Supporting information. Active site identification and modification of electronic states by atomic-scale. doping to enhance oxide catalyst innovation

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1 Supporting information Active site identification and modification of electronic states by atomic-scale doping to enhance oxide catalyst innovation Ying Xin, Nana Zhang, Qian Li, Zhaoliang Zhang,*, Xiaoming Cao,*, Lirong Zheng, Yuewu Zeng, ǁ James A. Anderson*, School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan , China Center for Computational Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai , China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing , China ǁ Center of Electron Microscopy and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou , China Surface Chemistry and Catalysis Group, Materials and Chemical Engineering, University of Aberdeen, AB24 3UE, United Kingdom *Corresponding author: Zhaoliang Zhang: Xiaoming Cao: James A. Anderson: S1

2 I. Experimental section i. Catalytic performance tests The steady state NH3-SCR and NO oxidation activity over Mox-Fe2O3 and Wy-Fe2O3 were tested in a fixed-bed quartz tube reactor (6.0 mm i.d.) with a thermocouple placed inside the catalyst bed in the temperature range of o C. In NH3-SCR reactions, the model flue gas consisted of 500 ppm NO, 500 ppm NH3, 5.3 vol.% O2, and balanced He. The total flow rate was kept at 300 ml/min corresponding to a gas hourly space velocity (GHSV) of h -1. Regarding NO oxidation, the feed consisted of 500 ppm NO, and 5 vol.% O2 with He as balance. The total flow rate was maintained at 100 ml/min and the same GHSV ( h -1 ) was used. Concentrations of NO and NO2 were monitored by a chemiluminiscence NOx analyzer (42i-HL, Thermo). N2O and NH3 were detected by a quadrupole mass spectrometer (MS, OmniStar 200, Balzers) using the m/z of 44 for N2O, and 17 for NH3. The data for steady-state activity and NO oxidation of catalysts were collected after about 1 hour on stream. The reaction rates were measured using the same facility as the steady-state reactions. Differently, the powder samples were pressed, crushed and sieved into mesh prior to use. The GHSV is estimated to be 200,000 h -1. The isothermal reactions at 140 o C were conducted at which a stable and small conversion of NOx ( 15%) was achieved in an approximate kinetic regime. At such a harsh condition, both Fe2O3 and MoO3 show negligible rate due to their low NOx conversion. ii. Characterization S2

3 X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500/PC diffractometer employing Cu Kα radiation (λ = Å) operating at 50 kv and 200 ma. The Brunauer-Emmett-Teller (BET) surface area and pore structure were measured by N2 adsorption/desorption using a Micromeritics 2020 M instrument. Before exposure to N2, the sample was outgassed at 300 o C for 5 hours. Inductively coupled plasma-atomic emission spectrometer (ICP-AES) experiments were carried out on the IRIS Intrepid IIXSP instrument from Thermo elemental. FESEM was performed on a Hitachi SU-70 microscope. Transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) was conducted on a JEOL JEM-2010 microscope at an accelerating voltage of 200 kv. High angle annular dark field (HAADF) images in scanning transmission electron microscopy (STEM) mode were also carried out on an FEI Titan ChemiSTEM transmission electron microscope operating at 200 kv, a spherical aberration corrector and an energy dispersive X-ray (EDS) analyzer with a spatial resolution as low as 0.8 nm. XPS data were obtained on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Kα radiation (225 W, 15 ma, 15 kv) and low-energy electron flooding for charge compensation. To compensate for surface charging effects, the binding energies were calibrated using the C 1s hydrocarbon peak at ev. X-ray absorption fine structure (XAFS) measurements for the Fe K-edge and the Mo K-edge were performed in the transmission mode and fluorescence mode at room temperature on the XAFS station of the 1W1B beamline of Beijing synchrotron radiation facility S3

4 (BSRF, Beijing, China), respectively. XAFS data were analyzed using IFEFFIT software package. 1 The temperature-programmed reduction with H2 (H2-TPR) experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H2 consumed. A 50 mg sample was pretreated in situ at 500 o C for 1 hour in a flow of O2 and then cooled to room temperature in the presence of O2. TPR was conducted at 10 o C/min up to 900 o C in a 30 ml/min flow of 5 vol.% H2 in N2. To quantify the total amount of H2 consumed, CuO was used as a calibration reference. The temperature-programmed desorption of NH3 (NH3-TPD) experiments were performed in a quartz reactor using 50 mg catalyst. NH3 was monitored using quadrupole mass spectrometer (MS, OmniStar 200, Balzers) with m/z=16. Prior to the experiments, the samples were pretreated at 500 o C for 30 min in 10 vol.% O2/He (50 ml/min) and cooled to 100 o C. NH3 adsorption was operated in 0.4 % NH3 (50 ml/min) until the outlet NH3 concentration was stable. Then, the samples were purged with He to remove any weakly adsorbed NH3. Finally, the samples were heated to 700 o C at 10 o C/min. The in situ FTIR spectra were recorded using a Bruker Tensor 27 spectrometer over the range cm -1, with 16 scans, at a resolution of 4 cm -1. Self-supporting wafers were pretreated in the IR cell at 500 o C in a flow of He for 30 min to remove any adsorbed species. After cooling to room temperature (RT) or 100 o C, the background spectrum was recorded. The FTIR spectra were recorded at RT or 100 o C in the flow of 500 ppm NH3 + He (150 ml/min) or 500 ppm NO ppm NH % O2 + He (300 ml/min). The samples were then heated to 450 o C at of 10 o C/min. S4

