Excess oxygen at metal‐oxide interfaces—a review of the segregation process based on the current state of knowledge

Abstract

AbstractAccumulation or depletion of excess oxygen atoms at metal‐oxide interfaces, which can handily be controlled by adjusting the oxygen activity, leads to modifications of the atomic structure and chemical composition of the heterophase boundaries. The established atomistic model, which also explains changes of the macroscopic material behavior, interprets the process as interfacial oxygen segregation. Structurally necessary vacancies at terminating close‐packed oxide lattice planes serve as oxygen traps. These point defects are reversibly refillable with excess atoms. Experimental findings on oxygen‐controlled alterations of metal‐oxide interfaces obtained by electron energy loss spectroscopy, quantitative gas volume measurement,18O‐16O isotope exchange experiments, and the electrochemical hydrogen probe are compiled and valued with regard to the basic modeling assumptions. Thermodynamics data derived from volumetrically determined oxygen isotherms show that the chemisorption segregation process is energetically comparable with the formation of a two‐dimensional oxide layer of the metal component at the phase boundary. Subsequent hydrogen accumulation occurs by the development of hydroxide bonds. For palladium‐based nano‐oxide dispersion microstructures under practical process conditions, diffusion of bulk‐dissolved oxygen to the precipitates represents the rate‐controlling substep of interfacial segregation, which can thus be suppressed by shock quenching. The spatial arrangement of the embedded particles is characterized by quantitative transmission electron image analysis. For these palladium‐oxide materials, an adapted model of the kinetics of interfacial oxygen segregation is established. The simulations are compared with results of electrochemical measurements and cooling curves determined contactlessly by infrared pyrometry.