International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 6; June 2017

Transcription

1 Effect of Sulfur Poisoning on the Catalytic Performance of Pt/Al 2 O 3 Diesel Oxidation Catalyst: Study on Sulfur Dioxide Adsorption and Sulfates Accumulation Zhengzheng Yang* College of Environmental Science and Engineering, China West Normal University, Nanchong , China. * Corresponding author: Zhengzheng Yang Abstract: In this work, the deactivation of Pt/Al 2 O 3 diesel oxidation catalyst results from sulfur poisoning was investigated. The catalytic performance measurements indicate that the presence of SO 2 in diesel exhaust conditions caused the competitive adsorption between SO 2 and CO, which repressed the catalytic activity of Pt/Al 2 O 3 for CO oxidation; and the SO 2 adsorption promoted the activity for C 3 H 6 catalytic oxidation. After long sulfur poisoning treatment, the sulfates accumulation on the surface of Pt/Al 2 O 3 catalyst results in an obvious deactivation of both C 3 H 6 and CO catalytic oxidation, which because of the surface active sites were covered by the accumulated sulfates. Keywords: Vehicular exhaust purification; Chemical aging; Sulfur poisoning; Sulfates accumulation 117

2 1. Introduction To efficiently purify the vehicular exhaust and fulfil the stringently regulated vehicle exhaust emissions according to standards Euro V/VI, innovative solutions for automotive exhaust gas emissions reduction must be developed. One solution for diesel emissions reduction is the use of diesel oxidation catalyst (DOC), DOC is a part of the diesel exhaust after treatment system and performs functions of removing CO, hydrocarbons and the soluble organic fraction (SOF) of particulates etc. [1, 2] In practice, Al 2 O 3 -based mixed oxides have long been used as commercial DOC support, and Pt/Al 2 O 3 -based DOC catalysts showed high activity. However, it was proved that Al 2 O 3 -based mixed oxides were readily deactivated due to the adsorbed SO 2 or the formation of sulfate on the catalyst surface. [3] Due to the fact that industrial desulfurization techniques hardly remove all the sulfur species in fuels and then sulfur species were present in almost all commercial diesel fuels. [4] Plentiful researches indicated that sulfur in the diesel fuel, almost entirely oxidized to sulfur dioxide (SO 2 ), as well as the presence of SO 2 in diesel emission gas could lead to catalysts deactivation and sulfur poison. [5] Moreover, in the presence of an oxidation catalyst, a significantly part of SO 2 can be oxidized into SO 3 at the exhaust temperature, which reacts with water vapor and other [6, 7] compounds in the exhaust and therefore forms sulfates. In the past, adsorption of SO 2 on the Al 2 O 3 -based catalysts and the catalytic properties have been studied. [8] In the presence of SO 2, O 2 and oxidation catalyst, surface sulfates are formed on Al 2 O 3 -based catalysts between 150 and 330 C, [9, 10] or above 400 C. [3, 11] Furthermore, the formed surface sulfates are stable even up to 800 C, [12, 13] and the bulk sulfates are also detected when the catalysts were exposed to SO 2 at high temperature, [14, 15] as well as the structure (Al-O) 3 S=O was proposed in the case of Al 2 O 3. [9, 16] However, the comparative study of SO 2 adsorption and sulfates accumulation on the deactivation of Pt/Al 2 O 3 catalyst was still scare. Hence, the goal of this work is to study the behavior of sulfur poisoning on Pt/Al 2 O 3 diesel oxidation catalyst caused by SO 2 adsorption and sulfates accumulation. 2. Experimental 2.1 Materials The Al(NO 3 ) 3 9H 2 O and ammonium hydroxide were punched form chengdu kelong chemical reagent factory (China), Chloroplatinic acid H 2 PtCl 6 solution was punched form Heraeus. All the chemicals were of analytical grade and used without further purification. 2.2 Catalyst preparation The porous Al 2 O 3 was prepared by using co-precipitation method. The Al(NO 3 ) 3 9H 2 O solution was slowly mixed with NH 3 H 2 O solutions under intense churning, and then filtered and washed the settling, followed by 120 C drying and 500 C roasting for 3 h under airflow, the porous Al 2 O 3 powder was obtained. The Pt/Al 2 O 3 catalyst was prepared by using impregnation method. The desired H 2 PtCl 6 solution was impregnated on the porous Al 2 O 3 powder with a mass ratio of 1.0 wt.%. After 120 C drying and 450 C roasting for 3 h at airflow, the Pt/Al 2 O 3 catalyst was obtained. To study the sulfur poisoning phenomenon caused by sulfates accumulation, the fresh catalyst was aged at 250 o C for 15 h in the aging gases of SO 2 : 50 ppm, C 3 H 6 : 600 ppm, CO: 1500 ppm, NO: 200 ppm, O 2 : 5%, CO 2 : 4%, vapour: 8%, N 2 : balance, flow rate: 800 ml/min, and marked as aged Pt/Al 2 O 3 catalyst. This laboratorial aging process had shown a correlation between this particular aging and vehicle aged catalysts. [17] 2.3 Catalytic performance measurements 118

3 Activity measurements were performed in a multiple fixed bed continuous flow reactor. The simulated diesel engine exhaust gases were controlled by mass flow controllers and contained a mixture of NO: 200 ppm, C 3 H 6 : 330 ppm, CO: 1000 ppm, O 2 : 10%, CO 2 : 8%, N 2 : balance and combinations of 7% H 2 O, whit/or without 50 ppm SO 2 (it amounts to diesel fuels with about 1000 ppm of sulfur), [18] the gas space velocity was 30,000 h -1. The outlet CO was detected using a FGA-4100 automotive emission analyzer (Foshan Analytical Instrument Co., Ltd, China), C 3 H 6 was analyzed with a GC2000II online gas chromatograph (Shanghai Analysis Instruments, China) using flame ionization detector (FID). 3. Results and discussion 3.1 Effect of SO 2 adsorption on catalytic performance of Pt/Al 2 O 3 catalyst Fig. 1 CO and C 3 H 6 conversion over Pt/Al 2 O 3 catalyst with or without SO 2 Reaction conditions: gas space velocity h -1 ; 330 ppm C 3 H 6, 1000 ppm CO, 200 ppm NO, 10% O 2, 8% CO 2, 7% vapor, N 2 balance, with or without 50 ppm SO 2. Before running the test, the catalyst was pre-treated at 500 C for 3 h under reaction conditions. CO and C 3 H 6 catalytic oxidation activity over the Pt/Al 2 O 3 catalyst under simulative diesel exhaust gases with or without SO 2 are shown in Fig.1. The CO and C 3 H 6 conversion rates are improved with the increase of temperature. The conversion of CO is faster than that of C 3 H 6, which caused by the competitive adsorption between CO and olefins on the Pt catalyst surface, due to the adsorption of CO on the Pt catalyst is stronger than the olefins, and therefore the adsorbed oxygen is easier to react with adsorbed CO. [19, 20] For CO catalytic oxidation, the T 50 (the temperature of reactant conversion reaches 50%) is about 198 C when SO 2 is not put in, and within SO 2 the T 50 is about 204 C; it can be seen that the presence of SO 2 decreased the catalytic performance of Pt/Al 2 O 3 for CO oxidation, which is mainly because of the competitive adsorption between SO 2 and CO. For C 3 H 6 catalytic oxidation, the T 50 is about 216 C under the SO 2 -free diesel exhaust conditions; and the SO 2 containing diesel exhaust emissions pronouncedly promote the activity of catalyst, the T 50 is about 212 C. Wilson et. al. have reported that SO 2 can enhance the oxidation of propane over Pt supported catalyst, since SO 2 chemisorption on oxygenated Pt surface enormously enhances the dissociative chemisorption and subsequent combustion of propane. [21, 22] Furthermore, Deshmukh et. al. proved that the presence of SO 2 can enhance the C 3 H 6 oxidation activity of oxide catalysts, this may be a result of the formation of hydroxysulfate species on the surface of oxide catalyst, an acid-base type interaction between surface hydroxysulfate and propene would result in an increase in propene surface coverage and hence in an increase in catalytic performance in C 3 H 6 oxidation. [23] 119

