Supporting Information
Cryogenic Carbon Monoxide Oxidation Cuprous Oxide
Burcu Karagoz,[a]† Tianhao Hu,[b]† Joakim Halldin Stenlid,[c, d, e, f]‡ Xiaoming Hu,[f] Markus Soldemo,[e,f] Frank Abild-Pedersen,[c] Kess Marks,[e] Henrik Öström,[e] Dario Stacchiola,[g] Jonas Weissenrieder,[g] Ashley R. Head,*[g]
[a] Diamond Light Source, Diamond House, Didcot, OX11 ODE, United Kingdom
[b] Department of Chemistry, Stony Brook University, Stony Brook, NY, 11974, United States
[c] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, United States
[d] SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University; Stanford, CA, 94305, United States
[e] Department of Physics, AlbaNova University Center, Stockholm University, Stockholm, SE-106 91, Sweden
[f] Light and Matter Physics, Applied Physics, KTH Royal Institute of Technology, Stockholm, SE-100 44, Sweden
[g] Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973 † These authors contributed equally to this work
‡ Present address: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, SE-412 96, Sweden
This PDF file includes:
Materials
Infrared Reflection Absorption Spectroscopy
Reflectivity Difference Calculations
X-ray Photoelectron Spectroscopy
Estimation of Surface Adsorbate Coverage Using XPS
Assignment of XPS C 1s Species
Comments on the Potassium and Adventitious Carbon Contamination
NEXAFS Data Collection and Analysis
Sum Frequency Generation Vibrational Spectroscopy
Computational Methods
Additional Computational Results
References
Experimental Section
Materials
The Cu2O(111) and Cu2O(100) single crystals were purchased from Surface Preparation Laboratory, The Netherlands. Both Cu2O single crystals followed the same pretreatment procedure before each experiment.[1] The Cu2O surface was cleaned by cycles of Ar+ sputtering (0.5 kV) for 20 min and annealing in O2 at 3 × 10-6 mbar at 600 °C for 10 min followed by annealing in UHV at 500 °C for 5 min. This treatment has been shown to give the PY surface reconstruction for Cu2O(111),[2] and we have characterized this surface over many experiments ourselves.[3–5] Fig. S1 shows a representative STM of the PY reconstructed surface.
Fig. S1. A representative (a) scanning tunneling microscopy image and (b) LEED image (34.3 eV) of an as-prepared Cu2O(111)-(3 3) surface. Minor regions of Cu2O(111)-(1 1) termination can be observed in the STM image. The STM image dimensions are 30 nm x 30 nm, and the scanning parameters are -1.0 V and 0.05 nA. (c) A representative LEED image (44.6 eV) of the Cu2O (100) surface.
Infrared Reflection Absorption Spectroscopy
IRRAS experiments were performed in Brookhaven National Laboratory using a Bruker 80v spectrometer.[6] IR light exits the spectrometer, travels through a wire grid polarizer, enters an ultra-high vacuum (UHV) chamber through a KBr window, is reflected off the sample, exits the chamber through a second KBr window, and is detected with a liquid-nitrogen cooled mercury-cadmium-telluride detector. The base pressure of the IRRAS chamber is better than 5 x 10-9 mbar. The spectrometer optics and detector are under a rough vacuum. The sample was cooled in the range of 115 K to 140 K under all conditions. Spectra were collected under UHV conditions and under increasing pressures of CO with both p- and s- polarized light. The spectra are an average of 10,000 scans (40 min collection time) and have 4 cm−1 resolution. The cleanliness of the sample was verified with in situ XPS prior to IRRAS experiments.
Reflectivity Difference Calculations
Starting with the equation for reflectance and absorbance, , reflectivity difference of the Cu2O surface before () and after () CO or CO2 adsorption as a function of the incidence angle () was calculated as follows:[7–9]
in which, represents each component of polarized light and can be computed by the following equations.
where
with the complex refractive index where is the refraction index and is the extinction coefficient
and the absorbance
The and subscripts refer to tangential (pt) and normal (pn) components of p polarization, respectively. The subscript corresponds to s polarization. is the thickness of the adsorption layer and is the wavelength of the respective light.
The following values of optical parameters are used in calculations: n1(vacuum) = 1, k1 = 0, n2(CO) = 1.0003[10], k2(CO) = 0.000655,[11] d2(CO) = 0.115 nm,[12] n2(CO2) = 1.0004,[13] k2(CO2) = 0.0001,[9] d2(CO2) = 0.208 nm,[14,15] n3(Cu2O) = 2.310,[16] k3 = 0.042.[16]
Fig. S2. An expanded wavenumber range of the IR data of Cu2O under 1 mbar of 13CO (top) and 1 mbar CO (bottom) shows no obvious carbonates around the expected region of 1600 to 1400 cm-1.
