Effect of SO and H O on the N O decomposition in the ... · Effect of SO2 and H2O on the N2O decomposition in the presence of ... the tolerance and the durability of Fe/ZSM-5

Applied Catalysis B: Environmental 46 (2003) 523–539

Effect of SO2 and H2O on the N2O decomposition inthe presence of O2 over Ru/Al2O3

George E. Marnellos∗, Evangelos A. Efthimiadis, Iacovos A. VasalosChemical Process Engineering Research Institute, 6th km. Charilaou-Thermi Road,

P.O. Box 361, 57001 Thermi, Thessaloniki, Greece

Received 8 March 2003; received in revised form 11 May 2003; accepted 10 July 2003

Abstract

Catalyst evaluation and kinetic analysis was performed on the catalytic decomposition of N2O to N2 over a Ru/Al2O3.An oxygen rich feed containing SO2 and H2O were used in order to examine their influence on the catalyst activity. AnAl2O3 support was used with various Ru loadings (1–4%). No enhancement in the activity was observed for Ru loadingshigher than 2%. The performance of the Ru/Al2O3 catalyst was tested using various pretreatment procedures. The activity ofthe catalysts changed with pretreatment procedure that was followed: calcinations at 600◦C in inert, reducing and oxidizingatmosphere. The initial temperature of each experiment affected the catalyst activity, as well. The effect of the presence ofSO2 (0–500 ppm) and/or H2O (0–10%) in the feed on the catalytic activity was also measured. A shift of the N2O conversionversus temperature curve towards higher temperatures was observed when SO2 and H2O were added, either separately orsimultaneously, into the feed. The inhibition caused by SO2 was attributed to the formation of sulphates while that caused bywater to the competitive chemisorption of H2O and N2O on the same active sites. Finally, kinetic and TPD studies showedthat N2O is catalytically decomposed to N2 both in absence and presence of poisonous gases.© 2003 Elsevier B.V. All rights reserved.

Keywords: N2O; SO2; H2O; Nitrous oxide; Decomposition; Reduction; Kinetics; Mechanism; Ru

1. Introduction

Nitrous oxide (N2O) contributes to the destructionof ozone in the stratosphere and it is considered asa strong greenhouse gas[1,2]. The major industrialsource of N2O is the production of nitric acid (400 ktN2O per year), which is a key raw material in the fer-tilizer industry [3]. N2O is also produced during thesynthesis of adipic acid, caprolactam, glyoxal, andfrom combustion processes of fossil fuels, biomass,and waste.

∗ Corresponding author. Tel.:+30-231-0498120;fax: +30-231-0498130.E-mail address: [email protected] (G.E. Marnellos).

Among different anthropogenic emissions thosethat can be reduced in the short-term are the onesassociated with combustion processes and chemicalproduction[1,2]. Up to now the European Union hasnot set specific emission limits for N2O, however, itis expected to do so in the near future. Thus, thereis a strong need for the development of new efficientand economical technologies, which will ensure theelimination of N2O from flue gases.

The catalytic decomposition and the selective cat-alytic reduction (SCR) of N2O to N2 are two candi-date technologies, which can be applied to reduce theN2O emissions. The selective catalytic reduction ofN2O has been studied over copper (Cu) and iron (Fe)deposited on various zeolites (ZSM-5, MFI)[2,4–9].

0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0926-3373(03)00292-3

524 G.E. Marnellos et al. / Applied Catalysis B: Environmental 46 (2003) 523–539

Both catalysts exhibited high activity under idealreaction conditions (absence of poisonous gases).Fe/ZSM-5 maintained most of its initial activity evenafter the addition of water (H2O) into the feed. TheH2O presence caused a shift of the N2O conversionversus temperature curve towards higher temperaturesover Cu/ZSM-5. Centi and Vazzana[7] examinedthe tolerance and the durability of Fe/ZSM-5 usinga feed that simulated flue gases. Their experimentalresults showed that about 60% of the N2O was re-duced to N2 for more than 600 h in the presence of500 ppm sulfur dioxide (SO2). SO2 was not oxidizedto SO3 over the catalyst and this caused the toleranceof this catalyst to the SO2 presence, according tothe above authors. The N2O reduction by CO[10]or CH4 [11] has been examined in the absence ofO2. NH3, as well, has been used as the reducingagent [12–14] resulting in high N2O conversionsover Ru and Rh zeolitic catalysts. However, the pres-ence of 3% O2 caused a significant inhibition of thereaction.

The catalytic decomposition of N2O has been stud-ied to a greater extent than SCR over a large variety ofcatalysts, which included supported and unsupportedmetals, pure and mixed oxides, and zeolitic systems[1,2]. Supported oxides are more effective in practi-cal applications due to the higher dispersion of activesites and the higher specific surface area of the sup-port. In most works alumina was used as carrier andPd, Cu, Co, Mn, Rh, Ru, Fe, Cr, etc. as active sites.Catalysts based on cobalt, copper, ruthenium andrhodium exhibit high catalytic activity for the N2O de-composition[15–22]. In our previous communication[22], the performance of various catalysts depositedon �-alumina, was examined. The experimental re-sults showed that both Rh/Al2O3 and Ru/Al2O3 wereactive catalysts leading to complete reduction of N2Oto N2 at high temperatures. The activity of other cata-lysts studied in the same work (Fe/Al2O3, Pd/Al2O3,In/Al2O3, Co/Al2O3) was significantly lower. Li andArmor [16,17] concluded that Rh- and Ru-exchangedZSM-5 are the most active catalysts for the N2O de-composition. These authors also reported that the ac-tivity of the Ru-ZSM-5 catalyst dramatically declinedin the presence of oxygen, which was attributed tocatalyst deactivation caused by oxygen, presumablydue to the formation of ruthenium oxides. Chang et al.[19] showed that Ru-HNaUSY is a highly active cat-

