of functional monolayers on surfaces of carbon materials
is inherently difficult because of the high bond strength of carbon
and because common pathways such as SN2 mechanisms cannot
take place at surfaces of solid materials. Here, we show that the
radical initiators can selectively abstract H atoms from H-terminated
carbon surfaces, initiating regioselective grafting of terminal alkenes
to surfaces of diamond, glassy carbon, and polymeric carbon dots.
Nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy
(XPS) demonstrate formation of self-terminating organic monolayers
linked via the terminal C atom of 1-alkenes. Density functional theory
(DFT) calculations suggest that this selectivity is at least partially
thermodynamic in origin, as significantly less energy is needed to
abstract H atoms from carbon surfaces as compared to typical aliphatic
compounds. The regioselectivity favoring binding to the terminal C
atom of the reactant alkenes arises from steric hindrance encountered
in bond formation at the adjacent carbon atom. Our results demonstrate
that carbon surface radical chemistry yields a versatile, selective,
and scalable approach to monolayer formation on H-terminated carbon
surfaces and provide mechanistic insights into the surface selectivity
and regioselectivity of molecular grafting.
Weakly bound noble gases (Ne, Ar, Kr, and Xe) are being utilized as probes to monitor the photocatalytic activity of the TiO 2 (110) surface. In this work, this adsorption problem is examined using different van der Waals-corrected DFT-based treatments on periodic systems. The assessment of their performance is assisted by the application of nonperiodic DFT-based symmetry adapted perturbation theory [SAPT(DFT)]. It is further verified by comparing with experimentally based determinations of the adsorption energies at one-monolayer surface coverage. Besides being dispersion-dominated adsorbate/surface interactions, the SAPT(DFT)based decomposition reveals that the electrostatic and induction energy contributions become highly relevant for the heaviest noble-gas atoms (krypton and xenon). The most reliable results are provided by the revPBE-D3 approach: it predicts adsorption energies of −118.4, −165.8, and −2231.7 meV for argon, krypton, and xenon, which are within 6% of the experimental values, and attractive long-range tails which are consistent with our ab initio benchmarking. Moreover, the revPBE density functional describes the short-range part of the potential energy curve more precisely, avoiding the exchange-only binding effects of the PBE functional. The nonlocal vdW-DF2 density functional performs well at the long-range potential region but largely overestimates the adsorption energies of noble gas atoms as light as argon. The Tkatchenko−Scheffler dispersion correction combined with the revPBE functional produces accurate estimations of the adsorption energies (to within 10%) but long-range attractive tails that decay too slowly as in first-generation nonlocal vdW-DF density functional. Lateral interactions between coadsorbate atoms contribute up to about 15−20%, being key in achieving good agreement with experimental measurements. The interaction with the noble-gas atoms reduces the work function of the TiO 2 (110) surface, agreeing to the experimental observation of an inhibited photodesorption of coadsorbed molecular oxygen.
Atomistic modeling of mineral-water interfaces offers a way of confirming (or refuting) experimental information about structure and reactivity. Molecular-level understanding, such as orbital-based descriptions of bonding, can be developed from charge density and electronic structure analysis. First-principles calculations can be used to identify weaknesses in empirical models. This provides direction on how to propose more robust representations of systems of increasing size that accurately represent the underlying physical factors governing reactivity. In this study, inner-sphere complex geometries of arsenate on hydrated alumina surfaces are modeled at the density functional theory (DFT)-continuum solvent level. According to experimental studies, arsenate binds to alumina surfaces in a bidentate binuclear (BB) fashion. While the DFT calculations support the preference of the BB configuration, the optimized geometries show distortion from the ideal tetrahedral geometry of the arsenic atom. This finding suggests that steric factors, and not just coordination arguments, influences reactivity. The O surf -As-O surf angle for the more favorable arsenate configurations is closest to the ideal tetrahedral angle of 109.5 • . Comparing the results of arsenate adsorption using a small cluster model with a periodic slab model, we report that the two model geometries yield results that differ qualitatively and quantitatively. This relates the steric factors and rigidity of the surface models.
affordable electrocatalysts to facilitate the reduction
of carbon dioxide (CO2) to high-value products with high
selectivity, efficiency, and large current densities is a critical
step for the production of liquid carbon-based fuels. In this work,
we show that inexpensive post-transition metals [tin (Sn) and lead
(Pb)] and their alloys (PbSn) are excellent cathode materials to reduce
CO2 in an ionic liquid/acetonitrile/water electrolyte media.
Electrochemical impedance spectroscopy measurements show that the
PbSn alloys exhibit lower charge-transfer resistance when compared
to the pure metal electrodes, as supported by electronic structure
calculations. Current densities as high as 60 mA/cm2 are
observed with optimal mixtures of ionic liquid, acetonitrile, and
water. Reduction product analysis identifies carbon monoxide (CO)
and formate (HCOO−) as primary reduced products, with higher
selectivity toward formate. Faradaic efficiency for formate on pure
Pb and pure Sn was determined to be 80 ± 4 and 86 ± 3%,
respectively. FE % improves as either Pb is incorporated into Sn or
vice versa, and there is a maximum FE of 91 ± 3% for both 50
and 40% Pb composition.
The inner-sphere adsorption of AsO 4 3− , PO 4 3− , and SO 4 2− on the hydroxylated α-Al 2 O 3 (001) surface was modeled with the goal of adapting a density functional theory (DFT) and thermodynamics framework for calculating the adsorption energetics. While DFT is a reliable method for predicting various properties of solids, including crystalline materials comprised of hundreds (or even thousands) of atoms, adding aqueous energetics in heterogeneous systems poses steep challenges for modeling. This is in part due to the fact that environmentally relevant variations in the chemical surroundings cannot be captured atomistically without increasing the system size beyond tractable limits. The DFT + thermodynamics approach to this conundrum is to combine the DFT total energies with tabulated solution-phase data and Nernst-based corrective terms to incorporate experimentally tunable parameters such as concentration. Central to this approach is the design of thermodynamic cycles that partition the overall reaction (here, inner-sphere adsorption proceeding via ligand exchange) into elementary steps that can either be fully calculated or for which tabulated data are available. The ultimate goal is to develop a modeling framework that takes into account subtleties of the substrate (such as adsorption-induced surface relaxation) and energies associated with the aqueous environment such that adsorption at mineral−water interfaces can be reliably predicted, allowing for comparisons in the denticity and protonation state of the adsorbing species. Based on the relative amount of experimental information available for AsO 4 3− , PO 4 3− , and SO 4 2− adsorbates and the well-characterized hydroxylated α-Al 2 O 3 (001) surface, these systems are chosen to form a basis for assessing the model predictions. We discuss how the DFT + thermodynamics results are in line with the experimental information about the oxyanion sorption behavior. Additionally, a vibrational analysis was conducted for the charge-neutral oxyanion complexes and is compared to the available experimental findings to discern the inner-sphere adsorption phonon modes. The DFT + thermodynamics framework used here is readily extendable to other chemical processes at solid−liquid interfaces, and we discuss future directions for modeling surface processes at mineral−water and environmental interfaces.
Among high-valence metal oxides, LiCoO2 and related materials are of environmental importance because of the rapidly increasing use of these materials as cathodes in lithium ion batteries. Understanding the impact...
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