Facet effect of hematite on the hydrolysis of phthalate esters under ambient humidity conditions

Phthalate esters (PAEs) have been extensively used as additives in plastics and wallcovering, causing severe environmental contamination and increasing public health concerns. Here, we find that hematite nanoparticles with specific facet-control can efficiently catalyze PAEs hydrolysis under ambient humidity conditions, with the hydrolysis rates 2 orders of magnitude higher than that in water saturated condition. The catalytic performance of hematite shows a significant facet-dependence with the reactivity in the order {012} > {104} ≫ {001}, related to the atomic array of surface undercoordinated Fe. The {012} and {104} facets with the proper neighboring Fe-Fe distance of 0.34-0.39 nm can bidentately coordinate with PAEs, and thus induce much stronger Lewis-acid catalysis. Our study may inspire the development of nanomaterials with appropriate surface atomic arrays, improves our understanding for the natural transformation of PAEs under low humidity environment, and provides a promising approach to remediate/purify the ambient air contaminated by PAEs.

The submitted work deals with the role of hematite crystals on the hydrolysis of phthalates in vapour phase, and especially the difference between the face orientations.
The title claims the "non-aqueous" hydrolysis, but the authors mean that the reaction take place in presence of hydrated vapour (relative humidity of 76%). So, the title should be changed into "(water) vapour phase" rather than "non-aqueous".
The concept of face-dependent reactivity has existed for decades, as in the earlier work by Knozinger in catalysis (doi 10.1080/03602457808080878 for example in 1978), or Hiemstra in environmental sciences (doi 10.1016/0021-9797(89) 90284-1 in 1989). Unfortunately, the bibliography of the submitted work only refer to the articles of 2000s (with very few exceptions). A few lines about the history of this approach would be welcomed.

Detailed comments
Li. 106. The orientation of faces has been characterized by microscopy. The surface reactivity depends on the purity of these surfaces, so a low amount of adsorbed ions/molecules can change it. It would be interesting to measure the isoelectric point of this solids by zetametry, and compares them to calculated ones to ensure that the difference in reactivity comes from the orientation and not from the presence of surface residual chemicals used in the synthesis. Li. 115. Has the presence of defects been investigated? They are expected to have a huge effect on reactivity.
Li. 121. The number of digits for the SSAs should be decreased (18.1 instead of 18.08 for example).
Li 141. The term "water content of 400%" is troublesome, as "over-wet condition". If the authors mean that liquid water is present, they could give the calculated value of water layers at the surface of the particles for example. Li. 165. Uncoordinated Fe sites are expected to be very reactive towards water molecules as a vapour or in a liquid. Thus, this type of sites may be not exist in the conditions of the synthesis (in water or at RH > 0%). This type of question arises in the surface chemistry of transition aluminas, and This investigation is more appropriate for some international journal covering the field of environmental protection.

I do not recommend this manuscript as publication in Nature communications.
Response: Thank you for the comments even though the reviewer did not recommend our paper as publication in Nature Communications. Herein, we would like to briefly explain the novelties of this manuscript. In this study, we investigated the specific facet effect of hematite on the hydrolysis of PAEs under ambient humidity/moisture condition, and found that the hematite {104} and hematite {012} exhibited 1-2 orders of magnitude higher catalytic activities than the hematite {001}. We proposed that the neighboring Fe atoms on the facet with a proper distance of 0.35-0.40 nm could form the bidentate coordination complex with PAEs, then inducing stronger Lewis-acid catalytic degradation. We think the novelties of this study stand on the following three aspects: 1) It is the first report to investigate the facet selectivity of minerals for "nonaqueous reaction" of organic contaminants. Currently, there are abundant studies about the mineral facet-dependent environmental processes, e.g., iron dissolution (Huang et al., 2017), adsorption of (in)organic substances (Huang et al., 2016;Shen et al., 2020), photocatalytic degradation (Guo et al., 2021;Zong et al., 2021;Zhou et al., 2012;), thermo-catalytic oxidation (Liu et al., 2011), and surface hydrolysis reaction , etc. However, almost all these facet-dependent reactions were conducted in aqueous solution. To the best of our knowledge, there are still no reports about facet-selective non-aqueous transformation of organic 2 contaminants. Although this process may be important in soil and air environments, as soil is only partially hydrated in most cases, and mineral particles in the air are exposed under ambient humidity, this transformation pathway is usually ignored. In this study, our results reveal that the relatively dry mineral surface with certain facet can perform much higher catalytic reactivity under ambient humidity/moisture conditions compared to the reaction occurring in solution.
2) The proposed bidentate binuclear complexation mechanism is new. The facet reactivity usually depends on the abundance and affinity of surface exposed reactive sites (Mei et al., 2020;Shen et al., 2020). While, our study reveals that the surface Fe atomic array is also important for certain reactions. It inspires researchers to select and design novel nanomaterials with proper surface atomic array (i.e., the active sites) to achieve high catalytic reactivity. For example, besides hematite, we also synthesized the {001} and {101} facet dominated titanium oxide (TiO2) nanoparticles (Li et al., 2015). Since the neighboring Ti-Ti site distance on both {001} and {101} facets is ideal for bidentate coordination (i.e., 0.38 nm), both minerals perform considerable hydrolysis rates for dimethyl phthalate (DMP) under RH 76% ( Figure R1).
While, the commercial amorphous TiO2 nanopowder exhibits poor catalytic activity due to its amorphous surface condition ( Figure R1). The results further emphasize the import roles of plane facet and bidentate-coordination for the hydrolysis reaction.
3) The facet dependent hydrolysis of PAEs is applicable for actual environmental remediation/purification. In this study, we also demonstrate that the facet specific hematite exhibits promising applicability for purifying PAE contamination in plastic greenhouse and indoor air, which is still lack of proper purification strategies up to date. Currently, most of the reported PAE degradation approaches, involving biodegradation (Di Gennaro et al., 2005), strong base catalyzed hydrolysis (Xu et al., 2010), radical based chemical oxidation (Yao et al., 2020), photocatalysis (Cai et al., 2021), etc., are designed for soil remediation or water treatment, while not adequate for air purification.
Based on the three justifications, we believe our study is scientifically valuable.

3
On the other hand, "Nature Communications" is a multidisciplinary journal covering all areas of biological, health, physical, chemical and earth sciences. We think our manuscript falls within the scope of Nature Communications, subjecting to the discipline of Earth and Environmental Science/Environmental Sciences/Environmental Chemistry. We carefully searched the published articles in Nature Communications within the scope "environmental chemistry + environmental protection + organic pollutant degradation", and found several articles since 2021, as listed below. It indicates that Nature Communications also welcomes investigations in the field of environmental protection.
(5) Chu, C., Huang, D., Gupta, S. et al. Neighboring Pd single atoms surpass isolated single atoms for selective hydrodehalogenation catalysis. Nat. Commun. 12, 5179 (2021). https://doi.org/10.1038. This research reported that the neighboring Pd single atoms can contribute to a superior performance for selective hydrodehalogenation of organohalides (e.g., 4-chlorophenol), and the DFT calculation also supports that the two neighboring Pd atoms are necessary for C-Cl bond cleavage.
