NH3 decomposition activity of Ru supported on hydrothermally derived carbon: Temperature effects on the morphological evolution

A hydrothermally derived carbon support was synthesized from the sustainable feedstock chitosan, with optional subsequent pyrolysis at 600 °C and 1000 °C, to explore its potential as a catalyst support material for ruthenium (Ru). The catalysts were prepared through wet impregnation using Ru nitrosyl nitrate as the precursor. Their catalytic performances in ammonia decomposition were investigated under conditions of 5 % NH₃ at 1 bar, within a temperature range of 300 °C to 600 °C, and a weight hourly space velocity of 15.000 mlN gcat-1 h-1. The analytical techniques employed in this study included elemental analysis, thermogravimetric analysis (TGA), gas adsorption measurements, Raman spectroscopy, X-ray diffraction (XRD), flame atomic absorption spectroscopy (F-AAS), scanning transmission electron microscopy (STEM), hydrogen-based temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). They unraveled that non-pyrolyzed supports showed a strong tendency for Ru agglomeration, whereas pyrolyzed supports exhibited improved metal distribution, which correlated with enhanced catalytic activity exceeding 50 % ammonia conversion at 450 °C. The surface chemistry of the carbon support was modified by varying the pyrolysis temperature, which affected the concentrations and types of oxygen and nitrogen surface groups. These changes altered the interaction between Ru and these surface groups. During the decomposition of the Ru precursor and the reduction of Ru oxides, the partial breakdown of oxygen and nitrogen surface groups led to surface reconstructions of the Ru nanoparticles, thereby affecting their crystallinity. This phenomenon was also observed during the catalytic testing, which was more pronounced on the HC-600 support. Modifying the surface chemistry of hydrochar via pyrolysis affects Ru distribution, reducibility, and crystallinity, thereby enhancing the NH₃ decomposition performance.