NH₃-SCR activity, hydrothermal stability and poison resistance of a zirconium phosphate/Ce_0.5Zr_0.5O₂ catalyst in simulated diesel exhaust

Abstract

A ZP/CZ (zirconium phosphate/Ce_0.5Zr_0.5O₂) catalyst exhibits over 80% NO_x conversion from 250 to 450 °C under a high GHSV of 300 000 h⁻¹ in the presence of H₂O, CO₂ and C₃H₈. Mixing with soot leads to a decrease in NO_x conversion of the catalyst at temperatures higher than 350 °C. After hydrothermal aging (760 °C for 48 h) and sulfur aging (400 °C for 48 h), ZP/CZ still possesses over 80% NO_x conversions in 289–450 °C and 297–466 °C respectively, which are significantly better than those of home made Cu-SAPO-34 and vanadium catalysts at higher temperatures. These results indicate that ZP/CZ is a promising catalyst for NO_x abatement for diesel engine exhausts.

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1. Introduction

As the diesel engine has high fuel efficiency, it reduces the fuel consumption and decreases the emission of the greenhouse gas – carbon dioxide (CO₂). However, NO_x, derived from the combustion of the fuel in oxygen-rich conditions, lead to various environmental problems such as acid rain, photochemical smog, ozone depletion and greenhouse effects.^1–3 Among various NO_x control strategies for diesel engines, selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) is one of the most promising technologies to meet the increasingly stringent standards for NO_x emissions.² V₂O₅–WO₃(or MoO₃)/TiO₂ catalysts have been widely used in diesel exhaust deNO_x due to their effectiveness and resistance to SO₂ poisoning.^2,4 However, the toxicity of vanadium restrained their application, particularly in mobile sources.⁵ Thus, great efforts have been made to develop environment-friendly NH₃-SCR catalysts with high SCR activity and N₂ selectivity in a wide temperature range, for controlling NO_x emissions from diesel engines.

The limited volume of vehicle requires deNO_x catalyst exerting high NH₃-SCR performance in case of high space velocity of exhaust. Therefore, transition metal-exchanged zeolites, particularly iron or copper-exchanged zeolites, have received increasing attention in recent years due to their high catalytic activities and selectivities at high space velocity.^6–8 While the insufficient low-temperature activity and/or high cost limit their industrial application. So it is valuable to find other catalysts with high de-NO_x ability at high space velocity. Massive reports about the SCR catalysts were only characterized in a simple mixture of composing NH₃, NO and O₂. However, the real work condition of SCR catalyst often includes other compounds such as CO, CO₂, H₂O and unburnt hydrocarbons (HCs). Especially, the hydrocarbons in the exhaust can always be adsorbed by the zeolite catalyst, leading to the poison of catalyst. Therefore, complex gas compounds, containing CO₂, H₂O and hydrocarbons (HCs), should be adopted to characterize the SCR catalyst objectively.⁹

CeO₂-based NH₃-SCR catalysts, including CeO₂–TiO₂ (ref. 10–12) and CeO₂–ZrO₂ solid acid catalysts,^13–16 have received much attention. In our previous report,¹⁴ we reported a novel phosphated ceria catalyst with high hydrothermal stability. But, the sulfur resistance of Ce_0.75Z_0.25-PO₄³⁻ still needed to be improved to meet the harsh conditions of diesel engine. Furthermore, we reported a zirconium phosphate@Ce_0.75Zr_0.25O₂ catalyst with enhanced NO_x conversions in a wider temperature window.¹⁶ However, the overall characterizations of this promising catalyst were not reported.

In this report, gas compounds including H₂O, CO₂ and HCs with a high gas hourly space velocity (GHSV) of 3 × 10⁵ h⁻¹ are introduced to study the “real” response of ZP/CZ catalyst (with/without soot). Meantime, the hydrothermal stability, sulfur resistance and anti-potassium poisoning ability of the catalyst were also tested to find if it is a suitable NH₃-SCR catalyst for commercial use of diesel engine.

2. Experimental

2.1. Catalysts preparation

All the materials (AR grade) for synthesizing the catalysts were from Aladdin Industrial Corporation, China.

