Strongly Coupled N-Doped CrOx/Ce0.2Zr0.8O2 ... - ACS Publications

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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A Rational Design for Enhanced Catalytic Activity and Durability: Strongly Coupled N‑Doped CrOx/Ce0.2Zr0.8O2 Nanoparticle Composites Wei Cai,*,†,‡ Qin Zhong,*,§ Dongyu Wang,§ Yunxia Zhao,‡ Mindong Chen,‡ and Yunfei Bu*,‡,§ †

Key Laboratory of Meteorological Disaster, Ministry of Education, Joint International Research Laboratory of Climate and Environment Change, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters and ‡Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China § School of Chemical and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China S Supporting Information *

ABSTRACT: As a classic catalyst for NO oxidation, CrOx/Ce0.2Zr0.8O2 has been widely researched to improve its intrinsic catalytic activity and stability under complex flue gas environments. Some strategies, such as nanosize reduction, composite catalysts, and transition metal or rare earth ion doping, have been reported to enhance the catalytic properties. However, the commercialization of CrOx/Ce0.2Zr0.8O2 is greatly hindered by its poor stability under complex flue gas environments. Herein, we reveal a new route to fabricate N-doped CrOx/Ce0.2Zr0.8O2 nanoparticles, which exhibit not only higher NO conversion but also H2O and SO2 tolerance. The morphology and structure were analyzed via X-ray diffraction, transmission electron microscope, et al., investigating the enhancement of N doping. Additionally, the formation of the Ce−O−N−Zr chemical bond and the possible catalytic mechanism were examined by in situ diffuse reflectance infrared Fourier transform spectroscopy, which provided insight into both the fabrication and the catalytic oxidation activity. Finally, density functional theory calculations were applied to expand the design and afford diverse functionality of the catalyst for use in various applications. KEYWORDS: N-doped Ce0.2Zr0.8O2, catalytic oxidation, enhanced adsorption capacity, strong H2O and SO2 resistance, superoxide radical

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INTRODUCTION

catalytic performance. However, SO2 easily reacts with Mn to form surface sulfates,11 leading to the irreversible deactivation of the catalyst. Cr-based catalysts have also attracted substantial attention due to their stronger resistance to SO2,12 and they also display high oxidation ability, like Mn-based catalysts. The existence of a mass of Brønsted and Lewis acid sites on the surface inhibits the adsorption of SO2.13 As one of the most commonly used mixed oxides in the investigation of three-way catalysts, the ceria-zirconia solid solution (CexZr1−xO2) has led to several studies examining its application to different

Nitrogen oxides (NOx), which mainly come from the combustion of fossil fuels, cause severe undesirable human health impacts.1 Among the NOx removal methods such as NOx storage and reduction (NSR), continuously regenerating trap (CRT), and selective catalytic reduction (SCR), the catalytic oxidation of NO is usually considered as the key step. Moreover, it is becoming the promising technology for NOx removal.2−5 When the oxidation efficiency (i.e., the ratio of NO/NOx) approaches ∼50%, the adsorption capacity of NOx is optimized.6 Among the NO oxidation catalysts, transition-metal oxides are typically investigated due to their advantages of high activity and low cost,7−10 and Mn-based catalysts exhibit the highest © XXXX American Chemical Society

Received: December 11, 2017 Accepted: February 6, 2018 Published: February 6, 2018 A

DOI: 10.1021/acsanm.7b00320 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 1. Illustration of the Synthesis Routes of N-Doped Ce0.2Zr0.8O2 and the Cr Impregnation Process

oxidation reactions14−16 due to the abundance of oxygen vacancies on its surface. However, the facile interactive reaction between SO2 and Ce17 largely restricts its commercial application. Recently, improving the tolerance of CexZr1−xO2 for SO2 has been considered as an effective solution. Nb-, Mo-, and Codoped CexZr1−xO2 have been reported to enhance the NH3− SCR activity, and the strong SO2 resistance of these materials was ascribed to the enhanced redox ability and the numerous Brønsted acid sites.18−20 Nonetheless, the generation of Brønsted acid sites not only inhibits the adsorption of SO2, strengthening the SO2 resistance, but also suppresses the adsorption of NO. For NH3−SCR reaction, the generation of Brønsted acid sites promotes the adsorption of NH3, thus increasing the catalytic performance. However, for NO oxidation, the generation of Brønsted acid sites decreases the catalytic performance. Hence, it is necessary to develop another way to enhance the SO2 resistance of CexZr1−xO2 for NO oxidation. Li et al.21 synthesized N-doped TiO2 for NH3−SCR, and the results showed that the increasing surface oxygen vacancies caused by N doping could heighten the SO2 resistance. This inspired us to examine whether the further generation of surface oxygen vacancies could improve the SO2 tolerance of the catalysts. In recent years, different N-doped catalysts, such as N-doped TiO222,23 and N-doped ZnO,24 have been widely explored for photocatalysis, and the promotional activity was ascribed to the increased number of defect sites (oxygen vacancies). Sousa et al.25 prepared N-doped activated carbon for the oxidation of NO to NO2, and the results showed that the existence of nitrogen-containing surface groups benefited from the oxidation of NO to NO2. Moreover, the Ce x Zr 1−x O 2 system has been widely investigated, but the synthesis for heteroatom doping and the resultant catalytic mechanism are still unclear. The obvious differences in the electronegativity and atomic radius of N and O should change the physicochemical properties and the catalytic mechanism. Hence, inspired by these benefits of N doping, N-doped CrOx/CexZr1−xO2 was first examined, aiming to systematically investigate the effects of N doping on NO oxidation and SO2 tolerance. On the basis of our previous work, 10%CrOx/Ce0.2Zr0.8O2 (10% Cr loading in mass) was chosen in this study, due to its highest NO conversion among CrOx/ CexZr1−xO2 series catalysts.26,27 Characterization methods, such as X-ray diffractin (XRD), Brunauer−Emmett−Teller (BET),

transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and UV−vis diffuse reflectance spectroscopy (DRS), were employed to evaluate the influence of N doping on the structural properties, surface components, NO and O 2 adsorption properties, catalytic performance, and SO2 tolerance. Moreover, the possible NO oxidation mechanism over Ndoped Ce0.2Zr0.8O2 was investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Density functional theory (DFT) calculations were performed, and a strong coupling between N-doped CrOx and Ce0.2Zr0.8O2 was demonstrated, which will expand the design and afford diverse functionality of the catalyst for use in various applications.

