- Email: [email protected]
ZnO nanowires catalyst
Atmospheric Pollution Research xxx (2016) 1e5
H O S T E D BY
Contents lists available at ScienceDirect
Atmospheric Pollution Research journal homepage: http://www.journals.elsevier.com/locate/apr
Original Article
Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst lu A. Osman Emirog AIBU, Department of Mechanical Engineering, 14100, Bolu, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 15 August 2016 Accepted 17 August 2016 Available online xxx
The main pollutant exhaust emissions from gasoline engines are HCs, CO, and NOx. Today, the removal of NOx known to cause the formation of ground-level ozone, acid rain and smog is the most significant issue. The stringent emission regulations require that the emission of NOx is limited. In this study, the catalyst was prepared by impregnating Rh on the ZnO NWs grown on monolith cordierite channels in order to investigate in particular its NOx reduction activity and potential of ZnO nanowire structures to be used as monolith catalyst carrier. ZnO nanowire arrays have different morphology and effective porosity. Furthermore, thickness of ZnO nanowire arrays are less than traditional wash coat. Therefore, the ZnO nanowire arrays can be used as alternative monolith catalyst carrier. The structure of the catalyst was examined by SEM and XRD. Activity tests were performed under lean, stoichiometric and rich conditions in order to examine specifically NOx reduction activity of the finished catalyst. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. This situation is expected in case of excessive oxygen in TWC reactions. Copyright © 2016 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Catalysts Exhaust emission Reduction Nitrogen oxide ZnO nanowire arrays Rhodium
1. Introduction Motor vehicles are the main source of air pollution reached dangerous levels. Under stoichiometric conditions three-way catalytic converters are used widely removing three main pollutant emissions of gasoline engines, hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) simultaneously. Today, the removal of NOx known to cause the formation of ground-level ozone, acid rain and smog is the most significant issue. Therefore, stringent emission regulations require the reduction of nitrogen oxide emissions to very low levels (Traa et al., 1999; Coskun et al., 2014). Catalytic reduction of NO using unburnt compounds such as CO, hydrogen (H2), and HCs included in the exhaust gas is the most convenient and practical way for removing of NO (Ueda and Haruta, 1999; Ding et al., 2005). The main components in a modern three way catalysts (TWC) are substrate (ceramic or metal base material), carrier (wash-coat), and very thin active metal layer (S¸en et al., 2016). Platinum (Pt) and palladium (Pd) are used as active metals for the oxidation of HCs
E-mail address: [email protected]. Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
and CO, while rhodium (Rh) are known to be active for NOx reduction during the TWC reactions (Heo et al., 2012). During the catalytic process, having a greater number of active sites where reactants are converted to product molecules in a certain time is very critical. For this reason, it is very widespread to increase the number of active sites by distributing the active particles onto a carriers having high surface area such as alumina (Al2O3), silica (SiO2), titania (TiO2), ceria (CeO2), zirconia (ZrO2), and lanthana (La2O3). Among them, Al2O3 is known to be best catalyst carrier in environmental applications. Generally, the carriers are not catalytically active materials, but they are very significant for improving the activity, durability and selectivity of the catalyst (Heck et al., 2009). Also, additional stabilizer substances such as CeO2 and ZrO2 play a significant role to increase the TWC activity and durability (Heo et al., 2012; Cuif et al., 1998). The catalyst loses its activity due to operation at high temperatures. As TWCs usually work at high temperature, they tend to deactivation due to active metal sintering and alloy formation between active metals themselves as well as thermal degradation of lez-Velasco et al., catalyst carrier (Heo et al. 2009, 2012; Gonza 2000). Especially, the Rh impregnated on Al2O3 forms Rh aluminate at high temperature and it leads to loss of NOx reduction activity (Heo et al., 2012; Zimowska et al., 2006). In this study, it is reported that Rh/ZnO nanowires (NWs) catalyst has the potential for removing of NOx. The catalyst was
http://dx.doi.org/10.1016/j.apr.2016.08.006 1309-1042/Copyright © 2016 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
2
prepared by coating Rh on the nanowires in order to investigate the potential use of ZnO nanowire arrays as carrier of monolith catalyst. ZnO nanowire arrays have different morphology and more effective porosity than traditional catalyst carrier as represented in Fig. 1. In the Fig. 1a and b, active metals are represented as black dots and flow path of the exhaust gasses are represented as green arrows (in the web version). Fig. 1a indicates traditional carrier which have random structure. However, nanowire arrays have more uniform structure than conventional catalyst carrier as seen in Fig. 1b. Moreover ZnO nanowire arrays allow more open frontal area (OFA). Because the thickness of ZnO NWs coating on the monolith cordierite is much less than the thickness of the conventional carrier coating. As a result, less exhaust pressure drop and engine power loss occurs in vehicle applications (S¸en et al., 2016; Nijhuis et al., 2001). Another advantage of the thinner ZnO NWs coating is that the active metals are closer to the catalyst surface. Many studies describing the growth of ZnO NWs on different supports have been reported (Eswar et al., 2013; Ameen et al., 2012; Zhu et al., 2012). The synthesis of well aligned nanowires on wide surfaces can be achieved uniformly at low temperatures by using the hydrothermal method that consists of two steps: seeding and growth (Greene et al., 2005, 2006). Recently, ZnO nanowire arrays have been investigated extensively as catalyst carrier in methanol steam reforming (MSR) applications (Danwittayakul and Dutta, 2012; Zhang et al., 2014). Baker et al. (2014) investigated the oxidation activity of the Pt/Pd/ZnO NWs catalyst. ZnO nanowires were grown on the Si-wafer and coated with Pt and Pd by sputtercoating. They reported that nanowire arrays improved the activity of the catalyst. Guo et al. (2013) observed that mechanical and thermal stability of ZnO NWs synthesized on monolith cordierite are very satisfying. S¸en et al. (2016) investigated the CO and propane (C3H8) oxidation activities of Pd/ZnO NWs catalyst. They reported that ZnO nanowire arrays have the potential of being used in catalyst applications, and further studies may be conducted on this topic. In this study, NOx reduction activity of the monolith Rh/ZnO NWs catalyst has been investigated. ZnO nanowires appear to be alternative catalyst carrier materials. 2. Experimental 2.1. Catalyst preparation The catalyst sample was prepared by cutting in 2,5 cm diameter and 2,5 cm length from monolith cordierite (2MgO$2Al2O3$5SiO2) which has 400 cells per square inch (cpsi). Hydrothermal method was used for growth of ZnO NWs on cordierite followed the procedure reported by Greene et al. (2005, 2006). Nanowire growth was carried out in the two steps: seeding and growth. For the seeding step, 5 mM seeding solution was prepared by dissolving zinc acetate dihydrate (Zn(Ac)2$2H2O, 98%, SigmaeAldrich) in ethanol (absolute, Merck). The catalyst sample was placed in 30 mL seeding solution and heated in the microwave for
Fig. 1. Schematic drawing of (a) conventional carrier structure (b) nanowires array.
