A new method is developed referred to as Gold Nanorod Optical Modeling Equations (GNOME) for determining the average aspect ratio of gold nanorods in solution. In this method, the observed inhomogeneously broadened optical spectrum is fitted to a number of calculated homogeneously broadened spectra with different aspect ratios having different contributions. From this method, the average aspect ratio is determined. This is a more accurate than the presently used method of TEM. The surface plasmon enhanced fluorescence spectra of gold nanorods are calculated as a function of the aspect ratio and compared to experimental spectra. In this calculation, the inclusion of both the aspect ratio distribution calculated from the GNOME method as well as the incorporation of the intrinsic fluorescence of bulk gold are found necessary to model the enhanced fluorescence spectrum of gold nanorods using previously published equations. The enhanced spectrum decreases rapidly as the aspect ratio increases and the surface plasmon band shift away from the gold interband absorption. Photochemical methods are used to synthesize silver nanoparticles on silica surfaces and gold nanoparticles in solution. The formation silver nanoparticles utilizes benzophenone as a photosensitizing agent to initiate the reaction. The effects of the light source and irradiation time are investigated. The presence of different forms of silica are investigated in the formation of metal nanoparticles. This method produced silver nanoparticles on silica that can be in the form of film or powder that are useful in heterogeneous catalysis. Direct photochemical methods are applied to generate gold nanoparticles from chloroauoroic acid in ethylene glycol in the presence of polyvinylpyrrolidone as a capping material. A detailed mechanism of the formation of the gold nanoparticle is determined. This is done by following the kinetics of formation of the gold nanoparticles after irradiation under different conditions. The disproportionation of the gold ions as well as their reduction by ethylene glycol is found to be important in the formation of the nanoparticles. Photochemical synthesis provides room temperature techniques to generate metal nanoparticles in a variety of environments.

Liquid organic chemical hydride is a promising candidate for hydrogen storage and transport. Methylcyclohexane (MCH) to toluene (TOL) cycle has been considered as one of the feasible hydrogen carrier systems, but selective dehydrogenation of MCH to TOL has only been achieved using the noble Pt-based catalysts. The aim of this study is to develop non-noble, cost-effective metal catalysts that can show excellent catalytic performance, mainly maintaining high TOL selectivity achievable by Pt based catalysts. Mono-metallic Ni based catalyst is a well-known dehydrogenation catalyst, but the major drawback with Ni is its hydrogenolysis activity to cleave C-C bonds, which leads to inferior selectivity towards dehydrogenation of MCH to TOL. This study elucidate addition of the second metal to Ni based catalyst to improve the TOL selectivity. Herein, ubiquitous bi-metallic nanoparticles catalysts were investigated including (Ni–M, M: Ag, Zn, Sn or In) based catalysts. Among the catalysts investigated, the high TOL selectivity (> 99%) at low conversions was achieved effectively using the supported NiZn catalyst under flow of excess H2. In this work, a combined study of experimental and computational approaches was conducted to determine the main role of Zn over Ni based catalyst in promoting the TOL selectivity. A kinetic study using mono- and bimetallic Ni based catalysts was conducted to elucidate reaction mechanism and site requirement for MCH dehydrogenation reaction. The impact of different reaction conditions (feed compositions, temperature, space velocity and stability) and catalyst properties were evaluated. This study elucidates a distinctive mechanism of MCH dehydrogenation to TOL reaction over the Ni-based catalysts. Distinctive from Pt catalyst, a nearly positive half order with respect to H2 pressure was obtained for mono- and bi-metallic Ni based catalysts. This kinetic data was consistent with rate determining step as (somewhat paradoxically) hydrogenation of strongly chemisorbed intermediate originating from TOL. DFT calculation indicated that Zn metal prefers to occupy the step sites of Ni where unselective C–C bond breaking was considered to preferentially occur, explaining suppression of hydrogenolysis activity. Additionally, it confirmed that the H-deficient species at methyl position group (C6H5CH2) was stable on the surface, making its hydrogenation being rate determining step, consistent with positive order in H2 pressure on TOL formation rate. This may explain the conclusive role by H2 in facilitating desorption of the H-deficient surface species that was produced through further dehydrogenation of TOL.

