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Photocatalytic degradation of acetic acid in gas phase in
the presence and in the absence of O2 using different
TiO2 and M-TiO2 : a comparative study on the
conversion, mineralization and intermediates’
selectivities
Ha Son Ngo
To cite this version:
Ha Son Ngo. Photocatalytic degradation of acetic acid in gas phase in the presence and in the absence of O2 using different TiO2 and M-TiO2 : a comparative study on the conversion, mineralization and intermediates’ selectivities. Catalysis. Université de Lyon, 2017. English. �NNT : 2017LYSE1230�. �tel-01721818�
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N°d’ordre NNT : 2017LYSE1230
THESE de DOCTORAT DE L’UNIVERSITE DE LYON
opérée au sein del’Université Claude Bernard Lyon 1 École Doctorale ED 206
École Doctorale de Chimie de Lyon
Spécialité de doctorat : CHIMIE
Soutenue publiquement le 08/11/2017, par :
Ha Son NGO
Photocatalytic degradation of acetic acid in gas
phase in the presence and in the absence of O
2using
different TiO
2and M-TiO
2: A comparative study on
the conversion, mineralization and intermediates’
selectivities
Devant le jury composé de :
M. Mohamed SARAKHA Professeur, Université Clermont Auvergne Rapporteur
Mme Nathalie HERLIN-BOIME Directrice de recherche, CEA Rapporteur
Mme Catherine PINEL Directrice de recherche, CNRS Examinatrice
Mme Valerie KELLER-SPITZER Directrice de recherche, CNRS Examinatrice
M. Jean-Marc CHOVELON Professeur, Université Lyon 1 Examinateur
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UNIVERSITE CLAUDE BERNARD - LYON 1
Président de l’Université
Président du Conseil Académique
Vice-président du Conseil d’Administration
Vice-président du Conseil Formation et Vie Universitaire Vice-président de la Commission Recherche
Directrice Générale des Services
M. le Professeur Frédéric FLEURY
M. le Professeur Hamda BEN HADID M. le Professeur Didier REVEL
M. le Professeur Philippe CHEVALIER M. Fabrice VALLÉE
Mme Dominique MARCHAND
COMPOSANTES SANTE
Faculté de Médecine Lyon Est – Claude Bernard
Faculté de Médecine et de Maïeutique Lyon Sud – Charles Mérieux
Faculté d’Odontologie
Institut des Sciences Pharmaceutiques et Biologiques Institut des Sciences et Techniques de la Réadaptation
Département de formation et Centre de Recherche en Biologie Humaine
Directeur : M. le Professeur G.RODE
Directeur : Mme la Professeure C. BURILLON Directeur : M. le Professeur D. BOURGEOIS Directeur : Mme la Professeure C. VINCIGUERRA Directeur : M. X. PERROT
Directeur : Mme la Professeure A-M. SCHOTT
COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE
Faculté des Sciences et Technologies Département Biologie
Département Chimie Biochimie Département GEP
Département Informatique Département Mathématiques Département Mécanique Département Physique
UFR Sciences et Techniques des Activités Physiques et Sportives Observatoire des Sciences de l’Univers de Lyon
Polytech Lyon
Ecole Supérieure de Chimie Physique Electronique Institut Universitaire de Technologie de Lyon 1 Ecole Supérieure du Professorat et de l’Education Institut de Science Financière et d'Assurances
Directeur : M. F. DE MARCHI
Directeur : M. le Professeur F. THEVENARD Directeur : Mme C. FELIX
Directeur : M. Hassan HAMMOURI
Directeur : M. le Professeur S. AKKOUCHE Directeur : M. le Professeur G. TOMANOV Directeur : M. le Professeur H. BEN HADID Directeur : M. le Professeur J-C PLENET Directeur : M. Y.VANPOULLE
Directeur : M. B. GUIDERDONI Directeur : M. le Professeur E.PERRIN Directeur : M. G. PIGNAULT
Directeur : M. le Professeur C. VITON
Directeur : M. le Professeur A. MOUGNIOTTE Directeur : M. N. LEBOISNE
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ACKNOWLEDGEMENTS
I wish to express my sincerely gratitude to my supervisor, Dr. Chantal GUILLARD for taking
me into her group. I cherish the time I worked with her whom I consider as my second
mother. She does not only give me the passion in research but also help me a lot in daily life
with every “tiny things” which appeared in my residence in France. I am particularly grateful
for her time, her advices, and the trust that she put on me during this work.
Another thank will be for Frederic DAPOZZE, the “indispensable” person of our group. He is
an expert of all the apparatus with unbelievable skills. Without him, this thesis could not be
finished.
Thanks to Dr. Catherine Pinel for having presided the jury. My sincere thanks also go to Dr.
Nathalie HERLIN-BOIME, Pr. Mohamed SARAKHA, Pr. Jean-Marc CHOVELON and Dr
Valerie KELLER-SPITZER for their time serving in my examination committee.
This work would not have been possible without the preciously help of many people. Special
thanks to: Lina Laama, Marième Bouhatmi, Marwa Hamandi, Chloé Indermuhle and Milena
Ponczek.
I am grateful to my parents and my wife, who have provided me through moral and emotional
support in my life. I am also grateful to my other family members and friends who have
supported me along the way.
A very special gratitude goes out to all down at Viet Nam Ministry of Education and Traning
as well as my university – Ha Noi University of Mining and Geology for helping and
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Cho niềm hy vọng của bố mẹ Cho giấc mơ của anh và em, vợ yêu...
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RÉSUMÉ
À l'heure actuelle, la pollution de notre environnement est devenue un problème
mondial menaçant la santé de l'humanité, mais une autre préoccupation de notre société est
également l'épuisement des ressources non renouvelables. Il est alors important d’une part de
trouver des solutions de remédiation de la pollution, efficaces et peu coûteuse
énergétiquement et d’autre part utiliser cette pollution pour élaborer de nouveaux produits ou
générer de l’énergie. La photocatalyse hétérogène est l'un des moyens prometteurs pour
répondre à ces problématiques. En effet, l’activation du photocatalyseur peut être réalisée
sous lumière solaire, en utilisant les 4-5% de photons UV activant TiO2, qui est le meilleur
catalyseur actuellement sur le marché pour traiter efficacement la pollution. De plus, afin
d’utiliser plus efficacement l’énergie solaire, un grand nombre de recherches sont dédiés à
l’élaboration de nouveaux matériaux activables sous visible. On peut citer par exemple l’ajout
de nanoparticules d’or ou d'argent qui peuvent absorber la lumière visible en raison de la
résonance des plasmons de surface. Le dopage par ce type de métaux est également
intéressant pour générer de l’hydrogène à partir de solution aqueuse contenant des piégeurs de
trous, en absence d’oxygène. Cependant, quel que soit l’atmosphère de travail, les
mécanismes sont toujours sujets à débat et les produits de dégradation peu étudié,
principalement en absence d’oxygène. En effet, dans ce cas, la majorité des études se focalise
sur la formation d’hydrogène.
L’objectif de la thèse est alors de mieux comprendre les mécanismes de dégradation
photocatalytique se produisant sous air ou sous azote en étudiant la disparition, la
minéralisation et les produits intermédiaires d’une molécule simple l’acide acétique. Les
réactions sont réalisées sous ces deux atmosphères afin de se placer dans des conditions de
dépollution ou de génération d’énergie. L’étude est réalisée en phase gazeuse et sous flux en
8
d’acide carboxylique comme molécule modèle a été choisie car une quantité importante de
polluants dans les eaux usées des industries des procédés chimiques, comme les colorants et
les pigments, les peintures, les produits pétrochimiques ... sont des acides carboxyliques. Mais
également car ces acides carboxyliques sont des polluants de l’atmosphère issus d’émission
anthropiques ou d’émission biogènes.
Dans un premier temps nous avons étudié la dégradation de l’acide acétique en
utilisant le photocatalyseur de référence, TiO2 P25. Quel que soit le flux gazeux de réaction,
air ou N2, nous avons montré que la réaction de décarboxylation est la première étape de la
disparition de l’acide acétique. Cependant, le devenir du groupe méthyle dépend du gaz
porteur et du débit molaire (en d’autre terme de la concentration du polluant en phase
gazeuse). Dans l'air, à faible concentration, le groupe méthyle est complètement minéralisé en
CO2, alors qu’il n’est que partiellement transformé en méthanol, formaldéhyde mais
également éthane si le débit molaire excède 0.8 µmoles par minutes. D’autres composés sont
également observés, diméthyl-éther, acétate de méthyl et formiate de méthyl. Dans tous les
cas, les sélectivités obtenues en formaldéhyde sont plus importante que celles observés en
méthanol. Ce résultat est expliqué en considérant la formation de HO2° et sa participation
majoritaire à la dégradation de l’acide acétique. Le mécanisme de dégradation se produisant à
la surface du photocatalyseur est alors représenté pour expliquer l’importance de ce
mécanisme comparé à celui faisant intervenir les radicaux hydroxyles. La schématisation du
mécanisme inclut la régénération du photocatalyseur et la formation possible de H2O2, lequel
a été observé dans la littérature. Il est également noté que la sélectivité d’éthane observé sous
flux d’air augmente avec la concentration en polluant et correspond à la réaction de deux
radicaux méthyl. Comme précédemment la formation de ce composé en surface du TiO2 est
proposée En atmosphère N2, trois nouveaux produits ont été détectés: le méthane (CH4),
l'acétone (CH3COCH3) et l'acétaldéhyde (CH3COH). L'étude des produits de dégradation de
9
que l'acétone et l'acétaldéhyde ne proviennent pas de la réduction du groupe carboxylique.
Les mécanismes de formation du méthane et de l’éthane sont proposés.
Dans une seconde étape l’impact du flux photonique et de l’humidité en présence de
TiO2 P25 et l'effet de différents TiO2 commerciaux sur la conversion et plus particulièrement
la distribution des produits intermédiaires ont été étudiés. La comparaison de l'efficacité de
différents TiO2 commerciaux a été discutée en considérant la présence de phase rutile, la
nature des espèces actives, la surface spécifique de TiO2, le nombre de groupes OH à la
surface des catalyseurs, la présence d'impuretés et la porosité des matériaux.
Notre étude s’est ensuite focalisée sur la détermination de l’efficacité d’échantillons de
TiO2 modifiés par ajout d’or afin d’améliorer la séparation des charges et ainsi la dégradation
de polluant en présence d’air ou la formation de produit en présence de flux d’azote.
Deux séries de Au/TiO2 avec les mêmes charges d'or (~ 0,16% en poids) ont été
préparées par les deux méthodes: pyrolyse laser et pyrolyse par pulvérisation de flamme
(Au-TiO2 LP et Au-TiO2 FSP). Les catalyseurs ont été premièrement caractérisés par RX, BET,
MET, UV-visible et analyse chimique. Tous ces catalyseurs ont été testés dans la dégradation
de l'acide acétique en phase gazeuse en présence ou en l'absence d'air puis comparés au TiO2
élaboré par la même méthode. L'impact du métal noble sur TiO2 a été discuté en considérant
l'efficacité photocatalytique, la minéralisation et la nature des intermédiaires formés. Les
résultats ont montré que la présence d'or améliore l'activité photocatalytique dans l'air dans le
cas des échantillons préparés par pyrolyse laser alors qu’aucun effet n’est observé avec les
catalyseurs préparés par pyrolyse à flamme (FSP). Ce résultat s’explique en considérant la
taille des nanoparticules d’or plus petite dans le cas des échantillons obtenus par pyrolyse
laser, 2-3 nm contre environ 5nm dans le cas de la méthode FSP. L’effet inverse est observé
sous atmosphère de N2, la présence d’or diminue de plus de moitié la dégradation de l’acide
acétique mais favorise la formation d’éthane. La formation de méthane est dans ce cas
10
cationique. Malheureusement, par XPS, il n’a pas été possible d’observer d’or probablement
dû à sa faible quantité.
L’impact du dopage à l'azote de TiO2 LP et Au-TiO2 LP a également été étudié. Ce
dopage diminue l'efficacité de cet échantillon. Enfin, les références Pt-TiO2 (0,5 et 5,0% en
poids de Pt) ont été testées en l'absence d'O2.
Finalement des études préliminaires ont été conduite d’une part sur l’efficacité de
textile lumineux photocatalytique pour dégrader l’acide acétique afin d’améliorer les
rendements quantiques et d’autre part sur les efficacités de catalyseurs Ag/TiO2 lesquels,
outre diminué la pollution organique permettrait également l’inactivation des
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TABLE OF CONTENTS
Introduction ... 25
CHAPTER I. Literature review ... 31
1 Semiconductor material ... 32
1.1 TiO2 material. ... 35
1.2 Metals loading on TiO2 ... 38
1.3 Photocatalyst support ... 42
2 Photocatalysis ... 43
2.1 Photocatalytic mechanism ... 44
2.2 Effects of different parameters on photocatalytic process ... 46
2.2.1 Pollutants initial concentrations ... 47
2.2.2 Light sources ... 48
2.2.3 UV intensity ... 49
2.2.4 Humidity ... 49
3 Carboxylic acids and photocatalytic degradation of carboxylic acids ... 50
3.1 Carboxylic acids in our environment ... 50
3.2 Carboxylic acid adsorption on TiO2 ... 53
3.3 Photocatalytic degradation reactions of carboxylic acids and acetic acid ... 57
3.3.1 Photocatalytic degradation of carboxylic acids in Air ... 57
3.3.2 Photocatalytic degradation of carboxylic acids in O2 free ... 58
References ... 63
CHAPTER II. Experimental part ... 83
1 Photocatalyst and pollutant ... 84
1.1 Photocatalyst ... 84
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2 Reactor and experimental set-up ... 85
3 Procedure ... 89
4 Analysis ... 89
4.1 Online analysis by PDHID/FID GC and Ar-PDPID GC ... 89
4.2 PTR-MS analysis ... 95
5 Photocatalytic evaluation ... 97
6 Characterization methods ... 98
6.1 X-ray diffraction (XRD) ... 98
6.2 Surface area measurement and porosity analysis ... 99
6.3 X-ray photoelectron spectroscopy (XPS) ... 101
6.4 Transmission electron microscopy (TEM) ... 102
6.5 TG – DTA – MS ... 103
References ... 106
CHAPTER III. Kinetics and mechanism of the photocatalytic degradation of acetic acid in absence or presence of O2. ... 107
1 Introduction ... 108
2 Experimental ... 109
2.1 Photocatalyst and pollutant ... 109
2.2 Reactor and experimental set-up ... 110
2.3 Procedure ... 110
2.4 Sampling and analysis of gaseous intermediates ... 111
3 Results and discussions ... 111
3.1 Adsorption of Acetic acid ... 111
3.2 Impact of oxygen on the disappearance and mineralization of acetic acid (AA) ... 113
3.3 Influence of the initial concentration of acetic acid ... 116
3.3.1 Influence of the concentration on the disappearance rate ... 116
13
3.3.3 Influence of the atmosphere on the nature of intermediate compounds ... 119
3.4 Evolution of intermediate compounds at different initial concentrations – mechanistic insights ... 120
4 Conclusions ... 129
References ... 130
Chapter IV. Impact of irradiance, humidity and residence time on the photocatalytic degradation of acetic acid under air or N2. Efficiency of different commercial TiO2 ... 135
1 Introduction ... 136
2 Experimental ... 139
2.1 Photocatalyst and pollutant ... 139
2.2 Reactor and experimental set-up ... 140
2.3 Procedure ... 141
2.4 Sampling and analysis of gaseous intermediates ... 141
3. Results and discussions ... 142
3.1. Effect of experimental conditions on the conversion and products of photocatalytic degradation of AA under continuous air flow. ... 142
3.1.1 Effect of photonic flux, humidity and residence time on the AA conversion ... 143
3.1.2 Effect of photonic flux, humidity and residence time on the formation of products and the mechanism of degradation ... 146
3.2. Effect of experimental conditions on the conversion and products of photocatalytic degradation of AA under continuous N2 flow. ... 153
3.2.1 Effect of photonic flux, humidity and residence time on the AA conversion ... 153
3.2.2 Effect of photonic flux, humidity and residence time on the formation of products under N2 ... 155
3.2.3 Discussion of the mechanism of photocatlytic degradation of AA under N2 ... 158
3.3 Comparison of the efficiency of different commercial TiO2 catalysts under air ... 160
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3.3.2 Nature and evolution of intermediates ... 167
3.4 Comparison of the efficiency of different commercial TiO2 catalysts under Nitrogen ... 169
4 Conclusions ... 170
References ... 172
Chapter V. Characterization and photocatalytic efficiency of TiO2 and Au-TiO2 prepared by laser pyrolysis and flame spray pyrolysis method. ... 181
1 Introduction ... 182
2 Experimental ... 184
2.1 Photocatalyst and pollutant ... 184
2.2 Characterization methods ... 184
2.3 Synthesis ... 185
2.4 Photocatalytic experiments ... 188
3 Results and discussions ... 188
3.1 Characterization of powders ... 188
3.1.1 Chemical anlysis ... 188
3.1.2 Morphology and structure ... 189
3.1.3 Optical properties ... 196
3.2 Efficiency of different TiO2 ... 198
3.2.1 Photocatalytic degradation of AA under Air. ... 198
3.2.2 Photocatalytic degradation of AA under N2. ... 200
3.3 Impact of gold deposition on TiO2 ... 203
3.3.1 Photocatalytic degradation of acetic acid in Air. ... 203
3.3.2 Photocatalytic degradation under N2. ... 206
3.4 Impact of doping nitrogen and nitrogen-gold deposition ... 211
4 Conclusions ... 215
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CHAPTER VI. Experiments for future developement ... 229
1 Introduction ... 230
2 Experimental ... 231
3 Results and discussions ... 233
3.1 Catalytic efficiency of TiO2 powder and TiO2 deposited on luminous textile ... 233
3.2 Impact of silver on photocatalytic activity ... 235
4 Conclusions ... 239 References ... 240 GENERAL CONCLUSIONS ... 243 APPENDICES ... 249 APPENDIX A. PTR-MS RESUTLS ... 250 APPENDIX B ... 253
APPENDIX C. TGA-MS analysis ... 256
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LIST OF FIGURES
CHAPTER IFigure I.1. Fermi level of different semiconductor types: a) intrinsic, b) n-type, c) p-type. Ea –
energy of acceptors; Ed – energy of donors. ... 34
Figure I.2. Estimated number of scientific publications on TiO2 photocatalysis per year (ISI Web Knowledge with the key words “TiO2” and “photocatalysis”). ... 35
Figure I.3. Formation of electron-hole pairs on TiO2 (rutile and anatase) 26. ... 37
Figure I.4. Photocatalytic activation scheme of TiO2 sensitized by noble metal particles. ... 39
Figure I.5. Photocatalytic cycle of the mechanism of H2 production from alcohols 40. ... 39
Figure I.6. Modes of formic acid adsorption on the surface of TiO2. ... 54
Figure I.7. a) Reversible adsorption, b) Irreversible adsorption 165. ... 56
Figure I.8. General mechanism of carboxylic acid degradation 12. ... 57
CHAPTER II Figure II.1. Experimental set-up used for degradation of AA in Air and N2. ... 85
Figure II.2. Gas – Liquid generator system. ... 87
Figure II.3. Photoreactor. ... 87
Figure II. 4. Emission of the PLL 18W lamp. ... 88
Figure II.5. Schema of FID-PDHID GC. ... 93
Figure II.6. Schema of Ar-PDID GC. ... 95
Figure II.7. Schematic representation of PTR-MS (adapted from ionic.com). ... 96
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Figure II.9. The principle of XPS. ... 101
CHAPTER III
Figure III.1. Amount of AA adsorbed per gram of catalyst as a function of the initial concentration of AA. ... 112 Figure III.3. Disappearance rate of AA as a function of initial concentration of AA. ... 116 Figure III.4. Mineralisation rate of AA as a function of AA concentration (a) and CO2
selectivity as a function of AA concentration (b). ... 118 Figure III.5. Selectivity of intermediate compound formed (a) under N2 and (b) under air. . 121
Figure III.6. Formation of methanol, formaldehyde and alkane (CH4 and C2H6) at different
AA initial concentration under Air (a) and under N2 (b). ... 123 CHAPTER IV
Figure IV.1. Molecules of AA disappeared per second as a function of the number of photon
generated per second in Air (AAo = 100, 280 and 800ppm). ... 143
Figure IV.2. Impact of residence time on the conversion of AA acid. [AA]= 800 ppm;
irradiance = 5 mW/m2. ... 145 Figure IV.3. C2H6 (a), HCHO (b) and CH3OH (c) evolution in Air at AAo = 100ppm (full
symbols) and 800ppm (empty symbols) as a function of the number of photon generated per second. ... 147 Figure IV.4. Selectivities of ethane, formaldehyde and methanol as a function of the number
of photons emitted at [AAo]= 800ppm (a) and [AAo]= 100ppm (b). ... 149
Figure IV.5. Selectivity of the different products observed under airflow as a function of
residence time. [AA]= 100 ppm; irradiance 5 mW/cm2. ... 152 Figure IV.6. Molecules of AA disappeared per second as a function of the number of photon generated per second (a) and of residence time (b) (AAo = 100ppm). ... 154
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Figure IV.7. Selectivity of intermediate products obtained in the degradation of AA ([AAo]=
100ppm) under N2 flow as a function of the number of photons/s ... 156
Figure IV.8. Selectivity of different products formed in the photocatalytic degradation of AA
under N2 flow as a function of residence time. ... 157
Figure IV.9. AA conversion (a) and mineralization (b) in Air at AAo = 800ppm, in presence
of different commercial TiO2 photocatalysts, as a function of specific surface area. In which:
cross symbol – PC series; diamond symbol – P25; round symbol – P90; triangle symbol – NA2 and square symbol– Hombikat. ... 161 Figure IV.10. XPS spectrum of UV100 (a) and PC500 (b) ... 165 Figure IV.11. Carbon balance of the degradation of AA in Air at AAo = 800ppm with
different commercial TiO2 as a function of specific surface area. ... 168
Figure IV.12. Carbon balance of the degradation of AA in N2 at AAo = 100ppm with different
commercial TiO2 as a function of specific surface area. ... 169
