Catalytic activity and reaction mechanism of autio2 for the reduction of nox by propene

Catalytic activity and reaction mechanism of Au/TiO2 for the reduction of NOx by propene TOKYO INSTITUTE OF TECHNOLOGY DEPARTMENT OF INTERNATIONAL DEVELOPMENT ENGINEERING Nguyen Quang Long Dissertation submitted in partial fulfillment of requirements for a doctoral degree of engineering Catalytic activity and reaction mechanism of Au/TiO2 for the reduction of NOx by propene By Nguyen Quang Long 06D51501 (Supervised by Professor Hirofumi Hinode) DEPARTMENT OF INTERNATIONAL DEVELOPMENT ENGINEERING GRADUATE SCHOOL OF SCIENCE AND ENGINEERING TOKYO INSTITUTE OF TECHNOLOGY 2009 Table of Content CHAPTER INTRODUCTION 1.1 Outline 1.2 Background 1.2.1 Automobile exhaust 1.2.2 Emission limitations 1.2.3 Lean-burn engine and effectiveness of three way catalysts 1.3 Treatment of NOx from lean-burn exhaust 1.3.1 NOx storage reduction 1.3.2 Selective catalytic reduction of NOx 1.4 Gold as a catalyst 1.4.1 A brief history of gold catalysis 1.4.2 Potential uses of gold as a catalyst 1.5 Objectives of the research and the structure of this thesis 10 References 12 CHAPTER REVIEW OF RELATED LITERATURE 13 2.1 Outline 13 2.2 Reduction of NOx by hydrocarbons 13 2.2.1 Zeolite-based as catalysts 13 2.2.2 Non-zeolitic catalysts 15 2.3 Mechanisms of the reduction of NOx by hydrocarbons 22 2.3.1 Adsorption-Dissociation mechanism 22 2.3.2 Oxidation-reduction mechanism 24 2.4 Gold catalysts for NOx reduction by hydrocarbons 25 References 28 CHAPTER CATALYTIC ACTIVITY OF Au/TiO2 30 3.1 Outline 30 3.2 Experimental 30 3.2.1 Preparation and characterization of the catalysts 30 i 3.2.2 Measurement of the catalytic activity 31 3.3 Results and discussion 32 3.3.1 Influence of preparation conditions 32 3.3.2 Influence of TiO2 crystalline type on NOx reduction 36 3.3.3 Influence of gold loading levels 38 3.3.4 Influence of the feed concentrations 40 3.4 Conclusions 43 References 44 CHAPTER EFFECT OF CeO2, Mn2O3 ON THE CATALYTIC ACTIVITY OF Au/TiO2 45 4.1 Outline 45 4.2 Experimental 45 4.2.1 Preparation and characterization of the catalysts 45 4.2.2 Measurements of the catalytic activity 46 4.2.3 Temperature program desorption experiments 46 4.3 Results and discussion 47 4.3.1 Effect of CeO2 addition 47 4.3.2 Effect of Mn2O3 addition 49 4.3.3 Catalytic activity for NO oxidation to NO2 50 4.3.4 Catalyst characteristics and NOx desorbed in TPD measurement 51 4.4 Conclusion 53 References 54 CHAPTER REACTION MECHANISM OVER Au/TiO2 55 5.1 Outline 55 5.2 Experimental 55 5.3 Results 57 5.3.1 Formation of adsobed species during co-adsorption of reactants 57 5.3.2 Formation of adsorbed species during SCR reaction 63 5.3.3 Consumption of adsorbed species 69 5.4 Discussion of reaction mechanism 73 ii 5.5 Conclusions 75 References 76 CHAPTER MECHANISTIC STUDY ON THE EFFECT OF CeO2 AND Mn2O3 77 6.1 Outline 77 6.2 DRIFTS results and discussion on CeO2-added Au/TiO2 catalyst 77 6.2.1 Co-adsorption of reactants on CeO2 77 6.2.2 Effect of CeO2 on the formation of adsorbed species 80 6.2.3 Effect of CeO2 on the consumption of adsorbed species 86 6.2.4 Conclusions of the influence of CeO2 on the reaction mechanism 88 6.3 DRIFTS results and discussion on Mn2O3-added Au/TiO2 catalyst 89 6.3.1 Co-adsorption of reactants on Mn2O3 89 6.3.2 Effect of Mn2O3 on the formation of adsorbed species 91 6.3.3 Effect of Mn2O3 on the consumption of adsorbed species 95 6.3.4 Conclusions of the influence of Mn2O3 on the reaction mechanism 97 6.4 Summary of the effect of CeO2 and Mn2O3 98 References 98 CHAPTER GENERAL CONCLUSIONS 99 ACKNOWLEDGEMENTS 101 LIST OF ORIGINAL PUBLICATIONS 102 iii List of Figures Figure 1-1 The emissions regulation for heavy-duty diesel vehicle [7] Figure 1-2 Fuel consumption and 3-way performance of a gasoline engine as a function of air-fuel (A/F) ratio [10] Figure 1-3 Possible mechanism of the NOx storage-reduction on NSR catalyst [10] Figure 1-4 The numbers of published articles concerning gold catalysts in recent years (Data from ISI) Figure 2-1 Effect of H2O on NO conversion to N2 and N2O on Pt/Na–ZSM-5 [7] 14 Figure 2-2 NO conversion and selectivity for Ga2O3/Al2O3 and Ga-ZSM-5 in the absence or presence of H2O [14] 16 Figure 2-3 Effects of the hydrothermal treatment on the activity of Cu-Al2O3 and CuZSM-5 [15] 16 Figure 2-4 Activities of different noble metal catalysts for the selective reduction of NOx [19] 18 Figure 2-5 Classification of the cooperation effect of catalytic species (a) Multiple-stage catalysts, (b) mechanical or physical