Non-equilibrium plasma has been found to have a synergistic effect on catalytic synthesis of NH₃. The non-equilibrium plasma and catalyst surface together could affect NH₃ synthesis through several mechanisms. Charging of the catalyst surface in the presence of non-equilibrium plasma is one such mechanism. We employed density functional theory (DFT) calculations to understand the effect of surface charge on surface reactivity of γ-Al₂O₃ supported single metal atom catalysts and a metal cluster. We investigated the effect of surface charge on adsorption energies of common adsorbates involved in NH₃ synthesis. It is found that adsorption energy of N, N₂, H, H₂, NH and NH₂ on metal atoms increases by up to ∼1.2 eV, whereas NH₃ desorption is increased by up to 0.45 eV upon surface charging. The present results provide a new mechanism of plasma enhanced catalysis potentially explaining why Ni, Pt and Co have better catalytic performance compared to Ru and Fe in ammonia plasma catalysis. Furthermore, we found that the correlations between adsorption energies of adsorbates change significantly with surface charging. These findings suggest that surface charging might play an important role in plasma synthesis of NH₃.