on vanadium exchanged natural zeolite using microwave irradiation

Transcription

1 212 Int. J. Vehicle Design, Vol. 59, Nos. 2/3, 2012 Catalytic reduction of NO x on vanadium exchanged natural zeolite using microwave irradiation A.O. Emiroglu* Department of Mechanical Engineering, Abant Izzet Baysal University, Bolu 14100, Turkey aosmanemiroglu@gmail.com *Corresponding author O. Eldogan Department of Mechatronics, Sakarya University, Sakarya 54187, Turkey eldogan@sakarya.edu.tr M. Teker Department of Chemistry, Sakarya University, Sakarya 54187, Turkey teker@sakarya.edu.tr A. Keskin Department of Automotive, Abant Izzet Baysal University, Bolu 14100, Turkey ahmkeskin@gmail.com I. Ekmekci Department of Industry Engineering, Istanbul Commerce University, Istanbul 34840, Turkey ismail.ekmekci@gmail.com Abstract: Zeolite-based catalysts are used commonly to control NO x emissions in selective catalytic reduction applications in lean combustion gasoline engines. In this study, a catalyst was prepared by ion-exchange of vanadium promoted using microwave irradiation, over the clinoptilolite type of natural zeolite to selectively reduce the nitrogen oxides, especially in lean combustion conditions with the HCs in the exhaust gas. According to experimental results, the maximum NO x conversion efficiency reached 28 % at temperature of 287 C and space velocity of 30,000 h 1. Copyright 2012 Inderscience Enterprises Ltd.

2 Catalytic reduction of NO x on vanadium exchanged 213 Keywords: zeolite; clinoptilolite; catalyst; exhaust emission; HC-SCR; microwave. Reference to this paper should be made as follows: Emiroglu, A.O., Eldogan, O., Teker, M., Keskin, A. and Ekmekci, I. (2012) Catalytic reduction of NO x on vanadium exchanged natural zeolite using microwave irradiation, Int. J. Vehicle Design, Vol. 59, Nos. 2/3, pp Biographical notes: Alaattin Osman Emiroglu received his BS from the Faculty of Technical Education at Gazi University in He received his MS degree in Mechanical Education from Gazi University in He received his PhD degree in Mechanical Education from Sakarya University in He worked at the Automotive Department of Abant Izzet Baysal University from 2001 to He was a visiting research scientist at the University of Texas at Austin between 2010 and He has been a faculty member at the Mechanical Engineering Department of Abant Izzet Baysal University since His main research interests include internal combustion engines, exhaust emissions and thermodynamic. Osman Eldogan received his BS from Mechanical Engineering Department at Istanbul Technical University in He received his MS degree in Mechanical Engineering from Selcuk University in He received his PhD degree in Mechanical Education from Marmara University in He worked at the Mechanical Enginering Department of Pamukkale University ( ) and Mechanical Education Department of Sakarya University ( ). Now he works at Mechatronic Engineering Department of Sakarya University from His main research interests include internal combustion engines, vehicle design and dynamics. Murat Teker received his BS from the Faculty of Chemistry of Istanbul University in He received his MS degree in Chemical Engineering from Istanbul University in He received his PhD degree in Chemical Engineering from Yildiz Technical University in He has been a faculty member at the Department of Chemistry of Sciences & Arts Faculty of Sakarya University since His main research interests include separation and adsorption of gases, alternative fuels, reaction kinetics in media microwave. Ahmet Keskin received his BS from the Faculty of Technical Education at Gazi University in He received his MS degree in Mechanical Education from Dumlupinar University in He received his PhD degree in Mechanical Education from Sakarya University in He has been a faculty member at the Automotive Department of Abant Izzet Baysal University since His main research interests include internal combustion engines, exhaust emissions, and alternative fuels. İsmail Ekmekci is a Professor at the Industrial Engineering Department of the Faculty of Engineering and Design at the Istanbul Commerce University, Turkey. He received BEng in Mechanical Engineering at 1980 and he is completing his MSc at 1984 and he is completing his PhD at 1995 in Thermal Science and Process Division of Mechanical Engineering Dept. of Yildiz Technical University, Istanbul-Turkey. He has published over 95 conference and journal papers. His major research areas are thermodynamics of power plants, exergy, computational fluid dynamics and renewable energy projects.

