Applied Catalysis B: Environmental 25 (2000) L75–L81

Letter

The effect of SO2 on the oxidation of NO over Fe-MFI and Fe-ferrieritecatalysts made by solid-state ion exchange

Richard Gilesa, Noel W. Canta,∗, Markus Kögelb, Thomas Turekb, David L. Trimmc

a Department of Chemistry, Macquarie University, Macquarie, NSW 2109, Australiab Institut für Chemische Verfahrenstechnik, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany

c School of Chemical Engineering and Industrial Chemistry, University of New South Wales, South Wales, NSW 2052, Australia

Received 20 September 1999; received in revised form 10 November 1999; accepted 11 November 1999

Abstract

The oxidation of NO and of SO2, separately and together, has been investigated over a Fe-MFI catalyst made by solid-stateion exchange. The catalyst is more active than Pt/SiO2 for the oxidation of NO alone but less active for SO2 oxidation. Theactivity for NO oxidation is greatly inhibited by the simultaneous presence of SO2 or water but complete recovery of theactivity is possible when they are removed subsequently. Exposure to H2O during NO oxidation results in displacement ofNO2 and NO to an amount which is equivalent to a NOx /Fe ratio of 2. A Fe-ferrierite catalyst also made by solid-state ionexchange is more active that Fe-MFI for NO oxidation but more susceptible to SO2 poisoning. ©2000 Elsevier Science B.V.All rights reserved.

Keywords:Fe-MFI; Fe-ferrierite; NO oxidation; SO2 inhibition; Water inhibition

1. Introduction

It is somewhat paradoxical that several of the cat-alytic methods for reducing nitric oxide to nitrogeninvolve an initial oxidation to NO2. This is clearlythe case with some zeolite catalysts for reduction us-ing hydrocarbons in the presence of excess oxygensince their performance can be enhanced by the inclu-sion of specific components active for the conversionof NO to NO2, either in a mixed bed [1–3] or sep-arately upstream of hydrocarbon introduction [4,5].Likewise, the intermittent storage reduction strategy

∗ Corresponding author. Tel.:+61-2-9850-8285;fax: +61-2-9850-8313.E-mail address:noel.cant@mq.edu.au (N.W. Cant).

used on some lean burn engines requires oxidation toNO2 since NO is not directly storable at high temper-ature [6–8]. The continuously regenerated trap (CRT)technology for the control of particulate emissionsfrom diesel engines also uses a catalyst to generateNO2 which is a much better oxidant for soot thaneither O2 or NO [9,10].

Current systems are seriously affected by the pres-ence of SO2 in the exhaust. Platinum, the most com-monly used NO oxidation catalyst, also converts SO2to SO3, which is emitted as sulfuric acid aerosols in theCRT situation or accumulates as sulfate (which blocksnitrate storage) during the intermittent lean burn op-eration [7,11–13]. Copper and cobalt-based transitionmetal zeolite catalysts are affected by ppm levels ofSO2, with Co-b the most resistant during long-term

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operations [14], but it is unclear if oxidation to SO3is part of the inhibition process. Particular types ofFe-MFI catalysts have been reported to be much moreresistant to SO2, at least when isobutane is the reduc-ing agent [15,16].

The ideal solution for these problems would be acatalyst with high NO oxidation activity, minimal ac-tivity for SO2 oxidation and resistant to SO2 poison-ing or, at the least, capable of easy regeneration af-ter sulfur poisoning. This is difficult to achieve giventhe need for NO oxidation at temperatures well below350◦C in some applications. In the present work wehave investigated Fe-MFI and Fe-ferrierite as possiblecatalysts for NO oxidation with the above features inmind. These systems exhibit some of the desired char-acteristics although they are not resistant to poisoningby high SO2 concentrations in simultaneous operation.

2. Experimental

The Fe-MFI samples used here were from batches,the preparation of which has been described in detailpreviously [17,18]. In essence, a starting NH4-MFI(Si/Al ratio of 11.4, AlSi-Penta Zeolithe Gmbh, Ger-many) was ball-milled for 1 h with FeCl2·4H2O in aknown ratio. It was then heated to 550◦C and main-tained there for 6 h. After cooling it was washed withwater and dried overnight at 110◦C. Most experimentswere carried out with a preparation having a Fe/Alratio of 0.75 (nominally 150% exchange based onthe FeCl2 used) which had been calcined in eitherair or nitrogen. This is designated subsequently asFe-MFI-150. Some experiments were carried out witha sample with a Fe/Al ratio of 0.25 (i.e. exchangelevel of 50%) which had been calcined in air (desig-nated Fe-MFI-50). The Fe-ferrierite was made in thesame way as Fe-MFI-150 from a base ferrierite withSi/Al = 8.4 (Tosoh, Japan) and then calcined in air(150% exchanged, designated Fe-FER-150). The frac-tion of iron lying within each zeolite is not known.Some is external, since XRD measurements showeddistinct lines due to Fe2O3 (hematite) in addition tounchanged lines due to the parent zeolite [18].

