Supporting Information. Bulk Hydroxylation and Effective Water Splitting by Highly Reduced Cerium Oxide: The Role of O Vacancy Coordination

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1 Supporting Information Bulk Hydroxylation and Effective Water Splitting by Highly Reduced Cerium Oxide: The Role of O Vacancy Coordination Filip Dvořák 1,a), Lucie Szabová 2), Viktor Johánek 1), Matteo Farnesi Camellone 3), Vitalii Stetsovych 1,b), Mykhailo Vorokhta 1), Andrii Tovt 1), Tomáš Skála 1), Iva Matolínová 1), Yoshitaka Tateyama 2), Josef Mysliveček 1,*), Stefano Fabris 3,*), Vladimír Matolín 1) 1) Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 18 Prague 8, Czech Republic 2) Center for Green Research on Energy and Environmental Materials (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 35-44, Japan 3) CNR-IOM DEMOCRITOS, Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Via Bonomea 265, Trieste, Italy *josef.myslivecek@mff.cuni.cz, *fabris@iom.cnr.it a) Present address: University of Pardubice, Faculty of Chemical Technology, Nám. Čs. Legií 565, 532 Pardubice, Czech Republic b) Present address: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 8, Czech Republic

2 Table S1: Parameters of the samples from Figure 1. Near-surface stoichiometry and Nearsurface oxygen vacancy (Ov) concentration refer to idealized bulk-terminated structures shown schematically in Figure 1 and identified experimentally based on the corresponding LEED pattern. 1 Experimental CeOx layer thickness has been determined from the attenuation of an XPS Cu 2p3/2 signal from the Cu(111) substrate by the CeOx layer. Experimental stoichiometry refers to the stoichiometry of the samples determined from the analysis of the XPS Ce 3d peak 2 yielding the stoichiometry averaged over the information depth of XPS. 1 Generally, the samples exhibit vertical gradient of the Ov concentration with Ov accumulating in the near-surface region. 1 Nearsurface Ov accumulation represents a lower estimate of the thickness of the near-surface Ov accumulation region. Near-surface Ov accumulation is calculated from a two-slab model of an ideal reduced slab (Ce7O12, CeO1.67, or Ce2O3) supported on a CeO2 slab so as to yield the experimentally determined XPS stoichiometry of the samples. 1 ML refers to a monolayer of CeO2(111), equivalent to.31 nm. Near-surface stoichiometry Near-surface Ov concentration (w.r.t. CeO2, %) Exp. LEED pattern Exp. layer thickness (ML) Exp. stoichiometry (XPS) CeOx Near-surface Ov accumulation (ML) CeO CeO CeO CeO

3 Figure S1: STM image of the coexistence of the c-ce2o3 (4 4) and CeO1.67 (3 3) phases on the oxidized c-ce2o3 sample. 4 4 nm 2. LEED of this sample showed a dominating 3 3 superstructure. Table S2 (next page): Equilibrium lattice parameter (a) and relative distance between Ce 3+ ions and O vacancies or OH groups [d(ce 3+ -Ov) and d(ce 3+ -OH)] in the set of supercells used to model the considered CeOx systems. E is the calculated energy difference between different Ce 3+ and Ov configurations that were sampled within the same stoichiometries.

