Zeolite Catalysts for Soot Oxidation: Influence of Hydrothermal

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Pt/zeolite Catalysts for Soot Oxidation: Influence of Hydrothermal Ageing Shuang Liu, Xiaodong Wu, Hui Luo, Duan Weng, and Rui Ran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04882 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 13, 2015

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The Journal of Physical Chemistry

Pt/zeolite Catalysts for Soot Oxidation: Influence of Hydrothermal Ageing Shuang Liu1, Xiaodong Wu*2, Hui Luo2, Duan Weng*1 and Rui Ran1 1

State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084 2

The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

* Corresponding authors: Email: [email protected]; Tel: 86-10-62792375; Fax: 86-10-62792375 Email: [email protected]; Tel: 86-10-62772726; Fax: 86-10-62772726

1 ACS Paragon Plus Environment


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ABSTRACT: Developments in diesel engines and gasoline direct injection (GDI) engines have spawned the requirement of soot oxidation catalysts that work well in both NO+O2 and O2. In this study, we found that supporting Pt on zeolites such as H-ZSM5 and USY may receive better soot oxidizers than commercial Pt/Al2O3 catalyst. Durability tests indicate that even after the hydrothermal ageing at 800 ºC, the aged Pt/H-ZSM5 is still a better soot oxidizer than Pt/Al2O3 in NO+O2, and the aged Pt/USY exhibits the best activity in O2. Further explorations reveal that the NO2 preferential adsorption on soot is dependent on acid sites both inside and outside micropores of zeolite, while the decomposition of surface oxygenated complexes (SOCs) can be promoted by strong acid sites on the external surface. These two factors contribute mainly to the high activity of the Pt/zeolite catalysts in NO+O2 and O2, respectively. Considering a relatively high external surface area is always essential to inhibiting the severe sintering of Pt, it is important to choose a zeolite support with a relatively large external surface area and a certain amount of acid sites after the hydrothermal ageing.

Keywords: Pt/zeolite; Soot oxidation; Hydrothermal ageing; Acid sites; External surface area.


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1. INTRODUCTION Diesel engines and gasoline direct injection (GDI) engines with lean burn technology are becoming more and more popular in the commercial market in recent years because of their high efficiency, durability and low CO and HC emissions. However, one of their major disadvantage with regard to environmental and health protection is the typically enhanced production of particulate matter (PM) comprising soot and unburned carbonaceous compounds.1 Therefore, the diesel particulate filter (DPF) and gasoline particulate filter (GPF) systems were developed for soot after-treatment and have been widely applied for the vehicles in Europe since the introduction of Euro 5 (2009) and Euro 6 (2014) legislation.1,2 In order to avoid filter blocking, one practical method is periodically raising the temperature of the DPF/GPF to above 600 °C if the pressure drop is too high. This can be achieved by malfunction of the engine and additional HC injection over the upstream oxidation catalyst. However, supplying heat costs fuel, and uncontrolled soot combustion can lead to extra high temperature and filter damage. Therefore, several so-called “passive regeneration” systems, such as the PSA (Peugeot-Citröen Societé d'Automobiles) system, the CRT® (Continuously Regenerated Trap, by Johnson Matthey) system, the CDPF/CGPF (Catalyzed Diesel/Gasoline Particulate Filter, or DPX®, by Engelhard) system and the DPNR (Diesel Particulate and NOx Reduction, by Toyota) system were developed.1-4 With the assistance of these systems, the trapped soot can be catalytically oxidized into CO2 at exhaust gas temperatures (about 200–400 °C) and the filter hereby is regenerated.5 In these passive regeneration systems, one key component is the soot oxidation catalyst, which should be both active and durable in the automobile exhaust environment. So far, platinum is the most widely applied soot oxidation catalyst because it is both highly active and durable. It is well known that Pt mainly promotes soot oxidation indirectly, i.e. by



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catalyzing oxidation of NO in exhaust gas to NO2, which is a powerful oxidant that can readily oxidize soot.6-10 Therefore, in absence of NOx (as in the condition of GPF), the advantages of Pt would be markedly weakened.1 Furthermore, since γ-Al2O3 as a commercial support is inert itself for both NO and soot oxidation, while some highly active supports like ceria may suffer from the fuel-derived sulfur oxides due to its basic nature,6 thus, the use of an appropriate acidic support may result in more practical Pt catalysts for soot oxidation. Actually, Oi-Uchisawa et al.10 have reported the potential of using non-basic supports (such as Ta2O5, Nb2O5 and WO3) to develop Pt-based catalysts with high soot oxidation activity. It has also been found that with the modification of acidic components such as WO3 and SO42-, the soot oxidation reaction on Pt catalysts could be significantly accelerated.11-14 These results implied the potential of acidic Pt catalysts as commercial catalysts for automobile exhaust soot purification. However, the low thermal stability of these acidic components makes significant deactivation of the catalysts unavoidable during application.11,15 Therefore, further exploration on stable acidic supports for Pt-based soot oxidation catalysts is still in demand. With plenty of acid sites, high surface area and good thermal stability, the H-ZSM and Y zeolites have been widely applied in many catalysis processes such as NOx reduction,16,17 hydroconversion,18,19 catalytic oxidation of volatile organic compounds (VOCs)20,21 and soot.22,23 Villani et al.22 investigated a series of Pt-loaded zeolites (ZSM-22, AlPO-11, Ba-Y and Na-Y) as catalysts for soot oxidation, and observed that these zeolites were comparable to the more common Al2O3 and TiO2 supports. In our previous study,23 it was further pointed out that the superior soot oxidation activity of Pt/H-ZSM5 came from the NO2 preferential adsorption and the catalytic formation and decomposition of surface oxygenated complexes (SOCs) over this catalyst. However, the influence of the specific amount, strength and location of zeolite acid sites


