Effects of Coadsorbed Water on the Heterogeneous Photochemistry of

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of Nitrates Adsorbed on TiO 2

Christopher J. Ostaszewski, Natalie M. Stuart, Daniel M. B. Lesko, Deborah Kim, Matthew J. Lueckheide, and Juan Gabriel Navea J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04979 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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The Journal of Physical Chemistry

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of Nitrates Adsorbed on TiO2 Christopher J. Ostaszewski, Natalie M. Stuart, Daniel M. B. Lesko, Deborah Kim, Matthew J. Lueckheide, and Juan G. Navea* Chemistry Department, Skidmore College, Saratoga Springs, NY, 12866-1632 * To whom correspondence should be addressed. Email: [email protected] (J. Navea) Abstract: Nitric acid, a well-known sink of NOx gases in the atmosphere, has been found to be photoactive while adsorbed on tropospheric particles. When adsorbed onto semiconductive metal oxides, nitrate’s photochemical degradation can be interpreted as a photocatalytic process. Yet, the photolysis of nitrate ions on the surface of aerosols can also be initiated by changes in the symmetry of the ion upon adsorption. In this study, we use quantum chemistry to model the vibrational spectra of adsorbed nitrate on TiO2, a semiconductor component of atmospheric aerosols, and determine the kinetics of the heterogeneous photochemical degradation of nitrate under simulated solar light. Frequencies and geometry calculations suggest that the symmetry of chemisorbed nitrate ion depends strongly on coadsorbed water, with water changing the reactive surface of TiO2. Upon irradiation, surface nitrate undergoes photolysis to yield nitrogen-containing gaseous products including NO2, NO, HONO and N2O, in proportions that depend on relative humidity (RH). In addition, the heterogeneous photochemistry rate constant decreases an order of magnitude, from (5.7±0.1)×10-4 s-1 on a dry surface to (7.1±0.8)×10-5 s-1 when nitrate is coadsorbed 1 ACS Paragon Plus Environment

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with water above monolayer coverage. Little is known about the roles of coadsorbed water on the heterogeneous photochemistry of nitrates on TiO2, along with its impact on the chemical balance of the atmosphere. This work discusses the roles of water in the photolysis of surface nitrates on TiO2 and the concomitant renoxification of the atmosphere.

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I.

Introduction Uncertainties in the photochemistry of atmospheric aerosols are largely due to the

multicomponent nature of these particles, with a wide variety of active chromophores within the solar spectral region of the lower troposphere. Among these components, titanium dioxide is a common trace metal oxide in atmospheric particulate matter (PM) generated from both natural and anthropogenic processes.1-3 The primary source of TiO2-containing aerosol particles is mineral dust transported to the atmosphere by winds.1,2 In addition, anthropogenic particulate matter, such as emissions from coal-fired power plants, contains trace amounts of TiO2.4 In all these cases, TiO2 minerals can be present in three major stable crystalline structures; anatase, rutile, and brookite. While the amount and structure of TiO2 present in atmospheric particulate matter varies with the source region, the presence of TiO2-anatase is particularly important because it has shown the highest photocatalytic activity.5-9 Furthermore, TiO2 has a band gap energy of 3.07 eV (corresponding to 404 nm), smaller than the energy cutoff of solar radiation in the troposphere.1,9 Thus, even trace amounts of TiO2 in atmospheric particulate matter can play a significant role in heterogeneous photochemical processes. Atmospheric particles have shown to adsorb and react with trace atmospheric gases such as nitrogen oxides, nitric acid, and ozone.10-13 Recently, heterogeneous photochemistry has been suggested as the underlying reason to explain the nearly steady-state concentration of nitrous acid (HONO) in the lower troposphere at daytime or in indoor environments.14-16 In fact, nitrate ion has been shown to react on clean surfaces of metal oxide, including TiO2, a mechanism responsible for the renoxification of the atmosphere and the formation of HONO.7,9 These studies suggest that photocatalytic activity of TiO2-containing atmospheric particles has important implications in the reactive nitrogen biogeochemical cycle. Yet, the role of TiO2 in the heterogeneous photochemistry 3 ACS Paragon Plus Environment


of atmospheric nitric acid in the HONO, N2O, and NOx atmospheric budget remains partially unresolved. Uncertainties are largely due to two complexities: First, the effects of coadsorbed water in the photocatalytic activity of TiO2; second, the large variety of adsorption modes of nitrates on the metal oxide surface, which can change due to the presence of coadsorbed water. Coadsorbed water can have opposing effects on the photolytic decomposition of surface NO-3 on TiO2: water can compete for active sites on the TiO2 surface, decreasing the coverage of nitrate ions and even blocking the incident radiation that initiates photocatalysis.17 Yet, TiO2 photocatalysis of adsorbed water leads to the generation of hydroxyl radicals and can open new 18 pathways for the reaction of surface species, such as nitrite ion (NO− 2 ). On the other hand, the

vibrational spectrum of nitrates on the surface of metal oxides, such as Al2O3, changes as a function of relative humidity.19,20 Recently, the photochemical conversion of nitric acid into gaseous phase NOx was shown via symmetry breaking of nitrates on the surface of Al2O3.21,22 At low relative humidity, these studies showed a higher rate of gas-phase photochemical conversion due to the readsorption of NO2 onto the Al2O3 surface.22 Ultimately, at higher relative humidity, the heterogeneous photochemical reaction of surface nitrate slows down because coadsorbed water blocks available surface sites.22 Such changes in relative humidity affect the symmetry, reactivity, photochemistry, and vibrational signatures of surface nitrates, yet little is known about the coordination modes of nitrates on TiO2 and the role of water in the photolysis of nitrate ion beyond symmetry breaking effects. In this work, the heterogeneous photochemistry of nitrate ion on TiO2 under simulated solar radiation is investigated as a function of relative humidity. In order to better understand the effects of nitrate adsorption on TiO2 and coadsorbed water, we combine attenuated total reflection FTIR spectroscopy with quantum chemical calculations in order to determine the vibrational frequencies 4 ACS Paragon Plus Environment

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of surface nitrate and the structural effects of competing coadsorbed water, allowing for a better understanding of nitrate ion coordination and solvation onto TiO2. II.

Experimental and computational methods

2.1

FTIR spectroscopy of chemisorbed nitrates

Much of the experimental apparatus and protocol used in the current study have been previously described.9 Briefly, photochemical reactions on TiO2 films are carried out on a horizontal attenuated total reflection (ATR) Ge crystal (PIKE Technologies) enclosed in a 54.0 cm3 Teflon chamber topped with a UV-transmission window, as shown in Figure 1. Gaseous HNO3, generated by bubbling purified air through a 3:1 mixture of sulfuric acid (96.0%) and nitric acid (79.5%), was flowed over the TiO2 film. Humidity of the gaseous nitric acid flow was varied from < 1% to 45% relative humidity (RH) by bubbling dry air through 18 MΩ purified water in the humidification chamber. Beyond the 45% RH, low coverages of surface nitrates lead to concentrations below the limit of quantification for kinetic analysis. Humid air and gaseous nitric acid were mixed on a mixing chamber before allowing the mixture to flow to the photochemical cell to be adsorbed onto TiO2. Gas generator

Photochemical cell

Dry air →

Solar simulator

Hygrometer HNO3→

HNO3/H2SO4

Gas analysis chamber

Heat absorber

0.00% RH

Long-path gas cell

IR source

Mixing chamber H2O→

H2O

Detector

Ge ATR crystal coated with TiO2

IR source

Detector To vacuum



Figure 1. Three component schematic diagram of the experimental-apparatus. The surface areas of TiO2 (Aldrich) powder used for all experiments reported here is (54.99 ± 0.04) m2 g-1, as determined by a seven-point N2-BET adsorption isotherm using a Quantachrome Nova 2200e surface area analyzer. The TiO2 band gap energy was measured as 3.07 eV (corresponding to 404 nm), well above the cutoff of our solar simulator (290 nm).9 Thus, the loss of chemisorbed nitrate from TiO2 can be a combination of light absorption and dissociation by surface nitrate and a surface reaction triggered by the electron-hole pairs. 2.2

Quantum Chemical Calculations

All electronic structure optimizations and vibrational frequency calculations were performed using the quantum chemical computational software Gaussian 09. The results were visualized using GaussView software. Quantum chemical calculations were carried out on binuclear clusters consisting on a geometrically restricted TiO2 surface site model with a nitrate ion coordinated in different configurations.19 Each cluster ground states were modeled using Becke’s three-parameter hybrid method with the LYP correlation functional (B3LYP). In this study, ground-states of nitrate adsorbed on TiO2 cluster ([Ti2O(OH)n(H2O)2(NO3)]-1), with n = 4 or 5, were optimized with a B3LYP/6-31+G(d) basis set, which was found suitable for geometry optimization.19 Excluding terminal hydrogens, the structures were optimized by freezing the titanium dioxide surface in a previously optimized (001) coordination.23 The different nitrate ion coordinations reported here are local minima of the potential energy surface, providing better understanding of the interface interaction between nitrate ions on TiO2.19 Vibrational frequency calculations were also performed at the B3LYP/6-31+G(d) level of theory. To account for anharmonicity, all calculated frequencies


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were scaled by a factor of 0.9632, as reported by Irikura et al.19,24,25 The scaling factor varies slightly with basis set under the same level of theory used in these calculations. III. 3.1

Results and discussion Vibrational spectroscopy

The attenuated total reflection FTIR spectra of TiO2 exposed to gaseous nitric acid followed by evacuation are shown in Figure 2 for five different relative humidity (RH) conditions:

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of

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