Investigation of the Facet-Dependent Catalytic Performance of Fe2O3

Subscriber access provided by GAZI UNIV

Article

Investigation of the Facet-Dependent Catalytic Performance of Fe2O3/CeO2 for the Selective Catalytic Reduction of NO with NH3 Jin Han, Jittima Meeprasert, Phornphimon Maitarad, Supawadee Namuangruk, Liyi Shi, and Dengsong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09834 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Investigation of the Facet-Dependent Catalytic Performance of Fe2O3/CeO2 for the Selective Catalytic Reduction of NO with NH3 Jin Han,† Jittima Meeprasert,‡ Phornphimon Maitarad,† Supawadee Nammuangruk,‡ Liyi Shi,† Dengsong Zhang†*

†

Research Center of Nano Science and Technology, Shanghai University, Shanghai, 200444, P. R. China ‡

National Nanotechnology Center (NANOTEC), NSTDA, Pathum Thani 12120, Thailand

ABSTRACT: The facet-dependent catalytic performance of Fe2O3/CeO2 catalysts for the selective catalytic reduction of NO with NH3 (NH3-SCR) has been investigated using combined experimental and density functional theory (DFT) methods. The structure and surface characteristics of the synthesized samples were characterized by XRD, XPS, TEM, ICP-AES, N2 sorption isotherms, Raman spectra, photoluminescence spectra, H2-TPR, NH3-TPD and NO+O2-TPD. It is found that the CeO2 nanorods and Fe2O3/CeO2 nanorods predominately exposed {110} and {100} facets rather than the stable {111} facets on CeO2 nanopolyhedra and Fe2O3/CeO2 nanopolyhedra. The influence of the 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

micro-morphologies and surface properties of CeO2 supports on the NO conversion and N2 selectivity has been compared. The Fe2O3/CeO2 nanorods achieve higher catalytic activity than the Fe2O3/CeO2 nanopolyhedra for NH3-SCR of NO. The synergetic effect between CeO2 supports and Fe2O3 species has been demonstrated. The insight into molecular facet dependence by the DFT method clearly showed that the Fe2O3/CeO2 {110} catalyst is more reactive to NO and NH3 gases than the Fe2O3/CeO2 {111} and naked CeO2 {110}, which agree well with the experimental results. As a result, the outstanding catalytic performance of Fe2O3/CeO2 nanorods is attributed to the adsorbed surface oxygen, oxygen defects and atomic concentration of Fe which are associated with their exposed {110} and {100} facets of nanorods.

Keywords: CeO2; Fe2O3, NH3-SCR; Structure-activity relationship; de-NOx

2


Page 2 of 43

Page 3 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION NOx, which comes from the combustion of fossil fuels and mobile sources, remains to be one of the most dangerous environmental pollutants, as they can cause acid rain, photochemical song, and have harmful effects on human health.1,

2

So far, such

technologies as NO decomposition,3, 4 selective non-catalytic reduction (SNCR)5, 6 selective catalytic reduction (SCR)

7, 8

and plasma storage-reduction

9, 10

have been

employed for NOx removal. Among these technologies, the SCR of NOx with NH3 (NH3-SCR) have been considered one of the most promising technologies in which the products mainly contain N2 and H2O.8, 11 Commercial NH3-SCR catalysts such as V2O5-WO3(MoO3)/TiO2, have been widely used for the elimination of NOx.7, 11

8,

However, there remains some inevitable problems unsolved, such as the volatility

and toxicity of VOx, the narrow operation temperature window, and especially the poor low-temperature operating activity.11-14 Moreover, the oxidation of SO2 to SO3 also could etch the equipment and block the pores of the catalysts. Therefore, it is urgent to search for more appropriate NH3-SCR catalysts with high catalytic activity, and great SO2-tolerance in low-temperature regions. 12, 15 Iron oxide is a typical active ingredient or promoter in NH3-SCR catalysts, which exhibits good NH3-SCR activity and N2 selectivity, because of its inherently environmentally friendly character, its prominent thermal stability and its outstanding H2O/SO2 resistance.12, 16-20 Iron oxides supported on Al2O3, TiO2, SiO2, ZrO2 and carbonaceous materials, are found to be the active sites for SCR of NO at medium temperature. Besides, the catalytic behaviors of Fe zeolites in the NH3-SCR of NO 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

have been investigated systematically. Qi and Yang13 found that the addition of Fe could improve the low temperature NH3-SCR activity of Mn/TiO2 in low temperature by enhancing the oxidation of NO. Wu et al.21 studied the effect of different transition metal as the addition over the manganese-based catalyst for SCR of NO and found that iron oxide could affect the catalytic activity through improving the dispersion of Mn and Ti, and thus enhanced the low temperature activity of Mn/Ti catalyst. Besides, Bruckner et al.22 investigated the role of Fe on catalytic performance over Fe-ZSM-5 catalysts and found that the accessible Fe3+ species were active sites in the SCR of NO with NH3. Therefore, iron oxide could be a suitable candidate to improve the NH3-SCR performance at low temperature. As one of the most familiar rare earth oxides, ceria (CeO2) have been widely investigated as catalysts and catalyst supports in many important catalytic reactions owing to its excellent redox properties, high oxygen storage capability and low cost characteristic.23-27 Recently, low dimensional CeO2 nanocrystals with various structures including nanocubes, nanorods, nanopolyhedra have been successfully synthesized and studied for catalytic reactions by us and other research groups.25, 28-32 Researchers usually obtained the objective shapes by terminating on different specific crystallographic planes; {110} and {100} for nanorods, {111} for nanopolyhedra.28 According to the density functional theory (DFT), the formation energy of an oxygen vacancy on the different facets follows the sequence {110} ˂ {100} ˂ {111}, while the chemical activity of these planes follow the opposite sequence.33, 34 These faceted nanomaterials offer the opportunity to observe the effect of surface structure upon the 4


Page 4 of 43

Page 5 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


activity and reducibility. CeO2 nanocrystals present a shape-dependent behavior in some catalytic processes. For example, nanorods and nanocubes exhibit a greater capacity to better capacity to sore oxygen than nanopolyhedra terminated primarily in {111} facets, and the rate of oxidation of CO catalyzed by CeO2 nanorods is greater than by traditionally grown ceria nanopolyhedra with undifferentiated shapes.35 Recently, our group found that the MnOx/ZrOx-CeO2 nanorods catalysts displayed a more excellent catalytic activity for NH3-SCR of NO at low temperature.29 The above result was according to the oxygen vacancies and the mobility of the lattice oxygen dependent on the shape of the CeO2 nano-support, which caused the different catalytic behavior. To date, the shape dependence of ceria nanomaterials in Fe2O3/CeO2 for NH3-SCR of NO has not been reported. Here, we design the synthesis of Fe2O3/CeO2 nanomaterials and investigate the effect of the micro-morphology of ceria on the catalytic activity of Fe2O3/CeO2 for the NH3-SCR of NO. The CeO2 nanorods and nanopolyhedra were prepared by a hydrothermal method while iron oxide supported on the CeO2 nanomaterials were prepared by a wet impregnation method. The catalytic performance of the CeO2 nanomaterials and Fe2O3/CeO2 nanomaterials with different morphology were examined in the NH3-SCR of NO.

2. EXPERIMENTAL SECTION Catalyst preparation. All the chemicals were purchased from Sinopharm Chemical Regent Company and were used without any further purification. 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 43

The CeO2 nanorods and nanopolyhedra were prepared by a hydrothermal method, using Ce(NO3)2·6H2O as the cerium source and NaOH as the precipitator. The morphologies of CeO2 nanoparticles were controlled by varying the concentration of NaOH, and the synthesis temperature and time. For CeO2 nanorods, the precipitated materials were thoroughly centrifuged by DI water till neutral. For CeO2 nanopolyhedra, the precipitated materials were thoroughly dialysed by DI water till neutral. Afterwards, all of the materials were dried overnight at 100 oC, and then calcined in static air at 400 oC for 4 h at a ramping rate of 2 oC min-1. The prepared CeO2 nanorods, and CeO2 nanopolyhedra were denoted as CeO2-NR and CeO2-NP. The Fe2O3/CeO2 catalysts were prepared by a wet incipient impregnation method. The Fe(NO3)3·9H2O was used as the source of ferric oxide, and the CeO2-NR, CeO2-NP were used as support materials. The loading value of ferric oxide was 3.0 wt. %. The Fe2O3/CeO2

nanorods,

and

Fe2O3/CeO2

nanopolyhedra

were

denoted

as

Fe2O3/CeO2-NR and Fe2O3/CeO2-NP, which were calcined at 400 oC for 4 h with a ramping rate of 2 oC min-1. Catalyst characterization. Nitrogen physisorption isotherms of the samples were accomplished on an Autosorb IQ (Quantachrom) system by N2 adsorption at 77 K, after degassing the samples at 300 oC for 3 h under vacuum. The specific surface area of the samples was calculated by the Brunauer-Emmett-Teller (BET) method. The morphologies of the as-prepared samples were characterized by using a field emission transmission electron microscope (TEM, JEOL JEM-2100F). The crystallographic structures were identified on an X-ray diffractometer (XRD, 6


Page 7 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Kigaku D/MAX2500V+/PC) at a scanning step of 8 ° min-1. The Raman spectra were recorded on a spectrometer (JY H800UV) equipped with an optical microscope at room temperature. The elemental and chemical composition was performed by means of X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA) using a monochromateic Mg Kα anode and a hemispherical energy analyzer. All of the binding energies were calibrated to the C 1s peak at 284.6 eV. The bulk compositions of the catalysts were determined by elemental analysis using an inductively coupled plasma atomic emission spectrometer (ICP-AES, PerkinElmer Optima 7300 DV). Each catalyst was dissolved by heating in a mixture of HNO3 and H2O2, and then the volume of the solution was fixed at 100 mL before the measurement. The ICP-AES working curves for Fe and Ce were plotted by employing Fe2O3 and CeO2 standard solutions, respectively. The photoluminescence (PL) measurements were obtained at room temperature in a fluorescence spectrophotometer (Shimadzu-RF 5301), and the samples were excited at 300 nm. The temperature programmed reduction with hydrogen (H2-TPR) experiments were performed with an auto-adsorption apparatus (tp5080, Tianjin XQ) with a thermal conductivity detector. For the analysis, 100 mg of powdered sample was pre-treated in nitrogen at 300 oC for 30 min. After cooling to room temperature, the H2-TPR was recorded in 5 % H2/N2, with a heating rate of 10 oC min-1 and final temperature of 650 oC. 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The temperature programmed desorption with ammonia (NH3-TPD) experiments were also performed with an auto-adsorption apparatus (tp5080, Tianjin XQ) with a thermal conductivity detector. For the analysis, 100 mg of powdered sample was pre-treated in nitrogen at 300 oC for 30 min. After cooling to 100 oC, the samples were saturated with high-purity anhydrous ammonia for 1 h; and subsequently flushed at the same temperature for 1 h to remove physically-adsorbed ammonium. Finally, the NH3-TPD was recorded from 100 oC to 850 oC with a heating rate of 10 oC min-1. The temperature programmed desorption with NO+O2 (NO+O2-TPD) experiments were performed with an auto-adsorption apparatus (tp5080, Tianjin XQ) with a thermal conductivity detector. For the analysis, 100 mg of powdered sample was pre-treated in nitrogen at 300 oC for 30 min. After cooling to 100 oC, the samples were saturated with high-purity anhydrous ammonia for 1 h; and subsequently flushed at the same temperature for 1 h to remove physically-adsorbed ammonium. Finally, the NO+O2-TPD was recorded from 100 oC to 510 oC, with a heating rate of 10 oC min-1. Catalytic performance tests. The NH3-SCR activity and ammonia oxidation tests were conducted in a heated fixed-bed quartz reactor using a 0.4 g catalyst (20 - 40 mesh). The gas mixture was composed of 500 ppm NO, 500 ppm NH3, 3 vol.% O2, 100 ppm SO2 (when used), 8 vol.% H2O (when used) and N2 balance. The gas hourly space velocity (GHSV) was about 20 000 h-1 while the total flow rate of the feed gas was approximately 250 mL min-1. The reaction temperature was from 50 oC to 375 oC. The concentration of NO of the inlet and outlet gases was measured by a SIGNAL MEASUREMEN analyzer. The concentration of NH3 was measured by an IQ 350 8


Page 8 of 43

Page 9 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ammonia analyzer. The concentration of N2O was measured by a Transmitter IR N2O analyzer. The NO conversion, N2 selectivity and NH3 oxidation conversion were calculated by the following equations: [] − [] × 100 [] 2[! ] N2 selectivity/%=1× 100 [] − [] + [#$ ] − [#$ ] [#$ ] − [#$ ] NH$ oxidation/% = × 100 [#$ ] NO conversion/% =

The relative turnover frequency (TOF) value was conducted to compare the activities of the different catalysts. The relative TOF (s-1) of NO over each Fe atom at different temperature was calculated by the following equation: TOF =