Water-soluble fluorescent metallic clusters at sizes comparable to the Fermi wavelength of an electron (~0.5 nm for gold and silver) were created and their photophysical properties were investigated at the bulk and single molecule levels. We employed biocompatible dendrimer and peptide to prepare a series of strong fluorescent gold and silver clusters with chemical or photo reduction methods. Facilitated by the well-defined dendrimer size, electrospray ionization mass spectrometry indicates that the fluorescent silver nanocluster size ranges from 2 to 8 Ag atoms. The correlation of emission energy with the number of atoms, N, in each gold nanocluster is quantitatively fit for the smallest nanoclusters with no adjustable parameters by the simple scaling relation of EFermi/N1/3, in which EFermi is the Fermi energy of bulk gold. The transition energy scaling inversely with cluster radius indicates that electronic structure can be well described with the spherical jellium model and further demonstrates that these nanomaterials are multi-electron artificial atoms. Fluorescence from these small metal clusters can be considered protoplasmonic, molecular transitions of the free conduction electrons before the onset of collective dipole oscillations occurring when a continuous density of states is reached. In addition, very strong single molecular Stokes and Antistokes Raman enhancement by fluorescent silver clusters was observed. Pushing to larger sizes, we also created ~ 2nm diameter glutathione encapsulated luminescent gold nanoparticles. Distinct from similarly sized but nonluminescent gold nanoparticles, these 2 nm gold nanoparticles show bright, long lifetime emission but no plasmon absorption. The emission might arise from charge transfer between gold atoms and the thiol ligand. Providing the missing link between atomic and nanoparticle behavior in noble metals, these highly fluorescent, water-soluble gold and silver nanoclusters offer complementary transition energy size scalings at smaller dimensions than do semiconductor quantum dots. The unique discrete excitation and emission and strong Stokes and antistokes Raman enhancement coupled with facile creation in aqueous solution open new opportunities for noble metal nanoclusters as biological labels, energy transfer pairs, and other light emitters in nanoscale electronics.

A strategy based on reticulating metal ions and organic ligands into atomically precise gold and silver nanoclusters (NCs) with high monodispersity has been advanced to a point that allows the design of NCs with strict stoichiometries, functionalities and valence. Of the Ag NCs discovered, Ag44 is the most studied, not only due to its high absorption that transcends the visible spectrum suitable for photovoltaics but also because of its long excited state lifetime, as revealed by nanosecond transient absorption spectroscopy. A major principle discovered in this dissertation is the ability to produce Ag44 in scalable amounts and with high stability in addition to modulation of the functional groups of the organic ligands via a fast and complete ligand exchange process. This new discovery has led to the development of synthetic designs in which new sizes were obtained by varying the reaction parameters (e.g., ligands functionality, reaction temperature and time), namely, Ag29 using dithiols and phosphines. The synthesized NCs possess tetravalent functionalities that facilitate their crystallization and characterization. Furthermore, Ag29 glows red and is therefore a possible candidate for sensing and imaging applications.

The effects of noble metal (NM) promotion and deposition order (co-deposition of NM with the final Co deposition [co-dep] or sequential deposition of NM after Co deposition [seq-dep]) on surface area, pore size, metal retention, crystallite size, noble metal distribution and bonding in Co Fischer Tropsch (FT) catalysts were studied as were the resulting Co reducibility and Fischer Tropsch activity/selectivity properties. Catalysts containing nominally 25wt% Co with either 0.3 wt% Ru, 0.58 wt% Pt, 0.55wt% Re, or no NM on a La-stabilized-Al2O3 support were prepared by wet deposition. The Co, Pt, and Re were uniformly dispersed, but Ru distribution and retention were problematic and deposition-order dependent—85% was lost with co-dep, but it was uniformly distributed while 54% was lost with seq-dep and it was concentrated at the pellet edge. The co-dep catalysts all have smaller reduced Co crystallite size than their corresponding seq-dep catalysts. The average crystallite diameters for all 3 co-dep catalysts are between 4.1 and 4.3nm and ~90% of the crystallites are < 6nm. XAFS measurements showed that after reduction at 360°C, Pt is bonded with Co even with mild calcination between the final Co and the Pt deposition. On the other hand, neither Ru nor Re formed direct bonds with Co. Ru remained in a separate metal phase after reduction even at low loadings. Re remained as Re2O7 and still promoted Co reduction well (e.g. 42% reduced to Co metal compared to none for the unpromoted catalyst). By all measures of reducibility (TPR, EOR, H2 uptake), all NM promoted catalysts were more reducible than the unpromoted catalyst. The co-dep catalysts have lower TPR peak temperatures, but lower extents of reduction than their corresponding seq-dep catalysts. The NM type effect on overall extent of reduction trend was Co/Pt-seq>Co/Re-seq>Co/Ru-seq=Co/Pt-co>Co/Re-co>Co/Ru-co>Co. The Co/Pt-co catalyst was the most active of all the catalysts both on rate per mass and per site basis. The co-dep catalysts were all more active than the corresponding sequentially deposited catalysts. The co-dep Pt and Re catalyst activity is greater due to higher activity per site, while co-dep Ru activity is greater due to a higher abundance of active sites.

