The influence of aging environment of model diesel oxidation catalyst Pt/Al2O3 on the NO oxidation activity is studied. The fresh catalyst Pt/Al/F (calcined in air at 500°C) is aged with or without phosphorus (P) poisoning (7.5wt%) at 800°C either in air (P/Pt/Al/O or Pt/Al/O) or in simulated diesel exhaust (P/Pt/Al/R or Pt/Al/R). Catalyst aged under diesel exhaust environment (Pt/Al/R) surprisingly presents the best NO oxidation activity under excess of O2 followed by the fresh (Pt/Al/F) and thermally aged (Pt/Al/O) catalysts. The activity difference between the catalysts is quite large, especially between Pt/Al/R and Pt/Al/O that are aged at the same temperatures but under different environments suggesting the importance of the aging environment for the catalytic activity. The NO oxidation activity of P poisoned catalysts P/Pt/Al/R and P/Pt/Al/O is minute as compared to their P free counter parts indicating that chemical aging is more detrimental for catalytic efficiency than thermal agin

Ferri, D.

Matam, Santhosh

Weidenkaff, A.

Winkler, A.

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ORIGINAL PAPERObservations on the Aging Environment Dependent NO OxidationActivity of Model Pt/Al2O3 Diesel Oxidation CatalystSanthosh Kumar Matam • A. Winkler •A. Weidenkaff • D. FerriPublished online: 26 February 2013 Springer Science+Business Media New York 2013Abstract The influence of aging environment of modeldiesel oxidation catalyst Pt/Al2O3 on the NO oxidationactivity is studied. The fresh catalyst Pt/Al/F (calcined inair at 500 C) is aged with or without phosphorus (P) poi-soning (7.5 wt%) at 800 C either in air (P/Pt/Al/O or Pt/Al/O) or in simulated diesel exhaust (P/Pt/Al/R or Pt/Al/R).Catalyst aged under diesel exhaust environment (Pt/Al/R)surprisingly presents the best NO oxidation activity underexcess of O2 followed by the fresh (Pt/Al/F) and thermallyaged (Pt/Al/O) catalysts. The activity difference betweenthe catalysts is quite large, especially between Pt/Al/R andPt/Al/O that are aged at the same temperatures but underdifferent environments suggesting the importance of theaging environment for the catalytic activity. The NO oxi-dation activity of P poisoned catalysts P/Pt/Al/R and P/Pt/Al/O is minute as compared to their P free counter partsindicating that chemical aging is more detrimental forcatalytic efficiency than thermal aging.Keywords Diesel oxidation catalyst  Aging environment Thermal aging  Chemical aging  Phosphorus  NO oxidationactivity1 IntroductionDiesel oxidation catalyst (DOC) comprising of Pt sup-ported on Al2O3 is typically used in combination with adiesel particulate filter (DPF) and NOx storage reduction(NSR) or selective catalytic reduction of NOx (SCR) cat-alyst to mitigate NOx emissions from lean burn dieselengines. In this system, the efficiency and durability ofDOC is the key as it catalyzes NO oxidation reaction thataffects the rate and selectivity of NOx reduction in SCR,NOx storage capacity in NSR and passive regeneration ofDPF by NO2 [1]. Therefore, the role of different factorssuch as aging (thermal and chemical) and Pt particle sizeand shape on the DOC performance has been a topic ofresearch [1–10].On the one hand, thermal aging, due to high temperatureexcursions in the DOC monolith, sinters Pt nanoparticlesand support as well as wash coat material. On the otherhand, chemical aging caused by contaminants like phos-phorus (P) interacting with the support, wash coat and Ptnanoparticles leads to occlusion of surface Pt sites. Thesetwo aging effects give rise to decreased catalytic efficiency[7–11]. Previous studies on a real DOC monolith of aheavy duty small truck diesel engine aged for 250 h underdifferent driving profiles have shown that chemical con-taminants P and Zn originating from the anti-wear andengine oil additives like zinc-dialkyldithiophosphates(ZDDP) deposit heavily with time on DOC [10]. It was alsoobserved that there is a concentration gradient of P, besidestemperature gradients, along the DOC monolith. Thehighest P concentration is at the inlet region (close to theengine) of DOC and it decreases along the length. Conse-quently, the aged DOC