A challenge to the Delta G~0 interpretation of hydrogen evolution

Platinum is a nearly perfect catalyst for the hydrogen evolution reaction, and its high activity has conventionally been explained by its close-to-thermoneutral hydrogen binding energy (G~0). However, many candidate non-precious metal catalysts bind hydrogen with similar strengths, but exhibit orders-of-magnitude lower activity for this reaction. In this study, we employ electronic structure methods that allow fully potential-dependent reaction barriers to be calculated, in order to develop a complete working picture of hydrogen evolution on platinum. Through the resulting ab initio microkinetic models, we assess the mechanistic origins of Pt's high activity. Surprisingly, we find that the G~0 hydrogen atoms are kinetically inert, and that the kinetically active hydrogen atoms have G's much weaker, similar to that of gold. These on-top hydrogens have particularly low barriers, which we compare to those of gold, explaining the high reaction rates, and the exponential variations in coverages can uniquely explain Pt's strong kinetic response to the applied potential. This explains the unique reactivity of Pt that is missed by conventional Sabatier analyses, and suggests true design criteria for non-precious alternatives

First-principles study fo the Fe-rich Fe_{x}Ni_{y}Al_{1-x-y} alloy system

Die eingereichte Diplomarbeit befasst sich mit der Identifikation und Beschreibung stabiler geordneter Phasen im ternären Eisen-Nickel-Aluminium System (Fe-Ni-Al). Die grundlegende Zielsetzung bestand darin eine konzentrationsabh\"angige Suche nach den Grundzustandsphasen eines tern\"aren Systems mit der Pr\"azision der Dichtefunktionaltheorie durchzuf\"uhren. Um dies zu erreichen wurde ein Cluster Expansion Ansatz in Kombination mit Monte Carlo Simulationen gew\"ahlt. Als grundlegende Gr\"o\ss{}e in der CE wird die Formationsenthalpie verwendet, welche Aussage \"uber die Stabilit\"at einer gebildeten Phase gibt. Durch Herausfiltern der Strukturen, welche die gr\"o\ss{}te Formationenthalpie besitzen k\"onnen durch ein Grundzustandsdiagramm die stabilen Phasen einer mehrkomponentigen Legierung identifiziert werden. In der vorliegenden Arbeit wurde das kubisch innenzentrierte Gitter (bcc) basierend auf der Fe-reichen Seitedes tern\"aren Systems Fe$_x$Ni$_y$Al$_{1-x-y}$ als Beispiel verwendet um die Anwendung der CE auf Systeme mit mehr Komponenten zu zeigen. Dieses Legierungssystem ist in wissenschaftlicher und technologischer Sicht wertvoll, da die bin\"aren Subsysteme (Ni-Al, Fe-Al, Fe-Ni) verschiedene Eigenschaften aufweisen, w\"ahrend das tern\"are System eine stabile B2 Phase über einen grossen Bereich im pseudo- bin\"aren Fe$_x$(NiAl)$_{1-x}$-Bereich zeigt. Auf der Basis von \textit{ab initio} Ergebnissen, welche mittels des \textit{Vienna Ab initio simulation packages} VASP f\"ur einige ausgew\"ahlte Strukturen berechnet wurden, wurden durch die Anwendung des \textit{UNiversal CLuster Expansion} UNCLE codes effektive Cluster Wechselwirkungen bestimmt. Nach einer ausgiebigen Grundzustandssuche konnten die stabilen Phasen im System identifiziert werden. Durch Kombination der Ergebnisse, die von den binären und ternären CE erhalten wurden, wurden mittels Monte Carlo Simulationen in kanonischen und grosskanonischen Ensembles die Phasendiagramme bei ausgew\"ahlten Temperaturen gezeichnet. Als Start fuer die Beschreibung der Formation von stabilen Phasen bei finiten Temperaturen wurde eine komplett ungeordnete Monte Carlo Box --entspricht unendlich hoher Temperatur-- verwendet und im Laufe der Simulation auf 100 Kelvin abgek\"uhlt. Durch langsames Aufheizen wurde der Effekt der Konfigurationsentropie studiert. Zur Identifikation der gebildeten Phasen wurde eine selbstentwickelte Methode verwendet, welche die Analyse der kurzreichweitigen Ordnung in einer Ausdehnung von 3x3x3 Atomen f\"ur bin\"are und tern\"are Phasen beinhaltet. Elementare Ausscheidungen wurden ab einer Clustergr\"o\ss{}e von 10 Atomen als solche gekennzeichnet. Die Untersuchungen an den bin\"aren Systeme f\"uhrten wie erwartet zu verschiedenen Ergebnissen. Die bin\"aren Phasen des Ni-Al Systems besitzen die h\"ochste Stabilit\"at im untersuchten tern\"aren System. Als stabilste Struktur wurde B2-NiAl identifiziert. Zwei weitere Phasen wurden gefunden. Im Gegensatz zu Ni-Al zeigt das Ni-Fe System keine Tendenz zur Bildung geordneter Strukturen im kubisch innenzentrierten Gitter. Die Phasenbildung im Al-Fe System ist energetisch sehr beg\"unstigt. Drei Grundzust\"ande wurden gefunden, welche auch bei h\"oheren Temperaturen stabil sind. Als stabilste Struktur im System bildet sich B2-NiAl weit in