<p> A rapid and simple radiochemical neutron activation technique involving tellurium coprecipitation of Au, lr, Pd and Pt was evaluated using the U.S.G.S. standards PCC-1 (peridotite) and DTS-1 (dunite) as well as a house standard, JHC-6 (spinifex zone of a komati itic peridotite). It was then applied to the analysis of samples of Archean greenstone from the Red Lake area.</p> <p> The technique gives detection limits of 0.01-0.02, 0.02, 0.1-0.4 and 1-2 ppb for Au, Ir, Pd and Pt respectively and precisions of <25-35%. The method can be successfully applied to the analysis of PGE levels in sulphur-poor rocks, and large numbers of samples can be processed in a short period of time.</p> <p> A comparison of PGE content between several types of mafic rocks was made as well as an investigation of possible secular trends (Archean vs. Phanerozoic) in PGE concentration.</p> / Thesis / Master of Science (MSc)

Climate change is one of the biggest global threats of the 21st century. Fossil fuels constitute by far the most important energy source for transportation and the different governments are starting to take action to promote the use of cleaner fuels. Biomass-derived fuels are a promising alternative for diversifying fuel sources, reducing fossil fuel dependency and abating greenhouse gas emissions. The research interest has quickly shifted from first-generation biofuels, obtained from food commodities, to second-generation biofuels, produced from non-food resources. The subject of this PhD thesis is the production of second-generation biofuels via thermochemical conversion: biomass is first gasified to synthesis gas, a mixture of mainly H2 and CO; synthesis gas can then be catalytically converted to different fuels. This work summarizes six publications, which are focused on the synthesis gas conversion step. Two processes are principally examined in this summary. The first part of the PhD thesis is devoted to the synthesis of ethanol and higher alcohols, which can be used as fuel or fuel additives. The microemulsion technique is applied in the synthesis of molybdenum-based catalysts, achieving a yield enhancement. Methanol cofeeding is also studied as a way of boosting the production of longer alcohols, but a negative effect is obtained: the main outcome of methanol addition is an increase in methane production. The second part of the PhD thesis addresses wax hydroconversion, an essential upgrading step in the production of middle-distillate fuels via Fischer-Tropsch. Bifunctional catalysts consisting of noble metals supported on silica-alumina are considered. The deactivation of a platinum-based catalyst is investigated, sintering and coking being the main causes of decay. A comparison of platinum and palladium as catalyst metal function is also carried out, obtaining a fairly different catalytic performance of the materials in terms of conversion and selectivity, very likely due to dissimilar hydrogenation power of the metals. Finally, a kinetic model based on the Langmuir-Hinshelwood-Hougen-Watson formalism is proposed to describe the hydroconversion reactions, attaining a good fitting of the experimental data. / Klimatförändringarna är ett av de största globala hoten under det tjugoförsta århundradet. Fossila bränslen utgör den helt dominerande energikällan för transporter och många länder börjar stödja användning av renare bränslen. Bränslen baserade på biomassa är ett lovande alternativ för att diversifiera råvarorna, reducera beroendet av fossila råvaror och undvika växthusgaser. Forskningsintresset har snabbt skiftat från första generationens biobränslen som erhölls från mat-råvaror till andra generationens biobränslen producerade från icke ätbara-råvaror. Ämnet för denna doktorsavhandling är produktion av andra generationens biobränslen via termokemisk omvandling. Biomassa förgasas först till syntesgas, en blandning av i huvudsak vätgas och kolmoxid; syntesgasen kan sedan katalytiskt omvandlas till olika bränslen. Detta arbete sammanfattar sex publikationer som fokuserar på steget för syntesgasomvandling. Två processer är i huvudsak undersökta i denna sammanfattning. Den första delen av doktorsavhandlingen ägnas åt syntes av etanol och högre alkoholer som kan användas som bränsle eller bränsletillsatser. Mikroemulsionstekniken har använts vid framställningen av molybden-baserade katalysatorer, vilket gav en höjning av utbytet. Tillsatsen av metanol har också studerats som ett sätt att försöka få en högre koncentration av högre alkoholer, men en negativ effekt erhölls: huvudeffekten av metanoltillsatsen är en ökad metanproduktion. Den andra delen av doktorsavhandlingen handlar om vätebehandling av vaxer som ett viktigt upparbetningssteg vid framställning av mellandestillat från Fischer-Tropsch processen. Bifunktionella katalysatorer som består av ädelmetaller deponerade på silica-alumina valdes. Deaktiveringen av en platinabaserad katalysator