(208a) Adsorption and Activation of O2 over Cu(I/II) Single-Site

Adsorption
and activation of O₂ over Cu(I/II) single-site on CeO₂
surface

Liqun Kang,¹ Bolun Wang,^1*
Qiming Bing,² Michal Zalibera,³ Robert Büchel,⁴Ruoyu Xu,¹Qiming Wang,¹ Yiyun Liu,¹Diego
Gianolio,⁵ Emma Gibson,⁶ Mohsen Danaie,⁷
Christopher Allen,⁷Qian He,⁸ Shaoliang Guan,^5,10 Anton Savitsky,⁹ Chris
Kay,¹ Sotiris E. Pratsinis,⁴Wolfgang
Lubitz,⁹
Jing-yao Liu,³ Feng Ryan Wang^1*

Abstract: Efficient and selective activation of molecular oxygen at
mild conditions is important for chemical transformation and energy conversion.
Here we report the selective O₂ adsorption on a
[Cu(I)O₂]^3- single-site over CeO₂
surface. Isolated single Cu sites achieve the lowest HOMO with maximised
electron withdrawing effect from Ce⁴⁺. O₂ is adsorbed on
the Cu single-site, forming electrophilic [Cu(II)O₂(h²-O₂)]^4-
at 298 K, and then dissociating into O^2- and forming nucleophilic
[Cu(II)O₄]^6- at 453 K. The evolution of adsorbed O₂
species on Cu is tracked by operando
XAS, EPR, NAP-XPS/NEXAFS and DFT simulation. The Cu(I)/(II)
single-sites are 10 times more active than that of the CuO clusters in
CO oxidation, showing a turnover frequency of 0.028
± 0.003 s^-1at 373 K and 0.01 bar P_CO. The transition between [Cu(I)O₂]^3-
and [Cu(II)O₂(h²-O₂)]^4-/[Cu(II)O₄]^6-sites
are found in CO rich and lean conditions, which is the key for the high
catalytic activity. The advantage to activate molecular O₂
into electrophilic species is important for selective oxidation
reactions, such as epoxidation of propene and partial oxidation of methane.

Discussion

The innovations in this
work are:

1.     We
use Ce⁴⁺ to reduce the HOMO energy of Cu(I/II) single-sites in order
to selectively adsorption of O₂, forming electrophilic
[Cu(II)O₂(h²-O₂)]^4- at
298 K and nucleophilic [Cu(II)O₄]^6- at
453 K.

2.     The advantage
in activating molecular O₂ into electrophilic species is
important for selective oxidation reactions, such as epoxidation of propene and
partial oxidation of methane.

3.     The
combination of operando Soft X-ray
(NEXAFS), Hard X-ray (XAS), EPR techniques and DFT simulation reveals the electronic
structure of adsorbed species and their evolution under different conditions.

4.     For the first time, we
quantify the absolute Cu(II) single-site loading over CeO₂ surface via EPR.
The highest absolute amount of Cu(II) single-site over CeO₂
surface is obtained in 1wt% CuO-CeO₂.

Molecular O₂ is the simplest oxidant for
combustion, oxidation and electrochemical reactions. In the search for
catalytic active centres for selective O₂ adsorption and activation,
here we report a Cu(I)/Cu(II) single-site system over a crystalline CeO₂
surface. The neighbour Ce⁴⁺ reduces the highest occupied molecular
orbital (HOMO) energy of Cusingle-sites, resulting in the lowered
work function (7.83 eV) comparing to that of the Cu₂O/CuO clusters
(6.07 eV), calculated from ultraviolet photoelectron spectroscopy (UPS). Such
electron withdrawing effect of Ce⁴⁺ is maximised with isolated
single Cu sites.

