Comparison of ¹²CO and ¹³CO TPD curves,showing that ¹³CO desorption islower than ¹²CO at any given temperature, in agreement with the TID experiments (Fig. 4). The TPD curves of both isotopologues were fitted with a single exponential growth function (R² > 0.997) to quantify the zero-order reaction behavior. Beyond 30 K, the partial pressure of CO deviates significantly from the exponential fit because at 30 K all the CO is sublimed into the gas phase and removal of gas-phase CO using conventional turbomolecular pumps is a slow process. The inset shows a zoomed-in view of the portion between 28.5 and 29.5 K.