Metal Exchanged ZSM-5 Zeolite Based Catalysts for Direct Decomposition of N 2 O

Transcription

1 Catal Lett (2009) 132: DOI /s y Metal Exchanged ZSM-5 Zeolite Based Catalysts for Direct Decomposition of N 2 O Suresh Kumar Æ S. Rayalu Æ Nunzio Russo Æ G. S. Kanade Æ H. Kusaba Æ Y. Teraoka Æ Nitin Labhsetwar Received: 18 April 2009 / Accepted: 8 July 2009 / Published online: 28 July 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Co?Pt/ZSM-5 and Ag?Pt/ZSM-5 type catalysts were prepared by ion exchange method followed by calcination. These Co and Ag based catalysts, promoted by a small amount of Pt have been studied for their catalytic activity towards N 2 O decomposition. Both the catalysts show high catalytic activity, however, cobalt platinum based catalyst shows relatively better activity at higher temperature. At 550 C almost 100% conversion of N 2 O is achieved over Co?Pt/ZSM-5 with a maximum of mmole of N 2 O decomposed per gram of the catalyst per unit time. These catalytic materials have been characterized for their structure, composition, morphology and other details, using XRD, SEM, EDX, ICP, BET techniques. Much improved catalytic activity for the bimetallic zeolite than the mono-metal containing compositions clearly demonstrate the synergistic effect of these transition metals, while high surface area of ZSM-5 is also responsible for the improved N 2 O decomposition activity. Keywords N 2 O decomposition Catalyst Zeolite ZSM-5 de-nox Co?Pt/ZSM-5 Ag?Pt/ZSM-5 S. Kumar S. Rayalu G. S. Kanade N. Labhsetwar (&) National Environmental Engineering Research Institute (C.S.I.R.), Nehru Marg, Nagpur , India nk_labhsetwar@neeri.res.in N. Russo Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, Turin, Italy H. Kusaba Y. Teraoka Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka , Japan 1 Introduction Control of N 2 O emissions has become one of the major challenges in environmental protection. N 2 O is a potential contributor to the green house gases with its global warming potential (GWP) as high as 310 [1]. Different methods reported for the N 2 O decomposition at the tail gas of industries includes thermal decomposition, selective catalytic reduction and catalytic decomposition. Thermal decomposition is an energy intensive method. Direct decomposition of N 2 O and selective catalytic reduction of N 2 O with hydrocarbons (HC-SCR) are extensively investigated as potential cost-effective technologies to remove nitrous oxide from industrial off-gases. Direct catalytic decomposition of N 2 O is a relatively simple method to convert nitrous oxide in to harmless products of N 2 and O 2. It is generally believed to be more cost-effective than HC-SCR as hydrocarbons are not often present on-site and an external supply of hydrocarbons is expensive, while catalytic reduction conventionally using ammonia involves the risk of reactant slip and unwanted combustion products. It is also associated with high cost for reductant introduction. Though, many catalysts have been studied for direct decomposition of N 2 O, search for the new catalysts with improved properties is a topic of continued interest. Transition and noble metals on various supports as well as perovskites have been widely studied, but most of these catalysts are not studied at higher concentration of N 2 O [2, 3]. Iwamoto et al. [4] reported the uniquely high and stable activity over exchanged Cu-ZSM-5 for the direct NO decomposition. Chen et al. [5], studied decomposition of N 2 O on Cu-ZSM-5 and found that N 2 O decomposition can occur on Cu-ZSM-5 at [350 C. The Cu ion is the active center for dissociative adsorption of N 2 O. Brink et al. [6]

2 Metal Exchanged ZSM-5 Zeolite Based Catalysts 249 reported that the catalytic decomposition of N 2 O on Fe-ZSM-5 takes place at temperature above 400 C and also observed that the addition of propane lowers the temperature for N 2 O conversion above 100 C. Iron-zeolites, especially Fe-ZSM5, are being studied as De-NOx catalysts in lean-burn gasoline and diesel engines and also for N 2 O decomposition and reduction. Also, high activity of Fe-ZSM5 was observed for selective oxidations using N 2 O as an oxidant (e.g., direct oxidation of benzene to phenol) [7, 8]. Platinum based catalysts on various supports have also been studied by various researchers for N 2 O decomposition. Huuhtanen et al. [9] studied the effect of Pt loading on the performance of Pt-ZSM-5 catalyst for De-NOx prepared by impregnation method. Baerns [10] also reported that Pt-ZSM-5 is very active for the decomposition of NO at low temperature. Chang et al. [11] reported