A low-crystalline ruthenium nano-layer supported on praseodymium oxide as an active catalyst for ammonia synthesis

Low-crystalline Ru nano-layers and the strong basicity of Ru/Pr2O3 synergistically accelerated the rate-determining step of ammonia synthesis.


Catalyst preparation
The Pr 6 O 11 support was prepared by precipitation at room temperature from a suspension formed by gradual addition of a solution of Pr(NO 3 )·6H 2 O (Kanto Chemical, Japan) to a 25 wt% NH 3 solution (Wako Pure Chemical, Japan). The precipitate was kept in suspension overnight with stirring, washed with distilled water, dried at 70 °C for more than 12 h, and calcined at 700 °C in static air for 5 h. CeO 2 prepared by the same precipitation methods using Ce(NO 3 ) 3 ·6H 2 O (Wako Pure Chemical, Japan) and MgO (the reference catalyst of the Catalysis Society of Japan, JRC-MgO-500) were also calcined at 700 °C. The supports were then impregnated with Ru 3 (CO) 12 (Tanaka Kikinzoku Kogyo, Japan) in a tetrahydrofuran (THF, Wako Pure Chemical, Japan) solution. The Ru loading was fixed at 5 wt% for each catalyst. The Ru 3 (CO) 12 -THF-support suspension was stirred for 12 h and dried in a rotary evaporator. The obtained powder was kept at 70 °C for 4 h under air. It was heated to 350 °C under a Ar stream and kept at 350 °C for 5 h to remove the CO ligand from the Ru 3 (CO) 12 .

Activity tests
The NH 3 synthesis rate was measured using a conventional flow system under either atmospheric pressure or high pressure. Powders of catalysts were pressed into pellets at 2.0 MPa for 5 min, crushed, and sieved to grains with diameters of 250-500 μm. Quartz wool was packed into a tubular Inconel reactor (i.d. = 7 mm), and 200 mg of catalyst was loaded. Research-grade gas was supplied from high-pressure gas cylinders. The catalysts were reduced in pure H 2 flow at 400 or 500 °C for 1 h at 0.1 MPa and then cooled to 310 °C in an Ar stream, and the pressure was then adjusted to 0.1, 0.9, or 1.0 MPa at 310 °C. An H 2 /N 2 gas mixture with an H 2 /N 2 molar ratio of 3 (gas hourly space velocity = 18,000 mL h -1 g -1 ) was then fed to the catalyst. The temperature of the catalyst was kept constant for 0.5 h to facilitate measurement of NH 3 synthesis rates. The catalyst was then heated in 20 °C increments to 390 °C. The NH 3 synthesis rate was determined from the rate of decrease of electron conductivity (CM-30R, DKK-TOA, Japan) of the dilute sulfuric acid solution that trapped the NH 3 produced under the experimental conditions. NH 3 yield was calculated as described below: where is molar flow rate of synthesized ammonia in effluent gas and is molar flow rate of N 2 which is supplied to catalyst. HSC Chemistry 6 software (ver. 6.12, Outotec Research, Finland) was used to calculate thermodynamic equilibrium.

Characterization of the catalysts
X-ray diffraction (XRD) analysis was performed using a SmartLab x-ray diffractometer (Rigaku, Japan) equipped with a Cu-Kα radiation source. For in situ XRD measurements, the sample was placed in a reactor chamber (XRK 900, Anton Parr) and treated at 400 °C for 1 h under a stream of H 2 . After treatment, the gas was switched from H 2 to N 2 , the sample was cooled to room temperature, and diffraction patterns were obtained. PDXL2 software (Rigaku) with ICDD, COD, [S1] and AtomWork [S2] databases was used to analyze the XRD patterns.
High-angle, annular, dark-field, scanning transmission electron microscopy (HAADF-STEM) and high-resolution STEM (HR-STEM) images were obtained on a JEM-ARM200F electron microscope (JEOL, Japan) operated at 200 kV. The samples were dispersed in ethanol, dropped onto a carbon-coated copper grid, and dried under vacuum at ambient temperature for 24 h.
The specific surface areas of the catalysts after N 2 treatment at 300 °C were determined by the Brunauer-Emmett-Teller method using a BEL-mini instrument (BEL Japan Inc., Japan).
The H 2 chemisorption capacity was measured to estimate the Ru dispersion of the catalysts. H 2 was fed to each sample at 30 mL min -1 , and the temperature was increased to 400 °C. The sample was maintained at 400 °C for 1 h, purged in a stream of Ar at 500 °C for 30 min, cooled to -78 °C, and flushed with Ar for 60 min. After this pretreatment, H 2 chemisorption was carried out at -78 °C in an Ar stream (30 mL min -1 ) using a pulsed-chemisorption technique.
Temperature-programmed desorption (TPD) measurements of CO 2 were performed in a TPD-1-AT apparatus (BEL Japan, Japan). Catalyst (100 mg) was loaded into a quartz reactor, reduced in a stream of H 2 at 400 °C for 1 h, purged in a stream of He for 30 min, and cooled to 50 °C. After 1% CO 2 in He gas (30 mL min -1 ) was fed to the catalyst for 30 min at 50 °C, the oven temperature was increased at 10 °C min -1 to 900 °C. The CO 2 desorption profile was monitored by quadrupole mass spectrometer at m/e = 44. CO 2 -TPD of catalysts not exposed to CO 2 at 50 °C was also measured.
The infrared spectra of adsorbed N 2 were collected by spectrometer (FT/IR-6600, Jasco, Japan) equipped with a mercury-cadmium-tellurium detector at a resolution of 4 cm −1 . Samples were pressed into self-supporting disks (10 mm diameter, about 20 mg). A disk was placed in a silica-glass cell equipped with CaF 2 windows and connected to a closed gas-circulation system. The disk was pretreated with circulated H 2 (26 kPa) passed through a liquid-nitrogen trap. The sample was heated from room temperature to 500 °C over 1 h and kept at that temperature for 3 h. Following reduction, the sample was evacuated at the same temperature for 2 h to remove the hydrogen. After this pretreatment, the disk was cooled to room temperature under vacuum. Pure N 2 (>99.9995%) was supplied to the system through a liquid-nitrogen trap. Isotopic nitrogen ( 15 N 2 , 98%) was used without purification. The infrared spectrum of the sample at room temperature before N 2 adsorption was used as the background, and difference spectra were obtained by subtracting the backgrounds from the spectra of N 2 -adsorbed samples. reaction temperature, 310 °C. Ammonia synthesis rates of electride-supported catalysts were reproduced from Ref. [S3]. Kitano et al. improved the method of Ru/C12A7:epreparation [S4] and developed a highly active Ru/HT-C12A7:e -. [S3] Ru/C12A7:e -(2 wt%) NH 3 synthesis rate (μmol g -1 h -1 )