Synthesis of CrO_x/C catalysts for low temperature NH₃-SCR with enhanced regeneration ability in the presence of SO₂

Abstract

Chromium oxide nano-particles with an average diameter of 3 nm covered by amorphous carbon (CrO_x/C) were successfully synthesized. The synthesized CrO_x/C materials were used for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR), which shows superb NH₃-SCR activity and in particular, satisfactory regeneration ability in the presence of SO₂ compared with Mn-based catalysts. The as-prepared catalysts were characterized by XRD, HRTEM, Raman, FTIR, BET, TPD, TPR, XPS and in situ FTIR techniques. The results indicated presence of certain amounts of unstable lattice oxygen exposed on the surface of CrO_x nano-particles with an average size of 3 nm in the CrO_x/C samples, which led to NO being conveniently oxidized to NO₂. The formed NO₂ participated in NH₃-SCR activity, reacting with catalysts via a “fast NH₃-SCR” pathway, which enhanced th NH₃-SCR performance of the CrO_x/C catalysts. Furthermore, the stable lattice of the CrO_x species made the catalyst immune to the sulfation process, which was inferred to be the cause of its superior regeneration ability in the presence of SO₂. This study provides a simple way to synthesize stable CrO_x nano-particles with active oxygen, and sheds light on designing NH₃-SCR catalysts with highly efficient low temperature activity, SO₂ tolerance, and regeneration ability.

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1. Introduction

Low-temperature NH₃-SCR (<200 °C), which can be located downstream of electrostatic precipitators and even desulfurizers, where most of SO₂ and dust have been removed, has been paid increasing attention in the past few decades.^1–4 Various transition metal oxides exhibit satisfactory activity for low-temperature NH₃-SCR, such as Mn, Fe, Cu, and Co.^2,5–9 Among them, Mn-based catalysts have become a focus for their excellent low-temperature activity and inherent environment-friendly nature.^4,10–12 According to literature, Mn-based catalysts have a unique advantage for low-temperature SCR (<200 °C) in contrast with other competitors. For example, Hu et al. reported a Co–Mn/TiO₂ catalyst with superior NH₃-SCR activity at 200 °C.¹¹ Mn–Ce–Ti mix-oxide catalysts were recorded to exhibit an operating temperature window from 150 °C to 200 °C.¹² We also reported Mn–Fe–Ti mix-oxide catalysts with satisfactory NH₃-SCR activity from 100 °C to 350 °C.⁵ However, the poor SO₂ resistance performance of Mn-based catalysts limits their practical application.

According to the previous reports,^4,13–15 the tolerance of metal oxide based catalysts to SO₂ depends on the type and oxidation state of the deposited metal, the nature of the support, and the type of reducing agent. In general, the rapid deactivation of NH₃-SCR catalysts involves two main mechanisms. One is the formation of ammonium salts.^14,16–22 The SO₂ in the feed gas can be oxidized to SO₃ on the surface of the catalysts, and the formed SO₃ would respond to NH₃ and water in feed gas transforming to NH₄HSO₄. The formed NH₄HSO₄ would deposit on the surface of the catalysts, cover active sites, block pores of the catalysts, and result in the deactivation of the catalysts. The other fact is the irreversible sulfation of the active phase.^{14,16,18–23} For most transition metal oxides usually reported as NH₃-SCR catalysts, such as Mn, Fe, Cu, and Co, all sulfating processes are spontaneous according to their Gibbs free energy values (Table S1†). In the sulfation process, formation of a metal sulfate requires breaking the metal oxide lattice. It is reasonable to predict that the more stable the metal oxides, the more difficult it is to break the metal oxide lattice and thus, harder is the sulfation of metal oxide. In general, the metal oxide with a high melting point has a stable crystal lattice. Thus, the melting point can be an indicator of the stability of the crystal lattice of metal oxides.²⁴ In the case of Mn-based catalysts, MnO₂ has a low melting point (Table S1†), indicating its unstable structure, which is the cause of the severe irreversible sulfation of MnO₂ in the NH₃-SCR process, particularly in the low temperature range (<200 °C).^{13,14,25–28} Cr₂O₃, which has the highest melting point among the transition metal oxides with NH₃-SCR activity (listed in Table S1†), was expected to have resistance to the sulfation process. Although the low NH₃-SCR activity of crystalline Cr₂O₃ is unsatisfactory, amorphous Cr₂O₃ exhibits superb low temperature NH₃-SCR activity, according to literature.^29–33 Thus, it appears to be a promising strategy to design a SCR catalyst with both low temperature activity and SO₂ tolerance via enhancing the catalytic activity of crystalline Cr₂O₃.

Compared with traditional metal oxide catalysts with a large particle size, nano-sized catalysts often have significant amounts of unsaturated-coordinated atoms exposed on their surfaces. Unsaturated-coordinated atoms are usually active; thus, nano-sized catalysts exhibit unique redox ability and enhanced catalytic activity.^34–36 Therefore, decreasing the size of catalysts appears to be a promising strategy to enhance low temperature NH₃-SCR performance of crystalline Cr₂O₃. However, nano-particle materials have the disadvantage of instability and tend to aggregate due to their high surface energy and their abundant surface unsaturated atoms. Recently, metal oxide nano-particles catalysts derived from MOFs material were reported.^37–39 Through a thermal decomposition process under controlled atmosphere, MOFs built from metal ions or nodes and polyfunctional organic ligands can transform into nano-materials, including nano-particles, single atoms, and metal oxide clusters. Wu et al.⁴⁰ synthesized Co nano-particles and single atoms from Co MOF and Co/Zn bimetallic MOF. Similar results were also reported by Li et al.³⁷ and Sun's group.⁴¹ The carbon from the organic ligands of MOFs remains in materials and can protect metal oxide nano-particles from aggregation.