Ce_0.5Zr_0.5O₂ powder was synthesized by a precipitation method which is similar with the routes reported in ref. 16. An appropriate amount of cerium(III) chloride heptahydrate and zirconyl chloride octahydrate were dissolved in deionized water to obtain the salt solution for precipitation. Afterwards, ammonia solution (25%) was added to the salt solution dropwise under magnetic stirring to obtain the precipitate until the pH of the mixed solution reaches 9–10. The mixture was aged at 60 °C for 20 h, followed by spray drying at 200 °C to get the spherical powders. The powders were calcined at 500 °C for 3 h to get the Ce_0.5Zr_0.5O₂ microspheres. ZP/CZ catalyst was prepared by impregnating zirconium phosphate (ZrP) (10 wt%) on Ce_0.5Zr_0.5O₂ powder according to the method of ref. 14. ZrP was prepared by co-precipitation method according to the ref. 17. To comprehensively evaluate the performance of the ZP/CZ, VWTi (V₂O₅–WO₃/TiO₂) and Cu-SAPO-34 were prepared as reference materials. The V₂O₅–WO₃/TiO₂ catalyst with 2 wt% V₂O₅ and 8 wt% WO₃ was prepared by conventional impregnation method using NH₄VO₃ and H₂C₂O₄·2H₂O as precursors and commercial WO₃/TiO₂ powders as the support. The sample was dried at 100 °C for 5 hours and then calcined at 500 °C for 3 h in air conditions. Cu-SAPO-34 was prepared by ion-exchange method. The commercial H/SAPO-34 molecular sieve (Al₂O₃/SiO₂/P₂O₅ = 1 : 0.3 : 0.8) powders were stirred in Cu(CH₃COO)₂ aqueous solution (0.1 mol L⁻¹, 10 mL of solution per g of zeolite) at 60 °C for 10 h. After that, the samples were filtered, washed, and dried at 110 °C for 12 h. Then, the powders were thermally treated in air at 550 °C for 4 h.

Hydrothermal aging of ZP/CZ, Cu-SAPO-34 and VWTi catalysts (ZP/CZ-TA, Cu-SAPO-34-TA and VWTi-TA) were performed at 760 °C in 10 vol% H₂O/air for 48 h. The sulfur-aged ZP/CZ, Cu-SAPO-34 and VWTi catalysts (ZP/CZ-SA, Cu-SAPO-34-SA and VWTi-SA) was obtained by treating catalyst in air containing 100 ppm SO₂ + 10% H₂O at 400 °C for 48 h. Potassium-doped catalysts were prepared by impregnation with KCl (0.6 wt%), followed by dehydration at 100 °C and calcination at 500 °C for 1 h.

2.2. Characterizations

H₂-TPR tests were performed on Micromeritics Auto Chem II. Prior to H₂-TPR experiment, 50 mg samples were treated by 10% O₂/He with a total flow rate of 50 mL min⁻¹ at 500 °C for 30 min. The reactor temperature was raised to 1000 °C at a heating rate of 10 °C min⁻¹ in H₂ (10 vol%)/Ar (50 mL min⁻¹).

2.3. Activity measurement

The activity measurement for NO reduction with ammonia was carried out in a fixed bed reactor made of quartz tube. To minimize the mass transfer limitations, the catalyst was sieved with 50–80 mesh size and then charged into the quartz reactor using quartz wool. The reaction gas mixture consisted of 500 ppm NO, 500 ppm NH₃, 5 vol% O₂, 5 vol% H₂O, [CO₂] = 5 vol% (when used), [C₃H₈] = 85 ppm (when used) and N₂ as balance. The total flow of the gas mixture was 0.5 L min⁻¹ at a gas hourly space velocity (GHSV) of 3 × 10⁵ h⁻¹. 0.1 mL oxidized catalysts with/without 1 wt% soot (mixed by a spatula for 2 min for “loose contact” conditions) were measured at 50 °C intervals in 30 min from 200 °C to 500 °C. The concentrations of nitrogen oxides and ammonia were measured at 200 °C by a thermo Nicolet 380 FTIR spectrometer equipped with a quartz tube (6 mm i.d.). The oxidized catalyst samples were obtained by thermally treating the samples at 500 °C in a 10% (v/v) O₂/N₂ flow (500 mL min⁻¹) for 30 min. The NO_x conversion was calculated by eqn (1).

3. Results and discussion

3.1. Anti-poisoning of catalyst

The NH₃-SCR activities of the catalysts are shown in Fig. 1. The fresh ZP/CZ catalyst shows above 80% NO_x conversions in a wide temperature region of 250–450 °C. N₂ selectivity of ZP/CZ catalyst is 100% before 350 °C, but it decreases after 350 °C. The lowest value is 91% at 500 °C. After hydrothermal aging, ZP/CZ-TA shows a deactivation in the temperatures lower than 400 °C. However, ZP/CZ-TA catalyst still owns over 80% NO_x conversions in 289–450 °C. In order to study the stability of phosphate at 800 °C, ZP/CZ catalyst was also pretreated at 800 °C in 10 vol% H₂O/air for 48 h (named ZP/CZ-800 °C). ZP/CZ-800 °C shows above 80% NO_x conversions in a wide temperature region of 291–420 °C. As a result, the phosphate in ZP/CZ catalyst has a good stability up to 800 °C. Compared with the fresh ZP/CZ catalyst, though the de-NO_x ability of ZP/CZ-SA catalyst decreases before 400 °C, it shows the highest NO_x conversions above 400 °C and owns the wide temperature window of 297–466 °C. The potassium poisoning results in the decrease in NO_x conversions in the whole temperature region.

