Structure-activity relationships of catalytic systems supported on oxide carriers

The present work focuses on the study of structural and catalytic properties of the dispersed phase of supported MoO3 catalysts. In situ Raman spectroscopy has been used to characterize the supported catalysts and provide fundamental information about the configuration and the molecular structure under various controlled gas atmospheres and operating temperatures. A strategy involving the combined use of in situ Raman and in situ FTIR vibrational spectroscopies and the combined use of in situ Raman spectroscopy and 18O2/16O2 isotopic exchange experiments has been applied to investigate the molecular structure of the molybdena dispersed phase. A series of supported molybdenum oxide catalysts supported on TiO2(anatase) has been extensively examined by means of in situ and operando Raman spectroscopy under oxidizing, reducing and ODH of ethane conditions. The molecular structure of the surface MoOx species has been assigned to the formation of isolated units, primarily as O=Mo(–O–Ti)3 with a characteristic Mo=O sretching frequency observed at 994 cm-1 both at low as well as at high coverage below monolayer (~6 Mo/nm2). A weak and broad band at ~925 cm-1 due to Mo–O– Mo functionalities becomes visible with increasing loading, indicating a low presence of associated (polymeric) molybdates with increasing loading on TiO2. When exceeding monolayer coverage, the Raman spectra revealed the formation of bulk crystalline MoO3 on the TiO2 support. Detailed catalytic studies of the supported molybdena catalysts on titania showed that the monolayer catalyst (15MoTi) provide better catalytic results. Additionally, the structural and catalytic properties of monolayer MoO3 catalysts supported on ZrO2, Al2O3, TiO2 and SiO2 were studied for the oxidative dehydrogenation (ODH) of ethane by in situ and operando Raman spectroscopy. The molecular structure of the dispersed surface species evolves from isolated monomolybdates (MoO4 and MoO5) to associated (MoOx)n units in polymolybdate chains, depending on the support. The nature of the oxide support material and of the Mo–O–support bond has a significant influence on the catalytic behavior of the molybdena catalysts with monolayer coverage while, the dependence of reactivity on the support follows the order ZrO2 > Al2O3 > TiO2 > SiO2. The vibrational properties of molybdena catalysts supported on titania, zirconia and alumina were studied by means of in situ vibrational (Raman and FTIR) spectroscopies and 18O/16O isotopic exchange experiments combined with in situ Raman spectra at 450 °C. The aim of this work was the discrimination between mono-oxo and di-oxo configuration for the deposited molybdena phase at low as well as at high coverage near monolayer. The vibrational isotope effects and the combined interpretation of the observed Raman fundamental, IR overtone, as well as calculated zero-order band wavenumbers and characteristics suggest a mono-oxo configuration for the deposited molybdena phase at low as well as at higher coverage, irrespective of the extent of association (polymerization). A “nextnearest- neighbor 18O/16O substitution” vibrational effect is observed, resulting in small red shifts (2–7 cm?1) of the Mo=16O Raman band wavenumber. This effect is found to be strongly related to the nature of the support, regarding the extent of the shift, and assigned to the different reducibilities of the various support materials.