undersöktes. Sintring och koksning var huvudorsakerna till deaktiveringen. En jämförelse mellan platina och palladium som funktionella metaller genomfördes också med resultatet att det var en ganska stor skillnad mellan materialens katalytiska egenskaper vilket gav olika omsättning och selektivitet, mycket sannolikt beroende på olika reaktionsmönster hos metallerna vid vätebehandling. Slutligen föreslås en kinetisk modell baserad på en Langmuir-Hinshelwood-Hougen-Watson modell för att beskriva reaktionerna vid vätebehandling. Denna modell ger en god anpassning till experimentella data. / El cambio climático es una de las mayores amenazas del siglo XXI. Los combustibles fósiles constituyen actualmente la fuente de energía más importante para el transporte, por lo que los diferentes gobiernos están empezando a tomar medidas para promover el uso de combustibles más limpios. Los combustibles derivados de biomasa son una alternativa prometedora para diversificar las fuentes de energía, reducir la dependencia de los combustibles fósiles y disminuir las emisiones de efecto invernadero. Los esfuerzos de los investigadores se han dirigido en los últimos años a los biocombustibles de segunda generación, producidos a partir de recursos no alimenticios. El tema de esta tesis de doctorado es la producción de biocombustibles de segunda generación mediante conversión termoquímica: en primer lugar, la biomasa se gasifica y convierte en gas de síntesis, una mezcla formada mayoritariamente por hidrógeno y monóxido de carbono; a continuación, el gas de síntesis puede transformarse en diversos biocombustibles. Este trabajo resume seis publicaciones, centradas en la etapa de conversión del gas de síntesis. Dos procesos se estudian con mayor detalle. En la primera parte de la tesis se investiga la producción de etanol y alcoholes largos, que pueden ser usados como combustible o como aditivos para combustible. La técnica de microemulsión se aplica en la síntesis de catalizadores basados en molibdeno, consiguiendo un incremento del rendimiento. Además, se introduce metanol en el sistema de reacción para intentar aumentar la producción de alcoholes más largos, pero los efectos obtenidos son negativos: la principal consecuencia es el incremento de la producción de metano. La segunda parte de la tesis estudia la hidroconversión de cera, una etapa esencial en la producción de destilados medios mediante Fischer-Tropsch. Los catalizadores estudiados son bifuncionales y consisten en metales nobles soportados en sílice-alúmina. La desactivación de un catalizador de platino se investiga, siendo la sinterización y la coquización las principales causas del problema. El uso de platino y paladio como componente metálico se compara, obteniendo resultados catalíticos bastante diferentes, tanto en conversión como en selectividad, probablemente debido a su diferente capacidad de hidrogenación. Finalmente, se propone un modelo cinético, basado en el formalismo de Langmuir-Hinshelwood-Hougen-Watson, que consigue un ajuste satisfactorio de los datos experimentales. / <p>QC 20160308</p>

Στη παρούσα εργασία μελετάται η ανάπτυξη και ο χαρακτηρισμός καινοτόμων υποστηριγμένων καταλυτών ευγενών μετάλλων για την αντίδραση μετατόπισης του CO με ατμό (Water Gas Shift, WGS) σε χαμηλές θερμοκρασίες καθώς και η κινητική της εν λόγω αντίδρασης. Εξετάστηκε η επίδραση των φυσικοχημικών και μορφολογικών χαρακτηριστικών της διεσπαρμένης μεταλλικής φάσης (Pt, Pd, Ru, Rh) και του φορέα (οξείδια μετάλλων) καθώς και της χρήσης προωθητών (αλκάλια, αλκαλικές γαίες) στην καταλυτική ενεργότητα. Μεγαλύτερη δραστικότητα παρατηρήθηκε για καταλύτες Pt υποστηριγμένους σε αναγώγιμα οξείδια, κυρίως TiO2 και CeO2. Η φαινόμενη ενέργεια ενεργοποίησης της αντίδρασης, Ea, είναι ανεξάρτητη από τη φύση του μετάλλου, όταν τα ευγενή μέταλλα διασπείρονται στους φορείς TiO2 και CeO2. Αντιθέτως για τους καταλύτες Μ/Al2O3, η φαινόμενη ενέργεια ενεργοποίησης της αντίδρασης, Ea, εξαρτάται από τη φύση του μετάλλου, υποδεικνύοντας ότι η αντίδραση WGS, σε καταλύτες ευγενών μετάλλων υποστηριγμένων σε μη αναγώγιμους φορείς, ακολουθεί διαφορετικό μηχανισμό. Για καταλύτες Pt/TiO2, Ru/TiO2, Pt/CeO2 και Pt/Al2O3 η μετατροπή του CO αυξάνεται με αύξηση της περιεκτικότητας του καταλύτη σε μέταλλο. Ωστόσο ο εγγενής ρυθμός της αντίδρασης ανά επιφανειακό άτομο μετάλλου και η φαινόμενη ενέργεια ενεργοποίησης της αντίδρασης, Ea, δεν εξαρτώνται από τη φόρτιση (0-5 wt.