Durability of ZSM5-supported Co-Pd catalysts in the reduction of NOx with methane

Applied Catalysis B: Environmental 39 (2002) 167–179

Durability of ZSM5-supported Co-Pd catalysts in the reduction of NOx with methane J.A.Z. Pieterse∗ , R.W. van den Brink, S. Booneveld, F.A. de Bruijn ECN Clean Fossil Fuels, P.O. Box 1, 1755 ZG Petten, The Netherlands Received 11 November 2001; received in revised form 19 April 2002; accepted 20 April 2002

Abstract Selective catalytic reduction (SCR) of NO with CH4 was studied over ZSM5-based cobalt and palladium catalysts in the presence of oxygen and water. Pore volume impregnation of cobalt was found to be more efficient and much simpler than the common (wet) ion-exchange method. In the case of Pd, wet ion-exchange was found to give superior activity. As compared to alternative catalytic systems reported in literature for CH4 -SCR in the presence of water, ZSM5-supported Co-Pd combination catalysts are very active and selective. The activity of the ZSM5-based Co-Pd combination catalysts, however, decreases strongly with time-on-stream. Strikingly, this deactivation is not (predominantly) caused by steam dealumination of the zeolites: loss of SCR activity with time-on-stream occurs irrespective of the presence or absence of water in the feed. The higher the temperature of calcination the lower the initial activity and the faster the deactivation. In addition to this, the deactivation is also more pronounced at higher reaction temperatures. These observations are consistent with a temperature-induced mechanism of ion migration and sintering as also confirmed by TPR analysis. The role of water in this migration process is not obvious. Hence, the limited thermal stability of ZSM5-supported metal (ion) catalysts leads to two demands, which have yet to be made for application of zeolites in CH4 -SCR: (1) stabilisation of the ionic phases in zeolite pores of different geometry; and (2) further improved activity and selectivity allowing one to operate at temperatures that do not exceed 350–400 ◦ C, where deactivation is not significant. © 2002 Elsevier Science B.V. All rights reserved. Keywords: NOx abatement; Methane; Selective reduction; Durability; ZSM5; Cobalt; Palladium

1. Introduction The emission of NOx during industrial, domestic and mobile activities still remains a major contributor to the acidification of the atmosphere and soil. Combustion of fossil fuels in the transport devices, power plants for electricity production, house heating and the chemical industry constitute the major sources of NOx emissions [1]. In this respect, catalytic meth∗ Corresponding author. Tel.: +31-224-564259; fax: +31-224-568615. E-mail address: [email protected] (J.A.Z. Pieterse).

ods to reduce NOx are of interest. In comparison to the non-catalytic solutions (e.g. the Exxon homogeneous gas phase reduction process [2]), catalytic methods offer lower operating temperatures. The selective catalytic reduction (SCR) with NH3 has found widespread application in the industry [3,4]. While high conversion (>95%) can be achieved at reasonably low temperatures of 300–400 ◦ C several disadvantages of this technique stand in the way of its application in other sectors than industry. A catalyst reactor system, NH3 injection unit and NH3 storage are necessary. To achieve high conversions, ammonia has to be injected into the flue gas before the catalyst and this requires

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 9 8 - X

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sophisticated distribution pipes and nozzles. Anhydrous ammonia needs to be transported and stored near the control unit. Altogether, the operation of a NH3 -SCR unit is complicated and expensive. While these difficulties are surmountable for larger installations, smaller and mobile applications are in need of an alternative DeNOx technology. In this respect, the use of hydrocarbons as reducing agents is under study. SCR with hydrocarbons (HC-SCR) is expected to be more cost-effective and safer than the existing SCR with NH3 . A variety of metal cation-containing zeolites have been studied as HC-SCR catalysts [5–7]. It was shown that appreciable SCR activity and selectivity can be achieved with zeolite-based catalysts. Nevertheless, the limited hydrothermal stability of zeolites has been a matter of concern. Structural degradation is reported to result by dealumination occurring at the tetrahedral Al sites. Leaching of the aluminium out of the framework by creating octahedral co-ordinated aluminium is known to be accelerated by water, usually present in exhaust gases, and is usually referred to as steam dealumination. Due to these problems, studies on metal-loaded oxide catalysts have found renewed interest. Unfortunately, the oxidic materials are clearly less active when compared to zeolites [8]. Presumably, the unique ability of the zeolite to generate highly dispersed metal ions at charge compensating sites (stabilised by the electrostatic field around the oxygen atoms in the pores) in the interior of the zeolite micropore system is of crucial importance in establishing high SCR activity. Therefore, research that focuses on stability improvements of zeolites next to activity improvements is important. Automotive catalysts are exposed to a wide range of temperatures from as low as 100 ◦ C during a cold start to as high as 1000 ◦ C for top-speed operation. Therefore, in analogy to conventional three-way catalysts, an excellent heat tolerance is required for SCR catalysts [9]. Application of zeolites in the field of the stationary pollution control, i.e. without large temperature excursions, seems less challenging. A few examples of potential markets for zeolite-based HC-SCR are treatment of flue gases stemming from nitric acid factories, small sized boiler installations, combined cycle devices and (lean-burn) gas engines and transformers. The high activity that can be accomplished with zeolites lowers the (required) minimum temperature of operation and therefore, poses fewer

