The effect of Ce on a high-efficiency CeO₂/WO₃–TiO₂ catalyst for the selective catalytic reduction of NO_x with NH₃

Abstract

In this study, W_0.1TiO_x and Ce_aW_0.1TiO_x (a = 0.1, 0.2, 0.5, 1.0) catalysts were prepared by a novel stepwise precipitation approach. The Ce_aW_0.1TiO_x catalysts showed much better activities and N₂ selectivity than W_0.1TiO_x. Particularly, the Ce_0.2W_0.1TiO_x (CeO₂/WO₃–TiO₂) catalyst showed excellent catalytic performance in a broad temperature range from 200 to 450 °C, under a high GHSV condition of 400 000 h⁻¹. Characterizations revealed that the novel preparation approach can achieve highly dispersed Ce species on the surface of the catalyst, and the Ce species could induce enhanced charge imbalance, superior redox functions, and outstanding adsorption and activation properties for NO_x and NH₃, which is the main reason for the highly efficient NO_x abatement capability of the CeO₂/WO₃–TiO₂ catalyst.

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Introduction

Selective catalytic reduction of NO_x with NH₃, using vanadium-based catalysts, has been widely applied for the removal of NO_x generated from stationary sources since the 1970s.¹ This technology has also been introduced into the market of diesel vehicles and has become the dominant technology for meeting the ever tightened emission standards.^2,3 Vanadium-based catalysts, especially V₂O₅–WO₃/TiO₂, were used as the first generation of NH₃-SCR catalysts for diesel engines.^2,4,5 However, the toxicity of the active vanadium species and the narrow operation temperature window are problems for their applications in diesel vehicles.

Increasingly stringent emission legislations for NO_x in mobile applications will require the use of intensification of NO_x reduction aftertreatment technology, which requires that the SCR catalyst should work efficiently under high space velocity conditions.⁴ For the purpose of obtaining higher deNO_x efficiency, great efforts have recently been focused on the optimization of the SCR catalysts for broadening the active deNO_x temperature window as widely as possible.²

Many types of vanadium-free catalysts, including oxides and zeolites, based on transition metals and/or rare earth metals have been studied for the NH₃-SCR reaction.^6,7 Several transition metals such as Fe,⁸ Mn,⁹ and Cu¹⁰ have been used in NH₃-SCR catalysts, while the investigation of rare earth metals for NH₃-SCR were mainly focused on Ce.¹¹ Ce has been shown to be an outstanding component in the support of NH₃-SCR catalyst.¹² Ce is also a very effective promoter for NH₃-SCR catalysts, such as V₂O₅–WO₃/TiO₂,¹³ Fe-ZSM-5,¹⁴ and Mn based low temperature SCR catalysts.^15–17 In addition, Ce-based composite oxide catalysts, such as Ce–Ti,¹⁸ Ce–W,¹⁹ Ce–Mo,²⁰ Ce–W–Ti,²¹ and Ce–Cu–Ti²² oxides, were shown to be very effective for NH₃-SCR reaction. In our previous study, a high-efficient Ce–W–Ti oxide catalyst was prepared by a novel stepwise precipitation approach.²³ This catalyst showed excellent catalytic performance, with superior low-temperature NO_x conversion, high N₂ selectivity and broad operation temperature window. Even under an extremely high space velocity condition of 1 000 000 h⁻¹, the catalyst could still be highly effective for NO_x conversion to N₂. In this study, we furtherly characterized this catalyst using various techniques and investigated the effects of Ce species on the catalyst for NH₃-SCR.

Experimental

Catalyst synthesis and catalytic performance test

The Ce_aW_0.1TiO_x (“a” represents the Ce/Ti molar ratio; a = 0.1, 0.2, 0.5, 1.0), with a fixed W/Ti molar ratio of 0.1, were prepared by an optimized precipitation method. Ce(NO₃)₃·6H₂O, (NH₄)₁₀W₁₂O₄₁ and Ti(SO₄)₂ were dissolved into distilled water together, with specific Ce/W/Ti molar ratio, and excessive urea was added into the mixed solution. Then, the solution was heated to 90 °C and hold there for 12 h under vigorous stirring. After that, the precipitated solid was collected by filtration, washed with distilled water, dried at 100 °C for 12 h, and calcinated at 500 °C for 5 h, orderly. The Ce_0.2W_0.1TiO_x catalyst with the best NH₃-SCR performance was denoted as CeO₂/WO₃–TiO₂. In addition, a W_0.1TiO_x catalyst was also prepared using the same method as a reference sample.

