Photooxidation of Ammonia on TiO₂ as a Source of NO and NO₂ under Atmospheric Conditions

Abstract

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NOx analyzer was used to study the kinetics of NH3 uptake and NOx formation from photochemistry initiated on irradiated (λ > 290 nm) TiO2 surfaces under atmospherically relevant conditions. The speciation of NH3 on TiO2 surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH3 concentration as expected for reactions proceeding via a Langmuir-Hinshelwood mechanism. The mechanism of NOx formation is shown to be humidity dependent: Water-catalyzed reactions promote NOx formation up to a relative humidity of 50%. Less NOx is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH4(+), and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH2 photoproduct and O2 in the presence of H2O supports the experimental conclusion that NOx formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH2 + O2 reaction and the reaction of NH2 + O2 in small water clusters were carried out. Solvation of NH2OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH3 on TiO2 surfaces represents a source of atmospheric NOx, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH3 on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.

Designing polymeric sensing materials: what are we doing wrong?

“…TiO 2 is commonly used to oxidize ammonia into nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ). [63] Therefore, ammonia will coordinate well with TiO 2 . In addition, TiO 2 was able to coordinate well with both PAA and PANI.…”

“…Multiple sensors listed in Table used titanium dioxide (TiO 2 ) nanoparticles to improve the sensitivity towards ammonia. TiO 2 is commonly used to oxidize ammonia into nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) . Therefore, ammonia will coordinate well with TiO 2 .…”

The monolithic cordierite supported V2O5–MoO3/TiO2 catalyst for NH3-SCR

“…As shown in Fig. 1b, the pores were mostly distributed in the range of 2-10 nm, especially at 3.4 nm, signifying that most of the pores morphology existed in the form of mesoporous which was conductive to provide suitable sites for the absorption of reactant molecular and then proceeded catalytic reaction [22]. Additionally, the free absorption in high specific pressure range symbolized a small amount of macropores presenting in the internal structure of catalysts, which made the mass transfer more easily [23].…”

Uptake of gaseous formaldehyde by soil surfaces: a combination of adsorption/desorption equilibrium and chemical reactions

“…However, the radial diffusion needs time and may result in a concentration gradient in the radial direction. Since we can only measure the averaged concentration instead of C wall , the determination of γ cannot be achieved unless the C profile is determined first (Behnke et al, 1997;Monge et al, 2010;Sassine et al, 2010;Kebede et al, 2013) (for details see reference Murphy et al, 1987). Therefore the factor of γ /(1 − γ /2) rather than simply γ is used by Murphy et al (1987) for a correction to the wall collision rate in the presence of a concentration gradient near the wall.…”