CHAPTER V
Figure V.1. TEM image of TiO2 LP. ... 191
Figure V.2. TEM of Pt-TiO2 LP (Pt 0.5%wt) sample. ... 191
Figure V.3. TEM (a) and HRTEM (a) of AuCl3-TiO2 LP sample. ... 192
Figure V.4. TEM of AuCl3-TiO2-FSP (a), (b) and Au acetate-TiO2 FSP (c), (d) samples. ... 193
Figure V.5. Diffractograms of LP series samples a) Pt loaded NPS b) Au loaded NPs (annealed powders). ... 194 Figure V.6. Diffractograms of FSP series samples. ... 195 Figure V.7. Transformed Kubelka-Munk (from diffuse reflectance spectra) with energy of excitation source for LPseries (a) and FSP series (b). ... 197
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Figure V.8. Conversion and mineralization of AA with TiO2 samples in Air ([AAo] =
800ppm). ... 198 Figure V.9. Carbon balance resulting from AA (CH3COOH) decomposition in presence of
different TiO2 samples in Air ([AAo] = 800ppm). ... 199
Figure V.10. AA conversion and mineralization with different TiO2 samples in N2 ([AAo] =
100ppm). ... 201 Figure V.11. Carbon balance resulting from AA degradation with different TiO2 samples in
N2. ... 202
Figure V.12. Efficiency and mineralization of pristine TiO2 samples and Au/TiO2 samples in
Air ([AAo] = 800ppm). ... 204
Figure V.13. Conversion and mineralization of AA with pristine TiO2 and Au/TiO2 (a); TiO2,
AuCl3-TiO2 LP and Pt/TiO2 LP (b) in N2. ... 207
Figure V.14. Carbon balance resulting from AA degrdation with : TiO2 LP, TiO2 FSP and
different Au/TiO2 (a) ; TiO2 LP, AuCl3-TiO2 LP and different Pt/TiO2 LP (b) in N2. ... 210
Figure V.15. Efficiency of TiO2 LP, N-TiO2, AuCl3-TiO2 LP and AuCl3-N-TiO2 LP in Air (a)
and in N2 (b). ... 212
Figure V.16. Carbon balance of AA degradation with N-TiO2 LP, TiO2 LP and Au-N-TiO2
LP in Air (a) and in N2 (b). ... 214 CHAPTER VI
Figure VI.1. Reactor and optical fibers. ... 232 Figure VI.2. Comparison of catalytic efficiency in Air ([AAo] = 280ppm) between TiO2 P25
powder and TiO2 P25 deposited on luminous textile. ... 234
Figure VI.3. Conversion and mineralization of AA in Air ([AAo] = 800ppm) using Ag/TiO2
20
Figure VI.4. Catalytic efficiency and mineralization of P25 and Ag series HP in N2 ([AAo]
=100ppm). ... 237 Figure VI.5. Carbon balance of AA degradation with P25 and Ag/TiO2 HP series. ... 238
21
LIST OF TABLES
CHAPTER ITable I.1. Band gap energies and corresponding irradiation wavelength for the excitation of
some semiconductor 316. ... 32 Table I.2. Range of volatile organic acids concentration in water and wastewater 145. ... 52
CHAPTER II
Table II.1. Target pollutant ... 85 Table II.2. Experimental conditions. ... 88 Table II.3. Response factor for FID detector. ... 90 Table II.4. Gas specifications. ... 95
CHAPTER III
Table III.1. Experimental conditions. ... 110 Table III.2. Acid acetic (AA) disappearance rate (r(AA)) and mineralization rate (r(CO2)) under
air and N2. ... 115
Table III.3. List of intermediates detected during the photocatalytic degradation of AA (range
of concentration between 0.4µmol/l to 120µmol/l) under Air or under N2 in presence of TiO2
P25 and an irradiance of 5 mW/cm2. ... 119 CHAPTER IV
Table IV.1. Experimental conditions ... 141 Table IV.2. Selectivity of intermediate products formed in the photocatalytic degradation of
22
Table IV.3. Selectivity of intermediates after the AA degradation in N2 ([AAo] = 100ppm).
... 157 Table IV.4. TG-TD-MS results ... 163 Table IV.5. XPS results for Hombikat UV100 ... 166
CHAPTER V
Table V.1. Main synthesis conditions of LP samples. ... 186 Table V.2. Main synthesis conditions of FSP samples. ... 187 Table V.3. Gold and N content in different materials. ... 188 Table V.4. Main characteristic of different catalysts. ... 189 Table V.5. Selectivities of intermediates from AA degradation in Air ([AAo] = 800ppm). .. 205
23
LIST OF ABREVIATIONS
AA: Acetic acidBET: Brunauer-Emmett-Teller CB: Conduction band
COD: Chemical oxygen demand DFT: Density functional theory EDTA: Diamine tetraacetic acid FID: Flame ionization detector
FTIR: Fourie transform infrared spectroscopy FWMH: Full width at half maximum
GC: Gas chromatography
ICP-AES: Inductively coupled plasma atomic emission spectroscopy ICP-OES: Inductively coupled plasma optical emission spectrometry NHE: Normal hydrogen electrode
NPs: Nanoparticles
NTA: Nitrolotriacetic acid
PDHID: Pulsed discharge helium ionization detector PDPID: Pulsed discharge photoionization dectector PID: Photoionization dectector
PTR-MS: Photon transfer reaction – Mass spectrometry PZC: Point zero charge
RH: Relative humidity SPB: Surface plasmon band
STP: Standard temperature and pressure TEM: Transmission electron spectroscopy
24
UV: Ultraviolet
XPS: X-ray photoelectron spectroscopy XRD: X-ray diffraction
VB: Valence band
VOAs: Volatile organic acids VOCs: Volatile organic compounds
25
26
At present, environmental pollution has become a global problem threatening the
health of humanity, but another concern of our society is also depletion of non-renewable
resources. So, it is important to find solutions for pollution remediation that are efficient,
inexpensive and use this pollution to develop new products or generate energy.
Among the various solutions proposed, heterogeneous photocatalysis is one of the
promising ways to address environmental issues since 1970s. This is inspired by the potential
of TiO2-based photocatalyst for the total destruction of organic compounds in polluted air and
wastewater 1,2.
Heterogeneous photocatalysis is based on the interaction between semiconductor
materials and light. Indeed, activation of the photocatalyst can be carried out under sunlight,
using the 4-5% UV photons activating TiO2, which is the best catalyst currently on the market
to effectively treat pollution. In photocatalytic process, photons with energies equal to or
greater than the semiconductor band gap are absorbed by the catalyst, which generates an
electron in conduction band and a hole in valence band. Then, electron (e-)/hole (h+) pairs can
then migrate to the surface of the catalyst and consequently involved in oxidation and
reduction reaction 3. By considering that we can get ‘free’ light from the sun, the idea of using
solar light energy as resource to clean up the environment is an ideal and extremely promising
approach. Moreover, in order to use solar energy more efficiently, a huge amount of research
is dedicated to the development of new materials that can be activated under visible
conditions. In particular, the addition of gold or silver nanoparticles, which can absorb visible
light because of the resonance of the surface plasmons, was often studied. This type of metals
is also useful for hydrogen production from aqueous solution containing hole scavengers in
the absence of oxygen 4 5 6 7 . However, in all working atmospheres, the mechanisms are
always subject to debate and degradation products rarely studied, particularly in the absence
27
9 and more recently, several studies focused on energy production and more specially on
hydrogen production 10.
