Hydrogen Surface Reactions and Adsorption Studied on Y₂O₃, YSZ, and ZrO₂

Abstract

The surface reactivity of Y2O3, YSZ, and ZrO2 polycrystalline powder samples toward H2 has been comparatively studied by a pool of complementary experimental techniques, comprising volumetric methods (temperature-programmed volumetric adsorption/oxidation and thermal desorption spectrometry), spectroscopic techniques (in situ electric impedance and in situ Fourier-transform infrared spectroscopy), and eventually structural characterization methods (X-ray diffraction and scanning electron microscopy). Reduction has been observed on all three oxides to most likely follow a surface or near-surface-limited mechanism involving removal of surface OH-groups and associated formation of water without formation of a significant number of anionic oxygen vacancies. Partly reversible adsorption of H2 was proven on the basis of molecular H2 desorption. Dictated by the specific hydrophilicity of the oxide, readsorption of water eventually takes place. The inference of this surface-restricted mechanism is further corroborated by the fact that no bulk structural and/or morphological changes were observed upon reduction even at the highest reduction temperatures (1173 K). We anticipate relevant implications for the use of especially YSZ in fuel cell research, since in particular the chemical state and structure of the surface under typical reducing high-temperature conditions affects the operation of the entire cell.

Toluene oxidation over ZrO2-based gasification gas clean-up catalysts: Part B. Kinetic modeling

“…A small shoulder in the water formation curves at 350 °C can be detected also over the doped catalysts. It has been reported by Kogler et al [27] that over Y 2 O 3 -ZrO 2 water desorption can be observed even after reduction and evacuation. The water desorption is related to the dehydroxylation of the Y 2 O 3 -ZrO 2 surface.…”

“…The model underestimates also the formation of water between 350-550 °C. Water desorption from reduced (at 400 °C) and evacuated (at room temperature) surface of Y 2 O 3 -ZrO 2 has been reported to have maxima at 427 °C by Kogler et al [27]. Thus, it can be suggested that in toluene oxidation experiments, the formation of water at 350-550 °C is due to the water desorption from the surface of Y 2 O 3 -ZrO 2 as well as from toluene oxidation as a product.…”

ZrO2 Acting as a Redox Catalyst

“…the sample with the highest degree of hydration, a new low-temperature release of water and CO is observed. Finally, all samples release concurrently CO and H 2 O at around 400°C, as well as CO 2 and H 2 at above 500°C. It can be noted that the effect of surface hydration is mainly seen in the low temperature window since the high-temperature processes do not differ greatly [14].…”

“…However, it is claimed that surface oxygen can be removed in vacuum at temperatures above 700°C [1,2]. Reduction of ZrO 2 has been suggested to follow a mechanism according to which surface OH groups formed by dissociatively activated hydrogen react to form water, and therefore the role of OH groups in directing the catalytic properties of the oxide should be considered [2]. So, reactivity of surface oxygen on ZrO 2 is connected to the formation of OH groups, which will always be present on the surface under conditions relevant to catalysis.…”

Water adsorption at zirconia: from the ZrO2(111)/Pt3Zr(0001) model system to powder samples

“…This is related to the fact that single crystals of pure ZrO 2 grown from the melt exhibit phase transformations upon cooling, thus only crystals of doped zirconia (e.g., YSZ) are available; these retain the hightemperature cubic phase. The surface chemistry of YSZ is much more complex than that of pure ZrO 2 , however, as shown for adsorption of H 2 O, 24 hydrogen, 25 or CO and CO 2 . 26 Therefore, the investigation of pure ZrO 2 , which is needed as a starting point of a well-grounded fundamental understanding, has to rely on thin lms.…”