Identification of Active Sites over Fe2O3-Based Architecture: The

Subscriber access provided by Kaohsiung Medical University

Letter

Identification of active sites over Fe2O3-based architecture: The promotion effect of H2SO4 erosion synthetic protocol Jie Zhang, Zhiwei Huang, Yueyao Du, Francisco Sánchez-Ochoa, Xiaomin Wu, and Guohua Jing ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00353 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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 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 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.

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 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

ACS Applied Energy Materials

Identification of active sites over Fe2O3-based architecture: The promotion effect of H2SO4 erosion synthetic protocol Jie Zhanga‡, Zhiwei Huanga,*‡, Yueyao Dua, Francisco Sánchez-Ochoab, Xiaomin Wua and Guohua Jinga,* a

Department of Environmental Science & Engineering, Huaqiao University, Xiamen 361021, P.

R. China. b

Instituto de Física, Universidad Nacional Autónoma de México - Apartado Postal 20-364, Cd.

de México C. P. 01000, México.

ACS Paragon Plus Environment

1

ACS Applied Energy Materials 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 2 of 23

ABSTRACT: Knowledge of tuning the composition and structure of the active sites is crucial for understanding and improving the properties of catalysts. Here, we report that selective catalytic reduction (SCR) of NOx using ammonia over Fe2O3 nanoparticle is boosted by a nanometric Fe2(SO4)3 shell, which is generated via a facile H2SO4 erosion synthetic protocol. The Fe2O3-H2SO4 erosion sample consists of a well-defined hexagon shape Fe2O3 core and a 1nm-thin Fe2(SO4)3 shell formed by H2SO4 erosion as evidenced by transmission electron microscopy measurements. A set of control experiments show that pure Fe2O3 or Fe2(SO4)3 alone cannot fully account for the high catalytic performance of Fe2O3-H2SO4 erosion sample. By combining electron microscopy, activity measurement, surface area titration, and temperature-programmed desorption and surface reaction, we show that reactants are activated within a zone at the surface interface of Fe2O3 and Fe2(SO4)3 on the as-prepared Fe2O3-H2SO4 erosion sample.

KEYWORDS: Fe2O3 nanoplates; selective catalytic reduction; H2SO4 erosion; NO removal; ferric oxides; vanadium-free catalyst; core-shell nanoplate; emission control


2

Page 3 of 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


Oxides of nitrogen (NOx), the ubiquitous byproducts of high-temperature combustion in power plants, automobiles and other combustion sources, are unwanted pollutants in the atmosphere. And their removal by selective catalytic reduction (SCR), using NH3 in the presence of oxygen (4NO + 4NH3 + O2 → 4N2 + 6H2O), is of major environmental interest. The most commonly used catalyst types are vanadium on titanium oxide monolithic catalysts.1,2 Although these vanadia-based catalysts are very active, the two main drawbacks of vanadium – the high toxicity and the volatility nature of vanadium – have definitely hindered the potential application of this technology.3-5 The replacement of vanadium by environmentally benign, stable and inexpensive oxide materials continues to be one of the most important research goals in the field of SCR catalysis.6-9 Among them, ferric oxides-based materials have elicited attention as promising SCR catalyst due to their lower toxicity, improved safety and lower cost for NOx removal.10-12 Particularly, ferric oxide with a hexagonal structure appears to be attractive because of its high thermal stability and mechanical robustness.13-16 However, their practical use has been constrained due to insufficient activity, which results in a narrow operating temperature window. The relatively low activity of ferric oxide group can be dictated by the low intrinsic acidity and strong oxidizing ability for the selective reduction of NO by NH3. Intense effort has been made to promote the overall efficiency and potential applications of ferric oxide group catalysts. For instance, a number of recent studies suggest that the crystalphase and morphology control of Fe2O3 nanomaterials are helpful for obtaining efficient metal oxide catalysts. This is because the chemical-physics surface properties are dependent on the surface termination of preferentially exposed planes.17-19 On the other hand, studies show that surface, structural defects and the presence of doping element in the Fe2O3 crystal lead to changes in the activation energies and cracking activity. These methods help to create a broad


3


Page 4 of 23

distribution of surface electronic states mediating the surface properties for adsorbed molecules.20-23 Nevertheless, it is still a challenge to effectively tailor the surface acidity, redox properties and catalytic performance of Fe2O3 crystal at the nanometric scale. Here, we report a facile H2SO4-erosion synthetic protocol allowed for tuning the strength of surface acidity/oxidizing ability over a well-defined hexagonal-shape Fe2O3 nanostructure. The enhanced acidity and optimal reducibility function cooperatively for the selectively reduction of NO with NH3. Because the Fe2O3 architecture used here is substantially uniform in size and shape, our work also shows how the catalyst design is possible at the nanoscale level. The H2SO4 eroded Fe2O3 oxides were labeled as Fe2O3-H2SO4 erosion samples. We prepared most of the Fe2O3-H2SO4 erosion catalysts using the same technique: H2SO4 (varying from 2.5-15 weight percent [wt%]) was used to immerse the parent Fe2O3 and then calcined in air at 500 °C during 4 hours (see the details in the Supporting Information). This treatment produced Fe2O3 eroded by H2SO4, with a surface area of 29.3 m2 g-1. The erosion step had a negligible effect on the total surface area of catalysts. For comparison, we prepared a Fe2O3Fe2(SO4)3 loading sample with a direct impregnation method. This experimental procedure involved mixing an aqueous solution of Fe2(SO4)3 with Fe2O3. Once it was dry, the Fe2SO4Fe2(SO4)3 were calcined in air at 500 °C for 4 hours. Fe2O3-HNO3 and Fe2O3-HCl erosion samples, by comparison, were prepared by a similar procedure of Fe2O3-H2SO4 erosion-5% catalyst as described above.


