Cette thèse porte sur l’étude de l’adsorption d’une molécule de dihydrogène sur différentes surfaces métalliques et plus particulièrement de la physisorption du dihydrogène sur des surfaces d’argent et de cuivre. Ce type d’adsorption met en jeu des interactions faibles de type van der Waals et se déroule relativement loin de la surface. A ce jour, la modélisation précise de ces systèmes (molécule physisorbée sur un métal) reste un défi. Deux types d’approches permettent d’obtenir une surface d’énergie potentielle. La première consiste à modéliser la surface métallique par un cluster et à utiliser des méthodes de calcul hautement corrélées et la seconde à considérer la périodicité de la surface et à utiliser la théorie de la fonctionnelle de la densité. Notre objectif était de coupler ces deux approches, qui ont chacune leurs avantages et leurs inconvénients, en utilisant une méthode d’incorporation. Nous avons utilisé le code VASP et la fonctionnelle vdW-DF2 pour obtenir une surface d’énergie potentielle considérant 5 des 6 degrés de liberté de la molécule de dihydrogène au dessus des différentes surfaces. Les caractéristiques de ces potentiels nous ont permis de réduire l’étude à 2 degrés de liberté (l’altitude du dihydrogène et sa rotation dans un plan perpendiculaire à la surface) pour l’approche par les clusters, qui a été réalisée avec le code MOLPRO. Nous avons comparé les données expérimentales et calculées afin de tester l’exactitude de la surface d’énergie potentielle. Pour cela, nous avons calculé les niveaux d’énergie ro-vibrationnelle de la molécule H2 physisorbée sur les différentes surfaces, en utilisant les différents potentiels 2D et nous les avons comparés avec les données des expériences de spectroscopie par perte d’énergie électronique. Pour les systèmes à dimension réduite, les niveaux d’énergie ro-vibrationnelle ont été obtenus en diagonalisant l’Hamiltonien nucléaire et en faisant une dynamique quantique par la méthode de Splitting. Pour simuler le spectre ro-vibrationnel, nous avons calculé la fonction de moment dipolaire du système suivant son orientation, par la méthode des champs finis avec des méthodes de calcul hautement corrélées. Le potentiel d’incorporation donne les résultats les plus proches de l’expérience pour les systèmes Cu(100)/H2 et Ag(100)/H2. En revanche, pour le système Ag(111)/H2 c’est le potentiel obtenu en modélisant la surface par un cluster qui donne les meilleurs résultats / This thesis concerns the study of the adsorption of a hydrogen molecule on various metallic surfaces and more particularly of the physisorption on silver and copper surfaces. This type of adsorption involves weak interactions of van der Waals type and takes place relatively far from the surface. To date, the accurate description of these systems (molecule physisorbed on a metal) is still a challenge. Two approaches allow obtaining a potential energy surface. The first one consists in modelling the metallic surface by a cluster and using highly correlated methods of calculation and the second one is considering the periodicity of the surface and using the density functional theory. Our objective was to couple these two approaches, which have their own advantages and their inconveniences, by using an embedding method. We have used the code VASP and the vdW-DF2 functional to obtain a potential energy surface considering 5 of the 6 degrees of freedom of the hydrogen molecule over the various surfaces. The characteristics of these potentials allowed to limit to 2 degrees of freedom (the height of the hydrogen molecule and its rotation in a perpendicular plan of the surface) for the study on clusters, which was achieved with the MOLPRO code. To check the accuracy of the potential energy surface we have compared the experimental data and the calculated ones. For that purpose, we have calculated ro-vibrational energy levels of the H2 molecule physisorbed on the various surfaces, by using the 2D potential determined with the various approaches and we have compared them with the experimental data obtained with electron energy loss spectroscopy. For the reduced systems, ro-vibrational energy levels were obtained by diagonalisation of nuclear Hamiltonian and with quantum dynamics by the Splitting method. To simulate the ro-vibrational spectrum, we calculated the dipole moment function of the system following its orientation, by the finite fields method with highly correlated methods of calculation. The embedding potential gives the closest results to experiment for the systems Cu(100)/H2 and Ag(100)/H2. On the other hand, for the system Ag(111)/H2, the potential obtained in modelling the surface by a cluster gives better results

No description available.

