Reduction of NO by propene over supported iridium catalysts under lean-burn conditions: an in situ FTIR study

Applied Catalysis A: General 263 (2004) 39–48

Reduction of NO by propene over supported iridium catalysts under lean-burn conditions: an in situ FTIR study Eduard Iojoiu1 , Patrick Gélin∗ , Hélène Praliaud, Michel Primet LACE-UMR CNRS 5634, Université Claude Bernard Lyon 1, bˆat. Chevreul, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France Received in revised form 28 November 2003; accepted 30 November 2003

Abstract The selective catalytic reduction (SCR) of nitric oxide by propene under lean-burn conditions over Ir/Al2 O3 and Ir/SiO2 was studied. Irrespective of the support, Ir supported catalysts were found to be active in the reduction of NO above 673 K, Ir/SiO2 being the most active. For both catalysts, the activity in the reduction of NO into N2 was shown to be strongly enhanced after their exposure at 873 K under the reaction mixture. In spite of varying initial Ir dispersions depending on the metal content and the support, Ir sintered after activation in the reaction mixture, which led to the same final Ir dispersion. An in situ FTIR study of both catalysts at various temperatures under the reaction mixture and under CO or NO used separately was carried out. It was shown that, under the reaction mixture, the surface of initially fully oxidised Ir particles progressively reduced with increasing temperatures into partially reduced Irδ+ surface species allowing the adsorption of both CO and NO (νCO at 2070–2050 cm−1 and νNO at 1870 cm−1 ). Additional species (formate, acetate, nitrate) formed only on the Al2 O3 support surface but these species are thought not to participate into the NO reduction. © 2003 Elsevier B.V. All rights reserved. Keywords: de-NOx ; NO SCR; Ir/Al2 O3 ; Ir/SiO2 ; FTIR study; Propene

1. Introduction Catalytic technologies for the control of NOx emissions include the non-selective reduction by CO and hydrocarbons over noble metal-based three-way catalysts, and the selective reduction by ammonia. The selective reduction of NO by hydrocarbons (HC-selective catalytic reduction, SCR) has attracted a lot of interest. Supported platinum catalysts have shown significant activity toward the HC-SCR at low temperature but large amounts of N2 O are produced with N2 and the range of temperatures within which the NO reduction proceeds is narrow [1–5]. Iridium-based catalysts have been also reported to be active and durable catalysts for HC-SCR of NOx [6–15]. N2 is formed between 623 and 873 K over Ir/Al2 O3 [16], i.e., at higher temperatures than over Pt-based catalysts which are active between 473 and 623 K. An advantage of Ir-based catalysts over Pt-based catalysts is that the selectivity of NO reduction toward N2 formation is high. ∗ Corresponding author. Tel.: +33-4-72-43-11-48; fax: +33-4-72-44-81-14. E-mail address: [email protected] (P. G´elin). 1 Present address: Institut de la Recherche sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne, France.

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.11.038

Recently, unsupported iridium catalysts were also found to exhibit high yield of nitrogen [17–19]. Wögerbauer et al. have investigated the behaviour of Ir black under HC-SCR conditions, examining the influence of the Ir crystallite size, the Ir/IrO2 ratio and the time on stream on the catalytic activity. The NO adsorption on Ir black of varying sizes is strongly favoured on larger crystallites, while the O2 adsorption would not depend on the crystallite size. This was thought to be responsible for improved yield in N2 [19]. The SCR of NO by propene over Ir/Al2 O3 under lean-burn conditions has been previously studied in our laboratory. The dispersion of supported Ir catalysts was also found to be an important factor influencing the activity in propene-SCR of NO. A dispersion in the 0.08–0.15 range was found to be required to obtain the best activity in NO reduction into N2 . Such a dispersion could be achieved after heating cycles in the reaction mixture up to 873 K. Highly dispersed Ir particles obtained after preparation of the fresh catalysts were found to sinter under reaction conditions, regardless of whether the catalyst was oxidised in O2 or in situ reduced in H2 [16] before reaction. Furthermore, it has been shown that, besides the size of Ir particles, the composition of the reaction mixture is another important factor influencing the activation of the Ir catalyst in the reduction of NO to N2 .

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E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48

The presence of both CO and O2 was found to be necessary for activating Ir/Al2 O3 while NO would not be [20]. In situ FTIR results revealed that initially fully oxidised Ir particles partially reduced in the reaction feed to form reduced Ir0 surface sites adsorbing CO at temperatures as high as 623–673 K [20]. But, in this study, no attention was paid to surface species present on the support. The present study aims at providing further understanding of the catalytic behaviour of supported iridium catalysts in the selective catalytic reduction of nitric oxide by propene under lean-burn conditions (1000 vpm NO, 2000 vpm C3 H6 , 500 vpm CO, 10 vol.% O2 and He as a carrier gas). In situ FTIR spectroscopy was used to study the ad-species formed at the surface of the metal sites and the support under the reaction mixture (containing NO, C3 H6 , CO and O2 ) at increasing temperatures. In order to better identify the ad-species formed, the interaction of the catalysts with each single component of the reaction mixture was also studied. Special attention was devoted to characterise the species formed on the support itself and to determine whether these species would play a role in the SCR activity. To this purpose, Ir catalysts were supported on two conventional supports, alumina and silica.