alyst for the N2O decomposition at low temperatures(120–200◦C). Kinetic rate measurements revealedthat the decomposition of N2O over Ru-HNaUSYand Ru-NaZSM-5 followed a first order reaction ratewith respect to the N2O partial pressure, and had anegative-power law dependence on the oxygen par-tial pressure. This strong oxygen inhibiting effectsuggested that the desorption of oxygen was the ratelimiting step in the N2O decomposition. Furthermore,Chang et al.[20] studied the effect of Ru loading onthe kinetics of the reaction over the Ru-NaZSM-5zeolite. Higher Ru-loadings led to lower turnover fre-quency rates and to higher apparent activation energy.They concluded that high and low Ru-loadings re-sulted in active catalysts in the presence and absenceof excess oxygen, respectively.

Zeng and Pang[23] examined the N2O decompo-sition reaction over Ru/Al2O3. They observed thatthe surface area of Ru/Al2O3 decreased with the in-crease of the metal loading and that O2 exhibitednegligible inhibition effect. The presence of CO2 inthe feed modified the surface acidity–basicity, whileH2O had a retardation effect, which was attributedto the change of the chemical constituent and, thus,of the acidity–basicity of the catalyst surface. Wangand Zeng[24] studied the decomposition of N2Oover Ru/Al2O3 catalysts. Excellent catalytic activitywas measured for both dry and wet N2O feeds. Pinnaet al. [25] examined the N2O decomposition over Rusupported on ZrO2. They concluded that the oxidizedsamples exhibited higher conversion than the reducedones. Surface Ru6+ interacted N2O. N2O acted as anoxidant of the ruthenium reduced sites and as a re-ductant of the overoxidized sites. Finally, Kawi et al.[26] studied the same reaction over a Ru/MCM-41catalyst and investigated the effect of the followingfactors on the catalytic performance of the catalyst:Ru precursors, amount of Ru loading on the support,impregnation method, presence of oxygen, carbonmonoxide (CO) and moisture in the feed stream.Ru/MCM-41 catalyst prepared from Ru(OH)3 wasthe most active for the catalytic decomposition ofN2O. Moreover, the catalyst activity was significantlyenhanced when CO was introduced into the feedstream. On the contrary, the presence of moisture andoxygen slightly decreased the catalyst activity, due tothe competitive adsorption between these species andN2O for the catalytic sites.

G.E. Marnellos et al. / Applied Catalysis B: Environmental 46 (2003) 523–539 525

The effect of compounds that typically exist in fluegas, such as O2, CO, CO2, SO2, and H2O on theactivity of catalysts used for the N2O decompositionhas been studied in previous works. However, the ef-fect of these gases on the reaction mechanism was notexamined. In this work we studied the performance ofRu/Al2O3, a catalyst that exhibit high N2O reductionrate when the feed is poison-free, in the presence andthe absence of SO2 and H2O in an oxygen-rich atmo-sphere. We measured the activity and we calculated ki-netic and adsorption data for the N2O decomposition.Finally, we used our experimental results to proposea reaction mechanism for the N2O decomposition.

2. Experimental

2.1. Materials

�-Alumina extrudates supplied by Engelhard (sam-ple code: Al-3992 E 1/8”) were used as the sup-port of the catalyst. The extrudates were crushedand sieved in order to be separated into particles ofsize 180–355�m. The dry impregnation techniquewas employed for the impregnation of Ru on the�-alumina carrier. The impregnation of the supportwas performed using a RuCl3·xH2O solution providedby next chimica. The catalyst was dried at 120◦C for2 h and calcined in air at 550◦C for 4 h prior to thereaction[27]. The heating rate throughout all stageswas kept constant at 10◦C/min. The metal loadingwas varied from 1 to 4 wt.%.

2.2. Catalyst characterization

The Ru/alumina catalyst was characterized by us-ing the nitrogen (N2) adsorption, the X-ray diffraction(XRD) and the EDS techniques. The N2 adsorptiondata measured in an Autosorb-1 (Quantachrome) ap-paratus were used for the calculation of the BETsurface area of the samples, while the pore structureof the samples was determined by an Autopore II(Micrometrics) porosimeter. Crystallographic infor-mation was collected using the powder XRD tech-nique. The diffraction intensity−2θ spectra weremeasured by a Siemens D 500/501 with Cu K� ra-diation (λ = 1.54178 Å) at a scanning rate of 0.04◦over 2 s. Finally, energy dispersive X-ray spectrom-

eter measurements were performed using a JEOLJSM-6300 spectrometer.