Konstanz, Univ., Diss., 2004.

Les recherches de cette thèse avaient pour but la compréhension de l’influence sur les parois de nanotubes de carbone de divers traitements post-synthèses : purification, recuit, découpage ultrasonique et fonctionnalisation. Si ce travail porte principalement sur la physisorption de gaz rares, plusieurs autres techniques d’études complémentaires ont également été utilisées telles que la microscopie électronique, l’analyse thermogravimétrique et la diffraction des rayons X. Une méthode systématique a été proposée pour déterminer le début et la fin des marches, souvent mal définies, qui sont contenues dans les isothermes représentant l’adsorption de gaz sur des tubes mono-parois. Nos résultats s'intéressent en particulier (i) à l'estimation du taux d’ouverture des tubes suivant certains des traitements précités, (ii) à l’applicabilité de la méthode BET au-delà de ses limites habituelles et (iii) dans le cas d'un traitement de fonctionnalisation, à la sélectivité des procédés de greffage sur les surfaces des nanotubes par rapport à celles correspondantes à d’autres espèces carbonées présentes dans un même échantillon. / The aim of this study was to understand the influence of various post-synthesis treatments on carbon nanotube surfaces: purification, annealing, ultrasonic cutting, and functionalization. This work deals mainly with rare gas physisorption, but other complementary techniques have also been used such as electron microscopy, thermogravimetric analysis and X-ray diffraction. A systematic approach has been proposed to determine the beginning and the end of the steps (which are usually not well-defined) on the isotherms corresponding to the gas adsorption on single-wall nanotubes. Our results focus in particular on (i) the estimation of the tube opening rate after several of the above mentioned treatments (ii) the validity of the BET method beyond its usual limits and (iii) in the case of functionalization, the selectivity of the grafting process on the carbon nanotube surfaces as opposed to that on the other carbon species present in a sample.