5 min at 70 W. Then it was calcined at 300 C in the furnace for 30 min. For the growth step, 600 mL growth solution was prepared by dissolving 25 mM zinc nitrate hexahydrate, (Zn(NO3)2$6H2O, 99%, SigmaeAldrich), 25 mM hexa methylene tetramine ((CH2)6N4, 99%, SigmaeAldrich) and 5 mM poly ethyleneimine (50% H2O, Fluka) in deionized water. And then, the catalyst sample was placed into plastic measuring cylinder which was holed at the bottom for circulation of the solution. The catalyst sample in measuring cylinder was placed into beaker and the growth solution was added to beaker and it was heated at 90 C for 3 h on a hot-plate. This growth stage was repeated three times with refreshed the solution. A peristaltic pump was used for circulation of solution throughout the cordierite channel. After that, the catalyst sample was washed with deionized water and calcined for 30 min at 300 C. The impregnation method was used to prepare the Rh/ZnO NWs catalyst. Rhodium(III) nitrate hydrate (Rh(NO3)3$xH2O, 36% rhodium, Sigma Aldrich) was dissolved in deionized water in order to prepare 5 mM Rh solution. Rh solution was dropped on catalyst sample and then washed and dried. The catalyst sample was calcined for 4 h at 400 C. All processes were performed under atmospheric conditions. 2.2. Catalyst characterization The scanning electron microscopy (SEM-Jeol JSM6390A) was used to assess the size, structure and dispersion of the ZnO NWs on cordierite. Crystallographic structure definition and composition quantification of the nanowires were carried out by using X-Ray Diffractometer (XRD-Rigaku Multiflex). 2.3. Catalyst activity tests The schematic image of the catalyst test system is shown in Fig. 2. The catalyst sample was placed at the center of the quartz tube which was inserted in a horizontal tube furnace (Protherm, PTF model). In order to prepare the gas mixture, nitrogen (N2, 99%), oxygen (O2, 99.9%), nitrogen oxide (NO, 10% balanced by N2), CO (99.5%) and C3H8 (99.5%) were sent into the quartz tube by mass flow control valves (Alicat, MC model). Lambda (l) value of the simulated gas mixture was calculated by using following formula (Gonz alez-Velasco et al., 1997).
l¼
2*ðO2 Þ þ ðNOÞ 10*ðC3 H8 Þ þ ðCOÞ
Fig. 2. Schematic view of the catalyst activity test system.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
For the stoichiometric condition, the gas compositions were arranged as follows: 1% CO, 0.05% NO, 0.975% O2, 0.1% C3H8 and balanced by N2. For lean and rich conditions, 1.075% and 0.875% O2 were sent to the gas mixture respectively. All tests were carried out at 15,000/h space velocity (SV, volumetric flow rate/catalyst volume). The catalyst activity tests were carried out at 25 C intervals between 200 C and 600 C to see the effect of the temperature on the catalyst performance. An exhaust emission analyzer (Horiba, Mexa584L) was used to measure the gas concentrations. The conversion efficiency was calculated by dividing the after catalyst gas concentration by the pre-catalyst gas concentration. 3. Result and discussion The SEM images of the catalyst sample taken in different resolution and scale are shown in Fig. 3a and b. Increasing the nanowires covering the cordierite channel surfaces will increase the surface area and the active site quantity. Therefore, coating the ZnO nanowires of entire surface is desirable. In Fig. 3a shows nanowires grown on all over the surface of the cordierite and nanowires uncoated areas on the cordierite channel surfaces are relatively minor. In Fig. 3b, the higher resolution SEM photograph of inner channel surface of the monolith catalyst shows that the nanowires have generally a homogeneous structure. The average length is 1 mm and the average diameter of the nanowires is 100 nm. The aspect ratio is about 10. SEM photographs of the cross-section of ZnO NWs and traditional catalyst wash-coat (g-Al2O3) were shown in Fig. 4a and b respectively. Fig. 4a indicates that the length of the ZnO NWs grown on surface of monolith cordierite. However, the 1 mm length of ZnO NWs, which were seen in the SEM photograph with higher resolution in Fig. 3b, cannot be visible in this resolution. As it is seen in
Fig. 3. SEM images of ZnO NWs on cordierite channels.
