Metal-organic framework-confined Co_3O_4 for humidity-immune ozone decomposition

Domain: biology
Published: 2026-03-23
Editorial outcome: major_revision

Abstract
Ground-level ozone (O ) is a major air pollutant, and catalytic decomposition represents a
3
promising strategy for its removal. However, maintaining high catalytic efficiency under humid
conditions remains a considerable challenge. In this study, we encapsulate ultrafine metal oxides
(UMOs; e.g., Co O , NiO) within the nanopores of an Fe O-cluster-based metal–organic
3 4 3
framework, PCN-333(Fe), for catalytic O decomposition. The optimized 30% Co O @PCN-
3 3 4
333(Fe) catalyst achieves sustained 100% O conversion for over 120 hours in a continuous airflow
3
containing 40 ppm O under high space velocity (1.75 × 105 Sh-1) and a broad range of humidity
3
(10-90% RH). Mechanistic investigations reveal that the exceptional performance originates from
an interfacial hydrogen-atom transfer process between Co O and the Fe O clusters of PCN-
3 4 3
333(Fe), as confirmed by in situ diffuse reflectance infrared Fourier-transform spectroscopy
(DRIFTS) and in situ Raman spectroscopy. This work proposes a general principle for designing
humidity-immune catalytic interfaces between metal oxides and porous materials, thereby
providing a practical foundation for sustainable control of pollutant emissions in complex
environments.

Introduction
Ground-level ozone (O ) has emerged as a major global air pollutant, posing serious threats to both
3
environmental quality and public health1, 2. Outdoor O is primarily generated through
3
photochemical reactions between nitrogen oxides and volatile organic compounds under sunlight,
while indoor O may accumulate from emissions produced by photocopiers, laser printers,
3
ultraviolet lamps, and disinfection devices3, 4. As a potent oxidant, O remains relatively stable at
3
typical ambient concentrations (ppb level)5; however, long-term exposure has been associated with
respiratory disorders, cardiovascular diseases, and even premature mortality6, 7. The World Health
Organization has recommended that the 8-h average O concentration exceeding 100 μg m-3 (≈ 51
3
ppb) poses health risks8. Alarmingly, O levels exceeding 150 ppb are frequently reported in many
3
regions, reflecting severe pollution episodes. These observations underscore the urgent need for
efficient, durable, and environmentally sustainable strategies for O removal.
3
Among various control measures, catalytic decomposition is regarded as the most promising
strategy owing to its high efficiency and absence of harmful by-products9, 10. Conventional
transition-metal oxides exhibit notable activity but suffer from rapid deactivation under humid
conditions, where water molecules compete strongly for active sites11-13. Dispersing ultrafine metal
oxides (UMOs) on carbonaceous or porous substrates can partially alleviate these issues14, yet such
systems still face challenges of weak host–guest interfacial coupling, limited control over particle
size, and heterogeneous distributions of active sites. In contrast, metal-organic frameworks (MOFs)
offer distinct advantages, including tunable organic linkers to balance
hydrophilicity/hydrophobicity, robust inorganic nodes to act as active sites, and hierarchical
porosity to facilitate mass transport15-19. Nevertheless, most reported UMO-MOF composites rely
on external loading, which often leads to non-uniform particle distributions, pore blockage, and
reduced accessibility of active sites20-22. Therefore, designing UMO-MOF catalysts with well-
defined structures, strong interfacial interactions, and high stability is of paramount importance,
offering new opportunities to overcome current limitations and realize highly efficient O
3
catalytic
decomposition.
Herein, we report a series of UMOs@PCN-333(Fe) composites (UMOs = ultrafine Co O or
3 4
NiO nanoparticles), in which the oxide nanoparticles are precisely encapsulated inside the
nanopores of PCN-333(Fe) rather than simply deposited on the external surface. Among them, 30%
Co O @PCN-333(Fe) achieves 100% O conversion across a wide humidity (10-90% RH) at room
3 4 3
temperature, and maintains long-term stability for over 120 h. DFT calculations combined with in
situ spectroscopy reveal that the exceptional catalytic performance originates from interfacial
hydrogen-atom transfer between Co O and Fe O clusters of PCN-333(Fe), which facilitates
3 4 3
stepwise O activation, improves O release efficiency, and accelerates active-site regeneration.
3 2
Furthermore, the remarkable humidity immunity arises from the MOF-confined protection of
active sites from water invasion, while water molecules actively participate in the reaction pathway
rather than competing with O . This work establishes a fundamental principle for engineering
3
interfacial hydrogen-atom transfer within MOF-confined oxide catalysts, thereby providing a
practical foundation for achieving sustainable pollutant control under complex and variable
environmental conditions.

Results
Syntheses and Structural Characterization of UMOs@PCN-333(Fe)
The synthetic route of UMOs@PCN-333(Fe) is depicted in Fig. 1. A sonication-microwave
confinement strategy was developed to achieve the precise incorporation of ultrafine metal oxide
nanoparticles within the nanopores of PCN-333(Fe). Initially, activated PCN-333(Fe) was
dispersed homogeneously in a hydrophobic solvent (e.g. n-hexane) by ultrasonication, preventing
aggregation and exposing the hierarchical pore system. Subsequently, controlled amounts of
UMOs precursors were introduced under 0°C, enabling their gradual penetration into the
nanopores of framework. After an aging process that facilitated precursor anchoring, the mixture
was carefully filtered and underwent microwave treatment, which induced rapid in situ nucleation
and growth of UMOs as highly dispersed clusters confined within the pores of PCN-333(Fe). This
step ensured both uniform nanoparticle distribution and preservation of the host crystallinity.
Finally, careful purification removed residual reactants without damaging the porous architecture.
This synthetic strategy thus provides a versatile platform for embedding nanoscale metal oxides
into MOFs, achieving strong interfacial coupling and well-defined hybrid architectures for
catalytic applications.
To elucidate the structural characteristics of UMOs@PCN-333(Fe), we selected 30%
Co O @PCN-333(Fe) as a representative material. The MTN-type zeolitic topology of PCN-333
3 4
(Fig. 1) consists of a dual-mesopore network formed by supertetrahedral building units, with two
types of mesocages (4.2 and 5.5 nm in diameter) interconnected by pore windows of ∼2.6 and 3.0
nm, thereby providing spatially well-defined compartments for nanoparticle encapsulation23, 24.
Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy
(AC-HAADF-STEM) was employed to probe the structural integrity and encapsulation behavior25.