%) και το μέγεθος των κρυσταλλιτών (1.3-16nm) του μετάλλου. Η επίδραση των μορφολογικών χαρακτηριστικών του φορέα στην καταλυτική ενεργότητα μελετήθηκε σε καταλύτες Pt/TiO2, και Pt/CeO2. Για τους καταλύτες Pt/TiO2 βρέθηκε ότι η μετατροπή του CO σε χαμηλές θερμοκρασίες βελτιώνεται σημαντικά όταν ο Pt διασπείρεται σε φορείς με μικρότερο μέγεθος κρυσταλλιτών. Η συχνότητα αναστροφής (TOF) του CO αυξάνεται κατά δύο τάξεις μεγέθους καθώς μειώνεται το μέγεθος των κρυσταλλιτών του TiO2 από 35 σε 16 nm, με παράλληλη μείωση της ενέργειας ενεργοποίησης από 16.9 έως 11.9 kcal/mol. Βρέθηκε, με χρήση τεχνικών θερμοπρογραμματιζόμενης αναγωγής (TPR) και φασματοσκοπίας Raman και FTIR, ότι η παρατηρούμενη αύξηση της ενεργότητας καταλυτών Pt/TiO2 οφείλεται σε αύξηση της αναγωγιμότητας του φορέα TiO2, η οποία αυξάνεται με μείωση του μεγέθους των κρυσταλλιτών του. Τα αποτελέσματα παρέχουν σημαντικές ενδείξεις για τη συμμετοχή του φορέα στο μηχανισμό της αντίδρασης WGS είτε άμεσα, μέσω του οξειδοαναγωγικού (redox) μηχανισμού, είτε έμμεσα, μέσω του συνδυαστικού (associative) μηχανισμού. Και στις δύο περιπτώσεις, φαίνεται ότι η παρουσία μερικώς ανηγμένων σωματιδίων TiO2 στην περιοχή κοντά στο διεσπαρμένο Pt, είναι απαραίτητη για την παραγωγή ενεργών κέντρων στη διεπιφάνεια μετάλλου/φορέα. Σε αντίθεση με τους καταλύτες Pt/TiO2, για τους καταλύτες Pt/CeO2 βρέθηκε ότι τόσο η συχνότητα αναστροφής του CO όσο και η ενέργεια ενεργοποίησης της αντίδρασης δεν εξαρτώνται σημαντικά από τα μορφολογικά χαρακτηριστικά του φορέα, τουλάχιστον υπό τις παρούσες πειραματικές συνθήκες. Η ενίσχυση του φορέα με κατάλληλη ποσότητα αλκαλίων (Na, K, Li, Cs) οδηγεί σε σημαντική αύξηση της ενεργότητας των καταλυτών Pt/TiO2. Βρέθηκε ότι σε όλες τις περιπτώσεις, η συχνότητα αναστροφής του CO περνάει από μέγιστο σε καταλύτες με περιεκτικότητα Pt:Αλκάλιο=1:1. Βέλτιστη συμπεριφορά παρουσίασε ο φορέας ενισχυμένος με Na, για τον οποίο παρατηρήθηκε ότι ο εγγενής ρυθμός της αντίδρασης ανά επιφανειακό άτομο Pt τριπλασιάζεται καθώς αυξάνεται η περιεκτικότητα σε Na από 0 σε 0.06 wt.%. Η προσθήκη αλκαλικών γαιών (CaO, SrO, BaO, MgO) στο φορέα οδηγεί σε σημαντική βελτίωση της καταλυτικής ενεργότητας των καταλυτών Pt/TiO2. Βέλτιστη συμπεριφορά παρουσιάζουν οι καταλύτες ενισχυμένοι με CaO και SrO σε περιεκτικότητα 2 wt.%, οι οποίοι έχουν υποστεί θερμική κατεργασία στους 600OC. Αύξηση της περιεκτικότητας CaO από 0 σε 4 wt.% έχει σαν αποτέλεσμα ο εγγενής ρυθμός της αντίδρασης να περνάει από μέγιστο, για το δείγμα με 2 wt.% CaO, του οποίου η συχνότητα αναστροφής του CO είναι ~2.5 φορές μεγαλύτερη συγκριτικά με το μη ενισχυμένο δείγμα. Τα αποτελέσματα των πειραμάτων Η2-TPD έδειξαν ότι, για καταλύτες ενισχυμένους με Na, Cs, CaO, WO3, καθώς και για καταλύτες M/TiO2 (M:Pt, Rh, Ru, Pd), ο ρυθμός της αντίδρασης ανά επιφανειακό άτομο Pt εξαρτάται από την ισχύ των θέσεων ρόφησης στη διεπιφάνεια μετάλλου/φορέα και περνάει από μέγιστο για μία ορισμένη τιμή της θερμοκρασίας εκρόφησης του υδρογόνου από τις θέσεις αυτές. Τα αποτελέσματα των πειραμάτων FTIR έδειξαν ότι η ενίσχυση των καταλυτών Pt/TiO2 με Na, Cs και CaO, οδηγεί σε αύξηση του πληθυσμού των ροφημένων ειδών CO στη διεπιφάνεια μετάλλου/φορέα. Το αντίθετο παρατηρείται για τον ενισχυμένο με WO3 καταλύτη. Για τους καταλύτες αυτούς καθώς και για τους Rh/TiO2 και M/Al2O3 (M: Pt, Ru, Pd), βρέθηκε ότι ο ρυθμός της αντίδρασης WGS αυξάνεται με ελάττωση της θερμοκρασίας διάσπασης των φορμικών ειδών. Τα αποτελέσματα υποδεικνύουν ότι η καταλυτική συμπεριφορά καθορίζεται σε μεγάλο βαθμό από τα φυσικοχημικά χαρακτηριστικά του φορέα, με τις καταλυτικά ενεργές θέσεις να εντοπίζονται στη διεπιφάνεια. Ο πληθυσμός και η ισχύς ρόφησης των ενεργών κέντρων και, επομένως, η καταλυτική ενεργότητα τροποποιούνται από τις αλληλεπιδράσεις μετάλλου/φορέα και από την ύπαρξη προωθητών. Η κινητική μελέτη της αντίδρασης WGS, σε καταλύτες Pt/TiO2 και Pt/0.34%Cs-TiO2, έδειξε ότι αύξηση της περιεκτικότητας του CO ή του Η2Ο στη τροφοδοσία οδηγεί σε αύξηση του ρυθμού, προσθήκη Η2 στην τροφοδοσία μειώνει σημαντικά τον ρυθμό ενώ το CO2 αφήνει το ρυθμό πρακτικά ανεπηρέαστο. Βρέθηκε ότι η αντίδραση είναι τάξης 0.5 ως προς CO, 1 ως προς Η2Ο, ~0 ως προς CO2 και ~-0.7 ως προς Η2. Τα κινητικά αποτελέσματα και για τους δύο καταλύτες προσαρμόζονται ικανοποιητικά σε εξίσωση ρυθμού που βασίζεται σε μηχανισμό ο οποίος περιλαμβάνει ρόφηση του H2O στο φορέα, ρόφηση των CO, Η2Ο, CO2 και Η2 στο μέταλλο, σχηματισμό ενδιάμεσων φορμικών ειδών στην επιφάνεια του φορέα και εκρόφηση των προϊόντων CO2 και H2. Τέλος μελετήθηκε η επίδραση του χρόνου επαφής στη συμπεριφορά καταλυτών 0.5%Pt/TiO2, 0.5%Pt/1%CaO-TiO2(Cal.600OC), 1%Pt/1%CaO-TiO2(Cal.600OC) και ενός εμπορικού καταλύτη και βρέθηκε ότι αύξηση του χρόνου επαφής (W/F) από 0.03 έως 0.20 × 3 g s/cm , οδηγεί σε σταδιακή αύξηση της μετατροπής του CO. Οι καταλύτες αυτοί υποβλήθησαν σε πειράματα μακροχρόνιας σταθερότητας, σε συνθήκες αντίδρασης, και από τα αποτελέσματα φαίνεται ότι η μετατροπή του CO παραμένει πρακτικά σταθερή για συνολικό χρόνο αντίδρασης περίπου 60 ώρες. Τα αποτελέσματα της παρούσας εργασίας μπορούν να χρησιμοποιηθούν για το «σχεδιασμό» και την ανάπτυξη καταλυτών οι οποίοι θα εκπληρώνουν τις προϋποθέσεις για χρήση σε εφαρμογές παραγωγής υδρογόνου για την τροφοδοσία κυψελίδων καυσίμου. / In the present study, a detailed investigation has been carried out in an attempt to identify the key physichochemical parameters which determine the catalytic