demands with regards to (hydro)thermal stability of the material. While LPG (C3 and C4 hydrocarbons) as the reductant in combination with iron-loaded zeolite as the catalyst was found highly effective for NOx conversion, poor logistics and abundance make its use rather expensive. In this respect, the use of methane— in the form of readily available natural or city gas—is very attractive. Cobalt-, platinum-, palladium-, indium- and gallium-loaded zeolites were found appreciably active for CH4 -SCR [5]. Especially the combination of cobalt and palladium seems promising due to remarkable high activity and appropriate selectivity [6,10]. The overall catalytic performance and therefore, the potential for practical use of the zeolite-based CH4 -SCR catalysts is, however, not easily drawn from literature. In particular, crucial information on the time-on-stream behaviour (i.e. the durability) under realistic exhaust conditions is lacking [5]. The current report aims at obtaining more insight in the potential of methane SCR by describing the overall catalytic performance of zeolite ZSM5-supported Co-Pd combination catalysts for CH4 -SCR as compared to the monometallic equivalents. Simulated feed gas is representative for lean-burn engine applications (including high concentrations of oxygen and water) and is also close to the off gas composition of nitric acid plants. Emphasis will be put on durability in the presence of water. 2. Experimental 2.1. Materials Pd-ZSM5 was prepared by conventional wet ionexchange in air (24 h at 80 ◦ C) from NH4 -ZSM5 powder (Alsi Penta SM27 (coded as AP) and Zeolyst (coded as CBV3024e)) and an acidified solution of palladium nitrate. Following the ion-exchange, the catalyst was filtered, washed thoroughly with demineralised water and dried for 16 h at 80 ◦ C. For the Co-Pd combination catalysts, cobalt was added to PdH-ZSM5 by either dry impregnation (added volume of cobalt precursor solution equals the pore volume) (coded as ‘Co(Imp)’) or by means of several successive wet ion-exchanges with 1 × 10−2 M Co nitrate solution (coded as ‘Co(WIE)’, [2∗ ] and [3∗ ] refers

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to repeating the ion-exchange). Co(WIE)-Na-ZSM5 was prepared by ion-exchange of Alsi Penta SN27 sample for 24 h at 80 ◦ C (i.e. the sodium form) with Co(NO3 )2 while the Co(Imp)-NH4 -ZSM5 sample was prepared by means of pore volume impregnation of the Co(NO3 )2 solution onto the NH4 form ZSM5 (i.e. SM27). Finally, the catalysts were dried for 16 h at 80 ◦ C and calcined at temperatures between 400 and 550 ◦ C in situ. This procedure ensured the transition of NH4 -ZSM5 into H-ZSM5 with the release of NH3 . 2.2. Characterisation The bulk composition of the catalyst was examined using ICP elemental analysis. SEM analysis, to study the morphology of catalysts, was performed with a JEOL-JSM-6330F microscope and structure determination was performed with X-ray diffraction (XRD) analysis. H2 -TPR spectra were recorded with an Altimira AMI-1 apparatus applying 30 ml min−1 flow of 10% H2 in Argon at a heating rate of 20 ◦ C min−1 . The infrared (IR) spectroscopic measurements were performed with a Bruker IFS-88 spectrometer equipped with a high vacuum cell. The high vacuum cell consists of a stainless steal chamber equipped with CaF2 windows and a resistance-heated furnace, in which the golden sample holder is placed. A sorption pump and a turbo molecular pump evacuated the cell to pressures below 10−6 mbar. The catalysts were pressed to self supporting wafers and placed in a golden sample holder in the furnace of the high vacuum cell. Spectra were recorded in the transmission absorption mode. To correct for the varying sample thickness of the different wafers, the spectra were normalised by the integral intensity of the overtones of the lattice vibrations. The spectra were recorded with a spectral resolution of 4 cm−1 . 2.3. Activity measurements Catalytic tests were carried out in a computer-controlled flow set-up. Gases were introduced by mass flow controllers and water by a saturator kept at the appropriate temperature. The tubing downstream the saturator was heated to 130 ◦ C to prevent condensation. The quartz reactor with an internal diameter of 0.6 or 1 cm is placed in an oven. The catalyst sieve

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fraction (0.25–0.5 mm) is placed on a quartz grid. The catalyst bed height ranges from 10 to 25 mm depending on the amount of catalyst used and the reactor diameter. Gas hourly space velocities (GHSVs) are reported at room temperature and atmospheric pressure. Unless stated differently, the feed consists of 500 ppm NO, 2500 ppm CH4 , 5% O2 and 5% H2 O in nitrogen. The quantitative analysis of the gas-phase components is performed using a Bomen MB100 Fourier transform infrared (FTIR) spectrometer equipped with a model 9100 gas analyser. At the start of each activity experiment, the reactor temperature is increased to 175 ◦ C at 3 ◦ C min−1 under N2 flow and flushed for 2 h. Subsequently, a background IR scan is made and the reaction gas mixture is then applied and fed to the catalyst. Pre-conditioning was set for 20 min at each temperature. Unless stated differently in the text, data are collected at ascending temperature using a ramp of 5 ◦ C min−1 to maximal 500 ◦ C. FTIR analysis averages 150 scans (resolution 1 cm−1 ) and is performed twice at each temperature. Conversion of NOx is defined as [1 − ((NO2 )t + (NO)t )/((NO2 )0 + (NO)0 )] × 100%, conversion methane is defined as [1 − (CH4 )t /(CH4 )0 ] × 100%, based on dry flow. The methane-based selectivity, in order to distinguish reaction with NOx via SCR and methane oxidation by oxygen, is defined by means of consumed methane and NO molecules, i.e. [CH4 /NOx ]consumed . 3. Results 3.1. Characterisation of fresh catalysts Two common ZSM5 zeolite batches from different origin were tested. SEM images revealed clear morphological differences among the samples. While the Alsi Penta SM27 sample has small 5 ␮m particles, Zeolyst CBV3024e reveals a non-uniform particle size with particles ranging up to 100 ␮m (Fig. 1). ICP elemental analysis revealed that the Si/Al ratio of the AP zeolite is about 13 (Table 1), which is in agreement with the Si/Al value for Alsi Penta SM27 reported (the producers specifications Si/Al ratio of 13.5). Zeolyst CBV3024e was also found to have Si/Al ratio = 13. Palladium was incorporated in the zeolites at a low loading in order to prevent excessive methane oxida-

