SiO2

Applied Catalysis B: Environmental 50 (2004) 73–82

Catalytic oxidation of nitrogen monoxide over Pt/SiO2 Joël Després, Martin Elsener, Manfred Koebel, Oliver Kröcher∗ , Bernhard Schnyder, Alexander Wokaun Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received 31 March 2003; received in revised form 10 December 2003; accepted 26 December 2003

Abstract The catalytic oxidation of NO was studied on a catalyst consisting of platinum supported on SiO2 . The kinetic behavior over Pt/SiO2 with a platinum loading of 2.5 wt.% was investigated in a feed containing 5% water and various concentrations of oxygen, nitrogen monoxide and nitrogen dioxide. The conversion of NO to NO2 increases when the oxygen concentration is increased from 0.1 to 10%, but levels off at higher concentrations. Increasing feed concentrations of NO lead to a decrease in the conversion to NO2 . The formation of NO2 is also depressed by the addition of NO2 to the feed. Both observations suggest that the oxidation of NO on Pt/SiO2 is autoinhibited by the reaction product NO2 . Further experiments have shown that the inhibition caused by NO2 is mostly persistent, i.e. a deactivation of the catalyst occurs. A pretreatment at 250 ◦ C in a feed containing 500 ppm NO2 causes a very strong decrease in activity. However, the initial activity can be restored either by a thermal regeneration at 650 ◦ C in air or by a regeneration under reducing conditions at 250 ◦ C, e.g. in a feed containing NH3 . This suggests that the deactivation by NO2 is due to the formation of a thin layer of platinum oxide covering the platinum surface at least partially. © 2004 Elsevier B.V. All rights reserved. Keywords: NO oxidation; NO2 ; SCR; DeNOx ; Platinum oxide

1. Introduction It becomes more and more evident that NO2 plays a decisive role in future exhaust gas aftertreatment techniques [1]. On the other hand, it is well known that the nitrogen oxides (NO+NO2 ) produced by combustion engines consist mainly of NO. The first example for this important role of NO2 is the NOx storage and reduction (NSR) catalyst presented first by Toyota [2]. This catalyst consists of a supporting oxide, typically ␥-Al2 O3 , doped with platinum metals (Pt, Rh), and a component showing basic properties (e.g. BaO) for the adsorption of NOx . The removal of nitrogen oxides is achieved by successive lean and rich phases. During the lean phase, NO is oxidized to NO2 on platinum and then stored on BaO as nitrate [3]. During a subsequent short rich phase, nitrogen oxides are released and reduced on Pt/Rh by the reducing species contained in the exhaust. The second example concerns the selective catalytic reduction (SCR) with N-containing reducing agents like am∗ Corresponding author. Tel.: +41-56-310-2066; fax: +41-56-310-2199. E-mail address: [email protected] (O. Kröcher).

0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.12.020

monia or urea over catalysts based on V2 O5 /WO3 /TiO2 . We have recently shown that the rate of the SCR reaction can be substantially increased if a fraction of the NO contained in the exhaust is converted to NO2 . The effect is most pronounced at low temperatures (200–300 ◦ C) and if the mixture contains equimolar amounts of NO and NO2 [4,5]. The third example is the continuously regenerating trap (CRT) proposed by Johnson Matthey for soot removal [6]. In this case, the strong oxidizing power of NO2 is used to continuously oxidize the soot collected on a particulate filter at comparably low temperatures. These temperatures are much lower than the ignition temperature of soot in air, which is typically around 550 ◦ C. All these processes require additional NO2 . It is usually obtained by using a strong oxidation catalyst which oxidizes NO to NO2 according to: NO + 21 O2
NO2

(1)

Bourges et al. [7] investigated the oxidation of NO over noble metal catalysts supported on ␥-alumina. Pt catalysts supported on ␥-Al2 O3 showed the highest activity. Xue et al. [8] investigated the role of the support and of the platinum loading on the oxidation of NO and the influence of

