Abstract
At temperatures below 600 K, the isotherms of hydrogen absorption by Pd exhibit hysteresis loops related to the first-order phase transition or, more specifically, to separation of a diluted phase and hydride. According to the experiments, addition of even small amount of the second metal, e.g. Au or Ta, can appreciably suppress hysteresis. This interesting effect is important in various applications, e.g., in the context of fabrication of efficient hydrogen sensors. To clarify its physical background, we present statistical calculations of the hydrogen absorption isotherms for a series of binary alloys of Pd with Mg, Cu, Ag, Ta, Pt, or Au by using the values of the H-metal interaction provided by the density functional theory (DFT). Aiming at the situations with small amount (≤15%) of the second metal, the metal atoms in an alloy are considered to be located at random or with short-range correlations. In the random alloy approximation, appreciable suppression of hysteresis is predicted for all the additives under consideration except Cu. Concerning the correlations, we show that the tendency of metals to mixing (as, e.g., predicted for the Pd–Au or Pd–Ta alloy) is in favour of additional suppression of hysteresis whereas the tendency to segregation (as, e.g., predicted for the Pd-Ag alloy) makes the hysteresis loops wider. For Au and Ta, our findings are in good agreement with available experimental data.
Highlights
Beside basic physics and chemistry [1], hydrogen absorption by binary alloys of Pd with other metals is of high current interest in the context of various applications related first of all to the so-called hydrogen economy [2], including hydrogen storage [3,4,5] and sen sors [6,7,8], and heterogeneous catalysis [9,10,11]
It results in the energetic heterogeneity of the absorption sites and can eliminate the phase transition
In our previous recent study [22] performed in the spirit of ab initio thermodynamics and first-principles microkinetics [23,24] and oriented to large NPs or supported nanofilms where the hydrogen uptake is determined primarily by the bulk, we have shown that in the case of Au the drop of Tc manifested in the suppression of the hysteresis loop in hy drogen absorption isotherms can be quantitatively explained by calculating these isotherms with the parameter values provided by density functional theory (DFT)
Summary
Beside basic physics and chemistry [1], hydrogen absorption by binary alloys of Pd with other metals is of high current interest in the context of various applications related first of all to the so-called hydrogen economy [2], including hydrogen storage [3,4,5] and sen sors [6,7,8], and heterogeneous catalysis [9,10,11]. Zhdanov because adding it to Pd should increase the energy of absorption at the interstitial sites containing the second metal It results in the energetic heterogeneity of the absorption sites and can eliminate the phase transition (in statistical physics, the corresponding general models, including e.g. the random-field Ising model, have been studied a few decades since the generic report by Imry and Ma [19]; the phenomenological models focused on hydrogen absorption by Pd alloys are available as well [20,21]). Despite different techniques and approximations, the conclusions drown in Refs. [22,25] are similar
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