Size-tunable Ni particles confined in the ordered mesoporous silica for catalytic H2 production from ammonia borane hydrolysis

Catalytic ammonia borane (AB) hydrolysis using supported Ni catalysts is a cost-effective way for H2 production. However, constructing highly active Ni catalysts is still challenging due to the low intrinsic activity and easy aggregation of Ni particles at high Ni content. In this study, Ni particles in SBA-15 (mi-Ni@SBA-15) were prepared using the template-assisted melt infiltration method. For comparison, Ni catalysts supported on SBA-15 were also prepared using conventional impregnation (imp-Ni/SBA-15-NT) and melt infiltration method (mi-Ni/SBA-15-NT). The catalysts were fully characterized and catalytic AB hydrolysis was carried out on the catalysts. The results showed that for imp-Ni/SBA-15-NT and mi-Ni/SBA-15-NT large Ni agglomerates were obviously observed and the mesopores of SBA-15 were partially blocked due to the formation of rodlike Ni particles. In contrast, for mi-Ni@SBA-15 the combination of the steric effect of template and tunable mesopore size of SBA-15 enables SBA-15 to effectively confine the Ni particles in its mesopores and regulate the particle size of Ni respectively. Increasing Ni content from 7.7 to 24.5 wt% only led to denser Ni particles in SBA-15 without marked growth of Ni particles for mi-Ni@SBA-15. The well-dispersed Ni particles on mi-Ni@SBA-15 with Ni content of 24.5 wt% enriched the active sites on the catalyst surface, thus leading to high catalytic activity for AB hydrolysis with a TOF of 37.8 molH2 molNi−1 min−1, which was at least 3.9 times higher than those of other tested catalysts. A comparison of the normalized catalytic activities by exposed Ni sites of catalysts with varied Ni particle sizes suggested that Ni sites on small Ni particles are more active than those on large ones. This work highlighted that mi-Ni@SBA-15 prepared using the template-assisted melt infiltration method are highly active catalysts for H2 production from AB hydrolysis.