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Effect of the Composition of Spinel-Type Ga2O3-Al2O3-ZnO Catalysts on Activity for the CH4-SCR of NO and Optimization of Catalyst Composition Masaru Takahashi,† Tetsu Nakatani,† Shinji Iwamoto,† Tsunenori Watanabe,‡ and Masashi Inoue*,† Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan, and The Kansai Electric Power Company, Inc., 3-11-20, Nyakuoji, Amagasaki 661-0974, Japan
Spinel-type Ga2O3-Al2O3-ZnO mixed oxides were prepared by the glycothermal reaction of zinc acetylacetonate hydrate, gallium acetylacetonate, and aluminum isopropoxide. With increasing Zn content, the crystallinity of the spinel structure of the glycothermal product increased, although excess Zn content resulted in the formation of zinc oxide as a byproduct. To optimize the Ga/Al/Zn ratio, the activities of catalysts prepared from various starting compositions and calcined at 700 °C were tested for the methaneSCR of NO under wet conditions. For this reaction, catalysts with compositions in a narrow range exhibited high activity. An increase in crystallinity of the catalyst decreased the amount of adsorbed water on the surface of the catalyst and improved the tolerance for water present in the feed gas, which inhibited the catalytic activity for the methane-SCR of NO. The optimal ratio for the spinel-type Ga2O3-Al2O3-ZnO catalyst was Ga/Al/Zn ) 1/5/0.75. This catalyst exhibited a higher activity than γ-Ga2O3-Al2O3 binary catalyst for the methane-SCR of NO under wet conditions. 1. Introduction Emissions of nitrogen oxides (NOx) in the exhaust gases of both mobile and stationary combustion devices must be regulated because these byproducts are toxic pollutants that lead to acid rain and photochemical smog.1 Currently, NOx emissions from stationary combustion sources are removed by the selective catalytic reduction (SCR) process using V2O5/TiO2-based catalysts and ammonia as a reductant.2 Although this reaction system has the advantage of its stability under oxygen-rich conditions, ammonia is very toxic and expensive. Therefore, safer and less expensive reductants such as hydrocarbons have been sought. Methane is one of the candidate reductants,3-7 because it is the main component of natural gas, an abundant resource, and a fuel for the power stations. Moreover, excess methane can easily be removed by catalytic combustion if methane leaks from the SCR device. It is well-known that gallium oxide is an effective catalyst for the dehydrogenation of paraffins to olefins8 and that Gacontaining zeolite catalysts are active for NOx removal using hydrocarbons as reducing agents.9 Shimizu et al. reported that γ-alumina-supported gallium oxide (Ga2O3/γ-Al2O3) catalyst prepared by the impregnation of γ-Al2O3 (JRC-ALO4) with gallium nitrate showed a high activity for the SCR of NOx with methane.3,4 Okimura et al. reported that Zn-Al-Ga complex oxide (Ga/Al/Zn ) 7/7/3) prepared by coprecipitation followed by calcination at 800 °C was very active for the SCR of NOx with hydrocarbons under dry conditions.10 In a previous article, we reported that a γ-Ga2O3-Al2O3 solid solution catalyst prepared by the glycothermal method showed an activity higher than that of impregnation catalysts for the methane-SCR of NO.11 However, the catalytic activity significantly decreased in the presence of water in the reaction gas.12 Therefore, we explored modifiers for the catalyst to improve * To whom correspondence should be addressed. E-mail: inoue@ scl.kyoto-u.ac.jp. † Kyoto University. ‡ The Kansai Electric Power Company, Inc.
