Mechanistic Studies On the Size Effect of Cobalt Nanoparticles On CO Oxidation

Recently, CO oxidation using cobalt has been the subject of attention due to the relatively cost of cobalt compared to the noble metal counterparts [1] [2] [3] [4]. A systematic understanding of the catalytic activity of cobalt metal nanoparticles, especially the role of the nanoparticle size, is lacking.  Experimental studies using stable cobalt nanoparticles of sizes 1 to 14 nm showed a clear size dependence of the kinetic parameters such as pre-exponential factor and activation energy on nanoparticle size. The surface of cobalt nanoparticles was oxidized to CoO by exposure to air prior to the kinetic studies using in-situ FTIR.  

In this theoretical study we investigate the CO oxidation on a CoO surface using Density Functional Theory (DFT) calculations.

 Three possible mechanisms were considered for CO oxidation on CoO, namely:

1. CO(ads) + O_L --> CO₂(g) + O_V

2. CO(ads) + O₂(ads) --> OOCO(ads) --> O(ads) + CO₂(ads)

3. CO(ads) + O₂(ads) --> O(ads) + OCO(ads) --> CO₂ (g) + O(ads)

The three mechanisms were investigated to calculate the barriers for CO oxidation and the mechanism with OCO intermediate was found to be in agreement with the experimental barriers. The size effect on the activation barriers will also be discussed. In this work, VASP (Vienna Ab Initio Simulation package) code [5-7] with Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA) [8] functional was used for the exchange and correlation functional. DFT+U method by Dudarev et al.[9] was used in spin polarized DFT-PBE calculations which accurately treats the strongly localized d or f electrons.

References

                (1)          Jiang, D.-e.; Dai, S. Phys. Chem. Chem. Phys. 2010, 13, 978-984.

                (2)          Chen, Y.-J.; Wu, D.-e.; Yeh, C.-t. Rev. Adv. Mater. Sci. 2003, 5, 41-46.

                (3)          Broqvist, P.; Panas, I.; Persson, H. J. Catal. 2002, 210, 198-206.

                (4)          Wang, C.-B.; Tang, C.-W.; Tsai, H.-C.; Kuo, M.-C.; Chien, S.-H. Catal. Lett. 2006, 107, 31.

                (5)          Kresse, G.; Furthmuller, J. Comp. Mater. Sci. 1996, 6, 15-50.

                (6)          Kresse, G.; Furthmuller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169-11186.

                (7)          Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558-561.

                (8)          Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868.

                (9)          Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B: Condens. Matter 1998, 57, 1505.