Paper IS+AS+SA+SS-MoA3
Catalysis on Singly Dispersed Bimetallic Sites on Oxide Support

Monday, October 19, 2015, 3:00 pm, Room 211C

Reaction events of heterogeneous catalysis occur on specific catalytic sites. Atoms of a catalytic site arrange in a specific geometric/electronic configuration for adsorbing/dissociating reactant molecules and subsequent coupling to form product molecules. Bimetallic catalysts play significant roles in chemical and energy transformations due to their tunable catalytic properties through ligand, geometric, bi-functional, or lattice strain effect.

When a bimetallic site (M₁A_n, M and A: metal elements, n ≥ 1) is one of the continuous sites on the surface of a bimetallic NP, this site is in a metallic state. However, when M₁A_n­ sites are separately anchored on a surface of a transition metal oxide support, these isolated bimetallic sites are in cationic state. Such change in electronic structure could cause these bimetallic sites to have stronger chemisorption to reactant or/and intermediate molecules, thus facilitating its dissociation and subsequent coupling. In addition, singly dispersion of metal M in M₁A_n minimizes the potential binding configurations of reactant molecules hence may enhance catalytic selectivity toward a specific reaction pathway. Here we present singly dispersed bimetallic catalyst Rh₁Co₃ prepared on Co oxide support, which exhibits 100% selectivity for the production of N₂ in NO reduction with CO.

Preparation of isolated bimetallic sites Rh₁Co₃ on Co₃O₄ nanorods begins with the formation of hydroxide species Rh(OH)_n on the surface of Co₃O₄, followed by calcination at 150ºC in O₂ to form Rh-O-Co bonds between singly dispersed Rh(OH)_n species and the surface of Co₃O₄, and concluded with a carefully controlled reduction to remove oxygen atoms between Rh and Co and thus a simultaneous formation of Rh-Co bonds. In-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to monitor the oxidation and reduction steps and to avoid over or under-reduction.

Formation of singly dispersed Rh atoms was visualized using HAADF-STEM. The bonding environment of Rh to three Co atoms was confirmed using in-situ EXAFS. For reduction of NO with CO, Rh₁Co₃/CoO exhibits high activity at 110 ºC with 100% selectivity toward N₂ production. In contrast, Rh-Co alloy NP/CoO has much lower activity and selectivity (10%) under the same condition. In-situ AP-XPS investigation shows that Rh atoms are at cationic state instead of metallic state. Along with this, DFT calculations suggest that a strong adsorption of intermediate N₂O molecules on Rh₁Co₃ site prevents its desorption as a byproduct and provides a dissociation pathway of N₂O to N₂ with a low activation barrier (~0.21 eV), thus leading to a 100% selectivity to N₂ production.