Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals

The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.

The sample was cleaned by a recipe which involved a flash desorption heating to 600°C, repeated sputtering by Ar + at 5 kV for 15 minutes and subsequent annealing to 950 °C for 3 minutes to ensure the desired surface. Despite careful preparation a small contaminant of Si (around 4 % of a monolayer) was detected throughout the measurements. No other contaminant was detectable in the XP spectra.
Before measurement start, the surface was chemically cleaned by O 2 and H 2 at 450°C, where oxygen removed adventitious carbon and hydrogen removed potential surface oxides created by the oxygen treatment. After this chemical cleaning the sample was cooled down in 0.16 L N /min H 2 until the intended measurement temperature was reached.
The sample gases used for the experiment were H 2 , CO, CO 2 , O 2 , and He. H 2 , CO 2 , O 2 and He were of 5N purity while the purity of CO was 99.997%. H 2 , CO 2 and He were delivered from 50 L 200 bar cylinders while CO and O 2 were delivered from similar 10 liter bottles to accommodate for the safety regulations at DESY. The gas cylinders were stored in safety cabinets ~20 meters of piping away from the point of the experiment. For CO 2 , O 2 , He and H 2 gas purifiers from SAES (model Nr. MC45) were implemented. In order to avoid formation of nickel carbonyls by CO interacting with stainless steel, a combination of copper piping and a gas purifier for CO (PALL, GLP8SIPVMM4) was used. The gases were dosed and mixed using a custom-built mixer. During operando measurements the flow was 0.20 L N /min CO or CO 2 and 0.4 L N /min H 2 . The gas mixing unit is not a UHV system.

Data analysis
Data analysis was performed using custom scripts for IGOR PRO 6.3 and Python 3. For adsorbate peak fitting, the specqp program and graphical user interface by Dr. Mikhail Shipilin was used. 4 The peaks were fitted to Voigt shapes. For the Rh 3p region a Tougaard background was utilized, where the metal and gas phase peaks were fitted as Voigt shapes, while the electron energy loss features were fitted as asymmetric Voigt shapes according to reference. 5

Spectra normalization and background subtraction
The O 1s and C 1s spectra presented in Figures 1 and 2 of the main text were normalized by the integration time and a constant background was subtracted. Inelastic scattering from photoelectrons scattering in the gas phase contribute to the background on the higher binding energy side, and can be modelled by a Shirley background, but the distinction between inelastic background and shake-up intensities from the adsorbates is hard to achieve, which is why it was left out of the presented figures in the main text. In order to do peak fitting, however, a modelling of the background is a technical necessity. Therefore, for peak fitting purposes of the C 1s spectrum, a Shirley background has been modelled in the fit and subtracted, neglecting the possibility for shake-up intensity to contribute.
In the combined O 1s and Rh 3p spectra, used for subtraction of the electron energy loss feature, a Tougaard background has been modelled for the fit of the spectrum. The Tougaard background has also been used for analysis of the Rh 3d spectra.

Calculations of surface coverage
The total coverage estimations are based on the comparison between the peak areas of the C 1s adsorbate region and the Rh 3d 5/2 component and for oxygen on the comparison between the area of the O 1s adsorbate region and the Rh 3p 1/2 component. An example of the overview spectra that were used for this calculation is given in figure S1. Figure S1. Overview XP spectrum utilized for the calculation of the surface coverage including the C 1s and Rh 3d 5/2 regions. This method has previously been utilized for similar coverage estimations for CO oxidation reactions. 6 In order to estimate the photoelectron intensity of a substrate of thickness we utilize Equation S2 In this equation is the number density of the atom in the substrate and, is the photoionization cross section, is the intensity 0 of the x-ray beam and is the effective probing depth which depends on both the mean free path of the photoelectrons and the ′ penetration depth of the x-rays (see Equation S1, in the "x-ray beam properties" section). The exponential term that follows models the attenuation of the photoelectrons in the gas phase. is the pressure, is the inelastical scattering cross section of the gas phase and is the travel distance of the photoelectrons in the gas phase. The integral over the thickness combines the effect of x-ray attenuation depth and the photoelectron attenuation dependent on the mean free path within the substrate.
The surface coverage calculations performed in this work utilize a model where the atomic coverage of one adsorbate species (C or O, denoted in the equation) of thickness t is compared to the semi-infinite Rh substrate denoted in the equation. This modifies the equation a bit as the treatment of two layers have to be performed separately, but simultaneously, some of the terms which are the same for both species cancel out. The result is Equation S3 The added exponentially decaying factors in the denominator are due to the attenuation of the Rh photoelectrons by the adsorbate layer. In order to extract the coverage, this equation was solved for the unknown thickness t simultaneously for all temperatures, and the resulting thickness t was compared to the thickness of a ML of the adsorbates on Rh. If the kinetic energy of the emitted photoelectrons of the adsorbate and the metal differ by a sufficiently large amount, we cannot cancel out the photoelectron attenuation term of the equation, and need to calculate it separately for the adsorbate and the metal. The utilization of this equation is possible thanks to the small energy difference between the utilized Rh and adsorbate peaks. In order to estimate the error of this method we duplicated one of the measurements at a different incidence angle resulting in a different penetration depth, but without changing the actual coverage. This resulted in a deviation of the estimated coverage by a factor 0.015. Consequently, the relative coverages should be well representative.
The coverage of each fitted component, was on the other hand determined by utilizing a detailed spectrum of only the C 1s region. The estimated relative error for the coverage calculations is 17.2 %. The largest errors stem from the CH and CH 3 peaks The total coverages of all carbon species during CO hydrogenation are given in Table S1.

