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Supplementary Information for ‘Catalyst Chemical State during CO Oxidation

Reaction on Cu(111) Studied with Ambient Pressure XPS and NEXAFS’

Baran Eren1, Christian Heine

1, Hendrik Bluhm

2, Gabor A. Somorjai

3, Miquel Salmeron*

,1,4

1, 2 Material and Chemical Sciences Divisions, Lawrence Berkeley National Laboratory, 1 Cyclotron

Road, Berkeley, California 94720, United States

3 Department of Chemistry, University of California, Berkeley, United States

4 Department of Materials Science and Engineering, University of California, Berkeley, United States

* Email: mbsalmeron@lbl.gov Phone: +1 510-486-6704

1. Calibration

1.1 Advanced Light Source Beamline 11.0.2 relative photon flux

The ratio between photon flux at 735 eV and 1150 eV is 2.19, and the ratio between photon flux at 490

eV and 1150 eV is 2.15.

1.2 Attenuation through gas phase

Attenuation through gas phase is obtained by measuring the C 1s component of a freshly cleaved HOPG

sample in UHV, and at near pressures of 0.3 Torr CO, 0.3 Torr CO + 0.03 Torr O2, 0.3 Torr CO + 0.1

Torr O2, and 0.3 Torr CO + 0.15 Torr O2 at room temperature. Neither CO nor O2 adsorbs on the HOPG

surface. The intensity is attenuated 1.51 times at 0.3 Torr CO, 1.56 times at 0.3 Torr CO + 0.03 Torr O2,

1.74 times at 0.3 Torr CO + 0.1 Torr O2, and 1.86 times at 0.3 Torr CO + 0.15 Torr O2

1.3 Normalization of the spectra for the concentration analysis

Photoionization cross sections are 0.25 Mb for O 1s, 0.6 Mb for Cu 2p and 0.3 Mb for C 1s at Ekin = 200

eV. All the Cu 2p, O 1s, and C 1s peaks are normalized to the photon flux given in 1.1, the gas phase

attenuation ratios given in 1.2, and the photoionization cross sections stated above.

For the concentration analysis, they are then divided by the total Cu 2p + O 1s intensity. This analysis is

necessary because Cu and Cu2O have very similar binding energies and widths in the Cu 2p region. Such

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an analysis results in 0.56 O:Cu atomic ratio for a bulk Cu2O sample, therefore the absolute error bar of

the intensity of our spectra is 12%.

2. Raw XPS and XAES spectra

2.1 Surface (1-3 layers) Sensitive spectra

2.1.1 O 1s spectra (Eph = 735 eV)

2.1.2 C 1s spectra (Eph = 490 eV)

The measurements in this study were performed with a spot size of 200 × 60 µm2 with a slightly

defocused beam as a compromise between signal intensity and beam damage. When no O2 was present in

the chamber, the beam induced peaks at around 283 eV (metal carbide) and around 284.4 eV (CHx

species) were also produced. The spot was constantly changed between each spectrum to minimize this

effect. This can be achieved also by further defocussing of the beam (as in our previous work)7 but is not

performed in this work because CO coverages on the (111) face of Cu is already very low, making it

difficult to get good signal to noise ratio. Once the reactant gases are pumped out, we can also observe

other contamination peaks with a total nominal coverage less than 0.1 ML. These peaks arise from

hydrocarbon contaminants desorbing from chamber walls when gases are introduced. Nonetheless, these

adventitious carbon peaks were negligible in the presence of O2 and did not affect current results.

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2.1.3 Cu 2p spectra (Eph = 1150 eV)

2.2 Near-surface (1-8 layers) sensitive spectra

These spectra are not used in the concentration analysis because NEXAFS spectra (shown below)

have better intensity and spectral resolution. Nevertheless, they are presented here so that they can be

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used as reference data for other studies. Moreover, the weak appearance of some peaks in the O 1s and C

1s regions at Ekin=610-620 eV supports the assignment to adventitious species adsorbed on the surface.

2.2.1 O 1s spectra (Eph = 1150 eV)

2.2.2 C 1s spectra (Eph = 900 eV)

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2.2.3 Auger Cu Cu L3M23M45 lines (Eph = 1150 eV)

2.2.4 Auger O KLL lines (Eph = 680 eV)

The intensity of the gas phase spectra are at least an order of magnitude lower than the intensity

of solid and condensed matter.

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3. Corrected NEXAFS spectra

3.1 O K edge (Ekin = 300 eV)

3.2 Cu L2,3 edge (Ekin = 300 eV)

4. XPS binding energy and normalized intensity tables of the O 1s region

Table S1 XPS binding energies in the O 1s region of gas matter. Values vary because they depend on the

sample work function.

CO(g) O2(g) O2(g) CO2(g)

Binding Energy (eV) 537.6 538.4 539.6 536.5

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Table S2 XPS binding energies and FWHM in O 1s region of solid and condensed matter.

Table S3 XPS binding energies and FWHM in C 1s region of solid and condensed matter. In the absence

of oxygen in the gas phase, intensities of the beam damage peaks (CHx and M-C) gradually increase.

CO/Cu has a complex structure due to strong satellites appearing at weak CO-Cu interactions. Besides

CO2δ-

/Cu2O, the peak at 289 eV also has a contribution from CO/ Cu2O.

5. Further discussion of the O K edge

Similar to the changes in observed during reduction by methanol exposure of the Cu2O in Ref.31

the

intensity ratio of resonances in the O K edge change depending on the stoichiometry of the oxide (Section

3.1 of the Supplementary Information). The higher ratio of 1’ to 3’ of the substoichiometric oxide was

interpreted to arise from a more metallic character than a thick Cu2O slab.31

The high resolution obtained

in our NEXAFS spectra makes it possible to distinguish multiple components in the white line, with one

component appearing as a shoulder at 533.6 eV in accordance with theoretical calculations.23

Interestingly, the ratio of this shoulder intensity to the white line peak intensity at 532.4 eV increases with

more metallic character. Especially at 298 K, the shoulder component at 533.6 eV has a stronger intensity

than the main component at 532.4 eV, which might also originate from the stronger metallic character of

substoichiometric oxides.

O/Cu or O/Cu2O Cu2O CO/Cu CO/Cu2O CO2δ-

/Cu2O

Binding Energy (eV) 529.4 530.2-.4 531.4-.6 534.2-.4 531.4-.5

FWHM (eV) 0.9-1 0.8-1 1-1.3 1.4-1.6 1-1.4

CO/Cu CO/Cu2O CO2δ-

/Cu2O CHx M-C

Binding Energy (eV) 286.2 287.9-288.0 288.9-9.1 ~284.4 ~283.1

FWHM (eV) 0.8-1 1.0-1.1 1-1.4 - -