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sulfur dioxide. Platinum supported on SiO2 exhibited a higher activity than Pt/Al2 O3 and Pt/ZrO2 at the same platinum loading. Oi-Uchisawa et al. [9] observed a similar influence of the support. Xue et al. [8] also investigated the influence of the platinum particle size for Pt/SiO2 samples. They concluded that the reaction of NO oxidation was strongly size-dependent. A higher NO oxidation rate was observed for larger platinum particles. The present work reports on the kinetic behavior of Pt/SiO2 for the oxidation of NO. The reaction was investigated with varying concentrations of oxygen, nitrogen oxide, and nitrogen dioxide. In order to elucidate the mechanism of the reaction, the catalyst was further pretreated in two feeds; one containing only oxygen, the other a mixture of oxygen and NO2 .

2. Thermodynamic equilibrium Fig. 1 shows the thermodynamic stability of NO–NO2 as a function of temperature for oxygen partial pressures between 0.01 and 0.3 bar. The calculated ratios of NO2 /NOx are restricted to the condition that the concentration of NOx is small as compared to the concentration of oxygen. The main conclusion is that NO2 is stable at low temperatures and NO at high temperatures. The dissociation of NO2 becomes evident at temperatures above 200 ◦ C. High partial pressures of oxygen increase the stability of NO2 in the gas phase.

3. Experimental 3.1. Sample preparation and characterization The sample containing 2.5 wt.% Pt on SiO2 was prepared by incipient wetness impregnation. The supporting silica

(Aerosil TX-200, Degussa, 200 m2 /g, 3 mm cylinders) was first crushed and sieved to obtain the fraction with particle sizes between 160 and 200 ␮m. 114 mg of Pt(NH3 )4 Cl2 ·H2 O (Alfa Aesar, Johnson Matthey Gmbh, 56.4 wt.% Pt) was then dissolved in 2 ml of deionized water and 2.5 g of the supporting oxide was added. After homogenization for 1 h at room temperature, the paste was dried at 120 ◦ C overnight. The dried powder was then first calcined in air at 300 ◦ C for 2 h and finally reduced in a flow of 5% H2 /N2 at 450 ◦ C for 1 h. The platinum particle size was estimated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). From the TEM analysis, an average particle size of 13 nm was determined. Using XRD line broadening analysis, the average Pt particle size was estimated to be 10 nm. The two measurements are in fairly good agreement. XPS spectra were recorded with an ESCALAB 220i XL (Thermo VGScientific), using non-monochromatic Mg K␣ radiation at 300 W. XPS spectra were recorded in the constant analyzer energy (CAE) mode with an analyzer pass energy of 50 eV for the survey scan and 20 eV for the detail scans of C 1s, O 1s, Pt 4f, and Si 2p. In order to correct for the sample charging, binding energies (BE) were referred to Si 2p at 103.5 eV. The composition of the samples was determined by quantitative analysis using the cross-sections given by Scofield [10]. The platinum particle size was estimated from the XPS intensities using the model of Kerkhof and Moulijn [11]. 3.2. Experimental setup The experimental setup has been described earlier [12]. Due to the presence of water in the feed gas, all gas lines were heated to 150–170 ◦ C. The reactor consisted of stainless steel and had an internal diameter of 16 mm. The test gas first passed a preheating section consisting of a coiled stainless steel tube (2 mm internal diameter, 70 cm length). Both the preheater and the reactor were inside a stainless

100 30 % O2 20 % O2 10 % O2 5 % O2 1 % O2

NO2/NOX (%)

80

60

40

20

0 0

200

400

600

800

1000

T (˚C) Fig. 1. The ratio NO2 /NOx according to the thermodynamic equilibrium as a function of temperature for various partial pressures of oxygen.

