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Konstantinos GIAPIS, et al.
Oxygen from Kinetic CO2
Oxygen from Kinetic CO2
https://www.nature.com/articles/s41467-019-10342-6
Nature Communications 10, Article number: 2294 (2019)
DOI: 10.1038/s41467-019-10342-6
Direct dioxygen evolution in collisions of
carbon dioxide with surfaces
Yunxi Yao, Philip Shushkov, Thomas F. Miller III & Konstantinos P. Giapis
AbstractYunxi Yao, Philip Shushkov, Thomas F. Miller III & Konstantinos P. Giapis
The intramolecular conversion of CO2 to molecular oxygen is an exotic reaction, rarely observed even with extreme optical or electronic excitation means. Here we show that this reaction occurs readily when CO2 ions scatter from solid surfaces in a two-step sequential collision process at hyperthermal incidence energies. The produced O2 is preferentially ionized by charge transfer from the surface over the predominant atomic oxygen product, leading to direct detection of both O2+ and O2-. First-principles simulations of the collisional dynamics reveal that O2 production proceeds via strongly-bent CO2 configurations, without visiting other intermediates. Bent CO2 provides dynamic access to the symmetric dissociation of CO2 to C+O2 with a calculated yield of 1 to 2% depending on molecular orientation. This unexpected collision-induced transformation of individual CO2 molecules provides an accessible pathway for generating O2 in astrophysical environments and may inspire plasma-driven electro- and photo-catalytic strategies for terrestrial CO2 reduction.
Introduction
Although plentiful in modern Earth’s atmosphere, molecular oxygen is extremely rare in space. Only trace amounts have been found elsewhere in our solar system1,2,3 and in interstellar clouds4,5. The recent discovery of abundant O2 in the coma of comet 67P/CG6 has rekindled interest in abiotic reactions, occurring in extreme environments, which release O2 from compounds, such as H2O, CO2, CO, silicates, and metal oxides. Such reactions may offer competing explanations for the origin of O2 in comets, in the upper atmosphere of Mars, and in Earth’s prebiotic atmosphere7,8,9. They may also present alternative ways for resource utilization related to space travel, such as generation of O2 from CO2 for making Mars habitable. Finally, new strategies for CO2 activation may be inspired by such reactions.
The dissociation of CO2 proceeds via multiple pathways depending on available energy. The partial dissociation reaction, CO2???CO?+?O (3P or 1D), has the lowest energy requirement (5.43 or 7.56?eV)10; it has been extensively studied in photochemistry and in heterogeneous catalysis under thermal activation conditions11,12. Full dissociation to C?+?O?+?O involves the cleavage of both C–O bonds and requires 16.46?eV. Other pathways may be possible at intermediate energies, such as the exotic reaction: CO2???C(3P)?+?O2(1Sg), which entails extensive intramolecular rearrangement of the CO2 molecule. Calculations have suggested that this reaction proceeds on the ground-state potential energy surface, by first forming a cyclic CO2 intermediate [c-CO2(1A1)], which then rearranges into a collinear COO(1S+) intermediate on its way to dissociation into C?+?O210. The first step in this channel involves bending of the CO2 molecule to bring the two O atoms in close proximity, which requires close to 6?eV of internal energy13.
Although inaccessible by thermal activation, transitions to electronically excited and anionic states of CO2 can bend the molecule as a first step to O2 production. Indeed, pioneering experiments employing VUV photo-excitation14,15,16 and electron attachment17,18 have shown that dissociation of CO2 into C(3P)?+?O2(X3Sg-) is possible, as evidenced by the detection of the complementary atomic C+ or C- fragment. Further confirmation of the exotic pathway, however, remained elusive as neutral or ionized O2 products were not detected. Using ion-beam scattering methods and numerical simulation techniques, we demonstrate here a different way to drive the direct reduction of CO2 to O2 with in situ detection of ionized O2 products. The process involves a previously unknown intramolecular reaction pathway, which occurs in energetic CO2 ion–surface collisions with a surprising lack of dependence on either the nature of the surface or the surface temperature. As such, the reaction may be relevant for astrophysical environments, such as comets, moons, and planets with CO2 atmospheres.
Results
Carbon dioxide scattering experiments and kinematic analysis
We first demonstrate the formation of O2 in hyperthermal CO2+/Au collisions by plotting kinetic energy distributions of three scattered molecular ion products: CO2+, O2+, and O2- for various CO2+ incidence energies (E0). Very weak signal of scattered CO2+ is detected for E0?<?80?eV (Fig. 1a). The CO2+ exit energy peak varies in proportion to E0, thus implying a ballistic or impulsive rebound from the surface and thereby precluding physical sputtering as its origin. Observation of this “dynamic” CO2+ signal is important, not only for proving that some CO2 survives the surface encounter but also for unraveling the collision sequence of the constituent atoms. Strong signal of scattered O2 ions is also observed (Fig.1b, c). The O2+ and O2- exit energies represent a large fraction of the incidence energy (57%) and increase monotonically with E0 over a larger range than scattered CO2+. The O2 ion signal intensity exhibits a maximum at E0?~?100?eV. Above that, only the O2+ distribution develops a shoulder (i.e., exit at ~30?eV) from physical sputtering.
The detection of fast O2 ion products is surprising. Neither sputtering of surface O2 nor O-atom abstraction reactions (Eley–Rideal) can explain their formation, because both mechanisms would produce O2 at much lower exit energies (see the section “Methods”). A remaining possibility to be explored here is dynamic formation of O2 through dissociation of CO2. Dynamic partial and full dissociation of CO2 is in fact consistent with the other detected products, including CO+, CO-, O+, O-, and C+ (Supplementary Fig. 1). The exit energy of the CO+, CO-, and O- fragments varies linearly with incidence energy, consistent with dynamic formation during the surface collision. In contrast, the O+ and C+ peaks show little dependence on E0, suggesting a different origin, i.e., sputtering19. Scattered C+ products appear at E0?>?80?eV, confirming full dissociation.
The presence of dynamically exiting CO2+ ions enables use of kinematics20 to clarify the scattering mechanism. Binary collision theory (BCT) allows calculation of the kinematic factor, defined as the fraction of incident energy retained by a scattered product exiting the surface. In the simplest possible model, CO2+ scatters as a whole molecule, i.e., a hard sphere with atomic mass of 44?Da. Under this assumption, BCT predicts a kinematic factor of 0.6349, which fits the data poorly (Fig. 2a) as may be expected given the quasi-linear nature of the triatomic CO2+ ion21,22. We consider next a kinematic model in which—as for diatomic molecules scattering on metal surfaces23—the leading O atom first collides with a surface Au atom, followed by a second collision of the CO moiety without prompt dissociation of the CO2 molecule. Applying BCT to this sequential-collision model yields a kinematic factor of 0.7870, which agrees very well with the CO2+ exit energy data (Fig. 2a, black line).
