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In this work, we performed a preliminary investigation on the redox behaviour of Sr2FeMo0.6Ni0.4O6-δ (SFMN) double perovskite in H2-H2O and CH4-H2O redox cycles in order to explore the potential use of this oxide as an Oxygen Carrier (OC) in fuel-assisted Chemical Looping Water Splitting (CLWS) processes driven by concentrated solar energy. The results were compared with our previous findings on the Reverse Water Gas Shift Chemical Looping (RWGS-CL) reaction. The improvement in performance due to the bimetallic exsolution on the OC surface is observed. This OC exhibits interesting activity and stability over CH4-assisted CLWS cycling. Future investigations are planned to examine the structural transformations that might impact the redox behaviour of this material in water splitting processes.

Summary: Developing active and stable catalysts for carbon-free hydrogen production is crucial to mitigate the effects of climate change. Ammonia is a promising carbon-free hydrogen source, as it has a high hydrogen content and is liquid at low pressure, which allows its easy storage and transportation. We have recently developed a nickel-based catalyst with a small content of ruthenium supported on cerium oxide, which exhibits high activity and stability in ammonia decomposition. Here, we investigate mechanochemical milling for its synthesis, a faster and less energy-consuming technique than conventional ones. Results indicate that mechanochemical synthesis increases catalytic activity compared to the conventional incipient wetness impregnation method. The interaction between the metal precursors and the support is key in fine-tuning catalytic activity, which increases linearly with oxygen vacancies in the support. Moreover, the mechanochemical method modifies the oxidation state of Ni and Ru species, with a variation depending on the precursors.

This work presents experimental data and modelling of the release of Mo from high-burnup spent nuclear fuel (63 MWd/kgU) at two different pH values, 8.4 and 13.2 in air. The release of Mo from SF to the solution is around two orders of magnitude higher at pH = 13.2 than at pH = 8.4. The high Mo release at high pH would indicate that Mo would not be congruently released with uranium and would have an important contribution to the Instant Release Fraction, with a value of 5.3%.Parallel experiments with pure non irradiated Mo(s) and XPS determinations indicated that the faster dissolution at pH = 13.2 could be the consequence of the higher releases from metallic Mo in the fuel through a surface complexation mechanism promoted by the OH− and the oxidation of the metal to Mo(VI) via the formation of intermediate Mo(IV) and Mo(V) species.

Abstract Understanding the behaviour of active catalyst sites at the atomic level is crucial for optimizing catalytic performance. Here, the evolution of Pt and Cu dopants in Au25 clusters on CeO2 supports is investigated in the water-gas shift (WGS) reaction, using operando XAFS and DRIFTS. Different behaviour is observed for the Cu and Pt dopants during the pretreatment and reaction. The Cu migrates and builds clusters on the support, whereas the Pt creates single-atom active sites on the surface of the cluster, leading to better performance. Doping with both metals induces strong interactions and pretreatment and reaction conditions lead to the growth of the Au clusters, thereby affecting their catalytic behaviour. This highlights importance of understanding the behaviour of atoms at different stages of catalyst evolution. These insights into the atomic dynamics at the different stages are crucial for the precise optimisation of catalysts, which ultimately enables improved catalytic performance.

A simple proportional integral (PI) controller with scheduled gain has been developed and implemented in a catalytic membrane reactor (CMR) to obtain pure hydrogen from a methanol steam reforming process. The controller is designed to track the setpoint of the pure hydrogen flow rate in the permeate side of the CMR via the manipulation of the fuel inlet flow rate. Therefore, the controller actuator is the liquid pump that provides the mixture of methanol and water to the reactor. Within the CMR, the catalytic pellets of PdZn/ZnAl2O4/Al2O3 have been used to facilitate the methanol steam-reforming reaction under stoichiometric conditions (S/C = 1), and Pd–Ag metallic membranes have been employed to simultaneously separate the generated hydrogen. The PI controller design is based on a mathematical model constructed using transfer functions acquired from dynamic experiments conducted with the CMR. The controller has been successfully implemented, and experimental validation tests have been carried out at 450 °C and relative pressures of 6, 8, 10, and 12 bar.

Abstract Metal clusters supported on TiO2 are widely used in many photocatalytic applications, including pollution control and production of solar fuels. Besides high photoactivity, stability during the photoreaction is another essential quality of high-performance photocatalysts, however systematic studies on this attribute are absent for metal clusters supported on TiO2. Here we have studied, both experimentally and with first-principles simulation methods, the stability of Pt, Pd and Au clusters prepared by ball milling on nanoshaped anatase nanoparticles preferentially exposing {001} (plates) and {101} (bipyramids) facets during the photogeneration of hydrogen. It is found that Pt/TiO2 exhibits superior stability than Pd/TiO2 and Au/TiO2, and that {001} facet-based photocatalysts always are more stable than their {101} analogous regardless of the considered metal species. The loss of stability associated with cluster sintering, which is facilitated by the transfer of photoexcited carriers from the metal species to the neighbouring Ti and O atoms, most significantly and detrimentally affects the H2-evolution photoactivity.