das tern\"are System hinein, wobei Eisen in diesem Fall als elementare Ausscheidungen formt. Diese Ausdehnung wird stark durch das Verh\"altnis der Konzentration von Nickel und Aluminium beeinflusst. W\"ahrend auf der aluminiumreichen Seite des untersuchten Konzentrationsbereichs B2-NiAl bis zu einer Konzentration von 80 \% Eisen gebildet wurde, wird die bin\"are Phase auf der nickelreichen Seite bereits bei 60 \% Eisen destabilisiert. Als Grund f\"ur dieses Verhalten wurde eine Stabilisierung des \"ubersch\"ussigen Aluminiums in der Eisenmatrix durch die Bildung von Fe$_3$Al gefunden. Da Nickel und Eisen keine geordneten Phasen im bcc Gitter bilden, wird die B2-Phase auf der nickelreichen Seite durch die Bildung von Ni-Antisites auf den Aluminiumpl\"atzen des Kristalls destabilisiert. Im Bereich des tern\"aren Systems, welcher aus nahezu reinem Eisen besteht wurde bereits ein Clustern von Nickel und Aluminium in einer B2-artigen Struktur identifiziert.The present diploma thesis deals with the identification and description of stable ordered phases in the ternary iron-nickel-aluminium system (Fe-Ni-Al) based on a first principle concept. The aim of the work was to perform a concentration dependant search for ground state phases of a ternary system with the precision of density functional theory calculations. For this the Cluster expansion in combination with Monte Carlo simulations was applied. As a basic quantity the enthalpy of formation was calculated, which is the responsible for the formation of stable phases in an alloy system. By including the enthalpies of formation of the calculated structures in a ground state diagram the stable phases could be identified. The body centered cubic (bcc) parent lattice was chosen, since we were interested in the Fe-rich Fe$_x$Ni$_y$Al$_{1-x-y}$ alloy system as an example for the application of a Cluster Expansion on a multicompound system. The Fe-Ni-Al system is of high scientific and technological interest since the binary subsystems (Ni-Fe, Al-Fe, Ni-Al) are of different characteristics (i.e. lattice types, phase stability) while the ternary phase diagram shows a stable B2-phase regime in the pseudo binary Fe$_x$(NiAl)$_{1-x}$ part. On the basis of \textit{ab initio} results for various configurations from DFT, calculated with the \textit{Vienna Ab initio Simulation Package} (VASP) effective cluster interaction energies were calculated employing the \textit{UNiversal CLuster Expansion} (UNCLE) code. After an extensive ground state search the stable phases in the bcc lattice were identified. By combining the results of the binary and ternary ground state searches to construct phase diagrams at finite temperatures the converged ECIs were used in both, canonical and grand-canonical Monte-Carlo simulations. The formation of ordered phases was simulated by starting from completely unordered systems --which represents an infinite high temperature-- and cooling down to 100 Kelvin. By slowly heating up again the effect of the configurational entropy was studied. The identification of the formed phases in the simulation boxes was done by a self elaborated method analyzing short range ordering with an extension of 3x3x3 atoms for binary and ternary phases and 10 atoms for elemental precipitations. As expected the investigations done on the binary systems lead to somehow different results. The binary Ni-Al system has shown to form the most stable phases in the ternary system, with B2-NiAl showing the lowest enthalpy of formation. Other two stable ground states have been identified. In contrast to Ni-Al the Ni-Fe system did not tend to form ordered structures in the bcc lattice. The ground states of the Al-Fe system turned out to be less stable then in the Ni-Al system, but the ordering is still energetically favourable. From the found phases at T=0K three have shown to be stable also at finite temperatures. B2-NiAl has shown to form deep into the ternary region, while Fe forms elemental precipitations. The ratio between nickel and aluminium concentration has a great influence in the extension of the B2 region. On the Al rich side of the investigated concentration range B2-NiAl is formed up to 80 \% iron, while on the Ni-rich side the phase is destablilized already at 60 \% Fe. As the reason for this behaviour a stabilization of the excessive aluminium in the iron matrix by Fe$_3$Al like ordering has been identified. Since nickel did not tend to form ordered structures with iron in a bcc lattice the excess of Ni destablilizes the B2-NiAl ordering by replacing Al in the B2 crystal. In the region of nearly pure iron nickel and aluminium still have shown to cluster in a B2-like way