Herein, novel CrO_x nano-particles covered by amorphous carbon (CrO_x/C) have been synthesized by a MOFs assisted process for low temperature NH₃-SCR. MIL-101, with a metal node of 3 Cr atoms,^42,43 was employed as a precursor. The results of the catalytic tests for NH₃-SCR showed that the prepared CrO_x catalyst exhibited satisfactory activity and superior regeneration ability. According to a series of characterizations, the CrO_x/C catalyst was observed to be composed of CrO_x nano-particles with an eskolaite phase and activated lattice oxygen. It was deduced that the activated lattice oxygen was closely related to the enhanced NH₃-SCR activity of the CrO_x/C catalyst. The stable lattice of the eskolaite phase-CrO_x inhibited the sulfating process, thus causing the SO₂ tolerance and regeneration ability. To the best of our knowledge, it is the first time that a non-Mn catalyst with excellent low temperature NH₃-SCR activity and remarkable regeneration ability has been reported. This study provides a simple route to synthesize stable CrO_x nano-particles with active oxygen and shed light on designing low temperature NH₃-SCR catalysts with SO₂ tolerance and regeneration ability.

2. Experimental details

2.1. Preparation of catalysts

Typically, MIL-101(Cr) was prepared by reacting terephthalic acid (332 mg, 2.0 mmol) with Cr(NO₃)₃·9H₂O (800 mg, 2.0 mmol) and de-ionized water (9.5 mL) at 220 °C for 8 h. Microcrystalline green powders of MIL-101(Cr) were produced during the reaction. The obtained powders were washed by ammonium hydroxide, water, and ethanol, in sequence, 3 times each. The powders were dried and calcined at a certain temperature for 4 h under N₂ flow, and the heating rate was set at 1 °C min⁻¹. Finally, the cooled sample was exposed to air and denoted as Cr₂O₃/C-X, in which X represents the calcining temperature.

As reported in ref. 44, a Cr₂O₃ sample was obtained by calcining Cr(NO₃)₃·9H₂O at 450 °C for 4 h. MnO₂ was purchased from Aladdin and was used without further purification. An active carbon supported Cr₂O₃ catalyst was synthesized through a wetness impregnation process. Active carbon (1.00 g) was dispersed into de-ionized water (50 mL) containing Cr(NO₃)₃·9H₂O (9.92 g). The turbid solution was oil-bath heated at 110 °C until the water was totally evaporated. The dried powders were calcined at 450 °C for 4 h under a N₂ flow and the obtained sample was noted as Cr₂O₃/C-WI.

2.2. Characterizations

The X-ray diffraction (XRD) patterns of the catalysts were studied using an XRD-6000 X-ray diffractometer (Shimadzu). X-ray fluorescence (XRF) analysis was performed on an ARL-900 X-ray fluorescence analyzer. FTIR analysis was carried out using a NEXUS870 spectrometer (NICOLET, America). Raman spectra were measured at a resolution of <1 cm⁻¹ using a JY Labram HR 800 spectrophotometer equipped with an argon-ion laser source and an air-cooled CCD detector. N₂ adsorption/desorption isotherms of the catalysts were obtained at −196 °C using an ASAP2020 physical adsorption instrument (Micromeritics) to calculate the BET surface area of the catalysts. TEM analysis was performed on a double-aberration corrected Titan™ cubed G2 60-300 S/TEM equipped with Super-X™ technology. X-ray energy dispersive spectroscopy (EDS) mappings were acquired using the Super-X EDS system, which is composed of four silicon drift detectors covering 0.7 s rad collection.

NH₃-temperature programmed desorption (NH₃-TPD) experiments were performed using a multifunction chemisorption analyzer, equipped with a thermal conductivity detector (TCD). Samples were pretreated under a NH₃–N₂ flow (NH₃ 1%) at 150 °C for 1 h and were heated under N₂ flow; the heating rate was set at 10 °C min⁻¹.

O₂-temperature programmed desorption (O₂-TPD) experiments were performed using a multifunction chemisorption analyzer, equipped with a thermal conductivity detector (TCD). Samples were pretreated under O₂–He flow (O₂ 25%) at 25 °C for 1 h and were heated under a He flow; the heating rate was set at 10 °C min⁻¹.

H₂-temperature programmed reduction (H₂-TPR) of the catalysts was recorded using a chemisorption analyzer. Samples were pretreated under a N₂ flow at 200 °C for 1 h, and were heated under a H₂–Ar flow (H₂, 7%); the heating rate was set at 10 °C min⁻¹.

