scholarly journals Variation of the sticking of methanol on low-temperature surfaces as a possible obstacle to freeze out in dark clouds

2020 ◽  
Vol 494 (3) ◽  
pp. 4119-4129
Author(s):  
K A K Gadallah ◽  
A Sow ◽  
E Congiu ◽  
S Baouche ◽  
F Dulieu
Keyword(s):  
Low Temperature ◽  
Surface Temperature ◽  
Gas Phase ◽  
Amorphous Solid ◽  
Vacuum System ◽  
Initial Value ◽  
Water Ice ◽  
Dark Clouds ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Temperature Programmed

ABSTRACT Sticking of gas-phase methanol on different cold surfaces – gold, 13CO, and amorphous solid water (ASW) ice – was studied as a function of surface temperature (7–40 K). In an ultrahigh-vacuum system, reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption methods were simultaneously used to measure methanol sticking efficiency. Methanol band strengths obtained by RAIRS vary greatly depending on the type of the surface. Nevertheless, both methods indicate that the sticking of methanol on different surfaces varies with surface temperature. The sticking efficiency decreases by 30${{\ \rm per\ cent}}$ as the surface temperature goes from 7 to 16 K, then gradually increases until the temperature is 40 K, reaching approximately the initial value found at 7 K. The sticking of methanol differs slightly from one surface to another. At low temperature, it has the lowest values on gold, intermediate values on water ice, and the highest values are found on CO ice, although these differences are smaller than those observed with temperature variation. There exists probably a turning point during the structural organization of methanol ice at 16 K, which makes the capture of methanol from the gas phase less efficient. We wonder if this observation could explain the surprising high abundance of gaseous methanol observed in dense interstellar cores, where it should accrete on grains. In this regard, a 30${{\ \rm per\ cent}}$ reduction of the sticking is not sufficient in itself but transposed to astrophysical conditions dominated by cold gas (∼15 K), which could reduce the sticking efficiency by two orders of magnitude.

scholarly journals Surface science investigations of the role of CO 2 in astrophysical ices

2013 ◽  
Vol 371 (1994) ◽  
pp. 20110578 ◽  
Author(s):  
John L. Edridge ◽  
Kati Freimann ◽  
Daren J. Burke ◽  
Wendy A. Brown
Keyword(s):  
Surface Science ◽  
Amorphous Solid ◽  
Temperature Programmed Desorption ◽  
Surface Data ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Temperature Programmed ◽  
Astrophysical Ices ◽  
Dust Grain ◽  
Science Investigations

We have recorded reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data for a range of CO 2 -bearing model astrophysical ices adsorbed on a graphitic dust grain analogue surface. Data have been recorded for pure CO 2 , for CO 2 adsorbed on top of amorphous solid water, for mixed CO 2 :H 2 O ices and for CO 2 adsorbed on top of a mixed CH 3 OH:H 2 O ice. For the TPD data, kinetic parameters for desorption have been determined, and the trapping behaviour of the CO 2 in the H 2 O (CH 3 OH) ice has been determined. Data of these types are important as they can be used to model desorption in a range of astrophysical environments. RAIR spectra have also shown the interaction of the CO 2 with H 2 O and CH 3 OH and can be used to compare with astronomical observations, allowing the accurate assignment of spectra.

scholarly journals Impact of oxygen chemistry on model interstellar grain surfaces

2018 ◽  
Vol 20 (8) ◽  
pp. 5368-5376 ◽  
Author(s):  
A. Rosu-Finsen ◽  
M. R. S. McCoustra
Keyword(s):  
Infrared Spectroscopy ◽  
Molecular Oxygen ◽  
Amorphous Silica ◽  
Amorphous Solid ◽  
Temperature Programmed Desorption ◽  
Crystalline Solid ◽  
Amorphous Solid Water ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Solid Water ◽  
Temperature Programmed

Temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O2) beams on amorphous silica (aSiO2) and water (H2O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW).

scholarly journals Segregation effect and N2 binding energy reduction in CO-N2 systems adsorbed on water ice substrates

2018 ◽  
Vol 619 ◽  
pp. A111 ◽  
Author(s):  
T. Nguyen ◽  
S. Baouche ◽  
E. Congiu ◽  
S. Diana ◽  
L. Pagani ◽  
...  
Keyword(s):  
Binding Energy ◽  
Gas Phase ◽  
Adsorption Energy ◽  
High Vacuum ◽  
Amorphous Solid ◽  
Abundant Species ◽  
Binding Energies ◽  
Ultra High Vacuum ◽  
Pure Species ◽  
Water Ice

Context. CO and N2 are two abundant species in molecular clouds. CO molecules are heavily depleted from the gas phase towards the centre of pre-stellar cores, whereas N2 maintains a high gas phase abundance. For example, in the molecular cloud L183, CO is depleted by a factor of ≈400 in its centre with respect to the outer regions of the cloud, whereas N2 is only depleted by a factor of ≈20. The reason for this difference is not yet clear, since CO and N2 have identical masses, similar sticking properties, and a relatively close energy of adsorption. Aims. We present a study of the CO-N2 system in sub-monolayer regimes, with the aim to measure, analyse and elucidate how the adsorption energy of the two species varies with coverage, with much attention to the case where CO is more abundant than N2. Methods. Experiments were carried out using the ultra-high vacuum (UHV) set-up called VENUS. Sub-monolayers of either pure 13CO or pure 15N2 and 13CO:15N2 mixtures were deposited on compact amorphous solid water ice, and crystalline water ice. Temperature-programmed desorption experiments, monitored by mass spectrometry, are used to analyse the distributions of binding energies of 13CO and 15N2 when adsorbed together in different proportions. Results. The distribution of binding energies of pure species varies from 990 K to 1630 K for 13CO, and from 890 K to 1430 K for 15N2. When a CO:N2 mixture is deposited, the 15N2 binding energy distribution is strongly affected by the presence of 13CO, whereas the adsorption energy of CO is unaltered. Conclusions. Whatever types of water ice substrate we used, the N2 effective binding energy was significantly lowered by the presence of CO molecules. We discuss the possible impact of this finding in the context of pre-stellar cores.

scholarly journals Experimental study of the penetration of oxygen and deuterium atoms into porous water ice

2019 ◽  
Vol 622 ◽  
pp. A148 ◽  
Author(s):  
M. Minissale ◽  
T. Nguyen ◽  
F. Dulieu
Keyword(s):  
Bulk Diffusion ◽  
Amorphous Solid ◽  
Surface Reactivity ◽  
Experimental Conditions ◽  
Limit Values ◽  
Water Ice ◽  
Chemical Tracer ◽  
No Consumption ◽  
Temperature Programmed ◽  
Accessible Surface

Context. Many interstellar molecules are thought to form on dust grains. In particular, hydrogenation is one of the major mechanisms of the formation of mantle ice. To date it is not clear if H atoms can penetrate the bulk of the ice mantle or if it only has chemical activity on the accessible surface of grains. Aims. We wish to study the efficiency of atoms deposited on the outer surface of the amorphous solid water to penetrate into the ice bulk. Methods. NO molecules react with O and H atoms. They are easily detected by infrared (IR) spectroscopy. These two properties make this molecule an ideal chemical tracer for the penetration of O and H atoms through water ice. In our experiments we first deposited a NO undercoat and covered this layer (at 40 K) with a variable amount of water ice. Then, we exposed this undercoat to D (10 K) or O (40 K) atoms, and we followed the NO consumption and the products that appeared via IR signatures, and we finally analyzed the desorption of all species through a temperature-programmed desorption technique. We experimentally characterize the accessible surface of the ice and provide a model to interpret quantitatively our measurements. Results. Water ice limits the destruction of tracer NO molecules. The thicker the ice, the more NO remains unreacted. H and O atoms lead to the same amount of NO consumption, pointing out that access to reactants for these two different atoms is identical. We discuss different possible scenarios of NO localization (in and/or on the ice) and determine how this affects our observables (IR data and desorption profiles). Conclusions. In our experimental conditions, it is not possible to measure any atom penetration through the bulk of the ice. The surface diffusion followed by reaction with NO or by self-reaction (i.e., H + H → H2) is faster than bulk diffusion. We propose lower limit values for penetration barriers. Therefore the building of astrophysical ice mantles should be mostly driven by surface reactivity.

scholarly journals Implications of Ice Morphology for Comet Formation

2005 ◽  
Vol 13 ◽  
pp. 491-494 ◽  
Author(s):  
M. P. Collings ◽  
J. W. Dever ◽  
M. R. S. McCoustra ◽  
H. J. Fraser
Keyword(s):  
Surface Science ◽  
High Vacuum ◽  
Binding Energies ◽  
Ultra High Vacuum ◽  
Water Ice ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Thermodynamic Conditions ◽  
Temperature Programmed ◽  
Additional Constraints ◽  
Ice Morphology

AbstractLaboratory surface science under ultra-high vacuum (UHV) conditions allows us to simulate the growth of ices in astrophysical environments. Using the techniques of temperature programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS) and micro-balance methods, we have studied binary ice systems consisting of water (H2O) and variety of other species including carbon monoxide (CO), at astrophysically relevant conditions of temperature and pressure. We present results that demonstrate that the morphology of water ice has an important influence on the behavior of such systems, by allowing processes such as diffusion and trapping that can not be understood through a knowledge of the binding energies of the species alone. Through an understanding of the implications of water ice morphology on the behavior of ice mixtures in the interstellar environment, additional constraints can be placed on the thermodynamic conditions and ice compositions during comet formation.

Kinetic Monte Carlo simulations of water ice porosity: extrapolations of deposition parameters from the laboratory to interstellar space

2018 ◽  
Vol 20 (8) ◽  
pp. 5553-5568 ◽  
Author(s):  
Aspen R. Clements ◽  
Brandon Berk ◽  
Ilsa R. Cooke ◽  
Robin T. Garrod
Keyword(s):  
Monte Carlo ◽  
Kinetic Monte Carlo ◽  
Monte Carlo Model ◽  
Amorphous Solid ◽  
Interstellar Space ◽  
Water Ice ◽  
Dark Clouds ◽  
Deposition Parameters ◽  
Experimental Laboratory ◽  
Kinetic Monte Carlo Simulations

Using an off-lattice kinetic Monte Carlo model we reproduce experimental laboratory trends in the density of amorphous solid water (ASW) for varied deposition angle, rate and surface temperature. Extrapolation of the model to conditions appropriate to protoplanetary disks and interstellar dark clouds indicate that these ices may be less porous than laboratory ices.

scholarly journals Using Surface Science Techniques to Investigate the Interaction of Acetonitrile with Dust Grain Analogue Surfaces

2021 ◽  
Author(s):  
Emily R. Ingman ◽  
Amber Shepherd ◽  
Wendy A. Brown
Keyword(s):  
Surface Science ◽  
Thermal Processing ◽  
Desorption Energy ◽  
Water Ice ◽  
Ice Surface ◽  
Coverage Analysis ◽  
Wide Range ◽  
Reflection Absorption Infrared Spectroscopy ◽  
Temperature Programmed ◽  
Dust Grain

Surface science methodologies, such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), are ideally suited to studying the interaction of molecules with model astrophysical surfaces. Here we describe the use of RAIRS and TPD to investigate the adsorption, interactions and thermal processing of acetonitrile and water containing model ices grown under astrophysical conditions on a graphitic dust grain analogue surface. Experiments show that acetonitrile physisorbs on the graphitic surface at all exposures. At the lowest coverages, repulsions between the molecules lead to a decreasing desorption energy with increasing coverage. Analysis of TPD data gives monolayer desorption energies ranging from 28.8 - 39.2 kJ mol-1 and an average multilayer desorption energy of 43.8 kJ mol-1. When acetonitrile is adsorbed in the presence of water ice, the desorption energy of monolayer acetonitrile shows evidence of desorption with a wide range of energies. An estimate of the desorption energy of acetonitrile from CI shows that it is increased to ~37 kJ mol-1 at the lowest exposures of acetonitrile. Amorphous water ice also traps acetonitrile on the graphite surface past its natural desorption temperature, leading to volcano and co-desorption. RAIRS data show that the C≡N vibration shifts, indicative of an interaction between the acetonitrile and the water ice surface.

Modeling of aerated upflow fixed bed reactors for nitrification

1999 ◽  
Vol 39 (4) ◽  
pp. 85-92 ◽  
Author(s):  
J. Behrendt
Keyword(s):  
Mathematical Model ◽  
Mass Transfer ◽  
Liquid Phase ◽  
Gas Phase ◽  
Fixed Bed ◽  
Fixed Bed Reactor ◽  
Bulk Liquid ◽  
Initial Value ◽  
Fixed Bed Reactors ◽  
Number Of Cells

A mathematical model for nitrification in an aerated fixed bed reactor has been developed. This model is based on material balances in the bulk liquid, gas phase and in the biofilm area. The fixed bed is divided into a number of cells according to the reduced remixing behaviour. A fixed bed cell consists of 4 compartments: the support, the gas phase, the bulk liquid phase and the stagnant volume containing the biofilm. In the stagnant volume the biological transmutation of the ammonia is located. The transport phenomena are modelled with mass transfer formulations so that the balances could be formulated as an initial value problem. The results of the simulation and experiments are compared.

scholarly journals Low Temperature Water-Gas Shift: Enhancing Stability through Optimizing Rb Loading on Pt/ZrO2

Catalysts ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 210
Author(s):  
Caleb Daniel Watson ◽  
Michela Martinelli ◽  
Donald Charles Cronauer ◽  
A. Jeremy Kropf ◽  
Gary Jacobs
Keyword(s):  
Low Temperature ◽  
Hydrogen Transfer ◽  
Water Gas Shift ◽  
Significant Shift ◽  
Catalyst Activation ◽  
X Ray ◽  
Temperature Programmed ◽  
Water Gas ◽  
X Ray Absorption ◽  
Temperature Water

Recent studies have shown that appropriate levels of alkali promotion can significantly improve the rate of low-temperature water gas shift (LT-WGS) on a range of catalysts. At sufficient loadings, the alkali metal can weaken the formate C–H bond and promote formate dehydrogenation, which is the proposed rate determining step in the formate associative mechanism. In a continuation of these studies, the effect of Rb promotion on Pt/ZrO2 is examined herein. Pt/ZrO2 catalysts were prepared with several different Rb loadings and characterized using temperature programmed reduction mass spectrometry (TPR-MS), temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), an X-ray absorption near edge spectroscopy (XANES) difference procedure, extended X-ray absorption fine structure spectroscopy (EXAFS) fitting, TPR-EXAFS/XANES, and reactor testing. At loadings of 2.79% Rb or higher, a significant shift was seen in the formate ν(CH) band. The results showed that a Rb loading of 4.65%, significantly improves the rate of formate decomposition in the presence of steam via weakening the formate C–H bond. However, excessive rubidium loading led to the increase in stability of a second intermediate, carbonate and inhibited hydrogen transfer reactions on Pt through surface blocking and accelerated agglomeration during catalyst activation. Optimal catalytic performance was achieved with loadings in the range of 0.55–0.93% Rb, where the catalyst maintained high activity and exhibited higher stability in comparison with the unpromoted catalyst.

scholarly journals The role of radiolysis in the modelling of C2H4O2 isomers and dimethyl ether in cold dark clouds

2020 ◽  
Vol 500 (3) ◽  
pp. 3414-3424
Author(s):  
Alec Paulive ◽  
Christopher N Shingledecker ◽  
Eric Herbst
Keyword(s):  
Gas Phase ◽  
Dimethyl Ether ◽  
Recent Observation ◽  
Cosmic Ray ◽  
Thermal Method ◽  
Dark Clouds ◽  
Ice Surface ◽  
Dust Grain ◽  
Complex Organic

ABSTRACT Complex organic molecules (COMs) have been detected in a variety of interstellar sources. The abundances of these COMs in warming sources can be explained by syntheses linked to increasing temperatures and densities, allowing quasi-thermal chemical reactions to occur rapidly enough to produce observable amounts of COMs, both in the gas phase, and upon dust grain ice mantles. The COMs produced on grains then become gaseous as the temperature increases sufficiently to allow their thermal desorption. The recent observation of gaseous COMs in cold sources has not been fully explained by these gas-phase and dust grain production routes. Radiolysis chemistry is a possible non-thermal method of producing COMs in cold dark clouds. This new method greatly increases the modelled abundance of selected COMs upon the ice surface and within the ice mantle due to excitation and ionization events from cosmic ray bombardment. We examine the effect of radiolysis on three C2H4O2 isomers – methyl formate (HCOOCH3), glycolaldehyde (HCOCH2OH), and acetic acid (CH3COOH) – and a chemically similar molecule, dimethyl ether (CH3OCH3), in cold dark clouds. We then compare our modelled gaseous abundances with observed abundances in TMC-1, L1689B, and B1-b.

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