Orientador: Jose de Alencar Simoni / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-07-21T15:09:50Z (GMT). No. of bitstreams: 1 Faria_CristianoGomesde_M.pdf: 2559018 bytes, checksum: 2691f93c5a4ec8deaf0efe618179583d (MD5) Previous issue date: 1996 / Mestrado

Surface chemistry is an emerging field that can give detailed insight about the elec- tronic properties and the interaction of complex material surfaces with their neigh- bors. This is for both solid-solid and solid-liquid interfaces. Among the latter class, the silica-water interface plays a major role in nature. Silica is among the most abundant materials on earth, as well in advanced technological applications such as catalysis and nanotechnology. This immediately indicates the relevance of a detailed understanding of the silica-water interface. In this study, we investigate the details of this interaction at microscopic level by analyzing trajectories obtained with ab initio molecular dynamic simulations. The system we consider consists of bulk liquid water confined between two β-cristobalite silica surfaces. The molecular dynamics were generated with the CP2K, an ab initio molecular dynamic simulation tool. The simulations are 25 picoseconds long, and the CP2K program was run on 64 cores on a supercomputer cluster. During the simulations the program integrates Newton’s equations of motion for the system and generates the trajectory for analysis. For analysis, we focused on the following properties that characterize the silica water interface. We calculated the density profile of the water layers from the silica surface, and we also calculated the radial distribution function (RDF) of the hydrogen bond at the silanols on the silica surface. The main focus of this thesis is to write the programs for calculating the atom density profile and the RDF from the generated MD trajectories. The atomic probability density profile shows that water is strongly adsorbed on the (001) cristobalite surface, while the RDF indicates differently ad- sorbed water molecules in the first adsorption layer. As final remark, the protocol and the tools developed in this thesis can be applied to the study of basically any crystal-water interface.

Philosophiae Doctor - PhD / Carbon dioxide emissions have become a major concern as they are one of the contributing factors to the “green-house” effect. Recently, much effort has been put into separating carbon dioxide (CO2) from flue gases linked to the combustion processes at fixed point sources. The development of solid sorbents for adsorption based on CO2 capture has attracted much attention. Ordered Mesoporous Silica (OMS) materials have recently attracted much attention as solid adsorbents for capturing CO2. OMS have been investigated for this purpose owing to their high pore volume, large surface area and ease of functionalizationIn this work we report on the synthesis of OMS viz. MCM-41 and SBA-15 along with amorphous silica as adsorbents for CO2 capture. MCM-41 was prepared with surfactants having different alkyl chain lengths (C14TABr, C16TABr and C18TABr) where TABr is trimethylammonium bromide. SBA-15 was prepared using a Triblock copolymer as a structure directing agent for the organization of polymerizing silica species. Commercial amorphous silica gel was used for comparative purposes. Initial characterization OMS with powder X-Ray diffraction (XRD) and small angle diffraction (SAXS) yielded diffraction patterns which may be associated with well-ordered structures of hexagonal mesoporous material. Ease of preparation for MCM-41 materials allowed for convenient scale- up, obtaining highly ordered mesoporous silica MCM-41 at room temperature. SBA-15 was also found to be scaled up with considerable ease through increasing the volume of the autoclave during hydrothermal treatment. Structural, morphological and textural properties of the adsorbents were characterized by N2 physisorption measurements, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Thermogravimetric Analysis (TGA). TEM confirmed the hexagonal structure; SEM showed that C14MCM-41 had spheroidal particle morphology whereas SBA-15 displayed rod-like structures. High surface areas of up to 1302, 1186, 1211 and 1024 m2/g for C14MCM-41, C16MCM-41, C18MCM-41 and SBA-15, respectively were obtained. The pore size of MCM-41 materials was increased from 2.6 nm for C14MCM-41 to 4.4 nm in diameter for C18MCM-41 using surfactants with different alkyl chain lengths. CO2 adsorption characteristics of OMS were studied using CO2-temperature programmed desorption (TPD). The results showed that C14MCM-41, C16MCM- 41, C18MCM-41 and SBA-15 desorbed 0.19, 0.16, 0.11 and 0.26 mmol/g respectively. The synthesized OMS were then further modified by grafting various amine moieties on their surfaces in order to increase their CO2 adsorptive capabilities. 3-(Aminopropyl)triethoxysilane (APS), N-[3-(trimethoxysilyl)propyl]ethylene- diamine (TPED), 3-chloropropyl)-trimethoxysilane (CPS), ethyl 2-bromopropanoate, tris(2-aminoethyl)amine (TREN) and guanidine. Several characterization techniques such as XRD, SAXS, HRTEM, HRSEM, TGA, Fourier Transform Infra-Red (FT-IR) spectroscopy, CO2 isotherms and CO2-TPD were used to analyze amine grafted solid sorbents for CO2 capture. The results revealed that the structural integrity of the amine modified sorbents was not compromised during the grafting process. The structural properties of the supports, such as surface area and pore size, nature of amine and the number of amine groups, affected the loading and CO2 adsorption capacity of chemically grafted sorbents. APS grafted amorphous silica gel adsorbed 0.67 mmol/g CO2, which proved to be the highest compared to C18MCM-41 and SBA-15 grafted with APS and TPED. C18MCM-41, SBA-15 and amorphous silica gel were also grafted with TREN and adsorbed 0.42, 0.51 and 0.27 mmol/g of CO2, respectively. A systematic study of guanidine grafted on C14MCM-41, C16MCM-41 and C18MCM-41 was investigated, for the first time, to the best of our knowledge. Structural properties like variation in pore size, proved to enhance the adsorption capacity of the adsorbent, coupled with the guanidine molecules grafted on MCM-41 materials. C18MCM-41-guanidine showed the highest CO2 uptake of the guanidine grafted MCM-41 materials, adsorbing 0.30 mmol/g. A novel synthetic route to TPED-TREN grafted C18MCM-41 and SBA-15, using ethyl 2-bromopropanoate as a linker, was investigated as a potential adsorbent for CO2 capture, for the first time. C18MCM-41-TPED-TREN and SBA-15-TPED-TREN CO2 adsorption capacity were found to be 0.14 and 0.14 mmol/g, respectively.