Ni-Mg/Al mixed oxide catalysts (Ni2Al, Ni2Mg1Al, and Ni1Mg1Al) obtained from layered double hydroxides (LDHs) were tested on the one-pot production of tetrahydrofurfuryl alcohol (TFA) from furfural (FF). Upon calcination at 400 °C and reduction at 500 °C, the LDHs gave catalysts containing small nickel crystallites (99.5%) was achieved on all the catalysts after 4 h at 190 °C and 5.0 MPa of H2 using 5 wt.% FF in ethanol and a furfural-to-catalyst mass ratio of 7.44 g/g. TFA evolved from the sequential hydrogenation of FF to furfuryl alcohol (FA) to TFA. Competing reaction routes involved decarbonylation of FF to furan (FUR) followed by hydrogenation to tetrahydrofuran (THF) or hydrogenolysis to n-butane (BU) and the hydrogenation of the carbonyl group in FF to form 2-methyl furan (mFUR) and its hydrogenation to 2-methyltetrahydrofuran (mTHF). A third competing route consisted of the nucleophilic addition of FF with ethanol and with FA to form acetals (such as 2-(diethoxymethyl)furan, FDA), which were later converted to difurfuryl ether (DFE) and tetrahydrofurfuryl ethyl ether (TFEE) as final products. Hydrogen pressure favored the production of TFA and diminished the formation of acetals, while temperature reduced the capacity of the catalyst to hydrogenate the furan ring, thus reducing TFA and increasing FA and FUR. An 80% yield to TFA was achieved with the Ni2Mg1Al catalysts after 6 h at 190 °C and 50 bar H2, but a variety of coproducts were present at low concentration. Testing of the catalysts in gas-phase hydrogenation conditions at atmospheric pressure revealed a poorer performance, with FA as the main product.

The catalytic wet air oxidation (CWAO) of p-hydroxybenzoic acid (p-HBA) was conducted in a batch reactor at 140 °C, and at a total air pressure of 50 bar over Ru-based catalysts. Four materials were selected as supports – TiO2, CeO2–TiO2, ZrO2–TiO2, and La2O3–TiO2 – all of which had mesopores in their texture and pollutant adsorption capacities. The supports were prepared by the sol-gel method, and then impregnated with 3 wt% of Ru precursor. Such characterization techniques as N2-sorption, XRD, XPS, H2–TPR, NH3-TPD, TEM, and HAADF-STEM were used to analyze the different solids. The correlation between catalytic activities and physicochemical properties was discussed. A significant specific surface area (SBET), a large amount of surface-active oxygen, and Lewis acidity sites were observed on cerium-containing catalysts (Ru/CeTi). Fresh Ru catalysts containing cerium showed higher activity than Ru/TiO2, Ru/ZrTi, and Ru/LaTi catalysts. It is assumed that the acidic sites and surface oxygen trap the p-HBA molecule, thus increasing the catalytic properties of the Ru particles which interact with the surface oxygen through the cerium redox process (Ce3+/Ce4+). As the presence of cerium increases surface-active oxygen, it inhibits the deposition of carbon on the surface of the Ru catalyst. The pseudo-second order (PSO) model adequately described the kinetic data of the p-HBA oxidation reaction using Ru catalysts.

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22−) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1−xO2−δ solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1−xO2−δ phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.

Mechanochemical synthesis for the preparation of bimetallic catalysts is gaining increasing interest, and in recent years, some important milestones have been reached. However, the complexity of mechanochemically prepared bimetallic supported catalysts still leaves many open questions that need a systematic approach to be solved. In this work, we summarize our experience of mechanochemically milling bimetallic catalysts, introducing some key parameters that should be taken into account, particularly the thermal stability and hydrophilicity of precursor salts, and the effect of the milling order, highlighting the differences with wet synthesis methods. Finally, we will provide some suggestions on the application of the design of experiments approach to the rationalization of the milling procedure for the preparation of supported bimetallic catalysts.

