Stability of Cobalt Particles in and outside HZSM‐5 under CO Hydrogenation Conditions Studied by ex situ and in situ Electron Microscopy

Encapsulated cobalt particles live another day: Heterogeneously catalyzed reactions like the CO hydrogenation exhibit harsh conditions leading to catalyst deactivation. The cobalt particles investigated via ex situ and in situ TEM are located inside and outside the ZSM-5 support, undergoing realistic and model conditions. The encapsulated cobalt particles are found to be highly resistant towards sintering. Designing stable materials for processes operating under harsh reaction conditions, like CO hydrogenation, is a challenging topic in catalysis. These may provoke several deactivation mechanisms simultaneously, like thermal sintering, oxidation or poisoning of the active sites. We report HZSM-5 supported cobalt catalysts, exhibiting cobalt nanoparticles encapsulated inside, or located at the exterior of the ZSM-5 support. The materials were studied by a combination of ex situ and in situ electron microscopy with respect to the growth of the cobalt particles. After 1200 h time on stream under CO hydrogenation conditions, the spent catalyst showed minimal sintering of encapsulated cobalt particles. In situ environmental TEM experiments under model reduction and CO hydrogenation conditions indicate the presence of cobalt nanoparticles, which appear highly resistant towards sintering even up to 700 °C. These results provide a first indication for the preparation of sinter stable catalysts suitable for operating in harsh reaction environments

Optimizing Ni-Fe-Ga alloys into Ni2_{2}FeGa for the hydrogenation of CO2_{2} into methanol

A screening study of the catalytic performance of ternary alloy nanoparticles containing nickel, iron and gallium supported on silica for methanol synthesis from CO$_{2}$ and H$_{2}$ was performed. Catalysts were prepared by incipient wetness impregnation and subsequently reduced in H$_{2}$ before catalytic testing. Ni$_{2}$FeGa showed the best performance of the tested catalysts in terms of methanol yield. An optimization of the preparation was done to improve activity and selectivity, reaching a performance close to that of commercially available Cu/ZnO/Al$_{2}$O$_{3}$/MgO at low reaction temperatures and pressure. Extensive in situ characterisation using environmental TEM, in situ XRD and in situ EXAFS of the formation of the Ni$_{2}$FeGa catalyst explains an optimal reduction temperature of 550 °C: warm enough that the three atomic species will form an alloy while cold enough to prevent the catalyst from sintering during the formation

In Situ Ptychography of Heterogeneous Catalysts using Hard X-Rays: High Resolution Imaging at Ambient Pressure and Elevated Temperature

A new closed cell is presented for in situ X-ray ptychography which allows studies under gas flow and at elevated temperature. In order to gain complementary information by transmission and scanning electron microscopy, the cell makes use of a Protochips E-chipTM which contains a small, thin electron transparent window and allows heating. Two gold-based systems, 50 nm gold particles and nanoporous gold as a relevant catalyst sample, were used for studying the feasibility of the cell. Measurements showing a resolution around 40 nm have been achieved under a flow of synthetic air and during heating up to temperatures of 933 K. An elevated temperature exhibited little influence on image quality and resolution. With this study, the potential of in situ hard X-ray ptychography for investigating annealing processes of real catalyst samples is demonstrated. Furthermore, the possibility to use the same sample holder for ex situ electron microscopy before and after the in situ study underlines the unique possibilities available with this combination of electron microscopy and X-ray microscopy on the same sample

Spin injection from Co2MnGa into an InGaAs quantum well

We have demonstrated spin injection from a full Heusler alloy Co2MnGa thin film into a (100) InGaAs quantum well in a semiconductor light-emitting diode structure at a temperature of 5 K. The detection is performed in the oblique Hanle geometry, allowing quantification of the effective spin lifetime and spin detection efficiency (22 +/- 4%). This work builds on existing studies on off-stoichiometric Heusler injectors into similar light-emitting-diode structures. The role of injector stoichiometry can therefore be quantitatively assessed with the result that the spin injection efficiency increases by a factor of approximately 2 as compared with an off-stoichiometric Co2.4Mn1.6Ga injector. (C) 2008 American Institute of Physics