Hydrogen electron energy loss features
The photoelectrons emitted from the sample all have some probability to cause electronic excitations of the gas phase H 2 molecules. This excitation is visible as a satellite, with a maximum appearing at 12.8-13 eV away from every metal peak on the higher binding energy side and is mainly caused by excitation of the H 2 molecule to the 1 ungerade state (sometimes just referred to as the 2p state,   ~ 12.5), 7 but excitations to higher states and into the vacuum also contribute. The multitude of excited states of the excited H 2 molecule give the peak a very broad and non-figurative shape. This hydrogen "ghost" peak has previously been observed and reported in XP spectra in ref. 8 Unfortunately for the case of studies on Rh catalysts, the Rh 3p 1/2 component, located at 521.4 eV produce a hydrogen electron loss peak with a maximum at 534.3 eV which overlaps with the adsorbate region of oxygen and the majority of the intensity in the O 1s region originates from this electron energy loss (EEL). In order to extract oxygen coverages, we follow the steps illustrated in Figure S2. To begin with, in Figure S2 a) we see the entire Rh 3p and O 1s region. It comprises two Rh 3p peaks in blue, their corresponding hydrogen electron loss peaks as well as the gas phase peak. We modelled the Rh peaks and the gas phase peaks as Voigt shapes, the electron loss peaks as asymmetric pseudo Voigt shapes using the method by Schmidt et al 5 and were able to subtract a Tougaard background. After the subtraction we obtain the spectrum in Figure S2 b). We cropped the spectrum for the Rh 3p 3/2 component and the 3p 1/2 component, in the region highlighted by green and black. From the 3p 1/2 component we subtract the dashed line which stems from the electron loss feature and from the 3p 3/2 component we subtract the dotted line, which is the contribution from the 3/2 component. The cropped spectrum of the Rh 3p 3/2 component is then scaled and shifted to the height and binding energy maximum of the 1/2 component. This results in Figure S1 c). The blue dotted data contains the overlapping adsorbate spectrum and the electron energy loss peak while the black line is a scaled and shifted peak of the 3/2 component and consequently does not have any adsorbates present. Subtracting these lines results in an isolated adsorbate spectrum.
The implementation of the Tougaard background modelling introduce a methodological error since areas of the electron loss peaks contribute to the modelled background, when they in reality do not contribute. Therefore, a quantification of the introduced error cannot easily be made. This is the reason why we prefer to not discuss the O 1s region in quantitative terms. Figure S2. Illustration of the removal procedure which eliminates the contribution of the EEL peak from the O 1s spectra.

