Localized surface plasmons generated on metallic nanostructures provide an efficient driving force for catalyzing chemical reactions, the kinetics of which cannot be understood properly by means of density functional theory, despite its wide use in simulating heterogeneous catalytic reaction mechanisms. Herein we report reaction pathways for the ammonia decomposition reaction on ruthenium-doped copper studied by the embedded correlated wavefunction method. Our computations provide a qualitative explanation for the experimentally observed change in the reaction order from thermal catalysis to hot-carrier-mediated photocatalysis, as reported very recently in Zhou, L.; et al. Science 2018, 362, 69.