Jonathan E. Mueller

Development of a ReaxFF reactive force field for aqueous chloride and copper chloride.

Copper ions play crucial roles in many enzymatic and aqueous processes. A critical analysis of the fundamental properties of copper complexes is essential to understand their impact on a wide range of chemical interactions. However the study of copper complexes is complicated by the presence of strong polarization and charge transfer effects, multiple oxidation states, and quantum effects like Jahn-Teller distortions. These complications make the experimental observations difficult to interpret. In order to provide a computationally inexpensive yet reliable method for simulation of aqueous-phase copper chemistry, ReaxFF reactive force field parameters have been developed. The force field parameters have been trained against a large set of DFT-derived energies for condensed-phase copper-chloride clusters as well as chloride/water and copper-chloride/water clusters sampled from molecular dynamics (MD) simulations. The parameters were optimized by iteratively training them against configurations generated from ReaxFF MD simulations that are performed multiple times with improved sets of parameters. This cycle was repeated until the ReaxFF results were in accordance with the DFT-derived values. We have performed MD simulations on chloride/water and copper-chloride/water systems to validate the optimized force field. The structural properties of the chloride/water system are in accord with previous experimental and computational studies. The properties of copper-chloride/water agreed with the experimental observations including evidence of the Jahn-Teller distortion. The results of this study demonstrate the applicability of ReaxFF for the precise characterization of aqueous copper chloride. This force field provides a base for the design of a computationally inexpensive tool for the investigation of various properties and functions of metal ions in industrial, environmental, and biological environments.

Nickel Cluster Growth on Defect Sites of Graphene: A Computational Study

The first extraction of graphene in 2004 led to a wide range of experimental and theoretical studies aimed at better understanding and exploiting the unique properties of this novel two-dimensional material. Among the many potential applications, which have been suggested, are uses of graphene as a substrate in high-performance catalysis and as a component in circuit-board technology. In particular, graphene s high surface area and conductivity have motivated proposals to use it as a substrate for growing and/or anchoring metal nanoparticles in high-performance catalysts and other electrochemical devices. However, the activity of such carbon-supported metal catalysts is strongly dependent on the dispersion and stability of the metal clusters on the support (i.e. the ability of the substrate to stabilize metal clusters of various sizes on its surface). Thus, vacancy defects are expected to play a vital role in making graphene suitable for these applications by supplying highly active binding sites for adsorbing and stabilizing metal clusters. Indeed, finite populations of single and double vacancy defects are thermodynamically stable in graphene, and have been studied extensively. Density functional theory (DFT) calculations revealed that vacancy defects resulting from the removal of up to five C atoms reconstruct to form non-hexagonal rings (models are shown in the Supporting Information: Figures S1.b–f). Even larger holes have been observed in electron microscopy experiments. Defects may also play a critical role in using graphene components for circuit fabrication. For example, taking advantage of the Dirac fermions in graphene requires opening up its band gap to convert it from a conductor into a semiconductor. This conversion can be achieved by doping graphene with either B or N atoms; however, another possibility for accomplishing this could be the adsorption of small metal clusters on the surface. Because the adsorption of such clusters can be used to tune additional magnetic and transport properties of the substrate, it might also provide a technique for controlling an additional set of electromagnetic properties. The catalytic nature of Ni is well established, and Ni nanoparticles are commonly used to catalyze the synthesis of carbon nanostructures. Owing to the strong affinity between Ni and C, the incorporation of Ni atoms into carbon nanostructures, grown using Ni catalysts, has been observed. Ushiro et al. reported that X-ray adsorption measurements detect Ni impurities in carbon nanostructures following nickel-catalyzed synthesis, which even treatment with acid is not able to remove. Moreover, Banhart et al. identified Ni impurities wrapped in onion-like graphenic particles by using electron microscopy. The work of Rinaldi et al. is even more supportive. Combining results from DFT calculations and high-resolution transmission electron microcopy measurements (HR-TEM) utilizing several in situ characterization techniques, they concluded that Ni atoms form very stable Ni– C compounds during nickel-catalyzed carbon nanotube (CNT) growth, which are incorporated into the final products. They also found unexpectedly strong adsorption of the Ni clusters on the CNT supports. However, despite the potential advantages of using Ni nanoparticles adsorbed on graphene, their catalytic and electromagnetic properties (with the exception of single and two Ni atoms adsorbates) remain mostly unexplored. Based on these findings, it would be expected that just as Ni nanoparticles might be used to tailor critical properties of defective graphene sheets, a graphene substrate might be used to modify the catalytic properties of nickel nanoparticles as well. To elucidate this potential interplay we employ DFT to study the adsorption of Nin nanoclusters on defective graphene (details in the Supporting Information). As substrate models we select graphene sheets with vacancy defects, resulting from the removal of x atoms (with x 5; see Figure S1 in the Supporting Information). To model the adsorbed Ni nanoparticles, we successively grew Nin clusters with n 10 and focused on the lowest energy adsorption configuration of each Nin cluster on each of these six graphene substrates (with and without vacancy defects). The binding energies (referenced against single Ni atoms and the graphene substrate) for the lowest energy configuration are summarized in Figure 1. The binding energies can be explained by three types of bond contributions. The first type of binding is between Ni atoms. As the cluster size increases the ratio of bulk to surface atoms increases so that the binding energy will asymptotically [*] Dr. W. Gao, Dr. J. E. Mueller, Dr. J. Anton, Prof. Dr. T. Jacob Institut f r Elektrochemie, Universit t Ulm 89081 Ulm (Germany) E-mail: timo.jacob@uni-ulm.de

