Enhanced Visible Light-Driven Photocatalytic Water-Splitting Reaction of Titanate Nanotubes Sensitised with Ru(II) Bipyridyl Complex

The ion exchange of Na+ cations was used to photosensitise titanates nanotubes (Ti-NTs) with tris(2,2'-bipyridine)ruthenium(II) cations (Ru(bpy)32+); this yielded a light-sensitised Ti-NTs composite denoted as (Ru(bpy)3)Ti-NTs, exhibiting the characteristic absorption of Ru(bpy)32+ in visible light. Incident photon-to-current efficiency (IPCE) measurements and the photocatalytic reduction of methyl viologen reaction confirmed that in the photosensitisation of the (Ru(bpy)3)Ti-NTs composite, charge transfer and charge separation occur upon excitation by ultraviolet and visible light irradiation. The photocatalytic potential of titanate nanotubes was tested in the water-splitting reaction and the H2 evolution reaction using a sacrificial agent and showed photocatalytic activity under various light sources, including xenon-mercury lamp, simulated sunlight, and visible light. Notably, in the conditions of the H2 evolution reaction when (Ru(bpy)3)Ti-NTs were submitted to simulated sunlight, they exceeded the photocatalytic activity of pristine Ti-NTs and TiO2 by a factor of 3 and 3.5 times, respectively. Also, (Ru(bpy)3)Ti-NTs achieved the photocatalytic water-splitting reaction under simulated sunlight and visible light, producing, after 4 h, 199 and 282 μmol×H2×gcat-1. These results confirm the effective electron transfer of Ru(bpy)3 to titanate nanotubes. The stability of the photocatalyst was evaluated by a reuse test of four cycles of 24 h reactions without considerable loss of catalytic activity and crystallinity.

Phosphate Recovery from Urine-Equivalent Solutions for Fertilizer Production for Plant Growth

This study presents a proof of concept for the recovery of phosphate from aqueous solutions with high phosphorus (PO4-P) initial contents to simulate the concentration of streams from decentralized wastewater systems. Solutions with ∼500 ppm phosphorus enable phosphate adsorption and recovery, in contrast to the highly diluted inlet streams (<10 ppm) from centralized wastewater treatment plants. In this work, Mg-Fe layered double hydroxide is used as a phosphate adsorbent, demonstrating its separation from aqueous streams, recovery, and use as a fertilizer following the principles of circular economy. We demonstrate that the mechanism of phosphate adsorption in this material is by a combination of surface complexation and electrostatic attraction. After the loss of crystallinity in the presence of water in the first cycle and its associated decrease in adsorption capacity, the Mg-Fe layered double hydroxide (LDH) is stable after consecutive adsorption/desorption cycles, where desorption solutions were reused to substantially increase the final phosphate concentration demonstrating the recyclability of the material in a semicontinuous process. Phosphate recovered in this way was used to complement phosphate-deficient plant growth medium, demonstrating its efficacy as a fertilizer and thereby promoting a circular and sustainable economy.

Role of the Deep Eutectic Solvent Reline in the Synthesis of Gold Nanoparticles

This work presents a mechanistic understanding of the synthesis of small (<3 nm) gold nanoparticles in a nontoxic, eco-friendly, and biodegradable eutectic mixture of choline chloride and urea (reline) without the addition of external reducing or stabilization agents. Reline acts as a reducing agent by releasing ammonia (via urea hydrolysis), forming gold nanoparticles even at trace ammonia concentration levels. Reline also affects the speciation of the gold precursor forming gold chloro-complexes, stabilizing Au⁺ species, leading to an easier reduction and avoiding the otherwise fast disproportionation reaction. Such a capability is however lost in the presence of large amounts of water, where water replaces the chloride ligands in the precursor speciation. In addition, reline acts as a weak stabilizing agent, leading to small particles (<3 nm) and narrow distributions although agglomerates quickly form. Such properties are maintained in the presence of water, indicating that it is linked to the urea stabilization rather than the hydrogen-bonding network. This work has important implications in the field of green synthesis of nanoparticles with small sizes, especially for biomedical and health care applications, due to the nontoxic nature of the components of deep eutectic solvents in contrast to the conventional routes.