2. Experimental 2.1. Catalysts A 1 wt.% Ir/Al2 O3 solid was prepared by impregnation of a mesoporous ␥-Al2 O3 (from Rhodia, 136 m2 g−1 , 117 ppm S, average pore radius 6 nm) containing small amounts of chlorine (Cl < 50 ppm) with Ir(C5 H7 O2 )3 dissolved in toluene. After drying under vacuum, the catalyst was calcined overnight in flowing O2 at 673 K (heating rate 1 K min−1 ) in order to decompose the acetylacetonate complex, being so called oxidised Ir/Al2 O3 . It could be reduced overnight in a flow of hydrogen at 673 K (heating rate 2 or 1 K min−1 ), being so called reduced Ir/Al2 O3 . The amount of H2 irreversibly adsorbed at room temperature on the reduced catalyst was 20.5 ␮mol H2 g−1 solid, which corresponded to a dispersion of 0.79 if assuming H/Irs = 1 (Irs : number of iridium surface atoms and H: number of adsorbed hydrogen atoms). Two sets of solids were also prepared with a mesoporous SiO2 (from Grace Davison, 596 m2 g−1 , Cl < 300 ppm, S < 300 ppm, average pore radius 3 nm). The first one (1 wt.% Ir/SiO2 ) was prepared by impregnation with Ir(C5 H7 O2 )3 and treated as above, leading to respectively oxidised and reduced Ir/SiO2 . The resulting H2 uptake was 4.4 ␮mol H2 g−1 solid (dispersion = 0.17). In order to obtain a higher dispersion, SiO2 was also exchanged with an aqueous solution of Ir(NH3 )5 (OH)2 . The exchange was performed overnight at room temperature. After successive washing centrifugation cycles and final drying, the solid was calcined in flowing air at 773 K (heating rate 1 K min−1 ) and reduced in flowing H2

at 573 K (heating rate 1 K min−1 ). The solid, thus obtained contained 2 wt.% Ir and the H2 uptake was 32.8 ␮mol H2 g−1 solid (dispersion = 0.63). 2.2. Catalytic activity measurements The selective catalytic reduction activity measurements were carried out in a down flow fixed-bed quartz reactor at atmospheric pressure, as already described [11,16]. Typically, 0.2 g of sample was used and the reactant gas mixture contained 1000 vpm NO, 2000 vpm C3 H6 , 500 vpm CO, 10 vol.% O2 and He as a carrier gas (total flow rate 10 l h−1 ). Prior to the reaction and depending on the catalyst, the solids were either re-oxidised under O2 (oxidised catalysts) or oxidised and in situ reduced in flowing H2 (reduced catalysts). The activity was measured as a function of temperature at increasing and decreasing temperatures following a heating cycle under the reaction mixture. Each experiment was conducted as follows: introduction of the mixture at 298 K, heating from 298 to 893 K at a rate of 2 K min−1 , plateau at 893 K for 20 min, cooling from 893 to 298 K (2 K min−1 ). Several successive temperature cycles could also be performed. Reactants and products (CO2 , N2 O, O2 , N2 , CO) were analysed by gas chromatography, with He as carrier gas, using a dual CTR1 column from Alltech (Porapak and molecular sieve) and a TCD detector. A Porapak column and a flame ionisation detector were employed for hydrocarbons. In addition, the concentrations of NO, NO2 , N2 O and CO2 were continuously measured on-line by means of Rosemount IR and UV analysers. In the present paper the amount of NO2 (around 60 ppm, i.e., NO conversion into NO2 of 6%) formed in the pipes of the apparatus was not subtracted. The conversions of NO into N2 , N2 O and NO2 and of CO and C3 H6 were calculated. Nitrogen and carbon balances were obtained within 5%. Several reactions are considered NO + 19 C3 H6 → 21 N2 + 13 CO2 + 13 H2 O NO + 21 O2 → NO2 C3 H6 + 29 O2 → 3CO2 + 3H2 O CO + 21 O2 → CO2 NO + CO → 21 N2 + CO2 Very low amounts of N2 O are formed on Ir-based catalysts and the corresponding reactions are not taken into account. 2.3. Physicochemical characterisation The catalysts were characterised before and after reaction by various techniques: powder X-ray diffraction, XRD (Philips PW1050/81 diffractometer with Cu K␣ radiation), TEM and EDX (high-resolution JEOL 2010 electron microscope), measurement of the irreversible chemisorption uptake of H2 at 298 K after reduction and evacuation at 673 K

E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48

3. Results 3.1. Catalytic activity measurements Table 1 shows the catalytic activity during the first and second heating-up steps over oxidised or reduced 1 wt.%

100 NO into N2 NO into NO2 CO C3H6

(A)

Conversion (%)

80 60 40 20 0 400

500

600

700

800

900

Temperature / K 100 NO into N2 NO into NO2 CO C3H6

(B)

80

Conversion (%)

(laboratory-made volumetric apparatus). The irreversible H2 chemisorption was measured by using the dual isotherm technique: after the determination of a first isotherm the sample was evacuated at 298 K for 30 min and a second isotherm was determined. The dispersion (ratio of the number of superficial Ir atoms to the total number of Ir atoms) was deduced from the H2 uptake assuming H/Irs = 1 [21–23]. For TEM and EDX analysis, samples of reduced catalysts were once again exposed to air, suspended in ethanol and a drop of this solution was deposited on a carbon-coated grid. IR spectra were recorded at a resolution of 4 cm−1 on a FTIR spectrometer Nicolet Magna 550 using self supported sample wafers. These samples were introduced into a home made cell with CaF2 windows allowing to record spectra at different temperatures under controlled atmosphere [24]. The cell was connected to a flow reaction system at atmospheric pressure. The temperature was monitored through a thermocouple in direct contact with the sample. The samples were activated in situ in flowing oxygen at 673 K and contacted with a mixture containing 1000 vpm NO, 2000 vpm C3 H6 , 500 vpm CO, 4 vol.% O2 and He balance (total flow rate 10 l h−1 ). Some FTIR measurements were recorded on solids pre-reduced under flowing hydrogen at 673 K for 1 h. The temperature was increased up to 773 K and then decreased to room temperature. The adsorption of CO or NO on oxidised or reduced solids was also performed by using 500 vpm CO/He and 1000 vpm NO/He. The spectra of the initial sample (after activation) and the gas phase were systematically subtracted from that obtained in various mixtures in order to focus on the changes of the catalyst surface. The gaseous phases are characterised by: a triplet at 1903, 1875, 1850 cm−1 for NO, a doublet at 1628 and 1601 cm−1 for NO2 , a doublet at 2173 and 2115 cm−1 for CO, a doublet at 2361 and 2338 cm−1 for CO2 and bands at 1683, 1652, 1636, 1472 and 1442 cm−1 for C3 H6 .

