In‐situ atomic scale studies of ammonia synthesis over ruthenium nanocatalysts

Abstract

Ammonia is an important chemical for the production of fertilisers and in chemical synthesis. Catalyst materials are employed to improve the rate of formation of ammonia from diatomic gas precursors. Historically, the industrial catalyst most widely used for this process is based on the magnetite phase of iron oxide, often promoted by alumina and other additives. 1 More recently, graphitic carbon supported ruthenium based catalysts have been developed, which can operate more efficiently and at lower pressures than iron catalysts. 2, 3 In this work, the sintering of Ru/C catalysts under ammonia synthesis conditions was studied by Environmental Scanning Transmission Electron Microscopy (ESTEM). 4 Samples were prepared by incipient wetness impregnation of Ru(NO)(NO 3 ) 3 on graphitic carbon. After drying at 150 °C, powdered samples were deposited onto 5 nm amorphous carbon coated MEMS chips supplied by DENS solutions. Additionally, model Ru samples were prepared by direct deposition of Ru precursor onto amorphous carbon MEMS chips. Ru nanoparticles were formed in the microscope by in‐situ reduction in H 2 gas. Suitable regions were identified and treated at 300 – 450 °C in H 2 , N 2 , or a mixed H 2 /N 2 gas atmosphere. Images were taken before and after treatment to study nanoparticle sintering. To limit beam exposure, the beam was blanked between images and during heating steps. Typical images of the Ru/C samples are shown in Figure 1, showing Ru particles around 1 – 5 nm in size in addition to smaller clusters. After initial reduction, electron diffraction patterns from particles on both supports can be assigned to hexagonal phase Ru metal, space group P 6 3 /mmc . An example heating series for Ru on amorphous carbon is shown in Figure 2. Particle migration and coalescence is observed following treatments at both 300 °C for 2 h and 450 °C for 1 h. For this image series the particle size distribution increased from 1.5 ± 0.38 nm before H 2 /N 2 treatment, to 1.57 ± 0.33 nm and 1.64 ± 0.4 nm after 300 °C/450 °C treatments respectively. The sintering behaviour of Ru nanoparticles will be investigated as a function of support, gas atmosphere and temperature. The authors would like to thank Mr Ian Wright and Dr Leonardo Lari for technical support, and the EPSRC (UK) for the strategic research grant EP/J018058/1.