Screening of TiO2-Supported Catalysts for Selective NOx Reduction

Detailed reaction kinetics for double-layered Pd/Rh bimetallic TWC monolith catalyst. Sung Bong Kang , Seok Jun Han , In-Sik Nam , Byong K. Cho , Chan...
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Ind. Eng. Chem. Res. 2004, 43, 7723-7731

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Screening of TiO2-Supported Catalysts for Selective NOx Reduction with Ammonia Mikaela Wallin,*,†,‡ Stefan Forser,†,‡ Peter Thorma1 hlen,† and Magnus Skoglundh†,‡ Competence Centre for Catalysis and Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden

In the present study the catalytic activities of several TiO2-supported metal and metal oxide catalysts have been tested simultaneously for the selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR) using high-throughput-screening equipment. Two different series of various catalyst samples were evaluated for different feed gas compositions, both with and without water, and before and after SO2 exposure. The first series of samples consisted of catalysts containing a single metal or metal oxide (Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Rh, Ag, W, Ir, or Pt), including a commercial SCR catalyst sample for comparison. Of the metals investigated in the first series, Cr, Mn, Fe, and Rh were found to have the most interesting catalytic properties, regarding activity and selectivity at high or low temperatures. A second series of samples were prepared containing combinations of the selected metals, and the results from the catalytic tests show a general trend of increased SCR activity at lower temperatures when Rh is added to the multiple-metal samples. The results further indicate that it is possible to widen the active temperature window by combining different metals in the catalyst formulation. 1. Introduction From a global warming perspective, diesel engines are presently preferred alternatives to conventional Otto engines in heavy-duty vehicles due to lower fuel consumption and hence lower emissions of CO2. Diesel engines operate at high air-to-fuel ratios, which give oxygen excess in the exhaust, and this makes it difficult to reduce the nitrogen oxides (NOx) formed during the combustion. The concern in our society to reduce environmental hazards has led to strict NOx emission regulations, which soon will be even stricter. To meet the forthcoming regulations, it is necessary to overcome the difficulty of reducing NOx under oxygen excess. An established technique for stationary units, which recently has gained increased attention for automotive applications, is selective catalytic reduction of NOx using ammonia or ammonia-releasing compounds (e.g., urea) as the reducing agent (NH3-SCR). Ammonia is added to the exhaust gas, where it reacts selectively with the NOx over a catalyst to form harmless nitrogen and water.1 A well-developed and frequently used SCR catalyst is vanadia supported on titania (V2O5/TiO2) with different promoters and stabilizers. This catalyst operates typically in the temperature interval of 250450 °C. However, introducing NH3-SCR in a vehicle, where the flow rate and temperature vary significantly, requires a catalyst that can operate in a wider temperature window, especially at lower temperatures. An additional disadvantage with the vanadia catalyst is the toxicity of vanadium. A vanadia-free catalyst is therefore preferable for vehicle applications. A wide range of different noble metals and base metal oxides have been the focus of many studies on NH3-SCR,

and many of the materials tested have shown promising catalytic properties for the NH3-SCR reaction.2-10 It is, however, difficult to compare results from different studies, due to varying preparation procedures and testing conditions. In our laboratory we have constructed a high-throughput-screening (HTS) reactor which enables testing of a variety of catalyst formulations, prepared similarly to real vehicle catalysts.11 The most important advantage with this reactor setup, besides the high-throughput capacity, is that all catalysts in a test series are exposed simultaneously to the reaction gas for equally long times and at the same temperature. It is thus possible to compare the activity of the catalysts in a fresh state and also during or after different deactivating treatments. The objective of the present study was to prepare a number of TiO2-supported metal and metal oxide samples and to compare the catalytic activity and selectivity of these materials for the NH3-SCR reaction using the HTS reactor. The selection of catalyst candidates has been based on catalytic performance, not regarding environmental impact. An important part of this study was to prepare the samples so that they are comparable to catalysts used in exhaust gas after-treatment, but also suitable for testing in the HTS reactor. The catalytic properties of the samples were evaluated in the HTS reactor for different gas compositions, in the fresh state and after sulfur exposure. Two series of catalysts were evaluated, where the first series of samples consisted of catalysts that, besides TiO2, contained a single metal or metal oxide and the second series contained a combination of metals or metal oxides based on the most promising results from the first series. 2. Experimental Section

* To whom correspondence should be addressed. Tel.: +46 31 772 29 59. Fax: +46 31 772 29 67. E-mail: mikaela@chem. chalmers.se. † Competence Centre for Catalysis. ‡ Department of Applied Surface Chemistry.