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on the catalytic soot oxidation remains unclear, and the universality of the above mechanism on other Pt/zeolite catalysts (such as Pt/USY) is still to be tested. More importantly, it should be always kept in mind that the presence of high amounts of water (> 2%), as well as the periodically high temperature (> 600 °C) in DPF may induce strong modifications of the physicochemical properties of the zeolite supports.6,24 It is well known that severe hydrothermal ageing causes strong dealumination of the zeolite framework, ultimately leading to the collapse of their structure and loss of the strong acid sites.16-20 Furthermore, the supported Pt particles may experience significant sintering and hereby deactivation during thermal ageing, especially in an oxidizing atmosphere like the automobile exhaust.6,22,25-28 However, the hydrothermal stability of Pt/zeolites as catalysts for soot oxidation has not been specifically studied so far.22,23 Therefore, it is necessary to examine the durability of Pt/zeolites as soot oxidation catalysts to fully evaluate their application potential. In this work, the catalytic behavior and durability on Pt/H-ZSM5, Pt/USY and Pt/Al2O3 was compared by measuring their activity with both soot temperature-programmed oxidation (soot– TPO) and isothermal soot oxidation. These catalysts were further characterized by a series of structural property measurements, NH3–TPD and NOx–TPD in order to explore the acid-assisted soot oxidation mechanism. Principles of choosing suitable acidic support for Pt-based soot oxidation catalysts with high activity and hydrothermal stability are proposed based on the results and discussions.

2. EXPERIMENTAL SECTION 2.1. Materials. Details about the preparation and characterization of the Pt/H-ZSM5 (PtZSM5), Pt/γ–Al2O3 (PtAl) catalysts, model soot (Printex–U) and “Nsoot” (the partially oxidized soot)



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have been previously reported.23 Additionally, another zeolite supported catalyst Pt/USY (PtUSY) with 1% Pt loaded was also synthesized in this work by incipient wetness impregnation of Pt(NO3)2 solution (27.82 wt.%, Heraeus) on a USY (NOVEL with a SiO2/Al2O3 ratio of 3.3 and a BET surface of 600 m2/g) support. After the impregnation, the powder was dried at 110 ºC overnight and calcined in air at 500 ºC for 2 h to obtain the as–received PtUSY. Moreover, to study the effect of high temperatures and water on the stability, the as–received catalysts were treated in an air flow (provided by an air pump) containing 10% water (provided by an injector followed by a pre-heater) with a total flow rate of 500 ml/min at 800 ºC for 10 h to obtain the hydrothermally aged catalysts with a suffix of “-A”. 2.2. Activity measurements. Temperature-programmed oxidation (TPO) of NO, soot and Nsoot in a fixed–bed reactor with the effluent gases monitored by an infrared spectrometer (Thermo Nicolet iS10) was performed to give a first screening of the catalyst activity. For (N)soot–TPO test, 10 mg (N)soot and 100 mg catalyst were mixed by a spatula in a reproducible way for ‘‘loose contact’’ conditions. To minimize the effect of hot spots, the samples were always diluted with 300 mg silica pellets. The reactant gas composition was 1000 ppm NO/10% O2/N2, 1000 ppm NO/10% O2/10% H2O/N2 or 10% O2/N2, and the gas flow was fixed at 500 ml/min with a gas hourly space velocity (GHSV) of 60,000 h-1. The reactor was heated from room temperature (RT) to 600 °C at a rate of 10 °C/min. The downstream CO2/(CO2+CO) ratio during soot oxidation was defined as the selectivity to CO2. To further investigate the intrinsic activity of the catalysts, the isothermal soot oxidation was performed at 300 ºC in 1000 ppm NO/10% O2/N2 and 400 ºC in 10% O2/N2, respectively, with the other reaction conditions similar to those in the soot-TPO tests. As mentioned in the previous works, the reaction regime in this study should be mostly controlled by the chemical kinetics.12,23


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The reaction rate (rsoot) was used to represent the intrinsic soot oxidation activity of the catalysts (Please refer to the Supporting Information for details). 2.3 Characterizations. X–ray diffractograms (XRD) of the catalysts were recorded in a diffractometer (D8 ADVANCE, Bruker, Germany) employing Cu–Kα radiation (λ = 0.15418 nm). The X–ray diffractograms were recorded between 20° and 80° (2θ) at an interval of 0.02°. The Brunner−Emmet−Teller (BET) surface area and external surface area of the catalysts were determined by physically adsorption–desorption of N2 at –196 ºC were obtained in a JW– BK122F (Beijing JWGB, China) instrument. The samples were degassed at 200 ºC under vacuum for 2 h before each analysis. X–ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 Xi system equipped with monochromatic Al Kα (1486.6 eV) X–ray source. The binding energy of C 1s (284.8 eV) was used as an internal standard. The high resolution Pt 4d spectra were fitted using the XPSPEAK program by the curve fitting with a Gaussian/Lorentzian ratio of 80/20 after subtraction of the Shirley–type background. Samples for high resolution transmission electron microscopy (HRTEM) analysis were prepared by dispersing the powder on holey carbon film supported on 3 mm Cu grids. A TECNAI G2 20 transmission electron microscope with a point resolution of 0.19 nm was used in this work. The size distribution of the platinum crystallites was determined by measuring 200 particles for each sample, and the mean Pt particle size was also calculated from these data. Pt dispersion was measured by CO titration in the same apparatus to that used in activity measurements. For each test, the catalysts were pre–oxidized in 30% O2/N2 at RT for 1 h. Afterwards, the flow was changed to N2 for degassing, and the reactor temperature was heated up to 150 °C. During the titration, 1% CO was introduced to the reaction atmosphere. The outlet