4

Page 5 of 23

(a) 100

EXPERIMENT GROUP

(b) 100

Fe2O3 Fe2O3-H2SO4 erosion-5% Fe2O3-H2SO4 erosion-2.5%

75

50

25

0 200

CONTROL GROUP Fe2O3-Fe2(SO4)3 loading Fe2O3-HCl erosion Fe2O3-HNO3 erosion

75

NO Conversion (%)

NO Conversion (%)

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


300

400

50

25

0 200

o

300

400 o

Temperature ( C)

Temperature ( C)

Figure 1. (a) NO conversion versus temperature data over experiment samples including parent Fe2O3 and Fe2O3-H2SO4 erosion catalysts. (b) The SCR activities of reference samples including Fe2O3-Fe2(SO4)3 loading, Fe2O3-HCl erosion and Fe2O3-HNO3 erosion samples in order to identify the active phase on H2SO4 erosion samples in (a). Reactant feed contains 600 ppm of NO, 600 ppm of NH3, 3 vol% O2, balanced with N2.

The catalytic activities of parent Fe2O3, Fe2O3-H2SO4 erosion catalysts under space velocity (SV = mass flow/catalyst mass) of 72000 cm3 h-1 g-1 are shown in Figure 1a. The reacting gas mixture simulated a thermal power plant flue gas composition: 600 ppm NO, 600 ppm NH3, 3.0 vol% O2, in a balanced N2 gas carrier. Elimination curves of NO to N2 and H2O (expressed in terms of XNO in the main text) were measured at several temperatures during temperatureprogrammed reaction mode with ammonia. A low activity and narrow temperature windows were observed over the pure Fe2O3 catalyst with a highest NO conversion efficiency of 48% at 330 °C. In particular, after the XNO of Fe2O3 reached the maximum value at 330 °C, the XNO showed a declined trend with further increased of temperature. The downward trend of abatement efficiency over Fe2O3 at high temperatures was due to the nonselective NH3 oxidation by O2 and N2O formation in selective reduction reaction over the catalyst (Figure S1).24 In


5


Page 6 of 23

contrast to the starting Fe2O3, the rates over the two Fe2O3-H2SO4 erosion catalysts (eroded by 2.5% and 5% H2SO4 in wt%) were much higher than the H2SO4-free Fe2O3 parent catalyst over the temperature range of interest. Moreover, Fe2O3-H2SO4 erosion catalysts were active over a much wider temperature window. Especially, the Fe2O3-H2SO4 erosion-5% (eroded by 5% H2SO4 in wt%) exhibited remarkable enhancement of NO conversion from 320 °C to 440 °C in which the observed NO conversion remained above 90%. Figure 1b shows the NO conversion efficiency measured over the Fe2O3-Fe2(SO4)3 loading, Fe2O3-HNO3 erosion and Fe2O3-HCl erosion samples - the control samples used to testify the surface active sites. Similar NO conversion curves were obtained over the Fe2O3-Fe2(SO4)3 loading and Fe2O3-H2SO4 erosion-5% sample. However, the NO conversion efficiencies in the whole temperature over the SO42- free samples of Fe2O3-HNO3 erosion and Fe2O3-HCl erosion samples were much lower than that over the Fe2O3-H2SO4 erosion-5%. The elimination curves for the NH3-SCR reactions were reproducible after samples were cooled down from the high end-point of the temperature scale. As seen from Figure 1b the reaction rate was significantly low for the catalysts of two SO42- free samples (by HNO3 or HCl erosion). Thus, the SO42- was important in the reaction, and the H2SO4 erosion process must have increased the number of active sites. Also, the SO42- species adsorbed on the Fe2O3 and Fe2(SO4)3 boundary defects (formed by H2SO4 erosion) must be associated with the active sites, because the extra mass percent of H2SO4 erosion on Fe2O3 did not increase the NO conversion efficiency (Figure S2). A comparative counterpart of Fe2(SO4)3/TiO2 catalyst was synthesized by an identical Fe2(SO4)3 loading with respect to Fe2O3-H2SO4 erosion catalyst eliminating the possible synergetic effect of Fe2O3 and Fe2(SO4)3 through boundary adjunctions. It could be seen here that, Fe2(SO4)3/TiO2 catalyst using conventional TiO2 as support had much poor catalytic performance relative to the


