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Multi-component noble metal catalysts prepared by sequential deposition precipitation for low temperature decomposition of dioxin
Applied Catalysis B: Environmental 41 (2003) 43–52
Multi-component noble metal catalysts prepared by sequential deposition precipitation for low temperature decomposition of dioxin M. Okumura a,∗ , T. Akita a , M. Haruta b , X. Wang c , O. Kajikawa d , O. Okada d a
National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda 563-8577, Japan National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onokawa, Tsukuba 305-8569, Japan c Kansai Research Institute, Kyoto Research Park 17, Chudo-ji Minami-machi, Shimogyo-ku, Kyoto 600-8813, Japan d Osaka Gas Co. Ltd., 6-19-9 Torishima Konohana-ku, Osaka 554-0051, Japan
b
Received 10 December 2001; received in revised form 5 April 2002; accepted 6 April 2002
Abstract To create a decomposition catalyst for the several pollutants with the necessary performance, we united noble metals of two and three different types by selective and successive deposition onto select mixed metal oxide supports. The catalytic oxidations of H2 , trimethylamine, o-chlorophenol and of dioxin derivatives over these multi-component noble metal catalysts were subsequently investigated. The ternary component catalyst, Au/Fe2 O3 -Pt/SnO2 -Ir/La2 O3 , showed greatly enhanced activity in the catalytic oxidation of o-chlorophenol and of dioxin at 423 K, indicating a catalytic activity enhancing (boosting) effect by the addition of the Ir/La2 O3 catalyst that by itself shows low activity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-component noble metal catalyst; Synergetic effect; Boosting effect; Low temperature oxidative decomposition of dioxin; Environmental catalyst
1. Introduction One of the most urgent needs in environmental protection today is the control of dioxin emission from incinerators [1,2]. There are several treatment techniques presently in use, such as the raising of the waste gas temperature to cause catalytic oxidative decomposition [3,4]. However, when considering the initial investment and mechanical complexity, this method is not practicable for small-scale incinerators. A novel solution for this task would be to decompose ∗ Corresponding author. Tel.: +81-727-51-9732; fax: +81-727-51-9714. E-mail address: [email protected] (M. Okumura).
dioxin and its derivatives at the dust filter using catalytic oxidation, preferably for practical reasons at temperatures <473 K. This particular figure is relevant since the waste gases are usually reduced to this temperature at the dust filter after efficient heat recovery. Although supported gold catalysts are remarkably active in the oxidation of CO and trimethylamine at temperatures <473 K [5–7], they do not exhibit sufficiently high catalytic activity for the oxidativedecomposition of dioxin. This is probably because a number of bondings, such as C–C, C–Cl, C–O, are broken at the catalyst surface. In order to develop a decomposition catalyst with improved performance, we attempted the creation of multi-component catalysts composed of various supported noble metal
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M. Okumura et al. / Applied Catalysis B: Environmental 41 (2003) 43–52
catalysts, all which have different catalytic properties. We speculated that an appropriate combination of a few noble metals deposited on different metal oxides (i.e. not on the same metal oxide support), might bring about a synergetic effect in the simultaneous decomposition of dioxin and other pollutants at temperatures between 473 K and the ambient temperature. Supported noble metal catalysts, such as Pd, Ir, Pt, and Au catalysts, are already used practically for environmental protection owing to their high catalytic activity in the oxidation or reduction of hydrocarbons, NOx and amine derivatives [8–11]. It is especially well known that Pt and Pd catalysts, such as Pt/SnO2 and Pd/SnO2 , are active in the combustion of hydrocarbons [12], and Au catalysts exhibit high catalytic activity in the oxidation of CO and of trimethylamine. Ir catalysts have recently been reported to be active in the reduction of NOx with hydrocarbon in the presence of oxygen [13]. Because incinerator exhaust gases contain a variety of pollutants—not only dioxin derivatives but also other compounds, such as odors, volatile organic compounds and the reaction intermediates of dioxins—it would be unreasonable to expect a single-component catalyst to achieve high catalytic performance for all of the exhaust gas pollutants. However, it can be assumed that if several supported single noble metal catalysts were encouraged to work in synergy, the overall catalytic activity would be greatly improved. The concept of this multi-component noble metal catalyst is to control the microscopic structure of the composition and organization of the noble metal catalysts at the nano-scale. In contrast to conventional bi-metal or multi-component catalysts that share the same metal oxide support, noble metal nano-particles deposited on different metal oxides, for example, Pt/SnO2 , Ir/La2 O3 and Au/Fe2 O3 , are prepared by depositing each metal component predominantly on select metal oxides that are well mixed as a result of preparation by coprecipitation. In this paper, the preparation of the multi-component catalysts by the sequential deposition precipitation method was carried out and the catalytic activity measurements in the oxidation reaction of hydrogen and trimethylamine over the several multi-component catalysts were examined in order to confirm whether or not this multi-component noble metal catalyst would improve the catalytic activity through a
synergetic effect. Finally, the oxidative decomposition of o-chlorophenol, and of dioxin derivatives over the multi-component catalysts were investigated. 2. Experimental 2.1. Catalyst preparation Single-component catalysts of Au/Fe2 O3 , Pd/SnO2 , and Ir/La2 O3 were prepared using the deposition precipitation (DP) method. Hydroxides of gold, palladium, or iridium were deposited on the metal oxide supports through pH-controlled chemical interaction in an aqueous solution of HAuCl4 , Pd(CH3 OO)2 or IrCl4 . These were then washed, dried and calcined in air at 673 K for 4 h. The support materials of Fe2 O3 , SnO2 , and La2 O3 were prepared by neutralizing aqueous solutions of iron(III) nitrate, stannic chloride, and lanthanum nitrate with sodium carbonate, respectively, and calcinating the resulting precipitates in air at 627 K. In order to prepare the binary component noble metal catalyst, Pd/SnO2 -Au/Fe2 O3 , the selective depositions of Pd and Au hydroxides onto a select metal oxide were carried out using the procedure described below. As shown in Fig. 1, the isoelectric points of Fe2 O3 and SnO2 in an aqueous solution are pH 4.5 and 8.0, respectively [14–16]. Adjusting the pH of the Pd(CH3 COO)2 solution with a suspension of Fe2 O3 -SnO2 mixture prepared by coprecipitation
Fig. 1. The relationship between the density of adsorbed ions and the isoelectric point of the support.