activity of supported noble metal catalysts for the water-gas shift (WGS) reaction. A kinetic model, has been also developed, which can describe the kinetics of the reaction. The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, Pd) for the WGS reaction investigated with respect to the structural and morphological properties of the dispersed metallic phase and the support. It has been found that Pt catalysts are generally more active than Ru, Rh and Pd, and exhibit significantly higher activity when supported on “reducible” (TiO2, CeO2, La2O3, YSZ) rather than on “irreducible” (Al2O3, MgO, SiO2) metal oxides. Titania-supported platimum is more active than the well-studied Pt/CeO2 catalyst, especially in the temperature range of 200-250oC. When noble metals are dispersed on “reducible” oxides, such as CeO2 and TiO2, the apparent activation energy (Ea) of the reaction does not depend on the nature of the metallic phase but only on the nature of the support. In contrast, Ea differs from one metal to another when supported on an irreducible oxide, such as Al2O3, indicating that a different reaction mechanism is operable. Conversion of CO at a given temperature, for all metal-support combinations investigated, increases significantly with increasing metal loading in the range of 0.1-5.0 wt.%. However, activation energy and specific activity (TOF) do not depend on the morphological and structural characteristics of the metallic phase, such as loading, dispersion and crystallite size. The effect of the morphology of the support on catalytic performance has been investigated over Pt catalysts supported on four commercial titanium dioxide carriers with different structural characteristics (surface area, primary crystallite size of TiO2). It has been found that conversion of CO at low temperatures (<300oC) is significantly improved when Pt is dispersed on TiO2 samples of low crystallite size. The turnover frequency of CO increases by more than two orders of magnitude with decreasing crystallite size of TiO2 from 35 to 16 nm, with a parallel decrease of activation energy from 16.9 to 11.9 kcal/mol. This is attributed to the higher reducibility of smaller titania crystallites, as evidenced from the results of temperature programmed reduction (TPR) techniques and in situ Raman and FTIR spectroscopies. H2 and CO-TPR experiments, demonstrated that the reducibility of titania, increases with increasing the specific surface area of the catalyst or, conversely, with decreasing the primary particle size ze ( TiO2 d ) of the support. This has been proven by the results of in situ Raman experiments conducted under hydrogen flow which showed that formation of substoichiometric TiOx species initiates at lower temperatures and is more facile over Pt/TiO2 catalysts with smaller titania particle sizes. FTIR experiments provide evidence that the reaction takes place via interaction between CO and hydroxyl groups of the support, with intermediate production of formates. Partial reduction of the support results in the creation of new sites for CO adsorption, probably located at the metal/support interface, which have been tentatively assigned to metallic Pt in contact with Ti3+ ions. The observed enhancement of the WGS activity of Pt/TiO2 catalysts with increasing the reducibility of the support (decreasing TiO2 d ) may be explained by both the “regenerative” and the “associative” mechanism of the reaction. In contrast to what has been found over Pt/TiO2 catalysts, catalytic activity of dispersed Pt and the apparent activation energy of the reaction do not depend on the structural and morphological characteristics of CeO2, at least in the range of surface areas (3.3-57 m2/g) and primary crystallite sizes (10-32 nm) investigated. The catalytic performance of titania-supported platinum catalysts for the WGS reaction can be significantly improved by addition of small amounts of alkali (Na, K, Li, Cs) promoters. The catalyst promoted with Na exhibits better catalytic performance, compared to Li-, Cs- and K-promoted samples. It has been also found that, at least in the case of Na- and Cs-promoted catalysts, the specific catalytic activity (TOF) goes through a maximum for alkali:Pt atomic ratios of 1:1. The catalytic activity of Pt/TiO2 catalysts can be also improved by addition of alkaline earth (CaO, SrO, BaO, MgO) promoters. Optimal results were obtained for the catalysts promoted with 2 wt.