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

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

mso-bidi-font-style:italic">Liqun Kang,¹ Bolun Wang,^1*
Qiming Bing,² Michal Zalibera,³ Robert Büchel,⁴ italic">Ruoyu Xu,¹ italic">Qiming Wang,¹ Yiyun Liu,¹Diego
Gianolio,⁵ Emma Gibson,⁶ Mohsen Danaie,⁷
Christopher Allen,⁷ italic">Qian He,⁸ Shaoliang Guan,^5,10 Anton Savitsky,⁹ minor-fareast;mso-ansi-language:EN-GB;mso-bidi-font-style:italic"> Chris
Kay,¹ Sotiris E. Pratsinis,⁴Wolfgang
Lubitz,⁹
Jing-yao Liu,³ Feng Ryan Wang^1*

text-align:justify;text-justify:inter-ideograph"> normal">Abstract: Efficient and selective activation of molecular oxygen at
mild conditions is important for chemical transformation and energy conversion.
Here we minor-fareast">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 normal;mso-bidi-font-style:italic">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 minor-fareast">, showing a turnover frequency of 0.028
± 0.003 s^-1at 373 K and 0.01 bar normal">P_CO. mso-fareast-theme-font:minor-fareast"> The transition between [Cu(I)O₂]^3-
and [Cu(II)O₂( " times new roman mso-symbol-font-family:symbol> Symbol">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. normal;mso-bidi-font-style:italic">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.

text-align:justify;text-justify:inter-ideograph"> normal"> italic">Discussion

text-align:justify;text-justify:inter-ideograph"> .15pt;font-style:normal;mso-bidi-font-style:italic">The innovations in this
work are:

text-align:justify;text-justify:inter-ideograph;text-indent:-18.0pt;mso-list:
l2 level1 lfo3"> font-style:normal;mso-bidi-font-style:italic">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( " times new roman mso-symbol-font-family:symbol> Symbol">h²-O₂)]^4- at
298 K and nucleophilic [Cu(II)O₄]^6- at
453 K.

text-align:justify;text-justify:inter-ideograph;text-indent:-18.0pt;mso-list:
l2 level1 lfo3"> mso-bidi-font-style:italic">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. italic">

text-align:justify;text-justify:inter-ideograph;text-indent:-18.0pt;mso-list:
l2 level1 lfo3"> mso-bidi-font-style:italic">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.

text-align:justify;text-justify:inter-ideograph;text-indent:-18.0pt;mso-list:
l2 level1 lfo3">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.

text-align:justify;text-justify:inter-ideograph">

page-break-before:always">

" times new roman>

inter-ideograph;line-height:normal"> " times new roman>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.

inter-ideograph;line-height:normal"> " times new roman>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 normal">via operando X-ray
adsorption fine structure (XAFS) (Fig. 1c-e). In comparison, CuO clusters are
reduced to metallic Cu under CO rich condition.

text-align:center;page-break-after:avoid">

inter-ideograph;line-height:normal">Figure 1. a, italic">CO conversion as a function of Cu(II) single-site EPR intensity. b, TOF and activation energy as a function of Cu
loading. normal">c, Gas concentration at the outlet of operando XAFS reactor
as a function of time. d italic">, 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. .15pt;font-style:normal;mso-bidi-font-style:italic">

inter-ideograph;line-height:normal"> " times new roman>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( symbol;mso-symbol-font-family:Symbol">h²-O₂)]^5/4-
is formed, showing the possible oxidation from Cu(I) to Cu(II). The adsorbed symbol;mso-symbol-font-family:Symbol">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 Symbol;mso-ascii-font-family:" times new roman mso-bidi-font-family: symbol>h²-O₂. The
adsorbed " times new roman>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-.

line-height:normal">

text-align:justify;text-justify:inter-ideograph"> normal">Figure 2 | Dynamics
in O₂ adsorption and activation SimSun;mso-fareast-theme-font:minor-fareast"> normal">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 minor-fareast"> 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. normal">c, DFT simulation mso-fareast-theme-font:minor-fareast"> 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. normal">

text-align:justify;text-justify:inter-ideograph"> .15pt;font-style:normal;mso-bidi-font-style:italic">The operando .15pt;font-style:normal;mso-bidi-font-style:italic"> 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 " times new roman mso-symbol-font-family:symbol> Symbol">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. normal">

text-align:justify;text-justify:inter-ideograph"> normal">