Visible light induced catalytic
sulfoxidation of alkanes
Den Naturwissenschaftlichen Fakultäten der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades
vorgelegt von Ayyappan Ramakrishnan aus Karaikal (Pondicherry), Indien
Als Dissertation genehmigt von den Naturwissenschaftlichen Fakultäten der Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 27.07.2006
Vorsitzender der Promotionskommission: Prof. Dr. D. -P.Hädler
Erstberichterstatter: Prof. Dr. H. Kisch
Zweitberichterstatter: Prof. Dr. U. Zenneck Die vorliegende Arbeit wurde von Mai 2002 bis Mai 2006 am Institut für Anorganische Chemie der Friedrich-Alexander Universität Erlangen-Nürnberg unter Anleitung von Herrn Prof. Dr. Horst Kisch durchgeführt.
I express my deep gratitude and thanks to my doctoral father, Prof. Dr. Horst Kisch for offering me an interesting research topic and meticulous guidance through out my research with helpful suggestions by his deep insight in the field. I also thank him for his solid hospitality and kindness through out my stay here in Erlangen.
I sincerely thank: Prof. U. Nickel for examining me for my physical chemistry exam related to my qualification for doctoral degree and unstinted support. Prof. U. Zenneck for examining me for my inorganic chemistry exam related to my qualification for doctoral degree and kindness. Dr. S. Sakthivel, my brother and his family for their continued strong support and several favours. Dr. Marc Gärtner, a good and valuable friend and a teacher of mine who always gave me a helping hand especially in chemistry and computers. Dr. G. Burgeth, Dr. W. Macyk, for their help in introducing me in various important experiments and discussions in settling my research problems. Dr. M. Moll and M. Clemens, for their assistance during NMR and HPLC measurements. C. Wronna for elemental analyses. Mr. P. Widlok for specific surface area measurements. Dr. F. W. Heinemann and Mr. P. Bakatselos for X-ray crystal structure determinations Mr. M. Bachüller for mass spectroscopy. Dr. J. Sutter for computer assistance. Mr. Uwe Reißer for electrical assistance. Mr. David Wunderlich, my dear friend who worked with me in some topics, for his close friendship and regular discussions regarding the work. Dr. R. Prakash, Dr. S. Shaban for all sorts of help and friendship. Mr. Radim Beranek and Mr. Joachim Eberl who were always helpful and friendly co-workers. Mr. M. Hausmann, Mr. W. Florian, and Mr. S. Sebastian, who were the students of this university, who worked with me in some topics.
All present and former co workers of Prof.Kisch and many other people from the Institute helping me in my research directly or indirectly.
Mrs. R. Jayanthi, Dr. V. Ramalingam, Dr. M. Palanichamy, excellent and brilliant teachers of chemistry at my various levels, who had introduced interest and fascination towards chemistry through their deep knowledge in this science, excellent teaching skills and kindness.
I thank my brother Eugenio and sister Shobana’ from Spain for their love and prayers. I also thank my parents, my brother Sivasundar Ramakrishnan and friends for their support.
Most of all, I thank God for everything.
Dedicated to
My Supreme Guru Ramalinga (Vallalar) and my loving parents
"A human being is a part of a whole, called by us universe, a part limited in time and space. He experiences himself, his thoughts and feelings as something separated from the rest... a kind of optical delusion of his consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty." Albert Einstein
1
TABLE OF CONTENTS
ABBREVIATIONS...... 5
1 INTRODUCTION...... 7
1.1 PHOTOCHEMICAL SULFOXIDATION...... 7 1.1.1 Introduction...... 7 1.1.2 Industrial importance of photosulfoxidation...... 7 1.1.3 History of sulfoxidation...... 8
1.2 MECHANISM OF SULFOXIDATION ...... 10 1.2.1 Photochemistry of sulfur dioxide...... 13 1.2.2 Secondary reactions in sulfoxidation ...... 14 1.2.3 Initiators, promotors, and inhibitors of sulfoxidation...... 15 1.2.3.1 Initiators ...... 15 1.2.3.2 Promotors...... 16 1.2.3.3 Inhibitors...... 17 1.2.3.4 Product composition of sulfoxidation of alkanes...... 18
1.3 VARIOUS TYPES OF SULFOXIDATION TECHNIQUES...... 18 1.3.1 Sulfoxidation in the presence of water (Light water process)...... 18 1.3.1.1 Method of operation...... 19 1.3.1.2 Process procedure...... 20 1.3.1.3 Separation of alkanesulfonates: ...... 20 1.3.1.3.1 Thermal separation...... 21 1.3.1.3.2 Solvent extraction...... 22 1.3.2 Sulfoxidation in the absence of water ...... 22 1.3.3 Goal of this work...... 23
2 VISIBLE LIGHT SULFOXIDATION...... 25
2.1 INTRODUCTION ...... 25
2.1.1 Unmodified TiO2 ...... 25
2.1.2 Metal Complex Modified TiO2 ...... 28 2.1.2.1 Desorption experiments ...... 30 2.1.2.2 Photostability ...... 31 2.1.2.3 Characterization techniques...... 32 2.1.2.3.1 Diffuse Reflectance Spectroscopy (DRS)...... 32 2
2.1.2.4 Quasi-Fermi level measurements: ...... 36
2.2 RESULTS AND DISCUSSION...... 40
2.2.1 Preparation of metal complex modified TiO2 ...... 40 2.2.2 Characterisation...... 41 2.2.2.1 Diffuse Reflectance Spectroscopy ...... 41 2.2.2.2 Photoelectrochemical properties...... 45 2.2.2.3 TEM, XRD, and BET surface area measurements: ...... 47 2.2.3 Photocatalytic properties ...... 51 2.2.3.1 4-chlorophenol degradation ...... 51 2.2.3.2 Kinetics ...... 52
2.2.3.3 General mechanism of action of TiO2 on organic pollutants...... 57
2.2.3.3.1 Mechanism of visible light degradation of 4-CP by 4%H2[PtCl6]/TH .59 2.2.3.3.2 Proposed mechanism...... 61 2.2.3.4 Visible light sulfoxidation of adamantane ...... 64 2.2.3.4.1 HPLC with Indirect photometric detection ...... 66 2.2.3.4.2 Principle ...... 67 2.2.3.4.3 Influencing factors for IPD ...... 70 2.2.3.4.4 Analysis by IPD with HPLC ...... 72 2.2.3.4.5 Isolation of 1-adamantanesulfonic acid...... 72 2.2.4 Results of adamantane sulfoxidation in methanol...... 74 2.2.5 Sulfoxidation of other alkanes...... 77 2.2.6 Mechanism of visible light sulfoxidation of adamantane in methanol by
4%[H2PtCl6]/TH ...... 78 2.2.7 Influence of metal complexing agents in visible light sulfoxidation ...... 81 2.2.7.1 Acetylacetone...... 81 2.2.7.2 Other complexing agents ...... 86 2.2.8 Mechanistic investigations for visible light sulfoxidation in the presence of acetylacetone...... 87 2.2.9 Mechanism of visible light sulfoxidation of adamantane in the presence of
acetylacetone by metal complex modified and unmodified TiO2 in methanol ..90 2.2.10 Experiments in acetic acid ...... 91 2.2.11 Mechanism of visible light sulfoxidation of adamantane in acetic acid...... 99
3 EXPERIMENTAL SECTION ...... 100 3
3.1 MATERIALS...... 100
3.2 SPECTROSCOPIC AND ANALYTICAL MEASUREMENTS...... 100 3.2.1 UV- vis spectroscopy...... 100 3.2.2 Diffuse Reflectance Spectroscopy ...... 100 3.2.3 NMR ...... 100 3.2.4 IR ...... 101 3.2.5 Mass spectroscopy...... 101 3.2.6 XRD ...... 101 3.2.7 BET...... 101 3.2.8 TEM...... 101 3.2.9 TOC ...... 101 3.2.10 Elemental Analysis ...... 101 3.2.11 HPLC...... 101 3.2.11.1 Analysis of 4-CP...... 101 3.2.11.2 Analysis of sulfonic acids...... 102
3.3 PREPARATION OF CATALYSTS ...... 102 3.3.1 Preparation of metal complex modified titania...... 102 3.3.2 Preparation of amorphous titania...... 102 3.3.3 Preparation of anatase titania (self prepared) ...... 102 3.3.4 Preparation of acetylacetone modified titania...... 103
3.4 VISIBLE LIGHT DEGRADATION EXPERIMENTS ...... 103 3.4.1 Degradation of 4-CP...... 103 3.4.2 Degradation of HCOOH ...... 105
3.5 PHOTOELECTROCHEMICAL MEASUREMENTS ...... 105
3.6 VISIBLE LIGHT SULFOXIDATION EXPERIMENTS...... 106 3.6.1 Photosulfoxidation procedure...... 106 3.6.2 Isolation of 1-adamantanesulfonic acid...... 107
3.7 CHARACTERIZATION OF THE ISOLATED 1-ADAMANTANESULFONIC ACID ...... 107 3.7.1 EA...... 107 3.7.2 IR ...... 108 3.7.3 13C NMR ...... 109 3.7.4 Mass spectra...... 109 3.7.5 Analysis by IPD with HPLC...... 110 3.7.6 Visible light sulfoxidation of n-heptane ...... 115 4
3.7.6.1 Isolation of sodiumheptanesulfonate in the presence of water ...... 119 3.7.6.2 Isolation of sodiumheptanesulfonate in the absence of water ...... 120
4 SUMMARY...... 122
5 ZUSAMMENFASSUNG ...... 133
6 REFERENCES...... 144
5
ABBREVIATIONS A acceptor abs. absorbance a.u. arbitrary units BET specific surface measurements according to Brunauer- Emmett-Teller theory CB conduction band 4-CP 4-chlorophenol D donor DRS diffuse reflectance spectroscopy E redox potential E energy
Ebg bandgap energy
EF Fermi level potential
F(R∞) Kubelka-Munk function FWHM full-width half maximum h+ hole in valence band Hacac acetylacetone HPLC high performance liquid chromatography IPC indirect photometric chromatography IPD indirect photometric detection I light intensity
Io incident light intensity
IA absorbed light intensity IFET interfacial electron transfer k rate constant ka apparent rate constant kmax / kmin momentum vectors of electrons
Kad adsorption rate constant L ligand LABS linear alkyl benzene sulfonates 6
λ wavelength LF ligand field LMCT ligand to metal charge transfer MLCT metal to ligand charge transfer MV2+ methyl viologen, 1,1’-dimethyl-4,4’-bipyridinium ion ε molar absorptivity n number of electrons NHE normal hydrogen electrode
P25 commercial name of TiO2 produced by Degussa * nEF quasi-Fermi level of electrons * pEF quasi-Fermi level of holes R diffuse reflectivity
Rt retention time S scattering coefficient SAS straight chain alkanesulfonates SAX strong anion exchanger SCE saturated calomel electrode SEM scanning electron microscopy
TH Titanhydrat-O, commercial TiO2 produced by Kerr-McGee TEM transmission electron micrograph TOC total organic content TON turnover number
τ 0 life time of the first excited electronic state U voltage UV ultraviolet VB valence band vis visible WAS wash active sulfonates XRD X-ray diffractogram
7
1 INTRODUCTION
1.1 Photochemical sulfoxidation 1.1.1 Introduction Photochemical sulfoxidation may be defined as the light induced reaction of alkanes or cycloalkanes with a mixture of sulfur dioxide and oxygen forming sulfonic acids (Eq. 1.1).[1]
1 hν RH + SO + O ⎯⎯→ RSO H (1.1) 2 2 2 3
Generally, photosulfoxidation refers to UV light induced sulfoxidation where SO2 is the ultraviolet light absorbing species. In industry 10 - 40 kW mercury lamps are used as the source of UV light.[2] This novel reaction was discovered in Germany by C. Platz of “IG Farben” in 1940.[1] Together with sulfochlorination and photochlorination this is one of the first photochemical reactions which have been developed on an industrial scale.[3, 4] Industrial scale sulfoxidation for preparation of alkanesulfonates was successfully developed by the German company Hoechst in the late 1940s.[3-5] Alkanesulfonates which are obtained by the photosulfoxidation are applied as effective surfactants, good wetting agents and emulsifiers.[2, 6]
1.1.2 Industrial importance of photosulfoxidation Alkanesulfonates have achieved greater significance as active detergent substances (WAS = wash active alkanesulfonates). So, these sorts of reactions producing straight chain alkanesulfonates (SAS) have growing importance owing to the increase in the demand for detergents.[7] Furthermore, the sulfonation of saturated aliphatic hydrocarbons is not possible by the current industrial method employing concentrated H2SO4 (oleum) for manufacturing the widely used surfactants, linear alkylbenzene sulfonates (LABS). The reason is the inertness of the alkanes and the significant lower solubility of the sulfonating agent (H2SO4) in the 8 alkanes, and additionally, the thermal decomposition of SAS under these reaction conditions. Another importance of photosulfoxidation is that the SAS produced by this reaction have significant advantages over LABS. They are as follows: SAS fulfil biodegradable criteria better than LABS. Though the detergent action is comparable, the better solubility of SAS in water is the reason for their preference in the liquid formulation of cleansing agents and detergents. Furthermore, the raw materials for SAS, alkanes with a given chain lengths, are available in cheaper rates due to the more economical techniques for separation and purification using molecular sieves like zeolites.[4]
Inspite of all these advantages, still the production of LABS is more economical as compared to that of SAS. More innovation and advancement in industrial photosulfoxidation process, development of novel catalysts with very high Turnover Numbers (TON), combined with the present soaring costs of petroleum products can in further years make the photosulfoxidation process equally or more economical than the sulfonation of alkylbenzenes. Moreover, this reaction falls under the category of C-H bond activation of unreactive alkanes, which is a field of great current interest.[8] Additionally this process utilizes the abundantly available alkanes which are generally inert.
1.1.3 History of sulfoxidation.
Year Area of research Details 1940 Discovery of Platz, (IG Farben industry, Germany) discovered Photosulfoxidation and patented this industrially important reaction.[1] 1950 Industrial Schimmelschmidt[9, 10] and Orthner[6] performed applications - Light- sulfoxidation of various alkanes, by the light- water water process process where water acts as a reactant as well as solvent to extract the alkanesulfonates. Schimmelschmidt had also contributed to the isolation of higher-molecular sulfonic acids from the reaction mixture and also for separating them from 9
the water insoluble constituents.[11]
Orthner also estimated the quantum yield of sulfoxidation of mepasine (a hydrocarbon obtained by the catalytic hydrogenation of hydrocarbon mixture obtained by Fischer-Tropsch process) in the presence of water to be 7-8.[6]
Asinger contributed significantly to the sulfoxidation of higher alkanes like dodecane and has deciphered the composition of the sulfoxidation mixture. He found an equimolar mixture of all theoretically possible isomers, excluding the terminal primary sulfonic acid which is formed in a lesser amount. The reason is the lower reaction rate of the H atoms in the CH3 groups as compared to the H atoms to the CH2 groups. This reaction has a great industrial value because only sulfonates of higher alkanes have detergent properties.[12] Asinger has also found that the relative reactivities of various C-H bonds in n-heptane also follows a similar trend like that of dodecane. Further Asinger has studied the substitutional properties of hydrocarbons and also contributed to the technological aspects of sulfoxidising alkanes.[4, 13]
Photosulfoxidation was made commercially successful on an industrial scale by the company “Hoechst” 1952 Mechanism of The mechanism of UV sulfoxidation is a free radical sulfoxidation type where SO2 absorbs light and initiates the sulfoxidaion. Graf is credited for clarifying the mechanism of sulfoxidation by the detailed studies on sulfoxidation of several alkanes.[9, 14] 1961 UV sulfoxidation of Adamantane was photosulfoxidised in the presence adamantane of H2O2 as a radical initiator at 70 °C affording 1-adamantanesulfonic acid monohydrate by Smith.[15] The yield in this UV light induced reaction was 15%. 1965 UV sulfoxidaton of The UV photosulfoxidation of n-hexane in the n-hexane presence of acetic anhydride as promoter was carried out by Ogata and the yield was found to be 26%.[16]
Ogata has also studied the relative reactivities of different C-H bonds in the photosulfoxidation of 10
n-hexane. The products, isomers of hexanesulfonates were converted to respective sulfonylchlorides by a similar method reported by Kirkland[17] and were analysed by gas chromatography. Ogata had reported that the relative reactivities of C1, C2, and C3 were 1:(0.8-1.3):(3.2-6.2) respectively. The SO2 / O2 ratio also had a strong influence on the reactivity of C3. Temperature’s role was also crucial as its increase, lowered the relative activities of the three bonds. The least activity of C3 in all of the three C-H bonds may be attributed to the intermediary six membered ring formed in its case alone.[18]
Further Ogata had also attempted photosulfoxidation of alkylbenzenes and had found that the yield in this case is lower than that with hydrocarbons like hexane or decane. The reason suggested is the inner filter effect of alkylbenzenes.[19] 1975 Industrial Boy developed a new solvent - extraction technique separation for isolation of the alkanesulfonates.[20] This process of alkanesulfonates technology employs treatment of sulfoxidation mixtures with weakly polar solvents such as CH3COC3H7, C2H5OC3H7 or [(CH3)2CH]2O to extract the sulfonates, followed by the separation of the solvent layer, neutralization with NaOH solution and evaporation to remove water, solvent and alkanes. 1991 Mercury Mercury photosensitized sulfination of alkanes with photosensitized sulfur dioxide produces sulfinic acids (RSO2H) and sulfination sulfinic esters which can be further oxidized easily to sulfonic acids with around 80% yield. This was achieved by Crabtree.[21] 2000 Thermal It was found by Ishii[22] that bis(acetylacetonato)- sulfoxidation of oxovanadium(IV) catalyses the transformation of alkanes. adamantane to 1-adamantanesulfonic acid at 40 °C and normal pressure. The selectivity of the reaction is 98% and the conversion was 43%.
1.2 Mechanism of sulfoxidation
The absorption spectrum of SO2 in n-hexane or isooctane (Figure 1.1) shows a maximum absorption at 290 nm which is due to an n → п* -1 -1 [16] transition (ε = 250 M cm ). Absorption of UV light by SO2 populates via 11 intersystem crossing the triplet state which abstracts hydrogen from the hydrocarbon producing an alkyl radical (Scheme 1.1, Eqs. 1.2, 1.3). An alternative C-H bond cleavage mechanism by energy transfer is unlikely since the energy of the first excited -1 singlet state of SO2 is less than 380 kJ mol , whereas a C-H bond dissociation requires about 400 kJ mol.-1
[23] Figure 1.1: Absorption spectrum of SO2 at 25 °C measured by McMillan.
12
ISC
1 RH SO2
3SO 2 • • R +HSO2
SO2
RSO3H SO 2
Scheme 1.1: Photosulfoxidation of alkanes where SO2 absorbs UV light and is excited 3 to SO2 and drives the reaction.
hν 3 SO2 ⎯⎯→ SO2 (1.2)
3 • • SO2 + R − H → R + HSO2 (1.3)
• • R + SO2 → RSO2 (1.4)
• • RSO2 + O2 → RSO2 − O − O (1.5)
• • RSO2 − O − O + R − H → RSO2 − O − O − H + R (1.6)
• • RSO2 − O − OH → RSO2 − O + OH (1.7)
• • RSO2 − O + R − H → RSO3 H + R (1.8)
RSO2 − O − O − H + SO2 + H 2O → RSO3 H + H 2 SO4 (1.9)
• • OH + RH → R + H 2O (1.10)
13
Subsequent radical addition reactions with sulfur dioxide and oxygen (Eqs. 1.4, 1.5) generate an alkylpersulfonyl radical which produces another alkyl starter radical and a persulfonic acid (Eq.1.6). Fragmentation of the latter and a hydrogen abstraction (Eqs. 1.7, 1.8) afford the alkanesulfonic acid. The sulfonic acid can also be produced through reductive hydrolysis (Eq. 1.9). Water necessary for this reaction step is probably formed according to Eq. 1.10. According to this mechanism, photosulfoxidation is a photoinduced chain reaction and therefore should proceed without further irradiation. This is true only in the case of lower alkanes (< C10) devoid of impurities.[24] However, long unbranched alkanes of insufficient purity require permanent irradiation, or addition of radical initiators or promotors like acetic or propanoic anhydrides. Based on the reaction conditions, esters, alcohols and traces of colored compounds are obtained as by-products.[2]
1.2.1 Photochemistry of sulfur dioxide
SO2 in the gas phase shows many electronic transitions in the spectral region of 180-390 nm. The transition to the first excited electronic state from the ground state starts already at 388 nm, as indicated by the very weak absorption not displayed in Figure 1. 1. But the transition to the second excited state from the ground state (n-π*) is much more intense. It starts at 337 nm with a maximum intensity at 294 nm. In n-hexane or isooctane maximum absorption is at 290 nm -1 -1 [16] (ε = 250 M cm ). A third, but less important absorption region of SO2 ranges from 240 to 180 nm. The nature of the excited states has been studied by Walsh[25] and [26] Mulliken. A detailed study of the fluorescence spectrum of SO2 reveals that the molecule fluoresces from all the three excited states. Nine strong transition bands of the second excited state (280-310 nm) can be found alone and the quenching of fluorescence takes place by a Stern-Volmer mechanism. In solid SO2 at 77° K, the phosphorescence life time of the first excited triplet state is 5 ± 1×10-4s. There is evidence for the triplet character of excited SO2 such as:
1. Observation of magetic field effect with SO2 excited in the first electronic state.
2. The radiative life time of the first excited state (τ 0 )found by integral absorption was between 1.3×10 -2 and 2.2×10-3 s suggesting a triplet state. 14
The photoreactions of pure SO2, SO2 / O2 and SO2 / hydrocarbons have been studied.
In pure SO2 at a wavelength of 313 nm, only S and SO3 are formed with quantum -2 efficiency of about 10 and in SO2 - O2 mixtures, only SO3 forms with about the same quantum yield as in the former case.
The chemistry of SO2 and hydrocarbons (RH) has been an area of much research. It was found by Dainton and Irvin[27] and later also verified by Calvert[28] that sulfinic acids (RSO2H) are formed by SO2 / RH reaction. The quantum efficiencies vary from 0.26 for pentane to 0.006 for methane. Crabtree[21] has also successfully conducted Hg-sensitized photosulfination followed by sulfonation.
Photochemistry of SO2 polluted atmospheres has also been studied. The disappearance of SO2 and hydrocarbons in the atmosphere has been an area of great environmental importance as the reaction pollutes the atmosphere. Regarding mechanistic investigations of these reactions, the involvement of the triplet state of SO2 is always favored. However, the mechanism is not very clear yet owing to its very high complexity as several factors in the atmosphere can play a role.[29, 30]
From the study of its photochemistry, it is evident that SO2 does not absorb light in the visible region (λ ≥ 400 nm). Only on UV irradiation it can absorb light and drive the sulfoxidation reaction.
1.2.2 Secondary reactions in sulfoxidation The alkylpersulfonic acids formed are not stable under sulfoxidation conditions, and decompose forming additional radicals (Eqs 1.6-1.8). Though sulfoxidation is a free radical type chain reaction, its quantum yield is greatly reduced by several secondary reactions. However, a larger amount of alkanepersulfonic acids is consumed by several secondary reactions, the typical one pointed out in Eq. 1.9, the key reaction which retards the chain reaction (Eq. 1.7.). The presence of sulfuric acid even in the case of very dry reactants confirms the formation of water in the reaction (Eq.1.10). It has been found [24] that with alkanes devoid of impurities, the decomposition of a part of the persulfonic acid is sufficient to compensate for the losses of radicals due to secondary reactions and termination 15 reactions. In this case sulfoxidation is autocatalytic and needs no further irradiation.
However with long unbranched alkanes (C10-C20), (which have industrial importance since only their sulfonic acids can be used as surfactants) and for those alkanes which lack sufficient purity, intermittent or continuous irradiation during the entire course of reaction is essential.
