Article pubs.acs.org/JPCC
NMR and EPR Studies of Free-Radical Intermediates from Experiments Mimicking the Winds on Mars: A Sink for Methane and Other Gases Hans J. Jakobsen,*,† Likai Song,§ Zhehong Gan,§ Ivan Hung,§ Henrik Bildsøe,† Jørgen Skibsted,† Ebbe N. Bak,‡ Kai Finster,‡ Per Nørnberg,‡ and Svend J. Knak Jensen† †
Danish Instrument Centre for Solid-State NMR Spectroscopy, Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, and ‡Department of Bioscience, Aarhus University, DK-8000 Aarhus C, Denmark § National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ABSTRACT: A new kind of solid−gas chemical reactions has been investigated using solid-state powder 2H, 13C, and 29Si NMR and EPR spectroscopies. These studies involve reactions between a silicate-created Si free-radical intermediate and a few ordinary gases such as isotopically 2H-, 13C-, and 17O-enriched methane (13CH4 and CD4), carbon dioxide (13CO2), hydrogen (2H2), and oxygen (17O2). The solid-state Si free-radical intermediate and gas reaction and silicate products are formed in a specially designed rotating apparatus, which by mechanical tumbling mimics the winds and collision speed of the mineral particles on Mars. It is shown that the “hard” quartz (SiO2) or corundum (α-Al2O3) grain particles, used to simulate the collision particles in a rotating Pyrex (borosilicate) reaction flask, act as an abrasive on the “soft” Pyrex flask and thereby create a silicate Si free-radical intermediate. EPR studies show that this radical is identical to the silicate radical intensively investigated by EPR from γirradiation of Pyrex and other glasses and shown to constitute a submicrostructure of either silicate chains or helices. The intensity of the silicate Si free-radical EPR signal for the reaction product is strongly reduced or even disappears by performing the tumbling of the abrasive grain particles in an atmosphere of methane or other gases. 13C{1H} and 29Si{1H} CP/MAS NMR experiments of the reaction product with methane gas show the presence of one-bond CH3−Si≡ and HO−Si≡ covalent bonds in its structure and that sieving the product leads to a sensitivity enhancement by a factor of ∼25. A flexible helical structure for the silicate Si free-radical intermediate is indicated by the preliminary results for the product resulting from the reaction with 13CO2 (encapsulation of the gas) and the indication of a congested methyl group in the product from reaction with methane.
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INTRODUCTION A recent investigation1 reported terrestrial experiments, which mimic the wind erosion of Si-containing minerals on Mars using an appropriate tumbling frequency of a Pyrex-glass apparatus setup. The experiments demonstrated that the wind action could mediate a reaction between Si-containing minerals and an atmosphere of methane (CH4). NMR experiments established the formation of a solid material containing both one-bond CH3−Si≡ and HO−Si≡ covalent bonds. This observation led to a proposal of a sink causing the disappearance of methane on Mars.1 As stated in that original study, methane has been observed in the atmosphere of Mars in the past 10−15 years (from a satellite orbiting the planet2 and from terrestrial telescopes3), which has caused speculations on its origin (methanogenesis,4 serpentinization,5 volcanism,6 or release from clathrates7). Moreover, the rapid methane disappearance after a methane plume eruption8,9 is unanticipated since comparable terrestrial processes8 would predict very much longer lifetimes. Therefore, difficulties in establishing appropriate mechanisms,10 which could account for the © 2016 American Chemical Society
rapid disappearance of the episodic and local releases of methane on Mars,8,9,11−13 have prompted the recent1 and ongoing research in this field. After submission of the original report,1 in situ measurements by the Curiosity team in the Gale crater on Mars found very little evidence, if any at all, for the presence of methane at this location (∼0.18 ± 0.67 ppbv).14 This observation led to the following statement by Webster et al.:14 “The very short lifetime of 0.4 to 4 years derived from the 2003−2006 observations requires powerful destruction mechanisms that have not been identified to date”. However, from new measurements in the Gale crater, Webster et al.15 recently reported that four sequential measurements during a period of 60 Martian days showed an increased level of methane of ∼7.2 ± 2.1 ppbv, i.e., a factor ∼40 increase compared to their twoyear earlier report. These measurements by Webster et al.14,15 on the detection and variability of methane at the Gale crater Received: September 1, 2016 Revised: October 25, 2016 Published: October 26, 2016 26138
DOI: 10.1021/acs.jpcc.6b08847 J. Phys. Chem. C 2016, 120, 26138−26149
Article
The Journal of Physical Chemistry C really illustrate that Mars is episodically producing methane from unknown sources on this planet. Thus, we agree that powerful destruction mechanisms (“sinks”) are required. Such a mechanism can be fully understood by the methane sink we recently proposed1 and which has now been further justified by a series of recent experiments. These experiments were intended to throw light on the reaction intermediates and products resulting from the overall tumbling process reported here. These new investigations on the reaction process, not considered previously in detail,1 have focused on (i) sample handling/processing, (ii) the material of the tumbling glass flask, (iii) reactions with other gases than CH4, and finally and most importantly (iv) characterization of the reaction intermediates and reaction products using solid-state NMR and EPR spectroscopies. The present article reports new preliminary experimental results that provide insight into the actual reaction happening in the tumbling apparatus/process and into the structure of the 13 CH3−Si≡/HO−Si≡ material recently isolated.1 Furthermore, these new results make our proposal, “A sink for methane on Mars? The answer is blowing in the wind”,1 much more plausible with respect to the actual different possible Si minerals present on Mars. Most importantly, the new result of a freeradical induced reaction from the tumbling experiments as observed from EPR could also contribute to a better understanding of several environmental issues taking place on planet Earth, such as environmental health problems, related to similar free-radical reactions. Specifically, these may include silicosis/asbestosis related to (i) workers being exposed to fine free-radical dust particles created by cutting/grinding asbestos or related Si-containing materials or (ii) the effect of wind erosions of hard materials (e.g., quartz sand) on Si-containing minerals in desert regions of the world.
