Contrasting Roles of Rare-Earth and Ga Oxide Additives in Spinel-Derived Water-Tolerant Ni-Co/Al2O3 Catalysts for Ammonia Decomposition

Abstract In this study, Ni-Co-Al mixed oxides were synthesized via continuous-flow hydrothermal synthesis (CFHS) in supercritical water (T ≥ 445 °C, p = 24 MPa) and combined with different loadings of La-, Ce-, and Ga-based oxides. From these precursors, catalysts for the decomposition of ammonia were generated through reductive pretreatment in hydrogen. The addition of Ga leads to less active catalysts that contain (Co,Ni)-Ga phases, which are transformed into (Co,Ni)4N during NH3 decomposition. In contrast, La- and Ce-based oxides strongly enhance the catalytic activity. The best performing catalysts, containing 13.7 at % La and 13.3 at % Ce, achieve H2 space-time yields of 25.1 and 21.5 mmol H2·gcat–1·min–1 at 500 °C, respectively, with only 22 wt % active (Co, Ni) metal. The Ce-containing catalyst shows a slightly higher turnover frequency of 0.13 s–1 at 400 °C and exhibits high stability and water tolerance under industrially relevant conditions, maintaining its performance after an accelerated aging test in NH3 that contains 2000 ppm H2O (p = 3 bar, GHSV = 1950 h–1) at temperatures ranging from 400 °C to 750 °C with a total time at 750 °C for 135 h. Operando XRD, in situ FTIR and temperature-programmed reduction and desorption experiments reveal that the promotional effect of La- and Ce-based oxides primarily originates from enhanced reducibility and increased metal dispersion rather than substantial electronic modification. This conclusion is supported by the similar TOF values observed under differential conditions and the unchanged onset temperature of surface NH3 decomposition (315 °C), despite large differences in catalytic activity.