In the energy field, most of the researches focused on the hydrogen production from
water using metal deposited on TiO2 or more especially from aqueous solution containing
methanol flushed under inert gas 6. Other promising applications of the photocatalytic process
could also be interesting such as the synthesis of alkane from organic acid substrate 11.
Actually, a significant amount of pollutants of chemical process industries like dyes and
pigment, paints, petrochemicals, …are carboxylic acids 12. Carboxylic acids can also be found
in different concentrations in a wide variety of oily wastes and form a significant part of
organic contaminants in the aluminum industry, which have a negative effects on the
production efficiency and increases raw material usage 13. Moreover, these carboxylic acids
are also pollutants of air issues from anthropogenic emissions and materials, biogenic
emissions, lacquer coatings, etc 14. Therefore, it could be interesting to salvagethese pollution
sources to obtain several valuable products that can be used as an energetic source and at the
same time solve environmental problems. This field is rarely explored.
The aim of the thesis is to better understand the mechanisms of photocatalytic
degradation occurring under air or under nitrogen and more especially the intermediate
products formed in presence of TiO2 but also M/TiO2 in order to evaluate their potentiality for
air treatment and for the production of useful hydrocarbons from pollution. The acetic acid
molecule, a simple carboxylic acid frequently found in air has been used as model molecule.
The reactions are carried out under these two atmospheres in dynamic mode to place
themselves under conditions of depollution or generation of energy. Up to now, some
publications mentioned the formation of methane and ethane from the photocatalytic
degradation of acetic acid under O2-free atmosphere. However, most of them are performed in
aqueous phase without oxygen, in presence of Pt/TiO2 and by using high concentration of
organic compounds. Moreover, the impacts of experimental conditions and of rutile phase are
28
of oxygen, the intermediate formation of H2O2 was proposed but its formation or its impact
was rarely studied. Being forced by these questions, the objective of our work is to propose a
mechanism of photocatalytic degradation of acetic acid in gas phase, by comparing the
disappearance, the mineralization and the selectivity of the intermediate products formed
under air or oxygen-free atmosphere using different concentrations of acetic acid in presence
of pure and modified TiO2.
The first part of this work, Chapter I, gives an overview of semiconductors,
particularly TiO2. It shows the advantages as well as the drawbacks of this material in
photocatalysis and different ways to improve its photoactivity. Then, the parameters, which
have important effects on photocatalytic reactions, were introduced. At the end, the choice of
the target pollutant acetic acid was explained and the issues in the photocatalytic of this
pollutant were discussed. Chapter II described the experimental set-up and procedures as well
as the techniques to characterize the photocatalysts. Also, the formula to calculate crucial
values was presented in these chapters.
For the results and discussions, in Chapter III: by using the data of the conversion
and the selectivity of the compounds that were formed during the degradation process in both
atmospheres, mechanisms would be suggested with the description of the participation of
different active species generated on the surface of the catalyst. Based on these results, the
role of the atmosphere as well as the favorite pathways in each case should be clearer.
In Chaper IV: a series of commercial catalysts were tested and compared to P25 for
better understanding the behavior of each to the reaction. The differences in behavior would
be studied and explained in the thesis considering the impact of the crystal phase, the surface
area or the presence of impurities on the catalysts and also their adsorption properties.
Furthermore, the effects of various factors such as humidity, UV intensity as well as the
contact time between AA and catalysts in the degradation process will be investigated.
In Chapter V, this thesis also takes the advantages of some typical noble metal/TiO2
29
well as N-doped TiO2 was also studied. The distribution of the products in the degradation
process was also taken into account in both atmospheres to better understand the potential in
environmental treatment and energy production.
Finally, for some perspectives, Chapter VI mentioned the work with luminous textile
as a support for TiO2 and the comparison in efficiency between the textile and traditional
TiO2 powder. Additionally, with the aim to evaluate the applying ability in microbiology and
30
31
32
1 Semiconductor material
Theoretically, photocatalyst is a material (mostly common TiO2) that is not consumed
and play as an assistant in pushing up reaction rate, and only activated by the energy of
photon absorbed. Their activities are affected by several factors including: structure, particle
size, surface properties, preparation etc. 15.
The size of semiconductor photocatalysts’ particles is commonly in the range of
micrometer to nanometer. In some cases, however, there are agglomerations of these particles.
Historically, plenty of works found out that the most suitable photocatalysts are metal
oxide semiconductors 3 106 due to their photo-corrosive resistance, photo-stability and their
band gap energies, which are able to work under near-UV/Visible radiation.
The gap between the valence band (VB) and the conduction band (CB) is defined as
band gap of a semiconductor. This band will determine what type of photons can activate a
photocatalyst, by exciting the electrons to make a jump from valence band to conduction band
and create a hole as a result. Some semiconductors with their band gaps are described in
Table I.1
Table I.1. Band gap energies and corresponding irradiation wavelength for the excitation of
some semiconductor 316.
Semiconductor Band gap energy (eV) Wavelength (nm)
ZrO2 3.87 320
ZnS 3.6 344
SnO2 3.5 354
33 ZnO 3.2 388 TiO2 (anatase) 3.2 388 α- Fe2O3 3.2 400 TiO2 (rutile) 3.0 413 WO3 2.8 443 Fe2O3 2.3 539 Cu2O 2.172 571
Semiconductors can be classified into two categories:
- Intrinsic semiconductors: pure material in which, electron and holes flow in opposite
directions in the presence of an electric field (i.e. TiO2)
- Extrinsic semiconductors: materials with impurities added by doping. In this case, it
will result in a change of relative concentrations of electron and holes in the material (i.e.
Metal-TiO2).
If impurities doped on the surface of the catalyst act as donors, semiconductors doped
can be of the n-type with n standing for a negative charge. If impurities doped in order to
attract and receive an electron from the semiconductor, they act as electron acceptors. Then,
they yield p-type semiconductors with p representing a positive charge. There is however, an
important parameter to describe the occupancy of electrons in the system so-called Fermi
Energy Level.
For an intrinsic (pure) semiconductor, the fermi level (EF), depicted in Figure I.1 as a
dotted line, is defined as the energy level in the middle of the band gap. The location of the
fermi level for an extrinsic or doped semiconductor varies based on the concentration and
34 to: 𝐸! = 𝐸! + 𝑘!𝑇𝑙𝑛( 𝑁! 𝑛!)
In which: EF - Fermi energy, Ei - initial energy (or the position of EF for an intrinsic
semiconductor), kB - Boltzmann constant, T - temperature (K), ND - concentration of donors,
and ni - intrinsic carrier density. As seen, for n-type semiconductors, the Fermi level is
slightly elevated towards the conduction band.
Figure I.1. Fermi level of different semiconductor types: a) intrinsic, b) n-type, c) p-type. Ea –
energy of acceptors; Ed – energy of donors.