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EXPERIMENTAL SECTION

Chemicals. All starting reagents came from Alfa Aesar, and they were used without any further treatment. Millipore system was adopted to generate the deionized water, which was used in the whole experiment. Synthesis. The precursor Ce0.2Zr0.8O2 was prepared according to our previous work.27 Briefly, Ce(NO3)3·6H2O and ZrOCl2·8H2O (molar ratio 1:4) were mixed in an aqueous solution. After it was stirred, refluxed, dried, and calcined at 500 °C, Ce0.2Zr0.8O2 was obtained. Then, Ce0.2Zr0.8O2 was treated at 600, 700, and 800 °C under 2 vol % NH3/N2 at a flow rate of 70 mL·min−1. After calcination, the N-doped Ce0.2Zr0.8O2 samples CZ4-N600, CZ4-N700, and CZ4-N800, respectively, were obtained. Then, the CrOx species were impregnated on N-doped Ce0.2Zr0.8O2 via stirring, drying at 120 °C for 12 h, and calcination at 500 °C for 4 h. It should be mentioned that the samples were calcined under pure N2 atmosphere to prevent the reoxidation of N-doped Ce0.2Zr0.8O2. Finally, a series of N-doped CrOx/Ce0.2Zr0.8O2 samples, denoted CZ4-N(x)-Cr (x = 600, 700, 800), was synthesized. The synthesis process of the catalysts was shown in Scheme 1. For comparison, Ce0.2Zr0.8O2 was first impregnated with CrOx and then treated at 700 °C under 2 vol % NH3/N2, denoted CrCZ4-N. The CrOx/Ce0.2Zr0.8O2 catalyst without NH3 treatment is denoted CrCZ4. Characterization Methods. XRD patterns of all the samples were recorded within the 2θ range of 10−80° on an XD-3 instrument (Beijing Purkinje General Instrument), and X-ray is emitted from Cu Kα radiation with the wavelength of 1.5418 Å. The scan speed is 8°· min−1, and the step value is 0.04°. Fourier transformation infrared (FT-IR) spectra of the samples molded into disks with KBr were analyzed on an IS10 FT-IR spectrometer (Nicolet), under the conditions of 32 coadded scans with 4 cm−1 resolution. B


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ACS Applied Nano Materials

Figure 1. N 1s XPS (a) and FT-IR (b) spectra of the CrCZ4 and CZ4-N(x)-Cr catalysts and (c, d) XRD patterns of the CrCZ4, CrCZ4-N, and CZ4-N(x)-Cr catalysts. The BET surface areas of all samples were obtained on an Autosorb-iQ Analyzer (Quantachrome Instruments). The pore structures were analyzed with the Barrett−Joyner−Halenda (BJH) method. Before the characterization, the samples were dried at 180 °C for 12 h with vacuum. High-resolution TEM (HR-TEM) of the samples was performed on a JEM-2100 (JEOL) under the accelerating voltage of 200 kV. XPS of the samples was characterized on an ESCALAB 250 spectrometer (Thermo) with Al Kα (hν = 1486.6 eV) radiation. Before the analysis, the samples were dried at 10 °C overnight with the vacuum. The adventitious C 1s peak at 284.4 eV was referenced for the calibration of the binding energies. UV−vis DRS of the samples was performed on a UV-2550 UV−vis spectrophotometer (Shimadzu) within the wavelength range of 200− 800 nm, and BaSO4 was the background sample. Mott−Schottky characterizations were performed with a fixed frequency of 100 Hz and a 10 mV amplitude for various applied potentials. All calculations were performed via the DFT with the DMol3 module.28,29 The exchange and correlation energy were calculated via the spin-polarized calculations applying the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional.30 The Brillouin zone was analyzed using a (4 × 4 × 1) Monkhorst−Pack grid, which ensures the convergence of the entire system. Fermi smearing was set as 0.1 eV, the convergence condition of the geometry optimizations was 1 × 10−5 eV, and the forces acting on the ions was 0.002 eV·Å−1. The bulk Ce0.2Zr0.8O2 was simulated and optimized. The initial crystal structure parameter a = 5.351 Å came from ref 31. The optimized lattice parameter was a = 5.285 Å, which is close to the experimental data. The (1 1 1) surface of the optimized bulk Ce0.2Zr0.8O2 was cleaved, and a 14 Å vacuum layer was set up. All atoms were adequately relaxed both in the bulk and slab calculations.

The binding energy (Ead) of the NO (or O2) molecule was calculated, containing all possible active acid sites on the surface, displayed as follows:

Ead = Esurface + E NO(or O2) − E NO(or O2)/surface

(1)

where Esurface is the energy of the surface, ENO(or O2) is the energy of an isolated NO (or O2) molecule, and ENO(or O2)/surface is the total energy of the same molecule adsorbed onto the surface. The obtained Ead with the positive value indicates the stable adsorption. Usually, a small adsorption energy within the range from 0.1 to 1 eV explains a weak interaction between the molecule and the surface. In comparison, the adsorption energy higher than 1 eV indicates a strong interaction between the molecule and the surface, which is expected to be coupled by a strong tension to the structure of the molecule and the surface. TPD was characterized on a ChemBET PULSAR automated chemisorption analyzer (Quantachrome Instruments). The detailed experimental process was that the 200 mg sample was dried at 120 °C for 0.5 h under the flowing He (70 mL/min). After it was cooled to 50 °C, the catalyst was exposed to 1%NO/He for 1 h with the flow rate of 70 mL/min. Next the atmosphere was switched to 70 mL/min pure He for 1 h, and then the temperature was heated to 800 °C with a heating rate of 10 °C/min. The desorption of NO was analyzed by a thermal conductivity detector (TCD). In situ DRIFTS spectra were acquired on an iZ10 FT-IR spectrometer (Nicolet) under the conditions of 32 coadded scans with 4 cm−1 resolution, which is equipped with the gas flow system. The KBr background was first recorded. The tested catalyst was pretreated at 300 °C for 2 h under pure He atmosphere and then cooled to 150 °C. Next the atmosphere was switched to 500 ppm of NO (when used) and 8 vol % O2 (when used), and the temperature was gradually increased to 300 °C. The spectra were collected at each temperature and each reaction time. In addition, the background spectrum at every temperature was subtracted. C