+,- ⁄./12NO 3cat 4Fe/5Fe

Where P is the standard atmospheric pressure (1.01×105 Pa); ν is the flow rate of NO (0.20 mL min-1); R is the proportional constant (8.314 J mol-1 K-1); T is the temperature (K); XNO is the NO conversion of the catalysts (%); mcat is the mass of the catalysts (0.4 g); βFe is the Fe loading (%) and MFe is the molar mass of Fe (55.85 g mol-1). DFT Calculations. Based on our catalyst preparations and characterizations, nanorods and nanopolyhedra morphologies of CeO2 contain the {110} and {111} as the dominant planes, respectively. Thus, we modelled the CeO2 with {110} and {111} planes to represent the nanorods and nanopolyhedra and the impregnated Fe2O3 over the CeO2 nanorods and nanopolyhedra are denoted as the Fe2O3/CeO2 {110} and Fe2O3/CeO2 {111}, respectively. Firstly, the CeO2 supporting models with a neutral stoichiometric plane of {110} or {111} were cleaved with 5 layers. The surface was 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

expanded to 3 × 2 dimensions to create the supercell. The calculations with periodic boundary conditions, a surface was represented by a thin slab, which was separated from its images in the direction perpendicular to the surface by a vacuum gap, in this case 15 Å. All calculations were performed using the DMol3 module of Material Studio Program package. DFT of DND basis function and GGA with PBE were used. The effective core potentials (ECP) and spin polarizations were applied. The Brillouin zone of Monkhorst-Pack grid was set at 2 × 2 × 1 and the cutoff radius was assigned at 4.2 Å. The SCF convergence energy was set as 1.0 e-5 Ha. The NO and NH3 adsorption abilities over the catalyst models of CeO2{110}, Fe2O3/CeO2{111} and Fe2O3/CeO2{110} were theoretically studied using the DFT calculations on the basis of theoretical investigation in terms of NO and NH3 adsorption energy over the surface models of the catalysts. The adsorption energy (Ed) is defined as: Ed = Esurface+gas – Esurface - Egas Where Esurface+gas is the total energy of the complex system, Esurface and Egas are the total energies of surface and gas, respectively. Therefore, the negative Ed value means more attractive adsorption energy whereas the positive value means more repulsive adsorption energy. 3. RESULTS AND DISCUSSION Texture Characterization. Fig. 1a illustrated the XRD patterns of the CeO2-NR, CeO2-NP, Fe2O3/CeO2-NR and Fe2O3/CeO2-NP. Typical diffraction peaks of fluorite cubic structure were observed in the both four as-synthesized materials. The 10


Page 10 of 43

Page 11 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diffraction peaks at 28.5 o, 33.0 o, 47.5 o, 56.4 o, 59.1 o, 69.4 o,76.9 o, and 79.4 o can be attributed to (111), (200), (220), (311), (222), (400), (331), (420), and (422) of the facets of CeO2 (JCPDS card NO. 43-1002, space group Fm3m), respectively. It was clearly observed that diffraction peaks of ferric oxide was not detected in the materials of Fe2O3/CeO2-NR and Fe2O3/CeO2-NP, which indicates that the ferric oxide are uniformly loaded on the surface of CeO2-NR or CeO2-NP. The specific surface areas, pore-volumes and average pore sizes of the catalysts were

analyzed

using

N2

adsorption-desorption

isotherms.

The

N2

adsorption-desorption isotherms of the four samples are shown in Fig. 1b. All of them imply the type IV isotherm with an inflection of nitrogen-adsorbed volume at P/Po=0.6, indicating the presence of mesopores in the samples consisting of agglomerates. Besides, the specific surface areas, pore volumes and average pore diameter of the samples were summarized in Table 1. As shown in Table 1, the surface area of naked CeO2-NR is higher than that of CeO2-NP, which is mainly because of their unique microstructure and good dispersion of nanorods. After loading with Fe2O3 on the CeO2 nano-supports, the surface areas of Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts are decreased to some extent and show no obvious difference between them. Since the addition of the active species could partly cover the stacking pores, so the average pore diameter of Fe2O3/CeO2-NR is decreased after the loading of Fe2O3, which is similar as the previous reports.36, 37 Structure and Morphology. The morphology and microstructures of the CeO2 nano-shapes and Fe2O3/CeO2 samples were investigated using TEM and high 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

resolution TEM (HRTEM) images. Fig. 2a shows the TEM image of CeO2-NR sample, displaying CeO2 in the morphology of nanorods with 28-150 nm in length and 5-10 nm in width. Besides, in Fig. 2b the CeO2 nanorods are grow along {110} direction. The structural model of CeO2-NR is shown in Fig. 2c. Fig. 2d-e show the TEM and HRTEM images of CeO2-NP. The TEM image of CeO2-NP indicates that the dimensions of CeO2-NP are about 10 nm. Fig. 2f shows the structure model of the CeO2–NP, which is enclosed by eight {111} and six {100} plans. Fig. 2g-h show the TEM and HRTEM images of Fe2O3/CeO2-NR. Compared with CeO2-NR, the microstructure of Fe2O3/CeO2-NR shows no obvious damage of the original morphology of CeO2-NR. The EDS spectra of Fe2O3/CeO2-NR (Fig. 2i) confirm the presence of Fe element, which indicates the Fe2O3 disperses on the surface of CeO2-NR. Fig. 2j-k show the TEM and HRTEM images of Fe2O3/CeO2-NP, respectively. Compared with CeO2-NP, the microstructure of Fe2O3/CeO2-NP shows no obvious change of the original nanopolyhedra structure of CeO2-NR. Fig. 2l shows the EDS spectra of Fe2O3/CeO2-NP with the presence of Fe, Ce, O elements, which indicates the Fe2O3 was highly dispersed on the surface of CeO2-NP. These results indicate that the desired nanostructure of CeO2 and Fe2O3/CeO2 catalysts with different exposed planes is successfully synthesized. Bulk and Surface Compositions and Defects of Catalysts. Raman spectroscopy was applied to further investigate the lattice vibrational states and defects of the materials.38 Fig. 3 displayed the visible (514 nm) Raman spectra of CeO2 and Fe2O3/CeO2 nanomaterials. As shown in the Fig. 3a, for the CeO2 nanostructures, the 12


Page 12 of 43

Page 13 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distinct F2g symmetry mode of the CeO2 phase centered at 460 cm-1 with weak bands at 250 cm-1, 599 cm-1 and 1180 cm-1 assigned to the second-order transverse acoustic (2TA) mode of CeO2, defect-induced (D) mode of CeO2, and second-order longitudinal (2LO) mode of CeO2, respectively.28,

39, 40

The Raman spectra for

Fe2O3/CeO2 nanostructures were similar to those of the supports. The I600 /I460 values (the ratio of the intensities of the D and F2g bands, Fig. 3b), which indicate the defect concentration, such as oxygen vacancies, over the four catalysts studied here follows the order: Fe2O3/CeO2-NR ˃ Fe2O3/CeO2-NP ˃ CeO2-NR ˃ CeO2-NP. After loaded the Fe2O3 on the surface of the CeO2 supports, the intensity ratios increased obviously. This observation demonstrates that the interaction between iron oxide and CeO2-NR is stronger than that with CeO2-NP. Therefore, the faceted CeO2 with different shapes could influence the synergistic interaction between Fe2O3 and CeO2 nanostructures. The XPS analysis was performed to clarify the surface atomic concentrations and valence states of all elements in CeO2-NR, CeO2-NP, Fe2O3/CeO2-NR and Fe2O3/CeO2-NP. The surface atomic concentrations of Fe, Ce and O were measured and summarized in Table 2, and the corresponding XPS spectra of Fe, Ce and O were shown in Fig. 4. As shown in Fig. 4a, the XPS spectra of O 1s were fitted into two peaks, in which the one at lower binding energies (529.5 eV) is corresponding to the lattice oxygen O2- (denoted as Oβ), and the one at higher binding energies (531.7 eV) is ascribed to the surface chemisorbed oxygen, such as O22- or O- (denoted as Oα).20, 41-43