6

Page 7 of 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


(a)

(b) 1 2

100 nm {001}

Fe2O3 Fe2(SO4)3

H2SO4 erosion

{001}

200 nm

(c)

(d) 2 X3 magnified inset

1 2 20 nm

2 nm

1

(e)

2

1 nm

1 2 5 nm

1.Fe2O3 2.Fe2(SO4)3

Figure 2. (a) Representative TEM images of the as-prepared Fe2O3-H2SO4 erosion-5% sample. (Inset) Magnified image. (b) Illustration of H2SO4 erosion on surface of Fe2O3 nanoplate, showing the formation of a Fe2(SO4)3 surface layer (denoted as 2) and a Fe2O3 core (denoted as 1). (c,d) HRTEM images taken from vertically aligned Fe2O3-H2SO4 erosion-5% plates, showing the 1-nm wall thickness of Fe2(SO4)3 surface layer. (d) is the enlarged portion of the region highlighted in white in (c). The inset in (d) is a higher-magnification view. (e) Representative TEM image of the asprepared Fe2O3-H2SO4 erosion-5% sample. (Inset) Magnified image. The interfacial structure between Fe2O3 and Fe2(SO4)3 can be seen more clearly in (e).

Fe2O3-H2SO4 erosion catalyst, although these two catalyst had a same loading amount of ACS Paragon Plus Environment

7


Page 8 of 23

Fe2(SO4)3 (Figure S3). Given that the Fe2(SO4)3/TiO2 exhibited higher surface area than the Fe2O3-H2SO4 erosion catalyst (315.8 m2 g-1 vs 29.3 m2 g-1), the Fe2(SO4)3 alone cannot fully account for the performance of the Fe2O3-H2SO4 erosion sample. Thus, it was concluded that the Fe2O3 and Fe2(SO4)3 placed at the surface interface must be associated with active sites for reaction of selective reduction of NO to N2 and H2O. Figure 2 showed representative microscopic analysis of transmission electron microscopy (TEM) images for the Fe2O3-H2SO4 erosion-5% sample. Erosion of surface layer of Fe2O3 took place in an aqueous solution of H2SO4 at room temperature. The hexagonal-shape nanoplate architecture of parent Fe2O3 could be well preserved during the H2SO4 erosion process (Figure 2a and the inset). It could be seen that the sample studied was substantially uniform in size, averaging 110 nm in edge length and 20 nm in thickness. It was hypothesized that the activity was engineered using the H2SO4 erosion experimental protocol to form a Fe2(SO4)3 thin film coating on the Fe2O3 nanoplate architecture, see Figure 2b. TEM and high-resolution TEM (HRTEM) imaging (Figure 2c and the magnified view in Figure 2d) gave conclusive evidence of the hypothesis. As shown in Figure 2d, after the H2SO4 erosion synthetic protocol, a nanometric Fe2(SO4)3 film was grown on the surface of Fe2O3 nanoplate. The thickness film of Fe2(SO4)3 was evaluated to be around 1 nm (or four atomic layers along the [001] direction). HRTEM images taken from the vertically aligned plate (Figure 2d) showed well-defined fringes with lattice spacing of 0.25 nm. This is consistent with the (001) facet for rhombohedral hexagonal phase α- Fe2O3 (space group R3-C, lattice constant a = 5.03±0.01 Å and c = 13.75±0.01Å).25,26 Thus, the H2SO4 erosion process did not affect the bulk crystal structure of Fe2O3. The HRTEM image presented in Figure 2e showed clear lattice fringes of 0.27 nm and 0.38 nm at zone 1 which were assigned to (-114) and (1-12) planes of Fe2O3, respectively. And a distinctively


8

Page 9 of 23

S 2p3/2 (Fe2(SO4)3)

Intensity (a. u.)

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


S 2p1/2 (Fe2(SO4)3)

Fe2O3-H2SO4 erosion-5%

Fe2O3

172

170

168

Binding Energy (eV)

Figure 3. S 2p XPS profiles of the as-prepared Fe2O3-H2SO4 erosion-5% sample and the parent Fe2O3, which confirm the existence of SO42- in Fe2O3-H2SO4 erosion sample.

