Suppression of hysteresis in absorption of hydrogen by a Pd-Au alloy

Published by the American Physical Society. Hydrogen absorption by Pd exhibits hysteresis loops (provided the temperature is not too high) and represents one of the classical examples of first-order phase transitions in metals. Experiments indicate that addition of even a small amount of Au is able to suppress hysteresis. From this perspective, we analyze the energetics of hydrogen in a Pd-Au alloy by using extensive density-functional-theory (DFT) calculations. The dependence of the hydrogen binding energy on the number (n) of Au atoms forming an adsorption site is found to be appreciably nonlinear. With the DFT input for statistical calculations, we reproduce special features of the hydrogen absorption isotherms and explain the rapid decrease of the corresponding critical temperature with increasing Au fraction. The key factor here is that the phase transition is related primarily to absorption in sites formed only by Pd. With increasing Au amount, the fraction of such sites rapidly decreases, the distance between H atoms located there becomes on average larger, the interaction between them becomes weaker, and accordingly the driving force for the phase transition decreases. It is of interest that all these effects can be illustrated by taking only the configurations with n≤2 into account. This means that in the context under consideration the fine details of the dependence of the hydrogen binding energy on n are in fact not too important

Partial or complete suppression of hysteresis in hydride formation in binary alloys of Pd with other metals

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

Pd single-atom sites on the surface of PdAu nanoparticles: A DFT-based Topological search for suitable compositions

Structure of model bimetallic PdAu nanoparticles is analyzed aiming to find Pd:Au ratios optimal for existence of Pd1 single-atom surface sites inside outer Au atomic shell. The analysis is performed using density-functional theory (DFT) calculations and topological approach based on DFT-parameterized topological energy expression. The number of the surface Pd1 sites in the absence of adsorbates is calculated as a function of Pd concentration inside the particles. At low Pd contents none of the Pd atoms emerge on the surface in the lowest-energy chemical orderings. However, surface Pd1 sites become stable, when Pd content inside a Pd-Au particle reaches ca. 60%. Further Pd content increase up to almost pure Pd core is accompanied by increased concentration of surface Pd atoms, mostly as Pd1 sites, although larger Pd ensembles as dimers and linear trimers are formed as well. Analysis of the chemical orderings inside PdAu nanoparticles at different Pd contents revealed that enrichment of the subsurface shell by Pd with predominant occupation of its edge positions precedes emergence of Pd surface species

Theoretical Study of the Methanol Dehydrogenation on Platinum Nanocluster

Методом функционала плотности изучена реакция дегидрирования метанола по механизму разрыва O-H-связи на нанокластере платины Pt79, проведено сравнение с идеальной поверхностью Pt(111). Найдено, что наиболее устойчивые комплексы образуются при адсорбции COНх (x = 1-4) частиц на низкокоординированных атомах нанокластера Pt79, при этом такой предпочтительности для атомов Н не обнаружено. Абсолютные значения энергии адсорбции на вершинах и ребрах нанокластера Pt79 выше на 0,2–0,7 эВ, чем на высококоординированных центрах регулярной поверхности Pt(111). Стабильность адсорбционных комплексов на поверхности нанокластера уменьшается от вершин к ребрам и затем к центру граней (111) нанокластера. Анализ энергетического профиля реакции показывает, что тепловой эффект образования ключевого интермедиата CH3O на кластере Pt79 становится нулевым в отличие от эндотермического (0,5 эВ) на регулярной поверхности Pt(111). Экзотермический эффект всех остальных реакционных стадий, за исключением десорбции СО, на нанокластере увеличивается на ~0,2-0,5 эВThe methanol dehydrogenation through the initial breaking of the O-H bond at Pt79 nanoparticle was studied with the DFT method. The comparison with an ideal surface of Pt (111) was carried out. The most stable complexes were found for COНх (x = 1-4) species adsorbed at low-coordinated atoms of nanocluster Pt79, whereas no preference for adsorption at corners and edges for Н atoms was found. The absolute adsorption energies of COНх species at corner and edge sites of platinum nanocluster increased by 0.2–0.7 eV in comparison with high-coordinated sites of the regular Pt(111) surface. The stabilization effect of adsorption at the nanoparticle decreases from corners to edges and then to the center of (111) facet. According to the reaction energy profile, the thermal effect of the formation of CH3O at the nanocluster becomes close to zero, in contrast to the endothermic effect (0.5 eV) on the regular Pt(111) surface. The exothermic effects for other reaction stages at the platinum nanocluster, excluding CO desorption, increase by ~0.2-0.5 e

Size-Dependence of the Adsorption Energy of CO on Pt Nanoparticles: Tracing Two Intersecting Trends by DFT Calculations

With density functional calculations, we studied the size dependence of adsorption properties of metal nanoparticles (NPs) via the example of CO as a probe on Pt_<i>n</i> clusters with <i>n</i> = 38–260 atoms. For the largest NPs considered, the calculated adsorption energies lie below the corresponding value for the (ideal) infinite surface Pt(111). For clusters of 38–116 atoms, we calculated a sharp increase of the adsorption energy with decreasing cluster size. These opposite trends meet in an intermediate size range, for clusters of about 200 atoms, yielding the lowest adsorption energies. These computational results suggest that a nanosized transition to a pronounced higher adsorption activity occurs for Pt NPs at notably larger nuclearities than for Pd NPs. We analyze the results by invoking the concept of generalized coordination numbers, adapted to the second-order level

Theoretical Study of the Methanol Dehydrogenation on Platinum Nanocluster

Методом функционала плотности изучена реакция дегидрирования метанола по механизму разрыва O-H-связи на нанокластере платины Pt79, проведено сравнение с идеальной поверхностью Pt(111). Найдено, что наиболее устойчивые комплексы образуются при адсорбции COНх (x = 1-4) частиц на низкокоординированных атомах нанокластера Pt79, при этом такой предпочтительности для атомов Н не обнаружено. Абсолютные значения энергии адсорбции на вершинах и ребрах нанокластера Pt79 выше на 0,2–0,7 эВ, чем на высококоординированных центрах регулярной поверхности Pt(111). Стабильность адсорбционных комплексов на поверхности нанокластера уменьшается от вершин к ребрам и затем к центру граней (111) нанокластера. Анализ энергетического профиля реакции показывает, что тепловой эффект образования ключевого интермедиата CH3O на кластере Pt79 становится нулевым в отличие от эндотермического (0,5 эВ) на регулярной поверхности Pt(111). Экзотермический эффект всех остальных реакционных стадий, за исключением десорбции СО, на нанокластере увеличивается на ~0,2-0,5 эВThe methanol dehydrogenation through the initial breaking of the O-H bond at Pt79 nanoparticle was studied with the DFT method. The comparison with an ideal surface of Pt (111) was carried out. The most stable complexes were found for COНх (x = 1-4) species adsorbed at low-coordinated atoms of nanocluster Pt79, whereas no preference for adsorption at corners and edges for Н atoms was found. The absolute adsorption energies of COНх species at corner and edge sites of platinum nanocluster increased by 0.2–0.7 eV in comparison with high-coordinated sites of the regular Pt(111) surface. The stabilization effect of adsorption at the nanoparticle decreases from corners to edges and then to the center of (111) facet. According to the reaction energy profile, the thermal effect of the formation of CH3O at the nanocluster becomes close to zero, in contrast to the endothermic effect (0.5 eV) on the regular Pt(111) surface. The exothermic effects for other reaction stages at the platinum nanocluster, excluding CO desorption, increase by ~0.2-0.5 e