Search

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.

Tantalum-oxy-nitridenanotubes(TaOxNyNTs) ordered in 1D were successfully synthesized through controlled anodicoxidationof metallic Ta foils followed by high-temperature annealing under NH3flow to prepare unconventional nanostructured photo-electrodes active with visible light. For the synthesis, a two-step anodization process was adopted: i) the first step inethylene glycolwith 0.33 % NH4F and 3 % H2O, ii) the second step in H2SO4(98 %) with 1 % HF and 4 % H2O, by applying different voltages and anodization times. Scanning Electron Microscopy integrated with Energy Dispersive X-rayspectroscopy(SEM-EDX) revealed the NTs length ranging from 2 to 5µm, and the inner tube diameter from 20 to 100nm, in good correlation with the synthesis parameters. The N-content (from 2.7 % to 19.1 %) was dependent on the nitridation temperature (from 500° to 900°C), and the conversion of bare Ta2O5into TaOxNystrongly influenced the absorption in the visible region with a bandgap shift from 4.0 to 2.1eV, respectively. Photo-electrochemical and catalytic performances were evaluated with chronoamperometric measurements and tests in different photo-reactions using AM 1.5G simulated sunlight: (i) degradation of methylene blue (MB), (ii) ethanol gas-phase photo-reforming and (iii) bias-assisted photoelectrochemical water splitting. The TaOxNyNTs film-type electrode obtained by nitriding at 800°C showed the highest photocurrent value (0.1mA∙cm-2), and the highest rates of MB degradation (k1of 0.0189min-1) and H2photo-production.

A major challenge for achieving energy transition and transforming the current energy model into distributed production is the development of efficient artificial leaf-type devices capable of directly converting carbon dioxide (CO2), water and sunlight into sustainable fuels and chemicals under ambient conditions. These devices should avoid using critical raw materials to be sustainable and cost-competitive. We report top-level results for the first time in converting CO2 and H2O to fuels (formate and H-2) using sunlight and electrodes based solely on earth-abundant materials. The cell provides a solar-to-fuel efficiency of >10% combined with world-record current densities to comparable devices operating at room temperature, without adding sacrificial donors or electrical bias. In addition, we present the novel concept of producing at the same time H-2 and an H-2-storage element (formate), the latter used to produce H-2 when light is absent. This solution allows continuous (24 h) hydrogen production using an artificial-leaf device. For the first time, we show the feasibility of this solution. The experimental results were obtained in an optimised, compact electrochemical flow cell, with electrodes based on Cu-S and Ni-Fe-Zn oxide (for CO2 reduction and oxygen evolution reactions, respectively) supported on gas-diffusion substrates, integrated with a low-cost Si-based photovoltaic module. The cell design allows for easy scale-up and low manufacturing and operating costs. The cell operates at a current density of about 17 mA cm(-2) and a full-cell voltage of 2.5 V (stable for at least ten hours and in on-off operations), providing formate productivity of 193 mu mol h(-1) cm(-2), paving the way towards the implementation of affordable artificial-leaf type systems in the future energy scenario., Energy & Environmental Science, 16 (4), ISSN:1754-5692, ISSN:1754-5706

International audience; Metal carbides M2C (MXenes) with two-dimensional (2D) structure have been indicated as promising materials for N2 fixation, with the activity being related to edge planes. Here, it is instead demonstrated that the transformation from a 2D- (nanosheets) to a 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N2 fixation activity due to the formation of exposed Ti−OH sites. A linear relationship is observed between ammonia formation rate and amount of oxygen on the surface of Ti3C2 MXene.

Surface-sensitive ambient pressure X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy combined with an electrocatalytic reactivity study, multilength-scale electron microscopy, and theoretical modeling provide insights into the gas-phase selective reduction of carbon dioxide to isopropanol on a nitrogen-doped carbon-supported iron oxyhydroxide electrocatalyst. Dissolved atomic carbon forms at relevant potentials for carbon dioxide reduction from the reduction of carbon monoxide chemisorbed on the surface of the ferrihydrite-like phase. Theoretical modeling reveals that the ferrihydrite structure allows vicinal chemisorbed carbon monoxide in the appropriate geometrical arrangement for coupling. Based on our observations, we suggest a mechanism of three-carbon-atom product formation, which involves the intermediate formation of atomic carbon that undergoes hydrogenation in the presence of hydrogen cations upon cathodic polarization. This mechanism is effective only in the case of thin ferrihydrite-like nanostructures coordinated at the edge planes of the graphitic support, where nitrogen edge sites stabilize these species and lower the overpotential for the reaction. Larger ferrihydrite-like nanoparticles are ineffective for electron transport.

International audience; The identification of the role of nano-morphology of electrodes for the reduction of CO2, particularly in determining the selectivity towards > C1 products of reduction, is still a challenge. We show here for the first time how a dendritic-type nano-morphology and related fractal dimension plays a crucial role in determining the selectivity for electrodes based on Cu and Fe. The electrodes, prepared by electrodeposition of Cu or Fe on metal substrates (Cu, Ti, Al, Fe, Fe-Cr-Ni alloy) or carbon-based gas diffusion layers (GDL), were tested in a continuous flow cell at constant potential (−1.5 V vs. Ag/AgCl) or at the onset potential of the first cathodic peak determined by cyclic voltammetry. Results showed that the dendritic-type nano-morphology influences positively not only the activity to formic acid, but especially the selectivity to gas products (i.e. CH4 and CO) and to higher carbon-chain liquid products (i.e. ethanol, isopropanol, acetic acid) when smoother edges and denser surface sites are present. Tests at the onset potential show very high total carbon Faradaic efficiency (FE) for Fe electrodeposited on GDL and Ti (about 92–95%). The lowest onset potential (−0.3 V) is observed for Cu on GDL, showing over 80% total carbon FE and almost 60% FE to formic acid.

Cu2 O/gas diffusion layer (GDL) electrodes prepared by electrodeposition were studied for the electrocatalytic reduction of CO2 . The designed electrode was also tested in solar-light-induced CO2 conversion in combination with a CuO/NtTiO2 photoanode using a compact photoelectrocatalytic (PEC) cell. Both PEC cell electrodes were prepared using non-critical raw materials and low cost, easily scalable procedures. In the PEC experiments, a total carbon faradaic selectivity of about 90 % to formate and about 75 % to acetate was obtained after 24 h of operations without application of potential/current or using sacrificial agents. In electrocatalytic tests of CO2 reduction at -1.5 V, the same electrode yielded high total faradaic selectivity (>95 %) but formed selectively formate (about 80 % selectivity) rather than acetate. The in situ transformation of the Cu2 O/GDL electrode leads to the formation of a hybrid Cu2 O-Cu/GDL system. Cyclic voltammetry data indicate that the potential and the presence of CO2 (not only of HCO3 - species) are both important elements in this transformation. Data also indicate that the surface concentration of CO2 (or of its products of transformation) on the electrode is an important factor to determine performance in the conversion of CO2 .

Electrocatalysts based on metal nanoparticles (metal = Ru, Fe and Pt), prepared by an organometallic (OM) route, and supported on carbon nanotubes (CNTs), have been studied in the electro-catalytic reduction of CO2 in comparison with those prepared using inorganic salts to deposit the metal nanoparticles (NPs) over CNTs (INORG route). The OM route allows to obtain electrocatalysts with superior performances in terms of productivity and especially turnover frequency, with an enhanced formation of C1 products (methanol and formic acid) with respect to C2+ products (acetic acid, acetone, isopropanol and ethanol). Particularly for Ru samples, there is an increase up to nearly two order of magnitude in the cumulative TOF with respect to a sample with an equivalent metal loading and mean particle size but prepared by the INORG route. Carbon selectivity to methanol reaches the interesting value of 65 %. The analysis of this intrinsic boosting of both activity and selectivity in the electrocatalytic reduction of CO2 suggests a role in this enhanced performance of small oxide patches on the metal nanoparticles formed as residues of the process of removal of the ligands present in the OM route.

Metal carbides M

The future feasibility of decarbonized industrial chemical production based on the substitution of fossil feedstocks (FFs) with renewable energy (RE) sources is discussed. Indeed, the use of FFs as an energy source has the greatest impact on the greenhouse gas emissions of chemical production. This future scenario is indicated as "solar-driven" or "RE-driven" chemistry. Its possible implementation requires to go beyond the concept of solar fuels, in particular to address two key aspects: i) the use of RE-driven processes for the production of base raw materials, such as olefins, methanol, and ammonia, and ii) the development of novel RE-driven routes that simultaneously realize process and energy intensification, particularly in the direction of a significant reduction of the number of the process steps.

