Di- and Tetrameric Molybdenum Sulfide Clusters Activate and Stabilize Dihydrogen as Hydrides

NaY zeolite-encapsulated dimeric (Mo2S4) and tetrameric (Mo4S4) molybdenum sulfide clusters stabilize hydrogen as hydride binding to Mo atoms. Density functional theory (DFT) calculations and adsorption measurements suggest that stabilization of hydrogen as sulfhydryl (SH) groups, as typical for layered MoS2, is thermodynamically disfavored. Competitive adsorption of H2 and ethene on Mo was probed by quantifying adsorbed CO on partly hydrogen and/or ethene covered samples with IR spectroscopy. During hydrogenation, experiment and theory suggest that Mo is covered predominately with ethene and sparsely with hydride. DFT calculations further predict that, under reaction conditions, each MoxSy cluster can activate only one H2, suggesting that the entire cluster (irrespective of its nuclearity) acts as one active site for hydrogenation. The nearly identical turnover frequencies (24.7 ± 3.3 molethane·h–1·molcluster–1), apparent activation energies (31–32 kJ·mol–1), and reaction orders (∼0.5 in ethene and ∼1.0 in H2) show that the active sites in both clusters are catalytically indistinguishable.


S1. Methodology for the Calculation of Thermochemical Properties
Standard thermodynamics equations were used for computing the thermochemical properties of the system. The free energy of a system at any given temperature (T) is defined as: where H is the enthalpy of the system and S is the total entropy of the system. The free energy of a gas-phase species at a non-standard partial pressure (p) was calculated according to the following relation: where G 0 is the standard free energy of the gas-phase species computed at 1 bar and kB is the Boltzmann constant.
The enthalpy of a system is defined as: where U is the internal energy of the system which was calculated according to the following equation: U = E el + E ZPE + E vib + E rot + E tr , (4) where Eel is the electronic energy of the system (obtained from single-point energy calculations), EZPE is the zero-point-energy (ZPE) correction to the internal energy of the system, and Evib, Erot, and Etr are the contributions to internal energy of the system due to its vibrations, rotation, and translation, respectively.
The contribution to the internal energy of the system due to its translation at a finite temperature is: The contribution to the internal energy of the system due to its rotation at a finite temperature is: E rot = RT, for a linear molecule, and (6a) E rot = 3 2 RT, for a general polyatomic molecule.
Finally, the contribution to the internal energy of the system from zero-point vibrational energy and molecular vibrations at a finite temperature were calculated using the following equations: where Θ ν,k = hν k /k B is the characteristic temperature of a vibrational mode k (obtained from vibrational frequency calculations), ν k is the frequency of the vibrational mode, and h is the Planck's constant. Each of the 3N -6 vibrational modes for minima (3N -5 vibrational modes for linear molecules and 3N -7 vibrational modes for transition states) were considered for calculating the EZPE and Evib. The frequencies obtained from DFT calculations were not scaled and were used as computed.
The total entropy of a system is defined as: where Sel, Svib, Srot, and Str are the contributions to the total entropy of the system from electronic, vibrational, rotational, and translational motions, respectively.
The entropy of a species due to its electronic degeneracy is defined as: where ω is the total electron spin-multiplicity of the system.
The entropy of a system due to its rotational motion can be obtained using the following equations: , for a diatomic molecule, and (11a) , for a general polyatomic molecule, where Θ r = h 2 8π 2 Ik B ⁄ is the characteristic rotational temperature of the system, σr is the rotational symmetry number, and I is the moment of inertia of the molecule.
The entropy of the system due to translational motion at a given temperature, T, and partial pressure, p, was obtained using the Sackur-Tetrode equation: Finally, the contribution to the entropy of a system due to its vibrational motion was determined using the rigid-rotor harmonic oscillator (RRHO) approximation: Following the approach by Cramer and Truhlar, 1 all frequencies below the 100 cm -1 were uniformly shifted up to the cut-off value for computing the vibrational entropy of the system.                 Spin-multiplicity (ω)          Spin-multiplicity (ω)

S11. X-ray Absorption Spectroscopy (XAS) Measurements
XAS measurements were performed at the P65 beamline of the German Electron Synchrotron (DESY) in Hamburg, Germany. 2 The storage ring operated at 6 GeV energy and 100 mA current. A water-cooled Si311 double crystal monochromator (DCM) was used for obtaining monochromatic X-rays. Two Rh-coated plane mirrors were installed in front of the DCM to reject higher harmonics. The DCM was calibrated to Mo K-edge energy by measuring a reference Mo foil and setting the first major inflection point to 20 000 eV. The beam spot-size was 200 μm (vertical) × 1.6 mm (horizontal) on the sample and the energy resolution of incident X-rays is estimated to be ~2 eV (ΔE/E ~1.0 × 10 -4 ) at Mo K-edge energy.
The carbonyl containing catalyst precursors were placed in a quartz capillary (WJM Glas, 1 mm outer diameter, 20 μm thickness) supported between two quartz wool plugs. The capillary was heated from below with a hot-air gas-blower (Oxford FMB). Gas flow rates were controlled using electronic mass flow controllers (Bronkhorst) and the pressure was monitored using a pressure gauge (Omega). All experiments were performed at ambient pressure. The catalyst precursors were sulfided in situ in a flow of 10 mL•min -1 H2S (10 vol.-% in H2) at 673 K for 2 h to form the sulfided Mo2S4/NaY catalyst. This sample was then treated in 10 mL•min -1 H2 at 673 K for 2 h to form the reduced Mo4S4/NaY catalyst. After performing sulfidation/reduction, the catalysts were exposed to ethene hydrogenation reaction conditions (T ≈ 473 K, pH₂ ≈ 950 mbar, pC₂H₄ ≈ 50 mbar) for at least 2 h. The capillary was then cooled down to room temperature and placed on the sample stage for XAS measurements, whilst maintaining the ethene/H2 gas flow. The data were monitored for any signs of X-ray beam damage. Several successive scans were averaged to reduce signal-to-noise ratio and improve the data quality.
The XAS spectra were measured between 19 800 and 21 000 eV in total fluorescence yield (TFY) mode using a passivated implanted planar silicon (PIPS) detector. For extended X-ray absorption fine structure (EXAFS) analyses, spectra were background subtracted, normalized, k 3 -weighted, and Fourier-transformed (FT) between k = 3 and k = 12 Å -1 . The EXAFS fitting was performed in k-space between 3 and 12 Å -1 simultaneously on the k 1 -, k 2 -, and k 3 -weighted data. The fitting was performed using Artemis software package. 3 E0 was set such that energy shift (ΔE0) obtained during the fit was less than 1 eV. A reference Mo foil was first fitted to Mo bcc structure and the Mo-Mo coordination number was set to 8 and 6 (theoretical values based on the bcc structure) for the first and second coordination shells, respectively, to obtain the amplitude reduction factor (S0 2 ) which was then used in the subsequent fits.