Effect of Soot on N2O Formation Over Pt Based Diesel Oxidation Catalyst Supported on Microporous TiO2
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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.
KeywordsDOC Microporous titania (m-TiO2) Platinum (Pt) Location
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 . 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 . 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 .
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.
2.1 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.
Textural property of soot and DOC catalysts
BET Surface area (m2g−1)
Pore volume (cc·g−1)
Average particle diameter (nm)b
Average particle diameter (nm)c
2 wt% Pt/Al2O3
2 wt% Pt/m-TiO2
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.
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 . Also, the presence of N in the soot is known to accelerate the N2O formation . 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.
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.
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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