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CO2 concentration was analyzed and integrated. By assuming a reaction stoichiometry CO+O– PtS→CO2+PtS, the Pt dispersion and mean Pt particle size were obtained.12,23 The acidity of the catalysts were measured by NH3 temperature–programmed desorption (NH3–TPD) in the same apparatus to that used in activity measurements. For each test, the sample powders were first treated in N2 at 500 °C for 30 min for proper degassing and then exposed in 1000 ppm NH3/N2 at RT for 30 min. Afterwards, the NH3 desorption profiles were obtained by ramping the reactor from RT to 600 °C at a heating rate of 10 °C /min in N2. The NOx temperature–programmed desorption (NOx–TPD) tests were performed in the same apparatus to that used in activity measurements. Prior to the test, the sample powders were exposed in 500 ppm NO2/10% O2/N2 (500 ml/min) at 350 °C for 30 min then cooled down to RT in the same atmosphere and flushed by N2. Afterwards, the NO and NO2 desorption profiles were obtained by ramping the reactor from RT to 600 °C at a heating rate of 10 °C/min in a 10% O2/N2 stream.

3. RESULTS 3.1. Solid properties. It has been widely accepted that the particle size of Pt has crucial influences on the catalytic performance of the Pt catalysts.6,22 Due to a good dispersion, no characteristic peaks of Pt species are observed in the diffraction patterns of the as–received catalysts in Fig. S1. Therefore, HRTEM was employed to characterize the Pt particles on the catalyst external surface. As shown in Figs. 1a, 1b and 1c and Table 1, the three as–received catalysts have similar average Pt particle size distributions. However, it should be noticed that PtUSY has a few of particles >12 nm ranging all the way up to 20 nm (not shown), indicating that the Pt aggregation is somehow more significant on this catalyst than on PtAl and PtZSM5.


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Furthermore, there is 38%~43% of the Pt located inside the micropores (Fig. S2) of H-ZSM5 which cannot be detected by TEM as reported in our previous work.23 In this study, by combination of the CO titration and TEM results, it is also observed that 48%~56% of the Pt species in PtUSY exist in the form of “inner” particles (Please refer to the Supporting Information for details), which will be restricted to the micropore diameter and have small particle sizes (< 1 nm).28 After the hydrothermal ageing, metallic Pt peaks arise on all the XRD patterns of the catalysts (Fig. S1). As shown in Table 1, for PtAl-A, the average Pt grain size calculated by Williamson– Hall’s equation from the XRD data correlates well with the particle size in the TEM result. However, remarkable differences are attained between these two results for PtZSM5-A and especially PtUSY-A. These differences can be explained by the limited statistics of large Pt particles (> 60 nm) in TEM. Since these large particles are so few in number, they are less likely to be fully observed and counted compared with the smaller ones. Therefore, the TEM results of the aged catalysts reflect actually a smaller average Pt particle size than accurate.26 In this sense, the Pt particles on PtZSM5-A and especially PtUSY-A have experienced far more severe sintering than on PtAl-A. Furthermore, the CO titration results reveal a negligible CO2 production over all the aged samples, indicating that those small “inner” Pt particles in the zeolite micropores have transferred onto the catalyst external surface and agglomerated upon the hydrothermal ageing.



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Figure 1. HRTEM images and size distributions of Pt particles on (a) PtAl, (b) PtZSM5, (c) PtUSY, (d) PtAl-A, (e) PtZSM5-A and (f) PtUSY-A.

Table 1. Overview of the structural properties of the catalysts. Pt dispersion

dPt from CO

SBET

SExt

(%) a

titration (nm) a

(m2/g)

(m2/g) d

PtAl

25.0

4.4

–

4.6

146

133

PtZSM5

40.5

2.7

–

4.4

258

86

PtUSY

45.6

2.5

–

4.8

591

37

PtAl-A

–e

–e

30.1

30.3

123

122

PtZSM5-A

–e

–e

41.5

26.4

250

92

PtUSY-A

–e

–e

82.2

37.5

554

59

Catalyst

dPt from

dPt from

XRD (nm) b TEM (nm) c

a

Average Pt particle size estimated from the CO titration results.

b

Average Pt grain size calculated by Scherrer equation from the XRD data.

c

Average Pt particle size estimated from the TEM results.

d

Obtained from the nitrogen physisorption at -196 ºC.

e

The CO titrated was too little to estimate the dispersions and particle sizes of Pt. 10 ACS Paragon Plus Environment

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Besides the particle size, the oxidation state of surface Pt is another important factor that affects the redox behavior and hereby catalytic performance. The metallic Pt ratio (Pt0/(Pt0+Pt2+)) of the surface Pt species on PtAl and PtZSM5 has been previously reported as 13% and 12%, respectively.23 XPS investigation indicates a Pt0/(Pt0+Pt2+) of 15% for PtUSY (Fig. 2), suggesting that the Pt species exist mainly in a +2 oxidation state on all the as–received catalysts. Moreover, transformation of the thermodynamically unstable PtO to Pt occurs during the hydrothermal ageing, resulting in the remaining of only metallic Pt on all the aged catalysts (Fig. 2).23,25,26 In any case, it seems that the electron density of the surface Pt species were not obviously affected by the supports.