decomposition of NO over Pr-, Cu-, Ga-, and Coexchanged ZSM-5. They reported that Co-ZSM -5 containing Co in the framework had the highest activity for NO decomposition. Recently we have reported bimetal exchanged zeolites towards N 2 O decomposition [12]. Recent years have also seen a large number of publications, which are concerned with reducing the N 2 O emissions caused by the anthropogenic activities. Use of these catalysts in the decomposition of N 2 O makes it possible to carry out the reaction at a temperature level substantially lower in comparison with the purely thermal decomposition, thereby reducing the energy penalty and more importantly CO 2 emissions [13, 14]. The present work is especially relevant to N 2 O emissions from nitric acid plants and, the temperature and N 2 O concentration used are opted accordingly. In this work, we have studied the activity of Co?Pt/ ZSM-5 and Ag?Pt/ZSM-5, for nitrous oxide decomposition. The main interest of this work was to explore the effect of co-cations on Pt, keeping its concentration quite low. Both Co and Ag are reported for the excellent adsorption and catalytic activity for NOx, and it was therefore, of our interest to investigate these in combination of Pt. In general, not much has been studied on combination of cations for catalytic N 2 O decomposition. These catalysts have been characterized with respect to their chemical composition, surface area, SEM, and XRD and their N 2 O decomposition activity has been studied at moderate N 2 O concentrations. 2 Experimental Commercial grade ZSM-5 with relatively high silica-alumina ratio (Si/Al = 280) was selected as the basic material considering its better hydrothermal stability. Co?Pt/ZSM- 5and Ag?Pt/ZSM-5 were synthesized with targeted loading of moles of platinum per gram and moles of silver or cobalt per gram of catalysts respectively. About 10 g of ZSM-5 was first exchanged with Pt salt containing g of Pt for 24 h at 30 C, followed by filtration and washing with deionized water, and drying at 110 C for 16 h. It was then exchanged with Co solution containing g Co, for 24 h followed by filtration and drying at 110 C. The catalyst was calcined at 450 C for 6 h. It was designated as Z-1. Ag?Pt/ZSM-5 was synthesized in similar manner. After exchanging with Pt, it was filtered, dried and then exchanged with Ag solution. This catalyst was designated as Z-2. Zeolite based catalysts containing only single cation (Pt or Co or Ag) have also been prepared for comparison of their activity with the bimetallic zeolite catalysts. These have been prepared in a similar way as described above. The catalyst samples were characterized for chemical composition using both wet chemical analysis and ICP- AES technique after acid digestion. The X-ray diffraction patterns have been recorded using X-ray diffractometer, (Rigaku: MiniflexII-DD34863) operated at 30 kv and 15 ma with a monochromator and using Cu-Ka radiation (k = nm). The radiations of Cu-K a were generated using X-ray generator of model of same make and the b radiation were filtered using monochromator. The sample was scanned for 2h ranges from 3 to 60. XRD data were analyzed for phase identification, presence of impurity phase, as well as for structural damage if any during the catalyst preparations. BET surface area of samples was determined following the standard nitrogen adsorption method using Micromeritics ASAP-2000 instrument. EDX analysis was performed using Philips XL 30 CP. The sample was placed on an adhesive C slice and coated with Au Pd alloy 10 nm thick layer. The aim of this analysis was to get information about the presence and distribution elements on the surface of the samples. The catalytic decomposition of N 2 O was carried out in a fixed-bed, steady state gas reactor. Catalyst sample (&0.1 g) was diluted with the same size and amount of quartz beads and placed in a quartz reactor for catalytic run. Prior to the catalyst testing, the catalysts were pretreated at 400 C for 2 h in a flow of helium to stabilize the catalysts. The flow rate of high purity N 2 O (5000-PPM in He) gas was measured by mass flow controllers (Albourg, USA). The total flow rate used for each run was 40 ml min -1 thereby yielding a space velocity of 24,000 h -1. After the catalyst had attained steady state over a period of 30 min at each temperature, the effluent gas was analyzed by a micro gaschromatograph (Varian CP-4900 TCD) using a Molecular Sieve 5A column (for the analysis of O 2,N 2 ) and a Porapak column as well as by a chemiluminiscence NOx analyzer (Environment SA) for N 2 O and NOx.

