Reactions of N 2O 4 with ice at low temperatures on the Au(111) surface

Abstract

Reactions of N 2O 4, formed by condensation of NO 2 gas, with adsorbed water films as models for ice surfaces were studied on a Au(111) substrate at low temperatures under ultrahigh vacuum (UHV) conditions. Two thermal reaction paths were found, primarily by using infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) techniques. One path evolved gas phase HONO (nitrous acid) and HNO 3 (nitric acid) below 150 K, independent of both the crystallinity of the ice film and the exposure of NO 2. In contrast, another reaction pathway depended strongly on these variables and was conveniently monitored by the formation of oxygen adatoms on the Au(111) substrate surface. The latter reactions only occurred following multilayer adsorption of NO 2 (present as N 2O 4) and only if the adsorption was on amorphous ice clusters and not crystalline ice. The extent of these reactions was proportional to the concentration of ‘free-OH’ groups on the ice film, indicating that water molecules with only two hydrogen bonds were required to initiate these reactions by strongly hydrating N 2O 4. Following adsorption of N 2O 4 multilayers on amorphous ice, we propose that isomerization of O 2N–NO 2 (D 2h–N 2O 4) to ONO–NO 2 (nitrite–N 2O 4) occurs upon heating to 130–185 K. Further heating to 200–260 K leads to formation of NO +NO − 3 (nitrosonium nitrate). This compound is stable on the Au(111) surface up to at least 275 K. Decomposition of NO +NO − 3 on the Au(111) surface at 275–400 K evolves gas-phase NO 2, and possibly NO (nitric oxide), and also produces oxygen adatoms that recombine and thermally desorb as O 2 at about 520 K. These investigations provide new data on reactions of nitrogen oxides and condensed phases of water, which are of general importance for several technologies and improved understanding of atmospheric chemistry. In addition, new details are revealed concerning a novel method for producing oxygen adatoms on Au(111) surfaces under UHV conditions.