The Ziff–Gulari–Barshad (ZGB) model is a simple Monte Carlo method for catalytic reactions of oxidation of carbon monoxide to carbon dioxide on a surface using Monte-Carlo methods which captures correctly the essential dynamics: phase transitions between two poisoned states (either CO2- or O-poisoned) and a steady-state in between.
It is named after Robert M. Ziff, Erdogan Gulari, and Yoav Barshad, who published it in 1986.
[2] When a gas-phase molecule touches an empty site, adsorption occurs immediately and the chemical reaction is also instantaneous.
While these requirements would still allow a large number of models and corresponding behaviors, the two special assumptions of the ZGB model are: (i) CO molecules are adsorbed "standing" with the O touching the surface, and require thus only one free lattice site; (ii) O2 molecules are adsorbed "flat" and require thus two adjacent free lattice sites for getting adsorbed.
The continuous transition belongs to the universality class of directed percolation.