Scaled and Dynamic Optimizations of Nudged Elastic Bands

We present a modified nudged elastic band routine that can reduce the number of force calls by more than 50% for bands with non-uniform convergence. The method, which we call "dyNEB", dynamically and selectively optimizes states based on the perpendicular forces and parallel spring forces acting on that region of the band. The convergence criteria are scaled to focus on the region of interest, i.e., the saddle point, while maintaining continuity of the band and avoiding truncation. We show that this method works well for solid state reaction barriers---non-electrochemical in general and electrochemical in particular---and that the number of force calls can be significantly reduced without loss of resolution at the saddle point

The fundamental drivers of electrochemical barriers

Reaction barriers dictate the rates of elementary reactions, and therefore are crucial to understanding electrochemical kinetics. Since these reactions tend to occur at heterogeneous surfaces, principles from catalysis can be expected to apply. Here, we use electronically grand-canonical calculations on a model system of the proton-deposition reaction on a range of metal surfaces to explore the extent to which these principles hold. First, we show that while reaction barriers are functions of potential, unlike endstates they tend to exhibit a nonlinear dependence, and this nonlinearity can be traced to differences in the electron transfer at the reaction barrier. We show that along a reaction path, this same electron-transfer difference forces the barrier to move earlier for downhill reactions, enforcing and explaining the Hammond-Leffler postulate for electrochemical reactions, as well as explaining the curvature in the Marcus-like relations. We further examine trends in barrier energies, at equivalent driving forces, for this reaction across metals. We find that the barrier energy correlates weakly to the hydrogen binding energy and the d-band center, but instead correlates strongly with the charge presented at the metal surface -- which is a direct consequence of the native work function of the material. This suggests that the energetics of the barrier are driven more strongly by the electrostatic, rather than the covalent, nature of the metal-adsorbate interaction, and suggests electrochemical barriers may have an independent driving force from electrochemical adsorbates

Field-induced Conductance Switching by Charge-state Alternation in Organometallic Single-Molecule Junctions

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These molecular intrinsic properties are usually addressed by varying the molecules electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high to low current ratios exceeding 1000 at voltage stimuli of less than 1.0 V. DFT calculations identify a localized, redox active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarised ground state unique to the Mo centre. This situation opens an additional slow and incoherent hopping channel for transport, triggering a transient charging effect of the entire molecule and a strong hysteresis with unprecedented high low-to-high current ratios.Comment: 9 pages, 4 figure

Solvation of furfural at metal–water interfaces: Implications for aqueous phase hydrogenation reactions

Metal-water interfaces are central to understanding aqueous-phase heterogeneous catalytic processes. However, the explicit modeling of the interface is still challenging as it necessitates extensive sampling of the interfaces' degrees of freedom. Herein, we use ab initio molecular dynamics (AIMD) simulations to study the adsorption of furfural, a platform biomass chemical on several catalytically relevant metal-water interfaces (Pt, Rh, Pd, Cu, and Au) at low coverages. We find that furfural adsorption is destabilized on all the metal-water interfaces compared to the metal-gas interfaces considered in this work. This destabilization is a result of the energetic penalty associated with the displacement of water molecules near the surface upon adsorption of furfural, further evidenced by a linear correlation between solvation energy and the change in surface water coverage. To predict solvation energies without the need for computationally expensive AIMD simulations, we demonstrate OH binding energy as a good descriptor to estimate the solvation energies of furfural. Using microkinetic modeling, we further explain the origin of the activity for furfural hydrogenation on intrinsically strong-binding metals under aqueous conditions, i.e., the endothermic solvation energies for furfural adsorption prevent surface poisoning. Our work sheds light on the development of active aqueous-phase catalytic systems via rationally tuning the solvation energies of reaction intermediates