For the majority of tailings substrates, flocculant adsorption proceeds through hydrogen bonding of the amide functionalities with neutral surfaces. However, flocculation of Bayer process residue solids takes place in highly caustic liquors, typically using high molecular weight polyacrylates. This represents an almost unique situation, and implies a totally different adsorption mechanism. Direct examination of polyacrylate adsorption within the complex matrix of real liquors and a mixture of residue phases is difficult, making it necessary to focus on a model substrate (hematite) and synthetic liquors (pH 13 at high ionic strength). Previous spectral studies have used ex situ techniques, with sample drying potentially altering the adsorbed species, leading to inconclusive results. This study presents for the first time direct in situ evidence of polyacrylate adsorption onto hematite obtained by Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) Spectroscopy. Adsorption and hence concentration of dilute polyacrylate solutions onto hematite-coated zinc selenide optics has provided spectra of adsorbed polymer under a range of conditions, unbiased by any contribution from the bulk polymer solution. Analysis of the polyacrylate carbon-oxygen stretching frequencies established differing modes of adsorption at low and high pH conditions. At pH 2 adsorption proceeded through bidentate chelation of the carboxylate to a surface ferric ion, facilitated through deprotonation of a carboxylic acid group and removal of a hematite surface hydroxyl group, i.e. chemisorption. Unshifted carboxyl peaks in the spectrum enabled the unadsorbed 'loops' and 'tails' to be distinguished from the adsorbed polymer, and represented at least 70% of the total polymer chain length. / In contrast, at pH 13 adsorption of polyacrylate occurs via physisorption and was only possible with the addition of electrolyte. This adsorption was enhanced with increasing electrolyte concentrations up to 1 M NaCl. The hematite surface charge was negative at high pH, however with increasing ionic strength the specific adsorption of Na+ ions decreased the negative surface charge, and at very high salt concentrations the surface became positively charged. For electrolyte with different monovalent cations, polymer adsorption increased in the order Li+ > Na+ > Cs+. The identity of the monovalent cation had no effect on the polymer solution dimensions but the ability to reduce the magnitude of the hematite surface charge followed the same trend. This finding is consistent with the 'structure making - structure breaking' model proposed by Berube and de Bruyn. At both high and low pH, polyacrylate exhibited adsorption isotherms that are best described by the Langmuir expression. Surface coverage was greatest at low pH due to more available surface sites and the adsorbed polymer conformation (a greater fraction of loops and tails). Adsorbed conformation and hence maximum adsorption was independent of molecular weight at low pH, however at high pH maximum adsorption increased with increased molecular weight, indicating an adsorbed polymer configuration exhibiting a greater degree of loops and tails. While the individual carboxylate-surface interaction was stronger at low pH than high pH, both were relatively weak. Despite this, none of the polymers could be removed by washing, demonstrating the strength and irreversibility of the multi-attached polymer molecules. / The stronger individual carboxylate-surface interactions at low pH is indicative of a higher activation energy of formation, and may be a contributing factor to the slower rate of adsorption at low pH. The rate of polyacrylate adsorption was dominated by mass transport limitations in all cases. The initial rate of adsorption was greater at lower polymer molecular weight, consistent with the relative polymer diffusion coefficients. This rate of adsorption was much less dependent on polymer molecular weight at high pH than at low pH, suggesting significantly different polymer-solvent interactions. It was clearly demonstrated that the sodium ion concentration within the high ionic strength Bayer liquors is more than sufficient to facilitate polyacrylate adsorption on bauxite residue. There is no need to postulate calcium bridging between the polymer and surface, as has been suggested in previous studies. Improved settling and clarity associated with the presence of calcium on residue surfaces is more likely due to enhanced particle coagulation prior to flocculation. FTIR-ATR has been shown to be an excellent tool for the in situ examination of polyacrylate adsorption onto hematite, and will be a powerful technique for the characterisation and subsequent comparison of the adsorption behaviour of other systems.

The immobilization of antibodies on silica surfaces has been a wide and common practice via cross-linking with the formation of covalent bonds between surface and antibody. The formation of antibody thin films on solid surfaces using pneumatic spray (PS) as the deposition technique and the analysis of the surface morphology of these films were investigated during this study. The pneumatic spray method was compared with the covalent bonding method Avidin-Biotin Bridge (ABB). The intensities and capture efficiency tests showed similar results for both techniques with a lower signal-to-noise ratio (SNR) for the PS deposited films. Specificity tests suggested that the bio-sensitivity of the antibody films that were pneumatically sprayed maintained their capture abilities after the immobilization process. Analysis obtained from the attenuated total reflectance Fourier transform infrared ATR-FTIR support these results indicating that the antibodies retained their native structure and chemical stability thorough the induced physisorption process. The pneumatic spray films also preserve mechanical stability by adhering to the surface after the rinsing procedures. Capture efficiency was tested for both immobilization techniques, the results of which were similar. The pneumatic spray technique was also tested using a diverse range of deposition times. It was shown that a 2 minute deposition time was sufficient to produce a film with similar capture efficiency to the avidin-biotin bridge technique. The surface density obtained for the 2 minute deposition was 9.05 ng/mm2, which is higher than the range of 2.2 to 4.74 ng/mm2 reported for the avidin-biotin bridge technique[1-3]. The contact angle measurements for the pneumatic spray films showed a higher hydrophobicity compared with the avidin-biotin bridge films. This is due to the higher surface roughness obtained for the avidin-biotin bridge films, a higher surface density for the PS and the random orientation of the antibodies in the pneumatic spray films. A study of shelf life showed that the pneumatic spray technique produces stable films that can be used for as long 100 days (study performed only up to 100 days) with similar capture efficiency to those prepared in the same day. To further understand the improvement in capture efficiency of the pneumatic spray films, the surface morphology was investigated to determine its influence in the cell adhesion process. The surface was characterized by several different techniques: ellipsometry to determine the thickness of the films, atomic force microscopy (AFM) to calculate the surface roughness, optical microscopy to identify particle formation during antibody immobilization process, fluorescent microscopy and sandwich fluorescent immunoassay to observe the immobilization patterns of antibodies and antigens on the surface, contact angle measurements to analyze the wettability of the antibody films and X-ray photoelectron spectroscopy (XPS) to confirm the presence of antibody on both deposition methods and to propose a growth model for the pneumatic spray deposition technique. A possible explanation for the similar results of capture efficiency for both techniques can be attributed to three main factors. First, the antibodies retained their native structure thorough the induced physisorption process allowing then to capture antigen normally. Second, the lack of orientation of the antibodies in the pneumatic films was compensated by high surface density thereby offering more binding sites to capture antigens. Third, hydrophobic surfaces are favorable to cell adhesion, therefore the high hydrophobicity of the pneumatic spray films increases the capture efficiency. It is important to mention that the time that it takes to produce the immunoassay surfaces was reduced dramatically from more than twenty four hours for the avidin-biotin bridge films to only a few minutes for the pneumatic spray films. In addition, pneumatic spray films significantly reduce the amount of materials and chemicals used in the deposition process. These factors make the pneumatic spray technique an excellent technique for the immobilization of antibodies on glass slides for commercial bio-sensor devices.

