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)

Oxidative Thermal Sintering and Redispersion of Rh Nanoparticles on Supports with High Oxygen Ion Lability

The thermal sintering under oxidative conditions of Rh nanoparticles supported on oxides characterized by very different oxygen storage capacities (OSC) and labilities was studied at 750 and 850 °C. Under sintering conditions, significant particle growth occurred for Rh/γ-Al2O3 (up to 120% at 850 °C). In striking contrast, Rh/ACZ (alumina–ceria–zirconia) and Rh/CZ (ceria–zirconia) exhibited marked resistance to sintering, and even moderate (ca. −10% at 850 °C) to pronounced (ca. −60% at 850 °C) redispersion of the Rh. A model is proposed based on a double-layer description of metal–support interactions assigned to back-spillover of labile oxygen ions onto the Rh particles, accompanied by trapping of atomic Rh by the resulting surface oxygen vacancies. This model accounts for the observed resistance to sintering and actual redispersion of Rh, consistent with both alternative sintering mechanisms, namely Ostwald ripening (OR) or particle migration and coalescence (PMC)

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

Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane

The effects of the metal oxide support on the activity, selectivity, resistance to carbon deposition and high temperature oxidative aging on the Rh-catalyzed dry reforming of methane (DRM) were investigated. Three Rh catalysts supported on oxides characterized by very different oxygen storage capacities and labilities (γ-Al 2O 3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) were studied in the temperature interval 400–750 °C under both integral and differential reaction conditions. ACZ and CZ promoted CO 2 conversion, yielding CO-enriched synthesis gas. Detailed characterization of these materials, including state of the art XPS measurements obtained via sample transfer between reaction cell and spectrometer chamber, provided clear insight into the factors that determine catalytic performance. The principal Rh species detected by post reaction XPS was Rh 0, its relative content decreasing in the order Rh/CZ(100%)>Rh/ACZ(72%)>Rh/γ-Al 2O 3(55%). The catalytic activity followed the same order, demonstrating unambiguously that Rh 0 is indeed the key active site. Moreover, the presence of CZ in the support served to maintain Rh in the metallic state and minimize carbon deposition under reaction conditions. Carbon deposition, low in all cases, increased in the order Rh/CZ < Rh/ACZ < Rh/γ-Al 2O 3 consistent with a bi-functional reaction mechanism whereby backspillover of labile lattice O 2− contributes to carbon oxidation, stabilization of Rh 0 and modification of its surface chemistry; the resulting O vacancies in the support providing centers for dissociative adsorption of CO 2. The lower apparent activation energy observed with CZ-containing samples suggests that CZ is a promising support component for use in low temperature DRM

Kinetics, electrokinetics behavior and electrocatalytic phenomena of innovative electocatalysts in fuel cells for reactions that are related with the control of pollutant emissions