From Table 2, the percentage of Oα species on the surface of Fe2O3/CeO2-NR is

much higher than that of Fe2O3/CeO2-NP, which could be attributed to the structural 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

differentiation of CeO2 supports. In addition, the percentage of Oα species on the surface of CeO2-NR is higher than that of CeO2-NP (see Fig. S1 and Table S1). Fig. 4b presents the Ce 3d XPS spectra of the two catalysts. The Ce 3d XPS spectra, corresponding to five pairs of spin-orbit doublets, were fitted into ten peaks: νo (880.5 eV), ν (882.5 eV), ν' (884.9 eV), ν'' (888.8 eV), ν''' (898.3 eV), µo (899.0 eV), µ (901.1 eV), µ' (903.5 eV), µ'' (907.5 eV) and µ''' (916.6 eV).29, 44, 45 The peaks labelled µ and ν represent the Ce 3d5/2 and Ce 3d3/2 states contribution, respectively. The marked νo, ν', µo, and µ''' peaks are assigned to the Ce3+ species, and the denoted ν, ν'', ν''', µ, µ'', and µ''' peaks are associated with the Ce4+ species.29, 44 The presence of Ce3+ is due to the reduction of Ce4+ in the oxide structure. The surface concentration of Ce3+ to the total Ce on the surface of CeO2-NR, CeO2-NP, Fe2O3/CeO2-NR and Fe2O3/CeO2-NP were summarized in Table 2 and Table S1. It is observed that the percentage of Ce3+ on the surface of CeO2-NR is much higher than that of CeO2-NP (see Fig. S2 and Table S1). After loading Fe2O3 on CeO2 nano-supports, the value of surface concentration of Ce3+ of Fe2O3/CeO2-NR was still higher than that of Fe2O3/CeO2-NP, which could be attributed to the structural differentiation of CeO2 supports as well. The oxygen vacancies could be produced by the electron transformation between Ce3+ and Ce4+, and the higher concentration of Ce3+ could form more oxygen vacancies. This indicated that the oxygen vacancies of the Fe2O3/CeO2-NR are more than that of Fe2O3/CeO2-NP. The XPS results for Fe peaks assigned to Fe species (Fe 2p3/2, Fe 2p1/2) were displayed in Fig. 4c. The Fe peaks of Fe2O3/CeO2 catalysts were assigned to oxidized 14


Page 14 of 43

Page 15 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe species, most likely Fe3+ type species.14, 46 The binding energies centered at about 709.8 eV and 711.2 eV may be assigned to Fe3+ in the spinel structure, and the binding energy centered at about 712.4 eV may be ascribed to Fe3+ bonded with a hydroxyl group, separately.47 It was interesting that the surface atomic concentration of Fe in Fe2O3/CeO2-NR was higher than that of Fe2O3/CeO2-NP, as given in Table 2. It has been reported that accessible Fe3+ can participate in the reversible redox cycle, which is beneficial for SCR activity. On account of the above results, the Fe2O3/CeO2-NR, which owns the higher surface atomic concentration of Fe, could display the better capacity for NH3-SCR performance than that of Fe2O3/CeO2-NP. The PL spectrum is a feasible technique for surveying the oxygen vacancies in the catalysts.48, 49 As shown in Fig. 4d, both samples displayed an obvious PL signal with a similar curve shape under the excitation at 330 nm. Usually, the stronger the PL signal, the higher the content of surface oxygen vacancy and defect. As shown in Fig. 4d, the intensity of the PL peaks for Fe2O3/CeO2-NR was much stronger than that of the Fe2O3/CeO2-NP over Fe2O3/CeO2 catalysts. And, the similar result was observed over CeO2 nano-structures (see Fig. S3). These observations indicate that the Fe2O3/CeO2-NR and CeO2-NR has higher concentration of oxygen defects than that of Fe2O3/CeO2-NP and CeO2-NP. These results imply that the nanorod structure own much higher concentration of oxygen defects than that of nanopolyhedra structure. The higher-density defect could absorb O2 to form chemisorbed oxygen, which is beneficial for the formation of nitro and nitrate groups to promote the SCR process at low-temperature. Namely, the more the oxygen defects, the more the chemical 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

adsorption oxygen was, as was verified by the XPS spectra of O 1s in Fig 4a. Redox properties. It is generally recognized that the redox activity of catalysts is crucial to the catalytic cycle in the NH3-SCR process. H2-TPR is an effective technique to evaluate the reducibility of the iron species supported on the different ceria supports catalysts. The results obtained during the H2-TPR experiments over various Fe2O3/CeO2 and CeO2 catalysts are shown in Fig. 5. For the CeO2 supports, the reduction profiles between 300 oC and 550 oC are attributed to the reduction of Ce4+ to Ce3+.20, 50 After loaded with a little amount of Fe2O3 on the surface of CeO2 supports, two obvious reductive peaks appeared at the same time over the two catalysts. The first reduction peak about 250 oC is due to the reduction of adsorbed oxygen on the surface of the catalysts. The second peak centered around 340 oC is attributed to the reduction of Fe2O3 to Fe3O4.51 Compared with the pure CeO2 supports, the reduction peaks of Fe2O3/CeO2 distinctly shifts to a relatively low temperature. The above results revealed that the addition of Fe2O3 on the surface of CeO2 nano-supports could enhance the redox properties of the naked CeO2. And, the CeO2 nanorods could own much stronger redox abilities than that of CeO2 nanopolyhedra. NH3 and NOx De-adsorption Analysis. The acid site distributions and changes of the surface acidity caused by the structure of the supports of CeO2 and the active species loaded on CeO2 supports investigated by NH3-TPD were illustrated in Fig. 6. It is clear that four catalysts present three desorption peaks at desorption temperature in the range 100-850 oC. The peaks around 160-180 oC were caused by the weak acid 16


