The effects of Bi₂O₃ on the selective catalytic reduction of NO by propylene over Co₃O₄ nanoplates

Abstract

Bi₂O₃/Co₃O₄ catalysts prepared by the impregnation method were investigated for the selective catalytic reduction of NO by C₃H₆ (C₃H₆-SCR) in the presence of O₂. Their physicochemical properties were analyzed with SEM, XRD, H₂-TPR, XPS, PL and IR measurements. It was found that the deposition of Bi₂O₃ on Co₃O₄ nanoplates enhanced the catalytic activity, especially at low reaction temperature. The SO₂ tolerance of Co₃O₄ in C₃H₆-SCR activity was also improved with the addition of Bi₂O₃. Among all catalysts tested, 10.0 wt% Bi₂O₃/Co₃O₄ achieved a 90% NO conversion at 200 °C with the total flow rate of 200 mL min⁻¹ (GHSV 30 000 h⁻¹). No loss in its C₃H₆-SCR activity was observed at different temperatures after the addition of 100 ppm of SO₂ to the reaction mixture. These enhanced catalytic behaviors may be associated with the improved oxidizing characteristics of 10.0 wt% Bi₂O₃/Co₃O₂. XRD results showed that Bi₂O₃ entered the lattice of Co₃O₄, resulting in the formation of lattice distortion and structural defects. H₂-TPR results showed that the reduction of Co₃O₄ was promoted and the diffusion of oxygen was accelerated with the addition of Bi₂O₃. XPS measurements implied that more Co³⁺ formed on the 10.0% Bi₂O₃/Co₃O₂ catalysts. The improved oxidizing characteristics of the catalyst with the addition of Bi₂O₃ due to the synergistic effect of the nanostructure hybrid, thus enhanced the C₃H₆-SCR reaction and hindered the oxidization of SO₂. Therefore, the 10.0% Bi₂O₃/Co₃O₄ catalyst exhibited the highest NO conversion and strongest SO₂ tolerance ability.

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1 Introduction

Lean burn engines, which are generally used in gasoline and diesel powered vehicles, are more fuel-efficient than the stoichiometric gasoline engines.¹ They also effectively reduce unburned hydrocarbons, CO₂ and CO in exhausts.² However, lean burn engines operate with a large excess of air, leading to a significant concentration of oxygen in the exhausts, where the noble-metal three-way catalysts cannot work well to reduce nitrogen oxides (NO_x).³ A large amount of NO_x produced by lean burn engines leads to serious air pollution and public health problems.

In order to control NO_x emission under the lean burn conditions, selective catalytic reduction of NO by hydrocarbons (e.g. propylene) has been undertaken and reported in the literatures as one potential application (HC-SCR). Many classes of catalysts, including supported noble metals (e.g. Pt,^4,5 Au^6,7), metal oxides (e.g. Ag₂O,^8,9 CuO,^10,11 SnO₂,^12–14 CoO_x (ref. 15–18)) and zeolite types (ZSM-5,¹⁹ MCM-41 (ref. 20)) have been investigated. In general, the noble metals are active and stable even at lower temperature, but the formation of N₂O is undesirable by using such precious metals, particularly, platinum-based catalysts. The zeolite-based catalysts were low thermal stability. Among metal oxides catalysts, cobalt oxides (e.g. Co₃O₄) are considered as one promising catalyst for HC-SCR due to its high catalytic activity.²¹ When combined with other oxides, such as CeO₂,¹⁶ ZrO₂,¹⁵ Al₂O₃,¹⁸ sulphated ZrO₂,²² the catalytic performances of the cobalt oxide catalyst could be improved as reported. These results implied that the chemical environment around cobalt oxide plays a crucial role in controlling the overall activity of cobalt containing catalysts in SCR reactions.

Bi₂O₃, a common oxide semiconductor, is widely used in the fields of chemical engineering and electronics such as NO detection²³ and the oxidation or ammoxidation of propylene.^24,25 In the oxidation/ammoxidation of propene over bismuth/molybdate catalyst, bismuth was thought to involve the rate-determining hydrogen abstraction from propylene,²⁶ exhibiting its mild oxidizing characteristics. This property might be also beneficial for the partial oxidation of propylene in C₃H₆-SCR for NO reduction. Therefore, it is of considerable interest to explore the application of Bi₂O₃ in the reduction of NO with propylene.

In the present study, Co₃O₄ nanoplates and Bi₂O₃/Co₃O₄ were prepared with the solvothermal and impregnation method respectively. Their catalytic performances in the NO reduction by C₃H₆ in the presence of O₂ were investigated. The catalysts were characterized with X-ray diffraction (XRD), temperature programmed reduction with hydrogen (H₂-TPR), and X-ray photoelectron spectra (XPS). The effects of Bi₂O₃ on the selective catalytic reduction of NO by propylene over Co₃O₄ nanoplates were expected to be elucidated.