The Role of Co-Adsorbed CO and OH in the Electrooxidation of Formic Acid on Pt(111)**

The electrooxidation of formic acid (HCOOH) on platinumgroupmetals has been widely studied for its great relevance to electrochemistry as a prototype reaction for the electrooxidation of small organics and its importance in understanding low-temperature fuel cells. 2] It is generally accepted that electrooxidation of HCOOH on Pt proceeds by a dual-path mechanism consisting of indirect and direct paths. In the indirect path, HCOOH is converted into adsorbed CO and then to CO2. In the direct path, HCOOH is converted into CO2 via a reactive intermediate, whose identity is still disputed. Unfortunately, the intermediates from both the indirect and direct paths compete with each another for adsorption sites and the opportunity to react with oxidizers (e.g. OH) on the surface. This competition couples these reaction paths kinetically, hampering the elucidation of their individual reaction mechanisms. In situ infrared reflection-adsorption spectroscopy (IRAS) identifies adsorbed CO, resulting from HCOOH dehydration, as the key reaction intermediate in the indirect path. However, a build-up of CO is observed to poison the system. In contrast, the identity of the reactive intermediate along the direct path is still controversial. Wilhelm and coworkers initially suggested either COH or CHO. Others have long assumed it to be COOH. Using IR spectroscopy, Osawa et al. and Feliu et al. found that formate (HCOO) is the reactive intermediate and that the oxidation of HCOO to CO2 is the rate-determining step for formic acid oxidation. In contrast, Behm et al. argue that weakly adsorbed HCOOH might be the key intermediate, leaving HCOO as a spectator. The electrochemical and spectral data obtained under both static and flow conditions, which provide the basis for these proposed reactive intermediates, are essentially identical. However, different interpretations of the non-linear relationship between the measured current and the formate coverage lead to different conclusions. Cyclic voltammograms (CVs) of HCOOH oxidation (Figure 1) show that the current (I) first peaks around 0.6 V as the potential (U) increases. The current then remains stable or decreases between 0.6 and 0.8 V in what is termed the negative differential resistance (NDR) region. A sharp, increasing around 0.95 V follows the NDR region. 10,12–14] During this process, IR spectroscopy measurements reveal that the polycrystalline Pt surface has a relatively constant coverage of CO below 0.8 V. Above 0.8 V, the coverage of adsorbed CO rapidly decreases due to oxidation, while HCOO quickly increases with increasing potential, until the surface is nearly saturated with HCOO at above 0.9 V. Once the CO coverage is almost completely depleted (ca. 0.95 V), the coverage of HCOO rapidly decreases with increasing potential up to 1.2 V. Thus, these IR measurements suggest that the CV curve can be understood in terms of CO adsorption, desorption, and oxidation by OH. Nevertheless, the detailed roles of adsorbed CO and OH have yet to be elucidated, substantially hindering our understanding of the mechanism of this fundamental reaction. First principles simulations have already been useful in studying HCOOH oxidation. Two density functional theory (DFT) studies indicate that HCOOH oxidation under electrochemical conditions either proceeds via intermediate COOH or initiates from a weakly adsorbed configuration of HCOOH, in which the C H bond is in a “down” configuration. However, several independent theoretical investigations report that the HCOOH adsorption models used in these studies do not correspond to the most energetically favorable structure. More recently, using a gas phase model (i.e. without treating solvation effects), we found that bidentate formate (HCOOB*) is the reactive intermediate in HCOOH oxidation. This result is fully consistent with Figure 1. The potential-dependent rate constants (R) and the experimental CV. The potential sweep rate is 50 mVs .

Novel transmetalation reaction for electrolyte synthesis for rechargeable magnesium batteries

A simple strategy for the synthesis of electrolyte solutions comprised of binuclear magnesium aluminate complexes without the need for organomagnesium compounds is established. The as-prepared phenolate based electrolyte exhibits an anodic stability of up to 3.4 V, good ionic conductivity and air-stability.