Before NH₃-SCR performance test, the powder samples were pressed, crushed and sieved to 40–60 mesh. The SCR performance tests of the sieved powder catalysts (0.06 mL, about 0.07 g) were carried out in a fixed-bed quartz flow reactor at atmospheric pressure. The performance test in this study was carried out under simulated conditions, which is not directly adapted to the NH₃-SCR process in realistic conditions. The reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH₃, 5 vol% O₂, N₂ balance, and 400 mL min⁻¹ total flow rate. The effluent gas, including NO, NH₃, NO₂ and N₂O, was continuously analyzed by an online FTIR gas analyzer (Nicolet Antaris IGS analyzer) equipped with a gas cell of 200 mL volume and 2 m path length and a liquid nitrogen cooled MCT-A detector. The concentration data were collected after 0.5 h when the SCR reaction reached a steady state.

Characterizations

The surface areas of the catalysts were obtained from N₂ adsorption/desorption analysis at −196 °C using a Micromeritics ASAP 2020. Prior to N₂ physisorption, the catalysts were degassed at 300 °C for 4 h. The surface areas were determined by BET equation in 0.05–0.35 partial pressure range.

Powder X-ray diffraction (XRD) measurements of the samples were carried out on a computerized Bruker-AXS D8 diffractometer with Cu Kα (λ = 0.15406 nm) radiation. The data of 2θ were collected at 8° min⁻¹ from 20 to 80°.

The XPS data were obtained on a Scanning X-ray Microprobe (ESCALAB 250, Thermo-VG Scientific) using Al Ka radiation (1486.7 eV). Binding energies of Ce 3d, W 4f, Ti 2p and O 1s were calibrated using C 1s peak (BE = 284.8 eV) as standard.

The surface morphology and elemental composition of the samples were studied using a scanning electron microscope (SEM, FEI Quanta 250F) combined with an energy dispersive X-ray (EDX) attachment. The accelerating voltage was 9.0 kV.

The H₂-TPR tests were carried out on a Micromeritics AutoChem_II_2920 chemisorption analyzer. The samples (100 mg) in a quartz reactor were pretreated at 400 °C in a flow of air (50 mL min⁻¹) for 1 h and cooled down to room temperature. Then H₂-TPR was performed in 10 vol% H₂/Ar gas flow of 50 mL min⁻¹ at a heating rate of 10 °C min⁻¹.

The NO_x-TPD and NH₃-TPD were performed using the same reaction system as catalytic performance tests. A typical experiment of NO_x-TPD used 300 mg sample and a gas flow rate of 200 mL min⁻¹. The experiment consisted of four stages: (1) degasification of the catalyst under N₂ at 350 °C for 1 h, (2) adsorption of 500 ppm NO and 5 vol% O₂ at 50 °C for 1 h, (3) isothermal desorption under N₂ at 50 °C for 1 h, and (4) temperature-programmed desorption under N₂ at 10 °C min⁻¹ up to 500 °C. A typical experiment of NH₃-TPD used 100 mg sample and a gas flow rate of 200 mL min⁻¹. The experiment procedure of NH₃-TPD was similar with that of NO_x-TPD, but in stage (2) the adsorption was carried out under 500 ppm NH₃ condition.

Results and discussion

NH₃-SCR performance

Fig. 1(A) shows the NO_x conversion over the catalysts with different Ce contents under a high GHSV of 400 000 h⁻¹. The W_0.1TiO_x just exhibited good catalytic performance in the high temperature region above 400 °C. When Ce was added, all of the Ce_aW_0.1TiO_x catalysts showed much better activities. Particularly, the Ce_0.2W_0.1TiO_x catalyst showed excellent catalytic performance in a broad temperature range from 200 to 450 °C, under such a high space velocity condition.

