selective nox reduction in h2 + no + o2 reaction under oxygen

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Selective NOx reduction in H2 + NO + O2 reactionunder oxygen-rich condition over Pt/rare earthoxide catalystsTo cite this article: M Itoh et al 2009 IOP Conf. Ser.: Mater. Sci. Eng. 1 012029

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https://doi.org/10.1088/1757-8981/1/1/012029

https://iopscience.iop.org/article/10.1143/JJAP.41.5951











1

Selective NOx reduction in H2 + NO + O2 reaction under oxygen-rich condition over Pt/rare earth oxide catalysts

M. Itoh, K. Motoki, M. Takehara, M. Saito, and K. Machida1 Center for Advanced Science and Innovation, Osaka University, Suita, Osaka, Japan

[email protected] (K. Machida)

Abstract. The Pt supported rare earth oxide (CeO2, Pr6O11, Eu2O3, Gd2O3) catalysts were prepared by a conventional impregnation method to evaluate their selective catalytic reduction properties of NOx with H2 under excess oxygen condition. Among them, good NOx reduction activity was obtained only on the Pt/CeO2 catalyst. Specific adsorption peaks assigned to NO3

- species were observed in its diffuse reflectance infrared Fourier transform spectrum (DRIFTs) for the Pt/CeO2 catalyst, indicating that NOx reduction proceeds via formation of such nitrate. DeNOx properties of Pt/CeO2 were further improved by using a solvothermal deposition technique. The catalyst prepared by physically mixing the CeO2 support with Pt particles deposited by a solvothermal method, consequently, gave the best deNOx performance (NO conversion: ca. 80%, N2 selectivity: ca. 75%).

1. Introduction Nitrogen oxides, NO and NO2 (referred to as NOx), are mainly exhausted from moving and stationary internal combustion. In recent years, the NOx emission amount has been severely restricted especially for automobiles. Diesel-fueled cars exhaust more NOx than gasoline-fueled ones, although diesel-fueled cars have a benefit to suppress CO2 emission due to high energy conversion efficiency of diesel engine. Some researches about selective catalytic reduction (SCR) of NOx by using NH3, CO, hydrocarbon, or H2 have been done to decrease the NOx emission from diesel cars.1-8 H2-SCR by using Pt catalysts is a promising deNOx technique under excess oxygen condition. This technique does not only provide high NOx conversion, but also works at relatively low temperature, which is relevant for diesel engine with low temperature exhaust gas. Hydrogen can be obtained on board by water gas shift reaction from the exhaust gas or using reforming and exhaust gas recirculation system.9 There are, however, an important disadvantage in the H2-SCR of NOx using the Pt catalysts. It tends to produce more N2O causing greenhouse effect rather than N2. The N2 selectivity has not been improved by changing Pt loading amount or H2 concentration in the feed gas, but affected by acidity/basicity characteristics of the oxides used as a support.10

In this study, rare earth oxides were used as a support of Pt metal for the purpose of controlling the solid acidity/basicity by lanthanide contraction. Moreover, cerium, praseodymium, and terbium form a nonstoichiometric oxide such as MO2-x, which can act as an electron donor to promote catalytic activity. For example, the ammonia formation rate was significantly enhanced by the reducing treatment because the partially reduced CeO2-δ promoted the Ru metal activity for N2 dissociation.11 In 1 To whom any correspondence should be addressed.

IUMRS-ICA 2008 Symposium “AA. Rare-Earth Related Material Processing and Functions” IOP PublishingIOP Conf. Series: Materials Science and Engineering 1 (2009) 012029 doi:10.1088/1757-8981/1/1/012029

c© 2009 IOP Publishing Ltd 1

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the present study, the Pt metal supported rare earth oxide (CeO2, Pr6O11, Eu2O3, Gd2O3) catalysts were prepared and their H2-SCR activity of NOx under excess oxygen condition was investigated from the viewpoints of the solid acidity/basicity and the electron donation influence of support on Pt metal. Furthermore, two different Pt loading techniques such as an incipient wetness method and solvothermal one were applied to the CeO2 support to modify the metal/support interface circumstance. Their distinct catalytic properties for NOx reduction were discussed with a focus on metal-support interaction.

