Shape effects of ceria nanoparticles on the water-gas shift performance of cuox /ceo2 catalysts

T1EDK-00094 UIDB/EQU/50020/2020 UIDB/00511/2020 CEECINST/00102/2018 UIDB/50006/2020 UIDP/50006/2020 DL 57/2017The copper–ceria (CuOx /CeO2 ) system has been extensively investigated in several catalytic processes, given its distinctive properties and considerable low cost compared to noble metal-based catalysts. The fine-tuning of key parameters, e.g., the particle size and shape of individual counterparts, can significantly affect the physicochemical properties and subsequently the catalytic performance of the binary oxide. To this end, the present work focuses on the morphology effects of ceria nanoparticles, i.e., nanopolyhedra (P), nanocubes (C), and nanorods (R), on the water–gas shift (WGS) performance of CuOx /CeO2 catalysts. Various characterization techniques were employed to unveil the effect of shape on the structural, redox and surface properties. According to the acquired results, the support morphology affects to a different extent the reducibility and mobility of oxygen species, following the trend: R > P > C. This consequently influences copper–ceria interactions and the stabilization of partially reduced copper species (Cu+ ) through the Cu2+ /Cu+ and Ce4+ /Ce3+ redox cycles. Regarding the WGS performance, bare ceria supports exhibit no activity, while the addition of copper to the different ceria nanostructures alters significantly this behaviour. The CuOx /CeO2 sample of rod-like morphology demonstrates the best catalytic activity and stability, approaching the thermodynamic equilibrium conversion at 350◦ C. The greater abundance in loosely bound oxygen species, oxygen vacancies and highly dispersed Cu+ species can be mainly accounted for its superior catalytic performance.publishersversionpublishe

Facet-Dependent Reactivity of Ceria Nanoparticles Exemplified by CeO2-Based Transition Metal Catalysts: A Critical Review

The rational design and fabrication of highly-active and cost-efficient catalytic materials constitutes the main research pillar in catalysis field. In this context, the fine-tuning of size and shape at the nanometer scale can exert an intense impact not only on the inherent reactivity of catalyst’s counterparts but also on their interfacial interactions; it can also opening up new horizons for the development of highly active and robust materials. The present critical review, focusing mainly on our recent advances on the topic, aims to highlight the pivotal role of shape engineering in catalysis, exemplified by noble metal-free, CeO2-based transition metal catalysts (TMs/CeO2). The underlying mechanism of facet-dependent reactivity is initially discussed. The main implications of ceria nanoparticles’ shape engineering (rods, cubes, and polyhedra) in catalysis are next discussed, on the ground of some of the most pertinent heterogeneous reactions, such as CO2 hydrogenation, CO oxidation, and N2O decomposition. It is clearly revealed that shape functionalization can remarkably affect the intrinsic features and in turn the reactivity of ceria nanoparticles. More importantly, by combining ceria nanoparticles (CeO2 NPs) of specific architecture with various transition metals (e.g., Cu, Fe, Co, and Ni) remarkably active multifunctional composites can be obtained due mainly to the synergistic metalceria interactions. From the practical point of view, novel catalyst formulations with similar or even superior reactivity to that of noble metals can be obtained by co-adjusting the shape and composition of mixed oxides, such as Cu/ceria nanorods for CO oxidation and Ni/ceria nanorods for CO2 hydrogenation. The conclusions derived could provide the design principles of earth-abundant metal oxide catalysts for various real-life environmental and energy applications

Σύνθεση και χαρακτηρισμός νανο-δομημένων καταλυτικών συστημάτων βασισμένων σε οξείδιο του δημητρίου (CeO2): σχέσεις δομής-δραστικότητας

The rational design and development of highly-active and cost-efficient catalysts for energy and environmental applications constitutes the main research pillar in the area of heterogeneous catalysis. In this perspective, the present thesis aims at the fine-tuning of noble metal (NMs)-free metal oxide catalysts, such as ceria-based transition metal catalysts (MxOy/CeO2, where M stands for Cu, Co, Fe, Ni) by means of advanced synthetic and/or promotional routes. It was clearly revealed that the adjustment of size, shape and electronic state of ceria-based metal oxides (MOs) can exert a profound influence on the reactivity of metal sites as well as on metal-support interfacial activity, offering extremely active and stable materials for various applications, such as CO oxidation, N2O decomposition and CO2 hydrogenation to value-added products. Through the present thesis, the pivotal role of support morphology and surface promotion on the solid state properties, metal-support interactions and in turn, on the catalytic performance of ceria-based mixed oxides was unambiguously revealed. More importantly, the fine-tuning of size, shape and electronic state can notably affect not only the reactivity of metal sites but also the interfacial activity (e.g., through the formation of oxygen vacancies and the facilitation of redox interplay between the metal and the support) offering a synergistic contribution towards the development of highly active composites. Through the proposed optimization approach extremely active and cost-efficient catalytic materials were obtained for CO oxidation, N2O decomposition and CO2 hydrogenation reactions, being among the most active reported so far in open literature. The general optimization framework followed in the present thesis can provide the design principles towards the development of earth-abundant metal oxides for various energy and environmental applications paving also the way for the decrease of noble metals content in NMs-based catalysts.Ο ορθολογικός σχεδιασμός και η ανάπτυξη αποτελεσματικών και συνάμα οικονομικών καταλυτικών συστημάτων για ενεργειακές και περιβαλλοντικές εφαρμογές αποτελεί τον κύριο ερευνητικό πυλώνα στον τομέα της ετερογενούς κατάλυσης. Προς αυτή την κατεύθυνση, στόχο της παρούσας διδακτορικής διατριβής αποτελεί η ανάπτυξη νανο-δομημένων καταλυτικών συστημάτων, απαλλαγμένων ευγενών μετάλλων, όπως μετάλλων μετάπτωσης υποστηριγμένων σε οξείδιο του δημητρίου (MxOy/CeO2, όπου M: Cu, Co, Fe, Ni), μέσω κατάλληλης τροποποίησης/βελτιστοποίησης σημαντικών σχεδιαστικών παραμέτρων, όπως αυτές του μεγέθους, του σχήματος και της ηλεκτρονιακής κατάστασης. Η παρούσα διδακτορική διατριβή ανέδειξε τον ρόλο κλειδί της μορφολογίας του φορέα και της επιφανειακής ενίσχυσης στις ιδιότητες στερεάς κατάστασης καθώς και στις αλληλεπιδράσεις μετάλλου-φορέα. Ειδικότερα, η κατάλληλη τροποποίηση του μεγέθους, του σχήματος και της ηλεκτρονιακής κατάστασης βρέθηκε να επηρεάζει σε σημαντικό βαθμό τόσο τη δραστικότητα των ενεργών κέντρων όσο και τη διεπιφανειακή ενεργότητα (π.χ. μέσω του σχηματισμού κενών θέσεων οξυγόνου και της υποβοήθησης των οξειδοαναγωγικών αλληλεπιδράσεων ανάμεσα στο μέταλλο και το φορέα) συνεισφέροντας συνεργιστικά προς την ανάπτυξη ιδιαίτερα ενεργών υλικών. Η προτεινόμενη προσέγγιση βελτιστοποίησης οδήγησε σε εξαιρετικά ενεργά και σταθερά καταλυτικά υλικά χαμηλού κόστους κατά την επιτέλεση σημαντικών αντιδράσεων, όπως η οξείδωση του CO, η διάσπαση του Ν2Ο και η υδρογόνωση του CO2, κατατάσσοντάς τα μεταξύ των πλέον ενεργών υλικών εν συγκρίσει με την τρέχουσα ερευνητική στάθμη. Το γενικό πλαίσιο βελτιστοποίησης που ακολουθείται στην παρούσα διδακτορική διατριβή μπορεί να παρέχει τις σχεδιαστικές αρχές προς την ανάπτυξη μεταλλοξειδίων χαμηλού κόστους για ποικίλες ενεργειακές και περιβαλλοντικές εφαρμογές, ανοίγοντας παράλληλα το δρόμο προς τη μείωση της φόρτισης σε ευγενή μέταλλα καταλυτικών συστημάτων βασισμένων σε αυτά

Recent Advances on Fine-Tuning Engineering Strategies of CeO₂-Based Nanostructured Catalysts Exemplified by CO₂ Hydrogenation Processes

Ceria-based oxides have been extensively involved in a wide range of catalytic applications due to their intriguing properties, related mostly to their superior redox features in conjunction with peculiar metal-support interaction phenomena. Most importantly, the fine-tuning of key interrelated factors, such as the size, morphology and electronic state of the catalyst’s counterparts, can exert a profound influence on the intrinsic characteristics and interfacial reactivity with pronounced implications in catalysis. The present review, while also elaborating our recent efforts in the field, aims to provide key fundamental and practical aspects in relation to the rational design and functionalization strategies of ceria-based catalysts, exemplified by the CO2 hydrogenation processes, namely, CO2 methanation and reverse water–gas shift (rWGS) reactions. Firstly, a description of the most prominent catalytically relevant features of cerium oxide is provided, focusing on reducibility and metal-support interaction phenomena, followed by a brief overview of the current status of ceria-based catalysts for various energy and environmental applications. Then, the main implications of fine-tuning engineering via either appropriate synthesis routes or aliovalent doping on key activity descriptors are thoroughly discussed and exemplified by state-of-the-art ceria-based catalysts for CO2 hydrogenation. It is clearly revealed that highly active and cost-efficient ceria-based catalytic materials can be obtained on the grounds of the proposed functionalization strategy, with comparable or even superior reactivity to that of noble metal catalysts for both the studied reactions. In a nutshell, it can be postulated that the dedicated fabrication of CeO2-based systems with augmented redox capabilities and, thus, oxygen vacancies abundance can greatly enhance the activation of gas-phase CO2 towards CO or CH4. Besides, the morphology-engineering of CeO2-based catalysts can notably affect the CO2 hydrogenation performance, by means of an optimum metal-ceria interphase based on the exposed facets, whereas doping and promotion strategies can effectively shift the reaction pathway towards the selective production of either CO or CH4. The conclusions derived from the present work can provide design and fine-tuning principles for cost-efficient, highly active and earth-abundant metal oxide systems, not only for the CO2 hydrogenation process but for various other energy and environmental applications

Ceria Nanoparticles’ Morphological Effects on the N2O Decomposition Performance of Co3O4/CeO2 Mixed Oxides

Ceria-based oxides have been widely explored recently in the direct decomposition of N2O (deN2O) due to their unique redox/surface properties and lower cost as compared to noble metal-based catalysts. Cobalt oxide dispersed on ceria is among the most active mixed oxides with its efficiency strongly affected by counterpart features, such as particle size and morphology. In this work, the morphological effect of ceria nanostructures (nanorods (ΝR), nanocubes (NC), nanopolyhedra (NP)) on the solid-state properties and the deN2O performance of the Co3O4/CeO2 binary system is investigated. Several characterization methods involving N2 adsorption at −196 °C, X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (ΤΕΜ) were carried out to disclose structure–property relationships. The results revealed the importance of support morphology on the physicochemical properties and the N2O conversion performance of bare ceria samples, following the order nanorods (NR) > nanopolyhedra (NP) > nanocubes (NC). More importantly, Co3O4 impregnation to different carriers towards the formation of Co3O4/CeO2 mixed oxides greatly enhanced the deN2O performance as compared to bare ceria samples, without, however, affecting the conversion sequence, implying the pivotal role of ceria support. The Co3O4/CeO2 sample with the rod-like morphology exhibited the best deN2O performance (100% N2O conversion at 500 °C) due to its abundance in Co2+ active sites and Ce3+ species in conjunction to its improved reducibility, oxygen kinetics and surface area

Hydrothermal Synthesis of ZnO–doped Ceria Nanorods: Effect of ZnO Content on the Redox Properties and the CO Oxidation Performance

The rational design of highly efficient, noble metal-free metal oxides is one of the main research priorities in the area of catalysis. To this end, the fine tuning of ceria-based mixed oxides by means of aliovalent metal doping has currently received particular attention due to the peculiar metal-ceria synergistic interactions. Herein, we report on the synthesis, characterization and catalytic evaluation of ZnO–doped ceria nanorods (NR). In particular, a series of bare CeO2 and ZnO oxides along with CeO2/ZnO mixed oxides of different Zn/Ce atomic ratios (0.2, 0.4, 0.6) were prepared by the hydrothermal method. All prepared samples were characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction (TPR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM). The CO oxidation reaction was employed as a probe reaction to gain insight into structure-property relationships. The results clearly showed the superiority of mixed oxides as compared to bare ones, which could be ascribed to a synergistic ZnO–CeO2 interaction towards an improved reducibility and oxygen mobility. A close correlation between the catalytic activity and oxygen storage capacity (OSC) was disclosed. Comparison with relevant literature studies verifies the role of OSC as a key activity descriptor for reactions following a redox-type mechanism

CO2 Hydrogenation over Nanoceria-Supported Transition Metal Catalysts: Role of Ceria Morphology (Nanorods versus Nanocubes) and Active Phase Nature (Co versus Cu)

