The instability of molecular electrodes under oxidative/reductive conditions and insufficient understanding of the metal oxide-based systems have slowed down the progress of H₂-based fuels. Efficient regeneration of the electrode's performance after prolonged use is another bottleneck of this research. This work represents the first example of a bifunctional and electrochemically regenerable molecular electrode which can be used for the unperturbed production of H₂ from water. Pyridyl linkers with flexible arms (–CH₂–CH₂–) on modified fluorine-doped carbon cloth (FCC) were used to anchor a highly active ruthenium electrocatalyst [Ru^II(mcbp)(H₂O)₂] (1) [mcbp²⁻ = 2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine]. The pyridine unit of the linker replaces one of the water molecules of 1, which resulted in RuPFCC (ruthenium electrocatalyst anchored on –CH₂–CH₂–pyridine modified FCC), a high-performing electrode for oxygen evolution reaction [OER, overpotential of ∼215 mV] as well as hydrogen evolution reaction (HER, overpotential of ∼330 mV) at pH 7. A current density of ∼8 mA cm⁻² at 2.06 V (vs. RHE) and ∼−6 mA cm⁻² at −0.84 V (vs. RHE) with only 0.04 wt% loading of ruthenium was obtained. OER turnover of >7.4 × 10³ at 1.81 V in 48 h and HER turnover of >3.6 × 10³ at −0.79 V in 3 h were calculated. The activity of the OER anode after 48 h use could be electrochemically regenerated to ∼98% of its original activity while it serves as a HE cathode (evolving hydrogen) for 8 h. This electrode design can also be used for developing ultra-stable molecular electrodes with exciting electrochemical regeneration features, for other proton-dependent electrochemical processes.

Green Chemistry has received widespread interest due to its capacity to meet environmental and economic objectives. The Twelve Principles were proposed to better perform Green Chemistry and have become the guideline for solving many environmental issues. Water contamination has become a major global challenge in the 21st century. Millions of people die from diseases caused by drinking contaminated water. Nitrate, metal ions and dye are the most frequent contaminants. Nitrate in drinking water, after ingestion, is reduced to nitrite by the gastrointestinal tract and threatens human health. Dye-polluted water is usually nonbiodegradable and poisonous: the main criticism is that it is harmful to human health and hampers the photosynthesis rate of aquatic life. Metal ions generally lead to biological and physiological complications when they bind to cellular macromolecules. Therefore, efficient and eco-friendly purification technology is pressing to provide solutions for water purification. 

This thesis is set out to investigate the electro-/photo- catalytical water purification techniques using different catalysts. Efficient nitrate electrochemical reduction was achieved by using NDC materials, and the active sites were determined with the help of a comprehensive solid-state NMR supported by theoretical calculation and DFT calculations. Furthermore, the photochemical dye degradation was performed using cellulose-based hybrid bio-inorganic catalysts. The intentional maintenance of the surface functional groups on cellulose-based materials can promote dye degradation performance and, most importantly, achieve simultaneous removal of heavy metal ions aside from photo dye degradation. Additionally, this thesis proposed two possible synthesis strategies to obtain electro-/photo- catalysts using cellulose-based materials as renewable resources. The Twelve Principles of Green Chemistry guided the optimization of the synthesis route and raw material selectivity. Notably, the low-temperature synthesis of hybrid photocatalysts maintained the surface functional groups and preserved the kinetic mechanism of contaminants' adsorption on bio-substrate.  This research is likely to contribute to a deeper understanding of renewable materials with green synthesis methods for catalysts targeting water contamination treatment.

The immobilization and structural analysis of platinum nanoparticles on a nitrogen-rich, bipyridine-containing covalent triazine framework (bpyCTF) having structural defects are disclosed by taking advantage of ¹⁵N solid-state nuclear magnetic resonance measurements at natural ¹⁵N isotope abundance and X-ray photoelectron spectroscopic analyses. The photocatalyst (Pt@bpyCTF) with structural defects reduces CO₂ to formic acid (FA) at a rate of 152 µmol h⁻¹g⁻¹ and a selectivity higher than 95% over CO and H₂ in water under simulated solar light. The presence of amine defects and the immobilization of Pt cause improvement in the photocurrent density and CO₂ capture capacity (≈8% by weight) despite the moderate surface area (0.54 cm³ g⁻¹)of the photocatalyst. Theoretical models and density functional theory calculations are employed to investigate the possible CO₂ reduction reaction (CO₂RR) mechanisms. Considering the exceptional CO₂ capture capacity and high FA production using only CO₂-bubbled water, this work highlights the great potential of nitrogen-rich CTFs for photocatalyzed CO₂RRs under green conditions.