We report here an easy and scalable synthetic route for the preparation of Cu-based nanoelectrodes active in C C coupling in the process of CO 2 electrocatalytic reduction. Copper nanowires (NWs) are used as the precursor for the deposition of small Cu nanoparticles (NPs) onto commercial carbon nanotubes (CNTs), preliminarily activated with HNO 3 to create oxygen functional groups on their external surface (o-CNTs). The Cu NWs, in contact with a suspension of o-CNTs in an ethanol-water mixture, transfer Cu fragments onto the o-CNTs. The result is a good dispersion of ultrafine Cu NPs (with an average size of less than 3 nm) preferentially deposited on the external part of the CNTs and evidencing a crystal phase structure of fcc (face-centered cubic) metal Cu (0) . The catalytic active phase (Cu NPs/CNTs) was assembled on a gas diffusion layer and tested in the electro-reduction of CO 2 , in presence of KHCO 3 aqueous electrolyte and under a continuous flow of pure CO 2 . For comparison, an electrocatalyst synthesized by conventional impregnation route (leading to higher size Cu NPs on o-CNTs and CuO monoclinic phase structure) was also tested. Results showed that the electrode with Cu NPs prepared by NW route was the best in terms of average current and carbon productivity, giving threefold higher formic acid and methanol production with respect to the conventionally prepared Cu/CNT sample. Moreover, C2-C3 products (ethanol, acetic acid, isopropanol, acetone) were obtained, showing the possibility of C C bond formation, probably due to the interfacial catalytic sites between the small Cu NPs and CNTs.

Fe2O3-CNT samples are studied for the room-temperature electrocatalytic synthesis of NH3 from H2O and N2 in a gas–liquid–solid three-phase reactor. A 30 wt % iron-oxide loading was found to be optimal. The performances greatly depend on the cell design, where the possibility of ammonia crossover through the membrane has to be inhibited. The reaction conditions also play a significant role. The effect of electrolyte (type, pH, concentration) was investigated in terms of current density, rate of ammonia formation, and Faradaic efficiency in continuous tests up to 24 h of time on stream. A complex effect of the applied voltage was observed. An excellent stability was found for an applied voltage of −1.0 V vs Ag/AgCl. At higher negative applied voltages, the ammonia formation rate and Faradaic selectivity are higher, but with a change of the catalytic performances, although the current densities remain constant for at least 24 h. This effect is interpreted in terms of reduction of the iron-oxide species above...

Photoanodes based on undoped TiO2 nanotube (TNT) thin films, fabricated by anodic oxidation of Ti foils modulating the anodization time (from 30 min to 5 h), were analysed and tested in a compact photo-electrocatalytic (PECa) device for H2 generation by water photo-electrolysis. The vertically aligned TNT films differ only in the film thickness (i.e. the length of the nanotubes), but they show: i) similar nanotube diameter, ii) uniform thickness and clean top surface and iii) same crystallinity degree in anatase phase. The TNT film becomes thicker by increasing the anodization time (up to 5.8 μm for TNTs anodized for 5 h). All the photoactive layers are able to almost completely absorb the UV part of irradiated light, while the thicker film evidences an enhanced visible light absorption. On the contrary, the photocurrent response decreases by increasing the film thickness. The most active photo-catalyst was the TNT sample anodized for 45 min, providing a H2 production rate of 22.4 μmol h−1 cm−2 and a STH efficiency as high as 2.5%. These values are among the best ever reported insofar as PECa cells use undoped TiO2 photoanodes and in absence of external bias or sacrificial agents.

A rapid and efficient electrochemical reaction requires active catalytic material, as well as proper electrode and cell design. Now, a gas diffusion electrode based on a stainless steel cloth successfully overcomes gas transport limitations for high-current ammonia electrosynthesis in non-aqueous solvents at ambient conditions.

Cu

Made available in DSpace on 2021-06-25T11:10:44Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-05-01 Ministero dell’Istruzione, dell’Università e della Ricerca Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) BiVO4 nanoparticles deposited onto TiO2 nanotube arrays (TNT) are used as heterostructured photoanodes in a compact-design photo-electrocatalytic (PEC) cell for solar-driven water splitting. No dopants, photosensitizers or other cocatalysts are added to enhance the catalytic activity but attention is focused on the relationship between TNT nanostructure (necessary for the novel compact-design PEC cell) and method of BiVO4 deposition. Three indicators are used to evaluate the catalytic performances: i) photocurrent density, ii) H2 production rate, and iii) solar-to-hydrogen efficiency (STH). Their dependence on photoanode characteristics (i.e. grade of TNT crystallinity) and operational parameters, such as anolyte concentration (NaOH in the range 0.1–1.0 M) and type of solar illumination (open spectrum or AM 1.5G filtered light), is analysed. While a linear relationship is observed between H2 production rate and photocurrent density, the behaviour of STH efficiency is more complex. An ordered and crystalline TNT film is necessary to maximize photocurrent density and H2 production rate, which can be further enhanced by depositing BiVO4. However, the methodology of BiVO4 deposition and the specific TNT nanoarchitecture have a marked influence in terms of light absorption, electronic conductivity and rate of reaction between photogenerated holes and OH−. This leads to an improvement or a depression of the photocatalytic behaviour as BiVO4 may in some cases favour charge recombination. Thus, understanding the role of the photoelectrodes in relation to the operational conditions may favour the preparation of scalable electrodes for improving performances of PEC cells in the generation of solar fuels. Departments of ChiBioFarAm and MIFT – University of Messina ERIC aisbl and CASPE/INSTM, V.le F. Stagno d'Alcontres, 31 Institute of Chemistry-Araraquara UNESP Rua Francisco Degni, 55, Bairro Quitandinha National Institute of Alternative Technologies for Detection Toxicological Evaluation and Removal of Micropollutants and Radioactive Substances (INCT-DATREM) São Paulo State University (Unesp) Institute of Chemistry Institute of Chemistry-Araraquara UNESP Rua Francisco Degni, 55, Bairro Quitandinha National Institute of Alternative Technologies for Detection Toxicological Evaluation and Removal of Micropollutants and Radioactive Substances (INCT-DATREM) São Paulo State University (Unesp) Institute of Chemistry Ministero dell’Istruzione, dell’Università e della Ricerca: 20179337R7 FAPESP: 2018/01934-0 FAPESP: 2018/16062-9

