UV/TiO2/H2O2 Photo Catalysis: A Case for Investigating the Magnetic field Dependence

1. UV/TiO2/H2O2 Photo Catalysis: A Case for Investigating the Magnetic field Dependence S.Aravamudhan Department of Chemistry North Eastern Hill University Shillong 793022 Meghalaya saravamudhan@hotmail.com Links for the full article:- http://www.nitrocloud.com/p/MwK5_4otYQIbrht32eUrze http:/www./ugc-inno-nehu.com/AOP2013-Article-SA.pdf

2. CONTENTS: INTRODUCTORY MATERIALS HAVE BEEN INCLUDED in SLIDES #3 to 6 OF THIS PRESENTATION FILE. 1. Enumerating the results from quantum chemical calculations. Slides # 7 to 13 2. Some of the spin system characteristics and associated magnetic field dependent features inherent to spin ensemble evolutions. Slides #14-17 3. Trends evident from the above results which have a bearing for the considerations of magnetic field effects. Slide # 18 & 19 4. The discussions of spin polarization effects on diffusing radicals. Slides # 20 to 22 5. Possible consequences on the oxidation processes (radical recombination) and the effects on efficiencies of the process of degrading the contaminants. Slides #23 http://nehuacin.tripod.com/AOP2013-full-article.pdf This presentation file: http://nehuacin.tripod.com/aop-SA.ppt

3. A POINT TO NOTE at the OUT SET: The electron spin orientations are mostly independent of orientations of the molecules where the spins are located. The orientations of the spins are influenced predominantly by applying relatively large external magnetic fields. Again, the orientations of the ensemble spins due to the magnetic fields (along or opposite to filed direction) are not so easily altered by the molecular tumbling molecular orientations instantaneously. That is the spin orientations do not follow the orientation changes of the molecules. But molecular degrees of freedom influences the spin alignment and population distributions only by relaxation processes. In order to facilitate this appreciation in the context of AOP few slides are included illustrating the various factors that have to be taken into considerations. Comprehending how is all this relevant for the radical recombination and consequent efficiency of AOP system is all about the objective of this presentation. The simplistic conclusions enlisted would have to be considered more quantitatively before actually finding the application to AOP. Introductory material on “Spins interacting with magnetic field “ are contained in the following internet video links. 1_NMR_ElementaryDefinition.WMV 2_NMR_SpinEnsemble.WMV

4. AOP2013-Article-15thSept2013-inv.doc

5. PROCESSES INVOLVING HYDROXYL RADICALS DURING PHOTOCATALYSIS: UV/TiO2/H2O2 TiO2 + UV → h+ +e− ------ Eq.4a H2O2 + e− → OH− +OH• ------ Eq.4b H2O2 →2OH• ------ Eq.4c . hv,λ<300nm

6. Thus this is the indirect photochemical degradation, where the electrons of the supplied H2O2 molecules are excited to form reactive radicals which react with organic molecules to decompose them. In the indirect photochemical oxidation radicals are formed by photolysis, which then attack the organic compounds. An example is the reaction of H2O2. Examples for possible reactions forming highly reactive radicals in the TiO2 Photo catalytic systems: h+ + H2O H+ + OH------ Eq.2 H2O2 2 OH-------- Eq.1 To assess the role of such reactive radicals, it became necessary to carry out Quantum Chemical theoretical studies. The studies were using the semi empirical method of PM3 approximation and Abinitio calculation using the SCF-STO 3G basis sets. O2 + e- O2- ------ Eq.3

7. iIn GO steps the coordinates change by energy minimization criterion In the medium molecular entities move around by thermal brownian fluctuations which is not included in energy minimization.

11. CONCLUSIONS FROM QUANTUM CHEMICAL RESULTS From the theoretical calculations it was evident that adding triplet oxygen (triplet-dissolved oxygen) or singlet oxygen or hydrogen per oxide did not make much difference. Not only that it is ineffective, but such additions also seem to make the OH radicals ineffective. What seems necessary is that required number of OH radical around the organic molecule with properly disposed towards the C-H bonds. The disposition of the O-H radical and the number of O-H radicals are to be ascertained by the diffusion characteristic s and the Brownian fluctuating motions. To some extent this inference could be made by manually altering the O-H coordinates in the structure editor and monitoring the progress in the visualization feature and the converging results. The next step is to inquire whether these diffusion charatcerics, particularly the correlation times associated with it, would favor in any way the O-H radical re-combinations which would reduce the availability of O-H radicals. This is the point where the magnetic fields can have a role in influencing the course of oxidation efficiencies..