J. Despr´es et al. / Applied Catalysis B: Environmental 50 (2004) 73–82

steel cylinder heated by a heating coil connected to a temperature controller. The catalyst temperature was measured by means of a thermocouple inserted in the quartz wool plug downstream of the catalyst bed. The gases at the reactor outlet were continuously analyzed by means of an FTIR spectrometer (Nicolet Magna IR 560, OMNIC® QuantPad software) equipped with a heated multiple pass gas cell (Graseby Specac G-2-4-BA-AU, path length 2 m) and a liquid nitrogen cooled MCT detector. The method developed allowed to quantify the concentrations of N2 O, NO, NO2 , HNO3 , NH3 and H2 O. The detection limits were 1 ppm for N2 O, NO2 and NH3 , 5 ppm for NO, 10 ppm for HNO3 , and 500 ppm for H2 O. 3.3. Catalytic tests All experiments were performed using 0.8 g of the powder sample, which was held between two quartz wool plugs. The gas flow rate was fixed at 150 lN /h and the feed contained 5% of water. The oxidation of NO (500 ppm) was tested in oxygen concentrations ranging from 0.1 to 30%. Experiments for testing the influence of the NO concentration were made at NO concentrations ranging from 100 to 1500 ppm in a feed containing 10% oxygen. In order to study the influence of NO2 , the oxidation of 500 ppm NO was investigated in a feed containing 10% oxygen and NO2 concentrations between 100 and 500 ppm. All these experiments were performed with a freshly reduced sample. In order to study the effect of changes of the platinum surface on the kinetic behavior, additional tests were performed with differently pretreated samples: • The standard reducing pretreatment given above (reduction in 5% H2 in N2 at 450 ◦ C for 1 h).

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• Pretreatment A: in 10% oxygen, 5% H2 O in N2 at 440 ◦ C for 2 h. • Pretreatment B: in 500 ppm NO2 , 10% oxygen, 5% H2 O in N2 at 250 ◦ C for 2 h. The oxidation activity of these samples was compared using a feed containing 500 ppm NO, 10% oxygen and 5% H2 O in nitrogen. 4. Results 4.1. Influence of the oxygen concentration Fig. 2 shows the oxidation of 500 ppm NO at various oxygen concentrations as a function of temperature. A maximum conversion is observed at ≈300 ◦ C for all oxygen concentrations. Above 300 ◦ C, the conversions decrease due to the thermodynamic equilibrium NO2 ⇔ NO + 1/2O2 , but are also depending on the oxygen concentration. The latter dependence is in accordance with Fig. 1, and a closer comparison of both figures reveals that the equilibrium is pretty well established above 300 ◦ C. Experimental values exceeding the theoretical curve should be attributed to inaccuracies of the temperature measurement. At temperatures below 250 ◦ C, the conversion increases with the temperature and the reaction is mainly kinetically limited. At the high oxygen concentrations, the conversion increases sharply with increasing temperatures. At low oxygen concentrations, the conversion increases only moderately with temperature. Fig. 3 shows the influence of the oxygen concentration on the oxidation rate at 150, 175 and 200 ◦ C. For oxygen concentrations below 10%, the reaction rate increases with increasing concentrations of oxygen but levels off for O2 > 20%.

100 30 % O2 20 % O2 10 % O2 5 % O2 1 % O2 0.1 % O2

Conversion (%)

80

60

40

20

0 100

200

300

400

500

600

Temperature (˚C) Fig. 2. Influence of the oxygen concentration on the conversion of NO as a function of temperature. Feed: 500 ppm NO, 0.1–30% O2 , 5% H2 O, and balance N2 .

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Oxidation rate (µmol/(g*s))

0.5 T = 200 ˚C T = 175 ˚C T = 150 ˚C

0.4

0.3

0.2

0.1

0 0

5

10

15

20

25

30

O2 concentration (%) Fig. 3. Influence of the oxygen concentration on the reaction rate of NO oxidation at 150, 175, and 200 ◦ C. Experimental conditions as in Fig. 2.