2.3. Reaction/analysis system

The experiments were carried out in a quartz,fixed-bed, micro-reactor (0.018 m i.d.)[27]. The cat-alyst was loaded onto a fine-quartz fritted disk andthe reaction temperature was continuously monitoredby a thermocouple inserted inside the catalyst bed.A second thermocouple, located next to the fritteddisk, was used to measure the temperature of theeffluent gases. The reaction unit was equipped withmass flow controllers and pressure indicators for theaccurate control of the flow and partial pressure ofeach reactant. Product analysis was performed witha series of on-line gas analyzers and a gas chro-matograph (Hewlett-Packard 6890 series) equippedwith TCD and FID [27]. A molecular sieve 13×column and a HayeSep N column were used forthe separation of the inorganic and organic species,respectively. The on-line analyzers were: a chemilu-minesence NO/NO2/NOx analyzer (42C-HL, ThermoEnvironmental), a non-dispersive infrared (NDIR), aCO and CO2 analyzer (NGA 2000, Rosemount), amagnetopneumatic O2 analyzer (MPA-510, Horiba), aN2O analyzer (VIA-510, Horiba) and a SO2 analyzer(NGA 2000, Rosemount). The signals received fromthe analyzers were continuously recorded in a PC.

2.4. TPD procedure

Temperature programmed desorption (TPD) stud-ies of the N2O adsorption experiments were carriedout in a quartz reactor using the same procedure de-scribed by Gao and Au[28]. The sample (0.5 g) wasplaced in a quartz reactor of 0.007 m i.d., where wasfirst calcined in situ at 700◦C for 1 h under a He flow(20 cm3/min) and then cooled down to room tempera-ture. Next, the sample was kept in an N2O-containingflow (20 cm3/min) for 1 h at 300◦C and, subsequently,cooled again down to room temperature. After beingHe-purged at room temperature for 1 h, the samplewas heated to 600◦C in helium (20 cm3/min) with aheating rate of 10◦C/min. The reactor exit was con-nected to the gas analysis system. The variation of thegas concentration with the reaction temperature wascontinuously recorded.


3. Results and discussion

3.1. Catalyst characterization

The Ru-impregnated Al2O3 was characterized bysurface area measurements, in order to investigate theeffect of pretreatment, Ru loading and presence of SO2in the gas feed. The surface of�-Al2O3 was initially180 m2/g. After impregnation with Ru, the surfacearea of the fresh material increased to about 192 m2/g.The most probable pore size was equal to 110 Å.After pretreatment (1 h under He flow at 600◦C),the surface area of the sample decreased to 135 m2/g(i.e. a 25% reduction). This was attributed to thermalsintering. The surface area of a fresh 4% Ru/Al2O3catalyst was 181 m2/g. Consequently, the depositionof Ru did not modify the internal surface area of thecatalyst.

Before evaluating the effect of the presence of SO2,the resulting surface area of a 2% Ru/alumina sampleafter reaction without poisons was measured. It wasfound that, when the catalyst was exposed to the react-ing mixture, a sharp decrease of the surface area oc-curred (i.e. 89 m2/g). The presence of SO2 in the feedcaused an increase of the surface area of the sample.A sample exposed to a gas feed containing 50 ppmSO2 had a surface area equal to 142.9 m2/g, while thesurface area of the same sample exposed to 200 ppmSO2 was equal to 140.3 m2/g. Thus, it was clear thatthe surface area increased in the presence of SO2 andwas almost independent of SO2 concentration. Thisspecific increase could be attributed to the resultingdecrease of the pore size (90 Å) as compared to thepore size of the fresh sample.

The surface Ruthenium species on�-Al2O3 wereidentified by X-ray diffraction (XRD). RuO2 domi-nated on the catalyst surface, as expected, when thecatalyst was pretreated with He or/and O2 gas mix-tures. This result was observed in other works, as well[22–26]. When the catalyst was pretreated with H2/He,only reduced Ru was identified on the surface. In allreacted samples ruthenium was in the form of RuO2,this was attributed to the presence of 5% O2 in the gasfeed. When the feed contained SO2 we did not observethe formation of Al2(SO3)4 on the catalytic surface.However, the presence of sulfates with concentrationlower than the resolution of the XRD technique can-not be excluded.

Finally, the EDS analysis showed that Ru was ho-mogeneously dispersed on the catalyst surface over allsamples (fresh and used). When the feed contained 50or 200 ppm SO2 we observed that sulfur was sparselyor homogeneously dispersed on the catalyst surface,respectively.

3.2. Effect of Ru loading

We varied the Ru loading on�-alumina (1, 1.5,2, 2.5, 3, 4% Ru/Al2O3) to choose the optimumloading for the N2O decomposition reaction in thepresence of excess oxygen. Screening tests were car-ried out for the N2O decomposition reaction usingthe following feed: 500 ppm N2O and 5% O2 in He(catalyst weight: 1 g,F = 500 cm3/min). The N2Oconversions versus the reaction temperature curvesover the Ru-based catalysts are presented inFig. 1.The temperatures where the curves become sharp donot necessarily decrease with the increase of the Ruloading. For a given conversion, the catalyst withthe lowest decomposition temperature was the 2.5%Ru/Al2O3. The experimental curves for Ru load-ings in the range of 1.5–3% almost coincide. Theinterpretation of these results requires metal disper-sion measurements that were not carried out in thiswork. Increase of the metal loading may lead to theaggregation of metal particles and thus, to lowerdispersion.