The anodic reactivity of UO2 and UO2 doped with Gd2O3 was investigated by electrochemical methods in slightly alkaline conditions in the presence of silicate and calcium. At the end of the experiments, the electrodes were analysed by X-ray photoelectron spectroscopy to determine the oxidation state of the uranium on the surface. The experiments showed that the increase in gadolinia doping level led to a reduction in the reactivity of UO2, this effect being more marked at the highest doping level studied (10 wt.% Gd2O3). This behaviour could be attributed to the formation of dopant-vacancy clusters (GdIII-Ov), which could limit the accommodation of excess O2− into the UO2 lattice. In addition, the presence of Ca2+ and SiO32− decreased the anodic dissolution of UO2. In summary, the Gd2O3 doping in presence of silicate and calcium was found to strongly decrease the oxidative dissolution of UO2, which is a beneficial situation regarding the long-term management of spent nuclear fuel in a repository.

Methane is a strong contributor to global warming. Here the authors demonstrate that mechanically prepared Pd-Pt supported on ceria catalysts abate methane better than conventionally impregnated ones, even under wet conditions, owing to a highly dynamic structure.

The selective oxidation of primary alcohols into their corresponding carbonyl compounds is challenging because of the easy over oxidization to acids and esters. The traditional reaction requires large amounts of solvent and oxidant, causing serious environmental issues. Recently, several efforts have been made to transform the reaction into a more sustainable process. Here, we investigated the solvent-free oxidation of benzyl alcohol using air as a green oxidant in the presence of ruthenium supported on alumina and zirconia, thereby meeting atom economy and environmental requirements. The materials were extensively characterized and, in addition to their activity, selectivity, and reusability, the environmental sustainability of the process was assessed according to green chemistry metrics. XRD, TEM, and XPS analyses suggest that the formation of metallic Ru on the support plays a key role in the catalytic activity. Ru supported on alumina, after a reduction treatment, achieves good activity (62% conversion) and a complete selectivity in a very sustainable process (without a solvent and with air as oxidant), as indicated by the very low E-factor value. The formulation is very stable and maintains high activity after recycling.

Four different nanoshapes of cerium dioxide have been prepared (polycrystals, rods, cubes, and octahedra) and have been decorated with different metals (Ru, Pd, Au, Pt, Cu, and Ni) by incipient wetness impregnation (IWI) and ball milling (BM) methods. After an initial analysis based on oxygen consumption from CO2 pulse chemisorption, Ni-like metal, and two forms of CeO2 cubes and rods were selected for further research. Catalysts were characterized using the Brunauer-Emmett-Teller formula (BET), X-ray spectroscopy (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR) and CO2 pulse chemisorption, and used to reduce of CO2 into CO (CO2 splitting). Adding metals to cerium dioxide enhanced the ability of CeO2 to release oxygen and concomitant reactivity toward the reduction of CO2. The effect of the metal precursor and concentration were evaluated. The highest CO2 splitting value was achieved for 2% Ni/CeO2-rods prepared by ball milling using Ni nitrate (412 µmol/gcat) and the H2 consumption (453.2 µmol/gcat) confirms the good redox ability of this catalyst.

The catalytic production of syngas by the partial oxidation of methane (POM) was investigated over Pd supported on ceria (0.5–2 Pd wt.%) prepared by incipient wetness impregnation and by mechanochemical methods. The performance of the Pd/CeO2 catalyst prepared by milling CeO2 and Pd acetate was superior to that prepared by milling CeO2 and Pd nitrate and to Pd/CeO2 prepared by impregnation from Pd acetate. The best catalytic activity of the Pd/CeO2 catalyst prepared from CeO2 and Pd acetate was obtained by milling at 50 Hz for 5 min. Two-step combustion and reforming reaction mechanism were identified. Remarkably, methane conversion increased progressively with Pd loading for the catalysts prepared by incipient wetness impregnation, whereas low metal loading showed better conversion of methane for the catalysts prepared by ball milling using Pd acetate. This was explained in terms of an impressive dispersion of Pd species with a strong interaction with the surface of ceria, as deduced from transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy characterization, which revealed a large quantity of highly oxidized species at the surface.

Abstract Chemical looping syngas production is a two-step redox cycle with oxygen carriers (metal oxides) circulating between two interconnected reactors. In this paper, the performance of pure CeO2/Ce2O3 redox pair was investigated for low-temperature syngas production via methane reduction together with identification of optimal ideal operating conditions. Comprehensive thermodynamic analysis for methane reduction and water and CO2 splitting was performed through process simulation by Gibbs free energy minimization in ASPEN Plus®. The reduction reactor was studied by varying the CH4/CeO2 molar ratio between 0.4 and 4 along with the temperature from 500 to 1000 °C. In the oxidation reactor, steam and carbon dioxide mixture oxidized the reduced metal back to CeO2, while producing simultaneous streams of CO and H2 respectively. Within the oxidation reactor, the flow and composition of the mixture gas were varied, together with reactor temperature between 500 and 1000 °C. The results indicate that the maximum CH4 conversion in the reduction reactor is achieved between 900 and 950 °C with CH4/CeO2 ratio of 0.7–0.8, while, for the oxidation reactor, the optimal condition can vary between 600 and 900 °C based on the requirement of the final product output (H2/CO). The system efficiency was around 62% for isothermal operations at 900 °C and complete redox reaction of the metal oxide.

