Effect of Surface Composition on Electronic Structure, Stability, and Electrocatalytic Properties of Pt-Transition Metal Alloys:  Pt-Skin versus Pt-Skeleton Surfaces

Abstract

The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and after immersion to an electrolyte. In contrast all sputtered surfaces formed Pt-skeleton outermost layers due to dissolution of transition metal atoms. Therefore, these three different near-surface compositions (Pt-skin, Pt-skeleton, and pure polycrystalline Pt) all having pure-Pt outermost layers are found to have different electronic structures, which originates from different arrangements of subsurface atoms of the alloying component. Modification in Pt electronic properties alters adsorption/catalytic properties of the corresponding bimetallic alloy. The most active systems for the electrochemical oxygen reduction reaction are established to be the Pt-skin near-surface composition, which also have the most shifted metallic d-band center position versus Fermi level.

Key Factors Affecting the Performance and Durability of Cathode Electrocatalysts in Polymer Electrolyte Fuel Cells Characterized by In Situ Real Time and Spatially Resolved XAFS Techniques

“…It should be noted that the rates of the structural transformation for the three cathode electrocatalysts were in the order of Pt 3 Co/C [ Pt 3 Ni/C [ Pt/C. Figure 3 (bottom A) shows the variation of the Pt L IIIedge XANES white line peak heights at 0.4 V with the specific power density [51], which is in agreement with the linear relationship between ORR activity and d-band center shift for Pt 3 Co, Pt 3 Ni, and Pt [70][71][72]. Although we did not investigate the particle size effect on the structural kinetics, the ORR kinetics may not be affected by the particle size of Pt cathode electrocatalysts (below 5 nm size) [73,74].…”

Synergetic Effect of Surface and Subsurface Ni Species at Pt−Ni Bimetallic Catalysts for CO Oxidation

“…14,17,20,[80][81][82] A fresh PtNi/CB sample without reduction was treated by the leaching process. In this sample, all Ni and Pt ions were completely washed away.…”

Supported Pd–Cu Bimetallic Nanoparticles That Have High Activity for the Electrochemical Oxidation of Methanol

“…[7,8] Moreover, alloying Pt with early transition metals could also decrease the amount of Pt used and, in some cases, lead to improved activity. [9][10][11][12] However, these catalysts remain very expensive, as a substantial fraction of Pt is still required. Alternatively, as a second approach, less-expensive metals, such as Pd, have been suggested as a good replacement for Pt.…”