3
Fig. 4b, the thickness of the alumina is about 20 mm in the middle of the cordierite channel walls and is about 100 mm in the corners of the channel walls. This causes a significant reduction in the OFA of the catalyst. OFA of the ZnO NWs grown on cordierite channels is less in comparison with the alumina. This causes less reduction in exhaust pressure in vehicle applications and increment of engine power. X-ray diffraction patterns of ZnO/Cordierite and Rh/ZnO/Cordierite catalyst are presented in Fig. 5. The XRD pattern of Rh/ZnO/ Cord indicated very intense peaks at 2q ¼ 21.5 , 26.1, 28.2 , and 29.2 , which corresponded to the cordierite and at 2q ¼ 31.5 , 33.7, and 36.1, which corresponded to the ZnO. The result shows that crystal structures of the cordierite and ZnO nanowires were maintained after the rhodium impregnation. But, Rh/ZnO NWs/ Cordierite catalyst exhibited additional peaks at 2q ¼ 34.2 , 62.7 which corresponded to the Rh2O3. The Rh in Rh/ZnO NWs catalyst represents Rh2O3. The mass percentage of deposited ZnO NW arrays on the cordierite monolith substrate was calculated by weighting method and found as 3.98 wt %. The amount of rhodium in the catalyst was estimated from the molarity of the prepared Rh solution and found approximately 0.43 wt %. Activity tests of catalyst were carried out under the three conditions of lambda. Fig. 6 exhibits the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under stoichiometric conditions.
Fig. 4. SEM images of the cross-section of monolith cordierite carrier: a) ZnO NWs, b) g-Al2O3.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
4
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
Fig. 8. Conversion efficiencies of the Rh/ZnO NWs catalyst at rich conditions. Fig. 5. XRD patterns for ZnO/Cordierite and Rh/ZnO/Cordierite.
Fig. 6. Conversion efficiencies of the Rh/ZnO NWs catalyst at stoichiometric conditions.
At low temperature, the catalyst starts to convert NO over 90% at about 300 C using CO as a reducing agent. It reaches the T90 temperature (90% conversion temperature) for CO at about 300 C. Because CO reacts with O2 instead of NO at temperature above 300 C, NO conversion efficiency drops rapidly. The interaction between NO and HC begins at about 400 C and both of them begin to transform simultaneously. HC reaches the T50 (50% conversion temperature) at 435 C and NO reaches the T50 at 520 C. Both gases reach the T90 at about 600 C. Fig. 7 shows that the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under lean conditions. In case of a lean mixture at low temperatures, NO uses the CO as reducing agent and both gases reach the T90 at about 300 C. In the meantime, CO reaches the T90 temperature at about 300 C. However, CO reacts with O2 instead of NO above 300 C as in stoichiometric condition and conversion of NO decreases rapidly. Because of excess oxygen in the gas mixture, HC reacts with O2 at high temperatures. NO conversion is not realized because NO cannot be reduced with HC. T50 temperature is reached for HC at about 430 C.
Fig. 7. Conversion efficiencies of the Rh/ZnO NWs catalyst at lean conditions.
Fig. 8 exhibits that the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under rich conditions. In case of rich mixture, NO uses CO as reducing agent at low temperature as in the stoichiometric and lean condition, and both gas reach the T90 at about 300 C. NO conversion efficiency decreases under rich conditions after reaching 95% at 325 C. At temperatures above 400 C, NO is reduced using HC as reducing agent and reaches the T50 at 440 C and reaches the T90 at 500 C. The conversion efficiency of NO under three conditions is shown Fig. 9. Since NO is reduced with CO at low temperatures, it reaches the T90 at about 300 C for all three conditions. For all conditions NO conversation of Rh/ZnO catalyst decreases between 300 C and 350 C. Because the catalyst selectivity of CO oxidation changes from NO to O2. However, reduction of NO with HC begins at temperature above 400 C again under stoichiometric and rich conditions. Under stoichiometric condition, NO reaches the T50 and T90 at 520 C and 600 C respectively. NO reaches the T50 and T90 at 440 C and 500 C respectively under rich condition. Due to the excess of oxygen in the gas mixture under lean condition, HC which will be used as reducing agent by NO reacts with O2, thus NO cannot be reduced. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. 4. Conclusions Rhodium is major metal for the reduction of NO. ZnO NWs were grown on monolith cordierite as catalyst wash-coat using the hydrothermal method and Rh/ZnO nanowire/cordierite catalyst was prepared. Activity tests were performed to determine the NO reduction capability under the stoichiometric, rich and lean conditions. As NO is reduced with CO at low temperatures, T90 has been achieved for NO at about 300 C for all three conditions. At higher temperatures, oxidation of CO with O2 accelerates and conversion efficiency of NO drops rapidly. At temperature above 400 C,
Fig. 9. Rh/ZnO NWs catalyst conversion efficiency of NO under different lambda conditions.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
reduction of NO with HC begins again under rich and stoichiometric conditions. NO reaches the T50 and T90 at 520 C and 600 C respectively under stoichiometric condition. T50 and T90 are achieved for NO at 440 C and 500 C respectively under rich condition. In case of excessive oxygen in the gas mixture under lean condition, HC which will be used as reductant by NO reacts with O2, thus NO cannot be reduced. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. This situation is expected in case of excessive oxygen in TWC reactions. Consequently, ZnO nanowire arrays have the potential of being used as catalyst carrier and further studies may be conducted on the Rh/ZnO NWs catalyst for the reduction of NO. Acknowledgments This study was supported by the Research Project Foundation of Abant Izzet Baysal University, Bolu, Turkey, under contact numbers: 2014.09.05.707 and 2013-09.03.604. References Ameen, S., Akhtar, M.S., Song, M., Shin, H.S., 2012. Vertically aligned ZnO nanorods on hot filament chemical vapor deposition grown graphene oxide thin film substrate: solar energy conversion. ACS Appl. Mater. Interfaces 4 (8), 4405e4412. Baker, C.A., Emiroglu, A.O., Mallick, R., Ezekoye, O.A., Shi, L., Hall, M.J., 2014. Development of an analytical design tool for monolithic emission control catalysts and application to nano-textured substrate system. J. Therm. Sci. Eng. Appl. 6 (3), 031014. Coskun, G., Soyhan, H.S., Demir, U., Turkcan, A., Ozsezen, A.N., Canakci, M., 2014. Influences of second injection variations on combustion and emissions of an HCCI-DI engine: experiments and CFD modelling. Fuel 136, 287e294. Cuif, J.P., Deutsch, S., Marczi, M., Jen, H.W., Graham, G.W., Chung, W., McCabe, R.W., 1998. High Temperature Stability of Ceria-zirconia Supported Pd Model Catalysts (No. 980668). SAE Technical Paper. Danwittayakul, S., Dutta, J., 2012. Zinc oxide nanorods based catalysts for hydrogen production by steam reforming of methanol. Int. J. Hydrogen Energy 37 (7), 5518e5526.