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Fig. 2. Pd(WIE)-H-ZSM5: origin of the ZSM5 samples. Calcination temperature (Tcal ) = 550 ◦ C; GHSV = 10,000 h−1 ; open symbols refer to methane and closed symbols to NOx .

tion activity [11]. The Pd content was 0.4 wt.%. The Co concentration of the catalysts varies between 2 and 3 wt.% depending on the number of successive ion-exchanges (coded [2∗ ] and [3∗ ] in Table 1). 3.2. Activity and selectivity Fig. 2 depicts conversion profiles as a function of the temperature obtained with AP- and CBV3024e-based Pd catalysts. NO2 formation was not observed. The AP-based catalyst reveals higher activity towards the selective reduction of NOx than the CBV3024e-based catalyst. In order to explore the impact of the preparation method, Pd-H-ZSM5 was prepared by two different methods: impregnation and wet ion-exchange from NH4 -ZSM5. Fig. 3 shows the catalytic performance as a function of the reaction temperature. On the ion-exchanged Pd catalyst, NOx conversion is higher

Fig. 1. Morphology of ZSM5 samples: (A) Pd-H-ZSM5 Alsi Penta; (B) Pd-H-ZSM5 Zeolyst.

Table 1 Physicochemical characterisation of the catalysts Catalyst

Si/Al ratio

Al/Pd ratio

Pd (%)

Co/Al ratio

Co (%)

Pd(WIE)-H-ZSM5 (AP) Pd(WIE)-H-ZSM5 (CBV3024e) Pd(Imp)-H-ZSM5 (CBV3024e) Co(WIE)-Na-ZSM5a Co(Imp)-H-ZSM5 Co(Imp)-Pd-H-ZSM5 Co(WIE)[2∗ ]-Pd(WIE)-H-ZSM5b Co(WIE)[3∗ ]-Pd(WIE)-H-ZSM5b

13 13 13 12.5 13 13 14 14

0.04 0.04 0.05 – – 0.04 0.04 0.04

0.4 0.4 0.5 – – 0.4 0.4 0.4

– – – 0.4 0.4 0.35 0.35 0.45

– – – 2.5 2.4 2.3 2.3 2.8

a b

Alsi Penta Na-ZSM5 SN27, WIE cobalt nitrate 12 h at 80 ◦ C (Na <0.5%). [2∗ ] and [3∗ ] WIE refers to repeating this procedure.

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catalytic performance. The impregnation method is believed to result in the formation of some Co3 O4 (see Section 3.4). Therefore, the activity of Co3 O4 was tested by physically mixing with quartz in order to achieve an overall cobalt loading equal to the Co-ZSM5 samples (not shown). NO2 was produced at low temperature, but no SCR activity was observed. Fig. 5 shows the conversion of NOx and CH4 as a function of the temperature for ZSM5 AP-supported Pd and Co-Pd combination catalysts. In the case of ion-exchanged cobalt, the increase of the cobalt loading from 2.3 (WIE[2∗ ]) to 2.8 wt.% (WIE[3∗ ]) did not result in a higher NOx removal efficiency. Increase of the cobalt loading only led to more conversion of NO to NO2 . Conversion of NO to NO2 appeared to be characteristic for ion-exchanged cobalt. The dry impregnated cobalt sample showed at similar loading somewhat better NOx removal efficiency. [CH4 /NOx ]consumed values as a function of the temperature are similar and therefore, independent on the preparation method. Comparing the conversion levels

Fig. 3. Pd-H-ZSM5: role of the preparation method. ‘Imp’ (impregnation) and ‘IonEx’ (ion-exchange); Tcal = 550 ◦ C; GHSV = 10,000 h−1 ; conversion (upper) and [CH4 /NOx ]consumed (lower).

than on the impregnated Pd catalysts. High methane combustion activity of the impregnated Pd catalyst is reflected in a high [CH4 /NOx ]consumed . Cobalt containing catalysts were prepared using Alsi Penta ZSM5 as the support material. Fig. 4 shows that Co(WIE)-Na-ZSM5 and Co(Imp)-H-ZSM5 catalysts with similar cobalt loading have identical

Fig. 4. Co-ZSM5: role of the preparation method. Tcal = 550 ◦ C; GHSV = 20,000 h−1 .

Fig. 5. Co-Pd-H-ZSM5 and Pd-H-ZSM5: role of the preparation method and cobalt loading. Tcal = 550 ◦ C; GHSV = 20,000 h−1 .

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Fig. 6. Influence of water on the NOx conversion and [CH4 / NOx ]consumed of Co(Imp)-Pd(WIE)-H-ZSM5. Temperatures 360 and 380 ◦ C were recorded with and without water; Tcal = 550 ◦ C, GHSV = 10,000 h−1 .