Zeng and Pang[23] studied the effect of Ru loadingon the same reaction in the absence of O2. They no-ticed that the surface area of Ru/Al2O3 decreased withthe increase of the metal loading. The optimum metalloading was equal to 0.2% (one order of magnitudelower than the results of the present study). However,the presence of oxygen affects the reaction mecha-nism that leads to the N2 formation from N2O. Kawiet al. [26] also examined the effect of Ru loading onMCM-41. The activity initially increased with the Ruloading (0.5–5 wt.%) and subsequently decreased asRu loadings further increased from 5 to 10 wt.%. Asa result, the maximum conversion of N2O was mea-sured when 5% Ru was deposited on MCM-41. Wechose to use the 2% Ru/Al2O3 in the experiments thatare presented in the following part of this work be-cause the metal loading is moderately high and theactivity is comparable to that of samples with highermetal loading.


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Fig. 1. Effect of Ru loading on the N2O conversion over Ru/Al2O3 catalysts. Feed: 500 ppm N2O, 5% O2, balance He. Flow rate:500 cm3/min, catalyst weight: 1 g.

3.3. Effect of the catalyst’s pretreatment procedure

The treatment of the catalyst prior to reaction af-fects its activity. As a result, we followed three typesof catalyst pretreatment. The feed during this stagewas either pure He, or 5% H2 in He, or 5% O2 in He,the flow rate was 500 cm3/min, the temperature was600◦C and the duration of the treatment was 1 h. Cat-alyst samples were evaluated both in the absence andpresence of poisonous gases.

Fig. 2shows the effect of temperature on the activ-ity of Ru/alumina for the N2O decomposition reactionin the presence of excess oxygen, for the three typesof pretreatment described earlier. These experimentswere performed using a different temperature se-quence. Specifically, the initial temperature was eitherthe highest or the lowest of the experiment, followedby a gradual decrease or increase, respectively, of thereaction temperature. The following reaction feed was

used throughout all experiments: 500 ppm N2O, 5%O2 in He (catalyst weight: 1 g,F = 500 cm3/min).A sigmoid-type conversion versus temperature curvewas observed in all experiments. The most activecatalyst was the one pretreated with He and whenwe started the experiment at the highest temperature,while the less active catalyst the one pretreated withO2 and when we started at the lowest temperature.When the sample was pretreated with H2 the variationof the N2O conversion with the temperature was notaffected by the starting temperature. The opposite wasobserved when the other two types of pretreatmentwere employed. If we combine the two experimentalcurves (temperature increase–decrease inFig. 2) for agiven type of pretreatment we observe the formationof a hysterisis-type dependence. In these experimentsfor a given temperature higher conversions weremeasured when the experiment started at the highesttemperature.


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Fig. 2. Effect of the pretreatment procedure on the N2O conversion over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, balance He. Flow rate:500 cm3/min, catalyst weight: 1 g.

The same series of experiments was repeated inthe presence of poisonous gases (addition of 50 ppmSO2 and 10% H2O to the feed).Fig. 3 depicts the ef-fect of temperature on the activity of the Ru/aluminacatalyst for different pretreatments. In most of theexperiments of this figure we observe a turning point,i.e. a temperature in the range of 450–470◦C, wherethe slope of the conversion versus temperature curvechanges abruptly. At temperatures higher than thisturning point we observe that the slope of the ex-perimental curves drops and complete conversion isreached at significantly higher temperatures whenthe experiment starts at the lowest temperature. Inall experiments ofFig. 2 and in the experimentsof Fig. 3 that start at the highest temperature com-plete conversion is reached immediately after theturning point. Moreover, the experimental curvespresented inFig. 3 are intercepted, while the cor-responding curves inFig. 2 are parallel to eachother.

Pinna et al.[25] studied the effect of the catalystpretreatment procedure on the catalytic decomposi-tion of N2O over Ru/ZrO2. In this work, the oxidizedsamples exhibited higher conversions than the reducedones. The formation of NO in the gas phase was de-tected. The authors proposed that N2O decompositionmainly occurred on the exposed reduced Ru sites. Fi-nally, they concluded that N2O acted as an oxidanton the reduced sample and as a reductant of the fullyoxidized ruthenium.

In our experiments, no NO molecules were detectedduring the reaction. When the experiment started athigh temperatures the feed O2 oxidized rapidly anyreduced Ru sites and this is illustrated to the smallvariations in the conversion versus temperature curvespresented inFigs. 2 and 3. It must be mentioned thatXRD analysis showed that Ruthenium oxide existedon all reacted and unreacted samples except in thecase of unreacted samples pretreated with H2, wheremetallic Ru prevails the catalyst surface.


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Fig. 3. Effect of the pretreatment procedure on the N2O conversion over Ru/Al2O3 when the feed contains SO2 and H2O. Feed: 500 ppmN2O, 5% O2, 50 ppm SO2, 10% H2O, balance He. Flow rate: 500 cm3/min, catalyst weight: 1 g.

3.4. Effect of poisonous gases on the activityof N2O decomposition in excess oxygen overRu/Al2O3

The presence of gases that do not participate in theN2O decomposition, such as SO2 and H2O on thekinetics and the mechanism of reaction was examined.We initially studied the effect of each of the abovecompounds on the extent of conversion separately andthen in combination.