The pristine PCN-333(Fe) framework displays periodically arranged mesopores on the (110) plane
(Fig. 2a). Upon encapsulation, the 30% Co O @PCN-333(Fe) sample preserves the parent pore
3 4
periodicity (Fig. 2b), yet exhibits locally elevated contrast, directly evidencing pore occupation by
Co O nanoparticles (Fig. 2c). EDS elemental mapping further confirms homogeneous Co
3 4
distribution across single MOF crystals (Fig. S1). The encapsulated Co O nanoparticles exhibit a
3 4
uniform size of 3.4-3.9 nm (average 3.6 ± 0.1 nm, Fig. S2), which matches the mesoporous cavity
dimensions of PCN-333. Scanning Electron Microscope reveals well-retained octahedral
morphology with smooth surfaces and absence of surface-deposited aggregates (Fig. S3),
corroborating true intracrystalline encapsulation. These observations unambiguously confirm that
Co O nanoparticles are formed in situ and precisely confined within the nanopores of PCN-
3 4
333(Fe), without compromising the structural integrity of its framework.
To probe the chemical state of Co element in 30% Co O @PCN-333(Fe), Co K-edge X-ray
3 4
absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)
spectroscopy were performed, revealing an absorption edge between Co foil and bulk Co O ,
3 4
closer to Co O , indicating a slightly reduced oxidation state due to nanoconfinement-induced
3 4
interface effects (Fig. 3a)26. Fourier-transformed EXAFS (FT-EXAFS, Fig. 3b) reveals a
prominent Co-O coordination at ~1.42 Å, marginally shorter than the 1.46 Å path in bulk Co O 27,
3 4
28, while subsequent Co-Co coordination shells are markedly attenuated, directly evidencing the
highly dispersed and size-confined nature of the encapsulated clusters. Quantitative EXAFS fitting
using crystallographic Co O parameters yields excellent agreement (Fig. 3c), unequivocally
3 4
confirming Co O as the dominant crystalline phase while exhibiting subtle oxidation state
3 4
reduction associated with elevated oxygen vacancy concentrations.
Powder XRD patterns, photograph, and FT-IR spectra (Fig. 3d and Figs. S4-S5) confirm the
preservation of PCN-333(Fe)'s crystalline integrity following Co O incorporation across various
3 4
loadings. The absence of distinct Co O diffraction peaks precludes large particle formation,
3 4
instead indicating highly dispersed, ultra-small Co O nanoclusters29, 30. Raman spectroscopy
3 4
identifies the characteristic Co O peak at 682 cm-1 (Fig. 3e), confirming the encapsulated phase’s
3 4
identity as Co O 31, 32. N adsorption isotherms (Fig. 3f) and pore size analysis (Fig. S6) reveal a
3 4 2
systematic decrease in uptake capacity with increasing Co O loading, consistent with progressive
3 4
pore filling. Notably, unlike previous reports where metal oxides (e.g., TiO ) were confined solely
2
to mesopores, Co O in this work also occupies micropores (∼11 Å diameter), attributable to the
3 4
smaller size of its precursors25. Inductively coupled plasma optical emission spectrometry (ICP-
OES) quantitatively verifies Co O mass percentages of 9.4%, 18.6%, and 27.3% (Table S1),
3 4
aligning with the designated loadings. Similarly, structure characterization of NiO (Figs. S7-S10)
confirms its successful encapsulation within the PCN-333(Fe) framework.
O catalytic decomposition on UMOs@PCN-333(Fe)
3
The O catalytic decomposition performance of UMOs@PCN-333(Fe) was evaluated with a
3
dynamic test at various humidity and temperature on a custom-built ozone analyzer at an inlet O
3
concentration of 40 ppm (Fig. S11). The effect of varying oxide loadings on catalyst under low
(10% RH) and high humidity (90% RH) conditions was first optimized (Figs. S12-S13).
Remarkably, 30% Co O @PCN-333(Fe) maintains complete O conversion for more than 160 h
3 4 3
at 40°C even under 10% RH (Fig. 4a), attesting to its outstanding durability in dry environments,
with a performance level comparable to the best catalysts reported to date13, 33-36. In addition, the
catalytic rate of O₃ decomposition over 30% Co O @PCN-333(Fe) was calculated to be 1.9
3 4
mmol·h-1·g -1. While performance exhibits humidity dependence, degrading to 120 hours at 90%
cat.
RH, this still represents exceptional longevity under high moisture stress compared to benchmark
materials. Furthermore, a pronounced positive temperature dependence of catalytic stability at
fixed low humidity (10% RH): 30% Co O @PCN-333(Fe) sustains 100% O conversion for 50
3 4 3
hours even at a low temperature of 10°C, underscoring its exceptional intrinsic activity and
robustness in O catalytic decomposition (Fig. 4b). Under 10% RH (Fig. 4c), the O conversion of
3 3
bulk Co O and pristine PCN-333(Fe) rapidly deactivate, reaching peak conversions of <10% and
3 4
<30% within 3 h, far below the sustained 100% activity of the composite. Even at 90% RH, 30%
Co O @PCN-333(Fe) preserves its pronounced advantage, underscoring its superior humidity
3 4
immunity (Fig. 4d). These results highlight the synergistic catalysis arising from the MOF-
encapsulated UMOs structure.
The robustness of 30% Co O @PCN-333(Fe) to dynamic environmental fluctuations is shown
3 4
in Figs 4e-f. When subjected to fluctuating humidity cycles between 30% and 90% RH, or
temperature shifts between 10°C and 40°C, the material consistently maintains 100% O
3
conversion for 50+ hours without performance decay. This robust adaptability underscores its
capability to handle real-world atmospheric variability where humidity and temperature rarely
remain static. Optimization of the active phase loading is critical for maximizing humidity
immunity. Fig. 4g demonstrates that Co O @PCN-333(Fe) with 10% loading sustains 100%
3 4
conversion for only 20 hours at 90% RH, whereas increasing the loading to 30% extends this
duration to more than 160 hours. This pronounced Co O loading dependence unequivocally
3 4
demonstrates that achieving an optimal Co O density within the PCN-333(Fe) framework is
3 4
indispensable for preserving abundant and accessible catalytic sites, particularly under wet
environments.
Critically, 30% Co O @PCN-333(Fe) exhibits industry-relevant performance even under
3 4
simulated extreme environments, underscoring its substantial potential for practical application.
