CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3

Journal Pre-proof MnOx -decorated VOx /CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3 Xiaomin Wu, Xiaolong Yu, Zhiwei Huang, Huazhen Shen, Guohua Jing

PII:

S0926-3373(19)31165-8

DOI:

https://doi.org/10.1016/j.apcatb.2019.118419

Reference:

APCATB 118419

To appear in:

Applied Catalysis B: Environmental

Received Date:

29 August 2019

Revised Date:

8 November 2019

Accepted Date:

11 November 2019

Please cite this article as: Wu X, Yu X, Huang Z, Shen H, Jing G, MnOx -decorated VOx /CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3 , Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118419

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MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3

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Xiaomin Wu, Xiaolong Yu, Zhiwei Huang, Huazhen Shen, and Guohua Jing.* Department of Environmental Science & Engineering, College of Chemical Engineering,

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Huaqiao University, Xiamen, Fujian 361021, P. R. China.

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Corresponding author: Tel.: +86-0592-6162300. E-mail: [email protected] (G. Jing).

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Corresponding Author

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AUTHOR INFORMATION

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* E-mail: [email protected].

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Graphical abstract

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Highlights

Highly dispersed surface monomeric vanadium species were the active sites over VOx-

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MnOx/CeO2-R, which increased the catalyst efficiency.

The CeO2{110} facet and surface oxygen vacancy defects had significant effects on the

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promotion of NH3-SCR performance.

NH3(L) species, NH4+(B) species and bridging nitrate species were the active intermediates, whereas bidentate nitrate were spectator species.

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The decoration of MnOx on VOx/CeO2{110} sped up the redox cycle of V5+/V4+ and

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increased the quantities of NH3(L) and bridging nitrate species.

Abstract:

Highly dispersed MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets were chosen to improve the low-temperature (200-250 °C) NH3-SCR activity. In this

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work, the catalytic data on NH3-SCR performance indicated that the improved catalytic activity were achieved when both VOx and MnOx were presented on the {110} facet of CeO2 nanorods. It could be deduced that the highly dispersed surface monomeric vanadium species were the reason for the high NH3-SCR activity from

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V-Nuclear Magnetic Resonance (NMR), Raman,

X-ray diffraction (XRD) and Transmission electron microscopy (TEM). It was also suggested that CeO2{110} facet and surface oxygen vacancy defects had significant effects on the

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promotion of NH3-SCR performance. Furthermore, the redox cycle of V5+/V4+ and the quantities

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of NH3(L) species and bridging nitrate species were increased by the introduction of Mn. The Langmuir-Hinshelwood mechanism is possible because abundant surface NH3(L), NH4+(B) and

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bridging nitrate species were produced as intermediates studied via in situ DRIFTs.

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Keyword: CeO2 {110} facets; VOx-MnOx/CeO2 catalysts; monomeric vanadium species; in situ

1. Introduction

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DRIFTs.

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Supported vanadium oxides (VOx) acted as heterogeneous catalysts are commercially important for the selective catalytic reduction of NOx with ammonia (NH3-SCR) [1-3], methanol oxidative dehydrogenation [4], and the partial oxidation of o-xylene [5], which are attributed to their good catalytic activity, high selectivity and SO2 resistance. In particular, the V2O5-

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WO3(MoO3)/TiO2 catalyst, as one of the supported vanadium-based catalysts, has been successfully applied to NH3-SCR in the temperature range of 300-400 °C [6-10]. The main reaction involved in NH3-SCR is as follows: 4NH3(g) + 4NO(g) + O2(g) → 4N2(g) +6H2O(l) (ΔHθ= -1629.8 kJ·mol-1)

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Extensive studies have demonstrated that the amount of V2O5 in conventional V2O5WO3(MoO3)/TiO2 catalyst is usually less than 1% to maintain the low conversion for SO2 oxidation to SO3 and prevent the formation of the undesirable byproduct of NH4HSO4 (ABS). In treating industrial exhaust streams, a high SO2 concentration can accelerate the deposition of NH4HSO4 on the catalyst surface and then poison the catalyst if a high amount of supported vanadium oxides are used as catalysts [11-13]. Thus, it is urgent to seek an efficient technique to

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solve the above problem. Recently, a tail-end NH3-SCR unit downstream of the desulfurization

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and dust collection devices is an efficient process for NOx abatement. At the tail-end unit, the low levels of SO2 and dust allow the application of commercial catalysts with high amounts of

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V2O5 [14-17]. The lifetimes of the vanadium-based catalysts are tremendously increased due to

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less ABS formation and dust deposition. Moreover, a high amount of supported vanadia catalysts will lower the NH3-SCR reaction temperature and save energy and cost. To the best of our

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knowledge, achieving good activity with such high amounts of supported vanadia catalysts for NOx abatement at low temperature (ie. 200-250 °C) is still a challenge [18, 19]. The relationship

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between V2O5 and catalytic activity, and the investigation of SO2 and H2O resistance have also grown increasingly urgent.

For vanadium-based catalysts, increasing the amount of V2O5 can significantly promote the catalytic activity of low-temperature NH3-SCR [13, 20, 21]. Currently, highly active V-based

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catalysts prepared by using deposition precipitation [21], sol-gel [22], and wet impregnation methods [23] have been developed. A method using V-based catalysts with consistent V=0.17 in flame spray pyrolysis yielded high catalytic activity of low-temperature NH3-SCR [24]. Djerad et al. [25] found that the isolated, but not well-dispersed, vanadium oxide species were less active for the NH3-SCR process. Kompio et al. [26] reported that 5 wt.%V2O5-10

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wt.%WO3/TiO2 catalysts with well-dispersed V2O5 exhibited high NH3-SCR activity. Therefore, a commercial catalyst with V2O5 loading of no more than 5 wt.% is suitable because such catalysts contain only one monolayer or submonolayer of V2O5 species [23, 27]. Note that such monolayer or submonolayer surface coverage of a high loading of vanadia (~8 V atoms/nm2) is highly dispersive and cannot generate crystalline V2O5 peaks detected by Raman spectra, which are far more active than isolated V2O5 crystals [28-30]. Loading of more than 5 wt.% V2O5 might

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lead to the formation of V2O5 crystals and exceed the monolayer capacity of the carrier, resulting

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in decreased efficiency of NH3-SCR activity. According to previous reports [25], highly dispersed monolayer or submonolayer vanadia oxides are probably surface monomeric vanadium

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species ( Fig. 1A), but not dimeric (Fig. 1B and Fig. 1C) or polymeric (Fig. 1D and Fig. 1E)

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O

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vanadium species, which seem to be the reason for good NH3-SCR activity. O

O

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O

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O

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O

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Support

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Support

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Fig. 1. Schematic drawing for: monomeric (A), dimeric (B and C), and polymeric (D and E) dispersed surface vanadia species supported over catalysts.

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On the other hand, the NH3-SCR performance of vanadium-based catalysts depends strongly on the composition of the supports. To date, VOx supported on ceria oxide (CeO2) has been widely investigated and has presented the enhancement for the NH3-SCR due to the excellent redox profiles of CeO2 and high oxygen storage capacity [31-33]. Wu et al. [34] prepared VOx/CeO2 catalysts and demonstrated a strong interaction between the labile surface oxygen of ceria and defect sites. Huang and Peng et al. found that CeVO4 was formed with a

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greater number of Brønsted acid sites after the introduction of Ce into VOx/TiO2 and effectively

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suppressed the formation of byproduct N2O [21, 35]. Schomäcker and coworkers reported that VOx catalysts with lower oxygen vacancy formation energy (i.e., VOx/CeO2) exhibited a lower

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apparent activation energy [36]. Recently, the successful synthesis of various morphologies of

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CeO2 nanocrystals allowed us to further investigate the influence of the facet-dependent CeO2support catalysts during the NH3-SCR reaction. This approach also allowed us to further study

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the effect of low oxygen vacancy formation energy on different CeO2 morphologies. The catalytic performance can be improved by exposing a well-defined CeO2 nanocrystal facet [37].