Page 16 of 43

Page 17 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sites; the peaks around 290-350 oC were caused by the medium acid sites; and the peaks around 440-700 oC were caused by the strong acid sites.52 It is generally accepted that Lewis acids attributed to coordinated NH3 molecules are more thermally stable than Brønsted acid sites attributed to NH4+,53 so it can be inferred that the desorption peak at a low temperatures is assigned to physisorbed NH3 and the partial ionic NH4+ bound to weak Brønsted acid sites, and the desorption peak at high temperature is assigned to ionic NH4+ bound to strong Brønsted acid sites and coordinated NH3 bound to the Lewis acid sites. It is observed from the NH3-TPD profiles that the intensity and the area of the desorption band of Fe2O3/CeO2-NR are remarkably larger than those of Fe2O3/CeO2-NP at high temperatures. The improvement in the NH3 desorption can elucidate the promotion of active facets over CeO2-NR. Besides, the above observation revealed that the loading of Fe2O3 species on the CeO2 nano-supports could improve the acid properties of the pure CeO2. The CeO2 nanorods could own much stronger acid properties than that of CeO2 nanopolyhedra. It is reported that adsorption of NOx species on the catalyst surface play a vital role in NO reduction. Fig. 7 shows the NOx desorption profiles of the catalysts. All the catalysts show similar curve shape between 100 oC and 700 oC. The former broad peak centered at 150-250 oC was ascribed to the decomposition of trans- and cis-hyponitrite species, and the latter broad one centered at around 420 oC was attributed to the decomposition of monodentate nitrate species, which mainly contributes to the formation of NO2 species. It is worth noting that the NH3-SCR 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalytic activities are related to the formation of the NO2 species, and the NO2 species could promote the “fast” SCR-progress, which was beneficial to enhance the SCR performance. Therefore, the NOx-TPD results prove that the Fe2O3/CeO2-NR have a large number of NO2 species than that of Fe2O3/CeO2-NP, which could beneficial for the NH3-SCR reaction. The obtained results demonstrated that the addition of Fe2O3 species on the CeO2 nano-supports could improve the adsorption abilities of the CeO2. Additionally, the CeO2 nanorods showed much stronger adsorption properties than that of CeO2 nanopolyhedra. DFT NH3 and NOx Adsorption Energies. The obtained Ed results and optimized complex structures of NO and NH3 adsorption over the naked CeO2{110}, Fe2O3/CeO2{110} and Fe2O3/CeO2{111} model catalysts have been shown in Fig. 8. Firstly, the NO adsorbed over the CeO2{110} by forming the ONO*, where O* is a top most surface oxygen, with the intermolecular distance of 1.30 Å, and its adsorption energy is -36.4 kcal mol-1 as seen in Fig. 8a. For the NH3, it preferred to adsorb over the Lewis site of Ce metal (see Fig. 8b) resulting in the adsorption energy of -22.3 kcal mol-1 and the intermolecular distance at 2.74 Å. For the Fe2O3/CeO2{110} catalyst model, its adsorption energy with NO or NH3 molecules is calculated to be -44.9 or -47.9 kcal mol-1, as displayed in Fig. 8c and 8d, respectively. It can be seen that the adsorption of NO and NH3 over the Fe2O3/CeO2{110} are significantly stronger than those over the naked CeO2{110} implying that the Fe2O3/CeO2{110} catalyst is more active to the two molecules of NH3-SCR of NO process than the naked CeO2{110} which agree well with the experimental result. In addition, the 18


Page 18 of 43

Page 19 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe2O3/CeO2{111} catalyst model was used to study the adsorption ability to NO and NH3 molecules. The optimized structure and adsorption energy results showed that an NO adsorbed over the Fe2O3/CeO2{111} resulted in -34.9 kcal mol-1 (see Fig. 8e) and the NH3 adsorption energy is -38.8 kcal mol-1 (see Fig. 8f). These obtained results clearly show that the NO and NH3 are preferentially adsorb over the Fe2O3/CeO2{110} surface than that over the Fe2O3/CeO2{111}. Therefore, these theoretical results are strongly supported the experimental results which have been mentioned that the NO and NH3 desorption showed the larger potential over the Fe2O3/CeO2-NR than the Fe2O3/CeO2-NP. Catalytic performance. Fig. 9a shows the NO conversion curves of the Fe2O3/CeO2 nanostructures with different shapes by ranging reaction temperature from 50 oC to 375 oC based on equal reactor sample weight. It can be clearly observed that the reaction temperature has a remarkable influence on the NO reduction efficiency over different Fe2O3/CeO2 catalysts. Fe2O3 loaded on CeO2 nanorods was the more active; with the Fe2O3 loaded on CeO2 nanoparticles was the lesser active. The light-off temperature (at which the conversion of NO reach 50 %, T50) of Fe2O3/CeO2-NR and Fe2O3/CeO2-NP were 175 oC and 200 oC, respectively. Since the two nano-shapes have well-defined exposed facets, it is possible to compare the NO conversion with NH3 on the basis of surface oxygen sites. Fig. 9b illustrates the N2 selectivity of the catalysts. As shown in Fig. 9b, the naked CeO2-NR and CeO2-NP show 100 % N2 selectivity below 275 oC. By increasing the reaction temperature, the N2 selectivity decreases slightly, but it is still higher than 90%. Additionally, after 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 43

loading Fe2O3 on the CeO2 nano-supports, the N2 selectivity almost keep unchanged during the whole test. The results indicate that all of the catalysts show excellent N2 selectivity for NH3-SCR of NO. Considering the different surface oxygen densities, the NO turnover frequency (TOF) on the basis of iron over the Fe2O3/CeO2-NR, Fe2O3/CeO2-NP catalysts with different temperature is compared in Fig 9c. To maintain differential conditions, the NO TOF is calculated only in the low temperature (≤ 175 oC). Similar to the NO conversion results, the TOFs of NO over two catalysts follows the trend: Fe2O3/CeO2-NR ˃ Fe2O3/CeO2-NP.

The

Fe2O3/CeO2-NR mainly has {110} and {100} facets, and Fe2O3/CeO2-NP has {111} facets. Therefore, the structure dependence of NH3-SCR of NO over the catalysts is clearly illustrated: {110} and {100} facets ˃ {111} facets. The oxidation of ammonia could occur in the NH3-SCR system and is detrimental because it lowers the effective NH3 available for NOx reduction. Therefore, it is necessary to quantify the extent of the ammonia oxidation for fully elucidating the NH3-SCR reaction. In order to evaluate the performance of the catalysts on the NH3 oxidation, the NH3 and O2 mixture gas was led over the catalysts at room temperature. After the saturated adsorption of NH3 and O2, the temperature is risen to 375 oC at a ramp of 5 oC min-1. The oxidation efficiency of NH3 over Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts is presented in Fig. 10. At the temperature below 225 oC over Fe2O3/CeO2 catalysts, the NH3 oxidation is very low and keeps steady-state. The further increase to 250 oC and leads a significantly higher conversion of NH3 over Fe2O3/CeO2-NP. A similar result was observed over CeO2-NR and CeO2-NP catalysts 20