3 214 A.O. Emiroglu et al. 1 Introduction Because of the increasing emission restrictions, the removal of nitrogen oxides in engine exhaust gases with excess oxygen (lean-burn and diesel engines) has gained importance. The widely used Three-Way Catalytic Converters (TWC) simultaneously control the three principal automotive pollutants, namely Carbon Monoxide (CO), Hydrocarbons (HC) and Nitrogen Oxides (NO x ). NO x emissions can be reduced up to 90% in stoichiometric exhaust, because there is essentially no excess oxygen following combustion. In this environment, NO x emissions are chemically reduced using exhaust CO, HC, Hydrogen (H 2 ) and other combustion products ( Bhattacharyya and Das, 2001). However the current three-way catalytic converters cannot remove NO x when there is excess of oxygen in the exhaust of lean combustion gasoline and diesel engines. The most important problem of TWCs is being efficient only in a very narrow region close to stoichiometric air/fuel ratio in addition to the high and rapidly rising price. For the fuel economy, the use of gasoline engines in the lean combustion area is desired. Today the fuel consumption of the developed lean mixture gasoline engines can be reduced up to 15% when compared to gasoline engines with three-way catalytic converters that have to work with stoichiometric air-fuel mixture. This ratio is even higher in direct injection gasoline engines (Das et al., 1997). While the engine is working with lean conditions, Selective Catalytic Reduction (SCR) systems may be used to reduce NO x to N 2 when there is excess oxygen in the exhaust. SCR is one of the most preferred methods due to its advantages like efficiency and the low working temperature. The reason why it is called selective is because the catalytic reducing reaction of NO x with reducing agents is more preferable to the reaction of oxygen with reducing agents. The noble metals, zeolites and metal oxides are used as catalyst for SCR. Ammonia, urea, carbon monoxide, hydrocarbons, methane, ethylene, propene, methanol and etc., are among the reducing agents. During the SCR applications in the lean burn gasoline engines for the reduction of NO x, unburned HC in the exhaust of the engine may be used as the reducing agent instead of oxygen. This process is named as HC-SCR of NO x (the selective catalytic reduction with HC). While implementing this system with the diesel engines, despite the necessity of HC addition (almost 2 to 3% of the fuel) as the reducing agent before catalyst, the existing HC quantity in the exhaust gas of gasoline engines, is sufficient for all the reduction of NO x. Zeolites and other porous materials are commonly used as catalysts greatly to reduce the exhaust emissions of lean burn gasoline engines and diesel engines. What is expected from industrial catalysts is the ability of activity, selectivity and endurance and all these are largely met by zeolites. Because of their rapid heating capacity and special electric field efficacy, microwaves are using widely in the field of synthesis chemistry in recent years (Gedye et al. 1998). In zeolite chemistry, microwave has been used mostly in the drying, cation-exchange and synthesis of zeolites (Whittington et al., 1992 and Arafat et al., 1993). As compared with conventional heating methods, the overall exchange speed upon the microwave radiation was over 60 times faster (Yin et al., 1998). Some studies related to the use of zeolites as catalysts with lean burn gasoline engines show that the metal ion exchanged zeolites are a potential alternative to reduce the NO x with HC, when there is excess oxygen. It is stated that ion exchange with the transition metals (i.e., Cu, Co, Fe, Ni) ZSM-5 and mordenite type catalysts especially, shows higher activity when compared to others (Petersson et al., 2005; Holma et al., 2004; Bhattacharyya et al., 2000; Subbiah et al., 2003; Mosqueda-Jimenez et al., 2003). It is stated that also the