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Recently, low dimensional CeO2 nanocrystals with various structures including nanocubes, nanorods, and nanopolyhedra, have been successfully synthesized and studied for some catalytic reactions [38-41]. Researchers have usually obtained the target morphologies by terminating on different specific crystallographic planes: {110} and {100} for nanorods, {111} and {100} for

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nanopolyhedra, and {100} for nanocubes [39]. In NH3-SCR, Han et al. [39] reported that Fe2O3/CeO2{110} afforded significantly higher NO conversion than naked CeO2{110} and Fe2O3/CeO2{111}, according to experimental results and density functional theory (DFT) calculations. Gao et al. [42] found that MnOx/ZrO2-CeO2 nanorods with preferentially exposed {110} facets were much more active toward NH3 and NO molecules than that of the nanocubes

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and nanopolyhedra. Zhang et al. [43] studied the VOx/CeO2 catalysts with predominately exposed {110}, {111}, and {100} facets of CeO2 to improve the activity of NH3-SCR. However, despite existing efforts, the origin of the facet-dependent CeO2 effect is still under debate. Furthermore, the optimal temperature of NH3-SCR over these VOx/CeO2 catalysts was high and cannot be used for the low-temperature (200-250 °C) NH3-SCR. Therefore, the exploration of the outstanding decorated VOx/CeO2 catalysts with predominately exposed {110} facets for low-

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temperature NH3-SCR must be further investigated.

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One of the most promising materials to lower the temperature of the NH3-SCR reaction appears to be Mn doped into or loaded on some mixed oxides [44-46]. The excellent redox cycle

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of superficial Mn3+/Mn4+ or Mn2+/Mn3+ in manganese oxides (MnOx) can lead to high catalytic

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activity. Studies have proposed that Mn-Ce/TiO2 [47], MnOx-CeO2/AC [48], Mn-Ce/Al2O3 [49], and MnOx/SAPO-34 [50] possess excellent NH3-SCR activity at low temperatures. For instance,

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Yang et al. [51] developed a MnOx-CeO2 catalyst prepared by coprecipitation that exhibited 95% NO conversion at 150 °C. Huang and coworkers found that 10 wt.% MnOx supported on

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multiwalled carbon nanotubes (MWCNTs) presented the best NO conversion in the range of 180-240 °C [52]. We have recently reported that the Ce-K-OMS-2 catalysts can afford ~100% NO conversion in a wide temperature range of 140-230 °C [53]. However, the above debates are mostly about the activation of NH3 and NO on the surface. The SO2 resistance and longer

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lifetime of low-temperature NH3-SCR catalysts are still challenging. Inspired by the above studies, VOx/CeO2 decorated with MnOx nanoparticles with preferentially exposed {110} facet might lower the catalytic temperature, improve the activity of NH3-SCR and prolong the catalyst lifetime. Moreover, a high distribution of active sites, such as V and Mn, on supports with

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monolayer or submonolayer surface coverage can promote the efficiency of NH3-SCR activity [54-56]. In this study, highly dispersed 5 wt.% MnOx-decorated 5 wt.% VOx/CeO2 support materials with three kinds of exposed CeO2 facets, CeO2 nanorods, CeO2 nanopolyhedra and CeO2 nanocubes, were synthesized by wet impregnation methods. The NH3-SCR performance of these catalysts was quite different, corresponding to their different active compositions and supports,

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which indicated an obvious correlation between the NH3-SCR activity and active phases and the

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CeO2-support. The promotional effect of MnOx, the activity of VOx species, the effect of CeO2 morphology as well as the oxygen vacancy defects (OVDs) were analyzed systematically by

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various characterizations. In addition, in situ DRIFTs were conducted to illustrate the reaction

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mechanism during the NH3-SCR reaction.

2.1 Catalysts Preparation

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2. Experimental Section

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CeO2 supports with nanorods, nanopolyhedra, and nanocubes were synthesized via hydrothermal methods [38]. The CeO2 nanostructures with different exposed facets were controlled by tuning the content of sodium hydroxide (NaOH) and the preparation time and temperature [4]. The prepared CeO2 nanorods, nanopolyhedra and nanocubes supports are

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denoted here as CeO2-R, CeO2-P and CeO2-C, respectively. For CeO2-R, 1.74 g of Ce(NO3)3·6H2O and 16.00 g of NaOH were dissolved in 35 mL and 30 mL deionized H2O, respectively. The mixture was stirred for 30 min to produce a milky slurry after being transferred to a Teflon bottle. Subsequently, the slurry was transferred into a 100 mL stainless steel vessel autoclave, and subjected to hydrothermal reaction at 100 °C for 24 h. The fresh white products

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were collected by filtration and washed with deionized water thoroughly, followed by drying at 60 °C in air overnight and calcination at 450 °C for 4 h. For the synthesis of CeO2-P and CeO2-C, a similar procedure was performed, except that the amount of NaOH for CeO2-P was 0.032 g and the hydrothermal temperature for CeO2-C was 180 °C, respectively. Monolayer or submonolayer 5 wt.% MnOx-decorated 5 wt.%VOx/CeO2 catalysts with different CeO2 morphologies were synthesized by the wet impregnation method [20, 56].

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Typically, a desired amount of NH4VO3 and Mn(NO3)2·4H2O was dissolved in the aqueous

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solution of oxalic acid with a mole ratio of NH4VO3 to oxalic acid of 1:2. After that, the three CeO2 supports powders with different morphologies, CeO2-R, CeO2-P, and CeO2-C, were added

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and stirred for 2 h, respectively. Then, they were dried at 100 °C overnight and calcined at

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450 °C for 4 h. The final products are denoted here as VOx-MnOx/CeO2-R, VOx-MnOx/CeO2-P, and VOx-MnOx/CeO2-C, respectively. For comparison, VOx/CeO2-R and MnOx/CeO2-R

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Mn(NO3)2·4H2O, respectively.

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materials were prepared by a similar method in which the precursors are NH4VO3 and

2.2 Catalyst Characterization

Transmission electron microscopy (TEM) and High-resolution TEM (HRTEM) were used to observe the morphologies of the catalysts by using FEI Tecnai G20. X-ray diffraction (XRD)

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was performed on a Bruker D8 Advance diffractometer between 10° and 90° operating at 40 kV and 30 mA using Cu Kα radiation. The textural characteristics of the samples were measured by N2 adsorption-desorption experiments by using an automatic volumetric apparatus. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) equation. X-ray photoemission spectroscopy (XPS) was performed by using a Thermo Scientific Escalab 250Xi

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electron spectrometer with a binding energy (B.E.) of 284.6 eV and calibrated using C 1s. The Raman experiment was performed by using an inVia-reflex Renishaw spectrometer equipped with an optical microscope at room temperature. 51

V MAS NMR were carried out on a Bruker AVANCE III 600 spectrometer at a resonance

frequency of 157.8 MHz using a 4 mm HX double-resonance MAS probe at a sample spinning rate of 14 kHz. The 51V MAS NMR spectra were collected using a 0.65μs pulse width (ca π/6 tip

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angle) with a spectra window of 1 MHz. For our samples, 24000 scans with a 0.3s recycle delay

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were used. The chemical shift of 51V was referenced to 1 M NaVO3 aqueous solution.

The temperature-programmed reduction of H2 (H2-TPR) was performed on a chemisorption

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analyzer equipped with a thermal conductivity detector (TCD). Samples (100 mg) were

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pretreated with He at 300 °C for 1 h to remove the adsorbed carbonates and hydrates. After cooling to room temperature, the TPR analysis was carried out in a reducing mixture of 5%

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H2/Ar with a flow rate of 50 mL/min. The programming temperature was tuned in the range of 30-800 °C, at a ramp of 10 °C/min.

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Ammonia temperature programmed desorption (NH3-TPD) was conducted on Micromeritics Autochem 2920 with a TCD detector. A total of 100 mg of sample was pretreated at 300 °C for 30 min. After that, it was saturated with NH3 (1 vol% NH3/He, 100 mL/min) for 1 h at room temperature (RT). After being purged with pure He at RT, NH3 desorption took place at a ramp

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of 10 °C/min in a He flow of 100 mL/min and a temperature range of 30-600 °C. In situ DRIFTs experiments were recorded on a Fourier transform infrared spectrometer

(Nicolet 6700) with a high-temperature reaction cell. In a typical experiment, the sample was pretreated in a He stream at 400 °C for 30 min prior to each experiment. After cooling to the desired temperature (220 °C), background spectra were collected in helium at 220 °C and

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automatically subtracted from the sample spectra. For the NH3-SCR reaction, gas (500 ppm NH3 and/or 500 ppm NO + 6% O2) was introduced into the cell at 100 mL/min. All spectra were recorded by accumulating 64 scans at 4 cm-1 resolution in the Kubelka-Munk format.

2.3 Catalytic Performance Evaluation The catalytic performance for NO removal was evaluated in a fixed-bed quartz reactor (i.d.

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= 6 mm) in the range of 100-400 °C at atmospheric pressure. The catalyst (200 mg, 40–60 mesh)

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was charged in the middle of the reactor between two quartz wool plugs for each run. The feed gas consisted of 500 ppm NO, 500 ppm NH3, 5.0% O2, 100 ppm SO2 (when used), 5.5 vol% H2O

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(when used) and balanced N2 with the gas hourly space velocity (GHSV) high to 160000 h-1. The

the following equations:

[NO]out ) 100% [NO]in

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NO conversion (%)  (1 

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NO conversion and N2 selectivity were calculated from the concentrations of the flue gas using

[NO2]out  2[N2O]out )  100% [NO]in  [NH3]in - [NO]out - [NH3]out

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N2 selectivit y (%)  (1 

(1)

(2)

where the subscript “in” and “out” represented the inlet (before the NH3-SCR reactor) and outlet

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(after the NH3-SCR reactor) gas concentration of NO, respectively.

3. Results and discussion 3.1 Comparison of NH3-SCR performance The NO conversion of the various catalysts as a function of reaction temperature is displayed in Fig. 2a. As shown in Fig. 2a, the VOx/CeO2-R catalyst afforded NO conversion higher than 95% at above 250 °C. To attain better low-temperature NH3-SCR performance, the 11

decoration of MnOx promoted the NO conversion of VOx-MnOx/CeO2-R at low temperature, achieving > 95% NO conversion at ~220 °C. With increasing reaction temperature, the NO conversion over VOx-MnOx/CeO2-R was slightly elevated, and > 95% NO conversion was achieved over a wide temperature span of 220-330 °C. Upon further increasing the reaction temperature, the NO conversion was slightly decreased due to the partial oxidation of NH3 [57]. For the VOx-MnOx/CeO2-R/P/C catalysts, the NH3-SCR performance of VOx-MnOx/CeO2-R was

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much better than those of VOx-MnOx/CeO2-P and VOx-MnOx/CeO2-C. The catalytic activity of

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the VOx-MnOx/CeO2 samples with different exposed facets decreased in the sequence of VOxMnOx/CeO2-R > VOx-MnOx/CeO2-P > VOx-MnOx/CeO2-C. Moreover, the NO conversion of the

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VOx-MnOx/CeO2-R catalyst was higher than that of the physic mixed catalyst of VOx/CeO2-R

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and MnOx/CeO2-R in the range of 100-350 °C, indicating the synergistic effect between VOx and MnOx in VOx-MnOx/CeO2-R (See Fig. S1a). The N2 selectivity and N2O formation of VOx-

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MnOx/CeO2-R/P/C during the NH3-SCR reaction are shown in Fig. S1 (b, c). All the catalysts had relatively high N2 selectivity except the physic mixed catalyst of VOx/CeO2-R and

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MnOx/CeO2-R in the temperature range of 100-300 °C. The best active VOx-MnOx/CeO2-R catalyst was tested to further investigate the effect of SO2 and H2O, and the results are shown in Fig. 2b. When 5.5% H2O was added to the reactor, the NO conversion of VOx-MnOx/CeO2-R decreased by only ~15% and was maintained at 80%

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during the 12 h test. After H2O removal, the NO conversion of VOx-MnOx/CeO2-R was restored to the original level. This result suggested that the competitive adsorption of H2O and NH3 on the active sites was reversible [58]. When SO2 (100 ppm) was introduced into the gas, the activity of NH3-SCR decreased slightly (NO conversion > 75%), which was similar to the effect of H2O addition. Note that NO conversion could not be recovered after the removal of SO2, indicating

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that the addition of SO2 resulted in irreversible deactivation of the catalyst. After 100 ppm SO2 and 5.5% H2O were introduced simultaneously, the NH3-SCR activity of VOx-MnOx/CeO2-R decreased much more, and ~60% NO conversion was maintained at 220 °C. The NH3-SCR activity of VOx-MnOx/CeO2-R decreased after 100 ppm SO2 and 5.5% H2O adding simultaneously was possibly due to the quickly generation of ammonium sulfate species (NH4HSO4) and subsequently partly blockage of the catalyst by NH4HSO4 (See Fig. S2) [13-15].

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That is to say, NH4HSO4 was much easier to produce in the presence of co-existence of SO2 and

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H2O compared to the presence of only H2O or SO2 at 220 ℃. Nevertheless, these results indicated that the VOx-MnOx/CeO2-R catalyst exhibited excellent low-temperature NH3-SCR

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MnOx/CeO2-R VOx/CeO2-R

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NO conversion (%)

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100

VOx-MnOx/CeO2-R

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5.5% H2O

100 ppm SO2

5.5% H2O + 100 ppm SO2

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VOx-MnOx/CeO2-P

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NO conversion (%)

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activity and displayed good resistance to SO2 and H2O at 220 ℃.

VOx-MnOx/CeO2-C

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Fig. 2. (a) NO conversion as a function of temperature in NH3-SCR, and (b) the effect of H2O and SO2 over the VOx-MnOx/CeO2-R catalyst at 220 °C. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 6%, [H2O] =

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5.5% (when used), [SO2] = 100 ppm (when used), N2 balance, GHSV = 160,000 h-1.

3.2 Morphology and Structure of the Catalysts 3.2.1 Morphological analysis

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TEM images and HRTEM images of the CeO2 nanorods (CeO2-R), nanopolyhedra (CeO2P), and nanocubes (CeO2-C) supports are shown in Fig. 3. Schematic illustrations of the particle morphologies are also illustrated in Fig. 3. CeO2 has three low-index facets: the {111}, {110}, and {100} facets. As depicted in Fig. 3a1, the CeO2-R sample contained CeO2 in the form of nanorods with 20-200 nm in length and 5-20 nm in width. Two interplanar spacings at 0.19 and 0.27 nm, assigned to the (220) and (200) lattice fringes, respectively, were observed in the

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HRTEM image in Fig. 3a2. The results revealed that the CeO2-R was enclosed by the {110} and

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{100} facets and grew along the {110} direction (Fig. 3a3) [38, 43]. Fig. 3b1-b2 shows the TEM and HRTEM images of the CeO2-P sample. According to Fig 3b1, the dimensions of CeO2-P

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included an average size of ∼15 nm with uniform CeO2 nanopolyhedra. Fig 3b2 displays the

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HRTEM image of CeO2-P indicating a truncated octahedron with interplanar spacings of 0.31 and 0.27 nm enclosed by the {111} and {100} facets, respectively. Fig 3b3 shows the structure

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model of the CeO2-P, which is enclosed by eight major {111} and six minor {100} plans [38, 59]. As shown in Fig. 3c1, CeO2-C exhibited a well-defined cubic morphology with a size range of 5-

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40 nm. The HRTEM image in Fig. 3c2 shows that a spacing of 0.27 nm was clearly observed. The results indicated that the cubes were only enclosed by six {100} facets, which is in accordance with previous reports [38, 41, 59, 60]. In addition, TEM and HRTEM micrographs of as-prepared VOx/CeO2-R, MnOx/CeO2-R, and VOx-MnOx/CeO2-R catalysts are illustrated in Fig.

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S3. For all of the supported CeO2-R catalysts, the nanorod shape was maintained after active phase loading and calcination. Furthermore, the VOx and/or MnOx nanoparticles were well dispersed in the CeO2 nanorod, which is also confirmed by XRD, as discussed in the following section. In addition, the EDS maps (Fig. S4) are further confirmed that V, Mn and O were uniformly distributed over the VOx-MnOx/CeO2-R sample, implying that vanadium and

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manganese species were highly dispersed on VOx-MnOx/CeO2-R. Combined with the NH3-SCR activity results, it is likely that the exposed {110} facet of CeO2 nanorods was active toward

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NH3-SCR while the {100} facet had little contribution to the NH3-SCR reaction.

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Fig. 3. The TEM images, HRTEM images, and schematic illustrations of (a1-a3) CeO2-R, (b1-b3) CeO2-P, and (c1-c3) CeO2-C samples.