Challenging summertime conditions representative of the Yangtze River Delta region in China,
where intense photochemical activity and high humidity converge to produce severe O pollution
3
episodes, thereby underscoring the urgent necessity of effective removal strategies. Remarkably,
under these accelerated degradation conditions, 30% Co O @PCN-333(Fe) sustains complete O
3 4 3
decomposition for 120 hours (Fig. 4h). This superior stability highlights not only the high intrinsic
activity of the encapsulated architecture but also its resilience against the simultaneous thermal
and humidity stresses that typically deactivate conventional catalysts. Beyond the single Co O
3 4
system, the encapsulation strategy proves broadly generalizable for enhancing the catalytic
efficiency of diverse transition metal oxides. As demonstrated in Fig. 4i, PCN-333(Fe)
incorporating NiO, or Co O nanoclusters achieves over 120 hours of continuous and complete O
3 4 3
conversion under optimized conditions, markedly outperforming state-of-the-art benchmarks
(Table S2). Furthermore, systematic evaluations of NiO@PCN-333(Fe) (Figs. S14-S16) confirm
its outstanding decomposition activity across a wide range of environmental conditions. Although
all encapsulated composites deliver transformative stability enhancements relative to their bulk
oxide counterparts, 30% Co O @PCN-333(Fe) consistently demonstrates the highest activity-to-
3 4
stability balance, firmly establishing it as a leading candidate for scalable and durable industrial
O removal.
3
O Conversion Mechanism over 30% Co O @PCN-333(Fe)
3 3 4
To elucidate the underlying mechanisms contributing to the excellent O catalytic
3
decomposition performance of 30% Co O @PCN-333(Fe), X-ray photoelectron spectroscopy
3 4
(XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy and in situ Raman
spectroscopy was performed. The chemical states of both Fe and Co underwent significant changes
after the encapsulation of Co O particles within the PCN-333(Fe) cages (Fig. 5a-b). Upon Co O
3 4 3 4
encapsulation, the Fe2+ content on PCN-333(Fe) increases from 45.1% to 61.9%, while the Co2+
content of Co O rises from 45.7% to 63.4%. Such concurrent enrichment of reduced Fe and Co
3 4
species underscores the emergence of an interfacial MOF-UMOs synergistic site, which is directly
promoted by the confined architecture. During O decomposition, oxygen vacancies are generated
3
to maintain electrostatic balance, and substantial evidence suggests that these vacancies act as the
dominant active sites, accounting for the high catalytic efficiency12, 37, 38. This conclusion is
corroborated by differential charge density calculation (Fig. S17), which reveals pronounced
charge transfer from MOF ligands to Fe and Co centers.
Electron Paramagnetic Resonance (EPR) analysis reveals that O exposure induces O •- species
3 2
in 30% Co O @PCN-333(Fe) when trapped by DMPO (Fig. S18). The in situ DRIFT spectrum
3 4
(Fig. 5c) exhibited prominent absorption bands at approximately 3660, 2125, 1650, and 1380 cm-
1. These bands are assigned to the O-H stretching vibration of doubly bridged hydroxyl groups (-
OHO-)39, coordinated O complexes adsorbed on Co sites40, the O-H bending vibrations in
3
adsorbed water and reaction intermediates41, and peroxide (O 2-) species11, respectively.
2
Meanwhile, new peaks were observed in the in situ Raman spectra (Fig. 5d), with the band at 526
cm-1 assigned to the O-O stretching vibration in Co-OOH42-44, and the bands at 804 and 1097 cm-
1 attributed to O 2- and O - species34, respectively, indicating the formation of new intermediates
2 2
on the catalyst surface during O decomposition. This finding is consistent with the literature
3
reports, suggesting that the peroxide intermediate is active for O catalytic decomposition.
3
Following a 24-hour exposure to an O gas flow, the Raman spectrum of 30% Co O @PCN-
3 3 4
333(Fe) showed no significant changes, which suggests that the structural integrity of the catalyst
remained intact and undamaged throughout this duration. This observation serves to confirm the
exceptional stability of the material. Furthermore, the accumulation of intermediate products was
found to be minimal, and it is noteworthy that the release of these intermediates at the active sites
constitutes the rate-limiting step in the O catalytic decomposition process, thereby providing
3
further evidence of the catalyst's outstanding performance.
Density functional theory (DFT) calculations were performed to elucidate the mechanism of O
3
decomposition over Co O @PCN-333(Fe) (Fig. 6). The model incorporates representative Fe O
3 4 3
clusters from PCN-333(Fe) and Co O nanoclusters to simulate the catalytic interface (Fig. S19).
3 4
The adsorption sites of H O and O in Co O @PCN-333(Fe) were calculated. H O was found to
2 3 3 4 2
preferentially occupy an open Fe site adjacent to the Co O , with a distance of 2.223 Å, whereas
3 4
O was located at a Co site in the vicinity of the Fe O cluster, with a distance of 1.825 Å,
3 3
highlighting the distinct roles of Fe and Co nodes in activating the two adsorbates. Subsequently,
the first O molecule preferentially adsorbs onto two adjacent open Co sites, with its two terminal
3
O atoms coordinated to them. Then, the coordinated H O at the Fe site dissociates, releasing two
2
free hydrogen atoms. One hydrogen atom reacts with the adsorbed O to generate a Co-OOOH
3
intermediate, while the other is coordinated at the interfacial O atom. The Co-OOOH species then
decomposes, releasing the first ·OOOH radical. The second O molecule subsequently adsorbs
3
onto the Co sites, during which the interfacial hydrogen-atom transfer toward the Co center and
reacts with the newly adsorbed O to form a second Co-OOOH intermediate. The decomposition
3
of this Co-OOOH intermediate releases the second ·OOOH radical, which subsequently re-adsorbs
onto Co centers to evolute the first O molecule, while leaving bridging O atoms shared by
2
neighboring Co sites. In view of the complexity of this step and its pivotal role in the overall
catalytic cycle, its kinetics were explicitly evaluated. The elementary step in which the *OOOH
intermediate forms the first O molecule and *OH (TS1), as well as the subsequent transfer of the
2
resulting *OH (TS2), was calculated. According to the DFT results, the Gibbs free energy barrier
(TS1) for the formation of *O and *OH intermediates from *OOOH is 0.47 eV, and the barrier
2
for the transfer of the generated *OH to the O atom of Fe-O-Co (TS2) is 0.64 eV. These DFT-
calculated energy barriers are moderate, indicating that the reaction process is kinetically feasible.
