Moniek Tromp

Single-site trinuclear copper oxygen clusters in mordenite for selective conversion of methane to methanol

Copper-exchanged zeolites with mordenite structure mimic the nuclearity and reactivity of active sites in particulate methane monooxygenase, which are enzymes able to selectively oxidize methane to methanol. Here we show that the mordenite micropores provide a perfect confined environment for the highly selective stabilization of trinuclear copper-oxo clusters that exhibit a high reactivity towards activation of carbon–hydrogen bonds in methane and its subsequent transformation to methanol. The similarity with the enzymatic systems is also implied from the similarity of the reversible rearrangements of the trinuclear clusters occurring during the selective transformations of methane along the reaction path towards methanol, in both the enzyme system and copper-exchanged mordenite.

Generating Highly Active Partially Oxidized Platinum during Oxidation of Carbon Monoxide over Pt/Al2O3: In Situ, Time-Resolved, and High-Energy-Resolution X-Ray Absorption Spectroscopy

High activity is generated by sudden formation of disordered oxidic platinum over a platinum catalyst supported on alumina (see picture). High temperature and low concentration of carbon monoxide are required to generate high activity.

Deactivation processes of homogeneous Pd catalysts using in situ time resolved spectroscopic techniquesElectronic supplementary information (ESI) available: EXAFS data and Fourier Transform. See http://www.rsc.org/suppdata/cc/b2/b206758g/

UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.

CoIII–Carbene Radical Approach to Substituted 1H-Indenes

A new strategy for the catalytic synthesis of substituted 1H-indenes via metalloradical activation of o-cinnamyl N-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co(III) carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1H-indene derivatives. The cheap and easy to prepare low spin cobalt(II) complex [Co(II)(MeTAA)] (MeTAA = tetramethyltetraaza[14]annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt(II) complex, and for the first time catalytic activity of [Co(II)(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1H-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C(sp(2))-H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co(III)-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) reveal the radical nature of the reaction.

Energy dispersive XAFS: characterization of electronically excited states of copper(I) complexes.

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Cr K-Edge XANES Spectroscopy: Ligand and Oxidation State Dependence - What is Oxidation State?

A series of Cr complexes varying in oxidation state, ligand and geometry were studied with Cr K‐edge XANES. The main absorption edge energy shift for an oxidation state change from Cr0 to Cr6+ is found to be similar to that for a series of Cr3+ complexes with different ligands. Theoretical XANES and density of states calculations using FEFF8.0 provided detailed insights in the origin of the XANES features for the series of distorted octahedral CrCl3L complexes. The geometry of the CrCl3L complex governs the position of the main absorption edge. Hard versus soft donor effects are overruled by the chlorine ligand for complexes with a facial geometry, whereas the chlorine ligand does not play a significant role in meridional geometry. The combined results call for a redefinition of generally used concepts like oxidation state.

Interaction of small gas phase molecules with alumina supported rhodium nanoparticles: an in situ spectroscopic study

Supported nanoparticulate Rh systems are studied as a model system for the important three way catalysts as used in the combustion engines of cars. Small Rh nanoparticles with a small particle size distribution can be easily synthesized and their morphology is studied using x-ray absorption fine structure (XAFS) spectroscopy. The interaction of the supported rhodium nanoparticles on ? -Al2O3 with small gas phase molecules like H2, O2, CO, NO, H2S and SO2 is investigated, in situ and time resolved, using a combination of techniques, i.e. XAFS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. The surface species formed upon exposure of the metal particles to the adsorbing molecules, and their sometimes disruptive interaction with the metal particles, are identified as a function of temperature and time. Dynamic equilibria are observed which change the oxidation state and the nuclearity of the supported rhodium particles under operational conditions. Rather than merely adsorb on a catalyst particle, these gases have a substantial role in defining the nature of the particle.

Local structure of reaction intermediates probed by time-resolved x-ray absorption near edge structure spectroscopy

A method for the analysis of time-resolved x-ray absorption near edge structure (XANES) spectra is proposed. It combines principal component analysis of the series of experimental spectra, multidimensional interpolation of theoretical XANES as a function of structural parameters, and ab initio XANES calculations. It allows to determine the values of structural parameters for intermediates of chemical reactions and the concentrations of different states as a function of time. This approach is tested using numerically generated data and its possibilities and limitations are discussed. The application of this method to a reaction with methylrhenium trioxide catalyst in solution, for which experimental data were measured using stopped-flow energy-dispersive x-ray absorption spectroscopy technique, is demonstrated. Possibilities and limitations of this experimental technique are also discussed.

Structure–performance relationships of Rh and RhPd alloy supported catalysts using combined EDE/DRIFTS/MS

Energy dispersive extended X-ray absorption fine structure spectroscopy (ED-XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS), have been combined for the structure-function study of Rh and RhPd supported catalysts for the reduction of NO by CO. The combined results show that although alloying of Rh with Pd prevents the dissociative oxidation of the Rh by NO, it does not prevent the extensive disruptive oxidation of Rh by CO. The influence of oxidative disruption by molecular CO in such systems may therefore be far more pervasive and catalytically important than has been previously observed. The overall metal particle size observed in the RhPd alloy system during the CO/NO reaction is significantly larger than for the Rh-only system for the entire temperature range employed. The catalytically active sites, however, are likely to be similar, with the overall activity of the alloy system to be reduced due to inactive RhPd alloy nanoparticles.

Transition metal dissolution and deposition in Li-ion batteries investigated by operando X-ray absorption spectroscopy

In Li-ion batteries the dissolution of transition metals from the cathode and their subsequent deposition on the anode are known to contribute to capacity fading. In this study, we investigate these processes using an NMC cathode and a graphite anode under operating conditions using X-ray absorption spectroscopy. The experiments are carried out in an operando cell, which allows both the time/voltage and spatially resolved determination of metal concentration and oxidation state of transition metal deposits on the graphite electrode. NMC shows a strong increase of the metal dissolution rate, if the upper cut off potential exceeds 4.6 V. Under operating conditions, the oxidation state of manganese, cobalt and nickel are found to be always +2 both on lithiated and delithiated graphite. In contrast, manganese is found to be present in the metallic state on lithiated graphite in the ex situ analysis, thus highlighting the importance of the operando characterization.