Type Ia polar stratospheric clouds (PSCs) are thought to consist of HNO 3 /H 2 O mixtures, usually assumed to be crystalline nitric acid trihydrate (NAT). However, it has recently been suggested that metastable nitric acid dihydrate (NAD) may form preferentially in the atmosphere due to a lower nucleation barrier. We have used Fourier transform infrared spectroscopy to investigate the crystallization kinetics of NAD aerosols. The crystallization rates were measured under two experimental regimes. In the first, we formed glassy 2:1 H 2 O: HNO 3 aerosols in a cryostat held at 77 K and measured the rates of crystalline NAD formation when the aerosols were warmed to stratospheric temperatures. The crystal growth rates were then used to estimate the activation energy for diffusional transfer of HNO 3 across the solid/liquid phase boundary, ∆g d . We found ∆g d ) 13.3 kcal mol -1 for the temperature range 190-202 K. We have also measured the crystallization rate of NAD aerosols nucleated at stratospheric temperatures. We used homogeneous nucleation theory and our estimate of ∆g d to determine the interfacial surface energy, σ, between NAD and a supercooled 2:1 H 2 O:HNO 3 solution. We found the interfacial surface energy for NAD to be σ ) 22 erg cm -2 , much lower than that estimated previously for NAT of σ ) 44 erg cm -2 . If the interfacial surface energy for NAD is indeed this much lower than that of NAT, nucleation of NAD from liquid HNO 3 /H 2 O aerosols may be an important step in the formation mechanism for crystalline type Ia PSCs.

We have attempted to hydrogenate adsorbed formate species on copper catalysts to probe the importance of this postulated mechanistic step in methanol synthesis. Surface formate coverages up to 0.25 were produced at temperatures between 413 and 453 K on supported (Cu/SiO2) copper and unsupported copper catalysts. The adlayers were produced by various methods including (1) steady-state catalytic conditions in CO2−H2 (3:1, 6 bar) atmospheres and (2) exposure of the catalysts to formic acid. As reported in previous work, the catalytic surface at steady state contains bidentate formate species with coverages up to saturation levels of ∼0.25 at the low temperatures of this study. The reactivity of these formate adlayers was investigated at relevant reaction temperatures in atmospheres containing up to 6 bar H2 partial pressure by simultaneous mass spectrometry (MS) and infrared (IR) spectroscopy measurements. The yield of methanol during the attempted hydrogenation (“titration”) of these adlayers was insignificant (<0.2 mol % of the formate adlayer), even in dry hydrogen partial pressures up to 6 bar. Hydrogen titration of formate species produced from formic acid also failed to produce significant quantities of methanol, and attempted titration in gases consisting of CO-hydrogen mixtures or dry CO2 was also unproductive. The formate decomposition kinetics, measured by IR, was also unaffected by these changes in the gas composition. Similar experiments on unsupported copper also failed to show any methanol. From these results, we conclude that methanol synthesis on copper cannot result from the direct hydrogenation of (bidentate) formate species in simple steps involving adsorbed H species alone. Furthermore, experiments performed on both supported (Cu/SiO2) and unsupported copper catalysts gave similar results, implying that the methanol synthesis reaction mechanism involves only metal surface chemistry. Pre-exposure of the bidentate formate adlayer to oxidation by O2 or N2O produces a change to a monodentate configuration. Attempted titration of this monodentate formate/O coadsorbed layer in dry hydrogen produces significant quantities of methanol, although decomposition of formate to carbon dioxide and hydrogen remains the dominant reaction pathway. Simultaneous production of water is also observed during this titration as the copper surface is rereduced. These results indicate that coadsorbates related to surface oxygen or water-derived species may be critical to methanol production on copper, perhaps assisting in the hydrogenation of adsorbed formate to adsorbed methoxyl.