Page 21 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Fig. S4. It was demonstrated that NH3 is mainly oxidized to NO at high temperature. By combining the NO conversion results in the NH3-SCR procedure with NH3 oxidation, the decline of catalytic activity at high temperature is caused by the obvious increasing of oxidation of NH3 with different shapes. The H2O resistance and SO2 tolerance performance of catalysts. As is wellknown, there is still residual H2O and SO2 in the flue gas even with the desulfurization before de-NOx measurement. Therefore, it is essential to investigate the effect of H2O and SO2 on the NH3-SCR activity of catalysts. Fig. 11a and b show the H2O resistance and SO2 tolerance performance of Fe2O3/CeO2-NR and CeO2-NR at 275 oC respectively. As shown in Fig. 11a, the values of NO conversion of Fe2O3/CeO2-NR and CeO2-NR remain unchanged with the presence of 8 vol.% H2O. The results show that presence of H2O could not affect the catalytic performance of Fe2O3/CeO2-NR and CeO2-NR. From Fig. 11b, the NO conversion value of Fe2O3/CeO2-NR still keeps at 94 % during the whole test period whether inlet or cease SO2. In contrast, the NO conversion over CeO2-NR decreases slightly after inducing 100 ppm SO2 in the feed gas, and it arrives at a steady state. After cutting off SO2 to the feed gas, the catalytic performance gradually recovers to its initial value without any fluctuations. The above results of H2O resistance and SO2 tolerance indicate that the Fe2O3/CeO2-NR could be an appropriate candidate for H2O resistance and SO2 tolerance. 4. CONCLUSIONS In conclusion, the Fe2O3/CeO2-NR obtained obviously higher NO conversion than 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

that of Fe2O3/CeO2-NP, CeO2-NR, and CeO2-NP in the SCR of NO with NH3. The Fe2O3/CeO2-NR also achieved a higher TOF of NO than that of Fe2O3/CeO2-NP at low temperature. Besides, the Fe2O3/CeO2-NR presents outstanding N2 selectivity, favorable H2O resistance and SO2 tolerance performance. The HRTEM images revealed that the CeO2 nanorods predominately exposed {110} and {100} facets rather than the stable {111} facets on CeO2 nanopolyhedra. The DFT calculation results showed that the Fe2O3/CeO2{110} surface is more reactive to NO or NH3 molecules compared with the Fe2O3/CeO2{111} surface and naked CeO2{110} surface. Furthermore, from the Raman, XPS and PL results, it can be concluded that the adsorbed surface oxygen, oxygen defects and atomic concentration of Fe which are associated with their exposed {110} and {100} facets at low temperature. Therefore, the structure dependence of catalyst for NO reduction is mainly on account of exposed facets of different nano-shape ceria supports and the synergistic effect between active species and supports. According to these excellent properties, the Fe2O3/CeO2-NR catalyst could be a promising candidate for the NH3-SCR of NO with NH3.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. The XPS spectra, PL spectra, NH3 oxidation and surface compositions of CeO2-NR and CeO2-NP. 22


Page 22 of 43

Page 23 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author *Phone: +86-21-66137152. E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the support of the National Basic Research Program of China (973 Program, 2014CB660803), the National Natural Science Foundation of China (U1462110), and the Shanghai Municipal Education Commission (14ZZ097). We thank National Nanotechnology Center (NANOTEC) in Thailand for computing resources.

REFERENCES (1) Hao, J. M.; Tian, H. Z.; Lu, Y. Q. Emission Inventories of NOx from Commercial Energy Consumption in China, 1995-1998. Environ. Sci. Technol. 2002, 36, 552-260. (2) Jarquin López, G.; Polupan, G.; Toledo Velázquez, M.; Lugo Leyte, R. Analytical and Experimental Research for Decreasing Nitrogen Oxides Emissions. Appl. Therm. Eng. 2009, 29, 1614-1621. (3) Groothaert, M. H.; Lievens, K.; Leeman, H.; Weckhuysen, B. M.; Schoonheydt, R. A. An Operando Optical Fiber UV-Vis Spectroscopic Study of the Catalytic Decomposition of NO and N2O over 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu-ZSM-5. J. Catal. 2003, 220, 500-512. (4) Smeets, P. J.; Groothaert, M. H.; Vanteeffelen, R. M.; Leeman, H.; Hensen, E. J. M.; Schoonheydt, R. A. Direct NO and N2O Decomposition and NO-assisted N2O Decomposition over Cu-Zeolites: Elucidating the Influence of the Cu-Cu Distance on Oxygen Migration. J. Catal. 2007, 245, 358-368. (5) Javed, M. T.; Irfan, N.; Gibbs, B. M. Control of Combustion-Generated Nitrogen Oxides by Selective Non-Catalytic Reduction. J. Environ. Manage. 2007, 83, 251-289. (6) Bae, S. W.; Roh, S. A.; Kim, S. D. NO Removal by Reducing Agents and Additives in the Selective Non-Catalytic Reduction (SNCR) process. Chemosphere 2006, 65, 170-175. (7) Forzatti, P.; Nova, I.; Tronconi, E. Enhanced NH3 Selective Catalytic Reduction for NOx Abatement. Angew. Chem. Int. Ed. 2009, 48, 8366-8368. (8) Kompio, P. G. W. A.; Brückner, A.; Hipler, F.; Auer, G.; Löffler, E.; Grünert, W. A New View on the Relations Between Tungsten and Vanadium in V2O5WO3/TiO2 Catalysts for the Selective Reduction of NO with NH3. J. Catal. 2012, 286, 237-247. (9) Shi, C.; Ji, Y. Y.; Graham, U. M.; Jacobs, G.; Crocker, M.; Zhang, Z. S.; Wang, Y.; Toops, T. J. NOx Storage and Reduction Properties of Model Ceria-Based Lean NOx Trap Catalysts. Appl. Catal., B 2012, 119-120, 183-196. (10) Lee, D. H.; Kim, K. T.; Kang, H. S.; Song, Y. H.; Park, J. E. Plasma-Assisted Combustion Technology for NOx Reduction in Industrial Burners. Environ. Sci. Technol. 2013, 47, 10964-10970. (11) Smirniotis, P. G.; Pena, D. A.; Uphade, B. S. Low-Temperature Selective Catalytic Reduction (SCR) of NO with NH3 by Using Mn, Cr, and Cu Oxides Supported on Hombikat TiO2. Chem. Inform. 2010, 40, 2479-2482. (12) Long, R. Q.; Yang, R. T. Superior Fe-ZSM-5 Catalyst for Selective Catalytic Reduction of Nitric

24


Page 24 of 43

Page 25 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Oxide by Ammonia. J. Am. Chem. Soc. 1999, 121, 5595-5596. (13) Qi, G. S.; Yang, R. T. Low-Temperature Selective Catalytic Reduction of NO with NH3 over Iron and Manganese Oxides Supported on Titania. Appl. Catal., B 2003, 44, 217-225. (14) Gao, R. H.; Zhang, D. S.; Liu, X. G.; Shi, L. Y.; Maitarad, P.; Li, H. R.; Zhang, J. P.; Cao, W. G. Enhanced Catalytic Performance of V2O5-WO3/Fe2O3/TiO2 Microspheres for Selective Catalytic Reduction of NO by NH3. Catal. Sci. Technol. 2013, 3, 191-199. (15) Li, H. R.; Zhang, D. S.; Maitarad, P.; Shi, L. Y.; Gao, R. H.; Zhang, J. P.; Cao, W. G. In Situ Synthesis of 3D Flower-Like NiMnFe Mixed Oxides as Monolith Catalysts for Selective Catalytic Reduction of NO with NH3. Chem. Comm. 2012, 48, 10645-10647. (16) Mou, X. L.; Zhang, B. S.; Li, Y.; Yao, L. D.; Wei, X. J.; Su, D. S.; Shen, W. J. Rod-Shaped Fe2O3 as an Efficient Catalyst for the Selective Reduction of Nitrogen Oxide by Ammonia. Angew. Chem. Int. Ed. 2012, 51, 2989-2993. (17) Long, R. Q.; Yang, R. T. Catalytic Performance of Fe-ZSM-5 Catalysts for Selective Catalytic Reduction of Nitric Oxide by Ammonia. J. Catal. 1999, 188, 332-339. (18) Balle, P.; Geiger, B.; Kureti, S. Selective Catalytic Reduction of NOx by NH3 on Fe/HBEA Zeolite Catalysts in Oxygen-Rich Exhaust. Appl Catal B-Environ 2009, 85, 109-119. (19) Long, R. Q.; Yang, R. T. Selective Catalytic Oxidation of Ammonia to Nitrogen over Fe2O3-TiO2 Prepared with a Sol-Gel Method. J. Catal. 2002, 207, 158-165. (20) Shu, Y.; Sun, H.; Quan, X.; Chen, S. Enhancement of Catalytic Activity Over the Iron-Modified Ce/TiO2 Catalyst for Selective Catalytic Reduction of NOx with Ammonia. J. Phys. Chem. C 2012, 116, 25319-25327. (21) Wu, Z. B.; Jiang, B. Q.; Liu, Y. Effect of Transition Metals Addition on the Catalyst of