Hydrogen production from urea in human urine using segregated systems

Removal of nitrogen compounds through biological processes represents the highest energy consumption in conventional centralised wastewater treatment facilities. Alternatively, segregated systems, where wastewater is treated at its source, present the potential to provide value to nitrogen-rich compounds contained in wastewater like urea. This paper demonstrates the feasibility of a novel process to recover energy from human urine based on the pre-isolation of urea to decrease the energy requirements for its thermal decomposition compared to the conventional thermal treatment when in solution, followed by its decomposition into hydrogen. Herein, urea is separated from an aqueous solution by adsorption onto activated carbon. Thermal urea desorption and decomposition into ammonia and CO₂ at 250 °C leads to full regeneration of the carbon, showing a constant adsorption capacity for at least 5 consecutive adsorption/desorption cycles. Finally, when the regeneration and urea decomposition step is coupled to an ammonia decomposition catalyst, hydrogen is produced to be used as an energy fuel. This process opens the door to a new way of circular economy by energy recovery from hydrogen-rich components in segregated wastewater streams. Preliminary energy balances show that the adoption of this energy recovery system in a city of 160,000 inhabitants would lead to a daily hydrogen production of 430 kg, with a net energy production of 2,500 kWh/day. In addition, such waste-to-energy process would lead to energy savings of 4,600 kWh/day in a conventional wastewater treatment plant reducing its energy consumption by around 35%.

Size Control in the Colloidal Synthesis of Plasmonic Magnesium Nanoparticles

Nanoparticles of plasmonic materials can sustain oscillations of their free electron density, called localized surface plasmon resonances (LSPRs), giving them a broad range of potential applications. Mg is an earth-abundant plasmonic material attracting growing attention owing to its ability to sustain LSPRs across the ultraviolet, visible, and near-infrared wavelength range. Tuning the LSPR frequency of plasmonic nanoparticles requires precise control over their size and shape; for Mg, this control has previously been achieved using top-down fabrication or gas-phase methods, but these are slow and expensive. Here, we systematically probe the effects of reaction parameters on the nucleation and growth of Mg nanoparticles using a facile and inexpensive colloidal synthesis. Small NPs of 80 nm were synthesized using a low reaction time of 1 min and ∼100 nm NPs were synthesized by decreasing the overall reaction concentration, replacing the naphthalene electron carrier with biphenyl or using metal salt additives of FeCl3 or NiCl2 at longer reaction times of 17 h. Intermediate sizes up to 400 nm were further selected via the overall reaction concentration or using other metal salt additives with different reduction potentials. Significantly larger particles of over a micrometer were produced by reducing the reaction temperature and, thus, the nucleation rate. We showed that increasing the solvent coordination reduced Mg NP sizes, while scaling up the reaction reduced the mixing efficiency and produced larger NPs. Surprisingly, varying the relative amounts of Mg precursor and electron carrier had little impact on the final NP sizes. These results pave the way for the large-scale use of Mg as a low-cost and sustainable plasmonic material.

Low Temperature and Pressure Single-Vessel Integrated Ammonia Synthesis and Separation using Commercial KATALCO Catalysts