5
Ding, X., Yang, J., Hou, J.G., Zhu, Q., 2005. Theoretical study of molecular nitrogen adsorption on Au clusters. J. Mol. Struct. THEOCHEM 755 (1), 9e17. Eswar, K.A., Ab Aziz, A., Rusop Mahmood, M., Abdullah, S., 2013. Surface morphology of seeded nanostructured ZnO on silicon by sol-gel technique. In: Advanced Materials Research, vol. 667, pp. 265e271. lez-Velasco, J.R., Botas, J.A., Ferret, R., Gonz Gonza alez-Marcos, M.P., Marc, J.L., rrez-Ortiz, M.A., 2000. Thermal aging of Pd/Pt/Rh automotive catalysts Gutie under a cycled oxidizingereducing environment. Catal. Today 59 (3), 395e402. lez-Velasco, J.R., Botas, J.A., Gonza lez-Marcos, J.A., Gutie rrez-Ortiz, M.A., 1997. Gonza Influence of water and hydrocarbon processed in feedstream on the three-way behaviour of platinum-alumina catalysts. Appl. Catal. B Environ. 12 (1), 61e79. Greene, L.E., Law, M., Tan, D.H., Montano, M., Goldberger, J., Somorjai, G., Yang, P., 2005. General route to vertical ZnO nanowire arrays using textured ZnO seeds. Nano Lett. 5 (7), 1231e1236. Greene, L.E., Yuhas, B.D., Law, M., Zitoun, D., Yang, P., 2006. Solution-grown zinc oxide nanowires. Inorg. Chem. 45 (19), 7535e7543. Guo, Y., Ren, Z., Xiao, W., Liu, C., Sharma, H., Gao, H., Gao, P.X., 2013. Robust 3-D configurated metal oxide nano-array based monolithic catalysts with ultrahigh materials usage efficiency and catalytic performance tunability. Nano Energy 2 (5), 873e881. Heck, R.M., Farrauto, R.J., Gulati, S.T., 2009. Catalytic Air Pollution Control: Commercial Technology, third ed. Wiley, Hoboken. Heo, I., Cho, B.K., Nam, I.S., Choung, J.W., Yoo, S., 2012. Activity and thermal stability of Rh-based catalytic system for an advanced modern TWC. Appl. Catal. B Environ. 121, 75e87. Heo, I., Choung, J.W., Kim, P.S., Nam, I.S., Song, Y.I., In, C.B., Yeo, G.K., 2009. The alteration of the performance of field-aged Pd-based TWCs towards CO and C3H6 oxidation. Appl. Catal. B Environ. 92 (1), 114e125. Nijhuis, T.A., Beers, A.E., Vergunst, T., Hoek, I., Kapteijn, F., Moulijn, J.A., 2001. Preparation of monolithic catalysts. Catal. Rev. 43 (4), 345e380. lu, A.O., Çelik, M.B., 2016. CO and C3H8 oxidation activity of Pd/ZnO S¸en, M., Emirog nanowires/cordierite catalyst. Appl. Therm. Eng. 99, 841e845. Traa, Y., Burger, B., Weitkamp, J., 1999. Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons. Microporous Mesoporous Mater. 30 (1), 3e41. Ueda, A., Haruta, M., 1999. Nitric oxide reduction with hydrogen, carbon monoxide, and hydrocarbons over gold catalysts. Gold Bull. 32 (1), 3e11. Zhang, H., Sun, J., Dagle, V.L., Halevi, B., Datye, A.K., Wang, Y., 2014. Influence of ZnO facets on Pd/ZnO catalysts for methanol steam reforming. ACS Catal. 4 (7), 2379e2386. Zhu, G., Gu, S., Zhu, S., Huang, S., Gu, R., Ye, J., Zheng, Y., 2012. Optimization study of metal-organic chemical vapor deposition of ZnO on sapphire substrate. J. Cryst. Growth 349 (1), 6e11. Zimowska, M., Wagner, J.B., Dziedzic, J., Camra, J., Borze˛ cka-Prokop, B., Najbar, M., 2006. Some aspects of metal-support strong interactions in Rh/Al2O3 catalyst under oxidising and reducing conditions. Chem. Phys. Lett. 417 (1), 137e142.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
H O S T E D BY
Contents lists available at ScienceDirect
Atmospheric Pollution Research journal homepage: http://www.journals.elsevier.com/locate/apr
Original Article
Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst lu A. Osman Emirog AIBU, Department of Mechanical Engineering, 14100, Bolu, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 15 August 2016 Accepted 17 August 2016 Available online xxx
The main pollutant exhaust emissions from gasoline engines are HCs, CO, and NOx. Today, the removal of NOx known to cause the formation of ground-level ozone, acid rain and smog is the most significant issue. The stringent emission regulations require that the emission of NOx is limited. In this study, the catalyst was prepared by impregnating Rh on the ZnO NWs grown on monolith cordierite channels in order to investigate in particular its NOx reduction activity and potential of ZnO nanowire structures to be used as monolith catalyst carrier. ZnO nanowire arrays have different morphology and effective porosity. Furthermore, thickness of ZnO nanowire arrays are less than traditional wash coat. Therefore, the ZnO nanowire arrays can be used as alternative monolith catalyst carrier. The structure of the catalyst was examined by SEM and XRD. Activity tests were performed under lean, stoichiometric and rich conditions in order to examine specifically NOx reduction activity of the finished catalyst. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. This situation is expected in case of excessive oxygen in TWC reactions. Copyright © 2016 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Catalysts Exhaust emission Reduction Nitrogen oxide ZnO nanowire arrays Rhodium
1. Introduction Motor vehicles are the main source of air pollution reached dangerous levels. Under stoichiometric conditions three-way catalytic converters are used widely removing three main pollutant emissions of gasoline engines, hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) simultaneously. Today, the removal of NOx known to cause the formation of ground-level ozone, acid rain and smog is the most significant issue. Therefore, stringent emission regulations require the reduction of nitrogen oxide emissions to very low levels (Traa et al., 1999; Coskun et al., 2014). Catalytic reduction of NO using unburnt compounds such as CO, hydrogen (H2), and HCs included in the exhaust gas is the most convenient and practical way for removing of NO (Ueda and Haruta, 1999; Ding et al., 2005). The main components in a modern three way catalysts (TWC) are substrate (ceramic or metal base material), carrier (wash-coat), and very thin active metal layer (S¸en et al., 2016). Platinum (Pt) and palladium (Pd) are used as active metals for the oxidation of HCs
