Understanding gas-phase reactions in model gas mixtures approximating preturbine heavy-duty natural gas engine exhaust compositions containing NO, NH 3 , NO 2 , CH 4 , CO, and C 2 H 4 is extremely relevant for aftertreatment procedure and catalyst design and is thus addressed in this work. In a plug-flow reactor at atmospheric pressure, five different model gas mixtures were investigated in the temperature range of 700-1 200 K, using species analysis with electron ionization molecular-beam mass spectrometry. The mixtures were designed to reveal influences of individual components by adding NO 2 , CH 4 , CO, and C 2 H 4 sequentially to a highly argon-diluted NO/NH 3 base mixture. Effects of all components on the reactivity for NO x conversion were investigated both experimentally as well as by comparison with three selected kinetic models. Main results show a significantly increased reactivity upon NO 2 and hydrocarbon addition with lowered NO conversion temperatures by up to 200 K. Methane was seen to be of dominant influence in the carbon-containing mixtures regarding interactions between the carbon and nitrogen chemistry as well as formaldehyde formation. The three tested mechanisms were capable to overall represent the reaction behavior satisfactorily. On this basis, it can be stated that significant gas-phase reactivity was observed among typical constituents of pre-turbine natural gas engine exhaust at moderate temperature. K E Y W O R D S ammonia, exhaust gas aftertreatment, gas-phase kinetics, natural gas engines, nitric oxide This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

years 1999 to 2002, several tons of AgI were annually introduced into the atmosphere worldwide for weather modifications such as hail suppression, precipitation enhancement, or drought operation. [8][9][10] Already in 1947, Vonnegut has identified AgI as an excellent ice nucleating agent by searching for crystals with lattice constants that resemble the lattice constant of ice as closely as possible. [11] Indeed, AgI nucleates ice at around -4 °C. [4,11] The importance of small lattice mismatches between the AgI surface and ice has been investigated by Evans. [12] He has studied ice nucleation in the presence of AgI particles at pressures up to 3000 bar, allowing the nucleation of ice I and ice III. In these experiments, it has been observed that the ice phase with the better lattice match nucleates preferentially. From these results, it has been concluded that lattice match is an important factor in the ability to nucleate ice.Heterogeneous ice nucleation using AgI particles has been studied in different nucleation modes (immersion, deposition, contact, and condensation freezing). [4] It is known that the ice nucleation ability of AgI is reduced by photolytic decomposition of the crystal. AgI decomposes mainly when exposed to ultraviolet light (λ < 440 nm). [13,14] When irradiated with light with wavelengths above 440 nm, the photolysis of AgI occurs to a lesser extent. [13] The photolysis also depends on the amount of adsorbed water on the AgI surface. When water is adsorbed on the surface, the photolysis is reduced. This is seen from the fact that the ice nucleation effectiveness is maintained for longer time when water is present. [15] The hydrogen bonding structure of adsorbed water molecules on AgI powder has been investigated with X-ray photoelectron spectroscopy and near edge X-ray adsorption fine structure spectroscopy. It has been found that the water molecules adsorbed on AgI indeed exhibit an ice-like hydrogen bonding structure. [16] Two polymorphs of AgI are stable at temperatures under 147 °C and at ambient pressures: βand γ-AgI. [17,18] While the metastable γ-AgI crystallizes in the zincblende structure, the thermodynamically stable β-AgI adopts the wurtzite crystal structure.Figure 1a shows a side view of the ( 0001) surface of β-AgI. The silver and purple spheres represent the silver cations and the iodide anions, respectively. The structure of β-AgI consists Silver iodide (AgI) particles are known for their outstanding ice nucleation ability. The effective ice nucleation has been explained by the structural similarity of the AgI surfaces and the basal plane of ice I h . However, the relevant AgI surfaces are polar, i.e., thermodynamically instable. This fact implies the existence of a stabilization mechanism. The nature of this stabilization mechanism remains, however, unknown. Additionally, calculations suggest that exclusively the silver-terminated and not the iodine-terminated surfaces nucleate ice. So far, no atomically resolved images at any AgI-water interface exist. This is most likely du...