2.1. Sample Preparation. The catalyst samples in the present study were different transition metals or oxides of transition metals supported on a TiO2 washcoat, which was coated on small steel disk substrates.

10.1021/ie049695t CCC: $27.50 © 2004 American Chemical Society Published on Web 10/23/2004

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The washcoat consisted of a high surface area anatase TiO2 powder (Hombikat UV 100, Sachtleben Chemie GmbH) and titania butoxide (Ti[O(CH2)3CH3]4; SigmaAldrich). Titanium butoxide reacts with water to insoluble titanium hydroxide, which forms titanium oxide during calcination. The titanium butoxide was thus used as a binder in the washcoat but also as a dispersing agent, which facilitated the spraying procedure, when the TiO2 powder was deposited on the substrates. Three different binder concentrations were tested, and after the tests were evaluated, it was decided to use 55 wt % TiO2 powder and 45 wt % titanium butoxide in the washcoat for most of the samples. The catalyst substrate disks were 7 mm in diameter and 0.03 mm thick, cut from a stainless steel foil (C, 0.02%; Cr, 20%; Al, 5.5%; rare-earth metals, 0.02%; balance Fe, Sandvik 0C404). Before the washcoat was deposited, the disks were cleaned in acetone and calcined in air for 24 h at 1000 °C. During calcination a thin layer of Al2O3 crystals was formed on the surface of the disks, making the surface adhesive for the washcoat. The substrate disks were coated by spraying a slurry, containing the TiO2 powder, the binder, and dry ethanol (99.5%, Kemetyl), onto the disks, which were kept heated to about 130 °C. The deposition followed a two-step procedure where first the heated disks were simultaneously sprayed with slurry and then calcined at 550 °C for 2.5 h in air. This procedure was repeated twice to give a final washcoat weight of 2 mg of TiO2 per disk. The active metals of the catalysts in this study were Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Rh, Ag, W, Ir, and Pt. The deposition of the metals or metal oxides was achieved by impregnating the TiO2 washcoat with an aqueous metal or metal oxide precursor solution. The concentrations of the precursor solutions were chosen so that the impregnation could be performed by adding equal volumes of solution to the disks, using a micropipet. The water was then subsequently evaporated by heating the disks to about 120 °C, and the samples were finally calcined at 500 °C for 1 h in air. Metal nitrate salts were preferably used as precursors to make the preparation of the different samples as equivalent as possible. However, in the case of tungsten, vanadium, molybdenum, and iridium other salts were used. A full description of all catalyst precursors is presented in Table 1. Two series of catalysts were prepared and tested. The first series consisted of samples with a single metal or metal oxide supported on TiO2 (Table 1). The concentrations of the precursor solution gave, after impregnation, 3.8 µmol of metal, corresponding to 10 wt % Mn, except for the Rh, Pt, and Ir solutions, which gave 0.32 µmol, corresponding to 3 wt % Pt. Two samples with a commercial SCR catalyst were prepared as reference samples by spraying slurries consisting of a commercial vanadia-based SCR catalyst powder sample, titanium butoxide binder (in two different concentrations), and ethanol onto two disks. A blank disk was also included in the test series, as well as a sample with only TiO2. Three samples were made in duplicate, and the effects of the different amounts of binder were tested for three catalysts. The second series of samples consisted of combinations of active metals or metal oxides, based on the catalytic properties observed for the samples in the first series. All samples were impregnated with precursor solutions, containing all metals involved, except Rh, with concentrations so that equal volumes gave equimo-

Table 1. Samples in the First Series of Catalysts

no.

sample name

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

blank Cr sample 1 Fe sample 1 Mo Ni W commercial 45 TiO2 Ag V Co Mg Pt Rh Ir commercial 20 Mn sample 1 Cr sample 2 Fe sample 2 Cu Mn sample 2 Mn65 Fe65 Mn20 Fe20

e

precursor

amount of metal (µmol)

Cr(NO3)3‚9H2Oa Fe(NO3)3‚9H2Oa (NH4)6Mo7O24‚4H2Ob Ni(NO3)2‚6H2Oa WO3c

0 3.8 3.8 3.8 3.8 3.8

AgNO3d VOSO4‚5H2Oa Co(NO3)2‚6H2Oa Mg(NO3)2‚6H2Oa Pt(NO3)2-solutiond Rh(NO3)3-solutiond IrCl3-solutiond

0 3.8 3.8 3.8 3.8 0.32 0.32 0.32

Mn(NO3)2‚4H2Oa Cr(NO3)3‚9H2Oa Fe(NO3)3‚9H2Oa Cu(NO3)2‚3H2Oa Mn(NO3)2‚4H2Oa Mn(NO3)2‚4H2Oa Fe(NO3)3‚9H2Oa Mn(NO3)2‚4H2Oa Fe(NO3)3‚9H2Oa

3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8

amount of binder (wt % in slurry) 0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 2.0 4.5 4.5 4.5 4.5 4.5 6.5 6.5 2.0 2.0

a Merck Eurolab. b Riedel-de Hae ¨ n. c Sigma-Aldrich. d Labassco. Johnson Matthey.

lar amounts of metals in the samples. For the Rhcontaining samples, rhodium nitrate solution was added in a second impregnation step, after the formation of the metal oxides. The compositions, corresponding to the molar ratios of metal or metal oxides in the samples, are presented in detail in Table 2. 2.2. Catalyst Characterization. The specific surface area was determined by N2 physisorption at 77 K according to the BET method. The crystalline phase was determined by powder X-ray diffraction (XRD) using a Siemens D5000 diffractometer with Cu KR radiation. The surface morphology and composition were studied using a scanning electron microscope (JEOL 880), equipped with an EDS detector (Link AN 10000). 2.3. Reactor Setup. The catalytic activity for NOx reduction with ammonia was investigated by experiments performed in a high-throughput-screening system, previously described in detail.11 The design of the system is shown schematically in Figure 1. The lower part consists of the sample positioning and heating unit, which contains an IR heater, the sample tray, the exhaust line, and the X-Y-Z unit. The latter is used to move the sample tray to the desired sample positions. At the top of the screening system, there is a circular hood working as the gas delivery/removal and gassampling probe unit. The gas sampling is performed for one sample at a time in the center of the hood. However, the construction of the hood enables exposure of reaction gas also to all the other samples not being analyzed. Thus, in the present study, the samples were exposed to the same gas mixture at the same temperature for equally long times. Feed gases were mixed from Ar, O2, NO (1.0% in Ar), NO2 (5000 ppm in Ar), NH3 (1.01% in Ar), and SO2 (0.25% in Ar) (all gases of 99.999% purity, Air Liquid) and introduced to the gas delivery unit via individual mass flow controllers (Brooks). The gassampling probe is a capillary inlet to a quadrupole mass spectrometer (Balzers QMA 125), which continuously

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7725 Table 2. Samples of the Second Series of Catalysts amount of metal (µmol)

no.

sample namea

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

blank Cr100 Fe100 Mn100 Cr75/Mn25 Cr50/Mn50 Cr25/Mn75 Cr50/Fe50 Cr25/Fe75 Mn75/Fe25 Mn50/Fe50 Mn25/Fe75 Mn50/Cr25/Fe75 Mn25/Cr50/Fe25 Mn25/Cr25/Fe50 commercial Cr100/Rh Fe100/Rh Mn100/Rh Cr50/Fe50/Rh Mn50/Fe50/Rh Mn50/Cr50/Rh Mn50/Cr25/Fe25/Rh Mn25/Cr50/Fe25/Rh Mn25/Cr25/Fe50/Rh

Cr

Mn

Fe

Rh

amount of binder (wt % in slurry)

0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12

0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5

3.8 3.8 2.85 1.9 0.95 1.9 0.95

0.95 1.9 0.95

3.8 0.95 1.9 2.85 2.85 1.9 0.95 1.9 0.95 0.95

3.8 3.8 3.8 1.9 1.9 0.95 1.9 0.95

1.9 1.9 1.9 0.95 0.95

1.9 2.85 0.95 1.9 2.85 0.95 0.95 1.9

1.9 1.9 0.95 0.95 1.9

a The name of the sample indicates the composition of metals in the sample; e.g., sample Mn50/Cr25/Fe25 contains 50% (mol %) Mn, 25% Cr, and 25% Fe based on a total of 3.8 µmol of metal.

