Low Energy Electron Induced CâH Activation Reactions in Methane

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Low Energy Electron Induced C–H Activation Reactions in Methane Containing Ices Sramana Kundu, Vaibhav S. Prabhudesai, and Erumathadathil Krishnakumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07348 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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The Journal of Physical Chemistry

Low Energy Electron Induced C–H Activation Reactions in Methane Containing Ices Sramana Kundu, Vaibhav S. Prabhudesai and E. Krishnakumar* Department of Nuclear and Atomic Physics Tata Institute of Fundamental Research 1 Homi Bhabha Road, Colaba, Mumbai 400005, India

ACS Paragon Plus Environment

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ABSTRACT Conversion of alkanes to functionalized compounds is a highly sought after goal. Under the high reaction temperatures, desired intermediate products like alcohols, aldehydes and carboxylic acids are more easily oxidized to CO2 than the starting alkane which makes it difficult to recover these functionalized compounds. Here, we use electrons with energy < 20 eV, to initiate reactions in pure methane and mixed layers of methane and oxygen condensed on a gold substrate at 15 K. Observation of ethane (in pure and mixed films) and methanol and formaldehyde (in mixed films) indicate methane activation by electrons. From the formation threshold and energy dependence of product yields, electronic excitation of reactant/s followed by dissociation into neutral radicals appears to be the reaction initiating step. The results demonstrate the utility of low energy electrons in bringing about functionalization in the simplest alkane.


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1. Introduction Alkanes (CnH2n+2), specifically methane (CH4), are the principal constituents of natural gas and petroleum, and as such are abundant on earth. However, there are very few processes for converting them directly to more valuable products. This is due to their relative inertness; the absence of low energy empty orbitals or high energy filled orbitals makes it difficult for them to participate in chemical reactions1. For CH4, the high C–H bond strength (439.28 kJ/mol = 4.55 eV)2, negligible electron affinity3, large ionization energy (12.6 eV), and low polarizability make it more like a noble gas in terms of reactivity4. In oxidation reactions, typically the oxidized products are more reactive than the starting alkanes and are consumed before recovery. Alkanes do react at high temperatures, such as in combustion, but usually proceed energetically downhill all the way to CO2 and H2O, which are thermodynamically stable and economically not useful. In such applications, alkanes are exploited for their energy content, but not as viable precursors for valuable chemicals. Since selective transformation to partial oxidation products of alkanes, such as alcohols, aldehydes and carboxylic acids is difficult, they are produced from unsaturated hydrocarbons, which are themselves obtained from alkanes through inefficient processes. Thus, processes for direct conversion of CH4 to methanol (CH3OH), formaldehyde (HCHO) or other liquid fuels and chemicals would lead to more efficient CH4 utilization1. More generally, due to the omnipresence of C–H bonds in all kinds of organic molecules, the ability to transform selectively, efficiently and in a predictable manner a specific C–H bond to a functional group, called C–H functionalization (or activation), is a highly sought after goal in chemistry. Such reactions could convert light alkanes to higher-value, functionalized chemical feedstocks1, or they could introduce functionality at specific positions of molecules already possessing one or many other functional groups5,6. Catalysts are specially designed and tested for


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their efficiency in the different types of activation reactions, as all of them require specific catalysts. There are excellent reviews and perspectives4–16 on the challenges, various approaches, methods, catalysts, etc. employed for C–H activation and will not be discussed here. Chemical reactions induced by electrons, even those with energies below 20 eV, which we refer to as low energy electrons (LEEs), in condensed molecular films can lead to the synthesis of bigger and complex molecules, as has been shown in a number of studies17–20. Processes like electron impact excitation and ionization and dissociative and non–dissociative electron attachment have been assigned as the reaction initiating step, providing fundamental insight into the mechanism of chemical modification. Energy dependent formation of products in specific cases highlight the ability to control chemical reactions by varying the electron’s energy. We want to explore the possibility of activating the C–H bond of CH4 using LEEs, first, in condensed films of pure CH4 and look for higher alkanes or alkenes. Next, we extend the study to condensed CH4 mixed with O2 to look for partial oxidation products of CH4. Electron driven reactions in a condensed medium containing CH4 is also relevant to interstellar chemistry, as it is one of the most common molecules discovered in the interstellar medium, various comets and meteorites. Along with CH4, its derivatives CH3OH and HCHO have been found in gas and ice phase in number of stellar objects, comets and planetary atmospheres21–24. While the formation of many interstellar molecules is reasonably explained by gas–phase reactions, solid-phase reactions must also be invoked for some molecules such as CH3OH, HCHO and saturated hydrocarbons, which are of paramount importance for the origin of life25–28. As astrophysical bodies are subjected to ionizing radiation in the form of cosmic rays, VUV radiation, energetic electrons, protons and ions, their feasibility in driving reactions in molecular ices accumulated on dust grains have been studied in laboratory simulations29–33. In pure CH4 ice


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irradiated by Lyman α photons (10.2 eV), MeV protons and α particles, keV electrons, the primary reaction step in all instances was determined to be the C–H bond cleavage to give CH3 and H radicals which subsequently drive further chemistry. Electrons with energies between 1 and 20 eV are the most abundant (5x104 per MeV) species produced by the interaction of highenergy ionizing radiation with a medium34,35. Thus, electron induced reactions in condensed films of CH4 and its mixture with O2 have significance as possible formation routes of important molecules of astrochemical interest. In this work, we explore the chemistry initiated by electrons of energy below 20 eV both in films of pure CH4 and CH4 mixed with O2 and deduce the pathways for the observed products from the energy dependence of their yield. Our observation of the next higher alkane, ethane (C2H6) and partial oxidation products, CH3OH and HCHO show that LEEs can drive CH4 activation reactions and suggest a possible low energy electron initiated formation route for the observed species on grain and dust surfaces in space.