X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5000 VersaProbe spectrophotometer. The contents of the metal ions were measured via an inductive coupled plasma emission spectrometer (Optima 5300DV, PE). Energy referencing was accomplished by setting the adventitious carbon peak to 284.6 eV. The ex situ XPS details are described below. The sample was treated under a certain atmosphere for a certain time in a reaction chamber connected with the intro chamber of the XPS instrument. Following this, the reaction chamber was vacuumized and the treated sample was transferred to the XPS instrument without exposure to air.

The in situ DRIFT experiments were performed on a Nicolet Nexus 5700 FTIR spectrometer using a diffuse reflectance attachment (HARRICK) equipped with a reaction cell (ZnSe windows). The number of scans was 32 at a resolution of 4 cm⁻¹ and the spectra were presented as Kubelka–Munk function, referred to the background spectra of the recorded catalyst in N₂.

2.3. NO oxidation tests

The NO oxidation tests were performed in a fixed-bed reactor with 0.2 g catalyst. The feed gas contained 500 ppm NO and 5 vol% O₂ with N₂ as the balance gas. The total flow rate of the feed gas was 100 mL min⁻¹, corresponding to a space velocity of approximately 30 000 h⁻¹. Including NO and NO₂, the effluent gases were continuously analyzed at 150 °C by an online Thermofisher IS10 FTIR spectrometer equipped with a 2 m path-length gas cell (250 mL volume).

2.4. NH₃-SCR activity, SO₂ poisoning, and regeneration tests

The NH₃-SCR activity tests were performed in a fixed-bed reactor with 0.2 g catalyst. The feed gas contained 500 ppm NO, 500 ppm NH₃, 5 vol% O₂, 50 ppm SO₂ (when used), 5 vol% H₂O (when used) and N₂ as the balance gas. The total flow rate of the feed gas was 100 mL min⁻¹, corresponding to a space velocity of approximately 30 000 h⁻¹. SO₂ poisoning and regeneration tests of catalysts were carried out at 150 °C. Including NO, NH₃, NO₂, and N₂O, the effluent gases were continuously analyzed at 150 °C using an online Thermofisher IS10 FTIR spectrometer equipped with a 2 m path-length gas cell (250 mL volume).

2.5. Regeneration of SO₂ poisoned catalysts

SO₂ poisoned catalysts were regenerated at 300 °C for 30 min, and then were cooled down to room temperature. All the heat treatments were carried out under N₂ atmosphere.

The sulfating of metal oxide catalysts during SO₂ poisoning was investigated by inductively coupled plasma-emission spectroscopy (ICP).

The SO₂-poisoned catalyst (0.2 g) was washed with deionized water for 5 times. The eluate was collected and diluted to 50 mL. The diluted eluate was investigated by ICP analysis. The contents of metal sulfate m_MSO4 were calculated using the equation below.

where, m_Mⁿ⁺ is the content of the metal ion, M_MSO4 was obtained via ICP analysis and M_M is the molar mass of metal sulfate, which also corresponds to the molar mass of the metal ion.

For Cr oxide, m_Cr2(SO4)3 = m_Cr³⁺ × 0.188

For Mn oxide, m_MnSO4 = m_Mn²⁺ × 0.137

3. Results and discussion

3.1. NH₃-SCR performance, SO₂ tolerance, and regeneration

The NH₃-SCR activities of the CrO_x samples derived from various precursors and MnO₂ are shown in Fig. 1 and S1†. As it was anticipated, all the catalysts derived from MIL-101 exhibited superior activities than pure Cr₂O₃. Remarkably, CrO_x/C-450, parallel to MnO₂, exhibited a wide operation temperature window from 125 °C to 200 °C. The activity of Cr₂O₃/C-WI was enhanced compared with pure Cr₂O₃, while it was much lower than CrO_x/C-450. This extraordinarily low temperature NH₃-SCR performance makes CrO_x/C-450 the best catalyst among all the samples derived from MIL-101. To evaluate the NH₃-SCR performances on the catalysts more precisely, the normalized rates per mole transition metal were calculated and the results are displayed in Fig. 2a. The catalytic activity order is CrO_x/C-450 > Mn₂O₃ > Cr₂O₃/C-WI > Cr₂O₃. In addition, the apparent active energies of CrO_x/C-450 and Cr₂O₃ based catalysts were obtained when NO conversions were limited to low conversion (Fig. 2b and Table S2†). The apparent active energy of NH₃-SCR on the CrO_x/C-450 catalyst was lower than those of SCR on Cr₂O₃/C-WI and Cr₂O₃, which further confirms the superb catalytic activity of the CrO_x/C-450 sample. Based on the kinetics data listed in Tables S3 and S4,† apparent kinetics equations of NH₃-SCR on CrO_x/C-450 and Cr₂O₃/C-WI catalysts were obtained (Fig. S2†). For CrO_x/C-450, r = [NH₃]^0.586[NO]^0.964 at 150 °C, while for Cr₂O₃/C-WI, r = [NH₃]^0.433[NO]^0.092. The different reaction rate equations of NH₃-SCR on CrO_x/C-450 and Cr₂O₃/C-WI catalysts indicated their different reaction mechanisms, which may result in different NH₃-SCR performance of CrO_x/C-450 and Cr₂O₃/C-WI catalysts. The N₂ selectivity of CrO_x/C-450 remained at a high level (over 90%) in its operation temperature window, while those of Cr₂O₃ and MnO₂ were very poor. This indicated that side reactions such as the formation of N₂O hardly occurred on the CrO_x/C-450 catalyst. SO₂- and H₂O-tolerance of CrO_x/C-450 was further tested and the results are presented in Fig. 1c and S1c.† CrO_x/C-450 exhibited over 80% NO conversion within 24 h in the presence of H₂O, indicating its satisfactory water tolerance. When SO₂ was introduced into the feed gas, the activity of CrO_x/C-450 gradually dropped to 60% within 20 h, while it dropped to 50% in 20 h when SO₂ and H₂O co-existed in the feed gas. The activity of CrO_x/C-450 could be recovered after a heat treatment at a temperature as low as 300 °C. This indicated that the poisoning effect of SO₂ on CrO_x/C-450 could be accelerated by H₂O, while H₂O hardly influenced the regeneration ability of CrO_x/C-450. In contrast, the MnO₂ catalyst deactivated rapidly upon SO₂ introduction in 6 h, and this process was irreversible. It was demonstrated that the CrO_x/C-450 catalyst has satisfactory SO₂-tolerance. The regeneration ability of CrO_x/C-450 was further studied. As shown in Fig. 1d, over 90% of the catalytic activity of CrO_x/C-450 could be recovered as compared to that of the fresh catalyst after 3 poisoning–regeneration cycles irrespective of whether H₂O was introduced by regeneration at 300 °C in flowing N₂, while that of MnO₂ catalyst dropped dramatically through only 1 poisoning–regeneration cycle, which indicated the remarkable regeneration ability of the CrO_x/C-450 sample.

