Designing a Precious Metal-Free Catalyst for Purification of Automotive Exhausts: The Dissociation of NO on Cu2O(111) and CuO(110) Surfaces

Nitrogen oxide (NO_x) is a by-product of fossil fuel combustion and forms nitric acid in the air, which is the main reason for acid rain and smog. The interaction of NO_xmolecule on catalysts is the rate-limiting step for its reduction process and is the motivation for recent investigations [1, 2]. Three-way catalysts, which are composed of precious metals such as Rh, Pd and Pt, work well for such purpose; however, their efficiency is compensated by their expensive cost. Therefore, a more readily available and inexpensive material with comparable, if not better, catalytic performance is a major concern among automobile companies and researchers.

In our recent work [3-6], we established the promising performance of Cu-based catalysts for the dissociation of NO by employing Computational Materials Design® based on density functional theory. Our results show that NO dissociation on Cu-terminated Cu₂O(111) and Cu-terminated CuO(110) surfaces is comparable with Rh(111) [4], which is characterized by a transition state lying below the reference energy (surface and gas phase NO). This phenomenon is attributed to the modified electronic structure of the surface Cu atoms of Cu oxide surfaces in comparison with that of Cu(111). Specifically, the initially low-lying density of states (d orbital) of the surface Cu atoms in Cu(111) are shifted towards the Fermi energy in the Cu oxide systems. In comparison with Rh(111), the calculated NO dissociation barriers are lower on Cu oxide catalysts. Furthermore, the binding energy of co-adsorbed N and O atoms is weaker on Cu oxides, which is favorable for subsequent reactions (Fig. 1). Likewise, our experimental collaborators have verified that Cu oxides can be better catalysts than Rh, Pd and Pt for the purification of exhaust gases.

Indeed, this study has gained recognition as it ranked first in the Technology Trend Survey (Nikkei Shinbun) and was also publicly acknowledged in Japan. The details of our work, as well as the oxidation of CO in the presence of dissociated NO on Cu oxide surface will be discussed in the meeting.

References:

[1] F. Oemry, H. Kasai et al., J. Nanosci. Nanotechnol.11 (2011) 2844.

[2] N.T. Quang, H. Kasai et al., Comput. Mater. Sci. 47 (2009) 111.

[3] H. Kasai, A.A.B. Padama et al., J. Jpn. Petrol. Inst. 56 (2013) 357.

[4] H. Kishi, H. Kasai et al., J. Phys.: Condens. Matter 24 (2012) 262001.

[5] A.A.B. Padama, H. Kasai et al., J. Phys.: Condens. Matter 24 (2012) 175005.

[6] J.L.V. Moreno, A.A.B. Padama, H. Kasai, CrystEngComm (2013) submitted.