2. Experimental Pt/MOx (M = Ce, Pr, Eu, Gd) catalysts were prepared by an incipient wetness method. The rare earth oxides used here were reagent grade produced by Shin-Etsu Chemical Co., Ltd. and their surface area values were not so high (ca. 10 m2). The detail preparation procedures were described in elsewhere.12 For Pt/CeO2, the solvothermal technique was also applied. Appropriate amounts of the CeO2 support and the Pt(NO3)2 aqueous solution were added to 80 mL of isopropyl alcohol in a test tube, and the test tube was placed in a 100 ml autoclave vessel. The sealed vessel was kept at 300 ˚C for 12 h to conduct a Pt deposition reaction completely. After cooling to room temperature, the products were collected by a centrifuge and repeatedly washed with acetone. The resultant powders were dried at 50 ˚C overnight, and subsequently calcined at 873 K for 2 h in air. For the solvothermal preparation, a commercially available CeO2 support with high surface area (ca. 120 m2) was also used as a reference. For convenience, the CeO2 support with high surface area is represented as h-CeO2.

Nitrogen adsorption isotherm was measured at 77 K to evaluate the specific surface area using a BET equation. CO2 temperature programmed desorption (TPD) experiments were carried out to estimate the solid basicity from the desorption temperature by using a gas chromatograph equipped with TCD. The sample surface area contacting CO2 roughly set to 10 m2 for all specimens by adjusting their weight to enhance a signal/noise ratio. Sample morphology was observed by a transmission or a scanning electron microscope. Catalytic testing was performed at atmospheric pressure in a conventional fixed-bed quartz reactor. Gas mixtures of 0.1 vol% NO, 0.4 vol% H2, 10 vol% O2, and balanced with N2 were fed to the catalyst sample with 0.15 g (W/F = 0.3 g·s·mL-1) placed in the reactor tube. The effluent from the reactor was analyzed by an online FT-IR spectrometer with a gas cell. Spectra were collected by scanning 5 times from 1000 cm-1 to 4000 cm-1 at resolution of 0.5 cm-1. Concentrations of NO, NO2, N2O, and NH3 were calculated from their calibration curves by integrating the peak area at respective 1911, 1586, 2214, and 1120 cm-1. The N2 production was evaluated by subtracting NO, NO2, N2O, and NH3 concentrations of outlet gas from the inlet NO concentration. DRIFTs measurements were performed by using the above spectrometer with a temperature controllable diffuse reflectance reaction cell to observe adsorption chemical species in situ.

3. Results and discussion Figure 1 shows the NO conversion with NO2, N2O, and N2 selectivity on the catalysts prepared in this study as a function of the reaction temperatures. Only Pt/CeO2 showed the good NO conversion with the N2 selectivity of ca. 40% in a series of the Pt/MOx (M = Ce, Pr, Eu, Gd) catalysts prepared by the impregnation method. Other catalysts provided poor deNOx activity, which is similar to the case without using a catalyst. Their BET surface area values are almost same to be ca. 10 m2/g. The DRIFTs measurements revealed that the different reduction activity is ascribable to the distinct adsorbed chemical species as shown in Fig. 2. On the spectrum for Pt/CeO2 (spectrum a), the peaks assigned to NO3

- were mainly appeared at 1036, 1316, 1435, and 1530 cm-1 and the other peaks at 1694 and 1766 cm-1 were due to Pt-NO stretching, and the absorption peak at 1617 cm-1 was responsible for adsorbed water.13, 14 Meanwhile, main peak at 1189 cm-1 was assigned to chelating nitrite (NO2

-) species and weak band at 1431 cm-1 was corresponded to the nitrate species observed on the DRIFT spectra for the other catalysts (spectrum b).15 A pattern of spectrum b is quite similar to those for the sole CeO2, Pr6O11, Eu2O3, and Gd2O3 supports without loading Pt metal, meaning that Pt metal hardly work on the Pr6O11, Eu2O3, and Gd2O3 supports. Sato et al. have been reported that rare earth oxides except for CeO2 possess solid basicity confirmed from the CO2-TPD (Temperature Programmed Desorption) analyses.16 The above


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results suggest that the rare earth oxide supports having solid basicity deactivate the Pt metal activity for the deNOx reaction by H2 probably due to specific interaction between the metal and the support.

The NOx reduction properties for Pt/ Pr6O11 were inferior, even though Pr6O11 includes the trivalent oxidation states working as an electron donor to promote the molecular dissociation. Furthermore, the Pt/CeO2 catalyst heated in H2 at 400 ˚C for 30 min before the catalytic test gave nearly same deNOx activity compared to the untreated Pt/CeO2 one. The reduction temperature of 400 ˚C is enough to generate trivalent cerium cation, because Pt/CeO2 starts to release oxygen from around 180 ˚C in a stream of H2 gas reported by Mattos et al.17 It was found that the electron donated from the partially reduced metal oxides does not affect the NOx reduction activity but the solid basicity rather has a significant influence on the deNOx activity. In fact, the catalytic properties for H2-SCR of NOx were improved when modifying the Pt/ZrO2 surface by an acidic functional group, especially in point of N2 selectivity.12

