We　investigated　the　catalytic　NH3decomposition　on　Ru　and　Ir　metal　surfaces　using　density　functional　theory.　The　reaction　mechanisms　were　unraveled　on　both　metals,　considering　that,　on　the　nano-scale,　Ru　particles　may　also　present　an　fcc　structure,　hence,　leading　to　three　energy　profiles.　We　implemented　thermodynamic　and　kinetic　parameters　obtained　from　DFT　into　microkinetic　simulations.　Batch　reactor　simulations　suggest　that　hydrogen　generation　starts　at　400　K,　425　K　and　600　K　on　Ru(111),　Ru(0001)　and　Ir(111)　surfaces,　respectively,　in　excellent　agreement　with　experiments.　During　the　reaction,　the　main　surface　species　on　Ru　are　NH,　N　and　H,　whereas　on　Ir(111),　it　is　mainly　NH.　The　rate-determining　step　for　all　surfaces　is　the　formation　of　molecular　nitrogen.　We　also　performed　temperature-programmed　reaction　simulations　and　inspected　the　desorption　spectra　of　N2and　H2as　a　function　of　temperature,　which　highlighted　the　importance　of　N　coverage　on　the　desorption　rate.　©　The　Royal　Society　of　Chemistry　2021.