Environ. Sci. Technol. 2006, 40, 2727-2733
Platinum Particle Size and Support Effects in NOx Mediated Carbon Oxidation over Platinum Catalysts KENNETH VILLANI,† WALTER VERMANDEL,† KOEN SMETS,† DUODUO LIANG,‡ GUSTAAF VAN TENDELOO,‡ AND J O H A N A . M A R T E N S * ,† Centre for Surface Chemistry and Catalysis, Catholic University of Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium, and Centre for Electron Microscopy for Materials Science, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Platinum metal was dispersed on microporous, mesoporous, and nonporous support materials including the zeolites NaY, Ba-Y, Ferrierite, ZSM-22, ETS-10, and AlPO-11, alumina, and titania. The oxidation of carbon black loosely mixed with catalyst powder was monitored gravimetrically in a gas stream containing nitric oxide, oxygen, and water. The carbon oxidation activity of the catalysts was found to be uniquely related to the Pt dispersion and little influenced by support type. The optimum dispersion is around 3-4% corresponding to relatively large Pt particle sizes of 2040 nm. The carbon oxidation activity reflects the NO oxidation activity of the platinum catalyst, which reaches an optimum in the 20-40 nm Pt particle size range. The lowest carbon oxidation temperatures were achieved with platinum loaded ZSM-22 and AlPO-11 zeolite crystallites bearing platinum of optimum dispersion on their external surfaces.
Introduction Trapping on filter followed by oxidation is an adequate means of elimination of carbonaceous particulate matter from diesel engine exhaust gas. With the help of catalysts the temperature of uncatalyzed oxidation of soot of over 500 °C can be significantly decreased. When dealing with solid reagents such as carbon, contact catalysis is rather difficult to achieve. One possibility to circumvent this problem is to build the catalyst in the individual soot particles by feeding a molecular catalyst precursor, such as a cerium compound together with the fuel into the diesel engine (1). Alternatively, the catalyst may act indirectly by generating a powerful gaseous oxidant. The continuously regenerated trap, CRT, is based on this principle as it exploits the strong oxidation power of nitrogen dioxide. In a CRT system, nitrogen dioxide is generated by a platinum catalyst in a position upstream of the particulate trap (2-4). Pt catalyzes oxidation of nitric oxide contained in the exhaust gas with molecular oxygen into nitrogen dioxide, which promotes the combustion of soot trapped on the filter downstream and is reduced back into NO (5, 6). The latest developments involve the incorporation of a NO oxidation catalytic function in the wash coat of the diesel * Corresponding author e-mail:
[email protected]. † Catholic University of Leuven. ‡ University of Antwerp. 10.1021/es051871h CCC: $33.50 Published on Web 03/22/2006
2006 American Chemical Society
particulate filter itself for facilitating regeneration, as it enables multiple regeneration of the reactive NO2 (7, 8). In experimental studies various types of oxides (9-11), eutectic mixtures of oxides (12-15), chloride containing mixtures (16-17), and silicon carbide supported catalysts (18) have been mixed with carbon particles and evaluated on their ability to lower the carbon oxidation temperature. Those studies revealed the superior catalytic activity of platinum metal in NOx mediated carbon oxidation. It was found that the support on which the platinum metal is dispersed plays an important role, e.g., by preservation of the accessibility of the Pt particles upon catalyst aging (18), by trapping of the sulfur (19), or by controlling the Pt dispersion (20). Xue et al. prepared Pt catalysts on silica, γ-alumina, and zirconia support and observed a higher NO oxidation activity on silica compared to the other two supports (20). In that study, Pt particle size was altered by altering the Pt loading. On silica, catalytic turnover frequencies per exposed Pt atom increased with Pt particle size in the range 1.3-21 nm, suggesting that NO oxidation is a structure-sensitive reaction (20). On γ-alumina and zirconia support, the influence of Pt particle size investigated in narrow ranges of 1.1-6.3 nm and 2.0-4.3 nm, respectively, was less conclusive (20). Zeolites are proven catalysts for a variety of chemical processes. Zeolites have a well-defined 3-dimensional micropore structure, a capability of cation exchange, and capability of acid-base catalysis. These properties can be fine-tuned for a desired application (21). Pt-loaded zeolites are, e.g., proven hydrocracking and skeletal isomerization catalysts in petroleum refining (22). In the area of environmental catalysis, Pt loaded zeolites can, e.g., be applied in catalytic oxidation of volatile organic compounds (23). Little attention has been paid to the potential of Pt loaded zeolite catalysts in the area of soot oxidation. In the present study zeolites with different pore structures, chemical composition, and particle size were plated with platinum metal and evaluated as catalysts for NOx mediated combustion of carbon black. The zeolites were compared to the more common alumina and titania supports.