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between 4.5 and 8.0, Pd2+ ions were adsorbed mainly onto the negatively charged SnO2 surfaces. This is because the Pd2+ ion and Fe2 O3 surfaces were positively charged in this condition and hence repelled each other. This speculation was confirmed by the results of the preparation of single component Au catalyst prepared by DP with such condition. In this condition, the loading amount of Au became extremely low. This intermediate was calcined in air at 673 K for 4 h. The calcined intermediate (e.g. Fe2 O3 -PdO/SnO2 ) was then added to the HAuCl4 solution whose pH had been adjusted to between 6 and 8, giving an intermediate (e.g. Au(OH)3 /Fe2 O3 + PdO/SnO2 ) with Au(OH)3 deposited mainly on Fe2 O3 . Every DP step involved the selective deposition of Au and Pd onto the different metal oxide supports by means of pH-controlled chemical interactions that were based on their different isoelectric points (IEP) in an aqueous solution of HAuCl4 and Pd(ac)2 , and subsequent washing, drying and calcination. The preparation of ternary component noble metal catalysts is also possible using a similar procedure, however, the noble metal ions available for the deposition precipitation procedure is limited. Consequently the coprecipitation method was also applied for the hybridization of the third noble metal catalyst, such as Ir/La2 O3 . As an example, for the preparation of Pd/SnO2 -Au/Fe2 O3 -Ir/La2 O3 , Ir(OH)4 and La(OH)3 were coprecipitated in the presence of Pd/SnO2 Au/Fe2 O3 prepared using the DP method. 2.2. Catalytic activity measurements The catalytic activities were measured in a fixed-bed flow reactor using 100, 500 and 1500 mg of catalyst samples for the oxidation of hydrogen, trimethylamine and o-chlorophenol, respectively. Multicomponent noble metal catalysts containing Pd, Pt, or Ir metals were reduced in 20% hydrogen in Ar (SV = 20,000 h−1 ml g−1 cat.) at 523 K for 150 min for the comparative measurements of catalytic activity. Hereinafter, the catalysts without H2 treatment are expressed as Au/Fe2 O3 -PdO/SnO2 -IrO2 /La2 O3 and those after H2 treatment are expressed as Au/Fe2 O3 Pd/SnO2 -Ir/La2 O3 . Reaction products were analyzed using gas chromatography. The reactant gases and the space velocities for the oxidation of hydrogen, trimethylamine and o-chlorophenol were 1 vol.% H2
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in air at the flow rate of SV = 20,000 h−1 ml g−1 cat., 100 ppm trimethylamine in air at the flow rate of SV = 10,000 h−1 ml g−1 cat., and 1500 ppm o-chlorophenol in 6 vol.% humidified air at the flow rate of SV = 10,000 h−1 ml g−1 cat., respectively. The dioxin derivatives used for the catalytic activity measurement over the catalysts are seven types of polychlorinated dibenzo-p-dioxin derivatives (PCDDs) and 10 types of polychlorinated dibenzofuran derivatives (PCDFs). Dioxins were dissolved into hexane keeping a concentration of 0.1 mg ml−1 hexane for each dioxin derivative. After the introduction of this solution containing 350 ng of dioxins into the catalyst bed of the fixed-bed flow reactor line, as shown in Fig. 2a, the catalyst bed was evaporated to dryness under ambient conditions by means of the passing of dry air. Activity measurements for the oxidative decomposition of dioxin derivatives were performed for 2 h by passing air (SV = 6000 h−1 ml g−1 cat.) preheated to 423 K. The conversion of dioxin was calculated from the following equation: Dad + Dtrap C = 1− × 100 Din where C is the conversion of dioxin, Din the amount of the introduced dioxin, Dad the amount of dioxins adsorbed in the catalyst bed, and Dtrap the amount of the dioxin adsorbed in the reaction line and XAD-2 resin trap, respectively. The oxidative decomposition of the incinerator exhausts gas over the ternary component noble metal catalyst was performed using a small batch type incinerator. First, a mixture of corrugated paper (19 kg) and vinyl chloride (1 kg) was incinerated for 90 min, and then a mixture of corrugated paper (4.75 kg) and vinyl chloride (0.25 kg) was added every 30 min. The catalytic reaction experiment shown in Fig. 2b was connected to the chimney of the incinerator. The activity measurements were performed by passing the exhaust gas (SV = 12,000 h−1 ml g−1 cat.) for 5 h at 423 K. The conversion of dioxin was determined from the difference in the amount of the adsorbed dioxin in the reference line and that in the catalyst line. The analysis of dioxins was carried out by the following procedure. The samples were extracted with 200 ml of toluene for 16 h by use of soxhlet extractor. Then, the extracts were concentrated to about 5 ml and were treated with sulfuric acid. After the purification, they
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Fig. 2. Experimental set-ups for the oxidative decomposition of (a) the laboratory reaction and of (b) the incinerator test. T, thermocouple; P, pump; M, massflow meter.
were analyzed by gas chromatograph–mass spectrometry (GC–MS).