Fig. 1 NH₃-SCR activity of catalysts: (a) NO conversion, (b) N₂ selectivity, (c) SO₂ poisoning test (step 1, 2, and 3 present SO₂ and H₂O (when used) on, SO₂ and H₂O (when used) off, and after regeneration), and (d) regeneration ability.

Fig. 2 (a) Normalized rates per metal mole of catalysts, (b) apparent active energy of CrO_x/C-450 and Cr₂O₃/C-WI catalysts.

3.2. Structural information

To investigate the NH₃-SCR on the catalysts, the structural information of catalysts was necessary. XRD, XRF, FTIR, Raman, and TEM analyses were carried out to investigate the structural properties of the catalysts synthesized from MIL-101(Cr). The XRD patterns of all samples are shown in Fig. S3.† In the XRD pattern of the precursor, sharp and distinct peaks attributed to the MIL-101 phase were detected, which is in agreement with the data reported by Jhung et al.⁴³ When MIL-101 was heated in N₂-flow, the XRD peaks of MIL-101 became weak and gradually disappeared with an increase in temperature. When the calcining temperature reached 450 °C, the peaks of the MIL-101 phase disappeared completely and replaced with wide and weak peaks at 24.5°, 33.5°, and 36.1°, which belonged to the (012), (104), and (110) plane, respectively, of the eskolaite phase (PDF#38-1479) as well as Cr₂O₃/C-WI.⁴⁵ This indicated that the MIL-101 structure could be destroyed through the calcining process, accompanied with the formation of the eskolaite phase. Furthermore, the CrO_x/C-450 sample was found to have a small particle size based on its broad XRD peaks.

FTIR and Raman analyses were carried out to study the carbon species in the catalysts. As shown in Fig. S4a,† the bands at 3300 cm⁻¹, 1600 cm⁻¹, and 1400 cm⁻¹ were identified in the FTIR spectra of MIL-101. The broad band at 3300 cm⁻¹ arises due to the stretching vibration of the surface –OH groups. The two bands at 1600 and 1400 cm⁻¹ could be attributed to the stretching vibration of the –COO group of the organic linkers of MIL-101. However, the intensity of bands at 1600 cm⁻¹ and 1400 cm⁻¹ decreased on increasing the calcination temperature. This indicates that the organic linker begins to decompose and carbonize when the calcining temperature increases. The carbonization process was further investigated by Raman analysis (Fig. S4b†). Wide bands at 1360 and 1590 cm⁻¹, corresponding to the D-band and G-band of MIL-101, were detected in the samples calcined at low temperatures, such as 350 and 400 °C. When the calcining temperature increased, the G-band gradually disappeared and the D-band still remained, which indicated the loss of the ordered structure of MOFs and the formation of amorphous carbon. In the spectrum of Cr-550, the G-band disappeared absolutely and only a wide D-band was observed. This illustrates that the organic linker carbonized and transformed to amorphous carbon during the calcining process, which accompanied with the destruction of the MOFs structure.

The elemental contents of catalysts were studied via XRF analysis. Since the organic species in MIL-101 completely transformed to amorphous carbon in CrO_x/C-450, as mentioned before, the ignition loss in XRF analysis of CrO_x/C-450 was believed to be the result of carbon species burning. Thus, the elemental contents of the catalysts could be calculated from the XPF data and the results are shown in Table 1. The CrO_x/C-450 catalyst consisted of 19.2% Cr, 28.7% O, and 52.0% C. As it was designed, the elemental contents of Cr₂O₃/C-WI were similar to those of CrO_x/C-450.

