Al2O3 in the presence of SO2 and H2O

Applied Catalysis B: Environmental 48 (2004) 1–15

Mechanistic and kinetic analysis of the NOX selective catalytic reduction by hydrocarbons in excess O2 over In/Al2 O3 in the presence of SO2 and H2 O George E. Marnellos∗ , Evangelos A. Efthimiadis, Iacovos A. Vasalos Chemical Process Engineering Research Institute and Department of Chemical Engineering, Aristotelian University of Thessaloniki, 6th km Charilaou, Thermi Road, P.O. Box 361, 570 01 Thermi, Thessaloniki, Greece Received in revised form 12 September 2003; accepted 12 September 2003

Abstract Mechanistic and kinetic experiments were carried out on the NOX selective catalytic reduction (SCR) by C3 H6 in excess O2 over In/Al2 O3 in the presence and absence of SO2 and H2 O. The In loadings in the catalyst varied in the range of 1–4%. The 2% In/Al2 O3 exhibited the highest activity. The performance of the aforementioned catalyst was measured using different pretreatment procedures. Pretreatment with O2 resulted in a more active catalyst than pretreatment with H2 or He either when the feed contained H2 O and SO2 or when it did not. The presence of H2 O (0–10%) enhanced the catalyst activity, while SO2 (0–500 ppm) acted as a poison by decreasing the NOX conversion as compared to the SO2 -free experiments. Kinetic and temperature programmed desorption (TPD) studies showed that a different reaction mechanism for the NOX reduction over In/Al2 O3 applied when H2 O and SO2 were present or absent in the feed. In the former case the reduction proceeds via the reaction of a reaction intermediate (CX HY OZ N), formed by the interaction of C3 H6 and NO2 , with other activated NOX species. In the latter case, NOX species adsorbed on In react directly with CX HY OZ to form nitrogen and combustion products. © 2003 Elsevier B.V. All rights reserved. Keywords: Nitric oxide; Selective catalytic reduction; Kinetics; Mechanism; Indium; Sulfur dioxide; Water

1. Introduction The reduction of nitrogen oxides (NOX ) emissions has become one of the greatest challenges in environment protection since NOX is a hazardous atmospheric pollutant. Nitric oxides may generate secondary contaminants through their interaction with other pollutants also emitted during the combustion of fossil fuels in stationary sources such as industrial boilers, power plants, waste incinerators and gasifiers, engines, and gas turbines [1]. Traditionally, NOX are reduced either adding three-way catalysts (TWC) under stoichiometric air/fuel conditions, or applying the selective catalytic reduction (SCR) with ammonia in lean conditions [1]. However, both methods have to overcome a variety of application problems. The use of TWC is limited by the inability of current catalytic convert∗ Corresponding author. Tel.: +30-231-0498120; fax: +30-231-0498130. E-mail address: [email protected] (G.E. Marnellos).

0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.09.011

ers to effectively reduce NO under excess oxygen conditions [2]. The use of ammonia (NH3 -SCR) requires the stoichiometric control of ammonia addition that is possible only for application in stationary NOX sources. The storage of NH3 and corrosion-resistant equipment for its use render the cost of NOX removal by this method expensive. The selective catalytic reduction of NOX with hydrocarbons (HC-SCR) is an attractive method to reduce the NOX emissions from combustion flue gases. This process may use unburned hydrocarbons that typically exist in flue gases as reductants, and thus, further reduce the cost of NOX removal. Following the initial work on the use of hydrocarbons for the NO reduction [3], a variety of catalysts such as metal-exchanged zeolites, transition metal- and noble metal-supported oxides have been tested [1]. Pure alumina has been used as support and as catalyst for the NO reduction. High NO reduction rates were measured over alumina at high temperatures (>500 ◦ C), but its activity drops in the presence of water in the feed [4,5]. The addition of active metals on alumina promotes the NO reduction by

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hydrocarbons. According to one reaction mechanism for the NO reduction to N2 [6], NO is oxidized and adsorbed on the catalyst surface as NO2 , which is then converted to molecular nitrogen after reaction with the reductant. The NO reduction over noble metal catalysts takes place at low temperatures (<300 ◦ C), but the N2 O formation is a side reaction, particularly, for platinum catalysts. Elements from the periodic table groups of 11 (Cu, Ag, Au), 13 (Ga, In) and 14 (Sn) are active for nitrogen oxides reduction by hydrocarbons when they are supported on various non-zeolite carriers such as alumina [7–14]. In previous works In deposited on various supports (zeolites [15,18], mixed oxides [19,23]) was used as the active site metal for the NOX reduction. Tabata et al. [15], studied the methane adsorption on In/ZSM-5 by UHV-temperature programmed desorption (TPD) and they calculated the adsorption heat that was equal to −132 kJ/mol. They also noticed that the amount of methane adsorbed on the catalyst surface was slightly decreased when water was preadsorbed on In/ZSM-5 surface. Kikuchi et al. [16] discussed the promotive effect of In deposited on H-ZSM-5 on the H2 O tolerance for the same reaction. They concluded that the addition of H2 O decreased the catalytic activity of In/H-ZSM-5 and this was attributed to the inhibition of the NO oxidation that takes place on the Lewis acid sites of zeolite. Zhou et al. [17] tried to identify the chemical nature of indium species on In/H-ZSM-5 catalysts used in the SCR of NOX by methane and the interaction between indium oxide and surface protonic acid sites of H-ZSM-5 zeolite. They postulated that only the one-dimensional network of In species was formed on H-ZSM-5 surface at very low In loading, whereas at In loadings higher than 10 wt.%, the formation of the surface crystalline In2 O3 occurred. They also claimed that the acid sites interacting with surface indium species were mainly protonic acid sites and that exchanges between In ions and protons of H-ZSM-5 occurred readily during the preparation of catalysts. Ogura et al. [18] examined the catalytic performance of In/H-ZSM-5 for the NO reduction by CH4 . They claimed that NO2 adspecies on InO+ sites concentrate in the zeolitic pore structure of In/H-ZSM-5. The catalytic activity of In-supported TiO2 –ZrO2 binary oxide for the NO reduction by propene in the presence of oxygen was investigated by Haneda et al. [19]. Indium promoted the reaction of NO2 with propene to form organic compounds containing oxygen and nitrogen, which was, probably, a reaction intermediate that leads to the N2 formation. Maunula et al. [20] have shown that Indium supported on sol–gel alumina is an active catalyst for the HC-SCR of NOX in the presence of oxygen. Enhancement in the activity was observed when the sol–gel alumina instead of a conventional ␥-alumina was used as support. The maximum activity of this catalyst was measured at temperatures close to 300 ◦ C. Haneda et al. [21] studied the reaction mechanism for the HC-SCR of NOX over In2 O3 /Al2 O3 . They revealed that organic nitro compounds are the initial reaction inter-

