have also been successfully tested in low temperature NH 3 Noble metals, especially platinum, have been reported to be active catalysts in NH 3

Transcription

1 46 Novel Pt/CNT and Pd/CNT catalysts for the low temperature ammonia and ethanol assisted selective catalytic reduction of NO Anna Avila 1 *, Mari Pietikäinen 1, Mika Huuhtanen 1, Anne-Riikka Leino 2, Tanja Kolli 1, Krisztián Kordás 2 and Riitta L. Keiski 1 1 University of Oulu, Department of Process and Environmental Engineering, P.O.Box 4300, FI University of Oulu, 2 University of Oulu, Department of Electrical and Information Engineering, P.O.Box 4500, FI University of Oulu 1 Introduction Since nitrogen oxides (NOx) are considered to cause global environmental problems (United Nations 1998), there is a demand for developing novel and more efficient catalytic materials for removing NOx components from flue and exhaust gases. Selective catalytic reduction (SCR) of NOx by ammonia (NH 3 - SCR) is one mature and well-proven technique for NOx abatement from stationary sources but also from exhaust gases of vehicles (Bosch & Janssen 1988). Besides ammonia, also hydrocarbons have shown ability to reduce nitrogen oxides in lean-burn conditions (Burch et al. 2002). Since the use of biomass-based fuels such as ethanol is increasing due to legislation such as the renewable energy directive, ethanol is one possible reductant also in the SCR process of passenger cars (European Parliament and Council 2009). Recently, carbon nanotubes (CNTs) have been found to be interesting catalyst support materials. CNTs are suitable for catalyst support materials because of their several advantageous properties such as electronic, adsorption, mechanical and thermal properties. (Serp et al. 2003) Metal decorated carbon nanotubes have already been reported to be effective e.g. in dehydrogenation of alkenes/alkanes and in Fischer-Tropsch synthesis of hydrocarbons (Wang et al. 2000, Tavasoli et al. 2008). CNTs decorated with V 2 O 5 have also been successfully tested in low temperature NH 3 -SCR (Huang et al. 2007). Noble metals, especially platinum, have been reported to be active catalysts in NH 3 -SCR of NO already in 1980s (Bosch & Janssen 1988). Noble metals are found to be efficient low temperature catalysts but, at higher temperatures, ammonia can be oxidised back to ammonia. There is also a possibility of forming undesired by-products such as NO 2 and N 2 O. Noble metals are shown to be active also in NO reduction by hydrocarbons in lean-burn conditions at low temperatures (Burch et al. 2002). 2 Objectives of the research In this study, platinum and palladium decorated carbon nanotube based catalysts were prepared, characterised and tested in NH 3 - and ethanol-scr. The objective was to gain new knowledge on the feasibility of the prepared noble metal loaded CNT catalysts in the abatement of nitric oxide. The studies simulate the NO emission mitigation at low temperatures from stationary sources by NH 3 -SCR, and from vehicles by ethanol assisted SCR. 3 Experimental methods Carboxyl functionalized multi-walled carbon nanotubes (CNTs) were decorated with platinum and palladium catalyst nanoparticles by wet impregnation method. The structure and composition of the synthesized catalysts were analysed by XRD, TEM, SEM and EDX spectroscopy. *Correspoding author, anna.avila@oulu.fi

2 ENERGY RESEARCH at the University of Oulu 47 The activity and selectivity of the prepared catalysts and parent carboxyl functionalized carbon nanotubes (CNT-COOH) were studied in the SCR model reaction. The experiments were carried out in a horizontal laboratory scale quartz tube flow reactor at atmospheric pressure and the product gas was analysed by a FTIR gas analyser (Figure 1). The used gas mixture was containing NO, O 2 and N 2 as balancing gas and NH 3 or C 2 H 5 OH as reductant. In the case of ethanol assisted SCR-reaction ethanol and water were fed to the reactor as liquids using a peristaltic pump while NH 3 was inserted as gaseous form. The total flow rate in both cases was 1000 ml/min which is corresponding to the gas hourly space velocity of h -1. The temperature was increased at a heating rate of 5 C/min from room temperature to 300 C in the case of NH 3 -SCR, and in ethanol-scr from 100 C to 300 C. Figure 1 Laboratory scale equipment for activity measurements. 4 Results and discussion 4.1 Catalyst characterisation Based on the TEM and SEM results, the metal particles are found to be nanosized and homogeneously distributed. The average particle sizes of Pt and Pd catalysts are detected to be below 4 nm. The metal loadings of the Pt and Pd catalysts are found to be 6.2±1.2 wt-% and 5.3±1.5 wt-%, respectively. In some locations also a higher Pd content is detected. XRD measurements show that Pd in Pd/CNT is actually in metallic/oxide (Pd/PdO) form. In Pt/CNT only metallic Pt is detected. 4.2 Activity measurements catalytic activity and selectivity The activity of parent CNT-COOH is relatively low and no significant NO conversion can be detected. There is a slight increase in NO reduction at ca. 125 C which may be claimed by physical adsorption of NO on the surface of the CNTs (Ahmed et al. 1993). Similar increase in NH 3 conversion at 125 C is also