mixture catalysts, and (c) multifunctional catalyst [21] 19 Figure 2-6 NO reduction conversion as a function of temperature mixtures of Ni–Ga oxide with different amounts of Mn2O3 [27] 20 Figure 2-7 Conversion of NO to N2 over Au/Al2O3, CeO2, mechanical mixture, and dual bed Au/Al2O3-CeO2 or CeO2-Au/Al2O3 [29] 21 Figure 2-8 Proposed mechanism for C3H6-SCR over Pt-containing catalysts [39] 23 Figure 2-9 Proposed mechanism of C3H6-SCR over Cu/Al2O3 [43] 24 Figure 2-10 Temperature dependence of NO conversion to N2 over Al2O3 and gold supported on a variety of metal oxides [47] 26 Figure 3-1 Schematic diagram of the catalytic activity test set-up 32 Figure 3-2 XRD patterns of rutile support Ti6 and Au(1wt.%)/Ti6 with various mass ratios PVA/Au 33 Figure 3-3 XRD patterns of Au/TiO2 with different TiO2 types and Au loading levels 34 iv Figure 3-4 TEM images of Au/TiO2 : Au(1wt.%)/Ti4 (a), Au(1wt.%)/Ti6 (b), Au(1wt.%)/Ti7 (c) 35 Figure 3-5 Size distribution of Au particles on different titania supports 35 Figure 3-6 The effect of titania type on the HC-SCR activity of Au/TiO2 37 Figure 3-7 The effect of Au content on the HC-SCR activity as a function of temperature 38 Figure 3-8 The effect of concentration of NOx and C3H6 on the activity of Au(0.1wt.%)/Ti7 41 Figure 3-9 The effect of H2O on the NOx reduction activity of Au/TiO2 catalysts: without H2O or with 5% H2O 42 Figure 3-10 The stability of 0.1%Au/Ti7 catalysts after dry-test and wet-test 43 Figure 4-1 Catalytic performance of the mechanical mixtures of Au/TiO2 and CeO2: (a) NO conversion to N2 and (b) C3H6 conversion to CO2 48 Figure 4-2 Catalytic performance of the mechanical mixtures of Au/TiO2 and Mn2O3: (a) NO conversion to N2 and (b) C3H6 conversion to CO2 49 Figure 4-3 Activity of the catalysts for the oxidation of NO to NO2 50 Figure 4-4 XRD patterns of singles and mixtures of 1%Au/TiO2 and MOx (M=Ce, Mn) 51 Figure 4-5 The TPD profiles of the NOx desorption from NO and O2 pre-adsorbed samples 52 Figure 5-1 Schematic diagram of the DRIFTS measurement 55 Figure 5-2 Experimental procedure of DRIFTS measurement 56 Figure 5-3 DRIFTS spectra of adsorbed species over Au/TiO2 after exposing in different gas flows for 40 at 150 oC 57 Figure 5-4 DRIFTS spectra of adsorbed species over Au/TiO2 after exposing in flow of NO/O2/He for 40 at different temperatures 59 Figure 5-5 Comparison spectra of surface adsorbed species between Au/TiO2 and TiO2 after exposing to NO/O2/He for 40 at 200 oC and 300 oC 60 Figure 5-6 DRIFTS spectra of adsorbed species over Au/TiO2 after exposing in flow of C3H6/O2/He for 40 at different temperatures 61 v Figure 5-7 DRIFTS spectra of adsorbed species over TiO2 (a) and Au/TiO2 (b) after exposing in C3H6/O2/He and Au/TiO2 after exposing inC3H6/He (c) for 40 at 200 oC 63 Figure 5-8 DRIFTS spectra of adsorbed species after 40 in SCR reaction at different temperatures 64 Figure 5-9 DRIFTS spectra of adsorbed species after 40 in reaction condition over TiO2 and Au/TiO2 66 Figure 5-10 DRIFTS spectra of adsorbed species during the SCR reaction at 200oC as a function of time 67 Figure 5-11 DRIFTS spectra of adsorbed species during the SCR reaction at 300 oC over TiO2, 0.1%Au/TiO2, and 1%Au/TiO2 for 5’ (a), 20’ (b), and 40’ (c) 68 Figure 5-12 DRIFTS spectra recorded over Au/TiO2 after flowing of C3H6/O2/He for 40 followed by purging He for 20 (a), then flowing of NO/O2/He 69 Figure 5-13 DRIFTS spectra recorded over Au/TiO2 after flowing of C3H6/O2/He for 40 followed by purging He for 20 (a), then flowing of NO/O2/He 70 Figure 5-14 DRIFTS spectra recorded over Au/TiO2 at 300 oC after flowing of C3H6/O2/He for 40 followed by purging He for 20 then flowing of He (a), O2/He (b), and NO2/He (c) 71 Figure 5-15 DRIFTS spectra recorded over Au/TiO2 after flowing of NO/O2/He for 40’ followed by purging He for 20’, then flowing of C3H6/O2/He 72 Figure 5-16 Schematic diagram of reaction mechanism over Au/TiO2 catalyst 75 Figure 6-1 DRIFTS spectra of adsorbed species over CeO2 after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures 78 Figure 6-2 DRIFTS spectra of adsorbed species over Au/Ti-Ce after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures 80 Figure 6-3 Comparison of DRIFTS spectra of adsorbed species over different samples after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at 200 0C 80 Figure 6-4 DRIFTS spectra of adsorbed species over Au-Ti-Ce after exposure to reaction condition NO/C3H6/O2 for 40 mins at various temperatures (a) and at 200 oC in various reaction times (b) 82 vi Figure 6-5 Comparison