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Figure 2. TEM, HR-TEM, and SAED images of the CrCZ4 and CZ4-N(x)-Cr catalysts: (a) CrCZ4, (b) CZ4−N600-Cr, (c) CZ4−N700-Cr, and (d) CZ4−N800-Cr. Activity Test. The fixed-bed quartz reactor of 6.8 mm inner diameter was applied for the catalytic oxidation of NO of all samples under atmospheric pressure. The reactant gas with a gas hour space velocity (GHSV) of 70 000 h−1 contained the mixture of 400 ppm of NO, 8 vol % O2, 6 vol % H2O(g) (when used), and 400 ppm of SO2 (when used), in which N2 was applied as the balance gas. The catalyst of 300 mg (sieve fraction of 40−60 mesh) was located near the thermocouple, which was fixed between two layers of pyrocotton. The temperature was gradually increased from 150 to 400 °C by every 50 °C, the system was kept steady for 90 min at each temperature, and then the gas products were analyzed on an Ecom-JZKN analyzer. NO catalytic oxidation efficiency is described as

equilibrium between the sample and the dye. The concentrations of methylene blue, acid orange, and rhodamine B were measured by UV−vis spectroscopy (T6, Beijing Purkinje General Instrument) at 554 nm.

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RESULTS AND DISCUSSION Confirmation of N Doping. To confirm that N was doped into the lattice of Ce0.2Zr0.8O2 and determine its chemical state, the N 1s XPS spectrum was recorded, as shown in Figure 1a. For the NH3-treated samples in the N 1s spectra, one peak at ∼399.6 eV was observed. Nitrogen is generally interstitially or/ and substitutionally doped into the lattice, similar to N-doped TiO2.32 As described by Fujishima et al., the N 1s peak at ∼400 eV is usually attributed to the nitrogen doping interstitially, while the peak at ∼396 eV is assigned to the nitrogen doping substitutionally.33 On the basis of the XPS results obtained here, it could be found that the NH3-treated samples all displayed the binding energy peak at 399.6 eV, which was ascribed to the nitrogen doping interstitially. Moreover, the additional peak at 396.4 eV was observed over CZ4−N700-Cr sample, indicating that the substitutional nitrogen doping also

NO catalytic oxidation efficiency(%) = (NOin − NOout )/NOin × 100

(2)

The degradations of acid orange, rhodamine B, and methylene blue under visible-light irradiation were applied to evaluate the photoactivity of CrCZ4 and CZ4−N700-Cr, for which a 500 W Hg lamp was used as the light source. In the experiment, the sample of 25 mg was dispersed in 25 mL of 10 mg/L above-mentioned solution. Prior to illumination, the mechanical stirring of the suspensions for 60 min in the dark was performed to reach the adsorption−desorption D


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ACS Applied Nano Materials existed in this sample. One could argue that the peak at ∼399.6 eV could also be ascribed to N−H or adsorbed NH3, due to its binding energy being close to that of NH3 (398.7−399.7 eV).34 Hence, the FT-IR characterizations over all samples were adopted, as shown in Figure 1b. The results showed that two peaks at 3430 and 1627 cm−1 were observed over all samples, which might be ascribed to the bands characteristic of N−H or NH3 (which generally appear at 3500−3300 cm−1 for N−H and 1650 cm−1 for NH3). However, because these two peaks were also found over the sample without NH3 pretreatment, the peaks could not be ascribed to the bands characteristic of N−H or NH3. Combined with the XPS results that the additional peak at 396.4 eV was observed over CZ4−N700-Cr sample, it could be concluded that N was doped into Ce0.2Zr0.8O2 successfully, and the main doping form was the interstitial nitrogen doping, which could be described as Ce− O−N−Zr or Ce−N−O−Zr. Moreover, the N 1s XPS spectrum of CZ4−N800-Cr showed a weaker peak intensity than that of the other two N-doped catalysts, indicating that the N-doping level was maximized with NH3 treatment at 700 °C. And the N 1s peak intensities of all NH3-treated catalysts were still weak relatively, indicating that only a small quantity of N element was doped into the lattice. Crystal Structure and Morphology Analysis. Wideangle XRD profiles were recorded to characterize the phase structure of the products, as shown in Figure 1c. All of the catalysts displayed four main peaks at ∼29.7°, 34.6°, 49.5°, and 59.2° related to the tetragonal structure of Ce0.16Zr0.84O2 (JCPDS No. 38-1437) and corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes.35 After NH3 treatment, the peaks of the catalysts shifted to lower angles, as seen in Figure 1d. Combined with the XPS and FT-IR results, it was confirmed that N was doped into Ce0.2Zr0.8O2, and the main doping form was the interstitial nitrogen doping, such as Ce− O−N−Zr or Ce−N−O−Zr. The interstitial nitrogen could expand the Ce0.2Zr0.8O2 lattice, and the peak shift could be explained based on the Bragg formulation (2d sin θ = nλ). One argument was that the peak shift might be ascribed to the Cr doping into Ce0.2Zr0.8O2 lattice. However, since the ionic radius of Cr3+ (rCr3+ = 0.0615 nm) is smaller than that of Ce4+ (rCe4+ = 0.097 nm), the doping form of Cr is generally the substitutional doping. Thus, the peak should shift to the high angle, based on the Bragg formulation. Hence, the Cr mainly existed in the form of CrOx over the surface of the catalysts. Among all samples, CZ4−N700-Cr exhibited the highest shift, indicating that the N-doping level was maximized under this condition, in accord with the N 1s XPS results. The CrCZ4-N and CZ4−N700-Cr displayed the similar XRD patterns, which was ascribed to the same NH3 treatment temperature. Additionally, the peaks at ∼28.5° and 54.9°, found in CZ4−N800-Cr, were ascribed to CrO2 (JCPDS No. 090332) and Cr2O3 (JCPDS No. 38-1479), respectively. This indicated that the CrOx species were not well-dispersed over the CZ4-N800 support. In comparison, no peaks corresponding to CrOx species were found over other catalysts obviously, indicating the well dispersion of CrOx species over the samples. A good diffusion of CrOx species can enhance NO oxidation, as reported in our previous study.36 TEM characterization was adopted to observe the morphologies of the four catalysts, as shown in Figure 2. In contrast to the other agglomerated samples, after NH3 treatment, the more distinct boundaries were observed. Further details in the HR-TEM images showed that the lattice fringes of