The formation of a laser-driven shock in fused silica was observed experimentally. Fused silica slabs were irradiated with 0.53 /µm laser light in a pulse of 2 ns FWHM at intensities ranging from 10¹² W/cm² to 5 x 10¹³ W/cm², producing a pressure pulse ranging from < 30 GPa to 500 GPa. Shock trajectories were observed using streaked shadowgram and schlieren methods. The experiment was modelled with a one-dimensional Lagrangean laser-plasma hydrocode. Comparison of the simulation results with the experimental observations indicate that the high pressure shock develops anomalously slowly at intensities > 1 x 10¹³ W/cm². Furthermore the shock displayed non-steady propagation for a transient period following its formation. The non-steady propagation is interpreted to be due to a relaxation process in the phase transformation of the fused silica to the high pressure stishovite phase which occurs in the shock front. The slow formation of the shock at high intensities is consistent with a significant volume collapse (phase transition) possibly induced by isentropic compression; however, this interpretation is uncertain due to the complications introduced by non-equilibrium thermodynamics and the possibility of two-dimensional motion. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A sílica obtida da queima de casca de arroz foi convertida nas fases &#945; e &#946;. A estabilização da fase &#946; foi obtida pela introdução de alumínio e cálcio na rede de sílica, 10001100 &#176;C, na proporção CaO:2Al2O3:38SiO2. Foi observado que a transição de fase &#945;&#8594;&#946; da cristobalita &#945; tem sua temperatura de transição reduzida de 175&#176;C bem como sua entalpia de transição, de 16,0 J/g para 7,0 J/g. Esse efeito é causado pela introdução de íon K+, presente na casca de arroz, na estrutura da sílica. O processamento empregado nessa conversão usa equipamentos normalmente encontrados em empresas de cerâmica tradicional moinho, atomizador, prensa e forno / Silica generated from burned rice shells has been converted to the &#946; and &#945 phases of cristobalite. The &#946; phase stabilization was acomplished by instilling aluminum and calcium into the silica structure, at 1000-1100 &#176;C, in the following proportion CaO:2Al2O3:38SiO2. The phase transition &#945;&#8594;&#946; of the &#945;-cristobalite has a lower transition temperature, as compared with cristobalite made from pure silica, from 270&#176;C to 170&#176;C. Also the transition entalphy was reduced from 16,0 J/g to 7,0 J/g. These changes were attributed to the K+ ion present in the silica structure of the burned silica. The process used in this convertion makes use of standard equipments, regularly used in ceramic producing companies mill, spray dryer, furnaces, and press.

The increasingly stringent environmental regulations worldwide demand the use of efficient methods for air purification. Moreover, the alarming effect of greenhouse gases on the world climate requires the removal and sequestration of large quantities of anthropogenic carbon dioxide (CO2). This work is contributed towards the development of efficient, amine-containing, lamellar structured silica adsorbents for CO2 removal. Seven different materials were prepared by impregnation of various amounts of PEI, over as synthesized, or partially extracted or calcined lamellar silica. Materials were characterized by powder XRD and SEM. CO2 adsorption capacity was measured by thermogravimetry. The effects of PEI loading, temperature, CO2 partial pressure and surface alkyl chains were investigated. PEI seems to be dispersed better in a consistent surface alkyl chain network, leading to enhanced CO2 uptake. VB-13, the material with 50 wt% of PEI, recorded the highest CO2 uptake at 75 °C, in the presence of both 15% CO2/N2 and 100% CO2 with values of 3.02 and 3.50 mmol/g respectively. The optimum temperature for CO2 uptake was found to be 75 °C for samples with high PEI loading. Moreover, higher uptake was recorded in the presence of 100% CO2 versus 15% CO2/N2 for all temperatures. Another objective of this study was to investigate the effect of humidity on the CO2 adsorption process. In that case use of the column-breakthrough technique coupled with mass spectrometry to discriminate between CO2 and water was considered. Complete understanding of the technique and the different effects of moisture on CO2 adsorption over amine-containing materials, namely promotion of CO2 uptake and stabilization of the adsorbent, were achieved, based on a thorough scrutiny of the literature. Nonetheless, because of the Covid-19 pandemic and several technical issues, some experiments could not be undertaken.

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<p>Microorganisms inhabiting silica solute-rich environments often show various degrees of Si02 mineralization as a consequence of exposure to SiOrsaturated waters. As such, it has been thought that microorganisms exert a prominent role in the immobilization of amorphous silica phases. While this intimate spatial relationship of microorganisms and amorphous Si02 phases are almost always observed in hot springs, the exact mechanisms by which microbes affect Si02 secondary mineral precipitation is still poorly understood. Further, available laboratory investigations to date consistently showed that microbes do not significantly impact Si02 immobilization, suggesting that microbial silicification is a mere consequence of exposure to a largely abiogenically-driven Si02 precipitation. </p> <p>This study demonstrates that discernible microbially-mediated silicification can occur under conditions where the potential for microbial opportunity to biomineralize is promoted. Identification of the key geochemical requirements for biosilicification to occur include thermodynamically favorable, but sluggish silica reaction kinetics associated with acidic conditions, and the necessity for colloidal silica rather than dissolved silicic acid species. This work provides the first results to bridge the apparent literature discrepancy between widespread, in-situ observations of microbial silicification, and the inability to demonstrate a detectable microbial effect in this process under well-constrained laboratory conditions.</p> <p>Acid conditions promote microbial silicification by overriding the dominant repulsive forces arising from charge similarities between Si02 and cell surfaces, via neutralization of deprotonated surface silanol and carboxylic groups, respectively. Mechanistic consideration for Si02 coordination to cell surfaces suggests direct chemical bonding of silanol to carboxylic groups forming stable inner-sphere complexes largely insensitive to environmental perturbations. This result indicates that microbially immobilized Si02 are more tenaciously-coordinated on cell surfaces and not simply electrostatically-held.</p> <p>Surface-dependent silicification showed higher Si02 mineralization propensities for unmineralized microbial cells compared to silica-encrusted cell matrices. Moreover, the extent and style of microbial Si02 mineralization is impacted by cellular level of metabolic activity. These results suggest that a biological overlay may be discernible in microbially induced biosilicification. </p> / Thesis / Doctor of Philosophy (PhD)

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