Surface Activation of Transition Metal Nanoparticles for Heterogeneous Catalysis: What We Can Learn from Molecular Dynamics

Many heterogeneous reactions catalyzed by nanoparticles occur at relatively high temperatures, which may modulate the surface morphology of nanoparticles during reaction. Inspired by the discovery of dynamic formation of active sites on gold nanoparticles, we explore theoretically the nature of the highly mobile atoms on the surface of nanoparticles of various sizes for 11 transition metals. Using molecular dynamics simulations, on a 3 nm Fe nanoparticle as an example, the effect of surface premelting and overall melting on the structure and physical properties of the nanoparticles is analyzed. When the nanoparticle is heated up, the atoms in the outer shell appear amorphous already at 900 K. Surface premelting is reached at 1050 K, with more than three liquid atoms, based on the Lindemann criterion. The activated atoms may transfer their extra kinetic energy to the rest of the nanoparticle and activate other atoms. The dynamic studies indicate that the number of highly mobile atoms on the surface increases with temperature. Those atoms with a high Lindemann index, usually located on the edges or vertices, attain much higher kinetic energy than other atoms and potentially form different active sites in situ. When the temperature passes the surface premelting temperature, a drastic change in the coordination number (SCN) of the surface atoms occurs, with attendant dramatic broadening of the distribution of the SCN, suppling active sites with more diverse atomic coordination numbers. The electronic density of states of a nanoparticle tends to “equalize”, due to the breaking of the translational symmetry of the atoms in the nanoparticle, and the d-band center of the nanoparticle moves further away from the Fermi level as the temperature increases. Besides Au, other nanoparticles of the transition metals, such as Pt, Pd, and Ag, may also have active sites easily formed in situ

Mechanisms of CO Activation, Surface Oxygen Removal, Surface Carbon Hydrogenation, and C–C Coupling on the Stepped Fe(710) Surface from Computation

To understand the initial steps of Fe-based Fischer–Tropsch synthesis, systematic periodic density functional theory computations have been performed on the single-atom stepped Fe(710) surface, composed by <i>p</i>(3 × 3) Fe(100)-like terrace and <i>p</i>(3 × 1) Fe(110)-like step. It is found that CO direct dissociation into surface C and O is more favored kinetically and thermodynamically than the H-assisted activation via HCO and COH formation. Accordingly, surface O removal by hydrogen via H₂O formation is the only way. On the basis of surface CH_<i>x</i> hydrogenation (<i>x</i> = 0, 1, 2, 3), surface CH_<i>x</i> + CH_<i>x</i> coupling and CO + CH_<i>x</i> insertion resulting in CH_<i>x</i>CO formation followed by C–O dissociation, surface C hydrogenation toward CH₃ formation is more favored kinetically than the formation of CH_<i>x</i>-CH_<i>x</i> and CH_<i>x</i>CO, as well as thermodynamically. Starting from CH₃, the formation of CH₄ and CH₃CO has similar barriers and endothermic reaction energies, while CH₃CO dissociation into CH₃C + O has low barrier and is highly exothermic. Therefore, turning the H₂/CO ratio should change the selectivity toward C–C formation and propagation

A Novel Color Modulation Analysis Strategy through Tunable Multiband Laser for Nanoparticle Identification and Evaluation

Creating color difference and improving the color resolution in digital imaging is crucial for better application of color analysis. Herein, a novel color modulation analysis strategy was developed by using a homemade tunable multiband laser illumination device, in which the portions of R, G, and B components of the illumination light are discretionarily adjustable, and hence the sample color could be visually modulated continuously in the RGB color space. Through this strategy, the color appearance of single gold nanorods (AuNRs) under dark-field microscopy was migrated from the spectrally insensitive red region to the spectrally sensitive green-yellow region. Unlike the traditional continuous-wave light source illumination, wherein the small spectral variations in the samples within a narrow spectral range are averaged by the whole spectrum of the light source, leading to little color difference, the application of sharp, multiband laser illumination could enlarge the color separation between samples, thus resulting in high spectral sensitivity in color analysis. By comparing the corresponding color evolution processes of different samples as the multiband combination of the laser illumination was changed, more efficient color separation of AuNRs was achieved. With this instrument and single Ag@AuNRs as the sulfide probe, we achieved high throughput and highly sensitive detection of sulfide at a detection limit of 0.1 nM, a more than 2 orders of magnitude improvement compared to the previous color sensing scheme. This strategy could be utilized for nanoparticle identification, evaluation, and determination in biological imaging and biochemical analysis

A Machine-Driven Hunt for Global Reaction Coordinates of Azobenzene Photoisomerization