X-ray Photoelectron Spectroscopy
The XPS measurements were conducted at the surface science high-resolution photoelectron spectroscopy end stations of the FlexPES[17] and I311[18] beamlines at MAX IV, Sweden. The analysis chambers of the end stations were equipped with a Scienta Omicron DA30-L and SES200 analyzers, respectively. The preparation chambers, in direct connection with the analysis chambers, were equipped with argon ion sputter guns, low energy electron diffraction optics, and precision leak-valves for gas dosing. The base pressures in both chambers were 1 × 10-10 mbar. The Cu2O(111) sample was mounted on a stainless-steel sample plate and the temperature was monitored using a K-type thermocouple spot-welded in proximity to the sample. The sample holder was mounted on a liquid nitrogen cryostat. The sample was prepared based on literature procedures previously demonstrated to give the (111) PY reconstructed surface
Estimation of Surface Adsorbate Coverage Using XPS
The inelastic mean free path (Cu2O) of electrons with 169 eV kinetic energy propagating in Cu2O is 6.1 Å according to the predictive model in ref [42]. In calculating Cu2O we use the following parameters for Cu2O: band gap 2.2 eV, density 6 g/cm3, 11 valence electrons for Cu, and 6 valence electrons for O. The O-O plane distance in the (111) direction of Cu2O is 1.23 Å. From the lCu2O and the atomic structure of Cu2O we calculate the intensity contribution from the O atoms residing at the surface of a bulk truncated O terminated clean Cu2O(111) surface (Fig. S3(a)) to 18.3% of the total O 1s intensity.
We estimate the COx coverage from its XPS O 1s intensity contribution in Fig. S3(b) to the total O 1s intensity. The COx contribution to the total O 1s intensity is 19% (after subtraction of overlapping Cu2O photoemission contribution). This translates to 3 oxygen atoms per surface unit cell. The CO C 1s peak intensities (including satellites) are twice the CO2 C 1s intensity (see Figure S4 below). The combined C 1s and O 1s information allows us to estimate an average coverage of ~0.75 CO2 molecules per surface unit cell (at the PY site) and ~1.5 CO molecules per surface unit cell.
Fig. S3. XPS O 1s core level spectra (h = 700 eV) collected at 93 K. (a) clean reconstructed Cu2O(111) surface, (b) after a 2.5 L dose of CO, and (c) at increasing temperature heating to the temperatures listed.
Fig. S4. Background subtracted C 1s spectra (h = 380 eV) from the Cu2O(111) surface after increasing doses of CO in units of Langmuir (L) at 93 K.
Assignment of XPS C 1s Species
Fig. S4 shows the evolution of the XPS C 1s region with increasing CO doses. The as-prepared clean surface (UHV) exhibits a trace of potassium, likely originating from K impurities in the bulk of the Cu2O crystal. The K 2p spin-orbit split peak areas (at 293.05 eV and 295.95 eV) were held constant in all spectral fits. We assign the peaks at 288.1 eV, 287.6 eV, and 286.7 eV to CO adsorbate. The peak at 289.0 eV is assigned to CO2 at the PY site, after reaction with the site’s oxygen atom. Remaining peaks at binding energies >289 eV are assigned to CO shake-up satellites. Note: although the peak at 291.3 eV is at a binding energy similar to physisorbed CO2 (spectrum CO2 in Fig. S6) its desorption characteristics follows CO, while physisorbed CO2 is found to desorb already <100 K. The relative C 1s intensity of CO (including satellites) to CO2 is approximately 2:1.
Comments on the Potassium and Adventitious Carbon Contamination
The relative spectral abundance of K compared to the COx species on the surface can be estimated from the K 2p and C 1s integrated intensities after considering the photoionization cross sections for K 2p (2.0160 Mbarn) and C 1s (0.5251 Mbarn).[20] This results in a relative concentration of 1.56% (cross section adjusted K 2p intensity / cross section adjusted C 1s intensity from COx lines and satellites) for the 2.5 L CO spectrum. Or ~0.03 K atoms per surface (√3x√3)30° unit cell. Carbonate species are known to form on Cu2O and other metal oxides when potassium is present,[21,22] and carbonate has an expected binding energy at 289.3 eV.[23] However, there is no evidence of carbonate in the NEXAFS (Fig. S7) or IRRAS data (Fig. S2), as mentioned in the main manuscript.