shows decreased catalytic effi-ciency for oxidation and reduction reactions (under totaldiesel exhaust composition) as compared to that of freshS. K. Matam (&)  A. Weidenkaff  D. FerriEmpa, Swiss Federal Laboratories for Materials Science andTechnology, Laboratory for Solid State Chemistry and Catalysis,Ueberlandstrasse 129, 8600 Du¨bendorf, Switzerlande-mail: santhosh.matam@empa.chA. WinklerEmpa, Swiss Federal Laboratories for Materials Science andTechnology, Internal Combustion Engines Laboratory,Ueberlandstrasse 129, 8600 Du¨bendorf, Switzerland123Top Catal (2013) 56:329–332DOI 10.1007/s11244-013-9975-7DOC. However, the role of concentration and temperaturegradients along the DOC during the aging process on theNO oxidation activity (under excess O2) of Pt/Al2O3 is notknown. In this work, we have studied the influence of agingenvironments of model DOC Pt/Al2O3 on NO oxidation(under excess of O2) by mimicking the aging process of areal DOC monolith. The results demonstrate that the agingenvironment affects the NO oxidation activity of thecatalysts.2 Experimental2.1 Catalysts and Aging Procedure2 wt% Pt/Al2O3 powder (a model DOC) was kindly pro-vided by Umicore AG. The catalyst was calcined in air at500 C for 4 h, labelled as Pt/Al/F. The fresh catalyst wasthen thermally aged in a muffle furnace flowing air at800 C for 5 h (denoted as Pt/Al/O). A portion of freshcatalyst was also treated in a plug-flow reactor undersimulated lean diesel exhaust gas mixture (feed composi-tion: 3,000 ppm CO, 250 ppm NO, 1,200 ppm C3H6,300 ppm C3H8, 11 vol% O2, 25 vol% CO2 in N2) with aflow rate of 200 ml/min at 800 C for 5 h (labelled as Pt/Al/R). To study the influence of chemical contaminantsnamely phosphorus (P) on DOC performance, the freshcatalyst powder was impregnated with (NH4)3PO4 anddried at 120 C over-night. Then, it was either thermallyaged in a muffle furnace in air (P/Pt/Al/O) or in a plug-flowreactor under simulated exhaust gas atmosphere at 800 Cfor 5 h (P/Pt/Al/R). The phosphorous content was deter-mined to be 7.5 wt% P by Energy Dispersive X-ray anal-ysis. The catalysts were characterized by N2-physisorptionat -196 C using a Micromeritics ASAP 2020c instru-ment. Prior to the experiments, the catalysts were pre-treated at 350 C for 2 h. The total surface area and porevolume of the catalysts were determined by the BETmethod and BJH desorption isotherm, respectively(Table 1).2.2 Activity TestsNO oxidation activity was determined in a quartz tubereactor (12 mm internal diameter and 400 mm length). Thereactor was loaded with 50 mg of catalyst (sieve fraction of150–200 lm) diluted with quartz particles of the same size.Prior to the reaction, catalysts were pretreated at 300 C in aflow of 10 vol% O2 in N2 for 1 h and subsequently cooled to50 C. Then, the reaction mixture (400 ppm NO, 10 vol%O2, balance N2 at 700 ml/min) was introduced at GHSV =92,840 h-1. Product evolution was followed by FTIR(Thermo Nicolet 5700 equipped with an Axiom gas cell).3 Results and DiscussionN2-physisorption data of the catalysts Pt/Al/F, Pt/Al/O andPt/Al/R show comparable surface area and pore volumesuggesting that high temperature treatment either in air orin lean exhaust environment does virtually not affect thephysical and textural properties of the support alumina ofthe catalysts (Table 1). However, P poisoned catalysts P/Pt/Al/O and P/Pt/Al/R show dramatic loss in both surface areaand pore volume as compared to their P free counter partsPt/Al/O and Pt/Al/R, respectively. These results indicatethat P is located on the surface and within the pores of thesupport alumina. Also, a fraction of P might be depositedon the surface of Pt nanoparticles [10, 11].The effect of aging environment of the catalysts on theirNO oxidation activity under excess O2 is shown in Fig. 1.NO oxidation is thermodynamically limited above 350 C(equilibrium between NO and NO2) and is kineticallycontrolled at lower temperatures, which are prominentlyseen in Fig. 1a. Kinetically controlled low temperature NOconversion is indeed dependent on the catalyst (Fig. 1b),whereas the equilibrium limited high temperature (espe-cially C400 C) NO conversion profiles