Αντικείμενο της παρούσας διατριβής ήταν η μελέτη των δομικών και καταλυτικών ιδιοτήτων καταλυτών ΜοΟ3 υποστηριγμένων σε οξειδικούς φορείς. Η χρήση της φασματοσκοπικής μεθόδου Raman υπό ελεγχόμενες in situ συνθήκες επέτρεψε το χαρακτηρισμό των καταλυτών κάτω από ένα εύρος συνθηκών που μελετήθηκαν. Η εφαρμογή της μεθοδολογίας Operando Raman-GC οδήγησε σε συμπεράσματα αναφορικά με δομικές αλλαγές και δεδομένα καταλυτικής αποτελεσματικότητας ενώ συνδυασμός των δονητικών φασματοσκοπιών Raman και FTIR με πειράματα ισοτοπικής εναλλαγής 18Ο/16Ο χρησιμοποιήθηκε για εκτενέστερη διερεύνηση της μοριακής δομής των καταλυτών. Ειδικότερα εξετάσθηκε μια σειρά καταλυτών ΜοΟ3/ΤiO2 με in situ φασματοσκοπία Raman κάτω από οξειδωτικές και αναγωγικές συνθήκες, καθώς επίσης και κάτω από συνθήκες αντίδρασης οξειδωτικής αφυδρογόνωσης του αιθανίου. Η μοριακή δομή των επιφανειακών ειδών μολυβδενίου βρέθηκε να σχετίζεται άμεσα με το σχηματισμό κυρίως απομονωμένων ειδών Μο, για χαμηλές και υψηλές φορτίσεις κοντά στη μονοστρωματική κάλυψη, ενώ για φορτίσεις που υπερβαίνουν το μονόστρωμα παρατηρήθηκε ο σχηματισμός κρυσταλλικού ΜοΟ3. Η αποτίμηση της καταλυτικής ενεργότητας κατέδειξε πως καταλύτες με πολύ καλή διασπορά του ΜοΟ3 πάνω στην επιφάνεια παρουσιάζουν τα βέλτιστα αποτελέσματα. Επιπροσθέτως, εξετάζεται η επίδραση του φορέα (ZrO2, Al2O3, TiO2 και SiO2) στη δομή και στην καταλυτική συμπεριφορά μονοστρωματικών υποστηριγμένων καταλυτών MoO3 για την αντίδραση της οξειδωτικής αφυδρογόνωσης του αιθανίου με την φασματοσκοπία Raman. Μελετήθηκε η επίδραση της φόρτισης, της θερμοκρασίας, της σύστασης της αέριας τροφοδοσίας και του χρόνου παραμονής των αντιδρώντων πάνω στα χαρακτηριστικά των φασμάτων Raman καθώς και στην καταλυτική ενεργότητα με στόχο την εξαγωγή σχέσεων δομής-ενεργότητας/εκλεκτικότητας υποστηριγμένων καταλυτών MoO3/MxOy (M=Zr, Al, Ti, Si) για την οξειδωτική αφυδρογόνωση του αιθανίου. Η επιλογή της φύσης του φορέα/υποστρώματος κρίθηκε ως σημαντικός παράγοντας που ελέγχει την αποτελεσματικότητα της λειτουργίας των καταλυτών για την ODH του αιθανίου. Τέλος, οι δονητικές φασματοσκοπίες Raman και FTIR κάτω από ελεγχόμενες in situ συνθήκες χρησιμοποιήθηκαν για τη διερεύνηση της μοριακής δομής των καταλυτικών συστημάτων MoO3/ZrO2, MoO3/Al2O3 και MoO3/TiO2. Η φασματοσκοπία Raman σε συνδυασμό με πειράματα ισοτοπικής εναλλαγής 18Ο/16Ο οδήγησε σε αξιόλογα συμπεράσματα αναφορικά με τη μοριακή δομή των διεσπαρμένων μολυβδενικών ειδών υπό το πρίσμα της διάκρισης μεταξύ mono-oxo και di-oxo δομών. Τα αποτελέσματα συνέκλιναν στην παρουσία mono-oxo μοριακών διαμορφώσεων για κάθε καταλυτικό σύστημα (MoO3/ZrO2, MoO3/Al2O3 και MoO3/TiO2), τόσο για χαμηλές όσο και για φορτίσεις κοντά στο μονόστρωμα. Η υποκατάσταση γειτονικών οξειδικών θέσεων των Μο=16Ο δεσμών βρέθηκε να συμβάλει στη σταδιακή μετατόπιση της κορυφής αυτών των δεσμών σε χαμηλότερες συχνότητες μέσω ενός φαινομένου που ονομάστηκε “δονητικό ισοτοπικό φαινόμενο υποκατάστασης 18Ο/16Ο επόμενων κοντινότερων γειτόνων” (Next – Nearest – Neighbor 18Ο/16Ο substitution vibrational isotope effect)