all catalysts exhibited the (1 1 1) plane of Ce0.2Zr0.8O2. However, after NH3 treatment, the crystalline interplanar spacing was increased and continued to increase with increasing NH3 treatment temperature. With further increase of the temperature from 700 to 800 °C, the increasing amplitude of the interplanar spacing was not distinct. The same phenomenon was observed in the selected-area electron diffraction (SAED) patterns. The above results were in accordance with the N 1s XPS and XRD analyses and confirmed that N was doped into the lattice. Moreover, the continuous rings of the (2 0 0) facet, with an interplanar spacing of ∼0.245 nm, indicated the presence of the tetragonal phase.37 The nitrogen adsorption and desorption isotherms of the catalysts are plotted in Figure S1a. The type IV isotherms were found over all catalysts, accompanied by type H3 hysteresis loops,38 indicating that mesopores with narrow slitlike shapes were the main pore structure.39 Thus, the relevant pore size scattergrams are shown in Figure S1b. All catalysts exhibited a sharp peak at ∼5 nm with a broad peak in the range of 10−15 nm. This indicated that the pore sizes of all catalysts were concentrated at ∼5 nm. The BET surface areas and pore structure parameters of all catalysts are given in Table 1. The Table 1. BET Surface Area and Pore Parameters of the CrCZ4 and CZ4-N(x)-Cr Catalysts samples CrCZ4 CZ4−N600Cr CZ4−N700Cr CZ4−N800Cr

BET surface area (m2/g)

pore volume (mm3/g)

pore size (nm)

42.1 32.9

57.6 46.4

4.9 4.1

17.9

45.9

4.1

14.7

43.1

4.1

surface areas of the N doping catalysts were smaller than that of CrCZ4, which may be a result of high-temperature treatment; thus, BET surface area of the N-doped samples continued to decrease with increasing NH3 treatment temperature. Moreover, the pore volume and pore size also decreased after NH3 treatment, which was attributed to the collapse of the pore structure at high NH3 treatment temperatures. Measurement of the Chemical State and Electronic Structure. XPS measurement was performed to investigate the changes in the chemical nature and elemental distribution on the surface of the catalysts before and after N doping, as shown in Figure 3. The Ce 3d XPS spectrum was fitted in two regions (3d5/2: v series, 3d3/2: u series) with four pairs of spin−orbit doublets. The eight peaks after Gaussian fitting were attributed to Ce4+ and Ce3+, in which the peaks labeled v, v″, v‴, u, u″, and u‴ were assigned to the Ce4+ state, while v′ and u′ were ascribed to the Ce3+ state.40 The Cr 2p spectra of all catalysts in the 2p3/2 region were numerically fitted to the Cr6+ and Cr3+ states, corresponding to CrO3 species and Cr2O3 species. The peaks at ∼578.1 eV were assigned to CrO3 species, while the peaks at ∼576.6 eV were ascribed to Cr2O3 species.41 It should be mentioned that CrO2 species were present in CZ4−N800Cr, according to the XRD results; thus, the peak at ∼576.1 eV, which was attributed to CrO2 species, was also fitted in the 2p3/2 region. In the Zr 3d spectra of all catalysts, the two peaks at ∼184.6 and 182.2 eV were ascribed to Zr 3d3/2 and Zr 3d5/ 2, respectively.42 The O 1s spectra mainly consisted of two components. The peak at low binding energy was assigned to E


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Figure 3. XPS spectra of the CrCZ4 and CZ4-N(x)-Cr samples. (a) Ce 3d, (b) Cr 2p, (c) Zr 3d, (d) O 1s.

According to the XPS spectra of all catalysts, after NH3 treatment, the binding energy (BE) of the Ce 3d and Cr 2p peaks was maintained, while the BE of the Zr 3d and O 1s peaks decreased. Combining the results that N was interstitially doped into the lattice and the fact that N atoms donate electrons, the electrons around the atom were increased, which could lead to the BE shift to the low value. Since the BE of Ce was maintained, while the BE of Zr decreased, it could be concluded that the Ce−O−N−Zr structure was constructed. The BE maintenance of the Cr 2p could be ascribed to the Cr species being loaded on CZ4 support via the impregnation process; thus, the N doping displayed little influence on the BE of Cr 2p. The elementary cation surface concentration of all catalysts is shown in Table 2. After NH3 treatment, the ratios of Cr6+, Ce3+, and Oβ were increased, and among all N-doped catalysts, CZ4−N700-Cr exhibited the highest value. In our previous study,1,40,41 we confirmed that high Cr6+, Ce3+, and Oβ concentrations assist the adsorption of NO and O2, which enhances the catalytic activity. Moreover, compared to the ratios of Cr6+ and Ce3+, the ratio of Oβ increased the most after NH3 treatment due to the interstitial doping of N atoms, which promoted further lattice distortion. Information about the surface electronic states and the band gap was obtained via UV−vis DRS measurement. As shown in Figure 4a, three typical peaks were found for all catalysts. For all catalysts, the peak at ∼270 nm was ascribed to O2→Ce4+ charge transfer and interband transitions,44 and the peaks at ∼463 and 590 nm were both attributed to Cr3+ in Cr2O3.45 The peak at ∼463 nm could also be assigned to the Cr6+ species anchored to the support surface.45 In addition, no clear