The average exit energies for all remaining scattered products are also plotted in Fig. 2a. Potential origins for such species include partial or full dissociation of CO2 and surface sputtering of adsorbed CO2 fragments. While some sputtering is indeed observed at high E0 (>140?eV), kinematic analysis of the exit energy data provides strong evidence for impulsive dissociation of the CO2 molecule24. Assuming delayed fragmentation of the CO2 parent24, the kinematic factors of the CO, O, and (possibly) O2 daughter products can be calculated from energy conservation to be 0.5724, 0.5008, and 0.2862, respectively. These factors are used as fixed slopes in one-parameter fittings of the respective data points (adjustable intercept). We find that the calculated slopes fit the O2±, CO±, and O- ion exit data very well (Fig. 2a lines), indicating that the latter ions are all dissociation products of CO2. On the contrary, the O+ and C+ data are not linear with respect to E0, suggesting formation by other processes.
Velocity analysis for the observed scattered species provides further evidence regarding the collision mechanism. Figure 2b compares the ion distributions of various peaks for E0?=?56.4?eV. The exit velocities of scattered CO+, O2+, O2-, and the slower part of the O- distributions overlap, suggesting a common origin. However, the O- distribution is noticeably broader, extending to higher exit velocities, which suggests alternative formation channels. The O2 ion products exit with velocities lower than CO2+ owing to inelasticity from breaking of chemical bonds and non-resonant surface ionization.
Although the kinematic analysis indicates conclusively that some CO2 scatters intact after a two-step sequential collision of the O and CO moieties, it leaves various aspects of the O2 formation mechanism unresolved. In particular, since the experiment is limited to observing ions, we are unable to assess how much neutral O2 is produced. Moreover, the kinematic analysis cannot shed light on whether O2 is formed via an electronically adiabatic or non-adiabatic mechanism, nor can it disentangle the collision-induced pathways that underlie the exit velocity distributions of the ionic fragments. To address these questions, we next turn to first-principles molecular dynamics (MD) simulations.
MD simulations of carbon dioxide collisions with gold
MD trajectories for the scattering of CO2 on Au(111) are performed in the experimental scattering geometry, with CO2 evolving on the ground singlet potential energy surface under the assumption that incoming CO2+ ions are neutralized before the hard collision. Facile neutralization occurs via resonant electron tunneling24,25,26 from the metal surface to the molecular cation because the molecular level of CO2 (-13.8?eV) lies well within the occupied band of Au (-5.3 to -15.3?eV). Electron transfer from and to the surface is explicitly included in the simulations to also account for ionization of neutral collision products. The calculated exit energies of the products are plotted in Fig. 2c along with linear two-parameter fits. The slopes obtained from this fitting procedure compare very well to those determined from BCT (Fig. 2a). For example, the exiting CO2+ has a calculated slope of 0.713 vs. the experimental value of 0.787. Negligible CO2 is found to survive for E0?>?80?eV, consistent with the lack of experimental signal at these energies. All other calculated slopes agree well with the experimental values; for instance, compare the slope of 0.54?±?0.02 vs. the experimental value of 0.57 for the O2- line. These results indicate broad agreement between the simulations and the scattering kinematics.
The formation of ions detected in the experiment requires surface ionization, which influences the yields of the ionic products. The MD simulations demonstrate a substantial enrichment of O2- ions over O-, resulting from the exponential dependence of the ionization probability on the coupling to the metal surface (Supplementary Fig. 2, red curve), which can reach ~30%, comparable to the experimentally derived yield of 33% (Supplementary Fig. 2, blue curve).
The agreement between experiment and simulations is further demonstrated by comparing the ion exit velocity distributions at E0?=?56.4?eV (Fig. 2d). Although the experimental peak positions appear systematically at somewhat larger velocities than the calculated ones, the distributions agree very well with respect to relative position of the peaks. In particular, both simulations and experiment find the CO+ and O- velocity distributions to be broadened, both find the O2+ and O2- distributions to be similar with the cation exiting slower than the anion, and both find CO2+ to exit with higher velocity than the ionized O2 products. The agreement suggests that the simulations provide a strong foundation for analyzing the reaction mechanism of the direct CO2 conversion to O2.
An ensemble of 20,000 CO2-on-Au collision trajectories were performed for each incidence energy, resulting in a variety of dissociation products, including O2 (Fig. 3a). Prior to the mechanistic ensemble analysis, it is instructive to review one representative trajectory that leads to collisional O2 formation (Fig. 3b). Select configurations are shown as insets, along with the CO2–Au interaction energy, ECO2
–Au, and the CO2 internal energy, ECO2, as a function of time. The incoming CO2 molecule is vibrationally excited (inset I). As the center-of-mass distance to the surface, ZCO2, decreases, the molecule penetrates the repulsive potential wall of the surface and ECO2–Au increases steeply. During this encounter, one of the O atoms of CO2 strikes a surface Au atom, giving rise to the first peak in the ECO2–Au curve (inset II). This collision occurs before ZCO2 reaches a minimum at the apsis point. As the O atom rebounds, the CO moiety collides with a different Au atom, causing a second peak in the ECO2–Au curve (inset III), which occurs after the apsis. As a result of the impulsive energy transfer during the collision, the rebounding CO2 undergoes substantial intramolecular rearrangement portrayed by the bond distance evolution in Fig. 3b. The O–O distance, rO2, decreases while the C–O distances, rCO, simultaneously increase, reaching a point along the trajectory where CO2 acquires a triangular configuration with nearly equal bond lengths (vertical dashed line). This strongly bent CO2 intermediate (inset IV) has a significant amount of internal energy, ECO2, and promptly dissociates to give a free C atom and a vibrationally hot O2 molecule (inset V). The complete CO2 collision trajectory discussed in Fig. 3b can be viewed in the Supplementary Video. The formation of O2 depicted by this representative trajectory proceeds by delayed fragmentation following the two-step sequential collision of CO2 with the surface. This mechanism is consistent with the assumptions of the kinematic model used earlier to explain the experimental data in Fig. 2a, b.
**
The calculated reaction yields of the various collision-induced dissociation channels of CO2 at E0?=?56.4?eV are shown in Fig. 3a. As expected for this low incidence energy, the partial dissociation channel dominates the reaction yield with 73% of all MD trajectories taking that pathway. The complete dissociation channel is second at 16%. A small fraction of the incoming CO2 (6%) survives the collision in correspondence with experimental detection. Approximately 5% of all trajectories lead to the strongly bent intermediate state—the precursor to O2 formation—which is characterized by C–O and O–O bond orders exceeding 0.7. This intermediate state fragments primarily via partial dissociation (51%) followed again by complete dissociation, albeit now with a higher yield (33%). Remarkably, one in eight (13%) of the strongly bent CO2 molecules produces O2. The overall neutral yield of the symmetric dissociation channel, CO2???C?+?O2, amounts to 0.6% at E0?=?56.4?eV. Upon increasing incidence energy, the neutral O2 yield obtained from the ensemble of isotropically oriented incident CO2 molecules reaches 0.8?±?0.2% for E0?~?70?±?15?eV (Fig. 4, blue line). Also it is clear from the figure that the fraction of O2-producing trajectories increases substantially once the strongly bent CO2 intermediate state is reached (Fig. 4, green line) and this fraction peaks at around 13% for E0?~?55?±?10?eV. The smaller total neutral O2 yield results from the small fraction of linear CO2 molecules reaching the strongly bent state (Fig. 4, red line). By preferentially orienting incoming CO2 molecules (axis parallel to the surface), the fraction of O2-producing trajectories increases to ~2% (Fig. 4, dashed blue line) resulting from an increase of the dissociation probability of the strongly bent state to O2 (Fig. 4, dashed green line). These findings suggest that activation of bending and symmetric stretching motion of CO2 prior to the collision may facilitate both the population of the strongly bent state and its dissociation to O2 leading to a significant increase in the total neutral O2 yield.