Experimental Section The origin and properties of the zeolites, alumina, and titania are provided in Table 1. The Pt content was 0.5 wt % unless indicated otherwise. Platinum was loaded via the incipient wetness impregnation method using an aqueous solution of Pt(NH3)4Cl2‚H2O (Alfa Aesar). Typically, for impregnating 1 g of support with 0.5 wt % Pt metal, 9 mg of Pt(NH3)4Cl2‚H2O was dissolved in 0.5 or 1 mL water, depending on the porosity of the support. The impregnated supports were dried in static air at 60 °C. The dry powders were compressed under a hydrostatic pressure of 40 MPa and the compacted tablets were crushed and sieved to obtain pellets of 0.25-0.50 mm. The catalyst pellets were loaded in a quartz tube, heated at 5 °C/min, and calcined at 400 °C for 1 h under a stream of oxygen gas (Air Liquide N45, 99.995% purity). Subsequently, the temperature was decreased to ca. 50 °C, the lines were flushed with nitrogen, and the catalyst was reduced under hydrogen (Air Liquide, 99.997% purity) in a temperature ramping at 5 °C/min to 400 °C and maintaining the catalyst at this temperature for 1 h. Two Pt/ZSM-22 catalyst samples with different platinum dispersion were prepared by altering the pretreatment scheme. Pt/ZSM-22 was obtained using the common pretreatment procedure resulting in a rather poor Pt dispersion of 3.6% (Table 1). In the preparation of Pt/ZSM-22*, the reduction with hydrogen was performed VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Characterization of Pt Catalysts
catalyst
pore type and pore diameters (nm)
origin of support
Pt/AlPO-11
synthesized according to ref. (25) Pt/ZSM-22 synthesized according to ref. (27) Pt/ZSM-22* synthesized according to ref. (27) Pt/Ferrierite commercial sample (TSZ-710, Toyo Soda) Pt/Na-Y Pt/Ba-Y Pt/ETS-10 Pt/Al2O3 Pt/TiO2
commercial sample (Zeocat) Na-Y, ion exchanged with BaCl2 courtesy of Engelhard Corp. La Roche Industries, γ-alumina A-201 commercial (808, Merck)
microporous 0.63 × 0.39 (26) microporous 0.55 × 0.44 (26) microporous 0.55 × 0.44 (26) microporous 0.54 × 0.42 and 0.48 × 0.35 (26) microporous 0.74 (25) microporous 0.74 (26) microporous 0.60 (28) mesoporous 2-10 nm nonporous
chemical composition
BET surface area (m2/g)
alumino-phosphate
157
0.070
22.3
4.2
20
alumino-silicate Si/Al ) 30 alumino-silicate Si/Al ) 30 alumino-silicate Si/Al ) 8.4
225
0.085
42
3.6
23
225
0.085
42
46
1.8
129
0.062
10.5
16
5.0
alumino-silicate Si/Al ) 2.71 alumino-silicate Si/Al ) 2.71 titano-silicate
618
0.320
6.5
20
4.1
565
0.292
6.3
37
2.2
285
0.145
7.9
44
1.9
alumina
340
0.000
390
47
1.8
titania
7.93
1.24 × 10-04
7.63
without cooling after the oxidative treatment. This procedure resulted in a high Pt dispersion of 46% in agreement with earlier work (27). Platinum dispersions were determined using CO chemisorption. Catalyst sample was loaded into a tubular reactor and pretreated in a stream of 20 mL/min of H2 (Air Liquide, 99.9997%) at 300 °C (heating rate 5 °C/min) for 1 h, and subsequently cooled to RT under a He (Air Liquide, 99.9997%) flow of 20 mL/min. For the titration of the Pt surface, the He flow was reduced to 10 mL/min and pulses of 5 µL of 100% CO were given with an interval of 2 min. The CO concentration in the outlet stream was followed with a Pfeiffer Omnistar quadrupole mass spectrometer. In the calculation of the dispersion, adsorption of 1 CO molecule per accessible Pt atom was assumed. The size of Pt particles, dPt, was derived from the Pt dispersion, DPt, assuming a cubic particle shape according to (24):
dPt ) 0.821
( ) 1 DPt
(1)