Supplementary Information
On binding energy corrections.
The reported binding energies in the main text are reported as measured after referencing to the Fermi edge, with no corrections to the presented data. Inherent to high energy (hard) x-ray experiments, some peaks may be shifted due to the recoil energy; i.e. the energy loss of the photoelectron due to the recoil from the emission process. This effect is less significant for conventional soft x-ray XPS performed with excitation energies of ~100 eV above the intended BE region. We have thus dedicated a section herein to estimate the magnitude of the said effect. The recoil energy experienced by an atom in a solid upon photoionization is approximated using is the kinetic energy of the electron and is the mass ratio of the electron and the atom. For a photoelectron ejected from a C 1s orbital of a carbon adatom (B.E. 284) with the instrumentation described in the article we have a kinetic energy of 4312.5 eV and the mass ratio for a carbon atom and an electron is 4.56*10 -5 . Since the binding energy is referenced to the Fermi edge, we also need to include the recoil energy of the valence band close to the Fermi level. The total shift due to recoil energy will thus be as shown in Equation S5: Here the index VB denotes the valence band of the metal. The photoelectrons of the Fermi edge have a kinetic energy of 4596.5 eV and the mass ratio of the photoelectron to the Rh atom is 5.33*10 -6 . The last piece of correction is to estimate the recoil shift of our references as they seldom have accounted for recoil energy in their calculations either. The observed change in binding energy due to recoil energy compared to a soft x-ray reference system will be the difference between the systems as shown in Equation S6.
where the indices Exp and Ref denote the experiment and reference respectively. The photon energy for measurements on the C 1s level used in reference 10 was 400 eV and will be used as an example. The kinetic energies assuming a 5 eV work function (a typical value as measured in our instrument) will be 106 eV for the C 1s photoelectron, and 395 eV for the valence band. Insertion of these values into equation S6 gives a shift of + 0.17 eV. The sign is significant; the recoil energy is a loss of photoelectron kinetic energy which means the binding energy is shifted in positive direction. As a carbon adatom is the smallest system considered in this paper the shift of all other species will be less than 0.17 eV. Furthermore, with the high-coverage-situation described in the main text, additional shifts due to coadsorption of several chemical species occur. The effect of coadsorption has for the case of CHx and CO on Rh(111) been investigated by DFT, which showed that the binding energies of CHx species can shift in either positive or negative directions up to 0.4 eV. 11

Justification for peak assignments C 1s peak assignments
The assignments of the peaks are made on a basis of experimental references from literature, calculated shifts from DFT and/or deductive reasoning. To begin with, we will discuss the C 1s peak assignments. In the literature there are a few different assignments to the CH x species with varying credibility. An often cited paper by Veselli and coworkers is one of few synchrotron-based reference studies which, amongst other, identifies two peaks around 283.3 eV and 283.7 eV. The 283.3 eV peak is assigned to atomic C, while the second one could be either CH or an inequivalent variant of atomic C. 10 Other references with far more noisy spectra originate from Iodine substituted methane in experiments by Solymosy and coworkers 12,13 , which tend to note a temperature and coverage dependence of the peak position. At the examined temperatures and coverages most relevant to our study they identify CH 3 at 284.6 eV and, the CH 2 specie at 283.7 eV. It is unanimously agreed upon that more hydrogen saturated hydrocarbons result in higher binding energy, as this has been confirmed by DFT calculations which are notoriously bad for calculating exact spectroscopic energies, but work well for examining trends. We have identified the two peaks clearly identifiable at 284.0 eV and 284.4 eV. From DFT calculations on the reaction pathway for CH 4 dissociation we know that the two reaction sinks are the CH 3 and CH species, which thus are the two most likely contributors. As was mentioned in the discussion segment of the main text, we know that multicarbon molecules also contribute to the C 1s spectra at the lower temperatures since the total C coverage exceeds 75 % of a ML, which is unlikely unless multicarbon species are present.
It is also worth mentioning that sp2-hybridized carbon, such as graphite, (poly-) aromatic compounds and triple-bond containing intermediates such as ethyildyne. There is a range between 284.7 eV and 285.3 eV in Figure 1 of the main text which clearly contains spectrum intensity, but is not highlighted with any identifier. At first glance it might look reasonable to assign this region to graphitic or aromatic carbon, as the binding energies would match, but we do not believe that there is a major contribution from these compounds. The reason is that the shape of the intensity in this region does not align with any aromatic or graphitic type of carbon. These molecules have several symmetric and spectroscopically equivalent carbons and would have resulted in a sharp and narrow peak, and this is the opposite of what we observe as the region does not contain any peak or shoulder at all. The region looks more like a tailing of the CH x peaks, which is a more plausible explanation considering that it is common for chemisorbed hydrocarbons on metal surfaces to produce shake-up features in XPS.
CO, on the contrary, is well-referenced in the literature and it has been agreed upon that there is an on-top peak with a maximum at 286 eV and a hollow peak at 285.3 eV. There is to the authors knowledge no reference spectrum for the methoxy species on Rh, as Rh is seldom used as a catalyst for methanol synthesis, but on other surfaces (i.e. copper) we have reliable references, which point toward a binding energy of 285.3 eV, and a shifting towards higher binding energy at higher coverages. 14 Normally, the Cu and Rh surfaces have fundamentally different electronic d-band filling which drastically change the binding energy of the adsorbates bound to them, but in the case of chemisorbed methoxy radicals this is less of an issue since the methoxy species are bound to the metal through the oxygen atom, rather than the carbon. We found a peak at 285.5 eV, which clearly has too high binding energy to be CO in the hollow site, moreover, the CO in hollow peak tends to emerge only at coverages above 33% of a ML, but the shoulder is still there even at coverages below 30%. We must thus assume another species and the methoxy radical is the next stable intermediate with a close-lying binding energy. Other potential species to fill this binding energy range would include CHO, COH, CHOH, but time-resolved pump-probe experiments have found that these intermediates are generally very short-lived and are consequently not expected to contribute significantly. 15

