- Email: [email protected]
Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5
Applied Catalysis B: Environmental 30 (2001) 259–265
Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2 O5 Shetian Liu a,∗ , Akira Obuchi b , Junko Oi-Uchisawa b , Tetsuya Nanba b , Satoshi Kushiyama b a
Chemical Engineering Department, Hebei University of Science and Technology, Scijiazhuang 050018, China b Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305-8569, Japan Received 27 June 2000; received in revised form 28 September 2000; accepted 28 September 2000
Abstract A series of SiO2 -supported MoO3 , V2 O5 , and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO2 -supported MoO3 , V2 O5 or Pt catalysts in the absence of SO2 in the reactant gas. Based on the effects of NO2 on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O2 to NO2 , while MoO3 and V2 O5 catalyze the oxidation of carbon with NO2 . Finally, the effects of SO2 on the carbon oxidation reaction were examined and discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Platinum; Molybdenum oxide; Vanadium oxide; Diesel particulate; Oxidation; Synergistic catalysis; NOx ; SO2
1. Introduction Soot and NOx emitted from diesel engines cause serious problems to human health and the environment. The removal of diesel soot can be achieved efficiently by using a particulate trap [1,2]. Such a trap, however, must be regenerated by continuously or periodically burning off the collected carbon to keep the exhaust backpressure within an acceptable limit. A plausible method for the continuous regeneration is to use a
∗ Corresponding author. Present address: Industrial Machine and Plant Development Center, Ishikawajima-Harima Heavy Industries Co., Ltd., Yokohama 235-8501, Japan. Tel.: +81-45-759-2164; fax: +81-45-759-2149. E-mail address: shetian [email protected] (S. Liu).
catalytic coating that promotes soot oxidation at relatively low temperatures. As a typical solid–solid–gas phase reaction, good contact between the carbon and catalyst is required to achieve catalytic oxidation. Regarding these aspects, catalyst design has been focused on how to improve the catalyst–carbon contact. Methods adopted to increase the contact include the use of metal fuel additives and catalytic compounds mobile on a support [3]. The use of a metal additive needs distribution and replenishment, and may also increase ash formation and cause clogging of the filter. Developing a particulate filter with a supported catalyst seems to be a more feasible solution. High mobility of the catalyst can be achieved by using oxides or their lowmelting-point eutectic mixtures. In this respect, MoO3 and V2 O5 [4] are recognized as being among the best
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 3 8 - 1
260
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
single-oxide catalysts to promote carbon oxidation by O2 . Their mixed oxides, such as CsVO3 –MoO3 , Cs2 MoO4 –V2 O5 [3,5,6] and Cs4 V2 O7 [7] have been reported to have higher catalytic activities than the single components. In addition, it has been reported that chloride-containing mixtures such as Cu/K/Mo/Cl [4,8,9] and Cu/K/V/Cl [10–12] have lower melting points than mixtures of only metal oxides and are highly active towards soot oxidation by O2 . However, chloride-containing catalysts are not thought to be practical because of their high volatility and potential toxicity [10,13]. The Johnson–Matthey group has developed another kind of catalytic system using Pt as the main active component, based on the concept of increasing the presence of a strong mobile oxidizer. Nitric oxide contained in the exhaust gas is oxidized on Pt to NO2 , an oxidizer much stronger than O2 , which then directly attacks the carbon surfaces [14,15]. Some recent studies [5,16] on the effect of NO also reveal that the presence of NO increases the rate of soot oxidation due to the oxidation of NO to form NO2 . Furthermore, we have recently found that the presence of SO2 as well as NO in the feed gas promotes Pt catalysis of carbon oxidation even more [17,18]. We attributed this increased carbon oxidation to SO3 , produced from SO2 on Pt, catalyzing the oxidation of carbon by NO2 . However, recent trends in diesel fuel demand a substantial decrease in the sulfur content, which will result in an extremely low concentration of SO2 in the exhaust gas. It is, therefore, desirable to develop a supported catalyst, which can substitute for SO3 and is active in promoting carbon oxidation by both O2 and NO2 , and combine this secondary catalyst with Pt. This secondary catalyst needs to attain high contact with the carbon surfaces so it must have a high mobility on the supporting material. In this paper, we report the activities of MoO3 and V2 O5 as the secondary catalyst and the effects of their combination with Pt.
