Equilibrium Hydrogen Pressure Research Articles

Low-temperature absorption equilibrium and chemisorption in the [formula omitted] system

Absorption isotherms for activated LaNi 5 H 2 have been determined in detail at 273 and 195 K. The variation of the two-phase plateau pressure with temperature has been measured from 174 to 273 K and extrapolates well with higher temperature data determined elsewhere; these data yield ΔH α β = −15.0 kJ/mol of H. Hysteresis loops have been examined in the two-phase coexistence region. Excess hydrogen sorption has been determined as a function of the equilibrium hydrogen pressures by differences between sorption by activated and unactivated (low surface area) samples. The enthalpy for the excess sorption has been found to be approximately −30 kJ/mol of H and becomes less exothermic with extent of sorption. The amounts of the excess sorbed hydrogen are larger than anticipated from the measured surface area (BET, krypton, 77.4 K) in order for excess sorption to correspond to chemisorption; nonetheless, arguments are presented to suggest this correspondence.

Equilibrium Pressure and Solubility of Hydrogen in Liquid Lithium

Equilibrium hydrogen pressures (P/sub H/sub 2//) were measured for the two-phase system, Li-LiH, for the liquid lithium of dilute solutions of 2.0 x 10/sup -3/ to 1.3 x 10/sup -2/ atomic ratio (H/Li), and for the liquid lithium controlled by a cold trap at temperatures between 200 and 450/sup 0/C. From the variation of P/sub H/sub 2// with temperature, the apparent heat of solution of hydrogen in liquid lithium was obtained as 34 and 19 kcal/mol for the Li-LiH and for the dilute solutions, respectively. For the liquid lithium controlled by the cold trap at 203/sup 0/C, the heat of solution was 19 kcal/mol. Using the Sieverts constant, which was obtained experimentally in this work, it was possible to determine the hydrogen concentration in liquid lithium from the P/sub H/sub 2//. The hydrogen-solubility in liquid lithium in the concentration <2 x 10/sup -2/ H/Li (corresponding to the temperature lower than 350/sup 0/C) was estimated from the hydrogen concentration data obtained through the P/sub H/sub 2// of the Li-LiH system. Although the cold trap could effectively control the hydrogen concentration in liquid lithium, this solubility was apparently lower than that obtained from the Li-LiH by factors of 2 to 3 atmore » the same temperature. The difference can be attributed to the interactions among hydrogen and other impurities and to compound formations such as Li/sub 2/H.« less

Hydrogen and Oxygen Behavior in Liquid Sodium (Experimental)

Hydrogen and oxygen behavior in a sodium test loop similar to the sodium-cooled fast breeder reactor were examined. Hydrogen partial pressure equilibrated to the sodium was measured by a membrane-permeation-type hydrogen-meter. An electrochemical oxygen-meter was used to determine the oxygen activity. The measured hydrogen pressure was converted to the hydrogen concentration through the Sieverts' constant, and the oxygen activity was also converted to the oxygen concentration through the characteristics of the oxygen-meter cells. Hydrogen and oxygen solubilities in liquid sodium were obtained as functions of the cold-trap temperature from 115 to 208/sup 0/C. Both data have a similar temperature dependence and were anomalously larger than the extrapolated values in the temperature region lower than 180/sup 0/C. At 300/sup 0/C, the addition of hydrogen to the liquid sodium, valved off from the purification apparatus, lowered the oxygen activity in the liquid sodium. The experimental results suggest a significant interaction between hydrogen and oxygen and the existence of (OH) species in liquid sodium. Using the solubility data of oxygen, hydrogen, and (OH) species, the free energy change of the reaction, (O)/sub in Na/ + (H)/sub in Na/ = (OH)/sub in Na/, was obtained and was ..delta..G/sup 0/(kcal/mole) = -28 + 0.043 T.more » From the temperature dependence of the equilibrium hydrogen pressure, the apparent heat of a solution of hydrogen in liquid sodium was obtained as 1.6 kcal/mole.« less