E-mail address: [email protected]. Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
and CO, while rhodium (Rh) are known to be active for NOx reduction during the TWC reactions (Heo et al., 2012). During the catalytic process, having a greater number of active sites where reactants are converted to product molecules in a certain time is very critical. For this reason, it is very widespread to increase the number of active sites by distributing the active particles onto a carriers having high surface area such as alumina (Al2O3), silica (SiO2), titania (TiO2), ceria (CeO2), zirconia (ZrO2), and lanthana (La2O3). Among them, Al2O3 is known to be best catalyst carrier in environmental applications. Generally, the carriers are not catalytically active materials, but they are very significant for improving the activity, durability and selectivity of the catalyst (Heck et al., 2009). Also, additional stabilizer substances such as CeO2 and ZrO2 play a significant role to increase the TWC activity and durability (Heo et al., 2012; Cuif et al., 1998). The catalyst loses its activity due to operation at high temperatures. As TWCs usually work at high temperature, they tend to deactivation due to active metal sintering and alloy formation between active metals themselves as well as thermal degradation of lez-Velasco et al., catalyst carrier (Heo et al. 2009, 2012; Gonza 2000). Especially, the Rh impregnated on Al2O3 forms Rh aluminate at high temperature and it leads to loss of NOx reduction activity (Heo et al., 2012; Zimowska et al., 2006). In this study, it is reported that Rh/ZnO nanowires (NWs) catalyst has the potential for removing of NOx. The catalyst was
http://dx.doi.org/10.1016/j.apr.2016.08.006 1309-1042/Copyright © 2016 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
2
prepared by coating Rh on the nanowires in order to investigate the potential use of ZnO nanowire arrays as carrier of monolith catalyst. ZnO nanowire arrays have different morphology and more effective porosity than traditional catalyst carrier as represented in Fig. 1. In the Fig. 1a and b, active metals are represented as black dots and flow path of the exhaust gasses are represented as green arrows (in the web version). Fig. 1a indicates traditional carrier which have random structure. However, nanowire arrays have more uniform structure than conventional catalyst carrier as seen in Fig. 1b. Moreover ZnO nanowire arrays allow more open frontal area (OFA). Because the thickness of ZnO NWs coating on the monolith cordierite is much less than the thickness of the conventional carrier coating. As a result, less exhaust pressure drop and engine power loss occurs in vehicle applications (S¸en et al., 2016; Nijhuis et al., 2001). Another advantage of the thinner ZnO NWs coating is that the active metals are closer to the catalyst surface. Many studies describing the growth of ZnO NWs on different supports have been reported (Eswar et al., 2013; Ameen et al., 2012; Zhu et al., 2012). The synthesis of well aligned nanowires on wide surfaces can be achieved uniformly at low temperatures by using the hydrothermal method that consists of two steps: seeding and growth (Greene et al., 2005, 2006). Recently, ZnO nanowire arrays have been investigated extensively as catalyst carrier in methanol steam reforming (MSR) applications (Danwittayakul and Dutta, 2012; Zhang et al., 2014). Baker et al. (2014) investigated the oxidation activity of the Pt/Pd/ZnO NWs catalyst. ZnO nanowires were grown on the Si-wafer and coated with Pt and Pd by sputtercoating. They reported that nanowire arrays improved the activity of the catalyst. Guo et al. (2013) observed that mechanical and thermal stability of ZnO NWs synthesized on monolith cordierite are very satisfying. S¸en et al. (2016) investigated the CO and propane (C3H8) oxidation activities of Pd/ZnO NWs catalyst. They reported that ZnO nanowire arrays have the potential of being used in catalyst applications, and further studies may be conducted on this topic. In this study, NOx reduction activity of the monolith Rh/ZnO NWs catalyst has been investigated. ZnO nanowires appear to be alternative catalyst carrier materials. 2. Experimental 2.1. Catalyst preparation The catalyst sample was prepared by cutting in 2,5 cm diameter and 2,5 cm length from monolith cordierite (2MgO$2Al2O3$5SiO2) which has 400 cells per square inch (cpsi). Hydrothermal method was used for growth of ZnO NWs on cordierite followed the procedure reported by Greene et al. (2005, 2006). Nanowire growth was carried out in the two steps: seeding and growth. For the seeding step, 5 mM seeding solution was prepared by dissolving zinc acetate dihydrate (Zn(Ac)2$2H2O, 98%, SigmaeAldrich) in ethanol (absolute, Merck). The catalyst sample was placed in 30 mL seeding solution and heated in the microwave for
Fig. 1. Schematic drawing of (a) conventional carrier structure (b) nanowires array.