3 250 S. Kumar et al. The catalytic activity was evaluated in terms of conversion (X) of N 2 O gas according to the following equation: X N2 O ¼ ðp N2 Oin P N2 O outþ=p N2 Oin: The catalyst samples were also kept at 500 C for 12 h in the air as well as feed flow to investigate their short term thermal and chemical stability and later evaluated for their catalytic activity. 3 Results and Discussion The original ZSM-5 has high SiO 2 /Al 2 O 3 = 280 ratio, which is responsible for its hydrothermal stability. However, the alumina content is enough for the cation exchange capacity to introduce small amount of catalytically active metal ions. The ICP-AES analyses confirm the targeted loading of Pt and co-cation on zeolites phase for both the catalysts. The X-ray diffraction patterns of Z-1 and Z-2 show intact structure of ZSM-5 after the ion-exchange experiments. It was not possible to identify the presence of metal/metal oxide due to its low content and high dispersion through ion exchange (Fig. 1). The scanning electron micrographs (SEM) (Figs. 2, 3) show the surface morphology of Co?Pt/ZSM-5 and Ag?Pt/ZSM-5 powder prepared by ion-exchanged method. The SEM micrographs show that the powders obtained by ion-exchange method contain particles typical of ZSM-5. EDX analysis of samples Co?Pt/ZSM-5 and Ag?Pt/ZSM-5 (Figs. 4, 5) confirm the presence and excellent dispersion of platinum, silver and cobalt on respective catalysts. As expected, there is practically no change in the morphology of zeolite samples after the platinum, silver and cobalt incorporation. The BET surface area for bare zeolite was 400 m 2 g -1, while there was no significant change observed after the incorporation of cations and calcination at higher temperature. The high temperature calcinations could oxidize the exchange metals in to oxides, while there are reports that the metal ions may also get reduced to zero valence state at higher temperature and charge compensation of zeolite framework through other mechanisms. There was no impact of metal incorporation on surface area, which, also indicate the high dispersion of metals on exchange sites of zeolites, and ruling out the possibility of pore blockage if any. In this way, ion-exchange followed by calcination is a simple method to incorporate metals and metal oxides in zeolites. The catalytic activity results for Co?Pt/ZSM-5 and Ag?Pt/ZSM-5 catalysts are presented in Fig. 6. Commercial ZSM-5 does not show any significant activity for N 2 O decomposition. The light-off temperature for Pt?Co/ZSM- 5(Z-1) is around 225 C and reaches T 90 at 450 C, Fig. 1 X-ray diffraction pattern of Co?Pt/ZSM-5, Ag?Pt/ZSM-5 and original ZSM-5 whereas for the Pt?Ag/ZSM-5, the reaction starts at around same temperature but reaches T 90 at about 550 C. This indicates the high catalytic activity of present catalysts and relatively better catalytic activity of Pt?Co/ZSM-5. The temperature range of catalytic activity is very crucial for industrial applications, where it is often difficult to observed more than 400 C temperature of the off-gas. Figure 7 shows change in TOF (Turn over frequency) vs temperature. TOF is calculated as millimoles of N 2 O converted per gram of catalyst per unit time. Maximum mmole of N 2 O are converted per gm of catalyst per min when Z-1 shows the maximum conversion at 550 C. Z-2 converts a maximum of mmole of N 2 O at about 550 C. In this way, both these catalysts show overall high catalytic activity for the N 2 O decomposition reaction. One peculiar observation is about the low temperature and light-off activity of these catalysts. Although, both the catalysts show good low temperature activity, the Ag Pt based catalyst shows better activity in the temperature region of C, however, the activity is relatively inferior beyond this temperature. Similar trend was also observed for the monometallic catalysts (Fig. 8). The Fig. 8 also confirms the relatively much lower activity of monometallic variants of ZSM-5, which clearly demonstrate the synergistic effect of metal combinations in the present catalysts. Such a substantial improvement in the activity is noteworthy and deserves further investigations. There are several reaction mechanisms proposed for the catalytic N 2 O decomposition. Decomposition of N 2 Oon the catalyst takes place probably based on a redox mechanism. N 2 O act as an oxidizing agent and oxygen is desorbed in the reducing step. During the pretreatment at high temperature, cobalt and silver are expected to be reduced to Co 2? and Ag o, which are then oxidized by N 2 OtoAg? and Co 3? and consequently N 2 O gets reduced to nitrogen and oxygen. Another possible mechanism for the decomposition of N 2 O on platinum site is donation of electrons from