are independent ofthe catalyst employed due to reverse decomposition of theproduct NO2 into NO and O [12]. Accordingly, the oxi-dation activity of the catalysts below 300 C is as follows:Pt/Al/R [ Pt/Al/F [ Pt/Al/O [ P/Pt/Al/R [ P/Pt/Al/O.Astonishingly, catalyst Pt/Al/R aged under lean exhaustenvironment exhibits the best NO oxidation activity fol-lowed by the fresh Pt/Al/F and thermally aged Pt/Al/Ocatalysts. This activity behaviour is intriguing and is notfollowing the literature that report increased NO oxidationactivity with increased Pt particle size. For example, NOoxidation activity (turnover rates) increased by a factor of100 on increase of Pt particle size from ca. 1.2 to 23 nm onAl2O3 [4]. Similarly, 4 times increased activity was alsoobserved on increase in the size of Pt particles from 2.4 to7 nm supported on Al2O3 [13]. According to these results,Pt/Al/R and Pt/Al/O that are aged at 800 C should inTable 1 N2-physisorption data of the catalystsCatalysts SBET (m2/g)a VTotal (cm3/g)bPt/Al/F 140 0.47Pt/Al/O 133 0.47Pt/Al/R 138 0.47P/Pt/Al/O 70 0.27P/Pt/Al/R 55 0.19a BET methodb Based on BJH desorption isotherm330 Top Catal (2013) 56:329–332123principle present comparable NO oxidation activity andtheir activity should be better than the fresh catalyst Pt/Al/Fthat is subjected to calcination only at 500 C. Moreover,the activity difference between the catalysts Pt/Al/R and Pt/Al/O is quite large (see Fig. 1b). For instance, Pt/Al/R isca. 15 times more active than Pt/Al/O at 150 C. At thistemperature, the fresh catalyst Pt/Al/F may contain anaverage Pt particle size of around 4 nm (similar to that on anew DOC monolith [10]) and is ca. 2 times more activethan the thermally aged catalyst Pt/Al/O which presumablycontains larger Pt particles [10]. Therefore, the activitybehavior of the catalysts Pt/Al/R, Pt/Al/F and Pt/Al/Ocannot be simply explained by Pt particle size dependenceas reported previously [4, 13], indicating the involvementof another factor in the activity determination. Besidesagglomeration of Pt particles during the high temperatureaging, there might plausibly be another important processoccurring during aging that may be governed by the agingenvironment, for example Pt particle morphology [10, 14].As a result of this the intrinsic electronic, geometric andcatalytic properties of surface Pt sites of Pt nanoparticlesevolved under these aging environments vary to differentdegrees in the catalysts. Hence, the catalysts exhibit sig-nificant activity differences.By comparison of Pt/Al/R and Pt/Al/O with the P poi-soned catalysts P/Pt/Al/R and P/Pt/Al/O, it is evident thatthe latter two show very poor NO oxidation activity(Fig. 1). Between these two catalysts, the P/Pt/Al/Rexhibits better NO oxidation activity than P/Pt/Al/O. Thisactivity trend is similar to the corresponding P free cata-lysts, again indicating the significant role of the agingenvironment in the activity determination. However, thepoor performance of P/Pt/Al/R and P/Pt/Al/O can be due toP that might be interacting with the support and Pt nano-particles. The interaction can modify the physico-chemicalproperties of the support as evident by N2-physisorptiondata, for example by forming AlPO4 like species [11]. Thiscould change the local chemical environment of Pt nano-particles, also P may perhaps occlude a fraction of surfacePt sites [11]. Consequently, chemical aging caused by Pbecomes more detrimental for NO oxidation activity thansimple thermal aging, in agreement with our previousresults reported on Pd/Al2O3 based three-way catalysts forCNG vehicle applications [11].By comparison of the results with the real DOCmonolith reported in [10] (as discussed in Sect. 1), it can besuggested that inlet (DOCin), middle (DOCmid) and outlet(DOCout) regions of the DOC monolith experience differ-ent aging environments. The DOCin and DOCout regionsexperience chemical and thermal aging, respectively.Whereas, aging environment at the DOCmid can be uniqueas it comprises of both exhaust pollutants (propene, pro-pane, CO, CO2 NO and O2) and their products such asunburned hydrocarbons, CO2, H2, NO2. It almost excludeschemical contaminants (such as P, Zn). The aging envi-ronment along the DOC monolith is similar to that ofmodel catalysts P/Pt/Al/R, Pt/Al/R and Pt/Al/O whichrepresent DOCin, DOCmid and DOCout, respectively. TheNO conversion efficiency (at ca. 238 C under simulateddiesel exhaust environment as reported in [10]) of thesethree regions of the DOC is compared with the corre-sponding NO oxidation activity of the model catalysts inFig. 