Fig. 1 (A) NO_x conversions, (B) N₂O productions and (C) NH₃ conversions over the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts. Reaction conditions: [NO] = [NH₃] = 500 ppm, [O₂] = 5 vol%, N₂ balance and GHSV = 400 000 h⁻¹.

The N₂O production over the samples during the catalytic performance test is shown in Fig. 1(B). We can see that, the addition of Ce remarkably inhibited the formation of N₂O during NH₃-SCR. At 350 °C, the N₂O production over W_0.1TiO_x reached 37.4 ppm. When a small amount of Ce was added, the N₂O production over Ce_0.1W_0.1TiO_x dropped to be just 2.7 ppm. The peak value of N₂O over W_0.1TiO_x at 350 °C was resulted from the competition between non-selective catalytic reduction reaction (NSCR reaction, 4NO + 4NH₃ + 3O₂ → 4N₂O + 6H₂O) and SCR reaction (4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O).^24,25

The NH₃ conversions over the samples during the tests are shown in Fig. 1(C). Below 350 °C, the NH₃ conversion over each Ce_aW_0.1TiO_x catalyst was very similar with the corresponding NO_x conversion, indicating that the NO and NH₃ were almost consumed with the same molar ratio. Above 350 °C, all of the NH₃ conversions over Ce_aW_0.1TiO_x catalysts kept to be 100%, which is associated with the oxidation of NH₃ in addition to SCR reaction. The NH₃ conversion over W_0.1TiO_x at 350 °C was almost the same as the NO conversion, indicating that the two N atoms in N₂O were mainly due to the coupling of one nitrogen atom from NH₃ and another one from NO.²⁶ A comparative investigation of the N₂O formation mechanisms during NH₃-SCR over Ce_0.2W_0.1TiO_x and W_0.1TiO_x demonstrated that CeO₂ is very effective for enhancing SCR reaction rate and thus depressing the NSCR reaction due to the competition between these two reactions.²⁴

NO oxidation performance

To investigate the NO oxidation properties of the catalysts, separated NO oxidation tests were carried out over the samples. NO₂ productions during the separated NO oxidations are shown in Fig. 2. NO oxidation to NO₂ is very important for low-temperature SCR to promote deNO_x efficiency by accelerating the “fast SCR” process (2NH₃ + NO + NO₂ → 2N₂ + 3H₂O). If a catalyst can effectively oxidize NO to NO₂ in situ under NH₃-SCR conditions, the NO_x can be efficiently removed at low temperature.^27–29 In this study, the order of NO₂ production over the catalysts matched perfectly with that of the low-temperature NO_x conversion. The NO₂ productions over Ce_aW_0.1TiO_x catalysts were clearly higher than that over W_0.1TiO_x, implying that the addition of Ce could effectively promote the oxidation of NO during NH₃-SCR reaction and thereby enhanced the low-temperature SCR performance.

Fig. 2 NO₂ production during separate NO oxidation reaction over W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts. Reaction conditions: [NO] = 500 ppm, [O₂] = 5 vol%, N₂ balance and GHSV = 400 000 h⁻¹.

BET surface area and XRD

As shown in Table 1, the BET surface area of W_0.1TiO_x, Ce_0.1W_0.1TiO_x and Ce_0.2W_0.1TiO_x were similar with each other. With further increase of Ce, the BET surface area decreased significantly from Ce_0.2W_0.1TiO_x to Ce_0.5W_0.1TiO_x and sharply from Ce_0.5W_0.1TiO_x to Ce_1.0W_0.1TiO_x. Among these samples, the BET surface area of Ce_0.2W_0.1TiO_x was the highest one, which probably be a reason for the highest NH₃-SCR performance of Ce_0.2W_0.1TiO_x.