The porosity and specific surface area of the support materials were determined using nitrogen adsorption at -196 °C on a Micromeritics TriStar 3000 gas adsorption analyzer. The total surface area was determined using the BET method. The surface area in mesopores and macropores was determined using the t-plot method. X-ray diffraction was performed on a STOE Stadi P instrument in transmission mode using Cu KR radiation. Transmission electron microscope (TEM) images were recorded on a Philips CM20 electron microscope equipped with an Oxford EDX attachment, operating at 200 kV. For the carbon oxidation experiments, carbon black (Degussa AG Printex-U) and Pt catalyst powder were carefully mixed with a spatula to obtain realistic contact conditions of carbon and catalyst (29). An aliquot of ethanol was added to obtain a paste, which was dried at 60 °C for 2 h and crushed. The catalyst/carbon ratio was 2:1 by weight. Combustion properties of Printex-U are similar to those of diesel soot (30). Carbon oxidation tests were performed in a magnetic suspension balance (Rubotherm). Quantities of 270 mg of catalyst/carbon mixtures were loaded in the suspended, perforated, stainless steel mini basket. The use of a stainless 2728
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specific surface area in meso- and Pt macropores dispersion (m2/g) (%)
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micropore volume (mL/g)
1.9
Pt particle size (nm)
43
steel mesh sample container and catalyst/carbon pellets ensured a sufficient contact of the gas with the solids and carbon oxidation in absence of mass-transfer limitation, a problem sometimes encountered in thermogravimetric equipment. The gas composition was 10% O2, 500 ppm NO, and 5% H2O in helium, the gas flow rate was 150 mL/min, and the heating rate was 5 °C/min. Weight loss curves were fitted with the sigmoidal function. The quality of the fitting was in most instances excellent (χ2 values from 0.5 to 1). The fitted curves were used for the kinetic analysis and determination of inflection temperature Tm and slope dm/dT. The NO into NO2 oxidation activity of the Pt catalysts was evaluated in a fixed-bed tubular reactor system. The test gas contained 10% O2, 500 ppm NO, and 5% H2O in helium at a total flow rate of 150 mL/min. The catalyst weight was 300 mg corresponding to a volumetric hourly space velocity of 15000 h-1. NO and NO2 in the reactor outlet were analyzed using an internally heated chemiluminiscence detector (Ecophysics 700 EL ht).
Results and Discussion Characterization of Platinum Catalysts. The zeolites that were selected for the present study comprise a variety of porosities and chemical compositions (Table 1). The aluminosilicate zeolite ZSM-22 and the aluminophosphate AlPO11 are structures with narrow micropores and relatively small micropore volumes of 0.07 and 0.09 mL/g, respectively. These zeolite samples have several tens of square meters per gram of surface area in meso- and macropores owing to the small particle size in the micrometer range and aggregation of these crystallites. Deposition of platinum on AlPO-11 and ZSM-22 zeolites led to a poor metal dispersion and formation of large Pt particles measuring ca. 20 and 23 nm, respectively. Given the pore size of 0.63 × 0.39 nm and 0.55 × 0.44 nm in AlPO11 and ZSM-22, respectively, such large particles are located on the external surface of the crystallites, as visualized with TEM on the Pt/AlPO-11 sample (Figure 1). TEM of the Pt/ AlPO-11 sample revealed the presence of a variety of particle sizes distributed as 10 ( 3 nm. This particle size according to TEM is somewhat smaller than that derived from CO chemisorption, viz. ca. 20 nm (Table 1). The discrepancy could have to do with the assumptions regarding CO adsorption stoichiometry and particle morphology made in
FIGURE 3. NO into NO2 conversion over Pt catalysts at different reaction temperatures. The thermodynamic equilibrium conversion is indicated with a dashed line. 9 Pt/AlPO-11, b Pt/TiO2, 2 Pt/ Ferrierite, ( Pt/Al2O3. Reaction conditions: 10% O2, 500 ppm NO, 5% H2O in helium. VHSV ) 15 000 h-1.
FIGURE 1. TEM picture of Pt/AlPO-11 catalyst.
FIGURE 4. NO into NO2 oxidation conversion at 250 °C against Pt particle size (“Pt” was left out of the sample notations for reasons of space limitation).
FIGURE 5. Turnover frequency of NO into NO2 oxidation at 250 °C against Pt particle size (“Pt” was left out of the sample notations for reasons of space limitation). FIGURE 2. X-ray diffraction patterns of (A) Pt/AlPO-11 thermally aged at 850 °C and (B) fresh Pt/AlPO-11. The 35-45° 2θ region with the Pt (111) diffraction indicated by arrow is enlarged in the insert. the interpretation of the CO adsorption data. With TEM only a limited number of specimens can be examined. When in the pretreatment the Pt/ZSM-22 catalyst was not cooled after the calcination and the reduction with hydrogen was performed while the catalyst was at 400 °C, a much higher Pt dispersion of 46% was obtained (sample ZSM22*, Table 1). A platinum metal dispersion is a consequence of Pt amine decomposition, oxidation, and reduction kinetics (31). For the preparation of highly dispersed Pt particles in Y type zeolites impregnation with Pt amine precursor and heating at 0.2 °C/min during calcination is recommended (32). In this work the aim was to prepare Pt particles of larger size. A faster heating rate of 5 °C/min was used. Na-Y and Ba-Y zeolite crystals present a significant intracrystalline micropore volume and little external surface area (Table 1). On Na-Y and Ba-Y zeolite Pt dispersions of 20 and 37% were obtained, respectively. These Pt dispersions are in the range of literature data. For instance, Pt dispersions in Y type zeolites of 34-50% were reported in ref 23. Pt particles a few nanometers large can be occluded inside the cages of zeolite Y micropore network (32, 33). The ETS-10 material is another large-pore zeolite favoring formation of
small Pt particles (Table 1). Ferrierite with its system of intersecting channels and smaller specific surface in mesoand macropores represents a transition of behavior between ZSM-22 and AlPO-11 and the large pore zeolites Na-Y, Ba-Y, and ETS-10. The Pt dispersion was 16%. The alumina sample having wider pores compared to zeolites enabled the realization of a high Pt dispersion of 47%. High Pt dispersions are typical of γ-alumina (23). On the poreless titanium oxide sample with its very small specific surface area of 7 m2 g-1, the dispersion was very poor, viz. 1.9%. The Pt/AlPO-11 catalyst was subjected to thermal aging in a muffle furnace at 850 °C for 12 h. X-ray diffraction (Figure 2) revealed the sharpening of the Pt(111) diffraction at ca. 40° 2θ, evidencing the sintering of the platinum. The Pt particle size determined by CO chemisorption grew from ca. 20 to ca. 135 nm. NO Oxidation Activity. The NO oxidation activity of the Pt catalysts was determined using a gas mixture comprising 500 ppm NO, 5% O2, and 10% water. The NO into NO2 conversion on Pt/AlPO-11, Pt/TiO2, Pt/Ferrierite, and Pt/ Al2O3 is plotted against reaction temperature in Figure 3. On each catalyst, the NO into NO2 conversion increased with temperature until the conversion curve reached the limit set by the thermodynamic equilibrium, represented by the dashed line in Figure 3. Although the catalysts have the same Pt content, the NO oxidation activity varied substantially. To VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Carbon weight loss and differential weight loss curves in temperature-programmed carbon oxidation experiments using different catalysts: (a) Pt/AlPO-11, (b) Pt/Na-Y, (c) Pt/Al2O3, (d) Pt/ZSM-22, (e) Pt/ZSM-22*, (f) Pt/Ba-Y, (g) Pt/ETS-10, (h) Pt/TiO2, and (i) Pt/Ferrierite. Curve (j) represents carbon oxidation without catalyst. Gas composition: 10% O2, 500 ppm NO, and 5% H2O in He. WHSV ) 15 000 h-1; heating rate 5 °C/min. limit the extent of backward reaction, for investigating the NO oxidation kinetics the reaction temperature was set at 250 °C. The NO into NO2 conversion obtained with the different catalysts at 250 °C plotted against Pt particle size can be fitted with a curve displaying a maximum in the size range 20-40 nm (Figure 4). The catalytic activity expressed as turnover frequency (TOF), i.e., mol NO converted per mol exposed Pt atoms per second, is presented in Figure 5. TOF increases with increasing Pt particle size in the range from ca. 2 to 5 nm and reaches a plateau for particles of 20 nm and larger. A possible explanation for the poor NO oxidation activity of very fine Pt particles smaller than 5 nm occluded in zeolite micropores could be electron deficiency as a result of electron transfer to the zeolite (34). Pt particles of similar small size supported on γ-alumina surface display low catalytic activity as well, showing that in this type of catalysis and at the particle sizes realized zeolite effects are probably less important. The Pt/ZSM-22* and Pt/ZSM-22 samples with 2730
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mean particle sizes of ca. 1.8 and 20 nm both fit with the observed trend of NO oxidation activity with Pt particle size (Figures 4 and 5). This indicates that the observed relationships are intrinsic to the Pt particle size and rather independent of support. All evidence is in favor of NO oxidation on platinum metal being a structure-sensitive reaction, in agreement with an earlier observation on silica support (20). The Pt/TiO2 sample with Pt particles measuring 43 nm is a little less active than the Pt/ZSM-22 sample with 23 nm Pt particles (Figure 4). A decrease of activity at very large particle sizes is to be expected, since too-large particles, although presenting the appropriate crystal faces, present too little Pt surface area. Carbon Oxidation Activity. Carbon oxidation experiments were performed under a gas stream with 10% O2, 500 ppm NO, and 5% H2O. Weight loss and differential weight loss curves of the carbon/catalyst mixtures owing to carbon combustion are presented in Figure 6. The representation is