4 Metal Exchanged ZSM-5 Zeolite Based Catalysts 251 Fig. 2 SEM photograph of Co?Pt/ZSM-5 catalyst Fig. 3 SEM photograph of Ag?Pt/ZSM-5 catalyst Fig. 4 EDX pattern for Co?Pt/ZSM-5 catalyst the surface to platinum sites, which causes the platinum cations to become electron rich, which in turn, lead to electron transfer to N 2 O resulting in weakening of the N O bond (10). Cleavage of N O bond may be the rate limiting step for this reaction, while desorption of oxygen from the catalyst surface may also require considerable energy. The better activity of Co based catalyst could be due to its ability to adsorb and activate N 2 O molecule. Therefore, a synergistic effect of Pt and Co is probably responsible for the overall activity for the decomposition of N 2 O. It was not possible to characterize the metal phases present on the Fig. 5 EDX pattern for Ag?Pt/ZSM-5 catalyst catalysts, as there could also be a possibility of the formation of nano-particles of metal oxides with preferential formation of certain crystal planes. These metal oxide particles are therefore, expected to have different electronic properties leading to different catalytic properties as well. 4 Conclusion Zeolite based bimetallic catalysts with compositions Co?Pt and Ag?Pt have been synthesized by Ion exchange

5 252 S. Kumar et al. Fig. 6 N 2 O decomposition versus temperature for Co?Pt/ZSM-5 and Ag?Pt/ZSM-5 Although, both the catalysts show high catalytic activity for the N 2 O decomposition reaction, however, Co?Pt based catalyst is relatively better and it was possible to get over 90% N 2 O decomposition at 450 C. The Ag?Pt based catalyst shows good activity at lower temperature range with lower light off temperature. The better activity of Co?Pt/ZSM-5catalyst could be due to the preferential adsorption of N 2 O and its activation on the cobalt sites, while presence of Pt shows promotional effect probably due to electron transfer. Hydrothermally stable ZSM-5 with high SiO 2 /Al 2 O 3 ratio appears to be an excellent support for the high dispersion of active metals like platinum, cobalt and silver while their catalytic properties can also be tailored by structural modifications. Their surface area is also equally responsible for their high catalytic activity, while their low cation exchange capacity could be useful for the dilute and uniform dispersion of active cations. The much lower activity of mono-metallic variants of these catalysts infer the very effective synergistic effect of present combinations of the metal ions for N 2 O decomposition activity. Acknowledgments This work was carried out under the CSIR supra-institutional project No. SIP-16 (3.1) as well as, under the research cooperation between NEERI, India and Kyushu University, Japan. Some characterization studies were carried out at VNIT, and JNARDDC, Nagpur India. Fig. 7 Turn-over-frequency (TOF) versus temperature for Co?Pt/ ZSM-5 and Ag?Pt/ZSM-5 N2O Conversion (%) Ag-ZSM-5 Co-ZSM-5 Pt-ZSM Temperture (Degree C) Fig. 8 N 2 O decomposition versus temperature for monometallic catalysts method, and studied for their N 2 O decomposition activity. Ion exchange method results in excellent dispersion of metals in ZSM-5 and does not affect its crystallinity, if ph of the solution is maintained properly during the synthesis. References 1. Inventory of US green house gases and sinks EPA, Washington 2. Nobukawa T, Yoshida M, Kameoka S, Ito S, Tomishige K, Kunimori K (2004) Catal Today 93 95: Perez-Ramırez J, Kapteijn F (2003) Catal Commun 4: Iwamoto M, Furukawa H, Mine Y, Vemura F, Mikuriya SC, kagawa S (1986) J Chem Soc Chem Commun Chen L, Chen HY, Lin J, Tan KL (1999) Surf Interface Anal 28: van den Brink RW, Booneveld S, Pels JR, Bakker DF, Verhook MJFM (2001) Appl Catal B: Environ 32: Chen HY, Sachtler WMH (1998) Catal Today 42:73 8. Panov GI, Uriarte AK, Rodkin MA, Sobolev VI (1998) Catal Today 41: Huuhtanen M, Rahkamaa-Tolonen K, Maunula T, Keiski RL (2005) Catal Today 100: RottEnder C, Andorf R, Plog C, Krutzsch B, Baerns M (1996) Appl Catal B: Environ 11: Chang Y, McCarty JG (1998) J catal 178: Labhsetwar N, Dhakad M, Biniwale R, Mitsuhashi T, Haneda H, Reddy PSS, Bakardjieva S, Subrt J, Kumar S, Kumar V, Saiprasad P, Rayalu S (2009) Catal Today 141: Pirngruber GD, Roy PK, Prins R (2007) J Catal 246: Sun K, Xia H, Feng Z, van Santen R, Hensen E, Li C (2008) J Catal 254: