― 28 ―SENAC Vol. 56, No. （3 2023. 7）[ 共同研究成果 ]Self-consistent van der Waals density functional study of NO-H2O
c o ad so r pt i on o n C u (1 11 )Thanh Ngoc Pham,* and Yoshitada Morikawa
Department of Precision Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka Suita, Osaka 565-0871
Email: thanh@cp.prec.eng.osaka-u.ac.jp
In this project, we have studied the reaction mechanisms in NOx purification catalysts using firstprinciple calculations. We have studied (i) atomistic insights into the co-adsorption behavior of NO with
H2O on Cu(111) [1] and (ii) metal-support interaction of supported Pd catalysts revealed by machinelearning enhanced global optimization [2]. In this report, we discuss the importance of intermolecularinteraction in co-adsorption of NO and H2O by means of self-consistent van der Waals density functional.
NOx purification catalysts are essential in three-way catalyst converters to reduce automotive exhaustNOx gas to non-toxic N2 gas [1]. Recently, Cu catalyst has been received much attention to replace scareand expensive platinum group metal (Pt, Pd, and Rh) catalysts due to its excellent performances in NOreduction. Elucidating reaction mechanism of NO reduction on Cu catalyst is important to improve thecatalytic activity and selectivity of this catalyst.
As the model for the Cu catalyst, NO adsorption and reaction on well-defined Cu surfaces have beenstudied. The interaction between NO and Cu surfaces are well studied but the interaction of NO with othercommon gases in TWC likes H2O, NH3, and H2 remains ambiguously, which hinders our design of selectiveNO reduction systems [3]. In particular, elucidating the interaction between adsorbed nitric oxide (NO) andwater (H2O) on metal surfaces is of paramount importance to uncover mechanistic details of theircompetitive coadsorption behavior and to guide the design of new NOx purification catalysts under wetcondition.
Recently, Koshida et. al observed that the H2O exposure to (NO)3-preadsorbed Cu(111) results in theformation of mixed NO-H2O complexes by scanning tunneling microscopy techniques [3]. We herein focus
on elucidating the NO-H2O interaction strength and compared that to NO-NO and H2O-H2O binding
strengths to prove that the mixed NO-H2O complexes is formed rather than separated NO and H2O clusters
on Cu(111). To this end, we constructed several mixed NO-H2O complexes and compared their binding
strengths to binding strengths of NO trimer and water hexamer. The adsorption energy of mixed nNOmH2O complex is defined as
𝐸𝐸𝐸𝐸ads = 𝐸𝐸𝐸𝐸 (𝑛𝑛𝑛𝑛NO − 𝑚𝑚𝑚𝑚H2 O) − [𝐸𝐸𝐸𝐸 (Cu) + 𝑛𝑛𝑛𝑛𝐸𝐸𝐸𝐸 (NO) + 𝑚𝑚𝑚𝑚𝐸𝐸𝐸𝐸(H2 O)],(1)
where 𝐸𝐸𝐸𝐸 (𝑛𝑛𝑛𝑛NO − 𝑚𝑚𝑚𝑚H2 O), 𝐸𝐸𝐸𝐸 (Cu), 𝐸𝐸𝐸𝐸 (NO), and 𝐸𝐸𝐸𝐸(H2 O) are total density functional theory (DFT) energies
of adsorbed nNO-mH2O, clean Cu(111), isolated NO, and H2O, respectively.
The DFT calculations were carried out by using the simulation tool for atom technology (STATE)
package [4]. We employed ultrasoft pseudopotentials to describe the electron-ion interactions with Cu 3d
4s, N 2s 2p, O 2s 2p, and H 1s valance states. Valance states were expanded by using plane wave basis-set
with cutoff energies of 36 and 400 Ry for wave functions and augmented charge density, respectively. We
used self-consistent van der Waals density functional (vdW-DF) method and the optB86b-vdW functional
is adopted [5]. This approach is proved to provide an reasonable accuracy for adsorption and reaction ofNO on three low-indexed Cu surfaces, namely Cu(100), Cu(110), and Cu(111) as indicated in our previousworks [6,7].
