Topics in Catalysis

, Volume 60, Issue 3–5, pp 361–366 | Cite as

Effect of Soot on N2O Formation Over Pt Based Diesel Oxidation Catalyst Supported on Microporous TiO2

  • Seung Gwan Lee
  • Hyun Jeong Lee
  • Inhak Song
  • Seunghee Youn
  • Do Heui Kim
  • Sung June Cho
Original Paper
  • 258 Downloads

Abstract

The N2O formation over the diesel oxidation catalyst supported on microporous TiO2 has been investigated in the presence of the soot to simulate cold start or regeneration condition. The platinum (Pt) catalyst entrapped in micropore was resulted in the increase of the catalytic activity compared to the Pt catalyst supported on alumina. Also, it was shown that the presence of the soot with the oxidation catalyst enhanced the N2O formation.

Keywords

DOC Microporous titania (m-TiO2Platinum (Pt) Location 

1 Introduction

N2O has an high greenhouse gas effect up to 300 times relative to that of CO2, thereby small amount of its emission from automobile source can impact the global warming largely [1, 2]. Most recent diesel engine emission has been reduced significantly using diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) and diesel particulate filter (DPF). SCR system using urea as a reducing agent has been considered as the state of art technology for the abatement of NOx emission [3]. Also, the N2O emission can be suppressed significantly under lean condition because the reduction condition was necessary for the N2O formation. In the meantime, the N2O emission can be increased when the reducing agent is injected in the SCR system following the reactions, such as 2NH3 + 2NO + O2 = N2O + N2 + 3H2O, 2NH3 + 2O2 = 2N2O + 3H2O and NH4NO3 = N2O + 2H2O [4]. Further, it seems that the N2O emission can be increased when the catalyst is aged and also the soot deposited on the diesel oxidation catalyst may accelerate the N2O formation when it is regenerated [5, 6, 7].

The hydrothermal transformation of commercially available TiO2 in the presence of alkaline hydroxides is resulted in the formation of various TiO2 nanostructures [8, 9, 10, 11]. Especially, the presence of LiOH in the hydrothermal synthesis medium was induced the formation of microporous TiO2 [8, 12, 13]. Very recently, the microporous TiO2 has been utilized as a support for vanadium catalyst for NH3 SCR catalyst to decrease the N2O emission [14].

In the present work, the microporous TiO2 has been employed as a support for diesel oxidation catalyst. Thus, the microporous TiO2 has been demonstrated as a potential catalyst support for platinum (Pt) based diesel oxidation catalyst. Also, the N2O emission from the present catalyst was studied in the presence of the soot to simulate the cold start or regeneration condition.

2 Experimental

2.1 The Preparation of Catalysts

TiO2 anatase (Aldrich, 98 %) of 2–8 g was added to the solution containing 10 M or more LiOH in the teflon lined autoclave for hydrothermal heating at 400–440 K for 72 h. under rotating condition at 40 rpm. After cool down to room temperature, the slurry was neutralized with 0.1 N HCl under stirring for 6 h. The solution was filtered and washed with deionized water thoroughly. The acidification and filtration was repeated three times to remove the residual trace metal hydroxides. The obtained product was dried at 330 K in an oven and calcined under flowing oxygen at 673 K for 4 h. The inductively coupled plasma analysis of the obtained sample showed that the residual Li was ~6 ppm level, indicating the complete removal of Li+ by the neutralization and subsequent thorough washing. The obtained TiO2 was designated as m-TiO2.

m-TiO2 was impregnated with an acetone/water solution of Pt(NH3)(NO3)2 (Johnson Matthey) overnight at room temperature (RT). The sample was dried at RT for more than 6 h. This sample was activated inside a Pyrex U-tube flow reactor by heating linearly to a given temperature at a rate of 0.44 K min−1 and maintaining at this temperature for 2 h in flowing O2 (dried through molecular sieve trap, >1 L min−1 g−1). Then the O2 gas was evacuated at 573 K. The sample was then reduced in flowing H2 (99.999 %, >200 mL min−1 g−1) with linear heating to 573 K over 4 h and maintaining at this temperature for 2 h. Desorption of chemisorbed hydrogen from the surface of the Pt nanoparticle was performed by increasing the temperature to 673 K over 2 h under 1 × 10−3 kPa and maintaining under this condition for 1 h. The sample was used in situ, or exposed to air at RT. The air-exposed samples were reduced again in H2 at 573 K, for further experiments. The Pt content of the sample was controlled to 2 wt% and the resulting sample was designated as 2 wt% Pt/m-TiO2. Similarly, the alumina was employed as a support for Pt loading for comparison which was also designated as 2 wt% Pt/Al2O3.

