Wojciech Grochala

Substantial emission of NH3 during thermal decomposition of sodium amidoborane, NaNH2BH3

Sodium amidoborane (SAB, NaNH2BH3), an ammonia borane (AB, NH3BH3) derivative, was recently reported as a novel solid-state hydrogen storage material. We reinvestigate its thermal decomposition using the best grade commercially available AB (98%) and technical quality (90%) substrates. We characterize evolved gases with mass and FT-IR spectroscopy and observe emission of hydrogen contaminated with significant amounts of ammonia making it undesirable for low-temperature fuel cell applications. We propose a reaction scheme which explains the evolution of NH3 from SAB.

SURFACE-ENHANCED RAMAN SCATTERING (SERS) AT COPPER(I) OXIDE

Surface-enhanced Raman scattering spectra of pyridine at a copper(I) oxide hydrosol and at Cu2O-covered copper electrode produced by ‘oscillating reaction roughening’ are reported. It is shown that on such surfaces pyridine molecules adsorb as pyridinium cations. The ‘oscillating reaction’ pretreatment yields a passivated Cu surface which does not undergo any reduction during the cathodic scan, producing a SERS spectrum of pyridinium cations even after decreasing the electrode potential to strongly negative values.

A multifaceted approach to hydrogen storage

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

ANION-INDUCED CHARGE-TRANSFER ENHANCEMENT IN SERS AND SERRS SPECTRA OF RHODAMINE 6G ON A SILVER ELECTRODE : HOW IMPORTANT IS IT ?

An additional anion-induced enhancement of surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) for Rhodamine 6G adsorbed on a silver electrode upon addition of chloride and citrate anions was determined as a function of the applied potential. The observed increase in intensities amounts to about 100- and 10-fold, for SERS and SERRS spectra, respectively. It was shown that the total increase in SERS intensity by added anions cannot be exclusively explained in terms of the potential-dependent charge-transfer effect and by the ‘active sites’ effect caused by thermodynamically unstable atomic-scale roughnesses. We suggest that this increase can be also ascribed to anion-induced reorientation of the dye molecule or to the potential-independent molecular resonance caused by the strong influence of coadsorbed chloride ions on the electronic levels of Rhodamine 6G.

Characterization of the copper surface optimized for use as a substrate for surface-enhanced Raman scattering

Abstract A new electrochemical procedure of roughening of a copper surface was developed, producing a surface with a high surface-enhanced Raman scattering (SERS) activity. The surface morphology and composition was characterized by energy dispersive/wavelength dispersive/X-ray microprobe analysis and compared to the Cu electrodes roughened by the commonly used oxidation–reduction reactions. The morphology of such a surface was completely different from those produced by traditional electrochemical procedures. Moreover, X-ray microprobe analysis revealed that our new surface remained unoxidized in contrast to the other SERS active Cu surfaces. The potential profiles of SERS intensity of the ring breathing mode of pyridine adsorbed on these surfaces were discussed and interpreted in terms of charge-transfer (CT) effect.

Electrooxidation of O-methoxyaniline as studied by electrochemical and SERS methods

Abstract The electrooxidation of o -methoxyaniline ( o -MA) has been studied by electrochemical and surface-enhanced Raman scattering (SERS) methods. The electrochemical measurements showed that the early stages of o -MA electropolymerization were affected by both the monomer to acid concentration ratio and by the acid concentration. The SERS experiments demonstrated the C-N coupling mechanism of o -MA dimerization or oligomerization. The results of both methods indicate that the intermediate redox couple observed in electrochemical response of poly ( o -methoxyaniline) can be assigned to the incorporated dimer probably having the cyclic structure.

Freezing in Resonance Structures for Better Packing: XeF2 Becomes (XeF+)(F−) at Large Compression

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.

Noble gas monoxides stabilized in a dipolar cavity: a theoretical study.

Encouraged by our previous theoretical results that indicated the stabilization of the HeO unit inside the ferroelectric cavity composed of two parallel LiF dipoles, we have now undertaken the theoretical study for the related noble gas systems, (NgO)(MF)2, Ng = Ar, Kr, Xe, M = Li, Na, K. The computational results indicate that all such molecules constitute local minima, which are protected by sizable energy barriers especially for M = Li, Na, and thus these systems might constitute interesting synthetic targets at low temperatures.

Phase transition induced improvement in H2 desorption kinetics: the case of the high-temperature form of Y(BH4)3

The high-temperature (HT) phase of Y(BH(4))(3) has been prepared by heating of the as mechanochemically synthesised low-temperature (LT) phase of Y(BH(4))(3) to 194-216 °C and subsequent rapid cooling to ambient temperature. Although the differences in the crystal structure and vibrational spectra for these closely-related polymorphs are rather small, yet the NMR MAS (1)H and CP MAS (89)Y spectra reveal clear differences in the chemical shifts for both nuclei. The thermal decomposition process of both forms differs noticeably below 260 °C, decomposition being faster and more facile for the HT phase. The activation energy for thermal decomposition, calculated according to the Kissinger equation, is nearly three times lower for the HT than for the LT polymorph for the first step of the thermal decomposition signalling giant improvement of kinetics of H(2) desorption.

The theory-driven quest for a novel family of superconductors: fluorides

Due to the enormous electronegativity of fluorine, the vast majority of binary and higher inorganic fluorides are high-melting large-band-gap electronic insulators, which are transparent in the visible region of the electromagnetic spectrum. Rare examples of metallic fluorides are known, but valence orbitals of F marginally participate in the electronic transport in these compounds. In this account we describe recent theory-driven attempts to turn unusual fluorides of divalent silver – called fluoroargentates(II) – into a novel class of high-temperature superconductors.