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The presence of some additives in the environment affects the kinetics of the hydrogen evolution reaction (HER) on the surface (see Eq. 1 and 2) and, consequently, the hydrogen absorption reaction (HAR) within metals (Eq. 3). While some compounds inhibit both reactions, for example, benzonitrile, benzotriazole, and pyrroles, 1-7 others inhibit the HER while promoting the HAR, 8-28 e.g., iodide ions, thiourea, and aminotriazole. 1,2,8,15,[18][19][20][21] The mechanism of action of these additives is attracting considerable attention. [1][2][3] Inhibitors of both the HER and HAR are compounds which adsorb preferentially on the metal surface and hence decrease the hydrogen surface coverage, which in turn decreases the rate of both the HER and HAR. 1,2,7,33,34 By similar argument, others 8,9,11,35 attributed the effect of promoters of the HAR (Eq. 3) to inhibition of the recombination reaction (Eq. 2). This leads to a decrease in the rate of the HER (Eq. 2) but to an increase in the hydrogen surface coverage and hence to an increase in the hydrogen absorption. It has also been suggested that the adsorbed species, including iodide in acid sulfate solution, weaken the metal hydrogen bond 1,10,15,17 and hence facilitate the entry of hydrogen into the metal. Another group of authors attributed the accelerating affect of these additives on the HAR to an electronic interaction between the adsorbed hydrogen atoms and the adsorbed additive. 24,26,36 While additives with electron donating nature, such as iodide, 37 are reasoned to accelerate hydrogen absorption, those with electron withdrawing nature have the opposite effect. 24,26,36 Recently, Qian et al. 38,39 investigated the effect of arsenic oxide and hydrogen sulfide in promoting hydrogen absorption into iron and steel in alkaline solutions. Their results revealed that arsenic oxide enhances hydrogen absorption through its reduction to arsenic hydride (AsH 3 ). These authors concluded that AsH 3 is desorbed from the metal surface, thereby increasing the number of adsorption sites for hydrogen. Thus, the hydrogen surface coverage and, consequently, the hydrogen absorption rate are increased 38 as reported earlier. 8 In the case of hydrogen sulfide in alkaline solution, they concluded that HS Ϫ adsorbs on the surface, thereby, decreasing both the hydrogen surface coverage and the hydrogen desorption rate, although enhancing hydrogen absorption. 39 In a prior study of the Fe/H 2 S-acid system, Iyer et al. 40 concluded that H 2 S caused an increase in the hydrogen surface coverage and, in turn, accounted for the increase in the hydrogen absorption rate.The aim of the present study is to analyze the effects of iodide ions on the kinetics of both the HER and HAR on iron in a more in-depth manner than in the past. Of particular importance is the effect of iodide ions on the hydrogen coverage of the iron surface and the rate constant of the hydrogen absorption step which affects the rate of hydrogen permeation within iron. The analysis of the results makes use of models that ha...

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The results presented show that benzotriazole (BTA) inhibits the absorption of hydrogen into iron which is cathodically polarized in an acid (pH 1.7) sulfate solution. Analysis of the results indicates that BTA inhibits also the hydrogen evolution reaction, although it does not change its mechanism, which is shown to be proton discharge-Tafel recombination. However, BTA shifts the position of the equilibrium, HfldS H, toward the left side and hence leads to a decrease in the concentration of Hat,s within the lattice. The extent of this decrease depends on the BTAconcentration, e.g., about fivefold all mMand tenfold at 10 mMBTA. This effect is opposite to that reported for inhibitors such as thiourea which inhibits the hydrogen evolution reaction and yet promotes hydrogen permeation.

Hydrogen absorption into iron (steel) membranes was studied using an electrochemical hydrogen permeation technique. The Iyer, Pickering, and Zamanzaden analysis of the measured steady-state hydrogen permeation rates, obtained for different membrane thicknesses, gives the rate constants of hydrogen absorption, k abs , and desorption, k des , in addition to those quantities which can be obtained from data for a single membrane thickness: i.e., the hydrogen surface coverage, H , exchange current density, i o , rate constants, k 1 and k 2 , of the hydrogen evolution reaction, and the important composite quantity called the kinetic-diffusion constant, k ϭ k abs /(1 ϩ k des L/D), where L is the membrane thickness and D is the diffusivity of hydrogen within the metal. The product of k and H determines the steady-state permeability of hydrogen within the metal. For large D and/or small L, k ϭ k abs , and the hydrogen permeation process is under surface control. From the analysis, we obtained k abs ϭ 2 ϫ 10 Ϫ10 mol cm Ϫ2 s Ϫ1 and k des ϭ 1.9 ϫ 10 Ϫ3 cm s Ϫ1 for iron in acidic sulfate solution.

Effects of thiosulfate (S 2 O 32-) and sulfite (SO 3 2-) on hydrogen evolution and permeation within iron have been studied in acid and neutral solutions. Results show that thiosulfate increases the hydrogen permeation rate and depolarizes the hydrogen evolution reaction at concentrations as low as 0.1 mM, in acid or neutral media. Sulfite showed similar and somewhat less pronounced effects in the acid medium and no effect on either process in the neutral medium. Results are discussed in light of the stability of the various sulfur species and their effects on the rates of both processes.

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This work is in direct support of GM’s ongoing effort in developing transformational technologies to reduce the barriers of onboard Natural Gas mass adoption. Alternative NG storage technologies seek to bypass the pressure and thermal challenges of traditional storage technologies by utilizing the chemical and physical properties of certain solid materials to store gas. While the gas adsorption/desorption properties, thermodynamics and kinetics of the adsorbent materials have been evaluated in depth there is, to our knowledge, no report of the compatibility of the same materials with the pressure vessel liner at (P, T) operating conditions. The stability of the benchmark Metal Organic Framework, Cu3BTC2, in the presence of Natural Gas contaminants and its impact on the metallic tank liner material has been investigated and will be addressed in this report.