Efficient ammonia synthesis over a Ru/La0.5Ce0.5O1.75 catalyst pre-reduced at high temperature

A Ru/La0.5Ce0.5O1.75 catalyst pre-reduced at an unusually high temperature (650 °C) catalyses ammonia synthesis at a high rate under mild conditions.

were pressed into pellets at 20 MPa for 5 min, crushed, and sieved to grains with diameters of 250-500 μm. A tubular Inconel reactor (i.d. = 7 mm) was packed with quartz wool, and then 100 mg of catalyst was added. Research-grade gases (>99.99%) were supplied from high-pressure cylinders without further purification. On the other hand, only for long-term test, inline gas purifier (MicroTorr MC50-904FV, SAES Pure Gas, USA) was used, and then concentration of impurities (H 2 O, O 2 , CO 2 ) in gas mixture was reduced to below 100 ppt. The catalysts were reduced in a flow of pure H 2 (60 NmL min −1 ) at 450, 500, 650, or 800 °C for 1 h at 0.1 MPa and then cooled to 300 °C in an Ar stream. After the pressure was adjusted to 0.1, 0.5, 1.0, or 3.0 MPa at 300 °C, a 3:1 (mol/mol) H 2 /N 2 mixture (space velocity = 72,000 NmL h -1 g -1 ) was fed to the catalyst. The temperature of the catalyst was kept constant for 0.5 h to facilitate measurement of ammonia-synthesis rates. The catalyst was then heated to 400 °C in 25 °C increments. The ammonia-synthesis rate was determined from the rate of decrease of electron conductivity (CM-30R, DKK-TOA, Japan) of a dilute sulfuric acid solution (1-100 M) used to trap the ammonia produced under the reaction conditions.

Kinetic analysis
Reaction kinetics were analyzed by a reported method. S3,S4 In brief, the reaction orders with respect to the N 2 , H 2 , and NH 3 were determined by measuring N 2 , H 2 , and NH 3 pressure dependence for the ammonia-synthesis rates with the assumption of the rate expression (1).
Equations (2) to (5) were also used for these analyses.
r, w, y 0 , q, C, and (1-m) denotes ammonia-synthesis rate, catalyst mass, ammonia mole fraction at the reactor outlet, flow rate, constant and a. Kinetic analyses were performed at 350 o C and 0.1 MPa. Other reaction conditions are described in Table S1 and Fig S1. To avoid the contribution of the reverse reaction, kinetic measurements were carried out at a GHSV value where the ammonia concentration at the reactor exit was far away from the thermodynamic equilibrium concentration.

Characterisation of the supported Ru catalysts
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and energy dispersive X-ray (EDX) elemental maps were obtained with a JEM-ARM200CF electron microscope (JEOL, Japan) operated at 120 or 200 kV. Electron energy loss spectroscopy was performed at an acceleration voltage of 80 kV to reduce damage to the sample by the electron beam. For STEM and electron energy loss spectroscopy, powdered catalyst samples reduced in a quartz reactor at 650 °C under a H 2 flow were crushed and deposited on TEM grids coated with a thin carbon film in a glove box. The samples thus prepared were transferred from the glove box to the inside of the TEM column without being exposed to the air by means of a special holder with a gas cell. For other measurements, samples were dispersed in ethanol under ambient conditions, and the dispersion was dropped onto a carbon-coated copper grid and then dried under a vacuum at ambient temperature for 24 h.
X-ray diffraction (XRD) analysis was performed with a SmartLab X-ray diffractometer (Rigaku, Japan) equipped with a Cu Kα radiation source. For in situ measurements, the sample was placed in a reactor chamber (XRK 900, Anton Parr, Austria) and heated at 500 or 650 °C for 1 h under a stream of H 2 or a stream of 1:4 (v/v) O 2 /N 2 , and then the diffraction patterns were observed at 500 or 650 °C under the corresponding gas stream. PDXL2 software (Rigaku) with ICDD, COD, S5 and AtomWork S6 databases was used to analyse the XRD patterns The specific surface areas of the catalysts after N 2 treatment at 300 °C were determined by the Brunauer-Emmett-Teller method using a BELSORP-mini instrument (BEL Japan Inc., Japan).
H 2 chemisorption capacity was measured to estimate the Ru dispersion of the catalysts. H 2 was fed to each sample at 60 NmL min -1 , and the temperature was increased from room temperature to 500, 650, or 800 °C. The sample was maintained at the desired temperature for 1 h in the H 2 flow and was then purged with a stream of Ar for 30 min, cooled to −78 °C, and flushed with Ar for 60 min. After this pretreatment, H 2 chemisorption analysis was carried out at −78 °C in an Ar stream (60 NmL min -1 ) by means of a pulsed-chemisorption technique.
The O 2 absorption capacity of the catalyst, which is a measure of the degree of catalyst reduction, was measured by means of a pulse injection method. O 2 absorption at 800 °C was measured by the same method.
Infrared spectra of adsorbed N 2 were measured with a spectrometer (FT/IR-6600, JASCO, Japan) equipped with a mercury-cadmium-tellurium detector at a resolution of 4 cm −1 as described previously. S2 Samples (~20 mg) were pressed into self-supporting disks (10 mm in diameter). Each disk was placed in a silica-glass cell equipped with CaF 2 windows and was then connected to a closed gas-circulation system. The disk was pretreated with circulated H 2 (80 kPa) that had been passed through a liquid N 2 trap. The disk was heated from room temperature to 500 or 650 °C and then kept at that temperature for 1 h. After the reduction, the cell containing the disk was evacuated at the same temperature for 0.5 h to remove the H 2 . After this pretreatment, the disk was cooled to room temperature under a vacuum. Pure N 2 (>99.9995%) was supplied to the system via a liquid N 2 trap. 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 background spectra from the spectra of the samples containing adsorbed N 2 .    increased with increasing pre-reduction temperature. Assuming that complete oxidation to La 0.5 Ce 0.5 O 1.75 and RuO 2 occurred, the values of the degree of reduction of Ce 4+ to Ce 3+ after pre-reduction at 500, 650, and 800 °C were determined to be 23%, 43%, and 63%, respectively.