(200c) DFT Study of Adsorption and Hydrogenated Dissociation of CO2 On MgO Surface

MgO was widely selected as support or promoter for catalyst in dry reforming of methane because of its basic properties, and showed favorable catalytic activity and stability. However, it was proved that CO₂ could only physical adsorb on MgO (1 0 0) surface which was dominated on MgO powder. The experiment researches showed Mg-O ion pairs of low coordination number which existed at corners, edges, vacancies of the MgO (1 0 0) plane or high Miller index surfaces were the basic sites and adsorbed CO₂ strongly. More kinds of MgO powder with various dominated surfaces were recently produced by different preparation methods. In this paper, first-principles calculations based on density functional theory (DFT) have been used to calculate the CO₂ adsorption on six widespread MgO surfaces, and the reaction pathways of the dissociation of adsorbed CO₂ with H have also been investigated. The results show that CO₂ could be chemisorbed on the perfect MgO (1 1 0), MgO (2 1 0), MgO (2 1 1) and Mg atoms polarized MgO (1 1 1) [MgO (1 1 1)_Mg] surfaces, and CO₂ is hard to direct dissociate. On the other hand, CO₂ could decompose to CO and O species on these four MgO surfaces if atomic H exists, and the hydrogenated dissociation mechanisms were diverse in the different MgO surfaces. When the free atomic H attacks adsorbed CO₂, the adsorbed COOH is the key intermediate on MgO (1 1 0), (2 1 0) and (2 1 1) surface, and it would only be dissociated to CO and OH on the (1 1 0) and (2 1 1) surface. On MgO (1 1 1)_Mg surface, the adsorbed COOH does not exist and CO₂ could directly dissociate when atomic H attacks. When free CO₂ adsorbed on the partial H covered MgO (1 1 1) surface, CO₂ could dissociated adsorb and became adsorbed CO and O. CO₂ could adsorb and transform into surface COOH on partial H covered MgO (1 1 0), (2 1 0) and (2 1 1), but the process is hard to occur on MgO (2 1 1) surface. Campare with the energy barriers (E_B) of the rate determining steps, CO₂ could easily adsorb and hydrogenated dissociated on MgO (1 1 1)_Mg surface (E_B = 0.37 eV), followed by MgO (1 1 0) (E_B = 0.65 eV), (2 1 1) (E_B = 0.67 eV) and (2 1 0) (E_B = 1.04 eV) surfaces.