In this work we report on the combined impact of active phase nature (M: Co or Cu) and ceria nanoparticles support morphology (nanorods (NR) or nanocubes (NC)) on the physicochemical characteristics and CO2 hydrogenation performance of M/CeO2 composites at atmospheric pressure. It was found that CO2 conversion followed the order: Co/CeO2 > Cu/CeO2 > CeO2, independently of the support morphology. Co/CeO2 catalysts demonstrated the highest CO2 conversion (92% at 450 °C), accompanied by 93% CH4 selectivity. On the other hand, Cu/CeO2 samples were very selective for CO production, exhibiting 52% CO2 conversion and 95% CO selectivity at 380 °C. The results obtained in a wide range of H2:CO2 ratios (1–9) and temperatures (200–500 °C) are reaching in both cases the corresponding thermodynamic equilibrium conversions, revealing the superiority of Co- and Cu-based samples in methanation and reverse water-gas shift (rWGS) reactions, respectively. Moreover, samples supported on ceria nanocubes exhibited higher specific activity (µmol CO2·m−2·s−1) compared to samples of rod-like shape, disclosing the significant role of support morphology, besides that of metal nature (Co or Cu). Results are interpreted on the basis of different textural and redox properties of as-prepared samples in conjunction to the different impact of metal entity (Co or Cu) on CO2 hydrogenation process

Facet-Dependent Reactivity of Fe₂O₃/CeO₂ Nanocomposites: Effect of Ceria Morphology on CO Oxidation

Ceria has been widely studied either as catalyst itself or support of various active phases in many catalytic reactions, due to its unique redox and surface properties in conjunction to its lower cost, compared to noble metal-based catalytic systems. The rational design of catalytic materials, through appropriate tailoring of the particles’ shape and size, in order to acquire highly efficient nanocatalysts, is of major significance. Iron is considered to be one of the cheapest transition metals while its interaction with ceria support and their shape-dependent catalytic activity has not been fully investigated. In this work, we report on ceria nanostructures morphological effects (cubes, polyhedra, rods) on the textural, structural, surface, redox properties and, consequently, on the CO oxidation performance of the iron-ceria mixed oxides (Fe2O3/CeO2). A full characterization study involving N2 adsorption at –196 °C, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) was performed. The results clearly revealed the key role of support morphology on the physicochemical properties and the catalytic behavior of the iron-ceria binary system, with the rod-shaped sample exhibiting the highest catalytic performance, both in terms of conversion and specific activity, due to its improved reducibility and oxygen mobility, along with its abundance in Fe2+ species

Effect of the Preparation Method on the Physicochemical Properties and the CO Oxidation Performance of Nanostructured CeO2/TiO2 Oxides

Ceria-based mixed oxides have been widely studied in catalysis due to their unique surface and redox properties, with implications in numerous energy- and environmental-related applications. In this regard, the rational design of ceria-based composites by means of advanced synthetic routes has gained particular attention. In the present work, ceria–titania composites were synthesized by four different methods (precipitation, hydrothermal in one and two steps, Stöber) and their effect on the physicochemical characteristics and the CO oxidation performance was investigated. A thorough characterization study, including N2 adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM) and H2 temperature-programmed reduction (H2-TPR) was performed. Ceria–titania samples prepared by the Stöber method, exhibited the optimum CO oxidation performance, followed by samples prepared by the hydrothermal method in one step, whereas the precipitation method led to almost inactive oxides. CeO2/TiO2 samples synthesized by the Stöber method display a rod-like morphology of ceria nanoparticles with a uniform distribution of TiO2, leading to enhanced reducibility and oxygen storage capacity (OSC). A linear relationship was disclosed among the catalytic performance of the samples prepared by different methods and the abundance of reducible oxygen species

Remarkable efficiency of Ni supported on hydrothermally synthesized CeO2 nanorods for low-temperature CO2 hydrogenation to methane

project code: T1EDK-00094 IF/01381/2013/CP1160/CT0007 UIDB/50020/2020 UIDB/50006/2020Nickel particles deposited on hydrothermally synthesized ceria nanorods (CeO2-NR) were found to be highly active and stable for CO2 methanation. A CO2-to-CH4 yield of 92% was achieved at 300 °C. The impact of various operational parameters was explored in conjunction with a thermodynamic analysis. The superior performance of Ni/CeO2-NR was demonstrated through a comparison with i) CeO2 and Ni/CeO2 commercial products, ii) various M/CeO2-NR lab-synthesized catalysts (M = Cu, Co, Fe), and iii) state-of-the-art literature catalysts. The results revealed that a unique combination of Ni with ceria nanorods is required for boosting the reducibility and in turn the methanation efficiency.authorsversionpublishe