the tolerance for water and found that Zn-modified Ga2O3Al2O3 catalyst had a high tolerance for water. In this work, therefore, the catalyst composition was optimized, and the effects of Zn modification on the performance of the γ-Ga2O3-Al2O3 catalysts were investigated. 2. Experimental Section 2.1. Preparation. The catalysts were prepared by the glycothermal method developed in our laboratory.13,14 Gallium triacetylacetonate [Ga(acac)3; Mitsuwa Chemical], aluminum triisopropoxide (AIP; Nacalai Tesque), and zinc diacetylacetonate hydrate [Zn(acac)2‚H2O; Nacalai Tesque] were mixed in various Ga/Al/Zn ratios and suspended in 80 mL of 1,5pentanediol (1,5-PeG; Wako) in a test tube; the test tube was then placed in a 200-mL autoclave. In the gap between the autoclave wall and the test tube, an additional 30 mL of 1,5PeG was added to carry the heat evenly. The autoclave was completely purged with nitrogen, heated to 300 °C at a rate of 2.5 °C/min, kept at 300 °C for 2 h, and cooled to room temperature. The product was washed with acetone by vigorous mixing and centrifuging and then air-dried. Ceramic yields of the catalysts were over 95%. In this article, the catalysts are specified by the Ga/Al/Zn ratio of the feed composition loaded for the glycothermal reaction. Note that ICP emission analysis indicated that the catalysts had essentially the same atomic ratios as the feed ratios. 2.2. Catalytic Activity Test. Catalyst tests for the SCR of NO with methane were carried out in a fixed-bed flow reactor. The glycothermal products in powder form were calcined at 700 °C for 30 min to remove the remaining organic impurities, tabletted, pulverized to 10-22 mesh size, and placed in the reactor. The catalyst bed was heated to 650 °C in a helium flow and held at that temperature for 30 min. Then, reaction gas containing 1000 ppm NO, 2000 ppm CH4, 6.7% O2, 0% or 2.5% H2O, and a balance of helium gas was introduced into the catalyst bed at W/F ) 0.3 g s mL-1 (SV ≈ 11 000 h-1). The reaction temperature was decreased from 650 to 450 °C at 5 °C/min and held for 15 min at each 50 °C interval to confirm
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Figure 1. XRD patterns of the products obtained by the reaction of Ga(acac)3, AIP, and Zn(acac)2‚H2O in 1,5-PeG at 300 °C for 2 h. The Ga/ Al/Zn ratios were (a) 0/1/0, (b) 1/3/0, (c) 1/1/0, (d) 3/1/0, (e) 1/0/0, (f) 1/1/1, (g) 3/1/1, (h) 1/3/1, and (i) 1/1/3 and are shown in the upper right inset. The peaks marked by b, 2, 9, and × are due to the spinel phase, GDB (glycol derivative of boehmite), boehmite, and ZnO, respectively.
Figure 2. XRD patterns of the catalysts obtained by calcination (700 °C) of the products whose XRD patterns are shown in Figure 1. The Ga/Al/Zn ratios were (a) 0/1/0, (b) 1/3/0, (c) 1/1/0, (d) 3/1/0, (e) 1/0/0, (f) 1/1/1, (g) 3/1/1, (h) 1/3/1, and (i) 1/1/3. The peaks marked by b, O, and × are due to the spinel phase, β-Ga2O3, and ZnO, respectively. Table 1. Properties of the Samples before and after Calcination of the Products Obtained by the Glycothermal Reaction before calcination
the stationary state. The effluent gases from the reactor were analyzed every 5 min with an on-line gas chromatograph (GL Science Micro GC CP-2002) equipped with 2-m Molsieve 5A (80 °C) and 10-m Poraplot Q (40 °C) columns. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a Shimadzu XD-D1 diffractometer using Cu KR radiation and a carbon monochromator. Crystallite size was calculated from the half-height width of the (440) diffraction peak of the spinel using the Scherrer equation. BET surface area was calculated using the single-point method based on nitrogen uptake measured at 77 K. The samples were pretreated in N2 flow at 300 °C for 30 min prior to the measurements. The Ga/Al/Zn ratios in the surface region of the catalysts were determined by X-ray photoelectron spectroscopy (XPS) performed on an ULVAC-PHI model 5500 spectrometer with 15 kV, 400 W Mg KR emission as the X-ray source. Temperature-programmed desorption of water (H2O-TPD) was also carried out in the fixed-bed flow reactor. The pretreatment conditions were the same as in the case of the catalyst tests (650 °C, 30 min in He). After the heat treatment in the helium flow, the temperature of the catalyst bed was decreased to 100 °C, and gas composed of 3.0% H2O and the balance helium was introduced into the catalyst bed at W/F ) 0.3 g s mL-1 and kept flowing for 1 h. After the treatment, the temperature was raised to 700 °C at 5 °C/min in a helium flow, and the effluent gases from the reactor were analyzed with a Pfeiffer Vacuum Omnistar GSD 301 O 1 quadrupole mass spectrometer. 