Atop Sites Research Articles

Differential patient travel burden to pediatric atopic dermatitis clinical trial sites.

Differential patient travel burden to pediatric atopic dermatitis clinical trial sites.

Reaction of hydrogen peroxide with amorphous Ti–O surfaces

Amorphous Ti–O thin films were synthesized by reactive magnetron sputtering and their affinity to H2O2 was studied electrochemically. They exhibit a pronounced affinity to H2O2, with TiO0.6 outperforming TiO1.5 and TiO2. Cathodic currents in the range of −211 μA/cm2 suggest that TiO0.6 is highly electroactive to H2O2. Molecular dynamics simulations based on density functional theory revealed rapid dissociation of H2O2 into OH on Ti–O surfaces, leading to diffusion of OH, facilitating both oxidation and reduction processes. These OH groups predominantly dock onto bridge and atop sites, oxidizing the surfaces. Notably, stronger interactions observed for lower oxidation states give rise to higher cathodic currents. Additionally, the ability of amorphous Ti–O thin films to generate free H atoms implies a possible reduction mechanism, likely leading to the desorption of H2O. Hence, amorphous TiO0.6 is more suitable for detection and monitoring of H2O2 than higher oxidation states in their crystalline forms, e.g., rutile TiO2, known as a benchmark for such applications.

Adsorption of chemical species onto nanographenes, pristine coronene and anchored systems as coronene−(TiO2)nn=1–4: Insights from ab initio molecular dynamics calculations

DFT (Density Functional Theory) calculations were carried out to evaluate the chemical interaction of CO (carbon monoxide) and NO (nitrogen monoxide) molecules onto nanographenes (NGs) n = 1–6 units; thus, for this range adsorption process is exhibited, excepting for n = 5 desorption is favored. On the other hand, dioxide titanium clusters (TiO2) n = 1–4 were proposed to adsorb CO molecule, atop and parallel adsorption sites were considered to attach CO molecule onto these clusters and four possible isolated models (C1-C4) are generated and as result of this chemical interaction, physisorption was obtained. In order to improve the adsorption of CO molecule, (TiO2)n−CO clusters are deposited onto (NG)2 to play the role as substrate, namely coronene (C) and then a set of nanocomposites are generated to analyze changes on adsorption tendency, for the above systems just CO molecule is considered. Some chemical and structural transition are obtained for largest sizes as well as adsorption energy values are more favored for C−(TiO2)3−CO system due to the close proximity between titanium atom of C-(TiO2)3 clusters and this chemical specie. Therefore, there are substantial chemisorption values in this instance due to the active sites. The values of the electronic gap do not exhibit significant variations, among these nanocomposites, however, the HOMO isosurfaces are shown in distinct ways, and for n ≤ 2 systems examined in this paper, LUMO isosurfaces are displayed at the center of the cluster. In all systems analyzed, the electronic transference goes from chemical species toward atoms at the adsorption site. These results suggest that these C−(TiO2)n nanocomposites are ideal candidates to storing or sense different sort of gases; thereby, these findings can be employed to design a kind of gases storage device for environmentally friendly energy applications.

Metadynamics simulations reveal alloying-dealloying processes for bimetallic PdGa nanoparticles under CO2 hydrogenation.

Supported bimetallic nanoparticles (NPs) often display improved catalytic performances (activity and/or selectivity). Yet, structure-activity relationships are difficult to derive due to the multitude of possible compositions, interfaces and alloys. This is notably true for bimetallic NPs used in the selective hydrogenation of CO2 to methanol, where the NPs respond dynamically to the chemical potential of the reactants and products. Herein, we use a combined computational and experimental approach that leverages ab initio Molecular Dynamics (AIMD) and Metadynamics (MTD) in conjunction with in situ X-ray absorption spectroscopy, chemisorption and CO-IR, to explore the dynamic structures and interactions with adsorbates under various CO2 hydrogenation conditions in highly active and selective silica-supported PdGa NPs. We find that PdGa alloying generates isolated Pd sites at the NP surface, changing the dominant binding modes of relevant adsorbates compared to pure Pd NPs: CO molecules mainly occupy atop sites and hydrides switch from mainly internal to atop and bridge sites. Under more oxidizing conditions, akin to CO2 hydrogenation, Ga is partially oxidized, forming a GaOX layer on the NP surface, with a partially dealloyed PdGa core and some remaining isolated Pd surface sites. Overall, these bimetallic NPs show high structural dynamics and a variable extent of alloying depending on the adsorbates relevant to CO2 hydrogenation. This work highlights that AIMD/MTD is a powerful approach to elucidate structural dynamics at a single particle level in complex catalytic systems.