3.2.2 XRD, BET surface area and pore volume The XRD patterns of CeO2-R and corresponding MnOx-decorated VOx/CeO2 samples are represented in Fig. 4. The XRD patterns of all the samples can be indexed to a typical fluorite

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cubic structure (space group Fm3m (225), a=b=c=5.411, JCPDS 34-0394). After vanadium and/or manganese loading on the CeO2-R/P/C, the diffraction peaks of VOx and/or MnOx were not present in any samples, suggesting that VOx and/or MnOx species were highly dispersed on the surface of the CeO2-R/P/C supports. Furthermore, the typical peaks of CeVO4 species (2θ = 24.0° and 32.5°) were not found in any of the samples after calcination [35]. In addition, it is worth noting that the larger BET surface area and total pore volume for VOx-MnOx/CeO2-R

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resulted in its observed broadened diffraction peaks compared with those of the VOx-

CeO2 (111)

Intensity (a.u.)

▼

CeO2-R

VOx-MnOx/CeO2-R

VOx/CeO2-R

VOx-MnOx/CeO2-P

MnOx/CeO2-R

VOx-MnOx/CeO2-C

(220)

(200)

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MnOx/CeO2-P and VOx-MnOx/CeO2-C samples (Table 1).

(311) ▼

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(222)

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(331) (422) ▼(420) ▼ ▼

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(400)

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Fig. 4. XRD patterns of CeO2-R and various corresponding MnOx-decorated VOx/CeO2 samples

Table 1 summarizes the specific surface area, pore volumes, and average pore sizes of the

samples. As shown in Table 1, the specific surface area of VOx-MnOx/CeO2-R (106.6 m2/g) was higher than those of VOx-MnOx/CeO2-P (92.5 m2/g) and VOx-MnOx/CeO2-C (73.1 m2/g), mainly due to the unique nanorod crystal morphology and good dispersion of active phases compared with those of the VOx-MnOx/CeO2-P and VOx-MnOx/CeO2-C samples [39]. This result implied

16

the large BET surface area and total pore volume of the VOx-MnOx/CeO2-R catalyst would promote the active sites for NH3-SCR. Regarding the comparison of VOx/CeO2-R and MnOx/CeO2-R, the BET surface area of VOx/CeO2-R slightly decreased to some extent (100.9 m2/g), which was partly ascribed to the stacking pore blockage of CeO2 nanorods by the vanadia species. Both the BET surface area and average pore size of VOx/CeO2-R decreased, which is similar to previous reports [39, 61]. In contrast, the BET surface area of MnOx/CeO2-R increased

of

to a high value (121.9 m2/g), which suggested the occurrence of a more defective CeO2 lattice

ro

and a strong intimate interaction between manganese and cerium oxides [62-64]. As mentioned above, decorating CeO2 nanorods with MnOx increased the BET surface area and total pore

-p

volume to contribute the enhancement of NH3-SCR. This result is consistent with the XRD

re

spectra. Table 1

lP

Quantitative data from the analysis of different samples. Exposed plane

BET surface area (m2/g)a

Total pore volume (cm3/g)

Average pore size(nm)

Crystallize size (nm)b

VOx-MnOx/CeO2-C

100

73.1

0.21

11.7

11.7

VOx-MnOx/CeO2-P

111, 100

92.5

0.13

11.5

11.8

VOx-MnOx/CeO2-R

110, 100

106.6

0.32

12.2

11.5

VOx/CeO2-R

110, 100

100.9

0.29

11.6

11.9

MnOx/CeO2-R

110, 100

121.3

0.46

15.1

15.6

Jo

ur na

Sample

a

Calculated by the BET method.

b

Crystallite size estimated by the Debye-Scherrer equation.

3.3 Redox Properties of the Catalysts The redox properties of catalysts are crucial to the catalytic cycle during the NH3-SCR reaction. H2-TPR is an effective method to estimate the reducibility of MnOx-decorated 17

VOx/CeO2 catalysts with different exposed facets. Thus, H2-TPR profiles were collected to investigate the reducibility of VOx and/or MnOx species supported on CeO2 catalysts with different exposed facets, and the results are shown in Fig. 5. As shown in Fig. 5a, when vanadium was supported on CeO2-R, the peaks at ~490 °C predominately corresponded to the reduction of V5+ [43, 65] and the shoulder peak at lower temperature (~370 °C) was assigned to the Ce4+/Ce3+ reduction of surface CeO2 [4, 25, 66]. However, the reduction patterns were

of

different when MnOx was loaded onto CeO2-R. Three reduction peaks were observed for

ro

MnOx/CeO2-R, which were attributed to MnOx as follows: MnO2 →Mn2O3 at ~207 °C, Mn2O3 → Mn3O4 at ~284 °C, and Mn3O4 →MnO at ~373 °C [67, 68]. An additional peak at ~717 °C was

-p

assigned to the reduction of bulk CeO2, which is in accordance with the previous reports [25, 66].

re

For the VOx-MnOx/CeO2-R catalyst, two main reduction peaks were observed. The peak at high temperature (~701 °C) could be attributed to the reduction of bulk CeO2; this result was much

lP

lower than the value for MnOx/CeO2-R. The broad peak at low temperature (~431 °C) was considered as an overlapped reduction peak, which was predominately associated with the

ur na

reduction of V2O5 species and the shoulder features were probably due to the reduction of CeO2 and multiple MnOx species [1, 43]. A precise discrimination of the reduction peak of multiple MnOx species on VOx-MnOx/CeO2-R was not possible. Note that the reduction temperatures of V5+ and Ce4+ both decreased drastically, suggesting that VOx and CeO2 became more reducible

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and more active surface oxygen was formed by the decoration of MnOx over the VOxMnOx/CeO2-R catalyst. The above result was attributed to the synergetic effect between the active phases and the CeO2 support. In other words, the presence of MnOx on the VOxMnOx/CeO2-R nanorods can significantly elevate the surface reduction and surface oxygen activity.

18

For the VOx-MnOx/CeO2-R/P/C catalysts, the reduction temperatures for the catalysts followed the trend: VOx-MnOx/CeO2-R < VOx-MnOx/CeO2-P < VOx-MnOx/CeO2-C. These low temperature shifts indicated that the reducibility of the catalysts was dependent on the morphology of CeO2 and due to the presence of more easily reducible highly dispersed surface monomeric vanadia species. The CeO2 nanorods possessed much stronger redox ability than the CeO2 nanocubes and nanopolyhedra. After VOx and MnOx loading, the VOx and MnOx species

of

on the surface of the CeO2 nanorods were easier to reduce, which benefited the redox ability and

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catalytic activity of NH3-SCR. This phenomenon might be due to much more active surface oxygen generated on the CeO2 nanorods and the strong interaction between active species and

-p

CeO2 nanorods [69]. Moreover, combined with the XRD and BET surface area results, the CeO2

re

nanorods exhibited elevated redox properties due to surface defects and a higher BET surface area than the CeO2 nanocubes and nanopolyhedra. VOx/CeO2-R

(b)

Intensity(a.u.)

VOx-MnOx/CeO2-R

431

701

223

ur na

207 284

373

370

100

200

300

400

VOx-MnOx/CeO2-R

lP

MnOx/CeO2-R

717

490

500

VOx-MnOx/CeO2-P

735 243

700

453

234

677

600

512

VOx-MnOx/CeO2-C

Intensity(a.u.)

(a)

722

431

701

223

800

100

o

200

300

400

500

600

700

800

o

Temperature ( C)

Jo

Temperature ( C)

Fig. 5. H2-TPR curves of (a) VOx and/or MnOx CeO2-R, and (b) VOx-MnOx/CeO2 with different exposed facets catalysts.