The aim of the present thesis is the efficient use of natural gas and biogas for electrical energy production. Two types of novel solid oxide fuel cells (SOFCs) were developed for the study of the CO₂ internal reforming of CH₄: i. High temperature fuel cell (Τ>800°C), based on YSZ (ZrO₂ stabilized 8 mol% Y₂O₃) solid electrolyte and Ni/YSZ cermet anode, and ii. Intermediate temperature fuel cell (Τ800°C) με στέρεο ηλεκτρολύτη βασισμένο σε YSZ και ανοδικό ηλεκτρόδιο Ni/YSZ και (ii) ενδιάμεσων θερμοκρασιών (Τ<750°C) με στέρεο ηλεκτρολύτη βασισμένο σε GDC και ανοδικό ηλεκτρόδιο Ni(Au)/GDC. Και οι δύο τύποι κυψελίδων εξετάστηκαν με απευθείας τροφοδοσία μιγμάτων προσομοιωμένου βιοαερίου (CH₄+CO₂) ως καύσιμο, ενώ μελετήθηκε τόσο η καταλυτική όσο και η ηλεκτροκαταλυτική συμπεριφορά τους. Τα πειραματικά αποτελέσματα έδειξαν ότι και οι δύο αυτοί τύποι κυψελίδων καυσίμου μπορούν να λειτουργούν αποδοτικά παράγοντας ηλεκτρική ενέργεια σε τροφοδοσία βιοαερίου οποιασδήποτε ποιότητας, ενώ η λειτουργία τους παρουσιάζει βέλτιστη συμπεριφορά στην περίπτωση τροφοδοσίας βιοαερίου ισομοριακής σύστασης. Η απόδοση των δύο κυψελίδων σε παραγόμενη ηλεκτρική ισχύ βρέθηκε αρκετά ικανοποιητική και εξαιρετικά σταθερή. Επίσης, στην συγκεκριμένη διατριβή αναπτύσσεται και ένα ηλεκτροχημικό κελί μονού διαμερίσματος για την μελέτη της αντίδρασης NO+C₃H₆+Ο₂ σε καταλύτη Ir ηλεκτροχημικά τροποποιημένου με επιφανειακό τροποποιητή Κ σε ένα ευρύ φάσμα θερμοκρασιών (250-400°C) συγκεντρώσεων οξυγόνου (0-5% Ο₂) και φορτίσεων προωθητή (Κ) με στόχο την μελέτη αυτού του καταλυτικού συστήματος κάτω από συνθήκες ηλεκτροχημικής προώθησης για την δυνατότητα ελέγχου ρυπογόνων εκπομπών που εμπεριέχουν υδρογονάνθρακες και οξείδια του αζώτου. Οι κινητικές μελέτες της αντίδρασης NO+C₃H₆+Ο₂ έδειξαν ότι σε χαμηλές συγκεντρώσεις οξυγόνου η επίδραση του καλίου στην ενεργότητα και στην εκλεκτικότητα του Ir είναι αμελητέα, ενώ σε υψηλές συγκεντρώσεις οξυγόνου το κάλιο προκάλεσε ισχυρή δηλητηρίαση στους ρυθμούς ανάστροφης κατανάλωσης τόσο του C₃H₆ όσο και του NO. Ακόμα, η προσθήκη του καλίου οδήγησε σε μια σημαντική υποβάθμιση της εκλεκτικότητας του μελετώμενου συστήματος ως προς άζωτο. Γενικά, η προσθήκη του καλίου στην επιφάνεια του καταλύτη Ir κατά τη διάρκεια της αντίδρασης C₃H₆+NO+Ο₂ εμφανίστηκε επιβλαβής και για την οξείδωση του C₃H₆ και για την δραστικότητα της ανάγωγης του NO καθώς και για την εκλεκτικότητα του καταλύτη ως προς το Ν₂ για ένα ευρύ φάσμα μελετώμενων συγκεντρώσεων οξυγόνου

Electropositive Promotion by Alkalis or Alkaline Earths of Pt-Group Metals in Emissions Control Catalysis: A Status Report

Recent studies have shown that the catalytic performance (activity and/or selectivity) of Pt-group metal (PGM) catalysts for the CO and hydrocarbons oxidation as well as for the (CO, HCs or H2)-SCR of NOx or N2O can be remarkably affected through surface-induced promotion by successful application of electropositive promoters, such as alkalis or alkaline earths. Two promotion methodologies were implemented for these studies: the Electrochemical Promotion of Catalysis (EPOC) and the Conventional Catalysts Promotion (CCP). Both methodologies were in general found to achieve similar results. Turnover rate enhancements by up to two orders of magnitude were typically achievable for the reduction of NOx by hydrocarbons or CO, in the presence or absence of oxygen. Subsequent improvements (ca. 30&#8315;60 additional percentage units) in selectivity towards N2 were also observed. Electropositively promoted PGMs were also found to be significantly more active for CO and hydrocarbons oxidations, either when these reactions occur simultaneously with deNOx reactions or not. The aforementioned direct (via surface) promotion was also found to act synergistically with support-mediated promotion (structural promotion); the latter is typically implemented in TWCs through the complex (Ce&#8315;La&#8315;Zr)-modified &#947;-Al2O3 washcoats used. These attractive findings prompt to the development of novel catalyst formulations for a more efficient and cost-effective control of the emissions of automotives and stationary combustion processes. In this report the literature findings in the relevant area are summarized, classified and discussed. The mechanism and the mode of action of the electropositive promoters are consistently interpreted with all the observed promoting phenomena, by means of indirect (kinetics) and direct (spectroscopic) evidences

