthesized catalysts showed higher activity as compared to those obtained with commercial Degussa P-25 TiO2 in presence of visible light. The higher photocatalytic activity of combustion synthesized TiO2 can be attributed to the lesser crystallite size, higher surface area, large amount of hydroxyl groups and decreased band-gap energy of the catalyst. The present study demonstrates the potential use of catalyst immobilized thin films for the photocatalytic inactivation of E. coli in the presence of UV light. The CS-TiO2 catalyst was immobilized on glass substrate by LbL deposition technique. The performance of immobilized CS-TiO2 was compared to commercial Degussa Aeroxide TiO2 P-25 (Aeroxide) catalyst. The effect of various operating parameters like catalyst loading, surface area and number of bilayers on inactivation has been investigated. It was observed that increasing the number of bilayers and the concentration did not influence the inactivation but increased surface area led to an increase in inactivation. It was observed that the catalyst immobilized on glass slides can be used for repeated experimental cycles with the same efficiency. It was observed that the inactivation process can be studied in continuous mode by using catalyst immobilized on glass beads. The work also focused attention towards understanding the microorganism inactivation mechanism and kinetic aspects. Various microscopy techniques such as optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the inactivation mechanism. From the images obtained, it was suggested that the inactivation is caused due to rupture of cell wall. The mechanism was also examined by carrying out degradation experiments on cell component such as protein and media component such as dextrose. UV alone was observed to degrade protein and the presence of catalyst showed no additional effect. On the other hand, dextrose does not respond to photocatalytic degradation even at a lower concentration. The photocatalytic degradation of Orange G dye was reduced by addition of dextrose sugar or protein which shows a possibility of competitive degradation. The kinetics of inactivation was studied by various models available in literature such as the power-law model, Chick-Watson model, modified Hom model, GInaFIT tool and a Langmuir-Hinshelwood type model. It was observed that power-law based kinetic model showed good agreement with the experimental data. A mechanistic Langmuir-Hinshelwood type model was also observed to model the inactivation reactions with certain assumptions.