Water-Assisted Growth of Cobalt Oxide and Cobalt Hydroxide Overlayers on the Pt3Co(111) Surface

Abstract

Bimetallic platinum-cobalt (Pt-Co) catalysts have highly enhanced performance for the oxygen reduction reaction (ORR), where this peculiar surface alloy structure contributes to efficient energy conversion processes. However, the detailed catalytic reaction steps and adsorbate-driven interactions on the surface morphology under practical operating conditions are still unclear. Here, we report the water-assisted surface reconstruction of the bimetallic Pt3Co(111) surface using near-ambient pressure scanning tunneling microscopy (NAP-STM) in a humid O2 environment. The segregated CoOx clusters are selectively transferred to the Co(II) oxide/hydroxide layer at 0.1 Torr of H2O/O2 gas mixture at elevated temperature. In contrast, this drastic phase transition is limited and dependent on the partial pressure of water because the nanoscale CoO islands are formed with 0.1 Torr of the N2/O2 gas mixture. Synchrotron radiation in-situ X-ray photoelectron spectroscopy measurements also support the water-assisted evolution of the Co(OH)2 species. These morphologic modulations not only explain the surface degradation process of the bimetallic Pt-Co catalysts but also indicate the active formation of CoOx/CoOOH intermediates during energy conversion.