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ZSM-5 catalysts
8:ENVIRONMENTAL
ELSEVIER
Applied Catalysis
B: Environmental
14 (1997) l-l 1
The effect of zeolitic protons on NO, reduction over Pd/ZSM-5 catalysts B.J. Adelmana,*, W.M.H. Sachtler” a VN. Ipatieff Laboratory,
Center for Catalysis and Surface Science, Department Evanston, IL 60208-3000, USA
Received 25 September
of Chemistry, Northwestern
University,
1996; received in revised form 19 November 1996; accepted 2 December 1996
Abstract Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 and Pd/NaZSM-5 after reduction and reoxidation at 500°C. Formation of Pd’+ ions is not observed in Pd/HZSM-5 in the absence of NO,, but in an atmosphere of NO;! + 02 the PdO particles are transformed into Pd ‘+ ions. Even in Pd/NaZSM-5, roughly 20% of the PdO particles are converted to ions in NO,; the protons needed for this process are mainly formed during reduction of Pd*+ to Pd’. When a reduced and reoxidized Pd/HZSM-5 catalyst is exposed to CHq + NO2 + 02, the rate of catalytic NO2 reduction to N2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd*+, but the rate of C& oxidation remains constant with time on-stream. .4pparently, Pd *+ ions are needed for SCR of NO,, but for methane oxidation with 02, Pd*+ ions and PdO particles have similar activity. Pd/HZSM-5 is active even when Pd is initially present as PdO particles, but Pd/NaZSM-5 remains inactive
also when part of the Pd is present as Pd 2+ ions. This shows that protons are not only required to convert PdO into Pd*+, but also that Pd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3Hs activation are discussed. 0 1997 Elsevier Science B.V. Keywords:
Effect of protons; NO, reduction; Pd/ZSM-5
1. Introduction
Metal-exchanged zeolite catalysts have been proven active in the selective catalytic reduction (SCR) of NO, by hydrocarbons [l-6]. The addition of O2 enhances the rate of NO, reduction [2,3]. In a previous work, it was shown that formation of adsorbed nitrogen oxides of the general formula NO, is the main cause of this enhancement [7-91. The NO, groups are the sites for H-abstraction from paraffin molecules.
*Correspondingauthor. 0926~860X/97/$17.000 1997 Elsevier Science B.V. All rights reserved. PZI SO926-3373(97)00007-6
However, the structure and reactivity of these groups depends upon the nature of the exchanged metal ions [lo]. With Cu/ZSM-5, the NO, sites are nitro and nitrate groups [lo]; they are reduced by C3+ paraffins or Cz+ olefins, but not by CH,; therefore, this material does not catalyze the reduction of NO, to N2 with methane [ 111. Conversely, over M/ZSM-5 catalysts with M = Co, Ni, Mn, or Rh, the NO, complexes abstract H atoms from CH,; consequently, these materials catalyze the SCR of NO, with CH4 and higher hydrocarbons [4-61. Recently, a different hydrocarbon specificity has been discovered over Pd exchanged ZSM-5
2
B.J. Adelman, W.M.H. Sachtler/Applied
catalysts [ 121. Palladium has been shown to be active when exchanged into H/ZSM-5, but not Na/ZSM-5. The cause of this difference is not fully understood. Two plausible explanations can be suggested. First, protons are required to convert Pd oxide particles into Pd2+ ions, and these ions are required for catalysis. This conversion of oxides into ions via reaction with zeolitic protons is well known and is called ‘protonolysis’ [13]. Recently, Resasco et al. showed by EXAFS that in their best Pd/HZSM-5 catalysts the Pd was mainly present as Pd2+ ions [ 141. An alternative hypothesis, not inconsistent with the former, assumes that a bifunctional mechanism is operating with this catalyst system. Indeed, Resasco et al. in a different paper showed that even physical mixtures of Pd/SiO:! and sulfated zirconia (SZ) were active for the SCR of NO,, though the conversion levels were lower than for metalexchanged zeolite samples [15]. Protons are, of course, undesirable in practical catalysts because they induce dealumination of the zeolite in the HzO-rich atmosphere of engine exhausts [ 16,171; but the observation that for some catalysts zeolite protons are necessary for SCR activity of NO, [ 12,18-201 is of possible relevance with respect to the reaction mechanism. Upon exposing Pd/HZSM-5 to a gas how containing N02, O2 and a hydrocarbon at an appropriate temperature, formation of N2 and CO2 is observed. When the hydrocarbon is varied, while the carbon flux is kept constant, a positive correlation has been observed between the formations of N2 and CO2 for a number of ZSM supported catalysts, including HZSM-5 [7]. However, no such correlation holds for Pd/HZSM-5. For instance, with 3000 ppm CH4 or 1000 ppm CsHs, the NO, reduction to N2 is significantly lower with C3H8 than with CH4, but the CO2 production is higher with CsHs. The same seems to be true for ZSM-5 supported Ga [ 16,171 and In [ 181 catalysts, which also require the acid form, HZSM5, of the support. This suggests that the reaction mechanism over these catalysts may differ significantly from that which is valid for other ZSM-5 supported catalyst. The objective of the present study is to investigate the relevant chemistry of Pd/ZSM-5 in order to get information on the reaction mechanism of SCR over Pd/HZSM-5.
Catalysis B: Environmental 14 (1997) l-11
2. Experimental 2.1. Catalyst preparation H/ZSM-5 was prepared via ion exchange at room temperature (RT) using a threefold excess of NHdNOs solution added to a slurry of Na/ZSM-5 (UOP lot #13023-60) in water. After 24 h, the slurry was vacuum filtered, washed with one-liter doubly deionized H20 and air dried. The sample was then calcined in UHP O2 to 500°C. Pd/HZSM-5 and Pd/NaZSM-5 were prepared via ion exchange at RT using a dilute (NH&Pd(NO& solution added to a HZSM-5 or a Na/ZSM-5 slurry, respectively. The slurries were stirred for 72 h prior to being vacuum filtered, washed with one-liter doubly deionized Hz0 and air dried. Exchange was performed over 72 h to ascertain a uniform distribution of Pd ions throughout the zeolite particles. In a previous work with Rh, it was found that after short exchange times a concentration gradient exists with more metal ions near the periphery than near the center of the zeolite analysis via inductively grains [ 131. Elemental coupled plasma spectroscopy (Therm0 Jarrel Ash, Atomscan 25 spectrometer) gave the following data: Pd/HZSM-5: Si/Al = 18, Pd/Al = 0.093, Na/Al = 0.0; Pd/NaZSM-5: Si/Al = 18, Pd/Al = 0.105, Na/Al = 0.72. 2.2. Reaction
studies
Pd samples were tested in a continuous-flow microreactor. The samples were pretreated as follows: calcination in UHP O2 to 500°C reduction in 5% H2 to 500°C and re-oxidation in UHP 02 to 500°C; purging with He was applied between these treatments. The results after the initial calcination differed from those obtained upon reduction and reoxidation, indicating different catalytic performances of Pd2+ ions and PdO particles. A series of tests was performed with feed compositions of 1000 ppm N02, 3000 ppm CH4 and 20000 ppm O2 with He as a diluent. In another test series, one of these components was omitted from the feed after steady-state conversion was attained. The activity was also measured upon subsequently reintroducing that component to the feed. The effluent gas was monitored with a Hewlett-Packard 5890 GC with Alltech 13X molecular sieve and Porapak Q columns.
B.J. Adelman, Wb4.H. Sachtler/Applied
Catalysis B: Environmental 14 (1997) I-11
Upon bypassing the catalyst bed, the initial concentrations of 02, NO2 and CH4 were determined. The conversion of NO, was calculated from the measured formation of NZ. The measured conversion of CH4 was checked against the measured formation of CO, in the effluent. 2.3. Temperature-programmed
reduction (TPR)
The procedure for monitoring temperature-programmed reduction (TPR) of zeolite supported catalysts has been described previously [21]. Hydrogen profiles for Pd/HZSM-5 and consumption Pd/NaZSM-5 were recorded after each of the following pretreatments: (1) calcination in UHP 02, (2) reoxidation in UHP O2 after reduction, and (3) thermal treatment in NO2 + 02 after reduction. All treatments were carried out at 500°C; in case of the NO, treatments, this temperature was held for more than 18 h.