The Pt/SiO2 was that designated 40-SiO2-PtCl-L inthe series prepared and characterised by Uchijima etal. [19] and contained 1.1 wt% Pt with a dispersion of40%.

Catalytic measurements were carried out using aflow system containing 140 mg samples of the cata-lyst pressed, crushed and sieved to 300–600mm andpacked in a 6 mm ID Pyrex tube mounted verticallyin a cylindrical furnace. Each sample was subjectedto a standard pre-treatment in 2% O2/He in a slowramp (3◦C/min) ending with 2 h at 500◦C and thencooled to reaction temperature. The reactant mixture(100 cm3(STP)/min, down flow) was prepared by com-bining analysed mixtures of O2/He, NO/He and/orSO2/He with helium carrier using electronic mass flowcontrollers (Brooks models 5850TR and 5850E).

The outlet stream from the reactor was accuratelydiluted with nitrogen to bring the concentrationswithin the range measureable by an atmospheric-typechemiluminescent analyser (Monitor Labs model9841). An internal drive logged signals correspond-ing to NO and NO+ NO2 (NOx) every 20–25 s. NO2was then obtained by the difference and conversionscalculated from both NO2 formed, and NO lost, eachas a percentage of the input NO concentration. Asdiscussed later, the two methods of calculation some-times gave very different values due to adsorption ofNO2 and, under some conditions with SO2 present, tothe formation of solid deposits on the cool outlet wallsof the reactor. Analyses for O2 and SO2 were carriedout with a micro-gas chromatograph (MTI Instru-ments) fitted with 5 A molecular sieve and PoraplotU columns. Conversions of SO2 were calculated bythe difference relative to inlet concentration.

3. Results and discussion

When first placed on-line the Fe-MFI catalystsreached three-quarters of their final activity for NOoxidation within an hour or two but, thereafter, con-versions continued to increase very slowly. Overnightoperation was required to reach a steady state whichwas independent of whether the initial calcinationwas carried out in air or nitrogen. Previously usedsamples responded more rapidly. Fig. 1 shows thebehaviour of a used sample of Fe-MFI-150 in anexperiment during which 500 ppm of SO2 was subse-quently added to the feed and then removed again. Oninitial exposure to NO/O2, conversions calculated byNO loss and by NO2 made are very different but trendto the same value after∼75 min (Fig. 1B). The initial

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Fig. 1. Effect of 500 ppm of SO2 on the oxidation of 1000 ppmNO in 5% O2 over 140 mg Fe-MFI-150 at 350◦C with flow rateof 100 cm3(STP)/min. A: measured concentrations; B: conversionsbased on NO lost and NO2 made.

difference is due to both uptake of NO and to storageof the NO2 produced by oxidation since, as may beseen from the measured concentrations in Fig. 1A, thecombined concentrations, NO+ NO2, are well belowthe input NO concentration during this period. Intro-duction of 500 ppm SO2 caused an immediate largefall in NO conversion, following which conversionscalculated by the two methods trend apart. This diver-gence was accompanied by the steady accumulationof solid white deposits at the reactor exit which, asexplained later, contained both NO and NO2. Whenthe flow of SO2 was stopped the activity of the cata-lyst recovered quite quickly. Conversions calculatedby NO loss are then close to the final value within2 min but those from NO2 production lag even morethan following the initial exposure. As before, this isdue to the uptake of NO2 as shown by a continuingdifference between input NO and output NO+ NO2in Fig. 1A. This longer lag probably arises becauseNO2 is needed to displace the sulfate stored throughthe oxidation of SO2.

Data for the temperature dependence of SO2 oxida-tion alone (determined subsequent to NO oxidation),

Fig. 2. Temperature dependence of the separate oxidations of1000 ppm NO, and of 500 ppm SO2, in 5% O2 over 140 mgof equilibrated Fe-MFI-150 (see text) with flow rate of 100 cm3

(STP)/min.

and for NO oxidation (after a series of co-feed exper-iments), over Fe-MFI-150 is shown in Fig. 2. The ac-tivity for SO2 oxidation is quite low—less than 12%at 350◦C and still only 28% at 450◦C. On the otherhand, the catalyst is very active for the oxidation ofNO in the absence of SO2. The methods of calculat-ing NO conversion then agree well with a maximumconversion of 67% at 315◦C and equilibrium reachedby 350◦C.

