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Selective reduction of nitric oxide by methane over Pd-exchanged H-ZSM-5: influence of activation
I. Kiricsi, G. P~il-Borb61y, J.B. Nagy, H.G. Karge (Editors) Porous Materials in Environmentally Friendly Processes Studies in Surface Science and Catalysis, Vol. 125 9 1999 Elsevier Science B.V. All rights reserved.
587
S e l e c t i v e r e d u c t i o n of n i t r i c o x i d e by m e t h a n e o v e r Pde x c h a n g e d H-ZSM-5: i n f l u e n c e of a c t i v a t i o n B. Pommier and P. G~lin Laboratoire d'Application de la Chimie h l'Environnement, UMR CNRS 5634, Universit~ Claude Bernard Lyon 1, F69622 Villeurbanne Cedex, France
This paper reports the influence of the precalcination step on the physicochemical and catalytic properties of a high Pd loading Pd-H-ZSM-5 catalyst prepared from exchange with Pd(NH3)4(NO3)2. Upon exchange, Pd(NH3)t 2§ decomposes into Pd(NH3)22§ and NH4 § which readily exchange protons and form Pd(II) ions or hydroxo complexes and H § upon calcination in 02 at 653 K. These Pd(II) species react with NO to produce Pd(I) mononitrosyl species and NO2. Pd(II) and Pd(I) species interact with the lattice 02- ions (IR bands at 975 and 940 cm-1). Pd(I) nitrosyl complexes are 100% selective for the reduction of NO by CHt in excess of 02, but not very active. Under reactants at 700-800 K, these species are irreversibly converted into sites considerably more active for NO reduction but also less selective. Precalcination in 02 at 773 K induces the partial sintering of Pd(II) into PdO, inactive for the reduction of NO, and this results in the lower conversion of NO into N2.
1. INTRODUCTION The catalytic performance of Pd-ZSM-5 catalysts in the NO reduction by CH4 in the presence of 02 was shown to be related to the dispersion of Pd [1-3]. The high selectivity and activity in Selective Catalytic Reduction (SCR) would be due to the presence of isolated Pd 2§ ions or complexes dispersed in the zeolite channels, while large PdO particles would be responsible for CHt combustion and not or to a small extent for SCR. It is generally accepted that the presence of protons is necessary for maintaining Pd in a high degree of dispersion. Protons could also play a role in the SCR mechanism themselves [4]. The Pd-H-ZSM-5 catalysts have been prepared in various ways, e.g. by impregnation or conventional exchange with varying Pd precursors. In all studies, high Pd loadings (1 wt % Pd or higher) were shown to decrease the Pd dispersion and, consequently, the SCR selectivity. The goal of the present study is to thoroughly reinvestigate the relevant chemistry of a Pd-H-ZSM-5 catalyst with high Pd loading prepared via exchange of H-ZSM-5 with tetraammine Pd(II) complex and
588 further reacted with 02, NO and NO/CH4/O2. It is demonstrated t h a t the activation is a key step for improved catalytic performance of Pd-H-ZSM-5 in the reduction of NO by CH4 in an excess of 02.
2. E X P E R I M E N T A L A Pd-H-ZSM-5 catalyst containing 1.4 wt % Pd was prepared by conventional exchange of a H-ZSM-5 sample (CBV 5020, Si/Al=25, from PQ Zeolites B.V.) in an aqueous solution of Pd(NH3)4(NO3)2 (Strem Chemicals) at 323 K. After thorough washing with deionized water, the catalyst was dried overnight at 393 K. The resulting material was referred to as Pd(NH3)-HZSM5. Pd(NH3)-HZSM5 was subsequently calcined in flowing 02 up to 653 and 773 K (heating rate of 0.5 K min -1) in order to decompose the Pd ammine precursor. The calcined materials were labelled Pd-HZSM5-653 and Pd-HZSM5-773 respectively. The a m o u n t of NH3 and NH4 § contained in Pd(NH3)-HZSM5 was determined by temperature-programmed decomposition (TPD) in 02 followed by mass spectrometry (typically 0.1 g catalyst, heating rate of 10 K min-D. For this purpose, a Balzers QMA 125 quadrupole was used to record the most relevant m/z signals as a function of time/temperature: 14 (N), 15 (NH), 18 (H20), 30 (NO), 44 (N20, CO2), 46 (NO2). Calibrations for NH3 and N2 were carried out. FTIR spectra of sample wafers (ca. 15 mg) were recorded at a resolution of 4 cm -1 on a FT-IR Nicolet Magna 550 spectrometer, using a greaseless cell equipped with KBr windows [5]. For in situ studies at varying temperatures under controlled atmosphere, a home-made cell was used [6]. The catalytic activities for the reduction of NO by CH4 over Pd-HZSM5-653 and Pd-HZSM5-773 catalysts were measured using a U-shaped quartz reactor (16 m m ID) operating in a steady-state plug flow mode. The samples (200 mg) were reactivated in-situ in flow of oxygen (linear ramp rate of 0.5 K min-D, purged in helium down to 523 K before being put into contact with the reactants. The reaction mixture was adjusted so as to examine the catalytic activity under lean conditions: 2000 vpm NO, 1000 vpm CH4, 6240 vpm 02; helium as balance; total flow rate = 167 cm3/min, GHSV -- 30,000 h -1. The catalytic activity was measured as a function of temperature in the range 523 - 873~ during two successive heating - cooling cycles (linear heating and cooling rate of 1 K min-1). The effluent gases were analyzed using two gas chromatographs equipped with TCD and FID detectors and NOx infrared analyzers. Carbon and nitrogen balances were checked. The NOx conversion was determined according to the following equation : NOx conversion % = ([NO]0 + [NO2]0 - [NO] - [NO2]) * 100/([NO]0 + [NO2]0) where [NO]0 and [NO2]0 are the inlet concentrations of NO and NO2 respectively and [NO] and [NO2] the outlet concentrations. The NO2 formation was low in the
589 whole range of t e m p e r a t u r e ([NO2]< 40 vpm), almost independent on the t e m p e r a t u r e and ascribed to the NO oxidation in the dead volume of the apparatus. The CH4 conversion was determined from the consumption of CH4.
3. R E S U L T S A N D D I S C U S S I O N
3.1. Stability of e x c h a n g e d a m m i n e Pd c o m p l e x e s in 02 In situ F T I R spectra of Pd(NH3)-HZSM5 in flowing 02 at increasing t e m p e r a t u r e s (linear ramp of 5 K min-D were recorded to investigate the decomposition of Pd precursors. Figure 1 shows the region of v OH and v NH vibrations. The main features are: (i) an intense band at ca. 3740 cm -1 attributed to terminal silanols, (ii) a band at ca. 3610 cm -1 ascribed to bridging hydroxyls (Brcnsted acid sites); its low intensity indicates the high level of cationic exchange, (iii) a broad complex massif with maxima at ca. 3340, 3270 and 3200 cm -], assigned to the v N H vibrations of NH3 ligands (in ammine Pd complexes) and surprisingly NH4 + cations. The coexistence of ammine Pd complexes and ammonium ions is confirmed by the presence of additional bands at 1630 cm -1 (for NH3) and 1450 cm -] (for NH4+), not shown for brevity. The presence of NH4 § ions is thought to arise from the partial decomposition of the ammine I A=0.2 Pd(II) salt during exchange. With increasing temperatures, the 3610 cm -1 band forms at the expense of the broad massif, indicating the decomposition of NH4 § cations into o acidic protons. The evolution of IR 423 K x~ spectra with t e m p e r a t u r e shows t h a t 523 K O the decomposition of both NH3 ligands w and NH4 § proceeds above ca. 573 K, 573 K being complete around 653 K. It must be noticed that, under 623 K these conditions, some Pd nitrosyl 653 K species ( v N O at 1880 cm-D form, I I I I I I which decompose at higher 3700 3500 3300 3100 2900 2700 temperatures in 02. When a slow heating rate in 02 up to 653 K is used, Wavenumber I c m "1 the complete decomposition of Pd complexes and ammonium ions is Figure 1. In situ IR spectra of Pd(NH3)- achieved without formation of nitrosyl HZSM5 heated in 02. compounds. Linear heating rate of 5 K min -1 TPR of the calcined Pd-HZSM5-653 L_
590
s a m p l e in H2 reveals a H2/Pd ratio equal to 1.0, consistent with a m e a n +2 oxidation s t a t e of Pd. The formation of isolated oxo or ,more likely, hydroxo Pd(II) complexes upon calcination in 02 was suggested [7]. The interaction of these species with the lattice will be f u r t h e r discussed (see p a r a g r a p h 3.4).
3.2. C h a r a c t e r i s a t i o n of the e x c h a n g e d a m m i n e Pd(II) c o m p l e x In the case of divalent Pd precursors whose charge should balance two A1 sites t h a t m i g h t be remote from each other in the ZSM-5 structure, it is valuable to d e t e r m i n e the actual valence of the exchanged Pd species. The i n t e n s i t y of the 3610 cm -1 v OH band was used to evaluate the n u m b e r of residual B r c n s t e d hydroxyls at each step of the preparation, i.e. firstly the ion exchange a n d secondly the calcination. The a m o u n t of NH4 § derived from the 1450 cm -1 intensity, was determined: n NH4 § -- 1.9 mol/uc. Upon exchange with Pd(NH3)4(NO3)2, the n u m b e r of Bronsted hydroxyls n H § is equal to 0.3 mol/uc. This allows to derive the n u m b e r of A1 sites neutralized by Pd complexes: 3.7 1.9 - 0.3 = 1.5 mol/uc, corresponding to 0.78 mol Pd/uc. It is concluded t h a t the exchanged Pd complex neutralises two A1 charges, being divalent as expected from the Pd precursor. Since the Pd precursor is partially decomposed during exchange into Pd(NH3)x type complexes (x<4), it is worthwhile to evaluate the m e a n value for x. This is achieved by m e a s u r i n g the total n u m b e r (n N) of NH3 ligands and NH4 § ions p r e s e n t after exchange by TPD in He followed by m a s s spectrometry: n N = 3.39 mol/uc. This allows to derive the n u m b e r of NH3 ligands (n NH3) involved in exchanged Pd complexes: n NH3 = n N - n NH4 § = 3.39 - 1.9 = 1.5 mol/uc. This corresponds to x = 2 in Pd(NH3)x. It is concluded t h a t Pd(NH3)42§ complexes readily exchange acidic protons of the ZSM-5 s t r u c t u r e with Pd(NH3)22§ complexes, which parallels the findings obtained upon exchanging a silica surface with the same Pd precursor [8].
3.3. R e a c t i v i t y w i t h NO Reacting NO at 523 K with Pd-HZSM5-653 results in the consumption of NO (NO/Pd=I.5) and the evolution of NO2 (NO2/Pd= 0.5). This result is in a g r e e m e n t with previous studies on low Pd loaded Pd-H-ZSM-5 catalysts [3, 9]. The formation of NO2 is consistent with the reduction of Pd(II) ions into Pd(I) and the overall consumption of NO indicates the formation of mononitrosyl Pd(I) complexes. The formation of Pd(I) mononitrosyl complexes is also revealed by the a p p e a r a n c e of an intense sharp IR band at 1880 cm -1 (not shown). Upon reaction of NO with Pd-HZSM5-773 at 523 K, the a m o u n t s of consumed NO a n d produced NO2 are m u c h lower t h a n after mild calcination: NO/Pd = 0.65 and NO2/Pd = 0.21. Accordingly, the 1880 cm -1 intensity is much lower, in the exact proportion of the NO consumption. This fairly agrees with the detection by XRD of large PdO particles inert toward NO [3], prompting to the poor dispersion of Pd in this sample. These results indicate t h a t the calcination step
591 is a key step to m a i n t a i n high Pd loaded Pd-H-ZSM-5 catalysts in a high degree of Pd dispersion.
3.4. I n t e r a c t i o n of P d c o m p l e x e s w i t h lattice o x y g e n ions Figure 2 shows the IR spectra, in the region of lattice vibrations (1000-800 cm -1) , of Pd(NH3)-HZSM5 after exchange, calcination in 02 at 653 K and reaction with NO at 523 K. The striking feature is the appearance of new intense bands at ca. 940 and 975 cm -1 depending on the t r e a t m e n t (spectra b, c and d). These bands are obviously due to the presence of Pd ions in extralattice positions in zeolite channels since, upon reduction of Pd ions into Pd metal particles in H2 (spectrum e), none of these bands can be observed. They can be attributed to T-O vibrations (T = Si, A]) distorted upon the interaction of Pd ions with lattice oxygen ions. Such interactions are expected to locally induce a lengthening of TO bonds, therefore causing a downward shift of T-O vibrations. The strength of the interaction should increase with the charge on Pd ions. This is clearly what is observed: the band at 975 cm -1 (spectrum d) appears with the m1 / I formation of Pd(I) nitrosyl species (see above section) while the band at ca. 940 - 930 G) 0ccm -1 (spectra b and c) is t~ correlated to the presence of .Q Im Pd(II) ions/complexes. Similar 0 w .O results were reported with Cu< ZSM-5 [10]. The same conclusions were already suggested for low Pd loading samples [3] but they can be drawn unambiguously in the present work because of " o~ I o , I the high intensity of the related IR bands. This is -1 consistent again with the fact Wavenum ber I cm t h a t all Pd ions are atomically dispersed in the zeolite channels. Figure 2. Lattice vibrations of Pd(NH3)It is worth noticing t h a t HZSM5: upon the exchange-drying (a) in air after exchange and drying, sequence followed by (b) t h e n evacuated under vacuum at 300 K, rehydration in air (spectrum (c) calcined in 02 at 653 K, a), no such interaction is (d) reacted with 0.1 Torr NO at 523 K, observed, which would suggest (e) reduced in H2 at 573 K.
~938cml
1000
IA =0.2 /I
900
800
592 the formation of hydroxo Pd complexes solvated by water molecules and not interacting with lattice 02- ions. 3.5. C a t a l y t i c a c t i v i t y f o r t h e r e d u c t i o n o f N O b y CH4 in e x c e s s o f 09
The conversions of CH4 and NO versus temperature over Pd-HZSM5-773 and Pd-HZSM5-653 samples are shown in figures 3 and 4, respectively.
5O
100
80 --
CH4
NO
4O
1st run - - o - 2nd run
c .o
60--
r 30 .o
L.
>
40--
O
o
c 0 o
1st run
20-
~2ndrun
20 10 0
0 500
600
700
800
900
!
500
600
=
I
700
e
I
800
=
900
Temperature / K
Temperature / K
Figure 3. Catalytic activity of Pd-HZSM5-773
50
100
CH4
80
40-
60
~ ~
o
"==
4o
NO
30
=> 20 ----- 1st run --o-- 2nd run
...... 1st run
20
n
ulO 0 +4
500
600
700
800
Temperature / K
900
500
600
700
8O0
Temperature / K
Figure 4. Catalytic activity of Pd-HZSM5-653
900
593 Both samples are revealed to be active in the reduction of NO by CH4 in the presence of 02 above 550 K (1 st run). However, the sample calcined at 653 K exhibits a much higher NO conversion t h a n the one calcined at 773 K. This is in a g r e e m e n t with the fact t h a t the activity for SCR is related to the amount of Pd(I) nitrosyl species. This amount was small for Pd-HZSM5-773, while in PdHZSM5-653 all exchanged Pd do form isolated Pd(I) mononitrosyl species upon reaction with NO (this was verified in situ before measuring the catalytic activity). Over Pd-HZSM5-653, the conversion of CH4 between 550 and 700 K during the 1st r u n is the same as the NO conversion (within experimental errors). This indicates t h a t the catalyst is totally selective for the reduction of NO by CH4. But the activity is low (less t h a n in the 2 nd run). NO is totally converted into N2 (no N20 formation). Since Pd(I) mononitrosyl species were the only Pd species formed before contacting the sample with reactants, it is concluded t h a t these species are 100% selective but not very active for the reduction of NO by CH4 and inactive for the direct oxidation of CH4 by 02. The striking feature for this sample is the steep increase of both NO and CH4 conversions above ca. 700 K, suggesting the activation of the catalyst under reactants. The activation is confirmed by the conversion curve of the 2 nd run, much higher t h a n in the 1st run. It was checked that the catalytic behaviour did not vary any more with subsequent runs, indicating the stabilisation of the catalyst. The selectivity for SCR, SSCR (defined as the fraction of m e t h a n e involved in the reaction of reduction of NO by CH4) in the 2 nd run is observed to vary from 0.6 down to 0.2 with increasing temperatures. It is deduced t h a t the activation u n d e r reactants has led to much higher activity rates in NO and CH4 conversions but lower selectivity for the reduction of NO. At the same time, formation of N20 is observed (up to 70 vpm at 770 K). The catalytic behaviour of Pd-HZSM5-773 under reactants contrasts m a r k e d l y with Pd-HZSM5-653. The 2 nd run does show a lower NO conversion t h a n the 1 st r u n in the whole range of temperatures while CH4 conversion is increased. This suggests a deactivation of the catalyst for SCR under reactants, a t t r i b u t e d to the sintering of Pd into PdO, only active for the CH4 combustion. In all cases, the NO conversion goes through a m a x i m u m around 800 K. This is accompanied by the total release of CH4. Therefore, the decrease of NO conversion above 800 K is attributed to the decrease of CH4 available for the NO reduction, reaction which competes with the total oxidation of m e t h a n e by 02 according to the value of SscR much lower t h a n unity. A previous study [3] had shown that, depending on the Pd loading, Pd-HZSM-5 catalysts would generate two types of Pd sites under CH~qO/O2 reaction mixture: (i) Pd cations atomically dispersed at exchange sites thought to be responsible for the reduction of NO by CH4 and this is confirmed in the present study; these sites are shown here to be low active but totally selective for SCR (no competition with the oxidation of CH4 by 02).
594 (ii) PdO aggregates able to catalyse the oxidation of methane. It is shown in the present work that another type of sites can be generated under reactants which can perform both the reduction of NO by CH4 and the oxidation of CH4 by 02 in competition. These sites are irreversibly formed under reactants and exhibit much higher activity for NO reduction than isolated Pd(I) nitrosyl complexes. Further studies are needed to identify the structure of these species.
REFERENCES
1. A. Ali, W. Alvarez, C.J. Loughran and D.E. Resasco, Appl. Catal. B: Environ., 14 (1997) 13. 2. A.W. Aylor, L.J. Lobree, J.A. Reimer and A.T. Bell, J. Catal. 172 (1997) 453. 3. P. G~lin, A. Goguet, C. Descorme, C. L~cuyer and M. Primet, Catalysis and Automotive Pollution Control IV, Brussels, April 9-11, 1997; Stud. in Surf. Sci. and Catal., 116, N. Kruse, A. Frennet and J.-M. Bastin (eds.), Elsevier, 1998, p. 275. 4. B.J. Adelman and W.M.H. Sachtler, Appl. Catal. B: Environ., 14 (1997). 5. M. Primet, J.C. V~drine and C. Naccache, J. Mol. Catal., 1978, 4, 411. 6. N. Echoufi and P. G~lin, J. Chem. Soc. Faraday Trans., 1992,88, 10671. 7. B. Pommier and P. G~lin, Phys. Chem. Chem. Phys., to be published. 8. A.L. Bonivardi and M.A. Baltan~is, Thermochim. Acta, 1991, 191, 63. 9. C. Descorme, P. G~lin, M. Primet and C. L~cuyer, Catal. Lett., 41 (1996) 133. 10. G.D. Lei, B.J. Adelman, J. Sarkany and W.M.H. Sachtler, Appl. Catal. B, 1995, 5,245.
587
S e l e c t i v e r e d u c t i o n of n i t r i c o x i d e by m e t h a n e o v e r Pde x c h a n g e d H-ZSM-5: i n f l u e n c e of a c t i v a t i o n B. Pommier and P. G~lin Laboratoire d'Application de la Chimie h l'Environnement, UMR CNRS 5634, Universit~ Claude Bernard Lyon 1, F69622 Villeurbanne Cedex, France
This paper reports the influence of the precalcination step on the physicochemical and catalytic properties of a high Pd loading Pd-H-ZSM-5 catalyst prepared from exchange with Pd(NH3)4(NO3)2. Upon exchange, Pd(NH3)t 2§ decomposes into Pd(NH3)22§ and NH4 § which readily exchange protons and form Pd(II) ions or hydroxo complexes and H § upon calcination in 02 at 653 K. These Pd(II) species react with NO to produce Pd(I) mononitrosyl species and NO2. Pd(II) and Pd(I) species interact with the lattice 02- ions (IR bands at 975 and 940 cm-1). Pd(I) nitrosyl complexes are 100% selective for the reduction of NO by CHt in excess of 02, but not very active. Under reactants at 700-800 K, these species are irreversibly converted into sites considerably more active for NO reduction but also less selective. Precalcination in 02 at 773 K induces the partial sintering of Pd(II) into PdO, inactive for the reduction of NO, and this results in the lower conversion of NO into N2.
1. INTRODUCTION The catalytic performance of Pd-ZSM-5 catalysts in the NO reduction by CH4 in the presence of 02 was shown to be related to the dispersion of Pd [1-3]. The high selectivity and activity in Selective Catalytic Reduction (SCR) would be due to the presence of isolated Pd 2§ ions or complexes dispersed in the zeolite channels, while large PdO particles would be responsible for CHt combustion and not or to a small extent for SCR. It is generally accepted that the presence of protons is necessary for maintaining Pd in a high degree of dispersion. Protons could also play a role in the SCR mechanism themselves [4]. The Pd-H-ZSM-5 catalysts have been prepared in various ways, e.g. by impregnation or conventional exchange with varying Pd precursors. In all studies, high Pd loadings (1 wt % Pd or higher) were shown to decrease the Pd dispersion and, consequently, the SCR selectivity. The goal of the present study is to thoroughly reinvestigate the relevant chemistry of a Pd-H-ZSM-5 catalyst with high Pd loading prepared via exchange of H-ZSM-5 with tetraammine Pd(II) complex and
588 further reacted with 02, NO and NO/CH4/O2. It is demonstrated t h a t the activation is a key step for improved catalytic performance of Pd-H-ZSM-5 in the reduction of NO by CH4 in an excess of 02.
2. E X P E R I M E N T A L A Pd-H-ZSM-5 catalyst containing 1.4 wt % Pd was prepared by conventional exchange of a H-ZSM-5 sample (CBV 5020, Si/Al=25, from PQ Zeolites B.V.) in an aqueous solution of Pd(NH3)4(NO3)2 (Strem Chemicals) at 323 K. After thorough washing with deionized water, the catalyst was dried overnight at 393 K. The resulting material was referred to as Pd(NH3)-HZSM5. Pd(NH3)-HZSM5 was subsequently calcined in flowing 02 up to 653 and 773 K (heating rate of 0.5 K min -1) in order to decompose the Pd ammine precursor. The calcined materials were labelled Pd-HZSM5-653 and Pd-HZSM5-773 respectively. The a m o u n t of NH3 and NH4 § contained in Pd(NH3)-HZSM5 was determined by temperature-programmed decomposition (TPD) in 02 followed by mass spectrometry (typically 0.1 g catalyst, heating rate of 10 K min-D. For this purpose, a Balzers QMA 125 quadrupole was used to record the most relevant m/z signals as a function of time/temperature: 14 (N), 15 (NH), 18 (H20), 30 (NO), 44 (N20, CO2), 46 (NO2). Calibrations for NH3 and N2 were carried out. FTIR spectra of sample wafers (ca. 15 mg) were recorded at a resolution of 4 cm -1 on a FT-IR Nicolet Magna 550 spectrometer, using a greaseless cell equipped with KBr windows [5]. For in situ studies at varying temperatures under controlled atmosphere, a home-made cell was used [6]. The catalytic activities for the reduction of NO by CH4 over Pd-HZSM5-653 and Pd-HZSM5-773 catalysts were measured using a U-shaped quartz reactor (16 m m ID) operating in a steady-state plug flow mode. The samples (200 mg) were reactivated in-situ in flow of oxygen (linear ramp rate of 0.5 K min-D, purged in helium down to 523 K before being put into contact with the reactants. The reaction mixture was adjusted so as to examine the catalytic activity under lean conditions: 2000 vpm NO, 1000 vpm CH4, 6240 vpm 02; helium as balance; total flow rate = 167 cm3/min, GHSV -- 30,000 h -1. The catalytic activity was measured as a function of temperature in the range 523 - 873~ during two successive heating - cooling cycles (linear heating and cooling rate of 1 K min-1). The effluent gases were analyzed using two gas chromatographs equipped with TCD and FID detectors and NOx infrared analyzers. Carbon and nitrogen balances were checked. The NOx conversion was determined according to the following equation : NOx conversion % = ([NO]0 + [NO2]0 - [NO] - [NO2]) * 100/([NO]0 + [NO2]0) where [NO]0 and [NO2]0 are the inlet concentrations of NO and NO2 respectively and [NO] and [NO2] the outlet concentrations. The NO2 formation was low in the
589 whole range of t e m p e r a t u r e ([NO2]< 40 vpm), almost independent on the t e m p e r a t u r e and ascribed to the NO oxidation in the dead volume of the apparatus. The CH4 conversion was determined from the consumption of CH4.
3. R E S U L T S A N D D I S C U S S I O N
3.1. Stability of e x c h a n g e d a m m i n e Pd c o m p l e x e s in 02 In situ F T I R spectra of Pd(NH3)-HZSM5 in flowing 02 at increasing t e m p e r a t u r e s (linear ramp of 5 K min-D were recorded to investigate the decomposition of Pd precursors. Figure 1 shows the region of v OH and v NH vibrations. The main features are: (i) an intense band at ca. 3740 cm -1 attributed to terminal silanols, (ii) a band at ca. 3610 cm -1 ascribed to bridging hydroxyls (Brcnsted acid sites); its low intensity indicates the high level of cationic exchange, (iii) a broad complex massif with maxima at ca. 3340, 3270 and 3200 cm -], assigned to the v N H vibrations of NH3 ligands (in ammine Pd complexes) and surprisingly NH4 + cations. The coexistence of ammine Pd complexes and ammonium ions is confirmed by the presence of additional bands at 1630 cm -1 (for NH3) and 1450 cm -] (for NH4+), not shown for brevity. The presence of NH4 § ions is thought to arise from the partial decomposition of the ammine I A=0.2 Pd(II) salt during exchange. With increasing temperatures, the 3610 cm -1 band forms at the expense of the broad massif, indicating the decomposition of NH4 § cations into o acidic protons. The evolution of IR 423 K x~ spectra with t e m p e r a t u r e shows t h a t 523 K O the decomposition of both NH3 ligands w and NH4 § proceeds above ca. 573 K, 573 K being complete around 653 K. It must be noticed that, under 623 K these conditions, some Pd nitrosyl 653 K species ( v N O at 1880 cm-D form, I I I I I I which decompose at higher 3700 3500 3300 3100 2900 2700 temperatures in 02. When a slow heating rate in 02 up to 653 K is used, Wavenumber I c m "1 the complete decomposition of Pd complexes and ammonium ions is Figure 1. In situ IR spectra of Pd(NH3)- achieved without formation of nitrosyl HZSM5 heated in 02. compounds. Linear heating rate of 5 K min -1 TPR of the calcined Pd-HZSM5-653 L_
590
s a m p l e in H2 reveals a H2/Pd ratio equal to 1.0, consistent with a m e a n +2 oxidation s t a t e of Pd. The formation of isolated oxo or ,more likely, hydroxo Pd(II) complexes upon calcination in 02 was suggested [7]. The interaction of these species with the lattice will be f u r t h e r discussed (see p a r a g r a p h 3.4).
3.2. C h a r a c t e r i s a t i o n of the e x c h a n g e d a m m i n e Pd(II) c o m p l e x In the case of divalent Pd precursors whose charge should balance two A1 sites t h a t m i g h t be remote from each other in the ZSM-5 structure, it is valuable to d e t e r m i n e the actual valence of the exchanged Pd species. The i n t e n s i t y of the 3610 cm -1 v OH band was used to evaluate the n u m b e r of residual B r c n s t e d hydroxyls at each step of the preparation, i.e. firstly the ion exchange a n d secondly the calcination. The a m o u n t of NH4 § derived from the 1450 cm -1 intensity, was determined: n NH4 § -- 1.9 mol/uc. Upon exchange with Pd(NH3)4(NO3)2, the n u m b e r of Bronsted hydroxyls n H § is equal to 0.3 mol/uc. This allows to derive the n u m b e r of A1 sites neutralized by Pd complexes: 3.7 1.9 - 0.3 = 1.5 mol/uc, corresponding to 0.78 mol Pd/uc. It is concluded t h a t the exchanged Pd complex neutralises two A1 charges, being divalent as expected from the Pd precursor. Since the Pd precursor is partially decomposed during exchange into Pd(NH3)x type complexes (x<4), it is worthwhile to evaluate the m e a n value for x. This is achieved by m e a s u r i n g the total n u m b e r (n N) of NH3 ligands and NH4 § ions p r e s e n t after exchange by TPD in He followed by m a s s spectrometry: n N = 3.39 mol/uc. This allows to derive the n u m b e r of NH3 ligands (n NH3) involved in exchanged Pd complexes: n NH3 = n N - n NH4 § = 3.39 - 1.9 = 1.5 mol/uc. This corresponds to x = 2 in Pd(NH3)x. It is concluded t h a t Pd(NH3)42§ complexes readily exchange acidic protons of the ZSM-5 s t r u c t u r e with Pd(NH3)22§ complexes, which parallels the findings obtained upon exchanging a silica surface with the same Pd precursor [8].
3.3. R e a c t i v i t y w i t h NO Reacting NO at 523 K with Pd-HZSM5-653 results in the consumption of NO (NO/Pd=I.5) and the evolution of NO2 (NO2/Pd= 0.5). This result is in a g r e e m e n t with previous studies on low Pd loaded Pd-H-ZSM-5 catalysts [3, 9]. The formation of NO2 is consistent with the reduction of Pd(II) ions into Pd(I) and the overall consumption of NO indicates the formation of mononitrosyl Pd(I) complexes. The formation of Pd(I) mononitrosyl complexes is also revealed by the a p p e a r a n c e of an intense sharp IR band at 1880 cm -1 (not shown). Upon reaction of NO with Pd-HZSM5-773 at 523 K, the a m o u n t s of consumed NO a n d produced NO2 are m u c h lower t h a n after mild calcination: NO/Pd = 0.65 and NO2/Pd = 0.21. Accordingly, the 1880 cm -1 intensity is much lower, in the exact proportion of the NO consumption. This fairly agrees with the detection by XRD of large PdO particles inert toward NO [3], prompting to the poor dispersion of Pd in this sample. These results indicate t h a t the calcination step
591 is a key step to m a i n t a i n high Pd loaded Pd-H-ZSM-5 catalysts in a high degree of Pd dispersion.
3.4. I n t e r a c t i o n of P d c o m p l e x e s w i t h lattice o x y g e n ions Figure 2 shows the IR spectra, in the region of lattice vibrations (1000-800 cm -1) , of Pd(NH3)-HZSM5 after exchange, calcination in 02 at 653 K and reaction with NO at 523 K. The striking feature is the appearance of new intense bands at ca. 940 and 975 cm -1 depending on the t r e a t m e n t (spectra b, c and d). These bands are obviously due to the presence of Pd ions in extralattice positions in zeolite channels since, upon reduction of Pd ions into Pd metal particles in H2 (spectrum e), none of these bands can be observed. They can be attributed to T-O vibrations (T = Si, A]) distorted upon the interaction of Pd ions with lattice oxygen ions. Such interactions are expected to locally induce a lengthening of TO bonds, therefore causing a downward shift of T-O vibrations. The strength of the interaction should increase with the charge on Pd ions. This is clearly what is observed: the band at 975 cm -1 (spectrum d) appears with the m1 / I formation of Pd(I) nitrosyl species (see above section) while the band at ca. 940 - 930 G) 0ccm -1 (spectra b and c) is t~ correlated to the presence of .Q Im Pd(II) ions/complexes. Similar 0 w .O results were reported with Cu< ZSM-5 [10]. The same conclusions were already suggested for low Pd loading samples [3] but they can be drawn unambiguously in the present work because of " o~ I o , I the high intensity of the related IR bands. This is -1 consistent again with the fact Wavenum ber I cm t h a t all Pd ions are atomically dispersed in the zeolite channels. Figure 2. Lattice vibrations of Pd(NH3)It is worth noticing t h a t HZSM5: upon the exchange-drying (a) in air after exchange and drying, sequence followed by (b) t h e n evacuated under vacuum at 300 K, rehydration in air (spectrum (c) calcined in 02 at 653 K, a), no such interaction is (d) reacted with 0.1 Torr NO at 523 K, observed, which would suggest (e) reduced in H2 at 573 K.
~938cml
1000
IA =0.2 /I
900
800
592 the formation of hydroxo Pd complexes solvated by water molecules and not interacting with lattice 02- ions. 3.5. C a t a l y t i c a c t i v i t y f o r t h e r e d u c t i o n o f N O b y CH4 in e x c e s s o f 09
The conversions of CH4 and NO versus temperature over Pd-HZSM5-773 and Pd-HZSM5-653 samples are shown in figures 3 and 4, respectively.
5O
100
80 --
CH4
NO
4O
1st run - - o - 2nd run
c .o
60--
r 30 .o
L.
>
40--
O
o
c 0 o
1st run
20-
~2ndrun
20 10 0
0 500
600
700
800
900
!
