Gas phase ¹H NMR studies and kinetic modeling of dihydrogen isotope equilibration catalyzed by Ru-nanoparticles under normal conditions: dissociative vs. associative exchange

Abstract

The equilibration of H2, HD and D2 between the gas phase and surface hydrides of solid organic-ligand-stabilized Ru metal nanoparticles has been studied by gas phase 1H NMR spectroscopy using closed NMR tubes as batch reactors at room temperature and 800 mbar. When two different nanoparticle systems, Ru/PVP (PVP ≡ polyvinylpyrrolidone) and Ru/HDA (HDA ≡ hexadecylamine) were exposed to D2 gas, only the release of HD from the hydride containing surface could be detected in the initial stages of the reaction, but no H2. In the case of Ru/HDA also the reverse experiment was performed where surface deuterated nanoparticles were exposed to H2. In that case, the conversion of H2 into gaseous HD was detected. In order to analyze the experimental kinetic and spectroscopic data, we explored two different mechanisms taking into account potential kinetic and equilibrium H/D isotope effects. Firstly, we explored the dissociative exchange mechanism consisting of dissociative adsorption of dihydrogen, fast hydride surface diffusion and associative desorption of dihydrogen. It is shown that if D2 is the reaction partner, only H2 will be released in the beginning of the reaction, and HD only in later reaction stages. The second mechanism, dubbed here associative exchange consists of the binding of dihydrogen to Ru surface atoms, followed by a H-transfer to or by H-exchange with an adjacent hydride site, and finally of the associative desorption of dihydrogen. In that case, in the exchange with D2, only HD will be released in the beginning of the reaction. Our experimental results are not compatible with the dissociative exchange but can be explained in terms of the associative exchange. Whereas the former will dominate at low temperatures and pressures, the latter will prevail around room temperature and normal pressures where transition metal nanoparticles are generally used as reaction catalysts.

Surprising Differences of Alkane C‐H Activation Catalyzed by Ruthenium Nanoparticles: Complex Surface‐Substrate Recognition?

“…Ligand separated nanoparticles catalyze a number of chemical reactions, e. g. hydrogenation of olefins and C−C activation . In combined 1 H gas phase and solid state 2 H NMR studies, some of us have shown that these particles contain surface hydrogens, which can be replaced by deuterons by exposure to D 2 gas, resulting in the release of HD. However, it was also observed that CH 2 groups of organic ligands such as hexadecylamine (HDA) were partially deuterated, a process which could only take place via C−H activation.…”

“…In the first step (see Scheme S1a), surface hydride of the latter – which contain a large number of defects such as edges, apexes and steps that offer coordinatively unsaturated metal atoms – are deuterated releasing HD when they are exposed to D 2 gas . In the second step, the alkane substrate forms a σ‐complex with surface Ru atoms and transfers an H from C to Ru (Scheme S1b) corresponding to an oxidative cleavage of a surface alkyl.…”

“…In the second step, the alkane substrate forms a σ‐complex with surface Ru atoms and transfers an H from C to Ru (Scheme S1b) corresponding to an oxidative cleavage of a surface alkyl. As surface H/D exchange is fast, there is a great chance that in the backward reaction a D is transferred to C. Finally, the deuterated product is released from the surface.…”

Reactions of D2 with 1,4‐Bis(diphenylphosphino) butane‐Stabilized Metal Nanoparticles‐A Combined Gas‐phase NMR, GC‐MS and Solid‐state NMR Study

“…These peaks are broader as compared to the liquid state because of efficient longitudinal and transverse spin‐rotation and dipolar relaxation mechanisms arising from the coherent and incoherent molecular rotation in the gas phase. This effect is stronger for small molecules such as HD, and even more pronounced for H 2 ,. In contrast to the Pt‐containing NPs, the spectra of the Ru‐containing NPs present an additional signal around 0.1 ppm.…”

“…For MNPs formed by hydrogenation of organometallic precursors it has been shown by a combination of 1 H gas phase and solid state 2 H NMR that hydrides are present on the surface of the particles and that they are mobile and exchangeable by deuterium . Recently, some of us have explored the kinetics of gas‐solid hydrogen isotope exchange of ruthenium metal nanoparticles (RuMNPs) in more detail . It was found that when D 2 is applied to RuMNPs, a direct reaction with surface hydrides occurs without D 2 dissociation leading to the formation of HD in the initial reaction stages.…”