12. The results of theoretical studie,s thus, indicated that the essential experimental finding on the significant role of OH radical is the dominating factor and all the other additions of Oxygen (dissolved oxygen or the singlet oxygen all play a role in some way or the other to replenish the depleting OH radical, and maintain the adequate level of the OH radical species in the medium. Since the theoretical investigations were not exhaustive to include the kind of change in the orientations, distances of the OH radical around the organic contaminant molecule (essentially an entropy consequence) the details of the results of theoretical studies are not reported in the present context. Thus, the presence of OH radicals in the medium is ensured, either by the surface reactions at the Ti O2 or in the medium itself, then the tendency for the OH radical to react with the C-H (aliphatic, or aromatic) protons get clearly established by the cluster geometry optimizations. The template examples of organic species considered were C4H8 O2 (alicyclic type), and C4H4O2 (aromatic type) with various numbers and dispositions of OH radicals around the organic molecule.

13. But the most important factor at this juncture is the OH• radicals diffusing into the solvent medium (water) and reacting with the organic solute pollutant molecules. If the OH• radicals undergo recombination by the collision of two hydroxyl radicals, then the availability of the hydroxyl radicals is reduced and the oxidative degradation of organic pollutants would be reduced.Since the recombination of radicals is affected by the spin states of the radical pair, the magnetic field can influence the radical recombination. Mostly it is the hydroxyl radicals which are diffusing in the system, which do not have much of spin state correlation for any given pair. It is to be remarked that the radical pairs generated by the photo excitation of a molecule would be a ‘intimate radical pair’ with the retention of a singlet state of the pair of electrons formed by the breaking of a bond.

14. The main factor to be taken into consideration is the spin dispositions for radicals to react and form bond. Considering the case of two O-H radicals represented below: Each spin is in a doublet state (uncoupled) Spin= 1/2 Spin= 1/2 These are each with an unpaired electron-corresponding to doublet spin state. This is the situation of the two spins not influencing each other and are uncoupled. No spin pairing can arise at this situation as required for the spins shared in a bond. These two spins are in a coupled singlet state. The coupling of spins brcomes possible when the two electrons are at such proximities for their orbital wavefunctions can overlap Coupled spins can be in triplet state also. This situation for spin-spin coupling would be considered in greater detail in a later slide after a few slides on the non-interacting ensemble of spins and the relaxation processes.

15. The diffusion characteristics do not allow enough time during collisions for effective spin-spin interactions Uncoupled spin ensemble Electron spin relaxation times are of the order of 10-6 Sec Spin-lattice relaxations and not due tospin-spin interactions Relaxation process Video internet link for illustration on relaxation 5_NMR_SpinRelaxation_FieldInhomogeneities.WMV Relaxation time T1 The descriptions here in may be more appropriate for Proton spins with positive gyro magnetic ratio. The logistics are similar for the electron spins with negative ratio. More detailed description of this phenomenon to follow in the next slide. Internet Video link ----- 8_NMR_ChemicalShift_Jcoupling.WMV Youtube Link for detailsSlide#3 contains distinction of Electron and Proton SPINs in Magnetic Field

16. Since experimental situations deal with ensemble of molecular entities and not single isolated specimen, it is first of all necessary to know what prevails phenomenally in an ensemble of spin systems. The non interacting spins making up the ensemble. What are the situations when the spins can interact and the delineation of the interacting regime from the non interacting regime. That is, what are the prerequisites for the onset of spin -spin coupling in an ensemble of spins. In the interacting spins regime description of spin states for the coupled spins and the associated consequences as different from the uncoupled spin systems. What are the spin states favoring the radical-radical recombination and how the overall magnetic field effects can influence the radical mediated chemical reactions. This consequence for the specific context of Oxidation processes involving O-H radicals.