3. Results and discussion 3.1. Hydrogen oxidation Fig. 3 shows the conversion vs. temperature curves in the oxidation of H2 over Au/Fe2 O3 and Pd/SnO2 ,
1:1 physical mixture of Au/Fe2 O3 and Pd/SnO2 , and the binary component noble metal catalyst, Pd/SnO2 Au/Fe2 O3 . The catalytic activity of Pd/SnO2 is much higher than that of Au/Fe2 O3 , and the catalytic activity of a physical mixture of Au/Fe2 O3 and Pd/SnO2 is situated in between those of the single-component catalysts. This shows that simple physical mixing does not bring about a synergistic effect. Conversely, the binary component noble metal catalyst, Pd/SnO2 -Au/Fe2 O3 , exhibits the highest catalytic activity and is more
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Fig. 3. Conversion of H2 as a function of catalyst temperature in H2 oxidation over single-component and multi-component noble metal catalysts. (䊏) 2 wt.% Au/Fe2 O3 ; (䊉) 2 wt.% Pd/SnO2 ; (䉱) mechanically mixed 2 wt.% Au/Fe2 O3 + 2 wt.% Pd/SnO2 ; (䉭) 2 wt.% Au/Al2 O3 -2 wt.% Pd/SnO2 ; reactant gas; H2 1 vol.% in air; SV = 20,000 h−1 ml g−1 cat.
active than Pd/SnO2 . It is surprising that the hybridization of single-component catalysts by the sequential deposition of noble metals on different metal oxide supports provides such significant synergy even in a simple reaction such as hydrogen oxidation. 3.2. Oxidative decomposition of trimethylamine Fig. 4 shows conversion of trimethylamine to CO2 as a function of catalyst temperature over the single-component catalysts, Au/Fe2 O3 , Pt/SnO2 and Ir/La2 O3 , and the ternary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 . Among the three single-component catalysts, the most active one is Au/Fe2 O3 and the most inactive is Ir/La2 O3 . The two binary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 , exhibit much higher catalytic activity than the single-component catalysts. The catalytic activities of the Au and Pt catalysts are greatly improved by the combination with the Ir catalyst, although the Ir catalyst by itself is much less active for this reaction. This
result indicates that Ir exhibits an boosting effect on the oxidative decomposition of trimethylamine over the other noble metal catalysts. 3.3. Oxidative decomposition of o-chlorophenol As a preliminary experiment, o-chlorophenol (o-CPh) was used instead of dioxin and the catalytic activities of the single-component, binary and ternary component noble metal catalysts were measured at 373 and 473 K. Fig. 5 shows the conversion of o-CPh during its oxidative decomposition to CO2 over 6 h duration. The single-component catalysts, except for Ir/La2 O3 , showed relatively high catalytic activity for the oxidative decomposition of o-CPh and each one has a characteristics feature. While Pt/SnO2 and Pd/SnO2 are more active in the conversion to CO2 , Au/Fe2 O3 is more active in the conversion of o-CPh. The binary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 , exhibits catalytic activity higher by 20% than the corresponding single-component catalysts. The conversions to CO2 over the ternary component noble metal catalyst,
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Fig. 4. Yield of CO2 as a function of catalyst temperature in the oxidation of trimethylamine over single-component and multi-component noble metal catalysts. (䉱) 2 atm% Ir/La2 O3 ; (䉬) 2 atm% Pt/SnO2 ; (䊏) 2 atm% Au/Fe2 O3 ; (䊊) 2 atm% Au/Fe2 O3 -2 atm% Ir/La2 O3 ; (䊉) mechanical mixture of 2 wt.% Pt/SnO2 and 2 wt.% Ir/La2 O3 ; reactant gas, trimethylamine 100 ppm in air; SV = 10,000 h−1 ml g−1 cat.
Fig. 5. Conversion of o-CPh and yield of CO2 in the oxidation of o-CPh over single-component and multi-component noble metal catalysts at 423 and 473 K. Pd, 2 atm% Pd/SnO2 ; Ir, 2 atm% Ir/La2 O3 ; Pt, 2 atm% Pt/SnO2 ; Au, 2 atm% Au/Fe2 O3 ; B-IC: binary component noble metal catalyst (2 atm% Au/Fe2 O3 -2 atm% Pd/SnO2 ); T-IC, ternary component noble metal catalyst (2 wt.% Pt/SnO2 -2 atm% Au/Fe2 O3 -2 wt.% Ir/La2 O3 ); mix, mechanically mixed catalyst (2 wt.% Pt/SnO2 + 2 atm% Au/Fe2 O3 + 2 wt.% Ir/La2 O3 ); reactant gas, o-CPh 1500 ppm in 6 vol.% humidified air; SV = 3000 h−1 ml g−1 cat.
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Au/Fe2 O3 -Pt/SnO2 -Ir/La2 O3 , at 423 and 473 K are comparatively high, 38 and 90%, respectively, whereas the activity of the physically mixed catalyst, Au/Fe2 O3 + Pt/SnO2 + Ir/La2 O3 , was much lower. This indicates that only the microscopic mixing of the single-component catalysts and/or multiple noble metal loading can improve catalytic activity. The large synergy observed for the ternary component noble metal catalyst suggest that each component interact with each other for mutual enhancement. Especially, it should be noted that Ir also exhibited an boosting effect on the oxidative decomposition of o-CPh over the other noble metal catalysts.
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3.4. Oxidative decomposition of PCDD and PCDF derivatives over the multi-component noble metal catalysts Fig. 6 shows that the binary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 present 78.8 and 69.9% conversion rates for the oxidative decomposition of PCDDs and PCDFs at 423 K, respectively. However, they show low activity in the decomposition of hexa- and octa- CDDs and CDFs, such as OCDD, 2,3,4,6,7,8-HxCDF, and 1,2,3, 4,6,7,9-HpCDF, OCDF. Especially, they have low activity in the decomposition of 1,2,3,4,6,7,9-HpCDF.
Fig. 6. Conversion from dioxin derivatives to CO2 in the oxidation of dioxin over binary and ternary component noble metal catalysts at 423 K in the laboratory reaction line. (A) 2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 ; (B) 2 atm% Ir/La2 O3 -2 atm% Au/Fe2 O3 ; (C) 2 atm% Ir/La2 O3 -2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 . SV = 6000 h−1 ml g−1 cat.; reaction time = 2 h.