TABLE 2. Carbon Oxidation Dataa T10 (°C)
T50 (°C)
T90 (°C)
Tm (°C)
Ea (kJ/mol)
ln k0
Pt/AlPO-11 Pt/ZSM-22 Pt/TiO2 Pt/Ferrierite Pt/Ba-Y Pt/Na-Y Pt/ETS10 Pt/Al2O3 Pt/ZSM-22* Pt/AlPO-11 850 °C Pt(5%)/AlPO-11 carbon
368 372 375 389 390 397 405 428 385 431 351 505
425 428 436 446 434 438 448 493 439 492 400 556
471 478 490 494 488 494 499 546 486 538 442 601
422 427 440 452 438 439 449 512 447 505 399 573
80 ( 5.8 77 ( 4.3 80 ( 3.5 105 ( 6 90 ( 4.7 101 ( 9.5 99 ( 10.5 125 ( 5.6 106 ( 7.4 148 ( 6.7 83 ( 5.1 164 ( 5.4
11.6 ( 1 10.9 ( 0.6 11.2 ( 0.6 15.3 (1.1 13.0 ( 0.8 14.9 ( 1.6 14.4 ( 1.8 17.1 ( 0.9 15.6 ( 1.3 20.9 ( 1 12.6 ( 1 21.3 ( 1.1
Pt/Na-Y (NOx absent) Na-Y (NOx absent)
465 493
528 546
562 573
543 556
218 ( 13.3 255 ( 8.6
30.6 ( 2 35.4 ( 2.9
catalyst
a T10, T50, and T90: temperatures at 10%, 50%, and 90% carbon weight loss; T : temperature at maximum carbon weight loss rate; E : m a Arrhenius activation energy; ko: preexponential factor in Arrhenius equation. Experimental error on T10, T50, T90, and Tm estimated at ( 2 °C.
FIGURE 7. Influence of Pt particle size on temperature at 10% carbon oxidation on Pt catalysts (“Pt” was left out of the sample notations for reasons of space limitation). limited to the temperature range from 300 to 650 °C ascribed to carbon oxidation. It was verified with blank experiments that in this temperature range there is little weight change of the catalyst. At lower temperatures there is a weight loss owing to desorption of moisture and eventual residual solvent from the catalyst/carbon mixture preparation. Temperatures at 10, 50 and 90% carbon loss, further denoted as T10, T50, and T90 were determined and collected in Table 2. The reproducibility of the thermogravimetric experiments was evaluated with Pt/Ba-Y catalyst. The reproducibility of the T10, T50, and T90 temperatures was within a (2 °C margin. In absence of catalyst, carbon oxidation set in around 505 °C (T10 value, Table 2). The presence of Pt catalyst lowered the carbon oxidation temperatures substantially. The temperature at 10% carbon combustion, T10, with Pt catalyst present was in the range of 368-428 °C depending on the support. The observed T10 temperature order was
Pt/AlPO-11 (368) < Pt/ZSM-22 (372) < Pt/TiO2 (375) < Pt/ZSM-22* (385) < Pt/Ferrierite (389) < Pt/Ba-Y (390) < Pt/Na-Y (397) < Pt/ETS-10 (405) < Pt/Al2O3 (428) (2) Based on the T10 values, the Pt/AlPO-11 and Pt/ZSM-22 based catalysts were most active. Surprisingly, the highest T10 values were obtained with the alumina support, which is a currently applied washcoat for catalyst monoliths. The T10 values were plotted against Pt particle size on the different catalysts in Figure 7. The T10 values can be fitted with a common trend line. The lowest T10 values were obtained at Pt particle sizes of ca. 20 nm, present in Pt/AlPO-11 and Pt/ZSM-22. The 43 nm large Pt particles present on Pt/TiO2 catalyst led to low carbon combustion temperatures as well.
FIGURE 8. Temperature at 10% carbon oxidation in catalyst/carbon mixtures in temperature-programmed experiments against temperature at 30% NO into NO2 oxidation in NO oxidation experiments in fixed bed reactor. Small Pt particles of a few nanometers reduce the carbon oxidation temperature less than larger particles of tens of nanometers. The Pt/ZSM-22* catalyst with 46% Pt dispersion is less active than the same zeolite support loaded with the same amount of Pt present as 20 nm particles. The role of NO in carbon oxidation was evaluated in reference experiments using a gas mixture of 10% O2 and 5% water (Table 2, lower part). The T10 temperature on Pt/Na-Y was 465 °C, which is substantially higher than the 397 °C determined in the presence of NO. Omitting the platina from the catalyst resulted in a further rise of the carbon oxidation temperature. On Na-Y, T10 was 493 °C approaching the T10 of 505 °C of carbon in absence of catalyst. To verify the relationship of carbon combustion with NO into NO2 oxidation activity of the Pt catalysts, the T10 values from carbon oxidation were plotted against the temperatures at 30% NO into NO2 conversion obtained in NO oxidation experiments (Figure 8). There is a positive correlation between carbon oxidation and NO oxidation activity. It confirms the catalytic role of platinum in carbon oxidation primarily is to oxidize NO into NO2. A plot of T10 temperatures against NO oxidation turnover frequency (Figure 9) reveals the trend toward lower T10 values with increasing activity of Pt atoms in NO oxidation. The influence of the Pt content was investigated for the AlPO-11 support. Catalysts were prepared with 0.5 and 5 wt % of platinum loading. The increase of the Pt content led to a moderate reduction of the carbon combustion temperatures (Figure 10). T10 decreased from 368 to 350 °C. An improvement was also found in NO into NO2 oxidation conversion experiments. At 250 °C, the NO into NO2 conversion increased VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 9. Temperature at 10% carbon oxidation in catalyst/carbon mixtures in temperature-programmed experiments against catalytic turnover frequency of NO oxidation of exposed Pt atoms at 250 °C determined in Figure 5.