Figure 2. XPS spectra of Pt 4d for (a) the as–received and (b) hydrothermally aged catalysts.

3.2. Catalytic activities for NO and soot oxidation. It is well known the oxidation of NO to NO2 is an important step in the NOx-assisted soot oxidation, especially for the reactions performed in a loose–contact mode.12,22-24 As shown in Fig. 3, for the as–received catalysts, the NO oxidation activity follows an order of PtAl > PtUSY > PtZSM5. It has been previously discussed that the low NO oxidation activity of PtZSM5 is mainly attributed to the existence of 11 ACS Paragon Plus Environment


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small Pt particles inside the H-ZSM5 micropores, which are quite inactive for the reaction.22,23,29 For PtUSY, although the proportion of “inner” Pt particles on this catalyst is even higher than that on PtZSM5, those large Pt particles (e.g. particles > 12 nm) contribute importantly to the reaction, making PtUSY a better NO oxidizer than PtZSM5.

Figure 3. NO2 profiles during temperature–programmed oxidation of NO. Reactant gas: 1000 ppm NO/10% O2/N2. Gas flow rate: 500 ml/min. Ramping rate: 10 oC/min.

After the hydrothermal ageing, with the sintering of Pt particles, PtZSM5-A and especially PtAl-A exhibit obviously higher NO oxidation activities than the as–received catalysts, while PtUSY-A presents a slightly higher activity after ageing. This can be explained by the competing effects between the size and amount of Pt active sites. As a structural sensitive reaction, the oxidation of NO is benefited by large Pt particles.29 Nevertheless, a too large Pt particle may have too few Pt active sites, which in turn hampers the reaction. Villani et al. reported a volcanoshaped dependence between the Pt particle sizes and their NO oxidation activity.22 They suggested that supported Pt particles with a size of around 30 nm are the best NO oxidizer, which is just the case of PtAl-A in this study. Meanwhile, the Pt particles on PtZSM5-A and especially 12 ACS Paragon Plus Environment

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PtUSY-A are too large (> 40 nm, as indicated by the XRD results) to provide enough active sites, resulting in lower NO oxidation activity. To compare the soot oxidation activities of the catalysts, data of the temperatures at which 50% of soot was catalytically oxidized (T50) and the soot reaction rate (rsoot) values over different catalysts are illustrated in Table 2. The corresponding TPO and isothermal oxidation profiles are shown in Figs. S3 and S4. All the as–received and aged catalysts yield a high CO2 selectivity (> 99.5 %). For the as–received catalysts, it is clear that PtUSY followed by PtZSM5 exhibits higher soot oxidation activity than PtAl, in both NO+O2 and O2. Moreover, the introduction of H2O benefits the NOx-assisted soot oxidation over all the catalysts significantly, with a decrease in T50 by about 20 oC. This agrees well with the results obtained by Oi-Uchisawa et al.13,14 and Jeguirim et al.30 who attributed the acceleration effect of H2O to the formation and catalysis behavior of HNO3 on the soot oxidation reaction. On the other hand, after the hydrothermal ageing, PtAl-A and PtZSM5-A exhibit even higher activities than the corresponding as–received catalysts in NO+O2, while their deactivation is fully revealed with the removal of NOx from the reaction atmosphere. Contrarily, PtUSY-A exhibits a worse activity than PtUSY regardless of the reaction atmospheres. Because the NO2–pretreated “Nsoot” contains larger amount of surface oxygenated complexes (SOCs) than Printex–U, the difference between rNsoot and rsoot was applied to reflect the ability of catalysts for SOCs decomposition, which is an important step for soot oxidation.9,12,13,23 As shown in Table 2, the rNsoot and rsoot values of Pt/Al2O3 are similar, both before and after the hydrothermal ageing. Nevertheless, the oxidation of Nsoot can be accelerated over PtZSM5 and PtUSY with (rNsoot-rsoot) > 0.03 µmol·s-1·gcat.-1. After ageing, PtUSY-A maintains this



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acceleration effect but PtZSM-A loses it. As a summary, PtZSM5, PtUSY and PtUSY-A have the ability to accelerate the decomposition of SOCs, while PtAl, PtAl-A and PtZSM5-A do not.

Table 2. T50 (ºC) and reaction rate (rsoot or rNsoot µmol·s-1·gcat.-1) of the catalysts for soot oxidation in different conditions. 1000 ppm NO+10% O2 (+10% H2O) a Catalyst

catalyst + soot

10% O2 catalyst + soot

catalyst + Nsoot

T50 a

rsoot b

T50

rsoot c

T50

rNsoot c

PtAl

463 (443)

0.206

546

0.149

545

0.152

PtZSM5

440 (424)

0.231

535

0.199

521

0.238

PtUSY

428 (406)