2 Experimental

2.1 Preparation of Co₃O₄ and Bi₂O₃/Co₃O₄

The Co₃O₄ support was synthesized via the solvent-thermal method, 50 mmol CoCl₂ solution (200 mmol L⁻¹, Sinopharm Chemical Reagent Co. China) and 25 mL of NaOH (2 mol L⁻¹, Sinopharm Chemical Reagent Co. China) were added to a round-bottom flask, ultrasonicated for about 20 minutes to obtain a light brown uniform suspension. And then the suspension was transferred into a stainless steel autoclave with Teflon liner. The autoclave was sealed and maintained at 120 °C for 12 h. The obtained product was collected after washing with deionized water for several times, finally calcined at 500 °C for 5 h in air (S1).

The Bi₂O₃/Co₃O₄ catalysts with different Bi₂O₃ contents were synthesized by impregnation method as followed: 0.0155 g Bi(NO₃)₃·5H₂O and 20 mL 3% NH₃·H₂O were added into a round-bottom flask, ultrasonicated for about 20 minutes to obtain a white uniform suspension. After that, the Co₃O₄ support (S1) were added in the suspension, ultrasonicated for about 15 minutes to make it homogeneously distributed in the suspension. The suspension was then dried at 80 °C with continuous stirring for 1 h, further heated at 120 °C for 12 h followed by calcination at 500 °C in air for 4 h, yielding the 5.0 wt% Bi₂O₃/Co₃O₄ catalyst (S2). 10.0 wt% Bi₂O₃/Co₃O₄ (S3) and 15.0 wt% Bi₂O₃/Co₃O₄ (S4) were prepared with 0.0310 g and 0.0465 g Bi(NO₃)₃·5H₂O respectively. In addition, the physical mixture of 10% Bi₂O₃ nanoparticles and Co₃O₄ support was also prepared and labeled as S5. As references, classical catalysts 4% Ag/Al₂O₃ and 2% Pt/Al₂O₃ were prepared to compare the catalytic performance of the Bi₂O₃/Co₃O₄ catalysts.

2.2 Catalytic activity tests

C₃H₆-SCR over the catalysts was carried out at atmospheric pressure in a fixed-bed quartz reactor (diameter = 10 mm). 0.1 g catalyst was used in each run with a reaction mixture composed of 200 ppm NO, 200 ppm C₃H₆, 100 ppm SO₂ (when needed) and 10 vol% O₂ in balance gas N₂. The total flow rate was 200 mL min⁻¹, corresponding to a GHSV 30 000 h⁻¹. Reaction temperature ranges from 100 to 500 °C. The concentration of NO was continuously measured by a NO analyzer (Thermo Environmental Instruments Inc., model 42c), which monitors NO, NO₂, and NO_x (NO_x represents NO + NO₂). The removal efficiency of NO was calculated as NO removal (%) = (1 − C/C₀) × 100%, where C and C₀ are concentrations of NO in the outlet and inlet, respectively.

2.3 Catalysts characterization

Scanning electron microscopy (SEM) images were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. X-ray powder diffraction (XRD) was carried out on Brukeraxs D8 Discover (Cu Kα = 1.5406 Å). The scanning rate is 1° min⁻¹ in the 2θ range from 20 to 80 degree. The reducibility of Bi₂O₃/Co₃O₄ catalysts was estimated by temperature programmed reduction with hydrogen analysis (H₂-TPR). The experiments were carried out with a Micromeritics 2910 apparatus using H₂/Ar (3/97, v/v) gas with a total flow rate of 15 mL min⁻¹. In each run, 0.030 g of the catalyst was previously activated at 500 °C for 30 min under air, and then cooled to RT. TPR started with the introduction of the mixture of H₂ and Ar. The catalyst was heated from room temperature (RT) to 1000 °C (10 °C min⁻¹). H₂ consumption was continuously monitored with the thermal conductivity detector. X-ray photoelectron spectra (XPS) of the catalysts were measured in a VG Multilab 2000 spectrometer by using Al Kα (1486.6 eV) radiation as the X-ray source. Photoluminescence (PL) measurement was carried out on a Shimadzu RF-5301 PC fluorescence spectrophotometer. Raman spectra were recorded using a Horiba Jobin-Yvon Lab Ram HR800 Raman microspectrometer, with an excitation laser at 514 nm.

3 Results

3.1 Scanning electron microscope (SEM) observation and XRD analysis

Co₃O₄ nanoplates with different dimensions were observed on the support Co₃O₄ (S1), shown in Fig. 1(a). Fig. 1(b–d) presented Bi₂O₃/Co₃O₄ catalysts with different Bi₂O₃ loading amounts. It was shown that Bi₂O₃ nanoparticles with dimension ca. 20 nm were supported on Co₃O₄. And the crystal size of Bi₂O₃ slightly increased with increasing Bi₂O₃ loading amount from 5% to 15%.

Fig. 1 SEM images of Co₃O₄ (a), 5.0% Bi₂O₃/Co₃O₄ (b), 10.0% Bi₂O₃/Co₃O₄ (c) and 15.0% Bi₂O₃/Co₃O₄ (d).