Vapor water typically exists in the flue gases. Ex-periments were, thus, carried out using a feed gaswith the following composition: 500 ppm N2O, 5%O2 and 0, 5 and 10% H2O in He. The reactor loadingwas 1 g of 2% Ru/Al2O3 and the total flow rate was500 cm3/min. The addition of water shifted the exper-imental curves to higher temperatures (approximately75◦C) as it is shown inFig. 4. The concentrationof H2O in the feed did not affect the activity ofthe catalyst. Similar behavior was also observed by

other researchers who studied the catalytic decom-position of N2O over Ru/Al2O3 in the presence ofwater [23,24]. We attributed the inhibition causedby the water presence in the feed to the compet-itive adsorption of H2O with N2O on the catalystsurface.

We performed step changes in the water concen-tration (0 or 10%) at a constant temperature (480◦C)and under reaction conditions identical to those usedin Fig. 4. The addition of 10% water after 1 h re-action in dry conditions caused initially a sharp de-crease of the N2O conversion followed by a rise upto approximately 90%. When the feed was switchedto dry the conversion further increased to 95% andremained constant even after the addition of 10%water. The same cycle was repeated once more andthe conversion remained in the vicinity of 100% inthe presence and the absence of water. Therefore, atthis relatively high temperature the presence of wa-ter practically does not affect the N2O conversion,


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Fig. 4. Effect of the inlet H2O concentration on the N2O conversion over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, 0–10% H2O, balanceHe. Flow rate: 500 cm3/min, catalyst weight: 1 g.

in agreement with the experimental results presentedin Fig. 4. We attribute this result to the tempera-ture of reaction (480◦C), where adsorption is notfavored.

The durability of Ru/alumina for 24 h operation wasalso tested in the presence of 10% water at 490◦C.These results were compared with the correspond-ing catalyst durability experiment in the absence ofwater. We selected to perform the durability tests atthis temperature because complete decomposition ofN2O was measured at this temperature in the pres-ence and the absence of water (Fig. 4). Under dryreaction conditions the catalyst maintains most of itsinitial activity even after 24 h of continuous opera-tion (Fig. 5). Under wet reaction conditions the ex-perimental curve initially followed the one measuredunder dry conditions. However, periodic oscillatoryphenomena were observed at higher reaction time andthe mean conversion at the end of the experiment wasabout 66%.

The presence of water in the feed does not alwaysaffect the extent of the reaction[1]. Given that theadsorption of water is exothermal, inhibition causedby water is observed at low temperatures, whereas theactivity is not affected at higher temperatures. As aresult, at low temperatures, competing adsorption ofH2O and N2O takes place, since they are both polarmolecules and use the oxygen end of the molecules tostrike a vacant Ru site[23,24,26]. On the other hand,high temperatures favor hydrothermal deactivation[1]. The reaction between the solid catalyst and H2Ocan lead to modification of the initial active catalyticphase[23,24]. Specifically, under wet reaction con-ditions we may have the formation of oxyhydroxides(RuO•(OH)2 and (RuO3(OH)2)2−) from the RuO2 (orRuOx). The presence of water in the feed may changethe surface acidity–basicity of the catalyst. The os-cillations in the N2O formation observed inFig. 5can be attributed to redox cycles taking place on thecatalytic surface. Another explanation for the same


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Fig. 5. Evolution of the N2O conversion with the reaction time over Ru/Al2O3 for H2O-free and H2O-containing feeds. Feed: 500 ppmN2O, 5% O2, 0 and 10% H2O, balance He. Temperature: 490◦C, flow rate: 500 cm3/min, catalyst weight: 1 g.

phenomenon is the periodic evaporation of water thataccumulates on the catalytic surface.

SO2 is frequently present in the flue gases derivedfrom combustion and chemical production plants[2].We, therefore, performed a series of experiments aim-ing at the SO2 effect on the extent of the N2O decom-position reaction at different temperatures rangingfrom 200 to 600◦C. The N2O conversion versus tem-perature curves over a 2% Ru/Al2O3 for 0, 50, 100and 200 ppm SO2-containing feed gas streams arepresented inFig. 6. It is evident that the presence ofSO2 caused a dramatic decrease of the N2O decom-position. An increase in the inlet SO2 concentrationleads to a shift of the experimental curve to highertemperatures. This shift is higher from 0 to 100 ppmSO2 as compared to a further concentration increase.Our experimental results agree with previous worksthat examine the catalytic N2O decomposition in thepresence of SO2 [2,29,30].

In the experiment presented inFig. 7, we intro-duced step changes using a variable inlet SO2 feed

concentration. The duration of each step was 1 hand the temperature 480◦C. A significant decreasewas observed when we inserted SO2 in the feed gas,while a 50% reduction in the N2O conversion wasmeasured, when 200 ppm SO2 were fed, comparedwith the corresponding conversion in the case of100 ppm SO2. Partial recovery of the activity wasnoticed in the successive SO2-free steps. The conver-sion dropped to about 20% when the feed contained500 ppm SO2.

The sensitivity of Ru/Al2O3 to the feed SO2 ispresented inFig. 8, where the effect of 50 ppm SO2on the activity is measured for 24 h of continuous op-eration at 490◦C. The conversion was initially 100%.One and a half hour later, the conversion started togradually decrease until it reached the final value(about 27%) after 11 h of operation. Afterwards, theconversion remained constant until the end of theexperiment.