(7) Min, Y., Zhou, X., Chen, JJ. et al. Integrating single-cobalt-site and electric field of boron nitride in dechlorination electrocatalysts by bioinspired design. Nat. Commun. 12, 303 (2021). https://doi.org/10.1038/s41467-020-20619-w. This article reported a single-atomic-site Co catalyst supported by carbon doped boron nitride for electro-catalytic reduction of chloramphenicol, which is a kind of halogen-containing antibiotics.   Kraushofer et al., 2018;Ovcharenko et al., 2016;Yan et al., 2020). In our original manuscript, we directly used the generally accepted surface termination model for the individual facet according to 9 these references. For example, for hematite {001} facet, the one-layer Fe-termination possesses the lowest surface energy under a wide range of oxygen chemical potentials ( Figure R2, Layer 1), while the double-layer Fe termination ( Figure R2, Layer 3) is usually energetically unfavorable Ovcharenko et al., 2016;Yan et al., 2020). Other investigations by scanning tunneling microscopy (SEM) proposed that both one-layer Fe-termination and O-termination might co-exist if the hematite {001} was prepared under high oxygen pressure (Bergermayer et al., 2004;Wang et al., 1998), or if the Fe atoms were to dissolve from an Fe-termination (Eggleston et al., 2003). Additionally, a ferryl (Fe=O) termination could also be observed under intermediate oxygen pressure, with the domain of onelayer Fe-termination (Lemire et al., 2005). While, the ferryl (Fe=O) termination exhibits higher surface energy than the one-layer Fe termination (Ovcharenko et al., 2016).
Although the exact surface termination of {001} facet is under debate among different experimental and theoretical investigations, the one-layer Fe-termination is more preferential, due to its lower surface energy and non-polar characteristic after its surface relaxation . For {012} facet, its surface termination is relatively simple. It possesses one plausible stoichiometric (1×1) surface termination with the arm-chair like topography as depicted in Figure R3, Layer 1 Kraushofer et al., 2018). In some conditions, the stoichiometric (1×1) surface can be reduced to (2×1) model by removing a raw of bridging oxygen ions (Kraushofer et al., 2018), but it is reversible via O2 treatment (Henderson, 2010).
However, the surface termination of {104} facet is less studied (Chan et al., 2015;Zhang et al., 2021), and additional experiments and theoretical calculations may be required.
Therefore, in the revised manuscript, we calculated the surface energies of different possible surface terminations for each facet using DFT+U method, performed in the VASP software package version 5.4. The generalized gradient approximation (GGA) approach with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was used for the energy calculation. The Coulomb repulsion parameter applied to the Fe 3d orbital of hematite was set as U = 5.0 eV. As shown in Table R1,   the terminations of {001}-layer 1, {104}-layer 1/layer 5 and {012}-layer 1 possess the   relatively lower surface energies. In detail, for the {001} facet, its layer 1 termination is the most stable configuration that is in accordance with the previous reports Ovcharenko et al., 2016;Yan et al., 2020). The outmost Fe atoms are relaxed inward to the underlayer plane of O atoms to reduce the surface energy ( Figure R2, Layer 1).
The inward relaxation is accompanied with significant charge redistribution, leading to the lowest empty surface state with Fe 3dz2 characteristic, which also significantly reduces the surface dipole and increases its stability (Ovcharenko et al., 2016). The Bader charge analysis indicates that the outmost Fe atoms remain 6.39 valence electrons (VEs), which means that 1.61 (calculated as: 8-6.39) VEs are transferred from Fe to the surrounding O anions ( Figure R5a). Therefore, although the outmost Fe atoms are three-fold coordinated with O ligands, their Lewis-acidity is moderate.
For the {012} facet, the DFT+U calculation suggests that the {012}-layer 1 is the most stable surface ( Figure R3, Table R1), which is also in coincide with the previous studies Kraushofer et al., 2018). After surface relaxation, the neighboring Fe distance is adjusted to 3.532 and 3.919 Å, which is suitable to form bidentate coordination with PAEs ( Figure R6a). The surface Fe on {012}-layer 1 has the average VEs of 6.28, suggesting the relatively weak Lewis-acidity. However, the diffused reflectance infrared Fourier transform spectroscopy (DRIFTs) measurement shows that the {012} facet has even stronger Lewis acidity than the {001} and {104} facets (Please see the response to the reviewer 3, Figure 26 or Supplementary Figure   15). This discrepancy might be ascribed to the existence of surface O vacancies.
Theoretically, the stoichiometric (1×1) surface of {012} facet is an ideal bulk termination. A certain degree of O defect must exist in actual sample (Kraushofer et al., 2018). We calculated the {012}-layer 1 termination with one O vacancy. The Fe atoms jointed to the O vacancy have the high VEs of 6.70 ( Figure R5b), indicating much stronger Lewis-acidity. The surrounding Fe atoms also possess higher VEs, due to surface charge re-distribution ( Figure R5b).
For the {104} facet, we calculated the five possible terminations ( Figure R4). The {104}-layer 1 possesses the relatively lower surface energy (γ = 1.94 J/m 2 ), following with {104}-layer 5 (γ = 2.12 J/m 2 , Table R1). In comparison with the {104}-layer 1, the layer 5 termination has an additional layer of bridging O atoms between the outmost  Figure R6b). This result is essential for calculating the bidentate site density.
Moreover, the bare exposed Fe atoms are reactive with water molecules. Therefore, the actual hematite surface must be hydroxylated and hydrated. Many theoretical calculations and experimental studies have investigated the interaction between water molecules and the hematite {001} and {012} facets (Kraushofer et al., 2018;Ovcharenko et al., 2016;. However, the surface hydroxylation and hydration conditions are complicated, and the reported results sometimes disagree with each other Ovcharenko et al., 2016;. By analyzing the O 1s XPS, the three hematite surfaces are partly hydroxylated and hydrated, with the bond -OH of 16-21.3%, and the chemisorbed H2O of 13.0 -17.3%, even under vacuum condition. Therefore, under ambient condition, the hematite surface should be more hydroxylated and hydrated. However, PAEs coordinate with the surface exposed Fe rather than -OH/H2O. If the Fe sites are occupied by hydroxyl groups or H2O molecules, PAE molecules could compete/substitute with the surface bond -OH/H2O for the uncoordinated Fe, especially under low surface moisture conditions. This can be evidenced by in-situ DRIFTS measurements that during Cl-DMA or DMP adsorbing onto the hematite surface, the surface bonded -OH/H2O is simultaneously substituted as indicated by the reversal peaks in the range of 3000-3700 cm -1 (please see  Table   1), which is in accordance with the previous reports 49-51 . The exposed Fe atoms on {001}-layer 1 termination are three-fold coordinated. However, according to Bader charge analysis, the valence electrons (VEs) remaining on the exposed Fe are relatively low (VEs = 6.39, indicating 1.61 VEs are transferred from Fe to the adjacent O) (Fig.   3a), since the exposed Fe atoms are relaxed inward to the underlayer plane of O atoms with significant charge redistribution.