Azobenzene is a very important system that is often studied for better understanding light-activated mechanical transformations via photoisomerization. The central C–NN–C dihedral angle is widely recognized as the primary reaction coordinate for changing <i>cis-</i> to <i>trans-</i>azobenzene and vice versa. We report on a <i>global</i> <i>reaction</i> <i>coordinate</i> (containing all internal coordinates) to thoroughly describe the reaction mechanism for azobenzene photoisomerization. Our global reaction coordinate includes <i>all</i> of the internal coordinates of azobenzene contributing to the photoisomerization reaction coordinate. We quantify the contribution of each internal coordinate of azobenzene to the overall reaction mechanism. Finally, we provide a detailed mapping on how each significantly contributing internal coordinate changes throughout the energy profile (from <i>trans</i> to transition state and subsequently to <i>cis</i>). In our results, the central C–NN–C dihedral remains the primary internal coordinate responsible for the reaction coordinate; however, we also conclude that the disputed inversion-assisted rotation is <i>half</i> as important to the overall reaction mechanism and the inversion-assisted rotation is driven by four adjacent dihedral angles C–C–NN with very little change to the adjacent C–C–N angles

High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis

Water adsorption and dissociation on the Fe(100) surface at different coverages have been calculated using density functional theory methods and ab initio thermodynamics. For the adsorption of (H₂O)_<i>n</i> clusters on the (3 × 4) Fe(100) surface, the adsorption energy is contributed by direct H₂O–Fe interaction and hydrogen bonding. For <i>n</i> = 1–3, direct H₂O–Fe interaction is dominant, and hydrogen bonding becomes more important for <i>n</i> = 4–5. For <i>n</i> = 6–8 and 12, structurally different adsorption configurations have very close energies. Monomeric H₂O dissociation is more favored on the clean Fe(100) surface than that on H₂O or OH precovered surfaces. O-assisted H₂O dissociation is favorable kinetically (O + H₂O = 2OH), and further OH dissociation is roughly thermo-neutral. With the increase of surface O coverage (<i>n</i>O, <i>n</i> = 2–7), further H₂O dissociation has similar potential energy surfaces, and H₂ formation from surface adsorbed H atoms becomes easy, while the desorption energy is close to zero for <i>n</i> = 7. The calculated thermal desorption temperatures of H₂O and H₂ on clean surface agree well with the available experiment data. The characteristic desorption temperatures of H₂O and H₂ coincided at 310 K are controlled by the kinetics of disproportionation (2OH → O + H₂O) and dissociation (2OH → 2O + H₂) of surface OH groups. The dispersion corrections (PBE-D2) overestimate slightly the adsorption energies and temperatures of H₂O and H₂ on iron surface. At 0.5 ML coverage (6 × OH), the adsorbed OH groups at the bridge sites do not share surface iron atoms and form two well-ordered parallel lines, and each OH group acting as donor and acceptor forms hydrogen bonding with the adjacent OH groups, in agreement with the experimentally observed surface structures. At 1 ML coverage of OH (12 × OH) and O (12 × O), the adsorbed OH groups at the bridge sites share surface iron atoms and form four well-ordered parallel lines; and the adsorbed O atoms are located at the hollow sites. Energetic analysis reveals that 1 ML OH coverage is accessible both kinetically and thermodynamically, while the formation of 1 ML O coverage is hindered kinetically since the OH dissociation barrier increases strongly with the increase of O pro-covered coverage. All these results provide insights into water-involved reactions catalyzed by iron and broaden our fundamental understanding into water interaction with metal surfaces

Hydrogen Evolution Reaction on Hybrid Catalysts of Vertical MoS₂ Nanosheets and Hydrogenated Graphene

Two-dimensional (2D) molybdenum sulfide (MoS₂) is an attractive noble-metal-free electrocatalyst for hydrogen evolution (HER) in acids. Tremendous effort has been made to engineer MoS₂ catalysts with either more active sites or higher conductivity to enhance their HER activity. However, little attention has been paid to synergistically structural and electronic modulations of MoS₂. Herein, 2D hydrogenated graphene (HG) is introduced into MoS₂ ultrathin nanosheets for the construction of a highly efficient and stable catalyst for HER. Owing to synergistic modulations of both structural and electronic benefits to MoS₂ nanosheets via HG support, such a catalyst has improved conductivity, more accessible catalytic active sites, and moderate hydrogen adsorption energy. On the optimized MoS₂/HG hybrid catalyst, HER occurs with an overpotential of 124 mV at 10 mA cm^–2, a Tafel slope of 41 mV dec^–1, and a stable durability for 24 h continuous operation at 30 mA cm^–2 without observable fading. The high performance of the optimized MoS₂/HG hybrid catalyst for HER was interpreted with density functional theory calculations. The simulation results reveal that the introduction of HG modulates the electronic structure of MoS₂ to increase the number of active sites and simultaneously optimizes the hydrogen adsorption energy at S-edge atoms, eventually promoting HER activity. This study thus provides a strategy to design and develop high-performance HER electrocatalysts by employing different 2D materials