of DRIFTS spectra of adsorbed species over different samples after exposure to reaction condition NO/C3H6/O2 for 40 mins at 200 0C 83 Figure 6-6 Comparison of (-NCO) peak’s area on Au/TiO2 and Au/Ti-Ce at 250oC as a function of reaction time Reaction condition: NO: 1500 ppm, C3H6: 1500ppm, O2: 10% in He 84 Figure 6-7 Change of (-NCO) peak’s area on Au/TiO2 under streams of NO+O2or NO2 The sample was pre-exposed to the reaction mixture NO/C3H6/O2for 40 minutes followed by purging He for 20 minutes 85 Figure 6-8 DRIFTS spectra in (C-H) stretching region of adsorbed species over (a) Au/TiO2, (b) CeO2 and (c)Au/Ti-Ce after exposing in C3H6/O2 for 40 mins followed by purging He for 20 min, and then under flowing of NO/O2 for mins or 40 mins at 250 oC 87 Figure 6-9 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) CeO2 and (c)Au/Ti-Ce after exposing in NO/O2 for 40 mins followed by purging He for 20 min, and then under flowing of C3H6/O2 for mins or 40 mins at 250 oC 88 Figure 6-10 DRIFTS spectra of adsorbed species over Mn2O3 after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures 90 Figure 6-11 DRIFTS spectra of adsorbed species over Au/Ti-Mn after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures 91 Figure 6-12 Comparison of DRIFTS spectra of adsorbed species over different samples after exposing in (a) NO/O2 and (b) C3H6/O2 for 40 at 200 0C 92 Figure 6-13 DRIFTS spectra of adsorbed species over Au-Ti-Mn after exposure to reaction condition NO/C3H6/O2 for 40 mins at various temperatures (a) and at 200 C for various reaction times (b) 93 Figure 6-14 Comparison of DRIFTS spectra of adsorbed species over over different samples after exposure to reaction condition NO/C3H6/O2 for 40 mins at 200 0C 94 Figure 6-15 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in C3H6/O2 for 40 mins followed by purging He for 20 min, and then under flowing of NO/O2 for mins or 40 mins at 200 oC 95 vii Figure 6-16 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in NO/O2 for 40 mins followed by purging He for 20 min, and then under flowing of C3H6/O2 for mins or 40 mins at 200 oC 96 viii Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 6.3 DRIFTS results and discussion on Mn2O3-added Au/TiO2 catalyst The samples used in this part are Mn2O3, and mechanical mixture of Mn2O3 and Au/TiO2 at mass ratio 1:1 with wt.% Au loading on the anatase TiO2 (thereafter denoted Au/Ti-Mn) The preparations of these samples have been previously reported in Chapter and The DRIFTS procedure has been described in Chapter The amount of Au/Ti-Mn sample to be used in DRIFTS was approximately 25 mg, and about 45 mg of Mn2O3 was used in order to fill up the sample cell 6.3.1 Co-adsorption of reactants on Mn2O3 The spectra recorded during co-adsorption NO/O2 and C3H6/O2 over Mn2O3 for 40 minutes at various temperatures are shown in Fig 6-10 The bidentate species (1547, 1237, and 1010 cm-1) are the main nitrate type obtained in the co-adsorption NO/O2 as seen from Fig 6-10a When the adsorption temperature increased, the nitrate bands were slightly reduced.On the other hand, the exposure to C3H6/O2 led to the formation of formate (1565, 1371 and 1347 cm-1) and acetate (1428 cm-1) (Fig 6-10 b) In addition, acetate band was strong at low temperatures (150, 200 oC), although its intensity was decreased at increasing temperatures The formate band at 1565 cm-1 was also declined at high temperatures Furthermore, when purging He after 40 minutes co-adsorption, the intensities of the above mentioned bands were significantly reduced For both cases (co-adsorption NO/O2 and C3H6/O2), at 300 oC, the all bands were disappeared after purging He for 10 minutes At 250 oC, the band intensities were reduced more than half Thus, it can be concluded that the adsorbed species on Mn2O3 were easy to be desorbed This is an important feature in a catalytic reaction because the active sites are easily liberated 89 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 0.05 (b) 0.05 (a) 1547 1565 1237 o 300 C 1010 300 C Absorbance (a.u.) Absorbance (a.u.) 1428 o o 250 C 250 oC 200 oC 200 oC 150 oC 150 oC 2300 2100 1900 1371 1347 1700 1500 1300 -1 Wavenumber (cm ) 1100 900 2300 2100 1900 1700 1500 1300 -1 Wavenumber (cm ) 1100 900 Figure 6-10 DRIFTS spectra of adsorbed species over Mn2O3 after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures Table 6-2 Band assignment for adsorbed species on Mn2O3 Wavenumber /cm-1 (This work) 1547 1237 1010 2964,2950 2871 1563 1381 1347 1428 Surface species Interpretation Bridging NO3(M-O)2= NO