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Conversely, the ternary component noble metal catalyst, Ir/La2 O3 -Pt/SnO2 -Au/Fe2 O3 , exhibits high catalytic activity whose average conversion rate is 98.0% for all types of dioxin derivatives. The markedly enhanced catalytic activity in the ternary component noble metal catalysts was in good agreement with that observed in the decomposition of o-CPh. 3.5. Decomposition of the incinerator exhaust gases over the ternary component noble metal catalyst The catalytic activity of Pt/SnO2 -Au/Fe2 O3 -Ir/ La2 O3 for the decomposition of dioxin derivatives in the experimental setup shown in Fig. 2b is summarized in Fig. 7. The average conversions for the decomposition of PCDDs and PCDFs are 99.8 and 99.8%, respectively. When compared with the V2 O5 /TiO2 catalysts that are most widely used for dioxin decomposition in commercial processes, it can be seen that Pt/SnO2 -Au/Fe2 O3 -Ir/La2 O3 has appreciably higher catalytic activity for the decomposition of dioxin
derivatives at 423 K. Thus, the obtained catalytic activity for the decomposition of dioxins at 423 K is remarkably high and its TEQ conversion reaches 99.7%. Comparing the results of the laboratory reaction and the incinerator test for the decomposition of PCDD and PCDF derivatives, the result of the incinerator test is slightly better than that of the laboratory reaction. This might be because the procedure of dioxin introduction used in the laboratory reaction is different from that used in the incinerator test. 3.6. TEM observation of the multi-component noble metal catalyst TEM images of the binary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 , and the ternary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 -Ir/ La2 O3 , are shown in Fig. 8. Highly mixed Fe2 O3 -SnO2 support was observed in the image of Au/Fe2 O3 -Pd/ SnO2 shown in Fig. 8a. Although relatively large Au particles were observed, Pd particles were not found
Fig. 7. Conversion from dioxin derivatives to CO2 in the oxidation of dioxin over the ternary component noble metal catalysts (2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 -2 atm% Ir/La2 O3 ) at 423 K in the incinerator reaction line.
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Fig. 8. TEM images of the multi-component noble metal catalysts (a) 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 ; (b) Ir/La2 O3 region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 ; (c) Au/Fe2 O3 and Pd/SnO2 mixed region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 ; (d) Ir/La2 O3 and Pd/SnO2 mixed region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 .
as particle images. It can be assumed that the ultra-fine Pd particles were highly dispersed over the support, as the existence of Pd was confirmed by EDX analysis. TEM images of Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 are shown in Fig. 8b, c and d. In Fig. 8b, a Ir/La2 O3 region of the ternary component noble metal catalyst is shown. It was determined from this image that the small dark dots were Ir nano-particles, and these particles were dispersed over the La2 O3 support. This was also confirmed by EDX analysis. As the preparation of the Au/Fe2 O3 -Pd/SnO2 intermediate of Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 is identical to that of the binary component noble metal catalyst, the microscopic structure of Au/Fe2 O3 -Pd/SnO2 in the ternary component noble metal catalyst was almost the same
as that of the binary component noble metal catalyst. Furthermore, highly mixed La2 O3 -SnO2 support was also observed in the image of the ternary component noble metal catalyst shown in Fig. 8d. From these analyses, it was determined that the metal oxides were highly mixed at a nano-scale and relatively small noble metal particles were dispersed onto the metal oxides. It might, therefore, be concluded that the Au/Fe2 O3 -Pd/ SnO2 and Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 catalysts are primary models of the multi-component noble metal catalyst that we proposed. From the several catalytic activity measurements, it was found that the more highly multiplied noble metal catalysts showed higher catalytic activity for the oxidation of complex molecules. The simply mechanically
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mixed catalyst showed low catalytic activity for the decomposition of H2 and o-CPh. Thus, the catalytic activity is greatly dependent on the nano-level dispersion of the single-component catalysts. This suggests that the spillover of the active species and the intermediates migrate to other active sites on different components of the catalyst and proceed onto further reactions. In the preparation of a multi-component noble metal catalyst, it is difficult to completely prepare the multi-component noble metal catalyst that consists of pure single-component catalysts because small amounts of noble metals might be deposited onto metal oxide supports where not required. Therefore, a combined noble metal catalyst might be prepared unintentionally. Although the amount of this catalyst might be small, the synergetic effect of the catalytic activity could be affected.
metal catalysts are necessary to promote the higher catalytic activity necessary for the decomposition of complex molecules, such as trimethylamine, o-CPh, and dioxin derivatives. The stability of multi-component catalysts, especially for the deactivation by Cl, and the sub-ppm or ppb level analysis of other byproducts have to be studied furthermore. While the mechanism of the synergy evident in the decomposition of dioxin over multi-component catalysts needs further investigation, the validity for the improvement of the catalytic activity is considered to have been confirmed. It is especially noteworthy that the multi-component noble metal catalyst appears to be a promising material for achieving the depuration of the typical dioxin containing exhaust gases emanating from incinerators.
4. Conclusion
[1] K. Balldvhmiter, H. Buchert, R. Niemczyk, A. Munder, M. Swerev, Chemosphere 15 (1986) 901. [2] H. Buchert, K. Ballschmiter, Chemosphere 15 (1986) 1923. [3] U. Duwel, A. Nottrodt, K. Ballshmiter, Chemosphere 20 (1990) 1839. [4] J.R. Donnelly, Air Pollution Engineering Manual, Van Nostrand Reinhold, New York, 1992, p. 263. [5] M. Haruta, Catal. Today 36 (1997) 153. [6] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [7] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, M. Haruta, Catal. Lett. 51 (1998) 53. [8] D.S. Stark, M.R. Harris, J. Phys. E21 (1988) 715. [9] G. Dong, J. Wang, Y. Gao, S. Chen, Cat. Lett. 58 (1999) 37. [10] R. Burch, Catal. Today 35 (1997) 27. [11] M. Okumura, N. Masuyama, E. Konishi, S. Ichikawa, T. Akita, J. Catal. 208 (2002) 485. [12] D.R. Schryer, B.T. Upchurch, J.D. Van Norman, K.G. Brown, J. Schyrer, J. Catal. 115 (1989) 132. [13] K.C. Taylor, J.C. Schlatter, J. Catal. 63 (1980) 53. [14] C.A. Parks, Chem. Rev. 65 (1965) 177. [15] R.H. Yoon, J. Coll. Interf. Sci. 70 (1979) 483. [16] P.G. Johansen, A.S. Buchana, Austr. J. Chem. 66 (1962) 967.