The cost-effective conversion of low-grade heat into electricity using thermoelectric devices requires developing alternative materials and material processing technologies able to reduce the currently high device manufacturing costs. In this direction, thermoelectric materials that do not rely on rare or toxic elements such as tellurium or lead need to be produced using high-throughput technologies not involving high temperatures and long processes. Bi2Se3 is an obvious possible Te-free alternative to Bi2Te3 for ambient temperature thermoelectric applications, but its performance is still low for practical applications, and additional efforts toward finding proper dopants are required. Here, we report a scalable method to produce Bi2Se3 nanosheets at low synthesis temperatures. We studied the influence of different dopants on the thermoelectric properties of this material. Among the elements tested, we demonstrated that Sn doping resulted in the best performance. Sn incorporation resulted in a significant improvement to the Bi2Se3 Seebeck coefficient and a reduction in the thermal conductivity in the direction of the hot-press axis, resulting in an overall 60% improvement in the thermoelectric figure of merit of Bi2Se3.

The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process requires the use of highly active, stable and selective photocatalytic materials based on abundant elements and the proper adjustment of the reaction conditions, including temperature. In this work, Cu2O-TiO2 type-II heterojunctions with different Cu2O amounts are obtained by a one-pot hydrothermal method. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The Cu2O-TiO2 photocatalysts exhibit a high selectivity toward acetaldehyde production and up to tenfold higher hydrogen evolution rates compared to bare TiO2. We further discern here the influence of temperature and visible light absorption on the photocatalytic performance. Our results point toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy for solar energy conversion.

The fabrication of small anatase titanium dioxide (TiO2) nanoparticles (NPs) attached to larger anisotropic gold (Au) morphologies by a very fast and simple two-step microwave-assisted synthesis is presented. The TiO2/Au NPs are synthesized using polyvinylpyrrolidone (PVP) as reducing, capping and stabilizing agent through a polyol approach. To optimize the contact between the titania and the gold and facilitate electron transfer, the PVP is removed by calcination at mild temperatures. The nanocatalysts activity is then evaluated in the photocatalytic production of hydrogen from water/ethanol mixtures in gas-phase at ambient temperature. A maximum value of 5.3 mmol·gcat-1·h−1 (7.4 mmol·gTiO2-1·h−1) of hydrogen is recorded for the system with larger gold particles at an optimum calcination temperature of 450°C. Herein we demonstrate that TiO2-based photocatalysts with high Au loading and large Au particle size (≈50 nm) NPs have photocatalytic activity.

We modified and evaluated the performance of a CuO/Cu electrochemical electrode for chemical oxygen demand (COD) determination by covering it with a Nafion (Nf) film. The resulting modified CuONf/Cu electrode sensor was used for the electrochemical determination of COD in river, slaughterhouse and estuarine water samples in order to evaluate its performance for this particular task. It was compared with the CuO/Cu sensor with no Nafion. The main electrochemical characteristics of interest, resistance, sensitivity, accuracy and reproducibility, were assessed by means of Linear Sweep Voltammetry using glucose as a standard. Results of these essays indicate that the procedure used produced smooth and firmly attached Nf films covering the whole copper surface. This sensor was shown to be resistant to interferences and effective in electro-oxidation of a wide range of organic compounds and therefore very useful for COD determination. Using the newly developed CuONf/Cu electrode an analytical linear range of 50 to 1000 mg·L−1 COD, with a detection limit of 2.11 mg·L−1 (n = 6) COD was achieved. The comparison shows that the CuONf/Cu sensor is more appropriate for COD determination than its counterpart with no Nafion.

The manufacturing of semiconducting films using solution-based approaches is considered a low cost alternative to vacuum-based thin film deposition strategies. An additional advantage of solution processing methods is the possibility to control the layer nano/microstructure. Here, we detail the production of mesoporous CuGaS2 (CGS) and ZnS layers from spin-coating and subsequent cross-linking through chalcogen-chalcogen bonds of properly functionalized nanocrystals (NCs). We further produce NC-based porous CGS/ZnS bilayers and NC-based CGS–ZnS composite layers using the same strategy. Photoelectrochemical measurements are used to demonstrate the efficacy of porous layers, and particularly the CGS/ZnS bilayers, for improved current densities and photoresponses relative to denser films deposited from as-produced NCs.