Wavenumber /cm-1 (Literature) νs(ONO) 1600-1650 Bidentate NO3(M-O2NO) νs(ONO) νas(ONO) ν(N=O) 1500-1565 1260-1300 1010-1040 νas(COO)+δ(CH) νs(CH) νas(COO) δ(CH) νs(COO) νas(COO) νs(COO) 2930-2970 2890 1630-1550 1380 1350-1250 1540 1430 H-COO- CH3-COO- 90 References 11,12 13 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 6.3.2 Effect of Mn2O3 on the formation of adsorbed species 6.3.2.1 Co-adsorption conditions Figure 6-11 shows the spectra collected over Mn2O3-added Au/TiO2 catalyst after 40 minutes exposure in the stream of NO/O2/He (a) or C3H6/O2/He (b) The three types of nitrate species (monodentate, bidentate, and bridging) were observed in the Fig 6-11a Monodentate (1520 and 1285 cm-1) was detected at 150 oC, but disappeared at higher temperatures Bridging nitrate, however, was presented in a significant amount because the band at 1610 cm-1 was strong even at 350 oC When the temperature was increased, the nitrate bands were reduced as similar to that over Au/TiO2 During co-adsorption C3H6/O2, formate and acetate compounds were obviously observed on the surface of Au/Ti-Mn Acetate band (1427 cm-1) was slightly increased as elevating temperatures, but formate bands (1381, 1365 cm-1) were reduced at high temperatures (250-350 oC) (a) 0.1 0.1 (b) 1560 1610 1645 1381 1427 1365 350 oC Absorbance (a.u.) Absorbance (a.u.) 1584 1550 1285 1240 1520 150 oC 300 oC 250 oC 200 oC 200 oC o 250 C 300 oC 350 C 2300 2100 1675 150 oC o 1900 1700 1500 1300 -1 Wavenumber (cm ) 1100 2300 900 2100 1900 1700 1500 -1 Wavenumber (cm ) 1300 1100 900 Figure 6-11 DRIFTS spectra of adsorbed species over Au/Ti-Mn after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 at various temperatures 91 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 1610 (a) 0.1 1585 1550 1240 1565 1285 1645 1675 Absorbance (a.u.) Au/Ti-Mn Absorbance (a.u.) (b) 0.1 Au/TiO2 TiO2 Au/Ti-Mn Au/TiO2 1550 1565 1371 1347 Mn2O3 1547 1380 1430 1365 1237 Mn2O3 TiO2 2300 2100 1900 1700 1500 -1 Wavenumber (cm ) 1300 1100 900 2300 2100 1900 1700 1500 -1 Wavenumber (cm ) 1300 1100 900 Figure 6-12 Comparison of DRIFTS spectra of adsorbed species over different samples after exposing in (a) NO/O2 and (b) C3H6/O2 for 40 at 200 0C The comparative results at 200oC of Mn2O3, TiO2, Au/TiO2, and Au/Ti-Mn are shown in Fig 6-12 The surface areas of the four samples were 5, 115, 102, and 50 m2/g, respectively Thus, although the amount of Mn2O3 used in DRIFTS experiments was about 2- 2.25 times higher than the other samples, the available surface area of Mn2O3 was about 10 times smaller than that of Au/TiO2 or TiO2 and about times smaller than that of Au/Ti-Mn It is seen that the nitrate band (1547 cm-1) on Mn2O3 was much weaker than those over the other samples Morever, the Figure 6-12a also reveals that the bridging nitrate species, which are produced by chemical bonds of nitrate with two active sites on the solid surface, were predominant on the surface of Au/Ti-Mn The strong bands of this nitrate type at 1610 and 1240 cm-1 proved this statement Results obtained at different adsorption time over Au/Ti-Mn indicated that the bands of bridging nitrate were observed along with bands of bidentate nitrate after minute, and kept increasing until the end of the measurement Moreover, only bidentate type was observed on Mn2O3 surface leading 92 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 the fact that bridging nitrate species located on TiO2 surface Hence, the presence of Mn2O3 contributed to the change of type of nitrate species adsorbed on TiO2 surface Additionally, as reflected from Fig 6-12b, formate bands were observed strongly over Mn2O3 and Au/Ti-Mn Acetone, which was observed obviously at 1675 cm-1 over Au/TiO2 and TiO2, was not detected over Mn2O3 and Au/Ti-Mn The intensities of formate bands are comparably strong over Mn2O3 despite its low surface area Comparing the spectra of Au/TiO2 and Au/Ti-Mn and considering the surface area of these samples (102 m2/g for Au/TiO2 and 50 m2/g for Au/Ti-Mn), it is noted that formate species were concentratively presented on Au/Ti-Mn, possibly on Mn2O3 surface at the respective temperature 6.3.2.2 Reaction condition 1557 (a) 0.1 1645 2180 1718 0.1 1381 1430 1365 o 250 C 1585 20’ 1285 1245 200 C 1381 1430 1365 40’ Absorbance (a.u.) Absorbance (a.u.) 300 C o 1645 1718 2180 o (b) 1557 10’ 1658 1605 5’ 150 oC 2300 2100 1900 1700 1500 -1 Wavenumber (cm ) 1300 1100 900 2300 2100 1900 1700 1500 1300 -1 Wavenumber (cm ) 1100 900 Figure 6-13 DRIFTS spectra of adsorbed species over Au-Ti-Mn after exposure