3. Results and Discussion 3.1. Glycothermal Reaction of Ga(acac)3, AIP, and Zn(acac)2‚H2O. Figure 1 shows the XRD patterns of the products synthesized by the glycothermal method. For the product obtained by the reaction of AIP alone, the main product was identified as the glycol derivative of boehmite (GDB),13 and a small amount of boehmite was also formed (Figure 1a). On the other hand, γ-Ga2O3 was formed by the glycothermal reaction of Ga(acac)3 alone (Figure 1e).14 For the products obtained by the reaction of mixtures of AIP and Ga(acac)3, the XRD peaks were attributed to the spinel structure (Figure 1bd). With increasing Ga feed ratio, the peaks shifted toward the lower-angle side, indicating that the unit cell was enlarged
a b c d e f g h i
after calcination
Ga/Al/Zn charge ratio
crystallite size (nm)
surface area (m2/g)
crystallite size (nm)
surface area (m2/g)
0/1/0 1/3/0 1/1/0 1/0.33/0 1/0/0 1/1/1 1/0.33/0.33 1/3/1 1/1/3
3.8 3.7 4.1 7.0 8.5 8.9 6.5 -
244 251 255 195 101 150 140 174 130
3.5 4.4 4.2 4.7 13 9.4 7.9 -
209 190 184 127 88 50 94 129 36
because of incorporation of Ga3+ with an ionic radius larger than that of Al3+ in the spinel structure. Therefore, these results indicate that a γ-Ga2O3-Al2O3 solid solution was directly formed by the glycothermal reaction.15 For the products obtained upon addition of Zn(acac)2‚H2O to the starting materials for the glycothermal reaction, the peaks due to the spinel structure were still seen just as in the case of the γ-Ga2O3-Al2O3 solid solution (Figure 1f-i). These peaks became sharper and shifted toward the lower-angle side upon addition of Zn. For example, the (440) peak shifted from 65.7° (Ga/Al/Zn ) 1/3/0; Figure 1b) to 64.7° (1/3/1; Figure 1h), from 65.2° (1/1/0; Figure 1c) to 63.8° (1/1/1; Figure 1f), and from 64.4° (1/0.33/0; Figure 1d) to 63.8° (1/0.33/0.33; Figure 1g). These results indicate that Zn2+ ions were incorporated into the spinel structure. The Ga/Al/Zn ratio of 1/1/1 would give an ideal spinel structure with the tetrahedral sites occupied by Zn2+ ions and octahedral sites by Ga3+ and Al3+ ions, which might be the reason for the sharp diffraction peaks for the product (Figure 1f).16 On the other hand, Ga-rich or Al-rich starting compositions gave defect-containing spinel structures (Figure 1g and h), whereas a Zn-rich starting composition yielded a mixture of the ideal spinel and ZnO (Figure 1i). Figure 2 shows the XRD patterns of the samples obtained by calcination of the glycothermal products whose XRD patterns are shown in Figure 1. The glycol derivative of boehmite was transformed into γ-alumina, which exhibited broad diffraction peaks (Figure 2a). When γ-Ga2O3 was calcined at 700 °C, partial transformation into β-Ga2O3 occurred (Figure 2e). The crystallite sizes and BET surface areas of the samples before and after calcination are summarized in Table 1. Most of the oxides (i.e., catalysts having high activities, vide infra) synthesized by the glycothermal method maintained small crystallite sizes and large surface areas even after calcination at 700 °C.17 It must be noted,
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Figure 3. (a) NO conversion to N2 at 550 °C in the methane-SCR reaction on catalysts with various compositions under wet conditions. (Reaction conditions: NO, 1000 ppm; CH4, 2000 ppm; O2, 6.7%; H2O, 2.5%; He, balance; SV, 11 000 h-1.) (b) Groups categorized by the crystal structures of the products obtained by the glycothermal reaction with various Ga/Al/ Zn ratios. The products in regions B and C were contaminated with boehmite and GDB and zinc oxide as byproducts, respectively, whereas the products in region A are XRD-pure spinels. The dotted lines, o and t, represent the compositions in which all of the Zn2+ and Ga3+ ions occupy the tetrahedral sites and all of the Al3+ ions occupy the octahedral sites, calculated assuming that all of the cation defects are located at the octahedral and tetrahedral sites for dotted lines o and t, respectively. (c) NO conversion to N2 in the methane-SCR reaction on catalysts with an Al-rich composition under wet conditions. (Reaction conditions: NO, 1000 ppm; CH4, 2000 ppm; O2, 6.7%; H2O, 2.5%; He, balance; SV, 11 000 h-1.) The broken line represents the catalyst composition with a Ga/Al ratio of 1/3 irrespective of Zn content.
