Peer review of: Adsorption and activation of molecular oxygen over atomic copper(I/II) site on ceria (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this work, the authors have synthesized Cu doped CeO2 nanoparticles with different weight 
loadings via flame spray pyrolysis. They have tried to selectively activate O2 through nanoparticles 
with low weight loadings of Cu. Through operando XAS, EPR, NAP-NEXAFS, and DFT calculations, the 
authors have concluded that it is the well-dispersed [Cu(I)O2]3- site that plays an essential role in 
selective activation of O2. However, there are some inconsistent results reported in the manuscript. In 
addition, solid justifications for the main conclusion should be further provided. Thus, I have to 
reserve my recommendation for its publication in Nature Communications. Specifically: 

Major points: 

1.In line 75-81 (or Figure 1), the authors suggested that the increasing HOMO and LUMO gap from 
bulk CuO to CuO cluster and to atomic Cu(I/II) sites is due to the electron withdrawing effect from 
Ce4+. This may be right. However, the size effect of the CuO clusters can also influence the HOMO 
and LUMO gap. Although Ce4+ can withdraw electrons from metallic Cu(0) (ref 28), it doesn’t mean 
that Ce4+ can withdraw electrons from Cu(II). 

2.The causality between the HOMO energy and the activity of the Cu sites are not clear. Considering 
the MvK mechanism is the most widely accepted mechanism (Chem. Rev. 113, 4391–4427 (2013), 
the highest activity of the atomic dispersed Cu(I/II) sites would be due to the observation that “CeO2 
is easier to be reduced under UHV with atomic Cu site (line 254-255)”. To support the main conclusion 
of the current manuscript, the authors should provide more convincing experimental or theoretical 
investigations revealing the causality between the HOMO energy and the activity of the Cu sites. 

3.In line 74-75 (or Figure 1), the authors stated that “the surface atomic metal site has discrete 
electronic structures compared with the continuous band structures found in clusters.” And line 82-84 
the discrete electronic structures of Cu on Ag/Cu systems were mentioned again. Why are the discrete 
electronic structures important, which shall be emphasized? 

4.The authors assigned discrete HOMO as occupied 3d orbitals of Cu (Figure 1). However, the 
transitions of 1s → 3d shown in Figure 2e indicated that 3d orbitals are not fully occupied and/or not 
discrete. The authors should provide justification for their description of discrete HOMO. 

5.In Figure 4, the authors assigned the pure Cu(II) species as [Cu(II)O4]6-, where all oxygen atoms 
become lattice O2-. However, according to the DFT calculations shown in Supplementary Figure 15, 
this structure is 1.05 eV less stable than [Cu(II)O2(η2-O2)]4-. The authors should provide justification 
for the existence of the unstable Cu(II) species. 

6.The selective activation of O2 was not well defined and determined in this work. The selectivity can 
be among different activated oxygen species, like O2-, O2-, O22-. The selectivity also can be among 
the activation of O2 and the other reactants. 

Minor points: 

1.Line 61, “group 8” should be “group 10” or “group VIII”. 
2.Line 69, “d9 and d10” should be “d10 and d9”. 
3.Line 81, the author state that “the energy of the LUMO increases, weakening its propensity to form 
a bond with O2”. Because O2 often plays as an oxidant, this statement should be revised. 
4.Line 93, Refs. 31 and 32 were cited for the [Cu(II)O2(η2-O2)]4-. However, both references are 
related to Ag rather than Cu. The proper references should be cited here. 
5.In Figure 2h, the smallest wt% is less than 0, which should be corrected. 
6.Line 163, the “tetragonally distorted octahedral” structure should be clarified. 
7.Line 243, the Raman band located around 830 cm-1 in Supplementary Fig. 13 is associated with the 
η2-O2. What is the more intense shoulder band around 600 cm-1? 
8.In Supplementary Materials line 208, the method for PDOS was provided. However, there is no 
PDOS reported in this work. 
9.In Supplementary Materials line 369, is it “CeO2(111)” or “CeO2(110)”? 



Reviewer #2 (Remarks to the Author): 

The authors describe an interesting study on the activation of dioxygen at isolated Cu single sites 
supported on ceria by a multitechnique approach combining operando XAS, EPR, NAP-NEXAFS and 
DFT simulations. They provided evidence for the presence of an electrophilic [Cu(II)O2(η2-O2)]4- 
species as a key intermediate state that should account for the high catalytic activity and selectivity of 
related oxidation reactions. In particular, the authors proposed the concept of using electronic 
structures to control the catalytic performance. Overall, these results allow to gain a fundamental 
insight into the structural, electronic and reactive properties of ceria-supported Cu single site 
catalysts. This work could be interesting to a broad readership. However, I cannot recommend this 
manuscript for publication in Nature Commun. in its current form. I have the following major 
concerns: 

1) With regards to the stable structure of Cu single atoms on ceria, the DFT simulations are 
insufficiently described. It is known that the morphologies of different ceria surfaces play a crucial role 
in the interaction between metal and support. As mentioned in the paper, the surface Ce3+ species 
are involved in the activation of dioxygen. In this context, the oxygen vacancy formation energy 
depends strongly on the orientation of ceria surfaces (see e.g., Chem. Rev., 2013, 113, 3949). The 
surface structures of the ceria NPs should be taken into account. 
2) In addition to Cu adatoms, single Cu atoms also could be stabilized by substituting a cerium atom 
on the surface and/or in the bulk under oxidative conditions. 
3) A number of groups have reported that metal single sites are not active for oxidation reactions (see 
e.g. Nat. Commun. 2019, 10: 3808; Nat. Mater. 2019, 18, 746). A recent work showed that the 
ultrafine Cu clusters (bilayer) on ceria account for the high catalytic activity for the water gas shift 
reaction (Nat. Catal. 2019, 2, 334). 
4) For the CO oxidation reaction, a synergistic effect should be more important than the activation of 
single reactant, i.e., a site cooperation allowing the activation of both CO and O2.