ACS Earth and Space Chemistry最新文献

Methylamine (CH₃NH₂) has been detected in the interstellar media and is considered an important precursor of prebiotic molecules. In addition to gas-phase processes and energetic processing of ice, two nonenergetic pathways have been proposed for the production of CH₃NH₂ on icy grain surfaces: successive hydrogenation reaction of HCN and CH₃ + NH₂ reaction. In this work, the latter process was experimentally investigated using the Cs⁺ ion pickup method, which allowed us to detect reactants (CH₃ and NH₂) and the product (CH₃NH₂) in situ. The CH₃ and NH₂ radicals were produced on amorphous solid water photolyzed by ultraviolet photons, where OH radicals abstracted H atoms from CH₄ and NH₃, respectively. CH₃NH₂ was produced even at 10 K, most likely as a result of the transient diffusion mechanism, in which the NH₂ radical transiently diffuses a significant distance upon formation to encounter a CH₃ radical. During warm-up of samples, the CH₃NH₂ yield increased between 15–30 K, probably due to the thermal diffusion of CH₃ radicals facilitating the CH₃ + NH₂ → CH₃NH₂ reaction.

Fe oxides frequently exist in systems containing both Fe(II) and Fe(III), where their reactivity is enhanced and where interfacial electron transfer from Fe(II) adsorbed to the solids causes the transformation of metastable Fe oxides. Here, we contribute to the understanding of such a transformation using green rust sulfate (GR) synthesized in the presence or absence of Si or Al as the starting material. X-ray diffraction (XRD) and pair distribution function (PDF) analyses showed that (i) rapid oxidation by Cr(VI) caused transformation to Fe oxyhydroxide with short-range ordering, with a pattern identical to that reported for the oxidation of isolated GR hydroxide sheets (i.e., a trilayer of Fe with both edge- and corner-sharing polyhedra) and (ii) goethite formed at the expense of the short-range-ordered Fe oxyhydroxide when residual Fe(II) was present, particularly when Si was absent. This is consistent with the Fe(II)-catalyzed transformation of the short-range-ordered Fe oxyhydroxide. High-resolution transmission electron microscopy (TEM) showed that the two oxidation products coexisted within individual particles and that particle shape and the crystallographic orientation of both products were inherited from the original GR crystals, i.e., they had formed through topotactic transformation. We interpret that the structural reorganization to goethite occurred either in response to distortions caused by polaron movement or as a result of electron transfer reactions occurring at internal surfaces. Once nucleated, goethite growth can be sustained by dissolution–reprecipitation.

Holmquistite, a Li-rich orthorhombic amphibole with space group Pnma, was investigated under high-pressure conditions by using in situ Raman and Fourier transform infrared (FTIR) spectroscopies to assess its vibrational properties and structural evolution. The results demonstrate that holmquistite maintains its crystallographic integrity up to ∼17.6 GPa. Raman spectroscopy reveals resolvable OH stretching bands corresponding to different cation configurations at the C sites, with pressure-induced splitting suggesting increased differentiation of hydrogen site environments. FTIR spectra exhibit complementary trends and higher resolution in the OH region, supporting the identification of crystallographically distinct hydrogen environments. An additional transient absorption feature observed in the OH region may indicate a possible pressure-sensitive change in dipole orientation. These spectroscopic observations highlight the structural robustness of holmquistite and support the utility of vibrational spectroscopy in evaluating site-specific OH environments in complex amphiboles.

Hydrogen cyanide, HCN, is a fundamental building block in astro- and cosmochemical environments, known for its ability to form prebiotically relevant molecules such as nucleobases. Although its polymerization is inhibited under the cold, dilute conditions of the interstellar medium, the higher temperatures of more evolved rocky bodies, combined with the presence of mineral surfaces, can catalyze the reaction. In this study, we use atomistic simulations grounded on the density functional theory (DFT) to elucidate the complete tetramerization pathway of HCN to diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN), catalyzed by the crystalline Mg₂SiO₄ forsterite (120) surface. Results demonstrate that the intrinsic acid–base properties of the surface facilitate chemical bond formation/cleavage needed for HCN oligomerization, lowering activation barriers by ∼120–220 kJ mol^–1 with respect to the gas-phase. Kinetic analyses reveal that the reactions are feasible at temperatures above 300 K, particularly under conditions present in warm, rocky bodies such as asteroids, meteorites, and planetary surfaces. The presence of water further accelerates key steps by assisting proton transfer processes. These findings support a model in which Mg-rich silicate minerals (abundant in the early Solar System) may have directly catalyzed the formation of complex organic molecules, which, in turn, are precursors of more complex biomolecules, thereby contributing to the essential chemical inventory for the emergence of life on early Earth and other primitive planets with propitious conditions.

Phenolic compounds are some of the most abundant emissions of biomass burning during wildfires. Catechol, the most abundant isomer of benzenediol in biomass burning emissions, undergoes oxidation in the aqueous phase of cloud droplets to form secondary organic aerosol (SOA), including products that absorb light at visible wavelengths, called brown carbon (BrC) chromophores. After cloud evaporation, the remaining submicron SOA particles are susceptible to further oxidant- and light-driven processing. Here, we investigate the multiphase ozone oxidation of the reaction mixture from the aqueous OH-initiated oxidation of catechol, i.e., simulated OH-driven processing in clouds, using a coated-wall flow-tube apparatus. Reactive uptake of ozone was determined for thin films from the OH-driven processing of catechol with and without further irradiation of the thin films, i.e., simulated light-driven processing after cloud evaporation, at low and moderate relative humidity (RH). The experimental time series were reproduced using kinetic multilayer modeling, which, along with qualitative microscopy experiments, provided insights into the diffusivity of these materials. After OH-driven processing, the thin films exhibited uptake coefficients of 2 × 10^–6 and 9 × 10^–6 at 0 and 50% RH, respectively, and 4 h of exposure to 130 ppb of ozone. After OH- and light-driven processing, the uptake coefficients were lower, 2 × 10^–7 at 0% RH and 4 × 10^–6 at 50% RH, for the same ozone exposure. Consequently, the reaction mixture of catechol was plasticized upon uptake of water vapor but vitrified under UV irradiation. Kinetic multilayer modeling shows that slower ozone diffusion at low RH and after light-driven processing can lead to an increase in the atmospheric lifetime of reactive species from less than 1 h to more than a day.