1.2.3 Initiators, promotors, and inhibitors of sulfoxidation 1.2.3.1 Initiators The most common initiators of sulfoxidation are peracids, organic peroxides, ozone and γ-radiation. Saturated linear chain peracids such as peracetic acid and its homologues, aromatic peracids or persulfonic acids[31] have been proved to be very good initiators of sulfoxidation. It is required that these initiators are added continuously during the reaction. This may be achieved by mixing them with the reactant gases. Organic peroxides are another important class of initators. However, their utilization demands a higher reaction temperature, which is more risky on the industrial scale. It has been found that the cyclohexanepersulfonyl peracetate which decomposes at around 70 °C initiates sulfoxidation effectively (Eq 1.11).[14] This type of initiator is generally formed in situ when acetic anhydride is added to the alkane during sulfoxidation.
∆ • • C6 H11 − SO2OO − CO − CH 3 ⎯⎯→ C6 H11 − SO2O + CH 3 − COO (1.11)
Ozone (O3) is also a well known initiator which can initiate sulfoxidation. O2 gas is first introduced to an ozonizer and further to SO2 and alkanes. The yield of sulfonic acid is proportional to the amount of ozone introduced. γ radiation as another approach to initiate sulfoxidation using Co-60 sources has several advantages such as: 1. Initiation of sulfoxidation without water and therefore simple and less expensive separation of alkanesulfonates (see section 1.3.2). 16
2. Non - deposition of products on the walls surrounding the radiation source. 3. Only a relatively low intensity power source is required.
The reaction continues for a certain period of time even after the irradiation is stopped. The disadvantage of this system is that the yield of di- and polysulfonic acids is very high, up to 40% of the total mixture of sulfonic acids [24] and can be avoided by intermittent irradiation. Chlorine in a concentration of 2-3 wt% is also an effective initiator.[4] The initiation is the same as in the case of sulfochlorination, i.e. the dissociation of Cl2 as given in the following equations.
• Cl2 → 2Cl (1.13) Cl • + RH → R • + HCl (1.14)
• • R + SO2 → RSO2 (1.4)
• • RSO2 + O2 → RSO2OO (1.5)
Azocompounds,[32] metalalkyls (dimethylzinc), or leadtetraacetate,[33] are also used as initiators. The efficiency of the initiators is based on the exact reaction conditions and may have a wide range, while the concentration of the initiators range from 0.03 to 5 wt%.
1.2.3.2 Promotors The common promoters are acetic or propanoic [4, 6, 14] [34] anhydrides and SO3. Acetic anhydride traps the persulfonic acid formed during sulfoxidation as alkanesulfonyl acetylperoxide as given in the following equations.
RSO2O2 H + ()CH 3CO 2 O → RSO2O2COCH 3 + CH 3COOH (1.15)
∆ • • RSO2O2COCH3 ⎯⎯→ RSO2 − O + OCOCH3 (1.16)
• • RSO2 − O + R − H → RSO3 H + R (1.8) 17
• • R − H + OCOCH 3 → CH 3COOH + R (1.17)
Though alkanesulfonyl acetylperoxide plays the same role as persulfonic acid, the efficiency is much higher compared to that of the latter. The role of this promoter is well studied as this peroxide could be isolated in the sulfoxidation of cyclohexane in the presence of acetic anhydride.[14] A further advantage of alkanesulfonyl acetylperoxide is that it is not reduced in the presence of water and SO2 and therefore is always available in the required amounts to maintain the chain reaction. Continuous addition of alkanesulfonyl acetylperoxide for sulfoxidation of alkanes, which are not 100% pure could drive the reaction without the necessity of any additional radical initiators.[24] The problem with this promoter is the formation of acetic acid due to its addition. This is not desirable for the detergent property of the sulfonates and hence should be eliminated from the reaction mixture. This makes the industrial scale operation expensive and complicated. [34] SO3 also acts as a promoter and is often mixed with the hydrocarbons in gaseous form. Several other promoters like halogenated derivatives of methane, ethane and ethyne have also been employed. Pentachloromethane, dichloromethane, chloroform, and acetic anhydride / chloroform mixtures have also been found as promising.[35]
1.2.3.3 Inhibitors Branched alkanes like 2,3-dimethylbutane, olefins like 1-hexene are found to be potential inhibitors of photochemical and even γ-ray induced sulfoxidation.[16, 24, 36] The reason for this inhibitiory action of 2,3-dimethylbutane is the fact that the abstraction of tertiary hydrogen atom during the chain reaction is more favorable and the stability of the resulting tertiary carbon radicals inhibits the addition of SO2. Formation of allylradicals which are stabilized by resonance may explain the inhibitory action of 1-hexene. Due to this, the activation energy for the abstraction of 18 an allyl secondary hydrogen is lower than that of primary hydrogen (750 cal / mol)[16, 24] or secondary (450 cal / mol) and therefore the allyl radical is more easily formed from 1-hexene. Eventually, the stability of allyl radicals hinders the chain propagation. Aromatic compounds are also reported to act as sulfoxidation inhibitors.[24]
1.2.3.4 Product composition of sulfoxidation of alkanes The sulfoxidation mixture as obtained in the industrial production contains the following compounds:
• Monosulfonated isomers • Di- and polysulfonates • Sodium sulfate • Sulfuric acid • Unreacted alkanes • Water
The distribution of isomers has been determined for a few compounds like n-hexane,[18] n-heptane and n-dodecane.[12] When the proportion of di- and polysulfonic acids is higher than 13%, the detergent properties of the sulfonates are greatly diminished. For every 1% alkane conversion to sulfonic acid there is about 10% of di- and polysulfonic acids formation due to the complicated multi-phase nature of this reaction. However, higher proportion of di- and polysulfonic acid conversion is avoided by limiting the alkane conversion to ca. 1%.
1.3 Various types of sulfoxidation techniques 1.3.1 Sulfoxidation in the presence of water (Light water process) The term “Light water process” is ascribed to the type of sulfoxidation in which light acts as the reaction initiator and water acts as both reactant and solvent to extract the products of sulfoxidation (Scheme 1.2). 19
Scheme 1.2: Scheme for the Light Water Process.
The reaction is carried out in a cylindrical reactor into which the light source is immersed in a continuous mode operation.
1.3.1.1 Method of operation The reaction mixture is pumped through the reactor and the sulfonic acids are separated from the reaction mixture well before the degree of conversion reaches its maximum. Recovered paraffins are recycled and fed into the reactor. The reason for using this continuous mode is to avoid following two problems:
1. When the mixture of alkanes, SO2 and O2 is irradiated in an immersion reactor, the reaction medium becomes turbid and the sulfonic acids which are not very soluble in the alkanes separate at the bottom of the reactor owing to their higher density. Under these conditions di- and polysulfoxidations occur more rapidly. 2. Sulfonic acids stick to the wall surrounding the light source forming tarry deposits which block the passage of light. The scheme of sulfoxidation in continuous mode is shown in Scheme 1.3. 20
GAS EXIT RECYCLED GASES ALKANES
RECYCLED ALKANES
PHOTO- REACTOR SEPARATION CHAMBER
SO2 + O2 SULFOXIDATION MIXTURE ALKANESULFONATES
Scheme 1.3: Scheme for photosulfoxidation in continuous mode.
1.3.1.2 Process procedure The reactor is fed continuously with paraffin and water. From its bottom a gas mixture of SO2 and O2 in the ratio of 1:2 is introduced. A uniform dispersion of the gases in the alkane is very important since the solublility of the gases in the alkane is low. Generally a high pressure is applied on the reaction (up to 5 atm). The circulating gases also ensure intensive mixing of the reactor contents. Additionally, powerful stirrers are employed, which is very important because the aqueous and the alkane phases must be constantly mixed so that the alkanesulfoperoxy acid initially formed immediately comes into contact with water and SO2 and is eventually decomposed to alkanesulfonic acid. The reaction temperature is 10 – 40 °C. 60 kW mercury arc lamps are used as the light source to initiate and maintain the chain reaction.
1.3.1.3 Separation of alkanesulfonates: 21
The aqueous phase constantly extracts the desired alkanesulfonic acid and sulfuric acid from the alkane phase owing to their higher polarity. The reaction mixture collected from the reactor usually contains components with the following composition percentages:
• Sulfonic acids: 20-25% • Sulfuric acid: 7-8% • Alkanes: 30-35% • Water: complementary amount to 100%
1.3.1.3.1 Thermal separation
After sulfoxidation the reaction mixture is freed of SO2 by degassing and is concentrated by distilling off the part of the water under vacuum. Then the reaction mixture is allowed to settle down in a first fractionating column. The upper phase which contains mainly alkanes is recycled after drying. The lower, denser phase predominantly contains the sulfonic acids and sulfuric acid. This phase is heated to 60-120 °C in a second fractionating column. This operation leads to a new separation into two phases. The lower phase containing aqueous sulfuric acid (50-65%) is largely removed. Discolourations of sulfonic acids which can occur in this stage due to this heating can be overcome by the addition of hydrogen peroxide. The organic phase which remains after separating sulfuric acid, consists of roughly equal parts of alkanes and sulfonic acids. This is neutralized with NaOH.
The sodium sulfonate is freed from the residual alkane in a thin layer evaporator at 200 °C in vacuo and is further recycled into the reaction. Under these conditions the weight composition of the solution is in the range of:
• Alkanemonosulfonates: 55% • Alkanedisulfonates: 6% • Paraffins: 0.1-0.4% 22
• Sodium sulfate: 5% • Water: complementary amount to 100%
The sulfonate melt that is formed can be cooled on a rotating drum and converted to flakes or processed with water to 60-65% pastes. The light water process [6] is of high cost, mainly due to its complex installations needed for the separation of sulfuric acid and the extraction of the sulfonate.[37]
1.3.1.3.2 Solvent extraction This is an alternative method to thermal separation where the sulfonic acid together with the alkane can be extracted from the mixture with weakly polar solvents such as alcohols, ketones, or ethers, leaving behind a 20% aqueous sulfuric acid. The solvent must be separated after the neutralization in an additional distillation column.[20]
1.3.2 Sulfoxidation in the absence of water This process has several advantages as the amount of sulfuric acid formed is very low and so there is no necessity for the sulfuric acid separation which results in a substantial cost reduction. Additionally the problem of eliminating dilute sulfuric acid (20-30% in water) is avoided. The chain reaction lasts longer than that of the light water process and therefore low intensity fluorescent lamps, emitting between 300-400 nm can be used. This is also a continuous mode operated process which however has not yet been commercialized. It is preferable that all the reactants are anhydrous since water induces the termination reations of the free radicals. Interestingly under application of high pressure it is observed that the sulfonic acids are less coloured.[38] Similar to the light water process the conversion must not exceed 50%. There are several methods of separating the alkanesulfonates. In most cases the sulfoxidation mixture is degassed of SO2 and then extracted with water or a water / methanol mixture. The remaining alkanes are extracted from the water / methanol phase with a volatile solvent (cyclohexane, petroleumether) by thin film evaporation 23 and reintroduced to the reactor.[12] Adour Entreprise developed an innovative separation process in which sulfonic acids are extracted with mono or triethylamine which simultaneously neutralizes the acids (see Scheme 1.4).[39]
RECYCLING OF ALKANES
NEUTRALISATION SO2 + O2 AND EXTRACTION
PHOTO- REACTOR RSO3MEA ALKANES DEGASSING EVAPORATOR
MEA RECYCLED
SO2
O2 MEA
Scheme: 1.4 Scheme for the innovative separation process using TEA (Triethylamine) designed by Adour Entreprise.[39]
1.3.3 Goal of this work We have investigated extensively in our group various metal complex modified photocatalysts which were very efficient in degrading pollutants like chlorophenols, azo dyes, dichloroacetic acid, and other common industrial pollutants including even cyanuric acid to benign products.[40-45] Additionally it was found that some of these photocatalysts not only achieved photocatalytic degradations but also synthetic organic reactions like sulfoxidation of alkanes under visible light.[46] 24
Unlike UV sulfoxidation with SO2 as the light absorbing species, here only the photocatalyst absorbs light and drives the reaction. Since these catalysts are able to utilize also visible light with λ ≥ 400 nm, this novel reaction is named visible light sulfoxidation. To our knowledge this is the first photocatalytic sulfoxidation to be reported. The aim of this present work was to explore and optimise the reaction conditions, to achieve an accurate detection and a quantitative isolation of the alkanesulfonic acids formed, and finally to comprehend the mechanism of this novel reaction (Eq. 1.18).
Metal complex modified TiO2 (photocatalyst) (light absorbing species)
RH + SO2 + O2 RSO3H Visible light sulfoxidation
λirr ≥ 400 nm (1.18) 25
2 Visible Light Sulfoxidation
2.1 Introduction We first report on the preparation, characterization and photocatalytic properties of these metal complex modified TiO2 materials which photocatalyse the sulfoxidation. To understand the selection of TiO2 as the semiconductor component in these photocatalysts, it is essential to discuss some of its unique properties.
2.1.1 Unmodified TiO2 Titanium dioxide is a white coloured widely used material with a high refractive index and great inertness. These qualities make it the principal pigment in paint industry and other applications including sun-block in suncreams, glossy coatings for magazines, the white colour of plastic forks etc.. TiO2 is known in three modifications namely rutile, anatase, and brookite. Rutile has a higher refractive index of 2.73 compared to that of anatase’s 2.55, which makes it preferable for the application as a pigment. Generally, anatase is found to be photocatalytically more active than rutile.[47] Anatase can be reverted to rutile by heating to higher temperatures ranging from 400-1200 °C.[48] Brookite is not very stable and therefore not a widely used modification. TiO2 has become the most successful commercial photocatalyst in various fields, since the first report on the ultraviolet light induced cleavage of water in 1972.[49] [50] TiO2 photocatalysis includes several applications like solar energy conversion, organic syntheses, for instance amino acids[51] and other organic compounds from photocatalytic oxidation of benzene, toluene and phenylmethylketones.[52] Further [53] [54] applications include CO2 reduction, cancer treatment, and cleaning of environment like degradation of halogenated compounds in air, sterilization,[55] degradation of surfactants,[56] and decomposition of oil spills in water surfaces.[57] This wide usage is due to its strong oxidative power, photostability and non-toxicity.[58-60]
However, one serious disadvantage of TiO2 is its large bandgap of 3.2 eV due to which it can absorb only 2-3% of solar light. To increase its photosensitivity from UV 26 to further visible region several attempts like doping with metal ions, especially transition metal ions,[61-65] coupling with narrow bandgap semiconductors,[66-68] mixing with organic dyes,[69-71] or doping with transition metal complexes[40-45, 72] have been made. To understand the reason for modifying TiO2 with various metal complexes, it is essential to discuss their photochemical behaviour.
2– 2– Photochemistry of [PtCl6] and [PtBr6] 2– 2– The photochemistry of [PtCl6] and [PtBr6] has been studied extensively.[73-79] Chloroplatinum(IV)complexes in aqueous solutions demonstrate thermal aquation[80] and photoaquation[81-83] as characteristic reactions. 4-x [PtClx(H2O)6–x] (x = 4-6) complexes, typically reveal a broad LMCT band (t2u → eg, maximum at 270 nm) (Figure 2.1) which extends from UV to the visible region and overlaps with singlet and triplet LF bands at ca. 380 and 480 nm.[75-79, 84] Upon irradiation in the range of λ = 270-450 nm, photoaquation of aqueous solutions of hexachloroplatinate(IV) to pentachloroaquoplatinate(IV) (Eq. 2.1), takes place with quantum yields of 0.87 to 13.4.[75]
2− − hν − hν − []PtCl 6 + 2H 2O []PtCl5 ()H 2O + Cl + H 2O [PtCl4 (H 2O)2 ] + 2Cl (2.1)
The initiation step is the homolytic bond cleavage of hexachloroplatinate(IV) to produce a chlorine atom and a labile Pt(III) species (Eq. 2.2). The further steps are the chain reactions (Eqs. 2.3, 2.4), whose growth is however broken by the reaction of the reactive intermediate chlorine atoms with Pt(III) species (Eq. 2.5). The same reaction is slow by several folds in the dark.[80]
IV 2− hν III 2− []Pt Cl6 ⎯⎯→ []Pt Cl5 + Cl (2.2)
III 2− III − − []Pt Cl5 + H 2O → []Pt Cl4 ()H 2O + Cl (2.3)
III − III 2− IV − III 2− []Pt Cl4 ()H 2O + [Pt Cl6 ] → [Pt Cl5 ()H 2O ] + [Pt Cl5 ] (2.4)
III − IV − []Pt Cl4 ()H 2O + Cl → []Pt Cl5 ()H 2O (2.5) 27
25000 1000
20000 800 -1 -1 15000 600 cm cm -1 -1 / M / M
ε 10000 400 ε
5000 200
0 0 200 300 400 500 600 λ / nm
Figure 2.1: UV-Vis-spectrum of a freshly prepared solution of H2[PtCl6] (6,75 x 10-5 M) in HCl (2 M). Major peak: 262 nm (LMCT). The visible region is zoomed 20 times to show the absorption shoulder of the complex in the visible region.
In contrast to the hexachloroplatinate(IV), photoaquation of hexabromoplatinate(IV) produces a bromine molecule in the initial step.[81, 85]
IV 2− hν II 2− []Pt Br6 ⎯⎯→ []Pt Br4 + Br2 (2.6)
Especially the strong absorption shoulder in the visible region for both H2[PtCl6] and
H2[PtBr6] made them a promising choice for surface modification of TiO2.
28
2.1.2 Metal Complex Modified TiO2 Previous investigations revealed that, in addition to amorphous titania doped in the bulk with Pt(IV)-, Au(III)- and Rh(III)- chloride [40-42] 2- [43-45] complexes, crystalline TiO2 surface-doped with [PtCl6] or [PtCl4] has also shown high activity towards the degradation of 4-chlorophenol. It’s structure can be IV n- - [45] written as {[Ti]-O-Pt Cl4L} , where L = OH , H2O, n = 1, 2. For the sake of simplicity the abbreviation H2[PtCl6]/TH is used since it indicates also the nature of the doping complex employed. It’s worth mentioning that the photocatalyst [45] 4%H2[PtCl6]/TH, which was prepared by surface modification of TiO2 with 2- [PtCl6] was the most active one among several catalysts prepared in our group like [86] [87, 88] [89] carbon, nitrogen or sulfur modified TiO2. It accomplished very fast photocatalytic degradation of various pollutants like 4-chlorophenol, dichloroacetic acid, lindane, trichloroethene and even cyanuric acid which is known to be the stable end product of atrazine decomposition, resistant even towards the attack of OH [90] radicals, produced under UV irradiation of unmodified TiO2. TH is Titanhydrat-O, a commercially available TiO2 in anatase modification (Figure 2.2) with a very high surface area of 334 m2 g-1. 29
80
60
40
Intensity / a.u. 20
0 20 30 40 50 60 70 80 2 Θ / °
Figure 2.2: XRD spectrum of Titanhydrat-O (TH), which correlates well with the theoretical anatase peaks (dotted lines) (ASTM file card No.21-1272).
4%H2[PtCl6]/TH is a much superior photocatalyst than P25, TH and [45] 1.1%H2[PtCl6]/P25 as corroborated by solar experiments too. P25 is the widely used commercial form of TiO2 produced by the German company Degussa, which exhibits excellent photocatalytic activity under UV irradiation and consists of anatase
(~ 70%) and rutile (~ 30%). 4%H2[PtCl6]/TH is one of the rare cases able to catalyze efficiently the photodegradation of pollutants even in diffuse indoor daylight. It was even more active than P25 upon UV irradiation.
30
2.1.2.1 Desorption experiments
The suspension of 4%H2[PtCl6]/TH in water showed a pH value of 3.4. Since it is known that fluoride ions irreversibly adsorb onto the TiO2 surface displacing OH groups,[91-93] desorption studies in the dark were conducted by stirring 4%H2[PtCl6]/TH suspensions in various concentrations of KF solution. No 2- desorption of [PtCl6] occurred within four days in the presence of 0.01 M KF solution.[94] At higher fluoride concentrations, i.e. 0.1 M and 0.5 M, desorption increased to 21 and 31%, respectively in the same time range. When the catalyst suspension in water was neutralised by NaOH and treated again with fluoride solution (0.5 M), desorption drastically reduced to 2%. This suggests that chemisorption of 2- [PtCl6] on the surface of TiO2 had occurred according to Eq. 2.7 given below:
IV n− III n− n− []TiO2 − OH + Pt Cl5 L → {[TiO2 ] − O − Pt Cl4 L } + HCl (2.7) L = Cl, OH, H 2O
2- Furthermore, the desorption of [PtCl6] which occurs in the absence of fluoride ions too, is an acid catalysed process. So under prolonged stirring in the dark or 24 h illumination (λ ≥ 455 nm) of the catalyst suspension in the presence of 0.1 M HCl, 2- complete desorption of [PtCl6] was observed. There was only 1% desorption, photochemically or thermally in 0.1 M HNO3 solution and no desorption in NaCl solution of the same concentration.
Based on the assumption that TH contains 5 OH groups per nm2, the highest limit of values reported for anatase,[95] estimation of the surface OH groups of TH which have 20 -1 reacted with H2[PtCl6] has been made. Since half of them, 8 × 10 g , possess basic character,[96] they could possibly displace a chloro ligand in the dissolved platinum complex. However, when compared to the surface concentration of platinum atoms, 20 -1 i.e. 1.2 × 10 g in 4%H2[PtCl6]/TH, only 15% of all the basic OH groups make this displacement.[45]
31
2.1.2.2 Photostability The initial pH value of the catalyst suspension in 4-CP solution was 3.4 before visible light irradiation and decreased to 3.0 after 120 min irradiation. This is due to the formation of HCl, CO2 and H2O as complete mineralization products of 4-CP. Since this lowering of pH enhances photodesorption in our catalyst, long term irradiations were performed in the presence of a mild base
NaHCO3 to neutralise the acid generated in the due course of the reaction. This addition was fruitful as there was no significant decline in the activity even after 19 cycles, when bicarbonate was present. While in its absence, the activity of the photocatalyst, drastically reduced to 50% of its original value already after three cycles.
1,0
0,8 0 0,6 / c/c
0,4 [4-CP]
0,2
0,0 0246810 time / d
Figure 2.3: Long term visible light degradation of 4-CP by 4%H2[PtCl6]/TH in the presence of 0.01 M NaHCO3 (λirr ≥ 400 nm). Around 20 cycles were made by equalising the initial concentration of 4-CP.[45]
32
Long time irradiation of aqueous suspensions of 4%H2[PtCl6]/TH with UV or visible light did not lead to metallic platinum formation or significant increase in chloride or 2- [PtCl6] concentration which clearly proves the high photostability of the catalyst.
2.1.2.3 Characterization techniques 2.1.2.3.1 Diffuse Reflectance Spectroscopy (DRS) One of the fundamental properties of semiconductors is their bandgap, the energy difference between the bottom of conduction band and top of valence band. DRS is used to determine the absorption characteristics of opaque semiconductor powders, from which their bandgap can be measured. Conventional transmission spectroscopy cannot serve this purpose due to extreme difficulties of preparing thin, transparent plates of these powders. When the incident beam is mirror like reflected off the surface of one particle, it is called specular reflectance. Diffuse specular reflectance is observed when the incident beam undergoes multiple reflections off the surfaces of several particles. Very significant is the case, when the incident beam penetrates into one or more particles. Then partial absorption and subsequent scattering occurs. This is called “diffusely reflected light” and this beam carries the vital data of the absorption characteristics of the material under investigation and is independent of the angle of the incident light beam. In Diffuse Reflectance Spectroscopy this so-called diffusely reflected light is collected by a special arrangement of mirrors and is transmitted to the detector. The ratio of the light scattered from an infinitely thick sample layer to that of an ideal non-absorbing reference (BaSO4, MgO, etc..) as a function of wavelength is recorded as the spectrum.[97-99] The fundamental theory of this phenomenon was first devised by Schuster in 1905 and his approach was employed and further developed by Kubelka and Munk in 1931.[98] The Kubelka-Munk theory proves that, assuming an infinitive thickness of the sample
(~ 5 mm for most of the materials), the diffuse reflectance of the sample (R∞) could be related to an apparent absorption (K) and apparent scattering coefficient (S) of the 33 sample, through the Schuster-Kubelka-Munk or Kubelka-Munk function or Remission
( F(R∞ ) ).