Table 1 BET surface area of the samples and NO_x/NH₃ desorption amounts during NO_x/NH₃-TPD

Sample	BET surface area (m^2 g^−1)	NOx desorption (μmol g^−1)	NH3 desorption (μmol g^−1)
W0.1TiOx	167.2	5.6	487.6
Ce0.1W0.1TiOx	160.5	14.0	419.7
Ce0.2W0.1TiOx	173.6	24.2	449.0
Ce0.5W0.1TiOx	127.5	6.7	391.9
Ce1.0W0.1TiOx	8.9	7.7	366.2

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The XRD results of the samples are shown in Fig. 3. Only anatase TiO₂ was detected in W_0.1TiO_x. With the addition of Ce, the crystallization of anatase TiO₂ got weaker and weaker, but still no Ce or W species was observed in Ce_0.1W_0.1TiO_x or Ce_0.2W_0.1TiO_x. With further increase of Ce, clear cubic CeO₂ was observed in Ce_0.5W_0.1TiO_x and Ce_1.0W_0.1TiO_x and Ce₂WO₆ was observed in Ce_1.0W_0.1TiO_x. The coexistence of Ce with W and Ti species in Ce_0.2W_0.1TiO_x could significantly inhibit the crystallization of Ti species, and the Ce and W species were probably existed as amorphous phase or crystallite phase with very small particle size, which will lead to highly dispersed active species.³⁰ In such case, it is reasonable to see high efficient NO_x conversion over Ce_0.2W_0.1TiO_x together with no clear XRD diffraction peak of Ce or W species.

Fig. 3 XRD patterns of the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts.

Overall structure of the catalyst

Fig. 4 shows the SEM images of the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts. All the samples consisted of small spherical particles with diameters in micrometer level. The particle size of these samples observed with SEM was much larger than the average crystallite sizes of TiO₂ calculated by XRD (in nanometer level), indicating that each spherical particle was not a single crystallite but an agglomerate of many single crystallites.

Fig. 4 SEM images of the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts.

To investigate the atomic distributions of the metal elements, two techniques (SEM-EDX and XPS) of elemental composition analysis were carried out. The resolution of EDX is over 1 μm in depth, so its result were used to represent the overall status of the sample. For XPS, the detection depth is less than 10 nm, so its result can be used as the surface information of the sample. The analysis results by EDX and XPS are shown in Fig. 5. From the EDX results, we can see that the measured Ce/Ti molar ratios were similar with the added molar ratios. However, the surface Ce/Ti molar ratios for Ce_0.2W_0.1TiO_x, Ce_0.5W_0.1TiO_x and Ce_1.0W_0.1TiO_x measured by XPS were much higher than the corresponding added molar ratios. The comparison of surface and overall Ce/Ti molar ratios clearly indicated the enrichment of Ce on the surface of the catalyst, which demonstrated the structure of the CeO₂/WO₃–TiO₂ catalyst with dispersion of CeO₂ on WO₃–TiO₂. An EDX mapping of the Ce for the Ce_0.2W_0.1TiO_x catalyst in Fig. 6 showed that Ce was distributed evenly.

Fig. 5 The measured Ce/Ti molar ratio of the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts by SEM-EDX and XPS.

Fig. 6 EDX mapping of the Ce for the Ce_0.2W_0.1TiO_x catalyst.

NO_x/NH₃-TPD

To investigate the NO_x and NH₃ adsorption/desorption capabilities of the catalysts, NO_x-TPD and NH₃-TPD were performed on Ce_aW_0.1TiO_x and W_0.1TiO_x (Fig. 7).

Fig. 7 (A) NO_x-TPD and (B) NH₃-TPD profiles of the W_0.1TiO_x and Ce_aW_0.1TiO_x catalysts.

The NO_x-TPD profiles are shown in Fig. 7(A). The first NO_x peak of Ce_aW_0.1TiO_x at ca. 100 °C was due to the desorption of physisorbed NO_x, while the second NO_x peak at ca. 275 °C was mainly associated with the decomposition of chemsorbed NO_x species.^31,32 The two NO_x peaks of W_0.1TiO_x were both observed at lower temperature than those of Ce_aW_0.1TiO_x, indicating that the adsorption of NO_x on W_0.1TiO_x was much weaker than those on Ce_aW_0.1TiO_x. The adsorbed NO_x on W_0.1TiO_x was obviously less than that on Ce_0.1W_0.1TiO_x or Ce_0.2W_0.1TiO_x, with similar BET surface area, indicating that NO_x was mainly adsorbed on Ce sites. The adsorbed NO_x on Ce_0.5W_0.1TiO_x and Ce_1.0W_0.1TiO_x were clearly less than that on Ce_0.1W_0.1TiO_x or Ce_0.2W_0.1TiO_x, indicating that the accumulation of Ce on the surface of catalysts with high Ce contents (as detected by XRD) was unfavourable for NO_x adsorption. Therefore, highly dispersed Ce species on the Ce_0.2W_0.1TiO_x catalyst is very important for the adsorption and activation of NO_x.