41

60 40 20 0 400

500

600

700

800

900

Temperature / K Fig. 1. Conversions of NO into N2 and NO2 and of CO and C3 H6 (%) vs. temperature (K) during the second heating-up step under the NO–CO–C3 H6 –O2 mixture containing 10 vol.% O2 over oxidised (A) 1 wt.% Ir/SiO2 and (B) in situ reduced 1 wt.% Ir/SiO2 .

Ir/SiO2 . The catalytic behaviour of 2 wt.% Ir/SiO2 samples (not shown) was found to be similar to that of 1 wt.% Ir/SiO2 samples. It can be observed that the catalysts activate, under the reaction mixture, with respect to the conversion of NO into N2 . Fig. 1 illustrates the conversions of NO into N2 and NO2 and the CO and C3 H6 conversions for the 1 wt.% Ir/SiO2 either oxidised (Fig. 1A) or reduced (Fig. 1B), during

Table 1 Catalytic activity during the first and second heating-up steps over oxidised or reduced 1 wt.% Ir/SiO2 1 wt.% Ir/SiO2

Max.a NO/N2 Max.a NO/NO2 T50 b CO T100 b CO T50 b C3 H6 T100 b C3 H6 a b

Oxidised

Reduced

1st heating-up

2nd heating-up

1st heating-up

2nd heating-up

35% at 803 K 54% at 657 K 503 K 513 K 558 K 573 K

56% at 783 K Small 568 K 603 K 618 K 663 K

14% at 793 K 49% at 698 K 423 K 473 K 503 K 508 K

42% at 733 K Small 543 K 578 K 618 K 643 K

Maximum NO conversion into N2 or NO2 (%). Temperatures at which CO and C3 H6 conversions reached 50 and 100%, respectively (K).

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E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48 100 NO into N2 NO into NO2 CO C3H6

Conversion (%)

80 60 40 20 0 400

500

600

700

800

900

Temperature / K Fig. 2. Conversions of NO into N2 and NO2 and of CO and C3 H6 (%) vs. temperature (K) during the second heating-up step under the NO–CO–C3 H6 –O2 mixture containing 10 vol.% O2 over oxidised 1 wt.% Ir/Al2 O3 .

the second heating-up step. The following remarks can be derived from data shown in Table 1 and Fig. 1: (i) The NO reduction into N2 is enhanced after the first temperature cycle under the mixture, regardless of the activation treatment, being oxidising or reducing. The NO oxidation decreases. The NO–N2 O conversion (not shown) remains always very low (much less than 10%). (ii) During the first heating-up step, the reduced solid is more active toward the oxidation of CO or C3 H6 than the oxidised solid. The catalytic activity in these oxidation reactions decreases along a temperature cycle but the decrease is larger with the reduced solid. It could be inferred that bulk metallic Ir0 particles would favour CO and C3 H6 oxidation into CO2 . (iii) NO–N2 reduction begins whenever C3 H6 and CO are totally consumed. The catalytic activity of iridium supported on Al2 O3 has already been reported [20]. Improved conversion of NO into N2 occurs after activation under reactants at 873 K, regardless of the oxidising or reducing pre-treatment. At the same time the NO conversion into NO2 decreases. The catalyst is selective toward the formation of N2 since the NO–N2 O conversion (not shown) remains very low (< 10%). Fig. 2 shows the catalytic activity of oxidised 1 wt.% Ir/Al2 O3 during the second heating-up step under the standard NO–CO–C3 H6 –O2 reaction mixture, i.e. after activation under reactants. The N2 formation proceeds within the

same temperature window as for the 1 wt.% Ir/SiO2 catalyst (Fig. 1A). Surprisingly, N2 forms when propene is totally converted. It can be noticed that 1 wt.% Ir/SiO2 samples are significantly more active in NO reduction than 1 wt.% Ir/Al2 O3 samples. Interestingly, the oxidation of NO to NO2 proceeds at lower temperature and with higher conversion levels on 1 wt.% Ir/Al2 O3 than on 1 wt.% Ir/SiO2 . No difference with respect to the conversion of propene and CO can be observed between these two types of catalysts. Metallic dispersion of the studied catalysts was measured by H2 chemisorption before and after reaction with the NO–CO–C3 H6 –O2 mixture at 873 K (Table 2). For samples containing 1 wt.% Ir, the dispersion measured after reaction was found to be equal to 10±2%, irrespective of the support, SiO2 or Al2 O3 . Taking into account the initial dispersion of these samples, this in turn involves a much higher variation of the dispersion upon reaction for the alumina-supported catalyst than for the silica-supported catalyst. Interestingly, the 2 wt.% Ir/SiO2 exhibits the same dispersion after reaction as the 1 wt.% Ir/SiO2 and 1 wt.% Ir/Al2 O3 , indicating that neither the preparation method nor the Ir content has any influence on the final Ir dispersion. The dispersion decrease of various samples upon reaction is confirmed by TEM observations exhibiting larger particles after reaction than before reaction. As already suggested for Ir/Al2 O3 catalysts, the sintering of iridium particles supported on SiO2 can be at least partly responsible for the activation of the catalyst for NO reduction upon reaction in the feed. This however would not explain more subtle variations of the catalytic behaviour between Al2 O3 -based and SiO2 -based catalysts since Ir dispersion is almost similar for both types of catalysts. In order to explain the different catalytic behaviour of the iridium supported on SiO2 and Al2 O3 , FTIR studies were performed to identify ad-species formed under reaction conditions. 3.2. In situ FTIR study of Ir/Al2 O3 and Ir/SiO2 3.2.1. Interaction of CO with Ir/Al2 O3 and Ir/SiO2 Fig. 3 shows the infrared spectra of the reduced 1 wt.% Ir/Al2 O3 catalyst contacted with 500 vpm CO in He. At room temperature CO adsorption leads to the appearance of a broad asymmetric band at ca. 2090 cm−1 together with weak bands below 1900 cm−1 . The 2090 cm−1 band can be attributed to CO linearly adsorbed on Ir0 surface sites of fully reduced supported particles [25]. Such a band has been observed upon CO adsorption on Ir surface sites obtained by decomposition and H2 reduction of Ir4 (CO)12 deposited

Table 2 Metallic dispersion for Ir/SiO2 and Ir/Al2 O3 catalysts measured by H2 chemisorption before and after reaction Sample

1 wt.% Ir/SiO2 2 wt.% Ir/SiO2 1 wt.% Ir/Al2 O3

Before reaction

After reaction

H2 adsorbed (␮mol g−1 catalyst)

Dispersion (%)

H2 adsorbed (␮mol g−1 catalyst)

Dispersion (%)

4.4 32.8 20.5

16.9 63 78.9

2.9 5.2 2.1

11.2 10 8

E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48 2000

43 423

773

2060

A = 0.02

673

573 473

Absorbance

Absorbance

A = 0.02 473

473

2100

373

373 298

298

2200

2000

1800

1600

1400

1200

1000

-1

Wavenumber / cm

2200

2000

1800

1600

1400

1200

1000

-1

Wavenumber / cm

Fig. 3. Infrared spectra of the reduced 1 wt.% Ir/Al2 O3 solid contacted with 500 vpm CO in He at 298, 373, 473, 573, 673 and 773 K (from the bottom to the top of the figure). The spectra of the initial sample and the gas phase have been subtracted.

Fig. 4. Infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid treated by 500 vpm CO in He at increasing temperatures 298, 373, 423 and 473 K (from the bottom to the top of the figure). The spectra of the initial sample and the gas phase have been subtracted.

on Al2 O3 , SiO2 or zeolites [26–29], and upon CO adsorption on various faces of Ir single crystals [30–32]. The exact position is function of the CO coverage. Increasing CO coverage yields marked progressive blue-shifts in the νCO band frequency mainly due to enhanced dipole–dipole coupling. The weak bands below 1900 cm−1 should be related to bicarbonate-type species formed on the support. These species are fully decomposed above 373 K. Upon heating the sample at 473 K, a doublet at ca. 2090 and 2000 cm−1 develops in addition to the band characteristic of CO on surface Ir0 sites. The formation of isolated Ir(I)(CO)2 complexes on the support is suggested as already evidenced by previous reports [33–35]. Further changes of the νCO spectra upon heating at higher temperatures would indicate the formation of other types of Ir carbonyl species. The formation of such Ir carbonyl complexes supported on alumina would be favoured when iridium particles are highly dispersed, which is the case in this sample. In the case of reduced Ir/SiO2 samples with lower dispersion, the formation of such species upon heating in CO atmosphere was much reduced. Fig. 4 shows the infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid contacted with 500 vpm CO in He. After adsorption of CO at room temperature, a weak broad band forms at ca. 2100 cm−1 together with intense features below 1900 cm−1 (1820, 1780, 1660, 1450). The latter bands can be unambiguously attributed to vibrations of bicarbonate species. Since such species were not observed upon the adsorption of CO on the support alone, their formation would result from the reduction of Ir oxidised species by CO into Irδ+ surface species leading to CO2 release. CO would then adsorb on the partially reduced Irδ+ surface species, forming Irδ+ -CO species characterised by the band at ca. 2100 cm−1 . The higher frequency of this band compared to ␯CO of Ir0 -CO is currently explained on the basis of CO bond strength variations induced by variations of the electron density at the metal adsorption site. Indeed, the decrease of the electron density at the metal site results in

the decrease of the electron back donation from the metal into the antibonding orbital of the CO molecule, therefore strengthening the C–O bond with respect to that of CO adsorbed on a reduced surface. Assuming no large difference of the extinction coefficient for CO adsorbed on Ir0 and Irδ+ , the low intensity of the Irδ+ -CO species band compared to that of CO adsorbed on the reduced sample would suggest fewer CO species formed on the oxidised sample. This would be consistent with the incomplete reduction of the Ir surface. Increasing the temperature of CO adsorption induces the increase of the νCO band indicating further reduction of the surface and the increasing number of Irδ+ -CO species. HCO3 − species are unstable with increasing temperatures as shown by the disappearance of the corresponding bands. It is concluded that CO can partially reduce the surface of fully oxidised Ir particles, leading to the formation of highly stable Irδ+ -CO species. The reaction is favoured by increasing temperatures of CO adsorption. 3.2.2. Interaction of NO with Ir/Al2 O3 and Ir/SiO2 Fig. 5 shows the infrared spectra of the reduced 1 wt.% Ir/Al2 O3 solid contacted with 1000 vpm NO in He at increasing temperatures. At room temperature mainly one band at 1878 cm−1 has been detected, being attributed to νNO on Ir0 in agreement with literature data [35] on single crystals [30–32,34] reporting a νNO band in the 1870–1800 cm−1 range for NO linearly adsorbed on Ir0 faces. A broad band of lower intensity is observed at ca. 1630 cm−1 . This band could be attributed either to multifold co-ordinated NO typically observed in previous studies [30,31] or to adsorbed water resulting from humidity incompletely removed from the gases. Upon heating at 373 and 473 K, bands at 1320 and 1230 cm−1 developed, which can be attributed to nitro/nitrito species bound to the alumina. This explanation is also supported by the fact that such species did not form when SiO2 was used as support. The band at 1880 cm−1

44

E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48

1870

A = 0.08

A = 0.04

573 473 373

298

Absorbance

Absorbance

673

473 773

473

1930

298 298

2000

1800

1600

1400

1200

1000

-1

shifted to lower frequencies and a sharp new feature at ca.1840 cm−1 formed. This very sharp band could be tentatively related to the formation of well defined Ir0 nitrosyl complexes [36]. These complexes decomposed upon further heating and only the νNO band at 1870 cm−1 characteristic of NO adsorbed on Ir0 particles can be observed at 673 K, without significant change of its intensity. This indicates that NO is strongly adsorbed on the Ir0 surface. At this temperature it must be mentioned that the nitro-nitrito species are fully decomposed. Fig. 6 shows the infrared spectra of the reduced 2 wt.% Ir/SiO2 solid contacted with 1000 vpm NO in He. The same behaviour was observed for NO adsorbed on reduced 1 wt.% Ir/SiO2 . NO adsorption at room temperature leads to a broad band at ca. 1850 cm−1 with a shoulder at 1880 cm−1 and a band of smaller intensity at ca. 1630 cm−1 . The latter band has disappeared after heating at 373 K. It could be attributed either to multifold co-ordinated NO or,

1870

Absorbance

A = 0.08

673 573 523 1850

473 373 298

1800

1600

1700

1500

1300

1100

Wavenumber / cm

Fig. 5. Infrared spectra of the reduced 1 wt.% Ir/Al2 O3 solid treated by 1000 vpm NO in He at 298, 373, 473, 573 and 673 K (heating-up step). The spectra of the initial sample and the gas phase have been subtracted.

2000

1900

-1

Wavenumber / cm

2200

2100

1400

1200

-1

Wavenumber / cm

Fig. 6. Infrared spectra of the reduced 2 wt.% Ir/SiO2 solid treated by 1000 vpm NO in He, at 298, 373, 473, 523, 573 and 673 K during the heating-up step (from the bottom to the top of the figure). The spectra of the initial sample and the gas phase have been subtracted.

Fig. 7. Infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid treated by 1000 vpm NO in He, at 298, 473 and 773 K during the heating-up step and at 473 and 298 K during the cooling-down step (from the bottom to the top of the figure). The spectra of the initial sample and the gas phase have been subtracted.

more likely, to physisorbed water resulting from humidity incompletely removed from the gases. According to data obtained on reduced Ir/Al2 O3 , the broad and multiple band at ca. 1850 cm−1 can be related to NO adsorbed on Ir0 surface sites of reduced iridium particles (broad feature centred at ca. 1880 cm−1 ) and well defined nitrosyl iridium complexes (sharp feature at 1850 cm−1 ). The formation of both species increases between 373 and 573 K. Above this temperature only NO adsorbed on Ir0 particles (broad band at 1880 cm−1 ) is still present. Contrarily to what is observed on the alumina catalyst, no adsorbed species formed on SiO2 (no bands between 1300 and 1200 cm−1 ). Fig. 7 shows the infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid contacted with 1000 vpm NO in He. Upon adsorption of NO at 298 K, a weak broad band at ca. 1930 cm−1 formed together with strong bands between 1700 and 1100 cm−1 . The band at 1930 cm−1 is attributed to the νNO vibration of NO adsorbed on Ir surface sites at least partial oxidised [37]. IrO2 particles are stable and can not be decomposed under the conditions used for activation. This fact involves the partial reduction of oxidised Ir species by NO leading to partially reduced Irδ+ species and NO2 release. NO2 can then react with basic O2− sites of the alumina surface to form nitrate-type species. This interpretation is corroborated by the appearance of bands below 1900 cm−1 , which can be attributed to nitrate species formed on the support. It is worthwhile mentioning that the reduction reaction is favoured upon increasing the temperature of NO adsorption since the intensity of the 1930 cm−1 band increased up to a maximum at 473–523 K. Above 473 K, all species formed on the support decomposed progressively, being fully decomposed at 773 K. On the contrary, the band at 1900–1930 cm−1 related to NO adsorbed on Irδ+ sites was still present. Upon decreasing the temperature, the intensity of the bands at 1900–1930 cm−1 was restored at 523 K and only bands responsible for nitrate