4 Catalytic reduction of NO x on vanadium exchanged 215 mordenite and clinoptilolite whose Si/Al ratio is above 5 has a high reduction efficiency at the HC-SCR of NO x (Xu et al., 2002; Saaid et al., 2002; Chen et al., 2000; Van Kooten et al., 2000). The studies show that the natural zeolite is in a level to compete with synthetic zeolite not only because it is cheaper but also because of its catalytic activities (Chung et al., 1999). The aim of this study is to prepare a new catalyst to be used in the exhaust of gasoline engine, that is also active in lean burn conditions, that can eliminate the negative specifications of the TWC and especially efficient in removal of NO x. 2 Experimental 2.1 Catalyst preparation Vanadium ion-exchanged natural zeolite catalyst (V-NZ) was prepared using clinoptilolite type natural zeolite mined from Manisa-Gordes, Turkey. To prepare catalyst, the microwave ion-exchange method was used. Firstly, zeolites were broken and turned in to 1 mm to 1.4 mm size by sifting, and to remove the water crystallines from its pores that are found in its structure it was kept in the 400 C stove for 6 hours. To eliminate the impurities in the natural zeolite it was kept for 16 hours in 110 C on the hot plate in the 2 M HCl solution (200 g zeolite in one litre of hydrochloric acid). After washing with distilled water and drying, it was calcined in 450 C stove for six hours (HCl-NZ). To increase the ion exchange capacity, it was kept in the 1 M NH 4 NO 3 (ammonium nitrate) solution 90 C drying oven for six hours. Later it was washed with distilled water and dried and then it was calcined in 450 C stove for six hours. This process was repeated twice under the same circumstances (NH 4 -NZ). Later in the 0.3 M VOSO 4 solution (10 g zeolite-100 ml VOSO 4 solution) it was kept in the microwave oven for 10 minutes. After filtering it was washed with distilled water and it was calcined in 500 C stove for six hours. Thus vanadium ion exchanged natural zeolite catalysts (V-NZ) was prepared. 2.2 Catalysts characterisation XRF analysis While preparing the catalysts, to be able to see the changes in the chemical compound of clinoptilolite and to be able to calculate the ratio of Si/Al, after every process, samples were taken to analyse elementally using the Philips Axios XRF device. In Table 1 the results of XRF analysis are seen. The results of XRF analysis show that in the V-NZ catalyst there is 1950 ppm (nearly 0.2%) vanadium. For the original sample the Si/Al ratio is 5.99 the ratio increases to in the V-NZ catalyst especially as a result of the acid process XRD analysis Both to define the zeolite type and to analyse the structural changes occured in the form of zeolite, X-ray diffraction patterns were obtained using a Rigaku MultiFlex Plus brand XRD device and it was seen that the d values were matching with the d values on the ASTM card of clinoptilolite. As a result of the chemical processes clinoptilolite protects its frame structure. In the Figure 1 the XRD pattern of vanadium ion exchanged clinoptilolite is seen.

5 216 A.O. Emiroglu et al. Table 1 The results of XRF analysis Elemental composition as oxides Clinoptilolite (wt %) HCl-NZ NH4-NZ V-NZ SiO 2,% Al 2 O 3,% Na 2 O,% MgO,% P 2 O 5,% K 2 O,% CaO,% TiO 2,% MnO,% Fe 2 O 3,% V, ppm 1950 H 2 O Si/Al ratio Figure 1 XRD pattern of V-NZ catalyst BET surface area The surface area measurement is performed by using the Micromeritics FlowSorb II-2300 device. The surface area measurement of clinoptilolite is carried out after the water molecules in the pores were removed at the beginning and the surface area is measured as m 2 /g. The results of the analysis show that the acid process increases the surface area of zeolite but other processes do not make any changing on the surface area. As a result of the ion exchange it was seen that the surface area of V-NZ catalyst is m 2 /g SEM photo The surface area of the catalyst was analysed with the Jeol 5600 LV model SEM device. This technique is used to discover the surface structure by determining the size of the particle and the surface texture. In the Figure 2 the SEM image of V-NZ catalyst is seen. 2.3 Catalyst test equipment The catalyst performance experiments were carried out in 4 cycle 4 cylinder spark ignition Renault engine. The data of the working conditions of the engine was transferred to a

6 Catalytic reduction of NO x on vanadium exchanged 217 Figure 2 SEM image of the V-NZ catalyst computer via a data logger. The schematic drawing of the experiment set up is seen in Figure 3. Figure 3 The schematic drawing of the catalyst test equipment For the reduction of NO x the existing HC in the exhaust of the engine was used, no additional reductant was injected to exhaust. During the emission measurement, after the probe of

