CeNi x Alloys as Catalysts for Ammonia Synthesis: Insights on Ni–CeN Surface Layer Formation and Its Impact

Abstract

Ammonia, which is widely used for the production of fertilizers, is becoming increasingly important as a hydrogen-containing energy vector. Typically, the ammonia synthesis activity of non-noble cheap metal-based catalysts (e.g., Ni) is well below that of ruthenium or cobalt. In this work, we unveil the performance of bulk CeNix alloys as compact bimetallic catalysts for ammonia synthesis. The spontaneous formation of a crystalline CeN surface layer was responsible for the higher activity of CeNi2 over CeNi5 (1.012 and 0.067 mmol g–1 h–1, respectively) at 400 °C and 0.9 MPa. The CeN layer was key since it served as a second active center for nitrogen dissociation, enhancing the ammonia synthesis rate to levels comparable to other rare earth-based alloys. Significant differences in the global kinetic mechanism were also found: CeNi2 showed significantly lower apparent activation energies than CeNi5 (55.3 vs 79.5 kJ mol–1, respectively). Furthermore, CeNi2 showed synthesis rates 1 order of magnitude higher than pure bulk CeN, thereby stressing the key role of Ni as an additional center for hydrogen and hydrogen-containing species (NHx) activation. We also demonstrated that the chemical state of cerium (oxide vs nitride) is key for enhancing the ammonia synthesis reaction. We found that Ce is required to be in the form of nitride for enhancing the activity of CeNi2, as revealed by the poor kinetic behavior (high activation energy, strong hydrogen poisoning, and poor affinity toward NHx species) and low synthesis rates found for a nanopowder Ni/CeO2 catalyst.

Es el preprint del artículo. Se puede consultar la versión final en https://doi.org/10.1021/acscatal.3c03654