Lignin has been, for a long time, treated as a low-value waste product. To change this scenario, high-value applications have been recently pursued, e.g., the preparation of hybrid materials with inorganic components. Although hybrid inorganic-based materials can benefit from the reactive lignin phenolic groups at the interface, often responsible for optimizing specific properties, this is still an underexplored field. Here, we present a novel and green material based on the combination of hydroxymethylated lignin nanoparticles (HLNPs) with molybdenum disulfide (MoS₂) nanoflowers grown via a hydrothermal route. By bringing together the lubricant performance of MoS₂ and the structural stability of biomass-based nanoparticles, a MoS₂-HLNPs hybrid is presented as a bio-derived additive for superior tribological performances. While FT-IR analysis confirmed the structural stability of lignin after the hydrothermal growth of MoS₂, TEM and SEM micrographs revealed a homogeneous distribution of MoS₂ nanoflowers (average size of 400 nm) on the HLNPs (average size of 100 nm). Regarding the tribological tests, considering a pure oil as reference, only HLNPs as bio-derived additives led to a reduction in the wear volume of 18%. However, the hybrid of MoS₂-HLNPs led to a considerably higher reduction (71%), pointing out its superior performance. These results open a new window of opportunity for a versatile and yet underexplored field that can pave the way for a new class of biobased lubricants.

Photoluminescent carbon nanoparticles (CNPs), such as carbon dots (CDs), have attracted much attention owing to a unique set of properties, like high and tunable fluorescence. In this way, the use of carbon-rich lignin has been demonstrated to be a sustainable approach to producing a broad range of photoluminescent CNPs. However, the valorization of this complex polyphenol is limited when it comes to green and efficient ways of conversion. In addition, the existing solvothermal approaches using lignin often result in CDs with low photoluminescence, while flammable and/or toxic solvents are employed. Here, we depolymerized technical lignins, i.e. kraft and soda, through electroreductive cleavage in two different sustainable media: deep eutectic solvent and levulinic acid. After depolymerization, lignin-derived monomers were generated, with a predominance of aryl ether and phenolic groups, which were further combined with 1,2-Phenylenediamine to produce N-doped CNPs in a solvent-free approach. Photoluminescent CNPs with varied sizes were generated (5–50 nm), which presented a wide photoluminescence emission, from blue to red, depending on solvent polarity. These results demonstrate a feasible and sustainable route for the solvent-free synthesis of photoluminescent CNPs using lignin-derived monomers as carbon source, which may find applications in a wide range of fields.

A straightforward method for the synthesis of a two-dimensional (2D) new copper(I) coordination polymer, namely Cu(bzpdc), containing the ligand benzophenone 4,4′-dicarboxylate, and its effective use as catalyst for the azide–alkyne click chemistry at room temperature is reported. Zig-zag formation caused by cuprophilic interactions resulted in an unprecedented crystal structure with a very high copper content (45.5% by weight). The catalyst was stable up until 300 °C and tolerant to various solvents, including water. Cu(bzpdc) showed excellent catalytic activity for click reactions of several organic azides and alkynes having different functional groups at room temperature and is comparable to its homogenous analogues. The recyclability of Cu(bzpdc) was also tested and proven to be effective.