Fig. 7. Influence of oxygen concentration on the NOx and CH4 conversion of Co(Imp)-Pd(WIE)-H-ZSM5 at 400 ◦ C. Tcal = 550 ◦ C; GHSV = 20,000 h−1 .

achieved with Co- (Fig. 4), Pd- and Co-Pd-ZSM5 (Fig. 5) it turns out that combining Co and Pd resulted in higher overall activity and higher maximum level of NOx removal. [CH4 /NOx ]consumed values increase upon introducing Co in Pd-H-ZSM5. The influence of water on the NOx conversion and [CH4 /NOx ]consumed was studied on Co(Imp)-Pd(WIE)ZSM5 and presented in Fig. 6. First the conversion curve was recorded in the presence of 5% water. Secondly, a fresh catalyst was used without water and the NOx conversion was seen to be higher. Subsequently, at temperatures of 360 and 380 ◦ C, the catalyst was subjected to water for 20 min in order to check the nature of the water inhibition. Water inhibits NOx and CH4 conversion, but the effect of water is reversible: the catalysts exposed to water at 360 and 380 ◦ C retains the original catalytic performance found without water when water is switched off. In a feed without methane water was also seen to inhibit NO oxidation reaction and, in a feed without NO, CH4 oxidation was inhibited (not shown). Fig. 7 presents the influence of oxygen concentration on the NOx and CH4 conversion levels. While NOx conversion increases nearly linearly with oxygen concentration the total methane conversion remains constant. This means that at higher oxygen concentrations less CH4 is needed to remove a certain amount of NOx . Fig. 8 presents NO oxidation activity in a feed without methane. Open symbols refer to the case without water. Co-H-ZSM5 is more active for NO oxidation reaction and much less sensitive to the presence of water than Pd containing H-ZSM5. No significant

NO oxidation activity is noticed over H-ZSM5 in the presence of water. 3.3. Stability of the catalytic performance Fig. 9 shows the time-on-stream behaviour of the Co-, Pd- and Co-Pd-ZSM5 catalysts. The Pd containing catalysts deactivate faster than catalysts without Pd. In order to clarify the role of the reaction conditions on the course of the deactivation the catalyst was kept under reaction conditions for 16 h after which the feed was changed to 150 ml min−1 nitrogen (Fig. 10). The exposure to nitrogen was maintained for 14 h after which the feed was changed back to the reactant stream. As can be seen in Fig. 10, the catalyst continued to degrade during the course of the nitrogen exposure.

Fig. 8. NO oxidation reaction at GHSV = 10,000 h−1 ; open symbols denote the reaction without water in the feed.

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Fig. 9. Time-on-stream behaviour at 450 ◦ C. Tcal = 550 ◦ C; GHSV = 20,000 h−1 . NOx conversion was normalised at initial conversion level; closed and black symbols and lines refer to the Pd containing catalysts.

Fig. 11 shows the influence of the reaction temperature on the stability of the SCR (after being calcined at 450 ◦ C). As the temperature of the reaction becomes higher the deactivation becomes more pronounced. Additional time-on-stream experiments (not shown) show furthermore that the mechanism of the catalysts degradation is not significantly affected by the temperature of calcination. The degradation of the catalysts in time can be fitted by an exponential decay function that shows the same trend irrespective of the calcination temperature (viz. 450 and 550 ◦ C). However, stability could be further improved by decreasing the calcination temperature toward 400 ◦ C, i.e. by preventing the catalyst from exposure to temperatures above 400 ◦ C.

Fig. 10. Influence of the temperature on the degradation of Co(Imp)-Pd(WIE)-H-ZSM5. Tcal = 550 ◦ C; Tmeasurement = 450 ◦ C; after 16 h time-on-stream the feed was switched for 14 h to nitrogen flow only, subsequently switched back; GHSV = 40,000 h−1 .

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Fig. 11. Influence of the reaction temperature (legend denotes “Tcal /Tmeasurement ”) on the stability of the SCR reaction. Co(Imp)Pd(WIE)-H-ZSM5; GHSV = 20,000 h−1 . Conversion is normalised at the initial conversion.

The course of the deactivation did not depend on the presence or absence of water. Fig. 12 presents the NOx conversion as a function of the time over Co(Imp)-Pd(WIE) with and without water in the feed. The presence of water has a strong inhibitory effect on the NOx removal. However, after switching to conditions without water the catalysts continues to deactivate in a similar manner. 3.4. Characterisation of fresh and spent catalysts 3.4.1. H2 -TPR Fig. 13 presents H2 -TPR spectra as recorded of the fresh samples after the calcination at 550 ◦ C. Also,

Fig. 12. Influence of water on the stability of NOx conversion of Co(Imp)-Pd(WIE)-H-ZSM5. GHSV = 60,000 h−1 ; Tcal and Tmeasurement = 450 ◦ C.

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J.A.Z. Pieterse et al. / Applied Catalysis B: Environmental 39 (2002) 167–179 Table 2 Hydrogen consumption by fresh and spent catalysts

Fig. 13. Hydrogen TPR spectra of fresh and spent samples, 10% H2 in argon, 30 ◦ C min−1 to 800 ◦ C. The sample code in the legend represents the spectrum next to it.

some spectra represent the samples used in the reaction at 450 ◦ C in the presence of water for at least 50 h on stream. These spectra do have the prefix ‘spent’ in the figure legend. A detailed hydrogen and CO (in order to distinguish between ions and oxides) TPR analysis of Co-zeolite is given by Sachtler and co-workers [12,13] and the peaks will be assigned accordingly. Co3 O4 can be found at around 380 ◦ C. Peaks at around 600–640 ◦ C were assigned to CoOx in the pores of the zeolite, peaks around 700 ◦ C and higher were assigned to Co2+ ions associated to charge compensating (exchange) sites. Strikingly, pore volume impregnation of cobalt precursor results, like ion-exchanged cobalt, in highly stabilised Co2+ cations. Nevertheless, at comparable loading the pore volume impregnation method gives some Co3 O4 as well. In the presence of Pd, the high temperature peaks assigned to ionic cobalt (Co2+ ) are shifted to about a 100 ◦ C lower temperature. Presumably, Pd is reduced at temperatures below 450 ◦ C and the presence of metallic Pd eases the reduction of cobalt species. Pd contributions can be found at 110–150 ◦ C (hydrated, i.e. Pd(H2 O)2+ ) and around 400 ◦ C (ions). Table 2 provides an overview of the hydrogen consumption per mole of metal. Reduction following the reaction Co3 O4 + 4H2 → 3Co + 4H2 O would lead to H/Co (mol/mol) = 8/3, complete reduction of Co2+ via Co2+ → Co0 +2H+ would result in H/Co = 2 and similarly H/Pd = 2 in case of PdO and Pd2+ . Therefore, the hydrogen consumption values in the table seem often too low. The reasons are: (1) the quantifi-

Catalyst

Co/Pd (wt.%)

H/M (fresh)

H/M (spent)

Co3 O4 (mix) Co(Imp) Co(Imp)-Pd(WIE) Pd(WIE) Co(WIE)[2∗ ]-Pd(WIE) Co(WIE)-Na

2/0 2.3/0 2.3/0.4 0/0.4 2.3/0.4 2.6/0

2.7 1.5 1.6 1.7 1.1 1

– 1.7 1.5 2 1.3 1.2

cation of the spectra suffer from incomplete reduction peaks at temperatures above 700 ◦ C; and (2) cations are located in positions that can not be reduced below 800 ◦ C. As a consequence, the values shown in the table contain valuable information but need to be interpreted with some special care. In fact, any increase in the hydrogen consumption indicates: (1) the formation of cobalt oxide species; and/or (2) migration of non-reducible cations [13,14] toward reducible locations. In some cases, it is possible to discern between options (1) and (2) by means of the evolution of reduction peaks at lower temperatures and if this process is paralleled by loss of high temperature cation intensity. Obviously the hydrogen uptake appears to increase on spent catalysts indicating the formation of species comprising higher oxidation state. For instance, after 40 h time-on-stream, the reduction profile of Co(Imp)-H-ZSM5 showed an increase of the CoOx phase (530 ◦ C) at the expense of Co3 O4 (350 ◦ C) and Co2+ (741 ◦ C). Pd and Co-Pd combination catalysts have mainly their PdOx contribution around 240 ◦ C increased at the expense of Pd2+ at 420 ◦ C. 3.4.2. IR analysis IR spectra of H-ZSM5, Co(Imp)-Pd(WIE)-H-ZSM5 and Co(Imp)-Pd(WIE)-H-ZSM5 used in the reaction for 40 h are shown in Fig. 14. Co(Imp)-Pd(WIE)-HZSM5 shows much less bridging hydroxy band intensity at 3610 cm−1 as compared to H-ZSM5. The loss of about 70% of the free bridging hydroxy bands is by far more than expected based on the Pd loading: in agreement with the TPR analysis, also after pore volume impregnation, cobalt ions are (partly) located on the charge compensating positions of the zeolite. After reaction, this situation is not significantly changed. No indications for steam dealumination could be observed, i.e. specific bands around 3640 and

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Fig. 14. IR spectra of H-ZSM5 and Co(Imp)-Pd(WIE)-H-ZSM5, samples were dehydrated at 400 ◦ C.

3680 cm−1 to be assigned to the formation of octahedral aluminium (hydroxy) species were not present.

4. Discussion 4.1. Activity of catalysts Cobalt zeolite catalysts have been known for a long time to be active for SCR of NOx with hydrocarbons [5]. In the case of CH4 -SCR, due to its potential to activate methane, palladium was expected and found to be active [15]. Gaz de France [10] and Ogura et al. [6,7] were the first to recognise the enhanced catalytic performance compared to the single metal systems of zeolite-supported Co-Pd combination catalysts. In order to understand and optimise the zeolitesupported Co-Pd catalyst for CH4 -SCR applications, it is helpful to define the catalytic properties of the single metal zeolite catalysts. To do so, picturing of the active species brought about by the preparation methods in relation to their function in CH4 -SCR is of primary interest. 4.1.1. Role of palladium There is no doubt that the incorporation of Pd into ZSM5 occurs preferably by means of ionexchange. Impregnation of Pd leads presumably to the co-existence of PdO agglomerates next to Pd2+ cations. PdO is known for the high methane combustion activity [16] and is believed to be responsible for the high methane oxidation activity at the

