Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars’ atmosphere

Abstract

We report gas-phase reactions of free iron-oxide clusters, Fe_nO_m⁺, and their Ar adducts with methane in the context of chemical processes in Mars’ atmosphere. Methane activation was observed to produce Fe_nO_mCH₂⁺/Fe_nO_mCD₂⁺ and Fe_nO_mC⁺, where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe₃O⁺ and Fe₄O⁺ were estimated to be 1 × 10⁻¹³ and 3 × 10⁻¹³ cm³ s⁻¹, respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 10⁷ cm⁻³ in Mars’ atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.

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1 Introduction

Clusters of metals and metal compounds are a group of substances with a variety of potential applications taking advantage of their size-specific properties. Among the numerous cluster studies, one of the most intriguing topics is cluster chemistry in space. Small mineral particles such as silicate (e.g., pyroxene (Mg, Fe)SiO₃ and olivine (Mg, Fe)₂SiO₄) are among the most abundant materials in space.^1,2 It is a prevalent hypothesis that such materials contribute to chemical processes in the planet-forming regions, e.g., formation of organic molecules.³ Size-selected gas-phase clusters provide a good model for this chemistry because it is possible to investigate reactions step by step with precise control over the number of atoms and molecules involved in the reaction,^4–6 providing molecular-level insights into metal- and metal compound-mediated catalytic reactions.⁷ Furthermore, spectroscopic studies in laboratories have been reported for silica and silicate clusters,^8–10 which were recently suggested to be highly abundant in the interstellar medium.^8,9 Therefore, clusters themselves may play important roles in chemical processes in space. In this context, we have reported reactions of gas-phase free silicate, Mg_lSiO_m⁻, and silica, Si_nO_m⁻, cluster anions with CO and H₂O molecules.^11,12 In addition, coadsorption and subsequent reaction of CO and H₂ molecules on cobalt cluster cations, Co_n⁺, have been examined to discuss the formation of organic molecules on the clusters.¹³

Another chemical process among recent topics in space is rapid methane loss in the atmosphere of Mars.¹⁴ Observation by the Curiosity rover found temporary spikes of methane and its rapid loss, but the mechanism of the loss has not been elucidated yet. Although some previous studies suggested a very local, small methane source near the landing site of the Curiosity rover,^15,16 an alternative possibility is the effective loss of methane via chemical reaction that is not considered in photochemical models. Mars’ soil is rich in iron oxides, where storms of iron-oxide particles (dust devils) occur very frequently.¹⁷ Hypervelocity impacts of small bodies on Mars would have also evaporated or made molten the surface materials.¹⁸ These surface processes may have formed very fine iron-bearing condensates on Mars. We thus hypothesize that iron-oxide particles/clusters, which are speculated to be abundant on Mars, are responsible for the rapid loss because the diiron-oxide cluster, Fe₂O₂, is known as an active center of methane monooxygenase (sMMO),^19–23 an enzyme that oxidizes C–H bonds in methane.

The C–H dissociation energy of methane is much larger than those of other alkanes as can be expected from its short bond length and the large HOMO–LUMO gap.²⁴ Numerous theoretical studies on the interaction of methane with model complexes involving a diiron-oxide core have been reported because methane activation is receiving considerable interest in modern chemistry.^25–36 These theoretical studies have revealed that the diiron-oxide core in sMMO plays an important role in C–H bond cleavage. Several mechanisms have been proposed for the hydroxylation of CH₄,^24,34,35 such as a mechanism via a CH₃ radical,^28,29,31 one not via radicals^26,27,33,35 and a non-synchronous concerted one.³²

As for experimental studies, gas-phase free FeO⁺ was reported to mediate the activation of methane.^37–41 Hydroxylation of methane by FeO⁺ was suggested to occur via a hydroxy intermediate HO–Fe⁺–CH₃.^37,38 This reaction mechanism has been supported by theoretical studies, where the pathway via the hydroxy intermediate is energetically more favorable than the one via a methoxy intermediate H–Fe⁺–OCH₃.^42,43 The methoxy intermediate was investigated experimentally by vibrational spectroscopy.⁴⁴ The reaction was also reported to be accompanied by spin inversion.^45–49 An activation mechanism of methane by neutral FeO and anionic FeO⁻ as well as cationic FeO⁺ was theoretically reported recently.⁵⁰

