Theoretical study of the influence of Na on CO adsorption and dissociation on Pd(1Â 1Â 1): Long-range or short-range interactions between co-adsorbates?

► Both short-range and long-range effects are demonstrated in (Na, CO) co-adsorption system. ► The long-range effect is insufficient to destabilize the C-O bond to an extent for CO dissociation. ► The Na-promoted CO dissociation is mainly ascribed to the short-range O-Na attraction.