The Effect of Co-Adsorbed Oxygen on the Photochemistry of Methane

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Effect of Co-Adsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water Sujith Ramakrishnan, Roey Sagi, Niharendu Mahapatra, and Micha Asscher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02155 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry

The Effect of co-Adsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water Sujith Ramakrishnan, Roey Sagi, Niharendu Mahapatra and Micha Asscher* Institute of Chemistry, Edmond J. Safra Campus, Givat-Ram The Hebrew University of Jerusalem, Jerusalem 9190401, Israel

*Corresponding author e-mail: [email protected]

Abstract The photochemistry of methane caged within amorphous solid water (ASW) is interesting as a model for studying interstellar and high-altitude atmospheric pathways for the formation of more complex hydrocarbons. Here we report on the photoreactivity of clean methane and in the presence of oxygen molecules, known as electron capture species, within two 50 ML thick D2O-ASW films adsorbed on Ru (0001) substrate under ultra-high vacuum (UHV) conditions. Irradiation by 248 nm UV photons (5.0 eV), where none of the involved molecules absorb these photons in the gas phase, leads to the formation of diverse hydrocarbons. In all cases, the presence of oxygen results in significantly enhanced reactivity. The dissociative electron attachment (DEA) mechanism with electrons generated within the metal substrate is thought to largely govern the photo-reactivity in this system. Methyl radicals as the primary photo-products, subsequently react with the surrounding water and neighboring methane as well as with the stable O2- anion. Post-irradiation temperature-programmed desorption measurements revealed cross sections for hydrocarbon formation in the range of 10-20–10-21 cm2. Possible mechanisms underlying the formation of various hydrocarbons and carbon dioxide as the final oxidation product are discussed.

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1. Introduction Chemical reactions on solid surfaces, which are initiated by light or charged particles, are of great interest in fields such as photo-catalysis, atmospheric chemistry, as well as astro-chemistry. Another relevant and very important field is radiation chemistry and biology, where ionizing irradiation (X-rays, alpha, beta, and gamma rays) lead to the emission of secondary electrons.1-5 These low-energy electrons (usually at less than 50 eV energy) may lead to molecules decomposition via dissociative electron attachment [DEA]. In space, the interstellar matrix (ISM)6 consists of very cold and dense regions generally known as molecular clouds (MCs)7-9 comprising various high-density gases (e.g., H2, CO, CH4, O2, and H2O) and dust particles (e.g., Si, Mg, C, and Fe). The temperature of these MCs ranges typically from 10 to 100 K and is often considered to be the source of various neutral and ionic fragments as well as more complex organic molecules.10-11 Interstellar methane in its condensed phase is ubiquitous and its abundance in this solid phase is estimated to be in the percentage range from 1 to 5% relative to H2O.12-15 Gas phase methane photolysis is considered to be the main source of more complex hydrocarbons in the atmospheres of Titan and the giant planets.16 In the interstellar environment, a considerable amount of methane remains embedded in a water-ice crystal lattice in the form of methane hydrate clathrate.17 For the past few decades, significant research on methane has been directed towards better understanding the chemistry within amorphous solid water (ASW)

following

excitation

by

ion-bombardment,18-21

electron-stimulated

reactions,22-23 and photolysis.23,24 However, the role of oxygen (a strong electroncapturing species) embedded inside an ASW/ CH4 mixture in the formation of organic moieties has not yet been reported. Very recent electron-induced reactions of solid mixtures of CH4/O2 (without water) have revealed the possible formation of small hydrocarbons.25-26 However, the caging of CH4 in water layers is more reminiscent of ISM conditions. In order to comprehend the mechanism underlying photon-induced chemistry and the formation of organic molecules in interstellar space, a detailed understanding of photochemical reactions of water ice-gas mixtures is imperative. Methane was chosen for our research, because of its inert characteristics27-29 with respect to thermal chemical reactivity under atmospheric conditions, which result in its accumulation in large quantities over the years underwater/underground30 in our planet and in the atmospheres of other planets in our solar system.31 2 ACS Paragon Plus Environment

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The mechanism underlying the electron-induced formation of formaldehyde and methanol within ASW by the hydrogenation of CO has been investigated extensively.32 The presence of these byproducts in the interstellar matrix suggests that these products may have been formed by the aforementioned reaction.33-35 It has been demonstrated that the CO hydrogenation reaction strongly depends on temperature.3639

Recently, it was proposed that O-atom diffusion is governed by quantum tunneling

up to 20 K, whereas above this threshold thermal diffusion dominates.40 Earlier studies by our group have demonstrated 41 that upon UV irradiation of caged methyl and ethyl chlorides inside ASW adsorbed on Ru (0001) substrate, the photo-reactivity stems from a dissociative electron attachment (DEA) process,42-43 where the hot electrons are excited at the metallic substrate. Weakly adsorbed methane on Pt (111) was shown to dissociate readily into methyl and hydrogen radical species upon irradiation with 193 nm (6.4 eV) photons.44 The authors of this work claim, however, that methane molecules adsorbed on Pt (111) are totally transparent to 248 nm photons (5 eV); hence, no photo-dissociation or photo-desorption should occur,44 which was further confirmed in their studies employing infrared reflection absorption spectroscopy (IRAS).45 Caging and other embedding methods may potentially pave the way to better understand the fundamental aspects of activating short alkanes such as methane in the presence of water molecules in well-controlled, model systems. Kang et al.46 reported that the irradiation of deep UV light (10−11 eV) onto an ice film produced metastable hydronium (H3O+) ions in the ice at low temperatures (53–140 K), which may lead to enhanced reactivity if other caged or sandwiched molecules co-exist. Kang et al. also studied the reactivity of nitrogen dioxide with ASW at low temperatures. They realized that the conversion efficiency of NO2 on the ice surface at a low temperature is significantly higher than on the surface of liquid water at room temperature.47-48 Here we focus mainly on the photochemical reaction and the excitation mechanism of CH4 sandwiched within two layers of ASW and ASW|CH4|O2|ASW on Ru (0001) substrate using excimer laser irradiation at 248 nm (5.0 eV photon energy). We demonstrate how methane conversion takes place near oxygen molecules entrapped in 3 ACS Paragon Plus Environment


ASW ice, analogous to conditions found in interstellar ice upon UV irradiation that exists in that medium. The results reveal the role of reactive radical species that are formed during such UV excitation and its potential in the presence of O2- ions, to activate the C-H bond of methane and to transform it into various longer-chain organic molecules in addition to total oxidation to CO2.

2. Experimental Procedures The experiments discussed here were performed in an Ultra-High Vacuum system (UHV) at a base pressure of 5 × 10-10 mbar (mostly water molecules) that was previously described.49-50 A sample holder was mounted on an X-Y-Z manipulator on a rotatable stage (McAllister). The various gases were introduced by backfilling the UHV chamber to the proper exposure through high-precision leak valves. The ruthenium single crystal (SPL Labs, NL) was prepared to within 0.5 degrees of the hexagonal (0001) orientation (dimension: 8 mm diameter disc, thickness: 1 mm). It was spot-welded between two 0.5 mm-diameter tantalum wires that were spot welded to two 3 mm diameter Ta rods attached at the bottom of a closed-cycle cryogenic cold head (Janis), enabling the sample to be cooled down to 25 K. The sample temperature was continuously monitored using a C-Type thermocouple (W26%-Re-W5%-Re), spot-welded to the side edge of the sample and electronically compensated and calibrated for room temperature. The sample was cleaned daily by employing an insitu Ne+ sputter gun, operated at 800 eV for 10 minutes, typically at a sample current of 5 μA. The sputter procedure was followed by annealing the sample to 1620 K, maintaining this temperature for 30 seconds, and then employing controlled cooling at a rate of 5 K/s after the annealing terminated. A Quadrupole mass spectrometer (SRS 200) was placed inside a glass shroud with a 5 mm aperture at its conical end located 1mm from the sample in order to monitor the gaseous products desorbed from the metal surface during temperature-programmed desorption experiments (TPD). An AC-current-based LabView algorithm enabled us to heat the sample at linear rates in the range of 0.5 to 10 K/s or temperature stabilized to within ± 1o K. The excitation of adsorbed or sandwiched molecules within the ASW layers utilized a UV light source at 5.0 eV photon energy generated from a pulsed (5 ns) KrF excimer


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laser (Coherent ExciStar-XS), operating at a repetition rate of 50-100 Hz and at a pulse energy of 0.2 mJ/pulse.

3. Results and Discussion The main objective of this study was to learn and better understand the interaction and the possible effect of oxygen molecules on the photochemistry of methane while they are co-embedded within two 50 monolayers (ML) thick films of ASW at 25 K. Our results are considered as model for reactivity pathways of these inert molecules and their conversion to hydrocarbons, which may occur under environmentally relevant conditions as well as on grains in interstellar space.

3.1 Calibration of methane monolayer and sandwich structures within ASW Before addressing the photochemistry of methane sandwiched within two ASW layers, it is important to calibrate its adsorption properties, namely, the relation between the exposure units (Langmuir (L), 1 L= 10-6 Torr × s) and the completion of a single monolayer, on a clean Ru (0001) substrate as observed via the corresponding TPD spectra. In addition, we would like to determine whether the methane molecules reside at the position within the ASW film as planned.


Area under the peak (CH4) (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

QMS signal intensity (Mass 16, CH4) (a. u.)


Exposure (L) Monolayer of CH4

Temperature (K)

Figure 1. Temperature programmed desorption (TPD) of CH4 from a clean Ru (0001) substrate at the indicated exposures in Langmuirs (L) units (1 L=10-6 Torr∙sec). The inset shows the corresponding area under the desorption peaks as a function of exposure in Langmuirs. The heating rate was 0.5 K/s.

TPD spectra following a gradually increasing exposure (indicated as the color code in Figure 1) of CH4 on a clean Ru (0001) substrate at 25 K are presented; a constant heating rate of 0.5 K/s was employed. Three TPD maxima are observed at the highest exposure of 7.0 L and 7.6 L. The high-temperature desorption peak centered at 47±1 K corresponds to desorption from the first layer of CH4, where methane molecules are bound directly to the ruthenium substrate. The peak at 33 K is attributed to the second layer that emerges at 2.3±0.1 L. At higher exposures, a third peak appears at 28 K reflecting the third monolayer. The third monolayer keeps increasing in intensity at exposures above 7.0 L. In order to make sure that a monolayer equivalent is deposited, we chose 2.3 L throughout this work to represent the full monolayer (1 ML) exposure. A similar separation between the 2nd and the 3rd monolayers was 6 ACS Paragon Plus Environment

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previously found in the desorption behavior of methyl chloride from the same ruthenium substrate.51 By integrating the area under the methane desorption peaks, one can observe an increasing slope when plotted vs. exposure (inset in Figure 1). This may suggest a somewhat increased sticking probability of methane on top of the second and thicker layers. From the above TPD spectra we concluded that at an exposure of 2.3±0.1 L CH4 the onset of the second layer is defined, suggesting that the Ru substrate has been fully covered by a monolayer of CH4 at this exposure. Growth via island formation, however, cannot be ruled out; in such a case the separation between the 2nd and 3rd layers is not as continuous and smooth. Next, the interaction of methane with ASW will be described when attempting to form a sandwich of methane between two ASW layers. This will allow us to potentially position the methane at different distances from the substrate by varying the thickness of the lower water film. In this way one could study the effect of the underlying substrate thickness on the outcome of the photochemistry of embedded methane molecules that we planned to study. Furthermore, the role of the co-presence of oxygen molecules as an efficient low-energy electron-capturing molecule together with the methane can be studied. In order to prepare this sample, a 50 ML thick D2O layer was grown first on a Ru(0001) substrate at 25 K followed by 1 ML of CH4 (2.3 L exposure) deposited over the ASW at the same temperature. A sticking probability test of CH4 on the bare Ru (0001) (Figure 1) was compared to that on the metallic substrate that was covered by 50 ML D2O via TPD at mass 16 (CH4). Within our experimental uncertainty, the uptake of methane was the same. Another 50 ML thick D2O layer was then overlaid on the ASW/CH4 film, thereby sandwiching methane within 100 ML D2O. By preparing these structures, we are not only able to trap this non-polar molecule—it also enables us to activate the molecules by utilizing externally supplied UV photons. In this way (using sandwiched layers), we will avoid the low-temperature desorption of methane molecules trapped in the pores of the top ASW layers. (See the Supporting Information, Figure S1, which shows the TPD plot of methane trapped/sandwiched inside two 50 ML D2O prepared on the Ru (0001) substrate). The two hightemperature peaks at 146 K and 160 K clearly confirm that ASW layers stabilize 7 ACS Paragon Plus Environment


methane within the porous structure up to the water desorption temperature. The lowtemperature peak at 40 K is associated with the desorption of CH4 molecules adsorbed on the top surface layers of ASW and are not entrapped inside the pores. The 146 K peak is associated with caged methane molecules that are released and desorbed at the ASW to crystalline ice phase transition, the so-called "volcano" desorption53,54. To ensure that the pure methane molecules were kept where we deposited them, namely, between the two 50 ML layers of ASW, we performed a mild sputter experiment of the sandwich structure, by employing a 300 eV Ne+ ion beam, and subsequently, monitoring the remaining content of both the methane and the water molecules. These results are summarized in Figure 2.

Integrated area under CH4 and D2O TPD peaks (a.u.)

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Sputter time(minutes) Figure 2: Normalized TPD area under the TPD peaks of the remaining D2O and sandwiched methane on Ru(0001) at 25 K following a Ne+ (300 eV, 1 μA sample current) sputter at different times of exposure to the ion beam. The TPD of water was performed with a reduced quadrupole sensitivity (channeltron voltage) to avoid its saturation. The overall TPD signal of water is 3 orders of magnitude higher than that of methane.

The homogeneous distribution of the methane molecules throughout the ASW film, when adsorbed as a sandwich, as reflected in Figure 2, is in contrast with the behavior of polar molecules like ethyl chloride, which form a relatively well-confined cage.52 A 8 ACS Paragon Plus Environment

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complementary experiment with 1 ML CH4 and 2.5 L O2 sandwiched between two layers of 50 ML D2O-ASW on Ru(0001) at 25 K, following Ne+ (300 eV) ion sputter at different times of exposure to the ion beam, is shown in the Supporting Information, Figure S1B. The relative decay of methane, as shown in the Supporting Information, Figure S1B, is much faster than the one in Figure 2, which is associated with the reactivity of methane in the presence of oxygen. In contrast to our results, a well-stabilized sandwich was reported in the case of CO molecules embedded within ASW films,32 studied by employing 100 eV electron-induced reactivity. Evidently here, owing to the preparation mode as a sandwich and methane's inert characteristics (negligible interaction with D2O molecules), the molecules do not form a wellconfined sandwich layer but rather, they diffuse throughout the ASW film, as can be seen from Figure 2. A similar case was recently reported in the sandwich preparation of dichloromethane in ASW.53

3.2 Photo-chemistry of the ASW|CH4|ASW system The goal of this part of our study was to activate methane molecules that are embedded within the ASW film. Here we will describe the response of this system to 5.0 eV photons (248 nm) generated by a KrF excimer laser at a low fluence of 0.2 mJ/pulse, where we obtain close to a maximum product yield. We observed that at a constant frequency of 100 Hz and a laser power of 0.2-0.3 mJ/pulse, a maximum of product yield was obtained; therefore, this laser power (0.2 mJ/pulse) was employed throughout the present experiments. Details on the photo-product’s yield vs. laser power are shown in the Supporting Information, Figure S2. Since both methane and the water molecules are transparent to this energy level, the most probable mechanism of activation is expected to arise from photo-induced hot electrons generated within the ruthenium substrate. These low energy electrons subsequently interact with the methane molecules as well as (to some extent) with the water molecules. Figure 3 displays the various photo-products as detected via post (248 nm) irradiation TPD spectra, at various irradiation times, indicated by the color code.


C2DH

QMS signal intensity (Mass 29,CHO) (a.u.)

(A)

Number of photons/cm2

Temperature (K) (C)


H2CO

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(B)

CHO

Temperature (K)

QMS signal intensity (Mass 45,HCO2) (a.u.)

QMS signal intensity (Mass 30,H2CO) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

QMS signal intensity (Mass 27,C2DH) (a.u.)


Temperature (K)


(D)

HCO2

Temperature (K)

Figure 3 (A-D): TPD spectra of photo-products at the indicated masses (27, 29, 30, and 45) following the irradiation of a sandwich-structure of methane within deuterated ASW film by 248 nm photons at a 0.2 mJ/pulse at 100 Hz (2.5 × 1016 photons/s). The irradiation times are translated to the number of photons/cm2 striking the sample, indicated as color code. The TPD heating rate was 0.5 K/s. The Y axis reflects the relative uptake of the products and should be compared to the clean 1ML methane signal shown in Figure 1.

The overall yield of the various photo-products as a function of the number of 248 nm photons striking the sandwich structure at 25 K is displayed in the Supporting Information, Figure S3. Except for mass 45, all graphs display an increasing yield with gradual saturation above 3-5 × 1019 photons. A more detailed example that includes the cross-section analysis obtained from such a display is presented in Figure 4 below for mass 27. From the initial signal growth, one can extract a cross section for the formation of each of the photo-products.


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Relative area under TPD peaks (mass 27)

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(3.4 ± 0.1)×10-22 cm2/photon


Figure 4: Integrated area under the (initial) TPD signals at mass 27 relative to the parent CH4 molecules area under the 1 ML TPD signal is plotted vs. the number of irradiated photons / cm2. A linear fit (= derivative through the initial points in units of cm2/photon), (in red) is used to extract the cross section (σ) for the formation of mass 27, a photo-product of methane. More details are provided in the text. Similar plots for the other masses are given in the Supporting Information, Figure S3.

The linear curve assumes that the very initial growth of mass 27 properly represents the formation cross section (in units of cm2/photon = σ), since it is not influenced by possible processes that may affect the cross section, e.g further light absorption of the product and its decomposition that may occur at larger number of photons (longer irradiation times). An attempt to identify the product molecules that give rise to the observed TPD spectra in Figure 3 (masses 27, 29, 30, 44, and 45 amu; other masses contribute negligible signals) is based on stable molecules that were directly adsorbed within the



same sandwich structure (ASW| stable molecules |ASW) and their TPD will be analyzed (no photon irradiation for comparison) in a separate section below. Before the photo-products are analyzed, one can clearly see a different distribution of the TPD peaks, with the most important contribution near the water desorption at 145 K and 160 K. The first peak at 145 K can be attributed to the so-called "volcano" desorbing molecules54-55 that erupt via cracks and defects at the onset of the ASW crystallization temperature. The peak at 160 K belongs to molecules that are well "solvated" and strongly interacting with the ASW film, therefore tend to desorb together with the water molecules. In addition, masses with a significantly smaller contribution to the TPD spectra are at or below 100 K. This kind of TPD peak distribution resembles gas chromatography, namely, in addition to total yield, one can obtain information on the interaction of the just-formed photo-product molecules with their host molecules (ASW) by analyzing the ratio of the 145 K to 160 K peak intensities and the presence of very low temperature desorption peaks. From the profile analysis of all TPD spectra (clean methane and when co-sandwiched with oxygen, see below), one can conclude that the temperature of the peaks is independent of the photo-product coverage. This indicates that the desorption is a first order kinetics in nature, which means that all the recombination reactions of the formed radical species mentioned above take place prior to molecular desorption. Hence the rate-limiting step in all desorption events shown in Figure 3 cannot arise from molecular or atomic diffusion and recombination, but rather, from the molecular desorption itself. Another explanation, possibly a more realistic one, is that all desorption events are fully governed by the water matrix’s physical and structural properties, which dictates when a molecule can or cannot desorb. 3.3 Photo-chemistry of the (ASW|CH4+O2|ASW) system Once the interaction and photochemistry of pure methane with the surrounding ASW has been addressed following excitation at 5.0 eV photon energy, the effect of coadsorbed oxygen molecules and methane within the ASW matrix will be discussed. An interesting property of molecular oxygen lies in its high electron affinity to form O2- species, in particular, when surrounded by water molecules, as a solvated negative ion. The electrons are generated by the UV photons via the metallic ruthenium substrate, as mentioned above. 12 ACS Paragon Plus Environment

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The experiments were performed by adsorbing the (50 ML D2O|1 ML CH4|2.5 L O2|50 ML D2O) layer in a sequential order at 25 K on the Ru (0001) substrate. The 2.5 L oxygen exposure was assumed to represent 1ML adsorbed molecules. Next, the prepared sample was irradiated by the 248 nm photons at a constant laser pulse energy ( 0.2 mJ/pulse) at a fixed repetition rate of 100 Hz, as with the pure methane described above. Following irradiation, the samples were heated at a fixed heating rate of 0.5 K/s to monitor their TPD spectra. The results are displayed in Figure 5. Similar products (masses) were formed, in addition to mass 33, exclusively formed in the copresence of oxygen and methane. Inspection of the overall yield of the various masses produced following exposure to the same number of 5.0 eV photons revealed that the presence of oxygen increased the yield by a factor of 3-6 compared to the case of pure methane, with the addition of mass 33. Another difference, owing to the presence of oxygen, is that the maximum yield is obtained at 1-2 × 1020 photons dosage and then it sharply decreases at higher exposure to the 5.0 eV photons.

Temperature (K)

Temperature (K) (C)

H2CO

QMS signal intensity (Mass 33,CH3OD) (a.u.)

Number of

photons/cm2


Temperature (K)

QMS signal intensity (Mass 45)(a.u.)

QMS signal intensity (Mass 30,H2CO) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH3OD

Temperature (K)


(E) HCO2/CH3OCH2

Temperature (K)


(D)


Figure 5 (A-E): TPD spectra of masses emerging from the (ASW /(CH4+O2) /ASW/Ru(0001)) sandwich system following 248 nm of photon irradiation at the indicated number of photons (inset table) by a 0.2 mJ/pulse. A) Mass 27, B) mass 29, C) mass 30, D) mass 33, and E) mass 45. The sample temperature was 25 K and the heating rate was 0.5 K/s for all the spectra. Dimethyl ether is an additional possible product that may be assigned to mass 45, however only at higher initial methane concentration.

The TPD profiles of all the masses are generally the same in both the pure methane and the co-sandwiched systems, namely, the most important contributions are at 145 K and 160 K desorption peaks. At masses 29 and 30, however, the lowtemperature peak at 100 K is enhanced, in particular, when irradiated by 1.5 × 1020 photons/cm2. Finally, the profile of the relatively intense new peak at mass 33 differs considerably from all the others, and has a single desorption peak at 160 K, overlapping the water’s desorption. In order to attain a better understanding of the range of possible products formed as a result of the 248 nm photon irradiation, we prepared sandwich systems in which a potential product (a stable molecule) was inserted in between the two 50 ML D2O layers, but with no laser irradiation, with subsequent heating at 0.5 K/s to obtain the TPD spectrum. Figures 6 A and 6 B present the corresponding desorption profiles of pure formaldehyde and methanol in comparison with the major products

(A)

QMS signal intensity (a.u.)

formed after irradiation.

QMS signal intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CH3OD

Temperature (K)


(B)

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Figure 6: TPD spectra of pure molecules without laser irradiation: (A) Formaldehyde (inset shows H2CO formation upon irradiation). (B) Methanol (inset shows CH3OD formation upon irradiation).

As in the case of the pure methane, also here with the co-adsorbed system, the integrated area under the TPD peaks for all the masses shown in Figure 5 is depicted with the aim to obtain a cross section for the formation of each of the masses. This is shown in the Supporting Information, Figure S4. A summary of all the formation’s cross sections obtained by the linear, initial rate analysis, of all the masses described in Figures 3 and 5, for pure methane and for the co-sandwiched system is presented in Table T1 next. Table T1: Photo-production cross sections of the various masses observed following irradiation (248 nm photons) of (A) pure methane and (B) co-sandwiched 1 ML CH4+2.5 L O2 within two 50 ML D2O-ASW layers.

