Direct conversion of methane into value-added chemicals is vital for sustainable development. Achieving this goal requires a fundamental understanding of the role of active catalytic sites and reaction mechanisms to design optimized materials with enhanced performance. In this work, a multi-technique approach was employed to investigate Pd(Ag)-Fe3-δO4 nanocomposite catalysts for the low-temperature partial oxidation of methane. Through a combination of in situ X-ray Absorption Spectroscopy (XAS) and Grazing Incidence X-ray Diffraction (GI-XRD), on-site X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Energy-Dispersive X-ray Spectroscopy (EDX), Electron Energy-Loss Spectroscopy (EELS), and X-Ray Emission Spectroscopy (XES), new insights into the electronic and structural evolution of catalyst atoms under reaction conditions are provided. High catalytic activities were achieved at 250 and 300°C (ca. 1.5 and 6 molCH4 kgcat−1 h−1 respectively), along with excellent formaldehyde selectivity (>95%). Lattice contraction induced by iron oxidation and the withdrawal of Pd (and Ag) from interstitial sites, together with Pd complete reduction and agglomeration, were found to accompany catalyst deactivation and loss of catalytic performance under reaction conditions. While silver incorporation has a negligible effect at 300°C, a lower degree of Pd agglomeration is observed for the PdAg-containing catalyst at 250°C, correlating with its higher catalytic activity previously reported under these conditions.

Multicomponent MoVTeNbO catalysts containing the orthorhombic M1 phase are highly active for the oxidative dehydrogenation (ODH) of ethane using O2 as oxidant, but their performance with N2O is strongly limited by the low efficiency of N2O decomposition capacity of the M1 catalyst. To overcome this limitation, we propose a strategy based on decoupling N2O decomposition from ethane oxidative dehydrogenation in a tandem reactor; in which N2O is decomposed in an upstream bed prior to contacting ethane with the M1 catalyst. Two different N2O decomposition catalysts were evaluated, i.e., CeO2 and a Cs-doped cobalt oxide (Cs-Co3O4), along with M1 catalyst. While with CeO2 the catalytic performance of the M1 catalyst was improved (by efficiently generating reactive oxygen species from N2O), with the Cs-Co3O4 we observed near-complete N2O conversion, leading to significantly higher ethane conversion and enhanced olefin yields of the subsequent M1 catalyst. In addition, different reactor configurations show that physical separation of both processes is essential to maximize performance, enabling high ethane conversion and ethylene selectivity at moderate temperatures. Characterization of fresh and spent catalysts indicates that performance is governed by the dynamic redox state of the M1 surface, strongly dependent on the oxidizing environment. These results demonstrate that decoupling oxidant generation from hydrocarbon conversion is an effective strategy for N2O valorization and selective ethane ODH.

Residual sludges from both a wastewater treatment plant and a drinking water treatment plant have been used as catalytic supports of ruthenium for reactions of biomass valorization employing very low reaction temperatures. The biomass derived substrates used for the catalytic reactions have been levulinic acid and furfural. Interestingly, Ru catalysts deposited on the sludges are far more efficient for this reactions than analogous Ru catalysts deposited on the pure oxides that constitute the sludges and also than Ru supported on a HY zeolite. The sludge obtained from the drinking water treatment plant proved to be the most effective support, primarily due to its high capacity to disperse ruthenium on its surface while maintaining a high proportion of ruthenium in its metallic state. Notably, a yield of 93 % to furfuryl alcohol from furfural was obtained at 30 degrees C in just 5 h using the most efficient catalyst. Moreover, the ruthenium is highly anchored on the support resulting in high stability and minimal metal leaching. The slight decline in the catalytic performance observed after several catalytic uses is mainly due to the formation of residual carbon deposits on the catalyst surface. Overall, this work offers a dual environmental interest, as the catalyst material is derived from waste sludge, and the studied catalytic reaction focuses on the valorization of biomass.