% CaO and SrO, the specific activity (TOF) of which is about 2.5 times higher compared to that of the unpromoted catalyst. The results of H2-TPD experiments, over Na, Cs, CaO and WO3-promoted Pt/TiO2 catalysts and M/TiO2 (M:Pt, Rh, Ru, Pd) catalysts, demonstrated that the reaction rate (TOF) depends on the strength of the adsorption sites at the metal/support interface and goes through a maximum for a specific temperature of hydrogen desorption from theses sites. FTIR experiments provide evidence that the addition of Na, Cs and CaO over Pt/TiO2 catalysts results in an increase of the population of CO species adsorbed at the metal/support interface. It has also been found (CO-TPD experiments) that the turnover frequency of CO increases with decreasing the temperature of the decomposition of formate species, which may be produced by interaction between CO adsorbed on platinum with hydroxyl groups of TiO2 at the metal/support interface. The above results indicate that the catalytic performance of supported noble metal catalysts for the WGS reaction depends strongly on the physichochemical characteristics of the support. The population and the strength of the catalytic active sites, probably located at the metal/support interface, can be altered due the metal-support interactions and the presence of promoters. The kinetic investigation of the WGS reaction has being carried out over Pt/TiO2 and Pt/0.34%Cs-TiO2 catalysts. It was found that the reaction rate increases with increasing the partial pressure of CO or H2O in the feed composition. The addition of H2 in the reaction mixture results in a substantial decrease of the reaction rate, while the partial pressure of CO2 does not affect the reaction rate. It has also been found that the reaction order is 0.5, 1, ~-0.7 and ~0 for CO, H2O, H2 and CO2, respectively. The kinetic results were modelled by a rate expression based on a mechanism reaction, which includes H2O adsorption on the support, CO, H2O, H2 and CO2 adsorption on Pt, formation of intermediate formate species on the support and finally desorption of H2 and CO2. The effect of contact time on the catalytic performance has been investigated, under realistic reaction conditions, over 0.5%Pt/TiO2, 0.5%Pt/1%CaO-TiO2(Cal.600OC), 1%Pt/1%CaOTiO2 (Cal.600OC) and a commercial catalyst. It has been found that the conversion of CO at a given temperature increases with increasing W/F between 0.03 and 0.20 × 3 g s/cm. The conversion of CO of the above catalysts is remained constant, under reaction conditions, for about 60 hours. The results of the present study, can be used to develop active, selective and stable LT-WGS catalysts suitable for Fuel Cell applications.

ZnO thin films and nanostructures were grown by eclipse pulsed laser deposition (EPLD) for the first time. On bare sapphire held at 600 °C, a complex nanostructured surface was formed when ablating a metallic Zn target in an oxygen ambient. Nanorods grown by a vapor-solid mechanism clumped together in well separated, micron-sized regions. Nanoscale pyramids with 6 fold symmetry formed between the nanorod clumps by vapor-liquid-solid growth. Strong photoluminescence (PL) was observed from the EPLD grown samples, an order of magnitude stronger than PLD grown nanorods formed under similar growth conditions. Low temperature PL was dominated by the I₇ exciton, which still has an unknown origin. Excitation intensity dependence of I₇ was drastically different than the rest of the nearby excitonic features, behavior which has not been previously reported for bound excitons in ZnO. I₇ also showed large, seemingly random variations in intensity across the surface of each sample compared to the other nearby recombinations, suggesting a structural connection. Introduction of a buffer layer had a profound effect on the morphology and PL from EPLD grown ZnO from a metallic Zn target. Pt has a high melting temperature, which helped suppress the vapor-liquid-solid nanostructure growth resulting in thin-film formation. For standard PLD, the ZnO film showed large grains separated by cracks on the surface. Due to the reduced growth rate in the EPLD geometry, the ZnO layer had a high density of nanoscale pores, reminiscent of the porous Pt buffer layer. Strong PL emission, which was dominated by I₇, was observed from the ZnO/Pt/Al₂O₃ which showed unusual blue/violet emission when the EPLD geometry was used for growth. Thin ZnO buffer layers deposited at reduced temperature also had a profound effect on EPLD grown ZnO, resulting in a random array of nanorods with alignment which was dependent on the growth temperature of the buffer layer. Buffer layers offer another dimension in the control over epitaxial structures and show large potential for EPLD growth of ZnO. Pb was the dominant impurity in the Zn targets used for EPLD growth, hinting at a Pb-related origin for the I7 peak. To explore this idea, hydrothermally grown bulk ZnO was ion-implanted with Pb and then annealed in oxygen at 600 °C to repair damage to the crystal. PL emission intensity was substantially reduced in the Pb-implanted ZnO but the line widths were preserved. No evidence of an I₇ feature was seen for Pb concentrations of up to 0.10%, three orders of magnitude higher than the expected level in the EPLD grown ZnO. However, this does not rule out a Pb-related complex as the origin of I₇ since Pb has complicated interactions with the