however, that the crystallite size of the ideal spinel significantly increased upon calcination (see Table 1, entry f, and compare Figures 1f and 2f), whereas the crystallite sizes of the products with the defect spinel structure were scarcely affected by calcination. These results might suggest that crystal growth of the spinel structure is controlled by the mobility of anions, whereas cation mobility does not affect crystal growth.18 3.2. Activity of Ga2O3-Al2O3-ZnO Catalysts with Various Compositions in the Methane-SCR of NO. Figure 3a and c shows the results (NO conversion to N2) for the methaneSCR of NO on catalysts with various compositions under wet conditions. For this reaction, catalysts with compositions in a narrow range, specifically, catalysts prepared from relatively high Al ratios, exhibited high activities, whereas the catalysts with the compositions in the other range showed low activities. Note that the XRD patterns of the used catalysts, which exhibited high activities, were identical with those of the fresh catalysts, indicating robustness of the present catalysts. To provide a possible explanation for these results, the products synthesized by the glycothermal method were divided into three groups as shown in Figure 3b. The solid straight line dividing regions A and C in Figure 3b represents the ideal spinel composition, where the tetrahedral sites in the spinel structure are occupied by Zn2+ ions and the octahedral sites are occupied by Ga3+ and/or Al3+ ions. When the Zn ratio was above this ideal composition, zinc oxide was formed (Figure 1i), and the products in region C contained significant amounts of zinc oxide. On the other hand, the products in region B were contaminated with boehmite and the glycol derivative of boehmite (GDB), which yielded Al2O3 upon calcination. The curved line dividing regions A and B was determined empirically. The byproducts (ZnO and Al2O3 derived from boehmite and GDB) had no activities for the methane-SCR reaction, and therefore, the catalysts derived from the products in regions B and C showed
quite low activities for the methane-SCR reaction. On the other hand, the products in region A were essentially single phases having the defect spinel structure. However, not all of the catalysts in region A had high activities. 3.3. Active Site of the Ga2O3-Al2O3-ZnO Catalysts. In our previous work on γ-Ga2O3-Al2O3 binary catalysts,19 we found that the activity of the catalyst increased with increasing Al content up to an Al/(Ga + Al) ratio of 0.778, where the highest activity was attained, whereas further increasing the Al content resulted in an abrupt decrease in catalyst activity. 71Ga MAS NMR and 27Al MAS NMR spectra of the catalysts suggested that Ga3+ and Al3+ ions preferentially occupied the tetrahedral and octahedral sites, respectively.10,19 Therefore, the catalyst having the highest activity had the defect spinel structure in which the tetrahedral sites were occupied solely by Ga3+ ions and all of the Al3+ ions were located in the octahedral sites. However, neither γ-Ga2O3 nor γ-Al2O3 has a high activity, even though they have tetrahedral Ga3+ and octahedral Al3+ ions, respectively. Therefore, the active sites for the methane-SCR reaction can possibly be associated with tetrahedral Ga3+ ions with octahedral Al3+ ions in the next-nearest-neighbor sites.4,10,19 The previous results and the fact that Zn2+ ions prefer to occupy tetrahedral sites as compared to Ga3+ and Al3+ ions16 suggest the order of priority for occupying the tetrahedral sites in the Ga2O3-Al2O3-ZnO spinel structure is Zn2+ > Ga3+ > Al3+. The dotted lines, o and t, in Figure 3b represent the compositions in which all of the Zn2+ and Ga3+ ions occupy the tetrahedral sites and all of the Al3+ ions occupy the octahedral sites, calculated assuming that all of the cation defects are located at the octahedral sites (dotted line o) and the tetrahedral sites (dotted line t). When the Ga3+ ion content was larger