FIGURE 12. Ln k0 against Pt exposed surface for different Pt catalysts: (a) Pt/TiO2, (b) Pt/ZSM-22, (c) Pt/AlPO-11, (d) Pt/AlPO-11 with 5% Pt, (e) Pt/FER, (f) Pt/Na-Y, (g) Pt/ZSM-22*, (h) Pt/Ba-Y, (i) Pt/ETS-10, and (j) Pt/Al2O3. are independent of temperature and that carbon oxidation obeys first-order kinetics. Activation energy (Ea) and preexponential factor (k0) (Table 2) were determined from the temperature-programmed carbon oxidation experiments (Figure 5) by fitting with the following equations:
Ea RTm2 dm )n minf dT
FIGURE 10. Carbon weight loss against temperature on Pt/AlPO-11 with different Pt contents: (a) 5% and (b) 0.5%. Curve (c) represents carbon oxidation without catalyst. Gas composition: 10% O2, 500 ppm NO, and 5% H2O in He. WHSV ) 15 000 h-1; heating rate 5 °C/min.
FIGURE 11. Carbon weight loss in admixtures with Pt/AlPO-11 catalyst: (a) fresh; (b) calcined in air at 850 °C. Curve (c) represents carbon oxidation without catalyst. Gas composition: 10% O2, 500 ppm NO, and 5% H2O in He. from 32 to 40%. The Pt dispersion on the AlPO-11 sample with 5% Pt was near the limit of the CO chemisorption technique and estimated at ca. 0.6% corresponding to a particle size of the order of approximately 135 nm, which is larger than the particle size of 20 nm at 0.5% Pt content (Table 1). The moderate gain in activity at 5% Pt loading reflects the moderate gain in exposed platinum surface. Assuming a simple cubic shape for the Pt particles, it is estimated that the exposed Pt surface area in the 5% Pt/ AlPO-11 sample was only ca. 1.5 times larger than in 0.5% Pt/AlPO-11 sample. Platinum sintering in Pt/AlPO-11 catalyst at 850 °C leads to a significant increase in carbon oxidation temperatures (Figure 11). The Pt particle size increased from 20 to ca. 135 nm. At constant Pt content, this sintering corresponds to about a 7-fold reduction of exposed Pt surface area. The method proposed by Bokova et al. (35) was adopted to determine the apparent activation energies of the carbon oxidation process based on temperature-programmed carbon oxidation experiments. The method assumes that the Arrhenius activation energy and the preexponential factor 2732
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k0 )
(
(dm dT )
( )
inf
inf
(3)
)
e(Ea/RT) β
minfn
(4)
in which m is the carbon mass, Tm is the temperature at the maximum carbon combustion rate, R is the universal gas constant, β is the heating rate, viz. 5 °C min-1, and n is the reaction order. The subscript inf denotes the inflection point of the weight loss curve. A shrinking core model for carbon combustion requires a reaction order n of 2/3. When the combustion takes place internally as well as externally, the reaction order n is unity. In previous studies reaction orders of unity were found to be most appropriate for catalyzed carbon oxidation (36). The Ea value of 164 kJ/mol found in the present work (Table 2) for uncatalyzed carbon oxidation is in the range 140-170 kJ/mol often encountered in the literature (35, 36). A decrease of activation energy and/or increase of the preexponential value lead to an increase of the combustion rate (35). The pre-exponential factor represents a collision frequency of reactants with catalyst surface. The k0 values were plotted against exposed Pt surface in Figure 12. The exposed Pt surface was calculated from the Pt particle size determined via CO chemisorption and assuming cubic particle shape (although the particle shape resembles rather the hexagonal prism, e.g., in the TEM picture of Figure 1). There is an increase of k0 values with Pt surface, as expected. The scattering of the experimental points is likely due to the roughness of the kinetic analysis. Preexponential factors for Pt/Na-Y and Na-Y catalysts in absence of NO amount to 30.6 and 35.4, respectively (Table 2). These high values must pertain to non NOx mediated carbon oxidation reactions. The apparent activation energies exhibit a decreasing trend with increasing Pt particle