Citation: Gkatziouras, C.; Solakidou, M.; Louloudi, M. Efficient [Fe-Imidazole@SiO 2 ] Nanohybrids for Catalytic H 2 Production from Formic Acid. Nanomaterials 2023, 13, 1670. https://doi.org/10.3390/ nano13101670 Academic Editor: Vincenzo Vaiano Received: 28 April 2023 Revised: 14 May 2023 Accepted: 16 May 2023 Published: 18 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Efficient [Fe-Imidazole@SiO 2 ] Nanohybrids for Catalytic H 2 Production from Formic Acid Christos Gkatziouras, Maria Solakidou and Maria Louloudi * Laboratory of Biomimetic Catalysis & Hybrid Materials, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece; ch.gkatziouras@uoi.gr (C.G.); m.solakidou@uoi.gr (M.S.) * Correspondence: mlouloud@uoi.gr Abstract: Three imidazole-based hybrid materials, coded as IGOPS, IPS and impyridine@SiO 2 nanohybrids, were prepared via the covalent immobilization of N-ligands onto a mesoporous nano- SiO 2 matrix for H 2 generation from formic acid (FA). BET and HRTEM demonstrated that the immobi- lization of the imidazole derivative onto SiO 2 has a significant effect on the SSA, average pore volume, and particle size distribution. In the context of FA dehydrogenation, their catalytic activity (TONs, TOFs), stability, andreusability were assessed. Additionally, the homologous homogeneous counter- parts were evaluated for comparison purposes. Mapping the redox potential of solution E h vs. SHE revealed that poly-phosphine PP 3 plays an essential role in FA dehydrogenation. On the basis of per- formance and stability, [Fe 2+ /IGOPS/PP 3 ] demonstrated superior activity compared to other hetero- geneous catalysts, producing 9.82 L of gases (VH 2 + CO 2 ) with TONs = 31,778, albeit with low recycla- bility. In contrast, [Fe 2+ /IPS/PP 3 ] showed the highest stability, retaining considerable performance af- ter three consecutive uses. With VH 2 + CO 2 = 7.8 L, [Fe 2+ /impyridine@SiO 2 /PP 3 ] activity decreased, and it was no longer recyclable. However, the homogeneous equivalent of [Fe 2+ /impyridine/PP 3 ] was completely inactive. Raman, FT/IR, and UV/Vis spectroscopy demonstrated that the reduced recyclability of [Fe 2+ /IGOPS/PP 3 ] and [Fe 2+ /impyridine@SiO 2 /PP 3 ] nanohybrids is due to the reductive cleavage of their C-O-C bonds during catalysis. An alternative grafting procedure is proposed, applying here to the grafting of IPS, resulting in its higher stability. The accumula- tion of water derived from substrate’s feeding causes the inhibition of catalysis. In the case of [Fe 2+ -imidazole@SiO 2 ] nanohybrids, simple washing and drying result in their re-activation, over- coming the water inhibition. Thus, the low-cost imidazole-based nanohybrids IGOPS and IPS are capable of forming [Fe 2+ /IGOPS/PP 3 ] and [Fe 2+ /IPS/PP 3 ] heterogeneous catalytic systems with high stability and performance for FA dehydrogenation. Keywords: formic acid; hydrogen production; dehydrogenation of formic acid; iron nanocatalysts; imidazole; pyridine 1. Introduction The clean energy potential of molecular hydrogen (H 2 ) has garnered significant interest due to its favorable characteristics, such as its energy density, which is 2.6 times greater than that of gasoline, and the absence of toxic byproducts during the combustion process [1–3]. However, free H 2 does not exist on Earth and a primary energy source is required for its production. Within the concept of a cyclic economy, the production of H 2 that is fully reliant on renewable sources includes two independent processes. The first process involves the generation of H 2 through the dehydrogenation of a hydrocarbon substrate, while the second one involves the reduction of CO 2 to produce hydrocarbon fuels [4,5]. The technology in question has the potential to revolutionize the industry, as it is worth noting that a significant majority of H 2 generation, specifically 96%, currently relies on non-renewable sources such as fossil fuels [6]. Formic acid (FA) is a highly promising substrate for providing H 2 , owing to its favorable cost and simplicity of handling [7,8]. The Nanomaterials 2023, 13, 1670. https://doi.org/10.3390/nano13101670 https://www.mdpi.com/journal/nanomaterials