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expense of SCR activity observed with the impregnated Pd(Imp)-H-ZSM5 catalyst in this study. In contrast, the ion-exchange procedure guarantees the highest level of dispersion by co-ordination of Pd ions with charge compensating oxygen sites in the zeolite. Following the studies of Misono [15], these zeolite stabilised Pd2+ cations activate methane and catalyse the CH4 -SCR of NOx . Protons are believed to be important for NO oxidation over Pd-H-ZSM5 and NO2 rather than NO is believed to react with activated CH4 [11,15]. However, Fig. 8 shows that Pd-H-ZSM5 (only about 10% of protons replaced by Pd) and H-ZSM5 have low NO oxidation activity and the NO2 concentration is far from equilibrium (Fig. 8). Therefore, the role of the proton sites in NO oxidation under the test conditions applied here may not be very significant. Moreover, Fig. 8 also shows that in the absence of water the NO oxidation activity is much higher and NO2 approaches the equilibrium concentration. Therefore, whether or not NO oxidation plays an important role for Pd-H-ZSM5 catalysts may depend on the presence or absence of water. Recently, Shimizu et al. [17] showed that, over Pd-zeolite, a CH4 -SCR mechanism operate that does not necessarily include the NO oxidation reaction. 4.1.2. Role of cobalt Also for Co-ZSM5, the similar activity of protonic ZSM5 and Na-ZSM5 in Fig. 4 disfavours a significant role of the proton in the NO oxidation reaction. In contrast to the Pd-H-ZSM5, for ZSM5-supported Co catalysts, the cobalt incorporation method seems less important. As seen in Figs. 4 and 5, conversion and selectivity are not dependent on whether ion-exchange or impregnation was the method of incorporation. Strikingly, both cobalt incorporation methods led to the prevalence of the high temperature reduction peak in the H2 -TPR spectrum assigned to Co2+ ions at cation positions of the zeolite. H2 -TPR also revealed the presence of some Co3 O4 on the Co(Imp)-H-ZSM5 catalyst while this was absent on the Co(WIE)-Na-ZSM5. Close analysis of Fig. 13 also reveals the presence of ‘Co-oxo ions’ (earlier reported by Sachtler and co-workers [12,13]), represented by the reduction peak at around 520 ◦ C. The amount of ‘Co-oxo ions’ on both catalysts are about equal as deduced from the reduction peaks. While the formation of PdO should be avoided, oxide-like

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Co species were reported to oxidise NO, which is claimed to be an important reaction in the mechanism that leads to N2 in Co-zeolite catalysts [18]. In order to distinguish the activity of Co3 O4 , Co-oxo ions and Co2+ ions, we tested the activity of Co3 O4 by physically mixing it with quartz as to achieve an overall cobalt loading equal to the Co-ZSM5 samples. NO2 formation approached the equilibrium concentration already at 340 ◦ C, however, SCR activity was not observed. Apparently, Co3 O4 is not able to activate methane. In a parallel test, we also found that the NO oxidation activity over Co(Imp)-ZSM5 and Co(WIE)-Na-ZSM5 was higher with Co(Imp)-ZSM5, however, at temperatures sufficiently high to activate CH4 (above 375 ◦ C) the NO2 concentration was similar (near equilibrium). Therefore, at temperatures sufficiently high for CH4 -SCR to take place over Co-ZSM5, the Co-oxo ions are responsible for the formation of NO2 [13] and Co2+ for the activation of CH4 . This explains the similar activity for NOx removal over both Co-ZSM5 catalysts. Note that this situation can differ in case of Co-Pd-ZSM5 combination catalyst: methane activation occurs now at lower temperature and activated methane may recombine with NO2 produced at low temperature by Co3 O4 , i.e. at temperatures too low for Co-oxo ions and Co2+ cations to produce significant (equilibrium) amounts of NO2 . Moreover, IR analysis pointed at the presence of CN and/or NCO species on the surface of Co-ZSM5 [19,20]. The (hydrolysed) cyanide species are believed to react with NO2 rather than NO to produce N2 and CO2 . This way one can also understand the enhanced SCR reaction rate over Co-ZSM5 catalyst with NO2 as compared to NO [18,19]. In summary, NO oxidation reaction is important for CH4 -SCR over Co-zeolite catalyst. 4.1.3. Combination of cobalt and palladium Synergistic effects between cobalt and palladium were proposed to explain the enhanced performance compared to the single metal systems [6,7]. The enhanced selectivity for CH4 -SCR upon increasing the concentration of oxygen suggests an important role of the NO oxidation reaction in the ruling mechanism. Most likely, the combination catalyst shows higher overall activity than the single metal-based zeolite catalysts due to the enhanced formation of activated

methane on Pd (in comparison to Co-ZSM5) that reacts mainly with NO2 formed via NO oxidation over Co species (rather than with NO as is the case over Pd-ZSM5 in the presence of water). In the case of the Co-Pd-ZSM5 combination catalysts the pore volume impregnation method for Co has two distinct advantages (in addition to the possible contribution of Co3 O4 in NO oxidation as pointed out in Section 4.1.2). Firstly, incorporation of cobalt in Pd-H-ZSM5 by means of ion-exchange seems to deactivate (part) of the Pd activity for the formation of activated methane species. This is clearly indicated by the increased production of NO2 at the expense of N2 in the case of Co(WIE)-Pd(WIE) as compared to Co(Imp)-Pd(WIE). Secondly, in order to achieve the desired high cobalt loadings several successive ion-exchanges need to be carried out. Therefore, it is of practical interest that simple pore volume impregnation of the cobalt precursor leads to cation sites at the charge compensating sites of the zeolite, as is clearly evidenced by the consumption of hydroxy bands in the IR spectra. 4.1.4. Influence of brand of ZSM5 The activity difference found with Pd(WIE)-HZSM5 derived from Zeolyst and Alsi Penta is remarkable. The catalysts have similar Si/Al ratio, impurity level, surface area and Pd loading. SEM analysis on the two ZSM5 samples revealed the presence of rather large particles in the Zeolyst sample. The particle size is important in the case of mass transport limitations. However, while a certain contribution of pore diffusion limitations was reported in the case of propane-SCR it was ruled out in the case of CH4 -SCR [5]. Significant differences in crystallinity of the two brands were also not supported by some additional XRD analysis. Interestingly, upon comparing two brands of ZSM5, from Zeolyst and Degussa, Sachtler and co-workers found superior performance of the Degussa sample [13]. Earlier studies in our group with ZSM5-supported iron catalysts for NOx and N2 O abatement technology led to similar conclusions [21]. Unfortunately, the limited set of experiments does not enable us to clarify the differences observed between different brands of the same zeolite. Nevertheless, it is important to acknowledge these findings. It is not inconceivable that these subtle differences together with the presence or absence of water constitute part of