3. Results First, Pd/HZSM-5 catalysts were tested that had only been calcined. No induction period was observed
3
with these catalysts. For a Pd/HZSM-5 sample that had been reduced and re-oxidized after calcination, Fig. 1 shows the NOz and CH4 conversions at 500°C. Clearly, the conversions of both reactants depend in a strikingly different manner on the reaction time, the CH4 conversion remains at a constant level of SO%, but the NO, conversion to N2 rises from an initial value of 20% over the course of 24 h to a steady-state value of 50%. After attaining steady state, the feed was switched from CHq + NO2 + 02 to NO2 + 02. Neither N2 nor CO2 was detected, as is obvious from Fig. 2(A). After one hour, the three component mixture, namely NO2 + CH4 + 02, was reintroduced. Steady-state conversions were rapidly reattained. Fig. 2 (B) shows the effect of omitting NO2 from the feed. In this case N2 formation ceased, but the rate of CH4 combustion increased. A similar negative effect of NO, on the rate of hydrocarbon combustion has been found previously with Cu/ZSM-5 [lo]. Unlike the previous case, returning from a CH4 + 02 feed to the three-component feed did not immediately re-establish the steady-state activity; it took several hours to attain the original steady-state conversion of NO,. Similar to earlier observations by Misono et al. [ 121, our Pd/NaZSM-5 exhibits very low
100
a’ (b)
0
200
400
600
800
1000
1200
1400
Time in Minutes Fig. 1. Conversion [CHd] = 3000ppm,
of (a) - NO1 to Nz and (b) - C& to CO2 over Pd/HZSM-5, that had been reduced and re-oxidized, [NO*] = 1000 ppm, and [O,] = 20000 ppm. Reaction temperature = 5OOT, and GHSV = 36000 h,
vs. time.
B.J. Ad&mm,
A
Catalysis B: Environmental
14 (1997) 1-11
100 90
--
80 77
,I 5 !!!
W34.H. Sachtler/Applied
1
II
III
.
r---
70 -60 --
s ii
50 --
0
40 --
s
30 --
._
.
.
. .
20 --
= CH4 conversion = NO* conversion
?? ??
10 -0-i
.
-?-
I
0
50
100
150
I 200
250
300
1000
1200
Time in Minutes
B
100 90 80
g ._ i!z
70 60
g
50
s 0 s
40 30 20 10 0 0
200
400
600
800
Time in Minutes Fig. 2. (A) Conversion of (m) - NO, to N2, and (0) - CH4 to CO? over Pd/HZSM-5, after steady state had been reached, vs. time. Feed composition I: [CHd] = 3000ppm, [NO*] = lOOOppm, and [02] = 20000ppm; feed composition II: [NO21 = lOOOppm, and [Oz] = 20000ppm; feed composition III: [C&l = 3000ppm, [NO21 = lOOOppm, and [Oz] = 20000ppm. Temperature = 5OO”C, and GHSV = 36000 h’. (B) Conversion of ( ?? ) - NO, to N2 and (0) - CH4 to CO2 over Pd/HZSM-5, after steady state had been reached, vs. time. Feed composition I: [C&l = 3000ppm, [NO*] = lOOOppm, and [Oz] = 20000ppm; feed composition II: [Cb] = 3000ppm. and [Oz] = 20000ppm; feed composition III: [CHh] = 3000ppm. [NOz] = lOOOppm, and [02] = 20000ppm. Temperature = 500°C; GHSV = 36000 h'
B.J. Adelman,
WMH.
Sachtler/Applied
Catalysis B: Environmental
14 (1997) 1-11
8000
6000
4000
2000
0 100
200
Temperature Fig. 3. TPR of Pd/HZSM-5 reoxidation at 500°C.
after calcination
300
in “C
at 500°C of (a) - a fresh Pd/HZSM-5
activity for NO, reduction while it does catalyze the oxidation of methane. Fig. 3 shows the TPR profile of a freshly calcined Pd/HZSM-5 sample (a) and the TPR profile of a Pd/HZSM-5 sample after reduction and reoxidation (b). The peaks below 0°C in these profiles are artifacts caused by desorption of Ar. The peak at 20°C in trace (b) is ascribed to PdO particles; this assignment is confirmed by the absence of this peak upon reducing the sample with CO at 350°C prior to the TPR run (not shown). In trace (a) this peak is small; instead, a broad feature is observed ranging from 90” to 250°C. This is attributed to reduction of Pd*+ ions. A similar broad reduction profile was observed previously with Pd/H mordenite [22]. The Pd2+ ions are formed either by release of the NH3 ligands from the Pd(NH&*+ ions or by the sequence: autoreduction to Pd’, followed by oxidation to PdO, followed by protonolysis. The negative peak near 80°C in trace (b) is characteristic of the decomposition of the Pd-P-hydride; its appearance confirms that Pd was formed during reduction of PdO. The formation and decomposition of this hydride is a familiar feature in Hz-TPR of Pd [21]. Fig. 4 shows the TPR profiles of Pd/HZSM-5 after reduction and reoxidation at 500°C (a) as well as that
sample,
and (b) - after subsequent
reduction
and
of Pd/HZSM-5 (b) and Pd/NaZSM-5 (c) after pretreatment in NO2 + 02 streams at 500°C for 2 h. Remarkably, no formation of Pd2+ ions is observed in Pd/HZSM-5 after the reduction/reoxidation treatment, even when the temperature was held at 500°C for over 48 h (not shown). Notwithstanding the abundance of protons, no TPR peak is observed near 100°C in trace (a). However, trace (b) shows that exposure to NO2 induces the conversion of PdO particles to Pd2+ ions; and the H2 consumption peak is shifted upward by 100°C with respect to trace (a) in Fig. 3. In the absence of a high proton concentration, no protonolysis occurs, i.e. the mixture of NO, leaves the PdO particles apparently almost unaffected, as follows from the large TPR peak near 50°C in trace (c), which is only slightly shifted in comparison to the peaks in trace (b) in Fig. 3 (to facilitate comparison this profile is repeated in Fig. 4 as trace (a)). Possible causes for the absence of protonolysis in Pd/HZSM-5 and its occurrence when protons and NO, are both present will be discussed below. The Hz consumption at 200°C of the Pd/HZSM-5 that was exposed to NO, is slightly greater than that required for the reduction of the Pd’+/PdO mixture to Pd’. This indicates formation of Pd’+-NO, complexes and co-reduction of NO, and Pd2+.
B.J. Adelman, W2f.H. Sachtler/Applied
Catalysis B: Environmental 14 (1997) I-11
18000
_
.
16000
z -
14000
g '3 5 Q
12000
E
8000
s
6000
I"
4000
10000
2000 0 -100
0
100
200
Temperature Fig. 4. TPR profile of Pd/HZSM-5 (a) - after reduction and re-oxidation after NO2 + 0, exposure at 500°C.
In view of the observation that temporary exposure of the catalyst to an atmosphere of CH4 + 02 lowered the catalytic activity, it was of interest to study Pd/HZSM-5 samples treated in this manner by TPR and to compare them to samples that had been reduced and re-oxidized. These results are reported in Fig. 5. Profile (a) of the reduced and reoxidzed sample shows abundance of PdO particles. A sample which was brought to steady state shows the typical signature of the coexistence of PdO particles and Pdzf ions. Even if such a sample is afterward exposed to Ha0 for one hour, ions are not converted to oxide particles, as is clear from the TPR profile (b) in this figure. In contrast, when NO2 is omitted from the gas atmosphere, again a population of abundant PdO particles is identified in the TPR profile (c), which is quite similar to (a), lacking the TPR peak characteristic of Pd2+ tons. The data clearly indicate that PdO is formed during the catalytic process, but NOz is instrumental for the protonolysis transforming PdO particles back into Pdzf ions. To identify the interaction of hydrocarbons with protons in the absence of a metal or metal ions, a calcined H/ZSM-5 sample was exposed to dilute streams of propane (0.1 to 0.4%) or heptane. The latter hydrocarbon was used because isomerization
300
400
500
in OC
at 500°C; (b) - TPR profiles of Pd/HZSM-5;
and (c) - Pd/NaZSM-5
and cracking can be expected to be more pronounced than with methane. In both cases, formation of CH4 and CzH4 was found to be significant; with C,HiG a variety of hydrocarbons were detected. Whereas heptane cracking was significant below 4OO”C, cracking of propane could be detected only at temperatures >5OO”C. Table 1 summarizes the CsHs conversion data. After reaction, the sample was cooled to RT in a flow of hydrocarbon. The formation of coke deposits was evidenced by a change in color from white to dark gray. This change was more pronounced for heptane than for propane. Heating in oxygen after exposure to C3Hs led to evolution of CsHs at 92°C and CO formation at 650°C. Two desorption peaks were found after exposure to C7Ht6, one at 92”C, the other at 200°C; CO formation was again significant at 650°C.
4. Discussion 4.1. Conversion
of PdO particles
to Pd2’ ions
The increase in NO, conversion with time on stream over the reduced and reoxidized Pd/HZSM-5 catalysts indicates slow transformation of Pd from a state
B.J. Adelmn,
WM.H. Sachtler/Applied
0
Catalysis B: Environmental 14 (1997) l-11
100
200
Temperature
300
400
500
in ‘C
Fig. 5. TPR profiles of Pd/HZSM-5 (a) - after reduction and reoxidation; followed by 1 h exposure to (b) - Hz0 (1.5%) or (c) - to Cb + O2
and after steady state conversion
Table 1 CjHs Cracking
the protons, hence, is to maintain Pd in the preferred PdFL,i state. Interestingly, some protonolysis occurs even in the Pd/NaZSM-5 sample after NO, exposure. Presumably, protons, which are formed either upon autoreduction of the Pd(NHs)z complex during the initial calcination step or upon reduction of Pd2+ ions in Hz during the first reduction step, are responsible for this conversion. However, as the concentration of such protons is limited, PdO particles predominate in Pd/NaZSM-5 even after extended exposure to NO,. Protonolysis was not detected with the reduced and reoxidized Pd/HZSM-5 in the absence of NO, at 500°C. Apparently, large PdO particles are formed during reduction followed by reoxidation at 500°C. As shown previously with Pd/HY [23], protonolysis of large PdO particles in zeolites is controlled by the diffusion of protons from more remote regions toward these particles; this rate is, in turn, controlled by the rate of diffusion of Pd2+ ions away from the particle. It thus stands to reason that large PdO particles at the external surface of the zeolite grains undergo very limited protonolysis which stops when the immediate environment is depleted of protons and saturated with
over H/ZSM-5
a
(‘3H8
CXH8 Conversion
to CHq + Cz&
(%)
Cont. b (%)
500°C
525°C
550°C
575°C
625°C
0.1 0.2 0.3
* * 1.0
* 2.6 1.1
2.0 2.8 3.0
8.3 6.2 5.5
38.0 NR 32
a 200 mg sample calcined in UHP O2 at 500°C. ’ Balance He. Total flow rate: 200 ml/min. * Conversion below detection limit. NR: Not recorded.
of lower to higher SCR selectivity. Such a transformation is also manifest from the TPR profile in Fig. 4 for the reduced and reoxidized Pd/HZSM-5 (trace (a)) and the Pd/HZSM-5 and Pd/NaZSM-5 samples after NO, exposure at elevated temperatures (traces (b) and (c)). Clearly, PdO prevails in the reoxidized catalyst, but in the presence of NO,, large PdO particles are transformed into Pd2+ ions in the support. This ability of NO2 to promote protonolysis of PdO to Pd2+ ions has never been reported before. One obvious role of
is obtained with CH4 + NO* + O2
8
B.J. Adelman, W.M.H. Sachtler/Applied
Pd2+ ions. The TPR profile of the ion-exchanged and calcined catalyst shows only a small concentration of PdO particles, the Pd2+ ions prevail; in this sample, no large particles are detected by TEM. It follows that the large PdO particles were formed at a dater stage. Indeed, agglomeration to large (2 100 A) particles has been observed by TEM for samples that had been reduced at 500°C after one redox. cycle at the same temperature. Agglomeration seems to occur mainly when reduction is carried out at higher temperatures; the agglomerated particles are precipitated at the external surface of the zeolite. Similar agglomeration of Pt has been reported when Pt/mordenite samples were reduced at 500°C [24]. The beneficial role of nitric oxides to promote redispersion of PdO particles is a new observation. Previously, Che et al. had shown that redispersion of Pd” particles in HY zeolites occurred upon their exposure to NO at RT [25]. However, in that study the Pd” particles were of the order of 20 A and redispersion involved oxidation of Pd” to Pd2+ ions and reduction of NO to N20 accompanied by evolution of H20. In the present study, protonolysis applies to PdO, of which further oxidation is obviously not possible; moreover the particle sizes in the present study are is in the order of 100 A. On the basis of our previous study of redispersion in zeolites [23], we assume that the beneficial effect of NO, on the protonolysis is basically an acceleration of the diffusion through the zeolite channels. One can speculate that either small Pd(N0s)2 particles or Pd(NOa)+ ions are formed which can diffuse faster than Pd2+ ions. For a zeolite with rather large distances between the Al containing tetrahedra, it stands to reason that monopositively charged complexes will move faster than dipositive ions of similar size. If surface nitration of PdO particles, followed by interaction with zeolite protons, results in Pd(NOa)+ ions, this chemistry could lead to faster exchange with protons in the interior than a mechanism relying on Pdzt or formation of [PdOH]+ ions. As evidenced by the TPR data after 02 and NO, exposure, the state of Pd strongly depends on the pretreatment. This is also seen when one of the three reaction components, CHq + NO2 + 02, is omitted from the feed. Although there is little difference in the steady-state NO, conversion and the initial conversion after one hour of NO2 + 02 exposure, the
Catalysis B: Envimnmental 14 (1997) l-11
initial conversion after a one-hour exposure to CHJ + 02 is significantly lowered and is similar to the initial NO, conversion over the red/reox pretreated Pd/HZSM-5 sample. Exposure to CI& + 02 for one-hour converts Pd2+ ions into PdO particles, as is evidenced by the similarities in the TPR profiles of the reduced and reoxidized sample and of the sample exposed to CH4 + 02. The formation of PdO may be caused either by hydrolysis: Pd2+ + Hz0 --) 2H+ + PdO, or by reduction
followed
Pd2+ + Hydrocarbon
by reoxidation:
---f Pd” + 2H+ + . . .
and Pd” +;02
+ PdO.
In the present case, hydrolysis can be eliminated as the dominant route, since the TPR profile in Fig. 5, trace (b) after one-hour Hz0 exposure shows a marked peak indicative of Pd2+ ions, which is absent in the reoxidized sample (Fig. 5, trace (a)) while the TPR in Fig. 5, trace (c), after C& + 02 exposure does not reveal presence of Pd ions. Therefore, it appears that some intermediate of methane oxidation, CH,O, acts as the reductant of Pd2+ to form Pd” + 2H+. In the presence of 02, the Pd” is rapidly reoxidized to PdO. In the presence of NO,, the PdO/Pd2+ ratio is low for Pd/HZSM-5, but high for Pd/NaZSM-5. This difference in PdO/Pd2+ explains one aspect of the catalysis: protons are required to maintain Pd in the more selective ionic state. This conclusion is in excellent agreement with recent EXAFS results of Resasco [14]. It cannot, however, explain the fact that Pd/NaZSM-5 is inactive for NO, reduction while physical mixtures of PdO/SiOz + SZ are active for SCR [ 151. TPR data show that a small fraction (~20%) of the Pd in Pd/NaZSM-5 is present as ions after exposure to NO2 + 02, yet N2 formation is not detected. Moreover, there is initially 20% NO, conversion (40% of the steady-state value) over the reduced and reoxidized Pd/HZSM-5 sample where the Pd is present as PdO particles either inside or, more likely, outside the HZSM-5 support. Since steady-state conversion is not achieved for several hours, rapid conversion of the oxide to the ion can be excluded. It
B.J. Ad&man, WWH. Sachtler/Applied
now becomes quite clear that although Pd2+ ions are more selective than PdO particles, their presence is not the sole requirement for SCR: protons are also required. 4.2. Interaction
of methane with surface sites
The question arises whether zeolite protons activate the hydrocarbon in a step of kinetic significance for SCR. With higher hydrocarbons, such as heptane, this is a likely first step leading to intermediates which are not formed in the absence of protons. Zeolite protons are known to react with hydrocarbons to form carbonium ions with pentacoordinated carbon [26]. For methane, however, formation of a carbonium ion is unlikely. Still, since N2 formation is higher when NO2 is in the feed [12], one might consider hypothetical reactions such as: CHf + NO2 -+ CHaOH + NO + H+, and CH,f + NOz + CHsNO + HO’ + H+. As methanol is known to be a selective reductant over solid acids such as HZSM-5 [27], one might speculate that its formation from methane would lead to SCR also over the Pd sites. However, if the formation of CHsOH via carbonium ions were crucial for SCR with methane, the formation of N2 over Pd/NaZSM-5 should be much higher when CHaOH is substituted for CH4 in the feed. This hypothesis is discarded on the basis of a test run with methanol in the feed. It showed a N2 formation of only 14%, which is similar to that achieved with CI&, albeit at a much lower temperature, viz. 185°C. Increasing the temperature resulted in lower N2 yields and in complete combustion of methanol. Therefore, it is unlikely that methanol is a reactive intermediate that readily forms over Pd/HZSM-5 but not Pd/NaZSM-5. A possible role of nitrosomethane (CHsNO) was not investigated in the present study. We conclude that interaction of protons with methane is not a probable first step in SCR over these catalysts. The fact that NO, reduction does occur over metal-free HZSM-5 [28] is then rationalized by assuming that methane can react with NO, groups associated with protons; subsequent steps ultimately lead to the formation of NZ. The activity and selectiv-
Catalysis B: Environmental 14 (1997) l-11
9
ity of Pd/HZSM-5 [12] or of physical mixtures [14], are, however, higher than those of HZSM-5. It thus appears that interaction of methane with Pd is faster than the rate limiting step with HZSM-5. Acid sites, though not involved in the first step of H abstraction from methane, may be important in subsequent steps. In other words, this catalyst acts as a bifunctional catalyst. 4.3. Interaction
of propane with sur$ace sites
The cracking data show that significant C3H8 cracking to CHq + CzI& occurs even at low hydrocarbon concentrations. However, no reaction of C3Hs was detected below 500°C over HZSM-5. This sharply contrasts with the high reactivity of CsHs at 300°C over Pd/ZSMJ, in the presence of NO2 and 02. Apparently, the Pd sites are essential for the activation of CsHs and the rate of its oxidation with O2 is significantly larger than that of NO reduction. This trend is opposite to that found for CH4 conversion. The low reduction level of NO, to N2 with propane as compared to methane at similar CO2 formation levels thus results from a higher rate of the nonselective reaction between hydrocarbon and 02, which is facilitated by Pd. Over the Pd/HZSM-5 (or Ga/HZSM-5 and In/HZSM-5), the formation of N2 thus cannot be correlated with the formation of CO2 at constant carbon flux. This pattern contrasts to that of Cu/ZSM-5 and Co/ZSM-5 [7]. Even though the rate determining step for propane combustion may be Habstraction for all these catalysts, the absence of the said correlation indicates that different sites are used for propane and methane. Hamada et al. found that C3Hs conversion over HZSM-5 at 400°C was 26% [29]. As no propane cracking is observed at that temperature, this indicates that in the presence of NO, + 02 propane is activated on proton-associated NO, sites, NOT ‘naked’ zeolite protons. In this respect propane and methane share a similar fate. Experiments where methane was omitted from the feed, after steady-state conversion of CHq + NO2 + 02 had been attained, show no detectable formation of N2. This strongly suggests that surface-deposited hydrocarbonaceous layers are not a significant reductant under the conditions of NO, reduction with methane. However, in the case of propane formation, ‘coke’ does occur, although no
10
B.J. Adelman, WMH. Sachtler/Applied
cracking products are observed in the gas phase below 500°C. A role of ‘soft coke’ has been proposed in the literature as a reactive intermediate in the SCR of NO, [30]. Indeed, it appears likely that when higher hydrocarbons are used more reactive coke can be deposited. However, in the presence of Pd, the oxidation of this coke by NO, will have to compete with its Pd catalyzed oxidation by Oz. PdO particles and, to a slightly lesser extent, Pd2+ ions also catalyze combustion of propane with 02 to CO2 + H20. This combustion is somewhat inhibited by the adsorption of NO,, which competes with O2 for the same sites. Still, the oxidation rate is higher with propane than with methane. Since this oxidation does not produce N2, the SCR of NO, is lower with propane than with methane.
Catalysis B: Environmental 14 (1997) 1-11
free HZSM-5 induces only negligible catalysis with propane or methane. Obviously, protons in Pd/HZSM-5 do not induce the primary step of Habstraction, but they interact with reaction intermediates formed over Pd or NO, sites. Over Pd, propane is more easily oxidized than methane. Therefore, methane is a more selective reductant for the SCR of NO, to N2 over these catalysts.
Acknowledgements We gratefully acknowledge financial support from the Director of the Chemistry Division, Basic Energy Sciences, U.S. Department of Energy; Grant DEFG02-87ER13654, and a grant-in-aid from the Ford Motor Corporation.
5. Conclusions References Pd/HZSM-5 is an active and selective catalyst for the reduction of NO, to N2 with CH4 as reductant, but Pd/NaZSM-5 only catalyzes CH4 combustion. The protons in Pd/HZSM-5 affect the activity of this catalyst in two ways, besides promoting dealumination of the zeolite. First, they stabilize Pd*+ ions vs. PdO particles in an oxidizing atmosphere. The ‘protonolysis’ PdO + 2H+ = Pd2+ + H20 is not a facile reaction when the PdO particles are large; however, it is dramatically accelerated in an atmosphere of NO + 02 . For the SCR of NO, with methane, Pd *+ ions are more active sites than PdO particles; the slow protonolysis, therefore, implies a slow increase in SCR selectivity with time on-stream. In contrast, the combustion of CH4 to CO + CO2 and H20 is time-invariant. The second role of the zeolite protons resides in the reaction mechanism of SCR. It is an example of bifunctional catalysis, i.e. some reaction steps make use of protons. Pd/NaZSM-5 samples which contain a small fraction of Pd*+ ions, but very few protons, are virtually inactive for SCR of NO,. Conversely, Pd/ HZSM-5 samples that mainly contain Pd as PdO particles display significant NO, reduction activity. At temperatures where SCR of NO, or combustion of CsHs are important over Pd/HZSM-5, the metal-
111M. Iwamoto, H. Yahiro, Y. Yu-U, S. Shundo, N. Mizuno, Shokubai
32 (1990) 430.
121M. Iwamoto, Proc. Meeting of Catalytic Technology for the Removal of Nitrogen Monoxide, Tokyo, 1990, p. 17. [31 W. Held, A. Konig, T. Richter, L. Puppe, SAE Paper 900496, 1990. [41 Y. Li, J.N. Armor, Appl. Catal. B 1 (1992) L31. [51 Y. Li, J.N. Armor, Appl. Catal. B 2 (1993) 239. [cl R. Burch, S. Scire, Appl. Catal. B 3 (1994) 295. [71 F. Witzel, G.A. Sill, W.K. Hall, J. Catal. 149 (1994) 229. PI Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [91 T. Beutel, B.J. Adelman, G.-D. Lei, W.M.H. Sachtler, Catal. Lett. 32 (1995) 83. [lOI B.J. Adelman, T. Beutel, G.-D. Lei, W.M.H. Sachtler, J. Catal. 158 (1996) 327. Ull M. Iwamoto, H. Hamada, Catal. Today 10 (1991) 57. [17-l Y. Nishizaka, M. Misono, Chem. Lett. (1994) 2237. H. Trevitio, [I31 D.C. Tomczak, G.-D. Lei, V. Schlinemann, W.M.H. Sachtler, Micropor. Mater. 5 (1996) 263. H41 D.E. Resasco, W.A. Alvarez, A. Ali, C. Loughran, Proc. XV. Ibero-American Symp. Catalysis, Cordoba, Argentina, Sept. 16, 1996, p. 59. [I51 C.J. Loughran, D.E. Resasco, Appl. Catal. B 7 (1995) 113. Cl61 T. Sane, K. Suzuki, H. Shoji, S. Ikai, K. Okabe, T. Murakami, S. Shin, H. Hagiwara, H. Takaya, Chem. Lett. (1987) 1421. Cl71 J.Y. Yan, G.-D. Lei, W.M.H. Sachtler, H.H. Kung, J. Catal. 161 (1996) 43. [181 K. Yogo, M Ihara, I. Terasaki, E. Kikuchi, Chem. Lett. (1993) 229. Cl91 K. Yogo, M. Ihara, I. Terasaki, E. Kikuchi, Appl. Catal. B 2 (1993) Ll.
B.J. Adelman, W4.H.
Sachtler/Applied
(201 K. Yogo, M. Ogura, N. Aratani, Y. Suguira, E. Kikuchi, Abstracts, 2nd Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT 2). Tokyo, 1994, p. 399. [21] ST. Homeyer, W.M.H. Sachtler, J. Catal. 117 (1989) 91. 1221 B.T. Carvill, B.A. Lemer, Z.C. Zhang, W.M.H. Sachtler, J. Catal. 143 (1993) 314. 1231 0. Feeley, W.M.H. Sachtler, Appl. Catal. 67 (1990) 141. 1241 B.T. Carvill, B.A. Lemer, B.J. Adelman, DC. Tomczak, W.M.H. Sachtler, .I. Catal. 144 (1993) 1. 1251 M. Che, J.F. Dutel, P. Gallezot, M. Primet, J. Phys. Chem. 80 (1976) 2371.
Catalysis B: Environmental 14 (1997) l-11
11
[26] W.O. Haag, R.M. Dessau, Proc. 8th Int. Congr. Catal., vol. 2, 1984, p. 305. [27] H. Hamada, T. Kintaichi, T. Yoshinari, M. Tabata, M. Sasaki, T. Ito, Catal. Today 17 (1993) 111. [28] K. Yogo, M. Umeno, H. Watanabe, E. Kikuchi, Catal. Lett. 19 (1993) 131. [29] H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito, M. Tabata, Appl. Catal. 70 (1991) L15. [30] G.P. Ansell, A.F. Diwell, S.E. Golunski, J.W. Hayes, R.R. Rajaram, T.J. Truex, A.P. Walker, Appl. Catal. B 2 (1993) 81.
ELSEVIER
Applied Catalysis
B: Environmental
14 (1997) l-l 1
The effect of zeolitic protons on NO, reduction over Pd/ZSM-5 catalysts B.J. Adelmana,*, W.M.H. Sachtler” a VN. Ipatieff Laboratory,
Center for Catalysis and Surface Science, Department Evanston, IL 60208-3000, USA
Received 25 September
of Chemistry, Northwestern
University,
1996; received in revised form 19 November 1996; accepted 2 December 1996
Abstract Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 and Pd/NaZSM-5 after reduction and reoxidation at 500°C. Formation of Pd’+ ions is not observed in Pd/HZSM-5 in the absence of NO,, but in an atmosphere of NO;! + 02 the PdO particles are transformed into Pd ‘+ ions. Even in Pd/NaZSM-5, roughly 20% of the PdO particles are converted to ions in NO,; the protons needed for this process are mainly formed during reduction of Pd*+ to Pd’. When a reduced and reoxidized Pd/HZSM-5 catalyst is exposed to CHq + NO2 + 02, the rate of catalytic NO2 reduction to N2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd*+, but the rate of C& oxidation remains constant with time on-stream. .4pparently, Pd *+ ions are needed for SCR of NO,, but for methane oxidation with 02, Pd*+ ions and PdO particles have similar activity. Pd/HZSM-5 is active even when Pd is initially present as PdO particles, but Pd/NaZSM-5 remains inactive
also when part of the Pd is present as Pd 2+ ions. This shows that protons are not only required to convert PdO into Pd*+, but also that Pd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3Hs activation are discussed. 0 1997 Elsevier Science B.V. Keywords:
Effect of protons; NO, reduction; Pd/ZSM-5
1. Introduction
Metal-exchanged zeolite catalysts have been proven active in the selective catalytic reduction (SCR) of NO, by hydrocarbons [l-6]. The addition of O2 enhances the rate of NO, reduction [2,3]. In a previous work, it was shown that formation of adsorbed nitrogen oxides of the general formula NO, is the main cause of this enhancement [7-91. The NO, groups are the sites for H-abstraction from paraffin molecules.
*Correspondingauthor. 0926~860X/97/$17.000 1997 Elsevier Science B.V. All rights reserved. PZI SO926-3373(97)00007-6
However, the structure and reactivity of these groups depends upon the nature of the exchanged metal ions [lo]. With Cu/ZSM-5, the NO, sites are nitro and nitrate groups [lo]; they are reduced by C3+ paraffins or Cz+ olefins, but not by CH,; therefore, this material does not catalyze the reduction of NO, to N2 with methane [ 111. Conversely, over M/ZSM-5 catalysts with M = Co, Ni, Mn, or Rh, the NO, complexes abstract H atoms from CH,; consequently, these materials catalyze the SCR of NO, with CH4 and higher hydrocarbons [4-61. Recently, a different hydrocarbon specificity has been discovered over Pd exchanged ZSM-5
2
B.J. Adelman, W.M.H. Sachtler/Applied
catalysts [ 121. Palladium has been shown to be active when exchanged into H/ZSM-5, but not Na/ZSM-5. The cause of this difference is not fully understood. Two plausible explanations can be suggested. First, protons are required to convert Pd oxide particles into Pd2+ ions, and these ions are required for catalysis. This conversion of oxides into ions via reaction with zeolitic protons is well known and is called ‘protonolysis’ [13]. Recently, Resasco et al. showed by EXAFS that in their best Pd/HZSM-5 catalysts the Pd was mainly present as Pd2+ ions [ 141. An alternative hypothesis, not inconsistent with the former, assumes that a bifunctional mechanism is operating with this catalyst system. Indeed, Resasco et al. in a different paper showed that even physical mixtures of Pd/SiO:! and sulfated zirconia (SZ) were active for the SCR of NO,, though the conversion levels were lower than for metalexchanged zeolite samples [15]. Protons are, of course, undesirable in practical catalysts because they induce dealumination of the zeolite in the HzO-rich atmosphere of engine exhausts [ 16,171; but the observation that for some catalysts zeolite protons are necessary for SCR activity of NO, [ 12,18-201 is of possible relevance with respect to the reaction mechanism. Upon exposing Pd/HZSM-5 to a gas how containing N02, O2 and a hydrocarbon at an appropriate temperature, formation of N2 and CO2 is observed. When the hydrocarbon is varied, while the carbon flux is kept constant, a positive correlation has been observed between the formations of N2 and CO2 for a number of ZSM supported catalysts, including HZSM-5 [7]. However, no such correlation holds for Pd/HZSM-5. For instance, with 3000 ppm CH4 or 1000 ppm CsHs, the NO, reduction to N2 is significantly lower with C3H8 than with CH4, but the CO2 production is higher with CsHs. The same seems to be true for ZSM-5 supported Ga [ 16,171 and In [ 181 catalysts, which also require the acid form, HZSM5, of the support. This suggests that the reaction mechanism over these catalysts may differ significantly from that which is valid for other ZSM-5 supported catalyst. The objective of the present study is to investigate the relevant chemistry of Pd/ZSM-5 in order to get information on the reaction mechanism of SCR over Pd/HZSM-5.
Catalysis B: Environmental 14 (1997) l-11
2. Experimental 2.1. Catalyst preparation H/ZSM-5 was prepared via ion exchange at room temperature (RT) using a threefold excess of NHdNOs solution added to a slurry of Na/ZSM-5 (UOP lot #13023-60) in water. After 24 h, the slurry was vacuum filtered, washed with one-liter doubly deionized H20 and air dried. The sample was then calcined in UHP O2 to 500°C. Pd/HZSM-5 and Pd/NaZSM-5 were prepared via ion exchange at RT using a dilute (NH&Pd(NO& solution added to a HZSM-5 or a Na/ZSM-5 slurry, respectively. The slurries were stirred for 72 h prior to being vacuum filtered, washed with one-liter doubly deionized Hz0 and air dried. Exchange was performed over 72 h to ascertain a uniform distribution of Pd ions throughout the zeolite particles. In a previous work with Rh, it was found that after short exchange times a concentration gradient exists with more metal ions near the periphery than near the center of the zeolite analysis via inductively grains [ 131. Elemental coupled plasma spectroscopy (Therm0 Jarrel Ash, Atomscan 25 spectrometer) gave the following data: Pd/HZSM-5: Si/Al = 18, Pd/Al = 0.093, Na/Al = 0.0; Pd/NaZSM-5: Si/Al = 18, Pd/Al = 0.105, Na/Al = 0.72. 2.2. Reaction
studies
Pd samples were tested in a continuous-flow microreactor. The samples were pretreated as follows: calcination in UHP O2 to 500°C reduction in 5% H2 to 500°C and re-oxidation in UHP 02 to 500°C; purging with He was applied between these treatments. The results after the initial calcination differed from those obtained upon reduction and reoxidation, indicating different catalytic performances of Pd2+ ions and PdO particles. A series of tests was performed with feed compositions of 1000 ppm N02, 3000 ppm CH4 and 20000 ppm O2 with He as a diluent. In another test series, one of these components was omitted from the feed after steady-state conversion was attained. The activity was also measured upon subsequently reintroducing that component to the feed. The effluent gas was monitored with a Hewlett-Packard 5890 GC with Alltech 13X molecular sieve and Porapak Q columns.
B.J. Adelman, Wb4.H. Sachtler/Applied
Catalysis B: Environmental 14 (1997) I-11
Upon bypassing the catalyst bed, the initial concentrations of 02, NO2 and CH4 were determined. The conversion of NO, was calculated from the measured formation of NZ. The measured conversion of CH4 was checked against the measured formation of CO, in the effluent. 2.3. Temperature-programmed
reduction (TPR)
The procedure for monitoring temperature-programmed reduction (TPR) of zeolite supported catalysts has been described previously [21]. Hydrogen profiles for Pd/HZSM-5 and consumption Pd/NaZSM-5 were recorded after each of the following pretreatments: (1) calcination in UHP 02, (2) reoxidation in UHP O2 after reduction, and (3) thermal treatment in NO2 + 02 after reduction. All treatments were carried out at 500°C; in case of the NO, treatments, this temperature was held for more than 18 h.
3. Results First, Pd/HZSM-5 catalysts were tested that had only been calcined. No induction period was observed
3
with these catalysts. For a Pd/HZSM-5 sample that had been reduced and re-oxidized after calcination, Fig. 1 shows the NOz and CH4 conversions at 500°C. Clearly, the conversions of both reactants depend in a strikingly different manner on the reaction time, the CH4 conversion remains at a constant level of SO%, but the NO, conversion to N2 rises from an initial value of 20% over the course of 24 h to a steady-state value of 50%. After attaining steady state, the feed was switched from CHq + NO2 + 02 to NO2 + 02. Neither N2 nor CO2 was detected, as is obvious from Fig. 2(A). After one hour, the three component mixture, namely NO2 + CH4 + 02, was reintroduced. Steady-state conversions were rapidly reattained. Fig. 2 (B) shows the effect of omitting NO2 from the feed. In this case N2 formation ceased, but the rate of CH4 combustion increased. A similar negative effect of NO, on the rate of hydrocarbon combustion has been found previously with Cu/ZSM-5 [lo]. Unlike the previous case, returning from a CH4 + 02 feed to the three-component feed did not immediately re-establish the steady-state activity; it took several hours to attain the original steady-state conversion of NO,. Similar to earlier observations by Misono et al. [ 121, our Pd/NaZSM-5 exhibits very low
100
a’ (b)
0
200
400
600
800
1000
1200
1400
Time in Minutes Fig. 1. Conversion [CHd] = 3000ppm,
of (a) - NO1 to Nz and (b) - C& to CO2 over Pd/HZSM-5, that had been reduced and re-oxidized, [NO*] = 1000 ppm, and [O,] = 20000 ppm. Reaction temperature = 5OOT, and GHSV = 36000 h,
vs. time.
B.J. Ad&mm,
A
Catalysis B: Environmental
14 (1997) 1-11
100 90
--
80 77
,I 5 !!!
W34.H. Sachtler/Applied
1
II
III
.
r---
70 -60 --
s ii
50 --
0
40 --
s
30 --
._
.
.
. .
20 --
= CH4 conversion = NO* conversion
?? ??
10 -0-i
.
-?-
I
0
50
100
150
I 200
250
300
1000
1200
Time in Minutes
B
100 90 80
g ._ i!z
70 60
g
50
s 0 s
40 30 20 10 0 0
200
400
600
800
Time in Minutes Fig. 2. (A) Conversion of (m) - NO, to N2, and (0) - CH4 to CO? over Pd/HZSM-5, after steady state had been reached, vs. time. Feed composition I: [CHd] = 3000ppm, [NO*] = lOOOppm, and [02] = 20000ppm; feed composition II: [NO21 = lOOOppm, and [Oz] = 20000ppm; feed composition III: [C&l = 3000ppm, [NO21 = lOOOppm, and [Oz] = 20000ppm. Temperature = 5OO”C, and GHSV = 36000 h’. (B) Conversion of ( ?? ) - NO, to N2 and (0) - CH4 to CO2 over Pd/HZSM-5, after steady state had been reached, vs. time. Feed composition I: [C&l = 3000ppm, [NO*] = lOOOppm, and [Oz] = 20000ppm; feed composition II: [Cb] = 3000ppm. and [Oz] = 20000ppm; feed composition III: [CHh] = 3000ppm. [NOz] = lOOOppm, and [02] = 20000ppm. Temperature = 500°C; GHSV = 36000 h'
B.J. Adelman,
WMH.
Sachtler/Applied
Catalysis B: Environmental
14 (1997) 1-11
8000
6000
4000
2000
0 100
200
Temperature Fig. 3. TPR of Pd/HZSM-5 reoxidation at 500°C.
after calcination
300
in “C
at 500°C of (a) - a fresh Pd/HZSM-5
activity for NO, reduction while it does catalyze the oxidation of methane. Fig. 3 shows the TPR profile of a freshly calcined Pd/HZSM-5 sample (a) and the TPR profile of a Pd/HZSM-5 sample after reduction and reoxidation (b). The peaks below 0°C in these profiles are artifacts caused by desorption of Ar. The peak at 20°C in trace (b) is ascribed to PdO particles; this assignment is confirmed by the absence of this peak upon reducing the sample with CO at 350°C prior to the TPR run (not shown). In trace (a) this peak is small; instead, a broad feature is observed ranging from 90” to 250°C. This is attributed to reduction of Pd*+ ions. A similar broad reduction profile was observed previously with Pd/H mordenite [22]. The Pd2+ ions are formed either by release of the NH3 ligands from the Pd(NH&*+ ions or by the sequence: autoreduction to Pd’, followed by oxidation to PdO, followed by protonolysis. The negative peak near 80°C in trace (b) is characteristic of the decomposition of the Pd-P-hydride; its appearance confirms that Pd was formed during reduction of PdO. The formation and decomposition of this hydride is a familiar feature in Hz-TPR of Pd [21]. Fig. 4 shows the TPR profiles of Pd/HZSM-5 after reduction and reoxidation at 500°C (a) as well as that
sample,
and (b) - after subsequent
reduction
and
of Pd/HZSM-5 (b) and Pd/NaZSM-5 (c) after pretreatment in NO2 + 02 streams at 500°C for 2 h. Remarkably, no formation of Pd2+ ions is observed in Pd/HZSM-5 after the reduction/reoxidation treatment, even when the temperature was held at 500°C for over 48 h (not shown). Notwithstanding the abundance of protons, no TPR peak is observed near 100°C in trace (a). However, trace (b) shows that exposure to NO2 induces the conversion of PdO particles to Pd2+ ions; and the H2 consumption peak is shifted upward by 100°C with respect to trace (a) in Fig. 3. In the absence of a high proton concentration, no protonolysis occurs, i.e. the mixture of NO, leaves the PdO particles apparently almost unaffected, as follows from the large TPR peak near 50°C in trace (c), which is only slightly shifted in comparison to the peaks in trace (b) in Fig. 3 (to facilitate comparison this profile is repeated in Fig. 4 as trace (a)). Possible causes for the absence of protonolysis in Pd/HZSM-5 and its occurrence when protons and NO, are both present will be discussed below. The Hz consumption at 200°C of the Pd/HZSM-5 that was exposed to NO, is slightly greater than that required for the reduction of the Pd’+/PdO mixture to Pd’. This indicates formation of Pd’+-NO, complexes and co-reduction of NO, and Pd2+.
B.J. Adelman, W2f.H. Sachtler/Applied
Catalysis B: Environmental 14 (1997) I-11
18000
_
.
16000
z -
14000
g '3 5 Q
12000
E
8000
s
6000
I"
4000
10000
2000 0 -100
0
100
200
Temperature Fig. 4. TPR profile of Pd/HZSM-5 (a) - after reduction and re-oxidation after NO2 + 0, exposure at 500°C.
In view of the observation that temporary exposure of the catalyst to an atmosphere of CH4 + 02 lowered the catalytic activity, it was of interest to study Pd/HZSM-5 samples treated in this manner by TPR and to compare them to samples that had been reduced and re-oxidized. These results are reported in Fig. 5. Profile (a) of the reduced and reoxidzed sample shows abundance of PdO particles. A sample which was brought to steady state shows the typical signature of the coexistence of PdO particles and Pdzf ions. Even if such a sample is afterward exposed to Ha0 for one hour, ions are not converted to oxide particles, as is clear from the TPR profile (b) in this figure. In contrast, when NO2 is omitted from the gas atmosphere, again a population of abundant PdO particles is identified in the TPR profile (c), which is quite similar to (a), lacking the TPR peak characteristic of Pd2+ tons. The data clearly indicate that PdO is formed during the catalytic process, but NOz is instrumental for the protonolysis transforming PdO particles back into Pdzf ions. To identify the interaction of hydrocarbons with protons in the absence of a metal or metal ions, a calcined H/ZSM-5 sample was exposed to dilute streams of propane (0.1 to 0.4%) or heptane. The latter hydrocarbon was used because isomerization
300
400
500
in OC
at 500°C; (b) - TPR profiles of Pd/HZSM-5;
and (c) - Pd/NaZSM-5
and cracking can be expected to be more pronounced than with methane. In both cases, formation of CH4 and CzH4 was found to be significant; with C,HiG a variety of hydrocarbons were detected. Whereas heptane cracking was significant below 4OO”C, cracking of propane could be detected only at temperatures >5OO”C. Table 1 summarizes the CsHs conversion data. After reaction, the sample was cooled to RT in a flow of hydrocarbon. The formation of coke deposits was evidenced by a change in color from white to dark gray. This change was more pronounced for heptane than for propane. Heating in oxygen after exposure to C3Hs led to evolution of CsHs at 92°C and CO formation at 650°C. Two desorption peaks were found after exposure to C7Ht6, one at 92”C, the other at 200°C; CO formation was again significant at 650°C.
4. Discussion 4.1. Conversion
of PdO particles
to Pd2’ ions
The increase in NO, conversion with time on stream over the reduced and reoxidized Pd/HZSM-5 catalysts indicates slow transformation of Pd from a state
B.J. Adelmn,
WM.H. Sachtler/Applied
0
Catalysis B: Environmental 14 (1997) l-11
100
200
Temperature
300
400
500
in ‘C
Fig. 5. TPR profiles of Pd/HZSM-5 (a) - after reduction and reoxidation; followed by 1 h exposure to (b) - Hz0 (1.5%) or (c) - to Cb + O2
and after steady state conversion
Table 1 CjHs Cracking
the protons, hence, is to maintain Pd in the preferred PdFL,i state. Interestingly, some protonolysis occurs even in the Pd/NaZSM-5 sample after NO, exposure. Presumably, protons, which are formed either upon autoreduction of the Pd(NHs)z complex during the initial calcination step or upon reduction of Pd2+ ions in Hz during the first reduction step, are responsible for this conversion. However, as the concentration of such protons is limited, PdO particles predominate in Pd/NaZSM-5 even after extended exposure to NO,. Protonolysis was not detected with the reduced and reoxidized Pd/HZSM-5 in the absence of NO, at 500°C. Apparently, large PdO particles are formed during reduction followed by reoxidation at 500°C. As shown previously with Pd/HY [23], protonolysis of large PdO particles in zeolites is controlled by the diffusion of protons from more remote regions toward these particles; this rate is, in turn, controlled by the rate of diffusion of Pd2+ ions away from the particle. It thus stands to reason that large PdO particles at the external surface of the zeolite grains undergo very limited protonolysis which stops when the immediate environment is depleted of protons and saturated with
over H/ZSM-5
a
(‘3H8
CXH8 Conversion
to CHq + Cz&
(%)
Cont. b (%)
500°C
525°C
550°C
575°C
625°C
0.1 0.2 0.3
* * 1.0
* 2.6 1.1
2.0 2.8 3.0
8.3 6.2 5.5
38.0 NR 32
a 200 mg sample calcined in UHP O2 at 500°C. ’ Balance He. Total flow rate: 200 ml/min. * Conversion below detection limit. NR: Not recorded.
of lower to higher SCR selectivity. Such a transformation is also manifest from the TPR profile in Fig. 4 for the reduced and reoxidized Pd/HZSM-5 (trace (a)) and the Pd/HZSM-5 and Pd/NaZSM-5 samples after NO, exposure at elevated temperatures (traces (b) and (c)). Clearly, PdO prevails in the reoxidized catalyst, but in the presence of NO,, large PdO particles are transformed into Pd2+ ions in the support. This ability of NO2 to promote protonolysis of PdO to Pd2+ ions has never been reported before. One obvious role of
is obtained with CH4 + NO* + O2
8
B.J. Adelman, W.M.H. Sachtler/Applied
Pd2+ ions. The TPR profile of the ion-exchanged and calcined catalyst shows only a small concentration of PdO particles, the Pd2+ ions prevail; in this sample, no large particles are detected by TEM. It follows that the large PdO particles were formed at a dater stage. Indeed, agglomeration to large (2 100 A) particles has been observed by TEM for samples that had been reduced at 500°C after one redox. cycle at the same temperature. Agglomeration seems to occur mainly when reduction is carried out at higher temperatures; the agglomerated particles are precipitated at the external surface of the zeolite. Similar agglomeration of Pt has been reported when Pt/mordenite samples were reduced at 500°C [24]. The beneficial role of nitric oxides to promote redispersion of PdO particles is a new observation. Previously, Che et al. had shown that redispersion of Pd” particles in HY zeolites occurred upon their exposure to NO at RT [25]. However, in that study the Pd” particles were of the order of 20 A and redispersion involved oxidation of Pd” to Pd2+ ions and reduction of NO to N20 accompanied by evolution of H20. In the present study, protonolysis applies to PdO, of which further oxidation is obviously not possible; moreover the particle sizes in the present study are is in the order of 100 A. On the basis of our previous study of redispersion in zeolites [23], we assume that the beneficial effect of NO, on the protonolysis is basically an acceleration of the diffusion through the zeolite channels. One can speculate that either small Pd(N0s)2 particles or Pd(NOa)+ ions are formed which can diffuse faster than Pd2+ ions. For a zeolite with rather large distances between the Al containing tetrahedra, it stands to reason that monopositively charged complexes will move faster than dipositive ions of similar size. If surface nitration of PdO particles, followed by interaction with zeolite protons, results in Pd(NOa)+ ions, this chemistry could lead to faster exchange with protons in the interior than a mechanism relying on Pdzt or formation of [PdOH]+ ions. As evidenced by the TPR data after 02 and NO, exposure, the state of Pd strongly depends on the pretreatment. This is also seen when one of the three reaction components, CHq + NO2 + 02, is omitted from the feed. Although there is little difference in the steady-state NO, conversion and the initial conversion after one hour of NO2 + 02 exposure, the
Catalysis B: Envimnmental 14 (1997) l-11
initial conversion after a one-hour exposure to CHJ + 02 is significantly lowered and is similar to the initial NO, conversion over the red/reox pretreated Pd/HZSM-5 sample. Exposure to CI& + 02 for one-hour converts Pd2+ ions into PdO particles, as is evidenced by the similarities in the TPR profiles of the reduced and reoxidized sample and of the sample exposed to CH4 + 02. The formation of PdO may be caused either by hydrolysis: Pd2+ + Hz0 --) 2H+ + PdO, or by reduction
followed
Pd2+ + Hydrocarbon
by reoxidation:
---f Pd” + 2H+ + . . .
and Pd” +;02
+ PdO.
In the present case, hydrolysis can be eliminated as the dominant route, since the TPR profile in Fig. 5, trace (b) after one-hour Hz0 exposure shows a marked peak indicative of Pd2+ ions, which is absent in the reoxidized sample (Fig. 5, trace (a)) while the TPR in Fig. 5, trace (c), after C& + 02 exposure does not reveal presence of Pd ions. Therefore, it appears that some intermediate of methane oxidation, CH,O, acts as the reductant of Pd2+ to form Pd” + 2H+. In the presence of 02, the Pd” is rapidly reoxidized to PdO. In the presence of NO,, the PdO/Pd2+ ratio is low for Pd/HZSM-5, but high for Pd/NaZSM-5. This difference in PdO/Pd2+ explains one aspect of the catalysis: protons are required to maintain Pd in the more selective ionic state. This conclusion is in excellent agreement with recent EXAFS results of Resasco [14]. It cannot, however, explain the fact that Pd/NaZSM-5 is inactive for NO, reduction while physical mixtures of PdO/SiOz + SZ are active for SCR [ 151. TPR data show that a small fraction (~20%) of the Pd in Pd/NaZSM-5 is present as ions after exposure to NO2 + 02, yet N2 formation is not detected. Moreover, there is initially 20% NO, conversion (40% of the steady-state value) over the reduced and reoxidized Pd/HZSM-5 sample where the Pd is present as PdO particles either inside or, more likely, outside the HZSM-5 support. Since steady-state conversion is not achieved for several hours, rapid conversion of the oxide to the ion can be excluded. It
B.J. Ad&man, WWH. Sachtler/Applied
now becomes quite clear that although Pd2+ ions are more selective than PdO particles, their presence is not the sole requirement for SCR: protons are also required. 4.2. Interaction
of methane with surface sites
The question arises whether zeolite protons activate the hydrocarbon in a step of kinetic significance for SCR. With higher hydrocarbons, such as heptane, this is a likely first step leading to intermediates which are not formed in the absence of protons. Zeolite protons are known to react with hydrocarbons to form carbonium ions with pentacoordinated carbon [26]. For methane, however, formation of a carbonium ion is unlikely. Still, since N2 formation is higher when NO2 is in the feed [12], one might consider hypothetical reactions such as: CHf + NO2 -+ CHaOH + NO + H+, and CH,f + NOz + CHsNO + HO’ + H+. As methanol is known to be a selective reductant over solid acids such as HZSM-5 [27], one might speculate that its formation from methane would lead to SCR also over the Pd sites. However, if the formation of CHsOH via carbonium ions were crucial for SCR with methane, the formation of N2 over Pd/NaZSM-5 should be much higher when CHaOH is substituted for CH4 in the feed. This hypothesis is discarded on the basis of a test run with methanol in the feed. It showed a N2 formation of only 14%, which is similar to that achieved with CI&, albeit at a much lower temperature, viz. 185°C. Increasing the temperature resulted in lower N2 yields and in complete combustion of methanol. Therefore, it is unlikely that methanol is a reactive intermediate that readily forms over Pd/HZSM-5 but not Pd/NaZSM-5. A possible role of nitrosomethane (CHsNO) was not investigated in the present study. We conclude that interaction of protons with methane is not a probable first step in SCR over these catalysts. The fact that NO, reduction does occur over metal-free HZSM-5 [28] is then rationalized by assuming that methane can react with NO, groups associated with protons; subsequent steps ultimately lead to the formation of NZ. The activity and selectiv-
Catalysis B: Environmental 14 (1997) l-11
9
ity of Pd/HZSM-5 [12] or of physical mixtures [14], are, however, higher than those of HZSM-5. It thus appears that interaction of methane with Pd is faster than the rate limiting step with HZSM-5. Acid sites, though not involved in the first step of H abstraction from methane, may be important in subsequent steps. In other words, this catalyst acts as a bifunctional catalyst. 4.3. Interaction
of propane with sur$ace sites
The cracking data show that significant C3H8 cracking to CHq + CzI& occurs even at low hydrocarbon concentrations. However, no reaction of C3Hs was detected below 500°C over HZSM-5. This sharply contrasts with the high reactivity of CsHs at 300°C over Pd/ZSMJ, in the presence of NO2 and 02. Apparently, the Pd sites are essential for the activation of CsHs and the rate of its oxidation with O2 is significantly larger than that of NO reduction. This trend is opposite to that found for CH4 conversion. The low reduction level of NO, to N2 with propane as compared to methane at similar CO2 formation levels thus results from a higher rate of the nonselective reaction between hydrocarbon and 02, which is facilitated by Pd. Over the Pd/HZSM-5 (or Ga/HZSM-5 and In/HZSM-5), the formation of N2 thus cannot be correlated with the formation of CO2 at constant carbon flux. This pattern contrasts to that of Cu/ZSM-5 and Co/ZSM-5 [7]. Even though the rate determining step for propane combustion may be Habstraction for all these catalysts, the absence of the said correlation indicates that different sites are used for propane and methane. Hamada et al. found that C3Hs conversion over HZSM-5 at 400°C was 26% [29]. As no propane cracking is observed at that temperature, this indicates that in the presence of NO, + 02 propane is activated on proton-associated NO, sites, NOT ‘naked’ zeolite protons. In this respect propane and methane share a similar fate. Experiments where methane was omitted from the feed, after steady-state conversion of CHq + NO2 + 02 had been attained, show no detectable formation of N2. This strongly suggests that surface-deposited hydrocarbonaceous layers are not a significant reductant under the conditions of NO, reduction with methane. However, in the case of propane formation, ‘coke’ does occur, although no
10
B.J. Adelman, WMH. Sachtler/Applied
cracking products are observed in the gas phase below 500°C. A role of ‘soft coke’ has been proposed in the literature as a reactive intermediate in the SCR of NO, [30]. Indeed, it appears likely that when higher hydrocarbons are used more reactive coke can be deposited. However, in the presence of Pd, the oxidation of this coke by NO, will have to compete with its Pd catalyzed oxidation by Oz. PdO particles and, to a slightly lesser extent, Pd2+ ions also catalyze combustion of propane with 02 to CO2 + H20. This combustion is somewhat inhibited by the adsorption of NO,, which competes with O2 for the same sites. Still, the oxidation rate is higher with propane than with methane. Since this oxidation does not produce N2, the SCR of NO, is lower with propane than with methane.
Catalysis B: Environmental 14 (1997) 1-11
free HZSM-5 induces only negligible catalysis with propane or methane. Obviously, protons in Pd/HZSM-5 do not induce the primary step of Habstraction, but they interact with reaction intermediates formed over Pd or NO, sites. Over Pd, propane is more easily oxidized than methane. Therefore, methane is a more selective reductant for the SCR of NO, to N2 over these catalysts.
Acknowledgements We gratefully acknowledge financial support from the Director of the Chemistry Division, Basic Energy Sciences, U.S. Department of Energy; Grant DEFG02-87ER13654, and a grant-in-aid from the Ford Motor Corporation.
5. Conclusions References Pd/HZSM-5 is an active and selective catalyst for the reduction of NO, to N2 with CH4 as reductant, but Pd/NaZSM-5 only catalyzes CH4 combustion. The protons in Pd/HZSM-5 affect the activity of this catalyst in two ways, besides promoting dealumination of the zeolite. First, they stabilize Pd*+ ions vs. PdO particles in an oxidizing atmosphere. The ‘protonolysis’ PdO + 2H+ = Pd2+ + H20 is not a facile reaction when the PdO particles are large; however, it is dramatically accelerated in an atmosphere of NO + 02 . For the SCR of NO, with methane, Pd *+ ions are more active sites than PdO particles; the slow protonolysis, therefore, implies a slow increase in SCR selectivity with time on-stream. In contrast, the combustion of CH4 to CO + CO2 and H20 is time-invariant. The second role of the zeolite protons resides in the reaction mechanism of SCR. It is an example of bifunctional catalysis, i.e. some reaction steps make use of protons. Pd/NaZSM-5 samples which contain a small fraction of Pd*+ ions, but very few protons, are virtually inactive for SCR of NO,. Conversely, Pd/ HZSM-5 samples that mainly contain Pd as PdO particles display significant NO, reduction activity. At temperatures where SCR of NO, or combustion of CsHs are important over Pd/HZSM-5, the metal-
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