The efficiency of photo-electrocatalytic (PECa) devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiO2 nanotube (TNT) arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis (splitting) and ethanol photo-reforming processes. The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, H2 production rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91% were obtained without adding sacrificial reagents.

Ammonia is synthesized directly from water and N2 at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half-cell for the NH3 synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half-cell. A rate of ammonia formation of 2.2×10−3 gNH3 m−2 h−1 was obtained at room temperature and atmospheric pressure in a flow of N2, with stable behavior for at least 60 h of reaction, under an applied potential of −2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH3 electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N2, making it more reactive towards hydrogenation.

A non-enzymatic electrochemical sensor based on cuprous oxide nanocubes (Cu2O-NC) has been developed. Cu2O-NC samples have been synthesized using a wet precipitation technique under different precipitation temperatures (25 and 60 °C). The samples were characterized by XRD (X-ray Diffraction), UV–visible Diffuse Reflectance Spectroscopy (UV–Vis DRS) and Scanning Electron Microscopy (SEM). Screen-printed carbon electrodes (SPCE) have been modified by casting the synthesized Cu2O-NC. The fabricated Cu2O-NC-SPCE sensor prepared at 25 °C provided the best sensing performance toward glucose, with a sensitivity of 1040 μA/mM cm−2 in the linear range from 0.007 to 4.5 mM, and a detection limit of 3.1 μM (S/N = 3). Cu2O-NC-SPCE electrodes were also evaluated for the amperometric determination of glucose. Testing results showed a satisfactory stability toward glucose sensing and selectivity against other sugars and ethanol, suggesting that the SPCE modification with Cu2O-NC could be a simple way to fabricate inexpensive and reliable sensors to monitor glucose in bio-fermentation processes.