3 48 detected. Huang et al. (2008) have reported that nitric acid treated activated carbon is able to adsorb ammonia due to the acidic groups on the surface of carbon. This might explain the increase in NH 3 conversion at ca. 125 C. The addition of Pt and Pd as nanoparticles on the surface of the CNTs leads to a considerable increase in the activity of the studied catalysts. The maximum conversions of NO over the Pt/CNT and Pd/CNT catalysts are 86% and 55%, respectively. The NO conversion is detected to decrease at high temperatures. This can be due to the oxidation of ammonia to NO over the noble metal catalysts. Almost all of the ammonia reacted in reactions over Pt and Pd catalysts. The selectivity of the Pt/CNT and Pd/CNT catalysts towards N 2 was, unfortunately, not as high as assumed. Because of the formation of by-products such as NO 2, N 2 O, CO and CO 2 the maximum selectivities of the Pt and Pd catalysts are detected to be below 30%. The preliminary results of ethanol-scr show that both Pt and Pd loaded CNT catalysts are promising materials for the low temperature selective catalytic reduction of NO when using ethanol as a reductant. The activity of the support material (CNT-COOH) was detected to be poor in ethanol-scr but Pt and Pd addition increased the NO conversion to nearly 100 %. The maximum NO conversions for Pt and Pd/CNT catalysts were >99% and >98%, respectively. The NO conversion reached the maximum value at around 210 C, however, the T50 value was ~160 C over both studied catalysts. In addition, no notable formation of undesired N 2 O was detected in the experiments. 5 Relevance of the research This research gives new knowledge on the suitability of platinum and palladium decorated carbon nanotube based catalytic materials in selective catalytic reduction of NO by ammonia and ethanol at low temperatures. The information obtained from this study is feasible in developing novel and more efficient catalyst materials for removing NO from emissions of both stationary sources and transportation vehicles. The developed catalysts were found to be active under the applied reaction conditions studied in this research. According to these preliminary results, Pt- and Pd/CNT catalysts seem to be promising materials for NO abatement by NH 3 and ethanol in the laboratory scale, however, further experiments are needed to find out the suitability and real performance of these catalysts in the studied reactions. Acknowledgements The study was financially supported by the Academy of Finland and Fortum Foundation. In addition, the authors would like to acknowledge the support from the Graduate School of Energy Science and Technology (EST). References Ahmed SN, Baldwin R, Derbyshire F, McEnaney B and Stencel J (1993) Fuel Vol 72.(3): Bosch H and Janssen F (1988) Catalysis Today Vol 2.(4):

4 ENERGY RESEARCH at the University of Oulu 49 Burch R, Breen JP and Meunier FC (2002) Applied Catalysis B: Environmental Vol 39.(4): European Parliament and Council (2009) Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources. Official Journal of the European Union, L140/ Huang B, Huang R, Jin D and Ye D (2007) Catalysis Today Vol 126.(3-4): Huang CC, Li HS and Chen CH (2008) Journal of Hazardous Materials Vol 159.(2-3): Serp P, Corrias M and Kalck P (2003) Applied Catalysis A: General Vol 253.(2): Tavasoli A, Abbaslou RMM, Trepanier M and Dalai AK (2008) Applied Catalysis A: General Vol 345.(2): United Nations (1998) Kyoto Protocol. [Online]. [accessed 30th March 2010]. PDF-document. Available at: < Wang Y, Shah N and Huffman GP (2004) Energy&Fuels Vol 18 (5):

5 Reference to this article: Avila A., Pietikäinen M., Huuhtanen M., Leino A.-R., Kolli T., Kordás K. and Keiski R. L. (2010) Novel Pt/CNT and Pd/CNT catalysts for the low temperature ammonia and ethanol assisted selective catalytic reduction of NO. In: Pongrácz E., Hyvärinen M., Pitkäaho S. and Keiski R. L. (eds.) Clean air research at the University of Oulu. Proceeding of the SkyPro conference, June 3 rd, 2010, University of Oulu, Finland. Kalevaprint, Oulu, ISBN pp SkyPro conference: Proceedings: