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Operando infrared spectroscopy of the reduction of NO by H2 over rhodium based catalysts
Catalysis Today 191 (2012) 59–64
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Operando infrared spectroscopy of the reduction of NO by H2 over rhodium based catalysts S. Carré, C. Dujardin, P. Granger ∗ Unité de Catalyse et de Chimie du Solide UMR CNRS 8181, Université Lille 1, Sciences et Technologies, 59650 – Villeneuve d’Ascq cedex, France
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 16 December 2011 Accepted 19 December 2011 Available online 9 February 2012 Keywords: NO/H2 reaction Rhodium IR operando study N2 O-selectivity
a b s t r a c t An operando spectroscopic study of the NO/H2 reaction has been achieved to relate spectral features to kinetic data in order to identify key intermediates involved in the production of N2 , N2 O and NH3 as the main reaction products. Transient experiments show that different nitrosyl species, essentially neutral, positively and negatively NO adsorbed over metallic Rh sites exhibit different reactivity below the light-off temperature at low conversion and temperature. In particular, stable positively charged nitrosyl species inhibit the re-adsorption and further dissociation of N2 O predominantly formed during the cold start engine. This can be related to a major drawback in the use of Rh for further developments of compressed natural gas engine catalysts running typically in three-way conditions. Hydrogen was found to enhance the conversion of NOx and the transformation to nitrogen mainly by destabilizing those species further enhancing the subsequent reduction of N2 O. The participation of the fast H2 /O2 reaction at high temperature that restores the metallic character of rhodium could be an indirect effect in the selectivity enhancement changing the nature and the relative rate of steps related to nitrogen production. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays three-way catalysts (TWC), mostly composed of noble metals, run efficiently to convert under three-way conditions NO, CO and unburned hydrocarbons [1–3]. The recent emergence of compressed natural gas engines [4,5] led to reconsideration of the actual catalyst formulations, which usually contain high noble metal loadings such as Pd to convert methane [6] and Rh usually recognized as the most active noble metal toward NO dissociation [7,8]. However, remaining questions need to be solved essentially related to the efficiency of TWC during the cold start engine and also under three-way conditions with higher running temperature typically for natural gas vehicle (NGV) catalysts that induce faster deactivation processes. First, at low temperature and low conversion, hydrogen is usually recognized as the most efficient reducing agent to transform NO but also the less selective one related to the predominant production of N2 O. It is also worthwhile to note that ammonia formation above the light-off temperature can be also considered as a serious drawback in the use of Rh. We have examined in this study the reactivity of nitrosyl species especially in the presence of oxygen because of the competitive H2 /O2 reaction, which usually occurs and sometimes having a negative impact on the selectivity. Previous in situ IR investigations of the NO/CO
∗ Corresponding author. Tel.: +33 320 434 938; fax: +33 320 436 561. E-mail address: [email protected] (P. Granger). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.12.034
reaction at 300 ◦ C over Rh/Al2 O3 , in the absence of H2 , tried to identify the nature of nitrosyl intermediates involved in the formation of N2 and N2 O [9]. From the reaction mechanism illustrated in Fig. 1, it was suggested that negatively charged and neutral NO species are substantially more active than thermally stable positively charged NO species and could be involved in the formation of N2 and N2 O according to steps (5) and (6) much faster than the classical recombination of two adjacent chemisorbed N atoms via step (4) over Rh based catalysts [10]. On the other hand, the reactivity of positively charged NO species has not been clearly established near the light-off temperature at 300 ◦ C. This paper is mainly focused on the relative reactivity of those species at much lower temperature and mainly deals with the promotional effect of hydrogen which has been currently related to the H-assisted NO dissociation according to step (8) [11,12]. One of the objectives of this study is to elucidate possible indirect effects of hydrogen that might also originate a beneficial effect. NOads + Hads → Nads + OHads
(8)
Direct and indirect effects were examined in this present study showing that the destabilization of positively charged NO species can also govern the selectivity behavior enhancing the subsequent conversion of N2 O. It will be also demonstrated that the weak partial pressure dependency of N2 O selectivity earlier evidenced at 300 ◦ C with CO as the sole reducing agent is partly suppressed at lower temperature when CO is replaced by hydrogen.
NOads
(2)
CO + *
COads
(3)
NOads + *
Nads + Oads
(4)
Nads + Nads
N2 + 2*
(5)
NOads+ Nads
N2 + Oads + *
(6)
NOads+ Nads
N2O + 2*
(7)
COads + Oads
CO2 + 2*
Fig. 1. Mechanism scheme earlier proposed for the NO/CO reaction over noble metals [9].
2. Experimental Rh/Al2 O3 (0.2 wt.% Rh) was prepared by pore-filling impregnation of a ␥-alumina (100 m2 /g) with a rhodium trichloride solution. After drying at 100 ◦ C overnight, the precursor was calcined at 500 ◦ C and then reduced under hydrogen at the same temperature for 2 h. Rh dispersion calculated from H2 titration at room temperature (RT) on pre-reduced samples was equal to 0.81 assuming H/Rh = 1. Infrared spectroscopic measurements were performed in a DRIFT cell (Harrick) that enabled heating of the sample and upstream gas flowing through the sample. Catalyst temperature was recorded using thermocouple inside the catalytic bed. The space velocity was adjusted to 10,000 h−1 . FTIR spectra were recorded with a spectral resolution of 4 cm−1 on a Nicolet 6700 spectrometer equipped with a MCT detector. The reaction products (N2 , NH3 and N2 O) were quantified with a gas chromatograph Varian CP490 and a FTIR spectrometer (Thermo 380) equipped with an IR cell for gas analysis. Samples were heated under 20 vol.% O2 in helium at 400 ◦ C for 1 h and cooled down to RT. After reduction under H2 at 400 ◦ C (10 ◦ C/min) for 2 h, the catalyst was flushed under helium at the same temperature for 15 min and then cooled down to RT. The mass of catalyst was typically 30 mg with a total flow rate of 10 mL/min. For NO-temperature-programeddesorption (TPD) experiments, NO (0.5 vol.% in He) was passed through the catalyst for 1.5 h at RT and purged 1 h under helium then the temperature was gradually increased under He flowing with a ramp of 7.5 ◦ C/min until 400 ◦ C. Temperature programed reaction (TPR) was carried out in the presence of 0.095 vol.% NO, 0.3 vol.% H2 and 0.1025 vol.% O2 diluted in He or in the presence of 0.0475 vol.% N2 O, 0.3 vol.% H2 and 0.1025 vol.% O2 diluted in He. For transient experiments, the flow of NO (0.5 vol.% in He) was maintained during 2 min and then replaced by H2 (0.5 vol.% in He) during 40 s with a 4-way valve system and repeated 10 times at different temperatures. During these sequences, reactants and products were simultaneously detected by mass spectrometry and IR gas spectroscopy. 3. Results and discussion 3.1. Relative reactivity of nitrosyl species pre-adsorbed at RT on Rh/Al2 O3 Preliminary investigations of NO adsorption at RT and thermal desorption of pre-adsorbed NO species on pre-reduced samples in H2 at 400 ◦ C is illustrated in Fig. 2. Data collected are in agreement with previous assignments [9,13,14]. After NO adsorption at RT, spectrum 2.a is dominated by the 1715 cm−1 IR band ascribed to bent nitrosyl on metallic Rh sites, generally noted as Rh(NO)␦− [15]. Weak and broad IR bands also appear at 1811 and 1893 cm−1 corresponding to neutral and positively charged NO species on
1631
NO + *
1715
(1)
1811
S. Carré et al. / Catalysis Today 191 (2012) 59–64
1893
60
a b c
0.2 KM
i
2000
1900
1800
1700
1600
Wavenumber (cm-1)
1500
1400
Fig. 2. FTIR spectra of pre-adsorbed NO on Rh/Al2 O3 at RT after 10 min (a) and 90 min exposure (b) under flowing 0.5 vol.% NO diluted in He and in the course of temperature-programed desorption experiments in He with dT/dt = 7.5 ◦ C/min at 25 ◦ C (c); 50 ◦ C (d); 100 ◦ C (e); 150 ◦ C (f); 200 ◦ C (g); 250 ◦ C (h); and 300 ◦ C (i).
Rh0 which develop during NO exposition due to the usual electron withdrawing effect of oxygen coming from NO dissociation. Oxygen accumulation at the surface usually induces a lowering of the electron back-donation, which weakens the Rh-NO bond. It is also worthwhile to note that chlorine accumulation on the catalyst coming from the rhodium precursor might enhance the acidic properties of the support with possible electronic induced effects that can alter the adsorptive properties of Rh particles and then promoting the stabilization of Rh(NO)␦+ . Some discrepancies arose in the past two decades related to the development of gem-dinitrosyl species previously observed in the range 1700–1830 cm−1 that could reflect the formation of N2 O [13]. An additional signal also develops at lower frequencies (1631 cm−1 ) mainly ascribed to nitrates more probably in interaction with alumina. As seen in Fig. 3, a rise in temperature during TPD experiments induces changes in the relative intensity of IR bands due to NO desorption and the involvement of surface reactions producing N2 and N2 O detected in the outlet gas mixture (see Fig. 3b). Let us note that the intensity ratio between neutral and negatively charged NO species slightly changes and can be easily correlated to a predominant production of N2 O below 150 ◦ C. Hence, those IR observations do not provide decisive arguments to discriminate between the preferential formation of chemisorbed mono- or dinitrosyl NO species on Rh on the basis of the observation of the 1715 and 1811 cm−1 IR band. Above 150 ◦ C, the main observation is associated to an increase in concentration of stable Rh(NO)␦+ probably due to the electronwithdrawing effect of chemisorbed O atoms from an extensive NO dissociation that accumulate during TPD experiments. Above 250 ◦ C, their disappearance can be explained by subsequent decomposition correlated to a slight production of N2 . Finally, nitrate species related to the 1631 cm−1 IR band remain stable up to 250 ◦ C. 3.2. In situ temperature-programed experiments of the NO/H2 /O2 reactions on Rh/Al2 O3 Temperature-programed conversion and yield curves are collected in Fig. 4a. The characteristic IR bands related to nitrosyl species on rhodium and intensity changes recorded in the course of the reaction can be visualized in Figs. 4b and 5, respectively. Preliminary, we have checked for low conversion and low temperature ranges that mass and heat transfers did not occur predominantly by comparing reaction rates and apparent activation energy values from data collected in a Continuous Stirred Tank Reactor (CSTR) running in chemical regime and in the cell reactor used in this study. The estimates of the apparent energy corresponding to the
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Fig. 5. IR spectra recorded during the temperature-programed reaction under NO/H2 /O2 over Rh/Al2 O3 at 25 ◦ C (a); 50 ◦ C (b); 100 ◦ C (c); 150 ◦ C (d); 200 ◦ C (e); 250 ◦ C (f); and 300 ◦ C (g).
Fig. 3. Changes in the intensity of integrated IR bands of nitrosyl species vs. temperature over Rh/Al2 O3 during NO-TPD experiments (a). Correlative changes in concentration of NO, N2 O and N2 during NO-TPD vs. temperature (b).
Fig. 4. Temperature-programed reaction on Rh/Al2 O3 in stoichiometric conditions with 0.095 vol.% NO, 0.3 vol.% H2 , 0.1025 vol.% O2 (GHSV = 10,000 h−1 ; dT/dt = 2.2 ◦ C/min). Corresponding conversion and yield curves vs. temperature (a) changes in the integrated IR bands related to nitrosyls species (b).
NO/H2 reaction of approximately 44 kJ mol−1 according to the margin of error is in rather good agreement with the value calculated of ∼36 kJ mol−1 in CSTR regime under similar stoichiometric conditions. Hence, it is expected that IR data collected below the light-off curves would be representative of the chemical regime. Up to the light-off temperature, the IR cell still mimics the performances of the CSTR. As shown in Fig. 4a, the conversion of NO starts significantly above 100 ◦ C with the predominant formation of N2 O. A maximum is observed in N2 O production corresponding to a complete NO conversion correctly explained by the occurrence of the successive reduction of N2 O by H2 . This observation is currently observed on Rh with a successive N2 O transformation usually delayed due a strong NO adsorption preventing the re-adsorption of N2 O [9]. We have studied separately the single temperatureprogramed N2 O/H2 /O2 reaction on the same catalyst and verified that the conversion of N2 O starts at much lower temperature in the absence of NO (see Fig. 6) further supporting the strong inhibiting effect of adsorbed NO species on rhodium on the rate of the successive N2 O transformation. Hence, this sub-reaction during the overall NO/H2 reaction only takes place when NO depletes the surface at quasi-complete NO conversion. Returning to the catalytic behavior of Rh/Al2 O3 during the NO/H2 /O2 reaction in Fig. 4a, the predominant information above 150 ◦ C is mainly related to the formation of ammonia, coexisting with nitrogen, with a maximum production near 280 ◦ C. Interestingly, the decrease in NH3 production above that temperature can be correlated to an extra
Fig. 6. Temperature-programed reaction under N2 O/H2 /O2 over Rh/Al2 O3 (reaction conditions: 0.0475 vol.% N2 O, 0.3 vol.% H2 , 0.1025 vol. % O2 diluted in He) – the blue dotted line corresponds to the production curve of N2 O during the overall NO transformation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 7. Transient experiments with sequential NO (2 min) and H2 (40 s) exposures at 150 ◦ C. Comparison between IR spectra and relative outlet gas composition from mass spectrometry analysis: 1st cycle (a); and 9th cycle (b).
production of N2 which suggests that the reduction of NO by ammonia would predominantly occur in this temperature range. The examination of the corresponding IR data simultaneously recorded in the course of the reaction (see Fig. 5) shows no significant change in the nature of nitrosyl species in interaction with rhodium at low temperature and low conversion with the appearance of Rh(NO)␦− , Rh(NO)0 and Rh(NO)␦+ . On the other hand, remarkable changes are observable on IR spectra recorded above 150 ◦ C with strong perturbations on IR bands below 1650 cm−1 ascribed to adsorbates in interaction with alumina. As observed a maximum in intensity for the 1630 cm−1 IR band is distinguishable at 200 ◦ C. Correlatively to a decrease in ad-NOx species above that temperature an additional IR band develops at 1469 cm−1 assigned to asymmetric deformation of ammonium ions [16]. All those observations seem in rather good agreement with changes in the composition of the outlet gas mixtures earlier displayed with an extra production of nitrogen possibly formed through the classical ammonia SCR reaction as described elsewhere [17], involving ad-NOx stored on alumina. Particular attention was essentially paid on the kinetic and spectroscopic features involving Rh sites below 150 ◦ C. The comparison between TPR-curves and IR bands of nitrosyl species in Fig. 4b provides significant information that allows the identification of nitrosyl species possibly involved in the formation of N2 and N2 O and those ones which would originate inhibiting effects. As observed, the intensity ratio between the 1712 and 1814 cm−1 IR bands changes significantly contrarily to previous observations in
Fig. 3. In addition, a continuous shift toward lower wavenumber values of the 1712 and 1814 cm−1 IR bands is observable during TPR experiment with a raise in temperature (see Fig. 5). Such observations allow us to differentiate between mono-nitrosyl and gem-dinitrosyl species. As previously stated [13], the positions of the asymmetric and symmetric stretches and the relative intensity of the two IR bands should remain constant if they result from isolated Rh(NO)2 species. Such trends differ from that observed in Fig. 5 with significant shifts of the maxima and changes in the relative intensity which indicate that the assignment the 1712 and 1814 cm−1 IR bands to Rh(NO)␦− and neutral NO is more consistent. As illustrated below 100 ◦ C, a slight attenuation of the IR bands characteristic of Rh(NO)␦− arises during the first stage of the NO/H2 reaction and then sharply drops whereas the reverse trend characterize the IR band related to Rh(NO)0 which intensifies before decreasing. Similar observations also characterize Rh(NO)␦+ which seem to indicate that chemisorbed O atoms from O2 adsorption or NO dissociation accumulate at the surface apart from the presence of hydrogen in the feed gas composition. The competition for adsorption seems to be largely in favor of O2 and NO at the expense of H2 . Hence, the slow H2 /O2 reaction would be unable to remove completely oxygen on Rh at low temperature. As a matter of fact, this competition seems more in favor to H2 at higher temperature and correlated to the strong attenuation of the IR band of Rh(NO)␦+ above 130 ◦ C. It is remarkable that the disappearance of positively charged NO species coincides with the successive
S. Carré et al. / Catalysis Today 191 (2012) 59–64
63
Fig. 8. Transient experiments with sequential NO (2 min) and H2 (40 s) exposures at 200 ◦ C. Comparison between IR spectra and relative outlet gas composition from mass spectrometry analysis: 1st cycle (a); and 9th cycle (b).
conversion of N2 O. All those observations emphasize the fact that the stabilization of less reactive Rh(NO)␦+ must be avoided at low temperature to gain significant enhancement in the selectivity behavior toward N2 production by accelerating the successive reduction of N2 O. This supposes that Rh preserves its metallic character, which is partly lost when oxygen accumulates at the surface. The use of free chlorine rhodium precursors can be an alternative to minimize the impact of the acidic properties of the support on the adsorptive properties of metallic rhodium particles. 3.3. Sequential NO and H2 exposures on Rh/Al2 O3 We have investigated the responses of Rh/Al2 O3 after successive exposures of NO and H2 as described in the experimental section. Outlet gas mixture was simultaneously analyzed by mass spectrometry and IR gas spectroscopy and compared to changes in intensity of IR bands related to nitrosyl species on Rh at two different temperatures. Experiments performed at 150 ◦ C are reported in Fig. 7 during the first switch from NO to H2 and after 9 cycles. Switch to reducing conditions after NO exposure does not induce significant change in the intensity of IR bands ascribed to Rh(NO)0 and Rh(NO)␦+ contrarily to Rh(NO)␦− highlighting that negatively charged NO species would react predominantly to produce N2 O with a MS signal related to concentration slightly exceeding that of nitrogen. After 9 cycles similar spectral features are observable and it is noticeable that switches under reductive conditions have
no significant impact on the production of N2 and N2 O remaining unchanged during the total duration of the experiment while the partial pressure of NO changes. This kinetic feature is characteristic of Rh demonstrating the weak partial pressure dependency of NO to N2 O selectivity earlier observed [18,19]. This particular behavior has been explained by the involvement of similar intermediates for the production of N2 O and N2 according to steps (5) and (6) when the surface is essentially covered by NOads reflected in those experiments by the stabilization of less reactive Rh(NO)0 and Rh(NO)␦+ toward NO dissociation. Now regarding similar experiments performed at 200 ◦ C (Fig. 8), it is worthwhile to note that the kinetic behavior of Rh/Al2 O3 differs from that observed at 150 ◦ C. In these temperature conditions subsequent switch in flowing hydrogen alters significantly the IR bands related to Rh(NO)0 and Rh(NO)␦− whereas Rh(NO)␦+ still remain unchanged. Further examination of the MS response shows that H2 is quasi-completely converted associated to a greater extent of NO conversion to N2 O, N2 and NH3 . Activation of the catalyst is discernible between the first and the 9th switch from NO to H2 in favor of the production of ammonia. However, the most prominent observation is probably related to the fact that the molar N2 O to N2 ratio sharply varies as a function of the NO concentration in the exhaust gas further indicating that the selectivity to nitrogen production now depends on the NO partial pressure. Such an observation indicates that step (4) would occur significantly. In these operating conditions, the higher conversion of hydrogen underlines the occurrence of the H2 /O2 reaction
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further liberating available sites for the dissociation of NO and the dissociative hydrogen adsorption. This situation would be favorable for explaining the greater extent of NO reduction due to a lower inhibiting effect of NO and the occurrence of the H-assisted NO dissociation as described in Eq. (8). 4. Conclusion This study is focused on the operando study of the NO/H2 reaction on Rh/Al2 O3 by coupling infrared spectroscopy and mass spectrometry measurements in order to compare changes in adsorbate surface composition vs. the composition of the inlet gas and temperature conditions. NO-TPD and temperature-programed reaction NO/H2 in the presence of oxygen show that essentially mono-nitrosyl species are involved in the production of N2 , N2 O and NH3 . At low temperature and conversion the surface is mostly covered by oxygen further stabilizing neutral and positively charged NO species less reactive than Rh(NO)␦− toward NO dissociation. Transient experiments underlined that the predominant production of N2 O does not depend on the residual NO concentration which agree with the fact that the production of N2 O and N2 involves the same intermediates. Further increase in temperature favors the H2 /O2 reaction. In such conditions, nitrosyl species become more reactive with increasing surface N and H coverages which favors the production of ammonia. However, changes in adsorbates composition strongly modify the selectivity behavior because it depends on the NO partial pressure conditions
which means that the usual associative desorption of two adjacent chemisorbed N atoms cannot be neglected in the overall production of nitrogen. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
P. Granger, V.I. Parvulescu, Chem. Rev. 111 (2011) 3155–3207. M. Shelef, G.W. Graham, Catal. Rev. Sci. Eng. 36 (1994) 433–457. P. Granger, C. Dujardin, J.F. Paul, G. Leclercq, J. Mol. Catal. A 228 (2005) 241–253. M. Salaün, A. Kouakou, S. Da Costa, P. Da Costa, Appl. Catal. B 88 (2009) 386–397. F. Klingstedt, A.K. Neyestanaki, R. Byggningsbacka, L.-E. Lindfors, M. Lundén, M. Petersson, P. Tengström, T. Ollonqvist, J. Väyrynen, Appl. Catal. A. 209 (2001) 301–316. Y. Renème, F. Dhainaut, M. Trentesaux, B. Ravanbakhsh, P. Granger, C. Dujardin, L. Gengembre, P.L. De Cola, Surf. Interface Anal. 42 (2010) 530–535. T.W. Root, L.D. Schmidt, G.B. Fisher, Surf. Sci. 134 (1983) 30–45. R.J. Gorte, L.D. Schmidt, J.L. Gland, Surf. Sci. 109 (1981) 367–380. C. Dujardin, A.S. Mamede, E. Payen, B. Sombret, J.P. Huvenne, P. Granger, Top. Catal. 30/31 (2004) 347–352. P. Granger, P. Malfoy, G. Leclercq, J. Catal. 223 (2004) 142–151. W. Hecker, A.T. Bell, J. Catal. 92 (1985) 247–259. F. Dhainaut, S. Pietrzyk, P. Granger, J. Catal. 258 (2008) 296–305. R. Dictor, J. Catal. 109 (1988) 89–99. F. Solymosi, M. Pasztor, J. Phys. Chem. 89 (1985) 4789–4793. C. De La Cruz, N. Sheppard, Spectrochim. Acta 78 (2011) 7–28. G. Ramis, M.A. Larrubia, J. Mol. Catal. A 215 (2004) 161–167. P. Forzatti, L. Lietti, I. Nova, E. Tronconi, Catal. Today 151 (2010) 202–211. P. Granger, J.J. Lecomte, C. Dathy, L. Leclercq, G. Leclercq, J. Catal. 175 (1998) 194–203. P. Granger, F. Dhainaut, S. Pietrzik, P. Malfoy, A.S. Mamede, L. Leclercq, G. Leclercq, Top. Catal. 39 (2006) 65–76.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Operando infrared spectroscopy of the reduction of NO by H2 over rhodium based catalysts S. Carré, C. Dujardin, P. Granger ∗ Unité de Catalyse et de Chimie du Solide UMR CNRS 8181, Université Lille 1, Sciences et Technologies, 59650 – Villeneuve d’Ascq cedex, France
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 16 December 2011 Accepted 19 December 2011 Available online 9 February 2012 Keywords: NO/H2 reaction Rhodium IR operando study N2 O-selectivity
a b s t r a c t An operando spectroscopic study of the NO/H2 reaction has been achieved to relate spectral features to kinetic data in order to identify key intermediates involved in the production of N2 , N2 O and NH3 as the main reaction products. Transient experiments show that different nitrosyl species, essentially neutral, positively and negatively NO adsorbed over metallic Rh sites exhibit different reactivity below the light-off temperature at low conversion and temperature. In particular, stable positively charged nitrosyl species inhibit the re-adsorption and further dissociation of N2 O predominantly formed during the cold start engine. This can be related to a major drawback in the use of Rh for further developments of compressed natural gas engine catalysts running typically in three-way conditions. Hydrogen was found to enhance the conversion of NOx and the transformation to nitrogen mainly by destabilizing those species further enhancing the subsequent reduction of N2 O. The participation of the fast H2 /O2 reaction at high temperature that restores the metallic character of rhodium could be an indirect effect in the selectivity enhancement changing the nature and the relative rate of steps related to nitrogen production. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays three-way catalysts (TWC), mostly composed of noble metals, run efficiently to convert under three-way conditions NO, CO and unburned hydrocarbons [1–3]. The recent emergence of compressed natural gas engines [4,5] led to reconsideration of the actual catalyst formulations, which usually contain high noble metal loadings such as Pd to convert methane [6] and Rh usually recognized as the most active noble metal toward NO dissociation [7,8]. However, remaining questions need to be solved essentially related to the efficiency of TWC during the cold start engine and also under three-way conditions with higher running temperature typically for natural gas vehicle (NGV) catalysts that induce faster deactivation processes. First, at low temperature and low conversion, hydrogen is usually recognized as the most efficient reducing agent to transform NO but also the less selective one related to the predominant production of N2 O. It is also worthwhile to note that ammonia formation above the light-off temperature can be also considered as a serious drawback in the use of Rh. We have examined in this study the reactivity of nitrosyl species especially in the presence of oxygen because of the competitive H2 /O2 reaction, which usually occurs and sometimes having a negative impact on the selectivity. Previous in situ IR investigations of the NO/CO
∗ Corresponding author. Tel.: +33 320 434 938; fax: +33 320 436 561. E-mail address: [email protected] (P. Granger). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.12.034
reaction at 300 ◦ C over Rh/Al2 O3 , in the absence of H2 , tried to identify the nature of nitrosyl intermediates involved in the formation of N2 and N2 O [9]. From the reaction mechanism illustrated in Fig. 1, it was suggested that negatively charged and neutral NO species are substantially more active than thermally stable positively charged NO species and could be involved in the formation of N2 and N2 O according to steps (5) and (6) much faster than the classical recombination of two adjacent chemisorbed N atoms via step (4) over Rh based catalysts [10]. On the other hand, the reactivity of positively charged NO species has not been clearly established near the light-off temperature at 300 ◦ C. This paper is mainly focused on the relative reactivity of those species at much lower temperature and mainly deals with the promotional effect of hydrogen which has been currently related to the H-assisted NO dissociation according to step (8) [11,12]. One of the objectives of this study is to elucidate possible indirect effects of hydrogen that might also originate a beneficial effect. NOads + Hads → Nads + OHads
(8)
Direct and indirect effects were examined in this present study showing that the destabilization of positively charged NO species can also govern the selectivity behavior enhancing the subsequent conversion of N2 O. It will be also demonstrated that the weak partial pressure dependency of N2 O selectivity earlier evidenced at 300 ◦ C with CO as the sole reducing agent is partly suppressed at lower temperature when CO is replaced by hydrogen.
NOads
(2)
CO + *
COads
(3)
NOads + *
Nads + Oads
(4)
Nads + Nads
N2 + 2*
(5)
NOads+ Nads
N2 + Oads + *
(6)
NOads+ Nads
N2O + 2*
(7)
COads + Oads
CO2 + 2*
Fig. 1. Mechanism scheme earlier proposed for the NO/CO reaction over noble metals [9].
2. Experimental Rh/Al2 O3 (0.2 wt.% Rh) was prepared by pore-filling impregnation of a ␥-alumina (100 m2 /g) with a rhodium trichloride solution. After drying at 100 ◦ C overnight, the precursor was calcined at 500 ◦ C and then reduced under hydrogen at the same temperature for 2 h. Rh dispersion calculated from H2 titration at room temperature (RT) on pre-reduced samples was equal to 0.81 assuming H/Rh = 1. Infrared spectroscopic measurements were performed in a DRIFT cell (Harrick) that enabled heating of the sample and upstream gas flowing through the sample. Catalyst temperature was recorded using thermocouple inside the catalytic bed. The space velocity was adjusted to 10,000 h−1 . FTIR spectra were recorded with a spectral resolution of 4 cm−1 on a Nicolet 6700 spectrometer equipped with a MCT detector. The reaction products (N2 , NH3 and N2 O) were quantified with a gas chromatograph Varian CP490 and a FTIR spectrometer (Thermo 380) equipped with an IR cell for gas analysis. Samples were heated under 20 vol.% O2 in helium at 400 ◦ C for 1 h and cooled down to RT. After reduction under H2 at 400 ◦ C (10 ◦ C/min) for 2 h, the catalyst was flushed under helium at the same temperature for 15 min and then cooled down to RT. The mass of catalyst was typically 30 mg with a total flow rate of 10 mL/min. For NO-temperature-programeddesorption (TPD) experiments, NO (0.5 vol.% in He) was passed through the catalyst for 1.5 h at RT and purged 1 h under helium then the temperature was gradually increased under He flowing with a ramp of 7.5 ◦ C/min until 400 ◦ C. Temperature programed reaction (TPR) was carried out in the presence of 0.095 vol.% NO, 0.3 vol.% H2 and 0.1025 vol.% O2 diluted in He or in the presence of 0.0475 vol.% N2 O, 0.3 vol.% H2 and 0.1025 vol.% O2 diluted in He. For transient experiments, the flow of NO (0.5 vol.% in He) was maintained during 2 min and then replaced by H2 (0.5 vol.% in He) during 40 s with a 4-way valve system and repeated 10 times at different temperatures. During these sequences, reactants and products were simultaneously detected by mass spectrometry and IR gas spectroscopy. 3. Results and discussion 3.1. Relative reactivity of nitrosyl species pre-adsorbed at RT on Rh/Al2 O3 Preliminary investigations of NO adsorption at RT and thermal desorption of pre-adsorbed NO species on pre-reduced samples in H2 at 400 ◦ C is illustrated in Fig. 2. Data collected are in agreement with previous assignments [9,13,14]. After NO adsorption at RT, spectrum 2.a is dominated by the 1715 cm−1 IR band ascribed to bent nitrosyl on metallic Rh sites, generally noted as Rh(NO)␦− [15]. Weak and broad IR bands also appear at 1811 and 1893 cm−1 corresponding to neutral and positively charged NO species on
1631
NO + *
1715
(1)
1811
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1893
60
a b c
0.2 KM
i
2000
1900
1800
1700
1600
Wavenumber (cm-1)
1500
1400
Fig. 2. FTIR spectra of pre-adsorbed NO on Rh/Al2 O3 at RT after 10 min (a) and 90 min exposure (b) under flowing 0.5 vol.% NO diluted in He and in the course of temperature-programed desorption experiments in He with dT/dt = 7.5 ◦ C/min at 25 ◦ C (c); 50 ◦ C (d); 100 ◦ C (e); 150 ◦ C (f); 200 ◦ C (g); 250 ◦ C (h); and 300 ◦ C (i).
Rh0 which develop during NO exposition due to the usual electron withdrawing effect of oxygen coming from NO dissociation. Oxygen accumulation at the surface usually induces a lowering of the electron back-donation, which weakens the Rh-NO bond. It is also worthwhile to note that chlorine accumulation on the catalyst coming from the rhodium precursor might enhance the acidic properties of the support with possible electronic induced effects that can alter the adsorptive properties of Rh particles and then promoting the stabilization of Rh(NO)␦+ . Some discrepancies arose in the past two decades related to the development of gem-dinitrosyl species previously observed in the range 1700–1830 cm−1 that could reflect the formation of N2 O [13]. An additional signal also develops at lower frequencies (1631 cm−1 ) mainly ascribed to nitrates more probably in interaction with alumina. As seen in Fig. 3, a rise in temperature during TPD experiments induces changes in the relative intensity of IR bands due to NO desorption and the involvement of surface reactions producing N2 and N2 O detected in the outlet gas mixture (see Fig. 3b). Let us note that the intensity ratio between neutral and negatively charged NO species slightly changes and can be easily correlated to a predominant production of N2 O below 150 ◦ C. Hence, those IR observations do not provide decisive arguments to discriminate between the preferential formation of chemisorbed mono- or dinitrosyl NO species on Rh on the basis of the observation of the 1715 and 1811 cm−1 IR band. Above 150 ◦ C, the main observation is associated to an increase in concentration of stable Rh(NO)␦+ probably due to the electronwithdrawing effect of chemisorbed O atoms from an extensive NO dissociation that accumulate during TPD experiments. Above 250 ◦ C, their disappearance can be explained by subsequent decomposition correlated to a slight production of N2 . Finally, nitrate species related to the 1631 cm−1 IR band remain stable up to 250 ◦ C. 3.2. In situ temperature-programed experiments of the NO/H2 /O2 reactions on Rh/Al2 O3 Temperature-programed conversion and yield curves are collected in Fig. 4a. The characteristic IR bands related to nitrosyl species on rhodium and intensity changes recorded in the course of the reaction can be visualized in Figs. 4b and 5, respectively. Preliminary, we have checked for low conversion and low temperature ranges that mass and heat transfers did not occur predominantly by comparing reaction rates and apparent activation energy values from data collected in a Continuous Stirred Tank Reactor (CSTR) running in chemical regime and in the cell reactor used in this study. The estimates of the apparent energy corresponding to the
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Fig. 5. IR spectra recorded during the temperature-programed reaction under NO/H2 /O2 over Rh/Al2 O3 at 25 ◦ C (a); 50 ◦ C (b); 100 ◦ C (c); 150 ◦ C (d); 200 ◦ C (e); 250 ◦ C (f); and 300 ◦ C (g).
