Three lattice gas models for A2 + B2 -->2AB reaction system are studied by Monte Carlo simulation in two-dimensional triangular lattice surface. When both A2 and B2 adsorb and dissociate on the catalytic surface in the random dimer-filling mechanism or in the end-on dimer-filling mechanism, there is no reactive window in the reaction system and just a discontinuous phase transition appears from a "B+vacancy" poisoned state to an "A+vacancy" poisoned state. However, a reactive window appears when one of the two dimers, A2, dissociates in the random dimer-filling mechanism but another dimer, B2, is in the end-on dimer-filling mechanism and the system exhibits a discontinuous phase transition from the active reaction state to a B+ vacancy poisoned state and a continuous phase transition to an "A+vacancy" poisoned state. Furthermore, we show that the critical behavior of the continuous phase transition with infinitely many absorbing states belongs to the robust directed percolation universality class.