Experimental and theoretical studies on the mechanism of hydrogenation by η6-arene β-diketiminato -ruthenium and -osmium complexes

The β-diketiminate class of ligands is renowned for the stabilization of coordinatively unsatd. main-group and transition metal complexes. Moreover, the application and scope of transition metal β-diketiminate complexes in the catalytic transformation of org. substrates is rapidly increasing. We recently reported on the synthesis, characterization and hydrogenation capabilities of a series of cationic η6-arene Ru- and Os-β-diketiminate complexes, 1. This family of complexes readily undergoes thermo-reversible [4+2] cycloaddn. with alkenes, 2 alkynes and H2 3. DFT calcns., in adjunction with high pressure NMR-deuterium labeling expts., reveal a concerted heterolytic cleavage mechanism with a low energy barrier. Now, a complete mechanistic pathway for alkene hydrogenation for complexes such as 1 has been modeled and supported by exptl. results. Furthermore, in situ NMR studies show that η6-arene hydrogenation is essential for the formation of the active catalytic species. Through steric and electronic variation of both the arene and β-diketiminate ligand, the efficiency of styrene, cyclohexene and 1-methylcyclohexene hydrogenation has been shown to be tunable, with a high turnover rate obsd. Lastly the dehydrogenation application of this family of complexes towards other types of catalysis and substrates will be discussed.

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

Renewable energy supply and energy storage in a closed materials cycle are the urgent global challenges of the 21st century. Carbon dioxide (CO2) hydrogenation over catalysts is a method to produce synthetic fuels from renewable energy in a CO2 neutral cycle. Numerous catalysts from pristine to novel materials have been investigated to achieve high CO2 conversion and selective hydrocarbon. Ahead of developing innovatively active catalyst with a high selectivity, it is crucial to understand the active sites and the related reaction mechanisms. Despite the fact that CO2 + H2 is a rather simple chemical equation, many reaction paths on a solid catalyst are possible. However, the proposed mechanisms from literatures have been controversial due to two main difficulties in 1) evidently identifying the reaction species, especially some vital transient species on the pathway of carbon hydrogenation and C-C coupling, and 2) proving the relation between surface structure of the catalysts and the reactivities. To address these difficulties, we built a new instrumental setup and developed an analysis program in order to investigate the catalyst surface and analyze the reaction mechanism in operando. We built up a diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy-gas chromatography (DRIFTS-MS-GC) instrument for an operando study of the surface, gas, and liquid products using DRIFTS, MS, GC, and ex situ NMR. A bilevel evolutionary Gaussi-an fitting (BEGF) program was developed for dataset treatment including peak deconvolution and kinetic plot. Standard chemicals like carbonates, bicarbonates, formic acid, acetic acid, methanol, and ethanol, and isotopic spectra using 13CO2 and D2 in the reactions were em-ployed for supporting peak assignments. Based on these infrastructures, CO2 adsorption and hydrogenation reactions on pristine metals (Fe, Co, Ni, and Cu), metal hydride alloy (La-Ni4Cu), and metal-oxide (Co-CoO) were studied. We found out pure Fe, Co, Ni, Cu and La-Ni4Cu metals are not efficient for CO2 hydrogenation, and have no detectable adsorption spe-cies on the surface. High conversion (> 90%) of CO2 on Co-CoO surface emphasized the im-portance of oxide for CO2 chemisorption and for the observations of the surface species. The following investigation on oxide supported metal catalyst Ru/Al2O3 via an in situ control of the individual formation and hydrogenation of each adsorption species demonstrates that the oxide assists CO2 initial activation. CO2 methanation starts at the interface of the metal and the oxide and passes through the steps of CO2 ï® HCO3-* (and/or HCOO-* in CO2+H2 co-adsorption condition) ï® CO* ï® CH4. We further explored the relationship between met-al/(metal+oxide) ratio and their reactivities. We synthesized Co/(Co+CoO) catalysts in the DRIFTS-MS-GC instrument, and investigated the CO2 hydrogenation reaction on site. The results reveal that high concentration of the oxide in the catalyst boosts CO2 methanation, be-cause the CO2 binding is moderate and the adsorption is enhanced when the oxide concentra-tion increased. In short, we established equipment for an operando study of CO2 hydrogenation on catalyst surfaces, unraveled the surface reaction mechanisms and the active sites, developed a highly active catalyst, as well as initiated an efficient data analysis program.

EPFL2020

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

EPFL2020

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

EPFL2012