In recent years, the potential for “green” ammonia produced from renewable energy has renewed the pursuit for a low-pressure, low-temperature ammonia synthesis process using novel catalysts capable to operate under these conditions. In past decades, the trend of decreasing the pressure in the existing Haber-Bosch process to the de facto limit of condensation at 80 bar has been achieved through catalysts such as iron-based ICI’s KATALCO 74-1. By replacing the separation of ammonia via condensation by absorption, the process loop can be integrated into a single-vessel at constant temperature, and the operating region drastically shifts to lower pressures (<30 bar) and temperatures (<380°C) unknown to commercial catalysts. Herein, the low-temperature and low-pressure activity of KATALCO 74-1 and KATALCO 35-8A catalysts is studied and compared to Ru/Cs/CeO2 catalyst known to have low-temperature activity through resistance to hydrogen inhibition. Due to its low-temperature and high-conversion activity, KATALCO 74-1 can be deployed in an integrated reaction and absorptive-separation using MnCl2/SiO2 as absorbent. Although further catalyst development is needed to increase compatibility with the absorbent in a feasible reactor design, this study clearly demonstrates the need to re-evaluate the viability of commercial ammonia synthesis catalysts, especially iron-based ones, for their deployment on novel green ammonia synthesis processes driven exclusively by renewable energy.

Green, scalable, low cost and reproducible flow synthesis of biocompatible PEG-functionalized iron oxide nanoparticles

A continuous synthesis strategy enabling the large-scale and cost-effective synthesis and functionalization of iron oxide nanoparticles in a single setup is developed, leading to fully biocompatible and application-ready PEG coated nanoparticles.

XAS investigation of silica aerogel supported cobalt rhenium catalysts for ammonia decomposition

The implementation of ammonia as a hydrogen vector relies on the development of active catalysts to release hydrogen on-demand at low temperatures. As an alternative to ruthenium-based catalysts, herein we report the high activity of silica aerogel supported cobalt rhenium catalysts. XANES/EXAFS studies undertaken at reaction conditions in the presence of the ammonia feed reveal that the cobalt and rhenium components of the catalyst which had been pre-reduced are initially re-oxidised prior to their subsequent reduction to metallic and bimetallic species before catalytic activity is observed. A synergistic effect is apparent in which this re-reduction step occurs at considerably lower temperatures than for the corresponding monometallic counterpart materials. The rate of hydrogen production via ammonia decomposition was determined to be 0.007 mol_H2 g_cat^-1 h^-1 at 450 °C. The current study indicates that reduced Co species are crucial for the development of catalytic activity.

Morphological Control of Nanostructured V2O5 by Deep Eutectic Solvents

Herein, we show a facile surfactant-free synthetic platform for the synthesis of nanostructured vanadium pentoxide (V₂O₅) using reline as a green and eco-friendly deep eutectic solvent. This new approach overcomes the dependence of the current synthetic methods on shape directing agents such as surfactants with potential detrimental effects on the final applications. Excellent morphological control is achieved by simply varying the water ratio in the reaction leading to the selective formation of V₂O₅ 3D microbeads, 2D nanosheets, and 1D randomly arranged nanofleece. Using electrospray ionization mass spectroscopy (ESI-MS), we demonstrate that alkyl amine based ionic species are formed during the reline/water solvothermal treatment and that these play a key role in the resulting material morphology with templating and exfoliating properties. This work enables fundamental understanding of the activity-morphology relationship of vanadium oxide materials in catalysis, sensing applications, energy conversion, and energy storage as we prove the effect of surfactant-free V₂O₅ structuring on battery performance as cathode materials. Nanostructured V₂O₅ cathodes showed a faster charge-discharge response than the counterpart bulk-V₂O₅ electrode with V₂O₅ 2D nanosheet presenting the highest improvement of the rate performance in galvanostatic charge-discharge tests.

Indirect photo-electrochemical detection of carbohydrates with Pt@g-C3N4 immobilised into a polymer of intrinsic microporosity (PIM-1) and attached to a palladium hydrogen capture membrane