different lattice spacing of 0.20 nm observed at zone 2 arose from the (220) plane of a film formed by H2SO4 erosion. The inset in Figure 2a showed that small numbers of holes were present on flat surface of Fe2O3 nanoplate, suggesting a high coverage of Fe2(SO4)3 surface after the H2SO4 erosion synthetic approach. X-ray photoemission spectroscopy (XPS) revealed peaks from the S 2p core level corresponding to Fe2(SO4)3 film (Figure 3). Therefore, HRTEM imaging and XPS showed credible evidence of a core-shell nanoplate made of Fe2O3 (core) and Fe2(SO4)3 (shell) after erosion step. It was hypothesized here that the Fe2O3 core of Fe2O3-H2SO4 erosion sample was still accessible. We focused on the NO probe molecule temperature-programmed oxidation (TPO, NO+1/2O2→NO2) profiles performed on 3 samples: (1) parent Fe2O3 nanoplate synthesized by hydrothermal method, (2) Fe2O3-H2SO4 erosion sample synthesized by 5 wt% H2SO4 immersion, (3) Fe2(SO4)3/TiO2 synthesized by the identical SO42- loading. The NO molecule was used as a probe molecule with TPO technique to evaluate the accessible surface adsorption sites of Fe2O3 core in the Fe2O3-H2SO4 erosion sample (Figure S4). The Fe2(SO4)3/TiO2, synthesized by direct


9


(b) 10 3 -1 -1

Preexponential Factors (cm g s )

17

-1

Activation Energy (kJ mol )

(a) 160 140 120 100 80 60 40 0

5

10

14

10

11

10

8

10

5

10

2

10

-1

10

15

0

H2SO4 Erosion Amout on Fe2O3 (%)

(c) 240 -1

TPD

120

10

15

(d) TPSR H2 + Os

3.0

200 160

5

H2SO4 Erosion Amout on Fe2O3 (%)

Acid sites Ln (R)

Desorption Amount (umol g )

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 10 of 23

2.7

80 40

2.4 1.96

2.03

2.10 -1

1000/T (K )

Figure 4. (a,b) Measured standard SCR apparent activation energy and preexponential factors as a function of H2SO4 erosion amount over Fe2O3 nanoplate. (c) Total surface acid amount measured over the parent Fe2O3 (black) and the as-prepared Fe2O3-H2SO4 erosion-5% sample (red) using NH3TPD technique. (d) Arrhenius-type analysis of H2-TPSR to identify the redox ability of surface oxygen species (Os) over the parent Fe2O3 (black) and the as-prepared Fe2O3-H2SO4 erosion-5% sample (red).

impregnation of Fe2(SO4)3 on TiO2 and then dried in air at 120 °C for 10 hours, showed three oxidation peaks of NO with the increasing of temperature up to 350 oC. However these oxidation sites provide by Fe2(SO4)3 on the as-prepared catalyst are probably not implicated in the catalytic process in this instance, because the Fe2(SO4)3/TiO2 performed poorly below temperature of 350 o

C as shown in Figure S3. A comparative test of NO oxidation was performed on the as-prepared

Fe2O3-H2SO4 erosion-5% sample and the starting Fe2O3. The as-prepared Fe2O3-H2SO4 erosion-


10

Page 11 of 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


5% sample behaved analogously to the parent Fe2O3, where a single, broadened conversion curve of NO to NO2 was observed within the temperature range of 250 oC - 600 oC, suggesting that these two samples catalyzed NO oxidation in a similar way. The analogous NO oxidation profiles of as-prepared Fe2O3-H2SO4 erosion-5% sample and Fe2O3 (Figure S4), coupled with the high NO abatement performance of Fe2O3-H2SO4 erosion-5% sample (Figure 1a), illustrated that enough catalytic active sites for NO activation were present on Fe2O3-H2SO4 erosion-5% sample, despite the high coverage of Fe2(SO4)3 on the surface. To get a better understanding of the kinetics about the as-prepared Fe2O3-H2SO4 erosion samples, kinetic measurements were carried out on samples with a broader range of H2SO4 erosion mass fractions (0 - 15 wt%) relative to Fe2O3. In this case, detailed kinetic analysis was given by the Arrhenius equation, k = r/[NO]0 = Ae-Ea/RT, where k is the rate constant, r is the normalized SCR rate (mol NO g-1 s-1), and [NO]0 is the molar NO concentration in the inlet.27,28 Using the reaction rate in the kinetic regime (XNO < 15%), activation energies, Ea, and pre-exponential factors, A, were obtained and plotted in Figure 4a and 4b as a function of H2SO4 erosion mass fraction. As can be seen in Figure 4a, the activation energies of Arrhenius equation increased with the increasing H2SO4 erosion amount until the erosion mass fraction reached 5%; above this erosion amount, it seems that the activation energies became insensitive to the H2SO4 erosion mass fraction. Pre-exponential factor variations shown in Figure 4b yielded similar results: At low H2SO4 erosion fraction, the resulting Fe2(SO4)3 shell grown on Fe2O3 significantly altered the measured pre-exponential factors; above 5% erosion mass fraction, the prefactors remained nearly unchanged under the reaction conditions. We proposed here that at low H2SO4 erosion condition, standard NH3-SCR occurred with the participation of


