First-Principles Studies of NOxChemistry on Agn/α-Al2O3

NO x chemistry on Al 2 O 3 and Ag n /Al 2 O 3 ( n ≤ 4) is investigated by density functional theory calculations. Three different terminations of α-Al 2 O 3 (0001) are considered, namely Al-terminated, fully hydroxyl-terminated, and partially oxygen-terminated. Weak adhesion energies are reported for Ag n supported on Al- and hydroxyl-terminated surfaces, whereas Ag n supported on partly oxygen-terminated surfaces are strongly bonded. This is connected to the charge state of the clusters: Ag n clusters supported on Al- and hydroxyl-terminated surfaces remain close to an Ag 0 state, whereas Ag n clusters on partially oxygen-terminated surfaces are in an oxidized state. Adsorption properties of several reaction intermediates relevant for NO x chemistry, such as O, NO, NO 2, and NO 3, are investigated. Marked variations in adsorption energy are calculated as a function of cluster size. For Al- and partial O-termination there is a strong preference to adsorb at the edge of the Ag n cluster. On the basis of the present results, the active phase of the Ag/Al 2 O 3 selective catalytic reduction (SCR) catalysts is proposed to be small Ag n clusters incorporated into the alumina matrix or adsorbed at Al-defect sites. Such conformations allow the Ag n clusters to be in an oxidized state, which is beneficial for NO x chemistry.