2 (1− R∞ ) K F(R∞ ) = = (2.8) 2R∞ S
Rs R∞ = (2.9) Rref
R∞ is the ratio of light intensity reflected from the sample (Rs) to the light intensity reflected from the reference (Rref). The above equation can be applied only under specific conditions like monochromatic irradiation, uniform distribution with low sample concentrations, and non-fluorescence of the sample. The dilution of the sample with the white reference plays a crucial role in the accuracy of the spectrum since distortions due to differences of scattering coefficients between sample and reference are carefully avoided. An additional advantage of the dilution is the decrease in the total amount of regular reflectance. When we shine light on a semiconductor, a photon is absorbed which excites an electron from the valence band to conduction band. The different electronic states within each band are characterised both by their energy and momentum.[100] The selection rules for photon absorption allow only the transitions with no net momentum change.[101] Accordingly, the band structure of semiconductors also determines the magnitude and energy of the absorption process. If there is no change in momentum in case of excitation of an electron from the valence to conduction band, the absorption probability is high because this transition is orbitally allowed and the semiconductor is called a direct bandgap semiconductor.[102] The basic differences between a direct and indirect bandgap semiconductor are given below and also displayed in Figure 2.4.
34
CB
E CB E
VB VB
k k AB
Figure 2.4: Energy vs. wave vector diagram displaying the band structure of a direct bandgap (A) and an indirect bandgap (B) semiconductor.[103]
Direct bandgap semiconductors: Semiconductors whose direct band to band transition from the highest level in the valence band to the lowest level in the conduction band is possible, because
kmax = k min, where kmax, kmin are momentum vectors of electrons of the highest level in the valence band to the lowest level in the conduction band. When the semiconductor is photoexcited, electrons change their energy state owing to the absorption of photons and maintain the same momentum. Indirect bandgap semiconductors:
Semiconductors whose direct band to band transition is forbidden because kmax ≠ k min requires a change of electron momentum. The necessary electron momentum changes can be induced by the interaction of electronic subsystem with phonons (lattice vibrations).[103]
35
The relationship between the absorption coefficient K of semiconductors and bandgap [104] energy (Ebg) is:
()hν − E n K ∝ bg (2.10) hν
The exponent n depends on the nature of transitions and shows the values for crystalline semiconductors as noted below: n = 1 / 2 for allowed direct transitions (at k = 0) n = 3 / 2 for forbidden direct transitions (at k ≠ 0) n = 2 for allowed indirect transitions n = 3 for forbidden indirect transitions
When the scattering coefficient S is assumed to be independent of the wavelength and proportional to the absorption coefficient then,
F(R∞ ) ∝ K (2.11)
From Eqns. 2.8 and 2.10, the below equation is derived.
1 n ()F(R∞ ) hν ∝ hν − Ebg (2.12)
For indirect semiconductors like TiO2, the square root of the absorption coefficient is proportional to the energy difference between the bandgap and incoming light.[105] The mathematical expression is presented as Eq. 2.13.[106]
1 2 ()F(R∞ ) hν ∝ hν − Ebg (2.13)
36
1 2 For determining their bandgap, ()F(R∞ ) hν is plotted vs. the light energy (eV). The intersection of the extrapolated linear region of the graph, with the energy axis affords bandgap energy. For direct semiconductors the square of the absorption coefficient is proportional to the energy difference between the bandgap and incoming light[105] as shown in the equation:
2 1/ 2 ()F(R∞ ) ()hν ∝ hν − Ebg (2.14)
Determination of the corresponding bandgap is analogously done as previously described for indirect semiconductors.
2.1.2.4 Quasi-Fermi level measurements: The Fermi level is the free energy of electrons and holes in a semiconductor under equilibrium conditions. It is defined as the energy at which the probability of a level being occupied by an electron is 0.5. In other terms, the Fermi level is the chemical potential of electrons in a semiconductor. In case of intrinsic semiconductors, the number of mobile electrons and holes always remains equal in both cases, either in the dark under thermal excitation or under irradiation. Accordingly the Fermi level lies exactly in the middle energy level position of the bandgap (Figure 2.5).
37
E
ECB EF EF
EF
EVB
Intrinsic p-type n-type
Figure 2.5: Position of the Fermi level in intrinsic, p-type, and n-type semiconductors.
In case of extrinsic semiconductors the position of the Fermi level varies with the nature of doping. For n-doped semiconductors (for example Si doped with 5th group elements like N or As) there are excess electrons in the lattice which become the majority charge carriers and therfore the Fermi level is closer to the conduction band. Whereas for p-doped semiconductors (for example Si doped with 3rd group Al or Ga) with an excess of holes as the majority charge carriers, the Fermi level is closer to the valence band.
TiO2 and metal complex modified TiO2 are n-doped semiconductors due to their intrinsic oxygen deficiencies.[107] Upon illumination of the semiconductor, the Fermi level splits into two quasi Fermi levels, one for the electrons, nEF* and another for the holes, pEF*. nEF* is displaced towards the bottom of the conduction band and nEF* towards the top of the valence band. Significance of the measurement of quasi-Fermi levels: Quasi-Fermi level values of a semiconductor are necessary to estimate the redox potential of redoxactive surface centers. A measurement of EF and nEF* in semiconductors can be achieved by several methods such as capacity measurements (Mott-Schotty)[108] modulation spectroscopy,[109, 110] photocurrent,[111] and photovoltage 38 measurements.[112] However, the described methods are applicable usually only for single crystals and not for semiconductor powders.
One approach to estimate the nEF* of semiconductor powders is based on the “suspension method” originally reported by Bard et al.[111] and modified by Roy et al.[112] Bard measured the photocurrents with a three electrode setup using methylviologen (MV2+) as electron acceptor, and in the presence of a reducing agent to quench the photogenerated holes. Roy recorded the photovoltage, with a two electrode setup under similar conditions, but without a reducing agent. However, Roy’s method was more accurate and faster than that of Bard’s photocurrent measurements even though both the methods showed similar results within experimental errors for P25.[113] Accordingly, we have used Roy’s method for our quasi-Fermi level measurement of TiO2 and our metal complex modified TiO2 catalysts. However, we prefer to use the term quasi Fermi level of electrons, nEF*, since it is more correct as all measurements are made under illumination of the semiconductor. nEF* almost merges with the conduction band under illumination and so it is reasonably assumed that nEF* ≈ ECB. The measurement is based on the pH dependence of the quasi-Fermi level of electrons of TiO2 as given in Eq. 2.15.
∗ ∗ n E F ( pH ) = n E F ( pH = 0 ) − k pH (2.15)
Where k is a constant with a value of 59 mV for TiO2.
The suspension of the semiconductor powder in an electrolyte solution is irradiated during measurements. The pH of the suspension is varied and the photovoltage developed at the platinum working electrode with respect to the reference electrode was recorded. Since the band edge positions of a semiconductor are generally pH-dependent, three different situations, presented in Figure 2.6, are possible.
39
EEE
- Eo(MV2+/+•) - Eo(MV2+/+•) Eo(MV2+/+•) E - hν CB hν E * n F hν + * + pEF + EVB
pH < pH pH = pHo pH > pH o o
Figure 2.6: Band edge positions in an n-type semiconductor in contact with a redox system (MV2+) under illumination as a function of pH.
At low pH values the nEF* is more positive than the redox potential of the electron acceptor (E°). Excited electrons from the conduction band cannot reduce the acceptor.
It becomes thermodynamically possible at pH ≥ pHo. The sigmoidal photovoltage - pH curves obtained depend on the potential of the reference electrode, the 2+ +• [MV ] / [MV ] ratio, the pH value, k and nEF*. At the pH value of the inflection point (pH0) the nEF* is equal to the potential of methylviologen (the reversible, 2+ +• pH-independent reduction of MV to blue MV with E°MV2+/+•, –0.445 V vs. NHE) (Table 2.1).
EMV2+/+• / V vs. NHE Compound Structure ± 0.01 V
MVCl2 –0.450 H3C N N CH3
Table 2.1: Structure and redox potential of methylviologen. 40
∗ 0 E ( pH ) E 2+ / +• k pH pH n F = MV + ()o − (2.16)
Employing Roy’s method the pH0 values of the catalysts were determined and based on the Eq.2.16, the corresponding nEF* values were calculated.
2.2 Results and Discussion
2.2.1 Preparation of metal complex modified TiO2
The catalysts were produced by surface modification of TiO2 by mixing it with an aqueous solution of hexachloroplatinic acid or hexabromoplatinic acid followed by stirring for 12 h. Then the water was removed in a vacuo and the resulting residue was dried under vacuum at room temperature to obtain dry powders. These dry powders were calcined in air at 160 °C for 2 h and were washed five times with water after centrifuging and were dried again following the same procedure described above. Subsequently the resulting dry powders were calcined again at
160 °C for 2 h. In the case of 4%H2[PtCl6]/TH, the washings were checked for the 2- presence of [PtCl6] referring to the UV-Vis-spectrum of a freshly prepared solution -5 of H2[PtCl6] (6,75 x 10 M) in HCl (2 M) with major peak, 262 nm (LMCT) (Figure.2.1). From this the amount of adsorbed platinum complex was estimated to be 4% and is given as its wt.%.[45, 46] It was also observed that complete desorption of 2- [PtCl6] from the 4%H2[PtCl6]/TH occurred in HCl. Additionally the catalyst was stirred in the presence of NaOH in the dark and the amount of chloride released into solution was measured by Ion Chromatography. From these details, it’s structure can IV n- - [45] be written as {[Ti]-O-Pt Cl4L} , where L = H2O ,OH , n = 1, 2. Catalysts with non-semiconductor supports like silica or alumina were prepared analogously using them instead of TiO2. Various catalysts prepared by the described procedure are listed below. The percentage values given denote the amount of the metal complex chemisorbed on the surface of the support, while “cal.” describes the 41 weight percentage of the complex added to the support during preparation. Except
4%H2[PtCl6]/TH, all other catalysts were prepared during this work.
• 4%H2[PtCl6]/TH
• cal.1%H2[PtBr6]/TH
• cal.2%H2[PtBr6]/TH
• cal.3%H2[PtBr6]/TH
• cal.6%H2[PtBr6]/TH
• 4%RhCl3/TH
• cal.3%RhCl3/TH
• cal.4%H2[PtCl6]/SiO2
• cal.8%H2[PtCl6]/SiO2
• cal.32%H2[PtCl6]/SiO2
• cal.4%H2[PtCl6]/SiO2 (grinding in ball mill)
• cal.4%H2[PtCl6]/Al2O3
• cal.4%H2[PtCl6]/Al2O3 (grinding in ball mill)
2.2.2 Characterisation 2.2.2.1 Diffuse Reflectance Spectroscopy
The diffuse reflectance spectra of 4%H2[PtCl6]/TH and cal.6%H2[PtBr6]/TH exhibited absorption already at about 550 nm (Figure 2.7). cal.6%H2[PtBr6]/TH in accordance with its much deeper yellow colour compared to that of chloro-modified, exhibits a stronger absorption than the latter. Assuming that the above photocatalysts are indirect semiconductors, a plot of modified 1/2 Kubelka-Munk function [F(R∞)hν] versus the incident photon energy hν affords a bandgap energy (Ebg) of 3.21 eV for 4%H2[PtCl6]/TH (Figure.2.8) which is in good 42
[45] agreement with the results reported previously. For cal.6%H2[PtBr6]/TH a bandgap of 3.03 eV was measured. The narrowing of the bandgap is proportional to the increasing amount of H2[PtBr6] added for the catalyst modification (Figure.2.9). The unmodified TH employed showed a bandgap of 3.21 eV in excellent agreement with the literature value of 3.20 reported for anatase.[114] The bandgap measured for
4%RhCl3/TH and cal.3%RhCl3/TH was 2.97 and 3.1 eV respectively, exhibiting band narrowing in both the cases (Figure. 2.10).
0,16
0,12 ) ∞
F(R 0,08
0,04 c b 0,00 a 300 400 500 600 700 λ / nm
Figure 2.7: Diffuse reflectance spectra of a) TH, b) 4%H2[PtCl6]/TH, c) cal.6%H2[PtBr6]/TH. The Kubelka-Munk function F(R∞) is employed as an equivalent to absorbance. (50 mg of the catalyst powder was diluted with 2 g of BaSO4).
43
3 2,0
2 1/2 )E) 1,5 ∞ 1
(F(R a
1/2 0 1,0 2,0 2,5 3,0 3,5 4,0 )E)
∞ E / eV (F(R 0,5 c b a 0,0 2,0 2,5 3,0 3,5 4,0 E / eV
Figure 2.8: Plot of transformed Kubelka-Munk function vs. energy of light absorbed. a) TH, b) 4%H2[PtCl6]/TH, c) cal.6%H2[PtBr6]/TH. (50 mg of the catalyst powder was diluted with 2 g of BaSO4) 44
2,0 3
2 1/2 )E)
1,5 ∞ 1
(F(R a 1/2
)E) 0 ∞ 1,0 2,0 2,5 3,0 3,5 4,0 E / eV (F(R
0,5 c c3 c2 c1 0,0 2,0 2,5 3,0 3,5 4,0 E / eV
Figure 2.9: Plot of transformed Kubelka-Munk function vs. energy of light absorbed. a) TH, c1) 1%H2[PtBr6]/TH c2) 2%H2[PtBr6]/TH c3) 3%H2[PtBr6]/TH c)
6%H2[PtBr6]/TH. (50 mg of the catalyst powder was diluted with 2 g of BaSO4) 45
2,0 3
2 1/2
1,5 )E) ∞ 1 (F(R 1/2 a )E)
∞ 1,0 0 2,0 2,5 3,0 3,5 4,0
(F(R E / eV
0,5 d e 0,0 2,0 2,5 3,0 3,5 4,0 E / ev
Figure 2.10: Plot of transformed Kubelka-Munk function vs. energy of light absorbed. a) TH, d) 4%RhCl3/TH, e) cal.3% RhCl3/TH.
Though there is no significant change in the bandgap of chloro and bromo modified titania, the positions of their conduction and valence band edges and other photoelectrochemical properties were measured.
2.2.2.2 Photoelectrochemical properties
The pH0 values of the catalysts were determined using Roy’s method. (Figure 2.11, for the sake of clarity, only three measurements are depicted).
Further determination of the quasi-Fermi level of electrons for cal.6%H2[PtBr6]/TH 46 by pH dependent photovoltage measurements afforded a value of -0.24 ± 0.02 V (vs. NHE). This is in agreement with the previously reported quasi-Fermi level of [45] 4%H2[PtCl6]/TH. There was an anodic shift of ~ 300 mV as compared to –0.54 V of TH for both the chloro and bromo modified titania. Assuming that nEF* ≈ ECB, now the position of the valence band edges were obtained adding the bandgap values. The band edge positions and the band gap values are displayed in the Figure 2.12. These values are useful for evaluating the oxidising and reducing power of photogenerated charges in the semiconductors and therefore the thermodynamic feasibility of IFET reactions.
a 600 1000 pH = 5.33 c 0
0
a b ''(U) f 400 -1000
456 200 pH
700 c
U / mV 0 pH = 10.58 0
f ''(U) b 0 pH = 10.56 -200 0 -700 81012 pH -400 2 4 6 8 10 12 14 16 18
pH
Figure 2.11: Photovoltage-pH dependence recorded for a) TH b) 4%H2[PtCl6]/TH c) cal.6%H2[PtBr6]/TH suspensions. The positions of the inflection points (pH0) are shown in the insets. 47
-1
0 CB
1
2 3.03 eV 3.22 eV 3.21 eV 3.21 eV 3.12 eV 3.00 eV 3.21 eV 3.03 eV
3 VB E / V (NHE) TH P25 ]/TH ]/TH ]/TH ]/TH ]/TH 6 6 6 6 6 -Rutile 4 2 [PtCl [PtBr [PtBr [PtBr [PtBr 2 TiO 2 2 2 2
5 4%H cal.2% H cal.2% cal.1% H cal.1% cal.6% H cal.6% cal.3% H cal.3%
Figure 2.12: Estimated band edge positions (± 0.02V) at pH = 7 and bandgap (± 0.02 eV) values of TiO2 and metal complex modified TiO2.
2.2.2.3 TEM, XRD, and BET surface area measurements:
Trasmission electron micrographs of 4%H2[PtCl6]/TH (Figure 2.13) were reported to show 200 nm large aggregates consisting of 2-4 nm sized anatase crystallites.[45, 46] The X-ray powder diffraction of TH and
4%H2[PtCl6]/TH (Figure 2.14-2.16) revealed the presence of anatase pattern for both.
48
[45, 46] Figure 2.13: TEM pictures of 4%H2[PtCl6]/TH.
/ a.u. Intensity
20 30 40 50 60 70 80 2 Θ / °
Figure 2.14: XRD patterns of 4%H2[PtCl6]/TH (top) and unmodified Titanhydrat-O (TH) (bottom).[46] 49
80 Lorentzian fit 2 Θ = 25.31 80 60 FWHM = 1.04
40
Intensity / a.u. Intensity 20 60 0 15 20 25 30 35 2 Θ / °
40
20 Intensity / a.u. Intensity
0 10 20 30 40 50 60 70 80 2 Θ / °
Figure 2.15: XRD pattern of TH with a Lorentzian fit of the major peak as inset.
80 Lorentzian fit 2 Θ = 25.24 100 60 FWHM = 1.07
40
80
Intensity / a.u. / Intensity 20
0 15 20 25 30 35 60 2 Θ / °
40 Intensity / a.u.
20
0 10 20 30 40 50 60 70 80 2 Θ / °
Figure 2.16: XRD of 4%H2[PtCl6]/TH with a Lorentzian fit of the major peak as inset. 50
The crystallite sizes of the powders were calculated using the Scherrer equation:
λ ⋅ 0.9 Crystallite size = (2.17) FWHM ⋅ cosθ
Where FWHM is the full-width at half maximum, λ the wavelength of the CuKα1 X-ray radiation employed (1.54056 Å), and θ the angle of diffraction.
Contrary to this, crystallite sizes were found to be 7-8 nm for both TH and
4%H2[PtCl6]/TH. So the metal complex modification did not significantly change the crystallite size since it was only a surface modification. Additionally, since the band gap of 4%H2[PtCl6]/TH did not vary with that of TH, it is concluded that there is no quantum size effect. These values correlate well with the non-observation of quantum size effects and also with the crystallite size given by the manufactures of TH, Kerr McGee. BET analysis revealed that the specific surface area of TH decreased from 334 to 254 2 -1 and 214 m g on surface modification with H2[PtCl6] and H2[PtBr6], respectively. The BET surface areas of the photocatalysts are reported in Table 2.2.
Photocatalysts BET surface area [m2g-1] TH (anatase, Kerr-McGee) 334 P25( anatase / rutile, Degussa) 50
4%H2[PtCl6]/TH 254 cal.6%H2[PtBr6]/TH 214
4%RhCl3/TH 234 cal.3%RhCl3/TH 230
Table 2.2: BET surface areas, measured for various photocatalysts.
51
2.2.3 Photocatalytic properties 2.2.3.1 4-chlorophenol degradation Phenols and its derivatives form a class of toxic compounds which are present in waste water of petrochemical industries (oil / gas industry, refineries and production of basic chemicals), dye manufacturing and paper industries. Phenolic compounds, especially halogenated ones, have been found to have possible endocrine-disrupting effects, exerted by their interference with the transport of thyroid hormones.[115] Phenols are also toxic to individual cells, including bacteria, since they uncouple the cell’s respiration.[116-118] The exposure of human beings to phenolic compounds turns out to be real from the results of a recent study, where some 50 brominated and chlorinated phenols were found in the plasma from Swedish blood donors.[119] Especially 4-chlorophenol is an ubiquitous pollutant which is formed by the chlorination of waste water, from chlorine bleaching of pulp and breakdown of the phenoxy herbicide, 2-4 dichlorophenoxy acetic acid (2,4-D).[120] It is also formed by the anaerobic degradation of highly chlorinated phenols, such as pentachlorophenol [121, 122] which has been extensively used for the preservation of lumber.[123] Since 4-CP is a common test pollutant in several systems and its degradation pathway is well studied[124] and more importantly as it does not absorb visible light, we have selected it as model compound for degradation by metal complex modified TiO2 under visible light irradiation. The UV spectrum of 4-CP reveals characteristic peaks at 225 and 280 nm due to π-π* and n-π* transitions, respectively (Figure 2.17) which were monitored for following it’s degradation process. Additionally the 4-CP degradation was also followed by HPLC, where it shows a characteristic peak at 222 nm. More details on the HPLC setup is discussed in the experimental section.
52
0.8 π − π*
0.6
0.4
Absorbance 0.2 n − π*
0.0 200 250 300 350 400 450 λ / nm
Figure 2.17: UV-vis spectrum of an aqueous solution of 4-CP (0.83 × 10-4 M).
2.2.3.2 Kinetics The adsorption of substrates on a surface can be described by the Langmuir-Hinshelwood model (Eq. 2.18):
d[]4 − CP k K []4 − CP − = a ad (2.18) dt 1 + K ad []4 − CP
where ka and K ad describe the apparent reaction rate constant and adsorption coefficient of 4-CP, respectively. 53
The catalyst surface can be assumed to be fully saturated with 4-CP molecules when its concentration is very high and then the above equation is reduced to a zero-order rate equation (Eq. 2.19):
d[]4 − CP − = k (2.19) dt a
Under conditions of low concentrations of 4-CP ( K ad [4-CP] << 1), the equation transforms to a pseudo first order reaction (Eq. 2.20):
d[]4 − CP − = k ' []4 − CP (2.20) dt a
' where ka is the new rate constant obtained from the product of ka and K ad . The initial concentration of 4-CP, used for degradation was 2.5 × 10-4 M was also low and hence Eq. 2.20 was applied. Integration of the equation leads to Eq. 2.21.
' ln[]4 − CP t = ln []4 − CP 0 − ka t (2.21)
Where []4 − CP 0 and []4 − CP t are concentration of 4-CP at initial time and at a specific time t. From the above Eq. 2.21, the rate constants for the 4-CP degradation are calculated (Figure 2.18) and reported in Table 2.3.
54
-9.5 } f
t -10.0 c -10.5 ln [4-cp] ln
-11.0 e
-11.5 b 0 1000 2000 3000 4000 t / sec
' Figure 2.18: Plot for determination of rate constants, ka for the complete mineralization of 4-CP b) 4%H2[PtCl6]/TH c) cal.6%H2[PtBr6]/TH e) cal.3%
RhCl3/TH f) TH, P25, cal.8%H2[PtCl6]/SiO2, cal.4%H2[PtCl6]/Al2O3 which were not active and had similar almost straight slopes, upon 60 min irradiation (λirr ≥ 455 nm).
55
' -1 Photocatalysts Rate constants, ka / s × 10-5
4%H2[PtCl6]/ TH 47
cal.3%RhCl3/TH 39
4.0%RhCl3/TH 36
cal.6%H2[PtBr6]/TH 20
cal.3%H2[PtBr6]/TH 18
cal.2%H2[PtBr6]/TH 15
cal.1%H2[PtBr6]/TH 10
cal.8%H2[PtCl6]/SiO2 5.72
cal.4%H2[PtCl6]/Al2O3 2.82 TH 1.79 P25 1.09
Table 2.3: Rate constants for various photocatalysts in the degradation of 4-CP.
The initial concentration of 4-CP was 2.5 × 10-5 M and the concentration of catalysts were 0.5 g / L. More details are given in the experimental section. The plot of ct / co of various catalysts vs. irradiation time is shown in Figure 2.19.
56
a 1.0 h g 0.8 i
O 0.6 / c t c c 0.4
d 0.2 b
0 102030405060 t / min
Figure 2.19: Visible light (λirr ≥ 455 nm) degradation of 4-CP by various photocatalysts: a) TH b) 4%H2[PtCl6]/TH, c) cal.6%H2[PtBr6]/TH, d) 4.0%RhCl3/TH, g) P25, h) cal.4%H2[PtCl6]/Al2O3 i) cal.8%H2[PtCl6]/SiO2
The photocatalyst 4%H2[PtCl6]/TH displayed a superior activity in visible light degradation of 4-CP, while bromo complexes modification showed around 50% lesser activity. The lesser activity of bromo modifications may be due to the lower oxidation potential of Br atom compared to that of Cl.[40] Compared to these catalysts the unmodified TH or P25 were almost inactive. When the TiO2 semiconductor support was changed to insulators like silica or alumina, there was totally no activity. This confirms the role of the semiconductor in this reaction. Rhodium modified complexes also exhibited a similar trend of high activity like that of 4%H2[PtCl6]/TH. 57
The photocatalytic and photoelectrochemical properties of the catalysts are summarized in the Table 2.4.
[b] * Rate constants Ebg pH0 (nEF ) Catalyst -5 [a] [c] × 10 [eV] [V vs. NHE] [sec-1]
4%H2[PtCl6]/TH 3.21 10.56 -0.24 47
cal.1%H2[PtBr6]/TH 3.21 10.28 -0.25 10
cal.2%H2[PtBr6]/TH 3.22 10.31 -0.25 15
cal.3%H2[PtBr6]/TH 3.12 10.12 -0.26 18
cal.6%H2[PtBr6]/TH 3.03 10.58 -0.23 20
4%RhCl3/TH 2.97 - - 36
cal.3%RhCl3/TH 2.97 - - 39
TH 3.21 5.33 -0.54 1.79
P25 3.03 4.45 -0.58 1.09
[a] = ± 0.02 eV [b] = ± 0.1 [c] = ± 0.02 V (pH = 7).
Table 2.4: Photoelectrochemical data, bandgap energies, and rate constants of visible light (λirr ≥ 455 nm) degradation of 4-CP, of the photocatalysts.
2.2.3.3 General mechanism of action of TiO2 on organic pollutants
The general processes occurring in semiconductor like TiO2 interacting with the organic substances adsorbed or near to its surface under illumination are sequentially reported.
58
Generation of electron- hole Pair:
fs − + TiO2 + hυ ⎯⎯→ TiO2 (ecb , hvb ) (2.22)
Trapping of charge carriers:
− IV ps III ecb + { ≡ Ti OH} ⎯⎯→ {≡ Ti OH} (2.23)
+ IV ps IV • + hvb + {}{}≡ Ti OH ⎯⎯→ ≡ Ti OH (2.24)
Recombination of the photogenerated charges:
− IV • + ns IV ecb + {}{}≡ Ti OH ⎯⎯→ ≡ Ti OH (2.25)
+ III ns IV hvb + { ≡ Ti OH} ⎯⎯→ { ≡ Ti OH } (2.26)
Interfacial electron transfer (IFET):
+ ns • {}≡ Ti IV OH • + D ⎯⎯→ {}≡ Ti IV OH + (D + ) (2.27)
III ns IV − • −• {}≡ Ti OH + O2 ()A ⎯⎯→ {}≡ Ti OH + O2 (A ) (2.28)
Here, D represents “donor“ and A the “acceptor”.
It is postulated that the IFET (Eqs 2.27 and 2.28) is the rate determining step in the degradation of organic pollutants in this semiconductor photocatalysis.[125-127]
Furthermore, the reduction of O2 achieved by the photogenerated electrons generate several species with increasing oxidising powers, like superoxide radicals, superoxide anions, hydrogen peroxide, and hydroxyl radicals (Eqs. 2.29 - 2.35). These species breakdown the pollutants according to their oxidising powers. However, the OH 59 radicals generated which possess a very high oxidation potential of ~2.4V,[128] non-selectively oxidise a major number of pollutants, including 4-CP.
− + • O2 + H → HO2 (2.29)
• • HO 2 + HO 2 → H 2 O 2 + O 2 (2.30)
− • − O2 + HO2 → O2 + HO2 (2.31)
− + HO2 + H → H 2O2 (2.32)
hν • H 2O2 ⎯⎯→ 2OH (2.33)
− • − H 2O2 + O2 → OH + OH + O2 (2.34)
− • − H 2O2 + eCB → OH + OH (2.35)
The photogenerated holes also possess a high oxidising power especially in the case of
UV illuminated TiO2.
2.2.3.3.1 Mechanism of visible light degradation of 4-CP by 4%H2[PtCl6]/TH Evidences for the key species in the reaction To start with the mechanistic investigation of the visible light degradation of 4-CP by 4%H2[PtCl6]/TH experiments were conducted to trace out the key radicals. It was found that OH radicals and also intermediary Cl atoms are produced during this reaction and that they play a vital role in driving this reaction.[44] OH radicals Scavenging experiments with benzoic acid[44] in the irradiated catalyst suspension in the presence of oxygen, where salicylic acid was formed stand as an evidence for the formation OH radical. However, these radicals were observed only in the case of employing oxygen as electron scavenger. When other electron scavengers like tetranitromethane (10-2 M) were used, only traces of salicylic acid were detected along – with the C(NO2)3 anion (Eq. 2.36).
60
− − • C()NO2 4 + eCB → C ()NO2 3 + NO2 (2.36)
This anion is stable and yellow coloured and was detected by its typical absorbance at 350 nm.[129-131] Additionally, fast formation of salicylic acid occurred when the same experiment was conducted with UV light (λirr > 335 nm). This observation can be reasoned out to be due to oxidation of water or surface-hydroxyl groups to OH radicals by holes generated in the valence band of titania on UV excitation. From these experiments it was concluded that OH radicals are formed in the visible light excitation of the catalyst and the route is only through the reductive pathway.[44] Intermediary Cl atoms The experimental evidence for intermediary Cl atoms is the formation of chlorophenols, when phenol was used as the substrate instead of 4-CP under otherwise identical conditions.[44]
Another evidence is the degradation of HCOOH by 4%H2[PtCl6]/TH even under argon atmosphere in the presence of the electron scavenger AgNO3 under
λirr ≥ 455 nm (Figure 2.20). Control adsorption studies confirmed that the disappearance of the acid is not due to adsorption factors. A black colour which intensified with the increase in irradiation time was observed. This suggested that the
AgNO3 in the system was reduced to Ag by the electrons generated by the visible light excitation of 4%H2[PtCl6]/TH. Since the electrons were scavenged by AgNO3 and further there was no oxygen in the system, the possibility of formation of OH radicals through the reductive pathway seems to be completely blocked. Further, unlike titania, in the case of 4%H2[PtCl6]/TH, only visible light is employed for excitation and there exists no oxidative pathway where OH radicals are formed through reactive holes. Since under these conditions, the formation of OH radicals seems to be inhibited, even then we observe efficient HCOOH degradation, most likely by the light-induced Cl atom which has an oxidising potential of ~1.3- 2.3 V.[128] HCOOH, which has an oxidation potential of 1.9 V,[132] could be oxidised only by OH (~2.4 V) or the intermediary Cl atoms. It is noted that formation of Cl atoms was observed also in the titania catalysed photodegradation of trichloroethylene.[133]
61
1,0
Adsorption study, dark, (without AgNO3)
0,8 0
Adsorption study ,dark, (with AgNO3) /TOC
t 0,6 TOC AgNO /Ar/455 nm 0,4 3
455 nm 0,2 0246 Irradiation time / h
Figure 2.20: Efficient degradation of HCOOH by 4%H2[PtCl6]/TH, even under argon atmosphere in the presence of AgNO3 as electron scavenger (For details refer to text).
2.2.3.3.2 Proposed mechanism Two redox centers are formed due to the excitation of - 4%H2[PtCl6]/TH. The oxidative one is described as a kind of Cl / Cl pair weakly bound to a Pt center. It is assumed that the oxidation potential of the adsorbed Cl atom is in the range of 2.6 - 1.3 V,[128] i.e. weaker than that of Cl / Cl- pair in aqueous 0 [134] solution, (E Cl/Cl- = 2.6 V) and stronger than molecular chlorine 0 [135] 4+ 3+ (E 1/2Cl2/Cl = 1.3 V) and 4-CP (1.18 V). The reductive center is Pt / Pt , whose redox potential is not available in the literature since the fast reduction of Pt4+ to Pt3+ and Pt0 makes its direct measurement difficult or almost impossible.[128] Though some stable Pt(III) complexes have been isolated and characterized,[136] time resolved diffuse reflectance spectroscopy[137] show that the oxidation state III is reached only in unstable intermediates of photoaquation reactions,[76, 77, 79] and on 62
[44] surfaces of TiO2. Photocurrent measurements confirm that the injection of an electron in to the conduction band of TiO2 by 4%H2[PtCl6]/TH occurs at pH 7. So it is logical to conclude that the redox potential of the surface Pt4+ / Pt 3+ couple should be equal or more negative than -0.28 V, the quasi-Fermi level of 4%H2[PtCl6]/TH at pH 7.[44] Thus the redox potential of Pt4+/3+couple is estimated to be at ca. -0.3 to -0.4 V and that of EPt-(Cl/Cl-) as ca. -0.4 V + 2.0 V = 1.6 V. Based on these values, the experimentally obtained positions of valence and conduction band edges, and other [138] relevant potentials taken from literature, a potential diagram for 4%H2[PtCl6]/TH is constructed[44] (Scheme 2.1). Since it is postulated that the surface modification of
H2[PtCl6] with TH is a chemisorption of the platinum complex on titania affording a surface tetrachloroplatinate(IV) complex covalently linked to the titania surface IV n- through a [Ti]-O-Pt bond, the catalyst is represented as {[Ti]-O-Pt Cl4L} , where - L = OH , H2O in the potential diagram (Scheme 2.1).
~ -0.3 to -0.4 V
− III n- − {[Ti]-O-Pt -Cl4L} CB -0.28 V - O2/O2 hν 0.16V IV n- {[Ti]-O-Pt -Cl4L} - 4-CP ½Cl2/Cl TiO 1.3 V 1.18 V 2 + ~ 1.6 to 2.3 V HCOOH 1.9 V + 0 n- + {[Ti]-O-PtCl -Cl3L} ⋅OH/OH- - ~ 2.4 V free Cl/Cl : VB 2.93 V ~ 2.6 V
Scheme: 2.1: Potential diagram of 4%H2[PtCl6]/TH at pH = 7. All potentials are given for pH 7 vs. NHE.[44] 63
Based on the observation that degradation was only observed when the platinum complex was attached to the semiconducting metal oxide, but not to silica or alumina the following mechanism was proposed for visible light degradation of 4-chlorophenol [45] by 4%H2[PtCl6]/TH. Light absorption by the titania - halogenoplatinate complex leads to a homolytic Pt-Cl cleavage affording an adsorbed chlorine atom and a Pt(III) complex as primary intermediates[75, 80] (Scheme 2.2, A). Injection of an electron from the latter into the conduction band of titania and subsequent reduction of oxygen reforms Pt(IV) and reduces oxygen to superoxide (Scheme 2.2, B). The latter can be converted to the OH radical via well known reaction steps (Eqs. 2.46 - 2.52) The OH radicals may oxidise
4-CP which is eventually converted to CO2, H2O and HCl in analogy with the photomineralisation catalysed by unmodified titania [139, 140] (Scheme 2.2, C). The adsorbed chlorine atom oxidises 4-CP via an oxyl radical to the above mentioned final products. (Scheme 2.2, D). Through this oxidation of 4-CP, chloride ligand is regenerated and therefore also the catalyst. (Scheme 2.2, E). However, the possibility of an alternate mechanism where the excited platinate complex may be converted to a Pt(V) intermediate cannot be totally ruled out though it is thermodynamically less favoured.
64
III Ti O Pt Cl
O2 ArOH
B A E D VIS
- • O2 ArO + H+ IV H+ Ti O Pt Cl
• ArOH H2O2 , OH
C
• ArO + H+
Scheme 2.2: Proposed mechanism for visible light degradation of 4-CP by [45] 4%H2[PtCl6]/TH ( X = Cl or Br).
2.2.3.4 Visible light sulfoxidation of adamantane Adamantane is a highly symmetrical hydrocarbon and is unique for its diamond like structure. This white solid possesses six secondary carbons and four tertiary (bridgehead) carbons. A detailed study of various reactions of adamantane reveals its general preference for tertiary position for any attack.[15] It is easy to handle and can be dissolved even in polar solvents like methanol. The resulting solution owing to the high polarity of the solvent, forms an excellent and uniform suspension with our photocatalyst powders which is very essential for an effective heterogeneous catalysis. Moreover, the UV and thermal sulfoxidation of this 65 alkane has been well studied.[15, 22] Taking the above merits into consideration, adamantane was chosen as a model alkane for visible light sulfoxidation. In the experiment, adamantane (1 mmol, 136.24 mg) was dissolved in 15 mL of methanol and 4%H2[PtCl6]/TH (30 mg, i.e. 2 g / L which was the optimised concentration of the catalyst to produce a maximum yield of 1-adamantane sulfonic acid) was added. The resulting suspension was sonicated for 1 min and filled into a cuvette. Then the cuvette was sealed with silicone rubber and sulfur dioxide (30 mL, 1.3 mmol) and oxygen (30 mL, 1.3 mmol) were metered simultaneously into it. A cut–off filter of λ ≥ 400 nm was placed in front of the cuvette. The suspension was stirred magnetically and irradiated. Samples were taken at regular intervals and the photocatalyst was filtered through a microporous organic filter (Whatman with pore size of 0.45 µm). The filtrate was degassed of sulfur dioxide by purging with nitrogen for 10 min and was injected into HPLC and analysed using the technique of indirect photometric detection. To find out if only adamantane is sulfoxidised, a blank experiment was performed in the absence of this cyclic hydrocarbon under otherwise identical reaction conditions. Surprisingly no hydroxymethanesulfonic acid, the possible product of methanol sulfoxidation could be detected by HPLC (Figure 2.21).
66 absorbance / a.u
retention time / min
Figure 2.21: Chromatogram of the blank reaction in methanol in the absence of adamantane after 10 h irradiation which reveals no characteristic peak confirming the non-sulfoxidation of methanol.
2.2.3.4.1 HPLC with Indirect photometric detection[141] The alkanesulfonic acids are UV transparent and therefore cannot be detected by the conventional high performance liquid chromatographic method (HPLC) using a UV detector. Therefore, HPLC with indirect photometric detection (IPD) was employed. IPD or indirect photometric chromatography (IPC) is a technique which involves the detection of light transparent ionic species using photometers with strong light absorbing eluent ions. This facilitates the light transparent sample ionic species to appear as troughs (negative peaks) in the base line as they substitute the strong light absorbing displacing ions in the ion exchange column.[141] The elution time of these sample negative peaks vary with the nature of the sample ion injected and their areas are proportional to the amount of the samples injected. 67
In this method, the eluent is made light absorbing (generally UV light, also in our case of alkanesulfonic acids) by adding strong UV absorbing ions in the eluent. The important condition is the charge of the UV absorbing ions should be the same as that of the sample ions to be separated and detected. These strong UV light absorbing species perform a dual role as given below: 1. Selective displacement of the sample ions from the ion-exchange chromatographic column. 2. Indication of the sample ions in the chromatogram as negative peak.
2.2.3.4.2 Principle Let us consider an ion-exchange column, for example an anion exchanger which is pumped and equilibrated with an electrolyte represented as Na+E-. This results in the occupation of all sites in the exchanger by the eluent ions, E-. A detector which can accurately sense all the ionic species is placed at the outlet of the eluent. It reveals a steady level of Na+, E- when the input concentration of the eluent is kept constant. (A in Figure 2.22)
Na+ Na+
E- S-
Absorbance (a.u) Absorbance (a.u) S-
Elution Volume Elution Volume
A B
Figure 2.22: Principle of indirect photometric detection. A: Before injection of the sample and B: After injection of the sample. 68
Now let us inject a sample represented as Na+S-. Then the sample anion S- will be generally retarded by the stationary phase, and will exit at a characteristic elution volume and could be detected by a detector placed at its exit. The detector could detect the concentration of the S- to rise and fall in a similar fashion as it leaves the column (B in Figure 2.22). The elution volume is determined by certain factors, like capacity of the exchanger, concentration of the solution, affinity of the stationary phase for S- relative to E-. In the case of conventional ion-exchange liquid chromatography, the system is well devised employing suitable detectors to monitor directly the magnitude of these sample peaks S-. However what is not considered or forgotten is the fact that according to the principle of electroneutrality and equivalence of exchange, there must be a concerted and equivalent change in E- along with the appearance of S-, given that the total equivalent concentration of anions (S- and E-) should remain fixed as the concentration of sodium co-ions is fixed. Therefore the concentration of S- in the effluent could be indirectly monitored continuously by the level of the eluent ion E- and hence the name, indirect photometric detection or indirect photometric chromatography. Thus, this feature of the ion exchange mode may be usefully tapped in the case of problematic sample ions. If sample ions do not possess a particular property, for example, UV-light absorbance, one may exploit this deficiency by deliberately choosing an eluent ion that is strongly UV-light absorbing and monitoring the negative peaks generated in the base line absorbance when transparent sample ions elute. This technique is applied for detection of several important UV transparent ions like chloride, nitrate, sulfate etc. (Figure 2.23).
69
Figure 2.23: Separation and detection of non-light absorbing species in IPC.[141] a) chloride b) nitrite c) bromide d) nitrate e) sulfate.
The development of combined ion exchange and indirect photometric monitoring is the main concern of this technique developed by Small et. al [141] The example of how the alkanesulfonic acids are detected in our case may serve to illustrate how the method works. The column is a strong anion exchanger and water-acetonitrile (60 / 40, v / v) with 0.01 M potassium hydrogenphthalate as UV absorbing counter ion was employed as the eluent. The flow rate was 2 mL / min, and the baseline was set up relating to this eluent and detection was made employing a UV detector. When the 70 alkanesulfonic acid was injected for detection, it replaces the eluent ions in the column by the ion - exchange mode, and since it is non-UV absorbing it is quantified as a negative peak. This method for alkanesulfonic acids was first published by Larson.[141, 142]
2.2.3.4.3 Influencing factors for IPD[141] In IPD the sample ions are detected and quantified by the decrement they produce in the eluent concentration. Since the displacing species is usually in much greater abundance than the sample species, a feature of elution chromatography, these decrements would ordinarily represent rather small fractional changes in the eluent level. Thus the accuracy of IPC is directly related to how exactly we can measure these fractional differences (the noise) of the baseline response. The signal to noise ratio, i.e. the sensitivity is related to various parameters which are discussed below: 1. Concentration of the eluent The sensitivity of the IPC peak is given as:
Signal C ()A − A = S S E (2.37) Noise NCE AE
Where CS and CE are concentration of sample and eluent respectively, AS and AE are absorbance of sample and eluent respectively while N is the fixed fluctuation which is random and is represented by noise at a particular base line absorbance. In the case of transparent ions, AS = 0 and so the equation reduces to
Signal C = S (2.38) Noise NC E
This reduced expression confirms that when the concentration of the eluent is low, the sensitivity of IPC is higher. 71
However, use of too dilute eluent solutions results in peak broadening and hence loss of sensitivity. Also, for ideal resolution the run time of the chromatogram should not be longer than the time necessary for resolving the negative peaks satisfactorily. 2. The relative affinity of E- and S-[141] The displacing power of the eluent ion with respect to the sample ions is important factor in IPC. Since different ions vary widely in their displacing power, the selection of appropriate eluent is a long process. Several experiments had been made to categorise and arrive at easy choices for the analyst and reported that iodide, o-phthalate, 1,2 sulfobenzoate and 1,3,5-benzenetricarboxylate (trimesate) are effective displacing ions.[141] The general trend is that the polyvalent ions being more potent displacing species than monovalent ions. However the trend is not observed in all cases as the effect of charge on eluent and sample ions together with concentration of eluent are also influencing factors in this measurement. 3. Photometric factors All principles that apply to conventional spectrophotometric measurements also apply to IPC, since accurate determination of absorbance (concentration) of eluent is a very vital part of IPC. The concentration of eluent to be used will generally be dictated by such other considerations as column capacity and eluent ion affinity and cell path length. Since the cell path length is fixed, the general wavelength dependant nature of the molar absorptivity of a given eluent ion is exploited to tune the eluent absorbance. Therefore, appropriate selection of wavelength for detection results in the desired optical absorbance of the eluent. A diode array detector which can operate under wide range of wavelength is therefore a very useful accessory to IPC. However under certain conditions efficient performance of fixed wavelength devices is also observed. When appropriate detection wavelength is selected, “optimum absorbance” requirement of IPC was accomplished even under a large range of eluent concentrations from l0-4 M to 1 M. Advantages of IPC 1. Single column simplicity 2. Employment in detection of an extensive range of ionic species 72
3. Fundamentally greater sensitivity than those of single – column conductometric measurements.
2.2.3.4.4 Analysis by IPD with HPLC Concentration of the sulfonic acids were measured by HPLC, (SCL 10 AVP system controller, SP10 AVP model UV detector, column: (250 x 4.6 mm I.D Partsil 10 SAX (Whatman) which is a strong anion exchanger with + -N R3 functionality, and is Si-O-Si bonded to partisil). Water-acetonitrile (60 / 40, v / v) with 0.01 M potassium hydrogenphthalate as UV absorbing counter ion was employed as the eluent.[142] Detections were made at 304 nm where the mobile phase has very high absorbance and the sulfonic acid is transparent. The pH value of the eluent with the counter ion was 5.8. In the case of 1-adamantanesulfonic acid, first it was synthesized from visible light sulfoxidation of adamantane by 4%[H2PtCl6]/TH and isolated as per the procedure below as it is not commericially available, and then its calibration curve was made.
2.2.3.4.5 Isolation of 1-adamantanesulfonic acid The visible light sulfoxidated suspension of adamantane was degassed of sulfur dioxide by purging with nitrogen for 10 min. Then the photocatalyst was filtered through a microsporous filter (Whatman 0.45µm). The filtrate was concentrated to a pale yellow viscous residue which was dried in a vacuum desiccator. 1-adamantanesulfonic acid was isolated similar to the method described by Smith et al.[15]After addition of ethyl acetate, a white solid was obtained, which was filtered and carefully recrystallized with ethyl acetate to afford colourless crystals of 1-adamantanesulfonic acid (0.15 mmol, 21 mg, 15% yield). Composition and structure of 1-adamantanesulfonic acid were verified by standard characterization methods including X-ray crystallography. They were all in concordance with literature values.[22] The elemental analysis report, IR, NMR, and mass spectra of 1-adamantanesulfonic acid are given in Experimental section.
73
1-adamantanesulfonic acid was dissolved in methanol and was injected to HPLC producing a negative peak with retention time of Rt = 5.1 min (Figure. 2.24) at the flow rate of 2.0 mL / min.
0.06
0.04
0.02
0.00
-0.02
Absorbance (a.u) 5.18
-0.04
-0.06 0123456789 t /min
Figure 2.24: Chromatogram of 1-adamatanesulfonic acid in methanol, which is revealed as a negative peak at the retention time (Rt) of 5.18 min.
Further, a calibration curve (Figure 2.25) was made in order to measure the concentration of 1-adamantanesulfonic acid during visible light sulfoxidation of adamantane.
74
1000000
800000
600000
400000
peak area / a.u. 200000
0 0 5 10 15 20 [1-adamantanesulfonic acid] / mM
Figure 2.25: Calibration curve for 1-adamantanesulfonic acid in methanol.
2.2.4 Results of adamantane sulfoxidation in methanol
When to a suspension of 4%[H2PtCl6]/TH in a methanolic solution of adamantane were added sulfur dioxide and oxygen, followed by subsequent irradiation with visible light, the formation of 1-adamantanesulfonic acid was observed. The Turnover Number (TON) of the reaction after 10 h (which was the optimized irradiation time for maximum yield of sulfonic acid) was 21 (Eq. 2.40) In homogeneous catalysis the Turnover Numbers (TON) of the reaction[143] are determined employing the Eq .2.39.
75
Volume concentration of product formed TON in = homogeneous cataylsis Volume concentration of catalyst (2.39)
However, in the case of more complex heterogeneous catalytic systems like we have the calculation is ambiguous. We have used the following equation often employed in heterogeneous catalysis if the number of active sites is unknown.
Amount of product formed TON in = heterogeneous cataylsis Amount of active material (Pt in our case) in the catalyst (2.40)
There was no formation of 1-adamantanesulfonic acid in the absence of the catalyst, and the reaction ceases when the irradiation is stopped. The corresponding bromo complex was also active but induced a smaller TON of 8 after 10 h. Only traces of 1-adamantanesulfonic acid were observed when unmodified TH was employed, whereas 1-adamantanesulfonic acid was completely absent when hexachloroplatinic acid was supported onto silica, alumina or amorphous titania.[144] Hexachloroplatinic acid itself and amorphous titania were also inactive. The yield of 1-adamantane sulfonic acid produced by the visible light sulfoxidation of adamantane catalysed by various photocatalysts under various irradiation times is shown in Figure 2.26 and summarized in Table 2.5. 76
% yield of 1-adamantanesulfonic acid Photocatalyst Rt [min] TON 4 h 10 h
4%[H2PtCl6]/TH 5.2 8 12 21
Cal.6%[H2PtBr6]/TH 5.1 2 5 8
4%RhCl3 /TH 5.2 5 21 41
TiO2- C 5.1 3 10 - TH 5.1 1 1 -
TiO2 (anatase,TiCl4) 5.1 1 1 -
TiO2 (amorphous, - No reaction No reaction - TiOSO4)
Table 2.5: Formation of 1-adamantanesulfonic acid on visible light sulfoxidation of adamantane in methanol, at different irradiation times, employing various photocatalysts.
77
12 b
10
8
6 c
% yield of 4
2
adamantane-sulfonic acid a 0 f'
0246810 irradiation time / h
Figure 2.26: Yield of 1-adamantanesulfonic acid in visible light sulfoxidation of adamantane in methanol, at different irradiation times. a) TH b) 4%[H2PtCl6]/TH c) cal.6%[H2PtBr6]/TH f’) TiO2(amorphous), H2PtCl6 or H2PtCl6 supported on SiO2,
Al2O3 and TiO2 (amorphous).
2.2.5 Sulfoxidation of other alkanes Visible light sulfoxidation of other alkanes like n-heptane, n-hexadecane, DL-camphor were performed analogous to adamantane. The yields for n-heptane and n-hexadecane were relatively less compared to that of adamantane. The reason may be the suspensions of these alkanes in methanol or acetic acid with the photocatalysts were not uniform and good. Sulfoxidation of DL-camphor demonstrated identical conditions and yield as that of adamantane. This could be attributed to its similar structure and properties like that of adamantane. The results of 78 sulfoxidation of these alkanes along with adamantane for comparison, is shown in the Table 2.6. Quantitative isolation of the alkanes was performed in an immersion lamp apparatus and the experimental details and procedures for isolation are reported in the Experimental section.
Amount of Amount of R-H RH alkanesulfonic acid TON [mmol] [mmol]
1.0 0.11 21
O 1.0 0.10 20
CH3(CH2)5CH3 50.0 0.06 11
CH3(CH2)14CH3 25.0 0.03 8
Table 2.6: Yield and TON for various alkanes after 10 h of irradiation (λ ≥ 400 nm) in methanol by 4%[H2PtCl6]/TH.
2.2.6 Mechanism of visible light sulfoxidation of adamantane in
methanol by 4%[H2PtCl6]/TH Based on the observation that photosulfoxidation was only observed when the platinum complex was attached to the semiconducting metal oxide, we propose a similar mechanism as recently formulated for visible light degradation [45] of 4-chlorophenol by 4%[H2PtCl6]/TH. Light absorption by the titania - halogenoplatinate complex leads to homolytic Pt-X cleavage affording an adsorbed halogen atom and a Pt(III) complex as primary intermediates (Scheme 2.3, A). Injection of an electron from the latter into the conduction band of titania and subsequent reduction of oxygen to superoxide (Scheme 2.3, C), reforms Pt(IV) (Scheme 2.3, B). The latter can be converted to the OH radical via well known 79 reaction steps.[145-149] The OH radicals may abstract hydrogen from alkane to give alkyl radicals (Scheme 2.3, D). Additionally hydrogen abstraction from the alkane that may occur by the intermediary halogen atoms, also produces alkyl radicals (Scheme 2.3, E), thus regenerating the halide ligand (Scheme 2.3, F). The alkyl radicals formed attack sulfur dioxide and finally give rise to formation of the sulfonic acid (see Eqs 1.4 - 1.9) as described in the general mechanism of sulfoxidation produce 1-adamantanesulfonic acid.
III Ti O Pt X
O2 RH
C B A F E VIS
R + H+ O2 IV Ti O Pt X
RH H+ SO2
OH D
RSO2 H2O + R
RSO H 3
Scheme 2.3: Proposed mechanism for visible light sulfoxidation of adamantane by titaniachloro or bromo platinate. X = Cl or Br. 80
The proposal that the intermediary halogen atoms can undergo hydrogen abstraction to provide alkyl radicals is supported by inhibition experiments with silver nitrate (0.1 M). Under otherwise identical reaction conditions but in the presence of silver nitrate, one observes the formation of elemental silver as indicated by the black color formed on the photocatalyst particles. This indicates that the light generated electrons predominantly reduce silver ions instead of oxygen, thus inhibiting the formation of OH radicals. Surprisingly the yield of 1-adamantanesulfonic acid formed after 10 h of irradiation is almost the same as compared to the absence of silver nitrate. The same effect is observed when silver nitrate was replaced by tetranitromethane as electron scavenger. From these experiments it is concluded that the halogen atom is able to produce an alkyl radical necessary for sulfoxidation, whether this occurs through hydrogen abstraction or through oxidation cannot be decided on the basis of the present experimental evidence. The oxidation reaction of adamantane can be written as follows (Eq. 2.41) and oxidation potential of adamantane under standard conditions is given as 2.96V.[150, 151]
. Ad − H → []Ad − H +. + e− (2. 41)
Where Ad-H refers to adamantane However the real oxidation potential under reaction conditions for adamantane was calculated employing Nernst equation.
. 0 0.059 [Ad ] E = E Ad −H + log (2. 42) Ad −H n []Ad − H
‘n’ refers to the number of electrons transferred in this reaction and so in this case, n = 1 The concentration of adamantane employed in visible light sulfoxidation in methanol was 66.67 mM. Since it is well known that here the concentration of reduced form (Ad-H) is in large excess compared to that of oxidized form (Ad.), it is assumed that 81 the ratio is ~ 107:1. Applying this assumption the actual reaction oxidation potential of the adamantane is calculated to be 2.5V. Since the oxidation potential of the adsorbed Cl atom is estimated to be in the range of 2.6 - 1.3 V,[128] the oxidation of adamantane by Cl atom seems thermodynamically 0 [134] 0 [135] feasible. (E Cl/Cl- = 2.6 V, E 1/2Cl2/Cl = 1.3 V. ). The assumption that subsequent electron injection from Pt(III) into the conduction band of titania regenerates Pt(IV) is supported by the experimental evidence that there is no reaction when the support is changed from titania to non-semiconductors like silica or alumina. The lesser activity of Cal.6%[H2PtBr6]/TH compared to that of
4%[H2PtCl6]/TH is comparable with its photocatalytic activity in visible light degradation of the ubiquitous pollutant 4-chlorophenol. In both the reactions
Cal.6%[H2PtBr6]/TH is around 3 fold less active than 4%[H2PtCl6]/TH. The lower oxidation potential of the bromine atom (0.65 V vs. NHE)[40, 152, 153] may explain the difference.
2.2.7 Influence of metal complexing agents in visible light sulfoxidation 2.2.7.1 Acetylacetone Since it was observed that the Cl ligand in the metal complex chemisorped to TiO2 plays a vital role in its photocatalytic activity, we wanted to explore the role of better complexing agents other than Cl in our catalyst. It was found that direct addition of acetylacetone to TiO2 makes this white powder pale yellow. DRS shows a shift in absorption towards visible region for all commercially available
TiO2 on contact with acetylacetone (Figure 2.27). Furthermore, since it is well known that acetylacetone is a good transition metal chelating agent,[154] it was added in the system so that it could chelate with Pt to form a more stable and efficient complex replacing the Cl ligands and thereby increasing the efficiency of the catalyst to absorb visible light. As an experimental support for this hypothesis, a significantly higher activity was observed when acetylacetone was employed as an additive.
82
g a 0.05 k*a*
0.04
0.03 ) ∞
F(R 0.02
0.01 k
g* 0.00 300 400 500 600 λ / nm
Figure 2.27: DRS spectra of various titania powders in the presence and absence of acetylacetone. a) TH g) P25 k) TiO2 (sol-gel method). a*, g*, k* represent the catalysts in the presence of acetylacetone (10µL). In all cases 50 mg of catalyst powder / 2 g of BaSO4 were employed (see Experimental section).
4+ The well studied interaction between adsorbates and the Ti ions on TiO2 surfaces is coordinative covalent bonding (CCB). Due to the low lying empty t2g orbitals of the Ti4+ centers in octahedral environments, their interaction with adsorbates lead to appearance of LMCT bands. With electron rich adsorbates like enediols (catechol,[155] ascorbic acid,[156] dopamine,[157] alizarin[158] etc.), carboxylates (sulfanylacetic acid,[159] 4-methylsulfanylbenzoic acid[160, 161]), nitrile (ferricyanide[162, 163]) and alcohol (4-hydroxybiphenyl[164]), LMCT bands are displayed in visible 83 region. Less electron-rich adsorbates such as thiocyanate revealed the corresponding LMCT band in the UV region.[165] Since acetylacetone is also an electron rich adsorbate, it is proposed that its LMCT bands are displayed in visible region. DRS spectra clearly support this proposal since there is a shift in the absorption towards visible region when acetylacetone is added to
TiO2 (Figures 2.27 and 2.32) Control experiments, where acetylacetone was added to
BaSO4 or SiO2 did not show any shift in absorption or colour change of the powder; the strong absorption of acetylacetone appeared at 280 nm.
50 d'
40 b' c'
30
20 d
% yield of b 10 c adamantane-sulfonic acid 0
0246810 irradiation time / h
Figure 2.28: Increase in the yield of 1-adamantane sulfonic acid in methanol in the presence of acetylacetone (1mmol). b, c, d represent 4%H2[PtCl6]/TH, cal.6%H2[PtBr6]/TH, and cal.4%RhCl3/TH respectively, in the absence of acetylacetone and b’, c’, d’ represent the same catalysts in the presence of acetylacetone (1mmol). 84
j' 30 a'
25 k' 20
% yield of 15 j 10 adamantane-sulfonic acid adamantane-sulfonic
5 a, k l, l' 0 0246810 irradiation time / h
Figure 2.29: Increase in the yield of 1-adamantanesulfonic acid in methanol in the [86] presence of acetylacetone (1mmol). a, j, k, l, represent TH, C-TiO2, anatase (self prepared) and amorphous TiO2 respectively, in the absence of acetylacetone and a’, j’, k’, l’ the same catalysts in the presence of acetylacetone (1mmol).
The results of visible light sulfoxidations of adamantane in methanol by various photocatalysts in the absence and presence of acetylacetone are displayed in Figures 2.28 - 2.30 and summarized in Table 2.7. All catalysts exhibited an enhanced activity in the presence of acetylacetone.
Especially in the case of 4%[H2PtCl6]/TH, the yield of 1-adamantanesulfonic acid after 10 h increased from 12 to 39% which is more than three folds increase. cal.6%H2[PtBr6]/TH also exhibited similar trends. Carbon modified titania (TiO2-C) 85 also revealed an increase in yield of 1-adamantanesulfonic acid with acetylacetone, i.e. from 10 to 30%.
A special attention has to be given to the anatase modifications TH, TiO2 (anatase, self prepared) which are not active in the absence of acetylacetone, but displayed a prominent activity in its presence. It was observed that only anatase modifications of titania showed an significant activity in the presence of acetylacetone, while amorphous modifications were inactive both in the presence and absence of acetylacetone. However, when TH was premodifed with acetylacetone and employed for sulfoxidation in methanol, it turned out to be inactive. The DRS of the premodifed TH with acetylacetone also did not reveal any characteristic shift in absorbance towards visible region. The preparation procedure for this compound is given in Experimental section.
d
i
c
a
c 0
ni 5
o
f
f
l
o
u 0 e
d s n
l 4 to
e
e e i c
n la
y a y
0 t
t 3 ce n % a h a it 0 w ne m 2 to a ce
d a 0 yl
a t
1 e - ac
1 t 0 ou th a d b wi c j k l Photocatalysts
Figure 2.30: Yield of 1-adamantanesulfonic acid (after 10 h) in methanol in the presence and absence of acetylacetone. a) TH b) 4%[H2PtCl6]/TH c) cal.6%[H2PtBr6]/TH d) RhCl3/TH j) TiO2-C k) TiO2 (anatase) l) TiO2 (amorphous). 86
Photocatalyst % yield of TON 1-adamantanesulfonic acid without Hacac with Hacac without Hacac with Hacac
4%[H2PtCl6]/TH 12 39 16 76 cal.6%[H2PtBr6]/TH 5 37 8 72
4%RhCl3 /TH 21 45 41 118
TiO2-C 10 30 - - TH 1 29 - -
TiO2 (anatase,TiCl4) 1 22 - -
TiO2 (amorphous, - - - -
TiOSO4)
Table 2.7: Visible light sulfoxidation of 1-adamantanesulfonic acid in the solvent methanol in the presence (1 mmol) and absence of acetylacetone after 10 h irradiation.
2.2.7.2 Other complexing agents Unlike acetylacetone, other complexing agents like hexafluoroacetylacetone pyrophosphate, ethylene glycol, sodium- dihydrogen phosphate, acetic acid did not produce an significant enhancement in the yield in the visible light sulfoxidation of adamantane in methanol in the presence of
4%[H2PtCl6]/TH (Figure 2.31).
87
15
p
10 q r
% yield of 5 1- adamantanesulfonic acid 1- adamantanesulfonic
0 0 3 6 9 12 15 18 21 24 irradiation time / h
Figure 2.31: Influence of additives (1 mmol) in visible light sulfoxidation of adamantane in methanol in the presence of 4%[H2PtCl6]/TH. p) without additive q) with hexafluoroacetylacetone r) with pyrophosphate.
2.2.8 Mechanistic investigations for visible light sulfoxidation in the presence of acetylacetone It was observed that metal complex modified titania show an enhanced activity in sulfoxidation in the presence of acetylacetone. This may be justified by the fact that acetylacetone being a chelating agent complexing much better than the Cl or Br atom stabilizes the intermediate Pt(III) complex. It was interesting to find that unmodified titanias were also active in visible light sulfoxidation in the presence of acetylacetone and to investigate the reason, DRS of the TH in the presence of acetylacetone, acetic acid and methanol (Figure 2.32) and the bandgap determinations (Figure 2.33) were made. 88
a
0.05 }
0.04
0.03 ) ∞
F(R 0.02
0.01 a*
0.00 300 400 500 600 λ / nm
Figure 2.32: DRS of naked TH or in the presence of acetic acid or in methanol (a) (all the three were almost identical). a* refers to the DRS of TH in the presence of acetylacetone.
DRS of TH clearly shows a shift in absorption towards visible region only in the case of acetylacetone addition and also a bandgap narrowing from 3.21 to 3.11 eV. This explains the reason for visible light activity and it is proposed that the acetylacetone is complexed directly with titania and drives the reaction.
89
2,0 3 s a t 2 1,5 1/2 )E) ∞ 1 (F(R 1/2 a )E)
∞ 1,0 0 2,0 2,5 3,0 3,5 4,0
(F(R E / eV u 0,5
0,0 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 4,0 E / eV
Figure 2.33: Plot of transformed Kubelka-Munk function vs. energy of light absorbed. a) TH s) TH + Hacac t) TH + MeOH u) TH + AcOH. Only in the case where TH is in contact with acetylacetone, there is a significant narrowing of the band gap from 3.21 to 3.11eV.
Surprisingly TH and P25 were active even in the absence of acetylacetone and displayed an enhanced activity in its presence only in the case of the solvent acetic acid. It may be justified due to the good bridging or chelating nature of the acetic acid itself.[154] However, a shift in absorbance towards visible light or band gap narrowing could not be observed with acetic acid or methanol. The results of DRS of TH with methanol correlate very well with the observation of non-activity of titania in visible light sulfoxidation when acetylacetone is excluded. 90
2.2.9 Mechanism of visible light sulfoxidation of adamantane in the presence of acetylacetone by metal complex modified and
unmodified TiO2 in methanol
TiO2 powders instantly turn pale yellow on physical contact with acetylacetone. It is proposed that acetlyacetone complexes with the surface OH groups as given in Eq.2.43 below:
H H H H H H H H H H H H H H H H Hacac O O O O O O O O O O O O O O O O O O - Ti Ti Ti Ti Ti - H2O, OH Ti Ti Ti Ti Ti O O O O O O O O O O O O O O O O
(2.43) Based on the observation of DRS and shift in bandgap of titania and also an enhanced yield in the presence of acetylacetone, the mechanism of visible light sulfoxidation in the presence of acetylacetone is proposed by analogy with the sulfoxidation in the presence of metal complex modified titania. The main difference is that acetylacetone as an efficient ligand replaces Cl or Br. In the case of pure TiO2 catalysing the reaction, it is postulated that surface titania centers are complexed with acetylacetone directly and this complex is able to absorb visible light (Scheme. 2.4).
91
RH III[Ti] O O2 O R. + H+
O2 SO2 H+ Vis OH. . RH RSO2
IV[Ti] O . R + H2O O TiO 2
Scheme.2.4: Mechanism of visible light sulfoxidation of adamantane by TiO2 in the presence of acetylacetone.
2.2.10 Experiments in acetic acid Visible light sulfoxidation of adamantane was performed analogous to methanol in another solvent acetic acid. 1-adamantanesulfonic acid in acetic acid and was detected as a negative peak with retention time, Rt = 4.89 min (Figure. 2.34) at the flow rate 2.0 mL / min by HPLC with IPD. Similar to the case of methanol, visible light sulfoxidations of adamantane in acetic acid were performed with various catalysts in the presence (1mmol) and absence of acetylacetone and are displayed in the Figures 2.35 and 2.36 and summarized in the Table 2.8.
92
0.3
0.2
0.1
0.0 4.89
Absorbance (a.u) Absorbance -0.1
-0.2
0123456789 t /min
Figure 2.34: Chromatogram of 1-adamatanesulfonic acid in acetic acid, which is revealed as a negative peak at the retention time (Rt) 4.89 min. The negative peak at the Rt ~ 2 min corresponds to the solvent acetic acid.
93
45 40 a 35 30 a' 25 j 20 g 15
% yield of % yield m 10 j' 5 g' 1-adamantanesulfonic acid 0 m' -5 0246810 irradiation time / h
Figure 2.35: Increase in the yield of 1-adamantanesulfonic acid in acetic acid in the presence of acetylacetone (1mmol). a, g, j, m, represent TH, P25, TiO2-C and TiO2-N (urea modified)[89] in the absence of acetylacetone, respectively and a’, g, j, m’ represent the same catalysts in the presence of acetylacetone (1mmol).
94
d
i
c
a
c i
n 0
4
f
fo l
o e
u on d t
s 0 l e
3 c e
e la i
n ty
y e
c
ta a
0 h e n n % 2 it o t a w e
ac m l 0 ty a 1 ce d t a a ou - - 0 ith 1 a d b j w g c n m k Photocatalysts
Figure 2.36: Yield of 1-adamantanesulfonic acid in acetic acid in the presence
(1mmol) and absence of acetylacetone. a) TH b) 4%[H2PtCl6]/TH c) 6%[H2PtBr6]/TH [144] d) RhCl3/TH g) P25 j) TiO2-C k) TiO2 (sol-gel preparation) m) TiO2-N (urea [89] [88] modified) n) TiO2-N ((NH4)2CO3 modified)
It was observed that platinum modified catalysts were active only in the case of acetylacetone addition. 4%[H2PtCl6]/TH and cal.6%[H2PtBr6]/TH produced an yield of 18 and 11% of 1-adamantanesulfonic acid after 10 h irradiation.
4.0%RhCl3/TH was active in the absence of acetylacetone, produced an yield of 18% after 10 h of irradiation. No influence of acetylacetone was observed. Surprisingly, in the case of acetic acid, even TH, P25 were active and addition of acetylacetone in their case reduced the yield of 1-adamantane sulfonic acid. TiO2-C followed a similar trend. TiO2-N ((NH4)2CO3 modified) and TiO2-N (urea modified) were moderately 95 active, however, failed to produce 1-adamantanesulfonic acid on acetylacetone addition.
Photocatalyst % yield of % yield of TON TON 1-ASA 1-ASA Without With Without Hacac With Hacac Hacac Hacac
4%[H2PtCl6]/TH - 18 - 35 cal.6%[H2PtBr6]/TH - 11 - 14
4%RhCl3 /TH 18 18 35 35
TiO2-C 7 12 - - TH 40 30 - -
TiO2 (sol-gel preparation) 3 - - -
TiOSO4 - - - -
TiO2-N ((NH4)2CO3 3 - - - modified)
TiO2-N (urea modified) 7 - - -
TiO(acac)2 - - - - cal.8%[H2PtCl6]/SiO2 - - - - cal.8% [H2PtCl6]/Al2O3 - - - -
[H2PtCl6]/TiOSO4 - - - - P25 12 6 - -
Table 2.8: Yield of 1-adamantanesulfonic acid in acetic acid in the presence (1 mmol) and absence of acetylacetone. (1-ASA and Hacac, refer to 1-adamantanesulfonic acid and acetylacetone, respectively)
Furthermore, the influence of the amount of acetylacetone in the visible light sulfoxidation of adamantane in acetic acid by 4%[H2PtCl6]/TH and cal.6%[H2PtBr6]/TH were also investigated and are displayed in Figures 2.37 and
2.39. It was found in both cases of 4%[H2PtCl6]/TH and cal.6%[H2PtBr6]/TH, 96 addition of 1 mmol of acetylacetone to the suspension of the 30 mg (2g / L) of the catalysts in 66.67 mM solution of adamantane in acetic acid produced the highest yield of 1-adamantanesulfonic acid. Since it is hypothesised that acetylacetone complexes with Pt metal in the catalyst replacing the Cl ligand, it is very important to calculate the ratio of Pt to acetylacetone in the system. The amount of Pt in 30 mg of -3 4%[H2PtCl6]/TH employed is 6.15 × 10 mmol. The amount of acetylacetone employed which displayed maximum activity was 1 mmol, which is multifold higher compared to that of the Pt amount. Moreover, the reduction of the amount of acetylacetone lower than 1 mmol did not produce any increase in yield of 1-adamantanesulfonic acid, rather a decrease. From these results, one can conclude that atleast 1 mmol of acetylacetone (corresponding to a ~160 times excess of acetylacetone relative to Pt) is necessary to observe maximum acceleration. Higher concentrations rather induced slower reaction.