size (Figure 13). The apparent activation energy on the Pt particles of optimum size of above 20 nm is around 80 kJ/mol. A similar value was found on the Pt/AlPO-11 catalyst with 5% Pt loading (Table 2). Below 10 nm Pt particle size, the apparent activation energy rises significantly, e.g., to ca. 125 kJ/mol on Pt/Al2O3. The apparent activation energy of carbon oxidation in absence of NO on Pt/Na-Y and Na-Y catalysts is much higher and amounts to 218 and 255 kJ/mol, respectively (Table 2). From the strong dependence of the catalytic activity of Pt particles in NO into NO2 oxidation on particle size observed
FIGURE 13. Activation energy Ea against Pt particle size for different Pt catalysts: (a) Pt/Al2O3, (b) Pt/ETS-10, (c) Pt/Ba-Y, (d) Pt/Na-Y, (e) Pt/FER, (f) Pt/AlPO-11, (g) Pt/ZSM-22, and (h) Pt/TiO2. in this work, it is concluded that NO oxidation on platinum is a structure-sensitive reaction. The reaction rate per exposed Pt atom increases with increasing Pt particle size and reaches a maximum above ca. 20 nm. Pt particles of optimum size of 20-40 nm are most active in NO into NO2 oxidation and reduce the carbon combustion temperature the most. The apparent activation energy of NOx mediated and Pt catalyzed carbon oxidation on Pt particles of optimum size is ca. 80 kJ/mol. At the optimum Pt particle size the nature of the support has little influence on the catalytic activity. The optimum Pt particle size was achieved on the external surfaces of AlPO-11 and ZSM-22 zeolite crystals and on titania support. Thermal treatment at 850 °C of Pt/AlPO-11 gave rise to significant Pt sintering to above the optimum Pt particle size. Small platinum particles synthesized inside zeolite cavities surprisingly exhibit poor catalytic activity. The nature of the support is important in catalyst preparation aspects and in stabilization of optimally sized platinum particles.
Acknowledgments We acknowledge the European Union for the financial support through the COMET project (contract G3RD-CT2002-00811). J.A.M. acknowledges the Flemish government for financial support through GOA.
Literature Cited (1) Lahaye, J.; Boehm, S.; Chambrion, P.; Ehrburger, P. Influence of cerium oxide on the formation and oxidation of soot. Combust. Flame 1996, 104, 199. (2) Cooper, B. J.; Thoss, J. E. Role of NO in diesel particulate emission control. SAE Paper 1989, 890404. (3) Hawker, P.; Myers, N.; Hu ¨ thwohl, G.; Vogel, H. T.; Bates, B.; Magnusson, L.; Bronnenberg, P. SAE Paper 1997, 970182. (4) Hawker, P. N. Diesel Emission Control Technology, system containing platinum catalyst and filter unit removes particulate from diesel exhaust. Platinum Metals Rev. 1995, 39, 2. (5) Jacquot, F.; Logie, V.; Brilhac, J. F.; Gilot, P. Kinetics of the oxidation of carbon black by NO2 influence of the presence of water and oxygen. Carbon 2002, 40, 335. (6) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. The oxidation of soot: a review of experiments, mechanisms and models. Carbon 2001, 39, 2247. (7) Allansson, R.; Blakeman, P. G.; Cooper, B. J.; Hess, H.; Silcock, P. J.; Walker, A. P. Optimising the low temperature performance and regeneration efficiency of the continuously regenerating diesel particulate filter (CR-DPF) system. SAE Paper 2002, 200201-0428. (8) Go¨rsmann, C. Catalytic coating for active and passive diesel particulate filter regeneration. Monatsh. Chem. 2005, 136, 91. (9) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Diesel Particulate emission control, review article. Fuel Process. Technol. 1996, 47, 1. (10) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Catalysts for the oxidation of soot from diesel exhaust gases. I. An exploratory study. Appl. Catal., B 1996, 8, 57. (11) Neri, G.; Bonaccorsi, L.; Donato, A.; Milone, C.; Musolino, M. G.; Visco, A. M. Catalytic combustion of diesel soot over metal oxide catalysts. Appl. Catal., B 1997, 11, 217. (12) McKee, D. W. Mechanisms of the alkali metal catalysed gasification of carbon. Fuel 1983, 62, 170.