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Manganese/Titania for Low-Temperature Selective Catalytic Reduction of Nitric Oxide with Ammonia. Appl. Catal., B 2008, 79, 347-355. (22) Kumar, M. S.; Schwidder, M.; Grünert, W.; Bentrup, U.; Brückner, A. Selective Reduction of NO with Fe-ZSM-5 Catalysts of Low Fe Content: Part II. Assessing the Function of Different Fe Sites by Spectroscopic In Situ Studies. J. Catal. 2006, 239, 173-186. (23) Mamontov, E.; Egami, T. Lattice Defects and Oxygen Storage Capacity of Nanocrystalline Ceria and Ceria-Zirconia. J. Phys. Chem. B 2000, 104, 11110-11116. (24) Costa, C. N.; Efstathiou, A. M. Mechanistic Aspects of the H2-SCR of NO on a Novel Pt/MgO-CeO2 Catalyst. J. Phys. Chem. C 2007, 111, 3010-3020. (25) Zhang, D. S.; Du, X. J.; Shi, L. Y.; Gao, R. H. Shape-Controlled Synthesis and Catalytic Application of Ceria Nanomaterials. Dalton Trans. 2012, 41, 14455-14475. (26) Claude Binet, M. D., Jean Claude Lavalley. IR Study of Polycrystalline Ceria Properties in Oxidises and Reduced States. Catal. Today 1999, 50, 207-225. (27) Zhou, F.; Zhao, X. M.; Xu, H.; Yuan, C. G. CeO2 Spherical Crystallite: Synthesis, Formation Mechanism, Size Control, and Electrochemical Property Study. J. Phys.Chem. C 2007, 111, 1651-1657. (28) Du, X. J.; Zhang, D. S.; Shi, L. Y.; Gao, R. H.; Zhang, J. P. Morphology Dependence of Catalytic Properties of Ni/CeO2 Nanostructures for Carbon Dioxide Reforming of Methane. J. Phys. Chem. C 2012, 116, 10009-10016. (29) Gao, R. H.; Zhang, D. S.; Maitarad, P.; Shi, L. Y.; Rungrotmongkol, T.; Li, H. R.; Zhang, J. P.; Cao, W. G. Morphology-Dependent Properties of MnOx/ZrO2-CeO2 Nanostructures for the Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. C 2013, 117, 10502-10511. (30) Du, N.; Zhang, H.; Chen, B. D.; Ma, X. Y.; Yang, D. R. Ligand-free Self-Assembly of Ceria

26


Page 26 of 43

Page 27 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanocrystals into Nanorods by Oriented Attachment at Low Temperature. J. Phys. Chem. C 2007, 111, 12677-12680. (31) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, L.; Zhang, H. P.; Liu, H. C.; Yan, C. Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109, 24380-24385. (32) Lu, Y. H.; Chen, H. T. Computational Investigation of NO2 Adsorption and Reduction on Ceria and M-Doped CeO2(111) (M=Mn, Fe) Surfaces. J. Phys. Chem. C 2014, 118, 10043-10052. (33) Wang, Z. L.; Feng, X. D. Polyhedral Shapes of CeO2 Nanoparticles. J. Phys. Chem. B 2003, 107, 13563-13566. (34) Sayle, T. X. T.; Parker, S. C.; Sayle, D. C. Oxidising CO to CO2 Using Ceria Nanoparticles. Phys. Chem. Chem. Phys. 2005, 7, 2936-2941. (35) Li, M. J.; Wu, Z. L.; Overbury, S. H. Surface Structure Dependence of Selective Oxidation of Ethanol on Faceted CeO2 Nanocrystals. J. Catal. 2013, 306, 164-176. (36) Ma, C. Y.; Mu, Z.; Li, J. J.; Jin, Y. G.; Cheng, J.; Lu, G. Q.; Hao, Z. P.; Qiao, S. Z. Mesoporous Co3O4 and Au/Co3O4 Catalysts for Low-Temperature Oxidation of Trace Ethylene. J. Am. Chem. Soc. 2010, 132, 2608-2603. (37) Si, Z. C.; Weng, D.; Wu, X. D.; Li, J.; Li, G. Structure, Acidity and Activity of CuOx/WOx-ZrO2 Catalyst for Selective Catalytic Reduction of NO by NH3. J. Catal. 2010, 271, 43-51. (38) Hao, Y. F.; Wang, Y. Y.; Wang, L.; Ni, Z. H.; Wang, Z. Q.; Wang, R.; Koo, C. K.; Shen, Z. X.; Thong, J. T. L. Probing Layer Number and Stacking Order of Few-Layer Graphene by Raman Spectroscopy. Small 2010, 6, 195-200. (39) Wu, Z. L.; Li, M. J.; Howe, J.; Meyer, H. M.; Overbury, S. H. Probing Defect Sites on CeO2 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595-16606. (40) Li, Y.; Wei, Z. H.; Gao, F.; Kovarik, L.; Peden, C. H. F.; Wang, Y. Effects of CeO2 Support Facets on VOx/CeO2 Catalysts in Oxidative Dehydrogenation of Methanol. J. Catal. 2014, 315, 15-24. (41) Kang, M.; Park, E. D.; Kim, J. M.; Yie, J. E. Manganese Oxide Catalysts for NOx Reduction with NH3 at Low Temperatures. Appl. Catal., A 2007, 327, 261-269. (42) Han, J.; Zhang, D. S.; Maitarad, P.; Shi, L. Y.; Cai, S. X.; Li, H. R.; Huang, L.; Zhang, J. P. Fe2O3 Nanoparticles Anchored In Situ on Carbon Nanotubes via an Ethanol-Thermal Strategy for the Selective Catalytic Reduction of NO with NH3. Catal. Sci. Technol. 2015, 5, 438-446. (43) Maitarad, P.; Zhang, D. S.; Gao, R. H.; Shi, L. Y.; Li, H. R.; Huang, L.; Rungrotmongkol, T.; Zhang, J. P. Combination of Experimental and Theoretical Investigations of MnOx/Ce0.9Zr0.1O2 Nanorods for Selective Catalytic Reduction of NO with Ammonia. J. Phys. Chem. C 2013, 117, 9999-10006. (44) Zhang, L.; Zhang, D. S.; Zhang, J. P.; Cai, S. X.; Fang, C.; Huang, L.; Li, H. R.; Gao, R. H.; Shi, L. Y. Design of Meso-TiO2@MnOx-CeOx/CNTs with a Core-Shell Structure as DeNOx Catalysts: Promotion of Activity, Stability and SO2-Tolerance. Nanoscale 2013, 5, 9821-9829. (45) Larachi, F.; Pierre, J.; Adnot, A.; Bernis, A. Ce 3d XPS Study of Composite CexMn1-xO2-y Wet Oxidation Catalysts. Appl. Surf. Sci. 2002, 195, 236-250. (46) Yamashita, T.; Hayes, P. Analysis of XPS Spectra of Fe2+ and Fe3+ Ions in Oxide Materials. Appl. Surf. Sci. 2008, 254, 2441-2449. (47) Yang, S. J.; Wang, C. Z.; Ma, L.; Peng, Y.; Qu, Z.; Yan, N. Q.; Chen, J. H.; Chang, H. Z.; Li, J. H. Substitution of WO3 in V2O5/WO3-TiO2 by Fe2O3 for Selective Catalytic Reduction of NO with NH3. 28


