(121b) CuO/Co3O4: A Highly Active and Selective Low Temperature NO Decomposition Catalyst

The direct decomposition of NO is thermodynamically favorable within the temperatures encountered by an automotive catalytic converter; however, activity remains too low for practical application [1]. Platinum group metals (PGM), such as Pd, are active for NO decomposition at temperatures above 600 °C, and attempts to improve their activity below 600 °C have been unsuccessful due oxidation of the metal and subsequent deactivation [2]. Regarding non-PGM catalysts, Cu-ZSM5 is perhaps the most widely known material displaying low temperature (≤ 500 °C) direct NO decomposition activity, however it has propensity to produce an undesired side product: the greenhouse gas N₂O [3]. The spinel oxide Co₃O₄ does not exhibit direct NOx activity as high as Cu-ZSM5, but even at 350 °C it is able to decompose 50% of N₂O to N₂ [4,5]. To address the performance gap below 500 ËšC, we therefore propose combining the benefits of both CuOx and Co₃O₄ into a single NO decomposition catalyst. Supported copper oxide on cobalt oxide CuOx/Co₃O₄ was synthesized by incipient wetness impregnation. We hypothesized that this material can utilize the N₂O decomposition capabilities of Co₃O₄ to reduce the N₂O generated by NO decomposition over the CuOx species.

The CuOx/Co₃O₄ were synthesized with Cu surface densities ranging from 0.7 to 10.3 Cu/nm² and characterized by XRD, XPS, and TEM. The Cu surface density corresponding to the monolayer loading for CuOx/Co₃O₄ was found to be 6.7 Cu/nm², therefore, the catalysts set included sub-monolayer, monolayer, or 3D-microcrystalline CuOx species. Thus, this catalyst set formed a strong basis for identifying a direct NOx decomposition structure-activity relationship. The catalysts were evaluated for direct NOx decomposition activity using a packed bed reactor. The maximum activity for NO decomposition by CuOx/Co₃O₄ was found at the CuOx surface density corresponding to monolayer. There is a subsequent decrease in the activity at higher surface densities due to the presence of CuOx microcrystals. When compared to Cu-ZSM5, the specific activity from 300-400 °C is similar, however, the surface area normalized (areal) activity exceeds that of Cu-ZSM5 as the CuOx/Co₃O₄ has a specific surface area that is an order of magnitude lower. Additionally, at or below 400 °C, the N₂ selectivity is higher over CuOx/Co₃O₄ compared to Cu-ZSM5, as a result of decreased N₂O formation. The CuOx/Co₃O₄ catalysts maintain relatively good activity in the presence of O₂ and CO₂. Nitric oxide adsorption/desorption reveals an optimized adsorption strength over CuOx/Co₃O₄. Additional mechanistic insight from in situ FTIR will also be presented.

References:

1. H. Falsig, T. Bligaard, C.H. Christensen, J. K. Norskov, Pure Appl. Chem., Vol. 79, No. 11, pp. 1895–1903, 2007.

2. G.K. Reddy, C. Ling, T. Peck, H. Jia, RSC Adv., 2017, 7, 19645

3. B. Moden, P.D. Costa, D. K. Lee, E. Iglesia, J. Phys. Chem. B 2002, 106, 9633-9641

4. P.W. Park, J.K. Kil, H.H. Kung, M. C. Kung, Catal. Today, Volume 42, Issues 1-2, June 1998, 51-60.

5. L. Xue, H. He, C. Liu, C. Zhang, and B. Zhang. Environ. Sci. Technol., 2009, 43 (3), 890-895.