A CeO₂/ZrO₂-TiO₂ Catalyst for the Selective Catalytic Reduction of NO_x with NH₃

Abstract

In this study, CeZr_0.5Ti_aO_x (with a = 0, 1, 2, 5, 10) catalysts were prepared by a stepwise precipitation approach for the selective catalytic reduction of NO_x with NH₃. When Ti was added, all of the Ce-Zr-Ti oxide catalysts showed much better catalytic performances than the CeZr_0.5O_x. Particularly, the CeZr_0.5Ti₂O_x catalyst showed excellent activity for broad temperature range under high space velocity condition. Through the control of pH value and precipitation time during preparation, the function of the CeZr_0.5Ti₂O_x catalyst could be controlled and the structure with highly dispersed CeO₂ (with redox functions) on the surface of ZrO₂-TiO₂ (with acidic functions) could be obtained. Characterizations revealed that the superior catalytic performance of the catalyst is associated with its outstanding redox properties and adsorption/activation functions for the reactants.

1. Introduction

NO_x (mainly NO and NO₂) in the atmosphere plays critical roles in the formation of severe air pollution problems, such as haze, acid rain, and photochemical smog. In the last few decades, great efforts have been devoted to the development of NO_x emission control technologies [1,2,3]. Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) has been widely applied for the removal of NO_x generated from stationary sources for many years, and it has also been used for the control of NO_x emission from diesel vehicles [2,4].

Catalysts play an important role in the development of NH₃-SCR technology [5,6]. Vanadium-based catalyst (especially V₂O₅-WO₃/TiO₂), with excellent SO₂ resistance, is the most widely used NH₃-SCR catalyst for NO_x emission control from power plants, and it was also applied on diesel vehicles as the first generation of SCR catalyst [4]. However, this catalyst system still has some problems, including the toxicity of active V₂O₅, narrow temperature window, and low thermal stability [2].

There has been strong interest in developing a vanadium-free catalyst that can be used on diesel vehicles [5,6,7,8,9,10,11]. Ce is a key component in three-way catalysts for emission control in automobiles for gasoline. CeO₂ provides an oxygen storage function through redox cycling between Ce³⁺ and Ce⁴⁺. In recent years, Ce has also attracted great attention for applications as a support [12,13], promoter [14,15,16,17,18], or main active component [19,20,21,22,23,24,25,26] for NH₃-SCR catalysts.

Pure Ce oxide is not suitable for use as an NH₃-SCR catalyst [27,28]. When Zr oxide was introduced into Ce oxide, the thermal stability and the oxygen storage capacity of the oxide could be significantly improved. Therefore, Ce-Zr oxide was investigated for NH₃-SCR [12,13,29,30,31,32,33,34]. In the NH₃-SCR reaction, both redox functions and acidic functions of the catalyst are needed [4,35]. Therefore, a high dispersion of active sites and close coupling of redox with acid sites is the way to design a highly efficient NH₃-SCR catalyst.

In this study, starting from a preparation of Ce-Zr oxide by the co-preparation method, we developed a Ce-Zr-Ti oxide catalyst using a stepwise precipitation approach, under the theoretical guidance of the close combination of the Ce-Zr oxide with strong redox functions and Ti oxide with excellent acid properties [4,5]. This obtained catalyst showed superior catalytic performance for NH₃-SCR.

2. Results and Discussion

2.1. NH₃-SCR Activity

Figure 1A presents the NO_x conversion over the catalysts with different Ti contents under a relatively high gas hourly space velocity (GHSV) of 200,000 h⁻¹. The CeZr_0.5O_x just exhibited over 50% NO_x conversion in a narrow temperature range of 350–425 °C. When Ti was introduced, all of the Ce-Zr-Ti oxide catalysts exhibited much better activities. With the increase in Ti content, the low temperature firstly increased and then decreased. As a result, the CeZr_0.5Ti₂O_x catalyst presented the best activity in a low temperature range, together with a high NO_x conversion in a wide temperature range. On the other hand, the variation in high temperature activity with Ti content was contrary to that of low temperature activity, with the activity of CeZr_0.5Ti₂O_x slightly lower than those of the other Ce-Zr-Ti oxide catalysts in a high temperature range. In addition, adding Ti to the catalyst also enhanced the N₂ selectivity, and the Ce-Zr-Ti oxide catalysts all presented higher N₂ selectivity than CeZr_0.5O_x (Figure 1B).