Finally, the transient Co-OOH intermediate decomposes to release the third O molecule, after
2
which both hydrogen atoms return to the Fe site to reconstitute the coordinated water. Overall, the
key driving force of the O catalytic decomposition process on Co O @PCN-333(Fe) is hydrogen-
3 3 4
atom transfer mechanism (Tables S3-S4 and Fig. S20), in which hydrogen atoms was transferred
among the Fe O cluster, the interfacial oxygen and the Co sites, ensuring continuous regeneration
3
of active centers. In addition, the O desorption barrier on bare Co O was also calculated and
2 3 4
found to be 1.84 eV (Fig. S21), which is much higher than the corresponding value of 0.62 eV on
Co O @PCN-333(Fe), underscoring the advantage of the hydrogen-atom transfer nanoreactor.
3 4
The above results indicated that the hydrogen-atom transfer between the interface between Co
3 4
and Fe O cluster enable stepwise O activation, efficient O release, and accelerated active-site
3 3 2
regeneration, thereby accounting for the high catalytic activity observed experimentally.

Methods
Materials and reagents
Co(NO ) ·6H O, Ni(NO ) ·6H O, MnSO , (NH ) S O , CO(NH ) , FeCl ·6H O, and 2,4,6-
3 2 2 3 2 2 4 4 2 2 8 2 2 3 2
Tris(carboxyphenyl)-1,3,5-triazine (H TATB) were purchased from Shanghai Macklin
3
Biochemical Technology Co., Ltd. DMF, EtOH, Acetone was purchased from Shenyang Xinhua
Reagent Factory without further purification.
The ozone (O ) catalyst performance test system is self-made, and its main accessories are as
3
follows: UV lamp and ballast were bought from Beijing Aerospace Hongda Optoelectronic
Technology Co., Ltd. Ozone monitor (Model 202) was bought from 2B Technologies. Humidity
generator was bought from Shanghai Penghe electronic technology Co., Ltd. Air pump (SPB-3)
was bought from Beijing BCHP Analytical Technology Institute. Gas mass flowmeter (PIPG-
M102-01-2000) was bought from Shaanxi Yidu Intelligence Technology Co., Ltd.
Synthesis of PCN-333(Fe)
PCN-333(Fe) was synthesized according to the previously reported literature24. H TATB (50 mg)
3
and anhydrous FeCl (60 mg) were dissolved in 10 mL of DMF, followed by the addition of 0.5
3
mL trifluoroacetic acid. The resulting solution was transferred to a Teflon-lined autoclave and
heated at 150 °C for 12 h, yielding a brown precipitate. After centrifugation, the product was
repeatedly washed with fresh DMF and Acetone to remove any residual reactants.
Synthesis of UMOs@PCN-333(Fe)
Co O precursor solution: dissolve 600 mg cobalt(II) nitrate hexahydrate (Co(NO ) ·6H O) and
3 4 3 2 2
600 mg urea (CO(NH ) ) in 1 mL deionized water, then sonicate until completely dissolved.
2 2
NiO precursor solution: dissolve 600 mg nickel(II) nitrate hexahydrate (Ni(NO ) ·6H O) and 600
3 2 2
mg urea (CO(NH ) ) in 1 mL deionized water, then sonicate until completely dissolved.
2 2
UMOs@PCN-333(Fe) hybrids were prepared by a sonication–microwave confinement strategy.
Activated PCN-333(Fe) (50 mg) was ultrasonicated (15 min) in 20 mL anhydrous DMF to fully
expose its hierarchical meso- and micro-pores. Separately, 1 mL UMOs precursor solution (Co O
3 4
or NiO precursor solution) was added dropwise to the MOF dispersion under continuous stirring.
The mixture was aged at 0°C for 2 h to allow precursor diffusion and anchoring within the cavities.
Subsequent microwave irradiation induced rapid in-situ nucleation of ultrafine Co O , or NiO
3 4
clusters without disrupting the framework. After cooling, the product was collected by filtration,
washed repeatedly with DMF and ethanol, and vacuum-dried at 80°C to furnish the corresponding
UMOs@PCN-333(Fe) composite.
Synthesis of Co O
3 4
The Co O precursor solution was prepared by dissolving 30 mg of cobalt(II) nitrate hexahydrate
3 4
(Co(NO ) ·6H O) and 31 mg of urea (CO(NH ) ) in 50 μL of deionized water. The resulting
3 2 2 2 2
mixture was subjected to microwave irradiation for a duration of 5 minutes to facilitate the reaction.
After cooling to room temperature, the resulting powder was washed by H O and ethanol for 3
2
times, respectively. Finally, the product was dry at 85°C for 12 hrs.
Synthesis of NiO
The NiO precursor solution was prepared by dissolving 30 mg of nickel(II) nitrate hexahydrate
(Ni(NO ) ·6H O) and 31 mg of urea (CO(NH ) ) in 50 μL of deionized water. The resulting
3 2 2 2 2
mixture was subjected to microwave irradiation for a duration of 5 minutes to facilitate the reaction.
After cooling to room temperature, the resulting powder was washed by H O and ethanol for 3
2
times, respectively. Finally, the product was dry at 85°C for 12 hrs.
Structure characterization
The powder X-ray diffraction (PXRD) patterns were recorded on a Bruker Advanced D8 X-ray
Powder Diffractometer (λ = 1.54056 Å). N adsorption/desorption isotherms were measured by
2
using a Beishide 3H-2000PS2 Surface Characterization Analyzer at 77 K. An Thermo Fisher 5700
Fourier transform infrared spectrometer was used for recording Fourier transform infrared (FT-IR)
spectra. X-ray photoelectron spectroscopy (XPS) measurements were carried out using an
ESCALAB 250 instrument. Metal oxides contents of all catalysts were determined by ICP-OES
on an Agilent 5110(OES) atomic emission spectrometer. Raman spectra was recorded on Renishaw
inVia™ InSpect Raman system. Synchrotron-based X-ray absorption spectroscopy (XAS)
measurements, including XANES and EXAFS, were carried out at the BL14W1 beamline of the
Shanghai Synchrotron Radiation Facility (SSRF) operated at 3.5 GeV with a maximum current of
260 mA. Powdered samples were uniformly spread on Kapton tape and mounted in a liquid-
nitrogen-cooled cryostat (80 K) to minimize radiation damage. Spectra were collected in
transmission mode for the Co K-edge (7709 eV) and Fe K-edge (7112 eV), with energy calibration
performed simultaneously using Co and Fe metal foils as references. Data reduction and analysis
were performed with Athena and Artemis software packages following standard procedures.
Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) and high-
angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were
performed on a JEOL JEM-ARM300F Grand ARM double-Cs-corrected microscope operated at
300 kV (JEOL Ltd., Japan). Powder samples were ultrasonically dispersed in ethanol, drop-cast
onto a holey carbon-coated copper grid, and dried under ambient conditions before imaging.
O decomposition test
3
The O catalytic decomposition of UMOs@PCN-333(Fe) were evaluated through a homemade O
3 3
analysis system at room temperature. O was generated from compressed air manufactured from a
3
fully automatic air source (SPB-3, Beijing BCHP Analytical Technology Institute) by an ultraviolet
lamp (10-08100, Beijing UVCN). Before the test, all samples were vacuum-dried at 120°C for 12
hours to remove guest molecules from the structure. Then, the mixture of the 50 mg catalysts and
450 mg of quartz sand were loaded into the quartz tube reactor. The total inlet gas flow was kept
at 1000 mL/min, and the inlet O concentration was kept at 40 ± 1 ppm; the concentration of O
3 3
and RH were controlled by controlling the flow of blank air and O while the total gas flow
3
remained unchanged. The inlet and outlet O concentration as well as RH were measured by an O
3 3
detector (model 202, 2B Technologies) and a humidity and temperature meter. The O conversion
3
rate was calculated via the following formula:
O 3 conversion (%) = (C inlet - C outlet ) / C inlet  100% (1)
where C
inlet
and C
outlet
are inlet and outlet concentrations of O
3
, respectively.
Computational details
All DFT calculations were carried out using the CP2K code (V2024.1)45. The Gaussian-and-plane-
wave (GPW) scheme was employed. Core electrons were described by norm-conserving
Goedecker–Teter–Hutter (GTH) pseudopotentials46-48, and the valence electron wavefunctions
were expanded in double-ζ Gaussian basis sets with polarization functions49, together with an
auxiliary plane-wave basis set with a kinetic energy cutoff of 450 Ry. Exchange–correlation effects
were treated within the generalized-gradient approximation using the Perdew-Burke-Ernzerhof
(PBE) functional50. Geometry optimizations were performed with the Broyden-Fletcher-Goldfarb-
Shanno (BFGS) algorithm until the self-consistent-field (SCF) energy converged to 1.0 × 10-6 a.u.
Long-range van der Waals interactions between the adsorbates and the framework were accounted
for using the DFT-D3 dispersion correction with an empirical damping function51. For Fe- and Co-
containing models, the total spin multiplicities were set to 4 and 3, respectively. All calculations
were carried out in a cubic simulation cell of 35 × 35 × 35 Å3.
The Gibbs free energy change for each elementary step was calculated at 298.15 K, defining as
follows:
∆𝐺 = ∆𝐸 +∆𝐸 −𝑇∆𝑆 (2)
𝐷𝐹𝑇 𝑍𝑃𝐸
where ∆𝐸 is the difference of electronic energy calculated with CP2K, ∆𝐸 is the difference
𝑍𝑃𝐸
of zero-point energy (ZEP), and 𝑇∆𝑆 is the changed entropy value. 𝐸 and 𝑇𝑆 were calculated
𝑍𝑃𝐸
using the following equations for each reaction intermediates,
1
𝐸 = ∑ ℎ𝑣 (3)
𝑍𝑃𝐸 𝑖 𝑖
2
1
−ℎ𝑣𝑖⁄
𝑇𝑆 = ∑
𝑖
ℎ𝑣
𝑖
(
ℎ𝑣 𝑖⁄
) − 𝑘
𝐵
𝑇∑
𝑖
ln(1− 𝑒 𝑘𝐵𝑇) (4)
𝑒 𝑘𝐵𝑇
where ℎ , 𝑣 , and 𝑘 are Planck’s constant, vibrational frequencies, and Boltzmann constant,
𝑖 𝐵
respectively. For vibrational frequency calculations, only the adsorbates were included with
negligible contribution from the framework of the catalyst.
The Gibbs free energy of O or reaction species adsorbed on the substrates was obtained after
3
the geometry optimization for the adsorption and reaction process. And then, the Gibbs free energy
change (∆G) was calculated between the free energy for each reaction step.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript has reported the Ozone decomposition via MOF-Confined Co3O4 for Humidity-Immune. The result is\ninterest and it can be accepted after major revisions.\n1. The encapsulated Co3O4 nanoparticles exhibit a uniform size of 3.4-3.9 nm (average 3.6 ± 0.1 nm), how about the pore\nsize of PCN-333(Fe) is?\n2. Where are the adsorption sites for O3 and H2O ? in pore of PCN-333 or on surface Co3O4?\n3. What is the reason for high humid resistance? What is the interaction between Co3O4 and PCN-333, which can improve\nthe ozone decomposition humid resistance of the catalyst?", "reviewer_2": "This work presents interesting results, however, there is a lack of novelty and not suitable for publication in this journal.\nConsiderable works have reported ozone decomposition in humid conditions and well-performed catalysts have been\ndeveloped (e.g. Nat. Commun., 2021, 12, 4152; 10.1002/anie.202309158; 10.1038/s41467-022-32678-2). What is the\nsuperiority of Co3O4@PCN-333(Fe) presented in this work over the-state-of-the art-catalysts？\nOther comments are listed below:\n1. How does this work relate to the challenges in line 46 you've raised? There is a lack of characterization of the\nhydrophilicity/hydrophobicity of materials.\n2. Why is the O3 concentration set at 40 ppm?\n3. What is the catalytic rate of O3 decomposition of 30% Co3O4@PCN-333(Fe)?\n4. The stability of both Co₃O₄ and PCN-333(Fe) is poor. Why has the stability of 30% Co₃O₄@PCN-333(Fe) been improved\nby several dozen times?\n5. The diagram of DFT lacks a control sample in Fig. 6 a.", "reviewer_3": "In this manuscript, the authors encapsulated ultrafine metal oxides e.g., Co3O4) within the nanopores of an Fe3O-cluster-\nbased metal–organic framework, PCN-333(Fe), and used the resulting material as the catalyst for the O3 decomposition. A\nhigh efficiency has been obtained for the optimized 30% Co3O4@PCN-16 333(Fe) catalyst. The decomposition mechanism\nwas proposed based on the spectroscopies and DFT calculations. The manuscript is potentially suitable for the publication\nin Nature Communications, but the DFT study requires substantial revision and recalculation before it can support the\nproposed mechanism.\n(1)The computational details are insufficient. The authors should specify which computational software was used. The spin\nstates of Fe and Co centers are not discussed—this information is essential. Moreover, it should be clarified along which\nspin surface the reaction proceeds.\n(2)Only minima were characterized in the calculations. Without transition-state structures, the mechanistic understanding\nremains incomplete. For example, the formation of the first O2 molecule from OOOH, leaving H and O atoms bound to the\ncatalyst, involves multiple bond-forming and bond-breaking events that seems to be unable to achieve in a single step. The\ncurrent computational results fail to provide insights into this and other key elementary steps.