Photocatalytic removal of NOx using titania nanotubes

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 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 had the highest total NOx removal efficiency while the one washed at pH 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 years of study I also thank Prof Jeffrey C.S Wu, 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, Chengming, 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 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 i Acknowledgement iii Table of Contents iv List of Figures vii List of Tables xi Abbreviations xii Chapter I Introduction 1.1 Background 1.2 Research objectives and scope Chapter II Literature Reviews 2.1 NOx abatement 2.1.1 Nitrogen oxides 2.1.2 NOx abatement 2.2 Photocatalytic removal of NOx using TiO2-based materials 2.2.1 Direct decomposition to N2 and O2 2.2.2 NO oxidation 2.2.3 NO2 oxidation and/or reduction 10 2.2.4 NO and NO2 12 2.3 Titanium dioxide 14 2.3.1 Titanium dioxide 14 2.3.2 Titania nanotubes 16 2.4 Control of TNTs properties 25 2.4.1 Effects of Ti sources 25 2.4.2 Effects of hydrothermal conditions 28 2.4.3 Effects of calcination 34 2.4.4 Effects of doping 37 2.5 The washing process in TNTs synthesis 41 2.5.1 Non-acid washing 41 2.5.2 Washing with acid solution 44 2.5.3 Effect of acid washing 48 Chapter III Materials and Methods 52 3.1 Material synthesis and characterization 52 3.1.1 Chemicals 52 -iv- 3.1.2 Material synthesis 52 3.1.3 Material characterization 55 3.2 Photocatalytic experiment 59 3.2.1 NOx source and analysis 59 3.2.2 Experimental set-up 59 3.2.3 Result calculation 60 Chapter IV Photocatalytic Oxidation of NOx using Pure TNTs 62 4.1 Effect of calcination temperature 62 4.1.1 Motivation 62 4.1.2 Results and discussion 62 4.1.3 Summary 81 4.2 Effect of washing pH value 82 4.2.1 Motivation 82 4.2.2 Results and discussion 82 4.2.3 Summary 106 4.3 Inter-effect of NO and NO2 108 4.3.1 Motivation 108 4.3.2 Estimation of inhibition or promotion effects 108 4.3.3 Results and discussion 110 4.3.4 Summary 123 Chapter V Photocatalytic Reduction of NO2 using Metal-doped TNTs 126 5.1 Mo-doped TNTs for NO2 reduction – effect of doping method 126 5.1.1 Motivation 126 5.1.2 Results and discussion 127 5.1.3 Summary 142 5.2 Mo-doped TNTs for NO2 reduction – effect of washing pH 143 5.2.1 Motivation 143 5.2.2 Results and discussion 143 5.2.3 Summary 151 5.3 Effect of metal doping on the oxidation and reduction activity of TNTs 153 5.3.1 Motivation 153 5.3.2 Results and discussion 153 5.3.3 Summary 162 Chapter VI Conclusions and Recommendations 163 6.1 Conclusions 163 -v- ... 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. . .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... Medium Using Niobium and Titanium Oxides", Brazilian Journal of Petroleum and Gas, (2), pp 071-082, 2013 -179- [220] N.H Nguyen and H Bai, "Photocatalytic removal of NO and NO2 using titania nanotubes