Abstract

Despite the fact that catalytic ammonia decomposition constitutes an attractive alternative for renewable energy transport due to its high H2 content (17.8 wt%) and COx-free product stream, among other benefits, there are challenges that must be addressed before it can be consolidated as a hydrogen (H2) chemical carrier technology. In particular, the focus lies on developing efficient bimetallic catalytic systems as alternatives to the cost and scarcity of ruthenium. In this context, from the perspective of rational catalyst design, a kinetic study of monometallic and bimetallic Co-Ni catalysts supported on gamma-Al2O3, selected through prior screening, is presented. The catalysts were prepared by incipient wetness impregnation (IWI) yielding catalysts with similar metal dispersion (13-18 %). Due to product inhibition sensitivity, a Langmuir-Hinshelwood-Huge-Watson (LHHW) kinetic model was fitted under integral conditions. The apparent activation energy (Eapp) derived from the kinetic model, alpha B and k5, served as an experimental descriptor for activity, showing a strong correlation with the H2 formation rate (R2 > 0.99) and accurately predicting the observed activity order: Ni > Co > Co-Ni across the full temperature range (437-475 degrees C). A pronounced anti-synergistic effect was observed in the bimetallic catalyst, resulting in a reaction rate 4.3 times lower than that of nickel and half that of cobalt. Similar values for the activation energy of the kinetically relevant step (Ea) were observed on the three catalysts, however, the strength with which the relevant intermediate N* binds to the surface, properly represented by the lumped endothermic change of enthalpy (Delta Htotal), resulted significantly higher on the bimetallic phase, which would explain its higher apparent energy (Eapp) and ultimately its lower reaction rate. The kinetic analysis and mechanistic discussion provided in this study could be useful to establishing guidelines in the rational design of catalysts for ammonia decomposition.