\n(3)It is very difficult to correlate the states shown in Fig. 6 with those in Fig. S27 and Table S4. I suggest labeling the states\nin Fig. 6 (both 6a and 6b) with Roman numerals consistent with the supplementary data. Additionally, the molecular\nstructures in Figs. 6a and S27 are not clearly presented.\n(4)If ·OOOH radicals are indeed generated during the reaction, a hydrogen-atom transfer (HAT) or proton-coupled electron\ntransfer (PCET) process must occur rather than the “proton-shuttling” mechanism proposed by the authors. Resolving this\nissue requires accurate identification and analysis of the spin states along the reaction pathway.\nOverall, the above issues render the proposed mechanism unreliable and unconvincing in its current form. A more rigorous\ncluster-based QM model (for example, using the model shown in Fig. S26) could provide a better foundation for addressing\nthese mechanistic questions."}, "author_response": "Response to Referees Letter\nReviewer #1:\nThis manuscript has reported the Ozone decomposition via MOF-Confined Co O\n3 4\nfor Humidity-Immune. The result is interest and it can be accepted after major\nrevisions.\n1. The encapsulated Co O nanoparticles exhibit a uniform size of 3.4-3.9 nm\n3 4\n(average 3.6 + 0.1 nm), how about the pore size of PCN-333(Fe) is?\nReply: We sincerely thank the reviewer for the insightful feedback. For clarity, PCN-\n333 (MTN topology) exhibits a hierarchical pore system in which 1.1 nm tetrahedral\nmicroporous cages share vertices to form two mesocage types: ~4.2 nm\ndodecahedral and ~5.5 nm hexakaidecahedral. These mesocages communicate\nthrough ~2.6 nm pentagonal and ~3.0 nm hexagonal windows, yielding two well-\ndefined and uniform pore apertures (Nat. Commun. 2015, 6, 5979). This architecture\nprovides uniform confinement and efficient mass transport that are central to our\nencapsulation strategy and size control. The relevant information has been added to\nthe revised manuscript (Page 6, Lines 97-98).\n2. Where are the adsorption sites for O and H O? in pore of PCN-333 or on surface\n3 2\nCo O ?\n3 4\nReply: We appreciate the reviewer’s question. The adsorption sites of both H O and\n3\nbetween the Fe nodes and the encapsulated Co O cluster. Specifically, H O was\n3 4 2\nfound to preferentially occupy an open Fe site adjacent to the Co O cluster, with an\n3 4\nH O-Fe distance of 2.223 Å, whereas O is adsorbed at a Co site in the vicinity of\n2 3\nthe Fe O node, with an O -Co distance of 1.825 Å. These results highlight the\n3 3\ndistinct roles of Fe and Co centers at the Co O @PCN-333(Fe) interface in\n3 4\nactivating the two adsorbates, and confirm that the adsorption occurs in the pores of\nPCN-333(Fe) rather than on a separate Co O surface. The relevant information has\n3 4\nbeen added to the revised manuscript (Page 15, Lines 268-272).\n3. What is the reason for high humid resistance? What is the interaction between\nCo O and PCN-333, which can improve the ozone decomposition humid resistance\n3 4\nof the catalyst?\nReply: Thank you. the high humidity resistance of Co O @PCN-333(Fe) originates\n3 4\nfrom two cooperative effects. First, the ultrafine Co O clusters are fully confined\n3 4\ninside the nanopores of PCN-333(Fe), which prevents water from blocking active\ncenters and preserves gas diffusion channels under humid conditions. Second, there\nis a strong interfacial interaction between Co O and the Fe O clusters of PCN-\n3 4 3\n333(Fe). This interaction enables a hydrogen-atom transfer process during O\n3\ndecomposition, in which water does not poison the catalyst but instead participates\nin stepwise O activation, facilitates efficient O release, and accelerates active-site\n3 2\nregeneration. These interfacial and confinement effects together account for the\nexceptional humidity-immune O catalytic performance.\nThis work presents interesting results, however, there is a lack of novelty and not\nsuitable for publication in this journal. Considerable works have reported ozone\ndecomposition in humid conditions and well-performed catalysts have been\ndeveloped e.g. Nat. Commun., 2021.12.4152: 10.1002/anie.202309158;\n10.1038/s41467-022-32678-2). What is the superiority of Co3O4@PCN-333(Fe) presented in this work over the-state-of-the art-catalysts? Other comments are listed\nbelow:\nReply: We sincerely appreciate the reviewer’s clear comments and criticism. We\nunderstand the concerns about novelty and benchmarking. We have revised the\nmanuscript to more explicitly highlight the unique advantages of our hydrogen-atom\ntransfer nanoreactors (UMOs@PCN-333(Fe)):\n1) Uniform intracrystalline confinement of ultrafine transition-metal oxides\nThis work achieves precise encapsulation of ~3.6 nm Co O clusters inside the\n3 4\nhierarchical meso-cages of PCN-333(Fe) with homogeneous spatial distribution.\nThe pore system remains accessible without surface deposition or blocking. This\ntrue nanoconfinement distinguishes our strategy from conventional external loading,\nand ensures durable exposure of active sites even under humid conditions.\n2) Exceptional long-term resistance to fluctuating humidity and temperature\nThe catalyst maintains 100% O conversion for >120 h across 10-90% RH at a\n3\nhigh space velocity of 1.75 × 105 h-1, and remains fully stable under alternating\nhumidity and temperature cycles. Such humidity-immune performance under\nrealistic environmental fluctuations surpasses most state-of-the-art systems that are\nonly stable in moderate humidity or for short durations.\n3) Distinct interfacial hydrogen-atom transfer catalytic mechanism\nIn situ DRIFTS, in situ Raman, and DFT reveal a unique hydrogen-atom transfer\npathway at the Co O -Fe O interface, where water actively promotes stepwise O\n3 4 3 3\nactivation and rapid regeneration of active sites rather than poisoning them. This\nmechanistic advance differentiates our catalyst conceptually from existing oxide-\nbased O decomposition systems.\n3\n1. How does this work relate to the challenges in line 46 you've raised? There is a\nlack of characterization of the hydrophilicity/hydrophobicity of materials.