2. Experimental Methods The experiments were conducted in an ultra–high vacuum (UHV) chamber with a base pressure of 8 × 10-10 torr. The molecules are deposited on an Au (100) single crystal attached to the tip of a cold head which is cooled down to about 15 K with a closed cycle He refrigerator. The gaseous molecules are introduced into the chamber through a stainless steel tube of 1 mm inner diameter symmetrically placed in front of the Au substrate along its normal at a distance of about 50 mm. The electron gun, which is described in detail elsewhere36, faces the centre of the substrate at an angle of 22.5° to its normal. The electrons coming out of the gun are magnetically collimated and guided on to the substrate. The magnetic field of about 50 gauss at the substrate is generated with


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the help of magnet coils mounted concentric to the electron gun axis. We use reflection– absorption infrared (IR) spectrometry (RAIRS) measurements to detect and quantify the IR active species on the substrate. For this purpose, an IR beam from an externally placed Fourier– transform infrared (FTIR) spectrometer is made incident on the substrate at 45° through a ZnSe window on the UHV chamber and the reflected beam is collected by an external MercuryCadmium-Telluride (MCT) detector with appropriate optics. The temperature of the substrate is measured by a Si diode sensor and controlled by a heater, both of which are mounted on the substrate holder. The heater is operated by a Lakeshore 335 temperature controller, using which we can set the substrate temperature at a desired value and ramp the temperature at different rates. We perform temperature programmed desorption (TPD) to detect and analyse the products of electron irradiation using a SRS 300 residual gas analyser (RGA) attached to the chamber, in conjunction with the heater and temperature controller. A typical IR spectrum of condensed CH4 is shown in Fig. 1. We see the two fundamental C–H stretches, ν4 at 1304 cm–1 and ν3 at 3010 cm–1, respectively. The column density of CH4 ice is determined from the area of one of its characteristic features and its integral cross–section or A– value by using the formula: N=

ୱ୧୬ ஘

୅୰ୣୟ

ଶ

୅–୴ୟ୪୳ୣ

(A)

Here θ is 45°, A–values of the 3010 cm–1 and 1304 cm–1 features are 5.7x10–18 cm molecule–1 and 3.8x10–18 cm molecule–1, respectively37. The no. of monolayers (ML) is given by: ML = N / density

(B)

The density of solid CH433 is 0.53 g cm–3. The estimated thickness from both the features is about 7–8 ML. In the second molecular system (CH4 and O2), we look for the partial oxidation product CH3OH. Thus, we mix CH4 and O2 in the ratio 2:1 to follow the stoichiometry of the reaction


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2 CH4 + O2 → 2 CH3OH

(1)

Since O2 is IR inactive, the IR spectrum of the CH4 and O2 mixture is similar to Fig. 1. As the deposited film is a composite mixture of the two molecules, it is more accurate to consider the column densities instead of monolayers. The quantity of CH4 in the mixture is equal to that of pure CH4 films and that of O2 is half of that. Thus, the column densities of CH4 and O2 in the mixture are 5x1015 molecules cm–2 and 2.5x1015 molecules cm–2, respectively. This is also verified from the intensities of their characteristic peaks in the TPD spectra of m/z = 16 (CH4+) and 32 (O2+).

Fig. 1: IR spectrum of condensed CH4 as obtained by us. (a) ν4 and (b) ν3 stretch region The thickness measurement technique using IR absorption is a well–established method used by several other researchers25,33,38,39. It is a direct method since the IR signal strength is directly proportional to the thickness of the molecular layer deposited on the surface. It is important to note that the intensity of bands in a RAIR spectrum depends on the wavelength dependent refractive indices of the adsorbate and substrate and also the adsorbate thickness. The detailed discussion can be found elsewhere40. At larger thickness (of the order of a few µm), interference effects introduce additional uncertainties. Though the above formulae based on Beer–Lambert law are a simplification, it is found to hold for a few monolayer thick adsorbate as in the present


