Maria Louloudi

Plasmon-assisted molecular catalysis (PAMC) is a novel phenomenon offering new tools for green H₂ production via formic acid dehydrogenation (FADH). Herein, we introduce the all-solid-state PAMC concept, where plasmonic films are used to boost FADH/H₂ catalysis by heterogeneous molecular catalysts. An integrated {Ag@SiO₂ plasmonic film}/{FeL@SiO₂ catalyst} system has been developed, in which a Fe catalyst was immobilized on SiO₂ particles and Ag@SiO₂ core–shell photoplasmonics were embedded in a film. Using flame spray pyrolysis (FSP) technology, a library of Ag@SiO₂ films with a controlled thickness D in the range of 1.5–12 μm has been engineered. The integrated {Ag@SiO₂ plasmonic film}/{FeL@SiO₂ catalyst} hybrid was optimized as an efficient photoplasmonic booster of H₂ production by HCOOH, achieving a >450% enhancement of the H₂ production rate by the FeL@SiO₂ catalyst. Under selective photoplasmonic 405 nm LED excitation, we found that the Ag@SiO₂ film thickness is the key determinant to achieve significant H₂/FADH boosting. The optimal D was 2 μm, which allowed a photoplasmonic boost of up to 440%, corresponding to a maximum H₂ production rate of 7.4 mL min^–1 (μmol of Fe catalyst)⁻¹. The thermodynamic basis of this phenomenon is exemplified via an Arrhenius study, revealing that the Ag@SiO₂ plasmonic film can decrease the activation energy (E_a) by 74% under 405 nm illumination. Αnalysis of the chemical basis of the phenomenon (via study of the solution potential (E_h)) reveals a significant contribution from photoplasmonic hot carriers generated by the Ag@SiO₂ film to be the key factor boosting H₂/FADH. These findings demonstrate the strong potential of integrated all-solid-state PAMC systems to deliver efficient, scalable, and light-responsive H₂ production technologies based on earth-abundant Fe catalysts and HCOOH as a commodity liquid hydrogen carrier.

Renewable H₂ production emerges as a forward-looking technology towards green energy transition. Herein, we present a study on novel multimetallic nano-oxides used as co-catalysts for H₂ production via HCOOH dehydrogenation (FADH) by an Fe^II(Polyphosphine) molecular catalyst, under near-ambient P, T conditions. The co-catalyst nano-oxides consist of multimetallic {LaSrCrFeO} and {LaSrCrFeVO} perovskites, produced by flame spray pyrolysis (FSP) technology. Kinetic catalytic H₂ evolution data show that both {LaSrCrFeO} and {LaSrCrFeVO} significantly boost H₂ via co-catalytic action. Arrhenius analysis reveals that they decrease the rate-limiting activation energy, E_a. Specifically, E_a = 77.4 kJ mol⁻¹ of {Fe²⁺/PP₃} catalyst is decreased to E_a = 67.8 kJ mol⁻¹ in {LaSrCrFeO + Fe²⁺/PP₃} and E_a = 56.2 kJ mol⁻¹ in {LaSrCrFeVO + Fe²⁺/PP₃} catalyst. These significant thermodynamic effects are not observed when the simple parental oxides are used. The present findings are discussed in the context of a boosting role {LaSrCrFeO} and {LaSrCrFeVO} to the key catalytic intermediates of the Fe^II(Polyphosphine) catalyst. Technology-wise, this work exemplifies a novel strategy for the industrial production of co-catalysts using FSP technology within the in-situ H₂ production landscape.

Covalent grafting of Fe-imidazole on FSP-made graphitized carbon nanoparticles with a magnetic Fe-core yields a highly durable hybrid catalyst for H ₂ production.

The use of hybrids, developed by grafting homogeneous catalysts onto supporting materials, has already demonstrated significant potential in various catalytic processes. These systems combine the advantages of homogeneous catalysts, such as high activity and selectivity, with those of solid supports, including enhanced recyclability. Catalytic hydrogen (H ₂ ) production via dehydrogenation of C1 organic molecules targeting its use in fuel cells is a contemporary scientific issue directly connected with climate crisis. Here, Iminophospine hybrid [SiO ₂ @benzNP] and its reduced analogue [SiO ₂ @benzNHP] were synthesized, covalently grafted on colloidal SiO ₂ , fully characterized (FT‐IR, RAMAN, TGA, ssNMR, BET), and used for in‐situ synthesis of [SiO ₂ @benzNP‐Fe ^II ] and [SiO ₂ @benzNHP‐Fe ^II ] catalytic complexes for H ₂ production from formic acid (HCOOH) and formaldehyde (HCHO), at 80 °C. In HCOOH, both heterogenized catalysts exhibit high selectivity, producing H ₂ and CO ₂ in a 1 : 1 ratio, without CO contamination, making them ideal for fuel cell applications. [SiO₂@benzNHP‐Fe ^II ] catalyst demonstrated superior performance in both substates. In HCOOH dehydrogenation, over 82,000 turnover number (TONs) were achieved and retained its efficiency for over five cycles, without any further metal addition. In HCHO dehydrogenation, it showed excellent efficiency as well, achieving 1.3 L of pure H ₂ with TONs exceeding 7,000, in 3 consecutive uses. Advanced spectroscopic analysis confirmed the stability and structural integrity of the catalysts, linking the Schiff base reduction and N−H groups to enhanced activity, durability and reusability. This study demonstrates the potential of hybrid materials with non‐noble metals for cost‐effective and sustainable H ₂ production, paving the way for scalable renewable energy solutions.