Influence of the surface structure on the adsorption of hydrogen on platinum, as studied by field emission probe-hole microscopy

The adsorption of hydrogen on platinum was investigated with a field emission microscope, equipped with a probe-hole assembly to enable adsorption studies on individual emitter regions. Adsorption of hydrogen is markedly face-specific. At 95 K and a hydrogen equilibrium pressure smaller than 2 × 10 −9 Torr the work function decreased strongly on the (111) face but increased on the (110) and (210) regions. Three different adsorption states were observed: β-hydrogen which desorbed above 300 K, α-hydrogen which desorbed around 230 K and a very weakly bound γ-state with a maximum heat of adsorption of 6 kcal mole . The α- and γ-states caused a decrease, the β-state an increase of the work function. The results show that the relative contribution of these three states and their heat of adsorption depend strongly on the crystal face. The β-state appeared to be absent on a smooth (111) plane. Hydrogen bound in the αstate has a relatively high heat of adsorption on the (111) region. A model has been proposed for the nature of the sites on the different surfaces involved in the adsorption of hydrogen.

A comparative study of systems of heavier rare-earth metals with hydrogen [Gd, Tb, Dy, Lu

Hydrogen equilibrium pressures as a function of hydrogen content of the metal and temperature have been measured for gadolinium, terbium, dysprosium, and lutetium, in the primary solid-solution range up to and including the two-phase region with the dihydride phase. All systems show strong similarities, and all are marked by a decrease in heat of solution of hydrogen in the metal with decreasing hydrogen concentration.

Thermodynamics of solid solution of hydrogen in M2C type carbides of vanadium and niobium

Equilibrium hydrogen pressures in the systems V 2CH 2 and Nb 2CH 2 have been determined at several temperatures in the range 400–650°C. Hydrogen pressure data were treated to obtain various thermodynamic quantities for the solutions of hydrogen in V 2C and Nb 2C. It has been found that the enthalpies of the reaction H(S.S.) = 1 2 H 2( g) are similar but slightly less endothermic compared to the respective metal hydrogen solid solutions. Entropies of dissolved hydrogen in V 2C and Nb 2C are also found to be similar to those of hydrogen in the respective metals.

The mechanism of hydrogen sorption by thin palladium layers I. Desorption

The effects of the thickness of thin electrodeposited palladium layers on the kinetics of hydrogen desorption processes are reported. Measurements have been performed with a piezoelectric quartz crystal microbalance under dynamic conditions, at a constant temperature of 61.1 ± 0.05 °C, and three different hydrogen-equilibrium pressures (α-phase region). From the kinetic analysis of desorption data, the desorption rate constants for the first-order step (k1d) and for the second order step (kIIdNeqi) have been calculated. The effect of the thickness of palladium layers is rather small in the first-order step, and very great in the second-order step. This behaviour is consistent with a mechanism which takes into account the adsorption of H atoms on two different surface states: (a) a weak adsorption (“pre-dissolved”) state, and (b) a strong adsorption state. The weight of the second- and first-order steps in the desorption process depends on the ratio of sites (a) to (b).

Electrochemical methods for studying diffusion, permeation and solubility of hydrogen in metals

