Metal−organic framework (MOF) materials are a class of nanoporous materials that have many potential advantages over traditional nanoporous materials for adsorption and other chemical separation technologies. Because of the large number of different MOFs that exist, efforts to predict the performance of MOFs using molecular modeling can potentially play an important role in selecting materials for specific applications. We review the current state-of-the-art in the molecular modeling and quantum mechanical modeling of MOFs. Quantum mechanical calculations have been used to date to examine structural and electronic properties of MOFs and the calculation of MOF−guest interactions. Molecular modeling calculations using empirical classical potential calculations have been used to study pure and mixed fluid adsorption in MOFs. Similar calculations have recently provided initial information about the diffusive transport of adsorbed fluids in MOFs.

PtM alloys (M = Co, Ni, Fe, etc.) have been extensively studied for their use in fuel cells, both in well-defined extended surfaces, [1] as well as in nanoparticles. [2] After the report about exceptional activity of Pt 3 Ni(111)-skin surface [1a] for the oxygen reduction reaction (ORR) a lot of efforts have been made to mimic this catalytic behavior at the nanoscale. It has been shown that a Pt 3 Ni(111) crystal annealed in ultrahigh vacuum (UHV) shows an oscillating segregation profile, with the outermost layer consisting of pure platinum while the second layer is enriched in nickel compared to the bulk composition. [1a, 3] Such a surface we termed Pt skin, and owing to the presence of the non-noble metal in the subsurface layer it has altered electronic properties compared to the monometallic Pt single crystal with the same orientation. Accordingly, altered electronic properties induce a change in adsorption behavior, specifically a shift of surface-oxide formation to higher potentials. [1a,b] This adsorption behavior is believed to be the origin of the high activity for the ORR. On the opposite side of the potential scale, the adsorption of hydrogenated species, denoted as underpotentially adsorbed hydrogen (H upd ), is also largely affected on Pt-skin surfaces. [4] Despite numerous efforts dedicated to synthesize nanocatalysts with Pt-skin-type surfaces, [2c, 5] it still remains a challenge to claim their existence at the nanoscale. To systematically resolve this issue, we attempt to provide fundamental insight into the adsorption properties of well-defined Pt-skin surfaces under relevant electrochemical conditions and to transfer that knowledge to corresponding nanocatalysts.For that reason, we first examine the formation and composition of Pt-skin surfaces by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM) in UHV, and second we study the composition of the surfaces in an electrochemical environment to establish their adsorption properties. We demonstrate by cyclic voltammetry that the surface coverage of H upd on Pt skin is about half of that found on Pt(111), whereas the surface coverage of a saturated monolayer of carbon monoxide is similar for both surfaces. This is an important finding, which provides a link towards accurate determination of the electrochemically active surface area of nanoscale catalysts. The developed methodology provides additional evidence for the existence of Pt-skin surfaces on Pt-bimetallic nanocatalysts and can substantially diminish errors in the evaluation of the real surface area and catalytic activity.A thorough examination of the Pt-skin surfaces was performed in view of their importance in electrocatalysis as well as in response to recent questions and doubts in the Figure 1. Surface characterization of Pt 3 M(111) surfaces in UHV: STM images of A) sputtered and B) annealed Pt 3 Ni(111) surfaces. The brightness in colors is a measure of the depth profile, with each color change marking a single atomic step. Low-energy ion scattering spectra ...

Improving the efficiency of electrocatalytic reduction of oxygen represents one of the main challenges for the development of renewable energy technologies. Here, we report the systematic evaluation of Pt-ternary alloys (Pt3(MN)1 with M, N = Fe, Co, or Ni) as electrocatalysts for the oxygen reduction reaction (ORR). We first studied the ternary systems on extended surfaces of polycrystalline thin films to establish the trend of electrocatalytic activities and then applied this knowledge to synthesize ternary alloy nanocatalysts by a solvothermal approach. This study demonstrates that the ternary alloy catalysts can be compelling systems for further advancement of ORR electrocatalysis, reaching higher catalytic activities than bimetallic Pt alloys and improvement factors of up to 4 versus monometallic Pt.