gamma-Valerolactone (GVL) is a versatile platform molecule that can be produced from furfural (FF), a compound readily obtained from biomass. The transformation of FF into GVL can be achieved in a one-pot process through a cascade of reactions involving hydrogenation, ring opening, dehydration, and cyclization. Designing an appropriate catalyst capable of promoting all these steps in a single system is therefore essential. In this work, catalysts based on zirconium supported on Y-zeolite, as well as catalysts supported on Y-zeolite coated with a theoretical monolayer of Nb2O5, were synthesized and evaluated for the conversion of FF into GVL. Reactivity tests were conducted under both batch and continuous-flow conditions. The catalysts were characterized using various techniques, including XRD, FTIR, IR-Py, DR-UV-Vis, HR-TEM, XPS, and NH3-TPD, to determine their physicochemical properties. Among the catalysts studied, Zr supported on Nb-modified Y-zeolite exhibited the highest catalytic activity, achieving a GVL yield of 65 % after 3 h at 180 degrees C in batch mode using 2-propanol as both hydrogen source and solvent. Similarly, under continuous-flow conditions, this bimetallic catalyst delivered the highest GVL yield (approximately 40 %), representing the best performance reported to date for this reaction. Furthermore, the addition of water to the reaction medium enhanced catalytic performance. Notably, the bimetallic catalyst maintained stable activity throughout the tested time-on-stream.

The development of sustainable fabrication routes for advanced semiconductor materials is crucial for environmental technology. This work presents a green electrochemical anodization strategy to fabricate tungsten trioxide (WO3) nanostructures, employing aqueous extracts of two Mediterranean plants, Cistus monspeliensis and Cistus albidus, as environmentally benign electrolytes. Nanostructures synthesized with varying extract percentages were comprehensively characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and electrochemical impedance spectroscopy measurements. Their performance was evaluated in photoelectrocatalytic (PEC) tests under simulated sunlight. The WO3 nanostructures anodized in the presence of optimal concentrations (7%) of C. monspeliensis and C. albidus extracts demonstrated superior properties. These optimal samples were then applied to degrade ciprofloxacin, a persistent antibiotic pollutant. The C. albidus-derived photoelectrocatalyst exhibited markedly higher efficiency, achieving a degradation rate constant of 1.76 h−1, which is four times faster than the 0.43 h−1 constant obtained for the C. monspeliensis-based sample. This study confirms that plant extracts can effectively tune the properties of anodized WO3 and introduces Cistus albidus extract as a particularly effective green agent for fabricating high-performance PEC materials for the removal of emerging water pollutants, using a green fabrication process to create high-performance materials for environmental cleanup.

Sludge derived from a drinking water treatment plant has been used as a support for vanadium to be used as catalyst for the oxidative dehydrogenation (ODH) of ethylbenzene using CO2 as a mild oxidizing agent. The valorization of this sludge is particularly attractive, as it is abundantly generated during water treatment processes and constitutes a widely available residual material. The sludge was characterized by physicochemical and electrochemical techniques, which revealed that the optimal vanadium concentration, ca. 4 wt%, corresponds to the highest loading employed without the formation of unselective crystalline V2O5. As a novelty, the catalytic performance of these V/sludge catalysts has been correlated with their electrochemical properties, showing that high activity is obtained when the density of acceptors is high and the resistance to the charge transfer of the outer layer is low. Unfortunately, due to carbon deposition and slight reduction of V5 + to V4+, a minor deactivation occurs after 8 h. Finally, we want to mention that consistent yields to styrene of 75% can be obtained with a catalyst with intermediate V-loading (ca. 4 wt%) in non-optimized reaction conditions.

In this work, novel titanium dioxide nanotubes have been synthesized by electrochemical anodization using different protic ionic liquids (PILs) to study the effect on two different environmental applications. Namely, the photoelectro-reduction of nitrous oxide and the use as anode in Li-ion batteries. Morphological, structural, physicochemical and (photo)electrochemical characterization of the samples was carried out to reveal the best nanostructure for each application. Among all the ionic liquids tested, triethanolamine (TEA) resulted to be the most efficient PIL, due to the longer and wider resulting nanotubes (6.615 µm and 54 nm, respectively for IL-0.25 sample). Different IL concentrations and hydrodynamic conditions were tested during anodization. The best electrochemical response in both applications was obtained by the sample synthesized using a TEA concentration of 0.25% v/v and 500 rpm. The results revealed the good suitability of this nanostructure, as 97% of N2O was eliminated after only 2h of experiment and 100% was reached after 3h. When tested as an anode in Li-ion batteries, specific capacity for slow charging velocities reached almost theoretical values and, for faster velocities, it demonstrated a capacity ca. 50% higher than an analogous sample synthesized in static conditions and without ionic liquid.

