George E. Marnellos

The aim of this study is to provide a comprehensive analysis of the outcome of two separate techno-economic studies that were conducted for the scaled-up and industrially relevant processes of a) synthetic natural gas (SNG) production from captured (cement-based) CO₂ and green-H₂ (via renewable-assisted electrolysis) and b) combined electricity and crude biofuel production through the integration of biomass pyrolysis, gasification, and solid oxide fuel cells. As was found, the SNG production process seems more feasible from an economic perspective as it can be comparable to current market values.

Layered double hydroxide (LDH)-derived mixed oxides offer a promising approach for CO₂ hydrogenation to light hydrocarbons. Herein, we explore the impact of various transition metals (X = Mn, Co, Cu, and Zn) incorporated into the M-Al or M-(Al+Fe) LDH structures, with the aim of exploring possible synergistic effects. Structural and compositional analyses reveal that an abundance of Fe over Al (Fe/Al ratio ~4) leads to the formation of mixed oxide crystalline phases attributed to CoFe₂O₄, CuFe₂O₄, and ZnFe₂O₄. Catalytic evaluation results demonstrate that the X-Al LDH-derived oxides exhibit high CO₂ conversion yet are selective to CH₄ or CO. In contrast, Fe incorporation shifts selectivity toward higher hydrocarbons. Specifically, the yield to higher hydrocarbons (C₂⁺) follows the order Ζn-Al-Fe > Cu-Al-Fe > Mn-Al-Fe > Co-Al-Fe >> Mn-Al, Co-Al, Zn-Al, Cu-Al, highlighting the pivotal role of Fe. Moreover, Zn-Al-Fe and Mn-Al-Fe catalysts have been shown to be the most selective towards light olefins. Zn-based systems also exhibit high thermal and structural stability with minimal coke formation, whereas Co-, Cu-, and Mn-based catalysts, when modified with Fe, experience increased carbon deposition or structural changes that may impact long-term stability. This work provides insights into the combined role of Fe and a second transition metal in LDHs for modulating catalytic activity, phase transformations, and stability, underscoring the need for further optimization to balance selectivity and catalyst durability in CO₂ hydrogenation applications.

Herein, motivated by the excellent redox properties of rod-shaped ceria (CeO₂-NR), a series of TM/CeO₂ catalysts, employing the first-row 3d transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) as active metal phases, were comparatively assessed under identical synthesis and reaction conditions to decipher the role of active metal in the CO₂ hydrogenation process. Notably, a volcano-type dependence of CO₂ hydrogenation activity/selectivity was disclosed as a function of metal entity revealing a maximum for the Ni-based sample. Ni/CeO₂ is extremely active and fully selective to methane (Y_CH4 = 90.8% at 350 °C), followed by Co/CeO₂ (Y_CH4 = 45.2%), whereas the rest of the metals present an inferior performance. No straightforward relationship was disclosed between the CO₂ hydrogenation performance and the textural, structural, and redox properties, whereas, on the other hand, a volcano-shaped trend was established with the relative concentration of oxygen vacancies and partially reduced Ce³⁺ species. The observed trend is also perfectly aligned with the previously reported volcano-type dependence of atomic hydrogen adsorption energy and CO₂ activation as a function of 3d-orbital electron number, revealing the key role of intrinsic electronic features of each metal in conjunction to metal–support interactions.

Non-noble metal electrocatalysts for the oxygen evolution reaction (OER) have recently gained particular attention. In the present work, a facile one-step electrodeposition method is applied in situ to synthesize cobalt sulfide nanostructures on nickel foam (NF) electrodes. For the first time, a systematic study is carried out on the impact of the Co/S molar ratio on the structural, morphological, and electrochemical characteristics of Ni-based OER electrodes by employing Co(NO₃)₂·6 H₂O and CH₄N₂S as Co and S precursors, respectively. The optimum performance was obtained for an equimolar Co:S ratio (1:1), whereas sulfur-rich or Co-rich electrodes resulted in an inferior behavior. In particular, the Co_xS_y@NF electrode with Co/S (1:1) exhibited the lowest overpotential value at 10 mA cm⁻² (0.28 V) and a Tafel slope of 95 mV dec⁻¹, offering, in addition, a high double-layer capacitance (C_DL) of 10.7 mF cm⁻². Electrochemical impedance spectroscopy (EIS) measurements confirmed the crucial effect of the Co/S ratio on the charge-transfer reaction rate, which is maximized for a Co:S molar ratio of 1:1. Moreover, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) were conducted to gain insights into the impact of the Co/S ratio on the structural and morphological characteristics of the electrodes. Notably, the Co_xS_y@NF electrocatalyst with an equimolar Co:S ratio presented a 3D flower-like nanosheet morphology, offering an increased electrochemically active surface area (ESCA) and improved OER kinetics.