Gas hydrates and Metal Nanoparticles for the Energy Transition and Environmental Applications

The word energy outlook 2017 of the International Energy Agency (IEA) predicted an increase in natural gas energy demand until reaching a quarter of global energy demand in the new policies scenario by 2040, becoming the second-largest fuel in the global mix after oil.1 Nevertheless, the EIA annual report of 2022 reports a change in the scenario due to the war highlighting that the demand for natural gas will not follow the trend predicted in 2017, but it should slightly increase until 2050 (Figure 1). In this regard, the first two chapters of this thesis aim for two different solutions to contrast CO2 emissions and simultaneously obtain an energy source. Chapter 1 concerns the study of gas hydrates for CO2 storage and CH4 harvesting; the results reported in this chapter are also related to the PRIN 2017 (Progetti di Rilevante Interesse Nazionale) project. Gas hydrates are ice-like compounds present in nature that contain high amounts of methane; therefore, there is great interest in studying these compounds to understand how to exploit this energy resource without impacting the environment where they are present. The CO2 injection in CH4-hydrate reservoirs represents an attractive solution. This method allows the substitution of CH4, initially present, with CO2 by exploiting the greater stability of CO2-hydrates compared to methane ones. Accordingly, an imbalance of the natural deposits would not be created, and it would be possible to obtain CH4 to be used as an energy source and simultaneously store the CO2 to limit the greenhouse effect. Therefore, extracting natural gas from CH4-hydrate reservoirs and simultaneously storing CO2 can be considered a valid strategy for the energy transition phase. Natural gas hydrates are mostly found in sediments under the seabed or in the permafrost region, and the physical and chemical properties of marine water influence the phase behavior of NGHs due to the perturbation of the hydrogen bonds during the formation of hydrate structures. In this regard, Section 1.2 reports the chemical composition of leached water from marine sediment samples collected at the National Antarctic Museum in Trieste, was obtained by inductively coupled plasma-mass spectrometry (ICP-MS) and ion chromatography (IC) analysis. The ICP- MS and IC analysis identified sodium as the most abundant cation with a not negligible amount of calcium, potassium, and magnesium, whereas chloride and sulfate have been identified as the most abundant anions. The chemical characterization identified a total salinity of 47.54 ‰ in the leached water. The obtained values were used as starting point to determine the thermodynamic parameters and have provided helpful information regarding the chemical- physical parameter of water in which the gas hydrates occurred in nature. Section 1.3 reports the study of the thermodynamic conditions of methane and carbon dioxide clathrate hydrates in pure quartz porous sand at two different brine concentrations (NaCl = 0.030 and 0.037 wt%). The results revealed that, despite the presence of brine, the porous medium promotes the formation of hydrates due to an increase in the surface-to-volume ratio and the improvement of heat transfer. Interestingly, it has been demonstrated that the promoting effect of sand is a temperature-dependent process occurring at temperatures above 3 – 5 °C when the brine is present in the system. Exploiting the similarity of ice structure with gas- hydrate structures, Raman microscale measurements were performed to gather information about the influence of sediments, salt, and temperature on the OH-stretching vibrations of water. The obtained results clarified how the addition of NaCl and or sediments to water under different temperature conditions (288.15 K and 258.15 K) influenced the water hydrogen bonds. Specifically, the changes of OH-stretching vibrations, when correlated with the NaCl concentrations, demonstrated that the presence of sediments partially inhibited the salt effects in the ice water, probably due to hydrophilic interactions with the silanol groups of sediments. In addition, SEM measurements showed morphological information on sediments and on the arrangement of ice around the quartz porous sand particles. Section 1.4 focuses on synthesizing CO2-hydrate in ultrapure water, in presence of sand, and binary CO2 /CH4 hydrates and the relative Raman and SEM characterization. Specifically, the hydrates were produced in a small–scale reactor and ex-situ analyzed by Raman spectroscopy that confirmed the gas uptake in the hydrate structures by identification of the fingerprint of CH4 and CO2 occupancy in the hydrates. The analysis of water Raman spectra permitted the description of the relationship between symmetric and asymmetric OHs bands but also provided information about the characteristics of water inside the different GHs, showing that the least ordered water structure was that of GHs containing sand. In contrast, the most ordered one was present on binary CO2 /CH4 hydrates. The morphological features of GHs were analyzed by a Field Emission Scanning Electron Microscopy instrument which allowed to distinguish the hydrate phase from the ice phase. In Section 1.5 single CH4 and CO2 hydrates and mixed CH4/CO2-hydrates (obtained from replacement processes) prepared in ultrapure water, seawater, SDS, natural sand, and artificial sand were synthesized by the University of Chieti and were ex-situ analyzed in our laboratory by Raman spectroscopy and Scanning Electron Microscopy as the main context of this thesis. The morphological characterization of GHs is becoming more and more interesting for the study of the formation and dissociation of GHs; however, the SEM images of the GHs are limited and difficult to interpret, therefore SEM images of different samples