CO2 Assisted Ethane Oxidative Dehydrogenation over MoO3 and V2O5 Catalysts Supported on Reducible CeO2-TiO2

Supported MOx (M= Mo, V) on mixed CeO2-TiO2 were investigated for the oxidative dehydrogenation of ethane (ODHE) using CO2 as a mild oxidant. Raman spectroscopic characterization of the synthesized catalysts under dehydrated conditions suggest that surface MoOx species prefer to anchor on the crystalline domains of TiO2. Upon increasing the amount of CeO2 in the mixed oxide support, an intense and broad band at ~930cm-1 underscored that the prevalent species tend to be polymeric (MoOx)n domains. On the other hand, in the case of VOx catalysts, a gradual shift in the symmetric stretching of the vanadyl (V=O) Raman band with increasing CeO2 content was observed that points at the gradual anchoring of the surface vanadia species on both TiO2 and CeO2 thus highlighting the possible existence of the amorphous VOx to be located at the interface of the two mixed oxides. The catalytic behavior of Mo and V were distinct. As the ceria content in the support increased, MoOx catalysts promoted the ODHE via Mars van Krevelen mechanism while VOx catalysts appeared to favor ethane direct dehydrogenation. Investigation of structure-function relationships via in-situ Raman spectroscopic efforts revealed that adding ceria not only changed the redox properties of the support but also improved those of the deposited metal oxide. We also show that upon incorporation of ceria into the support, CO2 directly participates in the reoxidation of the dispersed MoOx species during catalysis. This effect was distinct from the reaction of CO2 in the reverse water gas shift reaction. Operando Raman spectra revealed that the presence of CO2 enhances the stability of the bridging Mo–O–Mo bond of polymeric molybdena domains, which is proposed to affect the relative contribution of oxidative versus non-oxidative pathways in ethane dehydrogenation.</p

Colloidal Plasmonic Nanostar Antennas with Wide Range Resonance Tunability

Gold nanostars display exceptional field enhancement properties and tunable resonant modes that can be leveraged to create effective imaging tags or phototherapeutic agents, or to design novel hot-electron based photocatalysts. From a fundamental standpoint, they represent important tunable platforms to study the dependence of hot carrier energy and dynamics on plasmon band intensity and position. Toward the realization of these platforms, holistic approaches taking into account both theory and experiments to study the fundamental behavior of theseparticles are needed. Arguably, the intrinsic difficulties underlying this goal stem from the inability to rationally design and effectively synthesize nanoparticles that are sufficiently monodispersed to be employed for corroborations of the theoretical results without the need of single particle experiments. Herein, we report on our concerted computational and experimental effort to design, synthesize, and explain the origin and morphology-dependence of the plasmon modes of a novel gold nanostar system, with an approach that builds upon the well-known plasmon hybridization model. We have synthesized monodispersed samples of gold nanostars with finely tunable morphology employing seed-mediated colloidal protocols, and experimentally observed narrow and spectrally resolved harmonics of the primary surface plasmon resonance mode both at the single particle level (via electron energy loss spectroscopy) and in ensemble (by UV-Vis and ATR-FTIR spectroscopies). Computational results on complex anisotropic gold nanostructures are validated experimentally on samples prepared colloidally, underscoring their importance as ideal testbeds for the study of structure-property relationships in colloidal nanostructures of high structural complexity.</div

Molybdenum(VI) Oxosulfato Complexes in MoO₃–K₂S₂O₇–K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

The structural and vibrational properties of molybdenum­(VI) oxosulfato complexes formed in MoO₃–K₂S₂O₇ and MoO₃–K₂S₂O₇–K₂SO₄ molten mixtures under an O₂ atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400–640 °C. The corresponding composition effects were explored in the <i>X</i>_MoO₃⁰ = 0–0.5 range. MoO₃ undergoes a dissolution reaction in molten K₂S₂O₇, and the Raman spectra point to the formation of molybdenum­(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo­(O)₂ configuration as a core. The stoichiometry of the dissolution reaction MoO₃ + <i>n</i>S₂O₇^2– → C^2<i>n</i>– was inferred by exploiting the Raman band intensities, and it was found that <i>n</i> = 1. Therefore, depending on the MoO₃ content, monomeric MoO₂(SO₄)₂^2– and/or associated [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– complexes are formed in the binary MoO₃–K₂S₂O₇ molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO₂²⁺ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high <i>X</i>_MoO₃⁰), the observed spectral changes can be explained by partial dissociation of [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– by detachment of S₂O₇^2– and formation of a MoOMo bridge. Addition of K₂SO₄ in MoO₃–K₂S₂O₇ results in a “follow-up” reaction and formation of MoO₂(SO₄)₃^4– and/or associated [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>– complexes in the ternary MoO₃–K₂S₂O₇–K₂SO₄ molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum­(VI) oxosulfato complexes pertain to the Mo­(O)₂ stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm^–1 for the ν_s and ν_as Mo­(O)₂ modes of MoO₂(SO₄)₂^2– and [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– and (2) at 935 (polarized) and 895 (depolarized) cm^–1 for the respective modes of MoO₂(SO₄)₃^4– and [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>–. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO₂(SO₄)₃]^4– and [{MoO₂}₂(SO₄)₄(μ-SO₄)₂]^8– ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program