Table 1 Element contents and surface areas of catalysts

Sample	Contents^a (wt%)		Elements contents (at%)			Surface area^b (m^2 g^−1 catalyst)
	Cr2O3	Ignition loss	Cr	O	C	Sample	Carbon support^c	Active material
a Contents of catalysts were obtained from XRF analyses.b Surface areas of catalysts were obtained from N2 adsorption/desorption analyses.c Carbon support of CrOx/C-450 catalyst was obtained by washing CrOx/C-450 using hydrochloric acid. Carbon support of Cr2O3/C-WI is active carbon.
CrOx/C-450	70.2	29.8	19.2	28.7	52.0	256	784	20.8
Cr2O3/C-WI	69.5	30.5	18.9	28.4	52.7	279	897	9.9
Cr2O3	99.9	0.1	40.0	60.0	—	28	—	—

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TEM analysis was performed to investigate the structure of MIL-101, CrO_x/C-450, and Cr₂O₃/C-WI. As shown in Fig. S5a,† the MIL-101 with octahedron morphology can be observed distinctly, while it transformed to a hexagonal sheet after calcination (Fig. 3a). Further structural information was obtained via high-angle annual dark field (HAADF) imaging (Fig. 3b). The hexagonal-like sample observed in Fig. 3a was actually an aggregation of nano-particles with an average size of 3 nm. In addition, lattice fringes of d = 0.345 nm were observed (Fig. 3c), corresponding to the eskolaite Cr₂O₃ (012) crystal plane, which further confirmed that the Cr oxide in the CrO_x/C-450 catalyst has the eskolaite phase. From the EDX element mapping analysis (Fig. 3e–g), Cr element and O element were observed to be highly dispersed in the CrO_x/C-450 sample and the superimposed image of Cr and O matches the HAADF image. It was estimated that the CrO_x/C-450 catalyst was primarily formed by CrO_x nano-particles with eskolaite phase as designed in this study. The CrO_x/C-450 sample after 3 poisoning–regeneration cycles was also imaged to investigate the stability of the catalyst (Fig. S5b†). The used sample was primarily composed of CrO_x nano-particles, similar to the fresh sample, and no major difference in the particle size of Cr₂O₃ could be observed after 5 deactivation–regeneration circles, which evidently proved the stability of the sample CrO_x/C-450. Cr₂O₃ and Cr₂O₃/C-WI, however, had a bulk-like shape (Fig. S5c and d†) with average particle sizes of over 100 nm. In the structure of the MIL-101 precursor, metal nodes containing 3 Cr atoms were covered by organic linkers. Therefore, it is reasonable to suggest that CrO_x nano-particles in the CrO_x/C-450 catalyst stabilized by covering carbon species transformed from organic linkers of MIL-101 after the calcining process.

Fig. 3 (a) TEM image, (b–d) high resolution HAADF-STEM image of CrO_x/C-450 catalyst. (e, f) 2D STEM EDS elemental maps of Cr-Kα (Green) and O (Blue) elements, respectively. (g) A superimposed image of (e) and (f).

The feasible mechanism of catalyst synthesis is displayed in Fig. 4. During the calcination process, organic linkers covering Cr nodes carbonized and the structure of MIL-101 gradually destroyed. The amorphous carbon from organic linkers limited the growth of Cr nodes. Finally, when the calcined sample was exposed to air, the remaining Cr nano-particles were oxidized to CrO_x nano-particles with eskolaite phase, forming the structure of amorphous carbon covered CrO_x nano-particles (CrO_x/C).

Fig. 4 Possible mechanism of catalyst synthesis (green spheres present CrO_x nano-particles, brown skolen present amorphous carbon).

3.3. Textural properties of catalysts

As mentioned before, the CrO_x/C-450 catalyst was primarily formed by CrO_x nano-particles with eskolaite phase and exhibited enhanced NH₃-SCR activity and satisfying regeneration ability. To explore the relationship between the structure of CrO_x/C-450 and the NH₃-SCR performance, the properties of the catalysts were characterized by BET, XPS, H₂-TPR, O₂-TPD, and NH₃-TPD analyses.

The surface areas of the catalysts can influence the number of active sites on the catalyst surface, which is considered as an important fact affecting the catalytic activity of NH₃-SCR catalysts. The surface areas of CrO_x/C-450, Cr₂O₃, and Cr₂O₃/C-WI obtained from BET analysis are listed in Table 1. The BET surface areas of CrO_x/C-450 and Cr₂O₃/C-WI are similar and larger than that of Cr₂O₃. Due to the enhanced NH₃-SCR performances of CrO_x/C-450 and Cr₂O₃/C-WI (Fig. 1), it is believed that the enlarged surface area increased the activity of the Cr oxide catalyst. According to the reaction rates normalized by the surface areas of the active material of the catalysts, CrO_x/C-450 exhibited comparatively superior NO conversion than Cr₂O₃/C-WI at 125 °C and 150 °C. This infers that a large surface area is not the only reason for the excellent NH₃-SCR activity of CrO_x/C-450.