lattice oxygen (denoted Oα), while the peak with high binding energy was contributed by the chemisorbed oxygen (denoted Oβ), such as O2− and O−.43 The atomic ratios were calculated from XPS peak table, as shown in Table 2. It could be found Table 2. XPS Elementary Cation Surface Concentration of the CrCZ4 and CZ4-N(x)-Cr Catalysts atomic ratios

cation ratios 6+

samples CrCZ4 CZ4− N600-Cr CZ4− N700-Cr CZ4− N800-Cr

Cr/(Ce+Zr)a Cr/(Ce+Zr)b

Cr / Cr(total)

Ce3+/ Ce(total)

Oβ/ (Oα+Oβ)

0.26 0.26

1.11 1.54

31.7% 35.8%

21.1% 23.9%

37.9% 39.1%

0.26

1.34

37.1%

28.9%

51.1%

0.26

2.31

34.7%

22.9%

46.3%

a

Values calculated from the bulk composition. bValues came from XPS results.

that the Cr/(Ce+Zr) atomic ratios calculated from the bulk composition were lower than that calculated from XPS results. Since the particle size observed in TEM images was bigger than the penetration depth of the XPS probe (10 nm), Cr species were concluded to be enriched on the surface, which furtherly confirmed that Cr mainly existed in the form of CrOx over the surface. Moreover, the Cr/(Ce+Zr) ratio over CZ4−N800-Cr was the highest among all catalysts, in accordance with the results that CrOx species were found in XRD patterns. F


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Figure 4. UV−vis absorption spectra (a) and band gap energy (b) of the CrCZ4 and CZ4-N(x)-Cr catalysts. MS plots (c) and schematic diagram showing the energy band structure (d) of the CrCZ4 and CZ4−N700-Cr samples.

adsorption band ascribed to O2→Ce3+ charge transfer was observed, which would have been located at ∼255 nm.46 On the basis of the adsorption band edge in Figure 4a, the band gap (Eg) of each catalyst was calculated, as shown in Figure 4b. The Eg of CrCZ4, CZ4−N600-Cr, CZ4−N700-Cr, and CZ4− N800-Cr was 2.08, 1.86, 1.63, and 1.57 eV, respectively. The Eg clearly decreased after NH3 treatment and continued to decrease with increasing temperature. The difference in the Eg of CZ4−N700-Cr and CZ4−N800-Cr was not clear, indicating that the treatment temperature of 700 °C was optimal, which is in accordance with the XRD and TEM analyses. Mott−Schottky (MS) measurements were conducted on CrCZ4 and CZ4−N700-Cr to explain the band structure of the N-doped catalysts. As shown in Figure 4c, reversed sigmoidal plots were observed with an overall shape consistent with that typical for n-type semiconductors. The flat-band potential (Vfb) of CrCZ4 and CZ4−N700-Cr, calculated from the x intercept of the linear region, was found to be −0.81 and −1.06 V versus Ag/AgCl at pH 6.8, respectively. The Vfb was −0.21 and −0.46 V versus reversible hydrogen electrode (RHE) after converting to the RHE potential, according to the equation E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.195 V.47 In general, the Vfb is close to the bottom of the conduction band (ECB) of n-type semiconductors, which is just 0−0.2 V versus RHE more negative than the Vfb.47 On the basis of the above analysis, the band structures of the catalysts before and after N doping are shown in Figure 4d. After NH3 treatment, Eg decreased, indicating that the electrons were readily excited to the conduction band. Moreover, the more negative ECB of the Ndoped catalysts led to the accumulation of e− in the conduction

band, which could interact with O2 to form chemisorbed oxygen O2−, thus enhancing the oxidizing ability of the catalysts. This is in accordance with the results of XPS analysis. The band structures of the CZ4 and N-doped CZ4 samples were calculated via DFT using the program package DMol3, shown in Figure S2, to find the reason for the position changes of valence band (VB) and conduction band (CB) caused by N doping. It could be found that after N doping, the valence band approached the Fermi level, indicating that N-doped sample possessed more electrons than the undoped one. This was because N element contains the lone-pair electrons, and the N was doped into the lattice in the interstitial form; thus, the N doping made the sample have more electrons. The band gaps of CZ4 and N-doped CZ4 were also calculated, shown in the left bottom of Figure 4. The band gaps of CZ4 and CZ4-N were 0.079 Ha (2.15 eV) and 0.054 Ha (1.47 eV), separately. The band gaps were close to the values obtained from UV−vis DRS results, which were 2.08 and 1.63 eV, separately. The difference between the actual value and the theoretical value might be due to the Cr loading. Moreover, the calculation results confirmed that N doping could decrease the band gap of the catalysts theoretically. Measurement of the Adsorption Properties. To investigate the effect of N doping on the adsorption of NO and O2, the Ead values of NO and O2 molecules for the Ce0.2Zr0.8O2 and N-doped Ce0.2Zr0.8O2 (1 1 1) models were calculated, and the obtained configurations are shown in Figure 5. After calculation, the Ead of NO on the Ce0.2Zr0.8O2 and Ndoped Ce0.2Zr0.8O2 (1 1 1) surface was 2.65 and 20.85 eV, respectively, and the Ead of O2 on the Ce0.2Zr0.8O2 and N-doped Ce0.2Zr0.8O2 (1 1 1) surface was 3.56 and 18.87 eV, respectively. G