Fig. 2(A) shows the XRD patterns of the synthesized Co₃O₄ and Bi₂O₃/Co₃O₄ catalysts. It can be seen that the Co₃O₄ support possessed the characteristic peaks of Co₃O₄ (JCPDS 73-1701, a = 5.45 Å). Besides the characteristic peaks of Co₃O₄, the diffraction peaks due to Bi₂O₃ (JCPDS 22-515, a = 10.94 Å and c = 11.28 Å) were also observed on Bi₂O₃/Co₃O₄ catalysts which became sharper with the increase of Bi₂O₃ loading amount. It was suggested that the crystal size of Bi₂O₃ was bigger on the catalyst with higher loading amount of Bi₂O₃, which was consistent with the SEM observations. Moreover, the diffraction peaks of Co₃O₄ over Bi₂O₃/Co₃O₄ catalysts behaved a slight shift towards lower degree in comparison with that of Co₃O₄, shown in Fig. 2(B). It indicated that the Bi³⁺ inserted the lattice of Co₃O₄ in the preparation process, and changed the lattice parameter of Co₃O₄ due to the different radius of Bi and Co atoms. At the same time, the characteristic peak of Co₃O₄ became weak obviously after the deposition of Bi₂O₃, which revealed the reduction of crystal size. It was implied that the insertion of Bi₂O₃ induced the structure defect of Co₃O₄ and suppressed the growth of crystal.

Fig. 2 XRD patterns of Co₃O₄ (S1), 5.0% Bi₂O₃/Co₃O₄ (S2), 10.0% Bi₂O₃/Co₃O₄ (S3) and 15.0% Bi₂O₃/Co₃O₄ (S4) in the wide (A) and narrow (B) ranges.

3.2 Temperature-programmed reduction by hydrogen (H₂-TPR)

The H₂-TPR profiles of Co₃O₄, Bi₂O₃ and Bi₂O₃/Co₃O₄ catalysts were shown in Fig. 3. For the reduction of Bi₂O₃, a sharp peak was observed at ca. 490 °C, implying the reduction of Bi³⁺ in a narrow temperature range. A broad reduction peak from 330 to 460 °C with a large shoulder at the lower reduction temperature (ca. 380 °C) appeared on Co₃O₄ support (S1). Many researchers reported that the reduction of Co₃O₄ was a two-step reduction process involving the intermediate reduction of CoO.^16,21,27 Two main clear reduction peaks respectively located around 186 °C and 310–480 °C were shown in the TPR spectra. The low temperature TPR peak was associated with the reduction of Co³⁺ to Co²⁺, and the peak at high temperature was the subsequent reduction of CoO to metallic cobalt. In the TPR spectrum of Co₃O₄ synthesized in the present work, there are no obvious two peaks probably due to an abroad particle size distribution as shown in SEM observation. The large shoulder at 376 °C (peak I) in Fig. 3 should be attributed to the reduction of Co³⁺ to Co²⁺, and main reduction peak (peak II) is ascribed to the reduction of CoO to metallic cobalt.

Fig. 3 H₂-TPR profiles of pure Co₃O₄ (S1), Bi₂O₃, 5.0% Bi₂O₃/Co₃O₄ (S2), 10.0% Bi₂O₃/Co₃O₄ (S3) and 15.0% Bi₂O₃/Co₃O₄ (S4).

The reduction process of Bi₂O₃/Co₃O₄ catalysts (S2–S4) became complicated with the introduction of Bi₂O₃. The reduction peak at 487 °C became wider and shifted towards to the lower temperature, especially on the Bi₂O₃/Co₃O₄ catalyst with the highest Bi loading amount (S4). In addition, two new peaks around 342 °C (peak I) and 402 °C (peak II) respectively ascribed to the reduction of Co³⁺ to Co²⁺, Co²⁺ to metallic Co appeared on Bi₂O₃/Co₃O₄ catalysts (S2–S4).²⁸ Compared with the bulk Co₃O₄ (S1), both the reduction peak of Co³⁺ and that of Co²⁺ shifted to lower temperature after the deposition of Bi₂O₃, implying the promoted reduction of Co₃O₄. Moreover, the larger reduction peak I than peak II on S2–S4 samples indicated that the ratio of Co³⁺/Co²⁺ was higher on S2–S4 samples than that on the Co₃O₄ (S1). It revealed that the deposition of Bi₂O₃ on Co₃O₄ affected the oxidized state of cobalt in the synthesized Co₃O₄, more Co³⁺ were present on the supported samples (S2–S4) than the pure Co₃O₄.