Cross sections (cm2/photon)

m/z (Product)

ASW|1 ML CH4|ASW/Ru

ASW|1 ML CH4+2.5 L O2|ASW/Ru

(×10-21 cm2/photon)

(×10-21 cm2/photon)

(A)

(B)

27

σ = 0.34 ± 0.1

σ = 0.53 ± 0.03

29

σ = 1.09 ± 0.02

σ = 3.46 ± 0.4

30

σ = 1.19 ± 0.04

σ = 2.86 ± 0.3

33

No product formed

σ = 3.3 ± 0.6

45

σ = (0.1 ± 0.003)

σ = 0.2 ± 0.003

Additional experiments were performed to better understand the role of excess O2 and methane trapped inside ASW at low temperatures, analogous to interstellar matrix material (ISM). Figure 7 shows the combined area under the peak of the TPD spectra of various photo-products desorbed from A) ASW|1 ML CH4|10 ML O2|ASW and B) ASW|2 ML CH4|2.5 L O2|ASW after irradiating at 248 nm with a 0/2 mJ/pulse. 15 ACS Paragon Plus Environment

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Mass 44 integration


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(B)

(A) CO2 Formation



Figure 7: Integrated area under the peak of the indicated photo-product arising from: A) ASW|1 ML CH4|10 L O2|ASW and B) ASW|2 ML CH4|2.5 L O2|ASW on Ru (0001) substrate, irradiated by 248 nm photons at 0.2 mJ/pulse. The irradiation times are translated to the number of photons (X-axis) and an integrated area under the TPD spectral peaks (Y-axis). The inset in (A) is the integrated signal at mass 44 (CO2) as a function of exposure to 248 nm photons.

A similar set of experiments (summarized in Fig. 7A) was performed with a higher coverage of oxygen, 10 L oxygen (maintaining CH4 at 1 ML and sandwiched between 50 ML D2O-ASW on top and at the bottom for all the experiments). This was performed to better evaluate the major role of oxygen as an electron-capturing molecule in affecting the cross section of the photo-product. The TPD plots are very similar to Figure 5 except for mass 27, mass 31, and mass 46, which are formed in a negligible amount. The corresponding TPD spectra of the photo-products are shown in the Supporting Information, Figure S5. The lower photo-product yield, obtained with four times higher exposure of oxygen molecules, should be due to the partial and complete oxidation of the fragments to more thermodynamically stable molecules such as CO and CO2. From the inset in the plot of 7A, we see almost a linear increase in the formation of mass 44 (CO2) as a function of the number of photons with a cross section of σ = (27 ± 3) × 10-21 cm2/photon (in the case of 10 ML O2 experiment). This value is an order of magnitude higher than all the cross sections obtained for the product species discussed in the previous section. The corresponding cross section values derived from Figure 7A are shown in the Supporting Information, Table 1. All


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the masses detected here (m/z=30,33,45) display a gradual increase, except for mass 29,

which

becomes

saturated

after

an

overall

photon

irradiation

of

9.2 × 1019 photons/cm2. This may be due to the formation of other oxygenated molecules such as HCOOH, CO, and CO2. From the photo-products’ distribution plot [Figure 7B], we can infer that, upon doubling the concentration of methane and fixing oxygen at 2.5 L (similar to the amount discussed in section 3.3), a higher yield of photo-products is obtained in comparison with both 1 ML CH4 and the case of the 10 ML O2 experiments, described above. Moreover, early saturation of masses 27, 45, and 46 is observed following a 2.3 × 1019 overall photons dose, whereas the other photo-products become saturated at twice the number of photons, 9.2 × 1019 photons/cm2. We can conclude from this plot that the formation of masses 29 and 30 is 5-7 times more probable than all the other photo-products (except CO2). The TPD spectra of various product molecules and the corresponding cross-sections for their formation are summarized in the Supporting Information, Figure S6 and the Supporting Information, Table 1, respectively.

3.4 Product yield while varying the bottom D2O layer thickness

(C)

the TPD peaks (mass 44,CO2) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Bottom (D2O) Layer thickness(ML)

Figure 8: A) Integrated area under the TPD signals of A) mass 29, B) mass 30, C) mass 44 vs the bottom D2O layer thickness; the rest is fixed: [ variable D2O layer thickness | 1 ML CH4 | 2.5 L O2 | 50 ML D2O]. The red dotted line shows the trend in the formation of products while varying the bottom layer thickness.



The results in Figure 8 (A-C) display the products’ formation yield while varying the bottom D2O-ASW layer thickness in the range (20 ML D2O up to 250 ML D2O) with methane (1 ML), oxygen (2.5 L) overlaid by fixed 50 ML D2O, followed by a laser irradiation time equivalent to 9.2 × 1019 photons/cm2 at a constant pulse energy of 0.2 mJ per pulse. The products’ formation with a maximum yield at a bottom layer thickness of about 100 ML is surprising. There is a combination of at least two effects here:

1. The methane molecules are diluted across a thicker layer as the bottom

layer becomes thicker (see Figure 2). Electrons seem to transport quite readily across the ASW layer (as indicated from current measurements performed between the ruthenium substrate and ground when electrons strike significantly thicker ASW films from the vacuum side – see ref. 42 in this manuscript). The electrons gradually become less energetic within layers closer to the vacuum interface due to inelastic scattering events within the ASW film. However, if the cross section for DEA is larger at lower electrons energy - as was demonstrated in the case of several alkyl halide molecules condensed on Kr film while interacting with low energy electrons (see P. Ayotte, J. Gamache, A. D. Bas, I. I. Fabrikant and L. Sanche, J. Chem. Phys., 106(2), 749 (1997) (ref. 56 ), then a maximum in the cross section is expected at thicker layers and not at minimum thickness, with an apparent maximum yield at 100 ML. 2. Since the flux of photo-electrons striking the ASW film is maximum at the ASW-Ru interface, it may lead to further fragmentation of the main photo-products (e.g. CH3 radicals). This results in a decrease in the products yield. The exponential fit used to derive (a too high) cross section for the photo-induced process actually takes such an event into account (see Figure 4). By gradually diluting the electrons' flux at thicker bottom layers, further decomposition probability of the main primary fragments (CH3) decreases, leading to a maximum fragment yield at 100 ML of bottom layers. At this point we cannot conclude what process is dominant.

3.5 Analysis of possible photo-products As mentioned before, in order to attain a better understanding of the nature of the molecular products formed upon irradiation by the 248 nm photons, we prepared sandwich systems in which a potential product (as a stable molecule without photon irradiation) was inserted in between the two 50 ML D2O layers with subsequent 18 ACS Paragon Plus Environment

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heating at 0.5 K/s to obtain their TPD spectrum. The TPD spectra results of the test molecules are displayed in Figure 6 and in the Supplementary Information, Figure S5. 1.

Ethane (C2H6) (Figure S5 E) It can clearly be seen that weakly interacting

molecules such as ethane desorb primarily via the "volcano" peak at 145 K with a minor fraction desorbing with the water molecules at 160 K. In addition, an additional low temperature peak is near 65 K. The overall profile of the ethane desorption is very similar to that of the parent methane molecule; however, it does not fit any of the post irradiation TPDs at masses 30, 29, or 27 in both Figures 3 and 5. We can conclude that the simple recombination reaction of two methyl radicals, the primary photo-product, apparently does not occur while surrounded by the water matrix. Another reason might be that the cross section for further dissociation of the methyl group to methylene (CH2 + H) is very high. This leads to a very low concentration of methyl radicals at the end of our irradiation, therefore, lowering the recombination reaction probability to form ethane. A similar conclusion holds for acetylene, where no TPD profile following irradiation was found to match a sandwiched stable acetylene molecule. 2. Formaldehyde (H2CO) (Figure 6 A and S5 D): The TPD profile of formaldehyde fits very well the spectra obtained at masses 29 and 30 in both Figures 3 and 5, where the ratio of the peak intensities at 145 K to 160 K matches those obtained in our irradiated experiments. In addition, a low temperature peak exists at 100 K, which is unique to formaldehyde; however, it does not appear at the same intensity as in Figure 6 A. Further to the discussion above on the absence of ethane in our irradiated spectra, it is reasonable to assume that methylene groups (CH2) react with the surrounding water, initially extracting the OD group to form an unstable CH2OD that rearranges to eliminate the extra D radical and stabilizes as formaldehyde (H2CO). 3) Methanol and Ethanol Figure 6 B: Since the density of water molecules is much higher in comparison with the dissolved O2 molecules, the probability of a primary CH3 radical to encounter OD groups is high, leading to the formation of methanol (CH3OD, mass 33). Nevertheless, since we did not observe any mass 33 in the absence of oxygen, the mechanism underlying the formation of methanol may not be due to direct recombination of methyl and hydroxyl radical ions; instead, it might be activated by the presence of O2- ions. The methanol TPD profile in Figure 6 fits exactly the spectra obtained in Figure 5 (mass 33) and correlates well with the TPD 19 ACS Paragon Plus Environment


profiles shown in the Supporting Information, Figure S4. Owing to the strong interaction of alcohol with water, it desorbs together with the desorption of water at 160 K. This unique single-peak desorption profile of pure methanol and ethanol is shown in the Supporting Information, in Figure S5 F and G, respectively. Since mass 45 in Figures 3, 5 (as well as in the Supporting Information, Figure S5 E and S6 F) and mass 46 in the Supporting Information, Figure S4 do not exhibit any unique characteristics resembling alcohol desorption, we can exclude ethanol as a possible product formed during the irradiation of methane (248 nm photons) in the presence or absence of oxygen. 4) Formic acid (Figure S5 H): The low intensity of mass 45 and the smaller cross section values in Figures 3 and 5 suggest a low probability for the formation of formic acid. The same is observed in the TPD spectra of masses 45 and 46 in Figure S6 E and F, which correspond to a fragment of formic acid (COOH+) and of the parent molecule (HCOOH), respectively. The TPD spectra of the pure (non-irradiated) molecules exactly match the ratio of peak intensities at 145 to 160 K and the unique minor peak at 71 K in our irradiated experiments. This supports the assignment of mass 45 as one of the fragments of formic acid (COOH+). The formation of formic acid might be attributed to further oxidation of formaldehyde molecules, whose absorption cross section is possibly higher at this energy (5 eV) of photons. Hence, we suggest that the drop in the formaldehyde formation yield beyond a critical photon flux is attributed to the partial and complete oxidation to CO and CO2, respectively. 4) Di-methyl ether (DME) (Supporting Information, Figure S5 A).The TPD spectra of mass 45 in the Supporting Information, Figure S4 exactly matches the pure di-methyl ether desorption profile from ASW. The ratio of the peak intensities at 145 K and 160 K is not identical to that observed for pure formic acid. Since the interaction of DME with ASW is weaker than that of formic acid, a significant fraction of the formed dimethyl ether is desorbed during the phase transition temperature of ASW at 145 K. DME formation is observed only when the methane concentration is doubled, which may give rise to a higher density of nearby methyl radicals as well as methoxy radicals. These radicals may further recombine to form dimethyl ether, albeit at a low probability.