As for multinuclear species, an iron-oxide cluster, Fe₂O₃,⁵¹ as well as iron clusters, Fe₂,⁵² Fe₂⁺⁵³ and Fe₄,⁵⁴ has been reported to show methane activation by theory. Furthermore, a number of theoretical studies present geometric and electronic structures of iron-oxide clusters such as FeO⁺,^55,56 FeO₂⁻,⁵⁷ FeO₂^+/0/−,⁵⁸ FeO_n^+/−,⁵⁹ Fe_nO_m⁻ (n = 3, 4),⁶⁰ Fe_nO_m⁻ (n = 1, 2, m ≤ 6),⁶¹ Fe_nO_m (n = 1–5),⁶² (FeO)_n (n = 2–5),⁶³ (FeO)_n (n >10),⁶⁴ (Fe₂O₃)_n (n = 1–6, 10),^65–67 (Fe₃O₄)_n (n = 1–5),⁶⁸ Fe_nO_m⁺ (n = 3–6),⁶⁹ Fe_nO_m (n = 33, 45, 113),⁷⁰ Fe₃O₁₈,^71–73 Fe₂₅O₃₀ and Fe₃₃O₃₂.⁷³ However, in contrast to these extensive theoretical studies, experimental reports are limited: geometric-structure studies of (FeO)_n⁺ (n = 2–9) by ion mobility mass spectrometry⁷⁴ and stability studies by photodissociation,⁷⁵ collision-induced dissociation⁷⁶ and mass spectrometry.^65,71 In particular, reaction with methane has been examined by experiment only for Fe₂O₂⁺.⁷⁷ Note that, because methane activation has been attracting strong attention, a significant number of methane-reaction experiments have been reported for metal and metal-compound clusters, including Fe_n⁺ (n = 2–15),⁷⁸ Ni_n⁺ (n = 2–15),⁷⁹ Rh_n⁺,⁸⁰ Pd_n⁺,^81–83 Ta_n⁺,⁸⁴ Au₂⁺,^81,82,85,86 FeC₆⁻,⁸⁷ W_nC_m⁺ (n =1–5, m ≤ 5),⁸⁸ W_nN_m⁺ (n = 1–6, m ≤ 2),⁸⁸ Al₂O₇⁺,⁸⁹ Al₄O₆⁺ and NiAl₃O₆⁺,⁹⁰ as well as vibrational spectroscopy of methane complexes with Fe_n⁺,^91–93 Cu_n⁺,⁹⁴ Ta₄⁺,⁹⁵ Pt_n⁺⁹⁶ and Au₂⁺.⁹⁷

On the basis of these backgrounds, the present study reports gas-phase reaction of size-selected iron-oxide cluster cations, Fe_nO_m⁺, with methane, CH₄, and deuterated methane, CD₄, to verify our hypothesis that iron-oxide particles/clusters are responsible for the rapid loss of methane in Mars’ atmosphere.

2 Methods

Experiments were performed using the setup described in detail previously.^98,99 Briefly, Fe_nO_m⁺ (n = 2–4, 0 ≤ m ≤ n) were generated by a magnetron-sputter cluster-ion source,¹⁰⁰ with sputtering of an iron plate (Kojundo Chem. Lab. Co., Ltd, 99.99%) in the presence of oxygen (99.99995%). After thermalization by collisions with helium gas at liquid-nitrogen temperature, the generated cluster cations were mass-selected with a quadrupole mass filter (MAX-4000, Extrel CMS, LLC); among the isotopologues for a given composition, the one with the highest abundance was selected. Note that the reactant cluster cations thus mass-selected contained a trace amount of an Ar adduct Fe_n−1O_m+1Ar⁺ as well, which has the same mass as Fe_nO_m⁺. The cluster cations were introduced into a reaction cell with a linear 40-cm-long rf quadrupole ion guide, where methane (CH₄ or deuterated CD₄) gas was continuously flowed at 298 K. Ions produced by reaction while passing through the cell were identified by a second quadrupole mass analyzer (MAX-4000, Extrel CMS, LLC) employing a channel electron multiplier (Channeltron 4720, Burle Electro-Optics, Inc.) for an ion detector to measure the yield of each product ion.