2.2 Characterization Techniques

The morphologies of the samples were observed with a field emission scanning electron microscope (FE-SEM, Hitachi S-4700) and a transmission electron microscope (TEM, Jeol Tecnai F20) at Gwangju Branch of Korea Basic Science Institute. The accelerating voltage and emission current on the FE-SEM were 15 kV and 20 A, respectively and the accelerating voltage on the TEM was 200 kV. The crystallinities of the titanium oxide were evaluated by x-ray diffraction (XRD, Rigaku D/Max Ultima) with Cu Kα radiation at a scan rate of 2.0 degree per min. (40 kV, 40 mA). Nitrogen adsorption–desorption and argon adsorption–desorption isotherms were measured at 77 and 87 K, respectively (Micromeritics Instruments ASAP 2020). The specific surface areas of the samples were determined with the Brunauer–Emmett–Teller (BET) method and the pore size distribution was analyzed using the Barret–Joner–Halenda (BJH) method and the Horvath–Kawazo (HK) method, respectively.

The hydrogen adsorption isotherms were measured at 298 K by using a volumetric gas adsorption apparatus. The adsorption temperature was controlled to within ± 0.1 K by a constant temperature circulation bath. Total hydrogen chemisorption was obtained on the fresh sample after the evacuation at 673 K to remove adsorbed hydrogen. Reversible hydrogen chemisorption was measured after the removal of physically adsorbed hydrogen with the evacuation at room temperature for 1 h.

2.3 Catalytic Activity Measurements

The diesel oxidation activity was obtained in a fixed-bed quartz tubular reactor in which the 50–100 mg sample of 30–40 mesh was placed. 45 ppm NO, 1.9 % propylene, 11 % O2 and balanced with N2 were introduced as reactants at a space velocity of 60,000 h−1. NOx concentration of outlet gas was measured using NOx chemiluminescence analyzer (Eco Physics CLD 822 M h). The N2O concentration in the gas was monitored using G200 (GeoTech) after the CO2 absorption. The soot was obtained from the diesel emission testing facility at Korea Automotive Technology Institute (Chonan, Korea) and mixed with the catalyst physically using agate mortar for more than 30 min.

3 Results

The soot collected from DPF was analyzed as shown in Fig. 1. The XRD pattern of the soot in Fig. 1a showed the diffuse peak at 22–26° corresponding to amorphous polymeric carbon. The background at angle, 2–3° was increased dramatically, implying the presence of macroporous or mesoporous structure. As shown in Fig. 1b, the soot had spherical morphology in which the particle size was in the range of 50–80 nm [6, 15]. These soot particles were also agglomerated to form network of the large aggregate, which was also resulted in the formation of macroporous network. The textural properties were measured using nitrogen adsorption–desorption measurement as listed in Table 1. The isotherm shape of the soot in Fig. 1c indicated the macroporous texture showing the large mesopore at P/P0 = 0.9, which was also consistent with the result of surface area and pore volume, 74.7 m2 g−1 and 0.33 cc g−1, respectively. The thermo gravimetric analysis of the soot in Fig. 1d showed that it was start to be decomposed below at 773 K and completed almost at 973 K which resulted in more than 90 % weight loss. The small weight decrease, 4.7 % below 673 K can be ascribed to the presence of low molecular weight polyaromatic hydrocarbon with less than 4 rings [5].
Fig. 1

a XRD pattern, b SEM micrograph, c N2 adsorption–desorption isotherm of nitrogen at 77 K and d TGA pattern in 5 % O2 with balanced nitrogen of the soot

Table 1

Textural property of soot and DOC catalysts

Sample

BET Surface area (m2g−1)

Pore volume (cc·g−1)

H/Pta

CO/Pt

Average particle diameter (nm)b

Average particle diameter (nm)c

Soot

74.7

0.33

γ-Al2O3

251

0.36

anatase TiO2

9.7

0.04

m-TiO2

244

0.22

2 wt% Pt/Al2O3

245

0.35

1.20

0.56

~1.00

1.79

2 wt% Pt/m-TiO2

227

0.18

0.99

0.35

1.01

3.25

aDifference between total hydrogen chemisorption and reversible hydrogen chemisorption at 273 K, based on the total amount of Pt

bCalculated from hydrogen chemisorption

cCalculated from CO chemisorption

Figure 2a shows the XRD pattern for the m-TiO2 and 2 wt% Pt supported on m-TiO2 with the corresponding the nitrogen adsorption isotherm in Fig. 2b, respectively. Figure 3 also shows the scanning electron micrographs and transmission electron micrographs of the microporous TiO2. The resulting microporous TiO2 particle contained similar morphology to the parent material but with sharp edges, suggesting the formation of the well-crystallized TiO2 [8, 12, 13, 14]. The obtained microporous TiO2 had the typical nitrogen adsorption–desorption isotherm consistent with the Langmuir isotherm type containing micropore in which the corresponding surface area and pore volume were controlled to be 250 ± 20 m2g−1 and 0.20 ± 0.05 cc g−1, respectively [8]. In the meantime, the presence of the mesopore was observed above P/P0 > 0.9, indicating the formation of disordered network containing micropore. Such disordered network formation was shown clearly in Fig. 3d. The mesopore was formed with several interconnecting crystalline TiO2 wall of 3–4 nm thickness containing micropore [8, 13]. There was no external Pt particle on m-TiO2 as shown in Fig. 3c, d.
Fig. 2