4.2. Influence of the NO concentration Fig. 4 shows the conversion of NO to NO2 as a function of temperature for various NO concentrations at a constant oxygen concentration of 10%. The conversion traces again a parabolic curve with the maximum of conversion depending on the NO concentration. For a concentration of 100 ppm NO, the maximum is observed at 275 ◦ C. At higher concentrations of NO, the maximum is shifted to higher temperatures. Experimental values exceeding the theoretical curve should be attributed to inaccuracies of the temperature measurement. Between 150 and 300 ◦ C, the conversion of NO to NO2 decreases with increasing concentrations of NO. For example, at 200 ◦ C the conversion is 55% at 100 ppm, but only 12% at 1000 ppm. In the absence of water in the feed, a similar behavior has been observed indicating that water cannot

be responsible for the decreasing conversions at increasing NO feed concentrations. Assuming differential conditions at low temperatures, reaction rates have been calculated from the conversions at low temperatures. Fig. 5 shows the dependence of the oxidation rate on the NO feed concentration at 150, 175, and 200 ◦ C. The rate increases for NO concentrations up to about 500 ppm, but levels off at higher concentrations. 4.3. Influence of the NO2 concentration The oxidation of 500 ppm NO was further studied in the presence of various concentrations of NO2 at a constant oxygen concentration of 10% in the feed. Fig. 6 shows the conversion at 200 and 225 ◦ C. No curves are reported for 150 and 175 ◦ C, because at these low temperatures, the oxidation of NO is completely suppressed in the presence of NO2 .

100 100 ppm NO 500 ppm NO 1000 ppm NO 1500 ppm NO Equilibrium

Conversion (%)

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0 100

200

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400

500

600

Temperature (˚C) Fig. 4. Conversion of NO to NO2 as a function of temperature for various NO feed concentrations. Feed: 100–1500 ppm NO, 10% O2 , 5% H2 O, and balance N2 .

J. Despr´es et al. / Applied Catalysis B: Environmental 50 (2004) 73–82

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Oxidation rate (µmol/(g*s))

0.4 T = 200 ˚C T = 175 ˚C T = 150 ˚C

0.3

0.2

0.1

0 0

200

400

600

800

1000

NO concentration (ppm) Fig. 5. Dependence of the oxidation rate on the feed concentration of NO. Experimental conditions as in Fig. 4.

pretreatment A at 440 ◦ C shows the same activity as the freshly reduced sample. However, the sample with pretreatment B shows a much lower activity. Therefore, treating Pt/SiO2 with NO2 causes a persisting deactivation of the catalyst. In comparable experiments with Pt/Al2 O3 and Pt/ZrO2 , which are worse oxidation catalysts, we could also observe a slight deactivation by NO2 . The deactivation caused by NO2 was mostly persistent for all three catalysts. The XPS spectra of freshly reduced and pretreated Pt/SiO2 samples are shown in Fig. 8. The binding energies have been derived from the peak positions of Pt 4f5/2 and Pt 4f7/2 for the various samples and are listed with the corresponding values for unsupported platinum and platinum oxides [13] in Table 1. The BE values for the sample after reduction are slightly higher than the values characteristic of bulk platinum due to the low coverage of platinum [14]. After pretreatment A with a feed containing O2 at 440 ◦ C, the BE values remain unchanged. The XPS spectrum after pre-

It is evident that the conversion decreases with increasing concentrations of NO2 in the feed. At 225 ◦ C, for NO2 = 0, the conversion is 42%. If 100 ppm NO2 are added to the feed, the conversion drops from 42 to 15% at 225 ◦ C and from 24 to 6% at 200 ◦ C. Therefore, the addition of a small quantity of NO2 to the feed has a very pronounced inhibiting effect on the oxidation of NO. 4.4. Effect of pretreatments using oxygen or a mixture oxygen + NO2 In order to better understand the effect of NO2 , the influence of pretreating the samples in an oxidizing atmosphere was investigated. These pretreatments have been described above and involved either oxygen at 440 ◦ C (pretreatment A) or a mixture of oxygen and NO2 at 250 ◦ C (pretreatment B). Fig. 7 shows the oxidation of NO over the freshly reduced sample and after the pretreatments. The sample with

50 T = 225 ˚C

40 Conversion (%)

T = 200 ˚C 30

20

10

0 0

100

200

300

400

500

NO2,in (ppm) Fig. 6. Influence of NO2 on the conversion of 500 ppm NO in 10% O2 , 5% H2 O, and balance N2 .

