Supplementary Information. Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts: Principal Differences in Stability and Reactivity

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1 Supplementary Information Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts: Principal Differences in Stability and Reactivity Armin Neitzel, a Alberto Figueroba, b Yaroslava Lykhach, a * Tomáš Skála, c Mykhailo Vorokhta, c Nataliya Tsud, c Sascha Mehl, a Klára Ševčíková, c Kevin C. Prince, d Konstantin M. Neyman, b,e Vladimír Matolín, c Jörg Libuda a,f a Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, Erlangen, Germany b Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1, Barcelona, Spain c Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, Prague, Czech Republic d Elettra-Sincrotrone Trieste SCpA and IOM, Strada Statale 14, km 163.5, Basovizza- Trieste, Italy e Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain f Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, Erlangen, Germany S1. Stability of 0.3 nm thick 7%Pd CeO 2 mixed oxide: Angle resolved XPS study The Pd 3d core level spectra obtained from 0.3 nm thick 7% Pd CeO 2 film deposited onto 1.5 nm thick CeO 2 (111) buffer layer at 110 K followed by annealing to 750 K in UHV are shown in Figure S1a. The spectra were obtained with Al Kα radiation ( ev) under a photoelectron emission angle of 20 0 with respect to the surface normal. Under these conditions, it was possible to explore the Pd 2+ distribution in the volume of the films during annealing. Similar to the Pd 3d S1

2 spectra obtained by SRPES (Figure 2d), we resolved two doublet peaks associated with Pd 2+ in the Pd CeO 2 solid solution (3d 5/2, ev) and in PdO particles (3d 5/2, ev). Figure S1. Stability of 7% Pd CeO 2 upon annealing in UHV. Pd 3d (a) spectra obtained with Al Kα radiation (hv= ev) at a photoelectron emission angle of The integrated intensities of the peaks associated with Pd 2+, PdO, and Pd Cu species (b) as a function of temperature. We observed a slight increase of the peak associated with Pd 2+ species in the solid solution at the expense of the signal from PdO species during annealing up to 600 K. This indicates dissolution S2

3 of PdO phase at the surface (see also Figure 2d) followed by the migration of Pd 2+ species into the bulk. Above 600 K, the decrease of the Pd 2+ signal is accompanied by the emergence of a new component at about 336 ev (Pd 5/2 ). This component has a higher binding energy than the Pd bulk metal (335.1 ev) and, therefore, is most likely associated with the formation of Pd Cu alloy. S1 Note that the corresponding peak was not detected in the Pd 3d spectra obtained with SRPES (Figure 2d). This means that the Pd Cu alloy forms only at the interface between Cu(111) substrate and the CeO 2 buffer layer. The integrated intensities of the peaks associated with the Pd 2+ species and the Pd Cu alloy are plotted in Figure S1b as a function of temperature. Based on the above observations, we conclude that the disappearance of the Pd 2+ species from the surface at 700 K observed by SRPES is caused by the diffusion of Pd 2+ into the bulk followed by the formation of a Pd Cu alloy. S2. Stability of 1.5 nm thick Pt CeO 2, Pd CeO 2, and Ni CeO 2 mixed oxides The Pt 4f, Pd 3d, and Ni 2p core level spectra obtained from 1.5 nm thick 12% Pt CeO 2, 7% Pd CeO 2, and 13% Ni CeO 2 films deposited onto 1.5 nm thick CeO 2 (111) buffer layer at 110 K, respectively, are shown in Figure S2. In contrast to the 0.3 nm thick 12% Pt CeO 2, 7% Pd CeO 2, and 13% Ni CeO 2 films discussed in Section 3.2, we observed destabilization and reduction of atomically dispersed Pt 2+ and Pd 2+ species followed by formation of metallic Pt 0 and Pd 0 particles (see Figure S2). In particular, we observed emergence of Pt 0 above 500 K (Figure S2a-b). Earlier we pointed out that the multilayer film cannot provide a sufficiently large number of surface {100} nanopockets to anchor all deposited Pt. S2 Therefore, those atoms which are located in the bulk or at unfavorable facets are bound too weakly to resist sintering to metallic particles. S2 On the 7% Pd CeO 2 film, the higher thickness yielded a higher amount of PdO. Annealing of this film gave rise to a decrease of the Pd 2+ species and the emergence of metallic Pd 0 above 300 K. In parallel, we observed an intermediate increase of PdO amount between 200 and 400 K. This may suggest that reduction of Pd 2+ species proceeds via intermediate formation of a PdO phase. Unlike on Pt CeO 2 and Pd CeO 2 films, we did not observe reduction of Ni 2+ on 1.5 nm thick 42% Ni CeO 2 film. S3

4 Higher concentration of Pt in the 1.5 nm thick Pt CeO 2 film significantly lowers the stability of the Pt 2+ species (see Figure S3a-b). We observed the emergence of an intermediate Pt* species at 350 K followed by the emergence of metallic Pt 0 at 400 K. We noted that the emergence of the intermediate Pt* species is accompanied by an increase of RER (Figure S3c). Figure S2. Stability of 1.5 nm thick 12% Pt CeO 2 (a-c), 7% Pd CeO 2 (d-f), and 13% Ni CeO 2 (g-i) films upon annealing in UHV. Pt 4f (a), Pd 3d (b), and Ni 2p 3/2 (g) spectra obtained with hv=180 ev, hv=410 ev, and hv=1000 ev, respectively. The integrated intensities of the surface species (b, e, h) and the resonant enhancement ratio (RER) (c, f, i) on 12% Pt CeO 2 (b-c), 7% Pd CeO 2 (e-f), and 13% Ni CeO 2 (h-i) as a function of temperature. The data in (a, b) are adapted from ref. S2 by permission (Copyright 2014 John Wiley & Sons, Inc.) S4

5 Interestingly, a higher concentration of Pd in thick Pd CeO 2 film does not cause a significant change in the stability of Pd 2+ species. We observed the onset of metallic Pd 0 at 400 K similar as in the case of 0.3 nm thick films. Figure S3. Stability of 1.5 nm thick 22% Pt CeO 2 (a-c), 20% Pd CeO 2 (d-f), and 42% Ni CeO 2 (g-i) upon annealing in UHV. Pt 4f (a), Pd 3d (b), and Ni 2p 3/2 (g) spectra obtained with hv=180 ev, hv=410 ev, and hv=1000 ev, respectively. The integrated intensities of the surface species (b, e, h) and RER (c, f, i) on 22% Pt CeO 2 (b-c), 20% Pd CeO 2 (e-f), and 42% Ni CeO 2 (h-i) as a function of temperature. S5

6 Again, we did not observe reduction of Ni 2+ species on the 1.5 nm thick Ni CeO 2 films upon annealing. However, we noted an increase of the intensity of satellite I which suggests a segregation of the NiO phase to the surface of the film above 600 K. References S1. Chiba, M.; Thanh, M. N.; Hasegawa, Y.; Obora, Y.; Kawasaki, H.; Yonezawa, T., Synthesis of Binary Solid Solution Cu Pd Nanoparticles by DMF Reduction for Enhanced Photoluminescence Properties. J. Mater. Chem. C 2015, 3, S2. Bruix, A.; Lykhach, Y.; Matolínová, I.; Neitzel, A.; Skála, T.; Tsud, N.; Vorokhta, M.; Stetsovych, V.; Ševčiková, K.; Mysliveček, J., et al., Maximum Noble Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum. Angew. Chem., Int. Ed. 2014, 53, S6