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case. In a previous work27 on condensed CO2, we had done a comparison between IR method and volumetric dosing method to determine the thickness. The volumetric method is based on leaking in a given volume of gas from a container through an aperture and estimating the no. of molecules from the corresponding pressure drop. Assuming a sticking coefficient of unity and the geometric factors like aperture size and shape, its distance from the substrate, the solid angle subtended by the substrate at the aperture and the angular distribution of molecules effusing out of the aperture, we calculated the no. of molecules deposited on the substrate. We found that the volumetric dosing method gives a value which is 1–1.6 times that given by the IR method. We estimate an error of about 50 % in thickness determination. The electron gun could be operated in pulsed mode or direct current (DC) mode. In the present experiments it is operated in the DC mode, with a typical current of 0.2 µA. The irradiated area is about 4 mm2. The electron dosage is optimized to 250 µC, for 10–15 minutes of irradiation. For each electron energy, irradiation is done on a freshly deposited film. The electron injection curve is obtained by measuring the current transmitted through the substrate as a function of the negative bias on the filament. The onset of electron transmission through the substrate is taken as the ‘zero’ of the electron energy scale. On exposure to the electron beam, the film develops a negative charge due to accumulation of negative ions and trapped electrons. This gradually shifts the current onset to higher voltages. Accordingly, the injection curve is recorded and the filament bias is adjusted every minute or more frequently if required, to maintain a constant energy of the electron beam. The uncertainty in defining the electron energy is about 1 eV. TPD is performed at a temperature ramp rate of 60 K per minute (K/min), unless otherwise mentioned. Electron impact ionization inside the RGA takes place at an energy of 70 eV. We ascertain the formation of new products by comparing the TPD spectra after electron irradiation to the ones without any


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electron irradiation. All depicted TPD spectra are obtained after subtracting the corresponding spectra for un–irradiated samples.

3. Results As the first step in our experiments, we deposit pure CH4 and perform TPD without electron irradiation. The TPD spectra of m/z = 16 (CH4+) and m/z = 15 (CH3+), which are the major fragments of CH4, peak at the desorption temperature of CH4 around 45 K. No higher masses are seen. Similarly, we deposit the CH4 and O2 mixture and perform TPD without irradiation. In addition to the peaks corresponding to the CH4 fragments, the TPD spectra of m/z = 32 (O2+) and m/z = 16 (O+), corresponding to O2, peak at the desorption temperature of O2 around 35 K. No other masses are seen. These un–irradiated spectra are compared to the corresponding spectra after electron irradiation to detect the presence of any new signal, which would indicate the formation of a new product. For the irradiation measurements on both types of films, we start with a higher electron energy of 100 eV, where due to better signal to noise ratio in the TPD spectra, observation of new features is easier and product identification is unambiguous. After conclusively identifying the new products at higher energy, we go down in energy and monitor the corresponding features. Expectedly, the signal intensities decrease, due to which the minor fragment ions of a parent product are not observable at lower incident energies. However, the presence of the product can still be established from its major fragment ion/s. In the following sections, we consider the fragments which are observable at the lowest energies. 3.1 Electron Impact on Pristine CH4 Ice The TPD spectra of the mass fragments detected after irradiation of pure CH4 film with 10 eV electrons are shown in Fig. 2. We detect the fragments with mass to charge ratios (m/z) = 25 to


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30. All fragments have similar TPD spectra, peaking at the same temperature of around 130 K which implies they are fragments of a single parent molecule. The only possible fragments corresponding to the detected ions are: C2H6+ (m/z = 30), C2H5+ (m/z = 29), C2H4+ (m/z = 28), C2H3+ (m/z = 27), C2H2+ (m/z = 26) and C2H+ (m/z = 25). The likely product is ethane (C2H6); to confirm we compare our mass spectrum to that reported in the NIST databasea, shown in Fig. 3.

Fig. 2: TPD spectra of the fragments with m/z (a) 25, (b) 26, (c) 27, (d) 28, (e) 29 and (f) 30 after irradiation of pristine CH4 film with 10 eV electrons, performed at the rate of 60 K/min. All the TPD curves peak at the same temperature of 130 K. These spectra are obtained after subtracting the corresponding spectra for un–irradiated samples. The spectra have been smoothed by applying FFT filter to the raw data to remove the electrical noise of the RGA.


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Fig. 3: The relative intensities of the mass fragments m/z = 25 to 30 (a) as measured by us in the TPD spectra following irradiation of pristine CH4 film with 10 eV electrons and (b) the mass spectrum of C2H6 as reported by NISTa. The intensities are normalized to that of m/z = 28. The spectra compare well indicating that the dominant irradiation product is C2H6 in pure CH4 films. The relative yields of the fragments closely match those in the reference spectrum; hence we conclude that the dominant irradiation product is C2H6. At energies above 50 eV, we see signal intensities in the TPD spectra of m/z = 26 and 28 peaking at 170 K, whose relative yields are close to that of ethylene (C2H4), which might be a secondary product of irradiation. However, these signals are very weak at lower electron energies and we can hardly distinguish them from the falling edge of the TPD signals corresponding to C2H6. Thus it appears that C2H4 could be a minor product of irradiation; but its signal is too weak to measure at low electron energies.


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However, it should be noted that according to reported TPD spectra in literature,33,41 C2H4 usually desorbs before C2H6 or along with it which makes our suggestion probably untenable. Further experiments are required to make a definite conclusion. We perform irradiation at different energies and calculate the C2H6 yield. The result is plotted as a function of energy in Fig. 4. The yield curve shows the threshold of production as 8 eV, after which it increases till 15–16 eV, remains constant till 40 eV and then rises steadily with energy.