Influence of Water Molecules on CO2 Reduction at the Pt Electrocatalyst in the Membrane Electrode Assembly System.

CO2 electroreduction using a Pt catalyst in an aqueous solution system is known to produce only H2. Recently, a remarkable result has been reported that CH4 can be obtained by reducing CO2 using a membrane electrode assembly (MEA) containing a Pt catalyst. A big difference that exists between the two systems is the number of water molecules. Therefore, this study investigated the influence of water molecules on the CO2-reduction process at the Pt electrocatalyst in the MEA system. As a result, cyclic voltammetry indicated that adsorbed CO (COads) was formed by CO2 reduction in the MEA system more preferably than the aqueous solution system. In detail, the ratio of COads at the atop sites (linear CO, COL) on Pt, which participates in the CH4 generation reaction, to the total COads formed by the CO2 reduction became higher as the lower relative humidity (RH) at 50 °C in the MEA system. Cyclic voltammetry combined with in-line mass spectrometry revealed that the amount of COL and CH4 generated by the CO2 reduction reached their maximums at 63.1% RH. CH4 production by the extremely low-overpotential CO2 reduction was significantly achieved under all the RH conditions. Consequently, the Faradaic efficiency of the CH4 production at 63.1% RH was improved by 1.35 times compared to that at 100% RH. These results would be mainly obtained based on the H2O-involved chemical equilibrium of the reactions for the COads and CH4 formation. Overall, the present study experimentally clarified that the formation of COads (particularly COL) and the following CH4 from the CO2 reduction at the Pt electrocatalyst in the MEA system was facilitated by appropriately controlling the water-molecule content.

Highly active bimetallic Pt–Cu nanoparticles for the electrocatalysis of hydrogen evolution reactions: Experimental and theoretical insight

Improving the activity and stability of Pt-based electrocatalysts is still crucial for hydrogen generation applications. In this study, we investigated the catalytic properties of Cu-doped Pt alloy nanostructures synthesized by the modified polyol method, and compared the performance of PtCu catalyst with that of commercially available Pt/C for the hydrogen evolution reaction (HER) at room temperature. Structural analyses revealed that the PtCu catalysts exhibited fcc-Fm3¯m crystal structure with an average particle size below 5 nm. The HER performance of the catalysts showed overpotential of around −1.0 V (vs. Ag/AgCl) and Pt0.25Cu0.75 and Pt0.75Cu0.25 catalysts exhibited enhanced performance in 1 M KOH. The Pt0.75Cu0.25 catalyst exhibited distinct performance, with the highest mass activity found to be 62.80 mA mg−1Pt. The most active Pt0.75Cu0.25 catalysis for the HER process had the lowest onset potential of 0.989 V and Tafel slope of 35.5 mV dec−1, which were an improvement compared to commercially available Pt/C catalysts. First principle DFT calculations confirmed the stabilities of Pt1Cu3, Pt1Cu1 and Pt3Cu1 compositions as ordered alloy structures. Nudged elastic band method calculations and the d-band model verified the higher catalytic activity of Pt–Cu nanoparticles compared to pure nanoparticles and point out the relevance of a synergistic effect of Pt and Cu atoms on water dissociation. According to DFT calculations, Cu and Pt sites significantly influenced the adsorption of H2O and H, respectively, for splitting water molecules. While the Pt59Cu20 cluster has the highest H2O adsorption energy at the (100) atop site of Cu atoms, H adsorption occurs at the (111) site of Pt atoms.