We report the first low-temperature kinetics measurements of an HCCN radical–radical reaction. We measured the bimolecular rate coefficients for the HCCN + O₂ reaction under pseudo-first-order conditions inside an extended Laval nozzle with detection by chirped-pulse Fourier-transform millimeter-wave (CP-FT-mmW) spectroscopy. The extended nozzle in which the reaction is performed is followed by a shock-free secondary expansion to low density and low temperature optimal for detection by rotational spectroscopy. We determined the bimolecular rate as (1.67 ± 0.06) × 10^–11 cm³ molecule^–1 s^–1 at 20 K and (7.59 ± 0.12) × 10^–12 cm³ molecule^–1 s^–1 at 70 K, showing a pronounced negative temperature dependence. The results are discussed in light of recent theoretical calculations and a single previous room-temperature measurement of the reaction rate.

We investigated the reaction of propene (C₃H₆) with the cyano radical (CN) in light of the recent detection of five cyanopropene isomers in TMC-1. To provide reliable branching ratios, we characterized the stationary points on the potential energy surface using the high-accuracy jun-ChS-F12 method. The resulting energetics were then employed to derive temperature-dependent rate constants. Our calculations show that the formation of all observed cyano derivatives is feasible through this reaction, although it is secondary compared to the dominant formation channel of vinyl cyanide (C₂H₃CN). The predicted branching ratios are in good agreement with the observations, with the discrepancies prompting further investigation on the destruction mechanisms of the different isomers. Overall, this work supports a scenario in which these cyano derivatives in TMC-1 arise primarily from pure gas-phase chemistry.

The interplay between crystal surface reactivity and surface topography is crucial for parametrizing reactive transport models, yet the multiscale effects of crystal surface characteristics on reactivity remain insufficiently constrained. Here, we employ power spectral density (PSD) analysis on a data set of 80 sequential surface topographies of a dissolving calcite crystal to statistically evaluate steady-state dissolution under far-from-equilibrium conditions. Despite pronounced microscale variability in the distribution and frequency of topographic building blocks, the overall dissolution rate variability remained bounded within 2–3 orders of magnitude. This stability indicates that while local reactivity is highly heterogeneous, bulk dissolution behavior is comparatively robust. For reactive transport modeling, two key parameter domains emerge: (i) the range of reaction rates and (ii) the local microtopographic variability quantified by the PSD of surface spatial frequencies. We propose that combining these descriptors provides a pathway to parametrize pore-scale reactive transport models with improved predictive capability.

The nanosilicate dust grains in the interstellar medium are mainly composed of stoichiometries that correspond to two of the most dominant mineral phases in Earth’s upper mantle, namely, pyroxene (MgSiO₃)_N and olivine (Mg₂SiO₄)_N (N = 1, 2, ...). Apart from Mg, the possible presence of Fe in these silicate nanoclusters has also been mentioned in multiple investigations. It is well-understood that the presence of Fe (a transition metal), even in small quantities, can significantly impact the physical and chemical properties of the silicate nanoclusters. Hence, using first-principles quantum chemical density functional theory, we have investigated the influence of varying concentrations of Fe on the structural, electronic, magnetic, and spectroscopic properties of olivine and pyroxene nanoclusters with N = 1 to 10. Our calculated formation energies indicate that Fe incorporation leads to an enhancement in the stability of the nanoclusters. Incorporation of Fe also introduces magnetism into the nanosilicates. However, our calculations at 0 K suggest that in the event wherein more than one Fe is present in the cluster, ferromagnetism and antiferromagnetism may compete, leading to a drastic reduction in the net magnetic moment of the nanocluster. The calculated infrared (IR) spectra show that in general for the 10 μm peak, Fe doping creates a blue-shift. Our theoretically obtained comprehensive data set may be scaled and appropriately implemented (by taking note of contributions from larger silicate grains) to make a gross estimate of the Fe concentration in different regions of the cold interstellar medium.

Green rust (GR) is a mixed-valent Fe-layered double hydroxide (Fe(II)–Fe(III)-LDH) mineral that is prevalent in reducing geochemical environments, where it exhibits high sorption and redox reactivity. Here, we examined the morphology and chemistry of individual GR particles with nanoscale resolution in the presence and absence of Ni(II)_aq at reaction times between 1 h and 3 months using scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDXS). During the first day of reaction, sorbed Ni(II) accumulated alongside Fe in ∼10 nm thick rims around the GR particle edges, consistent with the formation of mixed Ni(II)/Fe(II)–Fe(III)-LDH. After 3 months, the rims were thinner and contained less Ni(II) despite a doubling of the sorbed load, suggesting sequestration of Ni(II) sorbates into the bulk during aging. Sequential extractions similarly provided evidence for declining levels of sorbed Ni(II) at the GR surface with time, concurrent with increasing levels inside the mineral bulk. The combined STEM-EDXS and extraction results demonstrate chemical and structural variations across GR particle surfaces and redistribution of Ni(II) sorbates during aging over time scales of days–weeks. These findings provide new mechanistic insights into the processes controlling the partitioning and mobility of trace metals in reducing geochemical systems.