Multi-component noble metal catalysts were found to interact and work in synergy for several catalytic reactions. This was determined to be because the different metal oxide supports and the deposited different noble metal particles were mixed in nano-scale. The resulting multi-component catalysts showed extremely high catalytic activity for not only the oxidation of trimethylamine and o-CPh that have complex molecular structure with different types of chemical bonding, but also a simple reaction such as the oxidation of H2 . Although the Ir catalyst exhibits low activity in the decomposition of dioxin derivatives, by adding it with noble metal catalysts the catalytic activity is greatly improved. The Ir catalyst is considered to enhance the oxidative decomposition of dioxin over the multi-component catalyst. Experimental results showed that highly united multi-component noble
References
Multi-component noble metal catalysts prepared by sequential deposition precipitation for low temperature decomposition of dioxin M. Okumura a,∗ , T. Akita a , M. Haruta b , X. Wang c , O. Kajikawa d , O. Okada d a
National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda 563-8577, Japan National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onokawa, Tsukuba 305-8569, Japan c Kansai Research Institute, Kyoto Research Park 17, Chudo-ji Minami-machi, Shimogyo-ku, Kyoto 600-8813, Japan d Osaka Gas Co. Ltd., 6-19-9 Torishima Konohana-ku, Osaka 554-0051, Japan
b
Received 10 December 2001; received in revised form 5 April 2002; accepted 6 April 2002
Abstract To create a decomposition catalyst for the several pollutants with the necessary performance, we united noble metals of two and three different types by selective and successive deposition onto select mixed metal oxide supports. The catalytic oxidations of H2 , trimethylamine, o-chlorophenol and of dioxin derivatives over these multi-component noble metal catalysts were subsequently investigated. The ternary component catalyst, Au/Fe2 O3 -Pt/SnO2 -Ir/La2 O3 , showed greatly enhanced activity in the catalytic oxidation of o-chlorophenol and of dioxin at 423 K, indicating a catalytic activity enhancing (boosting) effect by the addition of the Ir/La2 O3 catalyst that by itself shows low activity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-component noble metal catalyst; Synergetic effect; Boosting effect; Low temperature oxidative decomposition of dioxin; Environmental catalyst
1. Introduction One of the most urgent needs in environmental protection today is the control of dioxin emission from incinerators [1,2]. There are several treatment techniques presently in use, such as the raising of the waste gas temperature to cause catalytic oxidative decomposition [3,4]. However, when considering the initial investment and mechanical complexity, this method is not practicable for small-scale incinerators. A novel solution for this task would be to decompose ∗ Corresponding author. Tel.: +81-727-51-9732; fax: +81-727-51-9714. E-mail address: [email protected] (M. Okumura).
dioxin and its derivatives at the dust filter using catalytic oxidation, preferably for practical reasons at temperatures <473 K. This particular figure is relevant since the waste gases are usually reduced to this temperature at the dust filter after efficient heat recovery. Although supported gold catalysts are remarkably active in the oxidation of CO and trimethylamine at temperatures <473 K [5–7], they do not exhibit sufficiently high catalytic activity for the oxidativedecomposition of dioxin. This is probably because a number of bondings, such as C–C, C–Cl, C–O, are broken at the catalyst surface. In order to develop a decomposition catalyst with improved performance, we attempted the creation of multi-component catalysts composed of various supported noble metal
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 2 0 0 - X
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catalysts, all which have different catalytic properties. We speculated that an appropriate combination of a few noble metals deposited on different metal oxides (i.e. not on the same metal oxide support), might bring about a synergetic effect in the simultaneous decomposition of dioxin and other pollutants at temperatures between 473 K and the ambient temperature. Supported noble metal catalysts, such as Pd, Ir, Pt, and Au catalysts, are already used practically for environmental protection owing to their high catalytic activity in the oxidation or reduction of hydrocarbons, NOx and amine derivatives [8–11]. It is especially well known that Pt and Pd catalysts, such as Pt/SnO2 and Pd/SnO2 , are active in the combustion of hydrocarbons [12], and Au catalysts exhibit high catalytic activity in the oxidation of CO and of trimethylamine. Ir catalysts have recently been reported to be active in the reduction of NOx with hydrocarbon in the presence of oxygen [13]. Because incinerator exhaust gases contain a variety of pollutants—not only dioxin derivatives but also other compounds, such as odors, volatile organic compounds and the reaction intermediates of dioxins—it would be unreasonable to expect a single-component catalyst to achieve high catalytic performance for all of the exhaust gas pollutants. However, it can be assumed that if several supported single noble metal catalysts were encouraged to work in synergy, the overall catalytic activity would be greatly improved. The concept of this multi-component noble metal catalyst is to control the microscopic structure of the composition and organization of the noble metal catalysts at the nano-scale. In contrast to conventional bi-metal or multi-component catalysts that share the same metal oxide support, noble metal nano-particles deposited on different metal oxides, for example, Pt/SnO2 , Ir/La2 O3 and Au/Fe2 O3 , are prepared by depositing each metal component predominantly on select metal oxides that are well mixed as a result of preparation by coprecipitation. In this paper, the preparation of the multi-component catalysts by the sequential deposition precipitation method was carried out and the catalytic activity measurements in the oxidation reaction of hydrogen and trimethylamine over the several multi-component catalysts were examined in order to confirm whether or not this multi-component noble metal catalyst would improve the catalytic activity through a
synergetic effect. Finally, the oxidative decomposition of o-chlorophenol, and of dioxin derivatives over the multi-component catalysts were investigated. 2. Experimental 2.1. Catalyst preparation Single-component catalysts of Au/Fe2 O3 , Pd/SnO2 , and Ir/La2 O3 were prepared using the deposition precipitation (DP) method. Hydroxides of gold, palladium, or iridium were deposited on the metal oxide supports through pH-controlled chemical interaction in an aqueous solution of HAuCl4 , Pd(CH3 OO)2 or IrCl4 . These were then washed, dried and calcined in air at 673 K for 4 h. The support materials of Fe2 O3 , SnO2 , and La2 O3 were prepared by neutralizing aqueous solutions of iron(III) nitrate, stannic chloride, and lanthanum nitrate with sodium carbonate, respectively, and calcinating the resulting precipitates in air at 627 K. In order to prepare the binary component noble metal catalyst, Pd/SnO2 -Au/Fe2 O3 , the selective depositions of Pd and Au hydroxides onto a select metal oxide were carried out using the procedure described below. As shown in Fig. 1, the isoelectric points of Fe2 O3 and SnO2 in an aqueous solution are pH 4.5 and 8.0, respectively [14–16]. Adjusting the pH of the Pd(CH3 COO)2 solution with a suspension of Fe2 O3 -SnO2 mixture prepared by coprecipitation
Fig. 1. The relationship between the density of adsorbed ions and the isoelectric point of the support.