There will be a shift of the conduction band (n-type) by doping it with an intrinsic
semiconductor with donor impurities and “the probability of electron encounters increases” as
a result. As well, the hole encounters augments when the valence band (p-type) is shifted by
being doped with acceptor impurities. Hence, with the presence of donor or acceptor
impurities, the Fermi level would be changed and some photocatalytic semiconductor
properties can be modified. As a result, the band gap energy of the photocatalyst will be
35
1.1 TiO2 material.
Among the available semi-conductors which can be used as photocatalysts, TiO2 is
generally considered to be the best photocatalyst at the moment 2 3 8 9 6 . From year to year,
the number of researches on TiO2 in photocatalysis increases drastically (figure I.2). Most of
them focused on the properties of TiO2, on its applications and on how to enhance its activity
in numerous processes such as environmental treatment, energy generation etc.
Figure I.2. Estimated number of scientific publications on TiO2 photocatalysis per year (ISI
Web Knowledge with the key words “TiO2” and “photocatalysis”).
The advantages to choose TiO2 as a semiconductor material are:
- High oxidizing ability at ambient temperature and pressure.
- High photocatalytic activity in degrading various environmental pollutants.
- Non-toxic
36
- Cheap and easy to find.
The commercial TiO2 are now diverse on the market and one of the most important
materials is Evonik P25 TiO2, which is considered as a common standard and often used to
compare with other materials for determining photocatalytic activity.
Natural titaniums are normally in form of ilmenite (FeTiO3), perovskite (CaTiO3),
sphene (CaTiSiO5), leucoxene (a mixture of TiO2, Fe2O3, ZrO2), brookite (TiO2), anatase
(TiO2) and rutile (TiO2). Ilmenite is the main raw material to produce commercial TiO2 18.
Via sulphate or chloride preparation technique, different ratio of crystalline structures of TiO2
can be obtained. The main usage in industry is as a pigment in a wide range of products
including paints, paper, textile as well as ceramics 18. Other usages include gas sensors 19,
biomaterials, ceramic membrane materials 20, and photocatalyst 21.
Rutile TiO2 is particularly popular for using in paints. The reason is that it has better
light scattering ability that make rutile more opaque and suitable for shading purposes 18. On
the other hand, in different photocatalytic reaction, anatase showed higher activity than rutile.
The mechanism responsible for this phenomenon is still in debate. At present, there are some
accepted hypotheses, that are: 1) The difference in crystal phase, surface area and porosity; 2)
O2 is more difficult to be reduced in rutile form due to lower CB energy and it results in less
hydroxyl groups on rutile surface (figure I.3); 3) Indirect band gap for anatase and direct for
rutile: an indirect bandgap material enables the excited electrons stay longer in conduction
37
Figure I.3. Formation of electron-hole pairs on TiO2 (rutile and anatase) 26.
Rutile, however, showed higher photocatalytic activity in some processes. In a
research on phenol photodegradation using different crystal TiO2 types, Sun et al. 27 found
that the rutile titania powder with the particle size of about 7nm had higher photocatalytic
activity than that of anatase titania with the same surface area. They explained this by the
abundance of surface hydroxyl groups, which provided more active sites for the degradation
reaction in the powder, by thermogravimetric analysis data. Another work introduced by
Ohno et al. 28 showed that large particle of rutile performed higher efficiency for oxidation of
2-propanol in water while commercial anatase TiO2 gave no activity. Because of a synergy
between the {110} and {011} faces, rutile is likely more efficient in some types of
photocatalytic reactions. In case of anatase, the effect of faces is small compared to obtained
results with rutile, probably because the difference of the energy levels between crystal faces
is not important.
While the efficiency of each crystal forms is still a controversial subject, in many
publications it could be seen that authors focused on the interactions between two phases:
anatase and rutile. Some reports recorded better degradation efficiency of Evonik P25 TiO2
(mixture 70% anatase, 30% rutile) than the other forms in the degradation process of certain
38
between the both phases and concluded that this effect is not the origin of the higher
efficiency compared to pure phases. So, crystal phase and its effect on catalytic activity are
very interesting points of view and becomes one of the most important factors to explain the catalytic behavior.
1.2 Metals loading on TiO2
In order to improve the photocatalytic activity of TiO2, a lot of researches focused on
the loading of metals on this catalyst using different deposition method such as precipitation,
doping, impregnation or photodeposition with transition metals 33.
The reason to choose transition metals such as platinum (Pt), gold (Au), palladium
(Pd), rhodium (Rh), nickel (Ni), copper (Cu) and silver (Ag) to deposit on the surface of TiO2
is that these compounds play a role as a shuttle and keep photogenerated electrons at the
acceptor 34 35 36 37 38. Based on this argument, different studies have proved that with the
presence of metal ion or metal deposited on semiconductors the Fermi Level will be shifted to
significantly negative energy potentials (figure I.4). This shift improves the interfacial charge
transfer process efficiency and narrow the semiconductor energy band gap 39. As the Fermi
Level of noble metals is normally lower than the level of TiO2, electrons formed by exciting
TiO2 under UV jumped from the conduction band (CB) of TiO2 to the noble metal particles
while there is no change of the valence band (VB) of the photogenerated holes. This will
39
Figure I.4. Photocatalytic activation scheme of TiO2 (a) and TiO2 sentisized by noble metals
(b). In which: Ec – conduction band energy; Ev – valence band energy; Ef, m – Fermi level
energy of metal; Ef,s – Fermi level energy of semiconductor; Ef, m-s – Fermi level energy of
semiconductor with deposited metal.
Considering that M/TiO2 particles have advantages compared to pure TiO2, they were
used for photodecomposition of organic polluting materials 10153940 and for energy 17343741
42 (figure I.5).
Figure I.5. Photocatalytic cycle of the mechanism of H2 production from alcohols 40.
As mentioned above, when noble metal is present on the TiO2 surface then it provides
a site for the more efficient reduction of protons 43. It is well known that in case of platinum
40
photocatalytic acitivity of titanium dioxide would be improved via the formation of a
phenomena so-called Schottky barrier which limits the recombination of photogenerated e-/h+
pairs at the TiO2/Pt interface. Also, the interfacial charge transfer and the efficiency of the
photocatalytic reaction to produce H2 are enhanced 33 41 44 42 45. Nevertheless, the substrate
used 46, surface conditions, particle size, the dispersion of these particles as well as on the
preparation method to deposit Pt 47 48 and platinum loading 33 have significant influences on
photocatalytic activity. For example, Emillio et al. 33 studied the efficiency of some platinized
TiO2 and found that with the loading of Pt from 0.5% to 1%, the optimum amount of Pt for
the degradation of nitrolotriacetic acid (NTA) was 0.5%. They also found that the efficiency is more dependent on the nature of the substrate to be degraded than on the physicochemical properties of the samples while they applied platinized P25, UV100 and PC50 in the degradation of ethylene diamine tetraacetic acid (EDTA) and NTA 3346. In addition, with two common preparation methods: thermal impregnation and photodeposition, most of the reported works showed the latter technique gave more active photocatalyst 4950.
Gold (Au) also decreases the band gap energy of TiO2 to a value less than the
common band gap energies of titania in the bulk anatase and rutile form 51. However, it
should be noted that decreasing in the band gap is related to various parameters, particularly
the noble metals/TiO2 nanocomposite synthesis method 52.
