Highly active earth-abundant ammonia cracking catalysts using long chain hydrocarbon stabilised colloidal nanoparticles.
Mahin Julien J, El-Kadi Joseph J, Torrente-Murciano Laura L
In this work, we report the synthesis of highly active non-noble metal catalysts with controlled nanoparticle size and dispersion through a new catalyst preparation strategy, combining the continuous-flow synthesis of size-controlled Fe, Co and Ni nanoparticles by thermal decomposition of zero-valent precursors with their direct impregnation via the so-called colloidal incipient wetness impregnation (CIWI). The use of long-chain hydrocarbon ligands provides not only size control and colloidal stability at high nanoparticle concentration but also the synergetic simultaneous removal and metal reduction at low temperatures. Indeed, we demonstrate that the removal of these ligands is catalysed by Co and Ni particles, enabling their simultaneous reduction at lower temperatures and thus, avoiding their agglomeration. This combination of controlled nanoparticle synthesis, high-concentration processing and in situ self-cleaning provides a route to size control that is difficult to access by conventional incipient wetness impregnation. The flexibility of CIWI further enables systematic investigation of nanoparticle size-activity relationships independently of support effects. Applied to ammonia decomposition, Ni/CeO2 exhibits a pronounced size dependence, with 2.4 nm Ni nanoparticles delivering an activity of 3130 molNH3 molmetal -1 s-1 at 500 °C, ranking among the most active earth-abundant catalysts reported to date and approaching the performance of state-of-the-art Ru-based systems. The CIWI therefore provides a versatile platform for designing highly active, size-controlled non-noble metal catalysts and is broadly applicable to other metal-support combinations and catalytic reactions.