The development of new technologies to convert solar energy into solar fuels through the synthesis of innovative photoactive materials is a topic of great interest nowadays. By manufacturing photo-electrodes designed with purpose to mimic the plant leaves, solar energy can be harvested and used to produce higher value-added compounds (methanol, ethanol, formic acid, methane, and others) from CO2 conversion or hydrogen from water splitting. So, the present work aims preparing, characterizing and testing semiconductor electrodes based on Ti/TiO2 nanotubes (TNT) modified with bismuth vanadate (BiVO4) or cuprous oxide (Cu2O) in the photoelectrocatalytic water splitting (water oxidation) and reduction of CO2, respectively. Considering that the photoelectrocatalytic process on the TNT surface is very small due to the lack of visible-light response and high recombination of electron-hole pair, that takes to low quantum efficiency of the process, its coupling with other semiconductors, such as BiVO4 or Cu2O can increase light absorption to the visible region and decrease the recombination of the electron-hole pair by the formation of heterojunction, therefore increasing both the processes. The TNT electrodes were prepared by electrochemical anodization of titanium sheets. BiVO4 was prepared by microwave-assisted hydrothermal method by using a commercial microwave synthesizer system [1]. Cu2O nanostructures with different morphologies were prepared as described by [2]. For water splitting, TNT electrodes were decorated with BiVO4 nanoparticles (TNT/BiVO4) by spin or spray coating method using Nafion® (Nafion® perfluorinated resin solution 5 wt. %) as an anchoring agent. On the other hand, for CO2 reduction, the Cu2O nanoparticles (cube, Nc; sphere, Ns; octahedron, No) layers on the catalysts were obtained using two sequential dip-coating procedures, after pre-coating the TNT surface with polydopamine (PDA). The water splitting was carried out using a PEC reactor homemade of Plexiglas and equipped with a quartz window. In the two compartments where placed the two electrodes acting as cathode (GDL-Pt) and photoanode (TNT/BiVO4), where the evolution of H2 and O2 evolution was monitored. The electrolyte in the anodic and cathodic compartment was 1 M NaOH and 0.5 M H2SO4, respectively. A Nafion® exchange membrane was used between the two electrodes configuration for the selective transport of protons towards the cathode for H2 production. It is noteworthy that for water oxidation no external bias was applied. Notwithstanding, CO2 reduction PEC sealed system (1 kgf cm−2), performed in low bias potential (0.2 V), using 0.1 M Na2SO4 electrolyte saturated with CO2 (pH 4.5) for 2h. The system was irradiated by either UV-Vis irradiation or solar simulator. The TNT/BiVO4 spray electrode presented a very low H2 production, maybe due to the excess of BiVO4 present on the surface of the TNT electrode. However, TNT/BiVO4 spin electrode produced 21.5 μmol of H2 after 125 min of irradiation, which is 57% higher when compared to the non-modified TNT electrode. Concerning the photoelectrocatalytic CO2 conversion, it was observed that the electrodes presented a significant performance in the conversion of CO2 to methanol, when compared with bare TNT electrode (1.5 mg L−1/1.2 mg L−1): 10 mg L−1/3.8 mg L−1 (Nc), 6.0 mg L−1/1.3 mg L−1 (No) and 5.4 mgL−1/1.7 mgL−1 (Ns) under UV-Vis radiation and solar simulator, respectively. The results demonstrated the facet-dependent performance of these nanostructures as photocatalysts and the use of PDA proved to be a good strategy to obtain p-n heterojunction semiconductors with the improvement of its response in visible light region. Therefore, these results demonstrated the potentiality of using TNT electrodes modified with nanoparticles of bismuth vanadate for water splitting, or cuprous oxide for CO2 reduction, contributing to the development of efficient technologies to produce solar fuels. References [1] S. Ulah, E.P. Ferreira-Neto, C. Hazra, R. Parveen, H.D. Rojas-Mantilla, M.L. Calegaro, Y.E. Serge-Correales, U.P. Rodrigues-Filho, S.J.L. Ribeiro, Appl. Cat. B: Environ., 2019, 243, 121–135. [2] F.A.C. Pastrián, A.G.M. da Silva, A.H.B. Dourado, A.P. de Lima Batista, A.G.S. de Oliveira-Filho, J. Quiroz, D.C. de Oliveira, P.H.C. Camargo, S.I. Córdoba de Torresi. ACS Catal., 2018, 8, 6265-6272.

Compte tenu du recent taux alarmant d'epuisement des reserves de combustibles fossiles et de l'augmentation drastique des niveaux de CO2 dans l'atmosphere qui a conduit au rechauffement de la planete et a des changements climatiques severes, l'exploitation de toutes sortes d'energies renouvelables a ete la Parmi les principales priorites de la recherche Champs a travers le monde. L'une des nombreuses voies de ce genre est la reduction du CO2 aux combustibles utilisant des energies renouvelables, plus communement appelees cellules photoelectro-catalytiques (PEC). Des essais experimentaux sur la reduction du CO2 ont ete realises sur differents types de catalyseurs dans les deux cellules (Concu par un laboratoire) afin de comprendre la selectivite, la productivite et les produits de reaction obtenus. Des essais experimentaux ont ete realises sur differents types de catalyseurs a la fois dans les cellules en phase gazeuse et en phase liquide pour comprendre la selectivite, la productivite et les produits de reaction obtenus. Pour les etudes sur la reduction EC du CO2 en phase gazeuse, une serie d'electrodes (a base de nanoparticules (NPs) de Cu, Fe, Pt et CuFe deposees sur des nanotubes de carbone ou de noir de carbone puis placees a l'interface entre une membrane Nafion et Une electrode a couche de diffusion de gaz). Les resultats demontrent le type divers de produits formes et leurs productivites. Dans des conditions sans electrolyte, la formation de produits ≥C1 tels que l'ethanol, l'acetone et l'isopropanol a ete observee la plus elevee etant pour Fe et suivie de pres par Pt. Pour ameliorer Combustibles nets, un ensemble different d'electrodes a ete prepare sur la base de revetements MOF de type imidazolate de type zeolitique substitue (SIM-1) (Fe-CNT, Pt-CNT et CuFe-CNT bases sur MOF) Et Pt-MOF a montre des carburants ameliores. En se reportant aux etudes sur la reduction EC du CO2 dans une cellule en phase liquide, un ensemble similaire d'electrodes a ete prepare (NP - Cu, Fe, Pt, Ru, Co deposees sur des nanotubes de carbone ou du noir de carbone ont). Pour les conditions de phase liquide, en termes de produits C nets, les electrodes catalytiques a base de Pt sont en tete de la categorie, suivies de pres par Ru et Cu, tandis que Fe a obtenu la position la plus basse. Le mecanisme reactionnel sous-jacent probable a egalement ete fourni. Afin d'ameliorer encore les performances, on a synthetise des NP de metal (Ru, Fe, Pt et Cu) de differentes tailles en utilisant differentes techniques de synthese: (i) l'itineraire d'impregnation (ImR) pour obtenir des NP dans la plage de tailles de 10 a 50 nm; (Ii) Approche organometallique (OM) pour synthetiser des NPs uniformes et ultrafines dans la plage de tailles de 1-5 nm. Fe ont ete synthetises par une nouvelle voie de synthese et des conditions pour atteindre des NP de 1 a 3 nm. (Iii) Approche de haut en bas de Nanowire pour obtenir des NP de cuivre ultrafin dans la plage de taille de 2-3,8 nm. Les ameliorations apportees a la productivite du carburant se sont revelees etre de 5 a 30 fois plus elevees pour les petites NP sur les NP plus importantes et, en outre, une charge reduite de 10 a 1-2% en poids. Un autre ensemble d'electrodes a base de nano-mousses (Cu NF et Fe NF sur Feuille de Cu, Feuille de Foie, Al Foil, Inconel foil et Al grid / mesh) prepares par electrodeposition ont egalement ete etudies afin d'ameliorer encore la conversion de CO2 / carburant. Apres, l'optimisation du depot et de la tension a l'aide de la voltametrie cyclique, les carburants se sont ameliores de 2 a 10 fois par rapport aux combustibles nets les plus eleves obtenus a l'aide d'electrodes CNT dopees a base de NP