The enhancement of photocatalytic activity of TiO2 supported Au-NPs can be
attributed to the different Fermi levels on TiO2 53 54 55 56. It should be, however, mentioned
that their effective role in photocatalytic oxidative reactions is still a controversial topic,
especially in the case of Au/TiO2 575859. Recent studies evidenced that the properties of such
metal/oxides composites depend on the preparation method, particularly on the conditions of
gold deposition, on gold loading, size and shape of the gold particle and also on storage
conditions 60 616263. For example, several researchers found that the smaller size of gold was
41
degradation of oxalic acid in liquid phase 65, or for the reduction of C60 66. However, very rare
work followed the disappearance of the scavenger and in those examples, only Iliev et al. 65
focused on the disappearance of oxalic acid on liquid phase. Besides, the gold loading as well
as the dispersion of gold on TiO2 surface are crucial. There are authors suggested that from
the photocatalytic point of view, small semiconducting gold nanoparticles up to a certain
surface coverage should be the most active Au/TiO2 photocatalyst 6768 that can minimize the
agglomeration.
Another interesting property of Au nanoparticles (Au-NPs) is the presence of a visible
band at around 560 nm named as surface plasmon band (SPB) due to the collective excitation
of electrons confined in the metal NPs 69 70 71 72. The presence of Au on TiO2 can serve to
introduce visible light photoresponse in the Au-modified TiO2.
It is noteworthy that the deposition of Au can even decrease the photocatalytic activity
of TiO2, in some reactions, such as photo-oxidation of cyclohexane 73. In this reaction,
titania’s surface hydroxyl groups should generate hydroxyl radicals, but after the gold
deposition, coverage of these groups by Au restrict the formation of hydroxyl groups 73.
Over the years, the use of silver deposited on semiconductors in photocatalytic process
have much interest (for example in degradation of organic pollutants, hydrogen production,
disinfection) because it can enhance the photocatalytic activity and extend the light absorption
to visible light region 74 75 76 77 78. Silver, like other noble metals, acts as a trap of
photogenerated electrons and as a result improves the charge carrier separation 79. Moreover,
silver nanoparticles can absorb visible light due to localized surface plasmon resonance 80,
leading to new applications such as antibacterial textiles, medical devices, food preparation
surfaces as well as air conditioning filters and coated sanitary wares 81.
However, the presence of metal ion in the TiO2 matrix has direct impact on the
42
83. Severals publications showed that noble metal ion could be easily reduced at irradiated
TiO2 nanoparticles 84 85 86. If the fraction of metal ions is inserted in TiO2 matrix,
photogenerated electrons will reduce the free metal ions. In this case, the metal ions exist in
the interface of TiO2 and metal could become recombination centers that decrease the lifetime
of (e-, h+) pairs resulting in a negative effect on photocatalytic activity 3828788.
Recently, promising alternative energy type such as biogas and others valuable
hydrocarbons, which derives from photocatalytic reactions, attracted a lot of attentions 89. So,
the utilization of metal loading on TiO2 to overcome the disadvantages of TiO2 itself and
applies in crucial aspect such as water/air purification or energy production, is a really need of
photocatalysis research.
1.3 Photocatalyst support
The solutions to minimize the aggregation of photocatalyst particles as well as
maximizing their adsorption ability are the most important mission for researchers,
particularly in industrial applications. In this case, the utilization of support is taken into
account. Activated carbon has been considered as a good support for TiO2 in
photodegradation of volatile organic compounds in gas phase 90 91 92 93 94. As explained,
activated carbon facilitates the adsorption of VOCs, lowers the competition of water and
pollutants in adsorption as well as enhances the quantity of pollutants contacting with TiO2 95 96. However, Thevenet et al. 97 showed that TiO
2 on activated carbon only increased slightly
the acetylene disappearance rate but reduced the acetylene mineralization because of
significant adsorption of acetylene on activated carbon. In another work, Yoneyama et al. 98
announced that support would slower the diffusion rate of adsorbed pollutants to the catalysts
surface and hence decrease the degradation rate. Also, Lillo-Rodenas et al. 90 and Bouazza et
al. 99 focused on the photocatalytic activity of P25 combined with various carbonaceous
materials (such as activated carbon, carbon nanotubes, and carbon nanofibers) and white
43
the bare TiO2 performed better efficiency than the others. Although their justifications were
unclear, they proposed that pure TiO2 could be mixed with the support in preparation steps.
Besides, Mo et al. 9 used SEM images to examine the coating of P25 on SiO2 and mordernite.
They found that TiO2 was adsorbed on the surface of mordenite and SiO2, which increased
the reaction area of TiO2. As a result, the removal efficiency of toluene by using SiO2 and
mordernite was higher than P25.
Lately, the interest in optical fibers and luminous textile rose up for better light
utilization and mass transportation. The first idea of using optical fibers was proposed by
Marinangeli and Ollis 100101 but the very first experimental research was carried out on 1994
for the water treatment of 4-chlorophenol by Hofstadler et al. 102. In our group, several works
on TiO2/optical fibers started in 2001 by Danion et al. 103 104. In these studies, the authors
showed the effect of fiber diameter, the thickness of coating TiO2 layer as well as the length
of this layer. Also, the number of fibers used and the optimization of an experimental design
were proposed. After that, in 2012, Bourgeois et al. 105 implemented a research on removing
formaldehyde from indoor air using luminous photocatalytic textile (optical fibers are
wrapped with normal fibers) and proved that it was efficient in this type of reaction. Based
on the positive results obtained with optical fibers, in the industrial point of view, this type of
photocatalyst could be the promising material for different objective including environmental
treatment and energy production.
2 Photocatalysis
Photocatalysis has been attracted a lot of attentions in the last decades. Photocatalytic
reactions may occur homogeneously or heterogeneously. Both homogeneous and
heterogeneous photocatalysis are promising technologies but heterogeneous photocatalysis
has been more extensively studied in recent years because it could be used flexibly in order to
44
2.1 Photocatalytic mechanism
In presence or absence of O2, the first step of the photocatalytic mechanism is the
absorption of photons by the semiconductors. The energy of the photon, which is superior or
equal to the energy of the band gap, will excite the catalyst to generate (e-, h+) pairs.
TiO2 + hv à h++ e-
Classically, in the presence of O2, photogenerated electrons (e-) trapped on surface or
next-to-surface defects can react with the adsorbed oxygen 106 to form O
2-.
(O2)ads + e- à O2 -
Then, in order to form the hydroxyl radicals, holes can react with OH- or H2O:
OH- + h+ àOH
H2O + h+ à H++ OH
Several reactions can occur after as listed below. Most of them, however, are very
slow: O2 - + H+ à HO2 pKA=4.85 ± 0.1 2HO2 à H2O2 +O2 k = 7.5x105 mol-1Ls-1 O2 - + HO2 à HO2- +O2 k = 8.5x107 mol-1Ls-1 2 O2 - + H2O à HO2- + O2 + OH- k = 102 mol-1Ls-1 2 O2 - à O22- + O2 k = 3.5x10-1 mol-1Ls-1 O2 - + H2O2 à OH + O2 + OH- O2- + OH à O2 + OH- k = 2x109 mol-1Ls-1
45 OH + HO 2 à O2 + H2O H2O2 + OH à H2O + HO2 k = 2.7x107 mol-1Ls-1 H2O2 + e- à OH + OH -H2O2 + 2h+ à O2 + 2H+ H2O2 + 2e- + 2H+ à 2H2O O2 +2H+ + 2e- à H2O2 2H2O + 2h+ à H2O2 + 2H+
Moreover, the ion O
2
- and OH play roles as oxidizing species that can transform the
initial products into aldehydes/ ketones or alcohols into carboxylic acid 106.