The highest Cu single-site loading is 0.55wt%, as
confirmed by quantifying the feature peak of Cu(II) monomer in electron
paramagnetic resonance (EPR) spectra. The kinetic
studies of CO oxidation demonstrate a clear difference in active species. Below
1wt% CuO loading, the similar turnover frequencies (TOFs) and activation
energies E_a are obtained
(Fig. 1a). The content of CuO clusters increases with CuO loading beyond 1wt%,
leading to larger E_a and
smaller TOF. The CO conversion exhibits a linear relationship with the Cu
monomer EPR intensity (Fig. 1b), indicating identical Cu single-sites as
dominant active species with CuO loading below 1 wt%. A dynamic shift between
the [Cu(II)O₄]^6- and [Cu(I)O₂]^3-
upon CO lean and rich conditions is observed at 453 K via operando X-ray
adsorption fine structure (XAFS) (Fig. 1c-e). In comparison, CuO clusters are
reduced to metallic Cu under CO rich condition.

Figure 1. a, CO conversion as a function of Cu(II) single-site EPR intensity. b, TOF and activation energy as a function of Cu
loading. c, Gas concentration at the outlet of operando XAFS reactor
as a function of time. d, Contour map of the first derivative XANES spectra. e, Corresponding change of
coordination number in Cu-O, Cu-Ce (1) and Cu-Ce (2) scattering as a function
of time.

The difference in catalytic behaviours between Cu
single-site and CuO clusters suggests the different O₂ activation
mechanism. Near edge X-ray absorption fine structure (NEXAFS) and Spin-polarised
density functional theory (DFT) simulations are performed to investigate the
surface chemistry of Cu single-site. Under reductive environment, a [Cu(I)O₂]^3-
site is identified by Cu L₃ edge and O K edge NEXAFS (Fig. 2a
right). DFT simulation shows that a single Cu(0) atom is oxidized into Cu(I)
which coordinates with two surface oxygen ions to form a [Cu(I)O₂]^3- site (Fig. 2a left).
Upon O₂ adsorption at 298 K, an electrophilic species [Cu(I/II)O₂(h²-O₂)]^5/4-
is formed, showing the possible oxidation from Cu(I) to Cu(II). The adsorbed h²-O₂ is
different from the lattice O^2-, as determined in the O K edge NEXAFS
(Fig. 2b right). Raman spectroscopy reveals an O-O stretch at 830 cm^-1,
confirming the presence of h²-O₂. The
adsorbed h²-O₂ can
dissociate into 2 O^2- at 453 K, resulting in completed oxidised [Cu(II)O₄]^6-
site and lattice O^2- (Fig. 2c). The temperature is in consistent
with the calculated 1.41 eV energy barrier for this transition calculated from
DFT simulation. Finally, CO reduces [Cu(II)O₄]^6-,
resuming the original [Cu(I)O₂]^3-.

Figure 2 | Dynamics
in O₂ adsorption and activation tracked by the Cu L₃ edge and O K edge of NEXAFS. a DFT simulation and NEXAFS
of [Cu(I)O₂]^3-,
showing Cu(I) and lattice O^2-. b,
DFT simulation and NEXAFS of [Cu(I/II)O₂(h²-O₂)]^5/4-upon
O₂ adsorption at 298 K. The undissociated h²-O₂
adsorbed on Cu single-site is stable in UHV. c, DFT simulation and NEXAFS of [Cu(II)O₄]^6-
with dissociated O^2-. The O K edge spectrum is similar to that of [Cu(I)O₂]^3-, while only Cu(II)
is visible at Cu L₃ edge adsorption.

The operando NEXAFS and Raman results
confirmed the existence and stability of the undissociated h²-O₂
species over Cu, which is predicted by the DFT simulation. We hypothesise that
the electrophilicity of the Cu single-site resulted by the maximised electron
withdrawing effect of CeO₂ support would facilitate the formation
and stabilisation of the h²-O₂ species adsorbed on Cu. Such
an electrophilic O species is considered as a better oxidant towards the
epoxidation of ethylene compared to nucleophilic O species.