Support Induced Effects on the Ir Nanoparticles Activity, Selectivity and Stability Performance under CO2 Reforming of Methane.

The production of syngas (H2 and CO)-a key building block for the manufacture of liquid energy carriers, ammonia and hydrogen-through the dry (CO2-) reforming of methane (DRM) continues to gain attention in heterogeneous catalysis, renewable energy technologies and sustainable economy. Here we report on the effects of the metal oxide support (γ-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) on the low-temperature (ca. 500-750 ∘C) DRM activity, selectivity, resistance against carbon deposition and iridium nanoparticles sintering under oxidative thermal aging. A variety of characterization techniques were implemented to provide insight into the factors that determine iridium intrinsic DRM kinetics and stability, including metal-support interactions and physicochemical properties of materials. All Ir/γ-Al2O3, Ir/ACZ and Ir/CZ catalysts have stable DRM performance with time-on-stream, although supports with high oxygen storage capacity (ACZ and CZ) promoted CO2 conversion, yielding CO-enriched syngas. CZ-based supports endow Ir exceptional anti-sintering characteristics. The amount of carbon deposition was small in all catalysts, however decreasing as Ir/γ-Al2O3 > Ir/ACZ > Ir/CZ. The experimental findings are consistent with a bifunctional reaction mechanism involving participation of oxygen vacancies on the support's surface in CO2 activation and carbon removal, and overall suggest that CZ-supported Ir nanoparticles are promising catalysts for low-temperature dry reforming of methane (LT-DRM)

Pronkjewails in distant places:Mortuary studies in the eastern Mediterranean by the GIA

The Greek Archaeology research group of the GIA specializes in mortuary archaeology, studying sites in the eastern Mediterranean and Middle East that date from the Bronze Age through to the Late Roman period. Our methodology includes theoretical approaches; cemetery excavations; the analysis of legacy data; studies of grave architecture, tombstones and grave goods; osteological analyses; digitization of datasets and digital applications; and DNA analysis, as well as isotopic and biomolecular studies, and we are focused on performing integrated studies with thorough contextual analyses. Our central question is how people dealt with death and what their funerary remains tell us about their lives and their world. Together with our local and international network of researchers and laboratories, our staff and students aim to perform innovative research, reach out to the public, and provide diverse perspectives on life and death in the ancient eastern Mediterranean

Stabilization of catalyst particles against sintering on oxide supports with high oxygen ion lability exemplified by Ir-catalyzed decomposition of N2O

Iridium nanoparticles deposited on a variety of surfaces exhibited thermal sintering characteristics that were very strongly correlated with the lability of lattice oxygen in the supporting oxide materials. Specifically, the higher the lability of oxygen ions in the support, the greater the resistance of the nanoparticles to sintering in an oxidative environment. Thus with γ-Al2O3 as the support, rapid and extensive sintering occurred. In striking contrast, when supported on gadolinia-ceria and alumina-ceria-zirconia composite, the Ir nanoparticles underwent negligible sintering. In keeping with this trend, the behavior found with yttria-stabilized zirconia was an intermediate between the two extremes. This resistance, or lack of resistance, to sintering is considered in terms of oxygen spillover from support to nanoparticles and discussed with respect to the alternative mechanisms of Ostwald ripening versus nanoparticle diffusion. Activity towards the decomposition of N2O, a reaction that displays pronounced sensitivity to catalyst particle size (large particles more active than small particles), was used to confirm that catalytic behavior was consistent with the independently measured sintering characteristics. It was found that the nanoparticle active phase was Ir oxide, which is metallic, possibly present as a capping layer. Moreover, observed turnover frequencies indicated that catalyst-support interactions were important in the cases of the sinter-resistant systems, an effect that may itself be linked to the phenomena that gave rise to materials with a strong resistance to nanoparticle sintering

Development and characterization of novel catalysts for the low temperature water-gas shift reaction and kinetic study

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 () 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 ) 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.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

Nanocatalysis for Environmental Protection, Energy, and Green Chemistry

Nowadays, nanoscience and nanotechnology depict cutting-edge areas of modern science and technology across an array of applications, including heterogeneous catalysis [...

Mechanistic Study of the Selective Methanation of CO over Ru/TiO₂ Catalysts: Effect of Metal Crystallite Size on the Nature of Active Surface Species and Reaction Pathways

The effect of metal crystallite size on the reaction mechanism of competitive methanation of CO and CO₂ is investigated over Ru/TiO₂ catalysts of variable metal content (0.5 and 5.0 wt %) and crystallite size (2.1 and 4.5 nm) employing <i>in situ</i> FTIR and transient mass spectrometry techniques. Results show that although both catalysts follow the same mechanistic pathways, the relative population of reaction intermediates differs. Specifically, the nature of Ru-bonded carbonyl species detected on the catalyst surface upon interaction with the CO/CO₂-containing gas mixtures is the same for both catalysts. However, their relative population depends significantly on ruthenium crystallite size and determines catalytic activity. The population of multicarbonyl species adsorbed on partially oxidized Ru sites (Ru^<i>n</i>+-(CO)_<i>x</i>) decreases significantly with increasing Ru crystallite size, whereas the opposite is observed for CO species linearly bonded on reduced Ru crystallites (Ru_<i>x</i>-CO). This is correlated with the higher catalytic activity found for the 5%Ru/TiO₂ catalyst, since evidence has been provided that surface species adsorbed on reduced ruthenium sites are direct precursors of methane

