A TDS study of oxygen, water and carbon dioxide coadsorption with potassium on graphite

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H2O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.

Context.Formamide (NH2CHO) and methylamine (CH3NH2) are known to be the most abundant amine-containing molecules in many astrophysical environments. The presence of these molecules in the gas phase may result from thermal desorption of interstellar ices.Aims.The aim of this work is to determine the values of the desorption energies of formamide and methylamine from analogues of interstellar dust grain surfaces and to understand their interaction with water ice.Methods.Temperature programmed desorption (TPD) experiments of formamide and methylamine ices were performed in the sub-monolayer and monolayer regimes on graphite (HOPG) and non-porous amorphous solid water (np-ASW) ice surfaces at temperatures 40–240 K. The desorption energy distributions of these two molecules were calculated from TPD measurements using a set of independent Polanyi–Wigner equations.Results.The maximum of the desorption of formamide from both graphite and ASW ice surfaces occurs at 176 K after the desorption of H2O molecules, whereas the desorption profile of methylamine depends strongly on the substrate. Solid methylamine starts to desorb below 100 K from the graphite surface. Its desorption from the water ice surface occurs after 120 K and stops during the water ice sublimation around 150 K. It continues to desorb from the graphite surface at temperatures higher than160 K.Conclusions.More than 95% of solid NH2CHO diffuses through the np-ASW ice surface towards the graphitic substrate and is released into the gas phase with a desorption energy distributionEdes= 7460–9380 K, which is measured with the best-fit pre-exponential factorA= 1018s−1. However, the desorption energy distribution of methylamine from the np-ASW ice surface (Edes= 3850–8420 K) is measured with the best-fit pre-exponential factorA= 1012s−1. A fraction of solid methylamine monolayer of roughly 0.15 diffuses through the water ice surface towards the HOPG substrate. This small amount of methylamine desorbs later with higher binding energies (5050–8420 K) that exceed that of the crystalline water ice (Edes= 4930 K), which is calculated with the same pre-exponential factorA= 1012s−1. The best wetting ability of methylamine compared to H2O molecules makes CH3NH2molecules a refractory species for low coverage. Other binding energies of astrophysical relevant molecules are gathered and compared, but we could not link the chemical functional groups (amino, methyl, hydroxyl, and carbonyl) with the binding energy properties. Implications of these high binding energies are discussed.

Water and water coadsorbed with potassium on the basal plane of graphite were studied with thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS) in the temperature range 85-900 K. Water alone adsorbs nondissociatively on the clean graphite surface at 85 K, forming hydrogen bonded aggregates. Its structure depends both on the coverage and on substrate temperature. With increasing coverage at 85 K(0.5-1.0 monolayer (ML)) the libration mode at similar to 86 meV shows a rapid upward shift, indicating a phase transition from a 2D to a 3D structure. The transition can also be induced by annealing the low coverage structure. Water coadsorption with potassium is nonreactive or reactive, depending on temperature and potassium coverage. The nonreactive coadsorption at T-s = 85 K occurs only below a critical potassium coverage of BK less than or equal to 0.3 ML. It is characterized by substantial symmetry changes of the adsorbed water molecules, compared to the pure water adsorption, and is attributed to formation of hydrated-ion species on the surface. The surface solvation number at the lowest K coverage is three to four H2O molecules per potassium atom. K and H2O react at submonolayer coverages at 120-160 K to form surface KOH, KH, KxOy, and volatile products. The surface species gradually transforms/decomposes at elevated temperatures (200-500 K) to first form potassium-oxygen complexes that then serve as precursors to graphite oxidation to CO2 at similar to 750 K.

The chemisorption and coadsorption of water and oxygen on Cs-dosed Cu(110)

Combined DFT and MD simulation approach for the study of SO2 and CO2 adsorption on graphite (111) surface in aqueous medium

A multitechnique surface analysis study of the adsorption of H 2, CO and O 2 on [formula omitted] surfaces

The reaction of submonolayer Li atoms with CH3Cl at 100 K on a highly oriented pyrolytic graphite (HOPG) surface has been studied under ultrahigh vacuum. We exploit the low defect density of the high quality HOPG used here (∼109 defects cm–2) to eliminate the effects of step edges and defects on the graphite surface chemistry. Li causes C–Cl bond scission in CH3Cl, liberating CH3 radicals below 130 K. Ordinarily, two CH3 species would couple to form products such as C2H6, but in the presence of graphite, CH3 preferentially adsorbs on the flat basal plane of Li-treated graphite. A C–CH3 bond of 1.2 eV is formed, which is enhanced relative to CH3 binding to clean graphite (0.52 eV) due to donation of electrons from Li into the graphite and back-donation from graphite to CH3. A low yield of C1, C2, and C3 hydrocarbon products above 330 K is found along with a low yield of H2. The low yield of these products indicates that the majority of the CH3 groups are irreversibly bound to the basal plane of graphite, and only a small fraction participate in the production of C1–C3 volatile products or in extensive dehydrogenation. Spin-polarized density functional theory calculations indicate that CH3 binds to the Li-treated surface with an activation energy of 0.3 eV to form a C–CH3 adsorbed surface species with sp3 hybridization of the graphite, and the methyl carbon atoms is involved in bond formation. Bound CH3 radicals become mobile with 0.7 eV activation energy and can participate in combination reactions for the production of small yields of C1–C3 hydrocarbon products. We show that alkyl radical attachment to the graphite surface is kinetically preferred over hydrocarbon product desorption.

Interactions of low energy (10–600 eV) noble gas ions with a graphite surface: surface penetration, trapping and self-sputtering behaviors

Scanning Tunneling Microscopic Studies of Epitaxial Films of Polyurethane, Polyester, and Polysiloxane Formed during Step Polymerization

The nature of the adsorption bond between graphite islands and iridium surface

Thermal desorption spectroscopy analysis of oxygen from Pd-rich surfaces of PdCu(110) single crystal

Coadsorption of water and ions on Cu(110): Models for the double layer

Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).

Adsorption of caesium and oxygen and co-adsorption of oxygen and caesium on the Mo(110) surface are studied at room temperature by Angle Resolved Ultraviolet Photoemission Spectroscopy (ARUPS) using synchrotron radiation. Similar to the high-lying surface state of the Mo(100) surface, the sharp peak observed for Mo(110) at normal emission angle 1.15 eV below the Fermi energy, is markedly attenuated by caesium adsorption and shifted by 0./55 eV to higher binding energies. In contrast, adsorption of oxygen on the Mo(110) surface does not affect the energy and linewidth of this peak even at oxygen exposures up to 8 L (corresponding to about 0.7 monolayer of oxygen). This behaviour provides evidence that on Mo(110), oxygen is bonded differently than on Mo(100). As in the case of Cs/Mo(100) a covalent bonding between Cs(6s) and Mo(d) states is assumed. Co-adsorption of oxygen on to a Cs-covered Mo(110) surface is observed to restore the initial energy of the peak at 1.15 eV.

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 crystalline ice (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.