Ferric sludge from a drinking water treatment plant, was valorized as a catalyst support for the propane dehydrogenation reaction. Vanadium was incorporated at loadings of 4 wt% (highly dispersed VOx species), 8 wt% (small V2O5 crystallites) and 16 wt.% (large V2O5 crystallites and mixed Al-V-O species) and evaluated in CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP). The results showed that vanadium incorporation markedly enhanced the propane and CO2 conversion compared to the non-oxidative direct dehydrogenation of propane (DHP) reaction. The superior catalytic performance was attributed to the predominance of monomeric VOx species, which strengthened Fe–V interactions, suppressed excessive reduction of the active phases, and enhanced oxygen mobility. Taking together, these effects promoted a balanced redox cycle by favoring the partial Fe reduction and increasing the lattice oxygen mobility, as evidenced by XPS analysis. As a result, the catalyst containing 4 wt.% V exhibited good stability, achieving propylene and ethylene selectivities of 80.8% and 19.2%, respectively, after 280 min on stream. In contrast, higher vanadium loadings (8-16 wt.%) favor the formation of polymeric VOx domains and crystalline V2O5 phases, resulting in a slight decrease in catalytic performance, especially propylene selectivity, and poor catalytic stability. CO2-ODHP conducted on monomeric VOx species supported on an inexpensive high surface area waste material yields a fairly stable catalyst for propylene production. This approach offers a doble environmental benefit: it valorize waste from water-treatment and promotes the CO2 valorization through the CO2-ODHP reaction.

The growing demand of energy and the increase in the greenhouse gases emissions has caused the search for new renewable energy sources, such as biomass, to replace fossil fuels. Thus, there are different compounds, i.e. gamma-valerolactone (GVL), which can be obtained from different biomass-derived substrates such as levulinic acid (LA). In the present work, the catalytic hydrogenation of levulinic acid to gamma-valerolactone has been studied, using as a hydrogen source that obtained by photoelectrocatalytic water splitting. The use of the photoelectrocatalytic hydrogen has been conducted in two ways: i) one set-up in two steps involving a cell for hydrogen production and a reactor for the LA to GVL transformation or ii) a novel set-up consisting of just one unit in which hydrogen formation and LA hydrogenation take place simultaneously. The photoelectrocatalyst used for the water to hydrogen reaction is a TiO2 nanostructure synthesized by electrochemical anodization, whereas several catalysts based on ruthenium have been studied for the LA hydrogenation. Optimization in the synthesis procedure of ruthenium catalysts supported on gamma-alumina has allowed high yields to GVL at low reaction temperature (30 degrees C). The most efficient ruthenium catalyst presenting high proportion of surface metallic ruthenium and high dispersion on the support was that synthesized by a simple precipitation method. Finally, the use of the novel one-unit set-up makes possible not only achieving high GVL yields but also the recyclability of the catalyst with just a scarce loss in productivity, conversely to what happens using the traditional hydrogenation with pressurized hydrogen.