When SO2 is present in the feed, most of the cata-lysts used for N2O decomposition rapidly deactivate,


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Fig. 6. Effect of inlet SO2 concentration on the N2O conversion over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, 0–200 ppm SO2, balanceHe. Flow rate: 500 cm3/min, catalyst weight: 1 g.

since they are also active for the oxidation of SO2 toSO3 [1]. SO3 then reacts with the catalyst to form astable sulfate (in this case aluminum sulfate), whichinhibits the catalyst activity[2]. These species are sta-ble on the catalytic surface up to 480–500◦C. Furtherincrease of the reaction temperature favors their de-composition and, consequently, the cleanup of the cat-alytic surface. Therefore, catalytic system can toleratethe SO2 presence in the feed only at significantly hightemperatures. When operation at high temperatures isnot feasible reduction of sulfates may lead to partial orcomplete regeneration of the initial catalytic activity.

In most flue gases SO2 and H2O coexist. There-fore, we performed a series of experiments where thefeed contained both SO2 and H2O. We chose to usea constant H2O concentration of 10% and we var-ied the SO2 concentration in the range of 0–500 ppm.The variation of the N2O conversion with the reac-

tion temperature is shown inFig. 9. The co-existenceof SO2 and H2O caused a shift of the N2O con-version curves towards higher temperatures that in-creased with the SO2 concentration. The inhibitioncaused by both gaseous compounds is not cumula-tive: for example, the addition of 10% water causeda 75◦C shift of the conversion curve (Fig. 4), whilethe addition of 50 ppm SO2 resulted in a 70◦C shift(Fig. 6). The simultaneous addition of SO2 and H2Oin the feed shifted the decomposition curve towardshigher temperatures by 85◦C (Fig. 9). The catalystactivity in the presence of the highest SO2 concen-tration (500 ppm) was limited to temperatures higherthan 500◦C. We have also performed step changes andtransient tests in the presence of both SO2 and H2Oand the behavior was similar as in the previous caseswhere we examined separately the effect of SO2 andmoisture.


Fig. 7. Effect of different inlet SO2 concentrations on the N2O conversion over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, 0 or 50 or 100or 200 or 500 ppm SO2, balance He. Temperature: 480◦C, flow rate: 500 cm3/min, catalyst weight: 1 g.

3.5. Kinetic and mechanistic analysis of N2Odecomposition over Ru/Al2O3

Preliminary kinetic experiments were performed inorder to determine the appropriate set of experimen-tal parameters that would ensure catalyst evaluationfor the decomposition of N2O based on intrinsic ki-netic measurements. We varied the space-time andthe catalyst particle size to determine any possi-ble influence of external and internal mass transferlimitations on the catalytic activity, respectively.Variable space-time experiments were performed at395◦C, and aW/F value equal to 0.018 g s/cm3 wasselected which corresponded to a catalyst weightof 0.25 g and a feed flow of 800 cm3/min. Further-more, in our previous work[31] we measured nosignificant influence of internal mass transfer lim-itations on the catalytic activity of samples withparticle of size<355�m. Finally, the apparent ac-tivation energy of the reaction was calculated equal

to 46.7 ± 4.3 kcal/mol, a value similar with that ofprevious studies[1].

The effect of reactive gas concentration on theN2O decomposition was examined, keeping the otherreaction parameters, such as reaction temperatureand space velocity constant. The reaction temper-ature was to ensure “differential” reaction kinetics(conversion less than ca. 20%). As a result, the ex-pressions developed in this work may not apply athigher temperatures. We initially varied the inlet N2Oconcentration in the range of 150–1500 ppm. TheN2O decomposition rate over Ru/Al2O3 was mea-sured at 385◦C. We used these experimental resultsto calculate the apparent reaction order with respectto the N2O concentration. We also varied the inlet O2concentration in the range of 0.5–10% over the samecatalyst and at the same temperature. The apparentorder of reaction for the N2O and the O2 concentra-tion was 1.17± 0.03 and−0.27± 0.02, respectively.We developed the following reaction rate expression


0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000 50000 60000 70000 80000

Time (s)

N2O

conv

ersi

on (

mol

%)

0ppm SO 2

50ppmSO2

Fig. 8. Evolution of the N2O conversion with the reaction time over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, 0 and 50 ppm SO2, balanceHe. Temperature: 490◦C, flowrate: 500 cm3/min, catalyst weight: 1 g.

for the N2O decomposition in the presence of oxygenover Ru/Al2O3.

r(N2Odec) = kdec[N2O]1.17

[O2]0.27(1)

The N2O decomposition is approximately propor-tional to the N2O concentration, while O2 inhibitsthe decomposition. The latter agrees with the re-sults of Zeng and Pang[23] over the same catalyst(Ru/Al2O3).

We repeated the above kinetic experiments in thepresence of SO2 and H2O. A typical feed compo-sition of 500 ppm N2O, 5% O2, 50 ppm SO2 and10% H2O in He were employed along with a cata-lyst weight of 0.25 g and a total flow of 800 cm3/min(W/F = 0.018 g s/cm3). The apparent activation en-ergy of the N2O decomposition reaction was equal to45.8± 5.7 kcal/mol. The presence of poisonous gasesslightly decreased the value of the activation energy.