Atomic-scale manipulation of single-polaron in a two-dimensional semiconductor

Polaron is a composite quasiparticle derived from an excess carrier trapped by local lattice distortion, and it has been studied extensively for decades both theoretically and experimentally. However, atomic-scale creation and manipulation of single-polarons in real space have still not been achieved so far, which precludes the atomistic understanding of the properties of polarons as well as their applications. Herein, using scanning tunneling microscopy, we succeeded to create single polarons in a monolayer two-dimensional (2D) semiconductor, CoCl2. Combined with first-principles calculations, two stable polaron configurations, centered at atop and hollow sites, respectively, have been revealed. Remarkably, a series of manipulation progresses — from creation, erasure, to transition — can be accurately implemented on individual polarons. Our results pave the way to understand the physics of polaron at atomic level, and the easy control of single polarons in 2D semiconductor may open the door to 2D polaronics including the data storage.

Theoretical insights into electronic structures and durability of single-atom Pd/TiN catalysts

Density functional theory (DFT) calculations have been performed to evaluate the metal-support interactions between palladium atom and titanium nitride surface (Pd–TiN). N vacancy sites on defective TiN surface can stabilize Pd single atom under strongly oxidizing conditions, and surface defect-mediated stabilization is accompanied by obvious charge redistributions between Pd and substrate. The adsorption of several gas species on stable Pd–TiN surfaces is also explored to understand catalytic reactions. It is found that O, H, OH, O2, and CO favorably adsorb on Ti atop site, while CO2 and H2 prefer the hollow and Pd atop site, respectively. Moreover, co-adsorption with either H or OH weakens the CO-surface interactions, indicating the CO poisoning effect would be alleviated under both acidic and alkaline conditions. This study provides a perspective to understand the support effect of Pd–TiN, which sheds light on the design of high-performance Pd-based electrodes for proton exchange membrane fuel cells (PEMFCs).

Electrocatalytic CO2-to-C2+ with Ampere-Level Current on Heteroatom-Engineered Copper via Tuning *CO Intermediate Coverage.

An ampere-level current density of CO2 electrolysis is critical to realize the industrial production of multicarbon (C2+) fuels. However, under such a large current density, the poor CO intermediate (*CO) coverage on the catalyst surface induces the competitive hydrogen evolution reaction, which hinders CO2 reduction reaction (CO2RR). Herein, we report reliable ampere-level CO2-to-C2+ electrolysis by heteroatom engineering on Cu catalysts. The Cu-based compounds with heteroatom (N, P, S, O) are electrochemically reduced to heteroatom-derived Cu with significant structural reconstruction under CO2RR conditions. It is found that N-engineered Cu (N-Cu) catalyst exhibits the best CO2-to-C2+ productivity with a remarkable Faradaic efficiency of 73.7% under -1100 mA cm-2 and an energy efficiency of 37.2% under -900 mA cm-2. Particularly, it achieves a C2+ partial current density of -909 mA cm-2 at -1.15 V versus reversible hydrogen electrode, which outperforms most reported Cu-based catalysts. In situ spectroscopy indicates that heteroatom engineering adjusts *CO adsorption on Cu surface and alters the local H proton consumption in solution. Density functional theory studies confirm that the high adsorption strength of *CO on N-Cu results from the depressed HER and promoted *CO adsorption on both bridge and atop sites of Cu, which greatly reduces the energy barrier for C-C coupling.

Catalytic dissociation of a water molecule on the (1 1 1) surfaces of Pt0.5Ni0.5 bimetal: Density functional theory (DFT) study

One of the biggest challenges for the transition towards renewable energy sources today is the development of affordable platinum (Pt) containing bifunctional bimetals as electrode materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, the optimal ratio of a bimetal consisting of Pt and nickel (Ni) was studied. This study included the impact of bimetal mixing on the electronic and magnetic properties of the bimetal. However, the catalytic properties of the bimetal have not been studied and compared to that of the pure metals. In this study, spin-polarized density functional theory calculations were employed to investigate the surface properties of Pt0.5Ni0.5 bimetallic surfaces. Relaxation of the surface terminations showed that the 1.5(111) and 0.5(111) surface terminations are energetically the most favourable surfaces, while the 2.0(111) surface termination is energetically the least favourable surface. However, the 2.0(111) surface termination is energetically more favourable than pure Ni. The adsorption of single water (H2O) molecule onto various (111) surface terminations (0.0, 0.5, 1.0, 1.5 and 2.0) of the bimetallic surfaces was investigated. The H2O molecule was adsorbed onto the atop site in the most stable orientation, namely parallel to the surface (a flat geometry). The d-band centre revealed that the bonding of the H2O molecule on the 0.5(111) surface termination is stronger than on the pure Pt surface but weaker than on the pure Ni surface. Bond elongation of the OH bonds and an increase in the HOH bond angle were observed in the H2O molecule adsorbed on the 0.5(111) surface. The results show that the 1.5(111) surface termination, where the water adsorbs to a Ni atom, could potentially be used as an electrocatalyst for the water-splitting reaction. However, experimental data are needed to confirm these results.

Hydrogen Generation from Water Splitting by 3H2O + Pd13 → 3H2 + Pd13O3 Reaction

AbstractThe Pd clusters are widely concerned because of their outstanding catalytic capacity. Here such a candidate of Pd13 cluster with the icosahedron symmetry and multiple active sites is shown. The first‐principles calculations reveal that this cluster can maximally catalyze the water‐splitting reaction to generate three H2 from three H2O molecules. The geometrical structures of the reactants, transition states, and products are optimized, and their stabilities are confirmed by the frequency analysis. The intrinsic reaction coordinates are constructed to confirm the rationality of the reaction. Firstly, one H2O is adsorbed on atop sites of the Pd13 cluster and releases an H2 through a reaction of H2O@Pd13 → H2@[O+Pd13]. Then, the remained O+Pd13 cluster can continually split H2O and generate additional H2, and the obtained 2O+Pd13 cluster can also dissociate H2O to generate H2. However, it is found that the 3O+Pd13 cluster cannot continue the splitting reaction. It is also observed that the O+Pd13 cluster can adsorb two molecules at the same time and split them one by one. These findings support that the Pd13 cluster, O+Pd13, and 2O+Pd13 are promising candidates for efficient generation of H2 from H2O splitting reaction.

Critical Role of Thermal Fluctuations for CO Binding on Electrocatalytic Metal Surfaces.

This work considers the evaluation of density functional theory (DFT) when comparing against experimental observations of CO binding trends on the strong binding Pt(111) and intermediate binding Cu(111) and for weak binding Ag(111) and Au(111) surfaces important in electrocatalysis. By introducing thermal fluctuations using appropriate statistical mechanical NVT and NPT ensembles, we find that the RPBE and B97M-rV DFT functionals yield qualitatively better metal surface strain trends and CO enthalpies of binding for Cu(111) and Pt(111) than found at 0 K, thereby correcting the overbinding by 0.2 to 0.3 eV to yield better agreement with the enthalpies determined from experiment. The importance of dispersion effects are manifest for the weak CO binding Ag(111) and Au(111) surfaces at finite temperatures in which the RPBE functional does not bind CO at all, while the B97M-rV functional shows that the CO–metal interactions are a mixture of chemisorbed and physisorbed species with binding enthalpies that are within ∼0.05 eV of experiment. Across all M(111) surfaces, we show that the B97M-rV functional consistently predicts the correct atop site preference for all metals due to thermally induced surface distortions that preferentially favor the undercoordinated site. This study demonstrates the need to fully account for finite temperature fluctuations to make contact with the binding enthalpies from surface science experiments and electrocatalysis applications.

Dual-metallic single Ru and Ni atoms decoration of MoS2 for high-efficiency hydrogen production

The limitation of single-atom catalysts hinders its promotion and application in the HER field. Herein, we fabricated a dimetallic single-atom catalyst where monoatomic Ru and Ni co-modify MoS2 using a mild method (Ru/Ni-MoS2). Experiments combined with theoretical calculations indicated that the single Ru atoms were anchored through single Ni atoms with strong electronegativity, and most of the Ru atoms coordinated with the S were distributed on the Ni atop sites. And the synergistic effect of the Ru/Ni-MoS2 with S atoms bonded with Ni as hydrogen acceptor and single-atoms Ru as hydroxyl acceptor, effectively reduced the intermediate energy barrier of the water dissociation step. The as-fabricated Ru/Ni-MoS2 exhibited a super-low overpotential of 32 mV at 10 mA cm−2 with the corresponding Tafel slope of 41 mV·dec-1 as well as negligible current attenuation over 20 h, far surpassing that of Ru-MoS2 and Ni-MoS2. This work not only provides a high-efficiency HER catalysts, but also a new idea based on electronegativity difference and bimetallic single-atom regulation to design catalyst.

Factors that influence hydrogen binding at metal-atop sites.

The d-band model has proven to be effective for understanding trends in the chemisorption of various adsorbates on transition metal surfaces. However, hydrogen adsorption at the atop site of transition metals and their bimetallic alloy surfaces do not always correlate well with the d-band center of the adsorption site. Additionally, the d-band model cannot explain the disappearance of the local minima for H adsorption at the hollow site on the potential energy surface of 5d single-atom element doped Au and Ag(111) surfaces. Here, we use a simple model with factors, including the d-band center, filling of the d-band, renormalized adsorbate states, coupling matrix elements, and surface-adsorbate bond lengths, to correlate with the density functional theory calculated H binding energies on both mono- and bimetallic (111) surfaces. Our results suggest that H adsorption at metal-atop sites is determined by all these factors, not only by the d-band center. The strong adsorption of H at the atop sites of 5d metal surfaces can be explained by their lower repulsive contribution.

IR Intensities of CO Molecules Adsorbed on Atop and Low-Coordinate Sites of Pd Nanoparticles: Analysis Using Natural Perturbation Orbitals

Abstract The natural perturbation orbital (NPO) computational method was applied to the analysis of infrared (IR) intensities of CO molecules adsorbed on the surfaces of Pd nanoparticles. Enhancement of the IR intensities for a CO bonded to a single low-coordinate metal adsorbed on the metal surface (low-coordinate model) was compared with those for a CO bonded directly to the metal surface (atop model). This enhancement was ascribed to the mixing between occupied and virtual orbitals induced by molecular vibrations. The occ-virt term, representing this mixing, was efficiently decomposed into contributions from three NPO pairs ($\sigma _{z} - \sigma _{z}^{*}$, $\pi _{x} - \pi _{x}^{*}$, and $\pi _{y} - \pi _{y}^{*}$) for each model. The main contributors were the $\pi _{x} - \pi _{x}^{*}$ and $\pi _{y} - \pi _{y}^{*}$ pairs, which were localized around CO in the atop model, but delocalized throughout the Pd cluster in the low-coordinate model. This variance in orbital delocalization comes from differing interactions between Pd and CO in the atop and low-coordinate models.

Interaction of CO with Pt nanoclusters on a graphene-covered Ru(0001) surface.

The adsorption of CO on Pt nanoclusters on a single layer of graphene epitaxially grown on the Ru(0001) surface [Gr/Ru(0001)] was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The graphene layer was grown through exposure to ethylene using a method that has previously been shown to completely cover the surface. As CO adsorbs on Ru(0001) but not on graphene, the complete coverage of the Ru(0001) surface by graphene was verified with TPD as no CO adsorption was detectable. Previous work has demonstrated that Pt nanoclusters nucleate in the moiré unit cells of the Gr/Ru(0001) surface. Exposure of the Pt/Gr/Ru(0001) surface to CO gives rise to strong RAIRS peaks at 2065-2085 cm-1 assigned to CO at Pt atop sites and at 1848 cm-1 due to CO at Pt bridge sites. The CO TPD peak areas were used to quantify the CO coverage, which allowed for the determination of the RAIRS peak areas per CO molecule. It was found that the RAIRS intensity for CO on Pt/Gr/Ru(0001) is as much as nine times the intensity of CO on Ru(0001) on a per molecule basis. A more modest intensity enhancement was observed compared to CO on Pt islands on the Ru(0001) surface.