2. Experimental 2.1. Catalyst preparation The catalysts used were prepared by impregnating the support (SiO2 ; Wakogel C-100, Wako Pure Chem. Ind., Ltd.) with the corresponding solutions
using the incipient wetness method. The precursors used were: [Pt(NH3 )4 ](OH)2 (aqueous solution, Pt content = 1.9 wt.%; Tanaka–Kinkinzoku); (NH4 )6 Mo7 O24 ·6H2 O (Kanto Chem. Co., Inc.) aqueous solution; and V2 O5 (Wako Pure Chem. Ind., Ltd.) ammonia water solution. The platinum loading of Pt/SiO2 was 1 wt.%, and for V2 O5 /SiO2 and MoO3 /SiO2 the metal loading was 0.575 mmol V or Mo per gram of SiO2 . After drying at 120◦ C overnight, the SiO2 -supported Pt precursor was reduced at 400◦ C in 4% H2 in N2 gas flow for 4 h. V2 O5 –Pt/SiO2 and MoO3 –Pt/SiO2 were prepared by impregnating the reduced Pt/SiO2 with the corresponding V- or Mo-containing solutions. The loading amount of V or Mo was 0.575 mmol/g of the support. All samples were finally calcined at 500◦ C for 2 h. The prepared catalysts were pressed at 500 kg/cm2 in to pellets and crushed to granules of 0.25 to 0.50 mm for testing the oxidation activity on model soot. 2.2. Catalytic reaction test The catalytic oxidation activity of each prepared sample was evaluated with a temperature programmed reaction (TPR) system. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F; primary particle size = 28 nm; specific surface area = 80 m2 /g; CHN analysis: C = 97.99 wt.%, H = 1.12 wt.%, N = 0.06 wt.%) was used as model soot. The catalyst granules (0.5 g) and CB powder (0.005 g) were mixed together with a spatula and placed in a tubular quartz reactor (10 mm i.d.). Mixing in this way results in a “loose” contact between the catalyst and carbon, which is thought to be close to that found in practical cases [19]. The reactant gases of 1000 ppm NO or NO2 + 0 or 100 ppm SO2 + 10% O2 + 7% H2 O in N2 (compositions simulating those of diesel exhaust gas) were passed through the mixtures of catalyst sample and CB at a flow rate of 500 ml/min. The catalyst temperature was monitored with a thermocouple directly inserted in to the center of the catalyst bed. The reactor temperature was raised by 10◦ C/min from 80 to 750◦ C. The concentrations of CO2 and CO emitted from the carbon oxidation were continuously measured and recorded by a non-dispersive IR gas analyzer (Shimadzu, CGT-7000). The concentrations of NOx and NO in the outlet gas were monitored with
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
two NOx analyzers (Yanaco ECL-77A and Yanaco ECL-88A). TPR curves were evaluated at the initial temperature (Ti ), defined as the temperature when the CO2 + CO concentration reached 100 ppm; peak temperature (Tp ), the temperature at maximum CO2 + CO concentration; and final temperature (Tf ), when the CO2 + CO concentration returned to 100 ppm.
261
TPR results of carbon oxidation on SiO2 -supported Pt and MoO3 or V2 O5 catalysts are shown in Figs. 1 and 2. The corresponding peak temperatures are listed in Table 1 for clarity. On a blank SiO2 sample under a reactant mixture composed of 10% O2 , 7% H2 O and 1000 ppm NO diluted with N2 , the TPR profile showed a Tp of around 687◦ C (Fig. 1 and Table 1), typical for non-catalyzed combustion of carbon soot using oxygen. The carbon oxidation was started at 521◦ C (Ti ) and completed at about 710◦ C (Tf ). When 1 wt.% Pt was supported on SiO2 , Ti , Tp and Tf were at 359, 562 and 612◦ C, lowered by 162, 125 and 98◦ C, respectively. The
selectivity to CO, defined as the ratio of CO produced to the sum of CO and CO2 formed during the reaction, was almost zero, most probably due to high catalytic activity of Pt toward the oxidation of CO to CO2 . On V2 O5 /SiO2 , carbon oxidation was also greatly promoted, although the selectivity to CO was not decreased as much as in the case for Pt/SiO2 . Ti , Tp and Tf were at 427, 555 and 607◦ C, which were 94, 132 and 103◦ C lower than for blank SiO2 . Interestingly, the coexistence of Pt and V2 O5 on SiO2 lowered the peak temperature to the largest extent. Ti , Tp and Tf were 353, 518 and 557◦ C, respectively. Tp was 44◦ C lower than on Pt/SiO2 and 37◦ C lower than on V2 O5 /SiO2 . The TPR profile of carbon oxidation on MoO3 /SiO2 showed a sharp, narrow peak (Fig. 2 and Table 1) similar to that of V2 O5 /SiO2 (Fig. 1). The initial temperature (T i = 437◦ C) was much higher than that of Pt/SiO2 (359◦ C); however, Tp and Tf were 27◦ C and 24◦ C lower, respectively. The combination of Pt with MoO3 showed an effect similar but more pronounced than was observed for V2 O5 –Pt (Fig. 2 and Table 1); among the set of SiO2 -supported MoO3 and Pt catalysts, MoO3 –Pt/SiO2 exhibited the highest activity. Ti , Tp and Tf were 340, 475 and 522◦ C, respectively. Tp was 87◦ C lower than on Pt/SiO2 and 60◦ C lower than on MoO3 /SiO2 .
Fig. 1. TPR profiles of CO2 (solid symbol) and CO (open symbol) evolution from carbon oxidation on SiO2 -supported V2 O5 and Pt catalysts. Reactant gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2 . (䉲, 5) SiO2 ; (䊉) Pt/SiO2 ; (䉬, 䉫) V2 O5 /SiO2 ; (䊏) V2 O5 –Pt/SiO2 .