2. Note that the comparison should be treated withcaution and is only qualitative due to differences in thereaction conditions. Having said that, the activity trend ofDOCin, DOCmid and DOCout coincides well with the cor-responding model catalysts as shown in Fig. 2. DOCmidexhibits the best NO conversion efficiency followed by theDOCout and DOCin which correspond to Pt/Al/R, Pt/Al/Oand P/Pt/Al/R, respectively. These results confirm that theFig. 1 NO conversion profiles over the catalysts (a): Pt/Al/F (triangle), Pt/Al/R (unfilled circle), Pt/Al/O (square), P/Pt/Al/R (filled circle), andP/Pt/Al/O (filled square). Magnified low temperature activity data of the catalysts (b). Reaction conditions as described in Sect. 2.2Top Catal (2013) 56:329–332 331123aging environment does play an important role in the cat-alyst activation and deactivation, further characterizationand kinetic studies are in progress to address the role.4 ConclusionsThe effect of aging environment of model DOC Pt/Al2O3on NO oxidation under excess O2 is studied by mimickingthe aging process of a real DOC monolith. The preliminaryresults show that the aging environment affects signifi-cantly the NO oxidation activity of the catalysts. The cat-alyst aged under diesel exhaust environment at 800 Cpresents the best NO oxidation activity followed by thefresh (calcined at 500 C) and thermally aged (in air at800 C) catalyst. Chemical aging caused by phosphoruscan be more detrimental than the thermal aging. Theactivity trend of the catalysts coincides well with that of anaged real DOC monolith performance that varied along theDOC length, confirming the aging environment dependentNO oxidation efficiency of the catalysts.Acknowledgments The authors are thankful to Empa for financialsupport and the Swiss National Science Foundation for fundingASAP2020 equipment within the R’Equip program (project No.206021_128741/1).References1. Hauff K, Tuttlies U, Eigenberger G, Nieken U (2010) Appl CatalB 100:102. Burch R, Millington PJ (1995) Catal Today 26:1853. Xue E, Seshan K, Ross JRH (1996) Appl Catal B 11:654. Lee JH, Kung HH (1998) Catal Lett 51:15. Galisteo FC, Larese C, Mariscal R, Lopez Granados M, FierroJLG, Fernandez-Ruiz R, Furio M (2004) Top Catal 30–31:4516. Vaccaro AR, Mul G, Pe´rez-Ramirez J, Moulijn JA (2003) ApplCatal B 46:6877. Andersson J, Antonsson M, Eurenius L, Olsson E, Skoglundh M(2007) Appl Catal B 72:718. Weiss BM, Iglesia E (2009) J Phys Chem C 113:133319. Beck DD, Carr CJ (1988) J Catal 110:28510. Winkler A, Ferri D, Aguirre M (2009) Appl Catal B 93:17711. Matam SK, Otal E, Aguirre M, Winkler A, Ulrich A, Rentsch D,Weidenkaff A, Ferri Catal D (2012) Today 184:23712. Boll W, Tischer S, Deutschmann O (2010) Ind Eng Chem Res49:1030313. Mulla SS, Chen N, Cumaranatunge L, Balu GE, Zemlyanov DY,Delgass WN, Epling WS, Ribeiro FH (2006) J Catal 241:38914. Cabie M, Giorgio S, Henry CR, Axet MR, Philippot K, ChaudretB (2010) J Phys Chem C 114:2160Fig. 2 NO conversion efficiency at 238 C of the DOC monolithalong the length (10 cm in length and 20 cm in diameter) is comparedwith the corresponding model catalysts (data taken at 238 C fromFig. 1). DOC samples were taken at different length positions of themonolith. The DOC monolith was aged on a heavy duty small truckengine for 250 h under different driving profiles. Reaction conditionsfor the DOC samples are reported in [10]332 Top Catal (2013) 56:329–332123

Observations on the Aging Environment Dependent NO Oxidation Activity of Model Pt/Al2O3 Diesel Oxidation Catalyst

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Observations on the Aging Environment Dependent NO Oxidation Activity of Model Pt/Al2O3 Diesel Oxidation Catalyst

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