11


Page 12 of 23

NH3 adsorption, which presumably was the rate-limiting step at low H2SO4 erosion fraction over the as-prepared Fe2O3. It was noted that as the H2SO4 erosion increased to high mass fraction above 5%, the linear relationship of activation energy and prefactor versus H2SO4 erosion fraction level was no longer observed; this suggested that molecular adsorption of NH3 was no longer the rate-limiting step. It was hypothesized here that the H2SO4 erosion synthetic approach resulted in formation of 1-nm-thin Fe2(SO4)3 shell with modified adsorption properties towards NH3 at the Fe2O3 surface. As evidenced from the temperature-programmed desorption (TPD) of NH3 shown in Figure 4c, the total desorption amount of molecular NH3 of as-prepared Fe2O3-H2SO4 erosion sample-5% was 1.7 times greater than the Fe2O3 reference (218 umol NH3 g-1 vs 127 umol NH3 g-1). Hence, the almost twofold difference in NH3 adsorption between the as-prepared Fe2O3-H2SO4 erosion-5% sample and the starting Fe2O3 clearly demonstrated the feasibility to control the surface acidity through H2SO4 erosion synthetic protocol. Original NH3-TPD profile was shown in Figure S5. A broaden NH3 desorption peak of the starting Fe2O3 plate was centered at 411 °C. While the asprepared Fe2O3-H2SO4 erosion-5% sample exhibited a higher NH3 desorption temperature at 460 °C, indicating a stronger bonding energy towards NH3.29,30 It is widely accepted that an optimal redox ability of surface oxygen plays an important role in the reaction pathway of selective catalytic reduction. The use of H2 molecular in temperature-programmed surface reaction (TPSR) identified surface oxygen reducibility of importance to the NH3-SCR reaction on the parent Fe2O3 and the asprepared Fe2O3-H2SO4 erosion-5% sample (Figure 4d).31,32 The reducibility of the surface oxygen species (Os) of as-prepared Fe2O3-H2SO4 erosion-5% sample was greatly lowered


12

Page 13 of 23

100

80

NO Conversion (%)

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


60

H2O+SO2 ON

40

H2O+SO2 OFF

20

0 0

5

10

15

20

25

Time (h)

Figure 5. Stability test of the as-prepared Fe2O3-H2SO4 erosion-5% sample at 370 oC. Reactant feed contains 600 ppm of NO, 600 ppm of NH3, 3 vol% O2, 300 ppm of SO2 (when used) and 10 vol% H2O (when used) balanced with N2.

compared to the starting Fe2O3. As shown in Figure 4d, the activation barrier associated with abstraction of surface oxygen species for the as-prepared Fe2O3-H2SO4 erosion-5% sample was observed to be six times than that of the starting Fe2O3 (30 kJ mol-1 vs 5 kJ mol-1). Hence, the H2SO4 erosion synthetic protocol greatly influenced the reducibility behavior, and as a result a wider operating temperature window was observed for the H2SO4 erosion sample in NO removal (Figure 1a). The aforementioned process was caused due to the enhancement of the selectivity and the suppression of unwanted side reaction for excessive NH3 oxidation. In contrast, the as-prepared Fe2O3 produced a significant amount of byproducts and showed a decrease in NO removal efficiency at temperatures above 300 oC (Figure 1a and Figure S1). We explained the observation as resulting from the excessive oxidation capacity of the starting Fe2O3 as clarified by TPSR (Figure 4d). The suitable redox ability over the as-prepared Fe2O3-H2SO4 erosion-5% sample may be due to the masking of sites by ultra-thin Fe2(SO4)3 shell after the H2SO4 erosion.


13


Page 14 of 23

A plausible reaction coordinate can be summarized as follows: *SO42- + NH3 → *SO42-(NH3)(I) *Fe3+(OH-) + NO → *Fe3+ (NOOH)(II) *SO42-(NH3)(I) + *Fe3+(NOOH)(II) → *Fe3+(NOO-)(IV) + *SO42-(NH4+)(III) *Fe3+(NOO-)(IV) + *SO42-(NH4+)(III) → *Fe3+(H2O)(V) + *SO42- + N2 + H2O *Fe3+(H2O)(V) + *Fe3+(O2-) → *Fe3+(OH-) + *Fe3+(OH-) The asterisk (*) in the equations represents the support. The result above demonstrated that the as-prepared Fe2O3-H2SO4 erosion-5% sample showed salient performance in NO removal at operating temperature windows ranges from 300 oC to 450 oC. The available temperature windows could fulfill the criteria for practical stationary NO removal. SO2 and water vapor are always present in power plant exhaust. In a further set of experiments designed to examine the stability of the asprepared Fe2O3-H2SO4 erosion-5% sample, we evaluated the catalyst in the reaction gases with appropriate catalyst loading of 0.3 g at 370 oC, a typical temperature in practical applications, as shown in Figure 5. It can be seen that the addition of SO2 and H2O gradually lowered the NO removal efficiency from 80% to a stable steady state of 60%, which has been attributed to the competitive adsorption of sulfur dioxide and water molecules against NO and NH3 at the surface.33,34 After cutting off the supply of SO2 and H2O, the conversion of NO was fully restored. This indicates that the effect of H2SO4


