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 CO and O2 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 states 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 CO2 is calculated as a function of the dilution of sites.

We study the model proposed by Ziff, Gulari, and Barshad to mimic the oxidation of carbon monoxide (CO) in the presence of fixed impurities distributed over the catalytic surface. Our focus is on the continuous phase transition between the active phase, where occurs the production of carbon dioxide (CO2), and the inactive phase, where all the non inert sites become filled with oxygen molecules. We employ Monte Carlo simulations to calculate the different ratios between moments of the order parameter at the critical point, as well as, we determine the critical exponents β and ν⊥ as a function of the concentration of impurities. We show that the presence of impurities over the catalytic surface changes the critical behavior of the system. The critical exponents depend on the concentration of impurities and the model does not belong to the directed percolation universality class.

We studied the continuous phase transition between the active and the absorbing state of the Ziff-Gulari-Barshad (ZGB) model. Through Monte Carlo simulations we determined all the moments of the order parameter up to fourth order and their ratios at the critical point. We show that the ratios we found are in agreement with those of the contact and pair contact processes in two dimensions, which give support to the idea that the ZGB model is in the directed percolation universality class in (2+1) dimensions.

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