Peer review of: Entropy-stabilized single-atom Pd catalysts via high-entropy fluorite oxide supports (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this work Xu and co-authors reported the synthesis and characterization of multicomponent oxide, 
the so-called high-entropy oxide, supported highly stable Pd single-atom catalyst (SAC) as well as its 
catalytic performance in CO oxidation. The SAC was synthesized by a ball milling method and 
characterized by a combination of aberration corrected STEM, EXAFS as well as DRIFT. Although the 
former two characterization results are less convincing due to the low contrast between Pd and support 
elements, the DRIFT spectrum is very impressive and convincing. Therefore, I have no doubt about the 
formation of highly stable isolated Pd single atoms on the high-entropy oxide support. The high activity 
and stability of the catalyst in CO oxidation is also attractive. However the experimental data as well as 
scientific insights into the catalyst preparation and stability are lacking and remain concern. In whole I 
think the results are interesting. I recommend it for publication after addressing the following questions 
and concerns. 
1. What does the term “intrinsically stable” mean? Do the authors mean that single atoms are more 
stable than their nanoparitlce/nanocluster counterpart or mean single atoms are stable at elevated 
temperatures? I think authors should clearly illustrate this. Actually the former had not been 
demonstrated as no comparison with NPs/NCs on identical support was performed in this work while 
the latter is only demonstrated in oxidative atmosphere. 
2. The Pd SACs were indeed successfully fabricated on a high-entropy oxide support which shows good 
stability. However, whether the good stability is due to the high configurational entropy remains 
unaddressed. For example, He et al have recently reported (Cell Reports Physical Science 2020, 1, 
100004) that single oxide supported SAC can be synthesized by the same method which suggests that 
the high-entropy oxide is not indispensable. Therefore, in this work the formation of stable Pd SACs 
might originate from the strong interaction between Pd and one of the five elements rather than the 
high configurational entropy. This may be just like the strong interaction between Pt-CeO2 (Science 
2016, 353, 150) and Pt-Fe2O3 (Nat Commun, 2019, 10, 234) 
3. In SACs prepared by this method I believe some of the Pd atoms located in the bulk of the support 
instead of on the surface. So the surface exposed Pt atoms should be quantified by chemical adsorption. 
In addition, how about the post-deposition of Pd atoms onto the pre-synthesized high-entropy oxide 
support? Actually it would be more attractive if the post-deposited Pd single atoms are still stable. 
4. How about the stability of this catalyst (or the post-deposited catalyst) in reductive atmosphere? 
What temperature can they stand for in reductive atmosphere? 
5. The high-entropy oxide dominated by four 4+ cation and one 3+ cation (Zr3+). The latter may result in 
the formation of vacancies/defects. Is it possible that the defects play a critical role in stabilizing the Pd 
single atoms? 
6. XANES result suggested Pd existed as 4+ while XPS characterization evidenced Pd existed in 2+ or 
lower. Authors should explain this discrepancy. 
7. EXAFS fitting results suggested that Pd bonding to Zr and Ce/La through O, why no Pd-O-Ti or Pd-O-Hf 
bonding? 
8. In Fig 1b, Pd1@HEFO should be Pd1@HEFO-1.0 
9. In the caption of Fig4, 4h should be 4f 



Reviewer #2 (Remarks to the Author): 

This is a generally interesting work, describing the synthesis of a single atom Pd catalyst on a high 
entropy fluorite, in particular mixed Ce, Zr, Hf, Ti, La oxide, doped with Pd ions. The material was 
characterized thoroughly and evaluated as a catalyst for the CO oxidation, which is of a high interest for 
the treatment of vehicle exhaust gas. 
The work highlights the use of the mixed fluorite as a novel matrix to stabilize the Pd ions in the lattice, 
avoiding sintering and formation of the less catalytically active clusters of PdOx species. It further 
highlights the activity of the catalyst for its ability to perform CO oxidation at low temperatures (50 % 
conversion at ca. 130 oC), in comparison to a similarly Pd-doped system based on CeO2 only. 
Nevertheless, the importance of the claimed novelties is significantly restricted by the fact that 
sintering-resistant single atom catalysts for various catalytic reactions have already been reported in the 
literature. Furthermore, single atom catalysts active for CO oxidation at very low temperatures have 
been also reported in several articles. The Pd loading of the present work of 0.25 at. % helps the 
stabilization of Pd in the fluorite lattice, but it makes difficult to highlight its superior stabilization 
properties with respect to previous works. 
In particular, in J. Catal. 1995, 153, 304, W. Liu and M. Flytzani-Stefanopoulos reported the mixed Ce-La 
fluorite doped in the lattice with Cu, which presented 50 % conversion even at 80 oC, for the same 
reaction. Au doping was also reported, which showed even room temperature activity. Although at that 
time the exact configuration was not clear, later studies showed that 0.2-0.9 at % contents of Au exist 
solely as dopant ions in the lattice of the La-containing ceria (Fu etal. Appl Catal B-Environ. 2005, 56, 57). 
The substitution of Ce+4 with the lower valence La+3, was also recognized for its importance in creating 
sites for lattice doping with metals ions. 
Stable Pd incorporation in the lattice, as in Ce0.93Pd0.02Cu0.05O2−δ (Catal. Sci. Technol. 2016, 6, 8104) 
has been also reported with excellent performance for CO oxidation. Further single metal atom catalysts 
for CO oxidation have been reported with very high activities (Rh in ceria, Chem. Mater. 2004, 16, 11, 
2317) (Pd and Pt in metal oxides, ACS catalysis 2019, 9, 1595) (Pd in La+3 containing alumina Nat 
Commun 2014, 5, 4885) (Pt, Ru and Co single atoms, on carbon, C3N4 and TiO2 substrates Nat. 
Nanotechnol.2019, 14, 851). 
Methods for preparing sintering-stable single atom catalysts have been also reported for other types of 
oxidation reactions (Nat. Commun. 2019, 10, 234; Nat Commun 2020, 11, 335; J. Am. Chem. Soc. 2019, 
141, 18, 7283). 
Therefore, it is not quite clear what are the new insights/findings which the present work conveys. 
Some other points: 
In line 144, electron deficient Pd+4 is found for the catalyst by XPS, and later an intermediate oxidation 
between 0 and +2 (line 159) is found from XANES spectra. Why is that? 
The stability of the catalyst was tested, but low conversion was obtained (30%, Figure 4f inset). How was 
the stability at higher conversions? The deactivation test by water vapour was performed ex-situ, but for practical reasons water could be present during the catalytic reaction. 
In the current form, the manuscript is not acceptable for publication in Nature Communications.