Oxophilicity and structural integrity in maneuvering surface oxygenated species on nanoalloys for CO oxidation

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.