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two oxygen species were found below and over 300 °C, the adsorbed NO3− (peak α) was generated by the interaction between the adsorbed NO and chemically adsorbed oxygen below 300 °C, while it (peak β) was generated by the interaction between the adsorbed NO and lattice oxygen over 300 °C. After NH3 treatment, the peak area was increased, especially for peak β. Among all catalysts, CZ4−N700-Cr exhibited the highest peak area, indicating that it possessed the strongest NO adsorption capacity. The TPD results confirmed that N doping enhanced the NO and O2 adsorption capacity, which was in accordance with the XPS and DFT calculation results. Catalytic Oxidation Activity. The NO catalytic oxidation performance of all catalysts was examined in the temperature range of 150−400 °C at a high GHSV of 70 000 h−1, as shown in Figure 7a. Under high GHSV, all catalysts exhibited low NO oxidation efficiency. However, all catalysts exhibited the best catalytic activity at 300 °C, and N doping clearly enhanced the catalytic activity. Among the catalysts, CZ4−N700-Cr displayed the highest NO conversion, reaching over 50% at 300 °C, which satisfies the following alkali absorption process.6 The NO conversion began to decrease when the temperature exceeded 300 °C. Generally, at low temperature, the conversion is limited by kinetics and mass transfer, whereas at high temperature, the conversion is limited by the thermodynamic equilibrium.49 To further confirm the positive influence of N doping on NO oxidation, the samples without Cr impregnation were also tested, as shown in Figure S3a. NO conversion by CZ4 and CZ4-N700 continued to increase with increasing temperature, and CZ4-N700 displayed a higher catalytic activity than CZ4 over the entire temperature range, especially at temperatures above 300 °C. However, the NO conversion over CZ4 and CZ4-N700 catalysts began to decrease while the temperature was over 400 °C. This indicated that the addition of Cr could benefit from the enhancement of NO conversion at low temperature. But the efficiencies over CZ4 and CZ4-N700 were also limited by the thermodynamic equilibrium at high temperature. The influence of different Cr loading over Ndoped Ce0.2Zr0.8O2 on NO conversion was also investigated, as shown in Figure S3b. The results showed that the NO oxidation efficiency increased with the increase of Cr loading, and the catalyst with 10% Cr loading exhibited the highest NO conversion within the whole temperature range. The results indicated that the Cr loading amount played the key role in NO oxidation.

Figure 5. Adsorption configurations of NO (a, b) and O2 (c, d) on Ce0.2Zr0.8O2 (a, c) and N-doped Ce0.2Zr0.8O2 (b, d) (1 1 1) models.

The results indicated that N doping promoted the adsorption of both NO and O2. TPD analysis was performed to test the reactant gasadsorption properties of the N-doped catalysts, as shown in Figure 6. The O2-TPD profiles, shown in Figure 6a, were deconvoluted into three oxygen species regions. Generally, peaks at temperatures below 350 °C can be ascribed to physically and weak chemically adsorbed oxygen (denoted Oγ), while peaks at temperatures above 600 °C can be assigned to lattice oxygen (denoted Oε). The peak between 350 and 600 °C was related to oxygen defects (denoted Oδ) and is generally caused by the lattice distortion, and it was reflected as the lattice oxygen in the absence of O2 or as the chemically adsorbed oxygen in the presence of O2.48 The lattice defect content was found to improve after N doping, and CZ4−N700Cr exhibited the highest Oδ peak area, while CrCZ4 displayed the highest amount of lattice oxygen. In the NO-TPD profiles shown in Figure 6b, two main desorption peaks were observed, which were both ascribed to the adsorbed NO3−. Combined with the O2-TPD results that

Figure 6. O2-TPD (a) and NO-TPD (b) of the CrCZ4 and CZ4-N(x)-Cr catalysts. H


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Figure 7. (a) Catalytic oxidation of NO over the CrCZ4, CrCZ4-N, and CZ4-N(x)-Cr catalysts. (b) NO conversion vs temperature plots over CrCZ4 and CZ4-N(x)-Cr catalysts. (c) H2-TPR profiles of CrCZ4 and CZ4-N(x)-Cr catalysts. (d) Arrhenius plots of the TOF with respect to the amount of active species over CrCZ4 and CZ4-N(x)-Cr catalysts.

On the basis of the Arrhenius equation (Eqn 3), the catalytic oxidation profiles were converted to the kinetic curves, as shown in Figure 7b. Because of the mass transfer limited higher than 300 °C, the first two temperature points (350 and 400 °C) were not converted.50 Moreover, since the reaction rate decreased when the temperature was higher than 300 °C, the obtained apparent activation energy would be negative, which was unreasonable. The similar results were observed in Jason’s work.51 Additionally, the point of conversion lower than 10% was also not converted, due to the large error of the reaction rate at low oxidation efficiency. According to the following Eqns 3−5, the activation energy (Ea) and pre-exponential factor (A) were obtained.52 K r = A exp( −Ea /RT ) ln K r = −

Ea + ln A RT

K r = (X ·YNO·V )/(1000 × 22.4·W )

Table 3. Reaction Activation Energy (Ea), the PreExponential Factor (A), and the Corresponding R2 of CrCZ4 and CZ4-N(x)-Cr Catalysts samples

Ea (kJ·mol−1)

A (cm−3·g−1·s−1)

R2

CrCZ4 CZ4−N600-Cr CZ4−N700-Cr CZ4−N800-Cr

36.1 21.2 26.1 35.6

9606 454 1574 10 026

0.9743 0.9919 0.9898 0.9779

activation energy, especially for CZ4−N600-Cr and CZ4− N700-Cr catalysts, indicating that N doping benefited from the NO oxidation. Notably, compared with CZ4−N600-Cr and CZ4−N700-Cr, it could be seen that the Ea of former is lower than the latter, but the opposite situation was observed for the A, and the value of latter is 3 times higher than that of the former. It was reported that the catalyst with high A value also displayed high catalytic activity,53 based on the premise of low Ea value. Hence, CZ4−N700-Cr exhibited the highest NO conversion. The H2 temperature-programmed reduction (TPR) characterization was adopted to find active site on the surface of the catalysts, as shown in Figure 7c. There are two obvious reduction peaks of all catalysts during the temperature range of 100−750 °C. The peak at low temperature (300−450 °C) was assigned to the reduction of surface ceria oxide, while the peak at high temperature (500−600 °C) was contributed by the combination of the reduction of Cr6+ (Cr6+ to Cr3+) and bulk ceria (Ce4+ to Ce3+). Among all catalysts, it could be found that the reduction temperature of the ceria oxide and Cr6+ decreased