Coverage Dependent Water Dissociative Adsorption on the Clean and O‑Precovered Fe(111) Surfaces

Water dissociative adsorption on the clean and O-precovered Fe(111) surfaces at different coverage have been studied using the density functional theory method (GGA-PBE) and ab initio atomistic thermodynamics. On the clean p(3 × 3) Fe(111) surface, surface H, O, OH, and H₂O species can migrate easily. Considering adsorption and H-bonding, the adsorbed H₂O molecules can be dispersed or aggregated in close energies at low coverage, while in different aggregations at high coverage, indicating that the adsorbed H₂O molecules might not have defined structures, as observed experimentally. On the O-precovered surface (<i>n</i>_O = 1–8), the first dissociation step, <i>n</i>O + H₂O = (<i>n</i> – 1)O + 2OH, has a very low barrier and is reversible; and the barriers of the sequential OH dissociation steps, (<i>n</i> – 1)O + 2OH = <i>n</i>O + H + OH and <i>n</i>O + H + OH = (<i>n</i> + 1)O + 2H, are close (0.9–1.2 eV). All of these barriers are coverage independent. For OH and H adsorption at 1/3 ML coverage, surface OH forms a trimer (OH)₃ unit, and surface O forms a regular linear pattern. At one ML coverage, there are three dispersed (OH)₃ units for OH adsorption and three well-ordered parallel lines for O adsorption. The average adsorption energies for OH and O adsorption are coverage independent. Desorption temperatures of H₂O and H₂ under ultrahigh vacuum conditions are computed. Systematic comparison among the Fe(110), Fe(100), and Fe(111) surfaces reveal their intrinsic differences in water dissociative adsorption and provide a basic understanding of water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces

Strain Engineering to Enhance the Electrooxidation Performance of Atomic-Layer Pt on Intermetallic Pt₃Ga

Strain engineering has been a powerful strategy to finely tune the catalytic properties of materials. We report a tensile-strained two-to-three atomic-layer Pt on intermetallic Pt₃Ga (AL-Pt/Pt₃Ga) as an active electrocatalyst for the methanol oxidation reaction (MOR). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy characterization showed that the AL-Pt possessed a 3.2% tensile strain along the [001] direction while having a negligible strain along the [100]/[010] direction. For MOR, this tensile-strained AL-Pt electrocatalyst showed obviously higher specific activity (7.195 mA cm^–2) and mass activity (1.094 mA/μg_Pt) than those of its unstrained counterpart and commercial Pt/C catalysts. Density functional theory calculations demonstrated that the tensile-strained surface was more energetically favorable for MOR than the unstrained one, and the stronger binding of OH* on stretched AL-Pt enabled the easier removal of CO*

Supramolecular Porphyrin Cages Assembled at Molecular–Materials Interfaces for Electrocatalytic CO Reduction

Conversion of carbon monoxide (CO), a major one-carbon product of carbon dioxide (CO₂) reduction, into value-added multicarbon species is a challenge to addressing global energy demands and climate change. Here we report a modular synthetic approach for aqueous electrochemical CO reduction to carbon–carbon coupled products via self-assembly of supramolecular cages at molecular–materials interfaces. Heterobimetallic cavities formed by face-to-face coordination of thiol-terminated metalloporphyrins to copper electrodes through varying organic struts convert CO to C2 products with high faradaic efficiency (FE = 83% total with 57% to ethanol) and current density (1.34 mA/cm²) at a potential of −0.40 V vs RHE. The cage-functionalized electrodes offer an order of magnitude improvement in both selectivity and activity for electrocatalytic carbon fixation compared to parent copper surfaces or copper functionalized with porphyrins in an edge-on orientation

Iron Carbidization on Thin-Film Silica and Silicon: A Near-Ambient-Pressure X‑ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

Model catalysts consisting of iron particles with similar size deposited on thin-film silica (Fe/SiO₂) and on silicon (Fe/Si) were used to study iron carbidization in a CO atmosphere using in situ near-ambient-pressure X-ray photoelectron spectroscopy. Significant differences were observed for CO adsorption, CO dissociation, and iron carbidization when the support was changed from thin-film silica to silicon. Stronger adsorption of CO on Fe/Si than that on Fe/SiO₂ was evident from the higher CO equilibrium coverage found at a given temperature in the presence of 1 mbar of CO gas. On thin-film silica, iron starts to carbidize at 150 °C, while the onset of carbidization is at 100 °C on the silicon support. The main reason for the different onset temperature for carbidization is the efficiency of removal of oxygen species after CO dissociation. On thin-film silica, oxygen species formed by CO dissociation block the iron surface until ∼150 °C, when CO₂ formation removes surface oxygen. Instead, on the silicon support, oxygen species readily spill over to the silicon. As a consequence, oxygen removal is not rate-limiting anymore and carbidization of iron can proceed at a lower temperature