0.242

531

0.203

522

0.235

PtAl-A

444

0.225

553

0.107

553

0.105

PtZSM5-A

433

0.238

558

0.095

556

0.096

PtUSY-A

448

0.219

552

0.112

543

0.141

a

Data in parentheses were obtained in 1000 ppm NO/10% O2/10% H2O

b

Calculated based on the amount of COx generated at 300 ºC

c

Calculated based on the amount of COx generated at 400 ºC

3.3. NH3 temperature–programmed desorption. To measure the acidity of the catalysts, NH3–TPD was performed and the corresponding results are shown in Fig. 4 and Table 3. The principles for specific division of the different acid sites have been previously reported.23 On one hand, the strength of the acid sites is evaluated by the NH3 desorption temperatures. The low– and high–temperature (200–230 ºC and 330–360 ºC) peaks in Fig. 4 are attributed to the “strong” and “weak” acid sites on the catalysts, respectively.21,23 On the other hand, only the acid sites on the external surface of the catalysts are really “accessible” for soot. The amount of these “surface 14 ACS Paragon Plus Environment

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acid sites” can be roughly estimated by considering the relationship between the total acid sites amount obtained by NH3–TPD ([NH3]total), the external surface area (SExt) and BET surface area (SBET) of the catalysts through an equation of [NH3]surf = [NH3]total×SExt/SBET.23,31,32 As shown in Table 3, PtZSM5 and PtUSY have similar total amounts of acid sites. However, the ratio of strong/weak acid sites is much higher on PtUSY than on PtZSM5. It should also be noted that PtUSY has much less “surface acid sites” than PtZSM5 since its SExt/SBET is rather small (Table 1), indicating that most of acid sites are located in the micropores of the USY support. After the hydrothermal ageing, the total acid sites amounts on PtZSM5-A and PtUSY-A are still similar to each other. It is noted that the former catalyst almost loses its strong acid sites, while the latter maintains its strong acid sites to a large extent. As for PtAl, this catalyst has only a small amount of acid sites, both before and after the hydrothermal ageing. In a word, it seems that strong acid sites are more abundant and stable on PtUSY than on PtZSM5, though they both have far more acid sites than PtAl.

Figure 4. NH3-TPD profiles of the (a) as–received and (b) hydrothermally aged catalysts.



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Table 3. The acid sites amounts of the catalysts.

Catalyst

Total NH3

Total NH3 desorption Surface NH3

Surface NH3 desorption

desorption

over strong acid sites

over strong acid sites

desorption

(µmol/g cat.) a (µmol/g cat.) b

(µmol/g cat.) c (µmol/g cat.) d

PtAl

160

6

146

5

PtZSM5

1020

339

340

113

PtUSY

1030

543

64

34

PtAl-A

84

–e

76

–e

PtZSM5-A 511

–e

188

–e

PtUSY-A

250

60

28

563

a

Calculated from the total NH3 desorbed in NH3–TPD.

b

Calculated from the high–temperature NH3 desorption in NH3–TPD.

c

Calculated from the total NH3 desorption considering the external surface area of the catalysts.

d

Obtained from the NH3 desorption at strong acid sites considering the external surface area of

the catalysts. e

The amount is too low to be calculated.

3.4. NOx temperature–programmed desorption. To find out the affinity towards NOx species of different catalysts, NOx–TPD tests were performed with a pretreatment in 500 ppm NO2/10% O2/N2. During this pretreatment, a small amount of soot (about 0.5%) was oxidized in all the soot–containing samples, which is small enough to be ignored when analyzing the NOx– TPD results. The obtained desorption profiles are shown in Fig. 5, and the corresponding qualitative results are listed in Table 4.


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Figure 5. (a), (b) NO2 and (c), (d) NO profiles in NOx–TPD of the (a), (c) as–received and (b), (d) hydrothermally aged catalysts (or catalyst + soot mixtures).

As shown in Fig. 5, all the catalysts exhibit two kinds of NOx desorption peaks: the low– temperature peaks (with the maximum below 180 ºC) assigned to NOx weakly adsorbed on the support, and the high–temperature peaks (above 180 ºC) attributed to the decomposition of nitrate/nitrite couples.23,33 PtZSM5 and PtUSY exhibit obviously smaller amounts of high– temperature desorbed NOx compared with PtAl (Table 4), because the high acidity of the zeolites suppresses the formation and stability of nitrates/nitrites on the Pt/zeolite catalysts.12,23 It seems that strong acid sites are more effective “NOx–storage inhibiters” than the weak ones, as almost no NOx desorption occurs at high temperatures on PtUSY with plenty of strong acid sites. By comparing the as–received and aged catalysts, it is clear that the latter exhibits smaller NOx 17 ACS Paragon Plus Environment


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desorption amount, which is probably due to the structural damage (collapse of micropores, reduction of surface area and so on) during the high–temperature hydrothermal ageing.16-20

Table 4. Amounts of the NOx desorbed from different samples in the NOx–TPD tests. Low–temperature

High–temperature

NOx weakly

NOx desorption

NOx desorption

adsorbed/chemisorbed

(µmol)

(µmol)

on soot (µmol)

PtAl

0.42

6.47

–

–

PtAl+soot

0.45

5.67

0.03/0

0.03

PtZSM5

2.25

1.33

–

–

PtZSM5+soot

3.01

1.28

0.76/0

0.76

PtUSY

0.64

0.04

–

–

PtUSY+soot

1.21

0.37

0.57/0.33

0.90

PtAl-A

0.12

3.75

–

–

PtAl-A+soot

0.13

3.50

0.01/0

0.01

PtZSM5-A

1.34

0.62

–

–

PtZSM5-A+soot

1.96

0.54

0.64/0

0.64

PtUSY-A

0.47

0.01

–

–

PtUSY-A+soot

0.92

0.26

0.45/0.25

0.70

Soot

0.52

0

0.52/0

0.52

Sample

a

NOx on soot (µmol) a

The amount of NOx both weakly adsorbed and chemisorbed on soot.