The acidity of the catalysts, which can influence the absorption of reaction agents, is an important factor affecting the NH₃-SCR performance of catalysts. This property of catalysts was investigated by NH₃-TPD analysis. As displayed in Fig. S6a,† the NH₃ desorption behaviours of Cr₂O₃ and CrO_x/C-450 were similar. No distinct NH₃ desorption peak was observed from 150 to 400 °C in the profiles of both Cr₂O₃ and CrO_x/C-450 samples, which indicated the weak acidity of these two samples. Therefore, the giant NH₃-SCR performance difference between Cr₂O₃ and CrO_x/C is not the result of acidity.

The redox ability of materials is another significant factor influencing the catalytic activity of the NH₃-SCR catalyst. H₂-TPR method was utilized to discuss this property of the synthesized catalysts. As illustrated in Fig. 5b, Cr₂O₃ exhibited a single-peak profile. The H₂ consumption peak at 343 °C was a result of one-step reduction from Cr⁶⁺ to Cr³⁺, accompanied by the loss of lattice oxygen atoms connected with Cr⁶⁺ ions.⁴⁶ For the CrO_x/C-450 sample, the H₂ consumption peak shifted to a low temperature, which indicated that CrO_x/C-450 exhibited stronger oxidation ability and a higher amount of active lattice oxygen than Cr₂O₃. In addition, O₂-TPD analysis was carried out to investigate the stability of the oxygen atoms of the catalyst; the results are shown in Fig. S6b.† In the profile of Cr₂O₃, no O₂ desorption peak was discovered in the temperature range from 50 °C to 450 °C, which indicates that the oxygen on the surface of Cr₂O₃ is stable and inert. However, the curve of CrO_x/C-450 exhibited an O₂ desorption peak from 200 to 350 °C, which was much higher than the desorption temperature of absorbed O₂ species recorded earlier.⁴⁷ Hence, the desorption peak from 200 to 350 °C was believed to correspond to the dissociation of lattice oxygen from CrO_x/C-450. This demonstrates that the lattice oxygen of CrO_x/C becomes more active and unstable than that of Cr₂O₃. Remarkably, the CrO_x/C catalyst has high surface atom/lattice atom rate for its ultrasmall size as mentioned before. It is reasonable to conclude that the unsaturated surface atoms of CrO_x/C cause the unique redox ability exhibited in H₂-TPR analysis and the instability of the lattice oxygen atom detected in O₂-TPD analysis. Unstable and activated oxygen atoms are inferred to enhance the activity of CrO_x/C catalyst.

Fig. 5 (a) Normalized rates per surface areas of active material of CrO_x/C-450 and Cr₂O₃/C-WI catalysts, (b) H₂-TPR result of CrO_x/C-450 and bulk Cr₂O₃.

In XPS analysis, Cr 2p spectra (Fig. 6a) of all the samples were comparable. The Cr 2p_3/2 peak could be divided into two peaks at 576.7 eV and 578.6 eV, belonging to Cr³⁺ and Cr⁶⁺, respectively.⁴⁸ The relative contents of Cr³⁺ and Cr⁶⁺ ions were analogous (Table 2), indicating their similar Cr state. The O 1s peak (Fig. 6b) could be separated into two peaks at 530.1 eV and 531.9 eV, attributed to the lattice oxygen and surface –OH groups, respectively.^49,50 The relative contents of these two types of oxygen species are also listed in Table 2. The CrO_x/C-450 sample exhibited more surface –OH groups than Cr₂O₃. Notably, the peak belonging to the lattice oxygen of CrO_x/C-450 shifts to the high binding energy side, contrasting with that of Cr₂O₃ and Cr₂O₃/C-WI. It is evident that the lattice oxygen of CrO_x/C-450 carries less negative charge than Cr₂O₃ and Cr₂O₃/C-WI. Materials with an ultrasmall size are deemed to have abundant dangling bands and their surface atoms are usually unsaturated-coordinated. In case of CrO_x/C-450, some surface oxygen atoms are inferred to be unsaturated coordinated for ultrasmall size of CrO_x nano-particles. This unsaturated oxygen is considered to have less negative charge and is expected to be more active than the saturated coordinated oxygen of bulk Cr₂O₃.

Fig. 6 XPS spectra of Cr₂O₃, fresh CrO_x/C-450 catalyst, SO₂ poisoned CrO_x/C-450 catalyst, and regenerated CrO_x/C-450 catalyst: (a) Cr 2p spectra, (b) O 1s spectra, and ex situ XPS analysis of fresh CrO_x/C-450 catalyst and CrO_x/C-450 catalyst heated under NH₃ + NO flow at 150 °C for 1 h: (c) Cr 2p spectra, (d) O 1s.

Table 2 Relative contents of Cr and O species on surfaces of catalysts

Sample	Cr 2p		O 1s
	Cr^3+^a (%)	Cr^6+^a (%)	OH groups^a (%)	Lattice oxygen^a (%)
a Relative contents of Cr and O species were calculated from peak areas ratio of divided peaks in XPS spectra.
Cr2O3	76.74	23.26	13.39	86.61
CrOx/C-450	78.86	21.14	18.36	81.64
CrOx/C-450-regeneration	79.37	20.63	18.7	81.3
CrOx/C-450 ex situ treatment	94.87	5.13	36.76	63.24
CrOx/C-450-SO2 poisoning	77.06	22.94	—	—