Fig. 3. Changes in the intensity of integrated IR bands of nitrosyl species vs. temperature over Rh/Al2 O3 during NO-TPD experiments (a). Correlative changes in concentration of NO, N2 O and N2 during NO-TPD vs. temperature (b).
Fig. 4. Temperature-programed reaction on Rh/Al2 O3 in stoichiometric conditions with 0.095 vol.% NO, 0.3 vol.% H2 , 0.1025 vol.% O2 (GHSV = 10,000 h−1 ; dT/dt = 2.2 ◦ C/min). Corresponding conversion and yield curves vs. temperature (a) changes in the integrated IR bands related to nitrosyls species (b).
NO/H2 reaction of approximately 44 kJ mol−1 according to the margin of error is in rather good agreement with the value calculated of ∼36 kJ mol−1 in CSTR regime under similar stoichiometric conditions. Hence, it is expected that IR data collected below the light-off curves would be representative of the chemical regime. Up to the light-off temperature, the IR cell still mimics the performances of the CSTR. As shown in Fig. 4a, the conversion of NO starts significantly above 100 ◦ C with the predominant formation of N2 O. A maximum is observed in N2 O production corresponding to a complete NO conversion correctly explained by the occurrence of the successive reduction of N2 O by H2 . This observation is currently observed on Rh with a successive N2 O transformation usually delayed due a strong NO adsorption preventing the re-adsorption of N2 O [9]. We have studied separately the single temperatureprogramed N2 O/H2 /O2 reaction on the same catalyst and verified that the conversion of N2 O starts at much lower temperature in the absence of NO (see Fig. 6) further supporting the strong inhibiting effect of adsorbed NO species on rhodium on the rate of the successive N2 O transformation. Hence, this sub-reaction during the overall NO/H2 reaction only takes place when NO depletes the surface at quasi-complete NO conversion. Returning to the catalytic behavior of Rh/Al2 O3 during the NO/H2 /O2 reaction in Fig. 4a, the predominant information above 150 ◦ C is mainly related to the formation of ammonia, coexisting with nitrogen, with a maximum production near 280 ◦ C. Interestingly, the decrease in NH3 production above that temperature can be correlated to an extra
Fig. 6. Temperature-programed reaction under N2 O/H2 /O2 over Rh/Al2 O3 (reaction conditions: 0.0475 vol.% N2 O, 0.3 vol.% H2 , 0.1025 vol. % O2 diluted in He) – the blue dotted line corresponds to the production curve of N2 O during the overall NO transformation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 7. Transient experiments with sequential NO (2 min) and H2 (40 s) exposures at 150 ◦ C. Comparison between IR spectra and relative outlet gas composition from mass spectrometry analysis: 1st cycle (a); and 9th cycle (b).
production of N2 which suggests that the reduction of NO by ammonia would predominantly occur in this temperature range. The examination of the corresponding IR data simultaneously recorded in the course of the reaction (see Fig. 5) shows no significant change in the nature of nitrosyl species in interaction with rhodium at low temperature and low conversion with the appearance of Rh(NO)␦− , Rh(NO)0 and Rh(NO)␦+ . On the other hand, remarkable changes are observable on IR spectra recorded above 150 ◦ C with strong perturbations on IR bands below 1650 cm−1 ascribed to adsorbates in interaction with alumina. As observed a maximum in intensity for the 1630 cm−1 IR band is distinguishable at 200 ◦ C. Correlatively to a decrease in ad-NOx species above that temperature an additional IR band develops at 1469 cm−1 assigned to asymmetric deformation of ammonium ions [16]. All those observations seem in rather good agreement with changes in the composition of the outlet gas mixtures earlier displayed with an extra production of nitrogen possibly formed through the classical ammonia SCR reaction as described elsewhere [17], involving ad-NOx stored on alumina. Particular attention was essentially paid on the kinetic and spectroscopic features involving Rh sites below 150 ◦ C. The comparison between TPR-curves and IR bands of nitrosyl species in Fig. 4b provides significant information that allows the identification of nitrosyl species possibly involved in the formation of N2 and N2 O and those ones which would originate inhibiting effects. As observed, the intensity ratio between the 1712 and 1814 cm−1 IR bands changes significantly contrarily to previous observations in
Fig. 3. In addition, a continuous shift toward lower wavenumber values of the 1712 and 1814 cm−1 IR bands is observable during TPR experiment with a raise in temperature (see Fig. 5). Such observations allow us to differentiate between mono-nitrosyl and gem-dinitrosyl species. As previously stated [13], the positions of the asymmetric and symmetric stretches and the relative intensity of the two IR bands should remain constant if they result from isolated Rh(NO)2 species. Such trends differ from that observed in Fig. 5 with significant shifts of the maxima and changes in the relative intensity which indicate that the assignment the 1712 and 1814 cm−1 IR bands to Rh(NO)␦− and neutral NO is more consistent. As illustrated below 100 ◦ C, a slight attenuation of the IR bands characteristic of Rh(NO)␦− arises during the first stage of the NO/H2 reaction and then sharply drops whereas the reverse trend characterize the IR band related to Rh(NO)0 which intensifies before decreasing. Similar observations also characterize Rh(NO)␦+ which seem to indicate that chemisorbed O atoms from O2 adsorption or NO dissociation accumulate at the surface apart from the presence of hydrogen in the feed gas composition. The competition for adsorption seems to be largely in favor of O2 and NO at the expense of H2 . Hence, the slow H2 /O2 reaction would be unable to remove completely oxygen on Rh at low temperature. As a matter of fact, this competition seems more in favor to H2 at higher temperature and correlated to the strong attenuation of the IR band of Rh(NO)␦+ above 130 ◦ C. It is remarkable that the disappearance of positively charged NO species coincides with the successive
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Fig. 8. Transient experiments with sequential NO (2 min) and H2 (40 s) exposures at 200 ◦ C. Comparison between IR spectra and relative outlet gas composition from mass spectrometry analysis: 1st cycle (a); and 9th cycle (b).