An "indirect" photo-electrochemical sensor is presented for the measurement of a mixture of analytes including reducing sugars (e.g. glucose, fructose) and non-reducing sugars (e.g. sucrose, trehalose). Its innovation relies on the use of a palladium film creating a two-compartment cell to separate the electrochemical and the photocatalytic processes. In this original way, the electrochemical detection is separated from the potential complex matrix of the analyte (i.e. colloids, salts, additives, etc.). Hydrogen is generated in the photocatalytic compartment by a Pt@g-C₃N₄ photocatalyst embedded into a hydrogen capture material composed of a polymer of intrinsic microporosity (PIM-1). The immobilised photocatalyst is deposited onto a thin palladium membrane, which allows rapid pure hydrogen diffusion, which is then monitored by chronopotentiometry (zero current) response in the electrochemical compartment. The concept is demonstrated herein for the analysis of sugar content in commercial soft drinks. There is no requirement for the analyte to be conducting with electrolyte or buffered. In this way, samples (biological or not) can be simply monitored by their exposition to blue LED light, opening the door to additional energy conversion and waste-to-energy applications.

Mechanistic insights of the reduction of gold salts in the Turkevich protocol

This paper presents fundamental understanding of the mechanism of the Turkevich protocol, the method recommended by the National Institute of Standards and Technology for the synthesis of gold nanoparticles using sodium citrate as reducing agent. Herein, we reveal that the Turkevich mechanism consists of two consecutive reduction steps (Au³⁺→ Au⁺→ Au⁰) rather than a reduction followed by the disproportionation reaction as conventionally believed. This new understanding has profound implications: i. the second reduction step (Au⁺→ Au⁰), rather than the previously postulated first reduction step, is the rate-limiting reduction step and ii. the formation of acetone dicarboxylate (DC^2-) as an intermediate product through the oxidation of citrate has a key role as stabilizer and as a reducing agent (stronger than sodium citrate). This knowledge enables the synthesis of monodispersed gold nanoparticles with sizes ranging from 5.2 ± 1.7 nm to 21.4 ± 3.4 nm, with the lower end considerably smaller than previously reported through the Turkevich route. This work provides fundamental guidance for the controllable synthesis of nanoparticles using DC^2- as a reducing agent directly applicable to other precious metals.

Continuous manufacturing of silver nanoparticles between 5 and 80 nm with rapid online optical size and shape evaluation

Flexible manufacturing technology of nanoparticles with sizes between 5 and 80 nm. This unique size flexibility is enabled by coupling rapid online spectroscopy and a mathematical Mie theory-based algorithm for size and shape evaluation.

Oxidant free conversion of alcohols to nitriles over Ni-based catalysts

Ni-Based catalysts converting various primary alcohols to nitriles in high yields under oxidant-free, low temperature conditions.

Continuous low temperature synthesis of MAPbX3 perovskite nanocrystals in a flow reactor

Perovskite nanocrystals prepared at room temperature using a simple flow reactor.

Zeolite Y supported nickel phosphide catalysts for the hydrodenitrogenation of quinoline as a proxy for crude bio-oils from hydrothermal liquefaction of microalgae

The catalytic activity of nickel phosphide catalysts on different zeolite Y supports is investigated for the upgrading of algal bio-oils.

Highlights from Faraday Discussion on Designing Nanoparticle Systems for Catalysis, London, UK, May 2018

The 2018 Faraday Discussion on “Designing Nanoparticle Systems for Catalysis” brought together leading scientists to discuss the current state-of-the-art in the fields of computational chemistry, characterization techniques, and nanomaterial synthesis, and to debate the challenges and opportunities going forward for rational catalyst design.

Continuous synthesis of tuneable sized silver nanoparticles via a tandem seed-mediated method in coiled flow inverter reactors

Size control of metal nanoparticles is essential to achieve accurate adjustment of their unique chemical and physical properties.

Continuous synthesis of hollow silver–palladium nanoparticles for catalytic applications

Hollow bimetallic nanoparticles exhibit unique surface plasmonic properties, enhanced catalytic activities and high photo-thermal conversion efficiencies amongst other properties, however, their research and further deployment are currently limited by their complicated multi-step syntheses.