After mixing the catalysts with soot, the low–temperature NO2 desorption peaks move slightly towards lower temperatures (< 140 ºC). More importantly, they exhibit a remarkable increase in intensity over all the Pt/zeolite+soot mixtures compared with the pure catalysts (Figs. 5a and 5b). This additional NO2 has been attributed to the weakly–adsorbed NO2 on soot, which can also be observed in the profiles of the pure soot.7,12,23,34 Nevertheless, PtAl+soot and PtAl-A+soot release no more NO2 than the catalysts alone. Therefore, it is suggested that the soot in the 18 ACS Paragon Plus Environment

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Pt/zeolite+soot mixtures weakly adsorbed more NOx than it did in Pt/Al2O3+soot. It is also noted that the amount of high–temperature (>140 ºC for catalyst+soot, >180 ºC for pure catalysts) NO2 desorption increases a lot after adding soot to PtUSY and PtUSY-A, indicating some more firmly bonded chemisorbed NOx species (nitro groups and/or surface nitrates) have formed on soot in the corresponding mixtures. However, no such an increase is observed over the other catalyst+soot mixtures, which suggests that the USY support improves the adsorption and reaction between NO2 and soot more effectively than H-ZSM5 and Al2O3.7,34 In our previous studies, the inhibition effect of the acidic supports (the sulfated Al2O3 and HZSM5) on NO2 adsorption has been observed and discussed.12,23 As a result, more NO2 formed on Pt active sites can be preferentially adsorbed on the surface of soot instead of on the catalysts. As shown in Table 4, the NOx adsorption on soot in different catalyst+soot mixtures was obtained by subtracting the amount of NOx desorption over pure catalyst from that over the catalyst+soot mixture. Based on this calculation, the amount of the soot–adsorbed NOx in the PtAl+soot mixture (0.03 µmol) is negligible, indicating that PtAl does inhibit the NOx adsorption on soot due to the relatively strong NOx storage capacity of the neutral alumina support.35,36 Contrarily, the total amounts of the soot–adsorbed NOx in the PtZSM5+soot and PtUSY+soot mixture (0.76 and 0.90 µmol, respectively) are obviously larger than that on pure soot (0.52 µmol), demonstrating the preferential adsorption of NOx on soot in presence of these acidic catalyst. After the hydrothermal ageing, the amounts of the soot–adsorbed NOx in the PtZSM5A+soot and PtUSY-A+soot become smaller. This indicates that both the zeolite-supported catalysts lose their inhibition effect for NOx adsorption to some extent, due to their weakened acidity.



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4. DISCUSSION 4.1. Role of acid sites on NOx utilization and soot oxidation. In the experimental procedure of this work, soot and catalysts were mixed in the so–called loose contact condition, in which the NOx oxidation/adsorption/desorption processes are crucial to the catalytic oxidation of soot.1-3 In this sense, it seems that both the Pt/zeolite catalysts with low NO oxidation activity would be bad catalysts. However, they instead exhibit obviously higher soot oxidation activity in the presence of NOx than PtAl, indicating that some other factors counteract the negative influence of their weakened NO oxidation activities. One of the above mentioned “factors” should be the inhibition effect of the acidic supports on NO2 adsorption and hereby the preferential adsorption of NO2 on soot. As shown in Figs. 5a, 5b and Table 4, the NOx-storage abilities of Pt/zeolite catalysts are remarkably weaker than that of PtAl. During a NOx-assisted soot oxidation, after produced through NO oxidation over Pt, the NO2 can hardly be stored as nitrates/nitrites on Pt/zeolite. These NO2 may alternatively spill onto the soot surface, first weakly and/or chemically adsorb on soot, and then react with soot to generate SOCs (such as anhydride, lactone, carbonyl and quinone species).7,12,23,34 This step is important for soot oxidation, since these SOCs are more reactive than the complexes that have already existed on soot. Gaseous O2 presenting abundantly can further react with them and accelerate the entire soot oxidation reaction.8,9


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Figure 6. Relationship between the NOx (especially NO2) preferential adsorption on soot and the catalyst acidity.

Actually, one can quantify this “NO2 preferential adsorption” effect in a catalyst+soot mixture by measuring the NOx asdorption on soot with NOx–TPD. Since the kinetic diameters of NO (0.32 nm) and NO2 (0.52 nm) are smaller than the micropore sizes of PtZSM5 (0.57 nm) and PtUSY (0.63 nm), they can reach all the catalyst external surface and “inner” sites. Therefore, one can expect the total acid sites (not only the acid sites on the external surface) will determine the adsorption/desorption behavior of NOx. As shown in Fig. 6, all the Pt/zeolite catalysts exhibit some “NO2 preferential adsorption” ability, since their data points are located above the data of pure soot–adsorbed NOx. It is also noted that although catalysts with higher amounts of acid sites promote the preferential adsorption of NO2 on soot to a higher degree, a quasi–parabolic curve rather than a linear relationship is obtained. This indicates that for catalysts with high acidity, all of the NOx (especially NO2) will be transferred onto soot once formed. Further enhancement of the catalyst acidity will not result in any increase in the amount of the soot–adsorbed NOx.



