Daniel Assenbaum

Dehydrogenation of Dodecahydro-N-ethylcarbazole on Pd/Al2O3 Model Catalysts

To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.

Catalytic production of hydrogen from glucose and other carbohydrates under exceptionally mild reaction conditions

A catalytic reaction system for the production of hydrogen from sugars and even water-insoluble biomass like cellulose is presented. The reaction system is based on an ionic liquid that has the role to dissolve the carbohydrate feedstock and a ruthenium catalyst. As hydrogen dissolves in this media at very low level, hydrogen consuming side reactions have been hindered, leading to a gaseous product mixture consisting mainly of hydrogen and carbon dioxide. Investigations with isotopic labelling suggest a reaction sequence in which glucose first thermally decomposes to formic acid followed by Ru-catalyzed decomposition of the latter to hydrogen and CO2.

Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro‐N‐ethylcarbazole on Pd(111)

Dodecahydro-N-ethylcarbazole (H12-NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H12-NEC on Pd(111) by using a surface-science approach under ultrahigh vacuum conditions. By combining infrared reflection-absorption spectroscopy, density functional theory calculations and X-ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H12-NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C-H bonds is observed within the five-membered ring. Rapid dehydrogenation occurs to octahydro-N-ethylcarbazole (H8-NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H8-NEC proceeds within the six-membered rings. Starting from clean Pd(111), C-N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature-programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H8-NEC as the main product desorbing from Pd(111). The onset temperature for H8-NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H8-NEC is observed even above the temperature threshold for H8-NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.

Simple and recyclable ionic liquid based system for the selective decomposition of formic acid to hydrogen and carbon dioxide

Exploitation of hydrogen as an energy carrier requires the development of systems for its storage and delivery. Formic acid has been proposed as valuable hydrogen carrier compound, due to its relatively high hydrogen content (53 g L−1), the latter being easily and cleanly released in catalytic reactions under mild conditions (HCOOH → H2 + CO2). Ionic liquids are interesting solvents for homogeneous catalyzed formic acid decomposition systems as their extremely low volatility avoids solvent contamination of the produced hydrogen stream. In this paper an outstandingly simple, robust and active catalyst system is presented, namely RuCl3 dissolved in 1-ethyl-2,3-dimethylimidazolium acetate (RuCl3/[EMMIM][OAc]). This system proved to be fully recyclable over 10 times. Turnover frequencies (TOF) of 150 h−1 and 850 h−1 were obtained at 80 °C and 120 °C, respectively.