Konstantina Gravvani, Maria Solakidou, Maria Louloudi

https://doi.org/10.1002/chem.202404440

This study presents the design and application of two Fe(II)-based hybrid catalysts, [SiO₂@benzNP-Fe^II] and [SiO₂@benzNHP-Fe^II], for efficient hydrogen (H₂) production via formic acid (HCOOH) and formaldehyde (HCHO) dehydrogenation. The reduction of Schiff base ligands to incorporate N−H groups significantly enhances efficiency, stability, and reusability. Achieving >80,000 total turnover numbers (TONs), the results highlight their potential for sustainable energy solutions.

Christos Gkatziouras, Maria Solakidou, Maria Louloudi.

Formic Acid Dehydrogenation over a Recyclable and Self-Reconstructing Fe/Activated Carbon Catalyst

Energy Fuels 2024, 38, 18, 17914–17926

Molecular Catalysis; FA Dehydrogenation

Abstract

A novel catalyst, denoted as AC_ox@ImFe, was synthesized using matrix-activated oxidized carbon (AC_ox) featuring a [Fe²⁺-imidazole]-based complex covalently bonded to the carbon surface through Si–O–C bonding. The catalytic system, distinguished by its innovative hybrid structure that includes Fe²⁺ and imidazole on an oxidized carbon matrix in the presence of a polydentate phosphine, demonstrated remarkable turnover numbers (TONs), reaching 428,880 and effectively decomposed 53 mL of formic acid (FA) over 8 cycles. This sustained performance underscores the effectiveness, stability, and durability of the catalyst, which was further evidenced by a cumulative H₂ production of 22.1 L over the same period. Structural analysis using Raman, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and electron paramagnetic resonance (EPR) spectroscopy revealed structural changes in the used catalyst compared to the pristine material. Despite the observed structural changes, such as Fe-site aggregation and matrix restructuring, the catalyst maintained a high efficiency, with enhanced activity noted with each reuse. The stability of the carbon-based radicals from the matrix was confirmed, which is crucial for the sustained performance of the catalyst. Notably, upon repeated use, the catalyst underwent a self-reconstruction process, which is linked to alterations in the hydrophobicity of the catalyst and the overall structure, resulting in enhanced water durability and improved performance, making AC_ox@ImFe a robust and effective system for sustainable H₂ production.

Marinos Theodorakopoulos, Maria Solakidou, Yiannis Deligiannakis, MariaLouloudi.

Energies 2024, 17(16), 3934; 

https://doi.org/10.3390/en17163934

Formic acid dehydrogenation; H₂ production; Double ligand; Iron catalysts; PNP ligands

Abstract

Two types of iron-based catalysts, [Fe/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/PP3], for the dehydrogenation of formic acid (FADH), were synthesized. These catalysts were developed using a double-ligand approach combining a PNP ligand and a PP3 ligand, demonstrating functionality without the need for additional cocatalysts or additives. Furthermore, hybrid catalysts [Fe/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/PP3] were created by covalently grafting PNP ligands onto SiO₂ particles. The hybrid [Fe/SiO₂@^iProPNP/PP3] exhibited enhanced recyclability, with turnover numbers (TONs) exceeding 74,000. In situ ATR-FTIR and UV-Vis spectroscopies were used to monitor the structure and dynamics of the catalysts under catalytic conditions, revealing the formation of active catalysts through the involvement of all components: [Fe (metal)/PNP (first ligand)/PP3 (second ligand)/FA (substrate)], which are crucial to FADH catalysis. An Arrhenius study revealed that the hybrid [Fe/SiO₂@^iProPNP/PP3] had a lower activation energy (E_a = 42.5 kJ/mol) compared to its homogeneous counterpart (E_a = 48.2 kJ/mol), indicating superior catalytic performance. Conversely, [Fe/SiO₂@^tBuPNP/PP3] showed an increased activation energy (E_a = 48.3 kJ/mol) compared to its homogeneous form (E_a = 46.4 kJ/mol). This study discusses the differing roles of ^tBuPNP and ^iProPNP in catalyst configuration, highlighting the potential of double-ligand catalysts to enhance the performance and recyclability of PNP ligands in FADH, offering significant implications for the development of efficient and reusable catalytic systems.

