Carbon dioxide scattering experiments and kinematic analysis

We first demonstrate the formation of O 2 in hyperthermal CO 2 +/Au collisions by plotting kinetic energy distributions of three scattered molecular ion products: CO 2 +, O 2 +, and O 2 − for various CO 2 + incidence energies (E 0 ). Very weak signal of scattered CO 2 + is detected for E 0 < 80 eV (Fig. 1a). The CO 2 + exit energy peak varies in proportion to E 0 , thus implying a ballistic or impulsive rebound from the surface and thereby precluding physical sputtering as its origin. Observation of this “dynamic” CO 2 + signal is important, not only for proving that some CO 2 survives the surface encounter but also for unraveling the collision sequence of the constituent atoms. Strong signal of scattered O 2 ions is also observed (Fig.1b, c). The O 2 + and O 2 − exit energies represent a large fraction of the incidence energy (57%) and increase monotonically with E 0 over a larger range than scattered CO 2 +. The O 2 ion signal intensity exhibits a maximum at E 0 ~ 100 eV. Above that, only the O 2 + distribution develops a shoulder (i.e., exit at ~30 eV) from physical sputtering.

Fig. 1 Dynamic production of O 2 ± in CO 2 + collisions on Au. Scattered product kinetic energy distributions of a CO 2 +, b O 2 +, and c O 2 − ion exits from CO 2 +/Au for various CO 2 + beam energies (E 0 ) as annotated on each panel. Signal intensities in b and c cannot be compared to each other due to differences in detector bias Full size image

The detection of fast O 2 ion products is surprising. Neither sputtering of surface O 2 nor O-atom abstraction reactions (Eley–Rideal) can explain their formation, because both mechanisms would produce O 2 at much lower exit energies (see the section “Methods”). A remaining possibility to be explored here is dynamic formation of O 2 through dissociation of CO 2 . Dynamic partial and full dissociation of CO 2 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 E 0 , suggesting a different origin, i.e., sputtering19. Scattered C+ products appear at E 0 > 80 eV, confirming full dissociation.

The presence of dynamically exiting CO 2 + 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, CO 2 + 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 CO 2 + 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 CO 2 molecule. Applying BCT to this sequential-collision model yields a kinematic factor of 0.7870, which agrees very well with the CO 2 + exit energy data (Fig. 2a, black line).

Fig. 2 Kinematics and velocity analysis of CO 2 + scattering on Au. a Experimental exit energies of CO 2 +, O 2 +, O 2 −, CO+, CO−, and O− ions from CO 2 /Au collisions as a function of CO 2 + incidence energy. All points represent the peak of the respective energy distribution obtained from Gaussian fitting of the experimental data. All solid lines represent one-parameter linear fittings with BCT-derived slopes. No fittings are shown for O 2 + and CO+ data because of overlap with their negative ion counterparts. b Experimental velocity distributions of select scattered product ions for E 0 = 56.4 ± 2.5 eV. c Calculated exit energies of CO 2 +, O 2 +, O 2 −, CO+, CO−, and O− ions from MD simulations of CO 2 /Au collisions as a function of CO 2 + incidence energy; slopes and intercepts listed in the inset are two-parameter best-fittings. The error bars represent one standard deviation across 10 samples of 2000 trajectories each from the ensemble of molecular dynamics trajectories. d Calculated velocity distributions from MD simulations for select scattered ion products at E 0 = 56.4 eV. Vertical dashed lines in (b) and (d) indicate alignment with respect to the CO 2 + (dashed black line) and O 2 − (dashed blue line) peaks Full size image

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 CO 2 and surface sputtering of adsorbed CO 2 fragments. While some sputtering is indeed observed at high E 0 (>140 eV), kinematic analysis of the exit energy data provides strong evidence for impulsive dissociation of the CO 2 molecule24. Assuming delayed fragmentation of the CO 2 parent24, the kinematic factors of the CO, O, and (possibly) O 2 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 O 2 ±, CO±, and O− ion exit data very well (Fig. 2a lines), indicating that the latter ions are all dissociation products of CO 2 . On the contrary, the O+ and C+ data are not linear with respect to E 0 , 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 E 0 = 56.4 eV. The exit velocities of scattered CO+, O 2 +, O 2 −, 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 O 2 ion products exit with velocities lower than CO 2 + owing to inelasticity from breaking of chemical bonds and non-resonant surface ionization.

Although the kinematic analysis indicates conclusively that some CO 2 scatters intact after a two-step sequential collision of the O and CO moieties, it leaves various aspects of the O 2 formation mechanism unresolved. In particular, since the experiment is limited to observing ions, we are unable to assess how much neutral O 2 is produced. Moreover, the kinematic analysis cannot shed light on whether O 2 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 CO 2 on Au(111) are performed in the experimental scattering geometry, with CO 2 evolving on the ground singlet potential energy surface under the assumption that incoming CO 2 + 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 CO 2 (−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 CO 2 + has a calculated slope of 0.713 vs. the experimental value of 0.787. Negligible CO 2 is found to survive for E 0 > 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 O 2 − 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 O 2 − 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 E 0 = 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 O 2 + and O 2 − distributions to be similar with the cation exiting slower than the anion, and both find CO 2 + to exit with higher velocity than the ionized O 2 products. The agreement suggests that the simulations provide a strong foundation for analyzing the reaction mechanism of the direct CO 2 conversion to O 2 .