First, we study the adsorption of small NO and H2O clusters on Cu(111). We obtain the most stable sites
for NO monomer and H2O monomer are fcc-hollow and atop site with 𝐸𝐸𝐸𝐸ads of –1.325 and –0.261 eV,respectively. We find that NO tend to form NO trimer ((NO)3) on Cu(111) and (NO)3 is more stable thanseparated three NO monomer by –0.123 eV. The adsorbed H2O also tends to aggregate to form small
clusters on Cu(111). The binding strength of water hexamer ((H2O)6) is –1.369 eV w.r.t. separated six H2O
monomer. Atomic structures of (NO)3 and (H2O)6 are shown in Figure 1.

Self-consistent van der Waals density functional study of NO-H₂O coadsorption on Cu(111) ― 29 ―Figure 1. Atomic structures of NO trimer (NO)3, water hexamer (H2O)6, and mixed 4NO-3H2O
complex. Color: Cu, orange; N, blue; O, red, and H, white.
Then, we investigate several mixed nNO-mH2O complexes (n = 1–4 and m = 1–3) and the largest mixed
complex, i.e. 4NO-3H2O is shown in Figure 1. The calculated 𝐸𝐸𝐸𝐸ads of 4NO-3H2O is −7.424 eV, being morestable than separated 4/3(NO)3 + 1/2(H2O)6 state (−6.933 eV) by −0.491 eV. We find that adsorbed NOtends to received two hydrogen bonds from two water molecules and those adsorbates aggregate to form
large mixed nNO-mH2O clusters on Cu(111). Our results imply that the mixed nNO-mH2O complex is morestable than NO trimer and H2O hexamer. Intermolecular interactions in nNO-mH2O surpass the NO-NO
and H2O - H2O interactions, leading to the formation of mixed nNO-mH2O complex adsorbed on Cu(111).
Our finding is consistent with experimental STM observation [3] and well explains their results.
Intermolecular interactions in nNO-mH2O composes of NO-H2O and NO - NO interactions. The former ishydrogen bond where negatively-charged NO driven by back donation process from Cu(111) interacts
attractively with H2O via H-bond. The latter is the covalent interactions where 2π* orbitals hybridized atnear Fermi level.
In summary, we employed Supercomputer AOBA installed at the Cyberscience Center, Tohoku
University to study atomistic insights into the co-adsorption behavior of NO with H2O on Cu(111). Detailed
results are published in reference [1].
[1] T. N. Pham, Y. Hamamoto, K. Inagaki, I. Hamada, and Y. Morikawa, Phys Rev Mater. 6, 075801 (2022).
[2] T. N. Pham, B. A. C. Tan ,Y. Hamamoto, K. Inagaki, I. Hamada, and Y. Morikawa, submitted forpublication.
[3] H. Koshida, S. Hatta, H. Okuyama, A. Shiotari, Y. Sugimoto, and T. Aruga, J. Phys. Chem. C 122, 8894
(2018).
[4] Y. Morikawa, Phys. Rev. B 51, 14802 (1995).
[5] Y. Hamamoto, I. Hamada, K. Inagaki, and Y. Morikawa, Phys. Rev. B 93, 245440 (2016).
[6] K. Kuroishi, M. R. Al Fauzan, T. N. Pham, Y. Wang, Y. Hamamoto, K. Inagaki, A. Shiotari, H.
Okuyama, S. Hatta, T. Aruga et al. Phys. Chem. Chem. Phys. 23, 16880 (2021).
[7] T. N. Pham, Y. Hamamoto, K. Inagaki, D. N. Son, I. Hamada, and Y. Morikawa, J. Phys. Chem. C 124,2968 (2020).