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Furthermore, to determine whether the lattice oxygen participates in the NH₃-SCR reaction, ex situ XPS analysis was carried out. The CrO_x/C-450 catalyst was heated under NH₃ + NO flow at 150 °C for 1 h; the XPS spectra of CrO_x/C-450 before and after treatment are displayed in Fig. 6c and d. In the Cr 2p spectra (Fig. 6c), the peak of Cr⁶⁺ at 578.6 eV disappeared after NH₃ + NO treatment, while the peak of Cr³⁺ at 576.7 eV enhanced. This infers that Cr⁶⁺ species on the surface of CrO_x/C can react with the reagent molecules and eventually get consumed. Moreover, in the O 1s spectra (Fig. 6d), the peak intensity of lattice oxygen at 530.4 eV decreased after treatment, while the intensity of the peak attributed to the surface –OH group at 531.7 eV increased, which indicates the loss of surface lattice oxygen. Therefore, lattice oxygen was believed to take part in the NH₃-SCR on the CrO_x/C catalyst. Thus, CrO_x/C-450, which has more active lattice oxygen, can exhibit enhanced NH₃-SCR activity than Cr₂O₃.

3.4. Mechanism of NH₃-SCR on CrO_x/C catalyst

In “fast NH₃-SCR”, NO catalytically reduced NH₃ in assistance of NO₂, which was reported to have lower activation energy and enhanced catalytic activity compared with the typical NH₃-SCR. Herein, the CrO_x/C catalyst was proved to have activated lattice oxygen. It is reasonable to deduce that NO can be oxidized to NO₂ by the activated oxygen on the CrO_x/C surface, making the NH₃-SCR on CrO_x/C proceed as the “fast NH₃-SCR” pathway. To verify this conjecture, NO oxidation on CrO_x/C-450 and Cr₂O₃/C-WI catalysts was carried out. As displayed in Fig. 7a, NO could be oxidized to NO₂ on both CrO_x/C-450 and Cr₂O₃/C-WI. However, the mass of formed NO₂ on CrO_x/C-450 exceeded as compared to that on Cr₂O₃/C-WI. Moreover, the normalized rate by surface area of NO oxidation on CrO_x/C-450 clearly surpassed that on Cr₂O₃/C-WI (Fig. 7b). This indicated that NO is more easily oxidized to NO₂ on CrO_x/C-450 catalyst with activated oxygen, which probably resulted in the superb NH₃-SCR performance of CrO_x/C-450.

Fig. 7 (a) NO oxidation performances and (b) normalized NO oxidation rate by surface area on CrO_x/C-450 and C–Cr₂O₃ catalysts.

In order to confirm whether the formed NO₂ participated in NH₃-SCR on the CrO_x/C catalyst, further information about the reaction mechanism was obtained using DRIFTS. In the spectra of the absorption NO + O₂ saturated CrO_x/C-450 sample (Fig. 8a), the bands centered at 1280, 1335, and 1520 cm⁻¹, and a wide band divided into bands at 1730, 1690, and 1660 cm⁻¹ were detected. As reported elsewhere,⁵¹ these IR bands were attributed to weakly bound NO₂ (1730 cm⁻¹), nitrite anion (1335 cm⁻¹), ν_s(N=O) and ν_as(N=O) of symmetric N₂O₃ (1690 and 1660 cm⁻¹), bidentate nitrates (1520 cm⁻¹), and monodentate nitrates (1280 cm⁻¹). When the feed gas was switched to NH₃, the bands belonging to NO₂ and bidentate nitrates gradually disappeared, replacing with the bands of NH₃ absorbed on Lewis acid sites (1620 and 1217 cm⁻¹),^52,53 while the bands corresponding to symmetric N₂O₃, nitrite anion, and monodentate nitrates were still present. Indeed, it is apparent that NO₂ and bidentate nitrates participated in the surface reaction on CrO_x/C-450 and were consumed by NH₃, which is a typical “fast NH₃-SCR” pathway.⁵⁴ To determine the role of NH₃, co-absorption of NO + O₂ on CrO_x/C-450 after pre-absorption of NH₃ was investigated (Fig. 8b). In the spectra of the absorption NH₃ saturated CrO_x/C-450 sample, only a weak band at 1620 cm⁻¹ corresponding to NH₃ on Lewis acid sites was detected, indicating the low acidity of CrO_x/C-450 as mentioned in NH₃-TPD analysis. With the addition of NO and O₂, this band gradually disappeared, accompanied by the appearance of bands belonging to weakly bound NO₂ (1732 cm⁻¹), nitrite anion (1333 cm⁻¹), ν_s(N=O) and ν_as(N=O) of symmetric N₂O₃ (1691 and 1657 cm⁻¹), and bidentate nitrates (1515 cm⁻¹).⁵¹ This indicated that NH₃ on Lewis sites was reacted and consumed. Therefore, the entire NH₃-SCR on CrO_x/C-450 followed a L–H mechanism, in which NO was oxidized by the unsaturated oxygen of CrO_x/C-450 catalyst and transformed to NO₂. The formed NO₂ further absorbed on the surface of CrO_x/C-450 as bidentate nitrates, and reacted with NH₃ on Lewis sites, forming N₂ and H₂O at last. Due to the formation of NO₂, the reaction proceeded as a “fast NH₃-SCR” pathway, causing the decrease in reaction activation energy, thus enhancing the low-temperature activity.