Fotini Fragou, Areti Zindrou, Yiannis Deligiannakis, Maria Louloudi.

ACS Appl. Nano Mater. 2024, 7, 9, 10552–10564

https://doi.org/10.1021/acsanm.4c00950

Abstract

Biocidal activity and radical scavenging capacity (RSC), two seemingly opposing concepts, can coexist in engineered nanoceria (CeO₂) materials. In the present study, a series of CeO_2–x (x = 0–0.75) nanoparticles have been engineered utilizing the anoxic-flame spray pyrolysis (A-FSP) technology. A-FSP allows for tuning of the physicochemical and structural properties of CeO_2–x arising from lattice defects (Ce³⁺ and V_os) while maintaining minimal carbon incorporation. Our study aimed to understand the complex relationships between the biocidal and antioxidant activities of CeO_2–x, concepts whose origin was not sufficiently detangled in the bibliography. The biocide profiles of CeO_2–x nanoparticles toward the marine bacterium Aliivibrio fischeri were studied in tandem with their reactive oxygen species (ROS) scavenging capacity. A key finding of the present study is that the A-FSP process allows selective engineering of cluster-type Ce³⁺ and V_o defects, while typical, nonanoxic nanoceria structures (code-named ox-CeO₂) present mainly monomeric Ce³⁺ defects. The type of Ce³⁺ defects directly impacts the ROS scavenging efficiency. In addition, structural modifications that occur from the presence of cluster-type Ce³⁺ defects, such as larger particle sizes, are directly associated with lower biocidal activity. Thus, the findings of this study indicate that biocidal and ROS antioxidant activities are not mutually exclusive properties.

CeO₂; Flame Spray Pyrolysis; Anoxic; Nanotoxicity; Aliivibrio fischeri; EPR Spectroscopy; Αcute Τoxicity; Ιnterfacial Ιinteractions.

Fotini Fragou Areti Zindrou
Yiannis Deligiannakis, Maria Louloudi.

Carbon–SiO2 Hybrid Nanoparticles with Enhanced Radical Stabilization and Biocide Activity

ACS Appl. Nano Mater. 2023, 6, 22, 20841–20854

Abstract

Nanocarbon and nanosilica are widely-used nanomaterials of significant industrial interest. Herein, a library of {carbon–SiO₂} hybrid nanoparticles, with controlled nanocarbon content and sp³/sp² profile, was engineered by a flame spray pyrolysis (FSP) process in one step. Their structure–function relationship was evaluated by studying their radical-generation and radical-stabilization activities, using electron paramagnetic resonance spectroscopy in tandem with their biocide activity toward marine bacteria Aliivibrio fischeri. The surface properties of the C–SiO₂ hybrids were studied by Raman spectroscopy and surface-charge analysis by zeta potential measurements. Raman spectroscopy indicates that the FSP process, as designed and used herein, allows the progressive incorporation of nanocarbon moieties into the SiO₂ nanostructure, which may lead to distortion of the siloxane matrix. Concurrently, the C–SiO₂ hybrids’ surface charge profile reveals a trend correlated with the acute biocide activity toward Aliiv. fischeri. Interestingly, we find no correlation between the stable radical population and the nanohybrids’ biocide activity. Within this context, we discuss the role of surface charge, radical properties, and SiO₂ lattice distortions by the nanocarbon, all of them parametrized via FSP process protocols. Thus, the present study’s findings provide critical insight into the structure–biocide activity relationship of carbon–SiO₂ hybrid nanoparticles. Technology-wise, the present work exemplifies a scalable FSP process for the industrial production of applied nanomaterials, such as C–SiO₂ nanostructures.

SiO₂; Νanocarbon; Nanohybrids; Flame Spray Pyrolysis; Nanotoxicity; Aliivibrio fischeri; EPR Spectroscopy; Biocide Activity