Figure 1. Schematic illustration of the high-throughput-screening reactor.

monitors the outlet gas composition. The ion currents analyzed in the present study were m/e ) 17 (NH3, H2O), 18 (H2O), 28 (N2, N2O), 30 (NO, N2O, NO2), 32 (O2), 40 (Ar), 44 (N2O), and 46 (NO2). In the test series performed in the present study the outlet gas composition was analyzed for 2 min over each sample; thereafter, the sample tray was automatically moved to the next sample. 2.4. Activity Measurements. The SCR activity measurements of the samples were performed using three different feed gas compositions presented in Table 4, i.e., feedNO (NO + NH3 + O2 + Ar), feedNO+NO2 (NO

+ NO2 + NH3 + O2 + Ar), and feedNO+H2O (NO + NH3 + O2 + H2O + Ar). The samples were exposed for each gas mixture under atmospheric pressure with a global gas flow of 500 mL/min and a local sample flow of 20 mL/min. The catalytic activity was sequentially analyzed, at constant temperature (starting at 450 °C), for 2 min over each sample, which was enough to reach a stable concentration level of the outlet gases. When all samples were analyzed, the temperature was lowered by 50 °C to the next temperature level. Prior to each experiment the samples were preoxidized by exposure of 10% O2 in Ar for 20 min at 450 °C, followed by exposure of the current gas mixture for 1 h at 450 °C to stabilize the samples. The effect of sulfur was additionally investigated. The samples were first exposed to the same feed gas mixture as in feedNO+H2O but also containing 25 ppm SO2 at a temperature of 250 °C for 1 h. The SO2 was thereafter removed from the feed, and the temperature was raised to 450 °C, after which the samples were exposed to the feedNO+H2O gas mixture for 1 h. Finally, the catalytic activity was measured during exposure of the feedNO+H2O gas mixture at two temperature levels, 300 and 200 °C. 3. Results and Discussion 3.1. Catalyst Characterization. The results from the BET surface area measurements for the TiO2 powder and four TiO2-coated samples containing 0, 20, 45, and 65 wt % titanium butoxide binder in the washcoat are presented in Table 3. The results show that the surface area of the TiO2 was somewhat decreased by adding titanium butoxide binder; however, equal surface areas were observed for the samples with 20 and 45 wt % titanium butoxide binder in the washcoat. The results from the X-ray diffraction measurements are shown in Figure 2. An attempt was made to investigate the crystalline phases in a disk coated with TiO2 and the oxides of the active metal in a disk containing Mn and TiO2. However, anatase TiO2 was the only crystalline phase revealed from the X-ray diffraction. The samples in the present study are thus only presented by the current metal, although this may be present as a crystalline or amorphous oxide. An SEM image of the surface of a sample with TiO2 washcoat, including binder, is shown in Figure 3. EDS analysis of the sample showed peaks corresponding to Ti and Cu, from the TiO2 washcoat and the copper sample holder. No Al was detected on the surface of the sample. 3.2. Activity Tests of the First Sample Series, Single Metal and Metal Oxides. The SCR activity of the samples in the first series was initially tested using feed gas mixture feedNO (containing NO, NH3, and O2) in the HTS reactor. The results are summarized in Figure 4, where the NOx conversion is plotted as one bar for each sample at each temperature level. At higher temperatures the commercial SCR catalyst showed considerably higher activity compared to the other catalysts. However, in the lower temperature region the Pt catalyst showed the highest activity. The samples containing manganese showed SCR activity already at 150 °C, and the activity remained high in a relatively wide temperature range. The samples containing Cr showed, with the exception of the commercial catalyst, the highest activity at higher temperatures. Many of

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Table 3. Specific Surface Areas sample description

specific surface area (m2/g)

TiO2 powder disk with TiO2 (no binder)

238 105

Table 4. Compositions of the Feed Gas feed gas abbreviation

[NO] (ppm)

[NO2] (ppm)

[NH3] (ppm)

[O2] (%)

[H2O] (%)

feedNO feedNO+NO2 feedNO+H2O

1000 500 1000

500

1000 1000 1000

10 10 10

5

Figure 2. X-ray diffractogram of the Hombikat UV 100 TiO2 powder, a sample containing only the TiO2 washcoat, and a sample containing Mn deposited on the TiO2 washcoat. The sticks represent the reference XRD peaks of anatase TiO2.