Fig. 4: C2H6 yield from the TPD spectra following irradiation of a pristine CH4 film as a function of electron energy. The error bars represent the standard deviation of repeated measurements. 3.2 Electron Impact on CH4 and O2 Ice Mixture It is important to note that the mass fragments corresponding to the production of C2H6 from CH4, as shown in Fig. 2, are also present in the TPD spectra of the mixed film. However, they peak at a temperature of 110–130 K corresponding to the desorption temperature of C2H6 (Fig. 5 (a) and (b)). At m/z = 28, we have a large back ground signal, which is probably due to ambient CO (a common contaminant in UHV chambers) desorbing from different parts of the cryostat, making it difficult to extract reliable data using it. Thus, in our analysis we do not use the m/z = 28 fragment for analysis. It should be noted that despite the high background, m/z = 28 could be used as a reliable indicator of a new product in pure CH4 films because the relevant signal


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strengths are high. This is not the case for mixed films where the signal strengths are relatively weak. The notable differences between irradiation of pure CH4 films and the CH4 + O2 mixed films are the observation of a new fragment (m/z =31) in the TPD spectrum at a higher temperature (around 190 K) and the appearance of new peaks in TPD spectra of m/z = 29 and 30 at around the same temperature (180 K). These are the only fragments which are present even at the lowest electron energies at the elevated temperature. The TPD spectra of these mass fragments at three electron irradiation energies (10, 55 and 100 eV) are shown in Fig. 5. The vertical red line at 130 K is to show that the temperature ramp is paused for 10 minutes at 130 K after which it is resumed again. This is done so that the higher temperature peaks are not affected by the ones at lower temperature, which would otherwise have merged to give a broad feature from which identifying a new product would have been difficult. The new peak in the TPD spectra of m/z = 29 and 30 indicate the formation of new species from the CH4 + O2 mixed films. And the difference in the peak temperature of m/z = 31 from the two lower masses indicate the production of yet another new species from the mixed film. We identify the two new species observed in the mixed film as HCHO and CH3OH by comparing the mass spectra. We assign the species peaking at 180 K and yielding m/z = 29 and 30 to HCHO as it is known to produce mainly the ions HCHO+ (m/z = 30) and CHO+ (m/z = 29). We confirm this by comparing our mass spectra to that reported by the NIST databasea, as shown in Fig. 6. The NIST value falls within the uncertainty limit of our measurement. M/z = 31 can only be assigned to CH3O+ or CH2OH+, which is the dominant mass fragment in the mass spectrum of CH3OH as shown in Fig. 6(c). CH3OH and HCHO can both contribute to the signals in m/z = 29 and 30 as seen from their mass spectra. The major CH3OH fragment, m/z = 31, has


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Fig. 5: TPD spectra of the fragments with m/z (a) 29, (b) 30 and (c) 31 obtained after irradiation of CH4 and O2 mixed film with three different electron energies (10 eV, 55 eV and 100 eV). The lower temperature peak in (a) and (b) corresponds to desorption of C2H6. The vertical red line at 130 K is to show that the temperature ramp is paused for 10 minutes at 130 K after which it is resumed again. These spectra are obtained after subtracting the corresponding spectra for un– irradiated samples. The spectra have been smoothed by applying FFT filter to the raw data to remove the electrical noise of the RGA.


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an intensity which is about 40% of that of m/z = 29, as measured by us. In the mass spectrum of CH3OH (Fig. 6(c)), m/z = 29 is about half as intense as m/z = 31. Hence, the contribution of CH3OH to m/z = 29 would be about 20% of the total measured intensity. The contribution of CH3OH to m/z = 30 is even less than that.

Fig. 6: (a) The relative intensities of the mass fragments m/z = 29 and 30 as measured by us in the TPD spectra following irradiation of mixed CH4 and O2 ice with 10 eV electrons. Electron ionization mass spectrum of (b) HCHO and (c) CH3OH as reported by NISTa. The intensities are normalized to that of m/z = 29 in (a) and (b) and m/z = 31 in (c).


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Thus, in the TPD spectra measured by us (Fig. 5(a) and (b)), we estimate the contribution from CH3OH to m/z = 29 and 30 to be within the error limits; the dominant contribution being from HCHO. The intensity of the signal for m/z = 31 is less (Fig. 5(c)) compared to that of m/z = 29 and 30, and the other mass fragments of CH3OH in its mass spectrum have even less intensity (Fig. 6(c)). Hence, although we monitor all the channels corresponding to the relevant mass fragments, they are likely to be below the detection level of the RGA. M/z = 32 also corresponds to O2+ from O2 and although O2 from the substrate desorbs below 50 K, unfortunately, we observe O2 desorption from other parts of the cold stem of the He cryostat in the temperature range of CH3OH desorption. So the signal for m/z = 32 is dominated by O2+ and we cannot resolve the contribution from CH3OH+. For m/z = 29, we see that the trailing edge of CHO+ from HCHO lies in the temperature range of CH3OH desorption, so we cannot infer anything from this channel either. M/z = 31 is the only signal which is not affected by any other source and since we observe a measurable signal in the range 180–260 K, we assign it to CH3OH. We note a slight shift of the TPD peak to higher temperature as we lower the electron energy. It is known that the bonding between CH3OH molecules and the substrate is stronger than their intermolecular bonding in a multilayer film. This is the reason the molecules directly in contact with the surface desorb at a higher temperature than the bulk film42,43. In our system, the smaller CH3OH quantity formed at 10 eV is likely to be directly in contact with the substrate once CH4 and O2 desorb by 60 K, which leads to stronger surface bonding and higher desorption temperature. At higher irradiation energy, the larger CH3OH quantity formed probably forms a multilayer film and the bulk desorption occurs at a slightly lower temperature. We do not observe the production of ethanol (C2H5OH), as seen from its characteristic m/z = 45 being absent in the TPD spectra.


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The yields of HCHO and CH3OH as a function of electron energy are plotted in Fig. 7. The production threshold for all the fragments is between 7 and 8 eV, following which they increase till 10–12 eV, remain steady till 25–30 eV and then start increasing again.

Fig. 7: Yields of (a) HCHO and (b) CH3OH from the TPD spectra following irradiation of mixed CH4 and O2 ice as a function of electron energy (see text). The error bars represent the standard deviation of repeated measurements.

4. Discussion We can obtain some information about the primary reaction initiating step from the energy dependence of the product yield. Dissociative electron attachment (DEA) to gaseous and condensed films of CH4 and O2 lead to the production of fragment anions and corresponding neutral radicals in specific energy ranges. In gas phase, H– and CH2– from CH4 appear within the energy range 7–13 eV44. In condensed films, due to constraints on desorption of heavier masses, only H– is observed within 8–13 eV45. Electron attachment to O2 clusters, yielding the series of ions O2–, (O2)2–, (O2)3– and so on, peaks close to 0 eV and the cross–section falls sharply with energy46. DEA to gaseous O2, yielding O–, occurs in the range 5–9 eV47,48. In condensed O2, ESD of O– occurs between 6–10 eV, and also, with less intensity, between 12–15 eV49,50. However, the actual onset of DEA in condensed phase has been found to occur at ~4 eV based


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on charge trapping measurements51. If DEA is the primary process for producing the reactive fragments which eventually give the observed products, their yield should show an energy dependence similar to that of the anions. As we do not see any resonant features in the product yield curves, we conclude that DEA does not play a significant role in the reactions. It is also to be noted that peak DEA cross–sections of CH4 (0.017 Å2 at 10 eV)44 and O2 (0.0154 Å2 at 6.4 eV)55 in gas phase are about two orders of magnitude smaller than dissociation and ionization cross–sections (see Fig. 8 below). In general, both the processes of electron impact dissociation and ionization show threshold behaviour with yields rising with energy above the onset. Ionization energies of gaseous CH4 and O2 are 12.6 and 12.1 eV, respectively. While the ionization energies in a condensed medium may be lowered by almost 1 eV due to polarization effects17,52,53, they still remain higher than the observed onset (~ 8 eV) of product yields. Hence, it seems unlikely that ionization is the primary reaction initiating step in this energy range. Considering neutral dissociation, the C–H bond dissociation energy of CH4 is 4.55 eV2. The heats of reaction for their formation from CH4 are given in equations (2 – 4). A positive value of heat of a reaction signifies the minimum energy required for converting the reactants into products, i.e. it is the thermodynamic threshold of the reaction. In our experiment, this energy is supplied by electrons; hence the reaction will not proceed if the electron’s energy is below this value. CH4 → CH3 + H, ∆H = 4.55 eV

(2)

CH4 → CH2 + H2, ∆H = 4.78 eV

(3)

CH4 → CH + H2 + H, ∆H = 9.19 eV

(4)

From the heats of reactions, we infer that only CH3 and CH2 can be produced from CH4 at 8 eV impact energy. An additional requirement for electron impact dissociation is the excitation of the


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molecule into an electronic state which is dissociative along the given bond. The accessibility of such a state from the molecular ground state determines the energetic threshold of dissociation. It is known that all electronically excited states of CH4 are dissociative54,55. The experimental threshold energy for the production of CH3 radicals by the decay of the lowest lying 13T2 state of CH4 by electron impact has been reported to be 7.5 ± 0.3 eV by Makochekanwa et al.56, who also reported negligible production of CH2 in this energy range. According to the theoretical modelling of Ziółkowski et al.57, all triplet states dissociate to give CH3 + H and all singlet states give CH, CH2 and CH3 in 0.15:0.54:0.45 ratio. They find that triplet excitation dominates over singlet excitation and hence conclude that CH3 is the dominant product at all impact energies compared to CH2 which is a minor product. This is in contrast to the results of Nakano et al.58, who reported CH2 production to be comparable to CH3 below 15 eV, and earlier theoretical results (see ref.

55,59

) and photodissociation experiments60,61 that report dissociation into CH2 as

the dominant channel. The experimental cross–sections for electron impact dissociation of gaseous CH4 into the neutral radicals CH3 and CH2 measured by various groups are shown in Fig. 8 (a). The bond dissociation energy of O2 producing ground state O atoms is 5.1 eV. However, this requires excitation of O2 into one of the repulsive states, c 1Σu–, A’ 3∆u, or A 3Σu–, which lie at 6 eV. Thus, based on the threshold energies of all the processes, neutral dissociation of CH4 and O2 appear to be the most likely reaction initiating step at 8 eV. Below, we consider various reaction channels which could generate the observed products in pure and mixed ices, along with the heats of the reactions. The heat of a reaction is calculated from the standard heats of formation of the reactants and productsa. We consider only exothermic reaction pathways since endothermic reactions anyway cannot proceed without external energy supply. Even exothermic reactions having an activation barrier require external energy supply, so


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some of the listed reactions may not be feasible. In general, since radicals are produced from the parent molecules, they are present in smaller concentrations compared to CH4 or O2, which implies that a radical is more likely to encounter a parent molecule than another radical. Hence radical–radical interactions are less probable than radical–parent interactions. But since we do not know the activation barriers for any of the reactions, we cannot rule out radical–radical pathways. Also, radical–radical reactions are considered barrierless63–69 at very low temperatures, which may make them more favourable compared to the radical–parent ones.