Oxygen concentration dependence on the N2, NH3, N2O, and NO2 yield amounts was measured at 100 ˚C on Pt/CeO2 in order to examine the present NOx reduction mechanism. The total NO conversion sharply declined by adding little amount of oxygen gas (0.5 vol%) and then gradually decreased with increasing the oxygen concentration. The N2, N2O, and NO2 yield amounts were not strongly dependent on the oxygen concentration (0.5-10 vol%). When the oxygen concentration was fixed at 0.5 vol%, increasing hydrogen concentration to 0.8 vol% almost completely recovered the NO conversion to ca. 100%, having respective N2 and N2O yielding amounts of 170 and 630 ppm, together with a trace of NO2. These results indicate that oxygen adsorption and hydrogen one are competitive and the NO reduction was retarded by the oxygen adsorption. Time dependence accumulation of nitrate was checked by the DRIFTs measurements for Pt/CeO2 with flowing gas mixtures of 0.1 vol% NO and 10 vol% O2. Initial exposure of reactants to the freshly treated catalyst yielded the adsorbed nitrite (~1190 cm-1) and nitrate (~1310 and ~1430 cm-1) species. The intensity of peak assigned to

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Temperature (°C) Figure 1. NOx conversion and product selectivity in a stream of 0.1 vol% NO, 0.4 vol% H2, and 10 vol% O2 as a function of temperature over the Pt/CeO2 catalysts; (a) prepared by impregnating with low surface area CeO2, (b) prepared by impregnating with high surface area CeO2, (c) prepared by solvothermal method with high surface area CeO2, and (d) prepared by mixing Pt metal particles (reflux method) with high surface area CeO2. Light gray, dark gray, and black bars represent the NO2, N2O and N2 selectivity, respectively.


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Figure 3. TEM images observed on (a) Pt/CeO2 prepared by impregnating with low surface area CeO2, (b)Pt/CeO2 prepared by impregnating with high surface area CeO2, (c) Pt/CeO2 prepared by solvothermal method with high surface area CeO2. SEM image observed on (d) Pt metal particles prepared by reflux method.

nitrite was gradually weakened with time and the absorption peaks derived from nitrate species alternatively grew. This is a piece of evidence that nitrate is formed by oxidation of nitrite on the catalyst. When hydrogen gas was added to the above feed gas, the new peaks (~1690 and ~1760 cm-1)


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assigned to Pt-NO stretching emerged. This suggests that oxygen poisoning on the Pt metal is somewhat relaxed by hydrogen existence to enable NO adsorption, which is well agreed with the selectivity characteristics observed on the oxygen concentration dependence. Taking the above results into consideration, the NOx reduction on the present Pt/CeO2 catalyst plausibly proceeds as followed: first, nitrite species are formed by reacting NO with oxygen in the lattice of CeO2 and then further oxidization proceeds to generate the nitrate species. The resultant nitrate species are reduced to N2 or N2O by hydrogen attacking. Similar reaction scheme was proposed by Machida et al. on the Pt/TiO2-ZrO2 system.18

For the purpose of improving the NO conversion, the Pt/CeO2 catalyst with high surface area (Pt/h-CeO2) was prepared by using the impregnation method and its deNOx activity was tested as shown in Fig. 1 (b). The deNOx activity for Pt/h-CeO2 was poor and its NO conversion was almost similar to that obtained without catalysts. Figures 3 (a) and (b) show the typical TEM images for Pt/CeO2 and Pt/h-CeO2, respectively. The mean size of Pt particles loaded on the low surface area CeO2 is 10 nm and that on the high surface area one is 3 nm. Though Pt metal dispersion (comparable to the number of active metal sites) got better by increasing surface area of the support, the catalytic activity was inversely declined. Additional obvious difference of adsorbed NOx species was not found in their DRIFT spectra. These results lead to the conclusion that the Pt metal loaded on the high surface was deactivated especially for hydrogenation activity. So different Pt metal loading technique, a solvothermal deposition, was applied. The Pt metal particles on the Pt/h-CeO2 catalyst prepared by the solvothermal method were slightly aggregated compared to that prepared by the impregnation method as seen in Fig. 3 (c). Figure 1 (c) shows the deNOx activity for the Pt/h-CeO2 catalysts prepared by the solvothermal method. It was found that the solvothermal thermal process is suitable to enhance the deNOx activity on the Pt/h-CeO2 catalyst system. Figure 4 shows the CO2-TPD profiles on the Pt/h-CeO2 catalysts prepared by individual impregnation and solvothermal methods. Since the respective BET surface area values on the Pt/h-CeO2 prepared by the individual impregnation and solvothermal methods were 119 and 135 m2/g, the number of solid base sites was found to be decreasing when using the solvothermal method. The results obtained above indicate that the deNOx activity is influenced by characteristics of the Pt metals and strength of solid basicity for the supports.