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100 fresh pretreatment A pretreatment B equilibrium

Conversion (%)

80

60

40

20

0 100

200

300

400

500

600

Temperature (˚C) Fig. 7. Oxidation of NO over reduced and pretreated samples. Feed: 500 ppm NO, 10% O2 , 5% H2 O, and balance N2 .

treatment B at 250 ◦ C with a feed containing both O2 and NO2 does not show a significant change of the Pt 4f peak. The intensity ratio of the XPS signal from Pt and the XPS signal from SiO2 is ≈0.045 and remains constant for all pretreatments. The comparable sample Pt/ZrO2 was investigated by XPS, too. Fig. 9 shows the spectra of the Pt 4f core-level for the reduced and pretreated samples. For the reduced sample, the peaks of Pt 4f core-level were observed at a binding energy of 74.65 eV for Pt 4f5/2 and 71.50 eV for Pt 4f7/2 . The peak positions are in good agreement with values reported for

bulk platinum metal [11]. After pretreatment A at 440 ◦ C in a feed containing oxygen, a shift of the Pt 4f peaks towards higher binding energies can be observed, suggesting the formation of PtO. Pretreatment B yields a further shift of the binding energies to 75.70 and 72.85 eV, respectively. A small shoulder at 76.80 eV in the Pt 4f5/2 peak is observed, indicating the possible formation of PtO2 .

Pt (4f) Pt 4f5/2

Pt 4f7/2

Pt (4f) PtO (4f)

Pt 4f5/2

(a)

Pt 4f7/2

Intensity (a. u.)

Intensity (a. u.)

(a)

(b)

(b)

(c)

(c)

PtO2 (4f)

80

75

70

65

Binding Energy (eV)

Fig. 8. XPS spectra of the Pt 4f core-level of Pt/SiO2 : (a) freshly reduced; (b) after pretreatment A at 450 ◦ C in the base feed; (c) after pretreatment B at 250 ◦ C with NO2 and base feed.

80

75

70

65

Binding Energy (eV)

Fig. 9. XPS spectra of the Pt 4f core-level of Pt/ZrO2 : (a) freshly reduced; (b) after pretreatment A at 450 ◦ C with the base feed; (c) after pretreatment B at 250 ◦ C with NO2 and base feed.

J. Despr´es et al. / Applied Catalysis B: Environmental 50 (2004) 73–82

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Table 1 XPS data of Pt/SiO2 and Pt/ZrO2 Sample

Pretreatment

Binding energy (eV)

Pt PtO PtO2

I (Pt 4f)/I (Si 2p)

dPt a (nm)

Pt 4f5/2

Pt 4f7/2

74.60 [13] 75.60 [13] 76.80 [13]

71.20 [13] 72.30 [13] 73.60 [13]

– – –

– – –

Pt/ZrO2

Reduction Oxidation in O2 b Oxidation in O2 + NO2 c

74.65 75.30 75.70

71.50 72.35 72.85

– – –

– – –

Pt/SiO2

Reduction Oxidation in O2 b Oxidation in O2 + NO2 c

74.80 74.75 75.00

71.60 71.40 71.50

0.045 0.046 0.049

7.0 6.8 6.8

a b c

Platinum particle size dPt determined using the model of Kerkhof and Moulijn [11]. Pretreatment A. Pretreatment B.