than the composition represented by dotted line o, some of the Ga3+ ions occupied the octahedral sites. Therefore, these catalysts exhibited low activities. In other words, when a small number of Al3+ ions are incorporated into the Ga2O3 structure, the Al3+ ions occupy the octahedral sites, and the ratio of tetrahedral Ga3+ ions to total Ga3+ ions increases with increasing Al content. Therefore, the catalyst activity increases with increasing Al content. However, when the Al content becomes greater than the proportion represented by the dotted line t, some of the Al3+ ions occupy the tetrahedral sites, and the proportion of Ga3+ ions occupying the tetrahedral sites becomes small, resulting in a decrease in the activity. In fact, the catalysts with compositions in the range between the dotted lines o and t exhibited higher activities as shown in Figure 3a, indicating that the active sites of the present catalysts for the methane-SCR reaction are tetrahedral Ga3+ ions with octahedral Al3+ ions in the next-nearest-neighbor sites. 3.4. Inhibitory Effect of Water. A detailed investigation of the composition range in which the catalysts exhibited high activities suggests that there were two activity maxima at any reaction temperature, as indicated by the arrows in Figure 3. The broken line in Figure 3c represents the catalyst composition with a Ga/Al ratio of 1/3 irrespective of the Zn content. Now, the effect of Zn modification is considered for the catalysts along this line. Note that, in the γ-Ga2O3-Al2O3 binary system, catalysts having a Ga/Al ratio of around 1/3 exhibited high activities in the reaction under dry conditions, although the optimal ratio was 1/3.5 [Al/(Ga + Al) ) 0.778].19 Figure 4 shows the NO conversions for the catalysts with Ga/Al/Zn ratios of 1/3/x (x ) 0, 0.5, 1) under dry and wet conditions as a function of temperature. At higher temperatures (600 and 650 °C), the Zn-modified catalysts exhibited lower activities than the binary catalysts. However, at 450 °C under
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Figure 4. NO conversion to N2 in the methane-SCR reaction on Ga2O3Al2O3-ZnO catalysts with Ga/Al/Zn ) 1/3/x as a function of temperature (open symbols, dry conditions; closed symbols, wet conditions). The catalysts with x ) 0, 0.5, and 1 are represented by 0, 2, and 9, respectively. Reaction conditions: NO, 1000 ppm; CH4, 2000 ppm; O2, 6.7%; H2O, 0% (- - -) or 2.5% (s); He, balance; SV, 11 000 h-1.
Figure 5. Ratio of the NO conversion to N2 in the presence of water to that in the absence of water.
Figure 6. Ratio of the NO conversion to N2 in the presence of water to that in the absence of water at 500 °C as a function of the crystallite size of Ga2O3-Al2O3-ZnO catalysts with various Ga/Al/Zn ratios.
dry conditions and at 500 °C under wet conditions, NO conversion increased with increasing Zn content. The ratios of the NO conversion under wet conditions to that under dry conditions are displayed in Figure 5, which clearly shows that the effect of water, i.e., the decrease in the NO conversion caused by the presence of water in the feed gas, was more significant at lower temperatures. Figure 5 also shows that the decrease in NO conversion caused by water was less significant for the catalysts with higher Zn contents. As described above, Zn modification increased the crystallite size of the spinel structure. Therefore, the ratios of the NO conversion under wet conditions to that under dry conditions at 500 °C are plotted against the crystallite size of the catalyst in Figure 6. The results indicate that the activities of the catalysts with larger crystallite sizes are less inhibited by water in the feed gas. Figure 7 shows the H2O-TPD profiles of catalysts with Ga/ Al/Zn ratios of 1/3/x. A large desorption peak appeared at around 200 °C for all of the catalysts. This peak is attributed to the
Figure 7. H2O-TPD profiles of Ga2O3-Al2O3-ZnO catalysts with the Ga/ Al/Zn ratios specified in the figure.
