Luận án Tiến sĩ: Quang xúc tác loại bỏ NOx dùng ống nano titania - Nguyễn N. Huy

National Chiao Tung University Institute of Environmental Engineering Doctoral Dissertation Photocatalytic Removal of NOx using Titania Nanotubes Student: Nguyen Nhat Huy Advisor: Hsunling Bai August 2015 Photocatalytic Removal of NOx using Titania Nanotubes Student: Nguyen Nhat Huy Advisor: Hsunling Bai A Dissertation Submitted to Institute of Environmental Engineering College of Engineering National Chiao Tung University in partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Environmental Engineering August 2015 Hsinchu, Taiwan, Republic of China Photocatalytic Removal of NOx using Titania Nanotubes Student: Nguyen Nhat Huy Advisor: Hsunling Bai Institute of Environmental Engineering National Chiao Tung University Abstract In this study, titania nanotubes (TNTs) were synthesized by hydrothermal method using commercial Degussa TiO2 (P25) as a precursor. The properties of TNTs were modified by controlling of different synthesis parameters, including calcination temperature, washing pH value, and doping with various metals. The materials were then characterized by BET, SEM, TEM, XRD, TPD, ICP, XPS, UV-Vis absorption, and FTIR analyses. The photocatalytic reactions of NO and NO2 under UVA irradiation were performed to evaluate the activity of the TNTs materials.

For the effect of calcination temperature on TNTs properties and activity, results showed that the highest total NOx removal efficiency was achieved by TNTs calcined at 500℃ (T-500). The high activity of T-500 in both NO and NO2 photocatalytic reactions could be attributed to its high anatase crystallinity and high surface area. These two factors affect primarily on the conversion of NO2 to nitrate, which was the rate-limiting step for photocatalytic removal of NOx. The high anatase crystallinity could be responsible for the high efficiency at the beginning, while the high surface area could be accounted for retaining this high efficiency from nitric acid poisoning during the test period.

This study also reports for the first time on the role of contaminant sodium to neutralize the acidic reaction products during the NOx photocatalytic reaction and prevent surface deactivation of TNTs washed at different pH values. In the photocatalytic removals of NO and NO2, it revealed that both of the Na content and the structure of TNTs materials play important roles on the NOx removal pathway, initial efficiency and decay rate. The highest efficiencies were achieved by TNTs washed at pH 3 to 5, which may be due to their high amount of crystalline anatase for photocatalytic reaction and high sodium content for neutralization of acidic products. The mechanism based on the neutralization of the HNO3 resultant from the NOx photo-oxidation was also proposed.

-i- In the inter-effect of NO and NO2 during the photocatalytic oxidation process, separated and simultaneous reactions of NO and NO2 were performed and compared using different NO/NOx ratios of 0, 25, 50, 75, and 100%. With the increase of NO/NOx ratio, the conversion of NO was stable with high efficiencies. On the other hand, NO2 conversion efficiency declined significantly while NOx removal efficiency decreased slightly. The reactions of NO and NO2 was assumed to be independent and the separated reaction results were then employed for calculation of the simultaneous reactions results.

When the actual results of simultaneous reactions were compared with their expected results, the presence of NO2 did not show any effect on the NO conversion. In contrast, the presence of NO considerably inhibited the conversion of NO2 and therefore the NOx removal. In the photocatalytic reduction tests using Mo-doped TNTs, Mo could be successfully doped by precipitation and impregnation methods, but not by the hydrothermal method. The NO2 photocatalytic reaction results showed that the doping of Mo sharply declined the oxidation ability of TNTs while enhanced its reduction ability.

Moreover, Mo-doped TNTs prepared by the precipitation method provided the highest reduction ability, which may be due to its chemical oxidation states of Mo4+ and Mo5+. Additionally, washing pH has a strong effect on the properties and activity of the Mo-doped TNTs materials. Mo-doped TNTs washed at pH 3 had the highest total NOx removal efficiency while the one washed at pH 1 provided the highest ability for NO2 reduction. It is likely that the effect of washing pH on the activity and the selectivity of Mo-doped TNTs is via the Mo and Na contents of the materials.