The temperature dependence of NO oxidation underco-feed conditions using 1000 ppm NO and 500 ppmSO2 in 5% O2/He is shown in Fig. 3. The two

Fig. 3. Apparent conversions of NO calculated by NO loss andNO2 made during the oxidation of 1000 ppm NO in 5% O2 with500 ppm SO2 present over 140 mg of Fe-MFI-150 with flow rateof 100 cm3(STP)/min.

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methods for calculating NO conversion now givedivergent values. The apparent conversion based onNO loss relative to input NO exceeds the expectedequilibrium value above 400◦C while that based onNO2 production is always low. As in the experimentof Fig. 1, white deposits formed continuously at theexit from the reactor. While the composition of thesedeposits is not known with certainty, previous workwith Pt/SiO2 has shown that they form when NO,NO2 and SO3 are all present [20]. The NO : NO2 moleratio in the deposit was found to be close to unity anddecomposition with evolution of brown NO2 is rapidfollowing exposure to atmospheric water vapour.

If the deposits contain equimolar amounts of NOand NO2 then it can be shown that true conversionshould be given simply by the average of the apparentconversions calculated by two methods. Fig. 4 showssuch ‘average’ NO conversions for a series of co-feedexperiments using different SO2 concentrations. Forall SO2 concentrations, the average NO conversiontrends to the equilibrium value, also reached in the ab-sence of SO2, by 450◦C. At lower temperatures NOoxidation is greatly inhibited by SO2. Reaction al-most ceases at 250◦C with SO2 present despite >50%conversion in its absence. It should be noted that theFe-MFI-150 catalyst always regained full activity forNO oxidation after exposure to NO/SO2/O2/He mix-tures, or to SO2/O2/He alone. However, complete re-covery took some hours and required the presence ofNO/O2. It could not be made quicker by overnight

Fig. 4. ‘Average’ conversions during the oxidation of 1000 ppm NOin 5% O2 over 140 mg of Fe-MFI-150 in the presence of differentSO2 concentrations with a flow rate of 100 cm3(STP)/min.

Fig. 5. Comparison of the effect of different SO2 concentrationson NO conversion during the oxidation of 1000 ppm of NO in5% O2 over 140 mg of Fe-MFI-150 or Pt/SiO2 with flow rate of100 cm3(STP)/min.

flushing with He or O2/He. This indicates that therecovery process requires displacement of adsorbedSOx , probably sulfate, by adsorbed NOx , probablynitrate.

Fig. 5 shows the relationship between the averageNO conversion at 350◦C and SO2 concentration forFe-MFI-150 in comparison with that of the same massof the Pt/SiO2 catalyst (which has an activity in linewith that used by Xue et al. [21]). While the iron zeo-lite is clearly more active than Pt/SiO2 in the absenceof SO2, the presence of 100 ppm SO2 is sufficient toreverse the order.

The oxidation of NO over the Fe-MFI catalysts wasalso greatly influenced by the presence of water in thefeed. Fig. 6 shows the effect of adding 9000 ppm waterafter 16 h of continuous operation at 350◦C. Introduc-tion of water causes an initial sharp evolution of NO2alone followed by a drop in concentration to a lowerlevel over a more prolonged period during which NOis also evolved (Fig. 6A). Individual concentrationsin the exit stream exceed the feed NO concentrationand their combined concentrations reach almost threetimes the feed NO for a period before declining to anew steady state. This gives rise to apparently nega-tive conversions, when based on NO loss, and onesin excess of 100%, based on NO2 formed (Fig. 6B).Nevertheless, the two methods of calculation show thesame apparent conversion,∼29%, when a steady stateis eventually reached with the water present. This may

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Fig. 6. Effect of∼9000 ppm of water on the oxidation of 1000 ppmNO in 5% O2 over 140 mg of Fe-MFI-150 at 350◦C with flow rateof 100 cm3(STP)/min. A: measured concentrations; B: apparentconversions based on NO lost and NO2 made.

be compared to∼64%, only 3% short of equilibrium,prior to the introduction of water. Conversions calcu-lated by NO loss recover quickly when water is re-moved from the feed (Fig. 6B) but NO2 formation lagsdue to adsorption as demonstrated by the continuingdeficit in NO+ NO2 in the output stream versus NOin the feed stream (Fig. 6A).