The use of biomass as a renewable energy source has been studied as an alternative to traditional and nonrenewable energy sources in order to reduce the greenhouse gas emissions and decarbonize the economy. Levulinic acid (LA) is an important biomass-derived compound, that can be easily obtained by acid hydrolysis from biomass. LA can be transformed into high valuable chemical compounds by catalytic route, as for example in gamma-valerolactone (GVL), which has important applications as additive or precursor of biofuels. The transformation of LA to GVL requires a hydrogen source and a proper catalyst. In this work, monometallic catalysts based on Ru and Ni as well as bimetallic RuNi supported on a zeolite have been synthesized and characterized by XRD, XPS and TEM. The hydrogenation of LA into GVL has been studied with these Ni/Ru catalysts using non-conventional hydrogen sources and the results have been compared with the traditional hydrogen source (pressurized molecular hydrogen). The alternative hydrogen sources used are: i) a system based on metallic Zn and water, where Zn can decompose water into hydrogen but it is oxidized to ZnO, ii) a combined system that uses hydrogen produced by photoelectrochemical (PEC) water splitting (WS) using a nanostructure of TiO2. The bimetallic catalyst led to the highest yield to GVL when Zn was used in the reaction, but the yields achieved are not remarkable (less than 25 %). Conversely, the monometallic catalyst based on Ru was the one that exhibited the highest yield to GVL when pressurized and PEC hydrogen were used. In this case, yields to GVL exceeding 95 % were achieved at 30 degrees C although the reaction time required was lower when pressurized hydrogen was used. Interestingly, a new system for PEC WS has been proposed, improving the contact between hydrogen and levulinic acid. This new system decreases the induction time, and enhances the GVL formation, especially at low reaction times. Finally, these catalysts are stable after reaction regardless of the source of hydrogen employed.

Green hydrogen was produced by photoelectrochemical water splitting to carry out the hydrogenation reaction of levulinic acid using a ruthenium catalyst under mild conditions (atmospheric pressure and only 30 degrees C). For the hydrogen production, novel WO3 nanosheets were synthesized by electrochemical anodization under hydrodynamic conditions using different concentrations of tartaric acid. The samples were morphologically characterized by Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy techniques. The electrochemical behavior was studied by Electrochemical Impedance Spectroscopy analysis, Mott-Schottky diagrams and water splitting tests. From the analyses performed, it was determined that the nanostructures synthesized with intermediate concentrations of tartaric acid had an optimum behavior for hydrogen production. In fact, using the optimal WO3 photocatalyst (anodized with 0.1 M of tartaric acid) together with the ruthenium catalyst supported on alumina a gamma-valerolactone yield of 72.1 % was achieved at only 30 degrees C and atmospheric pressure after 8 h.

Supported precious metal catalysts, containing either mono or bimetallic Au and Pt nanoparticles, anchored on a hierarchical porous UVM-7 silica structure, were prepared and evaluated for the removal of volatile organic pollutants by oxidation. The catalysts were investigated for the simultaneous total oxidation of three model compounds: propene, toluene, and CO, commonly associated with "cold start pollutants" in automotive exhausts. Only Au showed low catalytic activity, while Pt nanoparticles were active, with a greater concentration of accessible Pt nanoparticles corresponding with increased catalytic activity. Interestingly, having both metals present on the same catalyst together was preferential for producing higher activity in the total oxidation of CO, propene, and toluene. The loadings of Pt nanoparticles on the catalyst surface, as well as the synthesis method, were important controlling factors. The order of metal loading deposition was influential, depositing Au and Pt sequentially resulted in surface enrichment of the latter deposited metal, leading to enhanced catalytic performance. When Au and Pt were loaded simultaneously, alloy formation occurred, and the surface Pt enrichment was more moderate, but still maintaining better catalytic performance compared with the pure Pt catalyst.

This work presents for the first time the combined effect of Sn and Zr in the conversion of furfural (FU) to γ-valerolactone (GVL) by using a single liquid-phase continuous flow reactor. To address the high costs and environmental impact of this cascade reaction, catalytic transfer hydrogenation is a promising approach, utilizing alcohols as hydrogen donors in place of molecular H2. This process, which requires both Lewis and Bro̷nsted acidity, when coupled with heterogeneous catalysts, offers a potentially more cost-effective and environmentally friendly alternative. The production of GVL in one pot has been studied using Sn- and Zr-based catalysts supported on dealuminated zeolite Y. The bimetallic catalyst with a Sn:Zr at. ratio of 1:1 achieved the best performance, reaching a yield to GVL of ca. 45% at 180 °C using 2-propanol as a hydrogen source, with a 10 min contact time. Moreover, stability studies, including long-term catalytic tests under reaction conditions, were carried out to evaluate the durability and the deactivation. In addition, an efficient regeneration protocol was developed and optimized, enabling catalyst reuse across multiple cycles with performance in terms of conversion and selectivities comparable to those observed with the fresh materials.