Figure 8. NO conversion to N2 in the methane-SCR reaction on Ga2O3Al2O3-ZnO catalysts with Ga/Al/Zn ) 1/3/x under wet conditions as a function of x. Reaction conditions: NO, 1000 ppm; CH4, 2000 ppm; O2, 6.7%; H2O, 2.5%; He, balance; SV, 11 000 h-1.
desorption of physisorbed water. However, the peak was spread out toward higher temperature, and desorption of water was observed even in the temperature range of the methane-SCR reaction (450-650 °C). Because physisorbed water cannot be present at higher temperatures, these water molecules must reflect chemisorbed water or surface (and/or structural) hydroxyl groups. It is noteworthy that the amount of desorbed water decreased with increasing Zn content. Catalysts with lower uptakes of water showed a higher tolerance to the inhibition of water. These results suggest that water present in the feed gas was preferentially adsorbed on the surface of the catalysts, thus disturbing the adsorption of the reactants on the active sites of the catalysts. Preferential adsorption of water seems to be the cause of the decrease in NO conversion. Li and Armor reached a similar conclusion for the CH4-SCR reaction of NOx on Coferrierite catalysts.21 The increase in NO conversion caused by Zn modification at lower temperatures under dry conditions indicates that H2O formed by the oxidation of methane also has an inhibitory effect on NO conversion. As shown in Figure 3, there were two high-activity regions, and the results in Figure 3 are replotted in Figure 8, in which NO conversion is plotted as a function of x, where x stands for the relative proportion of Zn in Ga/Al/Zn ) 1/3/x. The NO conversion at any temperature decreased upon modification with a relatively small amount of Zn (x ) 0.1). However, a further increase in the Zn ratio increased the NO conversion. This increase in the activity became more marked at lower temperatures because the inhibitory effect of water was more significant at lower temperatures, as described above. A further increase in the Zn ratio resulted in a decrease in NO conversion. This might be mainly due to the decrease in the population of tetrahedral Ga sites. However, the change in activity as the Zn content increased from x ) 0 to x ) 1 in Figure 8 could not follow the amount of tetrahedral Ga3+ sites, which can be estimated from the Ga
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Figure 9. Crystallite sizes and BET surface areas of Ga2O3-Al2O3-ZnO catalysts with Ga/Al/Zn ) 1/3/x as a function of x.
Figure 10. Surface metal composition determined by XPS (solid line); the dotted lines show the metal compositions used for the glycothermal reaction.
ratio. This suggests that the number of tetrahedral Ga3+ sites in the surface region, which has an important effect on the activity, does not completely correspond to the bulk composition. As shown in Figure 9, the crystallite size increased with increasing Zn content, whereas the BET surface area decreased. Therefore, the inhibitory effect of water was more pronounced for the catalysts with smaller crystallite sizes or with larger surface areas. 3.5. Surface Composition of the Ga2O3-Al2O3-ZnO Catalysts. The surface Ga/Al/Zn ratio was determined by XPS, and the results are shown in Figure 10. There was apparent disagreement between the bulk and surface compositions, which indicates that the spinel crystals have a concentration gradient between the surface and the bulk. When Zn was not incorporated into the starting materials, Ga was abundant, and the Al content was low in the surface region. The surface Ga composition decreased markedly when a small amount of Zn was included in the starting materials, whereas the surface concentration of Zn was significantly lower than that in the bulk. When Zn was added to the glycothermal reaction mixture, the formation of ideal spinel, ZnGa2O4 and/or ZnAl2O4, proceeds more readily, and the thus-formed particles act as the nuclei of mixed oxides, thus lowering the surface Zn concentration. A detailed discussion of the nucleation phenomenon under glycothermal conditions will be presented in a separate work. The change in NO conversion as a function of x shown in Figure 8 can be discussed in relation to the change in the surface Ga content shown in Figure 10. At lower temperatures, the inhibitory effect of water was more significant for the catalysts with lower Zn contents. Therefore, the activities of catalysts with higher Zn contents were higher than expected from the surface Ga content. On the other hand, at higher temperatures such as 600 °C, a good correlation between NO conversion and surface Ga content was seen because of the low inhibitory effect of water. These results support the conclusion that the amount
Figure 11. NO conversion to N2 in the methane-SCR reaction on Ga2O3Al2O3 binary catalyst (Ga/Al ) 1/3) and Ga2O3-Al2O3-ZnO catalyst (Ga/ Al/Zn ) 1/5/0.75) as a function of temperature. Reaction conditions: NO, 1000 ppm; CH4, 2000 ppm; O2, 6.7%; H2O, 0% (- - -) or 2.5% (s); He, balance; SV, 11 000 h-1.
