(200c) DFT Study of Adsorption and Hydrogenated Dissociation of CO2 On MgO Surface
AIChE Annual Meeting
Monday, November 4, 2013 - 3:15pm to 5:45pm
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 (EB) of the rate determining steps, CO2 could easily adsorb and hydrogenated dissociated on MgO (1 1 1)Mg surface (EB = 0.37 eV), followed by MgO (1 1 0) (EB = 0.65 eV), (2 1 1) (EB = 0.67 eV) and (2 1 0) (EB = 1.04 eV) surfaces.