Biogas management: advanced utilization for production of renewable energy and added-value chemicals

Summarization: Biogas is widely available as a product of anaerobic digestion of urban, industrial, animal and agricultural wastes. Its indigenous local-base production offers the promise of a dispersed renewable energy source that can significantly contribute to regional economic growth. Biogas composition typically consists of 35-75% methane, 25-65% carbon dioxide, 1-5% hydrogen along with minor quantities of water vapor, ammonia, hydrogen sulfide and halides. Current utilization for heating and lighting is inefficient and polluting, and, in the case of poor quality biogas (CH4/CO2 < 1), exacerbated by detrimental venting to the atmosphere. Accordingly, innovative and efficient strategies for improving the management and utilization of biogas for the production of sustainable electrical power or high added-value chemicals are highly desirable. Utilization is the focus of the present review in which the scientific and technological basis underlying alternative routes to the efficient and eco-friendly exploitation of biogas are described and discussed. After concisely reviewing state-of-the-art purification and upgrading methods, in-depth consideration is given to the exploitation of biogas in the renewable energy, liquid fuels, transport and chemicals sectors along with an account of potential impediments to further progress.Presented on: Frontiers in Environmental Scienc

The Effect of WO3 Modification of ZrO2 Support on the Ni-Catalyzed Dry Reforming of Biogas Reaction for Syngas Production

The time-on-stream catalytic performance and stability of 8 wt. % Ni catalyst supported on two commercially available catalytic supports, ZrO2 and 15 wt.% WO3-ZrO2, was investigated under the biogas dry reforming reaction for syngas production, at 750°C and a biogas quality equal to CH4/CO2 = 1.5, that represents a common concentration of real biogas. A number of analytical techniques such as N2 adsorption/desorption (BET method), XRD, H2-TPR, NH3- and CO2-TPD, SEM, ICP, thermal analysis (TGA/DTG) and Raman spectroscopy were used in order to determine textural, structural and other physicochemical properties of the catalytic materials, and the type of carbon deposited on the catalytic surface of spent samples. These techniques were used in an attempt to understand better the effects of WO3-induced modifications on the catalyst morphology, physicochemical properties and catalytic performance. Although Ni dispersion and reducibility characteristics were found superior on the modified Ni/WZr sample than that on Ni/Zr, its dry reforming of methane (DRM) performance was inferior; a result attributed to the enhanced acidity and complete loss of the basicity recorded on this catalyst, an effect that competes and finally overshadows the benefits of the other superior properties. Raman studies revealed that the degree of graphitization decreases with the insertion of WO3 in the crystalline structure of the ZrO2 support, as the ID/IG peak intensity ratio is 1.03 for the Ni/Zr and 1.29 for the Ni/WZr catalyst

CO2 Methanation on Supported Rh Nanoparticles: The combined Effect of Support Oxygen Storage Capacity and Rh Particle Size

CO2 hydrogenation toward methane, a reaction of high environmental and sustainable energy importance, was investigated at 200&ndash;600 &deg;C and H2/CO2 = 4/1, over Rh nanoparticles dispersed on supports with different oxygen storage capacity characteristics (&gamma;-Al2O3, alumina-ceria-zirconia, and ceria-zirconia). The effects of the support OSC and Rh particle size on reaction behavior under both integral and differential conditions were investigated, to elucidate the combined role of these crucial catalyst design parameters on methanation efficiency. A volcano-type variation of methanation turnover frequency was found in respect to support OSC; Rh/ACZ, with intermediate OSC, was the optimal catalyst. The structure sensitivity of the reaction was found to be a combined function of support OSC and Rh particle size: For Rh/&gamma;-Al2O3 (lack of OSC) methanation was strongly favored on small particles&mdash;the opposite for Rh/CZ (high OSC). The findings are promising for rational design and optimization of CO2 methanation catalysts by tailoring the aforementioned characteristics