It is important to note that the higher VEs of Fe cations usually implies lower coordination state and stronger coordination ability. However, when we further applied the diffused reflectance infrared Fourier transform spectroscopy (DRIFTS), using the compound 2-chloro-N,N-dimethylacetamide (Cl-DMA) as the probe molecule, to identify the surface Lewis-acid sites 12 , the results are discrepant to the Bader charge analysis (Fig. 3, Supplementary Fig. 14). For example, HNC exhibits wider red-shift of νC=O, which denotes to the carbonyl stretching vibration (1621 cm -1 on HNC vs. 1628 cm -1 on HNP & 1625 cm -1 on HNR, Supplementary Fig. 15a-c), suggesting that the HNC surface is more Lewis-acidic. This discrepancy might be ascribed to the existence of surface O-defects on actual HNC surface 53 . By introducing a certain degree of Odefects on the surface of {012} facet model, the Fe atoms adjacent to the O-vacancies possess 6.70 -6.74 VEs (Fig. 3c, Supplementary Fig. 14d), corresponding to the stronger Lewis-acidity. Meanwhile, a better fit for the IR spectra was also obtained using the {012} facet with O-defect for theoretical modelling (more discuss is shown below). Further study shows that the HNC400 (i.e., HNC calcinated at 400 o C for 2 h) exhibits a new shoulder peak at 1582 cm -1 ( Supplementary Fig. 15f), probably stemming from the thermal desorption of -OH groups and H2O molecules at the defective sites 56 , suggesting a small amount of surface defective sites. For comparison, the νC=O peaks in both HNR and HNR400 samples are narrow and sharp 14 ( Supplementary Fig. 15b, e), indicating negligible surface defects on HNR.
Moreover, the surface undercoordinated Fe sites are reactive with water molecules.
Under low water partial pressure, H2O molecules would entirely or dissociatively bond to lattice Fe and O with interactive hydrogen bonding, leading to surface hydroxylation, hydration and protonation (Fig. 3, Supplementary Fig. 14) Fig. 4 -Fig. 6). Therefore, under ambient condition, their surface should be more hydroxylated and hydrated 61 . Although surface hydroxylation/hydration could shield the surface reaction, and affect the valency of the exposed Fe, organic ligands can still compete with the surface bond -OH/H2O for the undercoordinated Fe sites, especially under low moisture condition 62 , which is strongly evidenced by the desorption of the chemisorbed -OH/H2O (3000 -3750 cm -1 ) substituted by either Cl-DMA or DMP on the three facets (Supplementary Fig. 15 & 16). Therefore, the surface hydroxylation/hydration would not change the coordination mode and the catalytic mechanism. To simplify the facet model, surface hydroxylation/hydration was not involved for modelling. In current study, the {001}layer 1, {104}-layer 1 co-existing with layer 5, and the {012}-layer 1 with O-defects are deduced as the surface terminations for HNP, HNR and HNC, respectively (Fig. 3)." In the "Methods" part, the method for VASP calculation was added, in page 24, line 479-488: "DFT calculations were performed to obtain the surface energies, the adsorption configurations, and the corresponding theoretical IR spectra, using the VASP 5.4 software. The generalized gradient approximation (GGA) approach with the Perdew-Bruke-Ernzenrhof (PBE) exchange-correlation functional, and a cutoff energy of 550 eV for the planewave basis set were used for all the calculations. DFT + U method was adopted to treat the 3d orbital electrons of Fe with Ueff = 5.00 eV 51 . Core    a. The surface energy (γ) comprised by the cleavage energy ( E cut ) and the reconstruction energy (E relax ) . All the atoms are kept fixed except for the atoms in the top surface layer. Namely, In the revised manuscript, the relevant results and discussion have been added in page 14-15, line 272-284: "The catalytic activity of the facet also strongly depends on the affinity of the exposed Fe atoms. The in-situ DRIFTS measurement (Supplementary The TON was calculated after reacting for 1 d, t = 1 d; C0 is the initial concentration of 2 μmol·g -1 ; NA The TOF of the initial stage is calculated. What is more, how to wash the samples needs to be provided. Response: Thank you for the comments. The X-ray photoelectron spectra (XPS) of the three hematites (HNP, HNR and HNC) are shown in Figure R7-R9 and all present clear C 1s peak. However, it does not mean that the hematite surface is heavily contaminated by the organic ligands during synthesis, since the fitted C compositions (C-C/C-H: 73-77%, C-N/C-O: 12-18%, C=O: 9-11%) do not match the chemical compositions of the organic ligands (CH3COOfor HNP, H2NCONH2 and HCONH2 for HNR, CH3COONH4 for HNC). For example, H2NCONH2 and HCONH2 were used to synthesize HNR. While, there are no C-C and C-H bonds in the two ligands, and the synthesizing temperature of 160-180 o C is unlikely to cause carbonization of the organic ligands. For HNR and HNC, the organic ligands used for synthesis contain high amount of N element. Therefore, the detection of N 1s by XPS could also reflect whether the hematite surface was contaminated by the organic ligands or not. As shown in Figure R8 and R9, the N 1s XPS signal was not observed on HNR and HNC, suggesting that the hematite surfaces were not contaminated by the organic ligands from synthesis.
To further verify this, we washed the hematite nanoparticles with ethanol and water for more than 10 times, then calcinated them at 400 o C for 2 h. However, the additional washing and calcination treatments still cannot remove the C 1s signal.
Referring to the literatures from different researchers, the residual organic carbon is commonly detectable in XPS analysis. For example, the synthesized hematite {001}, {012}, {110} nanoparticles also presented clear C 1s XPS signal in the literatures ( Figure R10), demonstrating that carbon contamination is difficult to avoid (Guo et al.,25 2021; Huang et al., 2016;Shen et al., 2020). Since the C compositions on the three hematite are very similar, the carbon contamination might stem from extra sources after synthesis. However, the discussion for the exact source of the carbon contamination is beyond the scope of current study, further investigation should be conducted.
As suggested by the other reviewer, we also measured the isoelectric point of the Moreover, the discussion about surface characterization, based on the XPS and zeta potential analysis, has also been added to the revised manuscript, in page 8, line 129-139: "Their isoelectric points were measured as pH 7.0 -8.5 ( Supplementary Fig.   3), close to the reported values 17,47 . No N 1s signals in X-ray photoelectron spectra (XPS) were detected on HNR and HNC ( Supplementary Fig. 5 & Fig. 6    We firstly calculated the theoretical IR spectrum of pure DMP, as a benchmark. As shown in Figure R11, For DMP adsorbed on {001}-layer 1, both the trans-DMP and cis-DMP form mono-dentate coordination with the surface exposed Fe atom ( Figure R9). The optimized cis-DMP geometry is not ready to form bidentate coordination on {001}layer 1, since the distance to the neighboring Fe is 0.5063 nm that is too wide (Figure   33 R2). The calculated IR spectra show that the carbonyl stretching vibration (ν2) red-shifts to 1640-1650 cm -1 ( Figure R12), which is slightly weaker than the experimental result (ν2 = 1621 cm -1 , Figure 4e(2) & 4f(1) in the manuscript). This can be probably explained by the presence of O defects on actual HNP surface. Unless prepared with extreme care, the synthesized hematite surface generally possesses a wide variety of defects.