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the lack of consensus on the mechanism of CH4 -SCR suggested among the different research groups [17]. 4.1.5. Influence of water As mentioned before, the presence of water inhibits the reactions. Temperature programmed desorption studies on Co/Na-ZSM5 suggested that the competitive adsorption between NO and H2 O is the most plausible explanation for the inhibition by water [22]. Indeed, from Fig. 6 it follows that the inhibition effect of water is completely reversible. In a feed without methane water was also seen to inhibit NO oxidation reaction and, in a feed without NO, CH4 oxidation was inhibited. Therefore, water is likely to hinder the other reactants to adsorb on the catalyst surface. The presence of water does not affect the selectivity to SCR as opposed to methane combustion. Long-term effects on the catalytic performance related to the presence of water are of primary interest [5,9,24] and will be discussed below. 4.2. Stability of catalysts Recently, in a comprehensive overview of SCR of NOx with hydrocarbons, Traa et al. [5] stresses the need for tests on the time-on-stream behaviour of metal-loaded zeolites in real exhaust gas for a good evaluation of their potential for practical application. In this respect, the presence of water in the test conditions is a prerequisite. A major concern in the use of zeolites for catalysis in the presence of water is steam dealumination, i.e. migration of lattice aluminium towards extraframework positions. While several reactions can be put forward that obviously benefit on the (steam)dealumination of zeolites (for instant alkane conversion over thus-created Lewis acid sites [23]) a possible beneficial impact for the DeNOx chemistry is not expected. Steam dealumination results in less charge compensating sites (as necessary for the stabilisation of metal (ions)) and eventually decrease of surface area. Consequently, assigning the loss of activity with prolonged times on stream to the structural changes caused by dealumination, i.e. inferior hydrothermal stability, is a popular explanation in literature [9,24]. However, from the results compiled here, we infer that one should speak in terms of thermal stability

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rather than of hydrothermal stability, i.e. the durability was not found to relate to the presence of water. Apart from the inhibiting effect of water discussed above, from Fig. 12 it follows, that there is no reason to assume a different mechanism of deactivation depending on whether or not water is present in the feed. Moreover, steam dealumination would result to the formation of extra-framework aluminium hydroxide [23]. Formation of extra-framework aluminium is, however, not proved by our IR analysis of the spent catalyst. Figs. 10 and 11 show the temperature dependence of the deactivation and TPR analysis shows that a metal phase transition has taken place. This means that thermal effects cause the loss of activity due to migration of metal and concomitant loss of the active sites. Water does not seem to play a role in this migration process. Hence, it is clear that high temperatures cause the migration of highly active cations toward less active locations. Nevertheless, several questions on the deactivation mechanism remain to be answered. Firstly, H2 -TPR on fresh and spent catalysts clearly indicates the existence of cobalt species that are more easily reduced. The cause of the small deactivation of Co-ZSM5 as seen in Fig. 9 seems related to the generation of Co-oxo ions (the evolution of the reduction peak around 530 ◦ C). The formation of Co-oxo species at the expense of cobalt ions at the cation positions of the zeolite could be the reason of deactivation of Co-ZSM5. However, if one assumes that Co-oxo ions are active for NO oxidation ([13], this work) but Co2+ is necessary for methane activation, one would expect increased NO2 at the expense of SCR activity. This is not observed: both reactions deactivate simultaneously. Secondly, the Pd containing catalysts deactivate faster than the Co-ZSM5 catalysts. In case of Pd containing catalyst, catalyst degradation corresponds mainly to the evolution of a peak around 200–250 ◦ C, presumably characteristic for the formation of some PdO aggregates. The relation of the deactivation of Zeolyst-based Pd catalysts with the formation of PdO aggregates is reported in literature [6,7,24,25]. However, the formation of PdO is expected to result to more methane combustion activity [16]. As a consequence, an increase of the [CH4 /NOx ]consumed with time-on-stream is expected. This is once more not unambiguously observed.

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Apparently, additional information on the structure and nature of cations in zeolites seems necessary to develop more understanding on the mechanism(s) of deactivation. In this respect, the work of Kaucky et al. [18,26] is of special interest. Three geometrically and electronically different cation positions for cobalt in ZSM5, viz. ␣, ␤ and ␥ were distinguished. The cations of ␤-type are most active for CH4 -SCR of NO. This location bears the highest level of accessibility for adsorption and reaction. It is not inconceivable that a similar favoured siting found for cobalt ions is also to be found for palladium. The most active ␤ sites are of medium strength of bonding to framework oxygens of the zeolite (as compared to weaker bound ␣ and stronger bound ␥). In order to explain deactivation, one could speculate that some migration of cations from ␤ locations toward locations bearing a lower level of accessibility and reaction probability, i.e. the ␣ and ␥ sites, occurs. Reaction parameters and structure of zeolites in relation to the chemical potential controlling this migration process are yet to be investigated. Altogether, stabilisation of the active metal cations in the zeolite pores could be the key to improved durability of zeolite-based catalysts for CH4 -SCR. Acknowledging the tremendous impact the zeolite structure has on the SCR of NOx with hydrocarbons [5,18,24,27–31] together with the outstanding activity and selectivity of Co-Pd combination catalysts compared to alternatives, makes screening of other zeolite structures as support of the Co-Pd combination of primary interest.

