KINETIC PHASE-TRANSITIONS IN THE A-B-2-D SURFACE-REACTION MODEL

A monomer-dimer-monomer (A-B-2-D) lattice surface reaction model is pr oposed, in which two adsorption parameters X(A) and X(D) are involved. The phase diagram for this model is determined by the partial simulat ion method. For the case of no desorption being considered, six phases are found, which are the two-species saturated phases S-AD, S-BD, S-B E and S-AE (E denotes the intermediate product AB), the one-species sa turated phase Sg and the steady reactive phase R. The R and S-B phases are restricted to the line X(A) = X(D). Phase transitions between two -species phases or between R and S-B are all second order, while that between R and S-AD is first order. It appears that there exists a pent acritical point at X(A) = X(D) = 0.023 +/- 0.001. If D desorption is taken into account, the reactive phase R extends to a region of finite area.