Consequently, the surface electronic structure would also be changed by point defects associated with surface nonstoichiometry . in the manuscript). Since the {014}-layer 5 also has relatively low surface energy (Table   R1), it may co-exist as {104} surface termination. However, the subsurface Fe atoms on {104}-layer 5 are fully coordinated, thus, DMP molecule can only bidentatecoordinate with the outmost Fe. In this model, the outmost Fe atoms possess relatively low VEs (6.28, Figure R15b). Correspondingly, the obtained carbonyl vibration shows a relatively small red-shift to 1627 cm -1 ( Figure R13), compared to that on {104}-layer 1.
For DMP adsorbed on the {012}-layer 1, both the neighboring Fe sites with the distance of 0.36 nm and 0.39 nm can bidentate-coordinate with cis-DMP, and the corresponding carbonyl vibration red-shifts to 1607~1620 cm -1 ( Figure R14) Figure R15c). Taking the {012}-layer 1 with one O vacancy for modelling, the obtained IR peak of ν2 shifts to 1590 cm -1 ( Figure   R14), which can also fit with our experimental observation. We suppose DMP molecules at low surface concentration prefer to interact with the Fe sites around O vacancies.
To sum up, we additionally performed the VASP calculation to simulate the adsorption configurations and IR spectra of DMP on different slab models, which could represent the periodic surface. The calculations can fit well with the experimental results. In general, bidentate coordination can induce stronger Lewis-acid interaction than monodentate coordination, imposing wider red-shift of carbonyl vibration to lower than 1600 nm -1 . In addition, using the simplified Fe-hydroxyl clusters to simulate surface Lewis-acid interaction, carried out by Gaussian calculation, is also reliable.
In the revised manuscript, we added the results by VASP calculation and the corresponding discussion in page 15-18, line 297-353: "To obtain further insights into the adsorption mechanism, we applied two methods to calculate the theoretical IR spectra, and compared with the experimental IR spectra.
While, on the O-defective site of {012}, the νC=O appears at 1627 (ν1) and 1590 (ν2) cm -1 (Fig. 4c(3)). Therefore, the theoretical IR spectra of DMP on the slab models directly show the coordination modes. In general, bidentate coordination can induce stronger Lewis-acid interaction than monodentate coordination, imposing wider red-shift of νC=O even to < 1600 nm -1 .
Secondly, the simplified Fe-hydroxyl clusters were also introduced to complex with DMP/DnBP as the representatives of the surface coordination models (Fig. 4d), as the following hydrolysis pathways were calculated using the DMP-Fe-hydroxyl cluster models. This calculation was conducted by the Gaussian software. Taking DMP molecule itself as a benchmark, both the VASP and Gaussian calculations can precisely predict the IR spectrum of pure DMP ( Supplementary Fig. 19). In detail, the symmetrical carbonyls of trans-DMP possess one theoretical νC=O at ~1730 cm -1 ( Fig.   4d(1)). For cis-DMP, since its two carbonyls are asymmetric, the νC=O splits to 1724 and 1744 cm -1 (Fig. 4d(2)). When one carbonyl group of trans-DMP coordinates with Fe(OH)3, the un-complexed νC=O locates at 1700 (ν1) cm -1 , and the complexed one redshifts to 1645 (ν2) cm -1 (Fig. 4d(3)). When both carbonyls of cis-DMP bidentatecoordinate with Fe2O(OH)4, the νC=O groups exhibit much wider red-shift to 1645 (ν1) and 1596 (ν2) cm -1 (Fig. 4d(4)). Similar results were obtained for DnBP ( Supplementary   Fig. 20). The IR spectrum of DMP by Gaussian calculation is in accordance with that by VASP calculation. Therefore, using the simplified Fe-hydroxyl clusters to simulate surface Lewis-acid interaction is also reliable.
By in-situ DRIFTS measurement, DMP was blown into the system filled with the HNP/HNR/HNC nanoparticles, then the cumulative adsorption of DMP on the hematite particles was in-situ detected. As shown in Fig. 4f, two significant νC=O absorption peaks were observed at ~1718 and 1620 cm -1 on all the three facets. The former indicates the physical adsorbed or weakly bond DMP, and the later should represent for the Lewisacid coordinated one. After 130 min, two additional shoulder peaks at ~1565 and ~1590 cm -1 start to appear on HNR (Fig. 4f(2), Supplementary Fig. 22a-b), suggesting bidentate-coordinated adsorption configuration formed on HNR. Such shoulder peaks were also observed on HNC (Fig. 4f(3)), however, did not appear on HNP ( Fig. 4f(1)).
Both the ex-situ and in-situ experimental IR spectra are in good accordance with the theoretical IR predictions (Fig. 4c & 4d), thus, providing the strong evidence that DMP is adsorbed onto HNR and HNC via the bidentate coordination, while via the monodentate coordination on HNP. As indicated by the much wider red-shifts of νC=O, the bidentate coordination can induce stronger Lewis-acid interaction."

The size distribution of these nanoparticles may be needed because sizes seem not well distributed from the SEM images, especially for the pseudo-cube nanoparticles.
Response: Thank you for the comment. The size distribution of synthesized hematite nanoparticles was characterized by the laser particle size analyzer (ZEN 3500 Zetasizer), at 100 mg/L and pH = 4.0 without pH buffer. The nanoparticles were dispersed directly into water and sonicated for 10 min before the measurement. As shown in Figure R16, the three hematites have wide size distribution. The average particle sizes for HNP, HNR and HNC are 304, 462 and 370 nm, slightly higher than the apparent particle sizes from SEM and TEM imagines, probably due to the partial aggregation of the nanoparticles in water.
In the revised manuscript, the corresponding information was added in page 8, line

The reactivity of the nanoparticles needs to be compared to other metal oxides for the hydrolysis of phthalates.
Response: Thank you for the comment. In the revised manuscript, we applied another two common metal oxides, γ-Al2O3 and TiO2 for comparison. The γ-Al2O3 was prepared from active neutral alumina (Shanghai Ludu Chemical Research Co. Ltd., Shanghai, China). Briefly, the neutral alumina was initially ground to pass through a 100 mesh sieve, then oven-heated at 600 o C for 2 h to obtain the γ-Al2O3. While, the TiO2 nanopowder (CAS 13463-67-7), which is amorphous, was purchased from Shanghai Lingfeng Chemical Research Co. Ltd. (Shanghai, China). As shown in Figure   R17, the γ-Al2O3 and the amorphous TiO2 perform one-order of magnitude lower catalytic hydrolysis activity than HNR (0.232 day -1 ) and HNC (0.755 day -1 ), even slightly lower than HNP (0.0476 day -1 ). We suppose that the over hydroxylated surface of γ-Al2O3 is unfavorable for Lewis-acid catalytic process. Moreover, since the TiO2 nanopowder is amorphous, it is also unable to form the bidentate coordination with DMP.