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between 4.5 and 8.0, Pd2+ ions were adsorbed mainly onto the negatively charged SnO2 surfaces. This is because the Pd2+ ion and Fe2 O3 surfaces were positively charged in this condition and hence repelled each other. This speculation was confirmed by the results of the preparation of single component Au catalyst prepared by DP with such condition. In this condition, the loading amount of Au became extremely low. This intermediate was calcined in air at 673 K for 4 h. The calcined intermediate (e.g. Fe2 O3 -PdO/SnO2 ) was then added to the HAuCl4 solution whose pH had been adjusted to between 6 and 8, giving an intermediate (e.g. Au(OH)3 /Fe2 O3 + PdO/SnO2 ) with Au(OH)3 deposited mainly on Fe2 O3 . Every DP step involved the selective deposition of Au and Pd onto the different metal oxide supports by means of pH-controlled chemical interactions that were based on their different isoelectric points (IEP) in an aqueous solution of HAuCl4 and Pd(ac)2 , and subsequent washing, drying and calcination. The preparation of ternary component noble metal catalysts is also possible using a similar procedure, however, the noble metal ions available for the deposition precipitation procedure is limited. Consequently the coprecipitation method was also applied for the hybridization of the third noble metal catalyst, such as Ir/La2 O3 . As an example, for the preparation of Pd/SnO2 -Au/Fe2 O3 -Ir/La2 O3 , Ir(OH)4 and La(OH)3 were coprecipitated in the presence of Pd/SnO2 Au/Fe2 O3 prepared using the DP method. 2.2. Catalytic activity measurements The catalytic activities were measured in a fixed-bed flow reactor using 100, 500 and 1500 mg of catalyst samples for the oxidation of hydrogen, trimethylamine and o-chlorophenol, respectively. Multicomponent noble metal catalysts containing Pd, Pt, or Ir metals were reduced in 20% hydrogen in Ar (SV = 20,000 h−1 ml g−1 cat.) at 523 K for 150 min for the comparative measurements of catalytic activity. Hereinafter, the catalysts without H2 treatment are expressed as Au/Fe2 O3 -PdO/SnO2 -IrO2 /La2 O3 and those after H2 treatment are expressed as Au/Fe2 O3 Pd/SnO2 -Ir/La2 O3 . Reaction products were analyzed using gas chromatography. The reactant gases and the space velocities for the oxidation of hydrogen, trimethylamine and o-chlorophenol were 1 vol.% H2
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in air at the flow rate of SV = 20,000 h−1 ml g−1 cat., 100 ppm trimethylamine in air at the flow rate of SV = 10,000 h−1 ml g−1 cat., and 1500 ppm o-chlorophenol in 6 vol.% humidified air at the flow rate of SV = 10,000 h−1 ml g−1 cat., respectively. The dioxin derivatives used for the catalytic activity measurement over the catalysts are seven types of polychlorinated dibenzo-p-dioxin derivatives (PCDDs) and 10 types of polychlorinated dibenzofuran derivatives (PCDFs). Dioxins were dissolved into hexane keeping a concentration of 0.1 mg ml−1 hexane for each dioxin derivative. After the introduction of this solution containing 350 ng of dioxins into the catalyst bed of the fixed-bed flow reactor line, as shown in Fig. 2a, the catalyst bed was evaporated to dryness under ambient conditions by means of the passing of dry air. Activity measurements for the oxidative decomposition of dioxin derivatives were performed for 2 h by passing air (SV = 6000 h−1 ml g−1 cat.) preheated to 423 K. The conversion of dioxin was calculated from the following equation: Dad + Dtrap C = 1− × 100 Din where C is the conversion of dioxin, Din the amount of the introduced dioxin, Dad the amount of dioxins adsorbed in the catalyst bed, and Dtrap the amount of the dioxin adsorbed in the reaction line and XAD-2 resin trap, respectively. The oxidative decomposition of the incinerator exhausts gas over the ternary component noble metal catalyst was performed using a small batch type incinerator. First, a mixture of corrugated paper (19 kg) and vinyl chloride (1 kg) was incinerated for 90 min, and then a mixture of corrugated paper (4.75 kg) and vinyl chloride (0.25 kg) was added every 30 min. The catalytic reaction experiment shown in Fig. 2b was connected to the chimney of the incinerator. The activity measurements were performed by passing the exhaust gas (SV = 12,000 h−1 ml g−1 cat.) for 5 h at 423 K. The conversion of dioxin was determined from the difference in the amount of the adsorbed dioxin in the reference line and that in the catalyst line. The analysis of dioxins was carried out by the following procedure. The samples were extracted with 200 ml of toluene for 16 h by use of soxhlet extractor. Then, the extracts were concentrated to about 5 ml and were treated with sulfuric acid. After the purification, they
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Fig. 2. Experimental set-ups for the oxidative decomposition of (a) the laboratory reaction and of (b) the incinerator test. T, thermocouple; P, pump; M, massflow meter.
were analyzed by gas chromatograph–mass spectrometry (GC–MS).