14

Page 15 of 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


erosion is not short lived, and the as-prepared Fe2O3-H2SO4 erosion-5% sample was sulfur-resistant and the inhibition of reaction by water was reversible. In summary, we have introduced a tunable H2SO4 erosion synthetic approach for the facile control of catalytic activity of a well-defined Fe2O3 catalyst. The active phases, account for the yield enhancement, consist of a Fe2(SO4)3 shell formed by H2SO4 erosion and a core of the parent Fe2O3. We expect our synthetic protocol introduced here to provide an effectively method for the improving overall activity of the material toward that of less costly and non-toxic Fe2O3, as well as to aim in the design of better SCR catalyst for NO removal. METHODS Catalyst preparation All chemicals were of analytical grade and used as received without further purification. The hexagonal α-Fe2O3 nanocrystal was synthesized according to Chen’s work by a hydrothermal method.16 6.56 g FeCl3·6H2O was dissloved in 240 mL ethanol with a trace addition of 16.80 mL deionized water. Then 19.20 g CH3COONa was added under stirring. The mixture was transferred to the teflon-lined, stainless autoclave, and maintained at 180 oC for 12 h. The solid product was obtained by centrifugation, and then washed by ethanol and deionized water for several times. Finally, the solid product was dried overnight at 80 oC to get hexagonal α-Fe2O3 nanocrystal. To obtain acid eroded α-Fe2O3 samples, 1.5 g Fe2O3 was stirred with aqueous solutions of different acids (HCl, HNO3, and H2SO4), then the excess water was removed by rotary evaporation and the solids were dried at 80 oC overnight. These resulting samples were denoted as Fe2O3-H2SO4/HCl/HNO3 erosion-x%, and x% stands for the H2SO4 erosion amount


15


Page 16 of 23

expressed in terms of mass fraction. As comparative counterparts, an identical mass percent of Fe2(SO4)3 with respect to Fe2O3-H2SO4 erosion-5% sample was loaded on Fe2O3 or TiO2 by a standard immersion method. The samples were denoted as Fe2(SO4)3/Fe2O3 and Fe2(SO4)3/TiO2, respectively. Catalysts were heated to 500 oC at a ramp of 10 oC min-1 and maintained at 500 oC for 4 h before use. Catalyst characterization X-ray photoemission spectroscopy (XPS) experiments were performed on Thermo Scientific Escalab 250Xi. N2 adsorption-desorption isotherms were recorded on Quantachrome NOVA2000e at 77 K. The BET method was adopted to calculate the special surface areas using desorption data. The temperature-programmed surface reaction (TPSR) by H2 and temperatureprogrammed desorption (TPD) of NH3 was performed on a chemisorption analyzer (Quantachrome ChemBET TPR/TPD) equipped with a thermal conductivity detector (TCD). For TPSR, 100 mg samples was pretreated in a He flow at a rate of 30 mL min−1 at 200 oC for 40 min, and then the temperature was decreased to room temperature. Samples were analyzed in a H2 flow (5% in He, 30 mL min-1) to 850 oC with a ramp rate of 8 oC min-1. For TPD, 100 mg samples was pretreated in a He flow at a rate of 30 mL min−1 at 300 oC for 1 h, then the temperature was decreased to 100 oC, the adsorption of NH3 was carried out for 1 h and followed by purge in He for 30 min. The sample was then heated from 100 to 800 oC in He atmosphere at a rate of 10 oC min-1. The NO probe molecule temperature-programmed oxidation (TPO) of samples were carried out in a fixed bed glass reactor with gas mixture of 600 ppm NO, 3 vol% O2, and N2 as the balance. The oxidation efficiency of NO to NO2 was monitored by a testo 340 flue gas analyzer. Transmission electron microscopy (TEM) images were recorded on FEI Tecnai G2 F30, the acceleration voltage was 200 kV and electric current was 20 mA.


16

Page 17 of 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


ASSOCIATED CONTENT Supporting Information Supporting Information figures are available in a separate document. It provides additional experimental results, including N2 selectivity of parent Fe2O3 (Figure S1), additional catalytic performance (Figure S2, S3), NO-TPO (Figure S4), and NH3-TPD (Figure S5). AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected] Author Contributions ‡These authors contributed equally. Funding Sources This research was supported by the Scientific Research Funds of Huaqiao University (grant no. 600005-Z17Y0067), Opening Project of Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (contract no. FDLAP18002), and the Subsidized Project for Cultivating Postgraduates Innovative Ability in Scientific Research of Huaqiao University. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Professor Xingfu Tang at the Fudan University for his great support with N2 selectivity measurement.


