Scientific Highlights

DFT Modelling of Ru Nanoparticles Supported on Graphene and Graphite Surface: A Study of the B5 Active Sites Localisation

Carbon-supported Ruthenium catalysts exhibit very good catalytic properties, and are applied in the ammonia synthesis as well as for methanation processes e.g. in the gasification of wet biomass in super-critical water. Both reactions are known to be structure sensitive and require specific active sites at the catalyst. It has been found experimentally that mainly B5 sites are present and actively taking part in the catalytic reactions. Epitaxial growth of Ru on the carbon support can be considered following two orientations, one with the same orientation as the hexagonal carbon lattice, the other being rotated by 30° around the surface normal. We describe a DFT study on the self-organizing of Ru nanoparticles deposited on carbon cluster models. The carbon support has been modelled using a single carbon layer (graphene-type) and double carbon layer (graphite-type). Our DFT results indicate that only one of them results in a stable epitaxial adsorption configuration. Basing on that result we were able to build and stabilize three-dimensional Ru nanoparticles containing up to 17 atoms. The smallest three-dimensional Ru cluster exhibiting B5-type active sites (three sites) contains eleven Ru atoms and has a particle diameter of 0.55 nm. The calculations provide insights into the nature and structure of Ru B5-type sites supported on carbon. Further molecule adsorption and reaction mechanism studies will follow.

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Further publications: Laboratory for Bioenergy and Catalysis (LBK)

Supported gold as catalyst for the decomposition of ammonia precursors in the selective catalytic reduction of NOx

Titaniumdioxide supported gold was found to catalyze the hydrolysis of formate-based ammonia precursor compounds which are proposed for the selective catalytic reduction of nitrogen oxides (NOx) in combustion exhaust gas. In contrast to other noble metals, the supported gold does not oxidize the released NH3, while it maintains decomposition of intermediate formic acid. Efficient formic acid decomposition is essential as it avoids the formation of side products which previously restricted the use of formate-based ammonia precursors. The stability of the catalyst under operating conditions was investigated by an accelerated hydrothermal aging treatment and sulfur poisoning. Both procedures affected the catalytic decomposition performance only to a small extent, which is unprecedented for supported gold catalysts.

The novel catalyst was implemented into an up-scaled prototype system of an ammonia generator for automotive applications and successfully tested in an exhaust gas aftertreatment system of a 3.0 L Diesel engine on an engine test bench at the Technical University of Munich. The system was then transferred to an industrial company and is currently tested for commercial utilization.

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Further publications: CEG Publications

Influence of Methyl Halide Treatment on Gold Nanoparticles Supported on Activated Carbon

Gold particles supported on carbon when subjected to a flow of methyl iodide or bromide redisperse from large ensembles to single atoms and/or dimers of gold. Methyl halide oxidizes gold leading to gradual particle dissolution. The process could be carried out at temperatures as low as 50 °C. The excess of halide could be removed by a post-treatment of the material with 1%H2O/H2, which does not influence the metal dispersion. This remarkable transformation opens the possibility of re-activating gold catalysts that lost their performance due to metal particles sintering.

The finding was proposed based on a combination of in situ (XAS), ex situ characterization (aberration corrected HAADF-TEM, XRD and XPS) and kinetic measurements. The work was a combined effort from Queen’s University Belfast (UK), Paul Scherrer Institute (Switzerland), Lehigh University (USA) and Cardiff University (UK)

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Citation: J. Sá, A. Goguet, S. F. Rebecca Taylor, R. Tiruvalam, C. J. Kiely, M. Nachtegaal, G. J. Hutchings, C. Hardacre, Angew. Chem. Int. Ed. 50 (2011) 8912-8916.

Publication: http://dx.doi.org/10.1002/anie.201102066

Further publications: LBK Publications

Identification of the SCR active sites in Fe-ZSM-5

The identification of the SCR active sites in Fe-ZSM-5 is of utmost importance for the understanding and optimization of the catalyst performance. No method (e.g. UV/VIS, IR, EPR, EXAFS, XPS, XRD) can definitively distinguish between isolated iron species and iron oxide clusters of different nuclearity in the same sample. A statistical approach was used to solve this problem. From the correlation of the measured SCR activity with the calculated concentration of different species the temperature dependent activities of isolated, dimeric and oligomeric iron species and iron particles could be determined.

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Publications: S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff, Estimation of the fractions of different nuclear iron species in uniformly metal-exchanged Fe-ZSM-5 samples based on a Poisson distribution, Appl. Catal. A 2009, accepted manuscript.

Further publications: Exhaust Gas Aftertreatment Group

Vibrational Spectra of Adsorbates from DFT

The hydrolysis of isocyanic acid was studied experimentally and theoretically and a reaction mechanism on different catalysts was established. The decreasing NOx emission limits for diesel vehicles impel the further development of the existing NOx deactivation technologies, particularly the selective catalytic reduction (SCR) of nitrogen oxides with urea. In the urea-SCR process, urea is injected into the hot exhaust gas, where it thermally decomposes into isocyanic acid (HNCO) and ammonia. HNCO quickly hydrolyses on the surface of SCR catalysts and even faster on the surface of specialized hydrolysis catalysts. The theoretical Density Functional Theory (DFT) method with cluster model was used. The reaction path was studied by adsorption of water and isocyanic acid, followed by optimization of the adsorbates structure. Additionally, vibrational analyses of the adsorbates were made. In the experimental part activity tests and surface sensitive techniques (XRD, XPS and DRIFTS) were applied. Measured DRIFT spectra could be reproduced by theoretical calculations. Based on the computational screening of different catalysts we can confirm the experimental results, which show that TiO2 is the best catalyst for isocyanic acid hydrolysis. Presented methodology of DFT modeling and "virtual" catalyst screening can be used for successful combination with different experimental methods available at PSI and for variety of questions and complex catalytic systems.

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Further publications: Catalysis for Energy Group