It has been indicated that surface acidity plays an important role in the adsorption and activation of NH₃, and the SCR reaction in the high-temperature region is likely controlled by the surface acid properties.^33,34 Both of W and Ti can play as acid sites for NH₃ adsorption during NH₃-SCR reaction. Therefore, it is reasonable to see that the addition of Ce induced less NH₃ adsorption on Ce_aW_0.1TiO_x than that on W_0.1TiO_x (Fig. 7(B) and Table 1). Furthermore, the addition of Ce led to a shift of NH₃ desorption peak to the low temperature from W_0.1TiO_x to Ce_aW_0.1TiO_x. It indicates that the NH₃ adsorbed species could be more easily desorbed at low temperature with the addition of Ce. Considering BET surface area for Ce₁W_0.1TiO_x is just 8.9 m² g⁻¹, the amount of adsorbed NH₃ (366.2 μmol g⁻¹) is more than that for monolayer adsorption (considering ≈1019 sites per m² for a monolayer), indicating the existence of physisorption. In addition, NH₃ could be chemically adsorbed on both of the Lewis and Brønsted sites to form NH_3ads and NH₄⁺ species, which could both present in the NH₃-SCR reaction.^35–37 Ce_0.2W_0.1TiO_x exhibited much better NH₃ adsorption capacity than other Ce_aW_0.1TiO_x catalysts (Table 1). That was probably an important reason for the better NH₃-SCR performance of Ce_0.2W_0.1TiO_x.

H₂-TPR

The H₂-TPR profiles of W_0.1TiO_x and Ce_0.2W_0.1TiO_x are shown in Fig. 8. The three peaks at 535/544, 562/557, and 782/794 °C were associated with the multi-stage reduction process from WO₃ to WO₂ via two non-stoichiometric WO_x oxides.^38,39 In addition, two more TPR peaks appeared at 473 and 880 °C, respectively, on Ce_0.2W_0.1TiO_x. The peaks at 473 °C can be attributed to the reduction of surface Ce⁴⁺ to Ce³⁺, while the peak at 880 °C might be assigned to the reduction of bulk CeO₂.^13,40 The H₂-TPR profiles strongly suggest an enhancement of redox property by the Ce species on the catalyst.

Fig. 8 H₂-TPR profiles of the W_0.1TiO_x and Ce_0.2W_0.1TiO_x.

The redox property of catalyst is very important for NH₃-SCR. Topsøe et al. proposed a well-known catalytic cycle for the SCR reaction involving both acid-based and redox functions.⁴¹ Previous studies by Lietti et al. have demonstrated that the redox functions of NH₃-SCR catalyst govern the catalytic reactivity in the low-temperature region.^34,42 Therefore, the enhanced redox property of Ce_0.2W_0.1TiO_x due to Ce species would beneficial for the low temperature NH₃-SCR performance.

XPS results of O 1s

With the introduction of Ce, the presence of Ce³⁺ in the catalyst could induce a charge imbalance, which would lead to oxygen vacancies and unsaturated chemical bonds and generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of catalyst.^22,43,44

The XPS results of O 1s of W_0.1TiO_x and Ce_0.2W_0.1TiO_x are shown in Fig. 9. The peak of O 1s was fitted into three sub-bands by searching for the optimum combination of Gaussian bands with correlation coefficients (r²) above 0.99. The sub-bands at 529.9–530.4 eV could be attributed to the lattice oxygen O²⁻ (denoted as O_β). Two shoulder sub-bands at 531.4–531.6 eV and 532.9–533.0 eV are assigned to the surface adsorbed oxygen (denoted as O_α), such as the O₂²⁻ and O⁻ belonging to defect-oxide or hydroxyl-like group, and chemisorbed water (denoted as O′_α), respectively.^45,46 The O_α ratio of Ce_0.2W_0.1TiO_x (35.4%) calculated by O_α/(O_α + O′_α + O_β) was much higher than that of W_0.1TiO_x (12.3%), which means that the introduction of Ce indeed created more surface oxygen vacancies. O_α is usually more reactive than O_β for oxidation reactions due to its higher mobility.¹⁶ Therefore, the enhanced O_α of Ce_0.2W_0.1TiO_x would beneficial for the activation of NO and NH₃ in the SCR reaction, and thereby promote the NO_x conversion at low temperature.