As our study has indicated that the plane facet and the formation of bidentate coordination are the two important prerequisites for rapid hydrolysis performance, we then synthesized two TiO2 nanoparticles with the main {001} facet and {101} facet, respectively, according to the methods by Li, et al. (2015). From the SEM imagines (please see the response to Reviewer 1, Figure R1), the  Fig. 26c, f), which is appropriate for bidentate coordination with DMP, according to our proposed mechanism. As shown in Supplementary Fig. 26g

The iron atoms between two adjacent surfaces are also undercoordinated, and thus
the contribution of the reactivity needs to be evaluated.
Response: Thank you for the comment. The amount of edge Fe atoms between two adjacent facets should be far less than the amount of undercoordinated Fe atoms on the exposed facet, according to the relative surface areas ( Figure R19) .
Therefore, the possible contribution from the edge/corner sites should be insignificant.
The Fe atoms between two adjacent surfaces (namely, the Fe in edge sites) usually possess lower coordination number. So, they should have stronger Lewis-acidity, not only to the contaminants, but also to surface water molecules. According to Boily et al. (2015) and , water molecules are reactive with the edge and defective sites under low water vaper pressure condition ( Figure R20). When surface water molecule is more abundant than the contaminates, these undercoordinated sites would prefer to react with water molecules. Finally, the Lewis-acidity of the edge sites would be reduced to a homogenized level as the bulk surface sites. Therefore, the strong Lewis-acidity of the edge sites has the low probability to react with the contaminates, if the mineral surface contains more than one monolayer of water (at RH 15%).  127, it has been modified: "For HNC, it has a pseudo-cubic shape with edge length of ~150 nm (Fig. 1c, f), lattice fringe of 0.37 nm and lattice angle of ~86˚ (Fig. 1i), indicating the single {012} facet." 7. Figure 4, Please indicate what the atoms are in different colors.
Response: Thank you for the comment. In the revised manuscript, the atoms in different colors have been assigned in Figure 3 and Figure 4.

TOC art is not fully displayed.
Response: Thank you for the comment. In the revised manuscript, a new compact TOC is provided. Figure R21. The new TOC art.

Reviewer #3 (Remarks to the Author):
The submitted work deals with the role of hematite crystals on the hydrolysis of phthalates in vapor phase, and especially the difference between the face orientations.
The title claims the "non-aqueous" hydrolysis, but the authors mean that the reaction take place in presence of hydrated vapor (relative humidity of 76%). So, the title should be changed into "(water) vapor phase" rather than "non-aqueous".
Response: Thank you for the comments. It is indeed a tricky question how to define the concept of "non-aqueous reaction". Originally, the description of "non-aqueous reaction" is to distinguish it from the "aqueous reaction", because the majority of interfacial studies were conducted in water solution, without considering the actual moisture conditions even if necessary. The mineral surface under relatively dry conditions would exhibit significantly different surface properties in comparison with that in aqueous phase.
Under the exposure of ambient humidity, the mineral surface is not completely dry.
It was reported that the mineral surface retains several layers of water molecules under the relative humidity (RH) of 76% Wirth et al., 2016). It is difficult to describe such surface condition precisely and elegantly. We have used other expression, e.g., the "low-moisture condition", "near drought/dry condition", "waterunsaturated phase/condition". In addition, the expression of "(water) vapor phase" is appropriate for describing the reactions in the atmosphere. However, we think that it may not be suitable to represent the soil condition. After a comprehensive consideration, in the revised manuscript, we modified the article title to "The facet effect of hematite on the hydrolysis of phthalates under ambient humidity/moisture condition", and the expression of "non-aqueous" in the revised manuscript has also been modified.

References:
Boily However, almost all these facet-dependent reactions were found in aqueous phase 17, 24-31 , and so far, only one study reported the facet-mediated hydrolysis reaction by hematite 17 . Therefore, exploring the hematite facet-mediated hydrolysis of PAEs under ambient humidity/moisture conditions can extend our understanding for the 52 environmental fate of PAEs." Figure R22. The number of year-published articles with the key words of "facet" + "catalysis", or "facet" + "adsorption". The data were collected from Web of Science (https://www.webofscience.com/wos/alldb/basic-search).

Detailed comments
Li. 106. The orientation of faces has been characterized by microscopy. The surface reactivity depends on the purity of these surfaces, so a low amount of adsorbed ions/molecules can change it. It would be interesting to measure the isoelectric point of this solids by zetametry, and compares them to calculated ones to ensure that the difference in reactivity comes from the orientation and not from the presence of surface residual chemicals used in the synthesis.
Response: Thank you for the comments. The three hematite after synthesis were carefully washed by ethanol (> 98%) and pure water (18 MΩ) for at least 5 times for each washing step. Usually, such washing protocol is adequate to remove the surface residual organic ligands and ions. To verify this, as suggested by the reviewer, we measured the surface zeta (ζ) potentials of the three hematite in a wide pH range (2-11).
As shown in Figure R23, the measured isoelectric points for HNC, HNP and HNR are between pH 7.0 and 8.5, close to the values reported in the literatures (Chatman et al.,53 2013; Li et al., 2020). For example, the isoelectric point of hematite {001} was reported as ~7.8 by the measurement of ζ potential , and the measured equivalent point of zero potential for hematite {001} and {012} were in the range of 8.35~8.65 (Chatman et al., 2013). The isoelectric point of hematite can to some extent indicate its surface condition. If the hematite surface is contaminated by organic carboxylic acid, the surface electrostatic potential would be significantly reduced. For example, it was reported that the isoelectric point of hematite {001} would decrease to pH < 3 in the presence of 10 μM oleic acid (Quast, 2016). It is obviously not the case for HNP {001} in our study, as we used acetic acid for synthesis.
However, we did not find the theoretical isoelectric point of hematite from literatures. The pKa values of the surface hydroxyl groups on hematite {001} and {012} were once predicted to be 10.3~18.9 by Ab initio molecular dynamics modelling (Yan et al., 2020). While, it is difficult to calculate the isoelectric point from the theoretical pKa values, due to the surface heterogeneity.
