(226a) Revealing Catalytic Active Units of Ru for NH3 Decomposition By Operando XPS

Weiqing Zheng,¹ Jian Zhang,² Dang Sheng Su,³ Robert Schlögl⁴

1. Catalysis Center for Energy Innovation, University of Delaware, 221 Academy St. Newark, DE 19716

2. Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China

3. Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

4. Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, 4-6 Faradayweg, Berlin 14195, Germany

Ruthenium-based catalysts exhibit the most promising catalytic properties both in ammonia synthesis and decomposition. There is consensus in the literature that the B₅ sites on Ru particle strongly influence the activity of ammonia decomposition. Yet, the nature of the active state in the Ru system is still heavily debated, since the appearance of the B₅ sites does not solely depend on particle size, but also on particle shape. Herein we probe the metal-carbon support interaction focusing on the morphology (size and shape) of Ru nanoparticles by atomic resolution transmission electron microscopy (TEM) and high pressure X-ray photoelectron spectroscopy (in situ XPS).

We found the morphology of Ru NPs strongly depends on the local geometry of carbon. We have deposited Ru nanoparticles on different pretreated graphite surfaces and further investigated by atomic resolution TEM imaging carried out on a Cs-corrected TEM. The Ru atomic clusters are immobilized on the dispersed oxidized vacancies on carbon surface, and tend to sit on locally disordering sites on the graphene layers. These atomic Ru clusters have a much higher density of high-energy sites (defects), leading to a strong Ru-N interaction and a shortage of sites for adsorbed H atoms, which will result in a lower turnover frequency of ammonia decomposition copared to the well-order Ru nanoparticles. The core-level photoelectron N1s spectra obtained at 400 °C in 0.5 mbar NH₃ show a mixture of N states. The peak at about 400.2 eV has been assigned to a molecular adsorbed N₂,^[1] which only appears on the disordered Ru nanoparticles.

The combinative characterizations can lead to a direct conclusion that the performance difference of carbon supported Ru catalysts for ammonia decomposition is dependent on the local graphitic order of carbon supports. The highly graphitic carbon support can promote the formation of well-order Ru nanoparticles with large terraces for H₂ desorption and steps as active sites for NH₃ dissociation.

References

[1] R. Schlögl, Angew. Chemie, 2003, 42, 2004-2008