Electrochemical methods for the investigation of metal-hydro gen systems are superior to other techniques, e.g., gas-volumetric, because of their simple procedure and their flexibility towards variation of experimental conditions. Moreover, these techniques allow measurements at very low hydrogen equilibrium pressures which occur, for instance, in the V(a)-metal-hydrogen systems in the concentration range of ideal dilute solutions at normal temperatures.The removal of surface layers, which retard the hydrogen passage from the electrolyte into the metal and vice versa, is a necessary condition for electrochemical methods to be applicable. Using ultra-high-vacuum techniques, surface hindrances on base metals like V, Nb, Ta, Ti, and others, can be eliminated. Thus, it is possible to study diffusion, permeation, and solubility of hydrogen (isotopes), in these metals as well as in precious metals such as palladium and its alloys.Various electrochemical hydrogen-diffusion and permeation methods are presented and compared. Their applicability and the limits of application are demonstrated for pre-selected systems.A recently developed electrochemical technique for measuring hydrogen (isotope) solubility in transition metals is described. By means of this method, pressure-concentration equilibrium isotherms of V, Nb, and TaH systems have been obtained recently, at ordinary temperatures, from vanishingly low hydrogen concentrations, corresponding to extremely low equilibrium pressures, up to atomic ratios H/Me of 0.5.

Thermodynamics of solid solution of hydrogen in Ta 6S and Nb 21S 8

Equilibrium hydrogen pressures in the systems Ta 6SH 2 and Nb 21S 8H 2 have been measured for temperatures in the range 300–550°C. Data for five different temperatures were treated to obtain various thermodynamic quantities. The reactions H(S.S.) = 1 2 H 2(g) for the systems Ta 6SH 2 and Nb 21S 8H 2 are found to be less endothermic than the respective metal-hydrogen reactions. Standard entropies of dissolved hydrogen in the sulfides are similar to those of hydrogen in the metals.

Absorption of hydrogen by vanadium-palladium alloys

Pressure-composition isotherms (273–373K) have been determined for the absorption of hydrogen by a series of six palladium alloys (f.c.c.) in the composition range from 1 to 8 at.% vanadium. At a given hydrogen content, the equilibrium hydrogen pressure progressively increases with vanadium content. Thermodynamic parameters for the absorption of hydrogen are reported at infinite dilution of hydrogen, and for the formation of the nonstoichiometric hydride phase from the hydrogen-saturated alloy. The relative, partial molar enthalpy of solution of hydrogen at infinite dilution increases slightly with vanadium content, e.g., for V(5%)-Pd, Δ H ̄ ° H = −8,500 j H compared with −10,040 j H for palladium. The presence of vanadium, which absorbs hydrogen itself in its normal b.c.c. structure, greatly inhibits the ability of palladium to absorb hydrogen. For example, the isobaric solubility of hydrogen (1 atm, 298 K) decreases from H Pd = 0.7 (palladium) to 0.024 (V(6%)-Pd). The lattice expansion due to the presence of interstitial hydrogen has been determined by X-ray diffraction. From these data it can be concluded that the formation of two non-stoichiometric hydride phases does not occur at vanadium contents greater than 5 at.% (298 K). Electrical resistance has been measured as a function of the hydrogen content of the alloys. The electrical resistance increases more markedly with hydrogen content for these alloys than for any of the palladium alloys previously examined.

The solubility ofRH2−x in Gd, Er, Tm, Lu and Y from ambient to 850°C

The solubility of hydrogen in Gd, Er, Tm, Lu and Y was determined from 25 to 850°C when the metal was in equilibrium withRH2−x (x varies between 0.1 and 0.2 depending on the rare earth metal). The room temperature solubilities determined by the lattice parametric method were found to be <0.1, 3.6, 7.7, 20.6 and 19.0 at, pct H in Gd, Er, Tm, Lu and Y, respectively. The change in unit cell volume for each atomic percent hydrogen added was nearly the same for all metals. The solubility of hydrogen increases more rapidly with temperature in those metals with low solubility at room temperature. Thus the solubility of hydrogen at 850°C is nearly the same in all five of the metals studied, that is, 35.0, 36.2, 36.0, 36.0 and 37.3 at. pct H in Gd, Er, Tm, Lu and Y, respectively. The equilibrium pressure of H2 in these studies was the equilibrium pressure of hydrogen in contact withRH2−x at the temperature concerned. A change in slope was observed in the solubility curves of the Gd-, Er-, Tm- and Lu-H systems. The logC) (at. pct H inR) was plottedvs 1/T for each system. Straight lines were obtained at temperatures above and below the changes in slope of the solubility curves. A calculation of the approximate ΔH of solution ofRH2−x in the metal sfrom the slope of the lines gave 4.35, 1.88, 1.28, 0.61 and 0.55 kcal/mole for Gd, Er, Tm, Lu and Y, respecitively in the low temperature portion. The change in slope which occurs at some point between 350°C and 650°C, depending on the metal, indicates a lower heat of solution ofRH2−x in these metals at the higher temperatures. In Lu there appears to be yet another change in slope in the neighborhood of 250°C.

Hydrogen absorption in intermetallic compounds of thorium

The formation of ternary hydrides has been studied for 15 intermetallic compounds of thorium, (five compounds in each of the systems Th-Ni, Th-Co and Th-Fe). With the exception of the compounds richest in 3 d-metal, they all form hydrides by reaction with hydrogen gas at room temperature. The hydrides prepared include the compounds ThCoH 4, ThNi 2H 5 and Th 7Ni 3H 28- Some compounds (Th 2Co 7H 4, Th 2Fe 7H 5) have equilibrium hydrogen pressures near to 1 atmosphere at 300 K. The results are discussed in terms of a model that predicts the enthalpy of formation of a ternary hydride from the knowledge of ΔH for the corresponding binary hydrides and binary intermetallic compounds. In addition we discuss experimental results on hydrogen absorption in LaNi 5 and related compounds, published earlier. The agreement between model predictions and experiment is quite convincing.

Isotope effects in solutions of hydrogen and deuterium in niobium

Hydrogen and deuterium equilibrium pressures in the region of single-phase solid solutions (α phase) with niobium were collected. These measurements were within the temperature and composition ranges of 535 to 773 K and 0.02 to 0.20 hydrogen atomic fraction. Good partial molar enthalpy and entropy values were calculated from these data. A ground-state hydrogen vibration frequency of 1800±50 K (3.8×1013 Hz) was derived by assuming a harmonic oscillator model. Finally, the results are compared with those obtained for the Group VB metals by other authors.

Heats of hydrogen absorption and phase transitions in the palladium–hydrogen system

Among transition metal hydrides, which retain metallic characteristics, those of palladium and palladiumrich alloys are somewhat unique in substantially retaining the cohesiveness and mechanical robustness of the hydrogen-free metal—even over phase transition regions. This is a practical advantage in obtaining detailed relationships between hydrogen contents and physical properties such as electrical resistance. Latterly, increasingly detailed and accurate measurements have been obtained of equilibrium hydrogen pressures and related thermodynamic parameters over narrow ranges of hydrogen content. The present paper reports calculations from electrochemical measurements at closely spaced temperatures, of changes in the heats of absorption of hydrogen over ranges of hydride phase transitions—made in conjunction with electrical resistance measurements. Results suggest the possibility that, aided by the high mobility of hydrogen in these solids, phase transformations may be preceded by the formation o f a series of subphases which exist within narrow limits of closely similar nonstoichiometric composition.

Thermal transpiration correction of hydrogen equilibrium pressure measurements in metal/hydrogen solution

Experimentally determined results on the measurement of hydrogen equilibrium pressure as a function of hydrogen concentration and temperature in α-titanium have been corrected for thermal transpiration effects in the apparatus. An empirical relationship, which accurately describes the transitional thermal transpiration region over which the effect is diminishing in magnitude with increasing hydrogen pressure, has been developed. This relationship is both simpler in form and more convenient in application than earlier derived correction factors.

Diffusion of Hydrogen in β‐Palladium Hydride at High Pressures

AbstractA relaxation method was applied for the determination of the intrinsic diffusion coefficient of hydrogen in β‐phase palladium hydride up to 23 kbar of equilibrium hydrogen pressure. The activation energies and frequency factors for diffusion obtained from the temperature dependence of the diffusion coefficients at constant pressure exhibit maximum values between 300 and 400 bar indicating the set‐up of a diffusive motion which cannot be longer described by a single‐jump mechanism between adjacent octahedral interstitials in the palladium lattice.

Hydrogen absorption in LaNi 5 and related compounds: Experimental observations and their explanation

The results are given of a study of the change in hydrogen sorption properties caused by partly replacing La or Ni in La Ni 5 by other metals. Desorption isotherms at 40 °C have been measured for La Ni 4M, where M represents Pd, Co, Fe, Cr, Ag or Cu, and for La 0.8R 0.2Ni 5, where R represents Nd, Gd, Y and Er, also Th and Zr. The equilibrium hydrogen pressures obtained in the two phase region can be explained qualitatively by assuming that the more stable the original RNi 5 compound, the weaker is the tendency to form a RNi 5-hydride, i.e., the higher the equilibrium pressure obtained for the hydride. This assumption which connects the stability of ternary hydrides with the heat of formation of the original compounds can also be formulated in a more quantitative way. This leads to a fair description of the experimental observations for the compounds having the CaCu 5 type of structure.

Hydrogen absorption and magnetic properties of LaCo 5xNi 5−5x compounds

The lattice constants, the Curie temperature and the saturation magnetization of the pseudobinary system LaCo 5xNi 5−5x have been determined as a function of composition. The variation of the lattice constants suggests an anisotropic magneto-volume effect accompanying the paramagnetic-ferromagnetic transition. The hydrogen absorption capacity decreases markedly with x in the series LaCo 5xNi 5−5x. The nature of the hydride phases corresponding to the various plateau pressures of the absorption isotherms has been studied by X-ray diffraction. The hydrogen equilibrium pressure at the first hydride is strongly reduced when x increases. Dependent on x one can discern two regions where this equilibrium pressure varies as In p = Cx + q, the absolute value for C being largest for small x values.

Phase relations and hydrogen absorption in the lanthanum-nickel system

The LaNi system has been reinvestigated in the range 50–100 at. % Ni. The following compounds were observed: LaNi, LaNi 1.4, LaNi 2, LaNi 3, La 2Ni 7 and LaNi 5. For these compounds the lattice constants have been determined. The phase LaNi 5 has a relatively large homogeneity region at elevated temperatures. For the compounds LaNi x within this region the equilibrium hydrogen pressure, p, of the 40 °C isotherms was found to change from 2.75 atm for LaNi 4.9 to about 9.2 atm for LaNi 5.4 according to In p = Cx + q with C = 2.4, q = − 10.75.

Phase equilibria and kinetics of evolution of dilute solutions of hydrogen in niobium

Ultrahigh vacuum techniques have been used to measure the equilibrium pressure of hydrogen dissolved in niobium in dilute solutions over the pressure, concentration and temperature ranges, pe=10-7-10-2 Torr, H/Nb=0-0.035, and T=351-487 K. Sievert's law which indicates ideal solution behaviour was observed for hydrogen concentrations below about H/Nb=0.023; the solubility data are described by the expression square root pe=c(395.4+or-33.8)exp(-(4.345+or-0.161)*103/T) where c=H/Nb*100. The standard partial molar entropy and enthalpy of solution are Delta s0=49.6+or-2.3 J K-1 g-atom-1 and Delta h0=-36.12+or-1.34 kJ g-atom-1. The value of Delta s0 is compared with theoretical entropies calculated from configurational and vibrational contributions. The rate of evolution of hydrogen from a niobium filament has been studied over the pressure and temperature ranges 10-7-10-3 Torr and 465-553 K. The rate limiting process is identified as the recombination of hydrogen atoms on the metal surface rather than diffusion within the bulk provided the overall concentration is less than H/Nb approximately=0.02. The activation energy for the process is 67.56+or-1.26 kJ mol-1.