  We study by kinetic Monte Carlo simulations the catalytic oxidation of carbon
monoxide on a surface in the presence of contaminants in the gas phase. The
process is simulated by a Ziff-Gulari-Barshad (ZGB) model that has been
modified to include the effect of the contaminants and to eliminate the
unphysical oxygen-poisoned phase. The impurities can adsorb and desorb on the
surface, but otherwise remain inert. We find that, if the impurities can not
desorb, no matter how small their proportion in the gas mixture, the first
order transition and the reactive window that characterize the ZGB model
disappear. The coverages become continuous, and once the surface has reached a
steady state there is no production of CO$_2$. This is quite different from the
behavior of a system in which the surface presents a fixed percentage of
impurities. When the contaminants are allowed to desorb, the reactive window
appears again, and disappears at a value that depends on the proportion of
contaminants in the gas and on their desorption rate.