Modification of Ammonia Decomposition Activity of Ruthenium Nanoparticles by N-Doping of CNT Supports

The use of ammonia as a hydrogen vector has the potential to unlock the hydrogen economy. In this context, this paper presents novel insights into improving the ammonia decomposition activity of ruthenium nanoparticles supported on carbon nanotubes (CNT) by nitrogen doping. Our results can be applied to develop more active systems capable of delivering hydrogen on demand, with a view to move towards the low temperature target of less than 150 °C. Herein we demonstrate that nitrogen doping of the CNT support enhances the activity of ruthenium nanoparticles for the low temperature ammonia decomposition with turnover frequency numbers at 400 °C of 6200 LH2 molRu -1 h-1, higher than the corresponding value of unmodified CNT supports under the same conditions (4400 LH2 molRu -1 h- 1), despite presenting similar ruthenium particle sizes. However, when the nitrogen doping process is carried out with cetyltrimethylammonium bromide (CTAB) to enhance the dispersion of CNTs, the catalyst becomes virtually inactive despite the small ruthenium particle size, likely due to interference of CTAB, weakening the metal-support interaction. Our results demonstrate that the low temperature ammonia decomposition activity of ruthenium can be enhanced by nitrogen doping of the CNT support due to simultaneously increasing the support's conductivity and basicity, electronically modifying the ruthenium active sites and promoting a strong metal-support interaction.

Synthesis of narrow sized silver nanoparticles in the absence of capping ligands in helical microreactors

Microtubular helical reactors generate secondary flows promoting the synthesis of mono-sized silver nanoparticles in the absence of capping ligands.

γ-Al2O3 nanorods with tuneable dimensions – a mechanistic understanding of their hydrothermal synthesis

This paper reports mechanistic understanding of the hydrothermal synthesis of alumina (γ-Al₂O₃) nanorods, presenting an economic and reproducible route for their manufacture with tuneable sizes for a wide range of applications.

Low temperature total oxidation of toluene by bimetallic Au–Ir catalysts

Intimate contact between gold and iridium nanoparticles supported on TiO₂ provides a synergetic effect leading to low temperature VOC oxidation activity.

The importance of particle-support interaction on particle size determination by gas chemisorption

ABSTRACT

The interaction of the metal-support and particle shape has a key role on the determination of the particle size by gas chemisorption. This paper demonstrates mathematically that, assuming metal particles with hemispherical shapes (a common assumption in this type of characterisation) can provide misleading results of up to one order of magnitude. Thus, the metal particle sizes are underestimated when the metal strongly interacts with the support and overestimated when there is a weak metal-support interaction. Additionally, we also demonstrate that although the assumption of spherical shapes always underestimates the size of particles, this error is considerably lower with regular geometries than that associated to the effect of the metal-support interaction due to their effect on the particle shape. Herein, it is demonstrated the importance of introducing the particle-support interaction factor in the chemisorption particle size determination.

Effect of nanostructured ceria as support for the iron catalysed hydrogenation of CO2 into hydrocarbons

This paper demonstrates the key role of the property–structure relationship of the support on iron/ceria catalysts on the hydrocarbon selectivity and olefin-to-paraffin ratio for the direct hydrogenation of carbon dioxide into hydrocarbons.

Selective telomerisation of isoprene with methanol by a heterogeneous palladium resin catalyst

A heterogeneous (DVB-resin-PPh₃-Pd-dvds) catalyst presents high activity for the telomerization of isoprene with methanol with an unsusual regioselectivity towards tail-to-tail telomerization products.

Tandem isomerization/telomerization of long chain dienes

The first example of a tandem reaction involving double-bond migration in combination with telomerization is reported. Homogeneous and heterogeneous Ru catalysts were employed as isomerization catalysts, and telomerization was realized using a homogeneous Pd(0) precursor complex with a N-heterocyclic carbene (IMes) ligand. Overall conversions approaching 60% were achieved with the best selectivity to telomerization products of 91% attained at 11% conversion. Conversion was markedly higher in the presence of longer-chain alcohol (1-butanol) as the nucleophile (telogen).