Pt metal particles were prepared by the solvothermal method in a similar manner mentioned above without adding the CeO2 support and mixed with the h-CeO2 support. This mixed type catalyst also provided the good NOx reduction activity in a similar manner as observed in the Pt/h-CeO2 prepared by the solvothermal method, suggesting that feature of the Pt metal is an important factor to determine catalytic activity for deNOx. Figure 2 (c) shows the DRIFT spectrum for the Pt/h-CeO2 catalyst prepared by the solvothermal method. The spectrum pattern is almost same to the Pt/CeO2 catalyst prepared by the

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impregnation method, but the peak at 1618 cm-1 assigned to adsorbed water became higher than the Pt/CeO2 having low surface area because of increasing the surface area. Obvious enlargement of broad peak derived from O-H stretching was also observed at around 3500 cm-1. A DRIFT spectrum pattern obtained on the mixed Pt/h-CeO2 catalyst was quite different as seen in Fig. 2 (d). No peak arose from nitrate ion could be detected and strong absorption peak at 1200 cm-1 might be originated from nitrite ion and shoulder peaks at 1120 and 1380 cm-1 is assigned to hyponitrite ion (N2O2

2-).19 It has been proposed above that NOx transforms into N2 by hydrogen attacking to NO3

- ion species. Disappearance of NO3-

peak implies that the reaction between intermediate NO3- and hydrogen is too fast to detect by the

DRIFT spectroscopy and this behavior supposedly contributes to the N2 selectivity. Further investigation is needed to elucidate the reaction mechanism in detail. Moreover, different technique to prepare Pt metal particles was done. The Pt metal particles were deposited by refluxing 200 mL of a Pt(NO3)2 ethylene glycol solution at 470 K for 3 h. The solution color changed from yellow to black during the reaction, and the fine black powders were separately obtained by centrifuging the resultant product. The Pt metal particles prepared from the reflux method formed in a rectangular shape as shown in Fig. 3 (d). Figure 1 (d) shows the NO conversion on a catalyst by mixing the resultant Pt particles with the h-CeO2 support. The highest N2 selectivity of 75% was observed with relative high NOx conversion (ca. 80%). This result strongly bears out the above idea that NOx reduction activity is considerably dependent on Pt metal nature rather than support one. XRD patterns for both Pt particles prepared by individual solvothermal and reflux method were assigned to fcc phase, but oxidation state and/or atomic arrangement of Pt on the surface probably contribute to the above behavior.

4. Conclusions The Pt supported rare earth oxide catalysts were prepared and their NOx reduction activity was measured in order to investigate the relationship between solid acidity/basicity of the supports and NOx reduction activity, together with the electron donation effect of nonstoichiometric rare earth oxides such as CeO2-δ and PrO2-δ for the deNOx activity. For former, the acidity/basicity character of the supports strongly affects the deNOx activity and the metal oxide having weak basicity is found to be suitable as a support for deNOx Pt catalysts. The latter electron donation effect hardly influences on the catalytic properties for the NOx reduction on the present reaction system. The NOx reduction under oxygen presence proceeds by hydrogen attacking to the adsorbed nitrate species, which is derived from the oxidation of the adsorbed nitrite species. Hydrogenation ability of a catalyst contributes to N2 and N2O selectivity, in which higher hydrogenation activity is desired to obtain high N2 selectivity. Furthermore, the NOx reduction activity is significantly affected by the physicochemical characteristic of the metal particles, which was confirmed from the activity measurements on the Pt-CeO2 catalyst prepared by physically mixing the Pt metal particles and the CeO2 support. The DRIFT spectra indicate that the relevant intermediate adsorption species is NO3

- and fast hydrogenation to it may provides the high N2 selectivity. The excellent performance deNOx catalyst can be designed by selecting the supports with suitable basicity and the metal with high affinity against NO and H2 dissociation.

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area A of “Panoscopic Assembling and High Ordered Functions for Rare Earth Materials” from the Ministry of Education, Culture, Sports, Science, and Technology.

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[6] Shimizu K and Satsuma A 2007 Appl. Catal. B: Environ. 77 202 [7] Machida M and Ikeda S 2004 J. Catal. 227 53 [8] Costa C, Savva P, Fierro J, and Efstathiou A 2007 Appl. Catal. B: Environ. 75 147 [9] Abu-Jrai A, Tsolakis A, and Megaritis A 2007 Int. J. Hydrogen Energy 32 3565

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selective nox reduction in h2 + no + o2 reaction under oxygen

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