An ensemble of 20,000 CO 2 -on-Au collision trajectories were performed for each incidence energy, resulting in a variety of dissociation products, including O 2 (Fig. 3a). Prior to the mechanistic ensemble analysis, it is instructive to review one representative trajectory that leads to collisional O 2 formation (Fig. 3b). Select configurations are shown as insets, along with the CO 2 –Au interaction energy, \({E}_{{\mathrm{CO}}_{2}}\) –Au , and the CO 2 internal energy, \({E}_{{\mathrm{CO}}_{2}}\), as a function of time. The incoming CO 2 molecule is vibrationally excited (inset I). As the center-of-mass distance to the surface, \({Z}_{{\mathrm{CO}}_{2}}\), decreases, the molecule penetrates the repulsive potential wall of the surface and \({E}_{{\mathrm{CO}}_{2}}\) –Au increases steeply. During this encounter, one of the O atoms of CO 2 strikes a surface Au atom, giving rise to the first peak in the \({E}_{{\mathrm{CO}}_{2}}\) –Au curve (inset II). This collision occurs before \({Z}_{{\mathrm{CO}}_{2}}\) 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 \({E}_{{\mathrm{CO}}_{2}}\) –Au curve (inset III), which occurs after the apsis. As a result of the impulsive energy transfer during the collision, the rebounding CO 2 undergoes substantial intramolecular rearrangement portrayed by the bond distance evolution in Fig. 3b. The O–O distance, \({r}_{{\mathrm{O}}_{2}}\), decreases while the C–O distances, r CO , simultaneously increase, reaching a point along the trajectory where CO 2 acquires a triangular configuration with nearly equal bond lengths (vertical dashed line). This strongly bent CO 2 intermediate (inset IV) has a significant amount of internal energy, \({E}_{{\mathrm{CO}}_{2}}\), and promptly dissociates to give a free C atom and a vibrationally hot O 2 molecule (inset V). The complete CO 2 collision trajectory discussed in Fig. 3b can be viewed in the Supplementary Video. The formation of O 2 depicted by this representative trajectory proceeds by delayed fragmentation following the two-step sequential collision of CO 2 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.

Fig. 3 Product yields and energetics of CO 2 scattering on Au. a Calculated yields of neutral dissociation products from a statistical analysis of an ensemble of CO 2 scattering trajectories on Au(111) at E 0 = 56.4 eV. Inset: average delay times (T delay ) of the dissociation channels relative to the point of closest approach of CO 2 to the surface. b Energetics along an illustrative CO 2 collision trajectory, which leads to O 2 formation on Au at E 0 = 56.4 eV. Curve identification: CO 2 –Au interaction energy (light blue), CO 2 potential energy (red), CO 2 center-of-mass distance from the metal surface (orange), O–O bond length (dark blue), and C–O bond lengths (full and dashed green lines). Insets I–V: Select configurations of the CO 2 surface geometry along the scattering trajectory Full size image

The calculated reaction yields of the various collision-induced dissociation channels of CO 2 at E 0 = 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 CO 2 (6%) survives the collision in correspondence with experimental detection. Approximately 5% of all trajectories lead to the strongly bent intermediate state—the precursor to O 2 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 CO 2 molecules produces O 2 . The overall neutral yield of the symmetric dissociation channel, CO 2 → C + O 2 , amounts to 0.6% at E 0 = 56.4 eV. Upon increasing incidence energy, the neutral O 2 yield obtained from the ensemble of isotropically oriented incident CO 2 molecules reaches 0.8 ± 0.2% for E 0 ~ 70 ± 15 eV (Fig. 4, blue line). Also it is clear from the figure that the fraction of O 2 -producing trajectories increases substantially once the strongly bent CO 2 intermediate state is reached (Fig. 4, green line) and this fraction peaks at around 13% for E 0 ~ 55 ± 10 eV. The smaller total neutral O 2 yield results from the small fraction of linear CO 2 molecules reaching the strongly bent state (Fig. 4, red line). By preferentially orienting incoming CO 2 molecules (axis parallel to the surface), the fraction of O 2 -producing trajectories increases to ~2% (Fig. 4, dashed blue line) resulting from an increase of the dissociation probability of the strongly bent state to O 2 (Fig. 4, dashed green line). These findings suggest that activation of bending and symmetric stretching motion of CO 2 prior to the collision may facilitate both the population of the strongly bent state and its dissociation to O 2 leading to a significant increase in the total neutral O 2 yield.

Fig. 4 Reaction probabilities for symmetric dissociation of CO 2 on Au. Calculated total O 2 yields for isotropic (solid blue line) and parallel (dashed blue line) orientation of incident CO 2 as a function of incidence energy. Also shown: Fraction of all trajectories reaching the strongly bent CO 2 intermediate state (solid red line); and fraction of the latter trajectories, which dissociate symmetrically to C + O 2 for isotropic (solid green line) and parallel (dashed green line) orientations. Inset: average geometry of the intermediate state computed from the lowest energy, bent CO 2 configurations visited by the scattering trajectories Full size image