Fig. 8: (a) Cross–sections for electron impact dissociation of gaseous CH4 into neutral radicals measured by various groups. CH258–squares, CH358–circles, CH362– triangles, (b) Cross–sections for electron impact ionization and dissociative ionization of gaseous CH4 from reference 82. CH4+–square, CH3+–circle, CH2+–triangle, CH+–inverted triangle, C+–diamond. 4.1 Reaction pathways in Pristine CH4 Ice Below, we consider the reaction pathways for C2H6 production starting from CH3 and CH2 radicals. We see that the experimental thresholds for both CH3 and CH2 production (Fig. 5 (a)) are in the 8–9 eV range, coinciding with that of C2H6 production observed by us. Thus, most likely one or both of them are involved in C2H6 production via reactions (5)–(6). CH3 + CH3 → C2H6, ∆H = – 3.89 eV

(5)


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CH2 + CH4 → C2H6, ∆H = – 4.10 eV

(6)

The calculations by Bennett et al.33 have shown that the combination of two CH3 radicals to produce C2H6 proceeds without an activation barrier. However, it should be noted that the energy released in an exothermic barrierless reaction needs to be dissipated by the medium to prevent the reverse reaction from occurring. We also see similarities in the electron energy dependence of C2H6 yield and radical production cross–sections. All the cross–sections rise from threshold to about 15–20 eV, after which they increase slowly (Nakano–CH3), fall slowly (Motlagh–CH3) or fall sharply (Nakano– CH2) till 80 eV. Since in our measurements, C2H6 yield is almost constant from 10 eV till 40 eV and then increases with energy, it is likely that the dominant precursor for the production of C2H6 is the CH3 radical. In condensed media, a single electron with sufficient energy can participate in more than one reaction by losing its energy in multiple collisions. Hence, the number of reactive species produced by an electron of particular energy would increase with the energy of the electron. This would lead to steadily increasing product yields with increasing electron energy. Judging by our observed product yield, this multiplicative effect does not seem to occur till about 40 eV. Coming to processes other than neutral dissociation, ionization and dissociative ionization of CH4 occurs above 12.6 and 14 eV (Fig. 8 (b)). Examples of ionization driven pathways for C2H6 production are CH4+ + CH4 → C2H6 + H2+, ∆H = 3.51 eV

(7)

CH3+ + CH4 → C2H6 + H+, ∆H = 4.41 eV

(8)

CH4+ + CH4 → C2H6+ + H2, ∆H = – 0.4 eV

(9)

CH3+ + CH4 → C2H6+ + H, ∆H = 2.34 eV

(10)


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CH2+ + CH4 → C2H6+, ∆H = – 2.97 eV

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(11)

Considering only exothermic reactions (∆H negative), neutralization of C2H6+ by electron capture can yield C2H6, above the respective appearance thresholds of CH4+ and CH2+. Due to trapping or solvation of electrons in solid films, such neutralization reactions cannot be ruled out in a condensed medium. Also, CH4+, CH3+ and CH2+ may capture an electron, forming excited state neutral species, which may then undergo reactions (12)–(14) which become feasible due to the internal energy available in the excited neutrals. As stated earlier, electronically excited CH4 is not stable54,55, so reactions involving CH4* are unlikely. CH4* + CH4 → C2H6 + H2

(12)

CH3* + CH4 → C2H6 + H

(13)

CH2* + CH4 → C2H6

(14)

Although the contributions of all the stated mechanisms towards C2H6 production, above their threshold energies, cannot be separated in our measurement, we can estimate their likelihood of occurrence. In reactions mediated by ions, there is a lot of excess energy in the system (> 12.6 eV) following recombination of the cation and electron. To obtain a stable C2H6 molecule as the final product, this energy has to be released which would involve multiple inelastic collisions inside the condensed film. A very likely process competing with stabilization would be the dissociation of the excited C2H6 molecule, which can occur in a single step. Thus ionization driven pathways may not contribute directly towards stable product formation. Experimental cross–sections for ionization and dissociative ionization of gaseous CH4 have been measured by Straub et al.70 (Fig. 8(b)). As we can see, from 30 eV onwards, the cross– sections for CH4+ and CH3+ are comparable to those of CH3 as measured by Nakano et al. and Motlagh et al. The increasing yield of C2H6 beyond 40 eV could be due to contribution from


22

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reactions involving CH4+ and CH3+. However, it is more likely due to the multiplicative effect of primary energetic electrons. Thus, based on the observed onset of C2H6 production, the energy dependence of its yield and the energetics of different processes, we infer that the reaction proceeds primarily through the neutral dissociation pathway.

4.2 Reaction pathways in CH4 and O2 Ice Mixture The simplest pathway for CH3OH production is the insertion of O into a C–H bond of CH4, which is exothermic by almost 4 eV: CH4 + O → CH3OH, ∆H = – 3.93 eV

(15)

Starting from CH3, production of CH3OH would require an OH entity, which might be present due to a coupling of O and H radicals. However, such secondary reactions are less likely to occur. HCHO can be generated from the following pathways, all of which are exothermic: CH4 + O → HCHO + H2, ∆H = – 3.01 eV

(16)

CH3 + O2 → HCHO + OH, ∆H = – 2.31 eV

(17)

CH2 + O2 → HCHO + O, ∆H = – 2.62 eV

(18)

CH3 + O → HCHO + H, ∆H = – 3.03 eV

(19)

CH2 + O → HCHO, ∆H = – 7.79 eV

(20)

Ionization–driven mechanisms can also be operative above the relevant thresholds (> 12.6 eV) leading to the observed products. Reactions like (21)–(24) (given below) followed by electron capture to yield the observed products may occur in the condensed films. Also, CH4+, CH3+, CH2+, O2+ and O+ may capture an electron, forming excited state neutral species, which may react to form the products. CH4 + O+ → CH3OH+, ∆H = – 6.7 eV

(21)


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CH4+ + O → CH3OH+, ∆H = – 5.68 eV

(22)

CH4 + O+ → HCHO+ + H2, ∆H = – 5.74 eV

(23)

CH4+ + O → HCHO+ + H2, ∆H = – 4.72 eV

(24)

However, as discussed in section 4.1, the excess energy present in ionization driven reactions make the products more prone to dissociation. It appears that reactions driven by neutral dissociation (9–14) are dominant in the lower energy range. From the CH3OH and HCHO yields shown in Fig. 7, additional pathways may only become relevant above 30 eV, as yields are almost constant till 30 eV. From the reported mass spectra of C2H6, HCHO and CH3OH (Figs. 3(b), 6(b) and (c)) and their electron impact total ionization cross–sections (TICS), we can calculate their partial ionization cross–sections. Using this information, the relative yields of the three products can be calculated at each energy. The respective TICS of C2H6, HCHO and CH3OH at 70 eV are 6.7 Å2 71

, 5 Å2

72

and 4.07 Å2

73

. We find that the relative yields of C2H6 and HCHO with respect to

CH3OH varies between 15–20 times and 2–3 times, respectively, as a function of energy, in our experiments. It is well known that oxidation of an alkane occurs in steps, namely, alkane to alcohol, alcohol to aldehyde, aldehyde to carboxylic acid. Hence, going by their production ratios, it is possible that CH3OH acts as an intermediate in the production of HCHO under electron impact in the condensed phase. For example, any CH3OH formed in the matrix can further react with an O atom via (25) CH3OH + O → HCHO + H2O, ∆H = – 4.17 eV

(25)

which can yield HCHO. The quantity of C2H6 produced in the mixed films is much larger than that of both CH3OH and HCHO. This could be due to twice the quantity of CH4 compared to O2 present in the ice mixture, which increases the number of reactive fragments generated from


24

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CH4. Larger formation of C2H6 could also imply that the reactivity among CH4 and its fragments is higher than their reactivity with O2 or O. Our observation of the role of neutral radicals in the reactions may be contrasted with earlier electron irradiation experiments on condensed films containing CH4, Wada et al.25. They observed CH3OH as the major and HCHO as the minor product in mixed CH4 and H2O films irradiated by electrons above 30 eV. This was attributed to the direct production of OH radical (from the dissociation of H2O) in their films, which they identified as the main reactive fragment forming CH3OH. Ionization of H2O and CH4 followed by rapid ion–molecule reactions, and neutralization with excess electrons was suggested as the possible route to form the reactive radicals. In the experiments of Huels et al.74, electron stimulated desorption (ESD) of cations from pure CH4 ices yielded two carbon atom containing species like C2Hn+ (n = 2 to 5), whereas mixtures of CH4 and O2 ices yielded cations of the type HnCO+ (n = 1 to 3) with thresholds above 20 eV. It was suggested that cations produced by electron impact ionization initiate the reactions. Thus, both these studies attribute the electron induced reactions to the process of ionization, whereas, our observations suggest that neutral dissociation plays the major role in our reaction scheme.

5. Conclusion In conclusion, we demonstrate that using low energy electrons we can synthesize larger and complex molecules starting from smaller molecules like CH4 and O2. The reactions can be classified as CH4 functionalization since the observed products, namely, ethane, methanol and formaldehyde contain the CH3, OH and C=O functional groups, respectively. The energetic threshold (≈ 8 eV) and dependence of product yield on energy signifies that electron impact excitation and subsequent dissociation of CH4 and O2 into neutral radicals O, CH3 and CH2 is likely the reaction initiating step. While earlier studies25,74 suggested ionization driven


25


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mechanisms for such reactions with much higher thresholds (22 – 30 eV), our observations prove that electronic excitation followed by dissociation can synthesize the products at much lower energies. As mentioned earlier, HCHO and CH3OH are considered as precursors for more complex species in interstellar chemistry and their abundance in the ISM can only be explained by reactions occurring on surfaces of dust grains. LEE induced pathways may play an important role in their formation considering LEEs are present wherever ionizing radiation interacts with matter. Techniques for direct conversion of CH4 to functionalized products, especially HCHO and CH3OH are extremely important and useful, since most conversion processes are indirect and energy–intensive. Since LEE induced reactions do not require any external activation, they may be used as a viable alternative tool. The next step would be to explore the efficacy of electrons in activating the C–H bonds in molecules having different functional groups, since the functional groups are usually more reactive than the C–H bonds.