RH + O 2- à RO + OH -RH + OH à R+ H 2O R + OH à ROH
ROH + h+ à degradation products
However, if the mobile electron recombines with hole then the photocatalytic reaction
ends and energy will be released in form of heat.
h+ + e-àTiO2 + heat
In absence of O2, there is no O2 to react with e- to form oxidizing specie O2 -, the fate
of e- in this case is still not revealed. Generally, the mechanisms without the participation of
O2 are described below:
46
Then:
OH- + h+ àOH
H2O + h+ à H++ OH
From this point, Mozia et al. 89 suggested the formation of photogenerated oxygen to
continue photocatalytic reactions
2OH à H 2O2 H2O2 + 2h+ à O2 + 2H+ H2O2 + OH à H2O + HO2 HO2 + OH à H2O + O2 HO2 + H+ + e- à H2O2 2H2O2 à 2H2O + O2
In oxygen-free atmosphere, Muggli et al. 107108 proposed the participation O2 lattice,
reacted with e- and played a role of oxidizing specie.
e- can also react with H+109110:
H+ + e- à H
but this is not the only pathway and one of our most important targets is studying the role of e
-in these reactions.
2.2 Effects of different parameters on photocatalytic process
There are plenty of factors that have significant effects on photocatalytic efficiency.
Normally, they could be divided into two groups: internal and external parameters. In the first
47
chemical aspects. The latter one consists of extrinsic factors including operating conditions,
which are confirmed as the crucial parameters, particularly in gas phase. In this part, extrinsic
factors will be discussed.
2.2.1 Pollutants initial concentrations
At low concentrations of pollutants, the disappearance rate of pollutants increases
linearly with their concentrations due to the low coverage of the surface of the catalyst. The
kinetic followed the first order law and is independent on pollutant concentration 111112. Then,
if working at higher concentration of pollutants, there will be a saturation of the surface and a
zero order kinetic is observed. These phenomenons are due to the adsorption on the active
sites of the catalyst. In summary:
- At very low concentrations, the number of active sites on TiO2 does not limit
photocatalytic degradation and there is no competition of the by-products and main pollutant to adsorb on the active sites 111. It is important to note that if the initial concentration is too low, the conversion will be about 100% and the comparison of the efficiency of different
materials is not possible 113
.
- Langmuir-Hinshelwood (L-H) model has been widely applied when considering that
intermediates do not have any impact and no limit occurs with the mass transfer 114115.
Another crucial factor needs to be discussed is residence time. This time is the
amount of time that a flow spends inside the reactor. By changing the gas flow rate, residence
time will change and there would be an effect on conversion of pollutants but also on
mineralization. At high flow rate, the reactants have a shorter residence time for being
adsorbed on the surface of the catalyst. As a result, the contact of these molecules with active
sites is limited. The residence time will be decreased in the photocatalytic reactor when
increasing the gas flow rate. Also, the mass transfer rate will record an improvement. Base on
48
rate showed that the residence time of pollutant molecules with TiO2 is an important factor.
Namely, the increase of the conversion with residence time was observed in various papers 116
117118.
2.2.2 Light sources
The photocatalytic reaction is affected by the light source used, particularly the
wavelength of the irradiation due to the absorption spectra of the material 119. Moreover, the
suitable wavelength would change depending on the crystalline phase of photocatalyst
(anatase or rutile phase), elemental composition and photocatalyst doping. For example, in the
case of Evonik P25 TiO2, which has the ratio of anatase and rutile at 80/20, a light at the
wavelength smaller than 380 nm is enough for photoexcitation. The rutile TiO2 has a smaller
bandgap energy compared to the anatase TiO2 (3.0eV instead of 3.2eV). This means that a
light wavelength until 413nm would also has a potential to activate rutile TiO2.
For UV irradiation, it can be classified as UV-A (315 to 400nm, 3.10 - 3.94eV),
while UV-B has a wavelength range from 280 to 315nm (3.94 - 4.43eV) and UV-C ranges
from 100 to 280nm (4.43 - 12.4eV). In the literature it could be found papers mentioned that
using a shorter wavelength is more effective for photocatalytic degradation and in this case
the quantity for catalyst required is considerably smaller than applying higher wavelength 120.
This behavior is explained considering the Planck’s equation.
The energy of photon = E = h c /λ
Where λ is the wavelength of the light; h is Planck's constant = 6.626 × 10-34joule.s; c is the
speed of light = 2.998 × 108m/s.
Actually, the wavelength is inversely proportional to the energy; the higher the energy, the
49
2.2.3 UV intensity
Intensity of the irradiation source has a crucial effect on the rate of photocatalytic
reaction. If high photocatalytic reaction rate is needed, a relatively important light intensity is
required 121122. The rate of the photocatalytic reaction increases when the irradiation intensity
increases. In fact, on the TiO2 surface, there are more (e-, h+) pairs which would be generated
when higher irradiance is applied and more hydroxyl radicals are produced 123 124. This
phenomenon, however, does not last without limitation. At high irradiation intensity, the
reaction rate becomes independent of light intensity, because the balance between the
generation rate (e-, h+) and the recombination rate of electron-hole would be reached 125.
Previous investigations have shown that the degradation rate is proportional to the light
intensity. However, if the intensity is superior to a certain value, the reaction rate is
proportional to the square root of the light intensity 126 127 and then become independent on
the intensity 128.
The quantum yield (Ф) is the ratio of the quantity of molecules decomposed and the
number of photons absorbed by the catalyst (Na): Ф = Δn / Na
In which: Δn is the number of molecules degraded, Na is the number of photon adsorbed by
the catalyst and related to the power of the lamp, P and energy per photon, E. Na would be
derived from Planck’s equation as follows:
Na = PE = Phc/λ = IAhc/λ
where I is the irradiance in Watt per unit area (W/m2)and A is surface area of the reactor by
considering that all the photons are absorbed by the catalyst.
2.2.4 Humidity
50
of the pollutant species, the process exterior parameters and one of the most important factors
– water concentration. The role of water in photocatalytic degradation process depends on the
concentration and the nature of the pollutants as well as the percentage of humidity 9 111 114
129. From different authors the following conclusions can be extracted:
- There is a competition between water and pollutants as well as oxygen molecules to
be adsorbed on the active sites of the surface. Indeed, at low concentration of water, it is often
observed an increase of the catalytic efficiency due to the •OH formation. On the other hand,
the presence of important amount of humidity, the adsorption of pollutants recorded a
significant decrease because of the competition of adsorption between water and pollutant 114
129130.
- The adsorption of water on the TiO2 surface causes a decrease in the upward bending
of the band, resulting in the efficient recombination of the photo-generated electrons and
holes 131.
- Humidity limits the poisoning of the TiO2 surface because it could oxidize a large
part of intermediates staying on the surface.
In literature, it could be easy to find papers studied the effect of humidity on the
conversion of the pollutants, but the influence on the mineralization process as well as the
nature of the other products and the role in their formations has been rarely reported 129.
3 Carboxylic acids and photocatalytic degradation of carboxylic acids
3.1 Carboxylic acids in our environment
Carboxylic acids are an interesting class of compound in atmospheric chemistry
because they are products of oxidation of various organic trace substances. Major sources of
short-chain organic acids in air are:
- Ozone – olefin reactions