Fig. 2. TPR profiles of CO2 (solid symbol) and CO (open symbol) evolution from carbon oxidation on SiO2 -supported MoO3 and Pt catalysts. Reactant gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2 . (䉲, 5) SiO2 ; (䊉) Pt/SiO2 ; (䉬, 䉫) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
3. Results and discussion 3.1. Oxidation of carbon on SiO2 -supported Pt and MoO3 or V2 O5 catalysts
262
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
Table 1 Peak temperatures of TPR profiles of carbon oxidation on SiO2 -supported Pt and MoO3 or V2 O5 catalysts Catalyst name
TPR peak temperatures (◦ C) Feed gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2
SiO2 Pt/SiO2 V2 O5 /SiO2 V2 O5 –Pt/SiO2 MoO3 /SiO2 MoO3 –Pt/SiO2
Feed gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2
Ti
Tp
Tf
Ti
Tp
Tf
521 359 427 353 437 340
687 562 555 518 535 475
710 612 607 557 588 522
361 372 323 319 306 311
606 572 542 505 518 484
681 620 586 556 570 528
As already reported [14,15], the catalytic performance of Pt/SiO2 is attributed to NO2 formation by the oxidation of NO. In TPR profiles of NO oxidation under dry conditions (without water addition in the feed gas) on samples containing Pt and Mo (Fig. 3), the decrease in NO concentration corresponds to the formation of NO2 , since the total concentration of NO + NO2 was almost unchanged during these TPR experiments. It is evident that on Pt/SiO2 the oxidation of NO proceeds above 150◦ C and thermodynamic equilibria among NO, NO2 and O2 is reached above 270◦ C. On MoO3 –Pt/SiO2 the oxidation of NO starts at above 200◦ C and thermodynamic equilibria is reached at above 370◦ C. The presence of MoO3 on Pt/SiO2 strongly inhibited the NO oxidation reaction. Considering the fact that MoO3 was mobile on SiO2 during calcination at 500◦ C in the preparation of the
Fig. 3. TPR profiles of NO oxidation on SiO2 -supported MoO3 and Pt catalysts. Feed gas: 1000 ppm NO + 10% O2 in N2 . (5) SiO2 ; (䊊) Pt/SiO2 ; (䉬) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
catalyst, some of the Pt sites on the SiO2 surface may be covered by MoO3 , which may decrease the NO oxidation activity. MoO3 /SiO2 and SiO2 showed no apparent activities towards NO oxidation. These results indicate that carbon oxidation over MoO3 /SiO2 results mainly from the oxidation by O2 , which is different from carbon oxidation over the Pt/SiO2 catalyst. The catalytic performance of MoO3 /SiO2 is attributed to oxidation activity and mobility of MoO3 [3] on the support, enabling MoO3 to be a direct catalyst for particulate oxidation. 3.2. MoO3 /SiO2 and V2 O5 /SiO2 catalysis of carbon oxidation with NO2 As revealed in the above section, the combination of Pt with MoO3 or V2 O5 on SiO2 promoted the carbon oxidation reaction. We think that this promotion effect results from the formation of NO2 on Pt with subsequent carbon oxidation proceeding by NO2 with the aid of MoO3 or V2 O5 as a secondary catalyst. To confirm the catalytic effects of MoO3 and V2 O5 , NO was replaced by NO2 in the feed gas and the carbon oxidation reaction was performed on all the prepared samples. The results are shown in Figs. 4 and 5 and the peak temperatures are also summarized in Table 1. It is evident that on all samples except Pt/SiO2 the carbon oxidation rates at lower temperatures were increased by the use of NO2 . The combustion rates over MoO3 /SiO2 or V2 O5 /SiO2 were clearly higher than those over Pt/SiO2 or SiO2 . Furthermore, near the low temperature end of the TPR profiles, the catalytic performances of MoO3 /SiO2 and V2 O5 /SiO2 were almost identical to those of MoO3 –Pt/SiO2 and
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
263
Fig. 4. TPR profiles of CO2 +CO evolution from carbon oxidation on SiO2 -supported MoO3 and Pt catalysts. Reactant gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 . (5) SiO2 ; (䊉) Pt/SiO2 ; (䉬) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
Fig. 6. TPR profiles of CO2 +CO evolution from carbon oxidation on MoO3 /SiO2 with different feed gases. (䉬) 1000 ppm NO2 +7% H2 O in N2 ; (䊉) 10% O2 + 7% H2 O in N2 ; (䊏) 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 .
V2 O5 –Pt/SiO2 . This clearly indicates that MoO3 and V2 O5 catalyze the reaction of carbon oxidation by NO2 , and their catalytic activities are higher than Pt/SiO2 . The total TPR profiles of carbon oxidation on MoO3 /SiO2 and V2 O5 /SiO2 appear as a composite of a sharp peak with a shoulder at the lower temperature side. We thought these peaks resulted from oxidation of carbon by O2 and NO2 , respectively. This specula-
tion was confirmed by further experiments performed for carbon oxidation on MoO3 /SiO2 with different feed gases. When only NO2 was used as the oxidant in the feed gas (Fig. 6), the oxidation of carbon started at 308◦ C. As the temperature increased, the carbon oxidation rate leveled off at about 370◦ C, most probably due to the limited concentration of NO2 in the feed gas. On the other hand, when only O2 was used as the oxidant, a sharp peak appeared with Ti at 408◦ C and Tp at 540◦ C. Ti was 100◦ C higher than that for NO2 . When NO2 + O2 were used as oxidants, the lower temperature end of the TPR profile coincided with that for TPR with only NO2 , and the peak position was close to that for TPR with only O2 . In the case of Pt/SiO2 (Fig. 4), the replacement of NO by NO2 in the feed gas had no discernible effect on carbon oxidation. This means that the supplement of the oxidant NO2 from NO oxidation on Pt is analogous to directly supplying NO2 in the feed gas. After combination with MoO3 , the performance of Pt/SiO2 was greatly promoted. Unlike, MoO3 /SiO2 , the TPR profile of MoO3 –Pt/SiO2 showed no apparent shoulder on the lower temperature side. The carbon oxidation rate increased continuously with the rise in temperature and exceeded that of MoO3 /SiO2 at temperatures above 380◦ C. This clearly indicates that, with Pt, NO2 is repeatedly supplied from the re-oxidation of NO which was produced from NO2 reduction by carbon.
Fig. 5. TPR profiles of CO2 +CO evolution from carbon oxidation on SiO2 -supported V2 O5 and Pt catalysts. Reactant gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 . (5) SiO2 ; (䊉) Pt/SiO2 ; (䉬) V2 O5 /SiO2 ; (䊏) V2 O5 –Pt/SiO2 .