4 In general, SO 2 adsorption on the Pt/Al 2 O 3 catalyst would cause a gentle decrease of CO oxidation activity, which may be responsible for the competitive adsorption between SO 2 and CO. And in the presence of SO 2 in diesel exhaust conditions, an acid-based type interaction would be formation and result in an increase of C 3 H 6 oxidation. 3.2 Effect of sulfates accumulation on catalytic performance of Pt/Al 2 O 3 catalyst Fig. 2 CO and C 3 H 6 conversion over fresh and aged Pt/Al 2 O 3 catalysts under SO 2 containing conditions Reaction conditions: gas space velocity h -1 ; 330 ppm C 3 H 6, 1000 ppm CO, 200 ppm NO, 10% O 2, 8% CO 2, 7% vapor, 50 ppm SO 2, N 2 balance. Before running the test, the catalyst was pre-treated at 500 C for 3 h under reaction conditions. To investigate the effect of sulfur poisoning caused by sulfates accumulation, the fresh Pt/Al 2 O 3 catalyst has endured the sulfur-poisoning treatment, and marked as aged Pt/Al 2 O 3. And this sulfur poisoned aging process shows a correlation between this mimic aging and practical vehicle aged catalysts. For CO catalytic oxidation, the T 50 of fresh Pt/Al 2 O 3 catalyst is about 204 C, after sulfur-poisoning aging for 15 hours, the T 50 of aged Pt/Al 2 O 3 catalyst is about 216 C, the CO conversion of T 50 temperature increased 12 C. This deactivation phenomenon resulting from sulfur-poisoning aging is mainly because of catalyst surface sulfates accumulation, and the amount of sulfates accumulation after this sulfur poisoned process is about 4 wt.%. Likewise, the surface sulfates accumulation on catalyst leads to an obvious deactivation of C 3 H 6 catalytic oxidation, the C 3 H 6 T 50 of fresh Pt/Al 2 O 3 catalyst is about 213 C, and the C 3 H 6 T 50 of aged Pt/Al 2 O 3 catalyst is about 222 C. It can be seen that the long sulfur-poisoning treatment results in an obvious deactivation of Pt/Al 2 O 3 catalyst for both CO and C 3 H 6 catalytic oxidation, which mainly caused by the surface sulfates accumulation, [24] due to the accumulation of sulfates on catalyst surface would cover the active sites of catalyst. 120

5 Conclusion Based on the above results, it is feasible to conclude that both SO 2 adsorption and sulfates accumulation would lead to the sulfur poisoning phenomenon of Pt/Al 2 O 3 diesel oxidation catalyst. Compared with surface sulfates accumulation, the sulfur poisoning of catalyst by SO 2 adsorption is relatively mild and mainly caused by the competitive adsorption between SO 2 and CO. Meanwhile, surface sulfates accumulation would results in an obvious deactivation of Pt/Al 2 O 3 catalyst, which mainly caused by the surface accumulation of sulfates covered the active sites of catalyst. Acknowledgements This work was supported by the Doctor Startup Foundation of China West Normal University (15E012). References [1] O.H. Bailey, M. Hedgecock, Architectural diesel oxidation catalyst for enhanced NO 2 generator, US Patent, 2011/ (2011). [2] G. Grubert, T. Neubauer, A.H. Punke, T.W. Mueller-Stach, A. Siani, S.A. Roth, J.B. Hoke, S. Sung, Y. Li, X. Wei, M. Deeba, Diesel oxidation catalyst composite with layer structure for carbon monoxide and hydrocarbon conversion, US Patent, (2012). [3] C.C. Chang, Infrared studies of SO 2 on γ-alumina, J Catal, 53 (1978) [4] M. Riad, S. Mikhail, Oxidative desulfurization of light gas oil using zinc catalysts prepared via different techniques, Catal Sci Technol, 2 (2012) [5] F.C. Galisteo, R. Mariscal, M.L. Granados, M.Z. Poves, J. Fierro, V. Kröger, R. Keiski, Reactivation of sulphated Pt/Al 2 O 3 catalysts by reductive treatment in the simultaneous oxidation of CO and C 3 H 6, Applied Catalysis B: Environmental, 72 (2007) [6] G. Corro, Sulfur impact on diesel emission control-a review, React Kinet Catal L, 75 (2002) [7] G. Corro, A. Velasco, R. Montiel, SO 2 Reactions over γ-al 2 O 3, Pt/γ-Al 2 O 3, Sn/γ-Al 2 O 3 and Pt-Sn/γ-Al 2 O 3, Catal Commun, 2 (2001) [8] L. Limousy, H. Mahzoul, J.F. Brilhac, P. Gilot, F. Garin, G. Maire, SO 2 sorption on fresh and aged SO x traps, Applied Catalysis B: Environmental, 42 (2003) [9] M.B. Mitchell, V.N. Sheinker, M.G. White, Adsorption and Reaction of Sulfur Dioxide on Alumina and Sodium-Impregnated Alumina, The Journal of Physical Chemistry, 100 (1996) [10] A. Pieplu, O. Saur, J.-C. Lavalley, M. Pijolat, O. Legendre, A Kinetic Model for Alumina Sulfation, J Catal, 159 (1996) [11] H.G. Karge, I.G.D. Lana, IR studies of sulfur dioxide adsorption on a Claus catalyst by selective poisoning of sites, The Journal of Physical Chemistry, 88 (1984) [12] O. Saur, M. Bensitel, A.M. Saad, J. Lavalley, C.P. Tripp, B. Morrow, The structure and stability of sulfated alumina and titania, J Catal, 99 (1986)

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