Experimental and theoretical insights into the mechanism of spinel CoFe2O4 reduction in CO chemical looping combustion

The reaction mechanism of CoFe2O4 with CO during chemical looping combustion (CLC) was studied via thermogravimetric analysis (TGA) experiments and density functional theory (DFT) calculations. The CO temperature-programmed reduction revealed that the reduction of CoFe2O4 is a one-step reaction process. CoFe2O4 can be directly reduced into the Co-Fe alloy. The existence of Co evidently improves the reactivity of CoFe2O4 as compared to Fe2O3. Two types of reaction kinetics are involved in the isothermal reduction of CoFe2O4. DFT calculations were performed to study the adsorption and oxidation of CO on two terminated surfaces (surface A and surface B) of CoFe2O4 (110). The results indicated that CO preferentially chemisorbs at Co/Fe–O bridge sites on surface A and at Co/Fe atop sites on surface B. CO oxidation on surface A was more thermodynamically and kinetically favorable than that on surface B. Furthermore, the oxygen vacancy formation on surface B needs more energy consumption than that on surface A. The different reactivity between surface A and surface B may be responsible for the two reaction rate peaks observed in isothermal experiments. The synergistic effect of Co and Fe atoms on the reactivity of CoFe2O4 can be mainly ascribed to the oxygen atoms in different Co/Fe coordination environments. These results can provide fundamental insights for further improving the performance of spinel CoFe2O4 oxygen carrier.

Ab initio molecular dynamics study of adsorption of hydroxyl groups on graphene surface

Reduced graphene oxide is the precursor to produce graphene in a large scale; however, to date, there has been no consensus on the electronic structure of reduced graphene oxide. In this study, we carried out an ab initio molecular dynamics simulation to investigate the adsorption process of hydroxyl groups on graphene surface. During the adsorption process, the OH group needs to firstly pass through a physical adsorption complex with the OH above the bridge site of two carbon atoms, next to surmount a transition state, then to be adsorbed at the atop site of a carbon atom. With a 5×5 graphene surface, up to 6 hydroxyl groups can be adsorbed on the graphene surface, indicating the concentration coverage of the hydroxyl groups on graphene surface is about 12%. The simulation results show that the negative adsorption energy increases linearly as the number of adsorbed hydroxyl groups increases, and the band gap also increases linearly with the number of adsorbed hydroxyl groups.

Effect of Coverage on Adsorptions of CO on Ni(110) Surface

The adsorptions of CO on Ni(110) surface at different coverages have been investigated by first-principle calculations. The results show that CO will be bonded at short bridge site preferentially. And CO preferentially absorbs upright at short bridge sites and top sites at 1/6 monolayer coverage. With increasing coverage to 0.5 ML, the enhanced steric repulsion leads to the slope of CO. For 1/6 and 1/2 monolayer coverage, CO is mainly bonded at short bridge site and atop site simultaneously at a certain ratio. When CO coverage is 1 monolayer, CO absorbs at short bridge site and forms p2mg configuration. As the coverage of CO is increased to finally form the p2mg structure, there is a continuous frequency shift up to the value 1962.27 cm-1. The vibration frequencies we calculated are consistent with relative experimental results. DOS of CO molecules and Ni atoms are discussed too.

In situ surface-doped PtNiCoRh nanocrystals promote electrooxidation of C1 fuels

Heteroatom-doped Pt-based nanocrystals have generated considerable interest and hold great prospects in heterocatalysis. However, engineering the superficial atomic configurations of these nanocrystals via in situ surface doping remains exceedingly challenging. Herein, we propose a one-pot, in situ surface doping chemical synthesis protocol to prepare quatermetallic PtNiCoRh dendritic nanocrystals as versatile and active catalysts for the electrooxidation of C1 fuels. Leveraging the selective coordination effect between ascorbic acid and Rh3+ ions, the doping of trace Rh atoms can be guided specifically at the near-surface of PtNiCoRh nanocatalysts. Electrocatalytic tests indicate that Pt67Ni16Co16Rh1 nanocrystals with in situ trace Rh-doped surface exhibit substantially enhanced activity, durability, and CO tolerance for the electrooxidation of methanol, formaldehyde, and formic acid. In situ Fourier transform infrared spectroscopy provides molecular-level insight into the exceptional performance of these nanocatalysts. The surface incorporation of anti-corrosive Rh atoms enables the transfer of CO intermediates from the atop Pt sites to the bridged Rh-Pt surface sites, thereby facilitating the elimination of these poisoning species from the catalyst surface. This study presents an effective in situ surface doping strategy which can enable the design of more atom-economic heterocatalysts.