5 min at 70 W. Then it was calcined at 300 C in the furnace for 30 min. For the growth step, 600 mL growth solution was prepared by dissolving 25 mM zinc nitrate hexahydrate, (Zn(NO3)2$6H2O, 99%, SigmaeAldrich), 25 mM hexa methylene tetramine ((CH2)6N4, 99%, SigmaeAldrich) and 5 mM poly ethyleneimine (50% H2O, Fluka) in deionized water. And then, the catalyst sample was placed into plastic measuring cylinder which was holed at the bottom for circulation of the solution. The catalyst sample in measuring cylinder was placed into beaker and the growth solution was added to beaker and it was heated at 90 C for 3 h on a hot-plate. This growth stage was repeated three times with refreshed the solution. A peristaltic pump was used for circulation of solution throughout the cordierite channel. After that, the catalyst sample was washed with deionized water and calcined for 30 min at 300 C. The impregnation method was used to prepare the Rh/ZnO NWs catalyst. Rhodium(III) nitrate hydrate (Rh(NO3)3$xH2O, 36% rhodium, Sigma Aldrich) was dissolved in deionized water in order to prepare 5 mM Rh solution. Rh solution was dropped on catalyst sample and then washed and dried. The catalyst sample was calcined for 4 h at 400 C. All processes were performed under atmospheric conditions. 2.2. Catalyst characterization The scanning electron microscopy (SEM-Jeol JSM6390A) was used to assess the size, structure and dispersion of the ZnO NWs on cordierite. Crystallographic structure definition and composition quantification of the nanowires were carried out by using X-Ray Diffractometer (XRD-Rigaku Multiflex). 2.3. Catalyst activity tests The schematic image of the catalyst test system is shown in Fig. 2. The catalyst sample was placed at the center of the quartz tube which was inserted in a horizontal tube furnace (Protherm, PTF model). In order to prepare the gas mixture, nitrogen (N2, 99%), oxygen (O2, 99.9%), nitrogen oxide (NO, 10% balanced by N2), CO (99.5%) and C3H8 (99.5%) were sent into the quartz tube by mass flow control valves (Alicat, MC model). Lambda (l) value of the simulated gas mixture was calculated by using following formula (Gonz alez-Velasco et al., 1997).
l¼
2*ðO2 Þ þ ðNOÞ 10*ðC3 H8 Þ þ ðCOÞ
Fig. 2. Schematic view of the catalyst activity test system.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
For the stoichiometric condition, the gas compositions were arranged as follows: 1% CO, 0.05% NO, 0.975% O2, 0.1% C3H8 and balanced by N2. For lean and rich conditions, 1.075% and 0.875% O2 were sent to the gas mixture respectively. All tests were carried out at 15,000/h space velocity (SV, volumetric flow rate/catalyst volume). The catalyst activity tests were carried out at 25 C intervals between 200 C and 600 C to see the effect of the temperature on the catalyst performance. An exhaust emission analyzer (Horiba, Mexa584L) was used to measure the gas concentrations. The conversion efficiency was calculated by dividing the after catalyst gas concentration by the pre-catalyst gas concentration. 3. Result and discussion The SEM images of the catalyst sample taken in different resolution and scale are shown in Fig. 3a and b. Increasing the nanowires covering the cordierite channel surfaces will increase the surface area and the active site quantity. Therefore, coating the ZnO nanowires of entire surface is desirable. In Fig. 3a shows nanowires grown on all over the surface of the cordierite and nanowires uncoated areas on the cordierite channel surfaces are relatively minor. In Fig. 3b, the higher resolution SEM photograph of inner channel surface of the monolith catalyst shows that the nanowires have generally a homogeneous structure. The average length is 1 mm and the average diameter of the nanowires is 100 nm. The aspect ratio is about 10. SEM photographs of the cross-section of ZnO NWs and traditional catalyst wash-coat (g-Al2O3) were shown in Fig. 4a and b respectively. Fig. 4a indicates that the length of the ZnO NWs grown on surface of monolith cordierite. However, the 1 mm length of ZnO NWs, which were seen in the SEM photograph with higher resolution in Fig. 3b, cannot be visible in this resolution. As it is seen in
Fig. 3. SEM images of ZnO NWs on cordierite channels.
