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1 Mechanism of Olefin Hydrogenation Catalysis Driven by Palladium-Dissolved Hydrogen Satoshi Ohno,*, Markus Wilde,*, Kozo Mukai, Jun Yoshinobu, and Katsuyuki Fukutani Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo, Japan Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba, Japan * Corresponding authors: oono@iis.u-tokyo.ac.jp; wilde@iis.u-tokyo.ac.jp S-1

2 Supporting Information S-1: Butene decomposition on Pd(110) and hydrogen desorption above 300 K Butene (C 4 H 8 ) molecules adsorbed on Pd(110) partially desorb and decompose upon warming from the temperature of adsorption (115 K). According to Figure 1, desorption of C 4 H 4 ceases at 250 K. At this temperature, the remaining surface butene has completely dehydrogenated to butadiene (C 4 H 6 ). 1 Above 250 K, the residual butadiene further decomposes into carbonaceous deposits that cannot be removed only by thermal desorption. 1 The C 4 H 6 decomposition process is evidenced by hydrogen (H 2 ) desorption between 300~600 K, as shown in Figure S1. It is seen that the H 2 evolution above 400 K due to butadiene decomposition from the H surf -covered Pd(110) is reduced relative to the clean surface. This finding mirrors the partial suppression effect of H surf on the C 4 H 8 decomposition that is evidenced by the enhanced butene desorption in the H surf -induced 190 K feature (Figure 1, main text). The additional H 2 desorption between K from H surf -covered Pd(110) (Figure S1) is due to desorption of H surf. Although the shape of the H 2 TDP trace is modified after butene co-adsorption, H surf is seen to desorb in the same temperature range and with unchanged integral intensity as from clean Pd(110), 2 suggesting that residual C 4 H 6 does not largely influence the H surf desorption process. Supporting Figure S1. H 2 TPD after adsorption of 5 L butene at 115 K onto clean (dotted line) or H surf -covered (1 L H 2, solid line) Pd(110). [1] Katano, S.; Kato, H. S.; Kawai, M.; Domen, K., Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd(110) in the Balance of Π-Bonded C 4 Hydrocarbon Reactions. J. Phys. Chem. C 2008, 112, [2] Ohno, S.; Wilde, M.; Fukutani, K., Novel Insight into the Hydrogen Absorption Mechanism at the Pd(110) Surface. J. Chem. Phys. 2014, 140, S-2

3 Supporting Information S-2: Full range HREEL spectra of butene on Pd(110) and their temperature dependence We investigated the butene adsorption state on clean versus H surf -saturated and additionally H abs -loaded Pd(110) using HREELS. Traces a-c of Figure S2 show HREEL spectra of butene obtained for these three hydrogen pre-charge conditions. Table S2 summarizes the assignments of the energy loss peaks in each spectrum. It is noteworthy that there are two peaks corresponding to C=C stretching vibrations at around 190 and 205 mev, indicating the presence of two different butene species on the Pd(110) surface. Since the energy position of the latter peak almost coincides with the C=C vibrational energy of cis-2-butene in the gas, liquid, or solid phase, 1 we conclude that this peak originates from a weakly adsorbed butene species, presumably in the second adsorbate layer. Traces a -c of Supporting Figure S2 show HREEL spectra after heating each system (a-c) to 140 K and quenching to 90 K. These spectra demonstrate the disappearance of the peak at 205 mev, while the other peaks remain clearly discernible. Supporting Figure S2. HREEL spectra recorded at 90 K of cis-2-butene adsorbed on (a) clean, (b) H surf- saturated, and (c) H surf -saturated Pd(110) loaded with several monolayers of H abs at 115 K. (a -c ) HREEL spectra after subsequent heating the systems a-c to 140 K and quenching to 90 K. S-3

4 Table S2. Observed energy loss peaks for butene-adsorbed Pd(110) for different H 2 pre-exposure conditions. The vibrational energies are expressed in mev. a Ref. 1. b Ref. 2. c Ref. 3. d Ref. 4. Clean H surf -saturated H abs -loaded CCC deform a,b 19, 37, 68 17, 35, 69 18, 69 CH bend (o) a C-C stretch a H-Pd c CH 3 rock a 124, , , 138 CH bend (i) a CH 3 s-deform a CH 3 d-deform d C=C stretch a, b 187, , , 205 CH 3 s-stretch CH 3 a-stretch CH stretch [1] McKean, D. C.; Mackenzie, M. W.; Morrisson, A. R.; Lavalley, J. C.; Janin, A.; Fawcett, V.; Edwards, H. G. M., Vibrational Spectra of cis and trans But-2-enes: Assignments, Isolated CH Stretching Frequencies and CH Bond Lengths. Spectrochim. Acta, Part A 1985, 41, [2] Katano, S.; Kato, H. S.; Kawai, M.; Domen, K., Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd(110) in the Balance of Π-Bonded C 4 Hydrocarbon Reactions. J. Phys. Chem. C 2008, 112, [3] Jo, M.; Kuwahara, Y.; Onchi, M.; Nishijima, M., Adsorbed States of Hydrogen on Pd(110): Vibrational Electron Energy Loss Spectroscopy and Low-Energy Electron Diffraction Studies. Solid State Commun. 1985, 55, [4] Katano, S.; Kato, H. S.; Kawai, M.; Domen, K., Selective Partial Hydrogenation of 1,3-Butadiene to Butene on Pd(110): Specification of Reactant Adsorption States and Product Stability. J. Phys. Chem. B 2003, 107, S-4

5 Supporting Information S-3: Butane desorption from H-covered Pd(110) versus butane production in C 4 H 8 hydrogenation on H abs -loaded Pd(110) In order to verify that the butane desorption observed in the reaction TPD measurements of the H 2 and C 4 H 8 co-adsorption system on H abs -loaded Pd(110) reflects the production rate of the hydrogenation reaction and not possibly the simple desorption of a reactant impurity, we compare in Figure S3 a reaction TPD trace (identical to the spectrum for 3.7 ML H abs in Figure 4) with a reference TPD spectrum of C 4 H 10 adsorbed on Pd(110) pre-saturated with chemisorbed hydrogen. Figure S3 shows that whereas desorption of molecularly adsorbed butane ceases already below 200 K, the reaction TPD trace from the catalytic butene/hydrogen co-adsorption system still strongly evolves at this temperature and peaks at 210 K. This clearly higher peak temperature and the much larger width of the butane desorption from the co-adsorption system indicate that this desorption signal originates from catalytic butane synthesis during the TPD heating ramp and that the butane product should desorb immediately after its formation. Supporting Figure S3. Comparison of butane desorption spectra (m/e=58) from C 4 H 10 adsorbed on H surf -saturated Pd(110) (prepared by exposures to first 1 L H 2 and then to 1.5 L butane at 100 K, dashed line) and from the reactive H 2 and C 4 H 8 co-adsorption system on Pd(110) pre-loaded with 3.7 ML H abs at 115 K (solid line). S-5

6 Supporting Information S-4: Isotope-exchange between C 4 H 8 and chemisorbed surface deuterium on Pd(110) We investigated the possibility of hydrogen exchange between adsorbed butene molecules and chemisorbed hydrogen on Pd(110) through TPD measurements of butene species after co-adsorbing 1.25 L C 4 H 8 at 115 K onto a Pd(110) surface saturated with chemisorbed D instead of H. Figure S4 displays the resulting TPD traces that include the original C 4 H 8 (m=56) as well as partially deuterated butene molecules of higher masses. The butene desorption states at 135 K and 165 K contain at most one D atom (m=57). The mass 57 (C 4 H 7 D) trace includes 5% of the C 4 H 8 intensity due to the natural abundance of 13 C and D isotopes in butene. Subtracting 5% of the C 4 H 8 desorption yield from the C 4 H 7 D trace leaves no pronounced shoulder at 165 K in the intense feature at 190 K (solid line). We therefore conclude that the butene TPD state at 165 K does not exchange significantly with surface D. On the other hand, considerable quantities of (higher) deuterated butene molecules (m=57, 58, 59, ) are seen in the desorption states at 190 K and 225 K. Although not shown because of the very small yield, we even observed fully isotope-exchanged C 4 D 8 in the desorption features at 190 K and 225 K. Supporting Figure S4. Butene TPD spectra after co-adsorbing 1.25 L C 4 H 8 at 115 K onto Pd(110) saturated with surface deuterium. The solid line in the m=57 trace is obtained by subtracting 5% of the m=56 yield from the raw data. S-6