Ag2(0) dimers within a thioether-functionalized MOF catalyze the CO2 to CH4 hydrogenation reaction

Ultrasmall silver clusters in reduced state are difficult to synthesize since silver atoms tend to rapidly aggregate into bigger entities. Here, we show that dimers of reduced silver (Ag2) are formed within the framework of a metal–organic framework provided with thioether arms in their walls (methioMOF), after reduction with NaBH4 of the corresponding Ag+-methioMOF precursor. The resulting Ag2-methioMOF catalyzes the methanation reaction of carbon dioxide (CO2 to CH4 hydrogenation reaction) under mild reaction conditions (1 atm CO2, 4 atm H2, 140 °C), with production rates much higher than Ag on alumina and even comparable to the state-of-the-art Ru on alumina catalyst (Ru–Al2O3) under these reaction conditions, according to literature results.

Gas adsorption measurements. The N2 adsorption-desorption isotherms at 77 K were carried out on crystalline samples of Ag + @1 and Ag 0 @1 with a Belsorp Mini X instrument. Samples were evacuated at 70 ºC during 24 h under 10 -6 Torr prior to their analysis.
X-ray Powder Diffraction Measurements. Polycrystalline samples of Ag + @1 and Ag 0 @1, and also after catalysis for Ag 0 @1, were introduced into 0.5 mm borosilicate capillaries prior to being mounted and aligned on an Empyrean PANalytical powder diffractometer, using Cu Kα radiation (λ = 1.54056 Å). For each sample, five repeated measurements were collected at room temperature (2θ = 2-60°) and merged in a single diffractogram.
X-ray photoelectron spectroscopy (XPS) measurements. Samples of Ag + @1 and Ag 0 @1 were prepared by sticking, without sieving, the samples onto a molybdenum plate with scotch tape film, followed by air drying. Measurements were performed on a K−Alpha™ X−ray Photoelectron Spectrometer (XPS) System using a monochromatic Al K(alpha) source (1486.6 eV). As an internal reference for the peak positions in the XPS spectra, the C1s peak has been set at 284.8 eV.
Microscopy measurements. Scanning Electron Microscopy (SEM) elemental analysis was carried out for Ag + @1 and Ag 0 @1, using a HITACHI S−4800 electron microscope coupled with an Energy Dispersive X-ray (EDX) detector. Data was analyzed with QUANTAX 400. The images of the Ag 0 @1 before and after methanation reaction were obtained on a Jeol JEM−F2100 microscope operated at 200 kV in dark field scanning transmission electron microscopy (DF−STEM mode).