The partial pressure of CH₄ and CD₄ in the reaction cell was adjusted to be 0.1 Pa; the pressure was monitored outside the reaction cell by a residual gas analyzer (RGA100, Stanford Research Systems, Inc.) and was converted to the pressure inside the reaction cell in a way reported previously by referring to the reaction cross section of Cr₂⁺ with O₂.^101,102 The pressure corresponds to the collision rate of 2 × 10⁴ s⁻¹, where the collisional cross section is estimated by the Langevin–Gioumousis–Stevenson model,¹⁰³

σ = πe{2α/(4πε₀)²E}^1/2 (1)

Here, α is the polarizability of methane (2.56 ų), e is the elementary charge, ε₀ is the vacuum dielectric constant, k is Boltzmann's constant, T is the temperature and E is the collision energy in the center-of-mass frame. The collision energy of the clusters with a methane molecule was 12 eV in the laboratory frame.

3 Results and discussion

3.1 Reaction products

Fig. 1 shows mass spectra of ions upon reaction of Fe₃O_m⁺ (m = 0–3) with deuterated methane, CD₄; note that Fe₂O_m+1Ar⁺, which is coincident in mass with Fe₃O_m⁺, coexists in the reactant in our present experimental conditions. For m = 0, 1 and 3, mass spectra of ions upon reaction with CH₄ are also shown (panels a′, b′ and d′). The abscissa shows a mass shift, ΔM, from Fe₃O_m⁺; the peaks observed at ΔM ≥ 0 are assigned to ions produced from Fe₃O_m⁺, whereas those at ΔM < 0 are from Fe₂O_m+1Ar⁺ subject to desorption of an Ar atom. Note also that the mass spectra were not recorded in the vicinity of ΔM = 0 because the intensity of the reactant ion was much higher than those of product ions, which caused saturation in the ion detector; the peak at ΔM = −40, which should have been observed in panels a, a′, b and b′, was also excluded from recording by the same reason.

Fig. 1 Mass spectra of ions produced from the reaction of Fe₃O_m⁺ and Fe₂O_m+1Ar⁺ with (a)–(d) CD₄ or (a′, b′ and d′) CH₄. Reactant ions are m = 0, 1, 2 and 3 for (a) and (a′), (b) and (b′), (c) and (d) and (d′), respectively. The abscissa shows a mass shift, ΔM, from Fe₃O_m⁺. Peaks are assigned to adducts to Fe₃O_m⁺ in the ΔM ≥ 0 region, while adducts to Fe₂O_m+1⁺ after desorption of Ar in the ΔM < 0 region. Note that the following ions are excluded from recording due to saturation: Fe₃⁺ and Fe₂O⁺ in panels (a) and (a′), Fe₃O⁺ and Fe₂O₂⁺ in (b) and (b′), Fe₃O₂⁺ in (c) and Fe₃O₃⁺ in (d) and (d′).

For the ΔM ≥ 0 region manifesting reaction channels from Fe₃O_m⁺, peaks at ΔM = 12 and 16 are observed upon reaction with CD₄ as shown in Fig. 1a–d (m = 0–3), while ΔM = 12 and 14 are observed for CH₄ as shown in Fig. 1a′, b′ and d′ (m = 0, 1 and 3). These results suggest that Fe₃O_mC⁺ and Fe₃O_mCD₂⁺/Fe₃O_mCH₂⁺ are produced upon reaction, i.e., dehydrogenation from methane proceeds. Mass spectra upon the reaction of Fe₂O_m⁺ (m = 0–2) and Fe₄O_m⁺ (m = 0–3) with CD₄ are shown in the ΔM ≥ 0 region of Fig. 2, where methane activation to produce Fe_nO_mC⁺ and/or Fe_nO_mCD₂⁺ is observed as in the case of Fe₃O_m⁺. Note that Fe₂O⁺ and Fe₂O₂⁺ are exceptions; reaction products are hardly identified. In the reaction of Fe₄O₂⁺ and Fe₄O₃⁺, partially dehydrogenated products, Fe₄O_mCD₃⁺, are observed.

