Supporting Information Highly Active and Noble-Metal-Alternative Hydrogenation Catalysts Prepared by Dealloying Ni–Si Intermetallic compounds

Noble-metal-alternative Ni-Si catalysts more active than Pd in the hydrogen storage reaction were developed using a unique procedure, i.e., surface dealloying with hydrofluoric acid treatment. The combination of the structural analysis and the DFT calculation revealed a specific active site structure, a Ni cluster embedded in a SiO2 matrix, and its unprecedented role in the molecular conversion.

X-Ray Photoelectron Spectroscopy (XPS) analysis was conducted using ULVAC PHI Quantera SXM with monochromatic Al Kα X-rays at 1486.6 eV, 14 kV and 1500 W. Spectra were measured for the sample after heat treatment under a 5% H 2 /Ar flow (500 mL min −1 ) at 673 K for 0.5 h. The base pressure was set below 1.0 × 10 -7 Pa. The diameter of detection and the take-off angle (TOA) were 400 µmΦ and 45°, respectively. Here, the surface normal corresponds to a TOA of 90°. The pass energy and the energy step were set at 29.35 eV and 0.125 eV, respectively. The binding energies were calibrated based on the hydrocarbon C1s peak at 285 eV.
The angular-resolved Hard X-ray Photoelectron Spectroscopy (HAXPES) measurements were conducted on the SPring-8 BL16XU beamline as described in the previous report. 1 The photon energy was set at 7946.6 eV for Ni2p 3/2 and Si 2p core levels. The energies and angular distributions of the photoelectrons were assessed using a VG-Scienta R4000-HV hemispherical analyzer. The objective lens has an effective acceptance angle of approximately ±30º and an angular resolution of 1.32º. The stability of the system was confirmed using the Au 4f 7/2 photoelectron peak for an Au film on a Si substrate. The overall stability of the photoelectron energy was found to be within 50 meV.
The angular distributions of the photoelectrons were determined at photoelectron take-off angles at 85º. Here, a take-off angle perpendicular to the surface is defined as 90º. The analysis depths of the HAXPES measurements were calculated according to the previous report. 2 The X-ray adsorption fine structure (XAFS) spectra of Ni K-edge were recorded by fluorescence mode at SPring-8 BL16B2. The angle between the sample surface and the direction vector of incident X-rays was 45°, and the spectra were acquired by solid state detector. The X-ray irradiated area on the sample surface was 1 mm (vertical) x 2 mm (horizontal). The Near edge X-ray adsorption fine structure (NEXAFS) spectra of Si K-edge were recorded by fluorescence mode at Aichi SR BL6N1. The angle between the sample surface and the direction vector of incident X-rays was 90°, and the spectra were acquired by silicon drift detector. The X-ray irradiated area on the sample surface was 1 mm (vertical) x 2 mm (horizontal).
The microstructure of the samples was observed by scanning electron microscopy (SEM; Hitachi High-Technologies S-5500), transmission electron microscopy (TEM; JEOL JEM-2100F), and scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX; FEI Talos F200X, at 200 kV). For STEM observation, a cross-section of the sample was prepared using focused ion beam (FIB: FEI, Helios, thickness: 100 nm).
CO pulse chemisorption was performed using BELCAT II (Microtrac BEL) to estimate the Ni and Pd dispersion of the prepared catalysts. Prior to chemisorption, the catalyst was pretreated under a 5% H 2 /Ar flow (40 mL min −1 ) at 400°C for 0.5 h. Because of the low metal dispersion of bulk materials, the catalyst amount was typically 100~300 mg to quantify sufficient amount of CO chemisorbed. After the reduction pretreatment, S3 He was introduced at the same temperature for 10 min to remove the chemisorbed hydrogen, followed by cooling to room temperature. A 10% CO/He pulse was introduced into the reactor, and the supplied CO flow was quantified downstream by a TCD. For each sample, the CO chemisorption was performed at least three times (error was below 5%) and the averaged value was reported.

Computational details
Periodic DFT calculations were performed using the CASTEP code 3 with Vanderbilt-type ultrasoft pseudopotentials 4

Discussion on the effect of HF concentration
As shown in Figure S4, Ni dispersion and the reaction rate initially increased and then plateaued as HF concentration increased. This is probably because Ni is also dissolved by HF when the surface Ni content increases, resulting in a steady state with a certain surface Ni/Si ratio. The order of TOF at 1.0 M was Ni 3 Si >> NiSi 2 > NiSi > Ni 2 Si, which is in accordance with that of Si content except Ni 3 Si.
Considersing that (1) Ni 3 Si has a thick SiO 2 layer at the surface (Si-rich shell, Figure S9) and that (2) the formation of SiO 2 matrix surrounding Ni is important ( Figure 5), Si-rich Ni-Si composition (at least at the surface region) would be the key factor to obtain an appropriate Ni@SiO 2 structure and a high catalytic performance. Ni 3 Si showed a specifically high TOF at [HF] of 1.0 M and the TOF significantly dropped at the higher [HF] ( Figure S4c). A possible interpretation is that the thick SiO 2 layer of Ni 3 Si is completely removed and Ni-rich surface appears.