to reaction condition NO/C3H6/O2 for 40 mins at various temperatures (a) and at 200 0C for various reaction times (b) The adsorbed compounds on the catalyst surface collected after 40 minute exposure in reaction stream at various temperatures are reported in Fig 6-13a The existence of nitrate was detected only at 150 and 200 oC with bands at 1605, 1285 and 1245 cm-1 Acetaldehyde was observed on the catalyst surface up to 250 oC Acetate and formate 93 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 species were also presented on the sample at 1430 cm-1 and 2381, 1365 cm-1 respectively Isocyanate intermediate compounds were evidenced with a band at 2180 cm-1 This band became weaker at high temperatures Figure 6.13b shows the change in the adsorbed species against the reaction time at 200 oC When the reaction time was increased, the intensities of bands ascribed to isocyanate, acetaldehyde, acetate were gradually increased On the other hand, while formate bands was kept increasing until the end of experimental time over Au/TiO2 (Fig 5-8), these bands seem to reach maximum level after about 10 minutes over Au/Ti-Mn It is suggested that the consumption of these species started faster and sooner over Au/TiMn 0.025 2878 2956 (e) 1580 1553 0.1 3685 1645 2180 (c) (b) 1718 1245 Au-Ti-Mn (e) Absorbance (a.u.) Absorbance (a.u.) (d) 1381 1430 1365 Au-Ti (d) Ti-Mn (c) 1563 2871 2950 2964 (a) 3800 3600 3400 TiO2 (b) Mn2O3 (a) 3200 3000 2800 2300 2100 1900 -1 Wavenumber (cm ) 1347 1700 1500 -1 Wavenumber (cm ) 1300 1100 900 Figure 6-14 Comparison of DRIFTS spectra of adsorbed species over over different samples after exposure to reaction condition NO/C3H6/O2 for 40 mins at 200 0C Figure 6-14 shows the spectra obtained over different catalysts at 200 oC for 40 minute reaction It is noted that formate (1563, 1347 cm-1) and acetate (1430 cm-1) species are predominant on the surface of Mn2O3 The bands of nitrate species were not 94 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 detected over this sample in the reaction condition Formation of acetaldehyde was occurred mainly on the surface of TiO2 because the band at 1718 cm-1 was only strongly seen over TiO2 and Au/TiO2 Importantly, the isocyanate band (2180 cm-1) was observed only on Au-containing samples, i.e Au/TiO2 and Au/Ti-Mn This phenomenon again indicated the crucible role of Au for the creation of the isocyanate intermediate 6.3.3 Effect of Mn2O3 on the consumption of adsorbed species In order to understand the consumption of adsorbed oxygenated hydrocarbons, the samples were firstly exposed to a stream of C3H6/O2/He for 40 minutes followed by He purge for 20 minutes, and finally exposed to a stream of NO/O2/He The report in Fig 6-15 shows the spectra obtained at 200 oC after 40 minutes exposure in C3H6/O2/He, after He purge, after NO/O2/He flowing in minutes and 40 minutes (a) Au/TiO2 (1) C3H6/O2- 40’ (2) He – 20’ (3) NO/O2 – 5’ (4) NO/O2 – 40’ 2935 2954 (b) Mn2O3 (c) Au/Ti-Mn 2871 (1) 2875 2954 2875 0.01 0.01 0.01 2950 (1) (2) (2) (1) (3) (3) (2) (3) (4) (4) (4) 3000 2900 Wavenumber [cm-1] 2800 3000 2900 Wavenumber [cm-1] 2800 3000 2900 Wavenumber [cm-1] 2800 Figure 6-15 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in C3H6/O2 for 40 mins followed by purging He for 20 min, and then under flowing of NO/O2 for mins or 40 mins at 200 oC Over Au/TiO2 the reduction of acetone and formation of acetaldehyde were obviously recognized in the region 1800-1200 cm-1 (Fig 5-12 in chapter 5) In the (C-H) stretching region (Fig 6-15), the decrease of acetone band at 2935 cm-1 was clearly 95 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 observed On the other hand, the reductions of formate bands (at 2954 and 2975 cm-1) were observed over Au/Ti-Mn Therefore, the presence of Mn2O3 led to the increase in the consumption of adsorbed formate species (a) Au/TiO2 (c) Au/Ti-Mn (b) Mn2O3 0.1 1605 1585 (1) (2) (3) (4) 0.1 1675 NO/O2- 40’ He- 20’ C3H6/O2- 5’ C3H6/O2- 40’ 0.1 1585 1365 1380 1240 1245 (4) 1550 (4) 1565 (3) (3) 14281347 1237 (4) (2) (3) (2) (2) (1) 1700 1500 1300 1100 900 Wavenumber (cm-1) (1) 1547 1700 1500 1300 1100 900 Wavenumber (cm-1) 1610 1700 (1) 1500 1300 1100 900 Wavenumber (cm-1) Figure 6-16 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in NO/O2 for 40 mins followed by purging He for 20 min, and then under flowing of C3H6/O2 for mins or 40 mins at 200 oC The Figure 6-16 shows results recorded in the reverse experimental procedure in which the samples were firstly exposed to NO/O2/He for 40 minutes followed by