3.3 X-ray photoelectron spectroscopy (XPS)

XPS measurements were carried out on Co₃O₄ and 10.0% Bi₂O₃/Co₃O₄ catalysts to examine the influence of Bi₂O₃ on the surface electronic state of Co₃O₄. The Co 2p and O 1s XPS profiles are shown in Fig. 4. In the Co 2p (Fig. 7(a)), the main peaks located at 779.6–781.3 eV and 794.8–796.5 eV are ascribed to Co 2p_1/2 and Co 2p_3/2 spin-orbital peaks, respectively.^29,30 It was well-known that the spin–orbit splitting value for Co³⁺ compounds is 15.0 eV, 15.1–15.3 eV for the mixed-valence Co₃O₄. Here, the spin–orbit splitting values of Co 2p for CO₃O₄ and 10.0% Bi₂O₃/Co₃O₄ are the same, 15.2 eV, which is close to that of mixed-valence Co₃O₄. So the cobalt species on both S1 and S3 should be Co₃O₄, agreeable with the XRD and TPR results.

Fig. 4 XPS study of Co₃O₄ (S1) and 10.0% Bi₂O₃/Co₃O₄ (S3): (a) Co 2p spectra, (b) O 1s spectra.

Fig. 5 Room-temperature PL spectra of Co₃O₄ (S1), 5.0% Bi₂O₃/Co₃O₄ (S2), 10.0% Bi₂O₃/Co₃O₄ (S3) and 15.0% Bi₂O₃/Co₃O₄ (S4).

Fig. 6 Raman spectra of Co₃O₄ (S1) and 10.0% Bi₂O₃/Co₃O₄ (S3).

Fig. 7 NO conversions over the catalysts: S1: Co₃O₄; S2: 5.0% Bi₂O₃/Co₃O₄; S3: 10.0% Bi₂O₃/Co₃O₄; S4: 15.0% Bi₂O₃/Co₃O₄; S5: the physical mixture of Bi₂O₃ and Co₃O₄.

Based on the restriction that Co 2p_3/2 binding energies of Co²⁺ and Co³⁺ components are 781.3 eV and 779.6 eV, respectively, the spin–orbit doublet splitting is 15.2 eV with a fixed ratio of 2/1 for the 2p_3/2-to-2p_1/2 peak area,²⁹ the Co 2p spectra of S1 and S3 can be fitted to the Co²⁺ (peak II and IV) and Co³⁺ (peak I and III) components.^30,31 The satellite peak of Co²⁺ and that of Co³⁺ in Co₃O₄ were also respectively observed at 787.0 eV and 791.0 eV.^32,33Fig. 4 shows that the peak areas of peak I and III increased with the addition of Bi₂O₃, implying that the surface Co³⁺ and the surface content ratio of Co³⁺/Co²⁺ increased with the addition of Bi₂O₃. More Co³⁺ was present on 10.0% Bi₂O₃/Co₃O₄ (S3) than the bulk Co₃O₄ support (S1), as consistent with the TPR results.

The O 1s XPS spectra of Co₃O₄ and Bi₂O₃/Co₃O₄ catalysts are shown in Fig. 4(b). For the Co₃O₄ sample, there are two peaks (I and II). The peak I located at ∼530.1 eV is attributed to the surface lattice of Co₃O₄, and the peak II at ∼531.5 eV is associated with OH⁻ groups.³⁴ In the case of 10.0% Bi₂O₃/Co₃O₄, besides the peak I and II, one new peak at ∼532.8 eV appeared, which should be related to the contribution of the oxygen from Bi₂O₃.

3.4 Photoluminescence (PL) and Raman spectra

PL emission spectra originating from the recombination of free charge carriers are useful to reveal the migration, transfer and separation of photogenerated charge carriers. Fig. 5 shows photoluminescence emission spectra of different catalysts at room temperature. All samples show one luminescence peak center at about 358 nm, which can be attributed to the radiative recombination of charge carriers. The pure Co₃O₄ has the strongest PL emission peak. This charge recombination process of Co₃O₄ can be greatly inhibited by the deposition of Bi₂O₃ on Co₃O₄. 10.0% Bi₂O₃/Co₃O₄ (S3) has the lowest PL emission peak, which is associated with its structural imperfection. The structural imperfection originating from the insertion of the Bi³⁺ into the lattice of Co₃O₄, as evidenced by XRD, increased the number of structural defects (e.g., oxygen vacancies), which could capture the electrons or holes, thus resulting in low radiative PL emission.

Fig. 6 shows the Raman spectra of 10.0% Bi₂O₃/Co₃O₄ (S3), pure Co₃O₄ (S1) and Bi₂O₃. For the pure Co₃O₄, the Raman peak at 655 cm⁻¹ was corresponded to the symmetry of Co₃O₄.³⁵ 10.0% Bi₂O₃/Co₃O₄ gave the similar Raman spectra with Co₃O₄, while the characteristic peaks of Bi₂O₃ could not be detected. Compared with Co₃O₄, the Raman peaks on 10.0% Bi₂O₃/Co₃O₄ shifted to the lower frequencies with stronger intensities, which associated with the lattice distortion or residual stress of the spinel structure. The XRD results showed that part of Bi₂O₃ entered the lattice of Co₃O₄ over 10.0% Bi₂O₃/Co₃O₄, leading to the lattice distortion and lattice defect. The highly defective structure formed on 10.0% Bi₂O₃/Co₃O₄ could accelerate the adsorption and activation of O₂, which was suggested to be related to the better catalytic performance.

