Ultrathin metal films with well-defined atomic arrangements can be formed on surfaces of single crystal electrodes by underpotential deposition (UPD) technique in which metals can be deposited on a foreign metal substrate at potentials more positive than the reversible potential of the bulk deposition if the interaction between the deposited metal and the foreign metal substrate is stronger than that of the deposited metal itself. The structure of a Pd UPD layer prepared on a Au(111) surface has been studied with an atomic resolution in situ by scanning tunneling microscopy (STM) and surface x-ray scattering (SXS) and it was found that a pseudomorphic Pd monolayer is formed on the Au(111) surface.^1-5 In addition, the Pd UPD layer on Au(111) surface is known to exhibit unique catalytic activities for oxygen reduction reaction (ORR).⁶ In the present work, the geometric and electronic structure of the Pd UPD layer on Au(111) surface during ORR were investigated by in situ XAFS measurements in the polarization dependent total reflection fluorescence (PTRF) configuration.^7-12

1) Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102, 4366 (1998).

2) Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473, 2 (1999).

3) Takahasi, Y. Hayashi, J. Mizuki, K. Tamura, T. Kondo, H. Naohara, K. Uosaki, Surf. Sci. 461, 213 (2000).

4) Naohara, S. Ye, K. Uosaki, J. Electroanal Chem. 500, 435 (2001).

5) Takahashi, Y. Hayashi, J. Mizuki, K. Tamura, T. Kondo, H. Naohara, K. Uosaki, Surf. Sci. 461, 213 (2000).

6) Naohara, S. Ye, K. Uosaki, Electrochim. Acta, 45, 3305 (2000).

7) W.-J. Chun, K. Asakura, Y. Iwasawa, J. Phys. Chem. B 102, 9006 (1998).

8) Masuda, H. Fukumitsu, S. Takakusagi, W.-J. Chun, T. Kondo, K. Asakura, K. Uosaki, Adv. Mater, 24, 268 (2012).

9) Masuda, K. Uosaki, J. Electron Spectrosc. Relat. Phenom., 221, 88 (2017).

10) Masuda, Y. Sun, H. Fukumitsu, H. Uehara, S. Takakusagi, W.-J. Chun, T. Kondo, K. Asakura, K. Uosaki, J. Phys. Chem. C, 120, 16200 (2016).

11) Masuda, K. Uosaki, Chapter 6, “Novel In Situ Techniques”, Electrochemical Science for a Sustainable Society A Tribute to J.O’M Bockris, Springer International Publishing, 2017, 147-174.

12) Masuda, T. Kondo, K. Uosaki, Section 5.13, “Solid-Liquid Interfaces”, XAFS Techniques for Catalysts, Nanomaterials, and Surfaces, Springer International Publishing, 2016, 505-525.