\nReply: We sincerely appreciate the reviewer’s valuable comment. Our original\ndiscussion of hydrophilic/hydrophobic properties was motivated by the fact that our\nconfinement strategy exploits the difference in wettability between the inner and\nouter surfaces of the MOF. However, we agree that introducing this aspect at this\npoint in the text is not fully appropriate and may distract from the main line of\nargument. Accordingly, we have removed the corresponding sentences and revised\nthis part to instead provide a concise description of the challenges associated with\ndispersing ultrafine metal oxides (UMOs) nanoclusters on carbonaceous or porous\nsupports. The relevant information has been revised to the revised manuscript (Page\n3, Lines 46-48).\n2. Why is the O concentration set at 40 ppm?\n3\nconcentration was set to 40 ppm for the following reasons: (i) 40 ppm falls within\nthe commonly used 20-50 ppm range in O catalytic decomposition studies,\n3\nfacilitating direct comparison with literature reports; (ii) this concentration enables\ncatalytic activity and deactivation features to be clearly amplified on an\nexperimentally accessible timescale; and (iii) the experimental setup delivers a\nstable O output at 40 ppm, which is beneficial for ensuring data reproducibility.\n3\n3. What is the catalytic rate of O decomposition of 30% Co O @PCN-333(Fe)?\n3 3 4\nReply: Thank you. In this study, the total inlet flow rate was fixed at 1.0 L·min-1 at\n40°C, with an O concentration of 40 ppm. Under complete conversion, the mass-\n3\nnormalized ozone decomposition rate can be estimated to 1.9 mmol·h-1·g -1\ncat.\naccording to\n𝑄 × 𝐶\n𝑉 =\n𝑡𝑎𝑡𝑜𝑙 𝑂3\n𝑜3 𝑉 × 𝑚\n𝑚 𝑐𝑎𝑡.\nwhere 𝑄 is the total inlet flow rate (L·min-1), 𝐶 is the O concentration, 𝑉\n𝑡𝑎𝑡𝑜𝑙 𝑂3 3 𝑚\nis the molar volume of the gas (25.7 L·mol-1 at 40°C and 1 atm), and 𝑚 is the\n𝑐𝑎𝑡.\ncatalyst mass.\nThe relevant information has been revised to the revised manuscript (Page 9,\nLines 164-166) and revised SI (Page S17).\n4. The stability of both Co O and PCN-333(Fe) is poor. Why has the stability of\n3 4\n30% Co O @PCN-333(Fe) been improved by several dozen times?\n3 4\nReply: We sincerely thank the reviewer for this insightful comment and fully agree\nthat bulk Co O and pristine PCN-333(Fe) are intrinsically unstable toward O and\n3 4 3\nH O. However, 30% Co O @PCN-333(Fe) does not behave as a simple physical\n2 3 4\nmixture of two unstable components; instead, the MOF-confined architecture\ngenerates a new interfacial “nanoreactor” whose structure and reaction pathway\nfundamentally differ from those of Co O and PCN-333(Fe) alone, thereby leading\n3 4\nto an orders-of-magnitude enhancement in durability.\n(1) Structural confinement and suppression of deactivation pathways\nAC-HAADF-STEM show that Co O is encapsulated as 3.4-3.9 nm clusters\n3 4\nuniformly distributed inside nanopores of PCN-333(Fe), while the MOF\ncrystallinity and pore ordering are preserved. This confinement (i) prevents Co O\n3 4\nsintering or growth into large particles and (ii) avoids external surface deposition\nand pore blockage that typically cause fast deactivation in bulk Co O and externally\n3 4\nloaded oxides. As a result, the active sites remain highly dispersed and accessible\nduring prolonged operation.\n(2) Interfacial charge redistribution and stable redox cycling\nXPS and Co/Fe K-edge XANES analyses reveal concurrent enrichment of Co2+\nand Fe2+, indicative of strong electronic coupling and a high density of oxygen\nvacancies. These interfacial defects serve as robust, regenerable active sites. In bulk\nCo O or bare PCN-333(Fe), O tends to over-oxidize the surface or attack the\n3 4 3\nframework, leading to irreversible deactivation. In contrast, in 30% Co O @PCN-\n3 4\n333(Fe) the redox process is buffered by the synergy Fe O clusters between Co O ,\n3 3 4\nenabling reversible Co-O-Fe redox cycling rather than structural degradation.\n(3) Hydrogen-atom transfer mechanism that turns water from poison into\npromoter\nIn situ DRIFTS and Raman spectroscopy, combined with DFT, demonstrate that\nO decomposition on 30% Co O @PCN-333(Fe) proceeds via a hydrogen-atom\n3 3 4\ntransfer pathway: water coordinated to Fe O sites dissociates and the released\n3\nhydrogen atom transfer between the Fe O cluster, interfacial oxygen, and Co sites\n3\nto form and decompose oxygen-containing intermediates. Protons finally return to\nthe Fe site to regenerate the original Fe-OH motif, with the rate-limiting step being\n2\nregeneration, (ii) minimizes accumulation of peroxo species, and (iii) ensures that\nwater actively participates in the reaction instead of competitively poisoning active\nsites, as happens on bulk Co O surfaces.\n3 4\n(4) Humidity-immune microenvironment and mutual protection of the two\ncomponents\nThe organic linkers and hierarchical pores of PCN-333(Fe) create a\nmicroenvironment that moderates local hydrophilicity/hydrophobicity and screens\nCo O nanoclusters from direct water flooding, while Co O rapidly consumes O\n3 4 3 4 3\nnear the interface, protecting the MOF framework from oxidative damage. This\nmutual protection explains why 30% Co O @PCN-333(Fe) retains both structural\n3 4\nintegrity (Raman, PXRD) and full activity over >160 h at 40°C and 10% RH, and\n120 h even at 90% RH, whereas both bulk Co O and PCN-333(Fe) alone drop to\n3 4\n<10% and <30% conversion, respectively, within ~3 h under identical conditions.\n5.The diagram of DFT lacks a control sample in Fig. 6a.\nReply: We thank you for this valuable comment regarding the absence of\na DFT reference system. Using pristine Co O as a theoretical benchmark allows the\n3 4\nbenefits of the interfacial nanoreactor to emerge more clearly.\nTo address this point, we have now taken O desorption as a key reference, since\n2\nit has been widely identified in the literature as the rate-limiting step for O\n3\ndecomposition over metal oxide catalysts. We have therefore calculated the\ncorresponding activation barrier on pristine Co O at the same computational level\n3 4\nand using the same model conditions as in Fig. 6. The O desorption barrier on\n2\npristine Co O is 1.84 eV, whereas the barrier for the analogous step on the\n3 4\nCo O @PCN-333(Fe) interfacial nanoreactor is only 0.62 eV, which is markedly\n3 4\nlower than that of Co O systems.