AFFCK: Adaptive Force-Field-Assisted <i>ab Initio</i> Coalescence Kick Method for Global Minimum Search

Global optimization techniques for molecules, solids, and clusters are numerous and can be algorithmically elegant. Yet many of them are time-consuming and prone to getting trapped in local minima. Among the available methods, Coalescence Kick (CK) is attractive: it combines a nearly insulting simplicity with thoroughness. A new version of CK is reported here, called Adaptive Force-Field-Assisted Coalescence Kick (AFFCK). The generation of stationary points on the potential energy surface is tremendously accelerated as compared to that of the earlier, pure <i>ab initio</i> CK, through the introduction of an intermediate step where structures are optimized using a classical force field (FF). The FF itself is system-specific, developed on-the-fly within the algorithm. The pre-computed energies resulting from the FF step are found to be surprisingly indicative of energies in subsequent Density Functional Theory optimization, which enables AFFCK to effectively screen thousands of initial CK-generated structures for favorable starting geometries. Additionally, AFFCK incorporates the use of symmetry operations in order to enhance the diversity in the search space, increase the chance for highly symmetric structures to appear, and speed up convergence of optimizations. A structure-recognition routine ensures diversity in the search space by preventing multiple copies of the same starting geometry from being generated and run. The tests show that AFFCK is much faster than traditional <i>ab initio</i>-only CK. We applied AFFCK to the search for global and low-energy local minima of gas-phase clusters of boron and platinum. For Pt₈ a new global minimum structure is found, which is significantly lower in energy than previously reported Pt₈ minima. Although AFFCK confirms the global minima of B₅^–, B₈, and B₉^–, it proves to be less efficient for systems with nontrivial bonding

Ethylene Dehydrogenation on Pt_4,7,8 Clusters on Al₂O₃: Strong Cluster Size Dependence Linked to Preferred Catalyst Morphologies

Catalytic dehydrogenation of ethylene on size-selected Pt_<i>n</i> (<i>n</i> = 4, 7, 8) clusters deposited on the surface of Al₂O₃ was studied experimentally and theoretically. Clusters were mass-selected, deposited on the alumina support, and probed by a combination of low energy ion scattering, temperature-programmed desorption and reaction of C₂D₄ and D₂, X-ray photoelectron spectroscopy, density functional theory, and statistical mechanical theory. Pt₇ is identified as the most catalytically active cluster, while Pt₄ and Pt₈ exhibit comparable activities. The higher activity can be related to the cluster structure and particularly to the distribution of cluster morphologies accessible at the temperatures and coverage with ethylene in catalytic conditions. Specifically, while Pt₇ and Pt₈ on alumina have very similar prismatic global minimum geometries, Pt₇ at higher temperatures also has access to single-layer isomers, which become more and more predominant in the cluster catalyst ensemble upon increasing ethylene coverage. Single-layer isomers feature greater charge transfer from the support and more binding sites that activate ethylene for dehydrogenation rather than hydrogenation or desorption. Size-dependent susceptibility to coking and deactivation was also investigated. Our results show that size-dependent catalytic activity of clusters is not a simple property of single cluster geometry but the average over a statistical ensemble at relevant conditions

Boron Switch for Selectivity of Catalytic Dehydrogenation on Size-Selected Pt Clusters on Al₂O₃

Size-selected supported clusters of transition metals can be remarkable and highly tunable catalysts. A particular example is Pt clusters deposited on alumina, which have been shown to dehydrogenate hydrocarbons in a size-specific manner. Pt₇, of the three sizes studied, is the most active and, therefore, like many other catalysts, deactivates by coking during reactions in hydrocarbon-rich environments. Using a combination of experiment and theory, we show that nanoalloying Pt₇ with boron modifies the alkene-binding affinity to reduce coking. From a fundamental perspective, the comparison of experimental and theoretical results shows the importance of considering not simply the most stable cluster isomer, but rather the ensemble of accessible structures as it changes in response to temperature and reagent coverage

Diborane Interactions with Pt₇/Alumina: Preparation of Size-Controlled Borated Pt Model Catalysts

Bimetallic catalysts provide the ability to tune catalytic activity, selectivity, and stability. Model catalysts with size-selected bimetallic clusters on well-defined supports offer a useful platform for studying catalytic mechanisms; however, producing size-selected bimetallic clusters can be challenging. In this study, we present a way to prepare bimetallic model (Pt_<i>n</i>B_<i>m</i>/alumina) cluster catalysts by depositing size-selected Pt₇ clusters on an alumina thin film, then selectively adding boron by exposure to diborane, and heating. The interactions between Pt₇/alumina and diborane were probed using temperature-programmed desorption/reaction (TPD/R), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (ISS), plane wave density functional theory (PW-DFT), and molecular dynamic (MD) simulations. It was found that the diborane exposure/heating process does result in preferential binding of B in association with the Pt clusters. Borated Pt clusters are of interest because they are known to exhibit reduced affinity to carbon deposition in catalytic dehydrogenation. At high temperatures, theory, in agreement with experiment, shows that boron tends to migrate to sites beneath the Pt clusters, forming Pt–B–O_suf bonds that anchor the clusters to the alumina support

Microscopic Study of Atomic Layer Deposition of TiO₂ on GaAs and Its Photocatalytic Application

We report a microscopic study of <i>p</i>-GaAs/TiO₂ heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H₂ evolution and CO₂ reduction of these heterostructures shows a very strong dependence on the thickness of the TiO₂ over the range of 0–15 nm. Thinner films (1–10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl₄ atomic layer deposition (ALD) precursor, which is corrosive. Ti³⁺ defect states (i.e., O vacancies) in the TiO₂ film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO₂ molecules bind stably to these Ti³⁺ states. Thicker TiO₂ films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO₂ films are conducting

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cm_Pt^–2 at 0.9 V) and high surface areas (>90 m² g_Pt^–1). Platinum–nickel (PtNi) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m² g_Pt^–1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the PtNi nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications