Review pubs.acs.org/CR
Titanium Dioxide Photocatalysis in Atmospheric Chemistry Haihan Chen,†,‡ Charith E. Nanayakkara,†,§ and Vicki H. Grassian*,‡,§ ‡
Departments of Chemical and Biochemical Engineering and §Chemistry, University of Iowa, Iowa City, Iowa 52242, United States 1. INTRODUCTION Titanium dioxide (TiO2) is a relatively inexpensive, nontoxic, and easy to handle material, which is of great interest, as evidenced by the wide number of applications as well as the number of publications that have appeared in the past 20 years.1 Besides reports in the literature related to the utilization of TiO2 in solar cells,2 in environmental remediation,3 in sensing,4,5 and as coatings,6,7 recent studies have focused on the role of TiO2 photocatalysis in atmospheric chemistry.8 It has been reported that TiO2 surfaces facilitate interfacial photochemical reactions and that these reactions have the potential to change the chemical balance of the atmosphere.9−11 CONTENTS TiO2 is a semiconductor material with a band gap of ∼3.2 eV, corresponding to a wavelength of ∼390 nm. TiO2 has three 1. Introduction 5919 major stable polymorphs, namely anatase, rutile, and brookite. 2. Potential Sources of TiO 2 Surfaces in the Among those, anatase has generally shown the highest Atmosphere or in Contact with Atmospheric photocatalytic activity,12 with some crystallographic planes of Gases 5920 anatase being particularly reactive.13 The presence of rutile in 2.1. TiO2-Containing Mineral Dust Aerosol 5920 an anatase sample, however, has been suggested to increase 2.2. Other Sources of TiO2 Particles in the catalytic activity.14−16 Degussa P25 TiO 2 is a typical Atmosphere 5921 commercially available material composed of 75% anatase and 2.3. Stationary Sources of TiO2 Surfaces in 25% rutile. It is generally accepted to exhibit better photoContact with the Atmosphere 5922 catalytic efficiency compared with other forms, and it has been 3. Heterogeneous Interactions of Water, Oxygen, widely used as a reference material in laboratory studies.15,17 and Carbon Dioxide on TiO2 Surfaces 5923 It is often difficult to accurately compare the photoactivities 3.1. Water Adsorption 5923 of different polymorphs of TiO2 due to surface area differences 3.2. Roles of OH/H2O in TiO2 Photocatalysis 5925 and lack of knowledge regarding what fraction of a nanoparticle 3.3. Molecular Oxygen Adsorption 5926 surface is active. The differences in morphology, surface 3.4. Interaction of O2 with OH/H2O 5927 structure, and surface chemistry for different TiO2 samples, as 3.5. Carbon Dioxide Adsorption 5927 well as the properties of the target molecule, and the overall 4. Heterogeneous Reactions of Atmospheric Trace mechanism of a specific photocatalytic reaction also come into Gases on TiO2 Surfaces 5928 play when comparing the inherent photoactivities of anatase, 5928 4.1. Nitrogen Oxides (NO, NO2) rutile, and brookite. Although anatase is often noted as being 4.2. Nitric Acid 5933 the most photoactive polymorph, there are numerous studies 4.3. Sulfur Dioxide 5934 showing that rutile has comparable or even greater photo4.4. Ozone 5934 activities than anatase.18−20 4.5. Hydrogen Peroxide 5936 When excited with light of wavelength equal to or greater 4.6. Volatile and Semivolatile Organic Comthan the band gap energy, there is the generation of electronpounds 5937 hole pairs in the conduction and valence bands, respectively. 4.6.1. Carbonyls 5937 The photogenerated electron-hole pairs can then follow several 4.6.2. Alcohols 5941 pathways: (i) recombination of electrons and holes on the 4.6.3. Organic Acids 5942 surface or in the bulk of the semiconductor; (ii) reduction of 4.6.4. Aromatics 5943 electron acceptors by photogenerated electrons; and (iii) 5. Conclusions and Future Directions 5944 oxidation of electron donors by photogenerated holes. Author Information 5944 Electrons and holes transported to the particle surface can Corresponding Author 5944 therefore initiate redox chemistry.8 Some examples of oxidation Author Contributions 5944 and reduction processes following reaction with electron Notes 5944 acceptors (A) and donors (D) are shown in Figure 1. Biographies 5944 Photogenerated electrons and holes, for example, can react Acknowledgments References
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Received: May 25, 2012 Published: October 22, 2012 © 2012 American Chemical Society
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dx.doi.org/10.1021/cr3002092 | Chem. Rev. 2012, 112, 5919−5948
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with H2O and O2 molecules, leading to the formation of reactive oxygen species (ROS).
The greatest interest and focus of this review is the potential impact of TiO2 photocatalysis on the atmospheric environment. While short wavelength ( 300 nm), (d) 100 mTorr of SO2 and 100 Torr of O2 in the presence of irradiation (λ > 300 nm), and (e) 100 mTorr of SO2, 13 Torr of H2O, and 100 Torr of O2 in the presence of irrdiation (λ > 300 nm). The spectra are fit to two transitions, S2p3/2 and S2p1/2, for two species, adsorbed sulfite and sulfate. The individual components are shown in red (sulfite) and blue (sulfate) solid lines. The total peak fit is shown in green with circular markers. Reprinted with permission from ref 182. Copyright 2010 American Chemical Society.
in the atmosphere and a reactive oxidant to initiate chain reactions of atmospheric compounds. High levels of tropospheric ozone can irritate respiratory system and reduce lung function.42 Ground level ozone is not emitted directly into the atmosphere but is formed through complex chemical reactions between precursor emissions of VOCs and NOx under solar 5935
dx.doi.org/10.1021/cr3002092 | Chem. Rev. 2012, 112, 5919−5948
Chemical Reviews
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one of the terminal oxygen atoms, followed by the dissociation into a free molecular oxygen and a surface oxygen atom that remains attached to the surface Lewis acid sites.190,191 The reaction mechanism can be depicted as follows,
A kinetic study conducted by Mills et al.,189 measuring the initial rate of ozone decomposition on a thin film of TiO2, has shown that the rate of ozone decomposition is independent of ozone concentration at high levels of ozone, i.e. 100−1000 ppm. At lower ozone concentrations,