We have determined the structures of dense adlayers of glycine and alanine on the Cu(110) and Cu(100) surfaces using plane wave density functional theory. These calculations resolve several experimental controversies regarding these structures. Glycine exists on Cu(110) as a single adlayer structure, while on Cu(100) two distinct glycine adlayers coexist. The glycine structures serve as useful starting points for constructing alanine adlayer structures. We considered separately the adsorption of enantiopure alanine and racemic alanine on each surface. Adlayers of enantiopure alanine are found to be closely related to the adlayers observed for glycine. Racemic alanine adlayers on Cu(110) are structurally analogous to those observed for glycine on this surface and adopt a pseudo-racemate ordering. On Cu(100), in contrast to glycine, racemic alanine is found to adopt a single adlayer structure that is an ordered racemate. Spontaneous segregation of molecular enantiomers does not occur in racemic adsorbed mixtures on either surface. Consideration of the orientationally distinct domains that may exist for each adlayer on these surfaces provides important information for the interpretation of the adlayer domain boundaries that are commonly observed in scanning tunneling microscopy images of amino acid adlayers. Examining this set of amino acid adlayers provides useful insight into the range of subtle behaviors that can arise in these and related systems where chiral molecules form ordered adlayers on flat metal surfaces.

We have computed the adsorption and diffusion of CO2, N2, CH4, and H2 in zeolitic imidazolate framework (ZIF) materials ZIF-68 and ZIF-70 from atomistic simulations. These simulations were performed using geometries obtained from density functional theory (DFT) optimization of the experimental crystal structure of ZIF-68 and on four structures of ZIF-70, one based on the experimental crystal structure and three having different imidazole/nitroimidazole substitution ratios. The framework charges for charge−quadrupole interaction (CQI) terms in our simulations were parametrized with charges obtained from Bader charge decomposition (periodic DFT calculations) and ChelpG charges (cluster DFT calculations). The adsorption and diffusion of the quadrupolar fluids can be dramatically different when using the Bader and ChelpG charges. Agreement between simulations and experiments for the N2 adsorption isotherms in ZIF-68 and 70 is very good when CQI terms are included. In contrast, simulations overpredict the amount of CO2 adsorbed at 298 K compared with experiments.

We present a comprehensive, density functional theory based analysis of the direct synthesis of hydrogen peroxide, H2O2, on 12 transition metal surfaces. We determine the full thermodynamics and selected kinetics of the reaction network on these metals, and we analyze these energetics with simple, microkinetically motivated rate theories to assess the activity and selectivity of hydrogen peroxide production on the surfaces of interest. By further exploiting Brønsted–Evans–Polanyi relationships and scaling relationships between the binding energies of different adsorbates, we express the results in the form of a two-dimensional contour volcano plot, with the activity and selectivity being determined as functions of two independent descriptors: the atomic hydrogen and oxygen adsorption free energies. We identify both a region of maximum predicted catalytic activity, which is near Pt and Pd in descriptor space, and a region of selective hydrogen peroxide production, which includes Au. The optimal catalysts represent a compromise between activity and selectivity and are predicted to fall approximately between Au and Pd in descriptor space, providing a compact explanation for the experimentally known performance of Au–Pd alloys for hydrogen peroxide synthesis, and suggesting a target for future computational screening efforts to identify improved direct hydrogen peroxide synthesis catalysts. Related methods of combining activity and selectivity analysis into a single volcano plot may be applicable to, and useful for, other aqueous phase heterogeneous catalytic reactions where selectivity is a key catalytic criterion.

No abstract

The mechanism of nitric oxide electroreduction on Pt(111) is investigated using a combination of first principles calculations and electrokinetic rate theories. Barriers for chemical cleavage of N-O bonds on Pt(111) are found to be inaccessibly high at room temperature, implying that explicit electrochemical steps, along with the aqueous environment, play important roles in the experimentally observed formation of ammonia. Use of explicit water models, and associated determination of potential-dependent barriers based on Bulter-Volmer kinetics, demonstrate that ammonia is produced through a series of water-assisted protonation and bond dissociation steps at modest voltages (<0.3 V). In addition, the analysis sheds light on the poorly understood formation mechanism of nitrous oxide (N2 O) at higher potentials, which suggests that N2 O is not produced through a Langmuir-Hinshelwood mechanism; rather, its formation is facilitated through an Eley-Rideal-type process.