Cl-free Ru/CeO 2 spherical catalysts were prepared by incipient wetness impregnation and subsequent washing processes using RuCl 3 •3H 2 O and a commercial spherical CeO 2 support. Cs addition improved the catalytic performance for NH 3 decomposition, and the optimal Cs loading was a Cs/Ru molar ratio of 0.43. Transmission electron microscopy (TEM) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM−EDX) suggested that Cs was present around Ru metal particles and served as an anchor for forming small, well-dispersed Ru metal particles (∼2 nm). CO 2 temperatureprogrammed desorption results indicated that the Cs-Ru/CeO 2 catalyst had more strong basic sites than the Ru/CeO 2 catalyst. Kinetic analysis results clearly demonstrated that Cs addition promoted recombinative desorption of N atoms, which is the rate-determining step of NH 3 decomposition. Consequently, the Cs-Ru/CeO 2 spherical catalyst (Cs/Ru = 0.43) treated in Ar for 1 h at 773 K exhibited high catalytic activity for NH 3 decomposition (NH 3 conversion: 40% at 523 K and 80% at 573 K) at 2000 mL-NH 3 g cat −1 h −1 .

The Ru/CeO 2 (JRC-CEO-3) catalyst prepared by NaBH 4 reduction showed a high activity for NH 3 decomposition compared with the other catalysts prepared in this work; however, this catalyst gradually lost its activity due to the accumulation of adsorbed species produced during the reaction. Cs signi cantly improved the catalytic performance of the Ru/CeO 2 catalyst, and the optimum Cs/Ru molar ratio was 0.4 because excess Cs species gradually covered the exposed Ru metal active sites. Cs-Ru/CeO 2 -0.4 catalyzed the NH 3 decomposition reaction without H 2 pretreatment because the catalyst was activated by NH 3 or H 2 produced by NH 3 decomposition. Moreover, Cs not only increased the N-H bond dissociation rate but also decreased the e ect of the H 2 partial pressure on the catalytic performance. Consequently, the Cs-Ru/CeO 2 -0.4 catalyst exhibited a high, stable activity (NH 3 conversion: 92% at 623 K and 60% at 573 K) with a gas hourly space velocity of 2000 mL-NH 3 g-cat 1 h 1 .