Performance of Particulate and Structured Pt/TiO₂-Based Catalysts for the WGS Reaction under Realistic High- and Low-Temperature Shift Conditions

The water–gas shift (WGS) activity of Pt/TiO2-based powdered and structured catalysts was investigated using realistic feed compositions that are relevant to the high-temperature shift (HTS) and low-temperature shift (LTS) reaction conditions. The promotion of the TiO2 support with small amounts of alkali- or alkaline earth-metals resulted in the enhancement of the WGS activity of 0.5%Pt/TiO2(X) catalysts (X = Na, Cs, Ca, Sr). The use of bimetallic (Pt–M)/TiO2 catalysts (M = Ru, Cr, Fe, Cu) can also shift the CO conversion curve toward lower temperatures, but this is accompanied by the production of relatively large amounts of unwanted CH4 at temperatures above ca. 300 °C. Among the powdered catalysts investigated, Pt/TiO2(Ca) exhibited the best performance under both HTS and LTS conditions. Therefore, this material was selected for the preparation of structured catalysts in the form of pellets as well as ceramic and metallic catalyst monoliths. The 0.5%Pt/TiO2(Ca) pellet catalyst exhibited comparable activity with that of a commercial WGS pellet catalyst, and its performance was further improved when the Pt loading was increased to 1.0 wt.%. Among the structured catalysts investigated, the best results were obtained for the sample coated on the metallic monolith, which exhibited excellent WGS performance in the 300–350 °C temperature range. In conclusion, proper selection of the catalyst structure and reaction parameters can shift the CO conversion curves toward sufficiently low temperatures, rendering the Pt/TiO2(Ca) catalyst suitable for practical applications

Mechanistic insights into metal Lewis acid-mediated catalytic transfer hydrogenation of furfural to 2-methylfuran

Summarization: Biomass conversion to fuels and chemicals provides sustainability, but the highly oxygenated nature of a large fraction of biomass-derived molecules requires removal of the excess oxygen and partial hydrogenation in the upgrade, typically met by hydrodeoxygenation processes. Catalytic transfer hydrogenation is a general approach in accomplishing this with renewable organic hydrogen donors, but mechanistic understanding is currently lacking. Here, we elucidate the molecular level reaction pathway of converting hemicellulose-derived furfural to 2-methylfuran on a bifunctional Ru/RuOx/C catalyst using isopropyl alcohol as the hydrogen donor via a combination of isotopic labeling and kinetic studies. Hydrogenation of the carbonyl group of furfural to furfuryl alcohol proceeds through a Lewis acid-mediated intermolecular hydride transfer and hydrogenolysis of furfuryl alcohol occurs mainly via ring-activation involving both metal and Lewis acid sites. Our results show that the bifunctional nature of the catalyst is critical in the efficient hydrodeoxygenation of furanics and provides insights toward the rational design of such catalysts.Presented on: ACS Catalysi

Hydrogenation of CO2 over supported noble metal catalysts

Summarization: The catalytic activity of supported noble metal catalysts for the CO2 methanation reaction has been investigated with respect to the nature (Rh, Ru, Pt, Pd), loading (0.1–5.0 wt.%) and mean crystallite size (1.3–13.6 nm) of the dispersed metallic phase. It has been found that the turnover frequency (TOF) of CO2 conversion for TiO2-supported catalysts increases following the order of Pd < Pt < Ru < Rh, with Rh being about 3 times more active than Pd. Selectivity toward methane depends strongly on the noble metal catalyst employed and is significantly higher for Rh and Ru catalysts compared to Pt and Pd, which mainly promote production of CO via the RWGS reaction. Conversion of CO2 at a given temperature increases significantly with increasing Ru loading in the range of 0.1–5.0 wt.%. Results of kinetic measurements show that the CO2 hydrogenation reaction is structure sensitive, i.e., catalytic activity is strongly influenced by metal crystallite size. In particular, for Ru/TiO2 and Ru/Al2O3 catalysts, the normalized turnover frequency (TOF divided by the length of the perimeter of the metal-support interface) increases by two orders of magnitude with increasing ruthenium crystallite size in the range of 0.9–4.2 nm and 1.3–13.6 nm, respectively. FTIR experiments provide evidences that CO2 hydrogenation reaction occurs via intermediate formation of adsorbed CO species (Rux-CO, Run+(CO)x, (TiO2)Ru-CO) produced via the RWGS reaction. Part of this species interacts with adsorbed hydrogen atoms producing methane, whereas the remaining species desorbs to yield CO in the gas phase. The CO2 hydrogenation pathway does not change with variation of Ru crystallite size. However, the relative intensity of the band due to CO species linearly bonded on reduced Ru crystallites (Rux-CO) increases significantly with increasing Ru particle size, indicating that Rux-CO species could potentially being the reactive surface intermediates.Παρουσιάστηκε στο: Applied Catalysis A: Genera