5 iii. Computational methods All the spin polarized density functional theory calculations were carried out utilizing Perdew-Burke-Ernzerh (PBE) 2 generalized gradient approximation (GGA) functional implemented in VASP code. 3,4 The projector-augmented-wave (PAW) pseudopotentials 5 were utilized to describe the valence-core interactions and a plane-wave kinetic energy cut-off of 500 ev was employed. The on-site Coulomb repulsion correction term of U within Hubbard scheme (PBE+U) was also applied in the 3d electrons of Fe to address the strong correlation between the electrons in partially occupied 3d orbitals of Fe. The value of U was set to 4.0 ev, based on the previous computational work. 6,7 Moreover, the previous study suggested that the adsorption energy on Fe2O3 surface is not sensitive to the value of U of Fe. 8 Hematite (α-fe2o3) is the most thermodynamically stable iron oxide and was found in samples here based on XRD analysis. For α-fe2o3, it is antiferromagnetic and the rhombohedral primitive unit cell of α-fe2o3 was proven to be most energetically stable with the magnetic configuration (+ +) in previous work. 9 Hence, the (104) surface found in our TEM result was represented by a p(2 1) slab based on the hexagonal conventional unit cell of α-fe2o3 with the optimized lattice parameters (a = b = Å and c = Å) and the most energetically stable magnetic configuration (Figure S14). The repeated slabs were separated from their neighboring images by a 10 Å vacuum along the normal direction of the surface. A gamma-centered Monkhorst-Pack k-point mesh size of was utilized for the Brillion zone integration. Each slab S5

6 contains 9 layers of Fe-O planes. The adsorbates and the top 5 layer of the surface slab could be relaxed while the bottom 4 layers were fixed during the geometry optimization. The structure relaxation was carried out until the force on each relaxed atom is less than 0.02 ev/å. The transition state was searched with a constrained minimization technique with the same force convergence criterion The adsorption energy Ead was calculated as follows: Ead = E(g) + E(surf) - E(g * ) where E(g), E(surf), and E(g * ) respectively denote as the total energies of gaseous species g, the clean surface, and adsorbed g. The bonding strength was quantitatively confirmed by the integral of the contribution of energy bands up to Fermi level based on projected Crystal Orbital Hamilton Population analysis (IpCOHP) for bonding. 13,14 Since the negative IpCOHP value represents bonding, the more negative IpCOHP is, the stronger bonding exists between atom pairs. S6

7 II. Figures Figure S1. (a) XRD patterns, and (b) dependence of lattice parameter (a) for a range of Mo contents for Mox-Fe2O3. S7

8 Figure S2. N2 selectivity of Mox-Fe2O3 in SCR reactions as a function of reaction temperatures. S8

9 Figure S3. (a-c, e, and g) SEM and (d, f, and h) TEM images with SAED patterns (insets) of Fe2O3, MoO3, and Mox-Fe2O3. (a) Fe2O3, (b) MoO3, (c, d) Mo0.03-Fe2O3, (e, f) Mo0.06-Fe2O3, and (g, h) Mo0.07-Fe2O3 catalysts. S9

10 Figure S4. (a) XRD pattern, (b) HRTEM image viewed along the [201] orientation, and (c) the atomic configurations based on the HRTEM image of Fe2(MoO4)3. The lattice spacing labeled in the HRTEM image are about 4.00, 4.51, and 3.32 Å, corresponding to (2 1-4), (2 0-4), and (2 2-4) crystal planes of Fe2(MoO4)3 phase, respectively. S10

11 Figure S5. (a) NOx conversion and (b) N2 selectivity of Fe2(MoO4)3 and the mechanically mixed catalysts. The mechanical mixing of catalysts were obtained by combining Fe2O3 and MoO3 (or Fe2(MoO4)3) powders with the Mo contents according to the ICP data of Mo0.06-Fe2O3 (Mo content: 6.72 wt.%). S11

12 Figure S6. XPS spectra of (a) Fe 2p, (b) Mo 3d, (c) O 1s of Mox-Fe2O3 catalysts. S12

13 Figure S7. Aberration-corrected HAADF-STEM images from one-dimensional crystal lattice of Fe2(MoO4)3 crystalline (a) and Fe2O3 microcrystal (b) in Mo0.06-Fe2O3 (the circles represent isolated Mo ions). S13

14 Figure S8. Normalized Fe K-edge XANES spectra (a) and the radial structure function (RSF) curves (b) for Mox-Fe2O3 and the reference samples. S14