7 218 A.O. Emiroglu et al. the device was fixed on the measurement point, the data was recorded after the lambda value was constant. The NO x conversion efficiency was calculated using the measured NO x concentrations for the input and output of catalyst and calculated as: η cat = m in m out /m in η cat : Catalyst NO x conversion efficiency (%) m in : Before catalyst NO x concentration (ppm) m out : After catalyst NO x concentration (ppm) (1 ) The high exhaust temperature is reached by increasing the revolution. The Space Velocity (SV) is adjusted to the desired amount by changing the closing kleps on the exhaust pipe and exhaust flow passing from the catalyst bed. The space velocity of exhaust gas is calculated as: SV = Exhaust flow/catalyst volume Exhaust flow: (lt/h) Catalyst volume: (0.25 lt) (2) The air-fuel ratio is adjusted to the desired value by dismounting the oxygen sensor of the engine and the pressure of fuel gallery was changed with a pressure adjustment mechanism added to the system. 3 Results 3.1 The change in the emission of NO x according to the lambda The temperature of exhaust gas was fixed to 325 C and the SV was fixed to 30,000 1/h, lambda was changed from 0.7 to 1.4 and NO x emission values measured before and after V-NZ catalyst are shown in Figure 4. In Figure 5. according to the lambda change the conversion efficiency of V-NZ to NO x is seen. As it is also understood from the graph the catalytic activity is extremely low in the rich mixture area. When the lambda is between 1 and 1.1, the conversion efficiency was observed to reach to the maximum level. When the lambda is 1,034 the conversion efficiency was 24%. Even though the peak of the NO x conversion efficiency is much lower than TWC, on the contrary to TWC, in the lean mixture area its activity continues. The reason of less drop of the efficiency of V-NZ catalyst in the lean condition compared to TWC is because the zeolite catalysts reduc e NO x with HC selectively in the condition of excess oxygen. In other words, the NO x absorbed on zeolite reacts with HC instead of oxygen. The reason why the maximum NO x conversion efficiency of V-NZ is lower when compared to TWC may be presented as the channel structure and pore of clinoptilolite type natural zeolite used not being as regular as the wash-coat of TWC. Another reason may be because the vanadium used as an active ingredient is less active compared to rhodium used to reduce NO x with TWC.

8 Catalytic reduction of NO x on vanadium exchanged 219 Figure 4 According to the lambda the change in NO x concentration Figure 5 According to the lambda change of the conversion efficiency of V-NZ to NO x 3.2 The change of NO x emissions according to the temperature Temperature was changed from 125 C 425 C by keeping the lambda constant at 1, SV 30,000 1/h, to observe the effect of NO x emissions as a function of V-NZ temperature. In Figure 6, the curve of change of NO x emissions are seen according to the exhaust gas heat before and after V-NZ. Figure 6 The change of NO x emissions according to the exhaust gas temperature In Figure 7, the change of NO x conversion efficiency of V-NZ is seen as a function of exhaust gas heat. To activate the catalyst a certain temperature value must be reached, it is seen that in

9 220 A.O. Emiroglu et al. low temperature values the catalytic activity of catalyst is low. Above 200 C, the efficiency increases. The maximum conversion efficiency is reached with V-NZ 28% at 287 C. The NO x conversion efficiency decreases as the temperature continues to rise after the peak point. This is because at high temperatures the NO x concentration in the exhaust gas increases and the HC and CO emissions that are used as reducing agents decrease. Figure 7 The NO x exchange efficiency of V-NZ according to the exhaust gas temperature 3.3 The change of NO x emissions according to the SV The SV was changed between 20,000 1/h and 100,000 1/h, in lambda 1 and temperature was stabilised to 325 C for the purpose of viewing the effects on NO x emission as a function of the SV of V-NZ. In Figure 8 the change in NO x emissions, according to the SV before and after the V-NZ is seen. Figure 8 The change in NO x emissions, according to the space velocity In Figure 9 according to the SV, the change in NO x conversion efficiency of V-NZ is seen. In the graph it is seen that the conversion efficiency of V-NZ is extremely affected by the SV. It is observed that when the SV rises from 21,000 1/h to 101,000 1/h the conversion efficiency of V-NZ decreases from 27% to 15%. The reason for this is the decrease of contact time and thus the reaction time between the exhaust gas and zeolite catalysts in the high SV.

10 Catalytic reduction of NO x on vanadium exchanged 221 Figure 9 The NO x exchange efficiency of V-NZ according to space velocity 3.4 The pressure drop in V-NZ catalysts The pressure drop caused by V-NZ catalyst is calculated according to the change of SV. In Figure 10 the graph of the change of pressure drop caused by V-NZ catalyst as a function of the SV is seen. According to the graph, with the increase of the space velocity, the fall of the pressure increases. In the 101,000 1/h space velocity, the pressure fall is 21,543 mbar. Figure 10 According to the space velocity the pressure drop caused by V-NZ catalyst 4 Conclusions The results of the experiment show that the maximum NO x conversion efficiency of 28% for Ni-NZ is reached in the stoichiometric air/fuel ratio, in 287 C, when the SV is 30,000 1/h. This ratio is inadequate for the prepared catalyst to be used in commercial applications in this condition. However, although the peak of the NO x conversion efficiency of the prepared V-NZ catalyst is much lower than TWC, the continuation of its activity even in the lean region shows that on the contrary to TWC, zeolite catalysts promise future for the emission control of the engines that are intended to work in the lean region and for the fuel economy. The studies still continue on the catalysts that are more enduring, economic and active for reducing NO x in the normal exhaust temperature.

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