The influences of H₂O and space velocity on the NO_x conversion over CeZr_0.5Ti₂O_x were tested and the results are shown in Figure 2. The existence of 5% H₂O in the flow gas decreased the low temperature activity, but enhanced the high temperature activity. As a result, over 80% NO_x conversion could still be achieved from 250 to 450 °C. When the GHSV was decreased from 200,000 h⁻¹ to 100,000 h⁻¹, the activity of the catalyst at low temperatures was obviously improved.

2.2. Separated NO/NH₃ Oxidation

To analyze the effects of Ti on the catalyst, separated NO oxidation and NH₃ oxidation tests were carried out for the CeZr_0.5O_x and CeZr_0.5Ti₂O_x (Figure 3). The NO₂ production during NO oxidation over the CeZr_0.5Ti₂O_x was clearly higher than that over CeZr_0.5O_x at a low temperature. Since the presentation of NO₂ in the reaction gas could promote the SCR reaction at a low temperature by accelerating the fast SCR process (2 NH₃ + NO + NO₂ → 2N₂ + 3H₂O), the enhanced low-temperature activity by the introduction of Ti should be associated with the promoted oxidation of NO to NO₂ over CeZr_0.5Ti₂O_x [10,35]. In addition, the introduction of Ti also promoted NH₃ oxidation over the catalyst at a high temperature. The NH₃-SCR reaction route at a high temperature mainly follows the Eley-Rideal mechanism, and the activation of NH₃ to form NH₂ species by oxidation plays the key role for the reaction with NO to form N₂ and H₂O, owing to NH₂ + NO(g) → N₂ + H₂O. Therefore, promoted NH₃ oxidation would be beneficial for the improvement of high temperature activity.

2.3. XRD

The X-ray diffraction (XRD) results of the CeZr_0.5O_x and Ce-Z-Ti oxide catalysts are presented in Figure 4. Both CeO₂ and ZrO₂ were detected in CeZr_0.5O_x. With the increase of Ti, the peaks for CeO₂ and ZrO₂ became more and more weak, and only anatase TiO₂ was observed for CeZr_0.5Ti₁₀O_x. Only weak peaks for CeO₂ with cubic fluorite structures (PDF# 43-1002) were observed in the CeZr_0.5Ti₂O_x, indicating that the introduction of Ti had induced the structural change of the CeZr_0.5O_x, and the crystallizations of Ce, Zr and Ti oxides in CeZr_0.5Ti₂O_x were significantly inhibited. As a result, the CeZr_0.5Ti₂O_x (165.1 m²/g) showed a higher Brunauer–Emmett–Teller (BET) surface area than CeZr_0.5O_x (113.5 m²/g).

2.4. H₂-TPR

The H₂ temperature-programmed reduction (H₂-TPR) profiles of CeZr_0.5O_x and CeZr_0.5Ti₂O_x are presented in Figure 5. The CeZr_0.5O_x exhibited two peaks at 496 and 755 °C due to the surface and bulk reductions of CeO₂ (as detected by XRD), respectively [31,36,37,38]. During the test, coordinatively unsaturated surface oxygen anions are easily reduced by H₂ in the low temperature region, while the bulk oxygen species are reduced only after the transportation to the surface [39]. With the addition of Ti, a sharp H₂ consumption peak appeared at 567 °C, which indicates that another type of Ce species might be formed. Considering the XRD results, this sharp peak might be associated with the reduction of the highly dispersed Ce species from Ce⁴⁺ to Ce³⁺ [22,34]. In addition, the H₂ consumption of CeZr_0.5Ti₂O_x was much higher than that of CeZr_0.5O_x at a low temperature. The H₂-TPR results clearly indicated the enhancement of redox functions for CeZr_0.5Ti₂O_x.

Previous studies have indicated that the redox properties of NH₃-SCR catalyst play a dominant role in the low temperature activity [35,40,41]. Therefore, the enhanced redox function of CeZr_0.5Ti₂O_x would beneficial for low temperature activity.

2.5. NO_x/NH₃-TPD

To investigate the NO_x and NH₃ adsorption/desorption properties of CeZr_0.5O_x and CeZr_0.5Ti₂O_x, NO_x temperature-programmed desorption (NO_x-TPD) and NH₃ temperature-programmed desorption (NH₃-TPD) were performed for the catalysts (Figure 6).

The NO_x-TPD profiles are presented in Figure 6A. The first NO_x peak of CeZr_0.5Ti₂O_x was at ca. 110 °C, mainly due to the desorption of physisorbed NO_x, while the other NO_x peak was at ca. 300 °C and was associated with the decomposition of chemsorbed NO_x species [42,43]. On the other hand, two weak peaks were observed for CeZr_0.5O_x at ca. 270 °C and ca. 410 °C, respectively, which were due to the decomposition of different types of chemsorbed NO_x species. With the addition of Ti, the adsorbed NO_x on CeZr_0.5Ti₂O_x was obviously more than that of CeZr_0.5O_x. Particularly, the desorbed NO₂ of CeZr_0.5Ti₂O_x was much higher, owing to the enhanced low-temperature activity for NO oxidation (as shown by the separated NO oxidation results), which could facilitate the conversion of NO_x in NH₃-SCR.

Surface acidity plays a dominate role in the high-temperature SCR activity due to its effects on the adsorption and activation of NH₃ [35,41]. Previous studies have revealed that Ti species of NH₃-SCR catalysts mainly act as acid sites in the reaction for NH₃ adsorption [4]. Therefore, the adsorbed NH₃ of CeZr_0.5Ti₂O_x was much more than that of CeZr_0.5O_x, which might be an important reason for the better NH₃-SCR activity of CeZr_0.5Ti₂O_x at high temperatures.

2.6. XPS

The X-ray photoelectron spectroscopy (XPS) results for Ce 3d of the CeZr_0.5O_x and CeZr_0.5Ti₂O_x are shown in Figure 7. The sub-bands labeled with u’/v’ and u⁰/v⁰ represent the 3d¹⁰4f¹ initial electronic state corresponding to Ce³⁺ and the 3d⁹4f² state of Ce³⁺, respectively [44]. The sub-bands labeled with u’’’ and v’’’ represent the 3d¹⁰4f⁰ state of Ce⁴⁺, and the sub-bands labeled with u, u’’, v and v’’ represent the 3d⁹4f¹ state corresponding to Ce⁴⁺ [44]. The presence of Ce³⁺ would induce a charge imbalance, which could lead to unsaturated chemical bonds and oxygen vacancies. The calculated Ce³⁺ ratio of CeZr_0.5Ti₂O_x (36.0%) was higher than that of CeZr_0.5O_x (33.8%), indicating that more surface oxygen vacancies presented in CeZr_0.5Ti₂O_x. In addition, the Ce³⁺ ratio of the catalyst could influence the redox ability and reactant adsorption and activation functions, and thereby contribute to NH₃-SCR performance.

The surface oxygen vacancies of the catalysts might generate weakly-adsorbed oxygen species or additional chemisorbed oxygen on the surface of the catalyst [27,45]. The XPS results of O 1s of the CeZr_0.5O_x and CeZr_0.5Ti₂O_x are shown in Figure 8. The O 1s peak was fit into two sub-bands. The sub-bands at 531.2–531.5 eV and 529.1–529.6 eV were assigned to the surface adsorbed oxygen (O_α), such as the O₂²⁻ and O⁻ belonging to defect-oxide or a hydroxyl-like group, and the lattice oxygen O²⁻ (O_β), respectively [46]. The O_α ratios of the catalysts were calculated by O_α/(O_α + O_β), and the CeZr_0.5Ti₂O_x showed higher O_α ratio than CeZr_0.5O_x. The results confirmed that the addition of Ti indeed induced more surface-adsorbed oxygen, which would facilitate NO oxidation to NO₂ (as shown by the separated NO oxidation and NO_x-TPD results), and thus facilitates the conversion of NO by fast SCR effects.

2.7. Formation Process Analysis of the CeZr_0.5Ti₂O_x Catalyst

Figure 9 shows the pH variations of the mixed solutions for the preparation of the CeZr_0.5O_x and CeZr_0.5Ti₂O_x catalysts. During the preparation of CeZr_0.5O_x, the initial pH value of the solution was 1.6. With the hydrolysis of urea, the pH increased gradually to be 7.6 after heating for 12 h. Due to the increase in pH, suspended particles began to appear in the solution in the second hour. The particles with the precipitation time of 2 h, 4 h, 6 h, and 12 h were collected and then calcined to be catalyst samples. The activity tests of these samples showed similar NO_x conversions with each other.

Due to the acidity induced by the added Ti(SO₄)₂, the initial pH value of the mixed solution during the preparation of CeZr_0.5Ti₂O_x dropped to be 1.1. With the hydrolysis of urea, the pH increased gradually after heating, and some white particles generated in the first hour and suspended in the solution. With the increase of time, the particles gradually turned yellow. The pH reached ca. 7.0 after 12 h of reaction. The particles with the precipitation times of 1 h, 4 h, 6 h, and 12 h were collected and then calcined to be catalyst samples. Interestingly, the activity test showed a remarkable enhancement of NO_x conversions for the four samples with the increase in precipitation time.

The surface metal atomic concentrations of the CeZr_0.5Ti₂O_x samples with different precipitation times were analyzed using XPS, and the variations in Ce, Zr, and Ti concentrations with precipitation time are shown in Figure 10. For the 1-h precipitation sample, only Ti and Zr, without Ce, were detected. With the increase in precipitation time, surface Ce concentration increased gradually in the samples. At the same time, Ti and Zr concentrations gradually decreased with the increase in precipitation time. A TEM-EDS mapping image showed that Ce was highly dispersed in the CeZr_0.5Ti₂O_x catalyst (Figure 11).

Considering the variations in the solution pH value when preparing the CeZr_0.5Ti₂O_x, the formation process of the catalyst can be proposed as follows: The Ti and Zr species were first co-precipitated with the increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti species with the further increase in pH. Finally, a CeZr_0.5Ti₂O_x catalyst with a higher surface Ce concentration than Ti and Zr was obtained. Through control of the hydrolysis of urea, the variations in the solution pH can be controlled, and then we can control the precipitation process of the catalyst, which is very important for the formation of highly-dispersed CeO₂ on ZrO₂-TiO₂. Thus, the obtained catalyst can present excellent NH₃-SCR performance.

3. Experimental Section

3.1. Catalyst Preparation and Activity Test

The CeZr_0.5Ti_aO_x (a = Ti/Ce molar ratio = 0, 1, 2, 5, 10), with a Zr/Ce molar ratio fixed to be 0.5, was prepared using a precipitation method. Desired precursors of Ce(NO₃)₃·6H₂O (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Zr(NO₃)₄·5H₂O (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Ti(SO₄)₂ (>98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were dissolved together in distilled water, and urea (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added to the mixed solution as a slowly-releasing precipitator. Then, the solution was heated to 90 °C to facilitate the release of NH₃ and thereby raise the pH value gradually. The temperature of the mixed solution was held at 90 °C for 12 h under vigorous stirring (some samples with shorter precipitation times were also prepared). After that, the precipitated powders were collected via filtration, washed using distilled water, and dried for 12 h at 100 °C. Finally, the catalyst was obtained after calcination at 500 °C for 5 h.

The SCR activity of the catalysts (40–60 mesh) were tested in a fixed-bed quartz flow reactor. The reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH₃, 5 vol.% O₂, N₂ balance, and 400 mL/min total flow rate. Different gas hourly space velocities (GHSVs) were obtained by changing the volume of catalysts, i.e., 0.24 mL catalyst for a GHSV = 100,000 h⁻¹ and 0.12 mL catalyst for a GHSV = 200,000 h⁻¹. The concentrations of effluent N-containing gases (NO, NH₃, NO₂ and N₂O) were continuously measured by an online FTIR gas analyzer (Nicolet Antaris IGS analyzer, Thermo-Fisher Scientific, Waltham, MA, USA). NO_x conversion and N₂ selectivity were calculated using the following equations, respectively:

$${\mathrm{NO}}_x \mathrm{conversion}=(1-\frac{{\left[\mathrm{NO}\right]}_{out}+{\left[{\mathrm{NO}}_2\right]}_{out}}{{\left[\mathrm{NO}\right]}_{in}+{\left[{\mathrm{NO}}_2\right]}_{in}})\times 100\%$$

$${\mathrm{N}}_2 \mathrm{selectivity}=(1-\frac{2{\left[{\mathrm{N}}_2\mathrm{O}\right]}_{out}}{{\left[{\mathrm{NO}}_x\right]}_{in}+{\left[{\mathrm{NH}}_3\right]}_{in}-{\left[{\mathrm{NO}}_x\right]}_{out}-{\left[{\mathrm{NH}}_3\right]}_{out}})\times 100\%$$

3.2. Characterizations

X-ray diffraction (XRD) measurements were carried out on a computerized AXS D8 diffractometer (Bruker, GER), with Cu Kα (λ = 0.15406 nm) radiation, from 20 to 80° at 8°/min.

Surface areas were tested using an ASAP 2020 (Micromeritics, Norcross, GA, USA) at −196 °C by N₂ adsorption/desorption and calculated using a BET equation in the 0.05–0.35 partial pressure range.

The X-ray photoelectron spectroscopy (XPS) results of Ce 3d and O 1s were measured on an ESCALAB 250Xi Scanning X-ray Microprobe (Thermo-Fisher Scientific, Waltham, MA, USA) using Al Ka radiation (1486.7 eV) and a C 1 s peak, with BE = 284.8 eV as the calibration standard.

The transmission electron microscopy (TEM) image and energy-dispersive X-ray spectroscopy (EDS) mapping of Ce were obtained using a JEM-2100F equipment (JEOL, Tokyo, Japan), combined with a specimen tilting beryllium holder for energy dispersive spectroscopy. The accelerating voltage was 200 kV.

The H₂ temperature-programmed reduction (H₂-TPR) was tested using an AutoChem_II_2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA), and the temperature-programmed desorption of NH₃ and NO_x (NO_x-TPD and NH₃-TPD) were tested using the same reaction system as the activity tests. Experiment details can be found in Reference [42].

4. Conclusions

A series of Ce-Zr-Ti oxide catalysts were prepared using a stepwise precipitation approach for NH₃-SCR. CeZr_0.5O_x without Ti just showed a relatively low NO_x conversion. When Ti was introduced, Ce-Zr-Ti catalysts showed much better activities and N₂ selectivity. A CeZr_0.5Ti₂O_x catalyst, which contains moderate Ti amounts, showed the best performance, which is associated with its optimal ratios for the redox (CeO_x) and acidic (TiO₂) components.

CeZr_0.5O_x and CeZr_0.5Ti₂O_x catalysts were characterized using various methods and the formation process during preparation was investigated. CeZr_0.5Ti₂O_x catalyst showed superior redox properties (by H₂-TPR), good adsorption and NO_x/NH₃ activation functions (by NO_x-TPD and NH₃-TPD, respectively), and enhanced charge imbalance (by XPS).

During preparation, the Ti and Zr species were first co-precipitated with an increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti species with the further increase in pH. As a result, CeZr_0.5Ti₂O_x catalyst with a surface Ce concentration higher than those of Ti and Zr was obtained. This preparation process resulted in the formation of highly-dispersed CeO₂ on ZrO₂-TiO₂, and thus the catalyst can present excellent NH₃-SCR performance.