In-situ Ambient Pressure XPS Study of CO Oxidation Reaction on. Pd(111) Surfaces

Transcription

1 In-situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces Ryo Toyoshima, Masaaki Yoshida, Yuji Monya Yuka Kousa, Kazuma Suzuki, Hitoshi Abe, Bongjin Simon Mun,, Kazuhiko Mase, Kenta Amemiya, and Hiroshi Kondoh,* Department of Chemistry, Keio University, Hiyoshi, Kohoku-ku, Yokohama , Japan, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki , Japan Department of Applied Physics, Hanyang University, ERICA , Republic of Korea Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju , Korea * Corresponding Author kondoh@chem.keio.ac.jp S1

2 1. AP-XPS apparatus Schematic drawing of the ambient-pressure XPS system used in this study is shown in Fig. S1. The differential pumping system consists of four separated stages where are pumped with turbo molecular pumps with pumping speeds of 1 st 500 L/s, 2 nd 700 L/s, 3 rd 210 L/s and 4 th 210 L/s. The second differential-pumping stage is equipped with an electron energy analyzer modified for high pressure experiments (OMICRON, EA125HP). A quadrupole mass spectrometer (HIDEN, HAL201) is also mounted at the second differential-pumping stage to monitor the partial pressures of the reactant and product gases. The incident x-ray beam is introduced through a 100 nm-thick silicon nitride film which separates the beam line and the ambient-pressure (AP) cell. The sample is spot-welded to a transferable compact sample holder. The sample holder is heated with a ceramics heater from the backside. The temperature is measured using an alumel-chromel thermocouple attached to the holder. The distance between the sample and thermocouple is about 4 mm. The sample can be cleaned in a UHV preparation chamber which is connected to the AP cell. O 2 and CO gases are introduced to the AP cell using variable leak valves. Figure S1: Schematic drawing of the ambient-pressure XPS system. S2

3 2. Confirmation of the absence of metal carbonyl species in the CO gas Ambient pressure CO exposure sometimes causes contaminations of metal carbonyl species such as Fe(CO) 6 or Ni(CO) 4, which are coming from CO gas storage cylinders. Kaichev et al. reported that Fe(CO) 6 has been observed at ev in the C1s region and at about 710/722 ev in the Fe2p region under mbar CO at 400 K. Since it may influence the catalytic activity, we have checked the presence of the metal carbonyl contamination. Figure S2 shows Fe2p and Ni2p XPS taken under exposure to 0.5 Torr of CO at room temperature. Photon energies of 850 and 950 ev were used for Fe2p and Ni2p, respectively. The absence of Fe and Ni species is confirmed. Therefore, even at 0.5 Torr of CO, the surface was free from carbonyl species of Fe and Ni. hν=950 ev hν=850 ev P co =0.5 Torr Ni2p 3/2 Fe2p 1/2 Fe2p 3/ Binding Enegy (ev) Figure S2: Fe2p and Ni2p XPS taken under exposure to 0.5 Torr of CO at room temperature. S3

4 3. Gas-phase XPS peaks under the reaction condition Figure S3 shows an O1s/Pd3p 3/2 XP spectrum taken during CO oxidation reaction at 300 C. The photon energy used was 650 ev. The peak deconvolution procedure is the same as that described in experimental section. CO 2 and O 2 gases are observed at about 536 and 538/539 ev, respectively. Although a CO-associated peak should be appearing at 538 ev, it is not clearly discernible due to overlap with the lower-energy peak of O 2. The intensity ratio of the 538 ev peak to the 539 ev peak is slightly larger than that for pure O 2. This indicates that the 538 ev peak includes the contribution from the gas-phase CO. The gas induced peaks do not overlap with the surface oxide induced peaks. O1s O Pd3p 2 +CO gases 3/2 Pd3p CO 3/2 2 gas O(II) O(I) Binding Energy (ev) Figure S3: An example for O1s/Pd3p XP spectrum including the gas-phase peaks taken under the reaction condition (O 2 : 200 mtorr, CO: 20 mtorr, T: 300 C). The spectrum is the same one that shown in Fig. 4(b)-α, but indicated with a wider energy range. S4

5 4. XPS observation of the chemisorbed oxygen on a Pd(111) surface We observed the chemisorbed oxygen species on Pd(111) with O1s XPS using our experimental setup. The photon energy used here was 650 ev. Figure S4 shows the O1s/Pd3p XP spectrum taken under 200 mtorr O 2 pressure at room temperature. A single peak was observed at ev in addition to the broad Pd3p peak. The ev peak is ascribed to the chemisorbed oxygen species. 1 This peak position is the same as that of the lower-energy component for the Pd 5 O 4 surface oxide. O1s/Pd3p Chemsorbed O ev Binding Energy (ev) Figure S4: XP spectrum of O1s during the O 2 gas introduction of 200 mtorr at room temperature. (1) Leisenberger, F. P.; Koller, G.; Sock, M.; Surnev, S.; Ramsey, M. G.; Netzer, F. P.; Klötzer, B.; Hayek, K. Surf. Sci. 2000, 445, S5