A novel biodegradable ammonia control material was developed from steam exploded corn cob and its adsorption capacity was studied by packed column and titration method. The packed column studies showed that the maximum absorption capacities of the raw corn cob (RCC) and the steam exploded corn cob (SECC) were 10.45 mg NHB3B/gRCC and 59.80 mg NHB3B/gSECC respectively. However, the titration of the water slurries with a NHB4BOH showed that the capacity of the SECC was 14.4 times that of RCC. The large difference between the packed column (SECC/RCC = 5.7) and the slurry titration (SECC/RCC = 14.4) was probably because: (1) the initial ammonia reaction products blocked the pores of the SECC and reduced diffusion into the pore structure; (2) the ammonia gas flow rates were too high and therefore the gas did not penetrate the pores; (3) the gas contact time was below the equilibrium value; and (4) since interior pore surface area is usually larger than the external surface area of a particle, it appears the low column SECC/RCC ratio is due to reactions on the SECC particle surface whereas the slurry result was a combination of both. Fourier Transform Infrared, FTIR spectroscopy was conducted on RCC, SECC, ammonia adsorbed on RCC and ammonia adsorbed on SECC in the range 4000–700 cmP-1P. The FTIR bands in the region between 1500 and 2000 cmPâ 1P showed a considerable difference between RCC and SECC. When SECC was treated with ammonia, the carboxylic functional group peak at 1700 cmP-1P was reduced and a new peak was observed at 1584 cmP-1P. The adsorption, desorption test and the heat of adsorption results suggested combined physisorption and chemisorption of ammonia on SECC but chemisorption was found to play an important role in ammonia removal. The BET specific surface area of RCC was 3.4 m2/g whilst that SECC was less than 1 m2/g. Although SECC had a low surface area compared with RCC its adsorption capacity was found to be greater than that of RCC meaning the adsorption process is chemically controlled. Also, the pore size distribution showed that RCC exhibited both macroporosity and mesoporosity whilst SECC showed only mesoporosity. It is interesting to note that upon steam exploding RCC, the macropores within RCC collapsed to form more mesopores in SECC. The high uptake of SECC was determined to be its small pore width compared to that RCC. Simultaneous Differential Scanning Calorimetry, DSC and Thermal Gravimetric Analyzer, TGA, was used to determine the heat of adsorption of ammonia on SECC. The heat of adsorption of ammonia on SECC was 33.00 kJ per mole of NHB3B. This study shows that SECC could be potentially used to remove NHB3B from various emission sources. / Master of Science

