COx-free hydrogen generation via decomposition of ammonia over copper and zinc-based catalysts

- Ammonia decomposition over γ-Al2O3-supported Cu and Zn heterogeneous catalysts.
- Low-temperature production of pure hydrogen feedstock for fuel cells applications.
- Ultrasonic impregnation synthesis showed superior performance over wet impregnation.
- Hydrogen production rate of 36.2 mmol g−1 min−1 was achieved over the Cu/Zn/Al2O3.
- All catalysts showed excellent long-term run stability, especially above 400 °C.

Production of hydrogen without any carbon dioxide or monoxide is of utmost importance for the chemical energy storage and conversion in a future low-carbon economy. Alumina-supported copper and Cu/Zn/Al2O3 catalysts were prepared by incipient wetness- and ultrasonic impregnation, while Zn/Al2O3 catalysts were also prepared to compare the effect of Zn. The characterisation of catalysts was carried out by N2 physisorption, XRD, H2-TPR, N2O chemisorption, NH3-TPD, CO2-TPD, H2-TPD, XPS and TEM. Catalysts prepared via ultrasonic impregnation showed a high metal dispersion and a small crystallite size, whereas Lewis acidic and basic sites were predominant on all catalysts, while the acid-base strength was strongly influenced by the presence of zinc in the case of Cu/Zn/Al2O3 catalysts. A high catalytic activity, reflected through the almost total conversion of ammonia at 600 °C with the H2 production rate of 36.2 mmol g−1 min−1, was achieved over Cu/Zn/Al2O3 catalyst, prepared by ultrasonic impregnation. The dispersion of metal and its acidic or basic nature played an important role, even more so than the presence of the two or three potentially synergistic metals on the surface of a catalyst. Under the applied reaction conditions, the catalysts showed an excellent stability for more than 100 h.

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