3.4 Effect of CeO2 facet and surface defects

19

From the above redox properties of the catalysts, intuitively, the effect of ceria facets and the promotional effect of MnOx were two crucial factors in determining low-temperature NH3SCR. In addition, the strong interaction between active VOx, MnOx species and CeO2 nanorods needs to be investigated. According to density functional theory plus U (DFT+U), the catalytic activity of different exposed facets follows the sequence: CeO2{111} > CeO2{110} > CeO2{100} [70, 71]. If one

of

solely attributes the catalytic activity of NH3-SCR to CeO2 with different facets, it is difficult to

ro

explain why VOx-MnOx/CeO2-R nanorods with exposed {110} and {100} facets exhibited the best activity. Therefore, to further illuminate how the facets of CeO2 affect the catalytic activity

-p

of NH3-SCR, Raman spectroscopy was carried out. Fig. 6 shows the visible Raman spectra (514

re

nm) of VOx-MnOx/CeO2-C, VOx-MnOx/CeO2-P and VOx-MnOx/CeO2-R nanostructures. As depicted in Fig. 6a, all three materials display a strong peak at 462 cm-1, representing the

lP

symmetrical stretching of the Ce-O vibrational mode in octahedral coordination (F2g). The other weak band at 599 cm-1 was corresponded to the second-order transverse acoustic (2TA) mode of

ur na

CeO2 and defect-induced (D) mode of cubic CeO2 [39, 41, 56, 69, 72]. According to the literature [62], the relative contents of surface defects here, possibly OVDs, can be represented by the relative intensity ratio of peaks at 599 and 462 cm-1 (denoted as ID/IF2g). The more the OVDs formed, the more energetically active the material became in the NH3-SCR reaction. As

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shown in Fig. 6b, the ID/IF2g value obtained for VOx-MnOx/CeO2-R (9.5%), was much larger than those of VOx-MnOx/CeO2-P (6.7%) and VOx-MnOx/CeO2-C (3.2%). Combined with the HRTEM results, the relative intensity ratio of ID/IF2g decreased in the following sequence: VOxMnOx/CeO2{110} > VOx-MnOx/CeO2{111} > VOx-MnOx/CeO2{100}, which was consistent with the NH3-SCR activity. Herein, one can deduce that the CeO2 nanorods with preferentially

20

exposed {110} facets have the most oxygen vacancy defect sites, consequently contributing to the activity for NH3-SCR of NOx.

Intensity (a.u.)

VOx-MnOx/CeO2-R

D

200

300

400

500

600

6.7 5

3.2

0

700

9.5

10

of

VOx-MnOx/CeO2-P

(b)

ro

VOx-MnOx/CeO2-C

F2g

Intensity ratio (ID/IF2g) / %

(a)

VOx-MnOx/CeO2-C VOx-MnOx/CeO2-P

Raman Shift (cm-1)

VOx-MnOx/CeO2-R

Oxygen vacancy defects

-p

Fig. 6. (a) Raman spectra, and (b) the corresponding peak intensity ratios of ID/IF2g over the catalysts.

re

The oxygen vacancies can be conductive to absorbed oxygen species, and active surface

lP

absorbed oxygen species are crucial to the NH3-SCR reaction [47, 57]. Hence, the surface oxygen species of the catalysts were further determined by the O 1s XPS analysis, and the results

ur na

are summarized in Fig. 7 and Table 2. As shown in Fig. 7a, all samples exhibited a dominant peak (529.3-529.8 eV) together with a shoulder peak (531.1-531.3 eV). The dominant peak at 529.3-529.8 eV was attributed to lattice oxygen (Olatt) species, and the shoulder peak at 531.1531.3 eV was ascribed to surface adsorbed oxygen (Oads) species on the surface of the catalysts.

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In this study, a high Oads/(Oads+Olatt) ratio could elevate the catalytic activity for NH3-SCR because of its participation in the oxygen reaction [47, 73]. The relative ratio of Oads/(Oads+Olatt) was quantified according to the O 1s spectra, as demonstrated in Table 2. For the VOxMnOx/CeO2-R/P/C catalysts, The ratios of Oads/(Oads+Olatt) were approximately 34.9%, 30.6%, and 26.1% for VOx-MnOx/CeO2-R, VOx-MnOx/CeO2-P, and VOx-MnOx/CeO2-C, respectively. The percentage of Oads/(Oads+Olatt) over the VOx-MnOx/CeO2-R catalyst was higher than that of 21

VOx-MnOx/CeO2-C and VOx-MnOx/CeO2-P. This phenomenon suggested that the VOxMnOx/CeO2-R catalyst generated many more OVDs, which might be ascribed to the higher proportion of Ce3+ (See Table 2) in the CeO2 nanorods. The O 1s XPS results were in accordance with the NH3-SCR results and the Raman spectra. Thus, such results again demonstrated the significant effects of the CeO2{110} facet and surface OVDs on the promotion of the

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performance of NH3-SCR.

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3.5 The promotional effect of MnOx on VOx-MnOx/CeO2-R 3.5.1 Analyses of surface active species

-p

We confirmed, in the above H2-TPR study, that Mn in the VOx-MnOx/CeO2-R catalyst

re

showed dramatically increased reducibility, resulting in enhanced NO conversion. So, to further determine the promotional effect of Mn on NO conversion, the XPS spectra of V 2p, Ce 3d, and

lP

Mn 2p were collected to demontrate the surface active species and oxidation state, and the results are presented in Fig. 7 and Table 2. As shown in Fig 7c, the V 2p3/2 peak at lower bonding

ur na

energy can be split into two peaks for VOx/CeO2-R. The main peak at 517.3–517.5 eV was ascribed to V5+, and a shoulder peak at ~516.4 eV was attributed to V4+. The relative ratio of V5+/(V5++V4+) was 80.2%, which proved that the surface of the catalyst mainly existed in the form of V2O5. After Mn decoration, the ratio of V5+/(V5++V4+) increased. The bands of V4+

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vanished, which could not be separated by fitting deconvolution. From Table 2, the ratios of V5+/(V5++V4+) showed obvious regular: VOx-MnOx/CeO2-R > VOx/CeO2-R. The oxidation states of vanadium oxide have a great effect on the catalytic activity of vanadia-based catalysts [74]. Early studies proposed that the key and rate-determining steps were divided into two cycles. An acid cycle was assumed to occur: the adsorption of NH3 on the V5+-OH Brønsted acid sites of the

22

vanadia-based catalysts. After that, the adsorbed NH3 was activated by transfer of an H atom to V5+=O sites. A redox cycle then occurred: the activated NH3 reacted with NO on the V5+=O groups to release V4+-OH, which could be regenerated by O2. That is, the active V5+=O sites were very reactive toward to NH3. The strong redox cycle enhanced the gaseous NO reactivity, which involved the NH3 species adsorbed on the V5+-OH Brønsted acid sites [75-77]. Some

why could the V5+ in our work dramatically increase catalytic activity?

of

researchers have reported that the use of V4+ catalysts can enhance NO conversions [78]. Hence,

ro

To elucidate this phenomenon, the XPS spectra of O 1s and Ce 3d were collected to explain, as shown in Fig. 7b and Fig. 7d. By the peak-fitting deconvolution technique, the

-p

complex spectra of Ce 3d were deconvoluted into 8 groups. The bands labeled v, v’’, v’’’, u, u’’,

re

and u’’’ correspond to the 3d104f0 state of Ce4+ cations, whereas v’ and u’ represent the 3d104f1 states, respectively, representing Ce3+ cations [79, 80]. As illustrated in Table 2, the ratio of

lP

Ce3+/(Ce3++Ce4+) in the VOx-MnOx/CeO2-R sample was higher than that of the VOx/CeO2-R sample. Furthermore, the relative ratios of Oads/(Oads+Olatt) over the VOx-MnOx/CeO2-R catalyst

ur na

was higher than that of VOx/CeO2-R (see Fig. 7b and Table 2). Therefore, the superior ratio of Ce3+/(Ce3++Ce4+) and the excellent redox cycle of superficial Ce3+/Ce4+ on the CeO2-R support was the reason why V5+ could dramatically increase catalytic activity. In addition, the relative ratio of Mn4+/(Mn4++Mn3+) was slightly different in all of the samples and the morphology of

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CeO2 had little effect on the ratios of V5+/(V5++V4+) and Ce3+/(Ce3++Ce4+) (see Table 2, and Supporting Information Fig. S5). The superior ratio of Ce3+/(Ce3++Ce4+), and Oads/(Oads+Olatt) on the VOx-MnOx/CeO2-R

catalyst could originate from the charge rebalancing of ceria and oxygen vacancy defects [21, 35, 81]. After MnOx loading, the active monomeric V2O5 species increased dramatically probably

23

due to the copresence of the redox couples V5+/V4+ and Ce4+/Ce3+. Furthermore, oxygen vacancy defect sites play a vital role in the redox properties of the VOx-MnOx/CeO2-R catalyst. A cycle could occurr in which the redox couples V5+/V4+ and Ce4+/Ce3+ transfer electrons to each other. Moreover, oxygen vacancies would speed up the NH3-SCR reaction because the defects would activate gaseous NO and O2. The additon of MnOx brought a promoted growth to the ratios of V5+/(V5++V4+), Ce3+/(Ce3++Ce4+), and Oads/(Oads+Olatt), which facilitated the catalytic activity of

(b)

Olatt

Olatt

Oads

Olatt

Oads VOx-MnOx/CeO2-R

534

533

MnOx/CeO2-R

532

531

530

529

528

527

536

535

534

(c)

ur na

Jo

Intensity(a.u.)