Surface stability of Pt3Ni nanoparticulate alloy electrocatalysts in hydrogen adsorption.

Nanoparticles of Pt/Ni alloys represent state of the art electrocatalysts for fuel cell reactions. Density functional theory (DFT) based calculations along with in situ X-ray absorption spectroscopy (XAS) data show that the surface structure of Pt3Ni nanoparticulate alloys is potential-dependent during electrocatalytic reactions. Pt3Ni based electrocatalysts demonstrate preferential confinement of Ni to the subsurface when the electrode is polarized in the double layer region where the surface is free of specifically adsorbed species. Hydrogen adsorption triggers nickel segregation to the surface. This process is facilitated by a high local surface coverage of adsorbed hydrogen in the vicinity of the surface confined Ni due to an uneven distribution of the adsorbate(s) on the catalysts surface. The adsorption triggered surface segregation shows a non-monotonous dependence on the electrode potential and can be identified as a breathing of the catalyst as was proposed previously. The observed breathing behavior is relatively fast and proceeds on a time scale of 100-1000 s.

Surface Buckling and Subsurface Oxygen: Atomistic Insights into the Surface Oxidation of Pt(111).

Platinum is a catalyst of choice in scientific investigations and technological applications, which are both often carried out in the presence of oxygen. Thus, a fundamental understanding of platinums (electro)catalytic behavior requires a detailed knowledge of the structure and degree of oxidation of platinum surfaces in operando. ReaxFF reactive force field calculations of the surface energies for structures with up to one monolayer of oxygen on Pt(111) reveal four stable surface phases characterized by pure adsorbate, high- and low-coverage buckled, and subsurface-oxygen structures, respectively. These structures and temperature programmed desorption (TPD) spectra simulated from them compare favorably with and complement published scanning tunneling microscopy (STM) and TPD experiments. The surface buckling and subsurface oxygen observed here influence the surface oxidation process, and are expected to impact the (electro)catalytic properties of partially oxidized Pt(111) surfaces.

ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell

In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H2/Ni/YSZ and C4H10/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC.

Growth of Stable Surface Oxides on Pt(111) at Near‐Ambient Pressures

Detailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time-dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) we show that a time scale of hours (t≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF-GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430-680 K and an O2 partial pressure of 1 mbar.

Multiscale Modeling of Au-Island Ripening on Au(100)

We describe a multiscale modeling hierarchy for the particular case of Au-island ripening on Au(100). Starting at the microscopic scale, density functional theory was used to investigate a limited number of self-diffusion processes on perfect and imperfect Au(100) surfaces. The obtained structural and energetic information served as basis for optimizing a reactive forcefield (here ReaxFF), which afterwards was used to address the mesoscopic scale. Reactive force field simulations were performed to investigate more diffusion possibilities at a lower computational cost but with similar accuracy. Finally, we reached the macroscale by means of kinetic Monte Carlo (kMC) simulations. The reaction rates for the reaction process database used in the kMC simulations were generated using the reactive force field. Using this strategy, we simulated nucleation, aggregation, and fluctuation processes for monoatomic high islands on Au(100) and modeled their equilibrium shape structures. Finally, by calculating the step line tension at different temperatures, we were able to make a direct comparison with available experimental data.

First-Principles-Based Multiscale, Multiparadigm Molecular Mechanics and Dynamics Methods for Describing Complex Chemical Processes

We expect that systematic and seamless computational upscaling and downscaling for modeling, predicting, or optimizing material and system properties and behavior with atomistic resolution will eventually be sufficiently accurate and practical that it will transform the mode of development in the materials, chemical, catalysis, and Pharma industries. However, despite truly dramatic progress in methods, software, and hardware, this goal remains elusive, particularly for systems that exhibit inherently complex chemistry under normal or extreme conditions of temperature, pressure, radiation, and others. We describe here some of the significant progress towards solving these problems via a general multiscale, multiparadigm strategy based on first-principles quantum mechanics (QM), and the development of breakthrough methods for treating reaction processes, excited electronic states, and weak bonding effects on the conformational dynamics of large-scale molecular systems. These methods have resulted directly from filling in the physical and chemical gaps in existing theoretical and computational models, within the multiscale, multiparadigm strategy. To illustrate the procedure we demonstrate the application and transferability of such methods on an ample set of challenging problems that span multiple fields, system length- and timescales, and that lay beyond the realm of existing computational or, in some case, experimental approaches, including understanding the solvation effects on the reactivity of organic and organometallic structures, predicting transmembrane protein structures, understanding carbon nanotube nucleation and growth, understanding the effects of electronic excitations in materials subjected to extreme conditions of temperature and pressure, following the dynamics and energetics of long-term conformational evolution of DNA macromolecules, and predicting the long-term mechanisms involved in enhancing the mechanical response of polymer-based hydrogels.