Computational investigation of CO adsorbed on Aux, Agx and (AuAg)x nanoclusters (x = 1-5, 147) and monometallic Au and Ag low-energy surfaces

Density functional theory calculations have been performed investigating the use of CO as a probe molecule for determining the structure and composition of Au, Ag AuAg nanoparticles. For very small nanoclusters (x = 1-5), vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles showed a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are calculated for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, and this proves to be energetically favourable only on atop and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies for the CO molecule red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that vibrational frequencies cannot be relied upon solely in order to obtain accurate compositional analysis, but we do believe that elemental rearrangement in the nanocluster from Ag@Au (or Au@Ag) to an alloy would result in a shift in the vibrational frequencies that indicate the change in the surface composition

Evaluating the Role of Anharmonic Vibrations in Zeolite β Materials

The characterization of zeolitic materials is often facilitated by spectroscopic analysis of vibrations, which informs about the bonding character of the substrate and any adsorbents. Computational simulations aid the interpretation of the spectra but often ignore anharmonic effects that can affect the spectral characteristics significantly. Here, the impact of anharmonicity is demonstrated with a combination of dynamical and static simulations applied to the structures formed during the synthesis of Sn-BEA via solid-state incorporation (SSI): the initial siliceous BEA (Si-β), aluminosilicate BEA (H-β), dealuminated BEA (deAl-β), and Sn-BEA (Sn-β). Heteroatom and defect-containing BEA are shown to have strong anharmonic vibrational contributions, with atomic and elemental resolution highlighting particularly the prevalence for H atoms (H-β, deAl-β) as well as localization to heteroatoms at defect sites. We simulate the vibrational spectra of BEA accounting for anharmonic contributions and observe an improved agreement with experimental data compared to harmonic methods, particularly at wavenumbers below 1500 cm–1. The results demonstrate the importance of incorporating anharmonic effects in simulations of vibrational spectra, with consequences toward future characterization and application of zeolitic materials

Hydride pinning pathway in the hydrogenation of CO2 into formic acid on dimeric tin dioxide

Capture of CO2 and its conversion into organic feedstocks are increasingly needed as society moves towards a renewable energy economy. Here, a hydride‐assisted selective reduction pathway is proposed for the conversion of CO2 to formic acid (FA) over SnO2 monomers and dimers. Our density functional theory calculations infer a strong chemisorption of CO2 on SnO2 clusters forming a carbonate structure, whereas heterolytic cleavage of H2 provides a new pathway for the selective reduction of CO2 to formic acid at low overpotential. Among the two investigated pathways for reduction of CO2 to HCOOH, the hydride pinning pathway is found promising with a unique selectivity for HCOOH. The negatively‐charged hydride forms on the cluster during the dissociation of H2 and facilitates the formation of a formate intermediate, which determines the selectivity for FA over the alternative CO and H2 evolution reaction. It is confirmed that SnO2 clusters exhibit a different catalytic behaviour from their surface equivalents, thus offering promise for future work investigating the reduction of CO2 to FA via a hydride pinning pathway at low overpotential and CO2 capturing

A computational study of direct CO₂ hydrogenation to methanol on Pd surfaces

The reaction mechanism of direct CO2 hydrogenation to methanol is investigated in detail on Pd (111), (100) and (110) surfaces using density functional theory (DFT), supporting investigations into emergent Pd-based catalysts. Hydrogen adsorption and surface mobility are firstly considered, with high-coordination surface sites having the largest adsorption energy and being connected by diffusion channels with low energy barriers. Surface chemisorption of CO2, forming a partially charged CO2δ−, is weakly endothermic on a Pd (111) whilst slightly exothermic on Pd (100) and (110), with adsorption enthalpies of 0.09, −0.09 and −0.19 eV, respectively; the low stability of CO2δ− on the Pd (111) surface is attributed to negative charge accumulating on the surface Pd atoms that interact directly with the CO2δ− adsorbate. Detailed consideration for sequential hydrogenation of the CO2 shows that HCOOH hydrogenation to H2COOH would be the rate determining step in the conversion to methanol, for all surfaces, with activation barriers of 1.41, 1.51, and 0.84 eV on Pd (111), (100) and (110) facets, respectively. The Pd (110) surface exhibits overall lower activation energies than the most studied Pd (111) and (100) surfaces, and therefore should be considered in more detail in future Pd catalytic studies

Magnetic coupling constants for MnO as calculated using hybrid density functional theory

The properties of MnO have been calculated using generalised gradient approximation (GGA-) and hybrid (h-) density functional theory (DFT), specifically variants of the popular PBE and PBESol exchange–correlation functionals. The GGA approaches are shown to be poor at reproducing experimental magnetic coupling constants and rhombohedral structural distortions, with the PBESol functional performing worse than PBE. In contrast, h-DFT results are in reasonable agreement with experiment. Calculation of the Néel temperatures using the mean-field approximation gives overestimates relative to experiment, but the discrepancies are as low as 15 K for the PBE0 approach and, generally, the h-DFT results are significant improvements over previous theoretical studies. For the Curie–Weiss temperature, larger disparities are observed between the theoretical results and previous experimental results

Controlling structural transitions in AuAg nanoparticles through precise compositional design

We present a study of the transitional pathways between highsymmetry structural motifs for AgAu nanoparticles, with a specific focus on controlling the energetic barriers through chemical design. We show that the barriers can be altered by careful control of the elemental composition and chemical arrangement, with core@shell and vertex-decorated arrangements being specifically influential on the barrier heights. We also highlight the complexity of the potential and free energy landscapes for systems where there are low-symmetry geometric motifs that are energetically competitive to the high-symmetry arrangements. In particular, we highlight that some core@shell arrangements preferentially transition through multistep restructuring of lowsymmetry truncated octahedra and rosette-icosahedra, instead of via the more straightforward square-diamond transformations, due to lower energy barriers and competitive energetic minima. Our results have promising implications for the continuing efforts in bespoke nanoparticle design for catalytic and plasmonic applications

Bulk ionization potentials and band alignments from three-dimensional periodic calculations as demonstrated on rocksalt oxides

The position of the band edges of a material plays a key role in determining the properties for a range of applications, but fundamental band bending is an interface-dependent property that cannot be quantified without knowledge of bulk electron energy levels. We present a method for calculating the bulk position of the valence band maximum, and therefore the bulk ionization potential, from periodic plane wave calculations as shown for a range of rocksalt ionic oxides. We demonstrate that, for the popular “slab alignment” technique, explicit consideration of any surface induced electronic polarization is necessary to calculate accurate bulk ionization potentials. Our proposed method to quantify these surface effects, using polarizable-shell based interatomic potentials, is very computationally affordable, and our updated slab alignment method yields much improved agreement with the available experimental data

Highlights from faraday discussion on designing nanoparticle systems for catalysis, London, UK, May 2018

The 2018 Faraday Discussion on “Designing Nanoparticle Systems for Catalysis” brought together leading scientists to discuss the current state-of-the-art in the fields of computational chemistry, characterization techniques, and nanomaterial synthesis, and to debate the challenges and opportunities going forward for rational catalyst design. The meeting was a vivid discussion of how the communities accummulate knowledge and on how innovativeness can be combined to have a stronger scientific impact. In the following, we provide an overview of the meeting structure, including plenaries, papers, discussion points and breakout sessions, and we hope to show, to the wider scientific community, that there is great value in continued international discussion and scientific collaboration in these fields

Methanol loading dependent methoxylation in zeolite H-ZSM-5

We evaluate the effect of the number of methanol molecules per acidic site of H-ZSM-5 on the methoxylation reaction at room temperature by applying operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS), which capture the methoxylation reaction by simultaneously probing surface adsorbed species and reaction products, respectively. To this end, the methanol loading in H-ZSM-5 (Si/Al ≈ 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively. The operando DRIFTS/MS data show that the room temperature methoxylation depends on the methanol loading: the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules, as evident from the evolution of the methyl rock band of the methoxy species and of water as a function of time. Significantly, no methoxylation is observed with ≤1 molecule per Brønsted acidic site. However, hydrogen bonded methanol occurs across all loadings studied, but the structure of hydrogen bonded methanol also depends on the loading. Methanol loading of ≤1 molecule per acidic site leads to the formation of hydrogen bonded methanol with no proton transfer (i.e. neutral geometry), while loading ≥2 molecules per acidic site results in a hydrogen bonded methanol with a net positive charge on the adduct (protonated geometry). The infrared vibrational frequencies of methoxy and hydrogen bonded methanol are corroborated by Density Functional Theory (DFT) calculations. Both the experiments and calculations reflect the methoxy bands at around 940, 1180, 2868–2876 and 2980–2973 cm−1 which correspond to ν(C–O), ρ(CH3), νs(C–H) and νas(C–H), respectively

Dataset to support : FHI-aims benchmark for BEEF-vdW, vdW-DF2 and mBEEFvdW

We have performed a benchmark of the BEEF-vdW, mBEEF-vdW and vdW-DF2 implementations in the all-electron electronic structure code in FHIaims against the S22 benchmark set