S4
X-ray crystallographic data collection and structure refinement. Crystals of Ag + @1 and Ag2@1with 0.14 x 0.12 x 0.12 mm and 0.08 x 0.08 x 0.06 as dimensions were selected and mounted on a MITIGEN holder in Paratone oil and very quickly placed on a nitrogen or liquid helium stream cooled at 90 or 45 K for Ag + @1 and Ag2@1, respectively to avoid the possible degradation upon dehydration. Diffraction data for Ag + @1 were collected on a Bruker−Nonius X8APEXII CCD area detector diffractometer, using graphite−monochromated Mo−Kα radiation ( = 0.71073 Å) whereas for Ag2@1, were collected using synchrotron radiation at CRISTAL beamline of the SOLEIL ( = 0.67165 Å). The data were processed through SAINT[S2] and CrysAlisPro [S3], reduction and SADABS [S4] multi-scan absorption software. The structures were solved with the SHELXS structure solution program, using the Patterson method. The model was refined with version 2018/3 of SHELXL against F 2 on all data by full-matrix least squares [S5,S6].
As reported in the main text, the robustness of the 3D network, allowed the resolution of the crystal structure of both Ag + @1 and Ag2@1, adsorbates, being their crystals suitable for X-ray diffraction, even over one-and two-step process, after a crystal-tocrystal transformation. For these reasons it is reasonable to observe a diffraction pattern sometimes affected by expected internal imperfections of the crystals [likely at the origin of some Alert level A for Ag + @1 and Ag2@1in checkcifs related to U(eq) value of some atoms] and thus a quite expected difficulty to perform a perfect correction of anisotropy, mainly affected by highly flexible thioether chains as terminal moiety (vide infra).
In both samples, all non-hydrogen atoms of the MOF network, except some dynamically disordered fragments of the ethylenethiomethyl chains of the methox ligand, NO3 − anions in Ag + @1, and the thermally disordered Ag + and Ag 0 atoms, were refined anisotropically. The use of some C−C and C−S bond lengths restrains as well as with a solution of silver nitrate in water (15.7 mg in 1.9 ml), and the mixture was dried in an oven at 100 °C overnight to obtain Ag−Al2O3 (1 wt%).
Typical procedure for the catalytic methanation reaction. The reactions were performed in a 7 mL glass vial equipped with a valve and a manometer. The solid catalyst (0.008 mmol of metal in each case, 5 mol% respect to CO2) was added, and the glass vial was closed and purged for 3 times with a gas mixture of N2 (internal standard), CO2 and H2 (1:1:4), for three times. Then, the gas mixture was added through the valve, and pressurized to 5 bars. Reactions were set at 140 ºC for 24 h. Samples were extracted using a Hamilton SampleLock gas syringe and reaction products analyzed by micro−GC.
Reuse of the Ag@1 catalyst. The general reaction procedure above was followed.
After 24 h reaction time, the gas reagents were evacuated, the glass vial was purged with the gas mixture N2, CO2 and H2 (1:1:4) for three times, and the reaction was carried out again under the same reaction conditions.

S6
Isotopic experiment. The reactions were performed in a 7 mL glass vial equipped with a valve and a manometer. Solid catalyst (0.0048 mmol of silver) was added, and the glass vial was closed and purged three times with a gas mixture of N2 (internal standard), CO2 and H2 or D2 (1:1:4). Then, the gas mixture was added through the valve, pressurizing to 3.5 bars. Reactions were set at 140 ºC for 5 h. Samples were extracted using a Hamilton SampleLock gas syringe and reaction products analyzed by micro−GC.
Computational details. Periodic density functional theory (DFT) calculations were performed with the Vienna Ab-initio Simulation Package (VASP) code [S9], using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [S10]. The valence density was expanded in a plane wave basis set with a kinetic energy cutoff of 600 eV, and the effect of the core electrons in the valence density was taken into account by means of the projected augmented wave (PAW) formalism [S11]. Integration in the reciprocal space was carried out at the Γ k-point of the Brillouin zone. During geometry optimizations, the positions of all atoms in the system were allowed to relax without restrictions. Atomic charges were estimated using the theory of atoms in molecules (AIM) of Bader [S12]. The MOF was described by means of a hexagonal unit cell with parameters a = b =18.057, c = 12.800, containing 2 Ca, 12 C, 12 S, 12 N, 60 C, 42 O and 80 H atoms. One Ag atom (Ag1) and one Ag dimer (Ag2) were placed in two different positions, in the channel and in the interstitial region, and the geometry of the resulting system was optimized without restrictions.
Ag + @1 Ag2@1  developing along a axis is evident only in Ag + @1 and Ag2@1. It is likely due to the presence of most hindering NO3and Ag + ions, or Ag 0 and Ag 0 2 species, respectively.
S21 Figure S13. High−angle annular dark−field scanning transmission electron microscopy (HAADF−STEM) images and particle mapping analysis of the Ag 0 @1 catalyst (a) before and (b) after methanation reaction.
S22 Figure S14. FT−IR spectra in the 2100−1700 cm -1 region of Ag 0 @1 before the methanation reaction (blue) and after 5 uses (red). The band associated to Agx(CO)x species has been pointed out.