Physica A 268 (1999) 89–102 www.elsevier.com/locate/physa Kinetic phase transitions in dimer–dimer irreversible catalytic surface reactions of the type: A 2 + xB 2 → 2AB x K.M. Khan ∗ , K. Yaldram, N. Ahmad, Qamar-ul-Haque Nuclear Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 4 June 1998; received in revised form 15 January 1999 Abstract The kinetics of irreversible dimer–dimer surface reactions of the type A2 + xB2 → 2ABx (x takes values 1, 2 and 3), with desorption of dimer B2, has been investigated by means of Monte Carlo simulation. For a desorption probability (P) of B2 equal to zero, a single discontinuous transition separating one saturated state from another is obtained at a point where feed con- centration is exactly related to the stoichiometry of the particular reaction. With the increase in P, a steady reactive state which is separated from the poisoned states by two transitions is obtained for each reaction scheme. The position of the transition points depends upon the value of P. For P equal to one, a single continuous transition point separates a poisoned state from a steady reactive state for each reaction scheme. The window width, which denes the steady reactive state, shows an exponential behaviour with P. c  1999 Elsevier Science B.V. All rights reserved. 1. Introduction Catalytic reactions on solid surfaces are of great complexity and they are thus inher- ently very dicult to deal with. The detailed understanding of such reactions is very im- portant in applied research, but rarely has such a detailed understanding been achieved either from experiment or from theory. In recent years a number of computer models have been proposed to study catalytic surface reactions. One simple monomer-dimer (MD) model is that proposed by Zi et al. [1] and is known as the ZGB model. This model has been used to study CO–O 2 reaction system. In this model a monomer (CO) * Corresponding author. Fax: +92-51-9290275. E-mail address: pinstech@paknet2.ptc.pk (K.M. Khan) 0378-4371/99/$ – see front matter c  1999 Elsevier Science B.V. All rights reserved. PII: S0378-4371(99)00040-0