15 Figure S9. Normalized Mo K-edge XANES spectra for Mox-Fe2O3 and the reference samples (a); the relative amounts of the Fe2(MoO4)3 microcrystals and (Mo7O24) 6- species obtained by Mo K-edge XANES linear combination fitting for Mo0.03-Fe2O3 (b), Mo0.06-Fe2O3 (c) and Mo0.07-Fe2O3 (d), respectively. S15

16 Figure S10. (a) H2-TPR profiles of Mox-Fe2O3, and (b) NO conversions as a function of temperatures in NO oxidation reactions over Mo0.06-Fe2O3 and under thermodynamic equilibrium. S16

17 Figure S11. Experimental NH3-TPD profiles of Mox-Fe2O3. S17

18 (a) Fe 2 O (b) Mo Fe 2 O He-450 o C He-400 o C He-450 o C He-350 o C He-400 o C He-300 o C He-350 o C Absorbance He-250 o C He-200 o C He-150 o C He-100 o C He-50 o C He purge-rt -30 min -25 min -15 min -5 min -3 min -2 min -1 min He pretreatment 1437 Absorbance He-300 o C He-250 o C He-200 o C He-150 o C He-100 o C He-50 o C He purge-rt -30 min -25 min -15 min -5 min -3 min -2 min -1 min He pretreatment Wavenumber (cm -1 ) Wavenumber (cm -1 ) Figure S12. In situ FTIR spectra of NH3 adsorbed on (a) Fe2O3, and (b) Mo0.06-Fe2O3 at room temperature and subsequently heated to 450 o C in a flow of He. The bands at ~1440 cm -1 are attributed to the asymmetric bending,mode of NH4 + species on Brønsted acid sites, 15 while the band at ~1214 cm -1 is assigned to the asymmetric and symmetric bending vibrations of NH3 coordinately linked to Lewis acid sites The band at ~1366 cm -1 was ascribed to an oxidation product of adsorbed ammonia species. 16,18,20 S18

19 (a) Fe 2 O (b) Mo Fe 2 O He-purge 30 min 1240 Absorbance 25 min 15 min 10 min 5 min 3 min 2 min 1 min NO+ +O 2 0 min Absorbance He purge-30 min 25 min 15 min 10 min 5 min 3 min 2 min 1 min NO+ +O 2 0 min Wavenumber (cm -1 ) Wavebumber (cm -1 ) Figure S13. In situ FTIR spectra of (a) Fe2O3, and (b) Mo0.06-Fe2O3 heated at 100 o C in a flow of NO+NH3+O2 after pretreatment by He at 500 o C for 30 min. The spectra of Fe2O3 and Mo0.06-Fe2O3 under the SCR reactions are quite similar as those of NH3 adsorption. Highlighting differences, the red-shift of absorbance bands due to ammonia on Lewis acid sites was observed and corresponds to the greater donation from the adsorbed and negatively charged NH For the coordinated NH3 on Lewis acid sites, the greater the electron donation from NH3, the stronger the adsorption. 24 In addition, a new band at ~1328 cm -1 was detected for Mo0.06-Fe2O3, which might be assigned to intermediate species from the combination of surface adsorbed and activated NH3 on the active sites and NOx, suggesting the possible Eley-Rideal (E-R) mechanism. 16,18 S19

20 (a) (b) Figure S14. (a) Hexagonal unit cell, and (b) corresponding rhombohedral primitive cell of α-fe2o3 with the most energetically stable magnetism (+ +). Each Fe cation is located at the octahedral interstice of close-packed oxygen anions with 6-fold coordination. The Fe cations with positive and negative magnetism respectively are displayed in purple and blue. The red balls represent O anions. S20

21 O2 * TSO-O 2O * Fe2O3 (104) Ead(O2) = 0.13 ev Ea = 2.44 ev ΔH = 2.18 ev Mo-surf Ead(O2) = 0.21 ev Ea = 0.59 ev ΔH = ev Figure S15. Geometry structures of O2 * adsorption and the transition state and final state of O2 * dissociation on the pure Fe2O3 (104) and the surface Mo site at Mo-doped Fe2O3 (104). The corresponding O2 adsorption energies, the energy barriers and reaction energies of O2 dissociation are listed. S21

22 Figure S16. Geometry structures of NH3 adsorption on the surface Mo site at Mo-doped Fe2O3 (104). S22

23 III. Tables Table S1. Textural properties, ICP and XPS data of Fe2O3, MoO3, and Mox-Fe2O3 catalysts. Sample Surface area m 2 /g Pore volume cm 3 /g Pore size nm Lattice parameters Å a c Fe wt.% ICP XPS NH3 absorption Mo/Fe Mo/Fe Mo Fe Mo Oα/(Oα+Oβ) ( 10-6 ) molar molar wt.% wt.% wt.% % mol/m 2 ratio ratio Fe2O Mo0.03-Fe2O Mo0.06-Fe2O Mo0.07-Fe2O MoO S23

24 Table S2. Textural properties and ICP data for Wy- Fe2O3 catalysts. Sample Surface area m 2 /g Pore volume cm 3 /g Pore size nm Fe wt.% W wt.% ICP W/Fe molar ratio W 0.09-Fe 2O W 0.13-Fe 2O W 0.17-Fe 2O WO S24

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