3. Results and discussion 3.1. Hydrogen oxidation Fig. 3 shows the conversion vs. temperature curves in the oxidation of H2 over Au/Fe2 O3 and Pd/SnO2 ,
1:1 physical mixture of Au/Fe2 O3 and Pd/SnO2 , and the binary component noble metal catalyst, Pd/SnO2 Au/Fe2 O3 . The catalytic activity of Pd/SnO2 is much higher than that of Au/Fe2 O3 , and the catalytic activity of a physical mixture of Au/Fe2 O3 and Pd/SnO2 is situated in between those of the single-component catalysts. This shows that simple physical mixing does not bring about a synergistic effect. Conversely, the binary component noble metal catalyst, Pd/SnO2 -Au/Fe2 O3 , exhibits the highest catalytic activity and is more
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Fig. 3. Conversion of H2 as a function of catalyst temperature in H2 oxidation over single-component and multi-component noble metal catalysts. (䊏) 2 wt.% Au/Fe2 O3 ; (䊉) 2 wt.% Pd/SnO2 ; (䉱) mechanically mixed 2 wt.% Au/Fe2 O3 + 2 wt.% Pd/SnO2 ; (䉭) 2 wt.% Au/Al2 O3 -2 wt.% Pd/SnO2 ; reactant gas; H2 1 vol.% in air; SV = 20,000 h−1 ml g−1 cat.
active than Pd/SnO2 . It is surprising that the hybridization of single-component catalysts by the sequential deposition of noble metals on different metal oxide supports provides such significant synergy even in a simple reaction such as hydrogen oxidation. 3.2. Oxidative decomposition of trimethylamine Fig. 4 shows conversion of trimethylamine to CO2 as a function of catalyst temperature over the single-component catalysts, Au/Fe2 O3 , Pt/SnO2 and Ir/La2 O3 , and the ternary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 . Among the three single-component catalysts, the most active one is Au/Fe2 O3 and the most inactive is Ir/La2 O3 . The two binary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 , exhibit much higher catalytic activity than the single-component catalysts. The catalytic activities of the Au and Pt catalysts are greatly improved by the combination with the Ir catalyst, although the Ir catalyst by itself is much less active for this reaction. This
result indicates that Ir exhibits an boosting effect on the oxidative decomposition of trimethylamine over the other noble metal catalysts. 3.3. Oxidative decomposition of o-chlorophenol As a preliminary experiment, o-chlorophenol (o-CPh) was used instead of dioxin and the catalytic activities of the single-component, binary and ternary component noble metal catalysts were measured at 373 and 473 K. Fig. 5 shows the conversion of o-CPh during its oxidative decomposition to CO2 over 6 h duration. The single-component catalysts, except for Ir/La2 O3 , showed relatively high catalytic activity for the oxidative decomposition of o-CPh and each one has a characteristics feature. While Pt/SnO2 and Pd/SnO2 are more active in the conversion to CO2 , Au/Fe2 O3 is more active in the conversion of o-CPh. The binary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 , exhibits catalytic activity higher by 20% than the corresponding single-component catalysts. The conversions to CO2 over the ternary component noble metal catalyst,
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Fig. 4. Yield of CO2 as a function of catalyst temperature in the oxidation of trimethylamine over single-component and multi-component noble metal catalysts. (䉱) 2 atm% Ir/La2 O3 ; (䉬) 2 atm% Pt/SnO2 ; (䊏) 2 atm% Au/Fe2 O3 ; (䊊) 2 atm% Au/Fe2 O3 -2 atm% Ir/La2 O3 ; (䊉) mechanical mixture of 2 wt.% Pt/SnO2 and 2 wt.% Ir/La2 O3 ; reactant gas, trimethylamine 100 ppm in air; SV = 10,000 h−1 ml g−1 cat.
Fig. 5. Conversion of o-CPh and yield of CO2 in the oxidation of o-CPh over single-component and multi-component noble metal catalysts at 423 and 473 K. Pd, 2 atm% Pd/SnO2 ; Ir, 2 atm% Ir/La2 O3 ; Pt, 2 atm% Pt/SnO2 ; Au, 2 atm% Au/Fe2 O3 ; B-IC: binary component noble metal catalyst (2 atm% Au/Fe2 O3 -2 atm% Pd/SnO2 ); T-IC, ternary component noble metal catalyst (2 wt.% Pt/SnO2 -2 atm% Au/Fe2 O3 -2 wt.% Ir/La2 O3 ); mix, mechanically mixed catalyst (2 wt.% Pt/SnO2 + 2 atm% Au/Fe2 O3 + 2 wt.% Ir/La2 O3 ); reactant gas, o-CPh 1500 ppm in 6 vol.% humidified air; SV = 3000 h−1 ml g−1 cat.
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Au/Fe2 O3 -Pt/SnO2 -Ir/La2 O3 , at 423 and 473 K are comparatively high, 38 and 90%, respectively, whereas the activity of the physically mixed catalyst, Au/Fe2 O3 + Pt/SnO2 + Ir/La2 O3 , was much lower. This indicates that only the microscopic mixing of the single-component catalysts and/or multiple noble metal loading can improve catalytic activity. The large synergy observed for the ternary component noble metal catalyst suggest that each component interact with each other for mutual enhancement. Especially, it should be noted that Ir also exhibited an boosting effect on the oxidative decomposition of o-CPh over the other noble metal catalysts.
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3.4. Oxidative decomposition of PCDD and PCDF derivatives over the multi-component noble metal catalysts Fig. 6 shows that the binary component noble metal catalysts, Au/Fe2 O3 -Ir/La2 O3 and Pt/SnO2 -Ir/La2 O3 present 78.8 and 69.9% conversion rates for the oxidative decomposition of PCDDs and PCDFs at 423 K, respectively. However, they show low activity in the decomposition of hexa- and octa- CDDs and CDFs, such as OCDD, 2,3,4,6,7,8-HxCDF, and 1,2,3, 4,6,7,9-HpCDF, OCDF. Especially, they have low activity in the decomposition of 1,2,3,4,6,7,9-HpCDF.
Fig. 6. Conversion from dioxin derivatives to CO2 in the oxidation of dioxin over binary and ternary component noble metal catalysts at 423 K in the laboratory reaction line. (A) 2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 ; (B) 2 atm% Ir/La2 O3 -2 atm% Au/Fe2 O3 ; (C) 2 atm% Ir/La2 O3 -2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 . SV = 6000 h−1 ml g−1 cat.; reaction time = 2 h.