Intensity(a.u.)

V4+

518

516

514

u'

u''

V5+

520

530

529

528

527

u

u'''

VOx-MnOx/CeO2-R

522

531

Ce 3d

V 2p3/2

524

532

(d)

V 2p

VOx/CeO2-R

533

Binding energy(eV)

Binding energy(eV)

526

Olatt

Oads

VOx/CeO2-R

lP

535

Olatt

Oads

re

VOx-MnOx/CeO2-P

VOx-MnOx/CeO2-R

-p

Oads

Olatt

Oads

VOx-MnOx/CeO2-C

536

O 1s

ro

O 1s

Intensity(a.u.)

Intensity(a.u.)

(a)

of

NO conversion.

v''' v''

v'

v

VOx-MnOx/CeO2-R

VOx/CeO2-R

512

920

915

910

905

900

895

890

Binding energy(eV)

Binding energy(eV)

Fig. 7. XPS spectra for (a, b) O 1s, (c) V 2p, and (d) Ce 3d of various catalysts.

24

885

880

Table 2 The XPS results of various catalysts for O 1s, V 2p, Mn 2p, Ce 3d.

V5+/(V5+ +V4+)%

Mn4+/(Mn3+ +Mn4+)%

Ce3+/(Ce3++ Ce4+)%

-

15.5

45.7

23.3

Oads

VOx/CeO2-R

529.8

531.3

26.1

80.2

MnOx/CeO2-R

529.3

531.3

30.6

-

VOx-MnOx/CeO2-R 529.4

531.2

34.9

~100

VOx-MnOx/CeO2-P

529.7

531.2

32.2

~100

VOx-MnOx/CeO2-C 529.6

531.1

25.7

~100

46.9

25.3

47.1

24.8

45.0

23.7

re

3.5.2 Acidic sites distribution (NH3-TPD)

-p

Olatt

of

Oads/(Oads+ Olatt)%

ro

Sample

O 1s BE(eV)

lP

Acidity is considered as an important factor during the NH3-SCR process because it is a prerequisite for efficient NH3 adsorption and activation. NH3-TPD was conducted to study the

ur na

acid strength and amount on the catalysts, and the results are displayed in Fig. 8. All NH3 desorption profiles of the catalysts displayed two NH3 desorption bands at approximately 125200 °C and 200-300 °C, which corresponded to the weak acidic sites and acid sites of intermediate strength, respectively [82]. The amount of weak acidic sites over the VOx-

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MnOx/CeO2-R catalyst (0.578 μmol/m2) was larger than that of corresponding sites over the VOx/CeO2-R catalyst (0.338 μmol/m2), ascribed to the synergetic effect between VOx, MnOx, and CeO2. The NH3-TPD results indicated that the greater number of weak acidic sites over the VOx-MnOx/CeO2-R sample generated by the addition of MnOx were benefical towards the activation of NH3, which were important during the NH3-SCR process [82]. These results were in agreement with the following in situ DRIFTs spectra for the adsorption of NH3. In addition, 25

although NH3-TPD profiles can afford substantial information about the strength and quantity of acidic sites, the nature of acidic sites (i.e., Lewis or Brønsted acid sites) in metal oxide catalysts cannot be provided sufficiently determined [83]. Thus, in situ DRIFTs experiments were performed to confirm the nature of the acidic sites and will be discussed in the next section.

264

of

VOx-MnOx/CeO2-R 127

ro

215

MnOx/CeO2-R

237

137

-p

Intensity (a.u.)

169

100

200

re

VOx/CeO2-R

300

400

500

600

700

o

lP

Temperature ( C)

ur na

Fig. 8. NH3-TPD profiles of the VOx/CeO2-R, MnOx/CeO2-R, and VOx-MnOx/CeO2-R catalysts.

3.5.3 Surface vanadia species

Raman spectroscopy of the V=O stretching vibrational mode was conducted to further study the surface structures of the catalysts as displayed in Fig. 9a. No peak corresponding to CeVO4 at

Jo

864 cm-1 was detected for either of the two V-based samples. For the VOx/CeO2-R catalyst, a V=O vibrational band at ∼1003 cm-1 was assigned to isolated surface vanadium oxide species [24]. After the introduction of MnOx, the peak of isolated surface vanadium oxide species vanished, suggesting that VOx was well dispersed on the CeO2 nanorods. According to previous literature [4, 84], νV=O < 1020 cm-1 was assigned to monomeric vanadium oxide (see Fig. 1A).

26

These results demonstrated that the isolated monomeric vanadium species were better dispersed on VOx-MnOx/CeO2-R than the VOx/CeO2-R catalyst. This clearly indicated that decoration with MnOx could enhance the dispersion of isolated monomeric vanadium species, subsequently contributing to the enhancement of NH3-SCR. To confirm the Raman spectroscopy result, we conducted 51V-NMR measurements. The 51V NMR spectra (Fig. 9b) show that, the VOx/CeO2-R catalyst presented major peak at -566 ppm

of

with a shoulder downfield at -536 ppm. The shoulder peak at -536 ppm is often assigned to

ro

monomeric surface vanadium oxide species [85, 86]. It was noteworthy that the absence of signal at -630 ppm in the NMR spectra of VOx/CeO2-R confirmed the absence of polymeric vanadyl

51

V-NMR spectroscopy, suggesting that VOx was well dispersed on the CeO2

re

species from

-p

species [85]. For the VOx-MnOx/CeO2-R catalyst, it was not visible to identify vanadium oxide

nanorods. It was in accordance with the Raman results.

lP

(b)

-563

Zoom in VOx-MnOx/CeO2-R

VOx-MnOx/CeO2-R

ur na

Intensity (a.u.)

(a)

-536 -566

1003

VOx/CeO2-R Zoom in

MnOx/CeO2-R

700 750 800 850 900 950 1000 1050 1100 1150 1200

VOx/CeO2-R 0

Raman Shift (cm-1)

-400

-800

Jo

51V (ppm)

-1200

-350 -400 -450 -500 -550 -600 -650 -700

51V (ppm)

Fig. 9. (a) Raman spectra of the VOx/CeO2-R, MnOx/CeO2-R and VOx-MnOx/CeO2-R catalysts, and (b) 51VNMR spectra of the VOx/CeO2-R and VOx-MnOx/CeO2-R catalysts.

3.6 In situ DRIFTs

27

To clarify that the intermediates of NH3 and NO interact with the surface active sites over the catalysts, in situ DRIFTs experiments were carried out in this work. 3.6.1 NH3 adsorption The in situ DRIFTs spectra of NH3 adsorption over the VOx/CeO2-R and VOx-MnOx/CeO2R catalysts at 220 °C are presented in Fig. 10. For the VOx/CeO2-R catalyst (Fig.10a), with increasing time, the bands at 3343, 3261, 3162, 1235, and 1182 cm-1 could be attributed to NH3

of

bound to Lewis acid sites, which was denoted as NH3(L). The bands at 1665, and 1432 cm-1

ro

could be related to NH4+ ions bound to Brønsted acid sites, which are denoted as NH4+(B) [57]. The band at ~1583 cm-1 was assigned to bonds between the hydrogen from NH3 and the surface

-p

oxygen (here denoted as O-H3N) [87]. The peak at 1009 cm-1 was due to the characteristic

re

vibrational modes of monomeric vanadium species after NH3 adsorption, which indicated that the monomeric vanadium species was the active species. For the VOx-MnOx/CeO2-R catalyst,

lP

the band at 1233 cm-1 can be attributed to the symmetric bending vibrations of the N-H bonds in NH3 linked to Lewis acid sites. With increasing the time, the amount of Lewis acids with

ur na

adsorbed ammonia increased. The adsorption area of the band at 1233 cm-1 over the VOxMnOx/CeO2-R catalyst was obviously larger than that over the VOx/CeO2-R catalyst. These results clearly suggested that MnOx can enhance the adsorption activity of Lewis acid sites, and thus increase the NH3-SCR activity [88]. Moreover, Fig. 10 (c, d) illustrates that slightly less

Jo

NH4+(B) was generated over the VOx-MnOx/CeO2-R sample than over the VOx/CeO2-R sample. Herein, it was considered that loading MnOx could increase NH3(L) species, which is favorable for low-temperature NH3-SCR.

