In situ generation of CO_x-free H₂ by catalytic ammonia decomposition over Ru-Al-monoliths

Ru catalysts supported on alumina coated monoliths has been prepared employing three different precursor, which are ruthenium chloride, ruthenium nitrosyl nitrate and ruthenium acetyl acetonate, by an equilibrium adsorption method. The Ru particle sizes could be controlled varying the metal precursor salt. Among the prepared catalysts, Ru catalyst prepared from nytrosyl nitrate exhibited the highest activity which is concomitant to the largest mean Ru particle size of 3.5 nm. The values of the apparent activation energy calculated from the Arrhenius equation are according to the Temkin-Phyzev model, indicating that the recombinative desorption of N ad-atoms is the rate-determining step of the reaction. However, the ratio between the kinetic orders with respect to ammonia and hydrogen (−α/β), is not in agreement to the valued predict by Temkin formalism. This fact could be related to the different operational conditions used during the reaction, and/or catalyst nature, but not to any change on the controlling step of the reaction.