Cooperative effect induced by the mixing of Na-ZSM-5 and Pd/H3PW12O40/SiO2 in the selective catalytic reduction of NO with aromatic hydrocarbons

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Abstract

Pd was loaded on the dispersed H3PW12O40 (HPW) over the SiO2 surface, and the catalyst was applied to the selective reduction of NO with aromatic hydrocarbons. The catalyst exhibited high activity in the NO reduction when branched aromatic hydrocarbons, such as toluene and xylene, were used as reductants. The catalytic activity of Pd/HPW/SiO2 was improved remarkably by physically mixing it with Na-ZSM-5. From the temperature programmed desorption (TPD) of toluene and the analysis of the products, it was inferred that the activity was enhanced when Pd/HPW/SiO2 and Na-ZSM-5 were mixed. In other words, aromatic hydrocarbons were partially oxidized to yield oxygenated hydrocarbons, e.g., benzaldehyde and phthalic anhydride, over Pd/Na-ZSM-5; in this reaction, a part of Pd migrated from Pd/HPW/SiO2 to Na-ZSM-5 during the course of the physical mixing procedure. Subsequently, the oxygenated hydrocarbons reacted with NO entrapped with HPW over Pd to yield N2.

Introduction

From the viewpoint of conservation of the environment, it is extremely desirable to eliminate NO generated by internal combustion engines. This is because NO is a harmful gas that causes acid rain. The catalytic reduction of NO using hydrocarbons has been studied as an effective way to convert NO to N2. In previous studies, numerous kinds of catalysts such as Cu-ZSM-5 and Pt/Al2O3 have been found to exhibit a high activity in the reaction [1], [2]. In particular, many researchers have used Pd for catalytic reactions with various reductants using hydrocarbons, alcohols, CO, H2, etc. [3], [4], [5], [6], [7], [8], [9]. On the other hand, the adsorption of NO has been studied as an alternative method of eliminating NO in the gas phase [10], [11]. Several research groups employed Keggin-type heteropoly acids as the adsorbent of NO for this purpose [12], [13], [14], [15]. The adsorption of NO occurred via replacement with the structural water present between the Keggin units of heteropoly acids. The adsorbed NO was activated by O2 gas with H+ to yield H+NO2. Reversible elimination of NO could also be easily performed by heating the sample above 573 K [12], [13]. In our recent studies, we found that a selective reduction of NO was possible over Pd loaded on HPW/SiO2 using methane as a reductant [16]. The reaction proceeded at 523 K, which is a significantly lower temperature than that required by the conventional catalysts. Even in the presence of 10% water vapor, the catalyst activity remained at the same level. In the case of this catalyst, it was necessary to combine Pd, HPW, and SiO2 to obtain the NO reduction. It was considered that the reaction occurred between NO adsorbed in the acid sites of HPW dispersed over the SiO2 support and methane dispersed over the Pd center. In the present study, first, the selective catalytic reduction (SCR) of NO was performed over Pd/HPW/SiO2 with various hydrocarbons and alcohols as reductants in order to enhance the possibilities of using the catalyst. Second, Pd/HPW/SiO2 was physically mixed with zeolites, and the mixture was used in the NO reduction with toluene as the reductant. It is expected that mixing the respective adsorbents for hydrocarbons and NO would lead to a cooperative effect; the catalytic reaction would be promoted by a combination of the activation of NO over HPW and toluene entrapped in zeolites [17], [18]. To achieve this, toluene was used as the reductant for NO, and it was found to be effective in the SCR of NO at a relatively low temperature.

Section snippets

Preparation of the catalysts

30 wt.% of H3PW12O40·nH2O (Wako Chemicals) was impregnated on SiO2 (N602A, Nikki Chemicals, surface area = 281 m2 g−1) in methanol; this is denoted by HPW/SiO2. Pd was then impregnated on HPW/SiO2 using a toluene solution of palladium (II) acetate (Wako Chemicals). Under the typical condition, the loading of Pd was maintained at 1 wt.%.

Pd/HPW/SiO2 and zeolites were physically mixed using a mortar for 5–360 min. Na-ZSM-5 (Tosoh Co., Si/Al2 = 23.8), H-beta (PQ Corp., Si/Al2 = 20), Na-MOR (JRC-Z-M20,

Screening of reductants for SCR of NO over Pd/HPW/SiO2 catalyst

The SCR of NO was performed over the Pd/HPW/SiO2 catalyst using various hydrocarbons and alcohols as the reductants. A comparison of the conversions of NO and hydrocarbons measured at 573 K is shown in Fig. 1. The conversions of NO were less than 10% when alkanes and alkenes were used for the reaction. Exceptionally, 32% propene was converted. Alcohols were not found to be effective reductants since they were completely oxidized to carbon dioxide and water. In contrast, a relatively high

Discussion

In our study, we found that the physical mixing of Na-ZSM-5 with Pd/HPW/SiO2 is the effective way of improving the NO reduction activity. It is possible that the improvement in activity was a result of the cooperative effect of both components. The role of Na-ZSM-5 could be attributed to the strong adsorption of toluene and the successive partial oxidation of adsorbents to yield oxygenates such as phthalic anhydride and benzaldehyde. Further, the presence of methyl groups in benzene ring

Conclusions

Pd loaded on HPW/SiO2 as well as its mixture with zeolites were employed as catalysts for the selective reduction of NO with hydrocarbons. The Pd/HPW/SiO2 catalyst exhibited high activity when branched aromatic hydrocarbons, such as toluene and xylene, were used as reductants. It was found that the activity of Pd/HPW/SiO2 was enhanced when Na-ZSM-5 was physically mixed with it. The activity of the catalysts obtained by mixing remained at the same level even in the presence of 10% water vapor.

Acknowledgement

The present work is supported by the Grant-in-Aid for Scientific Research (KAKENHI) in Priority Area “Molecular Nano Dynamics” from Ministry of Education, Culture, Sports, Science and Technology.

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