Modification of Active Sites on YSZ(111) by Yttria Segregation

Abstract

The surface properties of YSZ (111) have been investigated by X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), temperature programmed desorption (TPD) of adsorbed formate, and computational studies using the ReaxFF reactive force field approach. XPS and computer simulations showed enrichment of the surface with yttria. STM studies indicated that a high density of step edges are readily formed with ∼35% of the surface sites located at steps. Step edges are identified as the primary adsorption sites for formate. The formate oxidizes in a dehydration reaction producing carbon monoxide and water at ∼600 K. This is contrasted to the reaction of formate on pure zirconia where formate reacts by both dehydration and dehydrogenation reactions. This shift in the selectivity between pure zirconia and yttria-doped zirconia is attributed to the modification of the active step edge sites by yttria segregation. Therefore, the modification of active sites by minority species in a mixed oxide can control the chemical surface functionality.

Atomistic study of segregation and diffusion of yttrium and calcium cations near electrolyte surfaces in solid oxide fuel cells

“…The obtained concentration is approximately twice as high as that estimated for Y segregation at ZrO 2 GBs using the same approach (∼25 atom%) [35], indicating that dopant segregation at (1 1) surface is more pronounced than at GBs. Furthermore, according to recent experimental observations of a YSZ (1 1 1) surface using high-resolution X-ray photoemission spectroscopy [33], the outermost surface has approximately 45 atom% Y enrichment, which is in good agreement with our estimation and strongly supports the validity of our calculations.…”

“…These results are supported by previous qualitative experiments [20,22,33]. However, the analysis here is focused on the dilute-concentration case of defects without considering defects interaction.…”

“…1); however, the surface area is scaled-up to 10.9 × 10.0 nm in both x and y directions. In this model, both the first and second layers are treated as surface layers based on a previous experiment [33] and our results. Initially, for a given local concentration of dopant (or impurity) at surface C s , the corresponding number of Y Zr or Ca Zr are replaced randomly to a zirconium site within the surface layers.…”

“…Lee et al [32] have successfully predicted yttrium concentrations as a function of the depth from YSZ (1 0 0) surface by developing a hybrid Monte Carlo-molecular dynamics algorithm. Besides, the effect of steps existing at the YSZ surface on segregation energetics has been revealed using a reactive force field approach [33,34]. Yoshiya et al [35] have examined yttrium segregation energy at various types of YSZ GBs using chemical potential concepts and an empirical potential framework to estimate the local equilibrium concentration of yttrium at GBs.…”

Atomic structure and composition of the yttria-stabilized zirconia (111) surface

“…Despite its technological importance, experimental investigations of the C-YSZ(111) surface are scarce. Only recently it was observed by scanning tunneling microscopy (STM) at elevated temperatures that this surface exhibits a high number of defects [6], which are known to be important for surface reactions. Furthermore, at typical operation temperatures above 1000 K, a variation of the Y to Zr ratio is reported to change the performance of fuel cell devices considerably [7].…”