Graphene as an Adsorption Template for Studying Double Bond Activation in Catalysis

Hydrogenated graphene (H-Gr) is an extensively studied system not only because of its capabilities as a simplified model system for hydrocarbon chemistry but also because hydrogenation is a compelling method for Gr functionalization. However, knowledge of how H-Gr interacts with molecules at higher pressures and ambient conditions is lacking. Here we present experimental and theoretical evidence that room temperature O2 exposure at millibar pressures leads to preferential removal of H dimers on H-functionalized graphene, leaving H clusters on the surface. Our density functional theory (DFT) analysis shows that the removal of H dimers is the result of water or hydrogen peroxide formation. For water formation, we show that the two H atoms in the dimer motif attack one end of the physisorbed O2 molecule. Moreover, by comparing the reaction pathways in a vacuum with the ones on free-standing graphene and on the graphene/Ir(111) system, we find that the main role of graphene is to arrange the H atoms in geometrical positions, which facilitates the activation of the O=O double bond.


S2a. Direct comparison: H-Gr exposed to O 2 , H 2 and C 2 H 4
Direct comparison of the C 1s curve fitting after graphene growth, after hydrogenation and after the following exposures: i) 60 s, 1.0 mbar O2, ii) 60 s, 1.0 mbar H2, and iii) 60 s, 1.0 mbar C2H4. Exposure to O2 and C2H4 results in the complete removal of H dimers (component Ca), while exposure to H2 leads to no change to the C 1s spectrum. The slightly different relative size of Ca component is caused by the use of different hydrogen sources at different beamtimes (see the experimental section in the main article).

S2b. Direct comparison: H-Gr exposed to O 2 , H 2 and C 2 H 4
Accumulative plots representing the intensity of each C 1s component after graphene growth, after hydrogenation, and after i) 60 s O2 exposure at 1.0 mbar, ii) 60 s H2 exposure at 1.0 mbar, and iii) 60 s C2H4 exposure at 1.0 mbar. The values are stated next to each plot. All fits are shown in figure S2a.

S3. Discussion of H clusters and interaction with the substrate with the analysis of Ir 4f 7/2
In the main text we have shown how the exposure to molecular oxygen of a H saturated Gr film results in the selective removal of the most weakly adsorbed H structures (the H dimers). We observed, however, that some of the more strongly adsorbed H atoms (which form H clusters in the FCC and HCP regions of the moiré) are also removed. While the removal of dimers is not reflected in the Ir4f7/2 spectra, the partial removal of H clusters is reflected by the relative intensities of the iridium components.
The Ir 4f7/2 spectra in figure S3 are fitted with three components: bulk iridium (Ibulk at 60.8 eV), surface iridium (Isurf at 60.3 eV), and surface iridium atoms binding to C atoms from the graphene film atop (Ic at 60.7 eV). For the fit, we have assumed that the total surface area (Isurf + Ic) is maintained, and only the relative intensities of Isurf and Ic change.
After the graphene growth, in panel a, we only observe the bulk and surface components, due to the weak interaction between the Ir surface and the graphene film. After hydrogenation, the presence of hydrogen After oxygen exposure (right spectrum in figure S3b), the Isurf component recovers to 55 %, meaning that only 45 % of the iridium surface atoms remain binding to C-atoms from the graphene atop. This is in agreement with the reduction of the cluster-related C 1s components (Cb and Cd), which decrease from 55% to 36% of the total C 1s intensity. Note that for every C-H bond breaking from the graphene-like clusters, 1 to 3 Ir surface atoms unbind to C atoms (assuming that H atoms at the edges of the H clusters are the ones being removed).
Similar observations can be done for the Ir 4f 7/2 spectra measured before and after exposure to other gases (figure S3c for molecular hydrogen and figure S3d for ethylene). After hydrogen exposure, where no change can be observed in the H coverage (see figure 3a, or S2a and S2b above), the Ic and Isurf components maintain almost the same intensity ratios after exposure. After ethylene, on the other hand, which is capable of removing the H dimers but much less efficient than oxygen at affecting the H clusters coverage (see figure 3b or S2a, panel (iii)), we observe a slight recover of the Isurf component (from 20 % to 35 %), which can be directly linked to the 5 % diminishing of the Cb and Cd components in the C 1s spectra shown in figure S2b, panel (iii).
Overall, the analysis of the Ir 4f 7/2 confirms the conclusions drawn in the main text: Exposure to oxygen results in the complete removal of H-dimers and reduction of graphane-like H-clusters, while exposure to molecular hydrogen does not affect the system, and exposure to ethylene results in the removal of Hdimers alone.

S5. Graphene supercell and adsorption sites used for the DFT calculations
The picture above shows a 6×6 graphene sheet with adsorption sites highlighted in brown color. The annotations are the indices of C atoms onto which adsorption is considered (the others are not used in the calculations). In the following figures S6 -S10, the different investigated adsorption configurations are illustrated with the indices of the adsorption sites and corresponding adsorption strength sorted after the most stable configuration found. The title of each configuration points out where the adsorbates are located in the initial guess structure. Upon DFT relaxation, we filter out all failed structures, e.g.