Fig. 8 In situ DRIFT spectra of CrO_x/C-450: (a) NH₃ reacted with pre-absorbed NO + O₂, (b) NO + O₂ reacted with pre-absorbed NH₃.

3.5. Reasons of excellent SO₂ tolerance of regeneration ability

Sulfation process is a transition from metal oxide to metal sulfate. Irrespective of which type of intermediate exists in the sulfation process, the metal–oxygen band must be broken. It is reasonable to deduce that a metal oxide with a strong metal–oxygen band is difficult to be sulfated. The Cr–O band energy of the eskolaite phase Cr₂O₃ was calculated (see the ESI†). As displayed in Table S5,† the band energy of the Cr–O band in the eskolaite phase Cr₂O₃ exceeds that of Mn–O band in MnO₂, indicating that the sulfation process of Cr₂O₃ proceeds with more difficulty as compare that of MnO₂. This is considered to be the result of the various NH₃-SCR performances of MnO₂ and CrO_x/C-450 catalyst in the presence of SO₂. Moreover, MnO₂ and CrO_x/C-450 catalysts deactivated by SO₂ were washed with deionized water and the metal-ion contents in the eluate were measured by ICP analysis. As presented in Table S6,† Mn²⁺ was detected in the eluate of the deactivated MnO₂ sample, while no Cr³⁺ was detected in the eluate of the deactivated CrO_x/C-450 sample. It is evident that the CrO_x/C-450 sample was protected from sulfation.

To further understand the SO₂ poisoning and regeneration processes, the XPS spectra of the fresh CrO_x/C-450 catalyst, SO₂-poisoned sample, and regenerated sample were studied. As displayed in Fig. 6a and Table 2, the Cr state and relative content of Cr⁶⁺ and Cr³⁺ of each sample were similar, which indicated that, as it is designed, CrO_x/C-450 catalyst was difficult to be sulfated for the high lattice energy of Cr₂O₃. Comparing the O 1s peak of the fresh CrO_x/C-450 sample and the regenerated sample (Fig. 6b), their peaks of lattice oxygen were similar and both shifted to the high binding energy side than that of bulk Cr₂O₃. Since activated lattice oxygen still remained on the surface, the regenerated catalyst exhibited high NH₃-SCR activity, similar to the fresh catalyst.

In XPS spectra of the SO₂ poisoned sample, the peaks of S 2p and N 1s were detected. The S 2p peak (Fig. S7a†) consists of two peaks at 168.5 eV (S 2p_1/2) and 169.7 eV (S 2p_3/2), which belonged to SO₄²⁻. The N 1s spectra (Fig. S7b†) can be divided into two peaks at 399.5 eV and 400.5 eV, contributed to NH₃ and NH₄⁺, respectively. According to the relative atom contents listed in Table S7,† atoms-ratio of SO₄²⁻ and NH₄⁺ was nearly 1 : 1 on the surface of the SO₂-poisoned sample. This indicated that NH₄HSO₄ deposited on the surface of CrO_x/C-450 during the SO₂ poisoning process, which causes the deactivation of the catalyst. After heat treatment, the deposited NH₄HSO₄ could easily decompose and the CrO_x/C-450 catalyst with exposed activated lattice oxygen regains the superior activity.

4. Conclusions

In this study, we successfully designed and synthesized a novel chromium oxide nano-particles catalyst with excellent NH₃-SCR activity at 150 °C and remarkable SO₂ regenerative ability. The obtained CrO_x/C-450 catalyst was composed of CrO_x nano-particles covered by amorphous carbon. A carbon species, which was derived from the organic linkers of the MOFs precursor, protected the CrO_x nano-particles from aggregation. CrO_x/C catalysts primarily have Eskolaite phase Cr₂O₃ with average size of 3 nm and exhibit a large surface area. Due to the small size of CrO_x nano-particles in CrO_x/C catalysts, the lattice oxygen atoms of CrO_x/C were activated, so that NO could be oxidized to NO₂ on the catalyst surface. The formed NO₂ participated in reaction and made NH₃-SCR on CrO_x/C proceed through a “fast NH₃-SCR” pathway. The large surface area and activated lattice oxygen of CrO_x/C catalysts caused the enhanced NH₃-SCR activities. Due to the stable lattice of Cr₂O₃, CrO_x/C catalyst could hardly be sulfated in the SO₂ poisoning process. Therefore, the regenerated catalyst still retained prominent activity when NH₄HSO₄ deposited on the surface of the catalyst was removed during the regeneration process.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial supports of National High-tech Research and Development (863) Program of China (2015AA03A401), National Natural Science Foundation of China (No. 21573105), Natural Science Foundation of Jiangsu Province (BK20161392), and Jiangsu Province Science and Technology Support Program (Industrial, BE2014130) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Band energy calculation, thermochemical data, band energy data, mass of metal sulfate formed on catalysts, surface atomic concentration of SO₂ poisoned CrO_x/C-450 sample, extra NH₃-SCR and water tolerance tests, XRD patterns, FTIR data, Raman data, extra TEM image, NH₃-TPD data, H₂-TPR data, O₂-TPD data, extra XPS spectra. See DOI: 10.1039/c7ra09680a

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