3.5 Catalytic performance

Fig. 7 depicted the NO conversions over the Bi₂O₃/Co₃O₄ catalysts with different Bi₂O₃ contents (S1–S4), the physical mixture of 10% Bi₂O₃ nanoparticles and Co₃O₄ support (S5), 4% Ag/Al₂O₃ and 2% Pt/Al₂O₃ reference catalysts within the reaction temperature range of 100–500 °C. Co₃O₄ and Bi₂O₃/Co₃O₄ catalysts showed higher NO reduction activity than those of Ag/Al₂O₃ and Pt/Al₂O₃, especially Bi₂O₃/Co₃O₄ catalysts. The conversion of NO over Co₃O₄ support (S1) firstly increased with reaction temperature, reached the maximum conversion (ca. 60%) at ca. 300 °C and then decreased at higher temperature. The NO conversion was further increased with the addition of Bi₂O₃ into Co₃O₄ with the activity order: Co₃O₄ (S1) < 5.0% Bi₂O₃/Co₃O₄ (S2) < 15.0% Bi₂O₃/Co₃O₄ (S4) < 10.0% Bi₂O₃/Co₃O₄ (S3). Among all catalysts tested, 10.0% Bi₂O₃/Co₃O₄ (S3) possessed the highest activity for NO conversion in the reaction temperature window, reaching ca. 90% NO conversion at 200 °C. NO conversion under the lower reaction temperature (100–250 °C) over S3 also reached the highest among the catalysts tested. In contrast, the mixture of 10% Bi₂O₃ nanoparticles and Co₃O₄ support (S5) showed lower activity than S1 and S3. It was indicated that the interaction between Bi₂O₃ and Co₃O₄ in S3 is not the simply physical mixture like S5. The chemical interaction between them took place in S3 and should contribute the admirable catalytic performance of S3 in the C₃H₆-SCR reaction.

SO₂ usually exists in the diesel engine exhaust. So it is necessary to investigate the SO₂ tolerance of the catalyst in C₃H₆-SCR. Fig. 8 exhibited the effects of 100 ppm SO₂ co-fed in the reaction gas on the NO conversions over the catalysts at the different reaction temperatures. NO conversion over the S3 (10% Bi₂O₃/Co₃O₄) catalyst clearly did not change in the wide reaction window. The steady-state NO conversion reached 90.3% on S3 at 250 °C in the presence and absence of SO₂. In contrast, NO conversion decreased from 65.8% to 35.7% at 300 °C on the Co₃O₄ support (S1) when 100 ppm SO₂ was contained in the feed gas. NO conversions at other reaction temperatures also reduced in the presence of SO₂. These results obviously suggested that 10% Bi₂O₃/Co₃O₄ exhibited good resistibility against SO₂ that coexists with NO and C₃H₆ in the reaction mixture.

Fig. 8 SO₂ resistibilities of Co₃O₄ (S1) and 10.0% Bi₂O₃/Co₃O₄ (S3) with the reaction temperature.

Fig. 9 shows the SO₂ durability of 10% Bi₂O₃/Co₃O₄ catalyst with the reaction time at the optimum reaction temperature 200 °C in the C₃H₆-SCR of NO. When NO conversion reached to the maximum (89.3%), 100 ppm SO₂ was added in the reaction system, NO conversion immediately decreased. It was probably due to the competitive adsorption of NO and SO₂ on the active site. 20 min later, NO conversion reduced to 63.6%. After that, NO conversion recovered to 85.7%, and maintained at ca. 88% through the whole reaction period of 90 min. This result further illustrates the outstanding SO₂ resistibility of 10% Bi₂O₃/Co₃O₄ in the long time-reaction.

Fig. 9 NO conversion over 10.0% Bi₂O₃/Co₃O₄ (S3) before and after the addition of SO₂ (reaction temperature: 200 °C).

4 Discussions

In this study, the deposition of Bi₂O₃ with the proper loading amount on Co₃O₄ nanoplates enhanced NO conversion over Co₃O₄, especially at low reaction temperature (<200 °C). 10.0% Bi₂O₃/Co₃O₄ catalyst also showed the strong resistibility against SO₂ in the feed gas. XRD results showed Bi₂O₃ could enter the lattice of Co₃O₄, and promote the formation of the lattice distortion and structural defect as demonstrated by PL spectra and IR spectra. The H₂-TPR and XPS results showed that more Co³⁺ appeared with the deposition of Bi₂O₃. These changes were probably related to the promotive effects of Bi₂O₃ in the C₃H₆-SCR reaction.

Scheme 1 illustrated the mechanistic investigations for the HC-SCR reactions in the previous literatures.^17,36,37 According to these findings, the reactants (C₃H₆, NO and NO₂) are supposed to be first adsorbed on the active sites over the catalyst surface. Subsequently, the adsorbed nitrates formed via NO oxidation by O₂. C₃H₆ was also activated to form C_xH_yO_z species such as formate, acetate and so on. As these C_xH_yO_z species become available, nitrates subsequently reacted with them to yield nitrogen-containing organic species, such as NCO species. The final step would be the interaction of NOC intermediates with NO_x (NO, NO_x), decomposing into N₂, CO_x and H₂O as final products. This proposed reaction process reveals the crucial role of O₂ in the feed gas and the importance of oxidizing characteristics of the catalyst surface.

Scheme 1 The promotive effects of Bi₂O₃ on NO reduction by propene over Co₃O₄ catalyst.

In our work, PL and IR results showed that more oxygen vacancies were produced on the Co₃O₄ after the doping of Bi₂O₃. The vacancy could accelerate the adsorption, activation and diffusion of oxygen, which was suggested to be available for the oxidation reactions involved in HC-SCR. The doped Bi₂O₃ also increased the Co³⁺ concentration on the surface. The richness of Co³⁺ could promote the adsorption and activation of NO and (or) C₃H₆. What is more is the mild oxidizing characteristics of bismuth oxide in the selective oxidation and ammoxidation of propene to acrolein and acrylonitrile. It will accelerate the formation of C_xH_yO_z species. In short, the addition of Bi₂O₃ into the Co₃O₄ in the present study probably influenced the oxidation process in the C₃H₆-SCR reaction, favored the activation of C₃H₆ and NO, and then enhanced the following NCO intermediate formation and its decomposition with reaction with NO_x to N₂.

About poisoning HC-SCR catalyst with SO₂, the previous studies reported that the suppression effect of SO₂ on the SCR catalyst could attributed to the formation of sulphate on the catalyst.³⁸ The presence of surface SO₄²⁻ groups blocked the formation of nitrate and decreased the amount of adsorbed nitrates. Thus it hindered the transformation of NOC species and decreased the catalytic activity. In our work, the presence of 100 ppm SO₂ in the feed gas decreased the NO conversion over Co₃O₄ catalyst, while the SO₂ tolerance of Bi₂O₃/Co₃O₄ was strong. 10% Bi₂O₃/Co₃O₄ showed the good stability when SO₂ was co-fed in the mixture gas during 90 min. This promotive role of Bi₂O₃ on the resistibility against SO₂ also could be explained by the oxidizing properties of 10% Bi₂O₃/Co₃O₄. The addition of Bi₂O₃ into Co₃O₄ promoted the partial oxidation of propene activation of C₃H₆. The activation of C₃H₆ over 10% Bi₂O₃/Co₃O₄ seems more competitive in the compassion with the oxidization of SO₂ to form surface sulfate. Subsequently, it is suggested that the addition of Bi₂O₃ into Co₃O₄ is one appropriate method for improving the SO₂ resistance of Co₃O₄.

5 Conclusions

The optimum Bi₂O₃ loading on the Co₃O₄ nanoplates for the C₃H₆-SCR of NO was about 10%, giving the best catalytic activity, especially at low reaction temperature, as well as the strongest SO₂ tolerance. The decoration of moderate Bi₂O₃ on Co₃O₄ influenced the oxidation state of Co₃O₄, facilitate the surface oxygen mobility and the partial oxidation of propene involved in the C₃H₆-SCR reaction. Therefore, the combination of Co₃O₄ with Bi₂O₃ is more active than Co₃O₄. The addition of 100 ppm SO₂ to the feed hardly affected the catalytic performance of 10% Bi₂O₃/Co₃O₄.

Conflicts of interest

There are no conflict to declare.

Acknowledgements

This work was supported by the China National Natural Science Foundation (No. 51178360) and Hubei Superior and Distinctive Discipline Group of “Mechatronics and Automobiles” (XKQ2019058).