He purge for 20 minutes and finally exposed to C3H6/O2/He At 200oC, only the band of acetone at 1675 cm-1 was detected as the only new adsorbed compound over NO/O2 pre-adsorbed Au/TiO2 Moreover, on this sample the decrease of nitrate bands was not significant Therefore the adsorbed nitrate species were extremely stable on the surface of Au/TiO2 at this temperature (200oC) Whereas, significant amount of nitrate species (bridging type) were released from the surface of Au/Ti-Mn as clearly seen at band 1240 cm-1 It is noteworthy that nitrate species are located mostly on TiO2 surface Thus, the presence of Mn2O3 made the adsorbed nitrate 96 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 species easier to be activated at low temperatures although they presented on the surface as the most stable type (bridging nitrate) Moreover, the formate bands (1365, 1380 cm1) were strongly appeared as be seen in Fig 6-16b As discussed in chapter 5, the oxygenated hydrocarbons, especially acetate and formate, can be formed over nano Au particles, or active sites of TiO2 Therefore, over Au/TiO2 the strong bond of nitrate species with TiO2 surface at low temperatures made difficult for C3H6 and O2 to access the necessary sites With the addition of Mn2O3, however, nitrate on the surface of Mn2O3 were quite easy to be released and replaced by adsorbed formate and acetate species Moreover, nitrate species were located mostly on TiO2 surface The intimate contact of Au/TiO2 and Mn2O3 by mechanical mixing led to the interaction of the nitrate and formate and also acetate As a result, the SCR reaction can be preceded at low temperatures Unlike the addition of CeO2, the SCR reduction activity of mechanical mixture TiO2 and Mn2O3 (without Au contained) was relatively high (Fig.4-2) It may be explained by the ability of Mn2O3 in the catalysis for some reaction steps such as formation of undetected isocyanate or cyanide and then converted to N2 However, the presence of Au promoted significantly the SCR activity of this mixture as seen in Fig 4-2 It is possibly due to the role of Au in the formation of isocyanate intermediate compounds 6.3.4 Conclusions of the influence of Mn2O3 on the reaction mechanism The presence of additional Mn2O3 contributed to the formation of additional formate and acetate particularly at low temperatures It also made the bridging nitrate became the predominant adsorbed nitrate species on the catalyst surface The significant increase of the NO reduction activity at low temperatures of Au/Ti-Mn may be due to the interaction of the additional formate and acetate species, which were probably located on the surface of Mn2O3, with the adsorbed nitrate species, which were mainly placed on the surface TiO2 Moreover, the formation of other intermediate such as isocyanate or/and cyanide which then convert to N2 may be catalyzed by Mn2O3 instead of only by nano Au particles over the Au/TiO2 catalyst 97 Chapter6: Mechanistic study of the effect of CeO2 and Mn2O3 6.4 Summary of the effect of CeO2 and Mn2O3 The additional CeO2 and Mn2O3 influenced the catalytic activity of Au/TiO2 for C3H6-SCR as presented in Chapter The MOx-added samples performed better activity at low temperatures that that of Au/TiO2 The additional CeO2 contributed to the formation of adsorbed nitrate, nitrite and oxygenated hydrocarbons, especially formate CeO2 promoted the formation of NO2 which then moved and adsorbed on TiO2 surface The conversion of (-NCO) intermediates was also promoted by CeO2, possibly due to the formation of NO2 On the other hand, additional Mn2O3 made the bridging nitrate became the predominant adsorbed nitrate species on the catalyst surface It contributed to the formation of additional formate and promotion of nitrate reactivity at low temperatures Unlike CeO2, Mn2O3 might catalyze the reaction between the additional formate species (probably on Mn2O3), with the adsorbed nitrate species (probably on the nearby TiO2) As a result, the catalytic activity was promoted over Mn2O3-added catalyst References [1] Martinez-Arias, J Chem Soc., Faraday Trans, 91 (1995) 1679-1687 [2] K.I Hadjiivanov, Catal Rev Sci Eng 41 (2000) 71-144 [3] M Haneda, K Shinoda, A Nagane, O Houshito, H Takagi, Y Nakahara, K Hiroe, T Fujitani, H Hamada , J Catal 259 (2008) 223-231 [4] C Li, K Domen, K Maruya, T Onishi, J Catal 125 (1990) 445-455 [5] O Pozdnyakova, D Teschner, A Wootsch, J Kröhnert, B Steinhauer, H Sauer, L Toth, F.C Jentoft, A Knop-Gericke, Z Pấl and R.Schlưgl, J.Catal 237 (2006), 1-16 [6] A Yee, S J Morrison, and H