Figure 3. Scanning electron micrograph of a sample containing only the TiO2 support.

the samples showed activity only at temperatures exceeding 250 °C, and samples containing W, Ni, Co, and Mg showed almost no NOx reduction. The Cocontaining sample showed negative NOx reduction due to the formation of NOx from the NH3 oxidation reaction, which competes with the SCR reaction. There can also be undesired formation of N2O in side reactions to the SCR reaction. In the present study substantial amounts of N2O were observed for many of the samples and for the Pt-containing sample in particular. However, only very small amounts of N2O were observed for the commercial catalyst and the Fe-containing samples. The samples containing Mn, which showed high activity for NOx reduction, seemed to produce only moderate amounts of N2O at lower temperatures; however, the N2O formation increased as the temperature increased. The commercial samples and the Mn- and Fe-containing samples were prepared using different amounts of

sample description

specific surface area (m2/g)

disk with TiO2 and 45 wt % binder disk with TiO2 and 65 wt % binder disk with TiO2 and 20 wt % binder

75 55 71

titanium butoxide in the washcoat slurry to investigate any effect of the binding agent on the catalytic properties. The effect on the catalytic activity was not significant. For the Fe-containing samples the activity was almost constant; however, slightly different activity was observed for the Mn-containing samples. With a higher amount of binder in the washcoat slurry (Mn sample 1 and Mn sample 2), there seemed to be slightly higher SCR activity (compared to that of Mn 20). This minor effect may be related to the fact that it was much easier to obtain an even distribution of the TiO2 powder for the higher amount of binder, which also facilitated the metal precursor deposition. For the commercial samples it can, however, be observed that a higher amount of binder in the slurry results in decreased activity. This affect is probably due to the fact that the active component in the commercial sample is already deposited on the support material and the binder might block some of the active sites. 3.2.1. Effect of Increasing the NO2/NO Ratio. In Figure 5 the NOx reduction is presented for experiments performed using gas mixture feedNO+NO2, which contained equimolar amounts of NO and NO2. Compared to the experiments with only NO (Figure 4), there were changes in activity for most of the samples with an increased NOx reduction at lower temperatures as a general trend. For samples containing Ni and Rh the increased NO2/NO ratio increased the activity considerably in the whole temperature window, and the Rhcontaining sample now showed by far the highest activity at 150 and 200 °C accompanied by very low formation of N2O. A widening of the active temperature window, especially toward the lower temperatures, is the usual behavior of V2O5-based SCR catalyst when the NO2/NO ratio is close to 1,12 and this observation has also been reported for zeolite-based catalysts.13,14 In the present study an evident widening of the activity window was observed for the commercial sample, and also a slight broadening was observed for the vanadiacontaining sample, prepared in the present study. However, this was not a trend observed for any of the other samples. Even though the activity increased at lower temperatures for the Mn-, Fe-, and Cr-containing samples, the activity decreased at higher temperatures. Hence, the activity window was not broadened but shifted toward lower temperatures for these samples. For the samples containing Pt, Ir, Ag, and Cu the activity decreased over almost the whole temperature range with increased NO2/NO ratio. 3.2.2. Effect of H2O and SO2. The effect of H2O present in the feed gas was investigated in an experiment with gas mixture feedNO+H2O, containing NO, O2, and H2O (Figure 6). The presence of water suppressed the NOx reduction for all catalysts, especially at lower temperatures. For some samples the influence of water was considerable and the activity for NOx reduction was completely lost. However, the commercial samples and the samples containing V, Mn, Cr, Fe, Pt, and Rh still showed reasonable NOx reduction activity in the presence of water. Compared to that of the corresponding experiments in the absence of water, the activity for NOx

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Figure 4. NOx conversion for the catalysts in the first series (see Table 1) with inlet gas composition 1000 ppm NO, 1000 ppm NH3, 10% O2, and Ar as the balance. Negative values are not presented.

Figure 5. NOx conversion for the catalysts in the first series with inlet gas composition 500 ppm NO2, 500 ppm NO, 1000 ppm NH3, 10% O2, and Ar as the balance.

reduction was about 50% lower. At higher temperatures the commercial samples showed the highest activity with water present and lost about 30% of the NOx reduction without water. The samples containing Mn and Pt were least affected by water at lower temperatures. To study the effect of sulfur on the SCR activity, the catalysts were exposed to 25 ppm SO2 for 1 h followed by an activity test performed at 200 and 300 °C using gas mixture feedNO+H2O. In this experiment the catalytic activity at 200 °C was lost for all samples except for the sample containing Pt, and at 300 °C some NOx

reduction was observed for the commercial and the Pt-, Rh-, and Cr-containing samples. 3.3. Activity Tests of the Second Sample Series, Mixed Metal and Metal Oxides. Mixed metal oxide catalysts have previously been reported to be suitable catalysts for the SCR reaction.3 Of the samples tested in the first series in the present study, Mn, Fe, Cr, and Rh showed the most interesting catalytic properties of the tested single metal or metal oxide based samples. Samples based on Mn, Fe, Cr, and Rh were prepared in a second series of samples, containing a mixture of metals. Mn and Rh were selected for the high NOx

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Figure 6. NOx conversion for the catalysts in the first series with inlet gas composition 1000 ppm NO, 1000 ppm NH3, 10% O2, 5% H2O, and Ar as the balance.

Figure 7. NOx conversions at 200 °C for the second sample series (represented by the height and color of the pin), without Rh (left) and with Rh (right) and with only NO (top) and with equimolar NO + NO2 (bottom) in the feed. The axes in the ternary diagrams represent the molar ratio (%) of Mn, Fe, or Cr in the samples.

reduction activity at lower temperatures. Fe was selected for the high selectivity to N2 formation, and Cr was selected for the relatively high resistance to deactivation. Pt, which showed very high activity at lower temperatures, was excluded due to the very high N2O formation over this sample. A commercial sample (this time prepared without the titanium butoxide binder) and a blank sample were included in the series for comparison (see Table 2).

The same experiments as performed using the first series were also performed with the second series of samples. Figures 7-9 present an overview of the performance of the different samples at three different temperatures, 200, 300, and 400 °C, respectively, for experiments performed with gas mixtures feedNO (containing NO, NH3, and O2) and feedNO+NO2 (containing NO, NO2, NH3, and O2). The NOx conversions of the different mixed metal samples have been plotted in

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Figure 8. NOx conversions at 300 °C for the second sample series, without Rh (left) and with Rh (right) and with only NO (top) and with equimolar NO + NO2 (bottom) in the feed.

Figure 9. NOx conversions at 400 °C for the second sample series, without Rh (left) and with Rh (right) and with only NO (top) and with equimolar NO + NO2 (bottom) in the feed.

ternary diagrams. Each dot in the diagrams represents one sample, where the metal content of the sample is determined by its position in the diagram. The color intensity of the dots (and the height of their stick) increases with increasing NOx reduction, according to the color scale. The Rh-containing samples are plotted

in a separate diagram (right diagrams). The first observation is that the NOx reduction generally seems to be slightly lower compared to that of the first series of samples. The three samples containing Mn, Fe, and Cr were freshly prepared for the second series, and it is therefore difficult to determine whether the change

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in activity is due to slightly different preparation conditions, or to small changes in the reactor setup. Having the changed conditions in mind, it is clear that the conversion values from the second series of samples should not directly be compared to the results from the first series. The effect of mixing different metals in the catalyst formulation is not obvious from the activity data in the present study (Figures 7-9). However, when the results were studied in more detail, an effect of the mixing was observed. In the experiment performed with gas mixture feedNO (Figures 7-9, top), the activity for several of the mixed metal samples increased compared to the predicted catalytic activity based on the activities of the single metal samples. The predicted NOx conversions of the mixed metal samples were estimated by adding the NOx conversions of the equivalent single-metal samples weighted by the corresponding molar fraction of the metals in the mixed sample. The observed increase in activity for some of the mixed metal samples indicates either a synergistic effect between the mixed compounds, or that the involved metals likely have formed a mixed metal oxide phase, where possible mixed oxides are spinels, i.e., CrxMnyFe3-x-yO4, where x + y e 3. However, what kind of oxide phase and to what extent was not possible to determine from the results in the present study. The activity increase was for many samples observed in the lower temperature region (Figure 7), without losing activity at higher temperatures (Figure 9), indicating that it was possible to widen the active temperature window by mixing different metals. An example of this is the sample containing equimolar amounts of Cr and Fe (Cr50/Fe50, middle right in the ternary diagram), which showed about the same activity as or a slightly higher activity than the Cr sample and the Fe sample over the whole temperature range. The selectivity to N2 is of course an important property for an SCR catalyst. In all mixed samples, the selectivity seemed to be related to the amount of Fe present in the sample, and a higher selectivity to N2 was observed with a higher content of Fe in the sample. For gas mixture feedNO+NO2 (containing NO, NO2, NH3, and O2) a considerable increase in activity could be observed in the lower temperature region for all samples (Figure 7, bottom). However, in the higher temperature region (Figure 9, bottom), the activity was observed to be more or less the same or slightly decreased compared to the activity with only NO in the feed (Figure 9, top). 3.3.1. Effect of Rh. In the second half of the sample series with mixed metals, a small amount of Rh was added to the samples (3 mol % Rh based on the metal content). A general trend of enhanced NOx reduction at lower temperatures was observed for Rh-containing samples (Figures 7 and 8), particularly when NO2 was included in the feed (Figure 8). At higher temperatures the addition of Rh did not seem to affect the activity noticeably for the samples containing only Fe or Cr, whereas the NOx reduction significantly decreased over the sample containing only Mn. As for the mixed metal samples, there was no observed trend in the effect of Rh addition. It was notable, however, that the decrease in NOx reduction was not related to the amount of Mn in the sample, as would be expected from the considerable decrease in activity observed in the sample with only Mn. An example of this is the Rh-containing sample

with equimolar amounts of Cr and Mn (Cr50/Mn50/Rh), which showed a higher NOx reduction compared to the sample without Rh and also compared to the samples with the corresponding single metals. Consequently, no general trend could be observed at higher temperatures for the mixed metal samples containing Rh. 4. Concluding Remarks Several transition-metal and metal oxides supported on TiO2 have been tested simultaneously using a highthroughput-screening reactor. The samples were prepared in a way that resembles the conventional way of preparing catalysts for laboratory tests, and the HTS equipment used is a flow reactor. Using this system, it was possible to evaluate and compare numerous samples for the catalytic reduction of NOx using ammonia as a reducing agent under the same reaction conditions in a short time. Of the materials tested, Fe, Mn, Cr, and Rh were found to have the most interesting catalytic properties for ammonia SCR, regarding activity and selectivity at high or low temperatures. Mn showed good activity for NOx reduction in the lower temperature range, whereas Cr showed high reduction at higher temperatures and a relatively high resistance to SO2 deactivation. The Fe sample showed relatively high NOx reduction in the higher temperature region in combination with a very low N2O formation. Rhodium-containing samples showed remarkably high activity for NOx reduction at low temperatures when NO2 was included in the feed. The presence of water in the feed during the SCR reaction clearly hampered the NOx reduction over all samples. A more severe deactivation was, however, observed when the samples were exposed to SO2, after which only a few samples showed noticeable SCR activity. A second generation of samples was prepared using selected metals from the first generation. The effect of mixed metals in the catalyst formulation proved to be hard to deduce from these experiments, and there was no observed general trend. However, the results indicate that a widening of the active temperature window can be achieved with the right mixture of metals in the catalyst formulation, exemplified in the present study by a sample with 50% of the metal content being Fe and 50% being Cr. This sample showed equivalent or slightly higher activity over the whole temperature range, compared to the samples with the current single metals. When a small amount of Rh was added to the samples, a general trend of increased NOx reduction in the lower temperature region was observed. Acknowledgment Calesco Foil AB, Sweden, is greatly acknowledged for providing a thin steel foil used in the preparation of the samples and Sachtleben Chemie GmbH for providing the Hombikat UV 100 TiO2 powder. This study was partly performed within the LOTUS-project, which is financially supported by the European Commission (Grant No. GRD1-1999-11135), and partly within the Competence Centre for Catalysis, which is funded by Chalmers University of Technology and the Swedish National Energy Agency and the member companies AB Volvo, Johnson Matthey-CSD, Saab Automobile Powertrain AB, Perstorp AB, Albemarle Catalysts BV, the Swedish Space Corp., and AVL MTC AB.

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Received for review April 14, 2004 Revised manuscript received August 18, 2004 Accepted September 10, 2004 IE049695T