The behaviour of photoanodes based on TiO2 nanotubes (TNTs) grown on Ti gauze (80 mesh) by controlled anodic oxidation was studied in a compact photo-electrocatalytic (PECa) device for H2 production by water photo-electrolysis. The 3D-type structured TNTs-Ti meshes evidenced: i) a mesoporosity due to the presence of highly ordered TiO2 nanotube arrays grown on the round surface of the regularly woven wires of the Ti gauze and ii) a macroporosity due to the open area (36%) of the mesh. The resulting hierarchical porous 3D structure allowed the TNTs-Ti meshes to act both as photoactive materials for efficient light absorption and as porous substrates for fast mass diffusion of protons. The synthesis of TNTs was investigated by varying the applied voltage (in the range 40–60 V) and the anodization time (from 20 min to 7 h). Results showed that two main opposite aspects influenced their performances in the PECa cell: i) the inner diameter and ii) the length of TNTs. While the increase of the inner diameter with the anodization time should favour H2 productivity, longer nanotubes negatively influence the catalytic activity due to increasing charge recombination phenomena. The result is an initial decreasing profile of H2 evolution vs. anodization time, reaching a minimum at 3 h and then increasing again for the 5 h-anodized sample (H2 production rate: 1.4 L m−2 h−1). The maximum photoconversion efficiency (0.7%) was obtained, instead, for the 20 min-anodized sample. It is to remark that tests of water splitting were performed with no applied bias and without adding sacrificial donors, opening the route for a sustainable use of these 3D-type meso/macro porous structured photoanodes in the production of solar fuels.

An electrochemical device for the reduction of CO2 back to liquid fuels is here presented. The key of this novel electrocatalytic approach is the design and development of the gas diffusion membrane (GDM), which is obtained by assembling (i) a proton selective membrane (Nafion), (ii) a nanocomposite electrocatalyst based on metal-doped conjugated microporous polymer (CMP) and (iii) a C-based support working as the gas diffusion layer. CMP is a very attractive material able to adsorb CO2 selectively with respect to other gases (such as H2, O2, N2, etc.), also in mild conditions (r.t. and atmospheric pressure). Particularly, tetrakis-phenylethene conjugated microporous polymer (TPE-CMP) was synthesized through Yamamoto homo-coupling reaction. TPE-CMP was modified by depositing noble (Pt) and non-noble (Fe) metal nanoparticles to create the active catalytic sites for the process of CO2 reduction directly on the polymer surface where CO2 is adsorbed. The metal-doped TPE-CMP electrocatalysts were fully characterized by infrared spectroscopy (IR), thermo-gravimetric analysis (TGA) and transmission electron microscopy (TEM). Then, the as-assembled GDM was tested in our homemade semi-continuous three-electrode electrochemical cell working in gas phase at 60 °C, coupled with a cold trap for the accumulation of the liquid products. Results showed the better performances of the metal-doped TPE-CMP in terms of total productivity (C1–C8 oxygenates) with respect to other kinds of materials that do not show high CO2 adsorption capacity.