(3)

(4) (5)

where Kr is the NO oxidation rate, mol·g−1·s−1; A is the preexponential factor, cm−3·g−1·s−1; Ea represents the reaction activation energy, J·mol−1; R represents the gas contant, 8.314 J·mol−1·K−1; T is the reaction temperature, K; X is the oxidation efficiency, %; YNO is the NO concentration, 1 × 10−6, which is 400 in this work; V is the gas flow rate, cm3·s−1, which is 1.67 in this work; W is the catalyst weight, g, which is 0.3 in this work. After calculation, Ea and A of all catalysts are shown in Table 3. It could be found that N doping helped decrease the I


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Figure 8. Independent (a) and simultaneous (b) effects of H2O(g) and SO2 on NO conversion over the CZ4−N700-Cr and CrCZ4 catalyst at 250 °C. XRD patterns (c) and FT-IR spectra (d) of the fresh and H2O(g) and SO2-tested CZ4−N700-Cr catalysts.

little influence on the structure and morphology of the catalysts. The Cr 2p XPS spectra showed that the fresh and used catalysts displayed the similar peak positions and peak areas, and the corresponding cation ratios were shown in Table S1. The results indicated that Cr species could not be taken off during the NO oxidation process, proving its friendly environmental effect. For O 1s XPS spectra, it could be found that the peak positions of the used catalysts shifted the higher value, compared to the fresh catalysts. This was ascribed to the participation of chemisorbed oxygen in NO oxidation. However, the Oβ ratios of the used catalysts were close to the fresh, indicating that the chemisorbed oxygen could be regenerated during the reaction. Effect of H2O(g) and SO2 on NO Oxidation. The independent effects of H2O(g) and SO2 on NO oxidation over CZ4−N700-Cr and CrCZ4 catalysts were examined, as shown in Figure 8a. The NO conversion of them both decreased when H2O(g) was added in the flue gas. The steady-state NO conversion of CZ4−N700-Cr was ∼42% in the presence of H2O(g), while the NO conversion of CrCZ4 was ∼15%. However, the inhibition effects of H2O(g) over them were both reversible, and NO conversion recovered to its original level after H2O(g) removal. This reversible process also occurred with the addition and removal of SO2, and the steady-state NO conversion of CZ4−N700-Cr was ∼38%, while the other was ∼10%. The results showed that N doping was confirmed to enhance not only the NO oxidation capacity but also the H2O(g) and SO2 resistance. The simultaneous effects of H2O(g) and SO2 on NO conversion over CZ4−N700-Cr and CrCZ4 catalysts were also tested, as shown in Figure 8b. For CZ4−N700-Cr, it could be

with the N doping. Among all catalysts, CZ4−N700-Cr exhibited the lowest reduction temperature and highest reduction peak area, indicating that there exists strong interaction between the Ce and Cr species, and CZ4−N700Cr catalyst possessed high Cr6+ and Ce3+ contents. Combined with the activity test, it could be concluded that Cr6+ and Ce3+ existed as the main active sites. On the basis of the TPR results, the Arrhenius plots of the turnover frequency (TOF) with respect to the amount of active species (Ce3+ and Cr6+) over all catalysts were calculated based on Eqns 5 and 6,54 as shown in Figure 7d. It could be found that the TOF value of all N-doped catalysts were close to each other, but they were all higher than that of CrCZ4. The results further confirmed that N doping benefited from the NO oxidation. TOF = K r /nactive species

(6)

In the above equation, nactive species is the amount of active species (Cr6+ and Ce3+), mol·g−1. The photodegradation of acid orange, rhodamine B, and methylene blue over CrCZ4 and CZ4−N700-Cr was also investigated to confirm the enhancement of the oxidation properties upon N doping. As shown in Figure S4a,b, both catalysts exhibited efficient photodegradation, and CZ4−N700Cr displayed higher performance, especially in the photodegradation of rhodamine B and methylene blue. Hence, N doping indeed enhanced the oxidation capacity. With the aim to investigate the changes of the structure and element contents before and after NO oxidation, the fresh and the used CrCZ4 and CZ4−N700-Cr were selected to be characterized by XRD, TEM, and XPS techniques, as shown in Figure S5. It could be seen that the NO oxidation test showed J


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Figure 9. In situ DRIFTS spectra of CZ4−N700-Cr upon exposure to sole 500 ppm of NO (a) or 8 vol % O2 (c) at different temperatures for various times and followed by exposure to 500 ppm of NO and 8 vol % O2 (b, d) at 300 °C for various times.

BET, and FT-IR, as shown in Figure 8c,d. After the SO2 test, the specific surface area was decreased, but the degree was not obvious. Moreover, there was also no obvious difference between the fresh and SO2-exposed catalysts in the XRD patterns and FT-IR spectra (bands assigned to sulfate generally appear in the range of 900−1200 cm−1). The above results indicated that N doping can suppress the formation of surface sulfate, thus enhancing the SO2 resistance of the catalyst. Reaction Mechanism Investigation. The NO oxidation mechanism over CZ4−N700-Cr was investigated by in situ DRIFTS. The catalyst was exposed to 500 ppm of NO only, then the temperature was increased to 300 °C, and measurements were taken at each 50 °C increment. The spectrum was recorded at each temperature at different times, as shown in Figure 9a. Three typical bands were observed. The band at 1555 cm−1 was ascribed to chelated bidentate nitrate.57 The band at 1387 cm−1 was contributed by the adsorbed NO3−, generated by the reciprocity between the adsorbed NO and O2−.58 The band at 1209 cm−1 was ascribed to monodentate nitrate.59 The intensities of monodentate nitrate and bidentate nitrate clearly decreased with an increase in temperature, but the intensity of adsorbed NO3− remained almost the same, as O2− was replenished by lattice oxygen. After O2 was used to supplement the system, as shown in Figure 9b, the intensity of monodentate nitrate recovered to its original level, while that of bidentate nitrate did not. The intensity of adsorbed NO3− increased after O2 addition, which could be attributed to the increased generation of O2− on the surface. Moreover, a new band at 1348 cm−1 appeared, which could be assigned to