97
20 18 a 16 b 14 12 10 8 % yield of
6 4 2
1- adamantanesulfonic acid c 0 d -2 0246810 irradiation time / h
Figure 2.37: Influence of the amount of acetylacetone in visible light sulfoxidation of adamantane using 4%[H2PtCl6]/TH in acetic acid. a) 1 mmol b) 5 mmol c) 0.2 mmol d) 0.04 mmol.
98
12 a 10 b
8
sulfonic acid 6 % yield of 4
2
1- adamantane c 0 d
0246810 irradiation time / h
Figure 2.39: Influence of the amount of acetylacetone in visible light sulfoxidation of adamantane using cal.6%[H2PtBr6]/TH in acetic acid. a) 1 mmol b) 5 mmol c) 0.2 mmol d) 0.04 mmol.
99
2.2.11 Mechanism of visible light sulfoxidation of adamantane in acetic acid. The outcome of the visible light sulfoxidation of adamantane in acetic acid is very different from that in methanol. While 4%[H2PtCl6]/TH and cal.6%[H2PtBr6]/TH are activite in the latter solvent , they were inactive in the acetic acid. On the other hand in both the solvents addition of acetylacetone increases the yield. Acetylacetone complexes efficiently with Pt and the mechanism in this case is similar to that proposed in the presence of acetylacetone in methanol. Titania itself is active in visible light sulfoxidation of adamantane in acetic acid even without acetylacetone, unlike methanol’s case where it is active only in the presence of acetylacetone. Here the mechanism is proposed in analogy with that of sulfoxidation by titania in the presence of acetylacetone in methanol, the only difference being acetic acid as the complexing agent with titania instead of acetylacetone. In the case of addition of acetylacetone, it is generally observed that titania displayed a decreased activity and this could be justified that there is an opposition between the two complexing agents acetylacetone and acetic acid and the result is a detrimental effect in the yield of 1-adamantanesulfonic acid.
100
3 Experimental section
3.1 Materials TH (Titanhydrat–O, Kerr-McGee, 100% anatase, specific surface area : 334 m2 g-1), P25 (Degussa, specific surface area : 50 m2 g-1), hexachloroplatinic acid (Degussa), hexabromoplatinic acid (Aldrich), adamantane, > 99%, potassiumhydrogenpthalate, acetonitrile for HPLC, 1-heptanesulfonic acid sodium salt, monohydrate, methanol, DL-camphor, DL-10-camphorsulfonic acid, hydroxymethanesulfonic acid (formaldehyde sodium bisulfite addition product) (all from Acros Organics), silica (Grace 432, specific surface area : 308 m2 g-1), neutral alumina, (Aldrich, specific surface area : 150 m2 g-1) and n-heptane (Fischer), glacial acetic acid (Fluka), n-hexadecane and 1-hexadecanesulfonic acid sodium salt, (both from Merck) were used as received.
3.2 Spectroscopic and analytical measurements 3.2.1 UV- vis spectroscopy UV-vis spectra were recorded on a Varian Cary 50 spectrometer.
3.2.2 Diffuse Reflectance Spectroscopy Diffuse reflectance spectra were measured using a Shimadzu UV-2401 UV-VIS recording spectrometer equipped with a diffuse reflectance accessory. The background reflectance of bariumsulfate (reference) was measured before. 50 mg of each photocatalyst powder was well ground with 2 g of bariumsulfate and spread onto the sampling plate prior to the measurement. Reflectance was converted by the instrument software to F(R∞) values according to the Kubelka-Munk theory.
3.2.3 NMR 1H and 13C-NMR were recorded on JEOL FT-JNM-EX 270 or JEOL FT-JNM-LA 400 spectrometers at room temperature.
101
3.2.4 IR Perkin-Elmer 16 PC FT-IR was employed using KBr pellets
3.2.5 Mass spectroscopy JEOL JMS 700 (EI 70 eV, FD 2 KV)
3.2.6 XRD
Huber-diffractometer with Cu-Kα radiation (λ = 1.5048 Å)
3.2.7 BET Gemini 2370
3.2.8 TEM Philips Microscope CM 200 (200 kV)
3.2.9 TOC Shimadzu Total Carbon Analyser TOC-500 / 5050 with NDIR Optical System Detector
3.2.10 Elemental Analysis Cario Erba Elemental Analyser Model 1108
3.2.11 HPLC 3.2.11.1 Analysis of 4-CP HPLC: SCL 10 AVP system controller Column: reverse –phase (Supelco Discovery C-18) Eluent: 2-propanol and water (50 / 50, v / v) Detector: SP10AVP model UV detector Detection: UV-VIS at 222 nm
102
3.2.11.2 Analysis of sulfonic acids HPLC: SCL 10 AVP system controller Column: (250 x 4.6 mm I.D Partsil 10 SAX (Whatman) which is a strong anion + exchanger with -N R3 functionality and is Si-O-Si bonded to partisil. Eluent: Water-acetonitrile (60 / 40, v / v) with 0.01 M potassiumhydrogenphthalate as UV absorbing counter ion. The pH value of the eluent with this counter ion was 5.8. Detector: SP10AVP model UV detector (304 nm)
3.3 Preparation of catalysts 3.3.1 Preparation of metal complex modified titania[45]
To a suspension of 1 g TH (Titanhydrat-0), in 10 mL of H2O were added of 0.16 g of hexachloroplatinic acid / (0.25 or 0.125 or 0.08 or 0.41 g) hexabromoplatinic acid. After stirring for 12 h, water was removed in vacuo and the residue was dried under vacuum at room temperature for 3 h. The resulting powder was calcined in air at 160 °C, washed five times with 30 mL portions of water, dried as described above and again calcined for 2 h at 160 °C. Silica / alumina or amorphous titania were used instead of TH, respectively in the case of preparation of photocatalysts with the above supports, following the same procedure as described above.
3.3.2 Preparation of amorphous titania [144]
Titanium hydroxide was precipitated at pH 8 from a 0.25 M TiOSO4 aqueous solution by the addition of sodium hydroxide (0.25M). After ageing the suspension for 24 h, the precipitate was filtered and dried under air at 343 K. The residue was ground to a fine powder and calcined in a muffle furnace at 673 K for 4 h. XRD measurement revealed that the material was in the amorphous phase.
3.3.3 Preparation of anatase titania (self prepared)
Titanium hydroxide was precipitated at pH 5.5 from a 0.25 M aqueous TiCl4 (0.25 M) by the addition of sodium hydroxide (0.25M). After ageing the suspension for 24 h, the precipitate was filtered and dried under air at 343 K. The residue was ground to a 103 fine powder and calcined in a muffle furnace at 673 K for 4 h. XRD measurement showed that the material was in the anatase phase.
3.3.4 Preparation of acetylacetone modified titania 1 g of TH (0.0125 mol) was added 5 mL of acetylacetone (0.05 mol) and 14 mL of triethylamine (0.01mol). The pale yellow suspension was stirred overnight at a temperature of 80 °C and was filtered. The wet powder obtained was washed with methanol 3 times and dried under vacuum at room temperature for 2 h to yield a pale yellow powder.
3.4 Visible light degradation experiments 3.4.1 Degradation of 4-CP The visible light degradation of 4-CP was carried out in a jacketed cylindrical quartz cuvette attached to an optical train. Irradiation was performed with an Osram XBO -6 -1 -2 150 W xenon arc lamp, (Io (400 nm - 520 nm) = 2 x 10 Einstein s cm ) installed in a light condensing lamp housing (PTI A1010S) on an optical train. A water cooled cylindrical quartz cuvette was mounted at a distance of 24.5 cm from the lamp. A cut–off filter of λ ≥ 455 nm was placed in front of the cuvette. The suspension was stirred magnetically. In the experiment, aqueous 4-CP (14 mL, 2.5 × 10-4 M) and catalyst (7 mg, i.e. 0.5 g / L) were added. The resulting suspension was sonicated for 1 min and filled into the cuvette. Then the cuvette was irradiated. Samples were taken at regular intervals and the photocatalyst was filtered through a micropore filter (Merck, 0.45µm). The filtrate was analysed by UV-Vis spectroscopy and HPLC.
104
quartz- round cuvette cooled by water and equipped with a cut-off filter xenon-arc lamp power supply (with water cooling)
magnetic stirrer IR filter
Figure 3.1: Experimental setup for all visible light degradation and visible light sulfoxidation experiments.
Figure 3.2: Spectrum of 150 W xenon-arc lamp. The intensity of the lamp was measured to be 1095 W / m2, when a cut off filter of λ ≥ 400 nm was placed before it.
105
3.4.2 Degradation of HCOOH In the experiment, aqueous HCOOH (14 mL, 10-3 M) and catalyst (14, i.e. 1g / L) were added. The resulting suspension was sonicated for 1 min and filled into the cuvette. Then the cuvette was irradiated (λ ≥ 455 nm). Samples were taken at regular intervals (0 h, 2 h, 4 h and 6 h ) and the photocatalyst was filtered through a micropore filter (Whatman 0.45µm). The filtrate was analysed by TOC. The lamp and other conditions were analogous to that of 4-CP degradation. In the case of experiments -2 with electron scavenger AgNO3 (10 M), argon bubbling was started 30 minutes before irradiation and was continued through out the reaction. Adsorption experiments were conducted in 25 mL Erlenmeyer flasks wrapped with aluminium foils, magnetically stirred. Samples were withdrawn at 0 h, 2 h, 4 h and 6 h and were analysed by TOC.
3.5 Photoelectrochemical measurements Quasi-Fermi level measurements: [112] Quasi-Fermi energies (nEf*) were measured according to Roy’s method. 30 mg of catalyst and 6 mg of methylviologen dichloride were suspended in a 100 mL two-necked flask in 50 mL of 0.1M KNO3. A platinum flag and Ag / AgCl served as working and reference electrodes and a pH meter for following the proton concentration. HNO3 (0.1 M) and NaOH (0.1 M) were used to adjust the pH value. The suspension was magnetically stirred and purged with nitrogen gas throughout the experiment. Initially the pH of the suspension was adjusted to pH 1 before measurement. The light source was the same as used in the photosulfoxidation. Stable photovoltages were recorded about 30 min after changing the pH value. The obtained pH0 values were converted to the Fermi potential at pH 7 by the equation nEf* (pH 7) [112] = - 0.445 + 0.059 (pH0-7). Reproducibility of pH0 values was better than 0.1 pH units.
106
V
N2 bubbling pH meter
reference working electrode electrode hν
catalyst suspension
magnetic bar
Figure 3.3: Experimental setup for Quasi-Fermi level measurements of semiconductor powders.
3.6 Visible light sulfoxidation experiments 3.6.1 Photosulfoxidation procedure The visible light sulfoxidation of adamantane was carried out in a jacketed cylindrical quartz cuvette attached to an optical train. Irradiation was performed with an Osram
XBO 150 W xenon arc lamp, (Io (400 nm - 520 nm) = 2 x 10 -6 Einstein s-1cm-2) installed in a light condensing lamp housing (PTI A1010S) on an optical train. A water cooled cylindrical quartz cuvette was mounted at a distance of 24.5 cm from the lamp. A cut–off filter of λ ≥ 400 nm was placed in front of the cuvette. The suspension was stirred magnetically. In the experiment, 107 adamantane (1 mmol, 136.24 mg) was dissolved in 15 mL of methanol and
4%H2[PtCl6]/TH (30 mg, i.e. 2 g / L which was the optimised concentration of the catalyst to produce a maximum yield of 1-adamantanesulfonic acid) was added. The resulting suspension was sonicated for 1 min and filled into the cuvette. Then the cuvette was sealed with silicone rubber and sulfur dioxide (30 mL, 1.3 mmol) and oxygen (30 mL, 1.3 mmol) were metered simultaneously into it. A cut–off filter of λ ≥ 400 nm was placed in front of the cuvette. The suspension was stirred magnetically and irradiated. Samples were taken at regular intervals and the photocatalyst was filtered through a microporous organic filter (Whatman with poresize 0.45µm). The filtrate was degassed of sulfur dioxide by purging with nitrogen for 10 min before and analysed by HPLC using the technique of indirect photometric detection. n-heptane, n-hexadecane and DL-camphor (1 mmol each) were sulfoxidised in methanol under same conditions like adamantane. Sulfoxidation experiments in glacial acetic acid were performed under analogous conditions like that of methanol.
3.6.2 Isolation of 1-adamantanesulfonic acid 1-adamantanesulfonic acid was isolated similar to the method described by Smith et. al.[15] The sulfoxidised suspension was degassed of sulfur dioxide by purging with nitrogen for 10 min. Then the photocatalyst was filtered through a micropore filter (Whatman 0.45 µm). The filtrate was concentrated to a pale yellow viscous residue which was dried in a vacuum desiccator. After addition of ethyl acetate, a white solid was obtained, which was filtered and carefully recrystallized from ethyl acetate to afford colourless crystals of 1-adamantanesulfonic acid (0.15 mmol, 21 mg, 15% yield). Composition and structure of 1-adamantanesulfonic acid were verified by standard characterization methods. They were all in concordance with literature values.[22] The elemental analysis report, IR, NMR and mass spectra of 1-adamantanesulfonic acid are given below:
3.7 Characterization of the isolated 1-adamantanesulfonic acid 3.7.1 EA 108
Elemental analysis for the isolated 1-adamantanesulfonic acid (C10 h18O4S) revealed that it was a monohydrate. % Calculated: C: 51.26, H: 7.74, S: 13.6 % Found: C: 50.02, H: 8.31, S: 13.13.
3.7.2 IR Infrared analysis supported the presence of hydrated sulfonic acid. IR spectrum of 1-adamantanesulfonic acid, 2912, 1170, 1007, 617 cm-1 obtained matched well with the IR spectrum of adamantane-1-sulfonic acid (2914, 1167, 1006, 616 cm-1) available in literature.[22]
10
8
6
%T 4
2
0 2912 1170 1007 617
4000 3500 3000 2500 2000 1500 1000 500 cm-1
Figure 3.4: IR spectrum of 1-adamantanesulfonic acid in KBr.
109
3.7.3 13C NMR Measuring 13C NMR was particularly useful in identifying 1-adamantanesulfonic acid. The data was very concordant with that of literature values.[22]
Figure 3.5: 13C NMR obtained for 1-adamantanesulfonic acid (δ: 57.5, 37.8, 37.4, 29.8).[22]
3.7.4 Mass spectra Mass spectra also confirmed the formation of 1-adamantanesulfonic acid. The signals m / z 135 corresponds to [adamantane]+, m / z 217 corresponds to + [1-adamantanesulfonic acid -H2O] , m / z 434 corresponds to product ions of + [1-adamantanesulfonic acid -H2O] and m / z 650 to triple-product ions of + [1-adamantanesulfonic acid -H2O] . 110
Figure 3.6: Mass spectra obtained for 1-adamantanesulfonic acid.
3.7.5 Analysis by IPD with HPLC 1-adamantanesulfonic acid in acetic acid was detected as a negative peak with retention time of Rt = 4.89 min at a flow rate 2.0 mL / min and detection at 304 nm. The concentration of 1-adamantanesulfonic acid formed in the visible light sulfoxidation of adamantane was calculated using the calibration curve obtained using the isolated 1-adamantanesulfonic acid as standard.
111
300000
250000
/ a.u / 200000 area
150000 peak
100000
50000
234567891011 [1-adamantanesulfonic acid] / mM
Figure 3.7: Calibration curve of 1-adamantanesulfonic acid in acetic acid obtained from IPC with HPLC.
Calibration curves of various sulfonic acids were made and are displayed in the figures below:
500000
400000
300000 / a.u area
200000 peak 100000
0
0,000 0,002 0,004 0,006 0,008 0,010 [sodiumheptanesulfonate] / M
Figure 3.8: Calibration curve of sodiumheptanesulfonate in methanol. 112
500000
400000
/ a.u 300000 area
200000 peak
100000
0 0246810 [sodiumhexadecanesulfonate] / mM
Figure 3.9: Calibration curve of sodiumhexadecanesulfonate in methanol.
350000
300000
250000
/ a.u 200000
area 150000
100000 peak
50000
0
0,000 0,002 0,004 0,006 0,008 0,010 [DL-10- camphorsulfonic acid] / M
Figure 3.10: Calibration curve of DL-10-camphorsulfonic acid in methanol.
113
400000
350000
300000
250000 / a.u 200000 area 150000
peak 100000
50000
0
0246810 [DL-10- camphorsulfonic acid] / mM
Figure 3.11: Calibration curve of DL-10 camphorsulfonic acid in water.
350000
300000
250000
200000 / a.u
150000 area
100000 peak 50000
0
0246810 [hydroxymethanesulfonic acid] / mM
Figure 3.12: Calibration curve of hydroxymethanesulfonic acid in methanol.
114
400000
350000
300000
250000 / a.u 200000 area
150000 peak 100000
50000
0 0246810 [hydroxymethanesulfonic acid] / mM
Figure 3.13: Calibration curve of hydroxymethanesulfonic acid in water.
To confirm that 1-adamantanesulfonic acid and the possible product of sulfoxidation of methanol (hydroxymethanesulfonic acid) have different retention times in HPLC columm, a mixture of 1-adamantanesulfonic acid and the commercial hydroxymethanesulfonic acid was injected and found to have different retention times as shown in Figure 3.14.
115 absorbance / a.u. absorbance
retention time / min
Figure 3.14: Chromatogram of a methanolic solution containing both 1-adamatane sulfonic acid and hydroxymethanesulfonic acid. The negative peak at the retention time (Rt) 4.7 min corresponds to 1-adamatanesulfonic acid and 5.6 min to that of hydroxymethanesulfonic acid.
3.7.6 Visible light sulfoxidation of n-heptane n-heptane was sulfoxidised using visible light with the photocatalyst 4%[H2PtCl6]/TH. n-heptane (0.4 mole, 60 mL), water (60 mL) and the photocatalyst, (0.8 g / L), were placed in the reaction vessel. SO2 and O2 were bubbled into the suspension simultaneously in the ratio of 1:1 and the suspension was irradiated using immersion type tungsten-halogen lamp (λ ≥ 300 nm) for 10 h. The yield of 1-heptanesulfonic acid sodium salt was 25%. The sulfonic acid formed was detected by HPLC with IPC.
116
gas outlet
water cooling
SO2 + O2 W-halogen lamp
suspension of catalyst in alkane with solvent
Figure 3.15: Immersion lamp set up employed in visible light sulfoxidation of alkanes to achieve quantitative isolation of sulfonic acids. 117 Relative spectral intensity spectral Relative
Wavelength / nm
Figure 3.16: Spectrum of 100 W tungsten-halogen lamp employed in the immersion apparatus. The intensity of the lamp was measured to be 1498 W / m2, when a cut off filter of λ ≥ 400 nm was placed before it.
118
Figure 3.17: Chromatogram obtained for heptanesulfonic acid at Rt 3.7 min after 4 h irradiation in an immersion lamp apparatus.
Concentration of sodiumheptanesulfonate was obtained using the calibration curve shown in Figure 3.18.
119
600000
500000
400000
300000
200000 peak area / a.u. 100000
0
0,000 0,002 0,004 0,006 0,008 0,010 [sodiumheptanesulfonate] / M
Figure 3.18: Calibration curve of sodiumheptanesulfonate in water.
Sulfoxidation was also performed analogously in the absence of water, however, there was not a good yield as there was a poor suspension of the catalyst. Separation procedures for sodiumheptanesulfonate from heptane sulfoxidation in the presence and absence of water is given below:
3.7.6.1 Isolation of sodiumheptanesulfonate in the presence of water 1. Degassing of the sulfoxidation mixture containing heptanesulfonic acid, sulfuric acid, water, unreacted heptane and dissolved gases by passing N2 for 30 min. 2. Extraction of the mixture with the weakly polar solvent di-isopropyl ether. 3. Generally the solvent layer extracts the long chain sulfonic acids due to their less polar nature. However, in the case of heptanesulfonic acid, it was extracted from the aqueous layer owing to its relatively higher polar nature and was neutralized with NaOH, and was evaporated to remove water, solvent and n-heptane. 4. The residue was cooled and ground to give the sodiumheptanesulfonate.
120
3.7.6.2 Isolation of sodiumheptanesulfonate in the absence of water
1. Sulfoxidation mixture was degassed of SO2 by passing N2 for 30 min. 2. Heptanesulfonic acid in the sulfoxidation mixture was extracted with water (calc.half the volume of n-heptane added). 3. The extract was brought to pH 3 with 10% NaOH. 4. The aqueous solution was evaporated to dryness. 5. The residue was extracted with 70% ethanol (same volume as n-heptane). 6. The extract was evaporated to give sodiumheptanesulfonate. 121
Figure 3.19: IR spectra of sodiumheptanesulfonate in KBr. The bottom spectrum refers to the authentic sodiumheptanesulfonate while the top and the middle ones refer to sodiumheptanesulfonate isolated from the sulfoxidation of n-heptane in the absence and presence of water, respectively. 122
4 Summary
One of the few photoreactions applied in chemical industry is sulfoxidation of alkanes. (Eq.1)
1 hν RH + SO + O ⎯⎯→ RSO H (1) 2 2 2 3
In the reaction SO2 is the light absorbing species and therefore low pressure Hg lamps have to be employed. During the previous work on the photocatalytic properties of chloroplatinate titania (4%H2[PtCl6]/TH, TH = Titanhydrat-O), it was found that this compound surprisingly catalyses the visible light sulfoxidation of n-heptane. It was now the aim of this work to investigate the mechanism of this first catalytic photosulfoxidation of an alkane and to search for further semiconductor catalysts.
In the first part of this work, in addition to 4%H2[PtCl6]/TH, also 1%H2[PtBr6]/TH,
2%H2[PtBr6]/TH, 3%H2[PtBr6]/TH, 6%H2[PtBr6]/TH, 4%RhCl3/TH, and
3%RhCl3/TH were prepared. For comparison also 4%H2[PtCl6]/SiO2,
8%H2[PtCl6]/SiO2, 32%H2[PtCl6]/SiO2, 4%H2[PtCl6]/SiO2 (grinding in ball mill),
4%H2[PtCl6]/Al2O3, and 4%H2[PtCl6]/Al2O3 (grinding in ball mill) were synthesised.
Both 6%H2[PtBr6]/TH and 4%H2[PtCl6]/TH exhibited absorption already at about
550 nm. The diffuse reflectance spectra of 6%H2[PtBr6]/TH in accordance with its much deeper yellow colour compared to that of the chloro-modified, exhibits a stronger absorption than the latter. Unmodified TH showed a bandgap of 3.21 eV in excellent agreement with the literature value of 3.20 eV reported for anatase.
4%H2[PtCl6]/TH also showed almost the same bandgap of 3.21 eV, proving that modification does not contribute to change in bandgap. However, a bandgap of
3.03 eV was measured for 6%H2[PtBr6]/TH (Figure 1) and the narrowing of the bandgap is proportional to the increasing amount of H2[PtBr6] added for the catalyst modification. Similarly, also for 4%RhCl3/TH and 3%RhCl3/TH the values of 2.97 and 3.1 eV respectively, indicate the bandgap narrowing. 123
2,5
2,0
1/2 1,5 )E) ∞ 1,0 (F(R
0,5 d a c 0,0 2,0 2,5 3,0 3,5 4,0 E / eV
Figure 1: Plot of transformed Kubelka-Munk function vs. energy of light absorbed. a) TH, c) 6%H2[PtBr6]/TH and d) 4%RhCl3/TH
Determination of the quasi-Fermi level of electrons for 6%H2[PtBr6]/TH by pH dependent photovoltage measurements afforded a value of -0.24 ± 0.02 V (vs. NHE). This is in agreement with the previously reported quasi-Fermi level of
4%H2[PtCl6]/TH. There was an anodic shift of ~ 300 mV as compared to –0.54 V of
TH for both 4%H2[PtCl6]/TH and 6%H2[PtBr6]/TH. As a first test on photocatalytic properties of these new materials, the degradation of
4-CP was investigated. The photocatalyst 4%H2[PtCl6]/TH displayed a superior activity (Table 1) while bromo complex modified titania showed around 50% less activity. The lesser activity of bromo modifications may be due to the lower oxidation 124 potential of the bromine atom compared to that of chlorine. Compared to these catalysts, the unmodified TH or P25 were almost inactive. When the TiO2 semiconductor support was changed to insulators like SiO2 or Al2O3, there was no activity. This confirms the role of the semiconductor in this reaction. Rhodium modified complexes exhibited a similar trend of high activity like 4%H2[PtCl6]/TH. The photocatalytic and photoelectrochemical properties of the catalysts are summarized in the Table 1.