28

Absorbance (a.u.)

(a) 1182

3343 3261 3162

1665 1583

1009

1235

1432

15 min 10 min 7 min 5 min 3 min 1 min 0 min 3500 3000 2000

1800

1600

1400

1200

1000

800

-1

of

Wavenumber (cm )

ro

1186 1233

3342 3259 3160

1009

1427 1674 1580 15 min

-p

10 min 7 min 5 min 3 min 1 min 0 min 3500 3000 2000

1800

1600

1400

1200

1000

-1

800

lP

Wavenumber (cm )

re

Absorbance (a.u.)

(c)

Fig. 10. In situ DRIFTs spectra of NH3 adsorption exposed to a flow of 500 ppm NH3 over the (a) VOx/CeO2-

ur na

R, and (c) VOx-MnOx/CeO2-R catalysts at 220 °C, and the corresponding mapping results over (b) VOx/CeO2R, and (d) VOx-MnOx/CeO2-R.

3.6.2 NO + O2 adsorption

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Fig. 11 shows the in situ DRIFTs spectra of adsorbed species for the coadsorption of NO + O2 over the VOx/CeO2-R and VOx-MnOx/CeO2-R catalysts. For VOx/CeO2-R, with increasing times, several new bands at 1599, 1578, 1545, 1295, 1235, 1213, and 1009 cm-1 could be found in Fig. 11a. The bands at 1599 and 1213 cm-1 were related to bridging nitrate species, which originated from the adsorbed NO2 on the surface of catalyst. The peaks at 1578, 1545, and 1235 cm-1 were attributed to bidentate nitrate. The band at 1295 cm-1 was belonged to asymmetric and

29

symmetric NO2 vibrations of monodentate nitrite [7, 8]. The weak peak appeared at 1009 cm-1 was related to the characteristic vibrational modes of monomeric vanadium species after NO + O2 adsorption. For VOx-MnOx/CeO2-R, the change of the spectra for NO + O2 adsorption was similar to that for the VOx/CeO2-R catalyst. Fig. 11 (c, d) illustrates that more bridged nitrate and bidentate nitrate species were generated over the VOx-MnOx/CeO2-R sample than over the VOx/CeO2-R sample, which is in accordance with the results of NOx-TPD (See Fig. S6). The

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enhanced NO activation over VOx-MnOx/CeO2-R could be explained by the abundant surface

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adsorbed oxygen arising from oxygen vacancies. (a) 1213 1235 1295

1009

1800

1600

1400

lP

re

15 min 10 min 7 min 5 min 3 min 1 min 0 min

2000

-p

Absorbance (a.u.)

1578 1545 1599

1200 -1

1000

800

(c)

ur na

Wavenumber (cm )

1233 1213

1295

15 min 10 min 7 min 5 min 3 min 1 min 0 min

Jo

Absorbance (a.u.)

1576 1545 1599

2000

1800

1600

1400

1007

1200

1000

800

Wavenumber (cm-1)

Fig. 11. In situ DRIFTs spectra of NO + O2 adsorption exposed to a flow of 500 ppm NO +O2 over the (a) VOx/CeO2-R, and (c) VOx-MnOx/CeO2-R catalysts at 220 °C, and the corresponding mapping results over (b) VOx/CeO2-R, and (d) VOx-MnOx/CeO2-R.

30

To further illustrate the influence of MnOx on the amount of intermediates, the areas of the characteristic intermediate peaks after NH3 adsorption and NO +O2 coadsorption at 220 °C for 15 min were integrated, and the results are presented in Fig. 12. As mentioned above, the band at 1182~1233 cm-1 can be attributed to the characteristic symmetric bending vibrations of the N-H bonds in NH3 linked to Lewis acid sites, whereas the band at 1427~1432 cm-1 are due to the characteristic asymmetric bending vibrations of NH4+ chemisorbed on the Brønsted acid sites

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[89]. Fig. 12a illustrates the values of the integral peaks area of the peaks at 1182~1233 cm-1, and

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1427~1432 cm-1. These represent the quantities of Lewis acid and Brønsted acid sites, respectively [89, 90]. In Fig. 12a, the total amount of adsorbed NH3 species for the VOx-

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MnOx/CeO2-R catalyst was the same as that for the VOx/CeO2-R catalyst. However, after MnOx

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loading, the percentage of NH3(L) over VOx-MnOx/CeO2-R increased, reaching 86.8%. For NO + O2 adsorption, the quantities of bridging nitrate and bidentate nitrate species were calculated

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by the integral area of the characteristic peaks at ~1599 cm-1, and 1545~1578 cm-1, respectively. As shown in Fig. 12b, a much greater quantity of nitrate species was produced over the VOx-

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MnOx/CeO2-R catalyst than the VOx/CeO2-R catalyst, implying the much energetically stronger capacity of the VOx-MnOx/CeO2-R catalyst toward NO activation. The percentage of bridging nitrate species over the VOx-MnOx/CeO2-R catalyst (24.2%) was also larger than that of the VOx/CeO2-R catalyst (20.0%). The enhanced generation of bridging nitrate species was

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demonstrated to consist of major intermediates (see below), which considerably quite benefited the catalytic activity of low-temperature NH3-SCR. According to previous reports [75, 91, 92], the coordinatively unsaturated cationic sites can generate Lewis acid sites. These Lewis acid sites acted as electron acceptors interacted with the NH3 molecule and produce the NH3(L) species, which in turn become “activated” and are facilitated to react with nitrate anions. After that, the

31

NH3(L) intermediate species combine with nitrate anions to produce N2 and H2O, according to the Langmuir-Hinshelwood mechanism. Herein, the more Lewis acid sites there are, the better the catalytic activity for NH3-SCR. The quantities of NH3(L) species and bridging nitrate species were increased by introducing Mn; these species could be two crucial intermediates, as well as enhancing the activity of NH3-SCR. 30

30 NH3(L)

(b)

Bridging nitrate Bidentate nitrate

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(a)

+

NH4 (B)

20

15

15

10

10 19.7%

5

13.2%

0 VOx/CeO2-R

75.8%

80.0%

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20

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86.8%

-p

80.3%

25

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Integral Area (a.u.)

25

5 20.0%

0 VOx-MnOx/CeO2-R VOx/CeO2-R

24.2%

VOx-MnOx/CeO2-R

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Fig. 12. Relative quantities of various (a) adsorbed NH3 species, and (b) NOx species over the VOx/CeO2-R and VOx-MnOx/CeO2-R catalysts.

3.6.3 Reaction of NO + O2 with preadsorbed NH3

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To clarify the reaction pathways and reaction mechanism of N2 generation by adsorbed NH3

species and adsorbed NO species over the VOx/CeO2-R and VOx-MnOx/CeO2-R catalyst, transient IR experiments were carried out. Fig. 13 presents the in situ DRIFTs spectra of the VOx/CeO2-R and VOx-MnOx/CeO2-R catalysts in the gas feeding of NO+O2 after preadsorbing NH3 for 15 min followed by He purging for 15 min at 220 °C. For the VOx/CeO2-R catalyst (Fig.

32

13a), NH3(L) (1235 and 1176 cm-1), NH4+(B) (1665 and 1432 cm-1) and O-H3N (1583 cm-1) species decreased quickly within 10 min after introducing NO+O2 into the feed gas, indicating that the NH3(L), NH4+(B), and O-H3N species were the dominant active intermediate species for NH3 adsorption. The peak of monomeric vanadium species at 1009 cm-1 was also disappeared within 10 min, which indicated that monomeric vanadium species were the active sites on the catalyst. Moreover, the introduction of excess NO+O2 led to the generation and accumulation of

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bridging nitrate (1599 cm-1) and bidentate nitrate (1573, 1541 and 1241 cm-1) on the surface of

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the catalysts. For the VOx-MnOx/CeO2-R catalyst, the in situ DRIFTs results were similar to those for the VOx/CeO2-R catalyst (Fig. 13c). As discussed above, more NH3(L) species were

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produced over the VOx-MnOx/CeO2-R catalyst arising from the coordinatively unsaturated

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cationic sites by MnOx loading, which was proven to elevate the low-temperature NH3-SCR activity.

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(a) 1599 1241

1009

1541 15 min 10 min 7 min 5 min 3 min NO+O2 1 min He

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Absorbance (a.u.)