**figure4
The timescales for bond breaking and formation in the collision-induced dissociation reactions were evaluated for E0?=?56.4?eV and are reported in the inset of Fig. 3a. The average delay times reveal that both partial dissociation and the first C–O bond-breaking event in complete dissociation occur promptly after the apsis. In contrast, the formation of the strongly bent CO2 intermediate state and its fragmentation to O2 occur on a longer timescale, to allow for the significant intramolecular rearrangement that precedes symmetric dissociation. This is again consistent with the assumption of delayed fragmentation used in the kinematic modeling. The second C–O bond breaking in the complete dissociation channel is also delayed, irrespective of the degree of bending of CO2. The different timescales of the collisional reactions explain the widths of the observed exit velocity distributions. For instance, O atoms produced in prompt partial and delayed complete dissociation, have different velocity profiles, giving rise to a considerably broader O- velocity distribution (Fig. 2d). In particular, prompt partial dissociation involves direct scattering of O atoms from the much heavier Au target, producing faster O-atom exits owing to inefficient momentum transfer. On the other hand, the second C–O bond breaking involves dissociation of the more massive, recoiling CO moiety of CO2, which gives off slower O atoms (Supplementary Fig. 3). Moreover, the narrow velocity profiles of the molecular O2 ions stem from CO2 scattering as a whole molecule, which breaks apart unimolecularly during the rebound from the surface.
Discussion
The convergent analysis and agreement among experiment, kinematics, and first-principles MD simulations presented in this work support a collision-induced mechanism for direct intramolecular conversion of CO2 to O2. Specifically, with the dynamics evolving on the ground electronic state of neutral CO2, we find that O2 is formed via delayed fragmentation, where the delay results from atomic rearrangement of the colliding CO2 molecules into a strongly bent geometry. This geometry provides access to the O2 dissociation product, without visiting other intermediates. Alternative mechanisms were also theoretically investigated, including the possibility of a collision-induced, non-adiabatic transition of the neutral CO2 molecules to electronically excited states (Supplementary Fig. 4), as well as collisional dissociation on the anionic CO2- surface following double electron transfer from the Au surface. Although these more complicated processes offer intriguing and potentially exploitable alternative avenues to O2 formation, they were not necessary for explaining the experimental observables and were calculated to be less likely under the current experimental conditions.
The mechanism reported here is distinct from previously proposed mechanisms for CO2???C?+?O2 conversion. Specifically, the mechanism differs from that of photochemical interconversion14 not only in terms of activation (collisional vs. photochemical) but also because the collisional mechanism occurs via a delayed fragmentation of a single CO2 intermediate, i.e., without visiting the linear COO state. The collisional mechanism also differs fundamentally from that taking place in electron-attachment experiments17, where the CO2 bends spontaneously on the anionic potential energy surface. Instead, the bent CO2 state is accessed on the neutral surface via collisional energy transfer. Furthermore, while the collisional interconversion of CO2 to O2 has comparable efficiency to activation via high-energy photons and higher efficiency than via electron attachment, it is a much simpler process. Importantly, our mechanism is independent of surface temperature and generic to surface composition (tested on Au, Pt, SiO2, In2O3, SnO2) as long as: (i) the surface contains atoms heavier than the constituents of CO2 and (ii) surface charging is mitigated when CO2 ions are used. Finally, we note that an analogous dissociation reaction: OCS???C?+?SO, previously reported27 for OCS+ collisions on Ag(111), was speculated to occur via a sharply bent excited state, such as the OCS(3A), activated either by neutralization prior to impact or by the energetic collision with the surface. However, the basic mechanistic features of the latter process—including whether it involves unimolecular collisions or Eley–Rideal reactions with surfaced-absorbed O or S atoms—were not addressed.
The intramolecular CO2 reaction may be relevant in astrochemical environments with abundant CO2 and prevalent solar wind. Solar ultraviolet light photo-ionizes CO2 molecules readily, producing ions which are then picked up by the solar wind and accelerated to hyperthermal energies28,29. Collisions of these fast ions with the surfaces of dust particles or other astrophysical bodies can activate the dissociation. Such interactions may affect dynamically the composition of cometary comae, contributing to the abundance of the super-volatiles O2 and CO. Production of O2 from CO2 was explicitly disregarded in the coma of comet 67P early on (pre-perihelion) during the Rosetta mission, owing to the low abundance of CO2 and poor correlation between O2 and CO2 fluxes6. However, the situation may warrant reexamination in the post-perihelion phase, when CO2 can reach abundancies as high as 32% relative to H2O, a 10-fold increase versus pre-perihelion30. The precise level of contribution to the O2 abundance in the coma cannot be determined without CO2 ion energy and flux data. Nevertheless, the number is likely small for collisional encounters on dust and cometary surfaces. Even at low yield, however, contribution to the measured O2 abundance may be disproportionate if the CO2 reaction occurs close to the point of measurement. For example, we have verified experimentally that the reaction takes place on indium–tin oxide (ITO), a man-made material found on Rosetta’s thermal insulation and solar panels. Thus, CO2 collisions on the spacecraft’s exposed surfaces can change the composition of the surrounding gaseous halo with unknown repercussions for mass spectrometric measurements31.
Similar collisional processes may have occurred in early Earth when projectiles, such as meteorites, traversed through its CO2-dominated atmosphere; likewise, orbiting satellites/spacecraft or high-speed space debris32 will encounter neutral or photoionized CO2 in Mars’ upper ionosphere. In these situations, the target surface is moving against a stagnant CO2 atmosphere with correspondingly high velocities, driving the partial transformation of CO2 into O2. Indeed, O2 abundances in the 1000’s of parts per million measured at Mars33 may contain significant contributions from such processes.
Finally, although the yield of O2 is relatively small in the current study, a combination of collisional activation with photoexcitation, electron attachment, and Eley–Rideal reactions in a plasma reactor may result in a process that could be promising for CO2 reduction strategies, as well as plasma-driven continuous O2 production in CO2 atmospheres.