[b] * Ebg pH0 nEF Rate constant Catalyst [a] [c] -5 -1 [eV] [V vs. NHE] × 10 [s ]
4%H2[PtCl6]/TH 3.21 10.56 -0.24 47
cal.1%H2[PtBr6]/TH 3.21 10.28 -0.25 10
cal.2%H2[PtBr6]/TH 3.22 10.31 -0.25 15
cal.3%H2[PtBr6]/TH 3.12 10.12 -0.26 18
cal.6%H2[PtBr6]/TH 3.03 10.58 -0.23 20
4%RhCl3/TH 2.97 - - 36
cal.3%RhCl3/TH 2.97 - - 39
TH 3.21 5.33 -0.54 1.79
P25 3.03 4.45 -0.58 1.09
[a] = ± 0.02 eV [b] = ± 0.1 [c] = ± 0.02 V ( pH = 7)
Table 1: Photoelectrochemical data, bandgap energies, and rate constants of visible light (λirr ≥ 455 nm) degradation of 4-CP, for various photocatalysts.
125
The second and major part of the thesis deals with the visible light sulfoxidation of alkanes in the solvents methanol and acetic acid in the presence of metal complex modified titania and other semiconductor photocatalysts. Furthermore, the influence of some complexing agents like acetylacetone was investigated. Adamantane was employed as the model alkane and the analysis of 1-adamantanesulfonic acid was made by HPLC using the technique of Indirect Photometric Detection. The Turnover Number (TON, the ratio of amount of product (1-adamantanesulfonic acid) to amount of active material (Pt)) of the reaction in methanol after 10 h (which was the optimized irradiation time for maximum yield of 1-adamantanesulfonic acid) was 21. Photosulfoxidation of methanol did not occur as indicated by HPLC analysis. There was no formation of 1-adamantanesulfonic acid in the absence of the catalyst and the reaction ceases when the irradiation is stopped. The corresponding bromo complex was also active but induced a smaller TON of 8 after 10 h. Only traces of 1-adamantanesulfonic acid were observed when unmodified TH was employed, whereas 1-adamantanesulfonic acid was completely absent when hexachloroplatinic acid was supported onto silica, alumina or amorphous titania. Hexachloroplatinic acid itself and amorphous titania were also inactive. The yield of 1-adamantanesulfonic acid produced by the visible light sulfoxidation of adamantane catalysed by various photocatalysts under different irradiation times is shown in Figure 2.
126
12 b
10
8
6 c
% yield of 4
2
adamantane-sulfonic acid a 0 f'
0246810 irradiation time / h
Figure 2: Yield of 1-adamantanesulfonic acid in visible light sulfoxidation of adamantane in methanol, as function of irradiation time a) TH b) 4%[H2PtCl6]/TH c) 6%[H2PtBr6]/TH f’) TiO2(amorphous), H2PtCl6 or H2PtCl6 supported on SiO2,
Al2O3 and TiO2 (amorphous). [Catalyst] = 2 g / L, 15 mL of methanol.
Visible light sulfoxidations of other alkanes were performed under same conditions as that of adamantane and are reported along with adamantane in Table 2.
127
Amount of R-H Amount of RH TON [mmol] alkanesulfonic acid [mmol]
1.0 0.11 21
O 1.0 0.10 20
CH3(CH2)5CH3 50.0 0.06 11
CH3(CH2)14CH3 25.0 0.03 8
Table 2: Yield and TON for visible light sulfoxidation of alkanes in methanol after 10 h of irradiation photocatalysed by 4%H2[PtCl6]/TH.
Based on the observation that photosulfoxidation was only observed when the platinum complex was attached to the semiconducting metal oxide, a similar mechanism is proposed as recently formulated for visible light degradation of
4-chlorophenol by 4%[H2PtCl6]/TH. Light absorption by the titania - halogenoplatinate complex leads to homolytic Pt-X cleavage affording an adsorbed halogen atom and a Pt(III) complex as primary intermediates (Scheme 1, A). Injection of an electron from the latter into the conduction band of titania and subsequent reduction of oxygen to superoxide (Scheme 1, C), reforms Pt(IV) (Scheme 1, B). The superoxide can be converted to the OH radical via well known reaction steps. The OH radicals may abstract hydrogen from the alkane to give alkyl radicals (Scheme 1, D). Additionally, hydrogen abstraction from the alkane that may occur by the intermediary halogen atoms also produces alkyl radicals (Scheme 1, E), thus regenerating the halide ligand (Scheme 1, F). The alkyl radicals formed attack sulfur dioxide and finally give rise to formation of the sulfonic acid.
128
III Ti O Pt X
O2 RH
C B A F E VIS
R + H+ O2 IV Ti O Pt X
RH H+ SO2
OH D
RSO2 H2O + R
RSO H 3
Scheme 1: Proposed mechanism for visible light sulfoxidation of adamantane by titaniachloro- or bromoplatinate. X = Cl or Br.
Since it was observed that the Cl ligand in the metal complex chemisorbed to TiO2 plays a vital role in the photocatalytic activity of 4%[H2PtCl6]/TH, we wanted to explore the role of better complexing agents other than Cl in our catalyst. As it is well known that acetylacetone is a good transition metal chelating agent, it was added in the system so that it could chelate with Pt to form a more stable and efficient complex replacing the Cl ligands and thereby possibly increasing visible light absorption. As an 129 experimental support for this hypothesis, it was found that complexing agents like acetylacetone when added to the sulfoxidation of adamantane in methanol had significantly increased the yield. However, other complexing agents like hexafluoroacetylacetone, pyrophosphate, ethylene glycol, sodium-dihydrogen phosphate did not display any enhancing effect in the yield of sulfoxidation in methanol. All catalysts exhibited an enhanced activity in sulfoxidation in methanol in the presence of acetylacetone (Figure 3), especially in the case of 4%[H2PtCl6]/TH, the yield of 1-adamantanesulfonic acid increased from 12 to 39% which is more than three-fold increase. 6%H2[PtBr6]/TH also exhibited similar trends.
Carbon modified titania (TiO2-C) also revealed an increase in yield of 1-adamantanesulfonic acid with acetylacetone i.e. from 10 to 30%. Special attention has to be given to anatase modifications of titania, TH and TiO2 (anatase) which are not active in the absence of acetylacetone, but displayed a prominent activity in its presence. It was observed that only anatase modifications of titania showed an significant activity in the presence of acetylacetone, while amorphous modifications were inactive both in the presence and absence of acetylacetone. However, when TH was premodified with acetylacetone and employed for sulfoxidation, it turned out to be inactive. Addition of acetylacetone to TiO2 makes this white powder pale yellow. The DRS shows a shift in absorption towards visible region for all commercially available TiO2 on contact with acetylacetone. 130
50 d'
40 b' c'
30
20 d
% yield of b 10 c 1-adamantanesulfonic acid 0
0246810 irradiation time / h
.
Figure 3: Yield of 1-adamantanesulfonic acid in methanol in the presence of acetylacetone; b, c, d represent 4%H2[PtCl6]/TH, cal.6%H2[PtBr6]/TH, and cal.4%RhCl3/TH respectively, in the absence of acetylacetone; b’, c’, d’ the same catalysts in the presence of acetylacetone (1mmol); experimental conditions like Figure 2; [acetylacetone ] = 66.67 mM.
The bandgap of titania (TH) also narrowed from 3.21 to 3.11 eV on addition of acetylacetone. Based on these observations and on the enhanced yield in the presence of acetylacetone, the mechanism of visible light sulfoxidation in the presence of acetylacetone is proposed by analogy with the sulfoxidation in the presence of metal complex modified titania. The main difference is that instead of a Pt-X (X = Cl or Br) cleavage , now a Pt-O of acetylacetonate occurs. In the case of naked TiO2 catalysing 131 the reaction, it is postulated that surface titania centers are complexed with acetylacetone directly and now a Ti-O bond is cleaved in the primary step. Sulfoxidation in acetic acid as the solvent instead of methanol was also performed (Figure 4).
d
i
c
a
c i
n 0
4
o
f
f l
o e
u on d t
s 0 l e
3 c e
e la i
n ty
y e a
c
t a
0 h e n n % 2 it to a w e
ac m l 0 ty a 1 ce d t a
a u
o - 0 ith 1 a d b j w g c n m k Photocatalysts
Figure 4: Yield of 1-adamantanesulfonic acid in acetic acid in the presence (1mmol) and absence of acetylacetone. a) TH b) 4%[H2PtCl6]/TH c) 6%[H2PtBr6]/TH d) RhCl3/TH g) P25 j) TiO2-C k) TiO2 (sol-gel preparation) m) TiO2 -N (urea modified) n) TiO2 -N ((NH4)2CO3 modified)
It was observed that platinum modified catalysts were active only in the case of acetylacetone addition. 4%[H2PtCl6]/TH and 6%[H2PtBr6]/TH produced an yield of 18 and 11% of 1-adamantanesulfonic acid after 10 h irradiation respectively. There 132
was no influence of acetylacetone in the 4.0%RhCl3/TH catalysed sulfoxidation as both in its presence and absence, produced an yield of 18% of 1-adamantanesulfonic acid after 10 h irradiation. Surprisingly, TH was found to be active in the visible light sulfoxidation of adamantane and there was a detrimental effect by the addition of acetylacetone. This may be justified due to the good bridging and chelating nature of the acetic acid itself. Generally addition of acetylacetone increased the yield of adamantane sulfonic acid in the case of metalcomplexes, while it had a detrimental effect on yields in the case of on unmodified titania. TiO2-C followed a similar trend to that of TH. TiO2-N ((NH4)2CO3 modified) and TiO2-N (urea modified) were moderately active, however, failed to produce 1-adamantanesulfonic acid on acetylacetone addition.
133
5 Zusammenfassung
Eine der wenigen Photoreaktionen, die in der chemischen Industrie Anwendung finden, ist die Sulfoxidation von Alkanen (Gl.1).
1 hν RH + SO + O ⎯⎯→ RSO H (1) 2 2 2 3
Da in beschriebener Reaktion SO2 die Licht absorbierende Spezies representiert, müssen Niederdruck-Quecksilber Lampen eingesetzt werden. Während vorausgegangenen Arbeiten zu photokatalytischen Eigenschaften von
Titandioxid-Hexachloroplatinat (4%H2[PtCl6]/TH, TH = Titanhydrat-O) zeigte sich überraschenderweise dessen katalytische Aktivität in der Sulfoxidation von n-Heptan mit sichbarem Licht. Photokatatlytische Sulfoxidationen waren bis dahin unbekannt. Ziel der voliegenden Arbeit war es nun, den Mechanismus dieser neuartigen photochemischen Aktivierung eines Alkans zu untersuchen und weitere Halbleiter- Katalysatoren zu entwickelen.
Im ersten Teil der Arbeit wurden daher zusätzlich zu 4%H2[PtCl6]/TH noch
1%H2[PtBr6]/TH, 2%H2[PtBr6]/TH, 3%H2[PtBr6]/TH, 6%H2[PtBr6]/TH, 4%RhCl3/TH und 3%RhCl3/TH synthetisiert. Zu Vergleichszwecken wurden außerdem
4%H2[PtCl6]/SiO2, 8%H2[PtCl6]/SiO2, 32%H2[PtCl6]/SiO2, 4%H2[PtCl6]/SiO2
(Verreibung in Pulvermühle), 4%H2[PtCl6]/Al2O3, und 4%H2[PtCl6]/Al2O3
(Verreibung in Pulvermühle) hergestellt. Beide Pulver, 6%H2[PtBr6]/TH und
4%H2[PtCl6]/TH, zeigten bereits Lichtabsorption im Bereich von 550 nm. Unmodifiziertes TH besitzt eine Bandlücke von 3.21 eV in sehr guter Übereinstimmung mit dem literaturbekannten Wert von 3.20 eV für Anatas.
4%H2[PtCl6]/TH zeigte ebenfalls eine Bandlücke von 3.21 eV, was einen Einfluß der
Modifikation auf die Bandlücke ausschließt. Für 6%H2[PtBr6]/TH (Abbildung 1) jedoch ergab sich eine geringere Bandlücke von 3.03 eV, wobei die Bandlücke proportional zur Konzentration an zugesetztem H2[PtBr6] abnahm. Für die 134
Bandlücken für 4%RhCl3/TH und 3%RhCl3/TH ergaben sich Werte von 2.97 und 3.1 eV, sie zeigen also ebenfalls eine Verkleinerung.
2,5
2,0
1/2 1,5 )E) ∞ 1,0 (F(R
0,5 d a c 0,0 2,0 2,5 3,0 3,5 4,0 E / eV
Abbildung 1: Auftragung der transformierten Kubelka-Munk Funktion gegen die
Energie des einfallenden Lichts. a) TH, c) 6%H2[PtBr6]/TH, d) 4%RhCl3/TH.
Das Quasi-Fermi Niveau der Elektronen ergab sich mit Hilfe pH–abhängiger Photospannungsmessungen zu -0.24 ± 0.02 V (vs. NHE). Dieses stimmt mit dem kürzlich berichteten Wert für 4%H2[PtCl6]/TH überein. Im Vergleich zu -0.54 V für
TH entspricht dies einer anodischen Verschiebung von ~ 300 mV für 4%H2[PtCl6]/TH und 6%H2[PtBr6]/TH. 135
Als erste Testreaktion für die photokatalytische Aktivität dieser neuen Materialien wurde der Abbau von 4-CP untersucht. Dabei zeigte 4%H2[PtCl6]/TH eine überlegene
Aktivität. Das mit dem Bromokomplex modifizierte TiO2 besitzt eine um die Hälfte geringere Aktivität. Eine mögliche Erklärung für die gesunkene Aktivität könnte die kleinere Oxidationskraft von Bromatomen im Vergleich zu Chloratomen sein. Im Vergleich zu diesen Katalysatoren waren unmodifiziertes TH oder P25 fast inaktiv.
Durch Austausch des Halbleiters TiO2 gegen Isolatoren wie SiO2 oder Al2O3 verschwand die Aktivität vollständig. Dieser Befund unterstreicht die wichtige Rolle des Halbleiters in diesen Reaktionen. Rhodium-modifizierte Komplexe zeigten
ähnlich hohe Aktivität wie 4%H2[PtCl6]/TH. Photokatalytische und Photoelektrochemische Eigenschaften der verschiedenen Katalysatoren sind in Tabelle 1 zusammengefaßt. 136
[b] * Geschwindigkeits Ebg pH0 nEF Katalysator [a] [c] -konstante [eV] [V vs. NHE] × 10-5 [s-1]
4%H2[PtCl6]/TH 3.21 10.56 -0.24 47
cal.1%H2[PtBr6]/TH 3.21 10.28 -0.25 10
cal.2%H2[PtBr6]/TH 3.22 10.31 -0.25 15
cal.3%H2[PtBr6]/TH 3.12 10.12 -0.26 18
cal.6%H2[PtBr6]/TH 3.03 10.58 -0.23 20
4%RhCl3/TH 2.97 - - 36
cal.3%RhCl3/TH 2.97 - - 39
TH 3.21 5.33 -0.54 1.79
P25 3.03 4.45 -0.58 1.09
[a] = ± 0.02 eV [b] = ± 0.1 [c] = ± 0.02 V ( pH = 7)
Tabelle 1: Photoelektrochemische Daten, Bandlückenenergien sowie Geschwindigkeitskonstanten für den Abbau von 4-CP mit sichbaren Licht
(λirr ≥ 455 nm) für verschiedene Photokatalysatoren.
Der zweite und zugleich umfangreichere Teil der vorliegenden Arbeit befasst sich mit der Sulfoxidation von Alkanen mit sichtbarem Licht in Methanol und Essigsäure in Gegenwart der Metallkomplex-modifizierten Titandioxide oder anderer Halbleiter- Photokatalysatoren. Im weiteren wurde der Einfluß von komplexierenden Agentien wie Acetylaceton untersucht. Als Modelsubstanz für Alkane wurde Adamantan eingesetz und die Detektion der entsprechenden 1-Adamatansulfonsäure erfolgte mit 137
HPLC unter Anwendung der indirekten photometrischen Detektion. Die TON (Turnover number, Menge gebildeter 1-Adamantanesulfonsäure pro Menge aktivem Material (Pt)) der Reaktion in Methanol nach 10 h, was der optimalen Belichtungszeit für bestmögliche Ausbeute an 1-Adamantansulfonsäure entsprach, betrug 21. Eine Bildung von 1-Adamantansulfonsäure in Abwesenheit des Katalysators wurde nicht beobachtet. Ebenso kam die Reaktion zum Erliegen, wenn die Belichtung gestoppt wurde. Der entsprechende Bromokomplex zeigte ebenfalls Aktivität, jedoch betrug die TON nach 10 h nur 8. Wurde hingegen unmodifiziertes TH eingesetzt, fanden sich lediglich Spuren von 1-Adamantanesulfonsäure. Keinerlei Spuren von 1-Adamantanesulfonsäure fanden sich hingegen, wenn Hexachloroplatinsäure mit
SiO2, Al2O3 oder amorphem TiO2 geträgert wurde. Die Ausbeuten an 1-Adamantansulfonsäure unter Einsatz verschiedener Photokatalysatoren und bei unterschiedlichen Belichtungszeiten sind in Abbildung 2 zusammengefasst.
12 b
10
8
6 c 4 % Ausbeute % Ausbeute
2
1-Adamantansulfonsäure a 0 f'
0246810 Belichtungszeit / h
Abbildung 2: Ausbeuten von 1-Adamantansulfonsäure aus der Sulfoxidation mit sichtbarem Licht in Methanol bei verschiedenen Belichtungszeiten a) TH; b) 4%[H2PtCl6]/TH; c) 6%[H2PtBr6]/TH; f’) TiO2 (amorph), H2PtCl6 oder H2PtCl6 geträgert auf SiO2, Al2O3 and TiO2 (amorph); [Katalysator] = 2 g / l, 15 mL Methanol. 138
Die Sulfoxidationen anderer Alkane mit sichtbarem Licht wurde unter vergleichbaren Bedingungen wie für Adamantan durchgeführt und sind zusammen mit Adamantan in Tabelle 2 beschrieben.
Stoffmenge von Stoffmenge von RH TON R-H [mmol] Alkansulfonsäure [mmol]
1.0 0.11 21
O 1.0 0.10 20
CH3(CH2)5CH3 50.0 0.06 11
CH3(CH2)14CH3 25.0 0.03 8
Tabelle 2: Ausbeuten und TON für die Sulfoxidation von Alkanen mit sichtbarem
Licht in Methanol nach 10 h Belichtung, photokatalysiert von 4%H2[PtCl6]/TH.
Gestützt auf die Annahme, dass Photosulfoxidierungen nur beobachtelt werden können, wenn der Platinkomplex auf halbleitenden Metalloxiden aufgebracht wurde, gehen wir von einem ähnlichen Mechanismus aus, wie kürzlich für den Abbau von
4-CP mit sichtbarem Licht an 4%[H2PtCl6]/TH beschrieben. Lichtabsorption durch den Titandioxid-Halogenplatinat-Komplex führt zu homolytischer Pt-X-Spaltung unter Bildung eines adsorbierten Halogenatoms und eines Pt(III)-Komplexes als primäre Zwischenprodukte (Schema 1, A). Übertragung eines Elektrons von Pt(III) in das Leitungsband von TiO2, gefolgt von der Reduktion des Sauerstoffs zu Superoxid (Schema 1, C) regeneriert den Pt(IV)-Ausgangskomplex (Schema 1, B). Aus Superoxid entstehen über gut bekannte Reaktionsschritte OH Radikale, welche Wasserstoffatome von Alkanen unter Bildung von Alkylradikalen abstrahieren können (Schema 1, D). Eine zusätzliche Wasserstoffabstraktion von Alkanen könnte durch intermediäre Halogenatome 139 erfolgen (Schema 1, E), wobei die Halogenliganden regeneriert werden
(Schema 1, F). Die gebildeten Alkylradikale greifen SO2 an und ergeben Sulfonsäuren.
III Ti O Pt X
O2 RH
C B A F E VIS
R + H+ O2 IV Ti O Pt X
RH H+ SO2
OH D
RSO2 H2O + R
RSO H 3
Schema 1: Postulierter Mechanismus der Sulfoxidation von Adamantan mit Titanchloro- oder Titanbromoplatinat und sichtbarem Licht (X = Cl oder Br).
Beobachtungen ergabe, dass der Cl-Ligand eine entscheidende Rolle in der photokatalytische Aktivität von 4%[H2PtCl6]/TH spielt, worauf hin die Rolle besser 140 komplexierender Liganden an unserem Katalysator untersucht wurde. Acetylacetonat ist ein guter Chelatligand für Übergangsmetallkomplexe und seine Anwesenkeit erhöhte die Ausbeute der Sulfoxidation in Methanol deutlich. Andere Chelatliganden wie Hexafluoroacetylaceton, Pyrophosphat, Ethylenglycol und Natriumdihydrogen- phosphat hatten keine positiv Auswirkung. Alle hergestellten Katalysatoren wiesen bei der Anwesenheit von Acetylaceton diese höhere Aktivität auf (Abbildung 3). Besonders im Fall von 4%[H2PtCl6]/TH stieg die Ausbeute von 12 auf 39%, was einer mehr als Verdreifachung des ursprünglichen
Wertes entspricht. 6%H2[PtBr6]/TH folgte einem ähnlichen Trend.
Kohlenstoffmodifiziertes Titandioxid (C-TiO2) zeigte ebenfalls eine Erhöhung der Ausbeute durch Acetylaceton von 10 auf 30%. Besondere Aufmerksamkeit wurde auf die Anatasmodifikation von Titandioxid gelegt. TH und TiO2 (Anatas), die bei Abwesenheit von Acetylaceton inaktiv sind, zeigten eine herausragende Aktivität bei der Zugabe von Acetylaceton, während amorphes TiO2 in beiden Fällen inaktiv war. Als TH mit Acetylaceton vorbehandelt wurde, war es bei der Sulfoxidation inaktiv.
Die direkte Addition von Acetylaceton an TiO2 färbte das weiße Pulver leicht gelb.
Das DRS zeigte für alle käuflichen TiO2-Pulver, die in Kontakt mit Acetylaceton waren, eine Verschiebung der Absorptionsbande in den sichtbaren Bereich. Die Bandlücke von TH wurde durch die Komplexierung von Acetylaceton von 3.21 auf 3.11 eV verkleinert.
141
50 d'
40 b' c'
30
20 d
% Ausbeute % Ausbeute b 10 c 1-Adamantansulfonsäure 0
0246810 Belichtungszeit / h
.
Abbildung 3: Einfluss von Acetylaceton auf die Ausbeute an
1-Adamantansulfonsäure in Methanol. b, c, d repräsentieren 4%H2[PtCl6]/TH,
6%H2[PtBr6]/TH und 4%RhCl3/TH bei Abwesenheit und b’, c’, d’ die Katalysatoren bei Anwesenheit von Acetylaceton; Experimentelle Bedingungen wie in Abbildung 2. [Acetylaceton] = 66.67 mM.
