Computational efforts towards a fundamental understanding of the underlying mechanistic pathways in synthesis gas conversion processes such as Fischer-Tropsch synthesis are exemplary for the developments in heterogeneous catalysis. Advances in transient kinetic analysis methods contribute to unraveling complex reaction pathways over nanoparticle surfaces. Tracing the activity and selectivity of Fischer-Tropsch catalysts to the individual events occurring at the active site remains difficult with experimental techniques. Here we provide simulations of transient kinetics at the scale of the active site by making use of the reaction energetics for CO hydrogenation to methane on stepped and terrace cobalt surfaces that are suitable models for cobalt FT nanoparticle catalysts. We investigate the hydrogen-deuterium kinetic isotope effect and simulate common steady-state and chemical isotopic transients. Comparison to experimental literature leads to important mechanistic insights. Direct CO dissociation is the main pathway for breaking the C[sbnd]O bond and it occurs exclusively on step-edge sites. While the experimentally observed hydrogen-deuterium kinetic isotopic effect is often used as evidence for H-assisted CO dissociation, we show that hydrogenation of C and O as partly rate-controlling steps provides an alternative explanation. The simulations of the chemical transients provide significant insight into the importance of the changing surface coverages that strongly affect the reaction rate. The reversibility of CO dissociation on cobalt step-edges is evident from simulations of 12C 16O/ 13C 18O scrambling being in good agreement with experimental data.