conversion of N2 O. All those observations emphasize the fact that the stabilization of less reactive Rh(NO)␦+ must be avoided at low temperature to gain significant enhancement in the selectivity behavior toward N2 production by accelerating the successive reduction of N2 O. This supposes that Rh preserves its metallic character, which is partly lost when oxygen accumulates at the surface. The use of free chlorine rhodium precursors can be an alternative to minimize the impact of the acidic properties of the support on the adsorptive properties of metallic rhodium particles. 3.3. Sequential NO and H2 exposures on Rh/Al2 O3 We have investigated the responses of Rh/Al2 O3 after successive exposures of NO and H2 as described in the experimental section. Outlet gas mixture was simultaneously analyzed by mass spectrometry and IR gas spectroscopy and compared to changes in intensity of IR bands related to nitrosyl species on Rh at two different temperatures. Experiments performed at 150 ◦ C are reported in Fig. 7 during the first switch from NO to H2 and after 9 cycles. Switch to reducing conditions after NO exposure does not induce significant change in the intensity of IR bands ascribed to Rh(NO)0 and Rh(NO)␦+ contrarily to Rh(NO)␦− highlighting that negatively charged NO species would react predominantly to produce N2 O with a MS signal related to concentration slightly exceeding that of nitrogen. After 9 cycles similar spectral features are observable and it is noticeable that switches under reductive conditions have
no significant impact on the production of N2 and N2 O remaining unchanged during the total duration of the experiment while the partial pressure of NO changes. This kinetic feature is characteristic of Rh demonstrating the weak partial pressure dependency of NO to N2 O selectivity earlier observed [18,19]. This particular behavior has been explained by the involvement of similar intermediates for the production of N2 O and N2 according to steps (5) and (6) when the surface is essentially covered by NOads reflected in those experiments by the stabilization of less reactive Rh(NO)0 and Rh(NO)␦+ toward NO dissociation. Now regarding similar experiments performed at 200 ◦ C (Fig. 8), it is worthwhile to note that the kinetic behavior of Rh/Al2 O3 differs from that observed at 150 ◦ C. In these temperature conditions subsequent switch in flowing hydrogen alters significantly the IR bands related to Rh(NO)0 and Rh(NO)␦− whereas Rh(NO)␦+ still remain unchanged. Further examination of the MS response shows that H2 is quasi-completely converted associated to a greater extent of NO conversion to N2 O, N2 and NH3 . Activation of the catalyst is discernible between the first and the 9th switch from NO to H2 in favor of the production of ammonia. However, the most prominent observation is probably related to the fact that the molar N2 O to N2 ratio sharply varies as a function of the NO concentration in the exhaust gas further indicating that the selectivity to nitrogen production now depends on the NO partial pressure. Such an observation indicates that step (4) would occur significantly. In these operating conditions, the higher conversion of hydrogen underlines the occurrence of the H2 /O2 reaction
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further liberating available sites for the dissociation of NO and the dissociative hydrogen adsorption. This situation would be favorable for explaining the greater extent of NO reduction due to a lower inhibiting effect of NO and the occurrence of the H-assisted NO dissociation as described in Eq. (8). 4. Conclusion This study is focused on the operando study of the NO/H2 reaction on Rh/Al2 O3 by coupling infrared spectroscopy and mass spectrometry measurements in order to compare changes in adsorbate surface composition vs. the composition of the inlet gas and temperature conditions. NO-TPD and temperature-programed reaction NO/H2 in the presence of oxygen show that essentially mono-nitrosyl species are involved in the production of N2 , N2 O and NH3 . At low temperature and conversion the surface is mostly covered by oxygen further stabilizing neutral and positively charged NO species less reactive than Rh(NO)␦− toward NO dissociation. Transient experiments underlined that the predominant production of N2 O does not depend on the residual NO concentration which agree with the fact that the production of N2 O and N2 involves the same intermediates. Further increase in temperature favors the H2 /O2 reaction. In such conditions, nitrosyl species become more reactive with increasing surface N and H coverages which favors the production of ammonia. However, changes in adsorbates composition strongly modify the selectivity behavior because it depends on the NO partial pressure conditions
which means that the usual associative desorption of two adjacent chemisorbed N atoms cannot be neglected in the overall production of nitrogen. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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