Page 28 of 43

Page 29 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catal. Sci. Technol. 2013, 3, 161-168. (48) Zhang, S. L.; Liu, X. X.; Zhong, Q.; Yao, Y. Effect of Y Doping on Oxygen Vacancies of TiO2 Supported MnOx for Selective Catalytic Reduction of NO with NH3 at Low Temperature. Catal. Commun. 2012, 25, 7-11. (49) Zhang, R.; Zhong, Q.; Zhao, W.; Yu, L. M.; Qu, H. X. Promotional Effect of Fluorine on the Selective Catalytic Reduction of NO with NH3 over CeO2-TiO2 Catalyst at Low Temperature. Appl. Surf. Sci. 2014, 289, 237-244. (50) Maitarad, P.; Han, J.; Zhang, D. S.; Shi, L. Y.; Namuangruk, S.; Rungrotmongkol, T. Structure-Activity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies. J. Phys. Chem. C 2014, 118, 9612-9620. (51) Minicò, S.; Scirè, S.; Crisafulli, C.; Maggiore, R.; Galvagno, S. Catalytic Combustion of Volatile Organic Compounds on Gold/iron Oxide Catalysts. Appl. Catal., B 2000, 28, 245-251. (52) Fang, C.; Zhang, D. S.; Shi, L. Y.; Gao, R. H.; Li, H. R.; Ye, L. P.; Zhang, J. P. Highly Dispersed CeO2 on Carbon Nanotubes for Selective Catalytic Reduction of NO with NH3. Catal. Sci. Technol. 2013, 3, 803-811. (53) Guan, B.; Lin, H.; Zhu, L.; Huang, Z. Selective Catalytic Reduction of NOx with NH3 over Mn, Ce Substitution Ti0.9V0.1O2−δ Nanocomposites Catalysts Prepared by Self-Propagating High-Temperature Synthesis Method. J. Phys. Chem. C 2011, 115, 12850-12863.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1. XRD patterns (a) and Nitrogen adsorption-desorption isotherms (b) of the catalysts.

30


Page 30 of 43

Page 31 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. TEM and HRTEM images of CeO2-NR (a, b), CeO2-NP (d, e), Fe2O3/CeO2-NR (g, h) and Fe2O3/CeO2-NP (j, k); the structural model of CeO2-NR (c) and CeO2-NP (f); Energy dispersive X-ray spectroscopy (EDS) spectra of Fe2O3/CeO2-NR (i) and Fe2O3/CeO2-NP (l), the inset was the corresponding elements content.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3. (a) Raman spectra and (b) the corresponding peak intensity ratios of I600/I460 over the catalysts.

32


Page 32 of 43

Page 33 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. (a) O 1s, (b) Ce 3d, (c) Fe 2p XPS spectra and (d) PL spectra of Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5. H2-TPR profiles of the catalysts.

34


Page 34 of 43

Page 35 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. NH3-TPD profiles of the catalysts.

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7. NO+O2-TPD profiles of the catalysts.

36


Page 36 of 43

Page 37 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 8. Optimized complex structures of NO or NH3 molecules adsorbed over the (a,b) CeO2 (110), (c,d) Fe2O3/CeO2(110) and (e,f) Fe2O3/CeO2(111).

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 9. (a) NO conversion versus reaction temperature over the catalysts; (b) N2 selectivity versus reaction temperature over the catalysts; (c) TOF profiles as a function of temperature over Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts. Reaction conditions: [NO]= [NH3]= 500 ppm, [O2]= 3 vol.%, N2 balance, GHSV= 20 000 h-1.

38


Page 38 of 43

Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 10. Oxidation conversion of NH3 over Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts. Reaction conditions: [NH3]= 500 ppm, [O2]= 3 vol.%, N2 balance, GHSV=20 000 h-1.

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 11. (a) NO conversion versus reaction time on stream in the presence of H2O and (b) NO conversion versus reaction time on stream in the presence of SO2 over Fe2O3/CeO2-NR and Fe2O3/CeO2-NP catalysts. Reaction conditions: [NH3]= 500 ppm, [O2]= 3 vol.%, [H2O]= 8 vol. % (when used), [SO2]= 100 ppm (when used), N2 balance, T= 275 oC, GHSV=20 000 h-1.

40


Page 40 of 43

Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Physical properties of the catalysts. Sample

Specific Surface area/m2 g-1

Pore volume/cm3 g-1

Average pore diameter/Å

CeO2-NR

142.33

0.52

87.6

CeO2-NP

112.18

0.22

96.1

Fe2O3/CeO2-NR

107.18

0.36

61.4

Fe2O3/CeO2-NP

98.13

0.23

123.8

41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

Table 2. The surface (XPS) and bulk (ICP-AES) compositions of the samples. Surface atomic concentration Catalysts

The relative concentration ratios by XPS / %

Weight ratio by ICP-AES / %

by XPS / % C

Ce

O

Fe

Oα/(Oα+Oβ)

Oβ/(Oα+Oβ)

Ce3+/(Ce3++Ce4+)

Fe

Fe2O3

Ce

CeO2

Fe2O3/CeO2-NR

49.27

36.66

13.20

0.86

52

48

27

2.04

2.91

97.96

97.09

Fe2O3/CeO2-NP

48.74

36.73

13.98

0.55

40

60

24

2.07

2.96

97.03

97.04

42


Page 43 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

43


Investigation of the Facet-Dependent Catalytic Performance of Fe2O3

Recommend Documents