impurities and native defects in ZnO. Instead of I₇, other sharp excitonic features were observed near the band edge. A bound exciton with a localization energy of 12.4 ± 0.2 meV was observed in the Pb-implanted samples and was attributed to neutral interstitial Pb donors. Pb-implantation produced a clear PL signature which is unique enough to unambiguously detect its presence in ZnO. EPLD also proved successful at depositing oxides of the noble metals. Ir, Pt, Pd, and Ru targets were ablated in oxygen and argon ambients and films were collected on room temperature substrates. Growth in argon resulted in pure metal while oxidized layers were obtained in oxygen. This was clearly evident by the semiconductor-like transmission spectra observed for the oxidized samples. The high fluence used for these growths promoted the oxidation of these resilient metals while the shadow mask blocked most of the molten particulates generated by the high fluence. EPLD is an excellent way to produce oxides from metallic targets, a technique which should be explored in more detail for many material systems.

ZnO thin films and nanostructures were grown by eclipse pulsed laser deposition (EPLD) for the first time. On bare sapphire held at 600 °C, a complex nanostructured surface was formed when ablating a metallic Zn target in an oxygen ambient. Nanorods grown by a vapor-solid mechanism clumped together in well separated, micron-sized regions. Nanoscale pyramids with 6 fold symmetry formed between the nanorod clumps by vapor-liquid-solid growth. Strong photoluminescence (PL) was observed from the EPLD grown samples, an order of magnitude stronger than PLD grown nanorods formed under similar growth conditions. Low temperature PL was dominated by the I₇ exciton, which still has an unknown origin. Excitation intensity dependence of I₇ was drastically different than the rest of the nearby excitonic features, behavior which has not been previously reported for bound excitons in ZnO. I₇ also showed large, seemingly random variations in intensity across the surface of each sample compared to the other nearby recombinations, suggesting a structural connection. Introduction of a buffer layer had a profound effect on the morphology and PL from EPLD grown ZnO from a metallic Zn target. Pt has a high melting temperature, which helped suppress the vapor-liquid-solid nanostructure growth resulting in thin-film formation. For standard PLD, the ZnO film showed large grains separated by cracks on the surface. Due to the reduced growth rate in the EPLD geometry, the ZnO layer had a high density of nanoscale pores, reminiscent of the porous Pt buffer layer. Strong PL emission, which was dominated by I₇, was observed from the ZnO/Pt/Al₂O₃ which showed unusual blue/violet emission when the EPLD geometry was used for growth. Thin ZnO buffer layers deposited at reduced temperature also had a profound effect on EPLD grown ZnO, resulting in a random array of nanorods with alignment which was dependent on the growth temperature of the buffer layer. Buffer layers offer another dimension in the control over epitaxial structures and show large potential for EPLD growth of ZnO. Pb was the dominant impurity in the Zn targets used for EPLD growth, hinting at a Pb-related origin for the I7 peak. To explore this idea, hydrothermally grown bulk ZnO was ion-implanted with Pb and then annealed in oxygen at 600 °C to repair damage to the crystal. PL emission intensity was substantially reduced in the Pb-implanted ZnO but the line widths were preserved. No evidence of an I₇ feature was seen for Pb concentrations of up to 0.10%, three orders of magnitude higher than the expected level in the EPLD grown ZnO. However, this does not rule out a Pb-related complex as the origin of I₇ since Pb has complicated interactions with the impurities and native defects in ZnO. Instead of I₇, other sharp excitonic features were observed near the band edge. A bound exciton with a localization energy of 12.4 ± 0.2 meV was observed in the Pb-implanted samples and was attributed to neutral interstitial Pb donors. Pb-implantation produced a clear PL signature which is unique enough to unambiguously detect its presence in ZnO. EPLD also proved successful at depositing oxides of the noble metals. Ir, Pt, Pd, and Ru targets were ablated in oxygen and argon ambients and films were collected on room temperature substrates. Growth in argon resulted in pure metal while oxidized layers were obtained in oxygen. This was clearly evident by the semiconductor-like transmission spectra observed for the oxidized samples. The high fluence used for these growths promoted the oxidation of these resilient metals while the shadow mask blocked most of the molten particulates generated by the high fluence. EPLD is an excellent way to produce oxides from metallic targets, a technique which should be explored in more detail for many material systems.