Cellulose Hydrolysis in Acidified LiBr Molten Salt Hydrate Media

We screened nine acidified molten salt hydrates (solutions with water to salt molar ratio equal or less than the coordination number of the cation) as reaction media for cellulose hydrolysis, and we found that cellulose can be efficiently hydrolyzed in LiBr acidified MSH under mild conditions (>90% yield to water-soluble products in 0.05 M H₂SO₄ at 85 °C for 30 min). The effect of various factors (temperature as well as acid and initial cellulose concentrations) on the kinetics of hydrolysis reaction was also investigated. At the lowest temperatures examined (70 and 85 °C) low amounts of degradation products have been observed, and glucose appears to be in equilibrium with its dimers and possibly other oligomers. Higher temperatures (100–115 °C) enhanced the formation of degradation products (organic acids and humins). Analysis of the kinetic data indicate that hydrolysis rates are first order in cellulose and in H₂SO₄ concentration, and the initial hydrolysis rates have an apparent activation energy ∼123 kJ/mol. X-ray diffraction, SEM, and FTIR were also used to study cellulose’s structural/morphological changes upon treatment in the LiBr MSH media, in an attempt to understand the effects of the cellulose–salt interaction. Analysis of the data indicates that the enhancement of the hydrolysis rates can be attributed to the enhancement of the acidity of reaction media through synergistic effect of dilute acid and MSH, the breaking of crystalline structure through swelling, and the interaction of the salt with cellulose chains affecting the conformation and flexibility of the glycosidic bonds

Changes in Polymorph Composition in P25-TiO₂ during Pretreatment Analyzed by Differential Diffuse Reflectance Spectral Analysis

Elucidating structural and compositional polymorph changes of multiphase semiconductors in a quick and quantitative manner is important for their manufacturing and applications in catalysis alike. Derivative peak fitting of diffuse reflectance UV–visible spectra (DPR) is presented as an inexpensive, fast, and quantitative method to estimate both the composition of a multiphase semiconductor sample as well as the band gap energies of each component semiconductor in the mixture. Degussa P25 TiO₂, a widely used catalyst support and semiconductor photocatalyst, is a mixture of anatase and rutile polymorphs. The as-received P25 samples were subjected to grinding, sieving, and calcination or a combination of these operations. Samples were analyzed via DPR, X-ray diffraction (XRD), and Raman spectroscopy to quantify the percentage of anatase and rutile polymorphs present in each sample. Compositional measurements from DPR were in good quantitative agreement with XRD and Raman analysis. The application of in situ UV–visible spectroscopy DPR allowed for the on-stream determination of the onset temperature for rutilization during calcination, which occurred just above 823 K. In situ analysis of the ground and sieved samples during calcination revealed that grinding and sieving (between 45 and 53 μm) lowered the onset of rutilization to 673 K, leading to an increase in the formation of rutile when calcined at 773 K compared to as-received P25 calcined at the same temperature. However, the same process reduced the overall extent of rutilization at a higher temperature of 973 K. Changes in the exposed facets and shapes of the anatase crystallite, as indicated by XRD, were implicated in both reducing the rutilization onset temperature as well as leading to an overall greater resistance to rutilization. The quantitative success of the DPR method in this study may lead to application in other systems of mixed semiconductors