E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48

species on alumina formed, similarly to what was observed on the reduced sample. While no NO adsorption occur on fully oxidised IrO2 surface, the partial reduction of the surface of oxidised Ir particles allows NO adsorption leading to ␯NO characteristic of NO on Irδ+ sites. 3.2.3. Interaction of the reaction mixture with Ir/Al2 O3 The FTIR spectra of the oxidised 1 wt.% Ir/Al2 O3 catalyst exposed to the reaction mixture containing 1000 vpm NO, 2000 vpm, C3 H6 500 vpm CO and 4 vol.% O2 (He as carrier) were recorded at increasing temperatures from 298 up to 723 K and reported in Fig. 8. Two regions were examined: the region between 2200 and 1800 cm−1 (Fig. 8A) characteristic of νNO and νCO vibrations of NO and CO adsorbed on Ir sites and the region between 1800 and 1100 cm−1 (Fig. 8B) characterising species adsorbed on the support (bicarbonate, nitrate, . . . ). Upon contact of the reaction mixture with the catalyst at 298 K, only one broad weak band at ca. 1945 cm−1 forms in

(A)

Absorbance

A = 0.04

723 673

1870

623 573 523 473 423

2060

1945

373 298

2200

2100

2000

1900

1800

1700

-1

Wavenumber / cm

A = 0.2

1580

1470

(B)

1310

Absorbance

723 673 623 573 523 473 423 373 298

1800

1600

1400

1200

1000

-1

Wavenumber / cm

Fig. 8. Infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid contacted with the (1000 vpm NO, 500 vpm CO, 2000 vpm C3 H6 , 4 vol.% O2 , He) mixture in the heating-up step, at 298, 373, 423, 473, 523, 573, 623, 673 and 723 K (from the bottom to the top of the figure). (A) 2200–1800 cm−1 range and (B) 1800–1200 cm−1 . The spectra of the initial sample and the gas phase have been subtracted.

45

the 2200–1800 cm−1 range. This band can be attributed to NO bonded to partially reduced Ir sites in agreement with the IR data of NO adsorbed on oxidised Ir/Al2 O3 (see Fig. 7). As proposed above, the NO adsorption would involve the following steps: the partial reduction of oxidised Ir surface species by NO into Irδ+ species and release of NO2 , NO2 reacting with basic alumina sites to form nitrate species; the adsorption of NO on Irδ+ species. The low intensity of the 1945 cm−1 band suggests that, at room temperature, only a small number of Irδ+ sites are formed at the surface of oxidised Ir particles and involved in the adsorption of NO. Upon heating, this band increased in intensity up to 423 K and shifted to lower frequency at the same time. This can be explained by the progressive increase of the number of reduced surface Ir species, possibly by formation of Ir ensembles having an increasing metallic character. Further heating between 423 and 573 K leads to the decrease of the intensity of the band at ca. 1930 cm−1 , which indicates that this species is thermally unstable. Interestingly, a new band at ca. 1870 cm−1 starts to develop upon further heating between 523 and 623 K. This band can be attributed to NO adsorbed on fully reduced Ir sites (see Fig. 5). Simultaneously a band at 2065–2060 cm−1 develops which corresponds to the adsorption of CO on reduced Ir sites. It could be concluded that at this stage, large domains of Ir particles have been fully reduced to the metallic state under the reaction mixture even in the presence of high amounts of oxygen. It is not clear whether CO and NO are co-adsorbed on the same sites or compete for the adsorption on different sites. These species are observed to desorb above 673 K. In the range 1800–1100 cm−1 , contacting the catalyst with the reaction mixture at 298 K led to the appearance of bands at 1616, 1584, 1426, 1311 cm−1 . Similar bands have been observed upon flowing Al2 O3 with a (NO + O2 ) mixture; they are assigned to different form of nitrates [38–41]. Upon reaction at 373 K, the complete disappearance of the 1426 cm−1 band would suggest the rearrangement of nitrate species into mainly unidentate complexes [38–41]. These bands decreased progressively above 373 K until full depletion at 623 K. Simultaneously a weak doublet at 1392 and 1376 cm−1 developed, being maximum at 573 K. This doublet is characteristic of adsorbed formate species [41,42]. Between 423 and 623 K two strong bands at 1576 and 1458 cm−1 developed, which are assigned to νas (COO) and νc (COO) of adsorbed acetate species [41,43]. From 673 K, all these bands are almost depleted. It can be concluded that during the heating step, nitrate species are progressively removed from the alumina surface with increasing temperature, being replaced by formate and acetate species originating from the transformation of C3 H6 molecules. It is worthwhile mentioning that acetate species are more thermally stable than formate species. After heating up to 773 K for 15 min the sample was cooled down to room temperature under the reaction mixture and the spectra were reported in Fig. 9. While almost no bands were observed at 673 K, a band at ca. 2050 cm−1 and

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(A) A = 0.02

A = 0.04

1870

2050

673 623

Absorbance

Absorbance

673 623 573 523 473 423

573 523 473 423 373

1900

2100

323

373 298

2200

2100

2000

1900

1800

1700

-1

Absorbance

1470

1800

1600

1400

1200

Wavenumber / cm (B)

1580

2000

-1

Wavenumber / cm

A = 0.2

2200

673

Fig. 10. Infrared spectra (2200–1200 cm−1 range) of the oxidised 2 wt.% Ir/SiO2 solid contacted with the (1000 vpm NO, 500 vpm CO, 2000 vpm C3 H6 , 4 vol.% O2 , He) mixture, in the heating-up step at 323, 373, 423, 473, 523, 573, 623 and 673 K (from the bottom to the top of the figure). The spectra of the initial sample and the gas phase have been subtracted.