(13) McKee, D. W. Carbon oxidation catalyzed by low melting point oxide phases. Carbon 1997, 25, 5587. (14) Jelles, S. J.; van Setten, B. A. A. L.; Makkee, M.; Moulijn, J. A. Molten salts as promising catalysts for oxidation of diesel soot: importance of experimental conditions in testing procedures. Appl. Catal., B 1999, 21, 35. (15) van Setten, B. A. A. L.; van Dijk, R.; Jelles, S. J.; Makkee, M.; Moulijn, J. A. The potential of supported molten salts in the removal of soot from diesel exhaust gas. Appl. Catal., B 1999, 21, 51. (16) Neeft, J. P. A.; Schipper, W.; Mul, G.; Makkee, M.; Moulijn, J. A. Feasibility Study Towards a Cu/K/Mo/(Cl) Soot Oxidation Catalyst for Application in Diesel Exhaust Gases. Appl. Catal., B 1997, 11, 365. (17) Ciambelli, P.; Palma, V.; Russo, P.; Vaccaro, S. The Effect of NO on Cu/V/K/Cl catalyzed soot Combustion. Appl. Catal., B 1999, 22, L5. (18) Pesant, L.; Matta, J.; Garin, F.; Ledoux, M. J.; Bernhardt, P.; Pham, C.; Pham-Huu, C. A high performance Pt/β-SiC catalyst for catalytic combustion of model carbon particles. Appl. Catal., A 2004, 226, 21. (19) Oi-Uchisawa, J.; Obuchi, A.; Enomoto, R.; Liu, S.; Nanba, T.; Kushiyama, S. Catalytic performance of Pt supported on various metal oxides in the oxidation of carbon black. Appl. Catal. B 2000, 26, 17. (20) Xue, E.; Seshan, K.; Ross, J. R. H. Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3. Appl. Catal., B 1996, 11, 65. (21) Arnor, J. N. Metal-exchanged zeolites as catalysts. Microporous Mesoporous Mater. 1998, 22, 451. (22) Martens, J. A.; Tielen, M.; Jacobs, P. A. Attempts to rationalize the distribution of hydrocracked products. III. Mechanistic aspects of isomerization and hydrocracking of branched alkanes on ideal bifunctional large-pore zeolite catalysts. Catal. Today 1987, 1, 435. (23) Tsou, J.; Magnoux, P.; Guisnet, M.; Orfao, J. J. M.; Figueiredo, J. L. Catalytic oxidation of volatile organic compounds. Appl. Catal., B 2005, 57, 117. (24) White, M. G. Heterogeneous Catalysis; Prentice Hall: Englewood Cliffs, NJ, 1991; p 88. (25) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flannigen, E. M. Aluminophosphate Molecular Sieves: a new class of microporous crystalline inorganic solids. J. Am. Chem. Soc. 1982, 104, 1146. (26) IZA Structure Commission. Homepage. www.iza-structure.org. (27) Ernst, S.; Weitkamp, J.; Martens, J. A.; Jacobs, P. A. Synthesis and shape-selective properties of ZSM-22. Appl. Catal. 1989, 48, 137. (28) Goa, Y.; Wu, P.; Tatsuli, T. Liquid-phase Knoevenagel reactions over modified basic microporous titanosilicate ETS-10. J. Catal. 2004, 224, 107. (29) van Setten, B. A. A. L.; Schouten, J. M.; Makkee, M.; Moulijn, J. A. Realistic contact for soot with an oxidation catalyst for laboratory studies. Appl. Catal., B 2000, 28, 253. (30) Neeft, J. P. A. Ph.D. Thesis, Delft University of Technology, 1995. (31) Reagan, W. J.; Chester, A. W.; Kerr, G. T. Studies of the thermal decomposition and catalytic properties of some platinum and palladium ammine zeolites. J. Catal. 1981, 69, 89. (32) de Graaf, J.; van Dillen, J; de Jong, K. P.; Koningsberger, D. C. Preparation of highly dispersed Pt particles in zeolite Y with a narrow particle size distribution: characterization by hydrogen chemisorption, TEM, EXAFS, spectroscopy, and particle modeling. J. Catal. 2001, 203, 307. (33) Ryoo, R.; Cho, S. J.; Pak, C.; Lee, J. Y. Clustering of platinum atoms into nanoscale particle and network on Na-Y zeolite. Catal. Lett. 1993, 20, 107. (34) Tong, Y. Y.; Laub, D.; Schulz-Ekloff, G.; Renouprez, A. J.; van der Klink, J. J. Indications from 195Pt NMR for a temperaturedependent metal-nonmetal transition of small platinum particles in zeolites. Phys. Rev. B 1995, 52, 8407. (35) Bokova, M. N.; Decarne, C.; Abi-Aad, E.; Pryakhin, A. N.; Lunin, V. V.; Aboukaı¨s, A. Kinetics of catalytic carbon black oxidation. Thermochim. Acta 2005, 428, 165. (36) Dernaika, B.; Uner, D. A simplified approach to determine the activation energies of uncatalyzed and catalyzed combustion of soot. Appl. Catal., B 2003, 40, 219.
Received for review September 21, 2005. Revised manuscript received January 17, 2006. Accepted March 1, 2006. ES051871H VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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