500
600
=
I
700
e
I
800
=
900
Temperature / K
Temperature / K
Figure 3. Catalytic activity of Pd-HZSM5-773
50
100
CH4
80
40-
60
~ ~
o
"==
4o
NO
30
=> 20 ----- 1st run --o-- 2nd run
...... 1st run
20
n
ulO 0 +4
500
600
700
800
Temperature / K
900
500
600
700
8O0
Temperature / K
Figure 4. Catalytic activity of Pd-HZSM5-653
900
593 Both samples are revealed to be active in the reduction of NO by CH4 in the presence of 02 above 550 K (1 st run). However, the sample calcined at 653 K exhibits a much higher NO conversion t h a n the one calcined at 773 K. This is in a g r e e m e n t with the fact t h a t the activity for SCR is related to the amount of Pd(I) nitrosyl species. This amount was small for Pd-HZSM5-773, while in PdHZSM5-653 all exchanged Pd do form isolated Pd(I) mononitrosyl species upon reaction with NO (this was verified in situ before measuring the catalytic activity). Over Pd-HZSM5-653, the conversion of CH4 between 550 and 700 K during the 1st r u n is the same as the NO conversion (within experimental errors). This indicates t h a t the catalyst is totally selective for the reduction of NO by CH4. But the activity is low (less t h a n in the 2 nd run). NO is totally converted into N2 (no N20 formation). Since Pd(I) mononitrosyl species were the only Pd species formed before contacting the sample with reactants, it is concluded t h a t these species are 100% selective but not very active for the reduction of NO by CH4 and inactive for the direct oxidation of CH4 by 02. The striking feature for this sample is the steep increase of both NO and CH4 conversions above ca. 700 K, suggesting the activation of the catalyst under reactants. The activation is confirmed by the conversion curve of the 2 nd run, much higher t h a n in the 1st run. It was checked that the catalytic behaviour did not vary any more with subsequent runs, indicating the stabilisation of the catalyst. The selectivity for SCR, SSCR (defined as the fraction of m e t h a n e involved in the reaction of reduction of NO by CH4) in the 2 nd run is observed to vary from 0.6 down to 0.2 with increasing temperatures. It is deduced t h a t the activation u n d e r reactants has led to much higher activity rates in NO and CH4 conversions but lower selectivity for the reduction of NO. At the same time, formation of N20 is observed (up to 70 vpm at 770 K). The catalytic behaviour of Pd-HZSM5-773 under reactants contrasts m a r k e d l y with Pd-HZSM5-653. The 2 nd run does show a lower NO conversion t h a n the 1 st r u n in the whole range of temperatures while CH4 conversion is increased. This suggests a deactivation of the catalyst for SCR under reactants, a t t r i b u t e d to the sintering of Pd into PdO, only active for the CH4 combustion. In all cases, the NO conversion goes through a m a x i m u m around 800 K. This is accompanied by the total release of CH4. Therefore, the decrease of NO conversion above 800 K is attributed to the decrease of CH4 available for the NO reduction, reaction which competes with the total oxidation of m e t h a n e by 02 according to the value of SscR much lower t h a n unity. A previous study [3] had shown that, depending on the Pd loading, Pd-HZSM-5 catalysts would generate two types of Pd sites under CH~qO/O2 reaction mixture: (i) Pd cations atomically dispersed at exchange sites thought to be responsible for the reduction of NO by CH4 and this is confirmed in the present study; these sites are shown here to be low active but totally selective for SCR (no competition with the oxidation of CH4 by 02).
594 (ii) PdO aggregates able to catalyse the oxidation of methane. It is shown in the present work that another type of sites can be generated under reactants which can perform both the reduction of NO by CH4 and the oxidation of CH4 by 02 in competition. These sites are irreversibly formed under reactants and exhibit much higher activity for NO reduction than isolated Pd(I) nitrosyl complexes. Further studies are needed to identify the structure of these species.
REFERENCES
1. A. Ali, W. Alvarez, C.J. Loughran and D.E. Resasco, Appl. Catal. B: Environ., 14 (1997) 13. 2. A.W. Aylor, L.J. Lobree, J.A. Reimer and A.T. Bell, J. Catal. 172 (1997) 453. 3. P. G~lin, A. Goguet, C. Descorme, C. L~cuyer and M. Primet, Catalysis and Automotive Pollution Control IV, Brussels, April 9-11, 1997; Stud. in Surf. Sci. and Catal., 116, N. Kruse, A. Frennet and J.-M. Bastin (eds.), Elsevier, 1998, p. 275. 4. B.J. Adelman and W.M.H. Sachtler, Appl. Catal. B: Environ., 14 (1997). 5. M. Primet, J.C. V~drine and C. Naccache, J. Mol. Catal., 1978, 4, 411. 6. N. Echoufi and P. G~lin, J. Chem. Soc. Faraday Trans., 1992,88, 10671. 7. B. Pommier and P. G~lin, Phys. Chem. Chem. Phys., to be published. 8. A.L. Bonivardi and M.A. Baltan~is, Thermochim. Acta, 1991, 191, 63. 9. C. Descorme, P. G~lin, M. Primet and C. L~cuyer, Catal. Lett., 41 (1996) 133. 10. G.D. Lei, B.J. Adelman, J. Sarkany and W.M.H. Sachtler, Appl. Catal. B, 1995, 5,245.