On beam induced effects
The CO and CO 2 hydrogenation reactions are inherently susceptible to beam induced effects, sometimes referred to as beam damage. Three mechanisms for beam damage have been considered for this experiment. Firstly, we have direct heating of the sample by the x-ray beam. This has been estimated to only account for a very small degree of heating, and is readily offset by the cooling of the gas stream. Secondly, we have direct excitations of the molecules by the x-ray beam. Even though a narrow focus and high flux of the beam was used we note that a double excitation of the adsorbate is unlikely. The third mechanism is excitation of gas and adsorbate molecules due to photoelectrons from the metal substrate. The excited gas phase molecules have substantially lower barriers for dissociation, and are thus more reactive. This can show in the adsorbate spectrum as a flux dependence of the line shapes and even as the addition of extra peaks with increased flux. The combination of hard x-rays, the high pressure and high flux makes this mechanism the most severe form of beam damage. We know from experience that the C 1s region is most sensitive to beam damage, and we have attenuated the beam until the C 1s spectrum stopped changing. With a base flux of ~13 10 photons/second and 10x10 m we noted that an attenuation factor of 5.5 was necessary to offset beam induced effects for the CO hydrogenation systems and a factor of 11 was necessary for the CO 2 hydrogenation systems.

Online mass spectrometer data
Mass spectrometer (MS) data for selected masses acquired during CO hydrogenation over the Rh(211) surface are shown in Figure  S3, and for CO 2 hydrogenation on the Rh(211) surface the corresponding data is available in figure S4. This is thus acquired during the acquisition of the spectra in Figure 1 b) and Figure 3 b) of the main text respectively. The m/z ratios 2, 28 and 44 correspond to hydrogen, CO and CO 2 respectively, while m/z= 15, 18, 30 and 31 all correspond to potential products of the CO or CO 2 hydrogenation reaction. m/z = 15 is the methyl radical, which is a prevalent part of the MS pattern for methane. We can unfortunately not use the strongest peak of the CH 4 mass spectrum (m/z = 16) as it overlaps with the oxygen radical which is a prevalent feature of the CO pattern. m/z = 18 is the molecular weight of water, and also the most prevalent peak in the water mass spectrum. m/z = 30 is the mass for ethane (C 2 H 6 ), but influences from the natural occurrences of 14 CO, 13 C 17 O and C 18 O may contribute to this peak. Once again, the strongest peak in the ethane pattern is m/z = 28, which cannot be used as it overlaps with the reactant CO. Lastly, m/z = 31 correspond to the methoxy radical (CH 3 O) indicative of methanol production. We cannot use the m/z = 32 signal due to overlap from a tiny contamination in the gas stream from atmospheric oxygen.
It is clear from Figure S3 that most product signals do not change drastically with sample temperature. The largest exception is the water signal at m/z = 18. It should be noted, however, that the water signal is affected by the accumulation onto the chamber walls during transport from the sample region to the mass spectrometer and thus tends to increase or decrease steadily over time as the reaction is running. Other than the water signal there is a small increase of the methoxy signal at m/z = 31.
In Figure S4, on the other hand, there is no significant change of the MS signal of any of the products, except for the water signal which -as discussed previously -does not necessarily relate to the actual activity of the reaction.  The large flows required in order to maintain the pressure at the sample region will create inconveniences for the detection of reaction products. We need to consider the minimum turnover frequency (TOF) required in order to detect products. This quantity can be estimated from known experimental parameters. The TOF is defined as the number of products ( ) per active site and second as written in equation S7.