623 573 523 473 1310

423 373 298

1800

1600

1400

1200

1000

-1

Wavenumber / cm

Fig. 9. Infrared spectra of the oxidised 1 wt.% Ir/Al2 O3 solid contacted with the (1000 vpm NO, 500 vpm CO, 2000 vpm C3 H6 , 4 vol.% O2 , He) mixture in the cooling-down step, at 673, 623, 573, 523, 473, 423, 373 and 298 K (from the top to the bottom of the figure). (A) 2200–1800 cm−1 range and (B) 1800–1200 cm−1 . The spectra of the initial sample and the gas phase have been subtracted.

a very weak broad band at 1870 cm−1 were detected from 623 K and below. These bands were already detected in the same temperature range during the heating-up step but with a much higher intensity. This indicates that the number of reduced Ir sites involved in the adsorption of CO and NO has much decreased after reaction at 773 K. This is in agreement with the sintering of Ir particles reported after activation in the feed [20]. Upon decreasing the temperature to 298 K almost no change in the spectrum was observed. This would suggest no further change of the surface of Ir particles. In the region below 1800 cm−1 (Fig. 9B) the band characteristic of acetate at 1576 and 1458 cm−1 and formate at 1392 and 1376 cm−1 re-formed below 623 K, while unidentate nitrate started to re-form below 523 K. At 298 K, acetate, formate and unidentate nitrate can be detected at the alumina surface. 3.2.4. Interaction of the reaction mixture with Ir/SiO2 The FTIR spectra of the oxidised 2 wt.% Ir/SiO2 catalyst exposed to the reaction mixture containing 1000 vpm NO, 2000 vpm C3 H6 , 500 vpm CO and 4 vol.% O2 (He as carrier)

were recorded at increasing temperatures from 298 up to 673 K and reported in Fig. 10. In the region of νCO and νNO, two bands at ca. 2100 and 1900 cm−1 , respectively progressively developed from 323 up to 523 K and shifted to lower frequencies as their intensity increased (ca. 2070 and 1870 cm−1 at 523 K). These bands are due to respectively CO and NO adsorbed on reduced Ir sites. As for Ir/Al2 O3 it may be inferred that the adsorption is made possible by the partial reduction of the Ir surface sites by CO and/or NO. The progressive shift on the bands towards lower frequency can be explained by the progressive increasing metallic character of the adsorption sites. As far as the position of these bands would be indicative of the extent of reduction of the iridium surface, the slightly higher frequencies observed for Ir/SiO2 compared with Ir/Al2 O3 would reflect a lower degree of reduction. An additional weak band at 1950 cm−1 is observed at temperatures in the range of 323–523 K with almost no change of its intensity. This band would be due to the adsorption of NO on oxidised Ir sites. This band is no longer observed after reaction at 773 K. The higher reducibility of Ir/Al2 O3 compared to Ir/SiO2 is expected to allow the direct oxidation of NO by O2 at low temperatures (T < 700 K). The formation of NO2 over Ir/Al2 O3 is observed at much lower temperatures than over Ir/SiO2 (Figs. 1A and 2), which fairly agrees with the higher reducibility of Ir/Al2 O3 suggested by IR results. On the contrary to Ir/Al2 O3 no nitrate, acetate or formate species formed on SiO2 . After reaction at 773 K, the spectra were recorded upon decreasing the temperature and reported in Fig. 11. The band at ca. 1870 cm−1 re-formed below 673 K. Its intensity was almost constant between 523 and 323 K. Simultaneously, the νCO band at 2070–2090 cm−1 was progressively restored. The upward shift of the band with the increase of its intensity could be attributed to the increase of CO coverage upon temperature decrease. As in

E. Iojoiu et al. / Applied Catalysis A: General 263 (2004) 39–48

A = 0.02

Absorbance

1870 673 623 573 523 473 423 373 2090 323

2200

2000

1800

1600

1400

1200

-1

Wavenumber / cm

Fig. 11. Infrared spectra (2200–1200 cm−1 range) of the oxidised 2 wt.% Ir/SiO2 solid contacted with the (1000 vpm NO, 500 vpm CO, 2000 vpm C3 H6 , 4 vol.% O2 , He) mixture, in the cooling-down step, at 673, 623, 573, 523, 473, 423, 373 and 323 K (from the top to the bottom of the figure). The spectra of the initial sample and the gas phase have been subtracted.

the case of IrAl2 O3 , the intensity of these bands was lower than upon the heating step. The sintering of Ir particles during the experiment would explain this observation. 4. Discussion Both Ir/SiO2 and Ir/Al2 O3 solids are active for the NO reduction into N2 regardless of the nature of the support, Al2 O3 or SiO2 . In all cases the activity in the selective reduction of NO is enhanced after exposure of the catalyst with the reaction mixture up to 873 K. This activation is accompanied by iridium sintering. It must be pointed out that the metallic dispersion after activation, irrespective of the support used, is almost the same. In spite of similar dispersions of Ir on Ir/Al2 O3 and Ir/SiO2 after activation, the catalytic behaviour of both catalysts is different. The conversion of NO to N2 is significantly higher on Ir/SiO2 than on Ir/Al2 O3 while the reverse is observed with respect to the NO oxidation. Only a small amount of NO2 is formed over Ir/SiO2 catalysts. Different mechanisms have been proposed in the literature for the SCR of NO by propene. Consecutive steps with organo nitro or nitrite species, RONO2 , RONO such as C3 H7 –ONO (1662 cm−1 ), have been considered [44–48]. Adsorbed nitrate and acetate have been detected by Shimizu et al. [41,49] on Al2 O3 , in a flow of (C3 H6 –O2 ) and during NO–C3 H6 SCR. For these authors, the process starts with the nitrate formation (NO oxidation and subsequent adsorption of NO2 on basic oxygen sites) followed by its reaction with C3 H6 to form acetate; the acetate is active as a reductant and takes part into the N2 formation. We did observe in our study the same types of species on Ir/Al2 O3 (nitrate, acetate, formate), these species being thermally stable up to 673 K, when the conversion NO to N2 appears. Nitrate species can form by reaction of NO2 with basic sites of the