In addition, we measured the C 1s and N 1s signals of the synthesized hematite using X-ray photoelectron spectroscopy (XPS), to explore whether the hematite surface was contaminated by organic molecules. The most likely source of carbon contamination is the residual organic ligands from synthesis. The organic ligands used for synthesizing HNR are H2NCONH2 and HCONH2, and CH3COONH4 for HNC. All the ligands contain high amount of N element. While, as shown in Figure R24a -b, no N 1s XPS signal was observed on HNR and HNC, suggesting that no organic ligands are left on the hematite surface after preparation. However, the C 1s signal shows slight carbon contamination ( Figure R24c-e). The C 1s spectra on HNP, HNR and HNC are similar (C-C/C-H: 73-77%, C-N/C-O: 12-18%, C=O: 9-11%), and the compositions of carbon species do not match the chemical compositions of the organic ligands. For example, H2NCONH2 and HCONH2 were used to synthesize HNR. While, there are no C-C and C-H bonds in H2NCONH2 and HCONH2, and the synthesizing temperature of 160-180 o C is unlikely to cause carbonization of the organic ligands. Therefore, such carbon contamination might be from some adventitious sources. Actually, carbon 54 contamination on hematite is difficult to avoid, which were easily detectable by XPS analysis, and were commonly reported in the literatures ( Figure R10) (Guo et al., 2021;Huang et al., 2016;Shen et al., 2020). The discussion for the exact source of the carbon contamination is beyond the scope of current study.
However, it is important to note, the composition of C=O species only accounts for 9-11% of the total carbon component on the hematite surface. Only the C=O species can compete for the Lewis-acidic sites. The C-C/C-H and C-O/C-N species have poor complexation capability. Therefore, the slight carbon contamination should have little influence on the hydrolysis reaction. It would not challenge the main conclusion of current study. Therefore, the isoelectric points and the XPS results suggest the synthesized hematite surface are clean with small amount of adventitious carbon contamination. The varied surface reactivity is unlikely due to the presence of surface residual chemicals. Since MB and DMP/DnBP process different reaction kinetics on the three hematite, this provides a more robust justification that the difference in reactivity comes from the atomic-array of the facet.
The discussion about the surface condition has been added to the revised manuscript in page 8, line 129-139: "Their isoelectric points were measured as pH 7.0 -8.5 ( Supplementary Fig. 3), close to the reported values 17,47 . No N 1s signals in X-ray photoelectron spectra (XPS) were detected on HNR and HNC ( Supplementary Fig. 5 & Fig. 6), suggesting that no organic ligands from synthesis remain on the hematite surfaces. Although the XPS shows clear C 1s signals, the C=O species, which is able to compete for the Lewis-acid sites, comprise only a low amount (9-11% of the total surface carbon content, Supplementary Fig. 4 -Fig. 6). Therefore, the isoelectric points and the XPS results suggest that the HNP, HNR, HNC surfaces are clean with small amount of adventitious carbon contamination, and the residual carbon is expected to have little influence on the surface reactions." More discussion is provided in Supplementary Text 1: "Characterization of the surface condition. The isoelectric point of hematite can to some extent indicate its surface condition, because the isoelectric point of hematite could be sensitive to the 55 adsorbed ions and ligands. Based on zeta (ζ) potential measurement ( Supplementary   Fig. 3), the isoelectric points for HNC, HNP and HNR nanoparticles were measured as 7.0 -8.5, close to the values reported in the literatures. For example, the isoelectric point of hematite {001} was reported as ~7.8 1 , and the measured equivalent point of zero potential for hematite {001} and {012} were in the range of 8.35~8.65 2 . If the hematite surface was contaminated by organic carboxylic acid, the surface electrostatic potential would be significantly reduced. It was reported that the isoelectric point of hematite {001} would reduce to pH < 3 in the presence of 10 μM oleic acid 3 . It is obviously not the case for HNP in our study, as we used acetic acid for synthesis. Thus, the isoelectric points suggest that our synthesized hematite nanoparticles were unlikely contaminated by the organic ligands used in synthesis.
X-ray photoelectron spectroscopy (XPS) can provide extra evidence to examine whether the hematite surface was contaminated by organic. The XPS results of the three hematite (HNP, HNR and HNC) are shown in Supplementary Fig. 4-Fig. 6. Although all presented clear C 1s peak, the fitted C compositions (C-C/C-H: 73-77%, C-N/C-O: 12-18%, C=O: 9-11%) do not match the chemical compositions of the organic ligands (CH3COOfor HNP, H2NCONH2 and HCONH2 for HNR, CH3COONH4 for HNC). For example, H2NCONH2 and HCONH2 were used to synthesize HNR. While, there are no C-C and C-H bonds in the two ligands, and the synthesizing temperature of 160-180 o C is unlikely to cause carbonization of the organic ligands. In addition, since the organic ligands used for synthesis contain high amount of N element in the cases of HNR and HNC, the detection of N 1s by XPS could also reflect whether the hematite surface was contaminated by the organic ligands or not. As shown in Supplementary Fig. 5 & Fig.   6, no N 1s XPS signal was observed on HNR and HNC. Therefore, we can conclude the hematite surfaces are not contaminated by the organic ligands from synthesis.
To further verify this, we washed the hematite nanoparticles with ethanol and water for more than 10 times, then calcinated them at 400 o C for 2 h. However, the additional washing and calcination treatments still cannot remove the C 1s signal.   (14), 8622-8631. Yan, L., Chan, T. and Jing, C. 2020 Response: Thank you for the comment. In our previous studies, we developed a diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) method to identify the Lewis-acid centers on iron mineral surface, using 2-chloro-N,Ndimethylacetamide (Cl-DMA) as the probe molecule , as Cl-DMA is volatile and its carbonyl group can complex with the exposed Fe sites. So, the Cl-DMA molecules could be purged into the DRIFTS system through a humidified N2 flow.
Inside the DRIFTS chamber (model HVC-DRP-5, Harrick Scientific, USA), the iron mineral particles were filled. The Cl-DMA molecules were cumulatively adsorbed by the exposed undercoordinated Fe sites on the mineral surface, then, the adsorption intensity and capacity could be recorded in-situ as indicated by the diffused reflectance IR signals. The varied interaction forces between surface and the carbonyl groups of Cl-DMA would induce different electron states of the carbonyl groups. Therefore, the surface Lewis-acidity can be precisely distinguished according to the extent of red-shift for ν(C=O). We have used this method to probe the surface Lewis-acid sites on different iron minerals (ferrihydrite, goethite, hematite, maghemite) . As shown in Figure R25, the adsorbed Cl-DMA exhibits four distinct absorption peaks at ~1643 cm -1 , 1628-1630 cm -1 , 1583 cm -1 and 1577 cm -1 , respectively. The corresponding shoulder peak at ~1643 cm -1 on goethite represents for the hydrogen-bonding induced ν(C=O) of Cl-DMA. While, the peak at 1628-1630 cm -1 on all the iron oxides indicates the octahedral Fe Ⅲ site complexed with Cl-DMA. Furthermore, the broad peak at 1583 cm -1 on maghemite is ascribed to the interaction from the tetrahedral Fe Ⅲ site, which is specific for maghemite. Finally, the less intensive peak at 1577 cm -1 is assigned to the Fe Ⅲ sites with O-defect, since the defective sites usually possess lower coordination 59 number and relatively stronger Lewis-acidity. This method can clearly show the distribution and relative abundance of surface Lewis-acid sites.