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As a summary of section 3.5, we suggest the following reactions mechanisms, based on the results obtained in the presence of oxygen. The reactivity of O2- superoxide anion was reported in the literature with respect to its rapid oxidation of hydrocarbons in contaminated soils in refs. 57 and 58. 1. CH4 + hν + 2O2 + n[D2O] → CO2 + 2H2O + n[D2O]- (full oxidation, the most probable reaction, O2- as an intermediate). 2. O2- + D2O → 2OD + O- (OD as an intermediate). 3. CH4 + hν + O2 + n[D2O] → CH3 + H + O- + (n-1) [D2O] + 2OD → CH3OD + DHO + O + (n-1) [D2O]- (methanol is formed only in the presence of O2). 4. CH4 + hν + O2 + n[D2O] → CH3 + H + O- + (n-1) [D2O] + 2OD CH3 + hν + O2 + n[D2O] → CH2 + H + O- + (n-1) [D2O] + 2OD CH2 + O- → CH2O (Formaldehyde as one of the most stable products). In all the above potential reaction mechanisms the products are expected to desorb as second order kinetics if intermediates adsorb first on the substrate prior to TPD substrate heating. If they are formed during irradiation they will appear as first order desorption kinetics. Practically all our photo-products desorb at first order TPD kinetics. The ASW matrix, however, may interact with the various intermediates leading to apparent first order kinetics.

4. Conclusions This study has demonstrated that oxygen molecules play an important role in the photo-induced (5.0 eV photon energy) activation of methane embedded within layers of ASW adsorbed on a Ru(0001) single crystal surface. Dissociative Electron Attachment (DEA) is the apparent mechanism of activation; therefore, the formation of stable O2- ions enhances transient electron transfer to methane, leading to methyl radical formation as the first step towards its activation. The thermodynamics of the entire DEA process needs to take the environment (water solvent molecules) into account as stabilizing element of the transient ionic products. This may lead to DEA 21 ACS Paragon Plus Environment


at very low electrons energy. In a related system, Sanche and coworkers (see ref. 56) have shown a maximum cross section for methyl chloride dissociative electron attachment mechanism when adsorbed on Kr film at electrons energy of near 1 eV, significantly lower than the gas phase C-Cl bond energy in this molecule. The main products were carbon dioxide, formaldehyde, and methanol, as revealed by post-irradiation TPD. The desorption pattern, namely, the ratio between "volcano" (145 K) and typical multilayer water (160 K) desorption peaks enabled us to identify the chemical nature of the photo-products. These patterns were compared to the TPD spectra obtained from stable molecules sandwiched within the same two ASW layers (without photo-irradiation) as a way to ensure the chemical identity of the photoproducts. Conversion of methane to methanol occurs only in the presence of oxygen molecules, whereas formaldehyde is formed with a small probability also in the absence of oxygen. The overall photo-conversion can be quantified by typical crosssections in the range of 10-21 - 10-22 cm2/photon with CO2 an order of magnitude more probable to form than formaldehyde or methanol.

Supporting Information TPD of trapped methane within 100 ML ASW film; Even distribution of both methane and oxygen in ASW film, Laser power dependence of the yield of photo-products upon 248 nm laser irradiation; Cross sections for the formation of various photo-products; Stable molecules TPD following trapping in ASW as calibration spectra.

Acknowledgements Partial support by the Israel Science Foundation (ISF) and the Einstein Foundation Berlin is acknowledged. The help of Dr. Edvard Mastov and Mr. Marcelo Friedman is greatly appreciated.


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TOC Graphic



O2-

O2CH3.

O O O 2 O2- CH4 2 CH4CH4 CH4 2 O2 O2 O2 O4 O O2 CH O2 2 CH4 O2 CH4 O2 2CH O2 4 O2 CH4O - 2 O42 OO2 2 OO22O2 O2CH4O2- O2 2CH CH4 CH .

O O2O O 2 CH4 2 CH4CH4 CH4 OO CH4 O2 O2 O2 O2 2CH O2 O2 CH4O2 CH4 O2 2CH4 2 OO2 2 - 4OO CH4 OO2 2- OO 2 OCH O2 2 2 4O2 2 2 O2 CH4 CH2.

2

Heating Ru(0001) Substrate

0.5 K/s

Ru(0001) Substrate


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The Effect of Co-Adsorbed Oxygen on the Photochemistry of Methane

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