The high Mo but low Na content would promote the reduction reaction. On the other hand, the low Mo but high Na content would enhance the oxidation while inhibit the reduction reaction. This study also reports for the first time the doping of metal as a novel and facile method for controlling the oxidation and reduction activities of TNTs for photocatalytic removal of NO2. Doping of Fe, Sr, and Zn enhanced the intrinsic oxidation activity of TNTs.

Doping of Al, Co, La, Mg, Sn, W, and Zr showed little effect while doping of Ag, Ce, Mn, Ni, Sb inhibited the oxidation activity. Particularly, although the doping of Cr, Cu, Mo, and V inhibited the oxidation activity, it surprisingly provided TNTs with high reduction ability, which successfully reduced NO2 to NO. Key words: nitrogen oxides; TNTs; NO2; indoor air pollution control; environmental photocatalysis; photocatalytic reduction -ii- Acknowledgement Praise the God for always being with me during the difficult times of my life. I would like to thank my excellent advisor Prof.

Hsunling Bai, a model that I want to become in the future for my professional career, for her support, guidance, and encouragement during 5 years of study. I also thank Prof. Chungsying Lu, Prof. Ruey-an Doong, Prof.

Hui-Hsin Tseng, and Prof. Sue-min Chang for their valuable comments not only for the revision of this thesis but also suggest many ideas for my work in the future. I want to thank all my friends in NCTU for their help with my experimental as well as my living in Taiwan. Special thanks to Hungyu and Bryan with their numerous helps.

I also thank Liangyi, Tina, Oscar, Shihwan, Meihua, Guohua, Chihcheng, Momo, Misaki, Ashley, Yiwen, Lika, Joy, Adam, Josephine, Jeremy, Sih-yu, Cheng-chi, Ching-ching, Jenyu, Cheng- ming, You-ren, Anchi and other students in IEV, language centers of English and Chinese, and the Vietnamese communities, whose names are too many to mention. I want to have special thanks to NCTU for granting me the scholarship during 5 years of study, and Faculty of Environment, HCMUT for giving me the study leave. Finally, thanks to my families, especially my mother and my wife, for the love, patience, praying and support that went above and beyond. -iii- Table of Contents Abstract.

iii Table of Contents. iv List of Figures. vii List of Tables. Research objectives and scope.

Photocatalytic removal of NOx using TiO2-based materials. Direct decomposition to N2 and O2. NO2 oxidation and/or reduction. NO and NO2.

Control of TNTs properties. Effects of Ti sources. Effects of hydrothermal conditions. Effects of calcination.

Effects of doping. The washing process in TNTs synthesis. Non-acid washing. Washing with acid solution.

Effect of acid washing. Materials and Methods. Material synthesis and characterization. NOx source and analysis.

Experimental set-up. Photocatalytic Oxidation of NOx using Pure TNTs. Effect of calcination temperature. Results and discussion.

Effect of washing pH value. Results and discussion. Inter-effect of NO and NO2. Estimation of inhibition or promotion effects.

Results and discussion. Photocatalytic Reduction of NO2 using Metal-doped TNTs. Mo-doped TNTs for NO2 reduction – effect of doping method. Results and discussion.

Mo-doped TNTs for NO2 reduction – effect of washing pH. Results and discussion. Effect of metal doping on the oxidation and reduction activity of TNTs. Results and discussion.

