Noble-metal free catalysts for ammonia dehydrogenation within permeation membrane reactors for improved conversion

Abstract

Ammonia is a carbon-free hydrogen carrier that can be easily transported, but ammonia decomposition catalysts have historically relied on ruthenium. However, less scarce nickel-based catalysts have difficulties in performing ammonia decomposition as efficiently as ruthenium-based catalysts, as nickel-based catalysts are passivated by the hydrogen produced during the reaction. One way to circumvent this issue is to utilize a permeation membrane reactor that removes the hydrogen product during reaction. Here, we report the ammonia decomposition activity of less costly nickel-based trimetallic catalysts at temperature ranges that are compatible with permeation membrane reactors. A variety of nickel-based catalysts were synthesized to test the effect of the alkali (Na, K, Rb, and Cs) and alkaline earth metal (Ca, Sr, and Ba) promoter on the ammonia decomposition efficiency. The performance of the catalyst was highly dependent on the alkali or alkaline earth metal promoter utilized, with Rb demonstrating the strongest promotion. The best performing catalyst, 12%Ni/4%Yb/12%Rb/Al₂O₃ was tested in a permeation membrane reactor and achieved >99.9% ammonia conversion at >30 cm³ NH₃ g_cat⁻¹ min⁻¹. Additionally, the 12%Ni/4%Yb/12%Rb/Al₂O₃ catalysts maintained >90% of its initial activity after exposure to air for six months without reactivation.