mediates, which are then hydrolyzed to several oxygen- and nitrogen-containing compounds. Two combinatorial systems were used in order to study the NO reduction [22,23]. Krantz et al. [22] studied the SCR of NOX by propene over 56 quaternary Pt–Pd–In–Na combination catalysts impregnated on ␥-Al2 O3 pellets. Indium exhibited low activity at the temperature range of 500–550 ◦ C. However, In was a useful promoter in Pt and Pd formulations. Richter et al. [23] also used combinatorial catalysis to test the effectiveness of 56 samples comprising of combinations of four elements, i.e., Ag, Co, Cu, In deposited on ␥-Al2 O3 for the above reaction. Synergistic effects were measured in the multi-component catalyst compositions yielding high NOX conversions at 450 ◦ C and negligible N2 O formation. In/Al2 O3 was the single-component catalyst with the lowest activity among samples tested in this work. In-supported catalysts exhibit promising activity for the SCR of NO [15–23]. Their activity is measured at temperatures higher those where noble metals are effective. Therefore, these catalysts can be used for the cleanup of flue gases derived from industrial plants operating at high temperatures such as power plants and incinerators. We chose to use ␥-Al2 O3 as the support of our catalysts because it is a carrier frequently used in commercial catalysts. The scope of this study was the evaluation of In/Al2 O3 catalysts for the selective catalytic reduction of NO by propene in the presence of excess oxygen and the effect of gases that typically exist in flue gases, such as SO2 and H2 O, on their activity. Emphasis was placed on the effect of gases that do not participate on the NO reduction directly, because in most previous works the catalyst evaluation was performed in the absence of these gases.

2. Experimental 2.1. Materials ␥-Alumina extrudates supplied by Engelhard (sample code: Al-3992 E 1/8 in.) were used as the support of the catalyst system. The extrudates were crushed and sieved in order to be separated into particles of size 180–355 ␮m. The dry impregnation technique was employed for the impregnation of In on the ␥-alumina carrier. The impregnation of the support was performed by using an indium nitrate penta-hydrate solution provided by Merck. The catalyst was dried at 120 ◦ C for 2 h and calcined in air at 600 ◦ C for 6 h prior to the reaction [24]. The heating rate in all stages was kept constant at 10 ◦ C/min. The metal loading was varied from 1 to 4 wt.%. 2.2. Catalyst characterization The In/Al2 O3 catalyst was characterized by using the N2 adsorption, the X-ray diffraction (XRD) and EDS

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

techniques. The N2 adsorption data measured in an Autosorb-1 (Quantachrome) apparatus were used for the calculation of the BET surface area of the samples, while the pore structure of the sample was determined by an Autopore II (Micrometrics) porosimeter. Crystallographic information was established with the aid of the powder X-ray diffraction technique. The diffraction intensity –2θ spectra were measured in a Siemens D 500/501 with Cu K␣ radiation (λ = 1.54178 Å) at a scanning rate of 0.04◦ over 2 s. Finally, the energy dispersive X-ray was measured using a JEOL JSM-6300 spectrometer. 2.3. Reaction/analysis system Experiments were carried out in a fixed-bed quartz micro-reactor (0.018 m i.d.) [24]. The catalyst was loaded onto a fine-quartz fritted disk and the reaction temperature was continuously monitored by a thermocouple inserted inside the catalyst bed. A second thermocouple, located after the fritted disk, was used to measure the temperature of the effluent gases. The reaction unit was equipped with mass flow controllers and pressure indicators for the accurate control of the flow and partial pressure of each reactant. Product analysis was performed with a series of on-line gas analyzers and a gas chromatograph (Hewlett-Packard 6890 series) equipped with TCD and FID [24]. A molecular sieve 13× column and a HayeSep N column were used for the separation of the inorganic and organic species, respectively. The on-line analyzers were: a chemiluminesence NO/NO2 /NOX analyzer (42C-HL, Thermo Environmental), a non-dispersive infrared (NDIR), a CO and CO2 analyzer (NGA 2000, Rosemount), a magnetopneumatic O2 analyzer (MPA-510, Horiba), a N2 O analyzer (VIA-510, Horiba) and a SO2 analyzer (NGA 2000, Rosemount). The signals received from the analyzers were continuously recorded in a PC. 2.4. TPD procedure Temperature programmed desorption studies for the NOX adsorption experiments were carried out in the same quartz reactor, loaded with 0.5 g of fresh catalyst (␥-Al2 O3 or 2% In/Al2 O3 ). The sample was first calcined in situ at 600 ◦ C for 1 h under a flow of He (500 cm3 /min) and then cooled to room temperature. Following that, the sample was heated at the adsorption temperature (450 ◦ C) under pure helium flow. At this temperature, a gas mixture of NOX and/or O2 , and/or SO2 diluted in He was sent to the reactor for 1 h and the reactor was then cooled down to 100 ◦ C. The feed was switched to pure He flow at 100 ◦ C for 1 h to desorb the weakly (physically) sorbed species. The temperature was then increased to 600 ◦ C using a heating rate of 10 ◦ C/min. The exit of the reactor was connected to the gas analysis system. The variations of the NOX , SO2 , and N2 O concentrations with the temperature were recorded continuously.

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3. Results and discussion 3.1. Catalyst characterization We performed the surface area measurements over our samples to investigate the effect of catalyst’s pretreatment, In loading and presence of SO2 in the gas feed on the physical properties of the catalyst. The surface of fresh ␥-Al2 O3 was 180 m2 /g. After the impregnation of ␥-Al2 O3 with 2% In the surface area remained approximately the same and it was equal to 184 m2 /g. The pore size with the highest frequency was equal to 110 Å. After pretreatment with He flow for 1 h at 600 ◦ C the surface area of the sample decreased to 135 m2 /g (25% reduction). We attribute this change to sintering occurring at high temperatures. We examined the effect of the In loading on the surface area of the sample. A fresh 4% In/Al2 O3 catalyst had a surface area of 167 m2 /g, i.e., 10% lower than that of the 2% In/Al2 O3 . As a result, the pretreatment procedure of In/Al2 O3 caused a higher reduction on the surface area than the increase of In loading from 2 to 4%. We measure the internal surface are of 2% In/alumina sample exposed to the reactive mixture in the presence and the absence of SO2 in the feed. Following the exposure of the catalyst to the reactive gas mixture the surface area dropped to 114 m2 /g. Other batches of the same sample, which was exposed to a feed containing 50 or 200 ppm SO2 , had a surface area of 145.4 or 135.2 m2 /g, respectively. Thus, the presence of SO2 in the feed decreased the surface area of the fresh sample to a lower extent as compared to the SO2 -free experiment. The higher inlet SO2 concentration was, the lower the surface area after reaction was. The oxidation state of Indium species on ␥-Al2 O3 was characterized by X-ray diffraction technique. Our results showed that In2 O3 dominated on the catalyst surface, as expected, when the catalyst was pretreated in an inert (He) and in an oxidizing (O2 /He) atmosphere. When the pretreatment took place under a reducing atmosphere (H2 /He), only In2 O was observed on the surface. In samples exposed to an SO2 -containing feed no Al2 (SO3 )4 was observed. This does not exclude the presence of sulfates on the catalytic surface of concentration lower than the detection limit (ca. 1% wt.) of our instrument. EDS observations showed that In was homogeneously dispersed on the catalyst surface of both fresh and reacted samples. Moreover, the same analysis showed that sulfur was homogeneously deposited on the catalyst surface when it was exposed to an SO2 -contaning feed. 3.2. Effect of In loading Volcano-type NOX conversion versus temperature curves were measured in all experiments, and complete C3 H6 conversion was reached at the temperature where the maximum NO reduction was measured. Minimal amounts of CO were measured in the temperature range where incomplete propene oxidation takes place. At higher temperatures, the

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NO2 formation was favored due to the oxidation of the reductant, while at even higher temperatures NO oxidation to NO2 was suppressed by equilibrium limitations. The effect of indium loading on the performance of In/Al2 O3 catalysts was initially investigated. A series of In-based catalysts supported on alumina were prepared varying the metal loading (1, 1.5, 2, 2.5, 3, 4% In/Al2 O3 ). Screening tests were carried out using the following feed: 1000 ppm NOX , 1000 ppm C3 H6 , 5%O2 in He (catalyst weight = 1 g, flowrate = 500 cm3 /min). The conversion of NOX and the combustion of propene versus the reaction temperature over In-based catalysts are presented in Fig. 1. The NOX reduction curves pass through a maximum that depends strongly (conversion and temperature) on the metal loading. When the indium loading increases the total activity increases, while after a certain critical metal loading (2%) the activity decreases with a further increase of metal loading. This behavior can be attributed to the variation of the active site dispersion with the metal loading, though no metal dispersion measurements were performed. Higher metal loadings lead to lower dispersions, which favor the propene combustion as shown in Fig. 1. The optimum In loading is the result of the two competing reactions over the In sites: the NO reduction and propene oxidation. Maunula et al. [20], examined the effect of In loading (from 0.43 to 10.4%) on ␥-Al2 O3 prepared by sol–gel method. The differences in the activity were relatively small, especially in dry mixtures. The higher In concentration was the lower the operation window was. In both dry and wet (8% water) feeds the highest activity was measured over 1.7% In/Al2 O3 . This result is consistent to our experimental results. 3.3. Effect of the catalyst’s pretreatment procedure Another significant parameter for the activity of a catalyst is the procedure that takes place prior to reaction. In the

present study, three different pretreatment procedures were followed and the activity data for the same catalyst pretreated differently were compared. The first one was the “inert” type during which the catalyst was exposed to He for 1 h flow (500 cm3 /min) at 600 ◦ C prior to reaction. The “inert” type represented the pretreatment procedure followed in most experiments. The second one was the “prereduction” type procedure during which a 5% H2 in He flow (500 cm3 /min) was introduced at 600◦ C for 1 h. Finally, the last one was the “preoxidized” pretreatment during which the catalyst was treated with 5% O2 in He (500 cm3 /min) at 600 ◦ C for 1 h. The samples were evaluated both in the absence and the presence of SO2 and water. Fig. 2 shows the effect of temperature on the activity of In/alumina for the NOX reduction by propene in the presence of oxygen, for the three types of pretreatment procedure described above. The initial reaction temperature in the experiments of this figure was either the highest or the lowest one. The following reaction feed was used throughout all experiments: 1000 ppm NOX , 1000 ppm O2 , 1000 ppm C3 H6 in He (Catalyst Weight = 1g, Flowrate = 500 cm3 /min). The variation of the NOX and C3 H6 conversion was qualitatively the same in all experiments. Comparison between the experimental curves in Fig. 2 shows that at low temperatures significantly lower conversions are measured when He and H2 are used in the pretreatment and the experiment starts at the highest and the lowest temperature, respectively. In these two experiments higher conversions were measured at higher temperatures and the temperature where the max. NO conversion is measured is shifted toward higher temperatures as compared to the other experiments. When fresh catalyst is preoxidized, the difference in the NO conversion between the experiment that started at the lowest and that at the highest temperature is significantly lower than in the other two pretreatment types. We attribute this result to the oxidation of the catalyst by the feed O2 that is faster at higher temperatures. As

100

Conversion (mol%)

90

1%In/Al2O3 1.5%In/Al2O3 2%In/Al2O3 2.5%In/Al2O3 3%In/Al2O3 4%In/Al2O3

C3H6

80 70 60 50 40 30

NOX

20 10 0 250

300

350

400

450

500

550

600

650

Temperature (˚C) Fig. 1. Effect of In loading on the NOX conversion vs. temperature curves; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , balance He, flowrate: 500 cm3 /min, and catalyst weight: 1 g.

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Fig. 2. Effect of pretreatment procedure and initial reaction temperature on the NOX conversion vs. temperature curves; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , balance He; flowrate: 500 cm3 /min; catalyst weight: 1 g.

a result, the maximum NOX conversion (74.7% at 430 ◦ C) was measured when the catalyst was “preoxidized” and the experiment started at the lowest temperature, while the lowest conversion for a given temperature was measured in the experiment that started at the lowest temperature and H2 was used in the pretreatment. The differences in the propene conversion in Fig. 2 as a result of the pretreatment procedure or the initial reaction temperature are less important than the corresponding differences in the NO conversion. The above series of experiments were also performed in the presence of 50 ppm SO2 and 10% H2 O in the feed to simulate the composition of flue gases (Fig. 3). In general, the experimental curves (NO and the C3 H6 conversion versus temperature) are wider and shifted to higher temperatures as compared to the corresponding curves in the absence of SO2 and H2 O. Moreover, the max. NO conversion for a given

type of pretreatment and a given initial reaction temperature is lower in Fig. 3 than the corresponding value in Fig. 2. The difference in the NO conversion in the experiments presented in Fig. 3 is more pronounced at temperatures lower than that where the max. NO conversion is measured, while at higher temperatures the experimental curves converge. We attribute this performance to the variation of the temperature where complete C3 H6 oxidation is measured with the pretreatment procedure and the initial reaction temperature. At temperatures lower than 450 ◦ C the highest NO conversions were measured when the catalyst was preoxidized and the initial reaction temperature was the lowest. The highest NO conversion was measured when the above pretreatment was applied and the initial temperature was the lowest (59.2% at 480 ◦ C). Comparison between the propene oxidation results in Fig. 2 shows that the oxidization is favored by the O2 pretreatment and the high initial reaction temperature. No

Fig. 3. Effect of pretreatment procedure and initial reaction temperature on the NOX conversion vs. temperature curves when the feed includes SO2 and H2 O; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 50 ppm SO2 , 10% H2 O, balance He; flowrate: 500 cm3 /min; catalyst weight: 1 g.