5. Conclusions CH4 -SCR of NO was studied over ZSM5-based cobalt and palladium catalysts in the presence of oxygen and water. Wet ion-exchange as the method to incorporate palladium into ZSM5 was found superior to the impregnation method. On the contrary, pore volume impregnation of cobalt was found to be more efficient and much simpler than the common ion-exchange method. As compared to alternative catalytic systems reported in literature for CH4 -SCR in the presence of water, ZSM5 Alsi Penta-supported Co-Pd combination catalysts are very active and selective. The stability of the ZSM5-based Co-Pd combination catalysts

is, however, insufficient for practical application. The deactivation occurs irrespective to the presence or absence of water in the feed. Therefore, steam dealumination of the zeolite and/or water-induced ion migration is not part of the predominant deactivation mechanism. Based on our results, the deactivation can be explained as a temperature-induced mechanism of ion migration and sintering. The limited thermal stability of ZSM5-supported metal (ion) catalysts calls for stabilisation of the ionic phases in zeolite pores of different geometry. Moreover, further improvements of SCR activity and selectivity are necessary to limit high temperature exposure. In a subsequent study, we will report on the influence of the zeolite structure on activity and stability of the Co-Pd catalysts for CH4 -SCR of NOx .

Acknowledgements The group of Prof. Dr. Ir. L. Lefferts at the University of Twente (Ing. J.A.M. Vrielink and Dr. M. Rep in particular) is kindly acknowledged for the transmission IR measurements. At ECN, Dr. J. Hooijmans, Ing. D. Börger and Ing. P. van Vlaanderen are thanked for the SEM, ICP and XRD measurements, respectively. References [1] J.N. Armor, Catal. Today 38 (1997) 163. [2] A.M Dean, A.J. Degregoria, J.E. Hardy, R.K. Richard, US Patent 4,682,468 (1987), to Exxon Research Engineering. [3] G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B 18 (1998) 1. [4] V.I. Parlescu, P. Grange, B. Delmaon, Catal. Today 46 (1998) 233. [5] Y. Traa, B. Burger, J. Weitkamp, Micropor. Mesopor. Mater. 30 (1999) 3 (and references therein). [6] M. Ogura, Y. Sugiura, M. Hayashi, E. Kikuchi, Catal. Lett. 42 (1996) 185. [7] M. Ogura, S. Kage, M. Hayashi, M. Matsukata, E. Kikuchi, Appl. Catal. B 27 (2000) L213. [8] L. Chen, T. Horiuchi, T. Mori, Catal. Lett. 72 (2001) 1. [9] T. Horiuchi, L. Chen, T. Osaki, T. Mori, Catal. Lett. 72 (2001) 77. [10] C. Hamon, O. Le Lamer, N. Morio, J. Saint-Just, US Patent 6,063,0351 (1998), to Gaz de France. [11] A. Ali, Y.A. Chin, D.E. Resasco, Catal. Lett. 56 (1998) 111.

J.A.Z. Pieterse et al. / Applied Catalysis B: Environmental 39 (2002) 167–179 [12] X. Wang, H. Chen, W.M.H. Sachtler, Appl. Catal. B 26 (2000) L227. [13] X. Wang, H. Chen, W.M.H. Sachtler, Appl. Catal. B 29 (2001) 47. [14] Z. Sobalik, J. Dedecek, L. Ikonnikov, B. Wichterlova, Micropor. Mesopor. Mater. 21 (1998) 525. [15] M. Misono, CatTech 6 (1998) 53. [16] J. Au-Yeung, K. Chen, A.T. Bell, E.J. Iglesia, J. Catal. 188 (1999) 132. [17] K. Shimizu, F. Okada, Y. Nakamura, A. Satsuma, T. Hattori, J. Catal. 195 (2000) 151. [18] D. Kaucky, A. Vondrova, J. Dedecek, B. Wichterlova, J. Catal. 194 (2000) 318. [19] A.D. Cowan, R. Dumpelman, N.W. Cant, J. Catal. 151 (1995) 356. [20] L.J. Lobree, A.W. Aylor, J.A. Reimer, A.T. Bell, J. Catal. 169 (1997) 188.

179

[21] R.W. van den Brink, J.A.Z. Pieterse, S. Booneveld, F.A. de Bruijn, to be published. [22] Y. Li, J.N. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [23] J.A. van Bokhoven, M. Tromp, D.C. Koningsberger, J.T. Miller, J.A.Z. Pieterse, J.A. Lercher, B.A. Williams, H.H. Kung, J. Catal. 202 (2001) 129. [24] H. Ohtsuka, T. Tabata, Appl. Catal. B 21 (1999) 133. [25] H. Ohtsuka, T. Tabata, Appl. Catal. B 26 (2000) 275. [26] D. Kaucky, J. Dedecek, A. Vondrova, Z. Sobalik, Collect. Czech. Chem. Commun. 63 (1998) 1781. [27] Y. Li, J.N. Armor, Appl. Catal. B 3 (1993) L1. [28] Y. Li, J.N. Armor, J. Catal. 150 (1994) 376. [29] A. Boix, R. Mariscal, J.L.G. Fierro, Catal. Lett. 68 (2000) 169. [30] Y. Li, J.N. Armor, Appl. Catal. B 5 (1995) L257. [31] C. Montes de Correa, F. Cordoba, F. Bustamante, Micropor. Mesopor. Mater. 40 (2000) 149.