This study investigates the influence of adding different carboxylic acids (citric, tartaric, and aspartic acids) on the anodization of WO3 nanostructures under hydrodynamic conditions and their performance for the degradation of organic emerging pollutants. A comprehensive physicochemical and electrochemical characterization was performed to correlate these properties with their photoelectrocatalytic (PEC) response. WO3 nanostructures anodized with tartaric acid exhibited superior PEC behavior, making them the most promising candidates for the degradation of three common organic emerging pollutants: two drugs (trimethoprim and ciprofloxacin) and one pesticide (imidacloprid). The results demonstrated high PEC-degradation efficiencies and kinetic rates for all the pollutants, showing a synergistic effect compared to the individual photo, photocatalytic, and electrocatalytic approaches. Furthermore, removal percentages after 6 h were higher than those reported in the literature (>90 %, >50 %, and >80 % for ciprofloxacin, trimethoprim, and imidacloprid, respectively). Finally, a degradation mechanism was proposed for the three emerging pollutants, revealing the formation of more polar and simpler products following PEC degradation.

WO3 nanorods were fabricated following electrochemical anodization of tungsten, under controlled hydrodynamic conditions, in electrolytes containing three distinct carboxylic acids: citric, tartaric and L-aspartic acids, to study the influence of these complexing agents on the morphology and arrangement of the oxide layers. The samples were characterized by FESEM, TEM and XRD, and electrochemical analyses (EIS and ECSA) to assess their potential as anode materials for lithium-ion batteries. This characterization showed the nanostructures anodized in the presence of tartaric acid exhibit uniform morphology and lower total charge transfer resistance associated with the nanostructured layer of WO3 and cycling stability, resulting in more efficient electrochemical processes, better conductivity and stability, making these nanostructures promising for anodes in lithium-ion batteries. The cycling of the batteries was also conducted to understand the behavior of the nanostructures as anodes against metallic lithium. The results showed that the nanostructures analyzed in the presence of tartaric acid exhibited the best initial specific capacity, improving the capacity provided by the graphite ones. These samples also showed a good recovery after faster cycling. These findings demonstrate the effectiveness of complexing-agent-assisted anodization as a strategy for tailoring WO3 nanostructures with enhanced electrochemical performance.

Catalysts based on Ni-Me (Me = Co, Nb) supported on γ-Al2O3 were prepared by the coprecipitation-chemical deposition method via reflux, and hydrothermal method for oxidative dehydrogenation of ethane. For coprecipitation method, ammonia and urea were used as precipitating agents. In both methods, the Ni content was 30 wt.%, whereas that of Me (Co, Ni) was 5 wt.%. The activity and selectivity towards ethylene appeared to depend strongly on the composition of the catalysts and, interestingly, on the preparation conditions. In the present article, it is shown that mixed hydrothermal samples using urea showed the best performance. Specifically, the hydrothermally prepared NiNb/γ-Al2O3 sample was the one that showed the highest values. The introduction of Nb to Ni framework allowed to enrich with nucleophilic oxygen sites displayed via X-ray photoelectronic spectra (XPS) which lead to the formation of low reducible and selective sites.

In this work, a nanostructured WO3 photoanode has been used for photoelectrochemical H-2 production and CO2 reduction. In particular, we provide a novel method to synthesize tungsten oxide catalysts by anodization of tungsten using an ionic liquid, [EMIN][BF4], as electrolyte in hydrodynamic conditions. We found that the use of appropriate hydrodynamic conditions (200-400 rpm) provides larger and more homogenous nanostructures with higher surface area. All this leads to better morphological and electrochemical properties. An excessive rotation during the synthesis (600 rpm) breaks the uniformity of the morphology of the nanostructures, thus hindering the photoelectrochemical performance. This way, the use of an optimized WO3 photoanode has shown excellent potential for photoelectrocatalytic water splitting. Moreover, these nanostructures also present good performance in the photoelectrocatalytic CO2 reduction, leading to the main formation of acetic acid, formic acid, and methanol. In fact, after only 6 h in continuous mode, a remarkable formic acid concentration of 190 mu mol/L at 6 h has been obtained. Moreover, we have illustrated the great importance of the anode during the PEC reaction, significantly influencing the concentration of the obtained products owing to the electrons and protons transferred from the oxygen evolution reaction (OER).

Non-pure primary Vanadyl petroporphyrins extracted from oil, i.e., PVP(Oil), have been impregnated on SBA-15 or on SiO2. For comparison, synthetic vanadyl octaethylporphine, VP(S), was also impregnated on the same siliceous materials. The characterization result of unsupported porphyrins and the catalysts, by FTIR and DR-UV-vis, confirm the successful incorporation of the corresponding vanadyl porphyrins. Electrochemical and photoelectrochemical properties of these catalysts have been evaluated through Electrochemical Impedance Spectroscopy (EIS), Mott-Schottky (MS) and voltammetry tests. According to the results, primary vanadyl porphyrin extracted from oil, PVP(Oil), show better photoelectrochemical response, regardless of the support used (either SiO2 or SBA15), than synthetic ones, VP(S). Additionally, this study reveals that PVP(Oil) can be used as photoelectrocatalysts for methyl red (MR) degradation (a common organic dye), showing almost a complete degradation of (MR) using PVP(Oil)-SBA15 catalyst after 3h. Interestingly, an increase of more than 25% in the kinetic constant for the photoelectrochemical degradation of MR is shown for the PVP(Oil) in comparison to the VP(S).

This study reports the bio-mediated synthesis of tungsten trioxide nanostructures from aqueous extracts of Melia azedarach (L.) dry leaves. The synthesis was carried out by electrochemical anodization of tungsten foils in the presence of different volume percentages of leaves extracts. These nanostructures were afterwards completely characterized by Field-Emission Scanning Electron Microscopy (FESEM), X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), impedance measurements, linear sweep voltammetry in the presence of simulated solar light and UV-Vis spectroscopy. Finally, these nanoelectrodes were used as (photo)electrocatalysts in the photoelectrochemical water splitting process to obtain green hydrogen and as anodes in Li-ion rechargeable batteries. Nanostructures fabricated with 5% M. azedarach leaves extract provided the best (photo)electrocatalytic behavior for both applications, since in that biogenic electrolyte, thin nanorods arranged in very porous and spongy layers were formed. Therefore, the significant increase in specific surface area found for nanostructures fabricated following this green route, in comparison with the blank sample (i.e., nanostructure formed following a non-green route), was essential to enhance their performance as promising efficient (photo)electrocatalysts.

This research examines the influence of adding a commercial ionic liquid to the electrolyte during the electrochemical anodization of tungsten for the fabrication of WO3 nanostructures for photoelectrochemical applications. An aqueous electrolyte composed of 1.5 M methanesulfonic acid and 5% v/v [BMIM][BF4] or [EMIM][BF4] was used. A nanostructure synthesized in an ionic-liquid-free electrolyte was taken as a reference. Morphological and structural studies of the nanostructures were performed via field emission scanning electron microscopy and X-ray diffraction analyses. Electrochemical characterization was carried out using electrochemical impedance spectroscopy and a Mott–Schottky analysis. From the results, it is highlighted that, by adding either of the two ionic liquids to the electrolyte, well-defined WO3 nanoplates with improved morphological, structural, and electrochemical properties are obtained compared to samples synthesized without ionic liquid. In order to evaluate their photoelectrocatalytic performance, the samples were used as photocatalysts to generate hydrogen by splitting water molecules and in the photoelectrochemical degradation of methyl red dye. In both applications, the nanostructures synthesized with the addition of either of the ionic liquids showed a better performance. These findings confirm the suitability of ionic liquids, such as [BMIM][BF4] and [EMIM][BF4], for the synthesis of highly efficient photoelectrocatalysts via electrochemical anodization.