264
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
The above results strongly suggest that the highest performance attained on the MoO3 –Pt/SiO2 catalyst is based on a synergistic catalysis between Pt and MoO3 in the carbon oxidation reaction. Pt catalyzes the oxidation of NO with gas phase O2 to form NO2 , while MoO3 catalyzes the oxidation of carbon with NO2 . This synergistic catalysis is also clearly shown for the V2 O5 –Pt catalyst system (Fig. 5), but the activity of V2 O5 seems to be lower than that of MoO3 . 3.3. Effect of SO2 on carbon oxidation As reported in our previous papers [17,18], SO3 produced from SO2 on Pt surfaces works as a catalyst that accelerates the oxidation of carbon by NO2 . With the addition of SO2 to the reactant gas, the TPR curve of carbon oxidation on Pt/SiO2 shifted substantially to lower temperatures (Fig. 7a). On the other hand, the
oxidation of carbon on MoO3 –Pt/SiO2 showed almost identical features under either the presence or absence of SO2 in the reactant gas (Fig. 7b). On V2 O5 –Pt/SiO2 (Fig. 7c), the addition of SO2 was effective in promoting carbon oxidation but was less pronounced than in the case of Pt/SiO2 alone. The reason for the negligible effect of SO2 on MoO3 -containing catalysts is still not clear. Similar to that of NO oxidation (Fig. 3), the covering effects of MoO3 on Pt sites on the SiO2 surface may also decrease the SO2 oxidation activity. Another possible explanation is that MoO3 may adsorb SO3 at lower temperatures to hamper its diffusion onto the carbon surfaces. On the other hand, V2 O5 is used as the main active component in SO2 oxidation catalysts. The addition of V2 O5 to Pt/SiO2 may also cover some of the Pt sites but, at least, the catalytic SO2 oxidation may still proceed on V2 O5 . The effects of MoO3 and V2 O5 in promoting catalysis of carbon oxidation with NO2 are very similar to that shown by SO3 . They appear to act as the secondary catalysts to promote carbon oxidation mentioned in the introduction of this paper. Moreover, by combining MoO3 with Pt catalysts, it may be possible to decrease the emission of SO3 in to the atmosphere. Further study is necessary to clarify these speculations. 4. Conclusion SiO2 -supported Pt and V2 O5 or MoO3 were evaluated as catalysts for the oxidation of model soot with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation performances than the single component V2 O5 , MoO3 or Pt ones in the absence of SO2 in the reactant gas. V2 O5 and MoO3 showed higher activities than Pt in carbon oxidation by NO2 . The combination effect was explained by a synergistic catalysis: Pt catalyzes the oxidation of NO with gas phase O2 to NO2 , while V2 O5 and MoO3 catalyze the oxidation of carbon with NO2 . References
Fig. 7. TPR profiles of CO2 evolution from carbon oxidation on SiO2 -supported catalysts with (solid symbol) or without (open symbol) SO2 addition in the feed gas. Reactant gas: 1000 ppm NO + 0 or 100 ppm SO2 + 10% O2 + 7% H2 O in N2 .
[1] P. Zelenka, W. Cartellieri, P. Herzog, Appl. Catal. B 10 (1996) 3. [2] H.J. Stein, Appl. Catal. B 10 (1996) 69.
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265 [3] S.J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Appl. Catal. B 21 (1999) 35. [4] J.P.A. Neeft, Ph.D. Thesis, Delft University of Technology, The Netherlands, 1995. [5] S.J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Stud. Surf. Sci. Catal. 116 (1998) 667. [6] B.A.A.L. van Setten, R. van Dijk, S.J. Jelles, M. Makkee, J.A. Moulijn, Appl. Catal. B 21 (1999) 51. [7] G. Saracco, C. Badini, N. Russo, V. Specchia, Appl. Catal. B 21 (1999) 233. [8] G. Mul, Ph.D. Thesis, Delft University of Technology, The Netherlands, 1997. [9] G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, J.A. Moulijn, Appl. Catal. B 6 (1995) 339. [10] C. Badini, G. Saracco, V. Serra, V. Specchia, Appl. Catal. B 18 (1998) 137. [11] G. Saracco, V. Serra, C. Badini, V. Specchia, Ind. Eng. Chem. Res. 36 (1997) 2051.
265
[12] P. Ciambelli, M. D’Amore, V. Palma, S. Vaccaro, Combust. Flame 99 (1994) 413. [13] J.P.A. Neeft, W. Schipper, G. Mul, M. Makkee, J.A. Moulijn, Appl. Catal. B 11 (1997) 365. [14] B.J. Cooper, H.J. Jung, J.E. Thoss, US Patent 4,902,487 (1990) to Jonhson–Matthey. [15] P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson, P. Bronnenberg, SAE Technical Paper Series, 970182 (1997). [16] P. Ciambelli, V. Palma, P. Russo, S. Vaccaro, Appl. Catal. B 22 (1999) L5. [17] J-O. Uchisawa, A. Obuchi, Z. Zhao, S. Kushiyama, Appl. Catal. B 18 (1998) L183. [18] J-O. Uchisawa, A. Obuchi, A. Ogata, R. Enomoto, S. Kushiyama, Appl. Catal. B 21 (1998) 9. [19] J.P.A. Neeft, O.P. Pruissen, M. Makkee, J.A. Moulijn, Appl. Catal. B 12 (1997) 21.
Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2 O5 Shetian Liu a,∗ , Akira Obuchi b , Junko Oi-Uchisawa b , Tetsuya Nanba b , Satoshi Kushiyama b a
Chemical Engineering Department, Hebei University of Science and Technology, Scijiazhuang 050018, China b Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305-8569, Japan Received 27 June 2000; received in revised form 28 September 2000; accepted 28 September 2000
Abstract A series of SiO2 -supported MoO3 , V2 O5 , and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO2 -supported MoO3 , V2 O5 or Pt catalysts in the absence of SO2 in the reactant gas. Based on the effects of NO2 on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O2 to NO2 , while MoO3 and V2 O5 catalyze the oxidation of carbon with NO2 . Finally, the effects of SO2 on the carbon oxidation reaction were examined and discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Platinum; Molybdenum oxide; Vanadium oxide; Diesel particulate; Oxidation; Synergistic catalysis; NOx ; SO2
1. Introduction Soot and NOx emitted from diesel engines cause serious problems to human health and the environment. The removal of diesel soot can be achieved efficiently by using a particulate trap [1,2]. Such a trap, however, must be regenerated by continuously or periodically burning off the collected carbon to keep the exhaust backpressure within an acceptable limit. A plausible method for the continuous regeneration is to use a
∗ Corresponding author. Present address: Industrial Machine and Plant Development Center, Ishikawajima-Harima Heavy Industries Co., Ltd., Yokohama 235-8501, Japan. Tel.: +81-45-759-2164; fax: +81-45-759-2149. E-mail address: shetian [email protected] (S. Liu).
catalytic coating that promotes soot oxidation at relatively low temperatures. As a typical solid–solid–gas phase reaction, good contact between the carbon and catalyst is required to achieve catalytic oxidation. Regarding these aspects, catalyst design has been focused on how to improve the catalyst–carbon contact. Methods adopted to increase the contact include the use of metal fuel additives and catalytic compounds mobile on a support [3]. The use of a metal additive needs distribution and replenishment, and may also increase ash formation and cause clogging of the filter. Developing a particulate filter with a supported catalyst seems to be a more feasible solution. High mobility of the catalyst can be achieved by using oxides or their lowmelting-point eutectic mixtures. In this respect, MoO3 and V2 O5 [4] are recognized as being among the best
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 3 8 - 1
260
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
single-oxide catalysts to promote carbon oxidation by O2 . Their mixed oxides, such as CsVO3 –MoO3 , Cs2 MoO4 –V2 O5 [3,5,6] and Cs4 V2 O7 [7] have been reported to have higher catalytic activities than the single components. In addition, it has been reported that chloride-containing mixtures such as Cu/K/Mo/Cl [4,8,9] and Cu/K/V/Cl [10–12] have lower melting points than mixtures of only metal oxides and are highly active towards soot oxidation by O2 . However, chloride-containing catalysts are not thought to be practical because of their high volatility and potential toxicity [10,13]. The Johnson–Matthey group has developed another kind of catalytic system using Pt as the main active component, based on the concept of increasing the presence of a strong mobile oxidizer. Nitric oxide contained in the exhaust gas is oxidized on Pt to NO2 , an oxidizer much stronger than O2 , which then directly attacks the carbon surfaces [14,15]. Some recent studies [5,16] on the effect of NO also reveal that the presence of NO increases the rate of soot oxidation due to the oxidation of NO to form NO2 . Furthermore, we have recently found that the presence of SO2 as well as NO in the feed gas promotes Pt catalysis of carbon oxidation even more [17,18]. We attributed this increased carbon oxidation to SO3 , produced from SO2 on Pt, catalyzing the oxidation of carbon by NO2 . However, recent trends in diesel fuel demand a substantial decrease in the sulfur content, which will result in an extremely low concentration of SO2 in the exhaust gas. It is, therefore, desirable to develop a supported catalyst, which can substitute for SO3 and is active in promoting carbon oxidation by both O2 and NO2 , and combine this secondary catalyst with Pt. This secondary catalyst needs to attain high contact with the carbon surfaces so it must have a high mobility on the supporting material. In this paper, we report the activities of MoO3 and V2 O5 as the secondary catalyst and the effects of their combination with Pt.
