The aim of the present work was to investigate steam reforming of ethanol with regard to H₂ production over transition metal catalysts supported on CeO₂. Various parameters concerning the effect of temperature (400–800 °C), steam-to-carbon (S/C) feed ratio (0.5, 1.5, 3, 6), metal entity (Fe, Co, Ni, Cu) and metal loading (15–30 wt.%) on the catalytic performance, were thoroughly studied. The optimal performance was obtained for the 20 wt.% Co/CeO₂ catalyst, achieving a H₂ yield of up to 66% at 400 °C. In addition, the Co/CeO₂ catalyst demonstrated excellent stability performance in the whole examined temperature range of 400–800 °C. In contrast, a notable stability degradation, especially at low temperatures, was observed for Ni-, Cu-, and Fe-based catalysts, ascribed mainly to carbon deposition. An extensive characterization study, involving N₂ adsorption-desorption (BET), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM/EDS), X-ray Photoelectron Spectroscopy (XPS), and Temperature Programmed Reduction (H₂-TPR) was undertaken to gain insight into the structure-activity correlation. The excellent reforming performance of Co/CeO₂ catalysts could be attributed to their intrinsic reactivity towards ethanol reforming in combination to their high surface oxygen concentration, which hinders the deposition of carbonaceous species.

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.

The increasing trend in global energy demand has led to an extensive use of fossil fuels and subsequently in a marked increase in atmospheric CO₂ content, which is the main culprit for the greenhouse effect. In order to successfully reverse this trend, many schemes for CO₂ mitigation have been proposed, taking into consideration that large-scale decarbonization is still infeasible. At the same time, the projected increase in the share of variable renewables in the future energy mix will necessitate large-scale curtailment of excess energy. Collectively, the above crucial problems can be addressed by the general scheme of CO₂ hydrogenation. This refers to the conversion of both captured CO₂ and green H₂ produced by RES-powered water electrolysis for the production of added-value chemicals and fuels, which are a great alternative to CO₂ sequestration and the use of green H₂ as a standalone fuel. Indeed, direct utilization of both CO₂ and H₂ via CO₂ hydrogenation offers, on the one hand, the advantage of CO₂ valorization instead of its permanent storage, and the direct transformation of otherwise curtailed excess electricity to stable and reliable carriers such as methane and methanol on the other, thereby bypassing the inherent complexities associated with the transformation towards a H₂-based economy. In light of the above, herein an overview of the two main CO₂ abatement schemes, Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU), is firstly presented, focusing on the route of CO₂ hydrogenation by green electrolytic hydrogen. Next, the integration of large-scale RES-based H₂ production with CO₂ capture units on-site industrial point sources for the production of added-value chemicals and energy carriers is contextualized and highlighted. In this regard, a specific reference is made to the so-called Power-to-X schemes, exemplified by the production of synthetic natural gas via the Power-to-Gas route. Lastly, several outlooks towards the future of CO₂ hydrogenation are presented.

The impact of carbon type on the performance of the direct carbon fuel cell (DCFC) or hybrid carbon fuel cell (HCFC) is investigated by utilizing bare carbon or carbon/carbonate mixtures as feedstock, respectively. In this regard, four different types of carbons, i.e. bituminous coal (BC), demineralised bituminous coal (DBC), anthracite coal (AC) and pine charcoal (PCC), are employed as fuels in a SOFC of the type: carbon (carbonate)|Cu–CeO₂/YSZ/Ag|Air. The results reveal that in the absence of carbonates (DCFC configuration) the optimum performance, in terms of maximum power density (P_max), is obtained for the charcoal sample, which demonstrated a power output of ∼12 mW cm⁻² at 800 °C, compared to 3.4 and 4.6 mW cm⁻² with the anthracite and bituminous samples, respectively. Demineralization treatment of bituminous coal is found to improve the DCFC performance resulting in a maximum power density of 5.5 mW cm⁻². A similar trend in terms of maximum power density, i.e., PCC > DBC > BC > AC, is obtained in the hybrid carbon fuel cell (HCFC) employing a eutectic mixture of lithium and potassium carbonates (62 mol% Li₂CO₃ + 38 mol% K₂CO₃) in the anode compartment at a carbon/carbonate weight ratio of 4 : 1. An enhancement of up to 185% in the maximum power density is achieved by admixing molten carbonates with carbon feedstock, with its extent being dependent on carbon type and temperature. The obtained results are interpreted on the basis of carbon physicochemical characteristics and their impact on DCFC performance. It is found that the observed trend in volatile matter, porosity and structure disorder is perfectly correlated with the achieved power output. In contrast, high ash and sulfur contents notably inhibit the electrochemical performance. The superior performance demonstrated by pine charcoal in conjunction with its availability and renewable nature, reveals the potential of biomass as feedstock in both DCFCs and HCFCs.

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.

Different lanthanide (Ln)-doped cerium oxides (Ce_0.5Ln_0.5O_1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N₂ adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H₂. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO₂ ≈ Ce_0.5Pr_0.5O_1.75 > Ce_0.5Sm_0.5O_1.75 > Ce_0.5Gd_0.5O_1.75 > Ce_0.5Nd_0.5O_1.75 > Ce_0.5La_0.5O_1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO₂ and Cu/Ce_0.5Pr_0.5O_1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen.