Visible light-driven CO2 photoreduction by a Re(I) complex immobilized onto CuO/Nb2O5 heterojunctions

The immobilization of Re(I) complexes onto metal oxide surfaces presents an elegant strategy to enhance their stability and reusability toward photocatalytic CO2 reduction. In this study, the photocatalytic performance of fac-[ClRe(CO)3(dcbH2)], where dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine, anchored onto the surface of 1%m/m CuO/Nb2O5 was investigated. Following adsorption, the turnover number for CO production (TONCO) in DMF/TEOA increased significantly, from ten in solution to 370 under visible light irradiation, surpassing the TONCO observed for the complex onto pristine Nb2O5 or CuO surfaces. The CuO/Nb2O5 heterostructure allows for efficient electron injection by the Re(I) center, promoting efficient charge separation. At same time CuO clusters introduce a new absorption band above 550 nm that contributes for the photoreduction of the reaction intermediates, leading to a more efficient CO evolution and minimization of side reactions.

Well-dispersed titanium dioxide and silver nanoparticles on external and internal surfaces of asymmetric alumina hollow fibers for enhanced chromium (VI) photoreductions

Heterogenous photocatalysis is a suitable alternative for wastewater treatment. The supporting of the solid catalyst in a porous material is suggested to facilitate catalyst recovery and reuse. Here we propose for the first time the evaluation of supporting silver (Ag)-decorated titanium dioxide (TiO₂) catalysts on internal and external surfaces of alumina hollow fibers with asymmetric pore size distribution. The produced catalysts were considered for Cr(VI) photoreductions. The ultrasound-assisted process potentialized the distribution of Ag nanoparticles on the TiO₂ surface. The loading of Ag nanoparticles at concentrations greater than 5 wt% was necessary to improve the TiO₂ activity for Cr(VI) photoreduction. The loading of Ag nanoparticles at 30 wt% improved the Cr(VI) photoreduction of the single TiO₂ catalyst from 40.49 ± 0.98 to 55.00 ± 0.83% after 180 min of reaction. Suspended and supported Ag-decorated TiO₂ catalysts achieved total Cr(VI) photoreduction after 21 h of reaction. The adjusted reaction rate constant with the externally supported Ag-TiO₂ catalyst was 3.57 × 10^-3 ± 0.18 × 10^-3 min^-1. Similar reaction rate constants were achieved with suspended and internally supported catalysts (approximately 2.70 × 10^-3 min^-1). After 10 sequential reuses, all catalysts presented similar Cr(VI) photoreductions of approximately 66%. Nevertheless, the use of the externally supported catalyst is suggested for Cr(VI) photoreductions due to its superior catalyst activity at least in the first reuse cycles.

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO₂ reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy^- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc⁺, attributed to consecutive 1 e^- reduction. Under CO₂ atmosphere, a catalytic wave is observed (E_onset = 2.1 V vs. Fc/Fc⁺), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k_obs = 1.5 s^-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TON_CO = 21 and k_obs = 2.0 s^-1) and, additionally, formate was also detected (TON_HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO^-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy^- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)₂(acpy)]⁺ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)₂(pic)]⁺ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru^II center. The reaction is one of the few examples of C-H activation at mild conditions using O₂ as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10^-2 h^-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O₂ binding to [Ru(bpy)₂(acpy)]⁺ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru^III-OO˙^- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG^‡ = 41.1 kcal mol^-1), thus the formation of a diruthenium μ-peroxo intermediate, [(Ru^III(bpy)₂(O-acpy))₂O₂]²⁺via interaction of a second [Ru(bpy)₂(acpy)]⁺ was examined as an alternative pathway. The dimer yields two Ru^IVO centers with a low ΔG^‡ of 2.3 kcal mol^-1. The resulting Ru^IVO species can activate C-H bonds in acpy (ΔG^‡ = 23.1 kcal mol^-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O₂ as the sole oxygen source.

Aluminum oxides as alternative building blocks for efficientlayer-by-layerblocking layers in dye-sensitized solar cells

All inorganic layer-by-layer (LbL) thin films composed by TiO₂nanoparticles and [Al(OH)₄]^-anions (TiO₂/AlO_x) as well as Al₂O₃and Nb₂O₅nanoparticles (Al₂O₃/Nb₂O₅) have been deposited to fluorine-doped tin-oxide coated glass (FTO) surfaces and applied as blocking layers in dye-sensitized solar cells (DSCs). Structural and morphological characterization of the LbL films by different techniques reveal that inTiO₂/AlO_xassembly, aluminate anions undergo condensation reactions on the TiO₂surface leading to the formation of highly homogeneous films with unique optical properties. After 25 depositions transmittance losses below 10% in relation to the bare FTO substrate are observed. Electrochemical impedance spectroscopy shows thatTiO₂/AlO_xlayers impose an effective barrier for the charge recombination at FTO/electrolyte interface with an electron exchange time constant 50-fold higher than that for bare FTO. As a result, an improvement of 85% in the overall conversion efficiency of DSCs was observed with the employment of TiO₂/AlO_xblocking layers.Al₂O₃/Nb₂O₅LbL films can also work as blocking layers in DSCs but not as efficient, which is associated with the poor homogeneity of the film and its capacitive behavior. The production of cost-effective blocking layers with a low light scattering in the visible region is an important feature toward the application of DSC in other Building-integrated photovoltaic applications.

Electrocatalytic water oxidation reaction promoted by cobalt-Prussian blue and its thermal decomposition product under mild conditions

Water oxidation studies with Co-Prussian blue and Co₃O₄. Figure adapted from ‘Under the Wave off Kanagawa’ (Kanagawa oki nami ura), also known as ‘The Great Wave’, from the series ‘Thirty-six Views of Mount Fuji’ (‘Fugaku sanjūrokkei’) by K. Hokusai.

Spectroscopic characterization of a new Re(i) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications

A novel Re(i) complex with a thiosemicarbazone derivative is described and fully characterized. Its was further explored as CO₂ reduction electrocatalyst, being the first complex with a thiosemicarbazone derivative applied to this goal.

Unraveling the photocatalytic properties of TiO2/WO3 mixed oxides

TiO2/WO3 heterojunctions are one of the most investigated systems for photocatalytic applications. However, distinct behavior can be found in the literature depending on the pollutant to be degraded and the photocatalyst preparation conditions. Some authors reported improved photocatalytic activities in relation to TiO2, while others a deleterious effect. Different factors have been identified to influence the activity of such systems. In this work, a systematic investigation of TiO2/WO3 samples with different W/Ti ratios (0-100%) was carried out using different pollutants as targets (gaseous NO, acetaldehyde and aqueous methylene blue solutions). A detailed structural investigation along with transient absorption studies and photoelectrochemical measurements allowed the rationalization of some of the previously reported factors that control the TiO2/WO3 photoactivity, i.e. the inability to reduce molecular oxygen, the stabilization of the anatase phase and the adsorption surface properties. The investigations also identified a factor not previously reported: in TiO2/WO3 systems, a fraction of long-lived holes do not take part in the interfacial charge transfer to efficient hole quenchers, such as methanol. This behavior seems to be related to the doping of the TiO2 matrix with W(vi) and plays a key role in the photocatalytic activity.

Influence of the preparation conditions on the morphology and photocatalytic performance Pt-modified hexaniobate composites

The preparation of lamellar nanostructures through exfoliation of stacked niobates is an interesting approach to the development of photocatalysts for energy conversion and environmental remediation. These materials exhibit a rich surface chemistry and several nanocomposites can be produced through intercalation or impregnation of suitable precursors. In this work, the influence of the physico-chemical preparation conditions on the photocatalytic activity of Pt-hexaniobate nanocomposites was investigated aiming at the establishment of the main factors that control their photoreactivities. Modification of hexaniobate layers were carried out by adsorption and impregnation methods, using [Pt(NH₃)₄]Cl₂ (Pt1) and H₂PtCl₆ (Pt2), respectively. The addition of platinum precursors (1% wt.) were performed in the presence of the exfoliating agent tert-butylammonium hydroxide, sNb, or after acidic precipitation followed by resuspension in plain water, eNb. All samples were submitted to photoirradiation to reduce the platinum precursors and the effect of a previous thermal treatment was also evaluated. It was observed that H₂ evolution from aqueous methanol solutions is more favored on hexaniobate nanosheets (eNb-Pt1 and eNb-Pt2) instead of scrolled layers (sNb-Pt1 and sNb-Pt2), independent on the platinum precursor. Moreover, residual tert-butylammonium can act as hole scavenger and decrease the degradation rates for methanol oxidation in sNb samples. The curled layers observed for sNb samples seem to favor the photodegradation of cationic species, such as methylene blue. Thermal treatment at 500 °C leads to morphological changes with a decrease of the specific surface area due to restacking of the individual layers along with some curling. As a result, the H₂ evolution rates strongly decreases in relation to the non-sintered samples, suggesting that the 'soft' photoreduction of platinum precursors is the best method for preparation of these photocatalysts. The correlations between the preparation conditions and the photocatalytic activity for different photoreactions can allow the development of optimized materials for specific applications.

Quenching Effects of Graphene Oxides on the Fluorescence Emission and Reactive Oxygen Species Generation of Chloroaluminum Phthalocyanine

The photophysical behavior and reactive oxygen species (ROS) generation by chloroaluminum phthalocyanine (AlClPc) are evaluated by steady state absorption/emission, transient emission, and electron paramagnetic resonance spectroscopies in the presence of graphene oxide (GO), reduced graphene oxide (RGO), and carboxylated nanographene oxide (NGO). AlClPc and graphene oxides form a supramolecular structure stabilized by π-π interactions, which quantitatively quenches fluorescence emission and suppresses ROS generation. These effects occur even when graphenes are previously functionalized with Pluronic F-127. A small part of quenching is due to an inner filter effect, in which graphene oxides compete with AlClPc for light absorption. Nonetheless, most of the (static) quenching arises on the formation of a nonemissive ground state complex between AlClPc and graphene oxides. The efficiency of graphene oxides on the fluorescence quenching and ROS generation suppression follows the order: GO < NGO < RGO.

Transient Absorption Studies on Nanostructured Materials and Composites: Towards the Development of New Photocatalytic Systems

Being part of the development of environmentally clean and safe sustainable technologies photocatalysis is attracting increasing attention. During the last decade, great attention has been paid to the synthesis of different photocatalysts possessing high photocatalytic activity, whereas fundamental studies concerning the underlying photocatalytic processes have rarely been executed. The knowledge of these processes is, however, of utmost importance for the understanding of the reaction mechanism and thus for a better design of photocatalytic systems. The transient absorption spectroscopy (TAS) is one widely used method to study such fundamental processes. The present review paper focuses on the application of TAS in the UV-Vis-IR regions to investigate the charge carrier dynamics in ultrafast and nano-to-millisecond time regime. Hereby, the photo induced processes occurring in different materials will be discussed. Moreover, further attention is also paid to nanocomposite-based systems, in which different materials are used concomitantly to promote more efficient photocatalytic processes.

Characterization of a highly efficient N-doped TiO2 photocatalyst prepared via factorial design

The preparation of a highly efficient N-doped TiO₂ photocatalyst was optimized by factorial design and the resulting powder was fully characterized.

Layer-by-layer TiO(2)/WO(3) thin films as efficient photocatalytic self-cleaning surfaces

New TiO2/WO3 films were produced by the layer-by-layer (LbL) technique and successfully applied as self-cleaning photocatalytic surfaces. The films were deposited on fluorine doped tin oxide (FTO) glass substrates from the respective metal oxide nanoparticles obtained by the sol-gel method. Thirty alternative immersions in pH = 2 TiO2 and pH = 10 WO3 sols resulted in ca. 400 nm thick films that exhibited a W(VI)/Ti(IV) molar ratio of 0.5, as determined by X-ray photoelectron spectroscopy. Scanning electron microscopy, along with atomic force images, showed that the resulting layers are constituted by aggregates of very small nanoparticles (<20 nm) and exhibited nanoporous and homogeneous morphology. The electronic and optical properties of the films were investigated by UV-vis spectrophotometry and ultraviolet photoelectron spectroscopy. The films behave as nanoscale heterojunctions, and the presence of WO3 nanoparticles caused a decrease in the optical band gap of the bilayers compared to that of pure LbL TiO2 films. The TiO2/WO3 thin films exhibited high hydrophilicity, which is enhanced after exposition to UV light, and they can efficiently oxidize gaseous acetaldehyde under UV(A) irradiation. Photonic efficiencies of ξ = 1.5% were determined for films constituted by 30 TiO2/WO3 bilayers in the presence of 1 ppm of acetaldehyde, which are ∼2 times higher than those observed for pure LbL TiO2 films. Therefore, these films can act as efficient and cost-effective layers for self-cleaning, antifogging applications.