In this study, we applied the same method to measure the surface Lewis-acid sites, including the defective sites, on the three hematites (HNP, HNR and HNC). The hematites were pre-treated at 105 o C for overnight to mimic the condition before the kinetic experiment. As shown in Figure R26a-c, two significant absorption peaks appear at 1716-1721 cm -1 and 1621-1628 cm -1 , respectively. According to our previous investigation, the peak at 1716-1721 cm -1 represents the physically adsorbed or weakly interacted ν(C=O). And the peak at 1621-1628 cm -1 corresponds to the ν(C=O) coordinated with the octahedral Fe III sites. The full width at half maximum (FWHM) for the peak at 1621-1628 cm -1 is relatively small (i.e., 45 cm -1 for HNP, 27 cm -1 for HNR, and 47 cm -1 for HNC), suggesting that the surface Lewis-acid sites are unitary.
No significant absorption peaks or bands were observed in the wavenumber range lower than 1620 cm -1 , indicating that the surface defective sites are not abundant.
However, the ν(C=O) on HNC appears at relatively low wavenumber position, i.e., 1621 cm -1 on HNC vs. 1625 cm -1 on HNR and 1628 cm -1 on HNP. This is inconsistent with the Bader charge analysis results, as the valence electrons (VEs) were calculated to be 6.29, 6.36-6.58 and 6.39 for the exposed Fe on HNC, HNR and HNP, respectively.
The Fe cations with higher VEs usually indicate lower coordination state and stronger coordination ability. The possible explanation is that the HNC surface contains a certain degree of O-defects. Then, the Fe adjacent to or beyond the O vacancy can remain higher VE (6.57-6.70) ( Figure R15).
The three hematites were further calcinated at 400 o C for 2 h. Thus, the surface -OH/H2O can be removed, especially for those on the defective sites. As shown in  Table 1), which is consistent with the prior studies 52,53 . Since its exposed Fe atoms are five-fold coordinated, the VE number is expectedly low (VE = 6.29, Supplementary Fig. 14d)." In page 11-12, line 198-217: "It is important to note that the higher VEs of Fe cations usually implies lower coordination state and stronger coordination ability.
However, when we further applied the diffused reflectance infrared Fourier transform spectroscopy (DRIFTS), using the compound 2-chloro-N,N-dimethylacetamide (Cl-DMA) as the probe molecule, to identify the surface Lewis-acid sites 12 , the results are discrepant to the Bader charge analysis (Fig. 3, Supplementary Fig. 14 (Supplementary Fig. 18e-f). While, on the O-defective site of {012}, the νC=O appears at 1627 (ν1) and 1590 (ν2) cm -1 (Fig. 4c(3)). Therefore, the theoretical IR spectra of DMP on the slab models directly show the coordination modes. In general, bidentate coordination can induce stronger Lewis-acid interaction than monodentate coordination, imposing wider red-shift of νC=O even to < 1600 nm -1 ." Thirdly, the additional kinetic experiments of DMP on HNC in a wide range of surface concentrations also provide another evidence that the HNC surface contains a certain degree of defects (0.07 sites·nm -2 ), page 20, line 397-403: "To further verify this hypothesis, we examined the concentration effect ([DMP]0 = 2 -20 μmol·g -1 ) on the reaction. As shown in Supplementary Fig. 25, the hydrolysis rates of DMP on HNC decrease greatly as the initial DMP concentration exceeds 2 μmol·g -1 , suggesting that the O-vacancy density on HNC surface might be ~2 μmol·g -1 (i.e., 0.07 sites·nm -2 ), accounting for ~2% of the total surface area. By comparison, the hydrolysis rates of DMP on HNR only slightly decreased in the applied concentration range, due to its stoichiometric surface condition."

Reference:
Wu, D., Huang, S., Zhang, X., Ren, H., Jin, X. and Gu, C. 2021. Iron minerals mediated interfacial hydrolysis of chloramphenicol antibiotic under limited moisture conditions. Response: Thank you for the correction. In the revised manuscript, the number of digits has been modified.
Li 141. The term "water content of 400%" is troublesome, as "over-wet condition". If the authors mean that liquid water is present, they could give the calculated value of water layers at the surface of the particles for example.
Response: Thank you for the comment. The term "water content of 400%" means that in the reaction, 50 mg hematite nanoparticles were mixed with 200 μL water, which represents the reaction occurring under the aqueous-like condition. It was reported that, on α-Fe2O3 (001), the coverage of water reaches 1 monolayer at ~15% RH, and increases to 1.5 monolayer at 34% RH (Yamamoto et al., 2010). Obviously, under the condition (50 mg nanoparticles in 200 μL water), the water content of hematite is oversaturated. As shown in Figure R27, the hematite particles are immersed into the water. To avoid ambiguity, in the revised manuscript, the expression "water content of 400%" has been modified to "water oversaturated condition".   (li.161-163) actually exist in their systems.
Response: Thank you for the comments. It is true that the surface exposed undercoordinated Fe sites are reactive to water molecules, leading to surface hydroxylation and hydration, which are complicated and vary with respect to the exposed water partial pressure (Yamamoto et al., 2010).  applied XPS analysis to investigate the reaction between water vapor and clean {001} surface of α-Al2O3 and α-Fe2O3, and found that water molecules would be mainly dissociatively adsorbed at the defective sites below the threshold water partial pressure (i.e., ~1 Torr for α-Al2O3, and ~0.0001 Torr for α-Fe2O3). The dissociated OHgroup would attach to the surface Fe 3+ ion and the proton to the surface O 2ion . Above the threshold pressure, extensive hydroxylation would occur within one monolayer . Further increase of water partial pressure would induce more water layers with interactive hydrogen bonding to surface coordinated -OH groups and structural O, namely the hydration layer. By the way, the RH 76% corresponds to the water partial pressure of ~18 Torr, there should be more than two water layers on hematite surface (Yamamoto et al., 2010). Trainor et al. (2004) proposed that the terminal Fe of hematite {001} would bond with three -OH groups to form the (HO)3-Fe-O3-R termination. The hematite {012} surface was reported to be more reactive with water molecules than the {001} surface , and there are three possible types of (hydr)oxo functional groups exposed on the surface: Fe-(OH)2, Fe-OH and Fe2-(OH)2 .
The surface hydroxylation and hydration would significantly affect the reactivity of the hematite, as the hydroxyl groups and the chemisorbed water molecules would compete with the target compounds for the reactive sites. The competition effect has been examined in our previous studies, that the hydrolysis rates of chloramphenicol antibiotic on iron oxides greatly decreased as the increase of water partial pressure (RH > 76%) . Likewise, in current study, the hydrolysis rate of DMP/DnBP in water is about 2-orders of magnitude slower than that under RH 76% by the hematite.