Equation S7
where is the number of active sites and is the dwell time of the molecules in the uptake volume. From the calibrated pressure and flow ratings we estimate that the 0.053 mm3 uptake volume contains circa 3.67*10 13 CO molecules at any given point and the dwell time is circa 1.29*10 -4 seconds. We assume that on the (211) surface the limiting active sites for the reaction are the fourfold sites required for CO dissociation, which occur once per surface unit cell. This means that the (211) surface contains 9.97*10 12 sites within the sampling region.
In order for a change in the MS signal to be detected it has to be changed by 3x the noise level, the noise level being defined as 2x the standard deviation of the signal. Out of the reactions examined in this paper the most active is supposedly CO hydrogenation on the (211) surface. From the MS data we see that the average CO signal at m/z = 28 when the reaction was turned on was 2.13*10 6 a.u. Investigating the signal for the potential products, we see that the noise level of m/z =15 was 162 a.u. and in order for the change in this signal to reach the limit of detection it has to increase by 162*3 = 486 a.u. We call the quotient between the lowest detectable amount of product signal and the reactant signal the "lowest detectable fraction" (LDF) and this quantity evaluates to 486/2.13*10 6 = 2.29*10 -4 . We work under the assumptions (1) that the products and the CO molecules are present to the same degree in the sampling volume and in the MS detector volume and (2) that the signal is proportional to the number of molecules, with the same proportionality for both CO and CH 4 . With these assumptions we can estimate the number of turnovers needed in order for the reaction to be detected as the LDF multiplied by the number of CO molecules in the sampling volume, i.e. 2.29*10 -4 * 3.67*10 13 = 7.49*10 10 . This we can insert into equation S7 which yields equation S8 = = 8.39 × 10 9 9.97 × 10 12 * 1.29 * 10 -4 = 6.5 The resulting value of 6.5 turnovers per site and second is higher than the expected turnover frequency from our system, considering that previous measurements on the TOFs for Rh single crystals and foils typically amount to 0.1 -1.5 per second and site, even when measured at considerably higher pressures than in this study. 16 Thus, while the change in signal is not expected to reach above the limit of detection, the slope of the signal may still be statistically significant. We have thus fitted the spectra with a linear model in order to clarify any increases or decreases in the signal. The slope and the corresponding standard deviation are shown in Table S3. In the case for m/z = 31 we noted an increase in the signal at lower temperatures, which in the table is shown as a positive slope. The increase of m/z signal coincides with the appearance of the methoxy species in the O 1s and C 1s spectra. While weak, the slope is statistically significant and opposes the slope of the reactant gases, indicating a possibility that some methanol is being produced. The same is true for the m/z = 15 signal, which corresponds to the methyl radical and is part of the mass spectra of all alkanes.
It is interesting to note that the lower temperatures are seemingly more active than the higher temperatures, which would indicate that the limiting factor for turnovers are not related to the energy barriers of the reaction but rather to the residence time of the reactants on the surface. We note that this might be different at higher pressures.