Fig. 2 Mass spectra of ions produced from the reaction of Fe_nO_m⁺ and Fe_n−1O_m+1Ar⁺ (n = 2 and 4) with CD₄. Reactant ions are (a)–(c) n = 2, m = 0–2; (d)–(g) n = 4, m = 0–3. The abscissa shows a mass shift, ΔM, from Fe_nO_m⁺. Peaks are assigned to adducts to Fe_nO_m⁺ in the ΔM ≥ 0 region, while adducts to Fe_n−1O_m+1⁺ after desorption of Ar in the ΔM < 0 region. Note that the following ions are excluded from recording due to saturation: Fe₂⁺ in panel (a), Fe₂O⁺ and FeO₂⁺ in (b), Fe₂O₂⁺ in (c), Fe₄⁺ in (d), Fe₄O⁺ in (e), Fe₄O₂⁺ in (f), and Fe₄O₃⁺ in (g). Several peaks marked by the value of ΔM are not simply assignable, originating probably from reactant ions containing more than two Ar atoms.

As for the ΔM < 0 region in Fig. 1, product ions from Fe₂O_m+1Ar⁺ are observed, where the major reaction channel is methane adsorption without dehydrogenation. For example, Fe₂O_m+1CD₄⁺ along with a small amount of Fe₂O_m+1C⁺ is observed in the reaction of Fe₂O_m+1Ar⁺ (m = 0, 1), as shown in Fig. 1a and b. Adsorption of CD₄ is accompanied by desorption of an Ar atom to form Fe₂O_m+1CD₄⁺. This is in contrast to the fact that no products are observed in the reaction of Fe₂O⁺ and Fe₂O₂⁺ (see Fig. 2b and c). The results suggest that the adsorption energy of CD₄ was released by the desorption of Ar, which stabilized the CD₄ adducts to survive longer. The peak at ΔM = −20 observed in Fig. 2a and b is also assigned to CD₄ adducts accompanied by Ar desorption from FeOAr⁺ and FeO₂Ar⁺. In particular, the result of FeOAr⁺ is in contrast to a previous study, which reported that the reaction of FeO⁺ with CD₄ proceeds along two reaction channels: either to FeOD⁺ + CD₃ or to Fe⁺ + CD₃OD.³⁷ This would also be due to stabilization of the CD₄ adduct by desorption of Ar as discussed in the previous reaction study of Rh_n⁺Ar_m with CH₄;⁸⁰ it was reported that the CH₄ adducts do not have a sufficient energy for activation of the C–H bond due to energy release by ligand exchange.

3.2 Rate coefficients of methane activation

The reaction-rate coefficients to produce the ions described above were estimated in the same way as reported previously¹² by assuming pseudo-first-order reaction kinetics because the density of methane can be treated as constant during the reaction. However, the peak intensity of the reactant is necessary to estimate the rate coefficients, which is missing in the present measurement due to saturation. We therefore performed additional recording of mass spectra with lower sensitivity of the ion detector, where only major peaks, such as those at ΔM = 0 and −40, were observed (see Fig. S1 of the ESI†). The rate coefficients were estimated by combining the relative intensities of the reactant and product ions obtained in this low-sensitivity mode with the intensity ratios of the product ions in Fig. 1 and 2 obtained in a high-sensitivity mode.

Fig. 3 shows rate coefficients thus evaluated for methane dehydrogenation of Fe_nO_m⁺ in the size range of n = 2–4, which summed up all the reaction channels to the formation of C, CD₂ and CD₃ adducts. The result suggests that larger and oxygen-poor clusters tend to exhibit higher reactivity. For example, the rate coefficients of O-free iron clusters, Fe₃⁺ and Fe₄⁺, are estimated to be 3 × 10⁻¹² and 4 × 10⁻¹² cm³ s⁻¹, respectively. These values are consistent with the previous report;⁷⁸ note that the collision energies in the present experimental conditions for Fe₃⁺ and Fe₄⁺ are 1.0 and 0.8 eV in the center-of-mass frame, respectively. On the other hand, the present results estimate that the rate coefficients of monoxide clusters, Fe₃O⁺ and Fe₄O⁺, are 1 × 10⁻¹³ and 3 × 10⁻¹³ cm³ s⁻¹, respectively; these values are an order of magnitude smaller than those of O-free iron clusters. The rate coefficients of Fe₃O₂⁺, Fe₃O₂⁺, Fe₄O₂⁺ and Fe₄O₃⁺, are even smaller by an order of magnitude than those of monoxide clusters. The extremely low reactivity of Fe₂O₂⁺ is consistent with the previous report.⁷⁷ The rate coefficients obtained for CH₄ are shown in Fig. S2 of the ESI,† which are in the same order of magnitude as those for CD₄.