Fig. 9 XPS results of O 1s of the W_0.1TiO_x and Ce_0.2W_0.1TiO_x.

O₂ shut off test

Oxygen plays an important role in the NH₃-SCR reaction.¹ To investigate the effect of the lattice oxygen of the catalyst, the NO_x conversion of the W_0.1TiO_x and Ce_0.2W_0.1TiO_x as a function of time was measured at 350 °C when the supply of oxygen was stopped during the NH₃-SCR reaction (Fig. 10).

Fig. 10 O₂ shut off test results of the W_0.1TiO_x and Ce_0.2W_0.1TiO_x at 350 °C.

Before the supply of oxygen was stopped, the NO_x conversion over Ce_0.2W_0.1TiO_x was kept at ca. 96% under the reaction conditions (GHSV = 400 000 h⁻¹ and temperature = 350 °C). After the supply of oxygen was stopped, the NO_x conversion rapidly decreased by ca. 10% in 2 min, and then the decrease rate changed to be steady. After the stop of O₂ for 270 min, the NO_x conversion gradually decreased to be ca. 12%. Upon resupply of gaseous oxygen, the NO_x conversions of Ce_0.2W_0.1TiO_x was recovered to the initial level immediately. This result demonstrated that the lattice oxygen of the catalyst has substantially participated in the NH₃-SCR reaction in the absence of gaseous oxygen.⁴⁷ Under the same reaction condition, the initial NO_x conversion over W_0.1TiO_x was kept at ca. 57%. As soon as the supply of oxygen was stopped, the NO_x conversion decreased sharply to be ca. 26%. After the stop of O₂ for 180 min, the NO_x conversion kept stably to be ca. 8%. Upon resupply of gaseous oxygen, the NO_x conversions of W_0.1TiO_x was also recovered immediately. The results of O₂ shut off experiments showed that the W_0.1TiO_x only contained small amount of lattice oxygen that could be reduced, and lattice oxygen just slightly affect the reaction. However, the introduction of Ce remarkably increased the amount of lattice oxygen that could be reduced, and enhanced lattice oxygen could participate significantly in the SCR reaction.

Conclusions

W_0.1TiO_x and Ce_aW_0.1TiO_x (a = 0.1, 0.2, 0.5, 1.0) catalysts were prepared by a novel stepwise precipitation approach. The catalysts were characterized by various techniques and the effects of Ce on the catalyst for NH₃-SCR were investigated.

The W_0.1TiO_x just exhibited good catalytic performance in the high temperature region above 400 °C. When Ce was added, all of the Ce_aW_0.1TiO_x catalysts showed remarkably improved catalytic performance and N₂ selectivity. Particularly, the Ce_0.2W_0.1TiO_x (CeO₂/WO₃–TiO₂) catalyst showed excellent catalytic performance in a broad temperature range from 200 to 450 °C, under a high GHSV condition of 400 000 h⁻¹.

Characterizations indicated that the CeO₂/WO₃–TiO₂ catalyst prepared by the stepwise precipitation approach can achieve highly dispersed active CeO₂ on WO₃–TiO₂ and intense interaction among Ce, W and Ti species. The Ce species on the catalyst could induce enhanced charge imbalance, superior redox functions, and outstanding adsorption and activation properties of the reactants. That is the reason for the excellent NH₃-SCR performance of CeO₂/WO₃–TiO₂ catalyst.

Acknowledgements

We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (51308296), the Fundamental Research Funds for the Central Universities (30920140111012) and the Qing Lan Project of Jiangsu Province, China.

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Footnote

† Current address: BASF Corporation, 25 Middlesex Essex Turnpike, Iselin, New Jersey 08830, United States.

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