found that, under the condition of 200 ppm of SO2, the simultaneous effect on activity was weak, and it still displayed strong resistance to H2O(g) and SO2 even in the condition of 400 ppm of SO2. The catalyst exhibited 49% and 40% NO conversion under 200 and 400 ppm of SO2, respectively, and the activity recovered to the origin level in the short time, while the H2O(g) and SO2 were cut off. However, the simultaneous effect on activity over CrCZ4 catalyst was obvious, even under the condition of 200 ppm of SO2. Moreover, the activity could not recover to the origin level after the H2O(g) and SO2 were cut off. Combined with the results of the independent effect of H2O(g) and SO2 on NO conversion, it could be concluded that the N doping enhanced the H2O(g) and SO2 resistance of the catalysts. To investigate the reason that N-doped catalysts displayed strong H2O(g) and SO2 resistance, the in situ DRIFTS was adopted to observe the change of surface species during the H2O(g) and SO2-contained reaction process, as shown in Figure S6. The DRIFTS spectra showed that a new peak appeared when H2O(g) and SO2 were injected into the reaction system. Generally, the peaks at 1104 and 613 cm−1 are contributed by the free SO42− ion,55 and the adsorbed inorganic sulfate displays a weak peak in the range of 960−1030 cm−1.1,56 Hence, the new appeared peak was ascribed to the adsorbed sulfate, which could occupy the active site to decrease the activity. However, the peak disappeared when H2O(g) and SO2 were cut off, indicating that the occupied active site could be released when H2O(g) and SO2 were removed. The crystal structure, specific surface area, and surface species of SO2-exposed CZ4−N700-Cr were investigated by XRD, K


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ACS Applied Nano Materials *E-mail: [email protected]. (Q.Z.) *E-mail: [email protected]. (Y.-F.B.)

adsorbed oxygen. A similar in situ test was also conducted to further confirm the above process, as shown in Figure 9c,d. The catalyst was exposed to 8 vol % O2 only, and then, the temperature was increased. Two bands appeared at 3738 and 1348 cm−1 when only O2 was fed into the cell, which were assigned to terminal Zr4+−OH60 and adsorbed oxygen, respectively. After the system was supplemented with NO, bands assigned to bidentate nitrate, adsorbed NO3−, and monodentate nitrate appeared at 1591, 1381, and 1212 cm−1, respectively, as shown in Figure 9d. In the in situ DRIFTS spectrum, the band assigned to monodentate nitrate always appeared in the presence of O2. Hence, monodentate nitrate was determined to be the main adsorption form of NO, which was then converted into adsorbed NO3− through interaction with O2−. On the basis of the above analysis, the possible NO oxidation mechanism over CZ4−N700-Cr can be described as follows. NO was adsorbed on the Cr6+ site to form monodentate nitrate NO+. The O2− generated on the surface of the N-doped catalyst interacted with NO+ to form adsorbed NO3− and O2, and then, the adsorbed NO3− continued to interact with NO+ to form NO2.

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NO+ + 2O2− → NO3− + O2

(7)

NO3− + NO+ → 2NO2

(8)

ORCID

Yunfei Bu: 0000-0002-1488-5925 Present Addresses ∥

Key Laboratory of Meteorological Disaster, Ministry of Education, Joint International Research Laboratory of Climate and Environment Change, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China. ⊥ Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China. # School of Chemical and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Jiangsu Province of China (BK20170938), Startup Foundation for Introducing Talent of NUIST (2243141601034), the Natural Science Foundation of Jiangsu Province of China (BK20150892), the Natural Science Foundation of Jiangsu Higher Education Institution of China (15KJB610012), the National Natural Science Foundation of China (51408309), the National Natural Science Foundation of China (51678291), and the Industry-Academia Cooperation Project of Datang Pro-environment (DNEPT_CZ_179_16). Y.F.B. was supported by the Natural Science Foundation of Jiangsu Province of China (BK20160834), The 59th Chinese Postdoctoral Science Foundation (2016M591850).

CONCLUSION In summary, N-doped Ce0.2Zr0.8O2 was successfully synthesized via NH3 treatment, and then, CrOx species were loaded via impregnation. The XRD and TEM results confirmed that N was doped into the lattice, and the XPS and FT-IR results proved the formation of the Ce−O−N−Zr structure. The catalytic performance over the N-doped catalysts was enhanced, and among all catalysts, the catalyst treated at 700 °C exhibited the best catalytic activity, which was reflected in the oxidation of NO and the photodegradation of methylene blue, acid orange, and rhodamine B. Moreover, CZ4−N700-Cr displayed excellent H2O and SO2 resistance during NO oxidation. The enhanced catalytic activity was attributed to the increased surface concentration of Cr6+, Ce3+, and Oβ after NH3 treatment, calculated from the XPS results. The generation of O2− over the N-doped catalysts may be due to the decreased Eg and ECB. The DFT calculations indicated that N doping enhanced the adsorption capacity of NO and O2, and the TPD results experimentally confirmed this enhancement. Furthermore, the NO oxidation mechanism was investigated by in situ DRIFTS, which indicated that the interaction of NO+ and O2− leads to the formation of adsorbed nitrate, and then, NO+ interacts with the adsorbed nitrate to generate NO2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.7b00320. N2 adsorption−desorption isotherms, DFT calculation, catalytic oxidation test, photodegradation, additional XRD, TEM, XPS, DRIFTS, and additional table (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (W.C.) L


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