Methods
Experimental
All experiments were carried out in a custom-made low-energy ion scattering apparatus34. The CO2+ ion beam was extracted from an inductively coupled plasma, struck in a reactor held at 2?mTorr using a CO2/Ar/Ne gas mixture supplied with 500?W RF power at 13.56?MHz. Ions were delivered to a grounded surface at 45° incidence angle; typical beam currents of 5–15?µA were spread over a ~3?mm spot. Beam energy was varied between 40 and 200?eV by externally adjusting the plasma potential. The beam energy distribution had a Gaussian shape with a FWHM of ~5?eV. Typical target surfaces were polycrystalline Au foils (5?N), sputter-cleaned with an Ar+ ion gun before each run. Scattered ion products, exiting at an angle of 45° in the scattering plane, were energy-resolved and mass-resolved using an electrostatic ion energy analyzer and a quadruple mass spectrometer, respectively. All ions were detected using a channel electron multiplier, biased as appropriate to detect positive or negative ions. Differences in detector bias precluded a direct comparison of signal intensities between product ions of different charge polarities. All collected signals were normalized to the beam current measured on the sample...
Inventor: YAO YUNXI // GIAPIS KONSTANTINOS
Applicant: CALIFORNIA INST OF TECHN [US]
Apparatus and methods for facilitating an intramolecular reaction that occurs in single collisions of CO2 molecules (or their derivatives amenable to controllable acceleration, such as CO2+ ions) with a solid surface, such that molecular oxygen (or its relevant analogs, e.g., O2+ and O2- ions) is directly produced are provided. The reaction is driven by kinetic energy and is independent of surface composition and temperature. The methods and apparatus may be used to remove CO2 from Earth's atmosphere, while, in other embodiments, the methods and apparatus may be used to prevent the atmosphere's contamination with CO2 emissions. In yet other embodiments, the methods and apparatus may be used to obtain molecular oxygen in CO2-rich environments, such as to facilitate exploration of extraterrestrial bodies with CO2-rich atmospheres (e.g. Mars).
APPLICATIONS
[0001] The application claims priority to U.S. Provisional Application No. 62/463,021, filed Feb. 24, 2017, entitled “Method for Splitting Carbon Dioxide into Molecule Oxygen and Carbon,” the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] Apparatus and methods for producing molecular oxygen (or its relevant analogs, e.g., O2<+> and O2<- >ions) directly from CO2 molecules are provided.
BACKGROUND OF THE INVENTION
[0003] Carbon dioxide (CO2) is a symmetric linear triatomic molecule, wherein carbon atom is covalently bound to two oxygen atoms via strong double bonds. Trace amounts of CO2, a colorless gas, occur naturally in the Earth's atmosphere, wherein its current concentration is ~0.04% (405 ppm) by volume, having risen from pre-industrial levels of 280 ppm. However, despite a seemingly low concentration, CO2 gas is the main contributor to the atmospheric greenhouse effect, and, therefore, is of major concern to humanity, especially in view of the alarmingly rapid increases of its atmospheric concentration in recent decades. Consequently, various technologies for CO2 sequestration from the Earth's atmosphere are currently being investigated and pursued, although no definitive solution has been found to date.
[0004] In contrast, dioxygen gas (O2) is the basis of life on Earth and is rather abundant in the atmosphere. However, an oxygen-rich atmosphere is quite unique to Earth, because, although elemental oxygen is the third most abundant element in the universe, its molecular dioxygen form is very rare. Specifically, in contrast to Earth, where oxygenic photosynthesis has made O2 abundant, interstellar and cometary oxygen atoms are chemically bound to other elements in compounds such as H2O, CO2, CO, silicates, and metal oxides, making the release of O2 from these reservoirs difficult and energetically very expensive. As such, only tenuous amounts of dioxygen are found elsewhere in our solar system, e.g., in the moons of Jupiter and Saturn and on Mars; in fact, the abundance of molecular oxygen has been suggested as a promising biomarker. Accordingly, efficient generation of O2 from CO2 is particularly desirable for space travel to Mars, Venus, and other planetary bodies with CO2-rich atmospheres.
SUMMARY OF THE INVENTION
[0005] Embodiments are directed to methods and apparatus for forming molecule oxygen from carbon dioxide molecules.
[0006] In many embodiments the methods for splitting carbon dioxide into molecular oxygen and carbon include accelerating carbon dioxide molecules against a solid surface at an incident angle such that the carbon dioxide molecules have kinetic energy E0 of between 10 and 300 eV at collision against the solid surface.
[0007] In other embodiments the method further includes accelerating the carbon dioxide molecules prior to the acceleration. In other such embodiments the carbon dioxide molecules are ionized by one of either photoexcitation or energetic electron bombardment.
[0008] In still other embodiments the accelerated carbon dioxide molecules have a kinetic energy of between 20 and 200 eV.
[0009] In yet other embodiments the carbon dioxide molecules subjected to acceleration are produced in a carbon dioxide plasma. In some such embodiments the plasma ionizes the carbon dioxide molecules to produce carbon dioxide ions, and wherein the potential of the plasma is externally adjusted to produce an electric field in the plasma such that the carbon dioxide ions are accelerated to the kinetic energy E0.
[0010] In still yet other embodiments the solid surface comprises grounded metal electrodes.
[0011] In still yet other embodiments the solid surface comprises one or more element selected from the group: any element of rows 4, 5, and 6 of the Periodic table, an oxide of any element thereof, any combination thereof.
[0012] In still yet other embodiments the solid surface comprises one or more element selected from the group: Ti, V, Cr, Mn, Fe, CO, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, Ce, Hf, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, an oxide of any element thereof, any combination thereof.
[0013] In still yet other embodiments the solid surface is selected from the group of silicon oxide or indium tin oxide.
[0014] In still yet other embodiments the acceleration occurs via application of an electric field.
[0015] In still yet other embodiments the conversion efficiency of carbon dioxide molecules to molecular oxygen is up to 33%.
[0016] In still yet other embodiments the conversion efficiency of carbon dioxide molecules to molecular oxygen is at least 5%.
[0017] Many other embodiments are directed to an apparatus for splitting carbon dioxide into molecular oxygen and carbon, including:
a source of a gaseous mixture comprising carbon dioxide gas,
a solid surface,
a molecular accelerator configured to selectively accelerate the carbon dioxide molecules against the solid surface at an incident angle, such that the kinetic energy of the carbon dioxide molecules at collision against the solid surface is between 10 and 300 eV.
[0021] In other embodiments the apparatus further includes an ionizer for ionizing the carbon dioxide molecules prior to the acceleration, and wherein the molecular accelerator comprises an electric field.
[0022] In still other embodiments the carbon dioxide molecules are ionized by one of either photoexcitation or energetic electron bombardment.
[0023] In yet other embodiments the accelerated carbon dioxide molecules have a kinetic energy of between 20 and 200 eV.
[0024] In still yet other embodiments the carbon dioxide molecules subjected to acceleration are produced in a carbon dioxide plasma. In some such embodiments the plasma ionizes the carbon dioxide molecules to produce carbon dioxide ions, and wherein the potential of the plasma is externally adjusted to produce an electric field in the plasma such that the carbon dioxide ions are accelerated to the kinetic energy E0.
[0025] In still yet other embodiments the solid surface comprises one or more grounded metal electrodes.
[0026] In still yet other embodiments the solid surface comprises one or more biased metal electrodes.
[0027] In still yet other embodiments the solid surface is selected form the group consisting of any element of rows 4, 5, and 6 of the Periodic table, an oxide of any element thereof, any combination thereof.