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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a

NIST database: http://webbook.nist.gov/chemistry/form-ser.html (accessed June 2017)

ACKNOWLEDGMENT We acknowledge Daly Davis for her contribution in building the experiment and Satej Tare and Yogesh Upalekar for various technical help. We also acknowledge Department of Atomic Energy, Govt. of India for financial support. REFERENCES (1)

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Fig. 1: IR spectrum of condensed CH4 as obtained by us. (a) ν4 and (b) ν3 stretch region 57x40mm (300 x 300 DPI)


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Fig. 1: IR spectrum of condensed CH4 as obtained by us. (a) ν4 and (b) ν3 stretch region 57x40mm (300 x 300 DPI)



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Fig. 2: TPD spectra of the fragments with m/z (a) 25, (b) 26, (c) 27, (d) 28, (e) 29 and (f) 30 after irradiation of pristine CH4 film with 10 eV electrons, performed at the rate of 60 K/min. All the TPD curves peak at the same temperature of 130 K. These spectra are obtained after subtracting the corresponding spectra for un–irradiated samples. The spectra have been smoothed by applying FFT filter to the raw data to remove the electrical noise of the RGA. 125x88mm (300 x 300 DPI)


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Fig. 3: The relative intensities of the mass fragments m/z = 25 to 30 (a) as measured by us in the TPD spectra following irradiation of pristine CH4 film with 10 eV electrons and (b) the mass spectrum of C2H6 as reported by NISTa. The intensities are normalized to that of m/z = 28. The spectra compare well indicating that the dominant irradiation product is C2H6 in pure CH4 films. 117x167mm (300 x 300 DPI)



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Fig. 4: C2H6 yield from the TPD spectra following irradiation of a pristine CH4 film as a function of electron energy. The error bars represent the standard deviation of repeated measurements. 56x38mm (300 x 300 DPI)


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Fig. 5: TPD spectra of the fragments with m/z (a) 29, (b) 30 and (c) 31 obtained after irradiation of CH4 and O2 mixed film with three different electron energies (10 eV, 55 eV and 100 eV). The lower temperature peak in (a) and (b) corresponds to desorption of C2H6. The vertical red line at 130 K is to show that the temperature ramp is paused for 10 minutes at 130 K after which it is resumed again. These spectra are obtained after subtracting the corresponding spectra for un–irradiated samples. The spectra have been smoothed by applying FFT filter to the raw data to remove the electrical noise of the RGA. 98x117mm (300 x 300 DPI)



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Fig. 5: TPD spectra of the fragments with m/z (a) 29, (b) 30 and (c) 31 obtained after irradiation of CH4 and O2 mixed film with three different electron energies (10 eV, 55 eV and 100 eV). The lower temperature peak in (a) and (b) corresponds to desorption of C2H6. The vertical red line at 130 K is to show that the temperature ramp is paused for 10 minutes at 130 K after which it is resumed again. These spectra are obtained after subtracting the corresponding spectra for un–irradiated samples. The spectra have been smoothed by applying FFT filter to the raw data to remove the electrical noise of the RGA. 56x38mm (300 x 300 DPI)


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Fig. 6: (a) The relative intensities of the mass fragments m/z = 29 and 30 as measured by us in the TPD spectra following irradiation of mixed CH4 and O2 ice with 10 eV electrons. Electron ionization mass spectrum of (b) HCHO and (c) CH3OH as reported by NISTa. The intensities are normalized to that of m/z = 29 in (a) and (b) and m/z = 31 in (c). 148x268mm (300 x 300 DPI)



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Fig. 7: Yields of (a) HCHO and (b) CH3OH from the TPD spectra following irradiation of mixed CH4 and O2 ice as a function of electron energy (see text). The error bars represent the standard deviation of repeated measurements. 57x40mm (300 x 300 DPI)


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Fig. 7: Yields of (a) HCHO and (b) CH3OH from the TPD spectra following irradiation of mixed CH4 and O2 ice as a function of electron energy (see text). The error bars represent the standard deviation of repeated measurements. 56x38mm (300 x 300 DPI)



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Fig. 8: (a) Cross–sections for electron impact dissociation of gaseous CH4 into neutral radicals measured by various groups. CH277–squares, CH377–circles, CH381– triangles, (b) Cross–sections for electron impact ionization and dissociative ionization of gaseous CH4 from reference 82. CH4+–square, CH3+–circle, CH2+– triangle, CH+–inverted triangle, C+–diamond. 56x38mm (300 x 300 DPI)


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Fig. 8: (a) Cross–sections for electron impact dissociation of gaseous CH4 into neutral radicals measured by various groups. CH277–squares, CH377–circles, CH381– triangles, (b) Cross–sections for electron impact ionization and dissociative ionization of gaseous CH4 from reference 82. CH4+–square, CH3+–circle, CH2+– triangle, CH+–inverted triangle, C+–diamond. 56x38mm (300 x 300 DPI)



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Low Energy Electron Induced CâH Activation Reactions in Methane

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