Thermodynamics and phase behavior of CO on Pt(100): Grand canonical Monte Carlo simulations of the 2D Ising model

This study investigates the thermodynamic properties of carbon monoxide (CO) adsorption on the Pt(100) surface using grand canonical Monte Carlo simulations of a two-dimensional square Ising model. Using finite-size scaling analysis for system sizes N = 10 − 50, we determine the pseudocritical temperature of the order–disorder transition to be T C 2.2856(7) (in units of |J|/k B ), in good agreement with Onsager’s exact solution. Subsequently, a lattice of size 50×50 with nearestneighbor repulsive interactions (J < 0) is employed to simulate adsorption isotherms and isobars and to characterize the phase behavior of the adlayer. The simulations successfully reproduce the formation of the ordered c(2×2) phase at half-coverage (θ = 0.5) in the low-temperature regime. Furthermore, the isosteric heat of adsorption, extracted from the simulated isotherms, exhibits a characteristic steplike decrease near the critical coverage, directly reflecting the effect of lateral repulsive interactions. These results provide a consistent microscopic description of the thermodynamic properties and phase behavior of the CO/Pt(100) system.