Metal hydrides for concentrating solar thermal power energy storage

The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost for Concentrating Solar-thermal Power (CSP). We focus on the underlying technology that allows metal hydrides to function as Thermal Energy Storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room-temperature and as high as 1100 oC. The potential of metal hydrides for thermal storage is explored while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature thermal energy storage are also addressed

Correlations between the ionic conductivity and cation size in complex borohydrides

The ionic conduction in the metal borohydrides is often linked to the energy barriers of BH4− reorientation and cation diffusion. However, the ionic conduction is a complex phenomenon, and limited reports are available to establish straightforward correlations with experimental trends in alkali metal borohydrides. This communication reported the correlations between ionic conductivity and cationic size in complex borohydrides. The ionic conductivities of LiBH4, NaBH4, and KBH4 were found to decrease monotonically with increasing cationic radius. This evidences that borohydrides follow trends observed for other ionic conductors and provide further ground toward designing better borohydride-based electrolytes

Electrodeposited magnesium nanoparticles linking particle size to activation energy

© 2016 by the authors; licensee MDPI. The kinetics of hydrogen absorption/desorption can be improved by decreasing particle size down to a few nanometres. However, the associated evolution of activation energy remains unclear. In an attempt to clarify such an evolution with respect to particle size, we electrochemically deposited Mg nanoparticles on a catalytic nickel and noncatalytic titanium substrate. At a short deposition time of 1 h, magnesium particles with a size of 68 ± 11 nm could be formed on the nickel substrate, whereas longer deposition times led to much larger particles of 421 ± 70 nm. Evaluation of the hydrogen desorption properties of the deposited magnesium nanoparticles confirmed the effectiveness of the nickel substrate in facilitating the recombination of hydrogen, but also a significant decrease in activation energy from 56.1 to 37.8 kJ·mol-1H2as particle size decreased from 421 ± 70 to 68 ± 11 nm. Hence, the activation energy was found to be intrinsically linked to magnesium particle size. Such a reduction in activation energy was associated with the decrease of path lengths for hydrogen diffusion at the desorbing MgH2/Mg interface. Further reduction in particle size to a few nanometres to remove any barrier for hydrogen diffusion would then leave the single nucleation and growth of the magnesium phase as the only remaining rate-limiting step, assuming that the magnesium surface can effectively catalyse the dissociation/recombination of hydrogen

Materials challenges for hydrogen storage

Undesirable climate changes due to excessive anthropogenic CO2 emissions are of critical concern. Hydrogen as a clean energy carrier holds great promise in mitigating the problems. However, storing sufficient amount of hydrogen safely and practically poses large technical challenges, associated with materials properties that depend strongly on structure, chemistry and reaction path. Mechanical milling and chemical additions are effective in modifying various hydride systems. Considerable progresses have been achieved in improving thermodynamic and kinetic properties for hydrogen sorption. A final step to meet the technical challenges may rest with hybrid systems that can make use of modified physi- and chemi-sorptions, guided by computational simulations

An alumina-supported ni-la-based catalyst for producing synthetic natural gas

© 2016 by the authors; licensee MDPI, Basel, Switzerland. LaNi5, known for its hydrogen storage capability, was adapted to the form of a metal oxide-supported (γ -Al2O3) catalyst and its performance for the Sabatier reaction assessed. The 20 wt% La-Ni/γ-Al2O3particles were prepared via solution combustion synthesis (SCS) and exhibited good catalytic activity, achieving a CO2conversion of 75% with a high CH4 selectivity (98%) at 1 atm and 300 °C. Characteristics of the La-Ni/γ-Al2O3catalyst were identified at various stages of the catalytic process (as-prepared, activated, and post-reaction) and in-situ DRIFTS was used to probe the reaction mechanism. The as-prepared catalyst contained amorphous surface La–Ni spinels with particle sizes <6 nm. The reduction process altered the catalyst make-up where, despite the reducing conditions, Ni2+-based particles with diameters between 4 and 20 nm decorated with LaOx moieties were produced. However, the post-reaction catalyst had particle sizes of 4–9 nm and comprised metallic Ni, with the LaOxdecoration reverting to a form akin to the as-prepared catalyst. DRIFTS analysis indicated that formates and adsorbed CO species were present on the catalyst surface during the reaction, implying the reaction proceeded via a H2-assisted and sequential CO2dissociation to C and O. These were then rapidly hydrogenated into CH4and H2O