All O 1s spectra
Due to space restrictions and focus on the relevant results we have omitted the O 1s spectra acquired during the measurement from the main text. In this section we discuss the O 1s spectra and how they support the findings of the main text. The adsorbate signal is very broad compared to the C 1s region, and the overlap with the EEL region makes the interpretation harder. Figure S5 displays all spectra of the O 1s region during CO hydrogenation on Rh(111). Subplot a) plots the spectra as acquired, including the overlapping EEL peak. For comparison, each of the spectra are compared to the 325°C spectrum. Subplot b) shows the difference spectrum between 325°C and each other temperature, with a Savitzky-Golay smoothening algorithm applied. This clarifies at which binding energies the spectra differ, and it removes the EEL contribution, but this treatment also subtracts the parts of the spectra that do not change with temperature. For this reason, we have also plotted an EEL subtracted spectrum in subplot c), using the method discussed in the "hydrogen electron energy loss" section. We make the following observations: in the difference spectra as well as in the raw spectra we note enhanced intensity around 530.7 eV for the lower temperatures. This corresponds well to the binding energy of the methoxy species, and the decreasing trend of the peak agrees with the similar temperature behavior of the methoxy peak in the C 1s spectrum. Around 532 eV where one would expect CO to appear, there is little-to-no difference between the spectra at different temperatures. But, as was seen in the C 1s spectra, this is expected, since the CO coverage was also not changing significantly in the C 1s spectra either. In the EEL-subtracted spectrum we note that the maximum intensity is located around 532 eV, indicating that the majority of the oxygen coverage stems from CO.
Similarly, in Figure S6 we have made the same treatment for the O 1s region on Rh(211). As was the case for the (111) surface orientation we note a that the most significant contribution to the O 1s spectrum in Figure S6c) once again stems from CO as seen by a peak maximum at 532 eV. This contribution from CO is lower than on the (111) surface, which is also consistent with the C 1s spectra. What is not consistent, on the other hand is the coverage of methoxy on the Rh(211) surface, as we cannot see the same trend in the 530.7 region as we did for the (111) surface. The very low coverage in general is likely the cause of this discrepancy. Using the Rh 3p ½ component as a standard, we estimate the total O 1s coverage on the Rh(111) to amount to 25% and the total coverage on the Rh(211) surface to around 15%.
In Figure S7, we have displayed the O 1s region during CO 2 hydrogenation. We note, similar to the C 1s region, a higher coverage on the (211) surface, most evident in Figure S7 c). We also note that the 530 (O ads ), 530.7 (CH 3 O eV) and 532 (CO) regions differ in coverage between the CO and CO 2 hydrogenation reaction as seen in Figure S7 d). Figure S5. XP spectra of the O 1s region during ongoing CO hydrogenation on the Rh(111) surface. The pressure was 150 mbar, the CO:H 2 ratio was 1:2, and the photon energy was 4.6 keV. Figure S6. XP spectra of the O 1s region during ongoing CO hydrogenation on the Rh(211) surface. The pressure was 150 mbar, the CO:H 2 ratio was 1:2, and the photon energy was 4.6 keV. Figure S7. XP spectra of the O 1s region during ongoing CO 2 hydrogenation on a) the Rh(111) surface, and b) the Rh(211) surface. Subplot c) illustrates the difference between the two surfaces for the measurements at 250°C, and subplot d) contains a comparison of the XP spectra recorded during the CO hydrogenation reaction and the CO 2 hydrogenation reaction at 250°C on the Rh(211) surface. The total pressure was 150 mbar, the CO 2 :H 2 ratio was 1:2, and the photon energy was 4.6 keV.

Spectra of the Rh 3d region
Spectra of the Rh 3d region are shown in Figure S8. As a function of temperature, minute changes to the Rh 3d region were observed, mainly at the lower binding energy side of the 5/2 component. At all examined conditions, the Rh spectrum remained metallic. For all systems, the intensity around 306.6 eV increases with temperature. Since the coverage is temperature-dependent, we conclude that these intensity changes occur due to a surface state which at higher temperatures becomes accessible due to a lower coverage of hydrogen, carbon and oxygen compounds and this is consistent with other experiments where the surface-core-level shifts have been resolved in vacuum. 17 The shoulder barely visible at 315.8 eV in the CO-containing spectra occurs due to electron losses from excitations of the CO molecule. Fits of the C 1s spectra during CO hydrogenation.
All fits of the CO hydrogenation peaks used for the graph in Figure 2 of the main text are shown in Figure S9 for Rh(111) and in figure S10 for Rh(211). The raw data (without the Shirley background subtraction) is displayed in Figure 1 of the main text. Figure S9. Fitted XP spectra of C 1s region during CO hydrogenation on Rh(111), which was used as the basis for the coverage calculations of the individual components as shown in Figure 2a) of the main text as well as Table S1 in this Supplementary Material. Figure S10. Fitted XP spectra of C 1s region during CO hydrogenation on Rh(211), which was used as the basis for the coverage calculations of the individual components as shown in Figure 2b) of the main text as well as Table S1 in this Supplementary Material.