Proton Conductivity of Simple Ionic Hydroxides Part I: The Proton Conductivities of Al(OH)₃, Ca(OH)₂, and Mg(OH)₂

Abstract

Elektrische Eigenschaften / Katalyse / Protonenleitung / ThermodiffusionProton conductivity is an outstanding feature of AI(OH),, Mg(OH),, and Ca(OH), powders between room temperature and their respective dehydration temperatures. It can be measured by using Pd-black electrodes in H, either by d. c. conductivity measurements, applying high injection voltages (> lo4 V/cm), or by thermopotential measurements. Two conductivity mechanisms are observed which can be assigned to proton conductivity uh and defect proton conductivity u,!,. The activation energies uh are 1.65, 2.0, and 2.2 eV for AI(OH),, Mg(OH),, and Ca(OH), respectively. The activation energies ug are lower, ranging between 0.5 -1.0 eV. 4 occurs 20-60 K below the on-set temperature of dehydration. It is enhanced by the adsorption of CH,COOH vapor and suppressed by NH, vapor acting as proton donator and proton acceptor or traps respectively. Protonenleitung uH tritt bei Al(OH),, Mg(OH), und Ca(OH), zwischen Raumtemperatur und der Entwasserungstemperatur auf. uH ist meRbar mittels Palladiummoor-Elektroden in H, als Gleichstrom-Leitfahigkeit, wenn hohe Injektionsspannungen, > lo4 V/cm, angelegt werden, oder als Thermospannung im Temperaturgradienten. Mit Cu-Elektroden tritt keine uH auf. Zwei Leitfahigkeitsmechanismen sind erkennbar: Exzefiprotonen-Leitfahigkeit u i (p-Leitung) und Defektprotonen-Leitfiihigkeit a,!, (n-Leitung). Die Aktivierungsenergien fiir u;l betragen 1,65, 2,O und 2,2 eV bei AI(OH)?, Mg(OH), und Ca(OH),, die fur u,!, liegen niedriger, 0,5 -1 ,O eV. ExzeRprotonen-Leitung tritt 20 -60 K vor dem Beginn der Entwasserung auf. Durch Adsorption von CH3COOH-Dampf wird 0; urn den Faktor 1 3 -3 erhdht, durch Adsorption von NH3 unterdriickt.

Thermodynamics of Fe oxides: Part I. Entropy at standard temperature and pressure and heat capacity of goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and maghemite (γ-Fe2O3)

“…3), is in good agreement with that of the adiabatic calorimetry over the common temperature region. This double peak resembles the transformation pattern of hydroxides of Al, Ca, and Mg (Freund and Wengeler 1980). At temperatures slightly higher than 400 K, a sharp rise in the DSC trace signals the onset of dehydration.…”

H-D interdiffusion in brucite at pressures up to 15 GPa

“…The hydroxyl protons in Mg(OH) 2 are tightly bound to the O 2-ions, because the O-H distance for brucite (less than 0.958 Å: Catti et al 1995) is shorter than the ionic radius of O 2-ions (1.33-1.40 Å: Freund and Wengeler 1980). To escape from the hydroxyl lattice, protons must in some way become mobile via OH -+ OH -↔ HOH + O 2-…”

“…It has been believed that the protons in Mg(OH) 2 diffuse as only one migration mechanism, presumably as ground-level tunneling assisted by lattice phonons. Later, proton conductivity in Mg(OH) 2 powders reported by Freund and Wengeler (1980) indicated two kinds of conduction mechanisms: n-type (holes as charge carriers) and p-type (electrons as charge carriers). Recently, Noguchi and Shinoda (2010) favored the extrinsic vacancy mechanism for proton migration in portlandite [Ca(OH) 2 ], another isomorph of brucite, in the direction along the proton layer.…”

Structural transformations preparing the dehydration of Ca(OH)2

“…It is still difficult to determine if this a, coefficient is a continuous or discontinuous function of the temperature. Nevertheless, two possible discontinuities could be evidenced respectively around 370 K and 450 K. The first one would correspond to an enhancement of proton conductivity previously observed between 350 and 390 K (Freund et al, 1980). The second one would correspond to the occurrence of cleavages in the crystal, as they have been evidenced by differential scanning calorimetry.…”

“…It results from the formation of water molecules preceding the decomposition. Prereactional effects have been evidenced by different experimental techniques, in a large temperature range before the dehydration temperature (570 K with PHZ0 = 1.73 torr) (Freund and Wengeler, 1980;Freund, Scheikh-01-Eslami and Gentsch, 1975). In particular, X-ray and neutron diffraction studies on powders and single crystals of Ca(OH), and Ca(OD), have already revealed structural discontinuities at different temperatures which depend on the water pressure conditions (Chaix-Pluchery, 1986; Chaix-Pluchery, Bouillot, Ciosmak, Niepce and Freund, 1983;Chaix-Pluchery, Pannetier, Bouillot and Niepce, 1987).…”