3
Fig. 4b, the thickness of the alumina is about 20 mm in the middle of the cordierite channel walls and is about 100 mm in the corners of the channel walls. This causes a significant reduction in the OFA of the catalyst. OFA of the ZnO NWs grown on cordierite channels is less in comparison with the alumina. This causes less reduction in exhaust pressure in vehicle applications and increment of engine power. X-ray diffraction patterns of ZnO/Cordierite and Rh/ZnO/Cordierite catalyst are presented in Fig. 5. The XRD pattern of Rh/ZnO/ Cord indicated very intense peaks at 2q ¼ 21.5 , 26.1, 28.2 , and 29.2 , which corresponded to the cordierite and at 2q ¼ 31.5 , 33.7, and 36.1, which corresponded to the ZnO. The result shows that crystal structures of the cordierite and ZnO nanowires were maintained after the rhodium impregnation. But, Rh/ZnO NWs/ Cordierite catalyst exhibited additional peaks at 2q ¼ 34.2 , 62.7 which corresponded to the Rh2O3. The Rh in Rh/ZnO NWs catalyst represents Rh2O3. The mass percentage of deposited ZnO NW arrays on the cordierite monolith substrate was calculated by weighting method and found as 3.98 wt %. The amount of rhodium in the catalyst was estimated from the molarity of the prepared Rh solution and found approximately 0.43 wt %. Activity tests of catalyst were carried out under the three conditions of lambda. Fig. 6 exhibits the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under stoichiometric conditions.
Fig. 4. SEM images of the cross-section of monolith cordierite carrier: a) ZnO NWs, b) g-Al2O3.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
4
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
Fig. 8. Conversion efficiencies of the Rh/ZnO NWs catalyst at rich conditions. Fig. 5. XRD patterns for ZnO/Cordierite and Rh/ZnO/Cordierite.
Fig. 6. Conversion efficiencies of the Rh/ZnO NWs catalyst at stoichiometric conditions.
At low temperature, the catalyst starts to convert NO over 90% at about 300 C using CO as a reducing agent. It reaches the T90 temperature (90% conversion temperature) for CO at about 300 C. Because CO reacts with O2 instead of NO at temperature above 300 C, NO conversion efficiency drops rapidly. The interaction between NO and HC begins at about 400 C and both of them begin to transform simultaneously. HC reaches the T50 (50% conversion temperature) at 435 C and NO reaches the T50 at 520 C. Both gases reach the T90 at about 600 C. Fig. 7 shows that the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under lean conditions. In case of a lean mixture at low temperatures, NO uses the CO as reducing agent and both gases reach the T90 at about 300 C. In the meantime, CO reaches the T90 temperature at about 300 C. However, CO reacts with O2 instead of NO above 300 C as in stoichiometric condition and conversion of NO decreases rapidly. Because of excess oxygen in the gas mixture, HC reacts with O2 at high temperatures. NO conversion is not realized because NO cannot be reduced with HC. T50 temperature is reached for HC at about 430 C.
Fig. 7. Conversion efficiencies of the Rh/ZnO NWs catalyst at lean conditions.
Fig. 8 exhibits that the NO, HC and CO conversion efficiencies of the Rh/ZnO NWs catalyst under rich conditions. In case of rich mixture, NO uses CO as reducing agent at low temperature as in the stoichiometric and lean condition, and both gas reach the T90 at about 300 C. NO conversion efficiency decreases under rich conditions after reaching 95% at 325 C. At temperatures above 400 C, NO is reduced using HC as reducing agent and reaches the T50 at 440 C and reaches the T90 at 500 C. The conversion efficiency of NO under three conditions is shown Fig. 9. Since NO is reduced with CO at low temperatures, it reaches the T90 at about 300 C for all three conditions. For all conditions NO conversation of Rh/ZnO catalyst decreases between 300 C and 350 C. Because the catalyst selectivity of CO oxidation changes from NO to O2. However, reduction of NO with HC begins at temperature above 400 C again under stoichiometric and rich conditions. Under stoichiometric condition, NO reaches the T50 and T90 at 520 C and 600 C respectively. NO reaches the T50 and T90 at 440 C and 500 C respectively under rich condition. Due to the excess of oxygen in the gas mixture under lean condition, HC which will be used as reducing agent by NO reacts with O2, thus NO cannot be reduced. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. 4. Conclusions Rhodium is major metal for the reduction of NO. ZnO NWs were grown on monolith cordierite as catalyst wash-coat using the hydrothermal method and Rh/ZnO nanowire/cordierite catalyst was prepared. Activity tests were performed to determine the NO reduction capability under the stoichiometric, rich and lean conditions. As NO is reduced with CO at low temperatures, T90 has been achieved for NO at about 300 C for all three conditions. At higher temperatures, oxidation of CO with O2 accelerates and conversion efficiency of NO drops rapidly. At temperature above 400 C,
Fig. 9. Rh/ZnO NWs catalyst conversion efficiency of NO under different lambda conditions.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006
A.O. Emiroglu / Atmospheric Pollution Research xxx (2016) 1e5
reduction of NO with HC begins again under rich and stoichiometric conditions. NO reaches the T50 and T90 at 520 C and 600 C respectively under stoichiometric condition. T50 and T90 are achieved for NO at 440 C and 500 C respectively under rich condition. In case of excessive oxygen in the gas mixture under lean condition, HC which will be used as reductant by NO reacts with O2, thus NO cannot be reduced. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. This situation is expected in case of excessive oxygen in TWC reactions. Consequently, ZnO nanowire arrays have the potential of being used as catalyst carrier and further studies may be conducted on the Rh/ZnO NWs catalyst for the reduction of NO. Acknowledgments This study was supported by the Research Project Foundation of Abant Izzet Baysal University, Bolu, Turkey, under contact numbers: 2014.09.05.707 and 2013-09.03.604. References Ameen, S., Akhtar, M.S., Song, M., Shin, H.S., 2012. Vertically aligned ZnO nanorods on hot filament chemical vapor deposition grown graphene oxide thin film substrate: solar energy conversion. ACS Appl. Mater. Interfaces 4 (8), 4405e4412. Baker, C.A., Emiroglu, A.O., Mallick, R., Ezekoye, O.A., Shi, L., Hall, M.J., 2014. Development of an analytical design tool for monolithic emission control catalysts and application to nano-textured substrate system. J. Therm. Sci. Eng. Appl. 6 (3), 031014. Coskun, G., Soyhan, H.S., Demir, U., Turkcan, A., Ozsezen, A.N., Canakci, M., 2014. Influences of second injection variations on combustion and emissions of an HCCI-DI engine: experiments and CFD modelling. Fuel 136, 287e294. Cuif, J.P., Deutsch, S., Marczi, M., Jen, H.W., Graham, G.W., Chung, W., McCabe, R.W., 1998. High Temperature Stability of Ceria-zirconia Supported Pd Model Catalysts (No. 980668). SAE Technical Paper. Danwittayakul, S., Dutta, J., 2012. Zinc oxide nanorods based catalysts for hydrogen production by steam reforming of methanol. Int. J. Hydrogen Energy 37 (7), 5518e5526.
