  A random distribution of inert sites is introduced in the Ziff-Gulari-Barshad
model to study the phase transitions between active and poisoned states. The
adsorption of ${\rm CO}$ and ${\rm O_2}$ molecules is not possible at the
position of the inert sites. This model is investigated in the site and pair
approximations, as well as through Monte Carlo simulations. We determine the
mean coverages of the elements as a function of the dilution and show that the
continuous transition between the active and O-poisoned state is slightly
affected by moderate values of dilution in the pair approximation and in the
simulations. On the other hand, from the analysis of the hysteresis curves, the
transition between the active and CO-poisoned states changes from first-order
to continuous as one increases the concentration of inactive sites. The
observed transition in the site and pair approximations is always of
first-order nature. We also found the lines of transition and spinodal points
as a function of the concentration of inert sites. Finally, the production rate
of ${\rm CO_2}$ is calculated as a function of the dilution of sites.