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Furthermore, although PtUSY and PtZSM5 have similar amounts of acid sites, the former catalyst exhibits stronger ability for NO2 preferential adsorption. This is also supported by the fact that, in NOx–TPD the chemisorption of NO2 on soot occurs only in the PtUSY+soot mixture (Fig. 5a). Since the adsorption process was performed in NO2+O2, the surface R–NO2 can be easily removed by reacting with O2 (as in the case of pure soot, soot+PtAl and soot+PtZSM5).9 Therefore, there must have been intense NO2–soot reactions during the adsorption process for PtUSY+soot, in which a large amount of soot–chemisorbing NO2 generates and reserves to some extent even after the successive reaction with O2. One possible explanation for this extra NO2 chemisorption is that the strong acid sites over PtUSY influence the electronic states of soot, making it more likely to react with NO2.12,13,23 However, this speculation is falsified, because most of the strong acid sites on PtUSY are inside the micropores of the support and inaccessible for soot (Table 3). A more reasonable explanation is that, the strong acid sites are able to generate more NO2+ species through a protonation effect. Since this species is highly active to the nitration of aromatics, it can further accelerate the formation of nitrates/nitrites over soot.37 Then PtUSY with a higher amount of strong acid sites results in more intense NO2–soot reactions than PtZSM5, and hereby a more significant NO2 preferential adsorption on soot. Besides the NO2 preferential adsorption, another factor that enhances the soot oxidation activity of Pt/zeolite catalysts may be the direct promotion effect of acid sites on SOCs decomposition, which has been proved over PtZSM5, sulfated Pt/Al2O3 and even the corresponding acidic supports.12,23 However, it should be further emphasized that only the strong acid sites on the catalyst external surface (accessible for soot) have this promotion effect. One implication is that among all the hydrothermally aged catalysts, only PtUSY-A with strong acid sites exhibits obviously higher activity in Nsoot oxidation than in soot oxidation. Although the


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amount of total surface acid sites on PtZSM5-A is much larger than that on PtUSY-A (Table 3), it exhibits almost no promotion effect on SOCs decomposition (Table 2). As shown in Fig. 7, there is a good relationship between the amount of strong acid sites on the catalyst external surface and their promotion effect on SOCs decomposition (represented by the differences between the rNsoot and rsoot at isothermal oxidation). It can be seen that for the catalysts with a high amount of surface strong acid sites, a constant SOCs decomposition promotion is expected. This is because on these catalysts, the total reaction may be limited by factors other than SOCs decomposition (such as SOCs formation).

Figure 7. Relationship between the promotion effect on SOCs decomposition and the amount of strong acid sites on the external surface area of the catalysts.

In summary, with the “NO2 preferential adsorption” ability and the acceleration effect on SOCs decomposition, both the as–received Pt/zeolite catalysts exhibit higher soot oxidation activity than PtAl. These two factors, in addition to the NO oxidation activity, are critical to the catalytic behavior of an acidic Pt catalyst for soot oxidation in NO+O2. 23 ACS Paragon Plus Environment


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4.2. Influence of hydrothermal ageing on the catalysts. It is generally considered that the combination of high temperatures and a wet atmosphere may cause severe deactivation of the Pt supported zeolite catalysts, and the influence of a reductant like soot has not been addressed yet.19,20,26,27 In this study, it is observed that the addition of H2O (Table 2) does not deactivate the Pt/zeolite catalysts for NOx–assisted soot oxidation. Meanwhile, the hydrothermal ageing at 800 ºC does change the structural properties and catalytic behavior of the catalysts significantly. On one hand, the hydrothermal ageing leads to Pt sintering on all the catalysts. Datye et al.26 have proved that the sintering of supported Pt in air occurs via an Ostwald ripening mechanism, in which the growth of Pt particles occurs due to the migration of Pt atoms or clusters from small particles to the large ones. Therefore, γ–Al2O3 with a large external surface area (133 m2/g) may lead to isolation of the primary particles and hereby hamper the Pt migration upon ageing.38 Contrarily, the sintering of Pt is severe on the USY support with the smallest external surface area (37 m2/g). As a consequence, the sintering degree of Pt follows an order of PtUSY-A > PtZSM5-A > PtAl-A. Another possible explanation for the larger Pt particles on the acidic supports is related with the Pt–support interaction. Nagai et al.27 indicated that the Pt sintering could be controlled by the electron density of oxygen in the support through the Pt-oxide– support interaction, and basic supports (like CeO2 and ZrO2) might lead to sintering inhibition of the Pt particles. However, no obvious differences in Pt chemical state are found among all the catalysts in this study, so the Pt sintering is more likely related to the catalyst external surface area rather than to their acidity. On the other hand, the hydrothermal ageing results in breaking of the Si–O–Al bonds in the zeolite supports, and hereby remarkably reduces the acidity of the Pt/zeolite catalysts.18,19 It is also noted that catalysts with lower amounts of strong acid sites


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(PtZSM5 and PtAl) almost lose their strong acid sites after the ageing treatment, while PtUSY-A remains its strong acid sites to some extent. The changes in Pt particle size and surface acidity further alter the catalytic activity significantly. With larger Pt particles and less exposed Pt active sites, all the aged catalysts exhibit worse soot oxidation activity in O2 compared with the as–received ones. However, due to the structural sensitivity of NO oxidation reaction,6,22 both PtAl-A and PtZSM5-A become better NO oxidizers than the as–received catalysts, while PtUSY-A with too large Pt particles exhibit almost no increase in NO oxidation activity compared with PtUSY. For NOx–assisted soot oxidation, the catalytic activity follows an order of PtZSM5-A > PtAl-A > PtUSY-A. Although the catalytic performance here is suggested to be controlled by the state (e.g. amount and particle size) of Pt active sites, the NO oxidation activity, “NO2 preferential adsorption” ability and the acceleration effect on SOCs decomposition together make the reaction mechanism quite complicated.12,23 Therefore, it is necessary to differentiate the contributions of the abovementioned factors in the catalytic oxidation of soot. 4.3. Comparison of contributions of different factors. As mentioned above, there are three major factors that may influence the catalytic performance of a Pt/zeolite catalyst for soot oxidation, which are: (1) the particle size of Pt (associating with the reactivity and amount of Pt active sites), (2) the catalyst total acidity (associating with the ability of “NO2 preferential adsorption”), and (3) the strong acid sites on the catalyst external surface (associating with the acceleration of SOCs decomposition). Since their contributions to the catalytic oxidation of soot vary with the reaction atmosphere, the following discussion will be based on reactions performed in O2 (as in the condition of GPF) and NO+O2 (as in the condition of DPF) separately.