Experiments in which lower water concentrationswere added gave similar results except that the peakconcentrations of the NO and NO2 evolved were lowerand spread over longer periods. Exposure to waterduring NO oxidation at lower temperatures also pro-ceeded similarly except that the recovery of activitywhen water was removed was considerably slower.The amounts of NO and NO2 evolved on exposure towater at two temperatures are shown in Table 1. Theratio, total NOx evolved to Fe, is approximately twoat both 350 and 300◦C while the amounts of the threeforms of NOx , rapidly displaced NO2 and more slowlyevolved NO and NO2, are approximately equal giventhe uncertainties in the deconvolution. The processeswhich give rise to the evolution of both NO and NO2are not known. The infrared results of Chen et al. [22]

Table 1Displacement of adsorbed NO and NO2 by H2O during NO oxi-dation over Fe-MFI-150a

Temperature, ppm H2O Amounts desorbed (mmol/g) 6NOx /Fe

NO2 fast NO slow NO2 slow

350◦C, 9000 ppm 458 715 791 2.03300◦C, 3000 ppm 662 587 759 2.08

a After 16 h operation using 1000 ppm NO and 5% O2 over140 mg of Fe-MFI-150 with flow rate of 100 cm3(STP)/min.

show that NO2 adsorbs in two forms on Fe-MFI. Oneis present as a nitrate, the second either as another ni-trate species on a different site or, more likely in theiropinion, in nitro form. Displacement of the latter byH2O might account for the initial rapid evolution ofNO2 alone (Fig. 6A) while a subsequent slower reac-tion with nitrate produces NO and NO2 together. Thelatter process can be represented stoichiometrically as

2NO−3 + H2 O → 2OH− + NO + NO2 + O2

However, known bulk iron hydroxides are too unsta-ble to be formed. Goethite (FeOOH) decomposes at136◦C. This points to stabilisation of hydroxy specieswithin zeolite cages. Voskoboinikov et al. [23] havepresented evidence to show that the active sites for NOoxidation on Fe-MFI are dihydroxo complexes with abridging oxygen, HOFeOFeOH2+. Possibly these areconverted to nitro/nitrate species when NOx is presentbut are forced into an inactive, more hydroxlated formby water. Further work is desirable on this point.

Fig. 7 compares the activity of Fe-FER-150, Fe-MFI(at the two different exchange levels) and Pt/SiO2 forthe oxidations of NO, and of SO2, when carried outseparately under dry conditions. Each test was carriedout with a fresh sample under which conditions theFe-MFI-150 catalyst is slightly less active than afterequilibration with NO/SO2/O2 mixtures as shown inFig. 2. The activity order for NO oxidation is clearlyFe-FER-150 > either form of Fe-MFI > Pt/SiO2. Onthe other hand, Pt/SiO2 is by far the most activefor SO2 oxidation with Fe-FER-150 and Fe-MFI-50having the lowest activities. However, while theFe-FER-150 catalyst is the most active catalyst forNO oxidation it is also the most sensitive to thepresence of SO2 in co-feed experiments. As may beseen from Fig. 8, that the activity in the presence of200 ppm SO2 is insufficient to achieve equilibrium at

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Fig. 7. Comparison of the temperature dependencies of the ox-idation of 1000 ppm NO in 5% O2 over 140 mg each of freshFe-MFI-150, Fe-MFI-50, Fe-FER-150 catalyst, and Pt/SiO2, witha flow rate of 100 cm3 (STP)/min. A: SO2 oxidation; B: NO oxi-dation.

450◦C whereas Fe-MFI-150 is capable of that with500 ppm SO2 present (Fig. 4).

A further difference is that the Fe-FER-150 cata-lyst did not recover its activity in full after exposureto SO2. The maximum conversion, initially 91% at

Fig. 8. ‘Average’ conversions during the oxidation of 1000 ppm NOin 5% O2 over 140 mg of Fe-FER-150 in the presence of differentSO2 concentrations with a flow rate of 100 cm3 (STP)/min.

250◦C, declined to 81% at 285◦C after an experimentin which SO2 was oxidised alone, and further to 71% at264◦C after the co-feed experiments. This implies thatwhile the stability of nitrate species within Fe-MFI issuch that adsorbed SOx can be displaced by NO2 thisprocess is either incomplete or extremely slow withFe-FER.

4. Conclusions

Fe-MFI made by solid-state ion exchange has highactivity for NO oxidation but low activity for SO2 ox-idation. Sulfur dioxide greatly inhibits NO oxidationbut full recovery occurs as adsorbed SOx (probablysulfate) is displaced by adsorbed NOx (nitrates). Wateralso inhibits the oxidation of NO in a process whichappears to involve displacement of two forms of ad-sorbed NOx to yield NO and NO2. The total amountof NOx displaced is approximately twice the Fe con-tent of the catalyst in molar terms. Fe-ferrierite madeby solid-state ion exchange is more active for NO ox-idation than its Fe-MFI counterpart but is also moresusceptible to SOx poisoning.

Acknowledgements

This work has been supported by the AustralianResearch Council. Travel funds were provided bythe Australian Department of Industry, Science andTourism (through their Bilateral Science and Technol-ogy collaboration program) and the German Ministryof Education and Research (BMBF).

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