2. Experimental 2.1. Catalyst preparation The catalysts used were prepared by impregnating the support (SiO2 ; Wakogel C-100, Wako Pure Chem. Ind., Ltd.) with the corresponding solutions
using the incipient wetness method. The precursors used were: [Pt(NH3 )4 ](OH)2 (aqueous solution, Pt content = 1.9 wt.%; Tanaka–Kinkinzoku); (NH4 )6 Mo7 O24 ·6H2 O (Kanto Chem. Co., Inc.) aqueous solution; and V2 O5 (Wako Pure Chem. Ind., Ltd.) ammonia water solution. The platinum loading of Pt/SiO2 was 1 wt.%, and for V2 O5 /SiO2 and MoO3 /SiO2 the metal loading was 0.575 mmol V or Mo per gram of SiO2 . After drying at 120◦ C overnight, the SiO2 -supported Pt precursor was reduced at 400◦ C in 4% H2 in N2 gas flow for 4 h. V2 O5 –Pt/SiO2 and MoO3 –Pt/SiO2 were prepared by impregnating the reduced Pt/SiO2 with the corresponding V- or Mo-containing solutions. The loading amount of V or Mo was 0.575 mmol/g of the support. All samples were finally calcined at 500◦ C for 2 h. The prepared catalysts were pressed at 500 kg/cm2 in to pellets and crushed to granules of 0.25 to 0.50 mm for testing the oxidation activity on model soot. 2.2. Catalytic reaction test The catalytic oxidation activity of each prepared sample was evaluated with a temperature programmed reaction (TPR) system. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F; primary particle size = 28 nm; specific surface area = 80 m2 /g; CHN analysis: C = 97.99 wt.%, H = 1.12 wt.%, N = 0.06 wt.%) was used as model soot. The catalyst granules (0.5 g) and CB powder (0.005 g) were mixed together with a spatula and placed in a tubular quartz reactor (10 mm i.d.). Mixing in this way results in a “loose” contact between the catalyst and carbon, which is thought to be close to that found in practical cases [19]. The reactant gases of 1000 ppm NO or NO2 + 0 or 100 ppm SO2 + 10% O2 + 7% H2 O in N2 (compositions simulating those of diesel exhaust gas) were passed through the mixtures of catalyst sample and CB at a flow rate of 500 ml/min. The catalyst temperature was monitored with a thermocouple directly inserted in to the center of the catalyst bed. The reactor temperature was raised by 10◦ C/min from 80 to 750◦ C. The concentrations of CO2 and CO emitted from the carbon oxidation were continuously measured and recorded by a non-dispersive IR gas analyzer (Shimadzu, CGT-7000). The concentrations of NOx and NO in the outlet gas were monitored with
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
two NOx analyzers (Yanaco ECL-77A and Yanaco ECL-88A). TPR curves were evaluated at the initial temperature (Ti ), defined as the temperature when the CO2 + CO concentration reached 100 ppm; peak temperature (Tp ), the temperature at maximum CO2 + CO concentration; and final temperature (Tf ), when the CO2 + CO concentration returned to 100 ppm.
261
TPR results of carbon oxidation on SiO2 -supported Pt and MoO3 or V2 O5 catalysts are shown in Figs. 1 and 2. The corresponding peak temperatures are listed in Table 1 for clarity. On a blank SiO2 sample under a reactant mixture composed of 10% O2 , 7% H2 O and 1000 ppm NO diluted with N2 , the TPR profile showed a Tp of around 687◦ C (Fig. 1 and Table 1), typical for non-catalyzed combustion of carbon soot using oxygen. The carbon oxidation was started at 521◦ C (Ti ) and completed at about 710◦ C (Tf ). When 1 wt.% Pt was supported on SiO2 , Ti , Tp and Tf were at 359, 562 and 612◦ C, lowered by 162, 125 and 98◦ C, respectively. The
selectivity to CO, defined as the ratio of CO produced to the sum of CO and CO2 formed during the reaction, was almost zero, most probably due to high catalytic activity of Pt toward the oxidation of CO to CO2 . On V2 O5 /SiO2 , carbon oxidation was also greatly promoted, although the selectivity to CO was not decreased as much as in the case for Pt/SiO2 . Ti , Tp and Tf were at 427, 555 and 607◦ C, which were 94, 132 and 103◦ C lower than for blank SiO2 . Interestingly, the coexistence of Pt and V2 O5 on SiO2 lowered the peak temperature to the largest extent. Ti , Tp and Tf were 353, 518 and 557◦ C, respectively. Tp was 44◦ C lower than on Pt/SiO2 and 37◦ C lower than on V2 O5 /SiO2 . The TPR profile of carbon oxidation on MoO3 /SiO2 showed a sharp, narrow peak (Fig. 2 and Table 1) similar to that of V2 O5 /SiO2 (Fig. 1). The initial temperature (T i = 437◦ C) was much higher than that of Pt/SiO2 (359◦ C); however, Tp and Tf were 27◦ C and 24◦ C lower, respectively. The combination of Pt with MoO3 showed an effect similar but more pronounced than was observed for V2 O5 –Pt (Fig. 2 and Table 1); among the set of SiO2 -supported MoO3 and Pt catalysts, MoO3 –Pt/SiO2 exhibited the highest activity. Ti , Tp and Tf were 340, 475 and 522◦ C, respectively. Tp was 87◦ C lower than on Pt/SiO2 and 60◦ C lower than on MoO3 /SiO2 .
Fig. 1. TPR profiles of CO2 (solid symbol) and CO (open symbol) evolution from carbon oxidation on SiO2 -supported V2 O5 and Pt catalysts. Reactant gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2 . (䉲, 5) SiO2 ; (䊉) Pt/SiO2 ; (䉬, 䉫) V2 O5 /SiO2 ; (䊏) V2 O5 –Pt/SiO2 .