Basierend auf diesen Beobachtungen kann in Analogie zum Mechanismus in Abwesenheit von Acetylaceton ein ähnlicher Reaktionsablauf formuliert werden. Der Hauptunterschied ist, dass anstelle einer Pt-X-Bindung (X = Cl oder Br) jetzt eine Pt-O-Bindung von Acetylaceton gespalten wird. Im Falle der durch unmodifiziertes Titandioxid katalysierten Reaktion wird postuliert, dass Titanzentren an der 142
Oberfläche direkt durch Acetylaceton komplexiert werden und jetzt in einem ersten Schritt eine Ti-O-Bindung gespalten wird.
In einem weiteren Teil der Arbeit wurde Sulfoxidation in Essigsäure anstatt Methanol als Lösungsmittel durchgeführt (Abbildung 4).
e
r
u
ä s
n 0
4
e
o
t
f
l u
u n e o
s 0 t
b 3 e n c s la
a y
u t
t ce
A
n 0
A a 2 t i n
% m o
m et
a c 0 la d 1 ety
A c A
- 0 e 1 a d b j ohn g c n m k Photokatalysatoren
Abbildung 4: Ausbeute von 1-Adamantansulfonsäure in Essigsäure bei An-, und
Abwesenheit von Acetylaceton: a) TH, b) 4%[H2PtCl6]/TH, c) 6%[H2PtBr6]/TH, d) RhCl3/TH, g) P25, j) TiO2-C, k) TiO2 (Sol-Gel Methode) m) TiO2-N (Harnstoff modifiziert) und n) TiO2-N ((NH4)2CO3 modifiziert).Experimentelle Bedingungen wie in Abbildung 2.
143
Es wurde beobachtet, dass platinmodifizierte Katalysatoren nur im Fall der Zugabe von Acetylaceton aktiv waren. 4%[H2PtCl6]/TH und 6%[H2PtBr6]/TH ergaben eine Ausbeute von 18 bzw. 11% an 1-Adamatansulfonsäure nach 10 h Belichtung. Es ließ sich kein Einfluss von Acetylaceton auf die Sulfoxidation mit 4.0%RhCl3/TH feststellen, da sowohl bei An- als auch bei Abwesenheit von Acetylaceton eine Ausbeute von 18% an 1-Adamatansulfonsäure nach 10 h erreicht wurde. Überraschenderweise zeigte unmodifiziertes TH einerseits Aktivität bei der Sulfoxidation von Adamantan mit sichtbarem Licht reagierte andererseits jedoch nachteilig auf die Zugabe von Acetylaceton. Im Allgemeinen erhöht die Zugabe von Acetylaceton die Ausbeute an 1-Adamantansulfonsäure im Fall der Metallkomplex- modifizierten Pulver, während es im Fall von unmodifiziertem Titandioxid einen negativen Effekt auf die Ausbeute zeigte. TiO2-C folgt einem ähnlichen Trend wie
TH. TiO2-N ((NH4)2CO3 modifiziert) und TiO2-N (Harnstoff modifiziert) waren zwar aktiv, konnten aber nach Zugabe von Acetylaceton keine 1-Adamantansulfonsäure mehr bilden. 144
6 REFERENCES
[1] C. Platz, K. Schimmelschmidt, (I.G. Farben Industrie), FRG Patent 7355096, 1943. [2] Ullmann's Encyclopedia of Industrial Chemistry. Vol. A25. Starch and Other Polysaccharides to Surfactants edited by Elvers, B., Hawkins, S., Russey, W, Vol. A25, VCH, 1994. [3] F. Asinger, Chemie und Technologie der Paraffin-Kohlenwasserstoffe, Berlin, 1956. [4] F. Asinger, Paraffins, Chemistry and Technology, 1968. [5] Chemical and Engineering News, 13/10/1975, 84. [6] L. Orthner, Angew. Chem. 1950, 62A, 302. [7] Chemical and Engineering News, 23/1/1984, 17. [8] E. Gretz, T. F. Oliver, A. Sen, J. Am. Chem. Soc. 1987, 109, 8109. [9] K. Schimmelschmidt, G. Cramer, R. Graf, (Farbwerke Hoechst AG vorm. Meister Lucius & Bruning), FRG Patent 840093, 1952. [10] K. Schimmelschmidt, (Farbwerke Hoechst AG vorm. Meister Lucius & Bruning), FRG Patent 910165, 1964. [11] K. Schimmelschmidt, (Farberke Hoechst AG vorm. Meister Lucius & Bruning), FRG Patent 907052, 1954. [12] F. Asinger, G. Geiseler, H. Eckoldt, Chem. Ber. 1956, 89, 1037. [13] F. Asinger, Oesterreichische Chemiker-Zeitung 1956, 57, 272. [14] R. Graf, Justus Liebig Ann. Chem. 1952, 578, 50. [15] G. W. Smith, H. D. Williams, J. Org. Chem. 1961, 26, 2207. [16] Y. Ogata, Y. Izawa, T. Tsuda, Tetrahedron 1965, 21, 1349. [17] J. J. Kirkland, Anal. Chem. 1960, 32, 1388. [18] Y. Ogata, Y. Izawa, T. Tsuda, Bull. Chem. Soc. Jpn. 1965, 38, 1984. [19] Y. Ogata, Y. Izawa, T. Tsuda, K. Takagi, Bull. Chem. Soc. Jpn. 1966, 39, 2438. 145
[20] A. Boy, R. Brard, H. Passedroit, Chem. Eng. 1975, 82, 84. [21] R. R. Ferguson, R. H. Crabtree, J. Org. Chem. 1991, 56, 5503. [22] Y. Ishii, K. Matsunaka, S. Sakaguchi, J. Am.Chem. Soc. 2000, 122, 7390. [23] J. G. Calvert, J. N. Pitts, Photochemistry, John Wiley & Sons, Inc, 1966. [24] C. Beermann, European Chemical News, 1966, 36. [25] A. D. Walsh, J. Chem. Soc. 1953, 2266. [26] R. S. Mulliken, Can. J. Chem. 1958, 36, 10. [27] F. S. Dainton, K. J. Ivin, Trans. Faraday Soc. 1950, 46, 374. [28] J. G. Calvert, H. W. Sidebottom, C. C. Badcock, B. R. Rabe, E. K. Damon, J. Am. Chem. Soc. 1971, 93, 3121. [29] H. S. Johnston, K. dev Jain, Science 1960, 131, 1523. [30] P. W. Jones, A. H. Adelman, Environ. Sci. Tech 1972, 6, 933. [31] L. Orthner, K. Schimmelschmidt, (Hoechst), FRG Patent 887503, 1953. [32] W. H. Lockwood, (E. I. du Pont de Nemours & Co.), US Patent 2503280, 1950. [33] H. W. Bradley, (E. I. du Pont de Nemours & Co.), US Patent 2507088, 1950. [34] T. G. Alston, W. D. Schaeffer, (Union Oil Co.), US Patent 3372188, 1968. [35] A. Kowalski, W. Bogus, J. Perkowski, Tenside detergents, 1980, 17, 27. [36] J. F. Black, (Esso Research and Engineering), US Patent 3325387, 1967. [37] K. Rehn, K. Schimmelschmidt, (Farbwerke Hoechst A.-G.), GB Patent 1194699, 1970. [38] J. Mackinnon, A. A. Austin, (Esso Research and Engineering), FRG Patent 1252666, 1967. [39] Dumas, Inchaupse, Photosulfonation of Normal Paraffins and Fatty Esters, Adour Entreprise, 1982. [40] L. Zang, C. Lange, I. Abraham, S. Storck, W. F. Maier, H. Kisch, J. Phys. Chem., B 1998, 102, 10765. [41] L. Zang, W. Macyk, C. Lange, W. F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 2000, 6, 379. [42] H. Kisch, L. Zang, C. Lange, W. F. Maier, C. Antonius, D. Meissner, Angew. Chem., Int. Ed. 1998, 37, 3034. 146
[43] W. Macyk, H. Kisch, J. Inf. Rec. 2000, 25, 435. [44] W. Macyk, H. Kisch, Chem. Eur. J. 2001, 7, 1862. [45] G. Burgeth, H. Kisch, Coord. Chem. Rev 2002, 230, 41. [46] G. Burgeth, PhD thesis, Friedrich-Alexander Universität (Erlangen-Nürnberg), 2002. [47] M. I. Litter, Appl. Catal., B 1999, 23, 89. [48] K. N. P. Kumar, K. Keizer, A. J. Burggraaf, J. Mater. Chem. 1993, 3, 1141. [49] A. Fujishima, K. Honda, Nature 1972, 238, 37. [50] B. O'Regan, M. Graetzel, Nature 1991, 353, 737. [51] T. Sakata, J. Photochem. 1985, 29, 205. [52] M. Fujihira, Y. Satoh, T. Osa, Nature 1981, 293, 206. [53] K. R. Thampi, J. Kiwi, M. Graetzel, Nature 1987, 327, 506. [54] R. Cai, Y. Kubota, T. Shuin, H. Sakai, K. Hashimoto, A. Fujishima, Cancer research 1992, 52, 2346. [55] C. Wei, W. Y. Lin, Z. Zainal, N. E. Williams, K. Zhu, A. P. Kruzic, R. L. Smith, K. Rajeshwar, Environ. Sci. Technol. 1994, 28, 934. [56] H. Hidaka, K. Nohara, K. Ooishi, J. Zhao, N. Serpone, E. Pelizzetti, Chemosphere 1994, 29, 2619. [57] M. Heller, in Photocatalytical Purification and Treatment of Water and Air (Eds.: D. F. Ollis, H. Al-Ekabi), Elsevier, Amsterdam, 1993, p. 139. [58] D. F. Ollis, H. Al-Ekabi, Eds., Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. [59] M. A. Fox, M. T. Dulay, Chem. Rev. 1993, 93, 341. [60] O. Legrini, E. Oliveros, A. M. Braun, Chem. Rev. 1993, 93, 671. [61] V. Brezová, A. Blazková, L. Karpinský, J. Grošková, B. Havlínová, V. Jorík, M. Ceppan, J. Photochem. Photobiol. A: Chem. 1997, 109, 177. [62] G. Blasse, P. H. M. de Korte, A. Mackor, J. Inorg. Nucl. Chem. 1981, 43, 1499. [63] E. Borgarello, J. Kiwi, M. Graetzel, E. Pelizzetti, M. Visca, J. Am. Chem. Soc. 1982, 104, 2996. [64] L. Kruczynski, H. D. Gesser, Inorg. Chim. Acta 1983, 72, 161. 147
[65] H. Yamashita, M. Honda, M. Harada, Y. Ichihashi, M. Anpo, T. Hirao, N. Itoh, N. Iwamoto, J. Phys. Chem. B 1998, 102, 10707. [66] N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, in J. Photochem. Photobiol., A, Vol. 85, 1995, pp. 247. [67] M. G. Kang, H.-E. Han, K.-J. Kim, J. Photochem. Photobiol. A: Chem. 1999, 125, 119. [68] D. Bahnemann, D. Bockelmann, R. Goslich, M. Hilgendorff, in Aquatic and Surface Photochemistry (Eds.: G. R. Helz, R. G. Zepp, D. G. Crosby), Lewis, Boca Raton, Ann Arbor, London, Tokyo, 1994, pp. 349. [69] K. R. Gopidas, P. V. Kamat, J. Phys. Chem. 1989, 93, 6428. [70] J. E. Moser, P. Bonnôte, M. Grätzel, Coord. Chem. Rev. 1998, 171, 245. [71] A. Hagfeldt, M. Grätzel, Acc. Chem. Res. 2000, 33, 269. [72] S. Klein, J. Martens, R. Parton, K. Vercuysse, P. A. Jacobs, W. F. Maier, Catal. Lett. 1996, 38, 209. [73] R. L. Rich, H. Taube, J. Am. Chem. Soc. 1954, 76, 2608. [74] R. Dreyer, K. König, H. Schmidt, Z. Phys. Chem. 1964, 227, 257. [75] L. E. Cox, D. G. Peters, E. L. Wehry, J. Inorg. Nucl. Chem. 1972, 34, 297. [76] R. C. Wright, G. S. Laurence, J. Chem. Soc. Chem. Commun. 1972, 132. [77] D. Rehorek, C. M. Dubose, E. G. Janzen, Inorg. Chim. Acta 1984, 83, L7. [78] A. Goursot, A. D. Kirk, W. L. Waltz, G. B. Porter, D. K. Sharma, Inorg. Chem. 1987, 26, 14. [79] W. L. Waltz, J. Lilie, A. Goursot, H. Chermette, Inorg. Chem. 1989, 28, 2247. [80] C. M. Davidson, R. F. Jameson, Trans. Faraday Soc. 1965, 61, 2462. [81] A. W. Adamson, W. L. Waltz, E. Zinato, D. W. Watts, P. D. Fleischauer, R. D. Lindholm, Chem. Rev. 1968, 68, 541. [82] P. C. Ford, D. Wink, J. Dibenedetto, Prog. Inorg. Chem. 1983, 30, 213. [83] O. Horvath, K. L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, New York, 1993. [84] L. E. Cox, D. G. Peters, Inorg. Chem. 1970, 9, 1927. [85] V. Balzani, F. Barigelletti, L. De Cola, Top. Curr. Chem. 1990, 158, 31. [86] S. Sakthivel, H. Kisch, Angew. Chem., Int. Ed. 2003, 42, 4908. 148
[87] S. Sakthivel, M. Janczarek, H. Kisch, J. Phys. Chem. B 2004, 108, 19384. [88] S. Sakthivel, H. Kisch, ChemPhysChem 2003, 4, 487. [89] S. Sakthivel, H. Kisch, unpublished results, 2006. [90] T. A. Tetzlaff, W. S. Jenks, Org. Lett., 1999, 1, 463. [91] M. Hermann, H. P. Boehm, Z. Anorg. Allg. Chem. 1969, 368, 73. [92] M. Herrmann, U. Kaluza, H. P. Boehm, Z. Anorg. Allg. Chem. 1970, 372, 308. [93] J. A. R. van Veen, Z. Phys. Chem. Neue Folge 1989, 162, 215. [94] H. Kisch, G. Burgeth, W. Macyk, Adv. Inorg. Chem. 2004, 56, 241. [95] H. P. Boehm, M. Herrmann, Z. Anorg. Allg. Chem. 1967, 352, 156. [96] R. Flaig-Baumann, M. Herrmann, H. P. Boehm, Z. Anorg. Allg. Chem. 1970, 372, 296. [97] G. Kortuem, W. Braun, G. Herzog, Angew. Chem. 1963, 75, 653. [98] R. M. Edreva-Kardjieva, Bulg. Chem. Commun. 1992, 25, 166. [99] B. M. Weckhuysen, R. A. Schoonheydt, Catal. Today 1999, 49, 441. [100] R. Memming, Top. Curr. Chem. 1994, 169, 105. [101] N. S. Lewis, M. L. Rosenbluth, in Photocatalysis - Fundamentals and Applications (Eds.: N. Serpone, E. Pelizzetti), Wiley Interscience, New York, 1989, p. 45. [102] N. S. Lewis, M. L. Rosenbluth, in Photocatalysis - Fundamentals and Applications (Eds.: N. Serpone, E. Pelizzetti), Wiley Interscience, New York, 1989, p. 99. [103] A. Hagfeldt, M. Graetzel, Chem. Rev. 1995, 95, 49. [104] J. Pankove, Optical Processes in Semiconductors, Engelwood Cliffs, New Jersey, 1976. [105] N. Serpone, E. Pelizzeti, Photocatalysis, Wiley, New York, 1989. [106] J. Tauc, R. Grigorovici, A. Vanuc, Phys. Stat. Sol. 1966, 15, 627. [107] D. Duonghong, J. Ramsden, M. Grätzel, J. Am. Chem. Soc. 1982, 104, 2977. [108] J. M. Bolts, M. S. Wrighton, J. Phys. Chem. 1976, 80, 2641. [109] J. Tauc, Z. Vardeny, Philos. Mag. B 1985, 52, 313. [110] S. F. Pond, Surf. Sci. 1973, 37, 596. [111] M. D. Ward, J. R. White, A. J. Bard, J. Am. Chem. Soc. 1983, 105, 27. 149
[112] A. M. Roy, G. C. De, N. Sasmal, S. S. Bhattacharyya, Int. J. Hydrogen Energy 1995, 20, 627. [113] W. Macyk, PhD thesis, Friedrich-Alexander Universität (Erlangen-Nürnberg), 2000. [114] L. Kavan, M. Graetzel, S. E. Gilbert, C. Klemenz, H. J. Scheel, J. Am. Chem. Soc. 1996, 118, 6716. [115] K. J. Van den Berg, Chem.-Biol. Interact. 1990, 76, 63. [116] B. I. Escher, M. Snozzi, R. P. Schwarzenbach, Environ. Sci. Technol. 1996, 30, 3071. [117] S. G. A. McLaughlin, J. P. Dilger, Physiolog. Rev., 1980, 60, 825. [118] H. Terada, Environ. Health. Persp, 1990, 87, 213. [119] L. Hovander, T. Malmberg, M. Athanasiadou, I. Athanassiadis, S. Rahm, A. Bergman, E. K. Wehler, Arch. Environ. Contam. Toxicol. 2002, 42, 105. [120] P. H. Pritchard, E. J. O'Neill, C. M. Spain, D. G. Ahearn, Appl. Environ. Microbiol. 1987, 53, 1833. [121] T. Madsen, J. Aamand, Appl. Environ. Microbiol. 1992, 58, 557. [122] S. L. Woods, J. F. Ferguson, M. M. Benjamin, J. Environ. Sci. Eng. 1989, 23, 62. [123] N. G. Swoboda-Colberg, Chemical contamination of the environment: Sources, types, and fate of synthetic organic chemicals, Wiley, Newyork, 1995. [124] J. Theurich, M. Lindner, D. W. Bahnemann, Langmuir 1996, 12, 6368. [125] A. Mills, S. Le Hunte, J. Photochem. Photobiol., A 1997, 108, 1. [126] C. M. Wang, A. Heller, H. Gerischer, J. Am. Chem. Soc. 1992, 114, 5230. [127] H. Gerischer, A. Heller, J. Phys. Chem. 1991, 95, 5261. [128] W. Macyk, G. Burgeth, H. Kisch, Photochem. Photobiol. Sci. 2003, 2, 322. [129] K. D. Asmus, H. Henglein, Ber. Bunsen-Ges. Phys. Chem. 1968, 68, 348. [130] H. Henglein, Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 241. [131] P. Boarini, V. Carassiti, A. Maldotti, R. Amadelli, Langmuir 1998, 14, 2080. [132] M. Mrowetz, W. Balcerski, A. J. Colussi, M. R. Hoffmann, J. Phys. Chem. B 2004, 108, 17269. [133] A. L. Pruden, D. F. Ollis, J. Catal. 82, 1983, 404. 150
[134] S. D. Malone, J. F. Endicott, J. Phys. Chem. 1972, 76, 2223. [135] CRC Handbook of Chemistry and Physics, 80 ed., CRC Press, Boca Raton, New York, London, Tokyo, 1999. [136] R. Usón, J. Forniés, M. Tomás, B. Menjón, K. Sünkel, R. Bau, J. Chem. Soc., Chem. Commun. 1984, 751. [137] R. B. Draper, M. A. Fox, Langmuir 1990, 6, 1396. [138] D. T. Sawyer, J. S. Valentine, Acc. Chem. Res. 1981, 14, 393. [139] A. Mills, R. H. Davies, D. Worsley, Chem. Soc. Rev. 1993, 22, 417. [140] A. L. Linsebigler, G. Lu, J. T. Yates, Chem. Rev. 1995, 95, 735. [141] H. Small, T. E. Miller, Jr., Anal. Chem. 1982, 54, 462. [142] J. R. Larson, J. of Chromatogr., 1986, 356, 379. [143] H. Kisch, in Photocatalysis. Fundamentals and applications (Eds.: N. Serpone, E. Pelizzetti), Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1989, pp. 1. [144] S. Sakthivel, M. C. Hidalgo, D. W. Bahnemann, S. U. Geissen, V. Murugesan, A. Vogelpohl, J. Phys. Chem. B 2006, 63, 31. [145] R. B. Cundall, R. Rudham, M. S. Salim, J. Chem. Soc., Faraday Trans. 1 1976, 72, 1642. [146] I. Izumi, F.-R. F. Fan, A. J. Bard, J. Phys. Chem. 1981, 85, 218. [147] R. W. Matthews, J. Chem. Soc., Faraday Trans. 1 1984, 80, 457. [148] M. Fujihira, Y. Satoh, T. Osa, Bull. Chem. Soc. Jpn. 1982, 55, 666. [149] K. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka, A. Itaya, Bull. Chem. Soc. Jpn. 1985, 58, 2015. [150] F. Recupero, A. Bravo, H.-R. Bjorsvik, F. Fontana, F. Minisci, M. Piredda, J. Chem. Soc., Perkin Trans. 2 1997, 2399. [151] M. Braun, M. T. Maurette, E. Oliveros, Photochemical Technology, John Wiley & Sons, Chichester, 1991. [152] N. Vlachopoulos, P. Liska, J. Augustynski, M. Graetzel, J. Am. Chem. Soc. 1988, 110, 1216. [153] K. Hara, T. Horiguchi, T. Kinoshita, K. Sayama, H. Arakawa, Sol. Energy Mater. Sol. Cells 2001, 70, 151. 151
[154] G. I. Spijksma, H. J. M. Bouwmeester, D. H. A. Blank, V. G. Kessler, Chem. Commun. (Cambridge, U. K.) 2004, 1874. [155] Y. Wang, K. Hang, N. A. Anderson, T. Lian, J. Phys. Chem. B 2003, 107, 9434. [156] T. Rajh, J. M. Nedeljkovic, L. X. Chen, O. Poluektov, M. C. Thurnauer, J. Phys. Chem. B 1999, 103, 3515. [157] N. M. Dimitrijevic, Z. V. Saponjic, D. M. Bartels, M. C. Thurnauer, D. M. Tiede, T. Rajh, J. Phys. Chem. B 2003, 107, 7368. [158] T. Rajh, L. X. Chen, K. Lukas, T. Liu, M. C. Thurnauer, D. M. Tiede, J. Phys. Chem. B 2002, 106, 10543. [159] T. Rajh, M. C. Thurnauer, P. Thiyagarajan, D. M. Tiede, J. Phys. Chem. B 1999, 103, 2172. [160] T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, Langmuir 2004, 20, 4327. [161] T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, J. Phys. Chem. B 2004, 108, 5859. [162] E. Vrachnou, M. Graetzel, A. J. McEvoy, J. Electroanal. Chem. 1989, 258, 193. [163] M. Khoudiakov, R. Parise Alejandro, S. Brunschwig Bruce, J. Am. Chem. Soc. 2003, 125, 4637. [164] T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, Langmuir 2004, 20, 2753. [165] P. V. Kamat, Langmuir 1985, 1, 608.
Curriculum vitae
Name: Ayyappan Ramakrishnan Date of Birth: 15. 09. 1975 Place of Birth: Karaikal (Pondicherry), India Marital Status: Unmarried Parents: Ramakrishnan Somasundaram, Chandra Ramakrishnan Nationality: Indian
Educational details Since 05/2002: PhD: Freidrich Alexander University of Erlangen Nürnberg Erlangen- Germany Title of the thesis: Visible light induced catalytic sulfoxidation of alkanes. Doctoral father: Prof. Dr. Horst Kisch
07/1996- 05/1998: M.Sc in Applied chemistry, Anna University, Chennai, India, First class with distinction Title of Thesis: Studies on perovskite additives to positive plates of lead-acid batteries in Exide Industries, Chennai, India.
07/1993- 05/1996: B.Sc in Chemistry, First class, Bharathidasan University, Trichy, India. 06/1990-04/1993: Secondary and Higher secondary education 06/1978-05/1990: Primary and elementary education.
Professional experience: 11/1998- 06/2001: Lecturer in Chemistry, Hindustan College of Engineering, Chennai, India. 05/98-08/98 Trainee-Quality control chemist, Technical Department, Exide Industries Ltd., Chennai.