We observe traces of adventitious carbon (adv. C) that grows with instrument time (3.89% compared to the COx species in the 2.5 L CO spectrum). The adventitious carbon is a result of adsorption from the residual gas in the analysis chamber.
Fig. S5. Background subtracted C 1s spectra (h = 380 eV) of the reconstructed Cu2O(111) surface before (UHV) and after a saturation dose (2.5 L) of CO at 93 K as well as at incrementally increased sample temperatures. All spectra were collected at the indicated temperatures to avoid potential re-adsorption. The fitted C 1s peak intensities of the CO (including satellites) and CO2 components were extracted to Fig. 3b of the main manuscript.
Fig. S6. XPS C 1s spectra (h = 380 eV) from Cu2O(111) at 93 K. The spectrum labeled CO2 (red) was collected under a CO2 total pressure of 5 × 10-9 mbar to stabilize physisorbed CO2 since desorption of CO2 was observed already at 93 K under UHV conditions. The peak at 291.2 eV is assigned to physisorbed CO2 on sites that have not been reduced by CO adsorption. The spectrum labeled CO (blue) was collected after dosing of 1L CO at 93 K.
NEXAFS Data Collection and Analysis
Near-edge X-ray absorption fine structure data was collected at B07B ES-1 at the Diamond Light Source, Didcot, UK [24]. The data were collected in Auger electron yield mode with the photon beam angle set to 0° and 60° relative to the surface normal. The spectra covered a kinetic energy range of 255 eV to 280 eV, capturing the carbon KLL Auger emission, while the photon energy was scanned from 278 to 310 eV in 0.2 eV steps. Measurements were taken for both the clean surface and after CO adsorption at 140 K. The 2D spectra were normalized by the ring current and emitted photoelectrons, using polished copper plates facing the two horizontal optical mirrors. The data shown below were integrated over a narrower kinetic energy range (260 eV to 272 eV) to eliminate artificial features caused by photoemission peaks, and the clean-surface spectra were subtracted after undergoing the same treatment. Finally, the spectra were normalized to the signal at 300 eV in high photon energy. The surface was saturated with CO via dosing ~5 × 10-8 mbar CO for several minutes in a preparation chamber; saturation coverage was confirmed by XPS. NEXAFS data were collected in the analysis chamber in a pressure better than ~1 × 10-8 mbar, with the background pressure being residual CO.
Fig. S7. C K-edge NEXAFS data of CO adsorbed on to the reconstructed Cu2O(111) surface at 140 K with an incident photon angle of 0° and 60° relative to the surface normal. The sharp peak at 288.1 eV is consistent with adsorbed CO and CO2. There is no carbonate peak at 290.4 eV.
Sum Frequency Generation Vibrational Spectroscopy
The SFG experiments were performed using a surface science instrument at Stockholm University. The laser light was produced using a Ti:Sa amplifier femtosecond laser, providing pulses at 800 nm with a pulse duration of 37 fs, repetition rate of 1 kHz and a pulse energy of 4 mJ. The IR light entered the UHV chamber via a BaF2 viewport and the SFG light exited via a CaF2 viewport. The remaining 800 nm light was spectrally filtered using a 750 nm short wave pass filter. The SFG was spectrally analyzed and detected using an optical spectrograph, equipped with an ICCD camera allowing simultaneous detection of the entire probed spectral region for each laser shot. The base pressure of the chamber was 5 x 10-10 mbar, and sample cleanliness was verified using LEED and SFG prior to experiments. The sample was mounted on a liquid nitrogen cryostat and heated at a constant rate of 3 K/min during the experiment to observe the desorption kinetics.
Computational Methods
Symmetric Cu2O(111) surface slab models were generated from an optimized (cubic) bulk structure with a lattice parameter of 4.29 Å, which is in close agreement with experimental reports of 4.27 Å.[25–27] The models comprise seven Cu2O layers with, the two top and bottom layers free to relax and the remaining atoms fixed at their lattice positions. Stoichiometric as well as Cu or O deficient surfaces where considered, as specified in the figures and tables below. The (√3 × √3)R30° surface reconstruction (PY) with CuCUS vacancies and a pyramidal Cu4O cluster attached at one of the CuCUS vacancies was recently identified as the most stable surface termination of Cu2O(111) at the synthesis conditions applied herein [2,28]. Therefore, the PY surface reconstruction is the primary surface termination considered herein with complementary data for other surface terminations found in the Fig. S8.