Fig. 2. TPR profiles of CO2 (solid symbol) and CO (open symbol) evolution from carbon oxidation on SiO2 -supported MoO3 and Pt catalysts. Reactant gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2 . (䉲, 5) SiO2 ; (䊉) Pt/SiO2 ; (䉬, 䉫) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
3. Results and discussion 3.1. Oxidation of carbon on SiO2 -supported Pt and MoO3 or V2 O5 catalysts
262
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
Table 1 Peak temperatures of TPR profiles of carbon oxidation on SiO2 -supported Pt and MoO3 or V2 O5 catalysts Catalyst name
TPR peak temperatures (◦ C) Feed gas: 1000 ppm NO + 10% O2 + 7% H2 O in N2
SiO2 Pt/SiO2 V2 O5 /SiO2 V2 O5 –Pt/SiO2 MoO3 /SiO2 MoO3 –Pt/SiO2
Feed gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2
Ti
Tp
Tf
Ti
Tp
Tf
521 359 427 353 437 340
687 562 555 518 535 475
710 612 607 557 588 522
361 372 323 319 306 311
606 572 542 505 518 484
681 620 586 556 570 528
As already reported [14,15], the catalytic performance of Pt/SiO2 is attributed to NO2 formation by the oxidation of NO. In TPR profiles of NO oxidation under dry conditions (without water addition in the feed gas) on samples containing Pt and Mo (Fig. 3), the decrease in NO concentration corresponds to the formation of NO2 , since the total concentration of NO + NO2 was almost unchanged during these TPR experiments. It is evident that on Pt/SiO2 the oxidation of NO proceeds above 150◦ C and thermodynamic equilibria among NO, NO2 and O2 is reached above 270◦ C. On MoO3 –Pt/SiO2 the oxidation of NO starts at above 200◦ C and thermodynamic equilibria is reached at above 370◦ C. The presence of MoO3 on Pt/SiO2 strongly inhibited the NO oxidation reaction. Considering the fact that MoO3 was mobile on SiO2 during calcination at 500◦ C in the preparation of the
Fig. 3. TPR profiles of NO oxidation on SiO2 -supported MoO3 and Pt catalysts. Feed gas: 1000 ppm NO + 10% O2 in N2 . (5) SiO2 ; (䊊) Pt/SiO2 ; (䉬) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
catalyst, some of the Pt sites on the SiO2 surface may be covered by MoO3 , which may decrease the NO oxidation activity. MoO3 /SiO2 and SiO2 showed no apparent activities towards NO oxidation. These results indicate that carbon oxidation over MoO3 /SiO2 results mainly from the oxidation by O2 , which is different from carbon oxidation over the Pt/SiO2 catalyst. The catalytic performance of MoO3 /SiO2 is attributed to oxidation activity and mobility of MoO3 [3] on the support, enabling MoO3 to be a direct catalyst for particulate oxidation. 3.2. MoO3 /SiO2 and V2 O5 /SiO2 catalysis of carbon oxidation with NO2 As revealed in the above section, the combination of Pt with MoO3 or V2 O5 on SiO2 promoted the carbon oxidation reaction. We think that this promotion effect results from the formation of NO2 on Pt with subsequent carbon oxidation proceeding by NO2 with the aid of MoO3 or V2 O5 as a secondary catalyst. To confirm the catalytic effects of MoO3 and V2 O5 , NO was replaced by NO2 in the feed gas and the carbon oxidation reaction was performed on all the prepared samples. The results are shown in Figs. 4 and 5 and the peak temperatures are also summarized in Table 1. It is evident that on all samples except Pt/SiO2 the carbon oxidation rates at lower temperatures were increased by the use of NO2 . The combustion rates over MoO3 /SiO2 or V2 O5 /SiO2 were clearly higher than those over Pt/SiO2 or SiO2 . Furthermore, near the low temperature end of the TPR profiles, the catalytic performances of MoO3 /SiO2 and V2 O5 /SiO2 were almost identical to those of MoO3 –Pt/SiO2 and
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
263
Fig. 4. TPR profiles of CO2 +CO evolution from carbon oxidation on SiO2 -supported MoO3 and Pt catalysts. Reactant gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 . (5) SiO2 ; (䊉) Pt/SiO2 ; (䉬) MoO3 /SiO2 ; (䊏) MoO3 –Pt/SiO2 .
Fig. 6. TPR profiles of CO2 +CO evolution from carbon oxidation on MoO3 /SiO2 with different feed gases. (䉬) 1000 ppm NO2 +7% H2 O in N2 ; (䊉) 10% O2 + 7% H2 O in N2 ; (䊏) 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 .
V2 O5 –Pt/SiO2 . This clearly indicates that MoO3 and V2 O5 catalyze the reaction of carbon oxidation by NO2 , and their catalytic activities are higher than Pt/SiO2 . The total TPR profiles of carbon oxidation on MoO3 /SiO2 and V2 O5 /SiO2 appear as a composite of a sharp peak with a shoulder at the lower temperature side. We thought these peaks resulted from oxidation of carbon by O2 and NO2 , respectively. This specula-
tion was confirmed by further experiments performed for carbon oxidation on MoO3 /SiO2 with different feed gases. When only NO2 was used as the oxidant in the feed gas (Fig. 6), the oxidation of carbon started at 308◦ C. As the temperature increased, the carbon oxidation rate leveled off at about 370◦ C, most probably due to the limited concentration of NO2 in the feed gas. On the other hand, when only O2 was used as the oxidant, a sharp peak appeared with Ti at 408◦ C and Tp at 540◦ C. Ti was 100◦ C higher than that for NO2 . When NO2 + O2 were used as oxidants, the lower temperature end of the TPR profile coincided with that for TPR with only NO2 , and the peak position was close to that for TPR with only O2 . In the case of Pt/SiO2 (Fig. 4), the replacement of NO by NO2 in the feed gas had no discernible effect on carbon oxidation. This means that the supplement of the oxidant NO2 from NO oxidation on Pt is analogous to directly supplying NO2 in the feed gas. After combination with MoO3 , the performance of Pt/SiO2 was greatly promoted. Unlike, MoO3 /SiO2 , the TPR profile of MoO3 –Pt/SiO2 showed no apparent shoulder on the lower temperature side. The carbon oxidation rate increased continuously with the rise in temperature and exceeded that of MoO3 /SiO2 at temperatures above 380◦ C. This clearly indicates that, with Pt, NO2 is repeatedly supplied from the re-oxidation of NO which was produced from NO2 reduction by carbon.
Fig. 5. TPR profiles of CO2 +CO evolution from carbon oxidation on SiO2 -supported V2 O5 and Pt catalysts. Reactant gas: 1000 ppm NO2 + 10% O2 + 7% H2 O in N2 . (5) SiO2 ; (䊉) Pt/SiO2 ; (䉬) V2 O5 /SiO2 ; (䊏) V2 O5 –Pt/SiO2 .