[0028] In still yet other embodiments the solid surface is selected form the group consisting of Ti, V, Cr, Mn, Fe, CO, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, Ce, Hf, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, an oxide of any element thereof, any combination thereof.
[0029] In still yet other embodiments the solid surface is selected form the group of silicon oxide or indium tin oxide.
[0030] In still yet other embodiments the conversion efficiency of carbon dioxide molecules to molecular oxygen is up to 33%.
[0031] In still yet other embodiments the conversion efficiency of carbon dioxide molecules to molecular oxygen is at least 5%
[0032] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying data and figures, wherein:
[0034] FIGS. 1a-c summarize scattering dynamics of the three molecular ion products (CO2<+> in FIG. 1a, O2<+> in FIG. 1b, and O2<- >in FIG. 1c) resulting from CO2<+> collisions with Au surface in accordance with embodiments of the application.
[0035] FIGS. 2a-d provide spectra confirming the presence of fragmentation products in CO2<+> collisions with Au surface in accordance with embodiments of the application.
[0036] FIG. 3a schematically illustrates the collision sequence of an accelerated linear CO2 molecule scattering on surface, in accordance with embodiments of the application. FIG. 3b provides data summarizing a kinematic analysis of the collision sequence depicted in FIG. 3a.
[0037] FIGS. 4a-b illustrate a velocity analysis (a) and estimation of the CO2* excitation energy (b) for scattered CO2 dissociation products in accordance with embodiments of the application.
[0038] FIG. 5 provides data showing the trend in O2 formation vs. partial dissociation in CO2<+>/Au collisions in accordance with embodiments of the application.
[0039] FIGS. 6a-c illustrate energy distributions of CO2<+> (a), O2<+> (b), and O2<- >(c) ion exits from CO2<+>/Pt collisions as a function of the respective product exit energy for various CO2<+> beam energies (E0), in accordance with embodiments of the application.
[0040] FIGS. 7a-d illustrate energy distributions of dissociation fragments from CO2<+>/Pt collisions, in accordance with embodiments of the application.
[0041] FIG. 8 illustrates energy distributions of O2<- >ions scattered from CO2<+>/SiOx collisions, as a function of the CO2<+> incidence energy, in accordance with embodiments of the application.
DETAILED DISCLOSURE
[0042] Turning to the drawings and data, methods and apparatus for the facile conversion of CO2 to molecular oxygen are provided. It will be understood that the embodiments of the invention described herein are not intended to be exhaustive or to limit the invention to precise forms disclosed. Rather, the embodiments selected for description have been chosen to enable one skilled in the art to practice the invention.
[0043] Despite the pressing demand for effective removal of CO2 from the atmosphere and apparent benefits of converting CO2 specifically into O2, only one abiotic pathway to O2 was known until recently, namely, the three-body recombination reaction, O+O+M?O2+M, where the requisite atomic oxygen arises from CO2 photo-dissociation and M is a third body (Kasting, J. F., Liu, S. C., Donahue, T. M. Oxygen levels in the prebiological atmosphere. J. Geophys. Res. 84, 3097-3107 (1979); and Segura, A., Measows, V. S., Kasting, J. F., Crisp, D., Cohen, M. Abiotic formation of O2 and O3 in high-CO2 terrestrial atmospheres. Astron. Astrophys. 472, 665-679 (2007); the disclosures of which are incorporated herein by reference). This finding was superseded by the discovery of two new pathways for direct O2 formation from CO2: one via vacuum ultraviolet (VUV) photo-dissociation (as described in Lu, Z., Chang, Y. C., Yin, Q. Z., Ng, C. Y. & Jackson, W. M. Evidence for direct molecular oxygen production in CO2 photodissociation. Science 346, 61-64 (2014), the disclosure of which is incorporated herein by reference) and one via dissociative electron attachment (DEA) (as described in Wang, X. D., Gao, X. F., Xuan, C. J. & Tian, S. X. Dissociative electron attachment to CO2 produces molecular oxygen. Nature Chem. 8, 258-263 (2016), the disclosure of which is incorporated herein by reference). In both of these latter studies, direct detection of the neutral O2 photoproduct was not possible because of background interference. Instead, experimental evidence for the dissociation reaction of a highly-excited CO2 electronic state (Suits, A. G. & Parker, D. H. Hot molecules-off the batten path. Science 346, 30 (2014), the disclosure of which is incorporated herein by reference) into the C(<3>P)+O2(X<3>Sg<->) products was based on detecting the complementary atomic C<+> or C<- >fragment. In addition, O2<+> ions from the photo-dissociation of CO2 were recently detected using strong laser fields to photo-produce the doubly-ionized CO2<++> state, which dissociates into C<+>+O2<+>, altogether a very inefficient process (Larimian, S. et al. Molecular oxygen observed by direct photoproduction from carbon dioxide. Phys. Rev. A 95, 011404 (2017), the disclosure of which is incorporated herein by reference). Direct production of O2<- >from dissociative attachment in CO2 has collision cross sections so small ( ~10<-24 >cm<2>) that “signal must be accumulated over several days” to observe it even with extremely sensitive detection systems (Spence, D. & Schulz, G. J. Cross sections for production of O2<- >and C by dissociative electron attachment in CO2: an observation of the Renner-Teller effect. J. Chem. Phys. 60, 216-220 (1974), the disclosure of which is incorporated herein by reference). Therefore, new methods for the efficient splitting of CO2 to produce O2 are highly desirable and sought after in areas ranging from atmospheric science to space travel.
[0044] Direct conversion of CO2 into molecular oxygen is an energetically very unfavorable reaction. In principle, direct dissociation of CO2 can proceed along three pathways (shown below with the indicated dissociation energies):
CO2?CO+O (5.5 eV) (I)
CO2?C+O2 (5.8 eV) (II)
CO2?C+2O (11.0 eV) (III)
Channel (I) describes the primary partial dissociation reaction, which has been widely studied in photochemistry and in heterogeneous catalysis under thermal activation conditions (as detailed, for example, in: Rosen, B. A. et al. Ionic liquid-mediated selective conversion of CO2 to CO at low overpotentials. Science 334, 643-644 (2011); and Liu, M. et al. Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration. Nature 537, 382-386 (2016), the disclosures of which are incorporated herein by reference). Channel (III) represents the energetically expensive complete dissociation of CO2 with cleavage of both C—O bonds. In contrast, channel (II) is an exotic pathway, which requires extensive intramolecular bond rearrangement within the triatomic CO2, despite the fact that its dissociation energy is only 0.3 eV larger than that of channel (I). However, simulations have shown a possible way to realize channel (II) by first forming the cyclic CO2 complex [c-CO2(<1>A1)], which then must transform into the collinear COO(<1>S<+>) intermediate on its way to dissociation into C+O2. The first step in this scheme requires bending of the linear CO2 molecule in order to bring the two O atoms in close proximity. Although inaccessible by thermal activation, the barrier to bending may be, in theory, overcome by other means of excitation, such as VUV photon irradiation or energetic electron bombardment.
[0045] It has now been discovered that, as described in the embodiments of this invention, channel (II) can also be activated by energetic collisions of CO2 molecules (or, in some embodiments, of their CO2<+> ion analogs) with solid surfaces. In many such embodiments, when CO2<+> ions are collided with a solid surface, O2 molecules and O2<±> ions evolve directly from a scattered excited state (CO2*) undergoing late fragmentation. Accordingly, this application is directed to embodiments of an unexpected and surprisingly efficient method for facilitating an intramolecular reaction that occurs in single collisions of CO2 molecules (or their derivatives amenable to controllable acceleration, such as CO2<+> ions) with a solid surface, such that molecular oxygen (or its relevant analogs, e.g., O2<+> and O2<- >ions) is directly produced. In many embodiments of the invention, the reaction is driven by momentum of the CO2 molecule or ion accelerated against a surface and incoming at an incident angle and will occur irrespective of the surface composition and temperature. However, in many embodiments, the yield of O2 production from CO2 splitting does depend on surface composition in so far as surfaces that would be reactive with CO2 or its fragmentation products can poison the reaction. In some embodiments, the method may be used to remove CO2 from Earth's atmosphere, while, in other embodiments, the method may be used to prevent the atmosphere's contamination with CO2 emissions. In yet other embodiments, the method may be used to obtain molecular oxygen in CO2-rich environments, such as to facilitate exploration of extraterrestrial bodies with CO2-rich atmospheres (e.g. Mars).
[0046] In many embodiments, the method for splitting carbon dioxide into molecular oxygen and carbon comprises accelerating molecules of CO2 to a specific desired velocity, such that the accelerated molecules collide with a solid surface with kinetic energies between 10 and 300 eV. It will be understood that, within this kinetic energy range, the actual amount of energy required for the optimum conversion to O2 is determined by such parameters as the angle at which the accelerated CO2 molecule or ion approaches the surface prior to the collision and the atomic mass of the surface atoms. In many embodiments, larger kinetic energies are required to facilitate the reaction for larger angles of incidence ? with respect to the surface normal vector, wherein the least amount of required energy corresponds to normal incidence. More specifically, in many embodiments, if E0 is the required kinetic energy for maximum conversion at an angle of incidence ?, then the corresponding energy for normal incidence would be E0 cos<2 >?.
[0047] Without being bound by any theory, the kinematic analysis of the collisional process suggests that CO2 collisions with the provided surface under the disclosed herein conditions extensively perturb the CO2 intramolecular triatomic geometry and produce a strongly bent CO2 excited state, which, subsequently, dissociates to yield molecular as well as ionized O2. The disclosed herein process is reminiscent of exotic photochemical pathways for CO2 decomposition, but, unlike any known pathway, which typically have very low O2 yields, the intramolecular CO2 decomposition conducted according to the embodiments of this invention has an estimated O2 yield of ~33±3%. For comparison, the yield of previously reported CO2 photo-dissociation pathways is ~5±2%.
[0048] Of course, it will be understood that any acceleration technique known in the art can be employed to bring CO2 molecules to the velocities and surface striking energies necessary to enable the method of this application. Several approaches are known in the art to produce positively charged CO2 ions and to controllably accelerate them with an applied electric field. Accordingly, in many embodiments, prior to acceleration, the CO2 molecules are ionized via photo-excitation with ultraviolet light. In some other embodiments, the CO2 molecules are ionized by means of energetic electron bombardment under low pressure. In yet other, preferred embodiments, CO2 plasma is used to produce CO2<+>, which are accelerated against a biased surface with appropriate energy. In yet another embodiment, the solid surface may be moving against heated CO2 molecules at the required velocity.
[0049] In many embodiments, O2 ions are directly produced in hyperthermal CO2<+> collisions against Au surfaces. Specifically, FIGS. 1a to 1c summarize exemplary scattering dynamics of such CO2<+>/surface collisions, by showing energy distributions for three of its molecular ion products: CO2<+> (FIG. 1a), O2<+> (FIG. 1b), and O2<- >(FIG. 1c) for various CO2<+> beam energies (E0) (as annotated on each panel), and shows that both O2 ions are detected at certain, relatively narrow energy ranges, along with some amount of survived CO2<+>. Accordingly, first, FIG. 1a shows that a very weak scattered CO2<+> signal is detected at beam energies (E0) below 100 eV. Furthermore, the scattered CO2<+> ion exit energy varies in proportion to the CO2<+> incidence energy, thus precluding physical sputtering as its origin. Observing dynamic CO2<+> signal is important because it proves that some CO2 survives the collision. In addition, the observed kinematics provide insight into the collision sequence of the constituent atoms of the triatomic molecule impinging onto the metal surface. Second, in FIGS. 1b and 1c a strong scattered O2 signal is observed in both charge polarities, O2<+> and O2<->. Here, in contrast to the O2<-> energy spectra, the O2<+> signal appears somewhat noisy, owing to detector (channeltron) operation. As observed for the scattered CO2<+>, the O2<+> and O2<-> ion exit energies increase monotonically with the CO2<+> incidence energy. For both polarities, the scattered O2 ion signal intensity goes through a maximum, then it dies out for E0>150 eV. Interestingly, the O2<+> energy spectra develop a shoulder at ~20 eV for beam energies greater than 100 eV due to physical sputtering. As noted above, the O2<+> and O2<-> spectra intensities cannot be directly compared due to differences in how the detector is biased.
[0050] Notably, the demonstrated O2 formation is unexpected, since collision-induced dissociation of CO2<+> is expected to occur via channel (I) at low collision energies, followed up by channel (III) at larger incidence energies. Indeed, FIGS. 2a-d show that all of the CO2<+> dissociation products: CO<+>, O<+>, O<->, and C<+>, are detected in CO2<+>/Au collisions. Furthermore, the exit energies of the CO<+> and O<- >peaks vary with incidence energy (FIGS. 2a and 2c), indicating they are produced dynamically within the surface collision. However, as will be shown below, these fragments originate from the same excited state as the one producing the O2 ions. In contrast, the position of the O<+> and C<+> peaks is almost invariant around 20 eV (Exit Energy), suggesting a different origin. The appearance of scattered C<+> products for E0>70 eV confirms that complete dissociation of CO2 via channel (III) also occurs.
[0051] Since scattered CO2<+> ions are detected along with other collision products, a fraction of the incident ions must survive the hard collision. The kinematics (described in Yao, Y. & Giapis, K. P. Kinematics of Eley-Rideal reactions at hyperthermal energies. Phys. Rev. Lett. 116, 253202 (2016), the disclosure of which is incorporated herein by reference) of the scattered CO2<+> ions can help elucidate the scattering mechanism, which in turn provides clues for the formation of O2. Accordingly, FIG. 3a schematically illustrates the collision sequence of a linear CO2 molecule scattering on a solid surface according to many embodiments of this invention. In this scheme, the leading O atom collides with the surface first, followed by collision of the complementary CO moiety, and further followed by the molecule bending during the hard collision and forming of an excited triangular state, which, in turn, undergoes late dissociation into C+O2.