The surfaces were modeled using periodic spin-polarized density functional theory (DFT) calculations with the Vienna Ab initio Simulation Package (VASP)[29–31] and the PBE functional[32] with D3(BJ) dispersion corrections.[33,34] Hubbard corrections[35] were employed using a U-j value of 3.6 eV applied to the d-states of Cu, as suggested by Yu et al.[36] A plane wave basis set with a cutoff of 600 eV was used for the valence states (Cu: 3d104s1; O: 2s22p4; C: 2s22p2), and standard PBE PAW (projector augmented wave) potentials[37,38] for the core states. A Γ-centered 3×3×1 k-point mesh was used for sampling of the Brillouin zone, and a 0.05 eV Gaussian electronic smearing. A minimum vacuum separation of 15Å was employed to avoid spurious interactions between surface slabs.
Thermal corrections were added from thermochemical analysis based on vibrational (surface and gas states), rotational (gas states only), and translational (gas) contributions from numerical frequency calculations using ±0.015 Å displacements analyzed using the Vaspkit code.[39] The analysis is based on the harmonic oscillator, rigid rotor, and ideal gas approximations. Energy corrections of 0.01 and 0.65 eV were added to CO(g) and CO2(g) to ensure that gas-phase reaction energies are reproduced[40,41] These corrections are based on established values for the PBE functional [41] and were aligned versus measured adsorption energies of CO on Cu(111)[42–44] and Cu2O(100).[45] Barriers were computed using the climbing image nudged elastic band (CI-NEB) method[46] with nine intermediate images on smaller asymmetric surface models (including four Cu2O layers) and a plane wave cutoff of 400 eV.
Reactivity evaluation via the use of surface electrostatic potential maps, VS(r), was carried out as proposed and detailed by Stenlid and Brinck,[47–49] aided by the VESTA software.[50] Bader partial charges were computed using analysis scripts from the Henkelman and co-workers.[51] Bader charges were obtained as deviations from the valence electron count of the corresponding neutral atoms defined by the PAW potentials.
Optimized structures, with corresponding energies, have been uploaded to the Catalysis Hub database[52] and can be accessed directly via https://www.catalysis-hub.org/publications/KaragozCryogenic2025.
Additional Computational Results
Additional computational results regarding CO and CO2 adsorption onto various terminations of Cu2O(111) are included in Fig. S8 and Tables S1 and S2 below. Fig. S8 also includes surface electrostatic potential maps indicating sites susceptible to interactions with CO.
Fig. S8. Surface structures and corresponding 0.001 a.u. electrostatic potential maps of different surface terminations of Cu2O(111). ST = stochiometric, pristine; CuD = ST with 1ML CuCUS vacancies; ST-OS = ST with ⅓ ML OCUS vacancies; CuD-OS = CuD with ⅓ ML OCUS vacancies; PY = pyramidal reconstruction, i.e., CuD with ⅓ ML of Cu4O cluster adsorbed onto the CuCUS vacancies; PY-OS+SS = PY with 1 ML OCUS,PY and ⅓ ML of OCUS,SS vacancies. Color code atomistic model: Cu(orange), O(red).
Table S1
The CO and CO2 adsorption energies onto the surface terminations of Cu2O(111) displayed in Figure S8. At 140 K and pCO=103·pCO2=1 mbar. In the table, ‘vac’ indicates a vacancy.
Table S2
Calculated Bader charge (q) on the Cu atoms of different surfaces. * indicates adsorbed species.
a CO2 sits horizontally and interacts with 3 Cu atoms, giving a q ranging from 0.18 to 0.26 for Cu. The reported value is the average.
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Table extracted 1
| Surface | Site | ΔGad(CO), eV | ΔGad(CO2), eV |
| --- | --- | --- | --- |
| PY | CuCAS surface | -0.16 | |
| | CuCAS pyramid | -0.11 | |
| | CuCAS bridge long | -0.19 | |
| PY (reduced) | Opy-vac | -0.54 | -0.48 |
| | O-vac CuCUS-vac | -0.55 | -0.45 |
| | OPY | | -0.45 |
| ST | CuCUS | -1.11 | -0.47 |
| | CuCAS | 0.15 | |
| CuD | Cu-vac bridge | 0.16 | -0.46 |
| ST-OS | CuCUS | -1.07 | -0.40 |
| | O-vac bridge | -0.91 | |
| CuD-OS | O-vac hollow | -1.10 | -0.15 |
Table extracted 2
| Surface | q (Cu w/ CO(2)) | q (Cu w/o CO) |
| --- | --- | --- |
| PY | n/a | 0.53 |
| PY (CO*) | 0.60 | 0.53 |
| PY:O-vac | n/a | 0.22 |
| PY:O-vac (CO*) | 0.38 | 0.21 |
| PY:O-vac (CO2*) | 0.23a | n/a |