\n3 4\nThese results indicate that pristine Co O faces a substantially higher barrier for\n3 4\nO release and thus cannot efficiently convert the key oxygenated intermediates into\n2 3 4\npathway, dramatically lowers the O desorption barrier, thereby promoting stepwise\n3 2\ncatalytic activity and stability compared with the two control catalysts. The\ncorresponding energy barriers and the related discussion have been added to the\nrevised manuscript (Page 16, lines 297-300) and revised SI (Fig. S21).\nFig. S21. The O desorption energy barrier and the corresponding stationary-point\n2\nstructures of Co O .\n3 4\nReviewer #3 (Remarks to the Author):\nIn this manuscript, the authors encapsulated ultrafine metal oxides e.g., Co O )\n3 4\nwithin the nanopores of an Fe O-cluster-based metalorganic framework,PCN-\n3\n333(Fe), and used the resulting material as the catalyst for the O decomposition. A\n3\nhigh efficiency has been obtained for the optimized 30% Co O @PCN-333(Fe)\n3 4\ncatalyst. The decomposition mechanism was proposed based on the spectroscopies\nand DFT calculations. The manuscript is potentially suitable for the publication in\nNature Communications, but the DFT study requires substantial revision and\nrecalculation before it can support the proposed mechanism.\n(1)The computational details are insufficient. The authors should specify which\ncomputational software was used. The spin states of Fe and Co centers are not\ndiscussed this information is essential. Moreover. It should be clarified along which\nspin surface the reaction proceeds.\nReply: We thank the reviewer for pointing out this important issue. All DFT\ncalculations were performed using the CP2K package. In our models, the Fe- and\nCo-containing centers were treated in their high-spin configurations, with total spin\nmultiplicities of 4 and 3, respectively. These choices are consistent with the\nexperimentally inferred oxidation states and the weak-field O-donor environment of\nthe Fe O clusters and Co O -like sites, for which high-spin Fe(III) and Co(II/III) are\n3 3 4\ngenerally accepted as the ground states. All intermediates and transition states along\nthe reaction pathway were optimized on this high-spin surface, so the presented\nmechanism is computed consistently on a single, physically reasonable spin\nmanifold. We have clarified these points in the revised SI (Page S4-S6).\n(2)Only minima were characterized in the calculations. Without transition-state\nstructures, the mechanistic understanding remains incomplete. For example, the\nformation of the first O molecule from OOOH, leaving H and O atoms bound to\n2\nthe catalyst, involves multiple bond-forming and bond-breaking events that seems\nto be unable to achieve in a single step. The current computational results fail to\nprovide insights into this and other key elementary steps.\nReply: We thank the reviewer for this critical comment. In response, we have\ncalculated the kinetic mechanisms for the elementary step of *OOOH intermediate\nforming the first O molecule and *OH (TS1), as well as the subsequent transfer of\n2\nthe generated *OH (TS2). According to the DFT results, the Gibbs free energy\nbarrier (TS1) for the formation of *O and *OH intermediates from *OOOH is 0.47\n2\neV, and the barrier for the transfer of the generated *OH to the O atom of Fe-O-Co\n(TS2) is 0.64 eV. These DFT-calculated energy barriers are moderate, indicating that\nthe reaction process is kinetically feasible. The relevant information has been\nrevised to the revised manuscript (Page 15, Lines 283-291).\n(3)It is very difficult to correlate the states shown in Fig. 6 with those in Fig. S27\nand Table S4.l suggest labeling the states in Fig. 6 (both 6a and 6b) with Roman\nnumerals consistent with the supplementary data. Additionally, the molecular\nstructures in Figs. 6a and S27 are not clearly presented.\nReply: We thank you for this helpful suggestion. In the revised manuscript, we have\nnow labeled all intermediates and transition states in Fig. 6b and 6c with Roman\nnumerals that are fully consistent with those used in Fig. S20 and Table S4, enabling\na one-to-one correspondence between the main text and the Supplementary\nInformation. In addition, the molecular structures in Fig. S20 have been redrawn\nwith improved resolution to enhance their legibility. These changes should make it\nmuch easier for readers to track each elementary step and correlate the states across\nFig. 6, Fig. S20, and Table S4.\n(4)If ·OOOH radicals are indeed generated during the reaction, a hydrogen-atom\ntransfer (HAT) or proton-coupled electron transfer (PCET) process must occur\nrather than the proton-shuttling” mechanism proposed by the authors. Resolving this\nissue requires accurate identification and analysis of the spin states along the\nreaction pathway.\nReply: We thank the reviewer for this valuable comment. As you pointed out,\nif ·OOOH intermediates are involved, the elementary step is more appropriately\ndescribed as a hydrogen-atom transfer process rather than a proton-shuttling\nmechanism, and we have accordingly revised the mechanistic description\nthroughout the manuscript to use the term “hydrogen-atom transfer”. Regarding the\nanalysis of spin states along the reaction pathway, we fully acknowledge its\nimportance for radical-involving processes. However, a rigorous, quantitative\nmapping of all relevant spin manifolds and possible spin-crossing seams for surface-\nbound ·OOOH and related intermediates would require multireference or otherwise\nadvanced electronic-structure treatments together with extensive exploration of\nalternative spin configurations for each intermediate and transition state, which is\nbeyond the scope and computational budget of the present study. In this work, all\nkey intermediates and transition states were treated with spin-polarized DFT within\na single, internally consistent spin manifold, which is a standard and widely\naccepted protocol for comparative mechanistic studies of oxide-catalyzed O\n3\ndecomposition. Within this framework, the conclusion that the Co O @PCN-333(Fe)\n3 4\ninterface substantially lowers the reaction barrier relative to pristine Co O rests on\n3 4\ninternal like-for-like comparisons and is therefore expected to be qualitatively\nrobust, even though a fully exhaustive spin-state landscape is not available."}, {"round": 2, "reviewer_comments": {"reviewer_1": "After revised, this manuscript has clrifed questions and it can be accepted as current form.", "reviewer_3": "The authors have addressed the issues I raised. Now I recommend the acceptance of the revised manuscript."}, "author_response": ""}]

Decision letter