In order to check if the observed deactivation of Pt/SiO2 by NO2 can be attributed to a change in the size of the platinum particles, their size was estimated from the XPS intensities using the model of Kerkhof and Moulijn [11]. The model compares the experimental value of IPt /ISi (reported in Table 1) with the ratio IPt /ISi expected for a monolayer of platinum. If the experimental value is lower than the theoretical ratio for a monolayer, platinum is present in form of particles. The ratio IPt /ISi for a monolayer was calculated using the following equation:

 IPt Pt σPt β 1 + e−β = (2) ISiO2 monolayer σSiO2 2 1 − e−β SiO2 bulk β is the dimensionless support thickness, which is the ratio of the sheet thickness t (2/ρSiO2 SSiO2 ) of the supporting oxide and the escape depth of electrons from silica (λSiO2 ). λSiO2 is 3.8 nm [11], the BET surface area (SSiO2 ) is 160 m2 /g and the density of silica (ρSiO2 ) is 2.2×106 g/m3 [15]. The photoelectron cross-sections of silica (σSiO2 ) and of platinum

(σPt ) are 0.865 and 15.860, respectively [10]. (Pt/SiO2 )bulk is the bulk atomic ratio of platinum and SiO2 . The platinum loading of Pt/SiO2 is 2.5 wt.%, thus giving a value of 0.079 for (Pt/SiO2 )bulk . Therefore, the ratio of intensities expected for a monolayer (IPt /ISiO2 )monolayer is equal to 0.171. For the freshly reduced sample, the experimental ratio is 0.045. This value corresponds to about 26% of the monolayer. For a fraction equal to 0.26, the dimensionless crystallite size α (dPt /λPt ) is evaluated to be 3.7. Using the escape depth value λPt of 1.9 nm reported by Kerkhof and Moulijn [11], the particle size dPt of the freshly reduced sample is about 7.0 nm. Table 1 gives the particle size of the samples after pretreatments. It can be seen that the pretreatments do not affect the size of the platinum particles. 4.5. Regeneration of deactivated samples Two strategies were tested to restore the initial activity of the sample after deactivation with NO2 . The regeneration

100 fresh pretreatment B thermal regeneration regeneration with NH3 equilibrium

Conversion (%)

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0 100

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Temperature (˚C) Fig. 10. Oxidation of NO over freshly reduced sample, after pretreatment B with NO2 , and after regenerations. Experimental conditions as in Fig. 7.

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J. Despr´es et al. / Applied Catalysis B: Environmental 50 (2004) 73–82 250

NH3 (ppm)

200

150

100

50

0 0

2

4

6

8

10

Time (min)

Fig. 11. Evolution of NH3 during the regeneration of samples pretreated with NO2 (pretreatment B). Sample weight = 0.8 g, V ∗ = 150 lN /h, T = 250 ◦ C; feed: 200 ppm NH3 in N2 .

procedures were tested on samples that had previously been deactivated by NO2 applying pretreatment B. 4.5.1. Thermal regeneration The deactivated samples were regenerated in static air for 1 h at various temperatures ranging from 450 to 650 ◦ C in an oven. Fig. 10 shows that Pt/SiO2 recovers its initial activity after regeneration at 650 ◦ C. Lower temperatures were not sufficient to regenerate the catalyst. After regeneration at 550 ◦ C, the conversion of NO measured at 250 ◦ C was only 48% instead of 64% after a complete regeneration of the catalyst at 650 ◦ C. 4.5.2. Reducing treatment Another strategy to regenerate a deactivated sample consists of a reducing treatment. This involved treating the sample in the reactor at 250 ◦ C with a feed consisting of 200 ppm NH3 in nitrogen for 1 h. Fig. 11 shows the evolution of NH3 during the first minutes of the regeneration. During the regeneration, no formation of N2 O, NO, NO2 , or HNO3 was observed. Therefore, NH3 must have been oxidized to N2 , which is not detectable by the IR gas analyzer. As shown in Fig. 10, the deactivated sample was also fully regenerated by this treatment. Similar results were obtained with a treatment using NO in nitrogen.