1573

1176

1665 1583

2000

1800

1600

1432

1400

1235

1200

1000

800

-1

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Wavenumber (cm )

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(c)

Absorbance (a.u.)

1573 1546

1241

1601

1009

15 min 10 min 7 min 5 min 3 min NO+O2 1 min He 1674 1580 1427 2000

1800

1600

1400

1186 1233 1200

1000

800

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Wavenumber (cm-1)

Fig. 13. In situ DRIFTs spectra of NO + O2 reacted with pre-adsorbed NH3 species over the (a) VOx/CeO2-R,

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and (c) VOx-MnOx/CeO2-R catalysts at 220 °C, and the corresponding mapping results over (b) VOx/CeO2-R,

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3.6.4 Reaction of NH3 with preadsorbed NO + O2

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and (d) VOx-MnOx/CeO2-R.

The in situ DRIFTs spectra of the VOx/CeO2-R and VOx-MnOx/CeO2-R catalysts with

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preadsorbed NO + O2 when exposed NH3 at 220 °C are displayed in Fig. 14. For VOx/CeO2-R, the preadsorbed NO + O2 resulted in the presence of bridging nitrate species (1599 and.1213 cmbidentate nitrate species (1578, 1545, and 1235 cm-1), and monodentate nitrate species (1295

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1),

cm-1) with a weak-intensity shoulder band. Upon adding NH3 to the flue gas, the intensity of the peaks at ~1599 and 1213 cm-1 and the peak at 1295 cm-1 gradually decreased, related to bridging nitrate species and monodentate nitrate species, respectively. At the same time, coordinated

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NH3(L) (1235 and 1186 cm-1) and NH4+(B) species (1427 cm-1) formed. As displayed in Fig. 14(b), the intensity of bidentate nitrate species seldom varied after NH3 exposure for 15 min, suggesting that bidentate nitrate species were spectator species during the NH3-SCR reaction. For the VOx-MnOx/CeO2-R catalyst, the changes in the profiles on VOx-MnOx/CeO2-R were similar to those for VOx/CeO2-R, according to the Langmuir-Hinshelwood mechanism. The decreased rate of bridging nitrate species over the VOx-MnOx/CeO2-R catalyst was higher than 34

that of the VOx/CeO2-R catalyst, which exhibited good consumption of adsorbed NO species. This result implies that bridging nitrate species were the main reactive intermediate species during the NH3-SCR process. (a) 1235 1186 1009 1427

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15 min 10 min 7 min 5 min 3 min NH3 1 min He 1599 2000

1800

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Absorbance (a.u.)

1578 1545

1295 1213

1600

1400

1200

1000

800

1576

1186 1009

1427

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15 min 10 min 7 min 5 min 3 min NH3 1 min He 1599

1295 1213

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2000

1800

1600

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1233

1545

Absorbance (a.u.)

-p

Wavenumber (cm-1)

1400

1200

1000

800

Wavenumber (cm-1)

Fig. 14. In situ DRIFTs spectra of NH3 reacted with pre-adsorbed NO species over the (a) VOx/CeO2-R, and (c) VOx-MnOx/CeO2-R catalysts at 220 °C, and the corresponding mapping results over (b) VOx/CeO2-R, and (d)

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VOx-MnOx/CeO2-R.

3.7 Reaction mechanism 3.7.1 Correlation between structure and catalytic activity Our results revealed that the VOx-MnOx/CeO2-R catalyst can achieve better NH3-SCR performance at a lower temperature than the VOx/CeO2-R and MnOx/CeO2-R catalysts.

35

Obviously, the effect of the ceria facet and the promotional effect of Mn were two crucial factors in determining the low-temperature NH3-SCR performance. In addition, there was a strong correlative strong interaction between VOx and MnOx species and CeO2 nanorods. According to the results of HRTEM and catalytic performance analysis, the VOx-MnOx/CeO2-R nanorods had exposed {110} and {100} facets. The exposed {110} facet was active toward the NH3-SCR reaction, while the exposed {100} facet had little contribution to the NH3-SCR reaction. Further

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study by the results of Raman and O 1s XPS analysis demonstrated that the VOx-MnOx/CeO2-R

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sample with preferentially exposed {110} facet had the most oxygen vacancy defect sites compared with VOx-MnOx/CeO2-P and VOx-MnOx/CeO2-C, which was conductive to the

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activity of NH3-SCR toward NOx. The promotional effect of MnOx can be summarized as

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follows: (1) MnOx promoted the ratios of V5+/(V5++V4+), Ce3+/(Ce3++Ce4+), and Oads/(Oads+Olatt), which facilitated the catalytic activity of NO conversion. The cycle in which the redox couples

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V5+/V4+ and Ce4+/Ce3+ transfer electrons to each other could be elevated by the introduction of Mn. (2) The occurrence of more surface defects, possibly oxygen vacancy defects, in the {110}

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facet of the VOx-MnOx/CeO2-R catalyst and a strong intimate interaction between vanadium and cerium oxides was presented. (3) The relative quantities of Lewis acid sites and bridging nitrate species on the VOx-MnOx/CeO2-R catalyst were much higher than those of VOx/CeO2-R, which was positively related to the superior NH3-SCR activity.

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3.7.2 Proposed reaction mechanism

According to the mechanism proposed by Topsøe [75, 76] over V-based catalysts, the active

V5+ sites are very reactive toward NH3. A strong V5+/V4+ redox cycle can enhance gaseous NO reactivity. In this study, based on the above in situ DRIFTs discussions, the probable promotion mechanism of low-temperature NH3-SCR over the VOx-MnOx/CeO2-R catalyst is shown in

36

Scheme 1. The catalytic NH3-SCR performance over VOx-MnOx/CeO2-R was unambiguously correlated with the quantities of NH3(L), NH4+(B) species and the bridging nitrate species, whereas bidentate nitrate species were spectator species. During the NH3-SCR reaction, the well dispersed and superficial monomeric vanadium species on the {110} facet of CeO2 nanorods started with NH3 adsorption to generate the NH3(L) and NH4+(B) species. The activated NH3(L) and NH4+(B) species then quickly reacted with the bridging nitrate species to form N2 and H2O,

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according to the Langmuir-Hinshelwood mechanism. An redox cycle was also observed that V5+

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reacted with bridging nitrate species, releasing V4+, which was reoxidized by either NO or O2. The high ratio of Ce3+/(Ce3++Ce4+) sped up the redox cycle. Moreover, the presence of Mn

-p

induced a much greater quantity of OVDs and Ce3+, which contributed to the formation of more

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NH3(L) species and bridging nitrate species. These results also demonstrated the significant effects of MnOx on the promotion of NH3-SCR activity. As a result, the VOx-MnOx/CeO2-R

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catalyst exhibited the highest catalytic performance for low-temperature NH3-SCR.

Scheme 1. The possible mechanism over the VOx-MnOx/CeO2-R catalyst.

37

4. Conclusion We investigated in detail the MnOx-decorated VOx/CeO2 nanorods, whose surface was predominantly enclosed by {110} and {100} facets. It is likely that the exposed {110} facet was active toward the NH3-SCR reaction while the {100} facet had little contribution to the NH3SCR reaction. Moreover, there was a synergistic effect between the VOx and MnOx and the CeO2 with exposed {110} facets. More oxygen vacancy defect sites existed on the {100} facet of VOx-

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MnOx/CeO2-R than VOx-MnOx/CeO2-P or VOx-MnOx/CeO2-C, which was conductive to NH3-

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SCR activity. Furthermore, the redox cycle of V5+/V4+ and the quantities of NH3(L) species and bridging nitrate species increased by the introduction of Mn. The Langmuir-Hinshelwood

-p

mechanism is possible because abundant surface NH3(L), NH4+(B) and bridging nitrate species

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were produced as intermediates. During the NH3-SCR reaction, the well dispersed and superficial monomeric vanadium species on the {110} facet of CeO2 nanorods started with NH3

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adsorption to generate the NH3(L) and NH4+(B) species. The activated NH3(L) and NH4+(B)

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species then quickly reacted with the bridging nitrate species to produce N2 and H2O.

Daclaration of interest statement

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We declare that we have no financial and personal relationships with other people or

organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled

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MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3.

Notes

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The authors declare no competing financial interest

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ACKNOWLEDGMENT

This research was supported by the National Natural Science Foundation of China

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(21806045), the Scientific Research Funds of Huaqiao University (605-52418050), and the

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Scientific Research Funds of Huaqiao University (605-50Y17071). REFERENCES

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