The low stability of the electrocatalysts at water oxidation (WO) conditions and the use of expensive noble metals have obstructed large-scale H₂ production from water. Herein, we report the electrocatalytic WO activity of a cobalt-containing, water-soluble molecular WO electrocatalyst [Co^II(mcbp)(OH₂)] (1) [mcbp²⁻=2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine] in homogeneous conditions (overpotential of 510 mV at pH 7 phosphate buffer) and after anchoring it on pyridine-modified fluorine-doped carbon cloth (PFCC). The formation of cobalt phosphate was identified only after 4 h continuous oxygen evolution in homogeneous conditions. Interestingly, a significant enhancement of the stability and WO activity (current density of 5.4 mA/cm² at 1.75 V) was observed for 1 after anchoring onto PFCC, resulting in a turnover (TO) of >3.6×10³ and average TOF of 0.05 s⁻¹ at 1.55 V (pH 7) over 20 h. A total TO of >21×10³ over 8 days was calculated. The electrode allowed regeneration of∼ 85 % of the WO activity electrochemically after 36 h of continuous oxygen evolution.

Breaking down lignin into smaller units is the key to generate high value-added products. Nevertheless, dissolving this complex plant polyphenol in an environment-friendly way is often a challenge. Levulinic acid, which is formed during the hydrothermal processing of lignocellulosic biomass, has been shown to efficiently dissolve lignin. Herein, levulinic acid was evaluated as a medium for the reductive electrochemical depolymerization of the lignin macromolecule. Copper was chosen as the electrocatalyst due to the economic feasibility and low activity towards the hydrogen evolution reaction. After depolymerization, high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy revealed lignin-derived monomers and dimers. A predominance of aryl ether and phenolic groups was observed. Depolymerized lignin was further evaluated as an anti-corrosion coating, revealing enhancements on the electrochemical stability of the metal. Via a simple depolymerization process of biomass waste in a biomass-based solvent, a straightforward approach to produce high value-added compounds or tailored biobased materials was demonstrated. 

The development of smart and sustainable photocatalysts is in high priority for the synthesis of H₂O₂ because the global demand for H₂O₂ is sharply rising. Currently, the global market share for H₂O₂ is around 4 billion US$ and is expected to grow by about 5.2 billion US$ by 2026. Traditional synthesis of H₂O₂ via the anthraquinone method is associated with the generation of substantial chemical waste as well as the requirement of a high energy input. In this respect, the oxidative transformation of pure water is a sustainable solution to meet the global demand. In fact, several photocatalysts have been developed to achieve this chemistry. However, 97% of the water on our planet is seawater, and it contains 3.0–5.0% of salts. The presence of salts in water deactivates the existing photocatalysts, and therefore, the existing photocatalysts have rarely shown reactivity toward seawater. Considering this, a sustainable heterogeneous photocatalyst, derived from hydrolysis lignin, has been developed, showing an excellent reactivity toward generating H₂O₂ directly from seawater under air. In fact, in the presence of this catalyst, we have been able to achieve 4085 μM of H₂O₂. Expediently, the catalyst has shown longer durability and can be recycled more than five times to generate H₂O₂ from seawater. Finally, full characterizations of this smart photocatalyst and a detailed mechanism have been proposed on the basis of the experimental evidence and multiscale/level calculations. 

Electrochemical reduction of nitrate into ammonia is one potential strategy to valorize pollutants needed to close the nitrogen cycle. The understanding of carbonaceous materials as metal-free representatives of electrocatalysts is of high importance to ensure sufficient activity and target selectivity. We report on the role of defects in cellulose-derived nitrogen-doped carbon (NDC) materials, produced by ammonolysis at different temperatures, to obtain efficient electrocatalysts for the nitrate reduction reaction (NO₃RR). Carbon catalyst ammonolysis at 800 °C (NDC-800) yields the highest electrochemical performance, exhibiting 73.1% NH₄⁺ selectivity and nearly 100% NO₃⁻ reduction efficiency with a prolonged NO₃RR time (48 h) at −1.5 V vs. Ag/AgCl in a 0.1 M Na₂SO₄ electrolyte. We provide support to our findings by undertaking complementary structural analyses with scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, low-temperature N₂ adsorption, and theoretical studies based on multi-scale/level calculations. Atomistic molecular dynamics simulations based on a reactive force field combined with quantum chemistry (QC) calculations on representative model systems suggest possible realistic scenarios of the material structure and reaction mechanisms of the NO₃⁻ reduction routes.