Oxophilicity and Structural Integrity in Maneuvering Surface Oxygenated Species on Nanoalloys for CO Oxidation

Abstract

Platinum alloyed with transition metals at various compositions exhibits an enhanced catalytic activity for reactions involving oxygen activation, but little is known about the significance of oxophilicity that may play a role in maneuvering surface oxygenated species through the alloyed transition metal sites. Fundamental questions for sustainable high catalytic activity involve whether the oxygen activation or transfer occurring on the multicomponent particle surface induces any significant structural change of the nanoalloy in the catalytic process and how this is related to the oxophilicity. This paper addresses these questions by determining the catalytic activities of CO oxidation and the changes of the lattice parameters of nanoalloy particles in the catalytic processes as a function of the introduction of a third transition metal (M′ = V, Ni, Ir, etc.) into PtCo nanolloys. The results reveal an activity enhancement of M′ in the order of V > Ni > Ir, consistent with the order of oxophilicity of M′. In situ synchrotron high energy X-ray diffraction coupled to atomic pair distribution function analysis (HE-XRD/PDF) shows that the lattice parameter changes under CO oxidation conditions are very small for both PtVCo and PtNiCo nanoalloys. By comparing the changes in the lattice constant between PtVCo and PtNiCo in oxidation–reduction cycles under oxygen or hydrogen atmospheres, the former is also shown to exhibit a smaller change in lattice constant than those of the latter. The small change in the lattice parameter under CO reaction conditions is believed to reflect a surface self-regulation of the oxygenated species by the second and third transition metals in the nanoalloy (e.g., V and Co in PtVCo nanoalloy). Implications of the new insights into the unique integrity of the atomic-scale structure of platinum alloyed with transition metals are also discussed.

Nanoscale Alloying in Electrocatalysts

“…In comparison with other methods such as plasmatic cleaning or chemical cleaning [22], thermochemical processing strategy is not only effective in removing the encapsulation, but also in refining the nanostructural parameters. The combination of the molecular encapsulation based synthesis and thermochemical processing strategies typically involves a sequence of steps for the preparation of nanoalloy catalysts: (1) chemical synthesis of the metal nanocrystal cores capped with ligands, (2) assembly of the encapsulated nanoparticles on supporting materials (e.g., carbon powders, TiO2 or SiO2), and (3) thermal treatment of the supported nanoparticles [12][13][14][15][16][17]. The size and composition of the nanoparticles produced by thermochemical processing are controllable.…”

“…The size and composition of the nanoparticles produced by thermochemical processing are controllable. As shown for a series of binary and ternary alloy nanoparticle systems in Table 1 [12][13][14][15][16][17][23][24][25][26][27][28][29][30][31][32][33][34][35], the catalysts prepared by the molecularly-mediated synthesis and thermochemical processing methods have demonstrated enhanced catalytic and electrocatalytic properties for oxygen reduction reaction (ORR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR), etc. …”

“…The rational design of Pt-alloys involving transition metals (M/M′) could create a bifunctional (or multifunctional) synergy for the formation and removal of Pt-O or Pt-OH species. For ternary catalysts, the introduction of a second M′ into Pt-M alloy may further lead to a manipulation of the surface oxophilicity by maneuvering -O/-OH species over M and M′ sites through structural or compositional manipulation [12,15,17,[28][29][30][31][32][33][34]. The understanding of how Pt-OH and Pt-O binding energies can be tuned by the M/M′ oxophilicity would aid the design of the alloying metals for synergistic formation and removal of Pt-OH species in correlation with the structural and chemical complexity of the nanoalloys.…”

“…In addition to the obvious tunability in nanoscale alloying, the manipulation of surface oxophilicity is demonstrated by the introduction of a second metal M′ into Pt-M alloys. In many cases, ternary PtMM′ catalysts, where M, M′ = Ni, Co, Fe, V, Ir, etc., have demonstrated enhanced electrocatalytic activities and stabilities in comparison with commercial Pt/C and their binary counterparts [12,15,17,[29][30][31][32][33][34][35][36]. These aspects can be illustrated by studies of the ternary nanoalloy of PtIrCo in comparison with its binary counterparts [15,34].…”

“…Besides various nanostructural design parameters [11], the understanding of how these factors play an important role on catalytic properties is increasingly important. In this article, some of the recent findings in the exploration of nanoscale alloying degree for the preparation of the supported nanoalloy catalysts for catalytic and electrocatalytic reactions [12][13][14][15][16][17] will be highlighted. One important focus is the understanding of the structural correlation of nanoscale alloying properties with the electrocatalytic properties.…”