However, surface hydroxylation/hydration would not significantly change the reaction mechanism, since DMP/DnBP molecules mainly coordinate with the exposed Fe site instead of surface oxygen or hydroxyl groups. It could be evidenced by the insitu DRIFTS measurement of DMP on HNP/HNR/HNC. As shown in Figure R28, the clear reversal peaks at 3000 -3750 cm -1 represent the desorption of chemisorbed -OH/H2O, substituted by DMP. The PAEs can compete with surface water molecules and hydroxyl groups for the Fe sites. Therefore, to simplify the facet model, surface hydroxylation/hydration was not considered for modelling. Actually, many previous studies also used the bare Fe terminal slabs for modelling, without considering surface hydroxylation/hydration, when investigating the interaction between iron and organic compounds Han et al., 2021;Huang et al., 2017;Wu et al., 2020;Zhou et al., 2012).
In order to determine the facet surface termination and surface condition, we firstly 1640 cm -1 , which is slightly higher than the experimental observation (1621 cm -1 ).
Therefore, the actual {001}-layer 1 termination should contain a certain degree of Odefects, which can increase the Lewis-acidity of the exposed Fe. Likewise, for the stoichiometric {012}-layer 1 termination, the adsorbed DMP presents the ν(C=O) at 1645 (ν1) / 1625 (ν2) cm -1 and 1674 (ν1) /1607 (ν2) cm -1 ( Figure R30). While, DMP adsorbed on the O-defective site shows the ν(C=O) at 1627 (ν1) and 1590 (ν2) cm -1 ( Figure R30), which are much closer to the experimental results ( Figure R31). With the involvement of O vacancy, a better fit was obtained for the calculated IR spectra compared to our experimental results. While, the IR spectra of DMP adsorbed on the {104}-layer 1 and layer 5 can fit well with the experimental IR data (Please see the revised manuscript, Figure 4 and Supplementary Figure 18). Therefore, we can conclude that the actual {001} and {012} facets contain a certain degree of O-defects, however, the {104} facet has negligible defects.
Although the surface hydroxylation/hydration was not considered for modeling, the effect of hydration and hydroxylation on surface reaction was also discussed in the revised manuscript. The remaining valance electrons (VEs) of the exposed Fe atom can be used to indicate its catalytic activity. For {001}-layer 1, the VEs of the exposed Fe atoms are moderate even they are three-fold coordinated ( Figure R32a). With one Odefect on {001} facet, the average VEs of the exposed Fe are increased. While, with the surface hydroxylation, the VEs of the exposed Fe are reduced ( Figure R32a). For the {104}-layer 1, the dissociative bonding with a low extent of H2O molecules adventitiously reduces the VEs of the adjacent Fesubsurface, and increases the VEs of the adjacent Feoutmost ( Figure R32b). Furthermore, for {012} facet, the presence of O-defect increases the average VEs of the exposed Fe and more defects would contribute to the higher VEs of the Fe ( Figure R32d). Additionally, dissociative bonding with a low extent of H2O molecules can also induce surface charge redistribution ( Figure R32d).
Based on above discussion, we have carefully proposed the terminations of the facets. In the revised manuscript, we add one paragraph to discuss this aspect, in page 10-13, line 172-235: "Determination of facet terminations. The catalytic activity of hematite typically depends on its surface properties. However, the {001}, {104}, {012} facets have multiple theoretical surface terminations ( Supplementary Fig. 11 -Fig. 13).
To determine the exact termination, we calculated the surface energies of different possible terminations for each facet. It is generally accepted that the facet with the lowest surface energy is thermodynamically favorable. For the {001} facet, after surface relaxation, its layer 1 termination (Fe3-O3-Fe6-R, the subscript represents for the coordination number, Fig. 3a) possesses much lower surface energy (γ = 1.86 J·m -2 ) than the other terminations (Supplementary Table 1), which is in accordance with the previous reports 49-51 . The exposed Fe atoms on {001}-layer 1 termination are three-fold 69 coordinated. However, according to Bader charge analysis, the valence electrons (VEs) remaining on the exposed Fe are relatively low (VEs = 6.39, indicating 1.61 VEs are transferred from Fe to the adjacent O) (Fig. 3a), since the exposed Fe atoms are relaxed inward to the underlayer plane of O atoms with significant charge redistribution.
It is important to note that the higher VEs of Fe cations usually implies lower coordination state and stronger coordination ability. However, when we further applied the diffused reflectance infrared Fourier transform spectroscopy (DRIFTS), using the compound 2-chloro-N,N-dimethylacetamide (Cl-DMA) as the probe molecule, to identify the surface Lewis-acid sites 12 , the results are discrepant to the Bader charge analysis (Fig. 3, Supplementary Fig. 14). For example, HNC exhibits wider red-shift of νC=O, which denotes to the carbonyl stretching vibration (1621 cm -1 on HNC vs. 1628 cm -1 on HNP & 1625 cm -1 on HNR, Supplementary Fig. 15a-c), suggesting that the HNC surface is more Lewis-acidic. This discrepancy might be ascribed to the existence of surface O-defects on actual HNC surface 53 . By introducing a certain degree of Odefects on the surface of {012} facet model, the Fe atoms adjacent to the O-vacancies possess 6.70 -6.74 VEs (Fig. 3c, Supplementary Fig. 14d), corresponding to the stronger Lewis-acidity. Meanwhile, a better fit for the IR spectra was also obtained 70 using the {012} facet with O-defect for theoretical modelling (more discuss is shown below). Further study shows that the HNC400 (i.e., HNC calcinated at 400 o C for 2 h) exhibits a new shoulder peak at 1582 cm -1 ( Supplementary Fig. 15f), probably stemming from the thermal desorption of -OH groups and H2O molecules at the defective sites 56 , suggesting a small amount of surface defective sites. For comparison, the νC=O peaks in both HNR and HNR400 samples are narrow and sharp ( Supplementary Fig. 15b, e), indicating negligible surface defects on HNR.
Moreover, the surface undercoordinated Fe sites are reactive with water molecules.
Under low water partial pressure, H2O molecules would entirely or dissociatively bond to lattice Fe and O with interactive hydrogen bonding, leading to surface hydroxylation, hydration and protonation (Fig. 3, Supplementary Fig. 14) Fig. 4 -Fig. 6). Therefore, under ambient condition, their surface should be more hydroxylated and hydrated 61 . Although surface hydroxylation/hydration could shield the surface reaction, and affect the valency of the exposed Fe, organic ligands can still compete with the surface bond -OH/H2O for the undercoordinated Fe sites, especially under low moisture condition 62 , which is strongly evidenced by the desorption of the chemisorbed -OH/H2O (3000 -3750 cm -1 ) substituted by either Cl-DMA or DMP on the three facets (Supplementary Fig. 15 & 16). Therefore, the surface hydroxylation/hydration would not change the coordination mode and the catalytic mechanism. To simplify the facet model, surface hydroxylation/hydration was not involved for modelling. In current study, the {001}layer 1, {104}-layer 1 co-existing with layer 5, and the {012}-layer 1 with O-defects are deduced as the surface terminations for HNP, HNR and HNC, respectively (Fig. 3)."