17. -1/2 +1/2 Spin ensemble No external magnetic field. The energy levels are degenerate random The ensemble of spins, have equally distributed population between the two levels for the spin ½ protons -1/2 +1/2 No net magnetization On the application of field….. Splitting is instantaneous & population redistribution requires more time called the relaxation time -1/2 No radiations are present Not stimulated transitions: but spontaneous relaxation transitions Magnetic field +1/2 Net magnetization Degeneracy removed/Energy levels split Thermal equilibrium Boltzmann distribution ‘h ν = g β H’is the magnetic resonance condition

18. When the spins are in such proximity that the corresponding space- wave functions overlap, then the spin exchange interaction dominates. This makes possible the coupling of spins, when the two spins are describable by a single spins state with the total spin S=1. When the Total spin (of the composite coupled spins made up of two spin ½ particles) is equal to 1, the possible spin quantum number s are 1, 0 which are referred to as the Triplet and singlet states. In triplet state the allowed orientations are three with their magnetic quantum numbers ms = +1. 0, and -1 and in the singlet state the only possible ms value is 0. This is what is depicted in Fig-3. of slide #18 Besides the spin exchange interaction among the electron spins as described above, the electron spin density occurring at the nuclei of the radical/molecular entity can result in electron-nuclear hyperfine interaction which also requires a mechanism involving the electron wave functions, may be within a particular radical species instead of between two radical units. The electron nuclear hyperfine interaction causes a mixingof the SPIN States (the triplet and singlet) arising out of the electron spin exchange. This is depicted in Fig-2 of the slide# 18

19. Fig-2 Fig-1 Nucleus-electron interaction. The representation of singlet and triplet states of coupled spins of nucleus and radical electron: hyperfine interaction ‘a’. Below is the similar singlet and triplet situation for two interacting electron spins of radical pair when they are in favorable proximity. This can arise by the Spin exchange‘J’ interaction between the two electrons. Zeeman interaction in presence of external magnetic field The above mentioned hyperfine interaction drives the inter conversion of electron-electron singlet (S=0, Sz=0) and triplet states (S=1, & Sz= +1, 0, or -1) Fig-3

20. At such large intermolecular distances, the spins are in uncoupled doublet states. This is in spite of the fact that one may add up the spins to be equal to total 1 (for two spins) or 3/2 (for three spins). To specify the spin states, one must ascertain whether they are coupled are not. The hyperfine nuclear electron interactions can be present but is of no avail for the context of Coupled cases as it is non existent Spin= 1/2 Spin= 1/2 Spin= 1/2 Spin= 1/2 Spin= 1/2 Spin= 1/2 Spin= 1/2 Representing diffuse orbital extension around molecular hard sphere

21. + Coupled singlet Zero external field In solutions the zero field splitting changes with orientation and averages to zero but at every instant a non vanishing interaction present would favor radical recombinations. At high fields, the zeeman effect being large compared to zero field splitting values, the spins are almost in the doublet state; in an unfavorable situation for recombinations

22. Coupled triplet and singlet spin system description Doublet State of Radical Spins Uncoupled, independent, unpaired spins |αα > T+! |α> |α>  / 2 Coupled singlet 1/√2 |αβ-βα>  S0 1/√2 |αβ+βα> T0 |β> |β> - / 2 Uncoupled doublet |ββ > T-! Equivalent energy level description for doublet = Zeeman Energy in Magnetic Field

23. When the radical spins are far away from each other, they are non interacting spins. When the radicals get to such proximities to each other that the unpaired electron wave functions of the two radical spins can overlap, then there would be a coupling of the electron spins due to exchange interactions and this makes the recombination of the radicals possible. Depending upon the spin alignments relative to each other, the recombination product could be in a Triplet State (S=1) or a Singlet State (S=0). Either case can result in radical recombination. The triplet state recombination product would be in a slightly higher energy state than the singlet state since in the latter case the spins can be paired up in the product molecule. Moreover the hyperfine interactions can mix the singlet and triplet states and time dependent perturbations can cause transitions from triplet-singlet. Thus this happens if the collision parameters are favorable. Under such a favorable situation, if a high (compared to the zero field spin-spin interaction strengths) magnetic field is applied, then the ensemble spins tend to be more or less in the doublet (uncoupled) regime since the magnetic energy levels cannot mix and result in recombination. Only in magnetic fields up to 150 gauss the spin-spin interactions dominate and mix the spin state. Thus in a much higher fields of few Kilo gauss the spins may be nearly in the uncoupled situation and the colliding radical pairs may separate out again after collision without causing radical recombination.