References

1. C. R. Thomas, J. A. Pihl, M. J. Lance, T. J. Toops, J. E. Parks and J. Lauterbach, Effects of four-mode hydrothermal aging on three-way catalysts for passive selective catalytic reduction to control emissions from lean-burn gasoline engine, Appl. Catal., B, 2019, 244, 284–294.
2. Y. Rui, M. Ming, Z. Jing, L. Zheng, T. Hu and X. Li, A noble-metal-free SCR-LNT coupled catalytic system used for high-concentration NO_x reduction under lean-burn condition, Catal. Today, 2018, 347–356.
3. J. A. Lupescu, J. W. Schwank, G. B. Fisher, J. Hangas, S. L. Peczonczyk and W. A. Paxton, Pd model catalysts: effect of air pulse length during redox aging on Pd redispersion, Appl. Catal., B, 2018, 223, 76–90.
4. Y. Hu and K. Griffiths, Selective Catalytic Reduction of NO by Hydrocarbons on a Stepped Pt Surface: Influence of SO₂ and O₂, J. Phys. Chem. C, 2015, 119, 19789–19801.
5. M. Khosravi, C. Sola, A. Abedi, R. E. Hayes, W. S. Epling and M. Votsmeier, Oxidation and selective catalytic reduction of NO by propene over Pt and Pt:Pd diesel oxidation catalysts, Appl. Catal., B, 2014, 147, 264–274.
6. P. Miquel, P. Granger, N. Jagtap, S. Umbarkar, M. Dongare and C. Dujardin, NO reduction under diesel exhaust conditions over Au/Al₂O₃ prepared by deposition-precipitation method, J. Mol. Catal. A: Chem., 2010, 322, 90–97.
7. X. Wang, N. Maeda and A. Baiker, Synergistic Effects of Au and FeO_x Nanocomposites in Catalytic NO Reduction with CO, ACS Catal., 2016, 6, 7898–7906.
8. P. A. Kumar, M. P. Reddy, B. Hyun-Sook and H. H. Phil, Influence of Mg Addition on the Catalytic Activity of Alumina Supported Ag for C₃H₆-SCR of NO, Catal. Lett., 2009, 131, 85–97.
9. A. Musi, P. Massiani, D. Brouri, J.-M. Trichard and P. Da Costa, On the Characterisation of Silver Species for SCR of NO_x with Ethanol, Catal. Lett., 2008, 128, 25–30.
10. P. Namkhang and P. Kongkachuichay, Synthesis of Copper-Based Nanostructured Catalysts on SiO₂–Al₂O₃, SiO₂–TiO₂, and SiO₂–ZrO₂ Supports for NO Reduction, J. Nanosci. Nanotechnol., 2015, 15, 5410–5417.
11. J. Liu, X. Li, Q. Zhao, D. Zhang and P. Ndokoye, The selective catalytic reduction of NO with propene over Cu-supported Ti–Ce mixed oxide catalysts: promotional effect of ceria, J. Mol. Catal. A: Chem., 2013, 378, 115–123.
12. Z. Liu, J. Li and J. Hao, Selective catalytic reduction of NO_x with propene over SnO₂/Al₂O₃ catalyst, Chem. Eng. J., 2010, 165, 420–425.
13. M. Chen, J. Yang, Y. Liu, W. Li, J. Fan, X. Ran, W. Teng, Y. Sun, W.-x. Zhang, G. Li, S. X. Dou and D. Zhao, TiO₂ interpenetrating networks decorated with SnO₂ nanocrystals: enhanced activity of selective catalytic reduction of NO with NH₃, J. Mater. Chem. A, 2015, 3, 1405–1409.
14. L. Zhang, L. Li, Y. Cao, Y. Xiong, S. Wu, J. Sun, C. Tang, F. Gao and L. Dong, Promotional effect of doping SnO₂ into TiO₂ over a CeO₂/TiO₂ catalyst for selective catalytic reduction of NO by NH₃, Catal. Sci. Technol., 2015, 5, 2188–2196.
15. D. Pietrogiacomi, M. C. Campa, L. R. Carbone, S. Tuti and M. Occhiuzzi, N₂O decomposition on CoO_x, CuO_x, FeO_x or MnO_x supported on ZrO₂: the effect of zirconia doping with sulfates or K⁺ on catalytic activity, Appl. Catal., B, 2016, 187, 218–227.
16. Y. Yu, Q. Zhong, W. Cai and J. Ding, Promotional effect of N-doped CeO₂ supported CoO_x catalysts with enhanced catalytic activity on NO oxidation, J. Mol. Catal. A: Chem., 2015, 398, 344–352.
17. F. Huang, W. Hu, J. Chen, Y. Wu, P. Qu, S. Yuan, L. Zhong and Y. Chen, Insight into Enhancement of NO Reduction with Methane by Multifunctional Catalysis over a Mixture of Ce/HZSM-5 and CoO_x in Excess of Oxygen, Ind. Eng. Chem. Res., 2018, 57, 13312–13317.
18. L. Zhang, X. Yao, Y. Lu, C. Sun, C. Tang, F. Gao and L. Dong, Effect of precursors on the structure and activity of CuO-CoO_x/gamma-Al₂O₃ catalysts for NO reduction by CO, J. Colloid Interface Sci., 2018, 509, 334–345.
19. B. Dou, G. Lv, C. Wang, Q. Hao and K. Hui, Cerium doped copper/ZSM-5 catalysts used for the selective catalytic reduction of nitrogen oxide with ammonia, Chem. Eng. J., 2015, 270, 549–556.