Fig. 1 NO_x conversions vs. temperature over (a) ZP/CZ samples, (b) reference samples (reaction conditions: NO = NH₃ = 500 ppm, O₂ = 5 vol%, H₂O = 5 vol%, N₂ balance, GHSV = 3 × 10⁵ h⁻¹).

According to Fig. 1(b), the de-NO_x ability of Cu-SAPO-34 decreases after hydrothermal aging, sulfur aging and potassium poisoning. The NO_x conversions window for Cu-SAPO-34-TA, Cu-SAPO-34-SA and Cu-SAPO-34-KCl are 237–350 °C, 272–401 °C, 228–384 °C respectively. Cu-SAPO-34 owns better anti-potassium poisoning than ZP/CZ, while ZP/CZ have better performances in hydrothermal stability and sulfur resistance. As for VWTi catalyst, hydrothermal aging and potassium poisoning make the NO_x conversions lower than 80% in the whole temperature region. While, the de-NO_x ability of VWTi-SA is nearly the same with that of fresh VWTi sample, indicating the highest sulfur resistance of vanadium catalyst.

The evolution of N₂O and NO₂ during NH₃-SCR reaction is shown in Fig. 2. The outlet N₂O of catalysts are less than 10 ppm within the whole reaction time. NO₂ is detected at the temperatures higher than 350 °C due to the NH₃ oxidation to NO_x at high temperatures. The NO₂ generation at 350–500 °C on catalysts follows the sequence of ZP/CZ-KCl ≫ ZP/CZ > ZP/CZ-TA > ZP/CZ-SA, which is in coordination with the NO_x conversions at temperatures above 400 °C. Less than 10 ppm NO₂ is detected on ZP/CZ-SA at 500 °C, which indicates that the improved acidity by sulfation results in a significantly high N₂ selectivity of catalyst.

Fig. 2 NH₃-SCR activities of the catalysts: (a) N₂O generations and (b) NO₂ generations.

3.2. Effect of reaction conditions on catalyst

NH₃-SCR activities of the ZP/CZ catalyst under different reaction conditions are shown in Fig. 3. The SCR activity of Printex U (soot) was reported in the ref. 18. At higher temperatures (>280 °C), soot was able to selectively oxidize NH₃ to nitrogen while not active for NO_x reduction. Similarly, in the present study, lacking enough reductants (NH₃) for SCR reaction leads to a decreasing NO_x conversion after 350 °C when soot (1%) is mixed with the PZ/CZ catalyst (shown in Fig. 2(a)). When CO₂, C₃H₈ or the combination of the two species are added into the reaction gas, the NO_x conversions become higher after 350 °C. Our previous work reported that high NH₃ oxidation activity at high temperature led to a low N₂ selectivity and a decrease in NO_x conversions.^14,19–21 Since ammonia competes with HCs for the adsorption sites⁹ and CO₂ in the inlet also leads to a decreasing NH₃ conversion after 400 °C,²² less ammonia are able to participate in the deep oxidation reaction, leading to slightly higher NO_x conversions at temperatures higher than 400 °C. HCs have no effects on the NH₃ conversion but inhibit the NO₂ generations, especially at higher temperatures (25 ppm at 500 °C). It indicates that the inlet of HCs inhibit the NH₃ participating in the side reactions, leading to better selectivity and activity. The ZP/CZ catalyst exhibits over 80% NO_x conversion at 250–450 °C and at a high GHSV of 300 000 h⁻¹ in the presence of H₂O, CO₂ and C₃H₈, indicating that it is a promising catalyst for NO_x abatement for the diesel engine exhaust.

Fig. 3 NO_x conversions vs. temperatures over (a) ZP/CZ samples, (b) reference samples under different reaction conditions (reaction conditions: NO = NH₃ = 500 ppm, O₂ = 5 vol%, H₂O = 5 vol%, [CO₂] = 5 vol%, [C₃H₈] = 85 ppm, N₂ balance, GHSV = 3 × 10⁵ h⁻¹).

For Cu-SAPO-34, adding the combination of CO₂ and C₃H₈ or soot into the SCR reaction has little effect on NO_x conversion. The VWTi in similar reaction conditions presents a lower NO_x conversions than those of ZP/CZ. Moreover, mixing with soot leads to a serious deactivation of VWTi catalysts at the whole temperatures.