17


Page 18 of 23

REFERENCES (1) Sander, B. Properties of Danish Biofuels and the Requirements for Power Production. Biomass Bioenerg. 1997, 12, 177-183. (2) Peng, Y.; Li, J.; Chen, L.; Chen, J.; Han, J.; Zhang, H.; Han, W. Alkali Metal Poisoning of a CeO2-WO3 Catalyst Used in the Selective Catalytic Reduction of NOx with NH3: An Experimental and Theoretical Study. Environ. Sci. Technol. 2012, 46, 2864-2569. (3) Granger, P.; Parvulescu, V. I. Catalytic NOx Abatement Systems for Mobile Sources: from Three-Way to Lean Burn After-Treatment Technologies. Chem. Rev. 2011, 111, 31553207. (4) Liu, F.; Yu, Y.; He, H. Environmentally-Benign Catalysts for the Selective Catalytic Reduction of NOx from Diesel Engines: Structure-Activity Relationship and Reaction Mechanism Aspects. Chem. Commun. 2014, 50, 8445-8463. (5) Valverde, J. L.; Lucas, A. D.; Sánchez, P.; Dorado, F.; Romero, A. Cation Exchanged and Impregnated Ti-Pillared Clays for Selective Catalytic Reduction of NOx by Propylene. Appl. Catal. B: Environ. 2003, 43, 43-56. (6) Kato, A.; Matsuda, S.; Nakajima, F.; Imanari, M.; Watanabe, Y. Reduction of Nitric Oxide with Ammonia on Iron Oxide-Titanium Oxide Catalyst. J. Phys. Chem. 1981, 85, 1710-1713. (7) Qu, Z.; Fan, R.; Wang, Z.; Wang H.; Miao, L. Selective Catalytic Oxidation of Ammonia to Nitrogen over MnO2 Prepared by Urea-Assisted Hydrothermal Method. Appl. Surf. Sci. 2015, 351, 573-579. (8) Wang, Z.; Qu, Z.; Quan, X.; Li, Z.; Wang, H.; Fan, R. Selective Catalytic Oxidation of Ammonia to Nitrogen over CuO-CeO2 Mixed Oxides Prepared by Surfactant-Templated Method. Appl. Catal. B: Environ. 2013, 134, 153-166.


18

Page 19 of 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


(9) Zhang, Q.; Wang, H.; Ning, P.; Song, Z.; Liu, X.; Duan, Y. In Situ DRIFTS Studies on CuO-Fe2O3 Catalysts for Low Temperature Selective Catalytic Oxidation of Ammonia to Nitrogen. Appl. Surf. Sci. 2017, 419, 733-743. (10) Ren, Z.; Teng, Y.; Zhao, L.; Wang, R. Keggin-Tungstophosphoric Acid Decorated Fe2O3 Nanoring as A New Catalyst for Selective Catalytic Reduction of NOx with Ammonia. Catal. Today 2017, 297, 36-45. (11) Yan, L.; Liu, Y.; Zha, K.; Li, H.; Shi, L.; Zhang, D. Scale-Activity Relationship of MnOx-FeOy Nanocage Catalysts Derived from Prussian Blue Analogues for LowTemperature NO Reduction: Experimental and DFT Studies. ACS Appl. Mater. Interfaces 2017, 9, 2581-2593. (12) Han, J.; Meeprasert, J.; Maitarad, P.; Nammuangruk, S.; Shi, L.; Zhang, D. Investigation of the Facet-Dependent Catalytic Performance of Fe2O3/CeO2 for the Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. C 2016, 120, 1523-1533. (13) Liu, J.; Meeprasert, J.; Namuangruk, S.; Zha, K.; Li, H.; Huang, L.; Maitarad, P.; Shi, L.; Zhang, D. Facet–Activity Relationship of TiO2 in Fe2O3/TiO2 Nanocatalysts for Selective Catalytic Reduction of NO with NH3: In Situ DRIFTs and DFT Studies. J. Phys. Chem. C 2017, 121, 4970-4979. (14) Han, J.; Zhang, D.; Maitarad, P.; Shi, L.; Cai, S.; Li, H.; Huang, L.; Zhang, J. 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. (15) Bykova, E.; Dubrovinsky, L.; Dubrovinskaia, N.; Bykov, M.; McCammon, C.; Ovsyannikov, S. V.; Liermann, H. P.; Kupenko, I.; Chumakov, A. I.; Rüffer, R. Hanfland, M.; Prakapenka, V. Structural Complexity of Simple Fe2O3 at High Pressures and Temperatures. Nat. Commu. 2016, 7, 10661. (16) Chen, L.; Yang, X.; Chen, J.; Liu, J.; Wu, H.; Zhan, H.; Liang C.; Wu, M. Continuous Shape- and Spectroscopy-Tuning of Hematite Nanocrystals. Inorg. Chem. 2010, 49, 84118420.