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials., Trapping carbon dioxide within usable chemicals is a promising means to mitigate climate change, yet electrochemical C–C couplings are challenging to perform. Here, the authors prepared iron oxyhydroxides on nitrogen-doped carbon that efficiently convert carbon dioxide to acetic acid.

Titania nanotube (TNT)-array thin films well decorated by copper nanoparticles with average size of 3 nm were prepared by spray coating of a solution containing size-controlled Cu 0 nanoparticles. The consecutive calcination at 300 °C and 450 °C leads to the oxidation of these Cu NPs to CuO, with small amounts of Cu 2 O at the lower calcination temperature, but maintaining the high dispersion. Analogous materials prepared by copper electrodeposition lead to significantly larger Cu NPs. The TNT-array thin film shows significantly enhanced photocurrent (up to about 90%) with respect to a comparable thin film prepared by spin coating using a commercial TiO 2 P25 sample. The behavior is similar by applying different filters to cut part of solar light simulator radiation. Particularly, using an UV B/C blocking filter, which permits to pass light in the 350–550 nm range, an about two-fold intensification in the current-to-electrical energy conversion (normalized to the same total irradiance) is obtained. The presence of CuO nanoparticles decreases the photocurrent density with respect to the support alone (TNT-array 1h), but enhances the H 2 photogeneration rate in the gas-phase photoreactor experiments. The results indicate that in the tested experimental conditions, the main role of CuO nanoparticles is to act as co-catalyst to improve the H 2 photogeneration rate rather than to promote charge separation or other effects, which promote the photocurrent density.

Made available in DSpace on 2018-12-11T17:34:41Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-05-01 The role of CuO nanoparticles decorating TiO2 nanotubes (TNT) thin film photoanodes in the behavior of photo-electrocatalytic (PEC) cells for water splitting reaction is investigated. CuO is present mainly as small nanoparticles of few nanometer decorating the internal walls of the TiO2 nanotubes. Their presence improves i) the photocurrent behavior, ii) the H2 generation rate by water splitting in a full PEC device (without application of a bias) and iii) the solar-to-hydrogen (STH) efficiency. The increase is about 20% with respect to parent TNT photoanodes using open spectrum light from a solar simulator and about 50% increase using AM 1.5G filtered light from a solar simulator. An STH efficiency over 2% in the full PEC cell is observed in the best conditions. IPCE (incident photon to current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induce an enhanced IPCE in the 300–340 nm region. The increase in the performances in water splitting is mainly associated to the transient generation of a p–n junction between the CuxO nanoparticles and TNT upon illumination, which enhances photocurrent density by promoting charge separation. University of Messina ERIC aisbl and CASPE/INSTM Departments ChiBioFarAm and MIFT, viale F. Stagno d'Alcontres 31 Institute of Chemistry-Araraquara UNESP Rua Francisco Degni, 55, Bairro Quitandinha Institute of Chemistry-Araraquara UNESP Rua Francisco Degni, 55, Bairro Quitandinha

The electrocatalytic reduction of CO2 is studied on a series of electrodes (based on Cu, Co, Fe and Pt metal nanoparticles deposited on carbon nanotubes or carbon black and then placed at the interface between a Nafion membrane and a gas-diffusion-layer electrode) on two types of cells: one operating in the presence of a liquid bulk electrolyte and the other in the absence of the electrolyte (electrolyte-less conditions). The results evidence how the latter conditions allow productivity of about one order of magnitude higher and how to change the type of products formed. Under electrolyte-less conditions, the formation of >C2 products such as acetone and isopropanol is observed, but not in liquid-phase cell operations on the same electrodes. The relative order of productivity in CO2 electrocatalytic reduction in the series of electrodes investigated is also different between the two types of cells. The implications of these results in terms of possible differences in the reaction mechanism are commented on, as well as in terms of the design of photoelectrocatalytic (PEC) solar cells.

The addition of a CO

This essay analyses some of the recent development in nanocarbons (carbon materials having a defined and controlled nano-scale dimension and functional properties which strongly depend on their nano-scale features and architecture), with reference to their use as advanced catalytic materials. It is remarked how their features open new possibilities for catalysis and that they represent a new class of catalytic materials. Although carbon is used from long time in catalysis as support and electrocatalytic applications, nanocarbons offer unconventional ways for their utilization and to address some of the new challenges deriving from moving to a more sustainable future. This essay comments how nanocarbons are a key element to develop next-generation catalytic materials, but remarking that this goal requires overcoming some of the actual limits in current research. Some aspects are discussed to give a glimpse on new directions and needs for R&D to progress in this direction.