[0052] FIG. 3b provides ion exit energies of CO2<+>, O2<+>, O2<->, CO<+>, O<->, and C<+> ions as a function of CO2<+> incidence energy, along with corresponding one-parameter linear fittings as solid lines. Here, the slope for CO2<+> is calculated from binary collision theory (BCT), assuming two sequential collisions as proposed in the scheme depicted in FIG. 3a; while the other slopes are calculated by assuming a common excited CO2 precursor state fragmenting spontaneously into daughter ions. It should be noted, that since the O2<+> exit energy data overlaps with the O2 data, only the linear fitting for O2<-> is shown in FIG. 3b. In view of the data presented in FIG. 3b, first, if CO2<+> scatters as a whole molecule (i.e., a hard sphere with atomic mass of 44 Dalton), BCT predicts a kinematic factor of 0.6349, which does not fit well the energy data shown in FIG. 3b. This is not surprising given the quasi-linear triatomic nature of the CO2<+> ion (as explained in Walsh, A. D. The electronic orbitals, shapes, and spectra of polyatomic molecules. Part II. Non-hydride AB2 and BAC molecules J. Chem. Soc., 2266-2288 (1953); and Grimm, F. A. & Larsson, M. A Theoretical Investigation of the Low Lying Electronic States of CO2<+> in Both Linear and Bent Configurations. Phys. Scr. 29, 337-343 (1984); the disclosures of which are incorporated herein by reference). The next plausible scattering model assumes that parts of the molecular ion suffer distinct and successive collisions without molecular dissociation (as depicted in FIG. 3a): first the leading O atom collides with a surface Au atom, then the remaining CO moiety collides with the same Au atom. This model is similar to how diatomic molecules scatter on metal surfaces (as described, for example in Yao, Y. & Giapis, K. P. Direct hydrogenation of dinitrogen and dioxygen via Eley-Rideal reactions. Angew. Chem. Int. Ed. 55, 11595-11599 (2016), the disclosure of which is incorporated herein by reference). Applying BCT to the sequential collision scattering model yields a kinematic factor of 0.7870, which fits perfectly the CO2<+> exit energy data provided in FIG. 3b. This scattering behavior has been verified for another quasi-linear molecular ion, NO2<+>, when scattering on Au surfaces (described in Pfeiffer, G. V. & Allen, L. Electronic structure and geometry of NO2<+> and NO2<->. J. Chem. Phys. 51, 190-202 (1969), the disclosure of which is incorporated herein by reference).
[0053] Accordingly, in many embodiments, facilitating energetic CO2<+> scattering against a biased surface affects the angular configuration of the CO2 molecule during the collision and results in the unexpectedly efficient O2 production. Although not to be bound by theory, according to the disclosed herein mechanism and for many CO2 approach geometries, one of the O atoms of the CO2 molecule collides with the surface first and then rebounds in closer proximity to the other O atom in the resulting CO moiety. This mechanical deformation of the CO2 molecule is equivalent to a bending mode but occurs at much faster timescales than vibronic interactions (Renner-Teller effect). In addition, electronic excitation may also occur during the hard collision (as explained in Mace, J., Gordon, M. J. & Giapis, K. P. Evidence of simultaneous double-electron promotion in F+ collisions with surfaces. Phys. Rev. Lett. 97, 257603 (2006), the disclosure of which is incorporated herein by reference). Therefore, according to many embodiments, a strongly bent, highly excited CO2* state is produced in CO2/surface collision, which next decomposes preferentially into C+O2 on the rebound from the surface. Furthermore, in some embodiments, charge exchange of the CO2 dissociation fragments with the surface may aide the ionization of the O2 (FIG. 3a). In other embodiments, the excited state may split directly into ion pairs.
[0054] Furthermore, the very weak signal observed for CO2<+> scattered according to the embodiments of the invention implies a very low survival probability. CO2<+> fragmentation can occur before, during, or after the hard collision with the surface. Only delayed fragmentation, for example, of a rebounding highly excited CO2* precursor state, can explain dissociation products having the same exit velocity as the precursor. Therefore, according to some embodiments, the kinematic factors of O2, CO and O products can be calculated from energy conservation to be 0.5724, 0.5008, and 0.2862, respectively. The linear O2<->, CO<+> and O<- >ion exit data are fitted very well with these kinematic factors as slopes (FIG. 3b). However, the O+ and C<+> data cannot be fitted linearly, wherein their broader distributions suggest that other processes, such as surface sputtering, contribute to the measured peaks at lower energies, rendering de-convolution of the contribution from excited CO2* difficult.
[0055] According to many embodiments and the observed kinematics of the ion exits from CO2<+> scattering on Au, molecular O2 ions originate in surviving CO2 molecules or ions, possibly highly excited. To further illustrate this process, the distributions of the CO2<+>, O2<+>, O2<->, CO<+> and O<- >ion exits for E0=56.4 eV are re-plotted in FIG. 4a as a function of the corresponding ion exit velocity. As seen in FIG. 4a, the exit velocities of scattered CO<+>, O2<+> and O2<-> line up very well, thus confirming the single precursor origin, while the O<- >peak is somewhat broader than the latter three and overlaps partially with the O2 ion exit peaks. Surprisingly, the surviving CO2<+> is clearly faster than the O2 ion products. Therefore, the CO2* undergoing post-collisional dissociation according to the embodiments of the invention must become internally excited by inelastic processes, which, in turn, robs kinetic energy from the incident CO2<+> and results in its slower exit. Although the putative CO2* state cannot be directly detected, the additional energy needed to produce it can be estimated by assuming that the daughter O2<- >ion is emitted with the CO2* exit velocity. Then, the kinetic energy difference between CO2<+> and CO2* can be estimated and should be a measure of the relative excitation energy. Accordingly, FIG. 4b summarizes the results of this simple calculation as a function of E0. Notably, for E0<70 eV, the relative excitation energy is about 6 eV—a value remarkably close to that required for CO2 partial dissociation according to channel (I) mechanism, or for direct O2 formation according to channel (II) mechanism. Coincidentally, the energy penalty to form the triangular CO2 state (<1>A1) is also 6 eV (as reported in Lu, Z., Chang, Y. C., Yin, Q. Z., Ng, C. Y. & Jackson, W. M. Evidence for direct molecular oxygen production in CO2 photodissociation, Science 346, 61-64 (2014), the disclosure of which is incorporated herein by reference). For E0 above 70 eV, the relative excitation energy increases while the scattered CO2<+> signal dies out (FIG. 1a). Simultaneously, C<+> ions become detectable suggesting the onset of complete dissociation via channel (Ill).
[0056] Due to the violence of the surface collision, even the surviving CO2 molecular ions can be highly excited. The intercept of the CO2<+> data fitting in FIG. 3b reflects the inelastic energy loss for the CO2<+> ion exit, which amounts to 8.69 eV—most of it going to internal excitation. Adding the inelastic energy loss of the surviving CO2<+>, the absolute excitation energy for the CO2* precursor could be as high as 14 eV. This energy is comparable to the VUV-photon and electron energy needed for the direct formation of O2 from CO2.