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Figure 1. Tumbling apparatus used in the present research. Six separate compartments in the rotating wheel are able each to hold two large sample containers (length of 20 cm) as described in ref 1. In addition, the rotational frequency and the possibility of gas-pressure measurements within the individual sample containers during tumbling can be digitally controlled.
typical saltation collision speed close to threshold.1 The tumbling apparatus holds six separate compartments in the rotating wheel (Figure 1), each of which are able to accommodate two large reaction flasks (20 cm in length1), i.e., a maximum of 12 flasks per experiment. Following tumbling, the flasks from our initial experiments were all opened/handled at room temperature (RT) in an ordinary air atmosphere (STP) for the solid-state NMR experiments, whereas some of the more recent samples for the EPR experiments were opened, sieved, packed, and sealed in 3 mm o.d. quartz EPR sample tubes in a glovebox with an argon (Ar) atmosphere. Solid-State MAS NMR Spectroscopy. 13C{1H} and 29Si{1H} CP/MAS NMR experiments were performed at Aarhus University on a Varian INOVA-300 narrow-bore 7.05 T spectrometer at 75.43 and 59.59 MHz, respectively, using a home-built 5 mm CP/MAS probe and 5 mm o.d. zirconia rotors (∼110 μL sample volume). The spinning frequency νr varied for the different samples and for the kind of experiments performed but were generally in the range νr = 3−5 kHz. The 13 C{1H} CP/MAS NMR experiments used νr = 5.0 kHz, rf field strengths of γB1/2π ≈ γB2/2π = 60 kHz for 13C and 1H, respectively, during the CP contact time, γB2/2π = 100 kHz for the initial 90° 1H pulse and 1H TPPM decoupling, and a relaxation delay of 4 s. The 29Si{1H} CP/MAS experiments employed νr = 4.0 kHz, γB1/2π ≈ γB2/2π = 50 kHz for 29Si and 1 H during the CP contact time, γB2/2π = 100 kHz for the initial 90° 1H pulse and 1H TPPM decoupling, and a relaxation delay of 4 s. A single selected 29Si{1H} CP/MAS experiment on this 7.05 T spectrometer used a home-built 7 mm CP/MAS probe employing quite similar experimental conditions as described in the text and figure caption for this sample. Also, the single proton decoupled 13C{1H} MAS spectrum presented for the preliminary 13CO2 reaction product was obtained at 14.1 T as described in the figure caption for this reaction product
EXPERIMENTAL SECTION
Materials and Synthesis. The isotope-enriched gas samples used here were purchased from either Cambridge Isotope Laboratories (CIL), USA, or CortecNet, France. The tumbling apparatus shown in Figure 1 is used to simulate the wind erosion of surface materials on Mars by tumbling of the Pyrex glass (borosilicate, Simax 3.3) reaction flask depicted in Figure 1 of ref 1. A few tumbling experiments were also carried out using a quartz (SiO2) glass flask. The flasks were loaded with about 10 g of SiO2 grains, and most recently grains of αAl2O3 (corundum) have been used to elucidate the possible effect of SiO2 and α-Al2O3 acting as an abrasive on the glass flasks. These grains of SiO2 and α-Al2O3 have both been sieved to extract the fractions between 125 and 1000 μm. These fractions were ultrasonically treated and washed several times with water and finally dried before being loaded into the glass flasks. 13C-enriched methane (13CH4, >99% enrichment) is the main gas investigated in the tumbling experiments of the present work along with its 12C/2H isotopomer 12CD4 (>99% D-enrichment). However, preliminary results are also reported for some other gases: D2 (>99% D-enrichment), 13CO2 (>99% 13 C-enrichment), and 17O2 (70% 17O-enrichment). The glass flasks containing about 10 g of grain material were evacuated and thereafter loaded with the appropriate gas to a pressure in the range ∼500−700 mbar. The reaction flasks were then tumbled end-over-end at 30 rpm (corresponding to a rotational frequency of 0.5 Hz) for 3−7 months. Thus, the collision speed of the SiO2 grains as they tumble is about 1 m/s, which is a 26139
DOI: 10.1021/acs.jpcc.6b08847 J. Phys. Chem. C 2016, 120, 26138−26149
Article
The Journal of Physical Chemistry C (caption to Figure 9). 13C and 29Si chemical shifts are relative to neat liquid TMS, (CH3)4Si for the two nuclei. As described in Results and Discussion section, we found that sieving the isolated reaction products, using a sieve
