Dissociative adsorption of molecular oxygen on the Cu(001) surface: a density functional theory study

The presence of atomic oxygen on catalytic surfaces is essential for initiating the oxidation of hydrogen chloride to produce chlorine via the so-called Deacon process. This process provides molecular chlorine for the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in combustion. In this paper, the dissociative adsorption of molecular oxygen on the Cu(001) surface has been studied using density functional theory. A periodic p(3X2) 4 layer slab was adopted to simulate the adsorption of both molecular and atomic oxygen at a number of adsorption sites. We have found that a bridge-bridge configuration is the most stable structure on Cu(001) with the O₂ molecule adsorbed horizontally. The activation barrier for the dissociative adsorption of O₂ resulting from this configuration was calculated to be 5.1 kcal/mol, with an equivalent transition temperature of ~66 K. This is in good agreement with the experimental value of 40 K obtained under ultra high vacuum conditions. We have also found that a less energetically favourable, vertically oriented, physisorbed structure leads to an almost negligible reaction barrier for the dissociative adsorption of O₂ on Cu(001) (1.5 kcal/mol), with an equivalent transition temperature of ~20 K.