4 a [Å] d(ce 3+ -O v) / d(ce 3+ -OH) E [ev] CeO Ce1 NNN(O v) Ce2 NNN (O v) Ce1 NN (O v) Ce2 NN (O v) +.15 CeO H 2O 11.6 Ce1 NN(O v) & NN(OH) Ce2 NN (O v) & NN(OH) Ce1 NNN(O v) & NNN(OH) Ce2 NNN (OH) & NNN(OH) +.12 Ce1 NN(O v 1) & NNNN(O v 2) Ce2 NNN(O v 1) & NN(O v 2) Ce3 NN(O v 1) & NNN(O v 2) Ce4 NNN(O v 1) & NN(O v 2) CeO Ce1 NNN(O v 1) & NNNN(O v 2) Ce2 NNN(O v 1) & NNNNN(O v 2) Ce3 NNNNN(O v 1) & NNN(O v 2) +.28 Ce4 NNNN(O v 1) & NNN(O v 2) Ce1 NNN(O v 1) & NN(OH) & NN(OH) CeO H 2O 11.6 Ce2 NN(O v 1) & NN(OH) & NN(OH) Ce3 NN(O v 1) & NN(OH) & NNN(OH) Ce4 NN(O v 1) & NNN(OH) & NNNN(OH) Ce1 NN(O v 1) & NN(O v 2) Ce2 NNNN(O v 1) & NN(O v 2) Ce3 NNN(O v 1) & NN(O v 2) +.4 Ce4 NN(O v 1) & NNN(O v 2) Ce1 NNNN(O v 1) & NN(O v 2) CeO 1.75(NNN) 4.6 Ce2 NNN(O v 1) & NN(O v 2) Ce3 NNN(O v 1) & NN(O v 2) +.22 Ce4 NN(O v 1) & NNN(O v 2) Ce1 NNNN(O v 1) & NN (O v 2) Ce2 NNN(O v 1) & NNN(O v 2) Ce3 NN(O v 1) & NNN(O v 2) Ce4 NN(O v 1) & NNN(O v 2) Ce1 NN(O v 1) & NN (OH) Ce2 NNNN(O v 1) & NN(OH) +.3 CeO 1.75(NNN)+H 2O 4.1 Ce3 NNN(O v 1) & NN(OH) Ce4 NN(O v 1) & NNN(OH) Ce1 NNNN(O v 1) & NN (OH) Ce2 NNN(O v 1) & NN(OH) Ce3 NNN(O v 1) & NN(OH) Ce4 NN(O v 1) & NNN(OH) Ce 3O 5 Hexagonal First layer: 12 reduced Ce a = Second layer: 8 reduced Ce c = 9.51 Third layer: 4 reduced Ce c-ce 2O

5 Figure S2: (a) TPD of H2O from c-ce2o3. Data from Figure 1 in the main text. (b) Evolution of the stoichiometry of the Ce2O3 sample upon water exposure at 11 K (marked with arrow) and subsequent heating to increasingly higher temperatures. The strongest oxidation of the sample can be associated with desorption of H2 above 5 K. Upon water exposure at low temperature, the change of the sample stoichiometry is comparably smaller. Figure S3: Dependency of T* on the partial pressure of H2O (left) and H2 (right) for the water dissociation and H2 evolution reactions. We define T* as the T at which the reaction G is, while G< for T<T*.

6 Figure S4: Examples of experimental data and evaluation of experimental values. The experimental data (black lines) are belonging to samples marked,, in Figure 2c. (a) Determination of sample stoichiometry. XPS Ce 3d signal excited by Al Kα radiation is measured at normal emission. The resulting intensities are fitted according to the procedure developed by Skála et al. 2 for Ce 4+ and Ce 3+ contributions [light green and dark green lines in (a), the resulting fit plotted in red] and the weight of the Ce 4+ and Ce 3+ contributions is converted to sample stoichiometry CeOx. The obtained stoichiometry CeOx represents an average over the information depth of the XPS Ce 3d signal. (b), (c) Determination of the TPD yield of H2O (b) and H2 (c) from the recombination peak. TPD signal was obtained using a quadrupole mass spectrometer (QMS, Pfeiffer PrismaPlus) placed behind a differentially pumped nozzle in order to suppress background signal in TPD spectra. H2O exposure took place at sample temperature 1 K and the temperature ramp during TPD was set precisely at 2 K/s rate. The raw current output of the QMS has been calibrated to molecular fluxes for several reference pure gases - either directly by combining known surface coverage [using a Pt(111) single crystal as a reference] with calculated time integral of the TPD signal, or indirectly by using relative ionization probabilities for two species for deriving new sensitivity factors from the established ones. TPD yields (red hatched areas) were obtained by integrating the QMS output between 36 and 67 K [H2 TPD, (b)], or between 36 K and 64 K [H2O TPD (c)], always after subtraction of corresponding QMS background (green lines). (d) Determination of OH concentration on the samples. SRPES O 1s signal excited by 64 ev synchrotron radiation was measured at normal emission. The resulting intensities are fitted for OH and CeOx lattice O contributions 3,4 [light green and dark green lines in (d), the resulting fit plotted in red], and the area belonging to the OH peak evaluated.

7 Appendix 1: Attenuation of bulk OH signal in SRPES To estimate the expected intensity of SRPES OH signal on the samples presented in Figure 2d we make following (extreme) assumptions regarding the distribution of OH species on the surface and in the bulk of the samples: On the ι-ce7o12 and CeO1.67 samples (denoted, in Figures 2c,e) exhibiting slow oxidation by water during TPD experiments, all OH species are assumed to reside on the surface of the samples. Upon 1 st TPD of water (Figures 2a, b), these samples produce cm -2 H2 molecules and cm -2 H2O molecules by recombination of surface OH at approx. 5 K. This requires a surface concentration of OH groups of cm -2. Performing SRPES measurement of the OH peak on these samples at 3 K (as in Figure 2d) the expected intensity of the surface OH SRPES signal will be IOH S = cm -2 SRPES sensitivity. [1] On the c-ce2o3 sample (denoted in Figures 2c, e), on the other hand, the effective oxidation during first TPD indicates a bulk OH reaction. We assume a homogeneous distribution of OH in the bulk and quantify the bulk OH density at temperatures below the H2 desorption temperature from the H2 production upon first TPD ( H2 molecules cm -2, see Figure 2a) and from the thickness of the c-ce2o3 sample (12.3 monolayer or 3.8 nm, see Table S1) as cm -3 of bulk OH. The expected intensity of the bulk SRPES OH signal will be obtained using the bulk concentration of OH groups in the sample and the mean free path of the O1s photoelectron mediating the SRPES measurement. According to the TPP-2 formula, 5 the mean free path is 4.8 Å for the photoelectron of 11 ev kinetic energy in c-ce2o3, and the resulting intensity IOH B = cm cm SRPES sensitivity = = cm -2 SRPES sensitivity, [2] is lower than the expected signal from the surface OH on the ι-ce7o12 and CeO1.67 samples (IOH S, Eq. 1). Our estimate of the intensity of the OH peak as measured by SRPES on the c-ce2o3, ι- Ce7O12 and CeO1.67 samples is in qualitative agreement with the experimental results presented in Figure 2.

8 References: (1) Duchoň, T.; Dvořák, F.; Aulická, M.; Stetsovych, V.; Vorokhta, M.; Mazur, D.; Veltruská, K.; Skála, T.; Mysliveček, J.; Matolínová, I.; et al. Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111). J. Phys. Chem. C 214, 118, (2) Skála, T.; Šutara, F.; Prince, K. C.; Matolín, V. Cerium Oxide Stoichiometry Alteration via Sn Deposition: Influence of Temperature. J. Electron Spectros. Relat. Phenomena 29, 169, (3) Mullins, D. R.; Albrecht, P. M.; Chen, T.-L.; Calaza, F. C.; Biegalski, M. D.; Christen, H. M.; Overbury, S. H. Water Dissociation on CeO2(1) and CeO2(111) Thin Films. J. Phys. Chem. C 212, 116, (4) Matolín, V.; Matolínová, I.; Dvořák, F.; Johánek, V.; Mysliveček, J.; Prince, K. C.; Skála, T.; Stetsovych, O.; Tsud, N.; Václavů, M.; et al. Water Interaction with CeO2(111)/Cu(111) Model Catalyst Surface. Catal. Today 212, 181, (5) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths (IMFPS). IV. Evaluation of Calculated IMFPs and of the Predictive IMFP Formula TPP-2 for Electron Energies between 5 and 2 ev. Surf. Interface Anal. 1993, 2,