The exponential increase in the demands of world’s energy and the devastating effects of current fossil fuels based sources has forced us to reduce our dependence on the current sources as well as finding cleaner, cheaper and renewable alternates. Being abundant, efficient and renewable, hydrogen can be opted as the best possible replacement of the diminishing and harmful fossil fuels. But the transformation towards the hydrogen-based economy is hindered by the unavailability of suitable storage medium for hydrogen. First principles calculations based on density functional theory has been employed in this thesis to investigate the structures modelling and thermodynamics of various efficient materials capable of storing hydrogen under chemisorption and physisorption mechanisms. Thanks to their high storage capacity, abundance and low cost, metal hydride (MgH2) has been considered as promising choice for hydrogen storage. However, the biggest drawback is their strong binding with the absorbed hydrogen under chemisorption, which make them inappropriate for operation at ambient conditions. Different strategies have been applied to improve the thermodynamics including doping with light and transitions metals in different phases of MgH2 in bulk form.  Application of mechanical strain along with Al, Si and Ti doping on MgH2 (001) and (100) surfaces has also been found very useful in lowering the dehydrogenation energies that ultimately improve adsorption/desorption temperatures. Secondly, in this thesis, two-dimensional materials with high surface area have been studied for the adsorption of hydrogen in molecular form (H2) under physisorption. The main disadvantage of this kind of storage is that the adsorption of H2 with these nanostructures likes graphane, silicene, silicane, BN-sheets, BC3 sheets are low and demand operation at cryogenic conditions. To enhance the H2 binding and attain high storage capacity the above-mentioned nanostructures have been functionalized with light metals (alkali, alkaline) and polylithiated species  (OLi2, CLi3, CLi4). The stabilities of the designed functional materials for H2 storage have been verified by means of molecular dynamics simulations.

Over the past few years, the need for a clean and renewable fuel has sharply risen. This is due to increasing fossil fuel costs and the desire to limit or eliminate harmful by-products which are created during the burning of these fuels. Hydrogen is the most abundant element in the universe and can be used in either fuel cells or traditional internal combustion engines to produce energy with no harmful emissions. One of the main obstacles facing the implementation of a hydrogen economy is its storage. Classical methods of storage involve either high and unsafe pressures or liquid storage involving a large amount of energy. Two alternative hydrogen storage methods are investigated - physisorption, which is the weak chemical bonding to a material, as well as chemisorption, which is a strong chemical bond of hydrogen to a host material. Polyaniline, a conducting polymer, is investigated in both its bulk form as well as in nanostructured forms, more precisely nanofibers and nanospheres, to store hydrogen via physisorption. It is found the bulk form of polyaniline can store only approximately 0.5wt.% hydrogen, which is far short of the 6wt.% required for practical applications. Nanofibers and nanospheres, however, have been developed, which can store between 4wt.% and 10wt.% of hydrogen at room temperature with varying kinetics. A new complex metal hydride comprised of LiBH4, LiNH2 and MgH2 has been developed to store hydrogen via chemisorption. While the parent compounds require high temperatures and suffer of slow kinetics for hydrogen sorption, the work performed as part of this dissertation shows that optimized processing conditions reduce the hydrogen release temperature from 250°C to approximately 150°C, while the addition of nano sized materials has been found to increase the kinetics of hydrogen sorption as well as further decrease the hydrogen release temperature, making this one of the first viable hydrogen storage materials available. This is the first time that nanostructured polyaniline has been investigated for its hydrogen performance. Additionally, the thorough investigation of the effects of nano sized additives and processing parameter optimization of the multinary hydride are first reported in this dissertation.

Freiburg i. Br., Univ., Diss., 2008.