5
Ding, X., Yang, J., Hou, J.G., Zhu, Q., 2005. Theoretical study of molecular nitrogen adsorption on Au clusters. J. Mol. Struct. THEOCHEM 755 (1), 9e17. Eswar, K.A., Ab Aziz, A., Rusop Mahmood, M., Abdullah, S., 2013. Surface morphology of seeded nanostructured ZnO on silicon by sol-gel technique. In: Advanced Materials Research, vol. 667, pp. 265e271. lez-Velasco, J.R., Botas, J.A., Ferret, R., Gonz Gonza alez-Marcos, M.P., Marc, J.L., rrez-Ortiz, M.A., 2000. Thermal aging of Pd/Pt/Rh automotive catalysts Gutie under a cycled oxidizingereducing environment. Catal. Today 59 (3), 395e402. lez-Velasco, J.R., Botas, J.A., Gonza lez-Marcos, J.A., Gutie rrez-Ortiz, M.A., 1997. Gonza Influence of water and hydrocarbon processed in feedstream on the three-way behaviour of platinum-alumina catalysts. Appl. Catal. B Environ. 12 (1), 61e79. Greene, L.E., Law, M., Tan, D.H., Montano, M., Goldberger, J., Somorjai, G., Yang, P., 2005. General route to vertical ZnO nanowire arrays using textured ZnO seeds. Nano Lett. 5 (7), 1231e1236. Greene, L.E., Yuhas, B.D., Law, M., Zitoun, D., Yang, P., 2006. Solution-grown zinc oxide nanowires. Inorg. Chem. 45 (19), 7535e7543. Guo, Y., Ren, Z., Xiao, W., Liu, C., Sharma, H., Gao, H., Gao, P.X., 2013. Robust 3-D configurated metal oxide nano-array based monolithic catalysts with ultrahigh materials usage efficiency and catalytic performance tunability. Nano Energy 2 (5), 873e881. Heck, R.M., Farrauto, R.J., Gulati, S.T., 2009. Catalytic Air Pollution Control: Commercial Technology, third ed. Wiley, Hoboken. Heo, I., Cho, B.K., Nam, I.S., Choung, J.W., Yoo, S., 2012. Activity and thermal stability of Rh-based catalytic system for an advanced modern TWC. Appl. Catal. B Environ. 121, 75e87. Heo, I., Choung, J.W., Kim, P.S., Nam, I.S., Song, Y.I., In, C.B., Yeo, G.K., 2009. The alteration of the performance of field-aged Pd-based TWCs towards CO and C3H6 oxidation. Appl. Catal. B Environ. 92 (1), 114e125. Nijhuis, T.A., Beers, A.E., Vergunst, T., Hoek, I., Kapteijn, F., Moulijn, J.A., 2001. Preparation of monolithic catalysts. Catal. Rev. 43 (4), 345e380. lu, A.O., Çelik, M.B., 2016. CO and C3H8 oxidation activity of Pd/ZnO S¸en, M., Emirog nanowires/cordierite catalyst. Appl. Therm. Eng. 99, 841e845. Traa, Y., Burger, B., Weitkamp, J., 1999. Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons. Microporous Mesoporous Mater. 30 (1), 3e41. Ueda, A., Haruta, M., 1999. Nitric oxide reduction with hydrogen, carbon monoxide, and hydrocarbons over gold catalysts. Gold Bull. 32 (1), 3e11. Zhang, H., Sun, J., Dagle, V.L., Halevi, B., Datye, A.K., Wang, Y., 2014. Influence of ZnO facets on Pd/ZnO catalysts for methanol steam reforming. ACS Catal. 4 (7), 2379e2386. Zhu, G., Gu, S., Zhu, S., Huang, S., Gu, R., Ye, J., Zheng, Y., 2012. Optimization study of metal-organic chemical vapor deposition of ZnO on sapphire substrate. J. Cryst. Growth 349 (1), 6e11. Zimowska, M., Wagner, J.B., Dziedzic, J., Camra, J., Borze˛ cka-Prokop, B., Najbar, M., 2006. Some aspects of metal-support strong interactions in Rh/Al2O3 catalyst under oxidising and reducing conditions. Chem. Phys. Lett. 417 (1), 137e142.
lu, A.O., Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst, Atmospheric Pollution Please cite this article in press as: Emirog Research (2016), http://dx.doi.org/10.1016/j.apr.2016.08.006