Conclusions and Recommendations. Recommendation for future works. 182 -vi- List of Figures Figure 2.1 Post-combustion methods for NOx abatement.2 Photocatalysis by TiO2.3 TNTs formation mechanism.4 Morphological phase diagrams of TNTs with different precursors and hydrothermal conditions.5 TNTs morphology and crystal phase transformation during thermal treatment.6 Comparison in morphology and crystal phase transformation of H-TNTs and Na- TNTs during hydrothermal treatment.7 Different types of washing process applied for TNTs fabrication.8 Effect of sodium content on the TNTs phase transformation during thermal treatment.1 Typical TNTs synthesis procedure.2 Experimental set-up for photocatalytic removal of NOx.1(a) Surface area, pore volume, and pore size of P25 and TNTs calcined at different temperatures.1(b) N2 adsorption-desorption isotherms of P25 and TNTs calcined at different temperatures.1(c) Pore size distribution (logarithmic scale) of P25 and TNTs calcined at different temperatures.2 SEM results of P25 and TNTs calcined at different temperatures.3 TEM results of P25 and TNTs calcined at different temperatures.4 UV-Visible light absorption curves of P25 and TNTs calcined at different temperatures.5 XRD patterns of P25 and TNTs calcined at different temperatures.6 Results of photocatalytic reaction of NO only: (a) NO conversion efficiency, (b) NOx removal efficiency, and (c) NO2 selectivity.7 NOx removal efficiency for photocatalytic reaction of NO2 only.8 Results for (a) NOx removal and (b) NO2 conversion (averaged over 4 hr) under different conditions: photolysis, adsorption (ads.), and photocatalysis (ph.9 NO and NO2 conversion efficiencies for photocatalytic reaction of NO and NO2, respectively (results averaged over 4 hr).10 Reaction rates of NO and NO2 in the photocatalytic reactions.11 TEM results of P25 and as-synthesized TNTs washed at different pH values.12 SEM results of P25 and as-synthesized TNTs washed at different pH values.13(a) Surface area, pore volume, and pore size of P25 and as-synthesized TNTs washed at different pH values.13(b) N2 adsorption-desorption isotherms of P25 and as-synthesized TNTs washed at different pH values.13(c) Pore size distribution (logarithmic scale) of P25 and as-synthesized TNTs washed at different pH values.14 TEM results of P25 and calcined TNTs washed at different pH values.15 SEM results of P25 and calcined TNTs washed at different pH values.16(a) Surface area, pore volume, and pore size of P25 and as-synthesized TNTs washed at different pH values.16(b) N2 adsorption-desorption isotherms of P25 and calcined TNTs washed at different pH values.16(c) Pore size distribution (logarithmic scale) of P25 and calcined TNTs washed at different pH values.17 XRD results of P25 and calcined TNTs washed at different pH values.18 Na/Ti molar ratio of calcined TNTs washed at different pH values.19 TPD results of P25 and calcined TNTs: (a) NH3-TPD, (b) CO2-TPD, and (c) acid and basic site amounts.20 Efficiency of (a) total NOx removal and (b) average NO2 conversion in photocatalytic reaction of NO2 using P25 and TNTs.21(a) Total NOx removal in NO reaction using P25 and TNTs.21(b) NO conversion in NO reaction using P25 and TNTs.21(c) NO2 selectivity in NO reaction using P25 and TNTs.21(d) Averaged NO conversion in NO reaction using P25 and TNTs.22 NOx removal efficiency in NO2 adsorption using P25 and TNTs (inset is the NO2 adsorption capacity over 4 hrs).23 Total NOx removal efficiency for separated removals of NO and NO2 with different materials (P25, T-1.24 NO and NO2 conversions in separated and simultaneous reactions.25 Results from Ao et al.26 Regression results from Ao et al.27 Conversion efficiencies for simultaneous removal of NO and NO2 using (a) T-3, (b) T-1.28 Conversion capacity for simultaneous removal of NO and NO2 using (a) T-3, (b) T-1. actual results for NO conversion capacity using (a) T-3, (b) T-1.

actual results for NO2 conversion capacity using (a) T-3, (b) T-1. actual results for NOx removal capacity using (a) T-3, (b) T-1.32 FTIR spectra of KBr, fresh T-3, and T-3 after 4 h reaction with NO, NO2, and simultaneous NO (50%) + NO2 (50%) .33 Mechanism for photocatalytic oxidation of NOx using TiO2 .1 SEM results for as-synthesized TNTs: (a) T-0, (b) Mo/T-H, (c) Mo/T-P, (d) P25; and calcined TNTs: (e) T-0, (f) Mo/T-H, (g) Mo/T-P, (h) Mo/T-I.2 TEM results for as-synthesized TNTs: (a) T-0, (b) Mo/T-H, (c) Mo/T-P, (d) P25; and calcined TNTs: (e) T-0, (f) Mo/T-H, (g) Mo/T-P, (h) Mo/T-I.3 XRD results of P25 and calcined TNTs.4 (a) NH3-TPD result, (b) CO2-TPD result, and (c) amounts of acid and basic sites of P25 and calcined TNTs.5 UV-Vis results of P25 and calcined TNTs.6 XPS results of calcined TNTs.