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NO2 was detected during the experiments in the presence of poisonous gases. This is attributed either to the competitive adsorption of H2 O and NO on Al2 O3 Lewis acid sites where the oxidation of NO to NO2 takes place, or to the blockage of these sites by aluminum sulfates [15]. The presence of SO2 and H2 O in the feed led to the decrease of the CO2 selectivity since significant amounts of CO were measured in the products. Preoxidized In/Al2 O3 resulted in the formation of In2 O3 on the catalytic surface according to the XRD analysis. This sample favored the NOX selective catalytic reduction both in the absence and presence of SO2 and H2 O in the feed. Prereduced sample resulted in the formation of In2 O that exhibited the lowest activity as compared to the other pretreatment types. 3.4. Effect of SO2 and/or H2 O on the NOX reduction The influence of the SO2 and H2 O presence in the feed on the kinetics and the mechanism of the selective catalytic NOX reduction by hydrocarbons in excess oxygen was studied adding separately or simultaneously these gases in the reactive gas mixture. 3.4.1. Effect of H2 O Experiments were carried out using a feed gas with the following composition: 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 and 0, 5 and 10% H2 O in He. The reactor loading was 1 g of 2% In/Al2 O3 and the total flowrate was 500 cm3 /min. In Fig. 4 the variation of both NOX and propene conversion with the temperature is presented either in the presence or in the absence of water in the feed. The presence of H2 O suppresses the propene combustion activity. This is, probably, linked with the enhancement of the NOX reduction (80% at 440 ◦ C, in the presence of 10% H2 O) since NO reduction and C3 H6 oxidation are competing reactions. Similar results were observed by Maunula et al. [20]. The concentration of H2 O did not affect significantly the extent of the NOX re-

duction. Water is responsible for the surface concentrations of hydroxyls on alumina. In the absence of water, the hydroxyl groups disappear at elevated temperatures leading to the activation of C3 H6 by weakly bonded hydroxyl groups or surface O2 . We performed step changes in the H2 O concentration keeping the other reaction conditions constant to examine the presence of any permanent changes in the activity caused by the water addition in the feed. The composition of the reactive gas, the catalyst weight and the flowrate were identical to those used in Fig. 4 and the reaction temperature was 433 ◦ C. We selected to perform the experiment at this temperature where we measured the maximum NOX conversion when the feed contained 10% water. The addition of 10% water 60 min after the initialization of the experiment caused an instantaneous sharp increase in the activity. Following that, the conversion decreased to approximately 80%, where it remained constant. When we returned to our initial feed (without water) the conversion did not change and remained stable. The same cycle was repeated twice and the conversion remained constant at about 80%. Therefore, the addition of 10% H2 O in the feed caused an irreversible enhancement in the activity. The durability of the In/Al2 O3 was tested using a H2 O-containing and a H2 O-free feed. The duration of each experiment was 24 h and the reaction temperature was the same. All the experimental variables were remained identical in both experiments. In Fig. 5 we observe that the NOX conversion was constant throughout the experiment when the dry feed was used. When the wet feed was used, the initial NOX conversion was equal to 65% approximately. The water addition in the feed caused a sharp increase in the conversion. However, after 2 h of operation periodic oscillatory phenomena were noticed. In spite of these oscillatory phenomena, the average conversion remained constant even after 24 h of operation. The oscillatory phenomena can be attributed to periodic vaporization of H2 O when the catalytic surface is saturated by water.

Conversion (mol%)

100 0 % H2O

80

5 % H2O 10 % H2O

60

C3H6 NOX

40 20 0 200

300

400

500

600

Temperature (˚C) Fig. 4. Effect of H2 O concentration on the NOX conversion vs. temperature curves; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0–10% H2 O, balance He; flowrate: 500 cm3 /min; catalyst weight: 1 g.

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Fig. 5. Stability tests over In/Al2 O3 catalyst in the presence and the absence of water; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0 and 10% H2 O, balance He; temperature: 433 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 1 g.

The enhancement in the activity caused by the water presence in the feed is consistent with the results presented in Fig. 4. All the above observations indicate that in contrary with Pt [25], the presence of H2 O enhances Indium activity towards NOX reduction. The support also affects the catalyst performance. For instance, In supported on ZSM-5 exhibited lower activity in the presence of water than in the absence [15,26]. We postulate that H2 O impedes the formation of carbonaceous deposits, which are known to play a significant role in the HC-SCR mechanism [1]. Furthermore, small quantities of NO2 were detected implying that the formation of NO2 is inhibited by water molecules (competitive adsorption), which are coordinated to alumina Lewis acid sites.

Conversion (mol%)

100 90

3.4.2. Effect of SO2 SCR experiments were carried out at different temperatures in the range of 200–600 ◦ C in order to study the effect of the SO2 concentration in the reactive gas mixture on the extent of NOX reduction and propene consumption. The NOX and propene conversion versus temperature curves over 2% In/Al2 O3 for 0, 50, 100 and 200 ppm SO2 -containing feed gas streams are presented in Fig. 6. The SO2 presence in the feed had a negative effect on both activities. The lower maximum conversion and the higher temperature at which maximum conversion was observed as compared to the SO2 -free experiments revealed the inhibition caused by the feed SO2 . We correlated the max. concentration and the temperature where it is measured with the feed SO2 concentration in Fig. 6. The latter correlation was approximately

0 ppm SO2

80 70

50 ppm SO2

60 50 40

200 ppm SO2

100 ppm SO2

30 20 10 0 200

250

300

350

400

450

500

550

600

Temperature (˚C) Fig. 6. Effect of SO2 concentration on the NOX conversion vs. temperature curves; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0–200 ppm SO2 , balance He; flowrate: 500 cm3 /min; catalyst weight: 1 g.

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Fig. 7. Variation of the NOX conversion with the reaction time for 0 or 50 ppm SO2 -containing feeds; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0 or 50 ppm SO2 , balance He; temperature: 510 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 1 g.

linear. Comparison between experimental results for feeds containing 0–200 ppm SO2 showed that the catalytic activity was severely inhibited for feeds containing 100 or 200 ppm SO2 . The propene conversion versus temperature curves are shifted to higher temperatures when the feed SO2 concentration increases. The SO2 presence in the feed decreased the selectivities for the NO2 and CO2 formation, similarly to the water-containing experiments. The NOX conversion was measured in the experiment presented in Fig. 7, where feed gas streams containing 0 or 50 ppm SO2 were interchanged (three cycles). The reaction temperature during the experiment was remained constant and equal to 510 ◦ C. Initially (absence of SO2 ), the concentration of NOX at the exit of the reactor was 487 ppm (conversion 49%). After 60 min of operation, 50 ppm SO2 were added in the feed and the conversion after an instantaneous increase, decreased linearly with the time at about 45%. In

the next 60 min, SO2 was removed from the feed and the conversion remained constant. When SO2 was added in the feed the conversion decreased down to 25%. In the last cycle, when SO2 was removed from the feed, NOX conversion increased slightly up to 28%, while in the presence of SO2 the conversion decreased down to 19%. Propene conversion was almost complete throughout the experiment, but as the conversion of NOX decreased (lower than 30%) the selectivity of CO2 decreased, as well. In Fig. 8, we present the effect of different SO2 concentrations on the extent of the NOX reduction over In/Al2 O3 . The catalyst was initially exposed to a SO2 -free feed gas stream for 1 h. Following that 50, 100, and 200 ppm of SO2 were introduced to the reactor at the same temperature (510 ◦ C) as in Fig. 7. The catalyst was exposed to the SO2 -containing feed for 1 h and before each SO2 concentration it was exposed in a SO2 -free feed for 1 h, as well. SO2 inhibited the

Fig. 8. Variation of the NOX conversion with the reaction time for 0–50–100–200 ppm SO2 -containing feeds; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0 or 50 or 100 or 200 or 500 ppm SO2 , balance He; temperature: 510 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 1 g.

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

NOX reduction irreversibly since the conversion remained low during the part of the experiment where the SO2 was removed from the feed. A gradual decrease in the conversion was observed and the conversion at the end of the experiment reached 15%. Finally, we also examined the variation of the NOX conversion with the reaction time for 24 h in the presence and the absence of 50 ppm SO2 in the feed at a constant temperature equal to 510 ◦ C. Following the addition of 50 ppm SO2 in the feed, an instantaneous sharp decrease in the NOX conversion (from 50 to 35%) was observed. The conversion remained constant at higher reaction times. After 16 h of operation, a gradual small increase in the conversion until 38% was noticed. The NOX conversion was about 20% higher in the SO2 -free experiment. It is important that the NOX conversion was stable during both experiments. This implies that in the presence of low SO2 concentrations in the feed, In/Al2 O3 is active for the NOX reduction. This inhibition effect caused by SO2 can be attributed to the strong chemisorption of SO2 and to the formation of stable sulfate species on the catalytic surface. We postulate that the exposure of the In/Al2 O3 catalyst to an SO2 + O2 mixture results in the formation of SO3 over In, which migrates to the alumina surface forming aluminum sulfates. The formation of sulfates results in the decrease of the ␥-Al2 O3 sites that are available to adsorb nitrate species. Nitrates can be reduced by propene to N2 . We noticed that the presence of SO2 in the feed gas stream decreased the NO2 selectivity. This result can be attributed to the reaction of SO2 with NO2 to form NO and SO3 . 3.4.3. Effect of SO2 and H2 O The next stage involved the determination of the effect of the presence of both poisonous gases (SO2 and H2 O) in the feed. For this purpose, a series of experiments were carried

9

out at different temperatures (i.e., 200–600 ◦ C) in order to study the effect of the SO2 concentration in the presence of 10% H2 O in the feed on the extent of the NOX conversion. The NOX and C3 H6 conversions versus temperature curves over In/Al2 O3 for 0, 50, 100, 200 and 500 ppm SO2 -containing wet (10% H2 O) feed are presented in Fig. 9. The coexistence of SO2 and H2 O caused a shift of both conversion curves to higher temperatures as the SO2 concentration increased from 0 to 500 ppm. If we consider the SO2 effect on the NOX conversion presented in Fig. 6 then it is evident that the SO2 presence in the feed inhibits the activity. However, for a given SO2 concentration and reaction temperature, the NOX conversion is higher in Fig. 9 than that in Fig. 6. This was attributed to the presence of 10% H2 O in the feed in accordance to the results presented in Fig. 4. The propene conversion versus temperature curves followed the same trend with the NOX curves. As shown in Fig. 10, we periodically changed the feed composition by consecutively adding and removing both 50 ppm SO2 and 10% water from the feed (three periodic cycles). After 1 h reaction in the absence of poisonous gases, both gases were introduced in the reactor and after some smooth fluctuations we reached the same conversion as in the absence of poisonous gases (51.7%). When the poisonous gases were removed, the conversion slightly increased, remained stable and it was equal to 53.9%. When SO2 and H2 O were added for the second time, the catalyst started to gradually decrease its activity and at the end of the 1 h the conversion became equal to 46.6%. The same behavior was observed when the same procedure was repeated, the conversion increased slightly when the poisonous gases were removed, while in their presence the conversion decreased in the same way as in the previous cycle and its final value was equal to 38%. Therefore, SO2 inhibits irreversibly the NOX reduction when a wet or dry feed (Figs. 7 and 8) is

100 90

Conversion (mol%)

80 70 60

10%H2O + 0ppmSO2 10%H2O + 50ppmSO2 10%H2O + 100ppmSO2 10%H2O + 200ppmSO2 10%H2O + 500ppmSO2

50 40 30 20 10 0 300

350

400

450

500

550

600

Temperature (˚C) Fig. 9. Effect of SO2 concentration on the NOX conversion vs. temperature curves; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0–500 ppm SO2 , 10% H2 O, balance He; flowrate: 500 cm3 /min; catalyst weight: 1 g.

10

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

Fig. 10. Variation of the NOX conversion with the reaction time for H2 O and SO2 -free and H2 O and SO2 -containing feeds; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 0 or 50 ppm SO2 , 0 or 10% H2 O, balance He; temperature: 510 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 1 g.

used. The presence of water in the feed moderates the inhibition effect. Finally, transient experiments were carried out for 24 h in order to test the durability of the catalyst when it was exposed to both poisonous gases (50 ppm SO2 and 10% H2 O). The time dependence of NOX conversion both in the absence and presence of poisonous gases is depicted in Fig. 11. When the poisonous gases were added to the feed gas the conversion started to decrease smoothly for approximately 5 h and then remained constant until the end of the experiment. The NOX conversion at the end of the experiment was 48%. Oscillatory phenomena were observed in this experiment in accordance to those observed in Fig. 5 where H2 O was added in the feed. As a result, we attribute these phenomena to the H2 O presence in the feed as discussed previously.

3.5. Kinetic analysis of NOX reduction and C3 H6 oxidation over In/Al2 O3 Preliminary kinetic experiments were performed to determine the appropriate set of experimental parameters that would ensure catalyst evaluation for the reduction of NOX based on intrinsic kinetic measurements. The experimental parameters that were varied were the space-time (expressed in terms of the ratio of catalyst weight to the total feed flow, W/F) and the catalyst particle size (expressed in terms of the average particle diameter, dp ). We, thus, estimated any possible influence of external and internal mass transfer limitations on the catalytic activity. In experiments carried out previously, no significant influence of internal transfer limitations was measured on the catalytic activities of samples with particle size of less than 355 ␮m [27]. In our

Fig. 11. Stability tests over In/Al2 O3 catalyst in the presence of H2 O and SO2 ; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 50 ppm SO2 , 10% H2 O, balance He; temperature: 510 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 1 g.

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

experiments we used catalytic samples with particle size in the range of 180–355 ␮m. The variable space-time experiments were performed with a typical feed composition of 1000 ppm NOX , 1000 ppm C3 H6 , and 5% O2 in He, catalyst weight of 0.25 g and total flow of 300–1000 cm3 /min (W/F = 0.015–0.05 g s/cm3 ). We examined the effect of W/F on the catalytic activity of In/Al2 O3 for NOX reduction and C3 H6 oxidation at 390 ◦ C. All W/F values resulted in a proportional relationship between conversion and space-time. This indicates that under these reaction conditions there is no influence of the external mass transfer limitations on the measured reaction rates for both NOX reduction and C3 H6 oxidation. A W/F value of 0.03 g s/cm3 (corresponding to a catalyst weight W = 0.25 g and a feed flow F = 500 cm3 /min) was selected for the rest of the kinetic experiments. In Fig. 12, a typical Arrhenius plot is presented. Both reaction rates are plotted versus reciprocal absolute temperature (T = 300–415 ◦ C). The apparent activation energies of both reactions were equal to 8.7 ± 2.5 kcal/mol for NOX reduction and 33 ± 5.3 kcal/mol for C3 H6 oxidation. Previous studies in other noble metals supported on ␥-alumina showed that the apparent activation energies for both NOX reduction and C3 H6 oxidation were approximately the same implying a possible common rate determining step for both reactions [28]. In this work the activation energy for NOX reduction is significantly lower than that of C3 H6 oxidation. The effect of varying the feed concentration of each of the reactants (NO, O2 , C3 H6 ) while keeping the other con-

11

centrations constant was also studied on the NOX reduction and C3 H6 oxidation activities over the In/Al2 O3 . Parameters such as reaction temperature and space velocity were kept constant, so that the variations in the reaction rates derived from the concentration changes. The reaction temperatures were chosen to be relatively low to approach “differential” conditions (conversion less than ca. 20%). The scope of this study was to develop kinetic expressions for low reaction temperatures. Initially, the effect of the inlet NO concentration (250–3000 ppm) on the NOX reduction and C3 H6 oxidation over In/Al2 O3 at 385 ◦ C was examined. The apparent reaction orders were −0.11 ± 0.15 for NOX reduction and −0.24 ± 0.02 for C3 H6 oxidation. When we measured the effect of the inlet O2 concentration (0.5–10%) on the NOX reduction and C3 H6 oxidation over In/Al2 O3 at 385 ◦ C, the apparent reaction orders were zero for NOX reduction and 0.54 ± 0.07 for propene oxidation. Finally, we studied the effect of the inlet C3 H6 concentration (250–3000 ppm) on NOX reduction and C3 H6 oxidation on In/Al2 O3 at 385 ◦ C. The apparent reaction orders were 0.19 ± 0.04 for NOX reduction and 1 for C3 H6 oxidation. We combined the above observations to develop the following overall low temperatures kinetic expressions: r(NOX red) = kred r(C3 H6 ox) = kox

[C3 H6 ]0.19 [NOX ]0.11

(1)

[O2 ]0.54 [C3 H6 ] [NOX ]0.24

(2)

21

NO reduction

20

C3H6 oxidation

-ln[rate]

19

Eact= 33 kcal/mol

18

Eact= 31.4 kcal/mol 17

Eact= 11.15 kcal/mol

Eact= 8.7 kcal/mol

16

15

14 0,0013

SO 2 + H 2 O

0,0014

0,0015

0,0016

0,0017

0,0018

1/T (K) Fig. 12. Arrhenius plots for the NOX reduction by hydrocarbons in excess oxygen over In/Al2 O3 in the absence and presence of SO2 and H2 O; 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 50 ppm SO2 , 10% H2 O, balance He; temperature: 300–481 ◦ C; flowrate: 500 cm3 /min; catalyst weight: 0.25 g.

12

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

We also performed kinetic experiments in the presence of gases that do not participate directly in the NO reduction (H2 O and SO2 ). A typical feed composition of 1000 ppm NOX , 1000 ppm C3 H6 , 5% O2 , 50 ppm SO2 and 10% H2 O in He, was employed along with a catalyst weight of 0.25 g and total flow of 500 cm3 /min (W/F = 0.03 g s/cm3 ). In Fig. 12 a typical Arrhenius plot for the above case is also shown. Both reaction rates were plotted versus reciprocal absolute temperature (T = 395–481 ◦ C). The apparent activation energies of both reactions were 11.15 ± 1.7 kcal/mol for NOX reduction and 31.4 ± 2.5 kcal/mol for propene oxidation. The presence of poisonous gases increased the activation energy for NOX reduction and slightly decreased the activation energy for propene oxidation. The effect of the inlet NOX concentration (250–3000 ppm) on NOX reduction and C3 H6 oxidation over In/alumina at 455 ◦ C was examined and the apparent reaction orders were found to be 0.36 ± 0.13 for NOX reduction and −0.29 ± 0.1 for C3 H6 oxidation. In the following, we measured the effect of the inlet O2 concentration (1–10%) on the NOX reduction and C3 H6 oxidation at 455 ◦ C and in this case the apparent reaction orders were −0.33 ± 0.33 for NOX reduction and 0.21 ± 0.06 for propene oxidation. The effect of the inlet C3 H6 concentration (250–3000 ppm) on NOX reduction and C3 H6 oxidation on In/alumina at 455 ◦ C was also studied and the apparent reaction orders were −0.32 ± 0.31 for NOX reduction and 0.26 ± 0.1 for C3 H6 oxidation. Furthermore, the dependence of NOX reduction and C3 H6 oxidation activity on the SO2 concentration was examined and in this case the apparent reaction orders were −0.62 ± 0.11 for NOX reduction and −0.46 ± 0.1 for C3 H6 oxidation. Two overall low temperature kinetic expressions were developed for the NO reduction and C3 H6 oxidation in the presence of SO2 and H2 O: r(NOX red) = kred r(C3 H6 ox) = kox

[SO2

[NOX ]0.36 0.32 [O ]0.33 3 H6 ] 2

]0.62 [C

[C3 H6 ]0.26 [O2 ]0.21 [NOX ]0.29 [SO2 ]0.46

(3) (4)

The presence of 50 ppm SO2 and 10% H2 O modified the apparent order of reaction of the reactive gases for both NOX reduction and C3 H6 oxidation over 2% In/Al2 O3 . Different apparent orders of reaction for a given compound and the same reaction can be attributed to different reaction mechanisms. 3.6. Mechanistic studies TPD studies were carried out in the same reactor loaded with 0.5 g of fresh catalyst (␥-Al2 O3 or 2% In/Al2 O3 ). The catalyst was initially heated at the adsorption temperature (450 ◦ C) under pure helium flow. At this temperature, 2000 ppm of NOX diluted in He was sent to the reactor for 1 h and the reactor was then cooled down to 100 ◦ C. The feed was switched to pure He flow at 100 ◦ C for 1 h to

desorb the weakly (physically) sorbed species. The temperature was then increased to 600 ◦ C using a heating rate of 10 ◦ C/min. Initially, the NOX sorption capacity of the support solid (␥-alumina) was examined. The feed was 2000 ppm NOX in He during the sorption stage and pure He during the desorption stage of the experiment. Most of the NOX was desorbed at 250 ◦ C, while a second smaller peak was detected at 490 ◦ C. The overall amount of the desorbed NOX was about 12.3 ␮mol. About 45% of the desorbed NOX was nitrogen dioxide (NO2 ), implying some oxidation of the sorbed NO on the sample. The high percentage of the desorbed NO2 does not necessarily imply that an equally high percentage of the inlet NO is oxidized. It is possible that NO2 is adsorbed on the catalyst more strongly than NO. The NOX released at 490 ◦ C was attributed to the decomposition of aluminum nitrate formed during the sorption stage of the experiment. The NOX desorption curves over In/Al2 O3 are shown in Fig. 13 along with the corresponding ones over alumina. The same reaction conditions as in the previous experiment were applied in Fig. 13. The NOX curve over In/alumina exhibited a peak at 275 ◦ C that was attributed to the support solid (alumina), and a second peak at 485 ◦ C. The NO2 /NOX mole ratio in both peaks was about 0.45, that is, a value close to the one measured on alumina. Integration of the NOX curve gave the total amount of the desorbed NOX from the In/Al2 O3 sample (16.7 ␮mol). This value was higher than that calculated by the integration of the first peak over the alumina sample. Therefore, the impregnated In changed the overall amount of the sorbed NOX (increase by 33%). Following the NO sorption experiments, 2% O2 were added to the NO/He sorption mixture (Fig. 13) and the TPD experiment was repeated (peaks at 175 and 325 ◦ C). More NOX (42 ␮mol) was desorbed from the sample when the feed contained O2 than in the absence of O2 (12.25 ␮mol). The presence of oxygen in the feed caused a large increase in the second NOX desorption peak (32.4 ␮mol) that was attributed to the formation of nitrates. The NO2 /NOX ratio (0.4) was slightly lower than that in the absence of O2 (0.45). The same experiment was carried out over In/Al2 O3 (Fig. 13). The overall amount of the desorbed NOX was 47 ␮mol (16.7 ␮mol in the absence of O2 ), the NO2 /NOX ratio in the first peak (240 ◦ C) was 0.41, while the corresponding ratio in the second peak (475 ◦ C) was 0.42. The height of the NOX desorption peaks were significantly larger in the presence of O2 . Also, the desorption of SO2 from the Al2 O3 and In/Al2 O3 samples was examined. The feed gas was 2000 ppm SO2 in He during the sorption stage and pure He during the desorption stage of the experiment. When the samples were exposed to the SO2 environment at 450 ◦ C a first peak at 150 ◦ C was detected, and a second peak at 240 ◦ C. At higher temperatures the SO2 emissions were not zeroed. This can be attributed to the decomposition of aluminum sulfates at elevated temperatures. Larger amounts of SO2 were desorbed from alumina than from In/Al2 O3 . A possible explanation is

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

13

160 γ-Al2O3 NOX

Concentration (ppm)

140 120

In/Al2O3 NOX

100 80 γ-Al2O3 NOX, O2

60 40

In/Al2O3 NOX, O2

20 0 100

150

200

250

300

350

400

450

500

550

600

Temperature (˚C) Fig. 13. Temperature programmed desorption experiment over the support (␥-Al2 O3 ) and In/Al2 O3 ; 2000 ppm NOX , balance He and 2000 ppm NOX , 2% O2 , balance He; flowrate: 500 cm3 /min; catalyst weight: 0.5 g.

that SO2 is adsorbed on the In sites and is subsequently oxidized to SO3 in a similar way as in the NOX oxidation. SO3 can either desorb dissociatively (formation of SO2 and O2 ) or remain in the gas phase as SO3 that cannot be detected by our analyzer. The simultaneous adsorption/desorption of NOX and SO2 in the presence of O2 over alumina and In/Al2 O3 samples (Fig. 14) was also investigated. The feed in these experiments was 2000 ppm NOX , 500 ppm SO2 , 2% O2 in He and the adsorption temperature was 450 ◦ C. The same experimental procedure for both samples was employed. The NOX desorption data for SO2 -free and SO2 -containing feeds over alumina were compared and it was noticed that the location of both NOX desorption peak (175 and 325 ◦ C) and

the NO2 /NOX mole ratio (40 and 45% in the second peak) in the absence and presence of SO2 were similar. The main difference between the two experiments was that the overall amount of the desorbed NOX was substantially higher in the absence of SO2 . The NOX desorption data from the In/Al2 O3 sample were, qualitatively, similar to those over alumina. In the presence of In the concentration of NOX was slightly higher than that in the absence of In (60% in the first peak and 45% in the second one of the emitted NOX was NO2 over In/Al2 O3 and alumina, respectively). The evolution of SO2 from the alumina and In/Al2 O3 samples is shown in the same figure. The SO2 emissions at high temperatures was attributed to the aluminum sulfate decomposition.

Concentration (ppm)

120 100 NOX (In/Al2O3) 80

SO2 (In/Al2O3)

60

NOX (γ-Al2O3)

SO2 (γ-Al2O3)

40 20 0 100

200

300

400

500

600

700

o

Temperature ( C) Fig. 14. Temperature programmed desorption experiment over the support (␥-Al2 O3 ) and In/Al2 O3 ; 2000 ppm NOX , 2% O2 , 500 ppm SO2 , balance He; flowrate: 500 cm3 /min; catalyst weight: 0.5 g.

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G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

3.7. Reaction mechanism The selective catalytic reduction of NO with hydrocarbons is a complex reaction that can be the result of several parallel and/or consecutive reaction steps. Different reaction mechanisms have been proposed for this reaction, which can be roughly classified into two general categories: the “decomposition mechanism” and the “reduction mechanism”. The later mechanism considers that the reduction of NO into N2 takes place via the formation of nitrogen and oxygen-containing intermediates, and it has been supported by many researchers [1]. In a recent study [21] where In/Al2 O3 was employed, formation of acrylonitrile was identified. Moreover, product analysis showed that the concentration of nitrile compounds was high. On the contrary, there was no evidence for the formation of –CN and –NCO species [21]. Our TPD experiments showed that the NO sorption on the catalyst was affected by the presence of In and by the reactive gas mixture. Deposition of In on the support resulted in a stronger NO–In bond as compared to the NO-support bond and the presence of O2 in the feed strongly enhanced the accumulation of N-containing species on the catalyst. The almost zero-order dependence of the NOX reduction rate over In/Al2 O3 on the concentration of NOX , as pointed out by Eq. (1), suggests that either do not participate in the rate-determining step or that NOX adsorption is the rate-determining step in the NOX reduction. The latter is supported by the low value of the apparent activation energy for NOX reduction. As a result, we believe that NOX reduction proceeds via a reaction mechanism that involves the formation of a N-containing reaction intermediate as the rate-determining step. The reaction of this species with adsorbed NOX forms N2 . On the other hand, the reaction order for the NOX concentration was negative for the C3 H6 oxidation as described in Eq. (2). This implies that strongly adsorbed NOX inhibit the reductant oxidation. For the C3 H6 concentration, a positive reaction order was determined for both rate expressions (NOX reduction and C3 H6 oxidation) over In/Al2 O3 indicating the rapid removal of the adsorbed molecular fragments of the reductant from the active sites. A fractional order was calculated for the NOX reduction that is consistent with a reaction mechanism where adsorbed species are involved and a first order of reaction for the combustion. The zero-order dependence for the O2 concentration on the NOX reduction rate over In/Al2 O3 can be the result of the following phenomena: a positive contribution due to the activation of the reductant by O2 and a negative contribution due to the competition between O2 and NOX species to oxidize the reductant. Finally, the half order positive dependence for the O2 concentration on the C3 H6 oxidation rate suggested activation of the oxygen species by dissociative adsorption. The kinetic results for the NOX reduction over In/Al2 O3 in the absence of poisonous gases, described previously, support the following reaction scheme: initially, NO adsorbs on

the catalyst surface where reacts with a dissociative adsorbed oxygen to form NO2 . The reductant interacts with NO2 to form an activated intermediate of the type CX HY OZ N (nitrile or nitrite or nitrate, etc.). The final step involves decomposition of this intermediate by reaction with other activated NOX species, yielding N2 and combustion products. The presence of poisonous gases (50 ppm SO2 , 10% H2 O) significantly affected the reaction system. The kinetic analysis results presented previously showed that both reactions are negatively affected by the presence of sulfur dioxide. Noble metals normally enhance the SO2 oxidation to SO3 resulting in the formation of sulfate species that were identified in the TPD experiments. These sulfate species remain stable on the catalytic surface up to 480–500 ◦ C, thus, resulting in lower catalytic activity. It was interesting to note that the presence of sulfur dioxide in the feed affected both NOX reduction and propene oxidation reaction, since we observed lower NO2 formation rates and CO2 selectivity. The low quantities of NO2 detected in the presence of poisonous gases can be attributed specifically to SO2 since through its oxidation to SO3 , poisons the sites where the NO oxidation take place. Also, it is possible that SO2 reacts with NO2 forming NO and SO3 . We believe that the NO reduction over In/Al2 O3 follows a different reaction pathway when poisonous gases are absent or present in the feed. The higher apparent activation energies measured in the poisonous-containing experiments imply that there is a less complicated reaction scheme than the one proposed for the case of the SO2 -free experiments. The addition of the SO2 in the feed causes sulfation of the support and, thus, deactivation of some of the oxidation sites. Consequently, by inhibiting the oxidation reaction, the concentrations of the compounds involved in the NO reduction are expected to be higher. Based on the above remarks, the following reaction mechanism was proposed for the NO reduction by C3 H6 in the presence of poisonous gases and excess O2 : the adsorbed NOX species on In react directly with CX HY OZ , previously produced by the partial oxidation of propene, and produce nitrogen and combustion products.

4. Conclusions Mechanistic and kinetic analysis has been performed for the selective catalytic reduction of NOX by C3 H6 in excess oxygen over In/Al2 O3 , in the absence and presence of 50 ppm SO2 and 10% H2 O. Various In loadings (1–4%) have been employed to determine the effect of metal loading on the catalyst performance. The 2% In/Al2 O3 catalyst exhibited the highest conversion as compared to the other In loadings. The performance of the above catalyst was tested using various pretreatment procedures (prior to reaction). These experiments showed that in the absence and the presence of poisonous gases and starting either from the lowest or the highest temperature, preoxidized samples were the most active ones.

G.E. Marnellos et al. / Applied Catalysis B: Environmental 48 (2004) 1–15

The activity of the In/Al2 O3 catalyst was also measured in the presence of H2 O (0–10%) and/or SO2 (0–500 ppm) in the feed. The presence of water enhanced the catalyst’s activity for NOX reduction, while SO2 acted as an inhibitor. We postulate that SO2 poisoned the catalyst surface through the formation of aluminum sulfates and consequently blocked the oxidation sites on the surface where the NO oxidation to NO2 takes place. A different reaction mechanism for the NO reduction over In/Al2 O3 applies when SO2 is absent or present in the feed. In the former case, CX HY OZ N is initially formed as a result of the C3 H6 reaction with NO2 , and it reacts with other activated NOX species. In the presence of SO2 , where the activation energies are higher as compared to the absence of poisonous gases, a less complicated mechanism was proposed. The adsorbed NOX species on In react directly with a CX HY OZ , an intermediate formed by the incomplete oxidation of C3 H6 , to produce nitrogen and combustion products. Acknowledgements This work was funded by the Commission of the European Community, under Contract ENK5-CT-1999-00001. References [1] V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. [2] M.D. Amiridis, T. Zhang, R.J. Farrauto, Appl. Catal. B 10 (1996) 203. [3] M. Iwamoto, H. Yahiro, S. Shundo, Y. Yuu, N. Misono, Appl. Catal. 69 (1) (1991) L15. [4] P. Ciambelli, P. Corbo, M. Gambino, G. Minelli, G. Moretti, P. Porta, Catal. Today 26 (1996) 33.

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