prepared under known conditions it is proposed to be used as a reference standard for the identification of GHs. Raman measurements confirmed the hydrate structure's gas uptake by identifying the CH4 and CO2 fingerprint, CH4 cage occupancies, hydration number, and structural changes in water molecules. Pure CH4-hydrates displayed almost (> 95 %) full occupation in the large cage and a significant change in the small cage occupation related to the different environments used. The cage occupancy calculation of CO2/CH4-hydrates showed that ultrapure water is the best environment to obtain a high replacement yield followed by natural sand and seawater by using the CO2 exchange method. Contrarily to the solutions proposed in Chapter 1, whose goal is to replace CH4 with CO2 in natural gas hydrates and store CO2 in the hydrate form, Chapter 2 aims to address the problem of CO2 emission and energy demand by the electro-reduction of CO2, also called CO2 reduction reaction (CO2RR), to valuable fuels and chemicals. The reduction process is governed by a multi-step-based coordination chemistry comprising two, six, eight, and twelve electrons for the formation of C1 products (CO, methanol, CH4, HCOOH) and more valuable C2+ products (C2H4, EtOH, C2H6, CH3COOH). The CO2RR is carried out at the electrode/electrolyte interface, acting as an electrocatalyst with the respective electrode and an electrolyte. The CO2RR needs a catalyst to break the C=O bonds and suppress the competing H2 evolution reaction. Cu is unique from the available catalysts due to the possibility of providing multi- carbon products, including many hydrocarbons and oxygenates. The use of copper foil is not enough to give selectivity toward a specific product; however, copper nanocrystals have proved to be helpful for driving product selectivity due to the increased surface-to-volume surface area and the possibility to have different facets. Indeed, the low and high-index facets have different amounts of low-coordinated atoms, such as edges, steps, and kinks, and hence they demonstrate different activities and selectivity during the CO2RR. Another approach to drive the catalyst's selectivity is using a bimetallic catalyst. In the alloy phase, the electronic and geometric structures of parent nanomaterials are modified by creating new reaction pathways, which lead to the formation of a specific product. In this regard, in chapter 2 is reported the work carried out at the EPFL in synthesizing CuAg nanoalloy and its application as a catalyst for the CO2RR. Heavy metal pollution is another global environmental problem, and Chapter 3 reports the application of Ag-nanoparticles for the colorimetric detection of metal ions. Heavy metals in contaminated wastewater pose several risks to humans through assimilation pathways such as the ingestion of plant material (in the food chain) and the development of colorimetric sensor- based nanoparticles allowing rapid identification of metals by exploiting the plasmonic properties of Ag-nanocrystals. Specifically, the study presented in Section 3.3 is focused on the synthesis of etched AgNPs by exploiting the oxidation etching property of H2O2. The obtained etched AgNPs were successively characterized by UV-Vis, dynamic light scattering, and scanning electron microscopy. Successively the synthesized triangular Ag-nanoplates have been investigated as plasmon sensors for Hg ions detection in solution due to the localization of stronger near-field enhancements at their vertices. The triangular Ag-nanoplates displayed high selectivity toward Hg2+ with respect to all the investigated metal ions (Zn2+ , Cu2+, Fe2+, Ca2+, Mg2+, Cd2+, Cr2+ , Co2+, K2+, Na2+, Pb2+, and Mn2+). Additionally, a linear blue shift of the SPAB (Surface Plasmon Absorption Band) was observed upon the addition of different concentrations of Hg2+ (0.14 to 0.5 mg l−1), allowing the qualitative detection of Hg2+ in this range with a LOD of 0.013 mg l−1 (64.9 nM). The mechanism proposed for this type of sensor is related to an additional etching of the triangular nanoplates which leads to the formation of hexagonal and pentagonal nanoplates due to the erosion effect of Hg2+ . The experimental work presented in Section 3.4 regards the functionalization of AgNPs with different density layers of Mercaptoundecanoic acid (11MUA) and their applications as a colorimetric sensor for the detection of metal ions. UV-Vis spectroscopy was used to monitor the functionalization processes and to investigate the aggregation behavior of each AgNPs@11MUA sensor upon titration with the metal ions of interest, namely Ni2+, Zn2+, Co2+ , Cd2+, Mn2+, and Cu2+. The resulting UV-Vis raw data obtained for each layer density were submitted to principal component analysis to dissect the role of the metal ions in NP aggregation and in establishing the sensitivity and selectivity of the AgNPs@11MUA sensor. Interestingly, we observed an increase in sensor sensitivity and selectivity at a lower density of the functionalizing agent on the AgNPs’ surface, which results in characteristic colors of the NP suspension upon titration with each metal ion. Section 3.5 is focused on studying the aggregation mechanism in the case of full-monolayered FL–AgNPs@11MUA with Ni2+. Specifically, the application of the Hill equation on the SPABs of FL-AgNPs@11MUA triggered by Ni2+ allowed the distinguishing of three different linearity zones, with individual slopes, suggesting the existence of a different average number of bridging Ni2+ (-Ni2+ -) between AgNPs@11MUAs at different concentrations of free Ni2+, with 1, 5/6, and 12 (-Ni2+ -) in the concentration ranges 0–3, 3–7 and 7–10 μM, respectively. Additionally, the intermediate zone (3–7 μM) corresponded to the linearity range of the proposed AgNPs@11MUA sensor, with the AgNPs@11MUA cluster linked by (-Ni2+ -) = 5/6, thus being inferred as the ideal sensing cluster.