Structural, Electronic and Catalytic Properties of Graphene-supported Platinum Nanoclusters

Carbon materials are predominantly used as catalytic supports due to their high surface area, excellent electrical conductivity, resistance to corrosion and structural stability. Graphene, a 2D monolayer of graphite, with its excellent thermal, electronic and mechanical features, has been considered a promising support material for next generation metal-graphene nanocatalysts. The main focus of this dissertation is to investigate the properties of such metal-graphene nanocomposites using computational methods, and to develop a comprehensive understanding of the experimentally observed enhanced catalytic activity of graphene-supported Platinum (Pt) clusters. In particular, we seek to understand the role of graphene supports on the ground-state morphology and the electronic structure of graphene-supported Pt nanoparticles, which correlate strongly with their catalytic activity. First, through a series of empirical potential and density functional theory (DFT) calculations, we determine low-energy isomers of Pt nanoclusters on pristine and defective graphene. Our results indicate that point defects in the graphene support enhance the cluster-support interaction, increasing their stability and significantly alter their electronic properties. Next, we investigate the support effects on CO and O adsorption on graphene-supported Pt13 nanoclusters. Defective-graphene-supported Pt13 nanoclusters bind CO and O more weakly than clusters on pristine graphene or unsupported clusters. Additional ab initio MD calculations on CO-saturated Pt13 nanoclusters show that support defects are crucial in stabilizing Pt13 clusters at high CO-coverages; in contrast, Pt13 clusters supported on pristine graphene desorb upon CO saturation, leading to potential catalyst loss. Finally, we examine the support effects on the CO oxidation reaction on graphene-supported Pt13 nanoclusters. A detailed study of the CO oxidation kinetics is undertaken in the high CO coverage regime, locating transition states and minimum energy pathways. The relevant kinetic mechanism is sampled at various surface sites on clusters bound at support defects and on unsupported clusters. The results of this study show that strong cluster-support interactions can substantially reduce the reaction barrier for CO oxidation on graphene-supported clusters compared to unsupported ones. Our studies suggest that defect engineering of graphene could serve to enhance the catalytic activity of ultra-small Pt clusters, opening up another dimension for rational design of catalytic materials

The role of oxygenated species in the catalytic self-coupling of MeOH on O pre-covered Au(111)

The oxidation of alcohols plays a central role in the valorisation of biomass, in particular when performed with a non-toxic oxidant such as O2. Aerobic oxidation of methanol on gold has attracted attention lately and the main steps of its mechanism have been described experimentally. However, the exact role of O and OH on each elementary step and the effect of the interactions between adsorbates are still not completely understood. Here we investigate the mechanism of methanol oxidation to HCOOCH3 and CO2. We use Density Functional Theory (DFT) to assess the energetics of the underlying pathways, and subsequently build lattice kinetic Monte Carlo (kMC) models of increasing complexity, to elucidate the role of different oxygenates. Detailed comparisons of our simulation results with experimental temperature programmed desorption (TPD) spectra enable us to validate the mechanism and identify rate determining steps. Crucially, taking into account dispersion (van der Waals forces) and adsorbate-adsorbate lateral interactions are both important for reproducing the experimental data

The Effect of Patent Grant on the Geographic Reach of Patent Sales

This paper examines whether patents increase the geographic reach of the market for ideas. By employing a dataset of 25,127 US patents traded between US located firms, we find that patents sold during application phase are less likely to be traded outside the seller‟s state than patents that have been issued. To tackle the endogeneity issues we employ coarsened exact matching techniques. We find that patent grant increases the likelihood of a patent to be traded across boundaries of the state. This evidence is stronger for patents originating from the less innovative US states

Chemical Bonding of Transition-metal Co13_{13} Clusters with Graphene

We carried out density functional calculation to study Co$_{13}$ clusters on graphene. We deposit several free isomers in different disposition respect to hexagonal lattice nodes, studying even the $hcp$ $2d$ isomer recently obtained as the most stable one. Surprisingly, Co$_{13}$ clusters bonded to graphene prefer $icosahedron-like$ structures where the low lying isomer is much distorted, because it is linked with more bonds than in previous works. For any isomer the most stable position binds to graphene by the Co atoms that can lose electrons. We find that the charge transfers between graphene and clusters are small enough to conclude that the Co-graphene binding is not ionic-like but chemical. Besides, the same order of stability among the different isomers on doped graphene is well kept. These findings could also be of interest for magnetic clusters on graphenic nanostructures such as ribbons and nanotubes.Comment: 12 pages, 6 figure

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics. Here, the authors explore the blue photoluminescence signal arising from the interface between graphene and h-BN arranged in in-plane heterostructures, and fabricate a blue light emitting device utilizing the heterojunction as the emitting layer

Density-Functional Tight-Binding Simulations of Curvature-Controlled Layer Decoupling and Band-Gap Tuning in Bilayer MoS2

Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to the presence of time-reversal symmetry and the absence of inversion symmetry, making them promising candidates for valleytronics. In contrast, in bilayer TMDCs both symmetries are present and these desirable valley-selective properties are lost. Here, by using density-functional tight-binding electronic structure simulations and revised periodic boundary conditions, we show that bending of bilayer MoS2 sheets breaks band degeneracies and localizes states on separate layers due to bendinginduced strain gradients across the sheets. We propose a strategy for employing bending deformations in bilayer TMDCs as a simple yet effective means of dynamically and reversibly tuning their band gaps while simultaneously tuning valley-selective physics.peerReviewe

Replication Data for: Machine Learning Prediction of H Adsorption Energies on Ag Alloys

The data underlying this published work have been made publicly available in this repository as part of the IMASC Data Management Plan. This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012573