We varied the inlet N2O concentration in the rangeof 250–3000 ppm and measured the N2O decompo-

sition rate over Ru/alumina at 410◦C. These experi-mental results were used to calculate the apparent re-action order that was equal to 1.19 ± 0.16. The sameprocedure was used for O2 concentration in the rangeof 1–10% at the same temperature. The apparent reac-tion order was calculated to be nearly zero. However,during these experiments small fluctuations were ob-served in the reaction rate. This was attributed to ad-sorption/desorption of the feed gases on the catalyst.Furthermore, the effect of different inlet SO2 concen-tration (50–500 ppm) on the N2O decomposition overRu/Al2O3 at 410◦C was measured. The apparent re-action order for SO2 was equal to−0.61± 0.24. Wecombined the above experimental data to develop akinetic expression for the N2O decomposition overRu/Al2O3 in the presence of O2, SO2 and H2O.

r(N2Odec) = kdec[N2O]1.19

[SO2]0.61(2)

In agreement with expression (1) the N2O decompo-sition is approximately proportional to the inlet N2O


0

10

20

30

40

50

60

70

80

90

100

200 250 300 350 400 450 500 550 600

Temprature ( oC)

N2O

con

vers

ion

(mol

%)

0ppmSO2+10%H2O50ppmSO2+10%H2O100ppmSO2+10%H2O200ppmSO2+10%H2O500ppmSO2+10%H2O

Fig. 9. Effect of the SO2 inlet concentration on the N2O conversion over Ru/Al2O3. Feed: 500 ppm N2O, 5% O2, 0–500 ppm SO2, 10%H2O, balance He. Flow rate: 500 cm3/min, catalyst weight: 1 g.

concentration. The inhibition effect of the inlet O2concentration in expression (1) is replaced by the oneof SO2. This can be attributed to the rapid SO2 ox-idation to SO3 over the Ru sites. Therefore, oxygenderived from the bulk phase and oxygen derived fromthe N2O decomposition is consumed for the SO2 oxi-dation. As a result, active sites are no longer availablefor the N2O decomposition. In order to fit all the abovedata we used the least squares method and we achievedalways a correlation coefficient (R2) value higherthan 0.98.

In the N2O TPD studies, we used the procedure de-scribed by Gao and Au[28]. Fig. 10shows the evolu-tion of N2O desorbed from�-Al2O3 and Ru/Al2O3 inthe presence and absence of 5% oxygen. Initially, theN2O sorption capacity of the support was studied. Thefeed consisted of 0.5% N2O in He during the sorptionstage (T = 300◦C) and of pure He during the desorp-tion stage of the experiment. One N2O peak was ob-served at 237◦C. The overall amount of the desorbed

N2O was calculated integrating the desorption peakand it was about 21.6×10−6 mol. The N2O desorptioncurve over Ru/Al2O3 exhibited a first peak at 147◦Cthat was attributed to Ru sites and a second peak at200◦C attributed to the support. This implies that thebond between the N2O and the support was strongerthan between the N2O and Ru. The total amount ofdesorbed N2O in both peaks was 12.6 × 10−6 mol.At higher temperatures, a small peak correspondingto NOx desorption was also detected. Following that,5% O2 was added to the N2O/He sorption mixtureand the TPD experiment was repeated (Fig. 10). Overthe alumina sample, a peak was observed at 295◦C(29.9 × 10−6 mol), while the corresponding peak inthe absence of O2 appeared at lower temperatures. Thesame experiment carried out over Ru/Al2O3, revealedagain two peaks: the first at 220◦C attributed to thepresence of Ru and the second at 295◦C attributed tothe support. The overall amount of desorbed N2O was16× 10−6 mol. It is probable that N2O is sorbed over


Fig. 10. Temperature programmed desorption experiment over the support (�-Al2O3) and Ru/Al2O3. Feed: 5000 ppm N2O, 5% O2, balanceHe. Flow rate: 20 cm3/min, catalyst weight: 0.5 g.

electron deficient Ru sites that exist on the catalyst atambient temperatures.

TPD experiments were also performed in orderto study the simultaneous adsorption/desorption ofN2O and SO2 in the presence of O2 (Fig. 11). Over�-Al2O3, two peaks corresponding to N2O and onesharp peak corresponding to NOx were noticed. Theseresults indicated the presence of two types of ad-sorbed N2O: one weakly (atT = 222◦C) and onestrongly-bonded (atT = 318◦C). The total amount ofadsorbed N2O was equal to 30.8 × 10−6 mol. On theother hand, NOx desorbed 436◦C exhibiting a verysharp curve (44.4 × 10−6 mol). It is interesting thatonly NO (no NO2) was desorbed. This implies that,in the presence of SO2, N2O formed NO over thesupport. Over Ru/Al2O3, two consecutive N2O peakswere observed, the first one on Ru atT = 195◦C andthe second one on alumina atT = 295◦C. Moreover,the total amount of desorbed N2O (21.7 × 10−6 mol)was lower than over the support. A small peak corre-sponding to NOx was detected at higher temperatures

(i.e. T = 505◦C). Comparing the N2O desorptiondata for the SO2-free and SO2-containing feeds overalumina, it was concluded that the presence of SO2weakened the N2O–Ru bond, while at the sametime strengthened the N2O–alumina. Finally, over�-Al2O3, when SO2 was added, an extensive releaseof NO strongly bonded on the surface was observed,which denotes that the presence of SO2 favored theoxidation of N2O to NO over�-Al2O3. Obviously,these oxidation sites were not present when Ru wasimpregnated on alumina because no NOx release overRu/Al2O3 was observed.

