Abstract
The ZGB model (Ziff M. R., Gulari E. and Barshad Y., Phys. Rev. Lett., 56 (1986) 2553) for a monomer-dimer catalytic reaction exhibits both second-order and first-order irreversible phase transitions. We report a numerical simulation study of the ZGB model close to coexistence, performed by using the constant-coverage monomer ensemble (CC). By means of CC stationary measurements we found that, in the super-saturated region, there is a phase-transition–like behaviour, for finite systems, between a super-saturated state and a phase where solid monomer droplets coexist with the super-saturated state. Also, we show that the transition point converges, according to a power law behaviour, towards coexistence, so that it is no longer possible to measure any (thermodynamic) spinodal point by using the CC stationary approach. However, by using a dynamic CC ensemble, evidence of the upper spinodal point can clearly be identified: scale invariance of the monomer-droplet size distribution and a maximum in the susceptibility. It is also discussed how to define a spinodal point in the thermodynamic limit for this model.