We show that catalyst pre-treatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotubes chemical vapor deposition. Using a SiO2-supported Cobalt model catalyst and pre-treatment in NH3, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of nanotube nucleation is of fundamental importance for chiral control, thus identifying the pre-treatment atmosphere as a key parameter for control of diameter and chirality distributions.