Structural effects of ice grain surfaces on the hydrogenation of CO at low temperatures

Abstract

Experiments on the hydrogenation of CO on crystalline and amorphous ice at 15 K were carried out to investigate the structural effects of the ice surface. The effective rate of H atom addition to CO on the amorphous ice was found to be larger than that on the crystalline ice, while CO depletion on crystalline ice became larger after long exposure. We demonstrated that the CO-coverage on the ice surfaces dominates the effective reaction rate rather than the surface structure. The larger depletion of CO on crystalline ice, as compared to amorphous ice, suggests easier desorption of CO and/or products by the heat of the reaction.

An FTIR study on the catalytic effect of water molecules on the reaction of CO successive hydrogenation at 3K

“…In addition, several attempts to demonstrate the catalytic effect of water molecules on CO hydrogenation have been assayed in the past decade both experimentally [44][45][46] and theoretically [7]. The experiments consisted of bombarding a CO or CO-H 2 O mixed-ice surface by H atoms to determine the effect of waterice structures [24,[47][48][49], temperature [50] and thickness on CO hydrogenation at 10-20 K. These experiments clearly proved that, for a same temperature and an identical H flux, the reaction rate of CO hydrogenation is strongly related to the ice composition and morphology. The addition of water molecules to CO transforms the ice surface into an irregular structure with many cracks.…”

Carbon Monoxide Hydrogenation on Ice Surfaces

“…We also prepared three types of ice, ferroelectric ice XI, antiferroelectric ice XI and amorphous ice, for the purpose of investigating the dependence of the structure of ice on the reaction observed by Hidaka et al . The initial unit cells of the ferroelectric and antiferroelectric ices are generated as shown in the report of Hirsch et al .…”

“…One of the interesting features of these studies is that there is a peculiar dependence on the structure of ice. Hidaka et al . deposited CO molecules on amorphous and crystalline ice surfaces with H atoms and compared the reaction rate on these ice surfaces.…”

Gas-surface interactions and heterogeneous chemistry on interstellar grains analogues

“…Consequently, one can fit the single exponential decay of the species X with time in order to obtain an effective rate constant k = k H n H as given in table 3 for the hydrogenation of various ices. In a series of experimental investigations, the effects of various parameters such as the surface temperature, the surface composition, surface thickness and morphology [104][105][106]108] and H-atom flux [109] were investigated. As shown in figure 3, the efficiencies for the hydrogenation are sensitive to ice temperature, composition and structure.…”

“…CO being a major component of ice mantle, its hydrogenation has been investigated in priority by exposing to partial H-atom beams various ices such as pure solid CO, CO-H 2 O or H 2 CO-H 2 O mixed ices below 20 K, [100][101][102][103][104][105][106][107][108][109][110]. These experiments clearly demonstrate that the formation of formaldehyde H 2 CO (D 2 CO) and methanol CH 3 OH (CD 3 OD) could proceed by successive addition of hydrogen (deuterium).…”