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First, in an O2 atmosphere, the strong acid sites on the catalyst external surface, as well as the particle size of Pt, are crucial for the oxidation of soot. This is proven by several experimental evidences: (1) PtZSM5 and PtUSY with higher amounts of surface strong acid sites exhibit obviously higher “SOCs decomposition rates” (Fig. 7) and hereby higher rsoot than PtAl (Table 2). (2) After the hydrothermal ageing, the promotion effect of PtZSM5-A on SOCs decomposition has gone with its strong acid sites. Without such an effect, PtZSM5-A with less Pt active sites exhibits even worse soot oxidation activity than PtAl-A (Table 2). (3) PtUSY-A, which preserves some surface strong acid sites, shows the highest rsoot among the aged catalysts, although the Pt sintering on this catalyst is the most severe. It means that the acceleration effect of acid sites on SOCs decomposition can even counteract the catalyst deactivation caused by Pt sintering. Thus, to own strong acid sites on the catalyst external surface is probably the most important factor that domains the catalytic oxidation of soot in O2. Second, in the presence of NOx, the acid-promoted SOCs decomposition is no longer as important as in O2. One implication is that as a good NO oxidizer (Fig. 3), although PtZSM5-A can hardly promote the decomposition of SOCs (Fig. 7), it still shows a higher soot oxidation activity than PtUSY-A (Table 2). Contrarily, the “NO2 preferential adsorption” ability derived from total acidity is more crucial. PtZSM5-A can make the produced NO2 preferntially adsobed on soot whereas PtAl-A cannot (Fig. 6). This makes PtZSM5-A exhibit obviously higher soot oxidation activity than PtAl-A (Table 2) in spite of its relatively lower NO oxidaiton activity (Fig. 3). In addition, it is reminded that beside NO2 preferetial adsorption, the NO oxidaiton activity is always crucial to the NOx–assisted soot oxdation. It is noted that PtUSY-A with low NO oxdiaiton acitivity but high “NO2 preferential adsorption” ability is a worse soot oxidizer than


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PtAl-A, indicating that there is no absolute conclusion that whether the NO2 prodcution or its transfer onto soot makes more contribution to the reaction. The relationship between these two factors can be summarized as: for the catalysts with “NO2 preferential adsorption” ability, their activity for soot oxiation is generally differentiated by their NO oxidation activity. Finally, based on the above discussion, it can be judged that for a good hydrothermally stable Pt/zeolite soot oxidaiton catalyst, a large external surface is always required, which is essential for preventing severe Pt sintering. Meanwhile, a certain amount of hydrothemrally stable acid sites is also indispensible. The specific requirements of acid sites depended on the operation conditions: The amount of total acid sites is crucial in the NOx–assisted soot oxidation (as in the condition of DPF), while the amount of strong acid sites on catalysts surface is more important for soot oxidation in O2 (as in the condition of GPF).

5. CONCLUSIONS Pt/H-ZSM5, Pt/USY and Pt/Al2O3 catalysts with similar surface Pt particle sizes and chemical states were prepared. The hydrothermal ageing at 800 ºC does change the structural properties and catalytic behavior of the catalysts significantly. From the obtained results and corresponding discussions, it can be concluded that: (1) The acid sites on Pt/zeolites bring about the ability of NO2 preferential adsorption as well as acceleration of the SOCs decomposition. The former ability is related to the total acid sites and is essential in the NOx–assisted soot oxidation. The latter ability correlates well with the strong acid sites on the catalyst external surface, and is important for soot oxidation in O2.



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(2) The hydrothermal ageing leads to Pt sintering and acidity loss on all the catalysts. Pt on the catalysts with larger external surface area experiences less severe sintering, making the catalysts maintain relatively larger amount of active sites and/or higher NO oxidation activity, and hereby better soot oxidation activity after ageing. (3) Pt/zeolites soot oxidation catalysts with high total acidity can be potential for DPF regeneration, while those ones with stable surface strong acid sites are promising catalysts for the soot removal of GPF.

ASSOCIATED CONTENT Supporting Information. Calculation process of surface Pt content based on CO titration and HRTEM data, calculation process of rsoot (rNsoot) and figures S1 to S4. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Wu) * E-mail: [email protected] (D. Weng) Notes The authors declare no competing financial interests.


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ACKNOWLEDGEMENT The authors would like to acknowledge Projects 2013AA061902 and 2015AA034603 by the Ministry of Science and Technology of China and Project 113007A by the Ministry of Education of China. Moreover, we would also thank the financial support from the Key Laboratory of Advanced Materials of Ministry of Education.

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Zeolite Catalysts for Soot Oxidation: Influence of Hydrothermal

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