“…Despite this deficiency, the diversity of the application of ZrO 2 -based materials is considerable and directly reflects its manifold specific properties. Applications in ceramic engineering, 10 as biomedical implants, 11 in sensor technology, 12 as microelectronic devices, 13 as solid electrolytes 14 and in catalysis 2 have been reported. Concerning the latter, ZrO 2 is used as both catalyst material and catalyst support.…”
The surface chemical properties of undoped tetragonal ZrO and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO, as well as low conductivity. The transformation into monoclinic ZrO is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO, even after a heating-cooling cycle up to 1273 K. Treatments in CO and H increase the amount of monoclinic ZrO upon cooling (>85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO.
“…Despite this deficiency, the diversity of the application of ZrO 2 -based materials is considerable and directly reflects its manifold specific properties. Applications in ceramic engineering, 10 as biomedical implants, 11 in sensor technology, 12 as microelectronic devices, 13 as solid electrolytes 14 and in catalysis 2 have been reported. Concerning the latter, ZrO 2 is used as both catalyst material and catalyst support.…”
The surface chemical properties of undoped tetragonal ZrO and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO, as well as low conductivity. The transformation into monoclinic ZrO is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO, even after a heating-cooling cycle up to 1273 K. Treatments in CO and H increase the amount of monoclinic ZrO upon cooling (>85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO.
“…[17,21,4,26,25,11,37] Moreover, zirconia finds its applications in, for instance, solid oxide fuel cells [32] and electrochemical sensors. [22] Zirconia exists in several polymorphic modifications, among which monoclinic (m), tetragonal (t), and cubic (c) are the most common examples. The m-ZrO 2 crystal form, also known as the mineral baddeleyite, is the most stable at the temperatures below 1480 K, whereas the t-and c-forms are stable at significantly higher temperature regimes and transform to the monoclinic modification upon cooling.…”
Under the water-rich pre-treatment and/or reaction conditions, structure and chemistry of the monoclinic zirconia surfaces are strongly influenced by oxygen vacancies and incorporated water. Here, we report a combined first-principles and atomistic thermodynamics study on the structure and stability of selected surfaces of the monoclinic zirconia. Our results indicate that among the studied surfaces, the most stable (111) surface is the least vulnerable towards oxygen vacancies in contrast to the less stable (011) and (101) surfaces, where formation of oxygen vacancies is energetically more favorable. Furthermore, we present a vigorous, systematic screening of water incorporation onto the studied surfaces. We observe that the greatest stabilization of the surfaces is achieved when a part of the adsorbed water molecules is dissociated. Nevertheless, the importance of water dissociation for achieving the greatest stabilization is high for the less stable (011) and (101) surfaces, while completely hydrated (111) surface is stabilized equally regardless of the water dissociation state. Analysis of the constructed phase diagrams reveals that the (111) surface remains preferably clean and the (011) and (101) surfaces have dissociated water at low coverage under the reactive conditions of T = 600-900 K and p(H 2 O) < 1 bar. Upon temperature decrease and/or pressure increase, all studied surfaces gradually uptake water until fully hydrated. All in all, our findings complement and broaden the existing picture of the structure and stability of the monoclinic zirconia surfaces under the pre-treatment and/or reaction conditions, enabling rationalization of the potential roles of zirconia as a heterogeneous support and a catalyst component.
“…The value of U is of course proportional with the difference between the partial pressure of oxygen on the two sides of the zirconia layer [5].…”
Section: The Principles Of Gas Sensingmentioning
confidence: 99%
“…In a simple zirconia cell (Figure 10a), the oxygen detection performed by applying the Nernst equation (relation 2) allows to calculate the partial pressure of oxygen, p 1 on one side of a zirconia layer, if the pressure p 2 at the other side is known, by measuring the electromotive force, E developed between the two sides of zirconia layer [2,5,13].…”
Section: Electrochemical Sensors Working At High Temperaturesmentioning
This chapter aims a comprehensive presentation of the most common electrochemical sensors used in the real monitoring applications of air purity testing. Oxygen, hydrogen, hydrogen sulfide, nitrogen oxides, carbon monoxide and carbon dioxide are gases, which can be accurately detected and measured. Too high or too low oxygen concentration levels make the air improper for breathing. Hydrogen sulfide and carbon monoxide are dangerous species; any leakage needs to be pinpointed. A calibrated network of sensors for monitoring gas detection makes it possible to easily locate the source of gas escape during indoor air monitoring. The air quality monitoring stations based on electrochemical sensors are nowadays used to determine the global pollution index of the atmospheric air, in order to prevent the risks toward the human health and damage of environment, especially in the highly populated and industrialized urban areas.
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