Two Fe-based hybrids, [SiO₂@NP(Ph)₂/Fe^II/PP₃] and [SiO₂@NP(t-Bu)₂/Fe^II/PP₃], were synthesized using the double-ligand approach by covalently grafting NP ligands onto the surface of SiO₂. Both catalytic systems were evaluated for H₂ production through formic acid dehydrogenation (FADH), revealing important efficiency without requiring additional additives and/or co-catalysts. During the continuous addition of FA, [SiO₂@NP(Ph)₂/Fe^II/PP₃] and [SiO₂@NP(t-Bu)₂/Fe^II/PP₃] demonstrated great stability, achieving total TONs = 20,636 and 20,854, respectively. FT-IR and Raman spectroscopy provided insights into the role of NP ligands, such as NP(Ph)₂ and NP(t-Bu)₂, on the assembly and structural configuration of active hybrid Fe catalysts and their ability to dehydrogenate formic acid. Additional studies, including in situ mapping of the solution potential (E_h) of the catalytic reaction and an Arrhenius study of the activation energy (E_a), revealed a correlation between E_a and H₂ production rates: the system [SiO₂@NP(Ph)₂/Fe^II/PP₃] with an E_a = 29.4 KJ/mol shows an H₂ production rate of 58 mL-H₂/min, while [SiO₂@NP(t-Bu)₂/Fe^II/PP₃] with a E_a = 50.6 KJ/mol shows an H₂ production rate of 55 mL-H₂/min. This is the first example of a heterogeneous FADH system where the original strategy of a “double-ligand” has been demonstrated for homogeneous FADH catalytic systems. Herein we demonstrate that we can engineer a decrease in the activation barrier Ea via two synergistic steps: (i) via grafting of the NP ligand onto SiO₂ and (ii) using PP₃ as double ligand. This strategy leads to a boost in the H₂ production efficiency of [SiO₂@NP(Ph)₂/Fe^II/PP₃] as a heterogeneous catalyst, which for the first time has been shown to be able to outperform the parental reference/homogenous catalyst [Fe^II/PP₃].

Hydrogen atom transfer (HAT) and single electron transfer (SET) are two fundamental pathways for antiradical/antioxidant processes; however, a systematic in-tandem operational evaluation of the same system is lacking. Herein, we present a comparative study of the HAT and SET processes applied to a library of well-characterized hybrid materials SiO₂@GA, SiO₂@GLA, SiO₂@GLAM, and the doubly hybrid material {GLA@SiO₂@GLAM}. Hydroxyl radicals (^•OH), produced by a Fenton system, react via the single electron transfer (SET) pathway and hydrogen atom transfer, through oxygen- and carbon-atoms, respectively, while the stable-radical DPPH via the HAT pathway through oxygen-atoms. Electron paramagnetic resonance spectroscopy (EPR), eminently suited for in situ detection and quantification of free radicals, was used as a state-of-the-art tool to monitor ^•OH using the spin-trapping-EPR method. We found that the SiO₂@GA hybrid exhibited the highest SET ^•OH-scavenging activity i.e., [2.7 mol of ^•OH per mol of grafted GA]. Then, SiO₂@GLA, SiO₂@GLAM, and GLA@SiO₂@GLAM can scavenge 1.2, 1.3, and 0.57 mol of ^•OH per mol of anchored organic, respectively. The HAT efficiency for SiO₂@GA was [2.0 mol of DPPH per mol of grafted GA], while SiO₂@GLA, SiO₂@GLAM, and GLA@SiO₂@GLAM exhibited a HAT efficiency of 1.1 DPPH moles per mol of anchored organic. The data are analyzed based on the molecular structure of the organics and their -R-OH moieties. Accordingly, based on the present data we suggest that for hydroxyl (^•OH) radicals, the mechanisms involved are SET from an oxygen atom and HAT from a carbon atom. In contrast, for DPPH radicals, the HAT mechanism is exclusively operating and involves hydrogen atom abstraction from OH groups.

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.

The utilization of plasmonic nanomaterials in catalytic technologies is an emerging research field with foreseeable applications in energy-catalytic technologies. On this front, the coupling of plasmonic nanomaterials with molecular catalysts is a newly approached, thus far unexploited field, that we discuss herein. In the present mini review, we contrast the case where the plasmonic particle itself is the catalytic center against the case where the plasmonic particle acts as a co-catalyst for an operational catalytic system. In the first part, we present an outline of the key phenomena in nanoplasmonics, and their potential implications in catalytic processes. The concepts of hot electrons, hot holes, and the dynamics of their generation and transfer are reviewed, as are the contribution of near-field and photothermal effects to catalytic processes. All these plasmonic-phenomena are then discussed in conjunction with representative catalytic systems from the literature.

In the current study, the production of novel antioxidants for hygienic disinfection against common pathogenic bacteria, based on the incorporation of bioextractant oils/waters from either lavender or oregano distillates is proposed in the framework of circular economy. For the first time, the main compounds found in distillation products (oils/waters), specifically of lavender Lavandula angustifolia (lynalyl acetate and linalool) and of oregano Oreganum vulgare (carvacrol, thymol, and p-cymene) are presented. The analyses of both the lavender and oregano essential oils/waters indicate excellent physicochemical properties and microbial absence. Moreover, the antioxidant activity of all distillates as DPPH radical scavengers is assessed. The results confirm that the essential oils of both oregano and lavender possess superior antioxidant activity to their corresponding waters, while the oregano oil exhibited far better antioxidant activity than the lavender oil, as 1 mL of oregano oil was able to consume 45 μmoles of DPPH radicals. Overall, our research findings suggest that the particular lavender and oregano bioextractants produced possess important potential to address the resistance of bacteria from the perspective of their wider exploitation in therapeutic or preventive medicine, thus contributing to enhancing public health.