The use of heterogeneous chemistry for the characterization of functional groups at the gas/particle interface of soot and TiO2 nanoparticles

Abstract

Six gases [N(CH(3))(3), NH(2)OH, CF(3)COOH, HCl, NO(2) and O(3)] were selected to probe the surface of seven different types of combustion aerosol samples (amorphous carbon, flame soot) and three types of TiO(2) nanoparticles using heterogeneous, i.e. gas-surface reactions. The gas uptake to saturation of the probes was measured under molecular flow conditions in a Knudsen flow reactor and expressed as a density of surface functional groups on a particular aerosol, namely acidic (carboxylic) and basic (conjugated oxides such as pyrone, N-heterocycle and amine) sites, carbonyl (R(1)-C(O)-R(2)) and oxidizable (olefinic, -OH) groups. The limit of detection was generally well below 1% of a formal monolayer of adsorbed probe gas. With few exceptions most investigated aerosol samples interacted with all probe gases to various extents which points to the coexistence of different functional groups on the same aerosol surface such as acidic and basic groups. Generally, the carbonaceous particles displayed significant differences in surface group density: Printex 60 amorphous carbon had the lowest density of surface functional groups throughout, whereas Diesel soot recovered from a Diesel particulate filter had the largest. The presence of basic oxides on carbonaceous aerosol particles was inferred from the ratio of uptakes of CF(3)COOH and HCl owing to the larger stability of the acetate compared to the chloride counterion in the resulting pyrylium salt. Both soots generated from a rich and a lean hexane diffusion flame had a large density of oxidizable groups similar to amorphous carbon FS 101. TiO(2) 15 had the lowest density of functional groups studied for all probe gases among the three TiO(2) nanoparticles despite the smallest size of its primary particles. The technique used enabled the measurement of the uptake probability of the probe gases on the various supported aerosol samples. The initial uptake probability, gamma(0), of the probe gas onto the supported nanoparticles differed significantly among the various investigated aerosol samples but was roughly correlated with the density of surface groups, as expected.

Interaction of NO<sub>2</sub> with TiO<sub>2</sub> surface under UV irradiation: measurements of the uptake coefficient

“…The value of γ 0 (dark) = 6×10 −6 determined in the present study using BET surface area, lies between the above two values. On the contrary, Setyan et al (2009) have not observed any NO 2 uptake on the TiO 2 surface. Although these authors did not give any estimation of the upper limit of the uptake coefficient, simple calculations show that the value of γ 0 from the present study would lead to an initial drop of NO 2 signal at least by a factor of 1.5 under their experimental conditions.…”

“…Although these authors did not give any estimation of the upper limit of the uptake coefficient, simple calculations show that the value of γ 0 from the present study would lead to an initial drop of NO 2 signal at least by a factor of 1.5 under their experimental conditions. The reason for the lower NO 2 uptake to TiO 2 surface in the study of Setyan et al (2009) compared with the present one, could be the mode of preparation of solid samples. Note that in the present work TiO 2 samples were heated under pumping prior to uptake measurements.…”

“…The dark reaction of NO 2 with TiO 2 surface was investigated in three studies using a Knudsen cell reactor coupled to a quadrupole mass spectrometer (Underwood et al, 1999(Underwood et al, , 2001Setyan et al, 2009). …”

Perspectives on the Future of Ice Nucleation Research: Research Needs and Unanswered Questions Identified from Two International Workshops

“…Different combinations of functional groups/structures at the interface may give rise to a different degree of hydrophilic or hydrophobic interaction ( [13] Figure 1). Two important concepts related to Question 2.1 concern contact angle and a common theoretical base for the four classical freezing mechanisms (deposition ice nucleation and condensation, immersion and contact freezing) proposed by Pruppacher and Klett [1].…”

Quantitative Aspects of the Interfacial Catalytic Oxidation of Dithiothreitol by Dissolved Oxygen in the Presence of Carbon Nanoparticles

“…Therefore, the chemical reactivity at the interface of carbonaceous NP's were characterized in terms of surface functional groups using chemical titration at molecular flow conditions in a Knudsen reactor, as described by Setyan et al 29 Briefly, a molecular flow of a reactive probing gas is passed through a reactor containing the sample to specifically interact with functional groups on the surface of the particle during its gasresidence time. The temporary decrease of the exiting gas flow is recorded and can be related to the number of reacting sites on the particle.…”