Laura Torrente-Murciano

H2 Production via Ammonia Decomposition Using Non-Noble Metal Catalysts: A Review

The wide-spread implementation of the so-called hydrogen economy is currently partially limited by an economically feasible way of storing hydrogen. In this context, ammonia has been commonly presented as a viable option for chemical storage due its high hydrogen content (17.6 wt%). However, its use as an energy carrier requires the development of catalytic systems capable of releasing hydrogen at adequate rates and conditions. At the moment, the most active catalytic systems for the decomposition of ammonia are based on ruthenium, however its cost and scarcity inhibit the wide scale use of these catalysts. This issue has triggered research on the development of alternative catalysts based on more sustainable systems using more readily available, non-noble metals mainly iron, cobalt and nickel as well as a series of transition metal carbides and nitrides and bimetallic systems, which are reviewed herein. There have been some promising cobalt- and nickel-based catalysts reported for the decomposition of ammonia but metal dispersion needs to be increased in order to become more attractive candidates. Conversely, there seems to be less scope for improvement of iron-based catalysts and metal carbides and nitrides. The area with the most potential for improvement is with bimetallic catalysts, particularly those consisting of cobalt and molybdenum.

Highly dispersed encapsulated AuPd nanoparticles on ordered mesoporous carbons for the direct synthesis of H2O2 from molecular oxygen and hydrogen.

AuPd nanoparticles (<3 nm) have been encapsulated on the pores of a nanostructured CMK-3 carbon prepared by a nanocasting procedure. This material has been shown to be an excellent catalyst for the direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen.

Synthesis of novel composite materials via the deposition of precious metals onto protonated titanate (TiO2) nanotubes

Abstract Methods for the deposition of precious metals (Au, Pt, Pd and ruthenium hydrated oxide) onto the surface of nanotubular titanates are considered. Viable techniques include preliminary ion exchange of precious metal cations onto the nanotubes followed by chemical, electrochemical or photochemical reduction to the metal. The morphology and size of the metal nanoparticles ranged from spheroidal particles of a few nanometres to larger, rod like particles. The deposits, which were densely loaded onto the surface and were uniformly distributed, had a high surface area and good chemical stability. The size of metal nanoparticles ranged from 1 to 50 nm.

Deep eutectic-solvothermal synthesis of nanostructured ceria

Ceria is a technologically important material with applications in catalysis, emissions control and solid-oxide fuel cells. Nanostructured ceria becomes profoundly more active due to its enhanced surface area to volume ratio, reactive surface oxygen vacancy concentration and superior oxygen storage capacity. Here we report the synthesis of nanostructured ceria using the green Deep Eutectic Solvent reline, which allows morphology and porosity control in one of the less energy-intensive routes reported to date. Using wide Q-range liquid-phase neutron diffraction, we elucidate the mechanism of reaction at a molecular scale at considerably milder conditions than the conventional hydrothermal synthetic routes. The reline solvent plays the role of a latent supramolecular catalyst where the increase in reaction rate from solvent-driven pre-organization of the reactants is most significant. This fundamental understanding of deep eutectic-solvothermal methodology will enable future developments in low-temperature synthesis of nanostructured ceria, facilitating its large-scale manufacturing using green, economic, non-toxic solvents.

Telomerisation of long-chain dienes with alcohols using Pd(IMes)(dvds) catalyst

Several homogeneous palladium catalysts based on PPh3 and nucleophilic carbene (NHC) ligands were screened in the telomerisation reaction of 1,3-pentadiene with methanol. A Pd(acac)2-3PPh3 system showed the highest activity; initially, poor activity was observed using the Pd(0)-NHC catalysts. However, when methanol solvent/nucleophile was replaced by longer chain alcohols 1-propanol and 1-butanol, improved activity and selectivity was observed. Similarly, 1,3-hexadiene was telomerised with 1-propanol and 1-butanol with good selectivity using Pd(IMes)(dvds) as the catalyst. Pd(II)-PPh3 systems were ineffective as catalysts for the telomerisation of 1,3-hexadiene.

Enhanced AuPd Activity in the Direct Synthesis of Hydrogen Peroxide using Nanostructured Titanate Nanotube Supports

Nanostructured supports with tubular morphologies can impose one dimensional external constraints to supported metal nanoparticles affording small sizes (<2 nm) with high dispersion. Specifically, the curvature and chemical environment of the external nanotube surface appears to play a key role in determining the morphology and stability of the supported AuPd nanoparticles. This strategy is presented as an alternative nanoparticle stabilisation approach rather than encapsulation within porous structures or the use of organic capping agents with associated diffusional limitations. Here, we report the enhancement achieved in the AuPd alloy reactivity for the direct synthesis of hydrogen peroxide when supported on titanate nanotubes (Ti‐NT) with a productivity above 11 600 mol  H 2O 2  kgmetal−1 h−1 afforded by the high metal–support interaction.

Selective Oxidation of Salicylic Alcohol to Aldehyde with O2/H2 using Au-Pd on Titanate Nanotubes Catalysts

Alcohol oxidation with 100 % selectivity to the aldehyde product (versus acid formation) is achieved at mild conditions by a tandem‐type system where the intermediate species of hydrogen peroxide formation from an O2/H2 mixture are formed and utilised directly over Au‐Pd/Ti‐NT catalysts. In situ reaction of the intermediate species minimises the parallel decomposition of hydrogen peroxide into water, overcoming one of the limitations on the use of hydrogen peroxide as a green oxidant.

Insights into biphasic oxidations with hydrogen peroxide; towards scaling up

Bi-phasic oxidations using hydrogen peroxide oxidant and a tungsten-based catalyst (e.g. Na2WO4) can proceed quickly and effectively without the need for organic solvents or added phase transfer agents in emulsified systems. Providing sufficient contact area between phases easily overcomes the physical limitations associated with the phase transfer of species, facilitating its scale-up, recyclability of the catalyst and easing product separation. Using the epoxidation of sunflower seed oil as a model reaction, we have also shown how the chemical and safety limitations associated with the parallel decomposition of hydrogen peroxide by Na2WO4 can be suppressed by the use of readily available water soluble organic carboxylic acids and careful consideration of the catalyst to acid ratio. The combination of chemical and engineering solutions with a near 100% hydrogen peroxide use, makes, for the first time, this green oxidation pathway attractive for large-scale industrial applications.

Single-step synthesis of nanostructured γ-alumina with solvent reusability to maximise yield and morphological purity

Insights of the chemical mechanism for the hydrothermal synthesis of nanostructured alumina are elucidated for the first time, demonstrating the effect of the NaOH : Al molar ratio not only on the resulting morphology of the material but also on the product yield. Highly uniform pure γ-Al2O3 nanorods are synthesised under acidic conditions (pH 9) lead to γ-Al2O3 nanoplates. Maximisation of the morphological purity leads to a decrease in the overall aluminium yield which can be overcome by successful reusability of the synthesis medium in an effort to increase efficiency (yield and productivity) and minimise waste production towards feasible large-scale manufacturing of nanostructured materials.

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. The effect is directly related to the reducibility of the different nanostructured ceria supports and their interaction with the iron particles. Herein, we demonstrate that the iron-based catalysts can be modified not only by the addition of promoters, commonly reported in the literature, but also by careful control of the morphology of the ceria support.