The behavior of Fe, Co, Cu (mono-, bi- and trimetallic) on carbon nanotube (CNT) electrocatalysts in the solventless conversion of CO 2 to C1–C3 hydrocarbons/organics (HO) has been studied as a part of the general objective of developing novel photoelectrocatalytic (PEC) solar cells to convert CO 2 to solar fuels. The comparison of commercial conductive carbon black support is also analyzed, as well as the role of CNT. FeCu bimetallic electrocatalysts double the productivity to C1–C3 HO with respect to iron monometallic electrocatalysts, and the use of CNTs as conductive carbon support allows to double productivity to C1–C3 HO with respect to commercial conductive carbon black supports. The parallelism between solventless operations at atmospheric pressure and high-pressure operations in the electrocatalytic reduction of CO 2 in liquid electrolyte was evidenced. The results show that Fe and other metals, which is different from Cu are inactive in forming HO during the electroreduction of CO 2 at atmospheric pressure in a liquid aqueous electrolyte, are instead active and superior to copper under solventless conditions. The results indicate that the inhibition of the surface reactivity toward H 2 formation rather than minimizing the energy barriers in CO 2 electroreduction is the key to improve the performances and realize selective electrocatalysts.

This perspective paper introduces the concept that nanocarbons and related materials such as carbon dots are an interesting intrinsic photocatalytic semiconducting material, and not only a modifier of the existing (semiconducting) materials to prepare hybrid materials. The semiconducting properties of the nanocarbons, and the possibility to have the band gap within the visible-light region through defect band engineering, introduction of light heteroatoms and control/manipulation of the curvature or surface functionalization are discussed. These materials are conceptually different from the “classical” semiconducting photocatalysts, because semiconductor domains with tuneable characteristics are embedded in a conductive carbon matrix, with the presence of various functional groups (as C O groups) enhancing charge separation by trapping electrons. These nanocarbons open a range of new possibilities for photocatalysis both for energetic and environmental applications. The use of nanocarbons as quantum dots and photoluminescent materials was also analysed.

Copper nanoparticles on carbon nanotubes are used in the reduction of CO2 to acetic acid (with simultaneous water electrolysis) in a flow electrocatalytic reactor operating at room temperature and atmospheric pressure. A turnover frequency of about 7000 h−1 and a carbon-based Faradaic selectivity to acetic acid of about 56% were observed, indicating potential interest in this approach for using renewable energy. The only other products of reaction detected were formic acid and methanol (the latter in some cases), besides H2. The reaction mechanism, particularly the critical step of C–C bond formation, was studied by comparing the reactivity in tests with CO2 or CO, where formic acid or formaldehyde where initially added. The results indicate the need for having dissolved CO2 to form acetic acid, likely via the reaction of CO2˙− with surface adsorbed –CH3 like species. The pathway towards formic acid is instead different from the route of the formation of acetic acid.

The addition of a CO2 adsorption component (substituted imidazolate-based SIM-1 crystals) to a gas diffusion layer (GDL) type catalytic electrode allows to enhance the activity and especially the selectivity to >C1 carbon chain products (ethanol, acetone and isopropanol) of a Pt-based electrocatalyst that is not able to form products of CO2 reduction involving C-C bond formation under conventional (liquid-phase) conditions. This indicates that the increase of the CO2 effective concentration at the electrode active surface is the factor controlling the formation of >C1 products rather than only the intrinsic properties of the electrocatalyst.

The conversion of solar energy into H2 via water splitting process is one of the most attractive ways to obtain clean and renewable energy. Unfortunately, the fast back reaction of recombination and high band gap needed to activate the photo-catalytic materials, strongly limit the performances in conventional slurry photo-reactors. In this context we present a new photoelectrochemical approach with a double-chamber reactor configuration for H2 production by water photo-electrolysis. The core of the photo-system is a membrane electrode assembly consisting of different layers which hold distinct two areas of the reactor where the generation of O2 and H2 occurs separately. Particular attention is given to the development, on a nano-scale level, of the materials to be used as photoanode and electrocathode: nanostructured TiO2 arrays and carbon nanotubes are used respectively in the form of thin films separated by a proton conductive membrane. Results showed 3.2 mmol h−1 g−1 of H2 evolution that is about one order of magnitude higher with respect to the activity obtained with conventional slurry photoreactors. Moreover, we present the opportunity to recycle CO2 back to liquid fuels by using the same photoelectrochemical approach.