CO2 hydrogenation on a metal hydride surface

The catalytic hydrogenation of CO 2 at the surface of a metal hydride and the corresponding surface segregation were investigated. The surface processes on Mg 2NiH 4 were analyzed by in situ X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS) and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). CO 2 hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that for the CO 2 methanation reaction, the dissociation of H 2 molecules at the surface is not the rate controlling step but the dissociative adsorption of CO 2 molecules on the hydride surface. © the Owner Societies 2012.

The Origin of the Catalytic Activity of a Metal Hydride in CO2 Reduction

Corsin Battaglia, Laetitia Bernard, Shunsuke Kato, Andreas Züttel

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time-of-flight secondary ion mass spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.

Wiley-V C H Verlag Gmbh2016

Surface and bulk reactions in borohydrides and amides

Elsa Callini, Shunsuke Kato

Complex hydrides such as LiBH4 and LiNH2 exceed the gravimetric hydrogen density of transition metal hydrides by one order of magnitude. However, hydrogen in complex hydrides is covalently bound and arranged in subunits e.g. [NH2]- and [BH4] - with a fixed stoichiometry. Along this line of thought, these compounds can be considered as ordinary ionic compounds. The corresponding pseudo-binary phase diagram comprises a great variety of phases with a high ionic mobility of Li-cations as well as pseudo-anions as measured by diffusion gradients. Using this equilibrium preparation method, we are able to discriminate between stable and meta-stable phases such as Li2BH 4NH2 formed during high-energy ball-milling. We identify Li2BH4NH2 as a mobile intermediate during phase formation. Hydrogen desorption as relevant for their potential use as hydrogen storage does not take place below 200°C as measured by gravimetry. In particular amides, but also borohydrides emit hydrogen, and nitrogen and boron, respectively, containing species (NH3 and BHx, respectively). We probe the bulk and surface exchange of hydrogen in such subunits in LiNH2 and Li2BH4NH2 by hydrogen-deuterium experiments. In contrast to bulk exchange, surface exchange processes occur at very low temperatures - lower than a significant decomposition rate but coincidentally with the appearance of Li2NH comprising high ionic conductivity. The rate limiting step is thus the bulk transport - in agreement with the empirical correlation that the mobility of ions is linked to the hydrogen desorption kinetics of a material (P. A. Anderson et al., Faraday Discuss., 2011, 151, 271-284). However, the bulk transport of ionic species competes with the diffusion of neutral species such as ammonia, being the origin of the unwanted emission of NH3. These results are discussed with respect to the ionic mobility of borohydride and amide materials. © 2012 The Royal Society of Chemistry.

2012

Periodic operation applied to the kinetic study of CO2 methanation

Albert Renken

Periodic operation was applied to CO2 hydrogenation to CH4 at 383 K on 2% Ru/TiO2. A continuous feed recycle reactor combined with a diffuse reflectance IR cell and a mass spectrometer was used to follow simultaneously the gas phase CO2 and CH4 as well as the adsorbed species. Expts. consisting of periodic variations of CO2 in the H feed showed response curves with max./min. shifted in time in the sequence CO2, (formate)x->COx->CH4. Similar delays measured for the COx formation and hydrogenation indicate that both of these processes limit the overall reaction rate. A kinetic model was proposed and verified under periodic conditions. The main exptl. trends, which are pronounced time lags between CO2, COx and CH4, could be described satisfactorily. [on SciFinder (R)]

1994

Surface and bulk reactions in borohydrides and amides

Elsa Callini, Shunsuke Kato

2012