Synergetic Effect of Ultrasmall Metal Clusters and Zeolites Promoting Hydrogen Generation

Abstract Taking advantage of the synergetic effect of confined ultrasmall metal clusters and zeolite frameworks is an efficient strategy for improving the catalytic performance of metal nanocatalysts. Herein, it is demonstrated that the synergetic effect of ultrasmall ruthenium (Ru) clusters and intrinsic Brønsted acidity of zeolite frameworks can significantly promote the hydrogen generation of ammonia borane (AB) hydrolysis. Ultrasmall Ru clusters are embedded onto the silicoaluminophosphate SAPO‐34 (CHA) and various aluminosilicate zeolites (MFI, *BEA, and FAU) with tunable acidities by a facile incipient wetness impregnation method. Evidenced by high‐resolution scanning transmission electron microscopy, the sub‐nanometric Ru clusters are uniformly distributed throughout the zeolite crystals. The X‐ray absorption spectroscopy measurements reveal the existence of Ru‐H species between Ru clusters and adjacent Brønsted acid sites of zeolites, which could synergistically activate AB and water molecules, significantly enhancing the hydrogen evolution rate of AB hydrolysis. Notably, the Ru/SAPO‐34‐0.8Si (Si/Al = 0.8) and Ru/FAU (Si/Al = 30) catalysts with strong acidities afford high turnover frequency values up to 490 and 627 min−1, respectively. These values are more than a 13‐fold enhancement than that of the commercial Ru/C catalyst, and among the top level over other heterogeneous catalysts tested under similar conditions.

Typically, 10.2 g of finely ground Al(O i Pr) 3 powder was mixed with 21 g of TEAOH solution and suitable amount of water, followed by a continuous stirring for 2 h. Then, 6.92 g of phosphoric acid was dropwise added into the above mixture, followed by a drastically stirring for 2 h. Finally, the suitable amount of colloidal silica was slowly added. The reaction mixture was stirred for 1 h and then transferred into a 100 mL Teflon-lined stainless steel autoclave. The crystallization was conducted in a conventional oven at 170 °C for 3 days under static conditions. The as-synthesized solid products were centrifuged, washed with water and ethanol for several times, and then dried at 80 °C in the oven overnight, followed by calcination at 550 °C for 6 h.

S3
The nanosheet-like AlPO 4 -34 zeolite was prepared from the starting gel with the molar composition of 1.0 Al 2 O 3 : 1.2 P 2 O 5 : 2.0 TEAOH: 33 H 2 O under the same condition for the synthesis of SAPO-34 zeolites except without adding the colloidal silica.
Synthesis of Ru/SAPO-34 and Ru/AlPO-34 catalysts. Ru/SAPO-34-xSi and Ru/AlPO-34 catalysts were prepared by the incipient wetness impregnation method. Typically, 1 g of calcined SAPO-34 or AlPO-34 zeolite was impregnated with RuCl 3 solution (0.23 mL, 0.19 M), and then the mixture was drastically stirred to allow the RuCl 3 solution absorbed into the zeolites. The obtained solid was dried at 80 °C in the oven overnight, and then reduced sequentially in flowing H 2 with linear heating to 400 °C for 2 h and holding for 2 h. The temperature-programmed desorption of ammonia (NH 3 -TPD) experiments were performed using a Micromeritics Auto Chem II 2920 automated chemisorption analysis unit equipped with a thermal conductivity detector (TCD) under helium flow. The 29 Si, 27 Al, 31 P, and 1 H MAS NMR measurements were performed on Bruker AVANCE III 400 WB spectrometer at a magnetic field strength of 9.4 T. The resonance frequencies were 79.5, 104.2, 161.9, and 400.1 MHz for 29 Si, S4 27 Al, 31 P, and 1 H, respectively. The spinning rate of all samples at the magic angle was 12 kHz.

Synthesis of micron-sized SAPO
The chemical shifts were referenced to 85% H 3 PO 4 solution for 31 P, 1 M Al(NO 3 ) 3 solution for 27 Al, 2,2-dimethyl-2-ilapentane-5-sulfonate sodium salt (DSS) for 29 Si and 1 H, respectively. the X-ray photoelectron spectroscopy (XPS) was measured by ESCALAB 250 spectrometer. To obtain the valence of Ru clusters in the catalysts, the dissolved samples were prepared. The brief steps are listed as follows: the Ru/SAPO-34-0.2Si catalyst was first dissolved in suitable amount of NaOH solution (10 M). After stirring, the residual solid was then isolated from the mixture by centrifugation, washed with water, and dried with vacuum freeze dryer. The released gas was analyzed using Agilent GC 6890N, equipped with thermal conductivity detector (TCD) and Plot-Q column (Agilent J&W GC Columns, HP-PLOT/Q 19095P-Q04, 30m × 530µm × 40µm).
Liquid NMR spectra were recorded on a BRUKER AVANCEIII 500 MHz spectrometer (500.13 MHz for 1 H NMR and 160.42 MHz for 11 B NMR). Liquid samples of the filtrates, in which D 2 O was included as solvent or a lock, were contained in sample tubes of 5 mm.
The X-ray absorption spectroscopy data were collected at the Sector 20-BM beamline of the Advanced Photon Source at Argonne National Laboratory. Sample powders were packed on Kapton tapes and folded multiple times to enhance the signal. The beamline was equipped with a double-crystal Si (111) monochromator. A 12-element Ge fluorescence detector was used to collect spectra of the Ru K-edge. Data processing and EXAFS fitting were performed using the WinXAS software in conjunction with scattering path amplitude and phase functions calculated using the FEFF8 program.
The FTIR spectra were scanned between 4000 and 1200 cm -1 after the adsorbed samples degassed at temperatures of 30, 100, 200, 300 °C for 1 h, respectively, using a Perkin-Elmer Spectrum TM GX spectrometer. Samples were first pressed into self-supporting discs with a diameter of 15 mm. Subsquently, the samples were pre-treated in the IR cell attached to a vacuum line at 100 °C (1.2 °C /min) for 1.5 h, and then at 450 °C (2 °C /min) for 2 h under 10 -6 Torr. The adsorption of the deuterated acetonitrile was performed at 30 °C. After establishing a pressure of 10 torr at equilibrium, in order to remove the physisorbed species, the cell was evacuated at 30 °C.
The concentrations of Brønsted and Lewis acid sites were determined from the quantitative analyses of the characteristic IR bands at 2292 and 2320 cm -1 , respectively. The molar extinction S5 coefficient obtained for Lewis acid sites was 3.6 cm·µmol -1 and for Brønsted acid sites was 2.0 cm·µmol -1 .

Catalytic Tests
Hydrolysis Reaction of Ammonia Borane. The hydrolysis of AB reactions were carried out with an apparatus containing a reaction unit and a gas collecting device. In general, 0.5 mL distilled water was first placed in a two-necked round-bottomed flask (25 mL Figure S16. GC spectra using Agilent GC 6890N equipped with TCD detector for the evolved gas from AB hydrolysis over SAPO-34-0.2Si catalyst at 50 °C as compared with pure H 2 .