Enhanced Ammonia Decomposition Performance of K-doped Ru/γ-Al2O3 Catalyst: The Role of Reduction Temperature and Potassium

For the Ru/γ-Al2O3-O400Rx catalyst (prepared through air oxidation calcination at 400 °C and reduction at different temperatures), as the reduction temperature increased from 400 °C to 800 °C, the catalyst activity decreased with the increase in particle size (Fig. S4). Among them, the Ru/γ-Al2O3-O400R200 catalyst had a low reduction temperature (Fig. 2a), resulting in incomplete reduction of RuOx species to active metallic Ru. Therefore, not only the effect of Ru particle size but also the electron density affects the ammonia decomposition activity. To further increase the electron density of Ru, we conducted dechlorination and added potassium to the Ru/γ-Al2O3-O400R200 catalyst and Ru/γ-Al2O3-R200 catalyst. The influence of potassium concentration on ammonia decomposition activity was optimized, and the optimal concentration was found to be 10 wt% K (Fig. S5). As shown in Figure 5e, through dechlorination and potassium addition, the binding energy position of Ru3p in the Ru/γ-Al2O3-O400R200W catalyst decreased from 461.75 eV to 461.66 eV, a decrease of 0.09 eV, and the ammonia decomposition activity increased from 76.3% to 100% at 500 °C. The Ru/γ-Al2O3-R200W catalyst exhibited outstanding activity for ammonia decomposition with a hydrogen formation rate of 19.5 mmol/gcat/min at 450 °C (Fig. S6). Moreover, the binding energy position of Ru3p in the Ru/γ-Al2O3-R200W catalyst decreased by 0.19 eV from 462.33 eV to 462.14 eV, and the activity increased from 65.2% to 100%, showing a significant increase of 34.8%.

Figure 6a shows the stability of the 10K-Ru/γ-Al2O3-O400R200W (K-O400R200W) and 10K-Ru/γ-Al2O3-R200W (K-R200W) catalysts tested in a fixed-bed reactor. The 10K-Ru/γ-Al2O3-R200W catalyst exhibited excellent long-term stability over 700 hours, while the ammonia decomposition activity of the 10K-Ru/γ-Al2O3-O400R200W catalyst decreased by 97.3% in a 450-hour lifetime test (Fig. 6a). In contrast, the 10K-Ru/γ-Al2O3-R200W catalyst showed outstanding stability. XRD and XPS analysis of the catalyst structure before and after the stability test showed no diffraction peak of Ru in the 10K-Ru/γ-Al2O3-R200W catalyst, and the Ru3p binding energy position was reduced to 461.1 eV (Fig. S7). However, the particle size of the 10K-Ru/γ-Al2O3-O400R200W catalyst increased from 14.2 nm to 17.4 nm, further indicating the excellent anti-sintering ability of the 10K-Ru/γ-Al2O3-R200W catalyst (Fig. 6b). Additionally, HRTEM and TEM-Mapping results demonstrated that Ru remained highly dispersed in the 10K-Ru/γ-Al2O3-R200W catalyst after 700 hours of lifetime, with a particle size of 2.32 nm (Fig. S8). In conclusion, the highly dispersed 10K-Ru/γ-Al2O3-R200W catalyst prepared through atmosphere-induced methods exhibited high activity and stability due to its stable structure.