[0057] An important question for the CO2<+> dissociation reaction into O2 is that of efficiency. One way to assess the efficiency of CO2 dissociation according to the embodiments of the application is in terms of selectivity of channel (II) versus the predominant channel (I). Kinematic analysis has shown that the CO<+>, O2<- >and O<- >products are directly formed from a common parent, the putative CO2* excited precursor state. At low CO2<+> incidence energies, the main dissociation pathways are: a) partial dissociation to form CO+O (channel (I)), and b) intramolecular reaction to form O2+C (channel (II)). Assuming that O2<-> and O<- >are formed by resonant electron transfer to the corresponding neutrals with the same efficiency, one can use the relative intensity of O2<- >and O<- >to estimate the selectivity for O2 formation, S(O2), defined as follows:
[mathematical formula]
where I is the intensity of the corresponding negative ion exits. Notably, the electron affinities of O atoms and O2 are 1.46 eV and 0.45 eV, respectively, wherein the difference implies that negative ion formation should be more efficient for 0 than for O2, due to the lower barrier to resonant electron attachment. Therefore, S(O2), as defined here, underestimates the actual O2 formation selectivity. Furthermore, at high CO2<+> incidence energy, channel (III) also opens up, doubling the number of O atoms and O<- >ions produced and, thus, further worsening the estimate for O2 formation selectivity. With these limitations in mind, one can obtain a conservative estimate of the O2 formation selectivity, which is plotted in FIG. 5 as a function of CO2<+> incidence energy. According to the data plotted in FIG. 5, the O2 formation selectivity increases with E0, goes through a maximum of ~33±3% at E0=70 eV, then decreases. The first increase is consistent with more energy available for direct O2 formation. Remarkably, the turnaround point coincides with the opening up of the complete dissociation channel.
[0058] Although the above discussed results are based on monitoring ionic products, it is well known in ion-surface collisions, that the majority ( ~98%) of the ions are neutralized by the surface prior to the collision and thus collide with the surface as neutrals, in this case neutral (uncharged) CO2. Likewise, many scattered CO2 molecules and products of CO2 dissociation will not be charged, nevertheless will be contributing to the O2 yield.
[0059] The direct O2 production by collisional activation of CO2<+ >according to the embodiments of the application is clearly more efficient than activation using other means, such as high-energy photons or electrons. Indeed, O2 formation by photo-excitation of CO2 has an estimated selectivity of only 5±2% vs. the partial dissociation channel, while DEA processes in CO2 have minuscule cross sections for O2 production. In many embodiments, the higher O2 selectivity in the collisional activation process is attributed to more facile structural rearrangement in the CO2 during the hard collision, which brings the two O atoms closer together.
[0060] In some embodiments, the atomic composition of the collision surface affects the O2 yield of CO2 splitting method of the application and must be optimized. Specifically, in many embodiments, surfaces that can be easily sputtered, i.e., where the surface erodes significantly at low incidence energy, or where carbon atoms stick preferentially to the surface to form coatings, are not desirable, as they can interfere with the surface excitation process that facilitates the reaction and poison the CO2 dissociation. Consequently, in many embodiments, the general requirement for the collision surface is that it contains an atom with atomic mass larger than 16-18 Dalton, which is the atomic mass of the elemental oxygen, including its isotopes. In many preferred embodiments, the atomic mass of the collision surface elements is between 20 and 200 Dalton. Furthermore, although surfaces comprising atoms with atomic mass larger than 20 Dalton are acceptable, surfaces comprising atoms with atomic mass heavier than 40 Dalton are preferred. In many embodiments, the collision surface comprises of one or more element found in rows 4, 5, and 6 of the Periodic table or such element's oxide. In some such embodiments, the collision surface is comprised of one or more element from the list: Ti, V, Cr, Mn, Fe, CO, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, Ce, Hf, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, an oxide of any element thereof, or any combination thereof. In contrast, in many embodiments, surfaces comprising certain elements that might interfere with the CO2 dissociation reaction must be avoided. For example, surfaces comprising tungsten (W) must be avoided in many embodiments, as such surfaces, when oxidized, form a volatile tungsten oxide that vaporizes and consumes the surface.
[0061] Furthermore, in many embodiments, the collision enabled CO2 splitting reaction is generic to metal surfaces (FIGS. 6 and 7), and occurs even on oxides (FIG. 8). Specifically, FIGS. 6 and 7 demonstrate that, in some embodiments, efficient splitting of CO2 into O2 can be achieved via CO2 collisions with Pt surfaces, while FIG. 8 shows that, in other embodiments, the similar behavior is observed in CO2 collisions with silicon oxide surfaces.
[0062] In many embodiments, the CO2 splitting method of the instant application may be exploited in plasma reactors, wherein ion/wall collisions occur spontaneously at energies determined by the plasma potential. Remarkably, past attempts at using CO2 plasmas were plugged by relatively low O2 conversion, ostensibly because of the slow kinetics for the two-step O2 formation process in gas-phase collisions (as detailed in Spencer, L. F. & Gallimore, A. D. Efficiency of CO2 dissociation in a radio-frequency discharge. Plasma Chem. Plasma Phys. 31, 79-89 (2010), the disclosure of which is incorporated herein by reference). However, according to the embodiments of the invention, the O2 yield is greatly improved with the following three modifications to plasma reactor methods of CO2 splitting: a) maximizing the CO2<+> ion density, b) tuning the plasma potential between 40 and 150 eV, and c) providing grounded metal electrodes to enable CO2<+> ion/surface collisions.
Exemplary Embodiments/Experimental Materials and Methods
[0063] The following example sets forth certain selected embodiments relate to the above disclosure. It will be understood that the embodiments presented in this section are exemplary in nature and are provided to support and extend the broader disclosure, these embodiments are not meant to confine or otherwise limit the scope of the invention.
[0064] All experiments described in the instant application were carried out in a custom-made low-energy ion scattering apparatus described in detail in Gordon, M. J. & Giapis, K. P. Low-energy ion beam line scattering apparatus for surface science investigations. Rev. Sci. Instrum. 76, 083302 (2005), the disclosure of which is incorporated herein by reference. The CO2<+> ion beam was extracted from an inductively-coupled plasma, struck in a reactor held at 2 mTorr using a CO2/Ar/Ne gas mixture supplied with 500 W RF power at 13.56 MHz. Ions delivered to a grounded surface at 45° incidence angle; typical beam currents of 5 to 15 µA were spread over a ~3 mm spot. Beam energy was varied between 40-200 eV by externally adjusting the plasma potential. Typical target surfaces were polycrystalline Au foils (5N), sputter-cleaned with an Ar+ ion gun before each run. Scattered ion products, exiting at an angle of 45° in the scattering plane, were energy- and mass-resolved using an electrostatic ion energy analyzer and a quadruple mass spectrometer, respectively. All ions were detected using a channel electron multiplier, biased as appropriate to detect positive or negative ions. Differences in detector bias precluded a direct comparison of signal intensities between product ions of different charge polarities. All collected signals were normalized to the beam current measured on the sample.