The role of electron localization in the catalytic function of cerium-oxide-based systems

Abstract

Cerium oxide (Ce02, ceria) surfaces are important for ma ny appl ications, particula rly catalysis. The importance relies to a large extent on its facile reducibility and the associated ability to release lattice oxygen. On removing an O atom, two electrons are left behind that form two reduced Ce3+ species. The electrons occupy split-off states of the initially empty Ce 4f band, Jying inside the 02p-Ce5d band gap of ceria and bei ng highly localized in space [l ]. In this work, we apply density-functional theory (DFT) with the DFT+U approach and show that the ability of ceria to stabilize reduced states is at the origin of the unexpected cata lytic activity of ceria-based systems such as ceria surfaces for the conversion of alkynes to olefins [2], and of ceria supported Ni nanoparticl es for H2 production [3]. DFT simulations of acetylene hydrogenat ion on Ce02(1 l l ) provide molecular-leve! insight into the active site and reaction mechanism, where the formation of highly reactive C2H2 radical species is found to be essential [4], and help rationalize the applicabi l ity of Ce02 as a catalyst for olefi n production. Theoretical Ni/Ce02(l l l) model catalysts revea!strong Ni-ceria interactions -leading to Ni2+ and Ce3 species- as the key factor responsible of a not too weaken C-0 bond upon CO adsorption and a low H20 dissociation barrier , both making the Ni/Ce0 2 system attractive for the production of hydrogen vía the water­ gas shift (CO+H20C02+H2) reaction .