5. Discussion Increasing the oxygen concentration at constant temperature and constant NO concentration in the feed leads to a strong increase of the oxidation rate (Fig. 3). However, at oxygen concentrations above 10%, the increase levels off indicating a saturation effect. This is consistent with the idea of a dissociative adsorption of oxygen on platinum. When the adsorption sites are saturated with atomic oxygen, increasing oxygen concentrations will no longer enhance the reaction rate.

At constant temperature and constant oxygen concentration, the conversion of NO is highest at low concentrations of NO, decreasing markedly with increasing NO concentration (Fig. 4). The conversion decreases significantly up to NO concentrations of 500 ppm. This behavior has been observed for both dry and humid feeds and is quite surprising. Burch and Watling [16] and Olsson et al. [17] proposed an Eley–Rideal mechanism, i.e. oxygen reacts from an adsorbed state and NO reacts from the gas phase. In this case, the conversion should be almost independent of the NO concentration under differential conditions. On the other hand, a dependence of the conversion on NO concentration could be explained by assuming a Langmuir–Hinshelwood mechanism where NO reacts from an adsorbed state. High concentrations of NO would then result in a saturation of the adsorption sites, and oxygen and NO could compete for the adsorption sites. In this case, we could expect an inhibiting effect of one reactant, leading to a parabolic shape for the curve oxidation rate versus partial pressure of oxygen. However, the Pt–NO bond is much weaker than the Pt–O bond [18,19]. Therefore, it is more likely that the reaction mechanism can be described by an Eley–Rideal mechanism. The influence of the NO concentration can best be understood by taking into account the strong negative effect of NO2 (Fig. 6). At constant concentration of oxygen and NO, the addition of NO2 strongly inhibits the conversion of NO. Seifert [20] observed a similar inhibition effect of NO2 on the oxidation of NO over MnO3 /FeO3 . This inhibition by NO2 can explain the negative effect of increasing NO in the feed. At low NO concentrations, the concentrations of NO2 produced are low and their effect on the oxidation of NO remains small. For example, with NOin = 100 ppm, only 55 ppm NO2 is formed at 200 ◦ C. However, higher input concentrations of NO lead to higher concentrations of NO2 that inhibit the oxidation of NO. Therefore, these observations suggest that the oxidation of NO on Pt/SiO2 is autoinhibited by the product NO2 . This autoinhibition could be confirmed by oxidation tests over samples pretreated with NO2 , showing that Pt/SiO2 (Fig. 7), Pt/Al2 O3 , and Pt/ZrO2 are deactivated. It is well known that NO2 has strong oxidizing properties and dissociates easily on platinum into NO and atomic oxygen [21]. Olsson and Fridell [22] have shown that NO2 can oxidize platinum supported on Al2 O3 . Therefore, the deactivation of Pt/SiO2 caused by NO2 could be due to the formation of platinum oxide. The surface sensitive X-ray photon electron spectroscopy seems to be the best method to prove this explanation. For Pt/ZrO2 , we could clearly see by XPS, the formation of pure PtO after pretreatment with oxygen and a further shift towards the formation of PtO2 after pretreatment with NO2 . The distinctive formation of platinum oxides corresponds to the reduced activity of Pt/ZrO2 . Since, in general Pt/SiO2 is a much better oxidation catalyst than Pt/ZrO2 , less unreactive platinum oxides should be formed on deactivated Pt/SiO2 resulting in much weaker XPS signals. However, it was unexpected that no platinum oxide