of surface Ga3+ ions has a strong effect on the activity for the methane-SCR of NO. It is certain that there was a difference in nature between the catalyst systems with and without Zn modification. Therefore, the activities of the two catalysts that exhibited the highest activities in the individual systems are compared in Figure 11. When water was not fed to the reactor, the maximum NO conversion was attained by the catalyst without Zn modification. However, under wet conditions, the Zn-modified catalyst showed a higher NO conversion over essentially the whole temperature range because of its higher tolerance for water. The catalyst with the optimal ratio (Ga/Al/Zn ) 1/5/0.75) showed the highest activity for the methane-SCR of NO under wet conditions because the number of active sites was larger than that predicted by the bulk composition and because the catalyst had a high tolerance for water. 4. Conclusions Spinel-type Ga2O3-Al2O3-ZnO mixed oxides were directly obtained by the glycothermal reaction of mixtures of zinc diacetylacetonate, gallium triacetylacetonate, and aluminum isopropoxide at 300 °C for 2 h. With increasing Zn content, the crystallinity increased, and the defect spinel structure of the product approached the ideal spinel structure, although excess Zn content resulted in the formation of zinc oxide as a byproduct. In the ternary Ga-Al-Zn composition-activity map, there were two ranges where the catalysts prepared by calcination of the glycothermal products at 700 °C for 30 min exhibited high activities for the methane-SCR of NO. One was found at Ga/ Al/Zn ) 1/3/0-1/3.5/0, and the other was found at Ga/Al/Zn ) 1/5/0.75. Catalysts having the former composition showed high activities under dry conditions, whereas the catalyst with the latter composition exhibited a high activity under wet conditions. Tetrahedral Ga3+ ions with octahedral Al3+ ions in next-nearest-neighbor sites were closely related to the active sites for the methane-SCR of NO, and the order of priority for occupying the tetrahedral sites of the spinel structure was Zn2+ > Ga3+ > Al3+. Therefore, these two active ranges were found near the composition where Ga3+ ions selectively occupy the tetrahedral sites and Al3+ ions occupy the octahedral sites of the spinel structure. A clear correlation between the inhibitory effect of water and the crystallite size of the catalyst was observed, and one of the important factors in Zn modification is the increase in crystallinity, which decreased the amount of adsorbed water on the surface of the catalysts and improved their tolerance for water in the feed gas. H2O-TPD profiles
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clearly showed that the amount of water adsorbed on the catalyst surface decreased with Zn modification. If one assumes that all of the components are homogeneously distributed in the crystals, the population of active sites should decrease with increasing Zn content. However, this was not the case: the surface Ga content did not decrease because the concentration gradient was more significant with increasing Zn content. This point also contributed to the high activity of the catalyst with the composition of Ga/Al/Zn ) 1/5/0.75, which showed a higher activity than the binary oxide system (Ga2O3-Al2O3, ZnGa2O4, and ZnAl2O4) or single-component catalysts (Ga2O3, Al2O3, and ZnO). Acknowledgment This work was supported in part by a Grant-in-And for Scientific Research from the Ministry of Education, Science Sports and Culture, Japan. The ICP emission analysis was carried out at the Electric Power Substrate R&D Department, The Kansai Electric Power Company, Inc. Literature Cited (1) Bosch, H.; Janssen, F. Formation and Control of Nitrogen Oxides. Catal. Today 1988, 2, 369. (2) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. ReV. 1995, 95, 209. (3) Shimizu, K.; Satsuma, A.; Hattori, T. Selective Catalytic Reduction of NO by Hydrocarbons on Ga2O3/Al2O3 Catalysts. Appl. Catal., B 1998, 16, 319. (4) Shimizu, K.; Takamatsu, M.; Nishi, K.; Yoshida, H.; Satsuma, A.; Tanaka, T.; Yoshida, S.; Hattori, T. Alumina-Supported Gallium Oxide Catalysts for NO Selective Reduction: Influence of the Local Structure of Surface Gallium Oxide Species on the Catalytic Activity. J. Phys. Chem. B 1999, 103, 1542. (5) Iwamoto, M.; Yahiro, H. Novel Catalytic Decomposition and Reduction of NO. Catal. Today 1994, 22, 5. (6) Armor, J. N. Catalytic Reduction of Nitrogen Oxides with Methane in the Presence of Excess Oxygen: A Review. Catal. Today 1995, 26, 147. (7) Li, Y.; Armor, J. N. Selective Catalytic Reduction of NO with Methane on Gallium Catalysts. J. Catal. 1995, 145, 1. (8) Schulz, P.; Baerns, M. Aromatization of Ethane over GalliumPromoted H-ZSM-5 Catalysts. Appl. Catal. 1991, 78, 15.
(9) Tabata, T.; Kokitsu, M.; Okada, O. Relationship between Methane Adsorption and Selective Catalytic Reduction of Nitrogen Oxide by Methane on Gallium and Indium Ion-Exchanged ZSM-5. Appl. Catal., B 1995, 6, 225. (10) Okimura, Y.; Yokoi, H.; Ohbayashi, K.; Shimizu, K.; Atsushi, S.; Hattori, T. Selective Catalytic Reduction of Nitrogen Oxides with Hydrocarbons over Zn-Al-Ga Complex Oxides. Catal. Lett. 1998, 52, 157. (11) Inoue, M.; Inoue, N.; Yasuda, T.; Takeguchi, T.; Iwamoto, S. DeNOx Reaction with Methane on γ-Ga2O3-Al2O3 Solid Solutions Prepared by the Glycothermal Method. AdV. Sci. Technol. 2000, 29, 1421. (12) Takahashi, M.; Iwamoto, S.; Inoue, M. Highly Active B2O3Modified Ga2O3-Al2O3 Catalyst for SCR of NOx with Methane. AdV. Technol. Mater. Mater. Proc. J. 2002, 4, 76. (13) Inoue, M.; Kominami, H.; Inui, T. Reaction of Aluminium Alkoxides with Various Glycols and the Layer Structure of Their Product. J. Chem. Soc., Dalton Trans. 1991, 3331. (14) Inoue, M.; Inui, T. Glycothermal Synthesis of Metastable Phases. AdV. Sci. Technol. 1999, 14, 41. (15) Takahashi, M.; Inoue, N.; Takeguchi, T.; Iwamoto, S.; Inoue, M. Glycothermal Synthesis of the γ-Ga2O3-Al2O3 Solid Solutions by the Glycothermal Method and Its Catalytic Performance for SCR of NO with Methane. J. Am. Ceram. Soc., in press. (16) Kodama, T. High-Vacancy-Content Zinc(II)-Bearing Ferrites from Iron(III) Tartrate in Strongly Alkaline Solutions. J. Mater. Chem. 1992, 2, 525. (17) Inoue, M.; Kominami, H.; Inui, T. Synthesis of Large-Pore-Size and Large-Pore-Volume Aluminas by Glycothermal Treatment of Aluminium Alkoxide and Subsequent Calcination. J. Mater. Sci 1994, 29, 2459. (18) Nunes, S.; Bradt, R. Grain Growth of ZnO in ZnO-Bi2O3 Ceramic with Al2O3 Additions. J. Am. Ceram. Soc. 1995, 78, 2469. (19) Takahashi, M.; Inoue, N.; Nakatani, T.; Takeguchi, T.; Iwamoto, S.; Watanabe, T.; Inoue, M. Selective Catalytic Reduction of NO with Methane on γ-Ga2O3-Al2O3 Solid Solutions Prepared by the Glycothermal Method. Appl. Catal., B, in press. (20) Inoue, M.; Otsu, H.; Kominami, H.; Inui, T. Synthesis of Double Oxides Having Spinel Structure (ZnAl2O4, ZnGa2O4) by the Glycothermal Method. Nippon Kagakukai-shi 1991, 7, 1036. (21) Li, Y.; Armor, J. Selective Reduction of NOx by Methane on CoFerrierites. J. Catal. 1994, 150, 376.
ReceiVed for reView September 19, 2005 ReVised manuscript receiVed March 1, 2006 Accepted March 7, 2006 IE051050L