a X-ray powder diffraction patterns of (bottom) m-TiO2 and (upper) 2 wt% Pt/m-TiO2 and b N2 adsorption–desorption isotherm plots of (filled circle) m-TiO2, (filled triangle) 2 wt% Pt/m-TiO2, and (filled circle) anatase TiO2. The solid and open symbols indicate the absorption and desorption isotherm, respectively

Fig. 3

SEM and TEM images of a anatase TiO2, bm-TiO2, c and d 2 wt% Pt/m-TiO2. The circle in (d) indicated the Pt particle

The Pt particle size was measured using hydrogen and CO chemisorption as listed in Table 1. Before and after the Pt loading, there was no significant change in the textural properties, indicating moderate thermal stability. The Pt particle sizes was controlled to 1 nm independent of the support while the CO chemisorption showed that the particle size on alumina was smaller than that on m-TiO2.

The catalytic activities of the NOx decomposition in the presence of hydrocarbon over 2 wt% Pt/m-TiO2 and 2 wt% Pt/Al2O3 were measured as a function of temperature as shown in Fig. 4. The NOx decomposition activity of both catalysts was increased readily above 523 K without emission of N2O. However, the incorporation of 10 wt% soot increased the emission of N2O up to 40 ppm from 2 wt% Pt/Al2O3, where the soot was mixed with the catalyst physically using agate mortar for more than 30 min. In the meantime, there was negligible N2O emission from 2 wt% Pt/m-TiO2 in the presence of the soot.
Fig. 4

Catalytic activity of over NOx reduction using propylene over a 2 wt% Pt/Al2O3 and b 2 wt% Pt/m-TiO2 without (open circle, filled circle) and with soot (open square, filled square), respectively. The open and solid symbols were corresponded to N2O formation and NOx conversion, respectively

4 Discussions

It was known that the soot from DPF was consisted of various polyaromatic hydrocarbons (PAH) such as naphthalene, pyrene, etc. [6, 15, 16]. The soot used in this work, of which the particle size was 50 nm was also found to contain macropore mostly.

The soot containing PAH can be oxidized readily through both active oxygen mechanism and NO2 assisted mechanism, producing N2, CO2 and water [15]. Also, the presence of N in the soot is known to accelerate the N2O formation [17]. The incorporation of the soot into the catalyst can provide the reduction environment locally to the Pt catalyst because of intimate mixing. In the present work, both catalysts did not produce the N2O without the soot, indicating its role for the N2O formation over DOC prepared in this work. In the meantime, in the presence of the soot, the Pt supported on m-TiO2 resulted in no emission of N2O while the Pt supported on alumina produced N2O significantly up to 40 ppm. It was of interest that NOx conversion was decreased probably because of the plugging effect of the soot of ~50 nm on mesorpore. In the meantime, there was no plugging effect on Pt catalyst supported on alumina.

The contrasting difference in N2O emission between both catalysts can be due to the location of Pt atom in each support. The Pt particle in m-TiO2 was entrapped in the micropore as shown in Fig. 3c, d while the Pt particle was likely to be spread on open alumina surface. Thus, the Pt catalyst may be contact with the soot providing the local reducing environment, resulting in the N2O formation in the present experimental condition. In m-TiO2, the Pt particle may interact with NO but not the soot because of the soot size, 50 nm.

5 Conclusion

In summary, it was shown that the intimate contact between the soot deposited on the diesel oxidation catalyst and the Pt catalyst play a major role in the N2O emission providing the reducing environment at locally. It was found that the Pt location entrapped in the micropore of m-TiO2 was crucial over N2O formation. Therefore, in the present work, it was possible that the location of the Pt catalyst can be controlled for the suppression of N2O formation.

Notes

Acknowledgments

This project is supported by the “R&D Center for reduction of Non-CO2 Greenhouse gases(0458-20140019)” funded by Korea Ministry of Environment(MOE) as “Global Top Environment R&D Program”.

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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  • Seung Gwan Lee
    • 1
  • Hyun Jeong Lee
    • 1
  • Inhak Song
    • 2
  • Seunghee Youn
    • 2
  • Do Heui Kim
    • 2
  • Sung June Cho
    • 1
  1. 1.Department of Chemical EngineeringChonnam National UniversityKwangjuSouth Korea
  2. 2.School of Chemical and Biological Engineering, Institute of Chemical ProcessesSeoul National UniversitySeoulSouth Korea

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