Fig. 3 Rate coefficients of activation of CD₄ by Fe_nO_m⁺ as a function of the numbers of iron, n, and oxygen atoms, m. White pixels at (n,m) = (2,1) and (2,2) indicate the compositions of the clusters, for which dehydrogenation of methane was not observed. The hatched pixel at (n,m) = (2,3) represents an unexamined composition.

The rate coefficients of Fe_nO_mAr⁺ obtained from the ΔM < 0 region are shown in Fig. S3 of the ESI,† which were examined for CD₄; Fig. S3a and b (ESI†) show rate coefficients of dehydrogenation and those of CD₄ adsorption, respectively. The rate coefficients of CD₄ adsorption are about two orders of magnitude larger than those of the corresponding compositions shown in Fig. 3. This would be due to the effect of ligand exchange as discussed in Section 3.1. The rate coefficients of dehydrogenation are also higher than those of the corresponding compositions. This result is also explainable by the effect of ligand exchange as discussed in the previous reaction study of Rh_n⁺Ar_m with CH₄.⁸⁰ The presence of Ar encourages CD₄ adsorption, causing dehydrogenation to proceed efficiently if the cluster–CD₄ complex retains a sufficient energy for methane activation.

3.3 Rapid methane loss in Mars’ atmosphere

As described in Section 3.2, the rate coefficients of dehydrogenation by iron and iron-oxide clusters were obtained from the present reaction experiment. Here we consider the case that methane with an initial concentration of 2 × 10⁹ cm⁻³ decreases by an order of magnitude in seven days (≈6 × 10⁵ s).¹⁴ We also assume pseudo-first-order reaction kinetics by regarding the density of the iron-oxide clusters/particles as constant; we focus here on iron-oxide clusters because Mars’ soil is rich in iron oxides. The pseudo-first-order rate equation, −d[CH₄]/dt = kn[CH₄], is thus employed to estimate the number density n of the iron-oxide cluster from the rate coefficients k of methane dehydrogenation. Based on the value of k = 1 × 10⁻¹³ cm³ s⁻¹ obtained for Fe₃O⁺ in the present study, we estimate that the presence of iron-oxide clusters/particles of 4 × 10⁷ cm⁻³ (≈10⁻⁷ Pa at −50 °C) would explain the mysterious loss of methane in Mars’ atmosphere.

4 Conclusions

Gas-phase reaction experiments were performed on size-selected free iron and iron-oxide cluster cations, Fe_nO_m⁺ (n = 2–4, 0 ≤ m ≤ n), with methane. Dehydrogenation of methane was observed; larger and oxygen-poor clusters tend to exhibit higher reactivity. We also revealed that the presence of Ar as a weakly bound ligand promotes the adsorption and dehydrogenation of methane. Based on the rate coefficient of methane dehydrogenation determined in the present study, e.g., 1 × 10⁻¹³ cm³ s⁻¹ for Fe₃O⁺, it is speculated that the presence of such iron-oxide clusters/particles in Mars’ atmosphere at a density as low as 4 × 10⁷ cm⁻³ (≈10⁻⁷ Pa at −50 °C) is sufficient to explain the rapid loss of methane observed recently by the Curiosity rover. If ligand exchange with, for example, Ar were plausible, the required cluster/particle density could be even lower by about two orders of magnitude. The chemical process mediated by iron-oxide clusters/particles may play important roles in chemistry not only on Mars but also widely in space because most iron exists as compounds, typically iron oxides.¹⁰⁴

Author contributions

M. A. contributed to conceptualization, investigation, formal analysis, writing – original draft, funding acquisition and project administration. S. K. contributed to investigation and formal analysis. Y. S. contributed to conceptualization and writing – review & editing. A. T. contributed to writing – review & editing and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The present study was supported by Grants-in-Aid for Scientific Research on Innovative Areas (JP17H06456) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and for Scientific Research (B) (JP22H01288) from the Japan Society for Promotion of Science (JSPS). This work was also supported by the GIMRT Program of the Institute for Materials Research, Tohoku University (Proposal No. 202212-CRKKE-0013 and 202212-SCKXX-0009).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp01337a

‡ Present address: Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. E-mail: arakawa@geo.kyushu-u.ac.jp

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