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Conversely, the ternary component noble metal catalyst, Ir/La2 O3 -Pt/SnO2 -Au/Fe2 O3 , exhibits high catalytic activity whose average conversion rate is 98.0% for all types of dioxin derivatives. The markedly enhanced catalytic activity in the ternary component noble metal catalysts was in good agreement with that observed in the decomposition of o-CPh. 3.5. Decomposition of the incinerator exhaust gases over the ternary component noble metal catalyst The catalytic activity of Pt/SnO2 -Au/Fe2 O3 -Ir/ La2 O3 for the decomposition of dioxin derivatives in the experimental setup shown in Fig. 2b is summarized in Fig. 7. The average conversions for the decomposition of PCDDs and PCDFs are 99.8 and 99.8%, respectively. When compared with the V2 O5 /TiO2 catalysts that are most widely used for dioxin decomposition in commercial processes, it can be seen that Pt/SnO2 -Au/Fe2 O3 -Ir/La2 O3 has appreciably higher catalytic activity for the decomposition of dioxin
derivatives at 423 K. Thus, the obtained catalytic activity for the decomposition of dioxins at 423 K is remarkably high and its TEQ conversion reaches 99.7%. Comparing the results of the laboratory reaction and the incinerator test for the decomposition of PCDD and PCDF derivatives, the result of the incinerator test is slightly better than that of the laboratory reaction. This might be because the procedure of dioxin introduction used in the laboratory reaction is different from that used in the incinerator test. 3.6. TEM observation of the multi-component noble metal catalyst TEM images of the binary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 , and the ternary component noble metal catalyst, Au/Fe2 O3 -Pd/SnO2 -Ir/ La2 O3 , are shown in Fig. 8. Highly mixed Fe2 O3 -SnO2 support was observed in the image of Au/Fe2 O3 -Pd/ SnO2 shown in Fig. 8a. Although relatively large Au particles were observed, Pd particles were not found
Fig. 7. Conversion from dioxin derivatives to CO2 in the oxidation of dioxin over the ternary component noble metal catalysts (2 atm% Pt/SnO2 -2 atm% Au/Fe2 O3 -2 atm% Ir/La2 O3 ) at 423 K in the incinerator reaction line.
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Fig. 8. TEM images of the multi-component noble metal catalysts (a) 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 ; (b) Ir/La2 O3 region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 ; (c) Au/Fe2 O3 and Pd/SnO2 mixed region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 ; (d) Ir/La2 O3 and Pd/SnO2 mixed region of 2 wt.% Au/Fe2 O3 -2 wt.% Pd/SnO2 -2 wt.% Ir/La2 O3 .
as particle images. It can be assumed that the ultra-fine Pd particles were highly dispersed over the support, as the existence of Pd was confirmed by EDX analysis. TEM images of Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 are shown in Fig. 8b, c and d. In Fig. 8b, a Ir/La2 O3 region of the ternary component noble metal catalyst is shown. It was determined from this image that the small dark dots were Ir nano-particles, and these particles were dispersed over the La2 O3 support. This was also confirmed by EDX analysis. As the preparation of the Au/Fe2 O3 -Pd/SnO2 intermediate of Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 is identical to that of the binary component noble metal catalyst, the microscopic structure of Au/Fe2 O3 -Pd/SnO2 in the ternary component noble metal catalyst was almost the same
as that of the binary component noble metal catalyst. Furthermore, highly mixed La2 O3 -SnO2 support was also observed in the image of the ternary component noble metal catalyst shown in Fig. 8d. From these analyses, it was determined that the metal oxides were highly mixed at a nano-scale and relatively small noble metal particles were dispersed onto the metal oxides. It might, therefore, be concluded that the Au/Fe2 O3 -Pd/ SnO2 and Au/Fe2 O3 -Pd/SnO2 -Ir/La2 O3 catalysts are primary models of the multi-component noble metal catalyst that we proposed. From the several catalytic activity measurements, it was found that the more highly multiplied noble metal catalysts showed higher catalytic activity for the oxidation of complex molecules. The simply mechanically
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mixed catalyst showed low catalytic activity for the decomposition of H2 and o-CPh. Thus, the catalytic activity is greatly dependent on the nano-level dispersion of the single-component catalysts. This suggests that the spillover of the active species and the intermediates migrate to other active sites on different components of the catalyst and proceed onto further reactions. In the preparation of a multi-component noble metal catalyst, it is difficult to completely prepare the multi-component noble metal catalyst that consists of pure single-component catalysts because small amounts of noble metals might be deposited onto metal oxide supports where not required. Therefore, a combined noble metal catalyst might be prepared unintentionally. Although the amount of this catalyst might be small, the synergetic effect of the catalytic activity could be affected.
metal catalysts are necessary to promote the higher catalytic activity necessary for the decomposition of complex molecules, such as trimethylamine, o-CPh, and dioxin derivatives. The stability of multi-component catalysts, especially for the deactivation by Cl, and the sub-ppm or ppb level analysis of other byproducts have to be studied furthermore. While the mechanism of the synergy evident in the decomposition of dioxin over multi-component catalysts needs further investigation, the validity for the improvement of the catalytic activity is considered to have been confirmed. It is especially noteworthy that the multi-component noble metal catalyst appears to be a promising material for achieving the depuration of the typical dioxin containing exhaust gases emanating from incinerators.
4. Conclusion
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Multi-component noble metal catalysts were found to interact and work in synergy for several catalytic reactions. This was determined to be because the different metal oxide supports and the deposited different noble metal particles were mixed in nano-scale. The resulting multi-component catalysts showed extremely high catalytic activity for not only the oxidation of trimethylamine and o-CPh that have complex molecular structure with different types of chemical bonding, but also a simple reaction such as the oxidation of H2 . Although the Ir catalyst exhibits low activity in the decomposition of dioxin derivatives, by adding it with noble metal catalysts the catalytic activity is greatly improved. The Ir catalyst is considered to enhance the oxidative decomposition of dioxin over the multi-component catalyst. Experimental results showed that highly united multi-component noble
References