19


Page 20 of 23

(17) Mou, X.; Zhang, B.; Li, Y.; Yao, L.; Wei, X.; Su, D.; Shen, W. Rod-Shaped Fe2O3 as an Efficient Catalyst for the Selective Reduction of Nitrogen Oxide by Ammonia. Angew. Chem. Int. Ed. 2012, 51, 2989-2993. (18) Zhou, X.; Lan, J.; Liu, G.; Deng, K.; Yang, Y.; Nie, G.; Yu, J.; Zhi, L. Facet-Mediated Photodegradation of Organic Dye over Hematite Architectures by Visible Light. Angew. Chem. Int. Ed. 2012, 51, 178-182. (19) Zhou, X.; Xu, Q.; Lei, W.; Zhang, T.; Qi, X.; Liu, G.; Deng, K.; Yu, J. Origin of Tunable Photocatalytic Selectivity of Well-Defined α-Fe2O3 Nanocrystals. Small 2014, 10, 674-679. (20) Qu, W.; Chen, Y.; Huang, Z.; Gao, J.; Zhou, M.; Chen, J.; Li, C.; Ma, Z.; Chen, J.; Tang, X. Active Tetrahedral Iron Sites of γ-Fe2O3 Catalyzing NO Reduction by NH3. Environ. Sci. Technol. Lett. 2017, 4, 246-250. (21) Yang, S.; Liu, C.; Chang, H.; Ma, L.; Qu, Z.; Yan, N.; Wang, C.; Li, J. Improvement of the Activity of γ-Fe2O3 for the Selective Catalytic Reduction of NO with NH3 at High Temperatures: NO Reduction versus NH3 Oxidization. Ind. Eng. Chem. Res. 2013, 52, 5601-5610. (22) Kleiman-Shwarsctein, A.; Huda, M. N.; Walsh, A.; Yan, Y.; Stucky, G. D.; Hu, Y.; AlJassim,

M.

M.;

Mcfarland,

E.

W.

Electrodeposited

Aluminum-Doped

α-Fe2O3

Photoelectrodes: Experiment and Theory. Chem. Mater. 2009, 22, 510-517. (23) Cheng, L.; Yang, R. T.; Ning, C. Iron Oxide and Chromia Supported on TitaniaPillared Clay for Selective Catalytic Reduction of Nitric Oxide with Ammonia. J. Catal. 1996, 164, 70-81. (24) Chen, L.; Yang, X.; Jian, C.; Jia, L.; Hao, W.; Zhan, H.; Liang, C.; Wu, M. Continuous Shape- and Spectroscopy-Tuning of Hematite Nanocrystals. Inorg. Chem. 2010, 49, 84118420.


20

Page 21 of 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


(25) Kwak, J. H.; Tonkyn, R.; Tran, D.; Mei, D.; Cho, S. J.; Kovarik, L.; Lee, J. H.; Peden, C. H. F.; Szanyi, J. Size-Dependent Catalytic Performance of CuO on γ-Al2O3: NO Reduction Versus NH3 Oxidation. ACS Catal. 2012, 2, 1432-1440. (26) Zhang, X.; Wang, P.; Wu, X.; Lv, S.; Dai, J. Application of MnOx/HNTs Catalysts in Low-Temperature NO Reduction with NH3. Catal. Commun. 2016, 83, 18-21. (27) Long, R.; Yang, R. Catalytic Performance and Characterization of VO2+-Exchanged Titania-Pillared Clays for Selective Catalytic Reduction of Nitric Oxide with Ammonia. J. Catal. 2000, 196, 73-85. (28) Long, R.; Yang, R. Selective Catalytic Reduction of NO with Ammonia over Fe3+Exchanged

Mordenite

(Fe–MOR):

Catalytic

Performance,

Characterization,

and

Mechanistic Study. J. Catal. 2002, 207, 274-285. (29) Niwa, M.; Suzuki, K.; Katada, N.; Kanougi, A. T.; Atoguchi, T. Ammonia IRMS-TPD Study on the Distribution of Acid Sites in Mordenite. J. Phys. Chem. B 2005, 109, 1874918757. (30) Xiang, L.; Li, J.; Yue, P.; Chang, H.; Tao, Z.; Shen, Z.; Si, W.; Hao, J. Mechanism of Arsenic Poisoning on SCR Catalyst of CeW/Ti and its Novel Efficient Regeneration Method with Hydrogen. Appl. Catal. B: Environ. 2016, 184, 246-257. (31) Liang, Z.; Li, T.; Kim, M.; Asthagiri, A.; Weaver, J. F. Low-Temperature Activation of Methane on the IrO2 (110) Surface. Science 2017, 356, 299-303. (32) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts. Science 2003, 301, 935-938. (33) Ding, S.; Liu, F.; Shi, X.; He, H. Promotional Effect of Nb Additive on the Activity and Hydrothermal Stability for the Selective Catalytic Reduction of NOx with NH3 over CeZrOx Catalyst. Appl. Catal. B: Environ. 2016, 180, 766-774.


21


Page 22 of 23

(34) Chen, M.; Yang, J.; Liu, Y.; Li, W.; Fan, J.; Ran, X.; Teng, W.; Sun, Y.; Zhang, W. X.; Li, G. Dou, S.; Zhao, D. TiO2 Interpenetrating Networks Decorated with SnO2 Nanocrystals: Enhanced Activity of Selective Catalytic Reduction of NO with NH3. J. Mater. Chem. A 2015, 3, 1405-1409.


22

Page 23 of 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


TOC


23

Identification of Active Sites over Fe2O3-Based Architecture: The

Recommend Documents