20. M. Sakmeche, A. Belhakem, R. Kessas and S. A. Ghomari, Effect of parameters on NO reduction by methane in presence of excess O₂ and functionalized AlMCM-41 as catalysts, J. Taiwan Inst. Chem. Eng., 2017, 80, 333–341.
21. F. Bin, C. Song, G. Lv, J. Song, X. Cao, H. Pang and K. Wang, Structural Characterization and Selective Catalytic Reduction of Nitrogen Oxides with Ammonia: A Comparison between Co/ZSM-5 and Co/SBA-15, J. Phys. Chem. C, 2012, 116, 26262–26274.
22. D. Pietrogiacomi, A. Magliano, P. Ciambelli, D. Sannino, M. C. Campa and V. Indovina, The effect of sulphation on the catalytic activity of CoO_x/ZrO₂ for NO reduction with NH₃ in the presence of O₂, Appl. Catal., B, 2009, 89, 33–40.
23. A. Cabot, A. Marsal, J. Arbiol and J. R. Morante, Bi₂O₃ as a selective sensing material for NO detection, Sens. Actuators, B, 2004, 99, 74–89.
24. J. F. Brazdil, M. A. Toft, S. S. Y. Lin, S. T. McKenna, G. Zajac, J. A. Kaduk and J. T. Golab, Characterization of bismuth–cerium-molybdate selective propylene ammoxidation catalysts, Appl. Catal., A, 2015, 495, 115–123.
25. Y.-H. Lei and Z.-X. Chen, Theoretical study of propene oxidation on Bi₂O₃ surfaces, Sci. China: Chem., 2015, 58, 593–600.
26. R. B. Licht and A. T. Bell, A DFT Investigation of the Mechanism of Propene Ammoxidation over α-Bismuth Molybdate, ACS Catal., 2016, 7, 161–176.
27. S. Kumar Megarajan, S. Rayalu, Y. Teraoka and N. Labhsetwar, High NO oxidation catalytic activity on non-noble metal based cobalt-ceria catalyst for diesel soot oxidation, J. Mol. Catal. A: Chem., 2014, 385, 112–118.
28. J.-W. Shi, G. Gao, Z. Fan, C. Gao, B. Wang, Y. Wang, Z. Li, C. He and C. Niu, Ni_yCo_1−yMn₂O_x microspheres for the selective catalytic reduction of NO_x with NH₃: the synergetic effects between Ni and Co for improving low-temperature catalytic performance, Appl. Catal., A, 2018, 560, 1–11.
29. D. Wang, Q. Wang and T. Wang, Morphology-controllable synthesis of cobalt oxalates and their conversion to mesoporous Co₃O₄ nanostructures for application in supercapacitors, Inorg. Chem., 2011, 50, 6482–6492.
30. J.-W. Shi, Z. Fan, C. Gao, G. Gao, B. Wang, Y. Wang, C. He and C. Niu, Mn–Co Mixed Oxide Nanosheets Vertically Anchored on H₂Ti₃O₇ Nanowires: Full Exposure of Active Components Results in Significantly Enhanced Catalytic Performance, ChemCatChem, 2018, 10, 2833–2844.
31. Z. Zhu, G. Lu, Z. Zhang, Y. Guo, Y. Guo and Y. Wang, Highly Active and Stable Co₃O₄/ZSM-5 Catalyst for Propane Oxidation: Effect of the Preparation Method, ACS Catal., 2013, 3, 1154–1164.
32. H. Hu, S. Cai, H. Li, L. Huang, L. Shi and D. Zhang, In Situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism over Mn-Doped Co₃O₄ for the Selective Catalytic Reduction of NO_x with NH₃, J. Phys. Chem. C, 2015, 119, 22924–22933.
33. G. Gao, J.-W. Shi, Z. Fan, C. Gao and C. Niu, MnM₂O₄ microspheres (M = Co, Cu, Ni) for selective catalytic reduction of NO with NH₃: comparative study on catalytic activity and reaction mechanism via in situ diffuse reflectance infrared Fourier transform spectroscopy, Chem. Eng. J., 2017, 325, 91–100.
34. L. Wang, S. Zhang, Y. Zhu, A. Patlolla, J. Shan, H. Yoshida, S. Takeda, A. I. Frenkel and F. Tao, Catalysis and In Situ Studies of Rh₁/Co₃O₄ Nanorods in Reduction of NO with H₂, ACS Catal., 2013, 3, 1011–1019.
35. Y. Lou, L. Wang, Z. Zhao, Y. Zhang, Z. Zhang, G. Lu, Y. Guo and Y. Guo, Low-temperature CO oxidation over Co₃O₄-based catalysts: significant promoting effect of Bi₂O₃ on Co₃O₄ catalyst, Appl. Catal., B, 2014, 146, 43–49.
36. X. Chen, X. Yang, A. Zhu, C. T. Au and C. Shi, In situ DRIFTS study during C₂H₄-SCR of NO over Co-ZSM-5, J. Mol. Catal. A: Chem., 2009, 312, 31–39.
37. M. C. Campa, D. Pietrogiacomi, C. Scarfiello, L. R. Carbone and M. Occhiuzzi, CoO_x and FeO_x supported on ZrO₂ for the simultaneous abatement of NO_x and N₂O with C₃H₆ in the presence of O₂, Appl. Catal., B, 2019, 240, 367–372.
38. X. Xiao, S. Xiong, Y. Shi, W. Shan and S. Yang, Effect of H₂O and SO₂ on the Selective Catalytic Reduction of NO with NH₃ Over Ce/TiO₂ Catalyst: Mechanism and Kinetic Study, J. Phys. Chem. C, 2016, 120, 1066–1076.

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