264
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265
The above results strongly suggest that the highest performance attained on the MoO3 –Pt/SiO2 catalyst is based on a synergistic catalysis between Pt and MoO3 in the carbon oxidation reaction. Pt catalyzes the oxidation of NO with gas phase O2 to form NO2 , while MoO3 catalyzes the oxidation of carbon with NO2 . This synergistic catalysis is also clearly shown for the V2 O5 –Pt catalyst system (Fig. 5), but the activity of V2 O5 seems to be lower than that of MoO3 . 3.3. Effect of SO2 on carbon oxidation As reported in our previous papers [17,18], SO3 produced from SO2 on Pt surfaces works as a catalyst that accelerates the oxidation of carbon by NO2 . With the addition of SO2 to the reactant gas, the TPR curve of carbon oxidation on Pt/SiO2 shifted substantially to lower temperatures (Fig. 7a). On the other hand, the
oxidation of carbon on MoO3 –Pt/SiO2 showed almost identical features under either the presence or absence of SO2 in the reactant gas (Fig. 7b). On V2 O5 –Pt/SiO2 (Fig. 7c), the addition of SO2 was effective in promoting carbon oxidation but was less pronounced than in the case of Pt/SiO2 alone. The reason for the negligible effect of SO2 on MoO3 -containing catalysts is still not clear. Similar to that of NO oxidation (Fig. 3), the covering effects of MoO3 on Pt sites on the SiO2 surface may also decrease the SO2 oxidation activity. Another possible explanation is that MoO3 may adsorb SO3 at lower temperatures to hamper its diffusion onto the carbon surfaces. On the other hand, V2 O5 is used as the main active component in SO2 oxidation catalysts. The addition of V2 O5 to Pt/SiO2 may also cover some of the Pt sites but, at least, the catalytic SO2 oxidation may still proceed on V2 O5 . The effects of MoO3 and V2 O5 in promoting catalysis of carbon oxidation with NO2 are very similar to that shown by SO3 . They appear to act as the secondary catalysts to promote carbon oxidation mentioned in the introduction of this paper. Moreover, by combining MoO3 with Pt catalysts, it may be possible to decrease the emission of SO3 in to the atmosphere. Further study is necessary to clarify these speculations. 4. Conclusion SiO2 -supported Pt and V2 O5 or MoO3 were evaluated as catalysts for the oxidation of model soot with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation performances than the single component V2 O5 , MoO3 or Pt ones in the absence of SO2 in the reactant gas. V2 O5 and MoO3 showed higher activities than Pt in carbon oxidation by NO2 . The combination effect was explained by a synergistic catalysis: Pt catalyzes the oxidation of NO with gas phase O2 to NO2 , while V2 O5 and MoO3 catalyze the oxidation of carbon with NO2 . References
Fig. 7. TPR profiles of CO2 evolution from carbon oxidation on SiO2 -supported catalysts with (solid symbol) or without (open symbol) SO2 addition in the feed gas. Reactant gas: 1000 ppm NO + 0 or 100 ppm SO2 + 10% O2 + 7% H2 O in N2 .
[1] P. Zelenka, W. Cartellieri, P. Herzog, Appl. Catal. B 10 (1996) 3. [2] H.J. Stein, Appl. Catal. B 10 (1996) 69.
S. Liu et al. / Applied Catalysis B: Environmental 30 (2001) 259–265 [3] S.J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Appl. Catal. B 21 (1999) 35. [4] J.P.A. Neeft, Ph.D. Thesis, Delft University of Technology, The Netherlands, 1995. [5] S.J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Stud. Surf. Sci. Catal. 116 (1998) 667. [6] B.A.A.L. van Setten, R. van Dijk, S.J. Jelles, M. Makkee, J.A. Moulijn, Appl. Catal. B 21 (1999) 51. [7] G. Saracco, C. Badini, N. Russo, V. Specchia, Appl. Catal. B 21 (1999) 233. [8] G. Mul, Ph.D. Thesis, Delft University of Technology, The Netherlands, 1997. [9] G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, J.A. Moulijn, Appl. Catal. B 6 (1995) 339. [10] C. Badini, G. Saracco, V. Serra, V. Specchia, Appl. Catal. B 18 (1998) 137. [11] G. Saracco, V. Serra, C. Badini, V. Specchia, Ind. Eng. Chem. Res. 36 (1997) 2051.
265
[12] P. Ciambelli, M. D’Amore, V. Palma, S. Vaccaro, Combust. Flame 99 (1994) 413. [13] J.P.A. Neeft, W. Schipper, G. Mul, M. Makkee, J.A. Moulijn, Appl. Catal. B 11 (1997) 365. [14] B.J. Cooper, H.J. Jung, J.E. Thoss, US Patent 4,902,487 (1990) to Jonhson–Matthey. [15] P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson, P. Bronnenberg, SAE Technical Paper Series, 970182 (1997). [16] P. Ciambelli, V. Palma, P. Russo, S. Vaccaro, Appl. Catal. B 22 (1999) L5. [17] J-O. Uchisawa, A. Obuchi, Z. Zhao, S. Kushiyama, Appl. Catal. B 18 (1998) L183. [18] J-O. Uchisawa, A. Obuchi, A. Ogata, R. Enomoto, S. Kushiyama, Appl. Catal. B 21 (1998) 9. [19] J.P.A. Neeft, O.P. Pruissen, M. Makkee, J.A. Moulijn, Appl. Catal. B 12 (1997) 21.