J. Despr´es et al. / Applied Catalysis B: Environmental 50 (2004) 73–82

could be found by XPS. The binding energies of the Pt 4f peaks after pretreatment in the feed containing NO2 remain close to the BE values of bulk platinum (Table 1). This is probably due to the fact that the fraction of PtO that may form on large particles is low compared to the fraction of platinum metal [23]. In order to illustrate this, the following calculations were performed. Assuming that a monolayer of platinum oxide was formed at the surface of the particle after deactivation at 250 ◦ C, the fraction of the XPS signal arising from platinum oxide is estimated from:
 d I(t) = 1 − exp − (3) I∞ λPt with t, thickness of platinum analyzed by XPS; d, thickness of a platinum oxide layer; λPt , escape depth of electrons in platinum (1.9 nm) [11]. The lengths of the unit cell edges in PtO are 0.305 ± 0.3 and 0.535±0.5 nm [24]. If a platinum oxide monolayer with a thickness of 0.535 nm is formed on the surface, the fraction of Pt XPS signal arising from PtO would be 24.5%. Taking into account the low platinum loading and the large platinum particle size of the sample (i.e. ≈7 nm), such a fraction of platinum oxide is close to the detection limit. It is even more likely that the surface of the platinum particle is only partly oxidized to platinum oxide [25], or that only a fraction of the total Pt particles is covered with this thin oxide layer [26]. Because of the combination of small concentration and low dispersion of platinum oxide the XPS signal falls below the detection limit, explaining that no change of the XPS Pt 4f peak was observed after pretreatment B in the presence of NO2 (Fig. 8). Berry [27] studied the surface oxidation of a platinum wire in a feed containing oxygen. He showed that platinum oxide can dissociate rapidly into platinum and oxygen in air at atmospheric pressure and temperatures between 600 and 650 ◦ C. Therefore, the thermal regeneration of the deactivated sample at 650 ◦ C in air permits the destruction of a thin oxide layer at the surface of the particle, thus restoring the active platinum surface. Based on these arguments, we propose the following mechanism which is in agreement with all the experimental observations: 2Pt + O2
2Pt–Oads

(4)

NO + Pt–Oads
Pt + NO2

(5)

NO2 + Pt
Pt–Oads + NO

(6)

NO2 + Pt
PtO + NO

(7)

Reaction (4) is the dissociative adsorption of oxygen on platinum, yielding atomic oxygen adsorbed at the surface of the Pt particle. Reaction (5) is the oxidation of NO involving the reaction between NO and adsorbed oxygen according to the Eley–Rideal mechanism. Reactions (6) and (7) represent the dissociation of NO2 on Pt with two possible products: oxygen atoms adsorbed on platinum or platinum oxide.

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Adsorbed oxygen atoms are formed first until a maximum permissible coverage is obtained. Above a certain limit, a deeper oxidation of platinum occurs leading to the formation of some form of platinum oxide. This mechanism is in agreement with the observations of Segner et al. [21] on Pt(1 1 1) under UHV conditions. Whereas the maximum coverage of platinum with Oad in oxygen is limited to 0.25, values up to ≈0.77 have been found in the presence of NO2 . In this case, platinum oxide will be formed causing the deactivation of the catalyst. Due to the fact that oxygen is bound stronger in platinum oxide than in Pt–Oads , its reactivity will be decreased. Another explanation for the irreversible deactivation of Pt/SiO2 and the other oxidation catalysts after pretreatment with NO2 could be the sintering of platinum. However, applying the model of Kerkhoff to calculate the Pt particle size from the XPS data, it is shown that the size of the particles is constant after each treatment. Therefore, the lower activity after pretreatment with NO2 cannot be attributed to the increase of the Pt particle size.

6. Conclusions The oxidation rate of NO over Pt/SiO2 depends strongly on the concentrations of oxygen in the feed at low concentrations, but levels off at concentrations above 10%. Increasing concentrations of NO lower the conversion to NO2 . This observation can be attributed to the higher concentration of NO2 produced in the reaction when the feed concentration of NO is increased. NO2 as the product of the reaction inhibits the oxidation of NO, thus suggesting that the oxidation of NO is auto-inhibited. This inhibition even causes a persistent deactivation of Pt/SiO2 . The catalyst can be regenerated by a thermal treatment at 650 ◦ C in air or by a reducing treatment. Therefore, the deactivation must be attributed to the formation of platinum oxide on the surface of the platinum particles.

Acknowledgements The financial support of the Swiss Federal Office of Energy (BFE) is gratefully acknowledged.

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