47

alumina surface while acetate and formate species would form due to the incomplete oxidation of propene. It is clear that the higher activity in SCR of NO for Ir/SiO2 compared to Ir/Al2 O3 could not be related to these species present on the support, since being absent on the SiO2 support. It could be inferred that ad-species present on alumina are spectators and not intermediates in the formation of N2 from NO. However, their formation may reduce the accessibility to the Ir active sites, which could explain the lower activity of Ir/Al2 O3 compared to Ir/SiO2 . Cant et al. [50] have detected bands at 1723 and 1663 cm−1 by reaction of nitromethane on Co-ZSM-5 and Cu-ZSM-5. They have considered that the 1723 cm−1 band was due to a cyanuric acid, the cyclic trimer of HNCO, while the 1663 cm−1 one was caused by another s-triazine. In the present work such species possibly formed on Ir/Al2 O3 but were not detectable with confidence. For Ir/SiO2 no such species could be detected at all. The problem to know whether Ir particles are oxidised, partially oxidised or reduced is of debate. From XRD data previously reported [20] only IrO2 is detected on Ir/Al2 O3 after activation under reaction mixture, irrespective of the initial state of the catalyst, oxidised or reduced. For Wogerbauër et al. [15], the presence of bulk Ir0 is detected and considered as a crucial parameter for high DeNOx activity. It must be mentioned that this statement is only based on XRD data and not on a study of the iridium surface state. The present FTIR study clearly establishes that even though bulk IrO2 is detected, and in spite of strongly oxidising conditions, the fully oxidised Ir surface partially reduces under the reaction mixture. Indeed, although no adsorption of NO and/or CO can occur on a fully oxidised IrO2 surface, CO and NO can partially reduce surface oxidised Ir species, allowing their adsorption on these partially reduced sites. The key point is that this reduction process can also proceed when O2 is present in excess in the feed in addition to NO and CO. It is not possible to evaluate the extent of reduction of IrO2 particles from only FTIR data. On a qualitative point of view, taking into account the higher frequencies of νCO and νNO in the activated samples compared to νCO and νNO in fully reduced ones, it can be suggested that Ir adsorption sites are not fully reduced and keep an Irδ+ character.

5. Conclusions On the basis of the presented data we can draw the following conclusions: • Supported Ir catalysts exhibit activation for the reduction of NO into N2 under reactants at 873 K. This activation is accompanied by a decrease of the Ir dispersion down to ca. 10%. • The support has some influence on the catalytic property of Ir catalysts in the reduction of NO by propene under

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lean conditions: in spite of identical Ir dispersion on both supports after activation under reactants, Ir/Al2 O3 is less active than Ir/SiO2 in the conversion of NO into N2 and more active than Ir/SiO2 in the oxidation of NO into NO2 . • In spite of strongly oxidising reaction conditions, the Ir surface is thought to be partially reduced into Irδ+ surface species allowing CO and/or NO to chemisorb. These species are still present under the reaction mixture at temperatures as high as 623 K. This would suggest that a possible role of CO is to prevent the complete oxidation of the Ir surface by O2 present in large excess in the reaction feed, therefore allowing the adsorption and reaction of NO at the catalytic site. • The presence of formate, acetate and nitrate species was revealed on the Al2 O3 support, while being totally absent on the SiO2 support. These species would not be related to the activity of Ir catalysts in the reduction of NO into N2 . However, they might contribute to the decrease of the accessibility of the active Ir sites to reactants, thus explaining the lower activity of Ir/Al2 O3 in the selective reduction of NO into N2 . Acknowledgements The authors are very grateful to EZUS-LYON 1 for its financial assistance. EZUS-Lyon 1 is a research pool in which ANVAR (Agence Nationale de Valorisation pour la Recherche) is implied. References [1] M.D. Amiridis, T. Zhang, R.J. Farrauto, Appl. Catal. B 10 (1996) 203. [2] M. Sasaki, H. Hamada, Y. Kintaichi, Y. Ito, M. Tabata, Catal. Lett. 15 (1992) 297. [3] R. Burch, P.J. Millington, Catal. Today 26 (1995) 185. [4] P. Denton, A. Giroir-Fendler, H. Praliaud, M. Primet, J. Catal. 189 (2000) 410. [5] R. Burch, P.J. Millington, A.P. Walker, Appl. Catal. B 4 (1994) 65. [6] European patent application EP 0,696,470 A1 (14/02/1996), Applicant MITSUBISHI JUKOGYO KABUSHIRI KAISHA, Tokyo. [7] European patent application EP 0,730,900 A1 (11/09/1996), N.E. CHEMCAT CORP., Minatu-Ku, Tokyo. [8] European patentschrift 0,559,021 B1 (25/10/95), DEGUSSA, Frankfurt. [9] T. Nakatsuji, Appl. Catal. B 25 (2000) 163. [10] G. Zhang, T. Yamaguchi, H. Kawakami, T. Susuki, Appl. Catal. B 1 (1992) L15. [11] Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, Appl. Catal. B 4 (1994) 199. [12] A. Okumura, M. Hori, M. Horiuchi, Catalysts Catal. 39 (1997) 117. [13] M. Hori, A. Okumura, H. Goto, M. Horiuchi, M. Jenkins, K. Tashiro, Soc. Automotive Eng. 97 (1997) 2850.

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