Direct measurements of water adsorption enthalpy on hafnia and zirconia

Ushakov, Sergey V.; Navrotsky, Alexandra

doi:10.1063/1.2108113

Cited by 157 publications

(223 citation statements)

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“…The specific details of the titration of Mn(III and IV) oxidation state and calculations are described in detail elsewhere (7). The enthalpy of water adsorption was measured by calorimetry at room temperature using a Calvet microcalorimeter, coupled with a Micromeritics ASAP 2020 analysis system as described previously (4). The percent surface coverage relates the nanophase sample surface area to the area that would be covered by water based on water molecule size (10.8 A 2 ) (46).…”

Section: Methodsmentioning

confidence: 99%

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Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Birkner

Navrotsky

2014

Proc. Natl. Acad. Sci. U.S.A.

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Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn 2 O 3 ) to hausmannite (Mn 3 O 4 ) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn 2 O 3 and Mn 3 O 4 ) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings.phase transformation | oxidation/reduction | water adsorption/desorption M anganese oxides are ubiquitous in the natural environment, often occurring as very fine grained (nanopahase) precipitates and coatings, and also find numerous technological applications, especially in catalysis. Easy variation of manganese oxidation state (Mn 2+ , Mn 3+ , Mn 4+ ) lends complexity to phase behavior and physical and chemical properties of manganese oxides. The thermodynamic stability of manganese oxide nanoparticles helps determine the robustness of such oxides and their behavior in soils and larger-scale geochemical cycles, as well as in applications in catalysis, renewable energy, and environmental remediation. The structure and energetics of nanoparticle surfaces are influenced by the phase present and its oxidation state and by the extent of surface hydration; thus dry surfaces are structurally and thermodynamically distinct from hydrated surfaces (1, 2). Using Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite), previously, we showed that the position in temperature−oxygen fugacity space, of oxidation−reduction (redox) phase equilibria, is shifted at the nanoscale due to differences in nanophase surface energy as a function of particle size and surface hydration and that this thermodynamic shift is in favor of the lower surface energy phase (3). In the manganese oxides, the surface energy increases in the order hausmannite, bixbyite, pyrolusite, so small particle size favors the more reduced oxide phase.This paper summarizes observations in the nanoscale Mn 2 O 3 − Mn 3 O 4 system, which is found to undergo rapidly reversible redox phase transformations at room temperature induced by the adsorption/desorption of surface water. It is expected that the degree of reduction is a function of both average particle size and particle size distribution, but this work focuses on only one representative material to provide proof of concept. The rapid response of activated surfaces to changing hydration condition implies thermodynamic control and has implications for geochemistry, planetary scie...

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Section: Methodsmentioning

confidence: 99%

“…3 shows measured differential enthalpy of water adsorption as a function of coverage (method from ref. 4) for samples prepared at 350°C and 700°C. The most significant differential enthalpy differences occur during the first few doses in the experiment (Fig.…”

Section: Significancementioning

confidence: 99%

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Birkner

Navrotsky

2014

Proc. Natl. Acad. Sci. U.S.A.

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“…Most of the studies on the interaction with CO have been carried out using infrared spectroscopy 5,18,19,[22][23][24] . However, studies using other techniques, such as temperature-programmed methods 20,21,28,30 , calorimetry 27,31,32 , and gravimetry 33,34 , as well as theoretical studies 22,28 have been published.…”

Section: Introductionmentioning

confidence: 99%

Review: monoclinic zirconia, its surface sites and their interaction with carbon monoxide

Kouva

Honkala

Lefferts

et al. 2015

Catal. Sci. Technol.

145

138

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“…The difference between the surface energies diminishes because the adsorption energy is proportional to the surface energy [10]; this concept was first postulated by Cerofolini [58]. Indeed, differential heats of adsorption of water on tetragonal zirconia reach a maximum of 150 kJ mol -1 at low coverages, whereas the heats on monoclinic zirconia can be as high as 275 kJ mol -1 [59].…”

Section: Effects Of Adsorbates On Oxide Surface Energy and Reactivitymentioning

confidence: 96%

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

Klose-Schubert

Jentoft

2011

Top Catal

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The stability of a series of sulfated zirconia catalysts, promoted with up to 2 wt% iron or manganese, in their calcined state was investigated. Phase composition, nature of surface sulfate species, degree of hydroxylation, and butane isomerization activity changed duri ng aging over months in various atmospheres and during milling. The metastability of small oxide particles is discussed, including literature data on alumina, titania and other oxides. Catalytically active fractions of a material easily transition into more stable, less active forms.

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Direct measurements of water adsorption enthalpy on hafnia and zirconia

Cited by 157 publications

References 15 publications

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Review: monoclinic zirconia, its surface sites and their interaction with carbon monoxide

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

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