Direct STM Elucidation of the Effects of Atomic-Level Structure on Pt(111) Electrodes for Dissolved CO Oxidation Junji Inukai,* ,† Donald A. Tryk, † Takahiro Abe, ‡ Mitsuru Wakisaka, † Hiroyuki Uchida,* ,†,§ and Masahiro Watanabe* ,† † Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu, 400-0021, Japan ‡ Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3 Takeda, Kofu 400-8511, Japan § Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan * S Supporting Information ABSTRACT: We sought to establish a new standard for direct comparison of electrocatalytic activity with surface structure using in situ scanning tunneling microscopy (STM) by examining the electrooxidation of CO in a CO-saturated solution on Pt(111) electrodes with steps, with combined electrochemical measurements, in situ STM, and density functional theory (DFT). On pristine Pt(111) surfaces with initially disordered (111) steps, CO oxidation commences at least 0.5 V lower than that for the main oxidation peak at ca. 0.8-1.0 V vs the reversible hydrogen electrode in aqueous perchloric acid solution. As the potential was cycled between 0.07 and 0.95 V, the CO oxidation activity gradually decreased until only the main oxidation peak remained. In situ STM showed that the steps became perfectly straight. A plausible reason for the preference for (111) steps in the presence of CO is suggested by DFT calculations. In contrast, on a pristine Pt(111) surface with rather straight (100) steps, the low-potential CO oxidation activity was less than that for the pristine, uncycled (111) steps. As the potential was cycled, the activity also decreased greatly. Interestingly, after cycling, in situ STM showed that (111) microsteps were introduced at the (100) steps. Thus, potential cycling in the presence of dissolved CO highly favors formation of (111) steps. The CO oxidation activity in the low-potential region decreased in the following order: disordered (111) steps > straight (100) steps > (100) steps with local (111) microsteps ≈ straight (111) steps. 1. INTRODUCTION Surface structure-activity relationships are crucial for study of surface reactions of all types, including those that are important in catalysis, 1-9 electrochemistry and electrocatalysis, 10-22 and metal oxide surface chemistry, including photocatalysis. 23-31 Scanning probe microscopies, particularly scanning tunneling microscopy (STM), have already produced exciting results in these areas, even to the point of showing chemical processes at the atomic or molecular level on surfaces in real time (see, for example, STM videos from the Besenbacher group). 32 However, to achieve this feat, it is usually necessary to employ very special experimental conditions, e.g., low pressure and/or low temperature. For a number of years, it has been possible to achieve atomic resolution in STM images in liquid electrolytes at ambient temperature and pressure, with potential control, so that ordered adsorbate layers can be observed. 11-22 It has also been possible to observe electrode surfaces even as electro- chemical reactions are occurring, for example, etching 10,14 and electrodeposition and reconstruction, 33-38 i.e., processes involving displacement of many atoms. The Itaya group reported in situ, real-time etching processes on semi- conductor 10,39 and metal 39-42 electrodes elucidating the reaction mechanism on the atomic scale. Some examples of real-time videos, including metal electrodeposition, have been presented by the Magnussen group. 43 Also, it has been possible to observe electrodes such as Pt single crystals simultaneously with a quasi-steady-state molecular-level reaction such as oxidation of dissolved carbon monoxide. 44-46 In the present work, two of these strategies have been used, first, to examine, in situ, progressive changes in step edge structures on Pt(111) and related stepped single crystals during potential cycling and, second, to observe the effects of such structures on the CO adsorption configuration, both short and long range, and on the macroscopic-level current for CO oxidation. Thus, we are able to establish extremely direct relationships between the prevailing surface structure and the relative magnitude of the anodic oxidation current. Of course, the actual electrochemical reaction occurs at such a rapid rate that no changes can be observed in the STM images. There is already a vast body of literature on the electrochemical oxidation of CO, particularly on the platinum surface. 47-49 Specific examples include the work of the research groups of Cuesta, 50-54 Feliu, 55-71 Koper, 57,65,67,72-75 Korze- Received: October 19, 2012 Published: January 7, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 1476 dx.doi.org/10.1021/ja309886p | J. Am. Chem. Soc. 2013, 135, 1476-1490