In our reaction kinetics studies we developed ex-pression (1), where the N2O decomposition rate wasproportional to the inlet N2O concentration. Thisimplies that N2O was molecularly adsorbed on thecatalyst surface in accordance to our TPD experi-ments. In the same reaction rate expression the inletoxygen concentration exhibited a moderate negativeeffect on the decomposition rate. This inhibitive ef-fect was also reported by Zeng and Pang[23] and can


0

0,005

0,01

0,015

0,02

0,025

0 100 200 300 400 500 600 700

T(oC)

Con

cent

rati

on (

mol

%)

γ-Al2O3 (N2O)

2Ru/Al2O3 (N2O)

γ-Al2O3 (NOx)

2Ru/Al2O3

Fig. 11. Temperature programmed desorption experiment over the support (�-Al2O3) and Ru/Al2O3 when the feed contains SO2. Feed:5000 ppm N2O, 5% O2, 500 ppm SO2, balance He. Flow rate: 20 cm3/min, catalyst weight: 0.5 g.

be attributed to the reversible dissociative adsorptionof oxygen, either directly or via molecular adsorptionof oxygen, as proposed to occur over metal surfaces[1]. Moreover, the negative reaction order of oxygendenotes that oxygen desorption might be the ratedetermining step of the reaction. The importance ofthe catalyst oxidation state was also demonstrated inexperiments where the catalyst was pretreated un-der different conditions (Figs. 2 and 3). Thus, forlow nitrous oxide coverage and high partial pres-sure of oxygen, the following reaction scheme isproposed, in which the desorption of oxygen (steps6 or 7) probably determine the overall reaction rate[1].

N2O + X ↔ N2O − X (3)

N2O − X → N2 + O − X (4)

O2 + X → O2 − XX−→2O− X (5)

2O− X ↔ O2 + 2X (6)

N2O + O − X ↔ N2 + O2 + X (7)

The above reaction scheme involves an adsorptionstep of N2O at the active center (Ru-site) (Eq. (3)),followed by a decomposition step that gives rise tothe formation of N2 and a surface oxygen (Eq. (4)).This surface oxygen can desorb either by combina-tion with another oxygen atom (Eq. (6)) that has alsobeen adsorbed (Eq. (5)) or by direct reaction with an-other N2O molecule (Eq. (7)). It is possible that boththe adsorption of N2O (Eq. (3)) and desorption of O2(Eqs. (6) or (7)) are reversible, while all the other stepsare irreversible.

The presence of SO2 in the feed inhibited the N2Odecomposition reaction without actually changing thereaction mechanism, since no changes were measuredwith respect to the activation energy and the apparentactivation order of N2O. The only difference is that, inthe presence of poisonous gases, O2 indirectly affected


the reaction mechanism. We calculated zero reactionorder in the presence of SO2, while in the absence ofSO2 the reaction order was−0.27. In the former caseoxygen is dissociatevely adsorbed on the catalyst, thus,inhibiting the N2O decomposition, but in the sametime it is consumed according to the reaction:

SO2 + O − X ↔ SO3 + X (8)

SO3 can react with the support to form surface sul-phates[1] via the following reaction:

Al2O3 + 3SO3 ↔ Al2(SO4)3 (9)

4. Conclusions

Catalyst evaluation and reaction kinetic analysiswas performed for the catalytic decomposition reac-tion of N2O to N2 in an O2-rich atmosphere overRu/Al2O3 in the absence and presence of SO2 andH2O gases that typically exist in flue gases and donot participate in the decomposition.

We loaded our Al2O3 support with Ru in the rangeof 1–4% and noticed that for Ru-loading higher than2% the activity remained the same. We varied the pro-cedure of the catalyst pretreatment using inert, reduc-ing and oxidizing atmosphere before the introductionof the reactive gas mixture. In this way we modifiedthe initial oxidation state of Ru. Experiments werecarried out starting from the lowest or the highest re-action temperature. The same series of experimentswere repeated adding SO2 and H2O in the feed. Thehighest activity was measured in the experiment thatstarted at the highest reaction temperature when thecatalyst was pretreated with He and the feed was SO2and H2O-free.

The activity of Ru/Al2O3 was measured in the pres-ence of SO2 (0–500 ppm) and/or H2O (0–10%) in thefeed. A shift of the N2O conversion versus tempera-ture curve towards higher temperatures was observedwhen SO2 and H2O were added, either separately orsimultaneously, to the feed. The inhibition caused bySO2 was attributed to the formation of sulphates, whilethat caused by water to the competitive chemisorptionof H2O and N2O on the same active sites.

Kinetic and TPD studies were carried out to exam-ine the reaction mechanism that leads to the N2 for-mation from N2O under the reaction conditions of this

work. Our results support a reaction path where N2Ois dissociatevely adsorbed in the absence and presenceof SO2 and H2O in the feed. The removal of oxygenfrom the catalytic surface is, probably, the controllingstep for the N2O decomposition.

Acknowledgements

This work was funded by the Commission ofthe European Communities, under the ContractENK5-CT-1999-00001.

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