3.3. H₂-TPR

Fig. 4 shows the hydrogen consumptions of catalysts versus temperatures. The reduction of Ce–Zr–O mixed metal oxides typically occurs in two steps, namely, surface reduction followed by subsurface and bulk reduction.²³ Accordingly, the H₂ consumption peaks of ZP/CZ catalyst at 538 and 648 °C are ascribed to the surface/sub-surface lattice oxygen and bulk lattice oxygen respectively.

Fig. 4 H₂-TPR cures of the samples.

The hydrothermal aging leads to less reductive surface and bulk oxygen as indicated by the hydrogen consumption peaks shifting from 538 to 619 °C and 648 to 692 °C. According to the literature,^14,23 CePO₄ may lead to the difficulty for the reduction of surface/subsurface oxygen due to the decrease in Ce⁴⁺ ions. The formation of CePO₄ may alter the kinetics of the reduction process of Ce(IV) to Ce(III), inhibiting the mobility of lattice oxygen to the surface of catalyst. In our previous study,¹⁶ zirconium phosphate was used for ceria modification rather than phosphate because that the strong pre-combination between zirconium and phosphate may help to reduce the interaction between cerium and phosphate. After hydrothermal aging, the interaction between cerium and phosphate becomes stronger, leading to a decrease in redox ability of catalyst. K atom could facilely bonded to the CeO₂ surface and reduce the degree of reduction of the cerium species.²⁴ However, KCl poisoning has little effect on the redox property of ZP/CZ catalyst, which can be ascribed to the special structure of ZP/CZ catalyst. The zirconium phosphate provides acidity sites on the surface of the catalyst and the cerium sites in the core act as the redox sites.¹⁶ The alkali metal is easier to be adsorbed by the acidic shell.

A sharp reduction peak with a tail appears around 585 °C on ZP/CZ-SA corresponding to the reduction of surface oxygen and subsurface oxygen,¹³ suggesting the occurrence of strong interaction between sulfate species and ZP/CZ. This strong interaction inhibits the mobility of oxygen from bulk to surface, leading to the deactivation of SCR catalyst at low temperatures.

3.4. NH₃ adsorption

Fig. 5 shows the FTIR spectra of the adsorbed species on the catalysts arising from contact with NH₃ at various temperatures. Bands at 1437–1442, 1676–1685 and 2967–3014 cm⁻¹ are assigned to the σ_as and σ_s of NH₄⁺ on the Brønsted acid sites, respectively. And bands at 1178–1213 and 1579–1597 cm⁻¹ are ascribed to the σ_s NH₃ and σ_as NH₃ on Lewis acid sites. In the N–H region, several bands of NH₃ on Lewis acid sites appear at 3195–3400 cm⁻¹.^6,10–12,25 On ZP/CZ catalyst, all the bands of ammonia derived species decrease in intensity as the temperature increases. Plenty of acid sites are still detected on ZP/CZ catalyst at 350 °C. The strong acidity and the moderate redox property of ZP/CZ may lead to the excellent activity of catalyst.

Fig. 5 FTIR spectra of the catalyst arising from contact of NH₃: (a) ZP/CZ catalyst, from 50 °C to 350 °C, (b) different catalysts at 150 °C.

According to the results of Fig. 5(b), after hydrothermal aging, ZP/CZ-TA owns less acid sites and lower acidity. The decreased redox ability and acidity results in the lower NO_x conversions of ZP/CZ-TA than the fresh catalyst in the whole temperatures. KCl poisoning leads to a serious poison on the acidity of catalyst as indicated by the nearly disappearance of FTIR spectra at 1437 cm⁻¹. The potassium ions can strongly bind to the Brønsted acid sites,^26–31 leading to a decrease in adsorption sites for ammonia and thereby decreased the deNO_x ability. The weakest acidity accounts for the lowest NO_x conversions of ZP/CZ-KCl.

For ZP/CZ-SA catalyst, the deposition of sulfates brings more acid sites on catalyst, which can help to improve ammonia adsorption on catalyst at high temperature and suppresses the NH₃ deep oxidation.^20,21,32 Thus, ZP/CZ-SA catalyst owns the highest NO_x conversions after 400 °C.

4. Conclusion

The fresh ZP/CZ catalyst shows above 80% NO_x conversions in a wide temperature region of 250–450 °C in simulated diesel exhaust. After hydrothermal and sulfur aging, ZP/CZ catalyst still owns better deNO_x performance than Cu-SAPO-34 and vanadium catalysts. Although mixed with soot leads to a decrease in NO_x conversion at temperatures higher than 350 °C, adding H₂O, CO₂ and C₃H₈ into the reaction gas components has little effect on SCR activity of ZP/CZ catalyst. These results show that ZP/CZ is a promising catalyst for NO_x abatement for diesel engine exhaust.

Acknowledgements

The authors would like to acknowledge Project 51202126 by the National Natural Science Foundation of China and Project 2013AA065302 supported by the Ministry of Science and Technology, PR China for financial support.

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