Plasmon enhanced selective electronic pathways in TiO<inf>2</inf> supported atomically ordered bimetallic Au-Cu alloys

Herein, we investigate the mechanisms involved in the selective oxidation of ethanol to acetaldehyde by localised surface plasmon resonance (LSPR) enhanced Au-Cu alloys. Temperature programmed oxidation results in tandem with quantitative in-situ DRIFTS of the surface species under different illumination conditions revealed that the cleaving of C[sbnd]C bonds at the Au-TiO2 interface were inhibited in the presence of Cu at temperatures 175 °C, lowering acetaldehyde selectivity. The work suggests that LSPR photo-enhancement is defined by the inherent electronic interactions within the bimetallic alloy and is facilitated by atomically ordering of the Au-Cu arrays. As such, in addition to performance enhancement, LSPR photo-enhancement can be used in combination with other characterisation techniques to ascertain the selective electronic pathways in bimetallic catalysts

Understanding Plasmon and Band Gap Photoexcitation Effects on the Thermal-Catalytic Oxidation of Ethanol by TiO<inf>2</inf>-Supported Gold

To date there has been a lack of understanding on how photoexcited electron charge transfer can be beneficially combined in a hybrid photo-thermal-catalytic reaction. The effect of different excitation wavelengths on photo-thermal-catalytic oxidation by Au/TiO2 and TiO2 nanoparticles was studied via the gas-phase oxidation of ethanol over a temperature range of 100-250 °C under either visible light or UV illumination. Catalytic performance was assessed by monitoring the CO2 yield. Despite being a weak thermal catalyst (5% catalytic enhancement in comparison to neat TiO2 under thermal catalytic conditions), Au/TiO2 displayed a considerable photo-thermal synergism in the photo-thermal regime (>175 °C), with over 50% and 100% increases in catalytic performance in comparison to neat TiO2 under visible light and UV illumination, respectively. Photo-thermal-catalytic results and detailed probing of postreaction surface carbon species on Au/TiO2 indicated that photo enhancement under UV illumination was due to congruent roles of the photo and thermal catalysis, while photo enhancement under visible light illumination was due to plasmonic-mediated electron charge transfer from the Au deposits to the TiO2 support

C-C Cleavage by Au/TiO<inf>2</inf> during Ethanol Oxidation: Understanding Bandgap Photoexcitation and Plasmonically Mediated Charge Transfer via Quantitative in Situ DRIFTS

Research into photoenhanced heterogeneous catalysis with Au/TiO2 has gained traction in recent years because of the potential for activity enhancement due to its localized surface plasmon resonance effects, including oxidation reactions. While others have observed and described the effects of C-C cleavage by Au/TiO2, how C-C cleavage occurs has not been reported to date. To elucidate the mechanism and to understand the fundamental impacts of visible and ultraviolet (UV) photoexcitation on the dynamics of gas phase ethanol oxidation, an in situ, quantitative diffuse reflectance infrared fourier transform spectroscopy analysis of the surface of Au/TiO2 and neat TiO2 was performed. Key findings from the study include (i) discovery of exclusive oxalate species, a critical precursor to C-C cleavage, which is also an indicator of selective ethanol adsorption at the Au-TiO2 interfacial perimeter, (ii) fortification of C-C bond cleavage by Au/TiO2 via detection of single-carbon species such as formate and carbon monoxide on Au/TiO2 in the dark under visible light illumination, (iii) validation of previous postulations regarding ethanol adsorption on TiO2 followed by oxygen activation at the Au-TiO2 interfacial perimeter, and (iv) in situ re-enactment of the different impacts by bandgap photoexcitation and plasmonically mediated charge transfer, under UV and visible light illumination, respectively, on ethanol oxidation by Au/TiO2 and neat TiO2. (Chemical Equation Presented)