Idriss, J Catal 186 (1999) 279-295 [7] J Rasko, J Kiss, Appl Catal A 287 (2005) 252-260 [8] M Kantcheva, O Samarskaya , L Ilieva , G Pantaleo, A.M Venezia, D Andreeva, Appl Catal B 88 (2009) 113-126 [9] Tamás Bánsági, Tímea Süli Zakar and Frigyes Solymosi, Appl Catal B 66 (2006) 147-150 [10] S Naito, T Kasahara, T Miyao, Catal Today, 74 (2002) 201-206 [11] G Busca, J Lamotte, J Lavalley, and V Lorenzelli, J Am Chem 109 (1987) 51975202 [12] M Kantcheva, M.U Kucukkal, S Suzer, J Catal., 190 (2000) 144-156 [13] G Buscaa, E Finocchioa, V Lorenzellia, G Ramisa, M Baldi, Catal Today 49 (1999) 453-465 98 Chapter 7: General conclusions CHAPTER GENERAL CONCLUSIONS This chapter describes the summary of all achievements in this research The research background, methods of NOx treatment and Au catalyst were introduced in Chapter while a survey of related literature was reported in Chapter The general objective of this research was to investigate the catalytic activity and reaction mechanism of Au/TiO2 catalyst for the selective catalytic reduction of NOx by propene (C3H6-SCR) Specifically, the following were the objectives of the present research: To prepare and test the catalytic activity of nano-sized Au/TiO2 for SCR of NOx by propene To investigate the effect of additional CeO2 and Mn2O3 to Au/TiO2 on the catalytic activity To investigate the reaction mechanism of the SCR by propene over Au/TiO2 To understand the mechanistic effect of additional CeO2 and Mn2O3 to the catalytic activity of Au/TiO2 The first objective was achieved in Chapter which was presented and discussed the preparation and catalytic activity of Au/TiO2 catalyst for C3H6-SCR The catalysts were successfully prepared by a metal sol method The anatase type was better than the rutile type in terms of NOx reduction to N2 and C3H6 oxidation The temperature at maximum NOx conversion shifted to lower temperature compared to Au/Al2O3 When the gold loading of the catalyst was lowered to 0.1 wt.%, the catalyst retained good catalytic performance and the operation temperature range was widened, but the peak temperature slightly shifted to higher temperature Only slight decreases in the activity of used catalysts were observed even after moisture-containing test The investigation of the effects of additional CeO2 and Mn2O3 on catalytic activity of Au/TiO2 for C3H6-SCR was comprehensively reported in Chapter by which the second objective was obtained The enhancement of the catalytic activity of Au/TiO2 for C3H6- 99 Chapter 7: General conclusions SCR was observed when it is mixed mechanically with CeO2 or Mn2O3 Addition of these metal oxides shifts the peak temperature of the catalytic activity curves to lower temperature than that of Au/TiO2 The reaction mechanism of C3H6-SCR over Au/TiO2 catalyst was studied and described in detail in Chapter in order to achieve the third objective The formation of oxygenated hydrocarbons (acetate, formate, acetone, and acetaldehyde) and nitrate (bridging, bidentate, and monodetate) species during the SCR of NO by C3H6 was confirmed by DRIFTS results The amount and types of nitrate were observed on the catalyst surface depending on the temperature Over Au/TiO2 catalyst, the interaction of oxygenates and nitrate or/and NO2 produced nitrogen-containing compounds such as detectable cyanide and isocyanate compounds which then possibly converted to N2 and other products The isocyanate compounds are the predominant intermediates observed by DRIFTS The presence of Au particles was necessary to form oxygenated hydrocarbons, especially acetate species, and crucial to the production of isocyanate intermediate compounds They may also contribute to the conversion of these compounds to N2 Chapter explained the mechanistic effects of additional CeO2 and Mn2O3 on the catalytic activity of Au/TiO2 based on DRIFTS results The significant increase of amount adsorbed nitrate, nitro and adsorbed oxygenates on surface of Au/Ti-Ce improved the catalytic activity of CeO2-added sample On the other hand, the promotive effect on the formation of formate, reactivity of nitrate at low temperatures, and the catalytic ability of Mn2O3 towards C3H6-SCR led to the better performance of Mn2O3-added catalyst at low temperatures The conclusion of this chapter is corresponding to the fourth objective 100 Acknowledgements ACKNOWLEDGEMENTS First of all, I would like to express my deepest gratitude to my thesis supervisor, Professor Hirofumi Hinode who always gave me a lot of important guidance and advice during the hard time of my experimental work Thank you for patiently reviewing my work and giving me many valuable comments Secondly, I want to give my sincerest gratitude to the members of my thesis examination panel: Professor Sachio Hirose, Professor Toshihide Baba, Professor Takayuki Komatsu, and Professor Naoya Abe Thank you for providing me many important comments and additional inputs during my thesis presentations Thirdly, I would like to thank Japan International Cooperation Agency (JICA) for financial support Network/Southeast for my Asia doctoral program under Engineering Education the ASEAN University Development Network (AUN/SEED-Net) project And my sincere thanks to: - My home university, Ho Chi Minh City University of Technology (HCMUT), most especially to Mr Hoang Minh Nam, former Dean, and Dr Pham Thanh Quan, Dean of the Faculty of Chemical Engineering Thank you for providing me the opportunity to pursue this graduate study Furthermore, I acknowledge Professor Tran Khac Chuong for his encouragement and advice in catalyst preparation - Dr Chris Salim, Assistance Professor of Hinode laboratory Thank you for your useful help and advices not only in my research work but also in daily life - All member of Hinode laboratory for their help and supports: Carl-san, Awansan, Makii-san, Ishida-san, Wang-san, Chandra-san, etc - My parents, for all your love and your understanding Thank you for constantly encouraging me And the last but not the least, my dearest wife, Tran Thi Minh Tho Thank you for ceaselessly encouraging and supporting me 101 List of original publications LIST OF ORIGINAL PUBLICATIONS I International Journal Papers Long Q Nguyen, Chris Salim, Hirofumi Hinode, “Performance of nano-sized Au/TiO2 for selective catalytic reduction of NOx by propene”, Applied Catalysis A: General, 347, 1, pp 94-99, September, 2008 Long Q Nguyen, Chris Salim, Hirofumi Hinode, “Promotive effect of MOx (M=Ce, Mn) mechanically mixed with Au/TiO2 on the catalytic activity for nitrogen monoxide reduction by propene”, Topics in Catalysis, 52, pp 779-783, June, 2009 II International Conferences Long Q Nguyen, Chris Salim, Hirofumi Hinode, “Activity of Mechanical Mixtures of Au/TiO2 and MOx (M=Ce, Mn) for NOx Reduction by Propene”, The International Symposium on Creation and Control of Advanced Selective Catalysis, Kyoto, Japan, July, 2008 Long Q Nguyen, Chris Salim, Hirofumi Hinode, “Ceria-added nano-sized gold on titania as a promising catalyst for reduction of NOx from automobile’s lean-burn exhaust”, Asia-Oceania Top University League on Engineering (AOTULE) 2008 Postgraduate Conference, Auckland, Newzealand, November, 2008 Long Q Nguyen, Chris Salim, Hirofumi Hinode, ”Selective catalytic reduction of nitrogen monoxide by propene using titania supported gold catalyst”, Regional Conference in Chemical Engineering (RCCE), Manila, Philippines, January, 2009 III Domestic conference Long Q Nguyen, Chris Salim, Hirofumi Hinode, “Treatment of NOx pollutant from automotive exhaust gas: Selective catalytic reduction by nano-size gold catalyst”, 102 List of original publications 9th Spring Conference of Japan Society of International Development (JASID), Tokyo, Japan, June, 2008 IV Others Long Q Nguyen, Leonila C Abella, Susan M Gallardo, and Hirofumi Hinode, “Effect of Nickel Loading on the Activity of Ni/ZrO2 for Methane Steam Reforming at Low Temperature”, Reaction Kinetics and Catalysis Letters, 93, 2, pp 227-232, April, 2008 Long Q Nguyen, Leonila C Abella, Susan M Gallardo, and Hirofumi Hinode, “Effect of catalyst loading on Ni/ZrO2 activity for Methane Steam Reforming at low temperature”, Regional Symposium on Chemical Engineering (RSCE), Singapore, December, 2006 Long Q Nguyen, Chris Salim, Hirofumi Hinode, Application of nano-sized gold catalyst for the treatment of NOx pollutant, JICA International symposium: “Strategic Partnership among Higher Education Institutions in ASIAN and Japan”, Tokyo, Japan, October 2008 103 ... fulfillment of requirements for a doctoral degree of engineering Catalytic activity and reaction mechanism of Au/TiO2 for the reduction of NOx by propene By Nguyen Quang Long 06D51501 (Supervised by Professor... on the catalytic activity To investigate the reaction mechanism of the SCR by propene over Au/TiO2 To understand the mechanistic effect of the additional CeO2 and Mn2O3 This thesis consists of. .. objectives of the present research are as follows: To prepare and test the catalytic activity of nano-sized Au/TiO2 for SCR of NOx by propene To investigate the effect of mechanical additions of CeO2 and