In the course of metal oxide synthesis (ZnO, TiO 2), characteristic spectroscopic changes are observed associated with particle growth that starts with molecular species (Zn(CO2CH3)2, TiC14) and produces measurable (by transmission electron microscopy) crystallites. Furthermore, the potential role of these minerals in the photochemical transformation of organic molecules was to be investigated.
General concepts of semiconductor redlox-chemistry
There are two conceptually different ways in which an overall redox reaction between a donor D and an acceptor A can be driven by illuminated semiconductor particles. If reaction (2) is exothermic (~GO < 0) but kinetically hindered by high activation energies (Ea), then semiconductors act as photocatalysts (see right side of Figure 2), resulting in an overall reduction in Ea.
SEMI CONDUCTORS AS
Chapter 5 which has been published in the ACS symposium series 349 gives a broad overview of the principles that govern the fate of hydrogen peroxide in the
Chapter 2
PREPARATION AND CHARACTERIZATION OF QUANTUM SIZE ZINC OXIDE: A DETAILED SPECTROSCOPIC STUDY
Chapter 3
PREPARATION AND CHARACTERIZATION OF QUANTUM SIZE TITANIUM DIOXIDE
Similar spectral changes were observed in -y-radiolysis of ethanol colloid without oxygen. The Arrhenius plot of the rate constant k (inset in Fig. 6) gives the activation energy for the dissolution of the TiO2 (Pl) colloid.
26- Chapter 4
R1R2RSC· + 02 - R1R2R3C(02)• (11) Depending on the nature and concentration of the peroxygen. Depending on the nature and concentration of the peroxy radicals, first- or second-order decay kinetics prevail. Depending on the other substituents on the carbon atom, two different reaction patterns have been observed and analyzed in terms of reaction kinetics and products by pulse radiolysis. However, from Figure 7 we see that higher concentrations of 2-propanol do lead to higher concentrations of organic peroxides.
While reactions of the strongly reducing a-isopropyl radical will ultimately give only acetone and H 20 2 , the reactivity of the {J radical is expected to be different. {The J radical can add 02 in a diffusion-controlled reaction ( 7 4) according to Eq. 17 in analogy with similar reactions of organic pero:z:y radicals (75). This radical can either be stabilized on the particle surface until further reduction to an alkyl hydroperoxide (eq 18) or it can dimerize and fragment to give several products, among which organic peroxides and H 2 O 2 are expected to be found.
Carbon-centered radicals add oxygen at diffusion-controlled rates (74) (eq. 11) to form the corresponding peroxy radical. Under ambient conditions, low steady-state concentrations are likely to favor first-order decay processes. It is worth noting that neither Figure 6 nor Figure 10 yield linear plots, but resemble adsorption isotherms.
If we were to interpret figure 10 as an adsorption isotherm, we would conclude that half of the surface sites were covered at the acetate concentration ::::::2 X 10-3 M. Increasing the oxygen content from 20% to 100% saturation did not result in a corresponding increase in the quantum yield for H2O2 production. At an O2 content of ::::::10%, where the quantum yield has reached half of its final value (Figure 8), we estimate on a numerical basis that half of the particles interact with only one O2 molecule each.
In fact, the extramolecular presence of acetate may be sufficient to induce the observed electronic effects, as will be demonstrated for oxygen absorption.
36- Chapter 5
Therefore, the present work addresses the issue of hydrogen peroxide formation in aqueous solutions, as catalyzed by metal oxides upon irradiation with visible and near-UV light. These materials can act as sensitizers for photoinduced redox processes due to their electronic structure, consisting of a valence band with filled molecular orbitals (MOs) and a conduction band with empty MOs. For example, absorption of a photon with an energy above the bandgap energy generally leads to the formation of an electron/hole pair in the semiconductor particle (Fig.
Nernstian behavior" which results in a shift of the surface potential by 59 mV in the negative direction with an increase in pH of ApH = 1. Consequently electrons are better reducing agents in alkaline solutions while holes have a higher oxidation potential in the range of acidic pH ( 26).
122 THE CHEMISTRY OF ACID RAIN
- BAHNEMANN ET AL. Photocatalytic Formation of Hydrogen Peroxide 123 various oxide surfaces is warranted in order to Judge its
- BAHNF.MANN ET AL
These particles have an average diameter of 30 nm with 30% of the surface covered by Co(II }TSP). The electrode surface was covered with a dialysis membrane {molecular weight restriction) to prevent any interference caused by catalyst particles. under illumination, an aliquot of the solution is taken and titrated with iodide in the presence of catalyst (75,76) to form 13.
Aberchrome 540 is used for the determination of the light flux to enable an absolute measurement of the quantum yields (80). The formation of hydrogen peroxide upon illumination of an air-saturated aqueous colloidal suspension of zinc oxide particles is shown in Figure 2. With dioxide bound in the form of superoxide, 02 - • , this complex was extremely stable but acted as an effective acceptor for conduction band electrons produced during irradiation of the bulk Ti02.
H202 is also produced in the absence of hole scavengers, provided the right catalyst is used. As in the case of ZnO, no formation or depletion of hydrogen peroxide is observed in the absence of light. An octahedrally coordinated surface complex, e.g. Ti-O--CO(IIl)TSP-02 - • , was identified as the catalytically active species in the TiO2--CO(Il)TSP system.
The low steady-state concentrations (H202](ss) achieved during these experiments (5 - 25 µM, depending on the nature of the catalyst) indicate that the oxidation of H202 via the photocatalytic generation of hydrogen peroxide 127 between zinc oxide and Ti02-C.O systems (II)TSPs are that the difference between the zinc oxide and Ti02-C.O(II)TSP systems is that the surface complex (Co(II)TSP) acts as an electron relay in one case, while the sacrificial electron donors, as acetate is a prerequisite for termination of e-/h+ recombination in the second case. Following studies of these rather well-defined model systems, aerated aqueous suspensions of desert sand particles were irradiated (A(ex) = 350 nm) in the presence of sodium acetate.
- BAHNEMANN ET AL. Photocaralytic Formation of Hydrogen Peroxide 129
Formation and depletion of H202 observed under illumination and in the dark in an aerated aqueous suspension of Death Valley desert sand (other ex . conditions are given in the figure). Photocaralytic formation of hydrogen peroxide 129 .. concentration of H202 is formed by the direct excitation of such rx,lecules. We have shown that hydrogen peroxide can be produced photocatalytically in the presence of semiconductor particles.
Submicron sand particles are very abundant in the atmosphere where they act as condensation nuclei {90). Their involvement as catalysts and/or photocatalysts in chemical transformations that occur in natural environments has so far been neglected even though they are rather persistent in the atmosphere with half-lives of several days before precipitation occurs. Even though the steady-state concentration of H202 observed in the desert sand experiment was quite low due to the metal-catalyzed dismutation, it can nevertheless be high enough to yield reasonable amounts of oxidation products such as H2SO4 or HN03. • Further experiments are underway to study the photocatalytic activity of natural systems.
50- Chapter 6
INTRODUCTION
In the work described below, the photocatalytic activity of a-Fe2O3 colloids is compared with the activities of ZnO and TiO2. Transparent α-Fe2O3 colloids were prepared by controlled hydrolysis of FeC!J, which was followed by membrane dialysis until the residual Cl - concentration was below 10-5 M. The concentrations used resulted in an absorption of more than 95% of the incident photons at 330 nm.
Illumination apparatus and actinometry have been described elsewhere.[ 14 ) Typically, a volume of 2 ml of suspension was illuminated with the collimated beam of an Osram XBO 450 W lamp through a 300 nm UV cutoff and an IR filter. In a typical illumination experiment, 25-50 µl aliquots were taken, diluted in 2500 µl water (in the case of the Fe2O3 colloid, 0.5 mM NaOH was used to precipitate the colloid) and then analyzed by HPIC.
RESULTS AND DISCUSSION
Chapter 7
ON THE REACTIVITY OF THE OXIDIZING SPECIES GENERATED BY PHOTOEXCITED TiO2 PARTICLES IN AQUEOUS SUSPENSION
70- INTRODUCTION
Research in this laboratory has provided strong evidence for oxidations occurring by OH• radicals at pH > pHzpc (pH of zero point charge), but it cannot be excluded that part of the observed oxidation rate is due to transfer of direct transfer of electrons from the organic donor molecule. , e.g. We decided to study the oxidative degradation of acetate molecules in illuminated TiO2 aqueous suspensions. While the oxidative degradation of anions such as acetate as a function of pH has been reported several times [l, 7], the oxidative degradation of cations has not been examined in detail.
The observed pH dependence of the acetate oxidation rate was explained in terms of a band shift of the valence band Evb pH [V, compared to our experimental findings, cannot be explained by conventional electrochemical and solid-state terms (e.g., the Nernst equation flat band potential). It is found that the effect of surface adsorption of reactant molecules on the oxide surface is the dominant factor in the electrochemical reactivity of TiO2 particles.
72- Background
The photo-degradation of acetate was performed by illuminating 2 ml aliquots in a 1 cm quartz slide with 300 nm light. A pH-stat titration technique was used to determine the rates of organic molecule degradation via the rates of OH consumption. At pH~9, a low hydroxide consumption was observed when suspensions of TiO 2 were illuminated in the absence of added organic molecules, or when suspensions containing electron donors were stirred in the dark (or in
At pH<9, no consumption of hydroxide due to oxidation of organic molecules was observed in the TiO2 suspension in the dark. 2 mM, [chloroacetate] = 1 mM) is illuminated with near-UV light in the presence of air, the quantitative decomposition of organic molecules can be immediately observed (initial quantum yield: about 0.6%. Since the quantum yield of product formation is lower than the quantum yield of acetate decomposition and is a 1 : 1 ratio of glyoxylate to glycolate, rather than implying that mechanisms other than those described in eqs 2-3 are at work.
76- photodegraded before they can dimerize and fragment
Phys. Chem
88- Chapter 8
RECOMMENDATIONS FOR FUTURE RESEARCH
This thesis also described several photochemical experiments: the photocatalytic formation and decomposition of H2O2 and organic peroxides, decomposition of chlorinated hydrocarbons using aqueous suspensions of TiO2 and ZnO. One of the most promising applications of these catalysts is the degradation of toxic molecules (e.g. chlorinated hydrocarbons or even N03) using cheap sunlight. The transparent colloidal materials produced as part of this thesis research are ideally suited for mechanistic studies using fast optical techniques.
A significant increase in the rate of electron transfer to oxygen was found and increased oxidation rates were assumed. Thus, it seems very promising to modify the colloidal catalysts prepared in this thesis research in a similar way in order to achieve a more efficient charge separation. Given the fact that OH• radicals are generated during the photoexcitation of Ti02 particles (Chapter 7), the attached electron relay must be resistant to oxidation by this species.
