Aravind Asthagiri

Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps†

On the right path: Based on DFT calculations (incorporating the role of water solvation) of the activation barriers of elementary steps, a new path that leads to methane and ethylene for CO(2) electroreduction on Cu(111) was identified. Methane formation proceeds through reduction of CO to COH (path II, see picture), which leads to CH(x) species that can produce both methane and ethylene, as observed experimentally.

Dimeric [Mo2S12]2− Cluster: A Molecular Analogue of MoS2 Edges for Superior Hydrogen‐Evolution Electrocatalysis

Proton reduction is one of the most fundamental and important reactions in nature. MoS2 edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS2 edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein we report a dimeric molecular analogue [Mo2 S12 ](2-) , as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo2 S12 ](2-) is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo2 S12 ](2-) exhibits a hydrogen adsorption free energy near zero (-0.05 eV). This work helps shed light on the rational design of HER catalysts and biomimetics of hydrogen-evolving enzymes.

First principles study of Pt adhesion and growth on SrO- and TiO2-terminated SrTiO3(100)

We use density functional theory to investigate Pt growth on SrTiO3(100) surfaces. We have determined the favored monolayer arrangement for Pt on both terminations of SrTiO3(100). The work of separation as a function of the number of monolayers of Pt has been determined for Pt films of up to 5 monolayers. To examine which surface growth mode will dominate Pt film growth on this substrate, we have also examined isolated Pt clusters on each termination of SrTiO3(100). Our results indicate that the TiO2-terminated surface is more favorable for epitaxial growth of (100) oriented Pt films than the SrO-terminated surface.

Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of σ-complex formation

We investigated the molecular adsorption of methane, ethane, and propane on a PdO(101) thin film using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The TPD data reveal that each of the alkanes adsorbs into a low-coverage molecular state on PdO(101) in which the binding is stronger than that for alkanes physically adsorbed on Pd(111). Analysis of the TPD data using limiting values of the desorption prefactors predicts that the alkane binding energies on PdO(101) increase linearly with increasing chain length, but that the resulting line extrapolates to a nonzero value between about 22 and 26 kJ/mol at zero chain length. This constant offset implies that a roughly molecule-independent interaction contributes to the alkane binding energies for the molecules studied. DFT calculations predict that the small alkanes bind on PdO(101) by forming dative bonds with coordinatively unsaturated Pd atoms. The resulting adsorbed species are analogous to alkane sigma-complexes in that the bonding involves electron donation from C-H sigma bonds to the Pd center as well as backdonation from the metal, which weakens the C-H bonds. The binding energies predicted by DFT lie in a range from 16 to 24 kJ/mol, in good agreement with the constant offsets estimated from the TPD data. We conclude that both the dispersion interaction and the formation of sigma-complexes contribute to the binding of small alkanes on PdO(101), and estimate that sigma-complex formation accounts for between 30% and 50% of the total binding energy for the molecules studied. The predicted weakening of C-H bonds resulting from sigma-complex formation may help to explain the high activity of PdO surfaces toward alkane activation.

Dispersion-corrected density functional theory calculations of the molecular binding of n-alkanes on Pd(111) and PdO(101)

We investigated the molecular binding of n-alkanes on Pd(111) and PdO(101) using conventional density functional theory (DFT) and the dispersion-corrected DFT-D3 method. In agreement with experimental findings, DFT-D3 predicts that the n-alkane desorption energies scale linearly with the molecule chain length on both surfaces, and that n-alkanes bind more strongly on PdO(101) than on Pd(111). The desorption energies computed using DFT-D3 are slightly higher than the measured values for n-alkanes on Pd(111), though the agreement between computation and experiment is a significant improvement over conventional DFT. The measured desorption energies of n-alkanes on PdO(101) and the energies computed using DFT-D3 agree to within better than 2.5 kJ/mol (< 5%) for chain lengths up to n-butane. The DFT-D3 calculations predict that the molecule-surface dispersion energy for a given n-alkane is similar in magnitude on Pd(111) and PdO(101), and that dative bonding between the alkanes and coordinatively unsaturated Pd atoms is primarily responsible for the enhanced binding of n-alkanes on PdO(101). From analysis of the DFT-D3 results, we estimate that the strength of an alkane η(2)(H, H) interaction on PdO(101) is ~16 kJ/mol, while a single η(1) H-Pd dative bond is worth about 10 kJ/mol.

Thermal fluctuations in the structure of naturally chiral Pt surfaces

The intrinsic chirality of metal surfaces with kinked steps (e.g., Pt(643)) endows them with enantiospecific adsorption properties. To understand these properties quantitatively the impact of thermally-driven step wandering must be assessed. We derive a lattice-gas model of step motion on Pt(111) surfaces using diffusion barriers from density functional theory. This model is used to examine thermal fluctuations of straight and kinked steps.

An ab initio study of adsorption of alanine on the chiral calcite surface

Density functional theory calculations has been used to examine the adsorption of d- and l-alanine (Ala) on the chiral calcite surface. We find negligible differences ( < 1 kcal/mol) in adsorption energies for the most stable minima of d- and l-Ala on the calcite surface. The strongest interaction in the adsorbed system is between the surface Ca and the O atom of the carboxyl group on Ala. The source of the weak enantiospecificity is the relative difference in dimensions of the Ala and the atoms on the calcite surface. The surface O and Ca atoms on the calcite surface are separated by 6 Å, while the Ala molecule is roughly 3–4 Å in size. This disparity in dimensions prevents the Ala molecule from making three strong points of contact with the surface, a prerequisite for strong enantiospecificity.

Theoretical insight on reactivity trends in CO2 electroreduction across transition metals

Density Functional Theory (DFT) based models have been widely applied towards investigating and correlating the reaction mechanism of CO2 electroreduction (ER) to the activity and selectivity of potential electrocatalysts. Herein, we examine the implications of the theoretical choices used in DFT models that impact the stability of the reaction intermediates and the limiting potential (UL) of the activity/selectivity determining steps in CO2 ER across transition metals. Three theoretical choices are considered: (i) the type of exchange-correlation (XC) functional, (ii) the surface facet of the metal electrocatalyst, and (iii) the effect of solvation. The impact of the theoretical choices is also studied in the context of deriving scaling relationships for electrocatalyst screening. The analyses reveal that the choice of XC functional (PBE versus RPBE) can alter binding energies of CO2 ER intermediates by 0.30 eV, but have little impact on surface reaction energetics. Surface termination has greater impact, as OH*-terminated adsorbates bind weaker on average by 0.26 eV on stepped facets. Including explicit local solvation stabilizes the OH*-terminated adsorbates, preferentially decreasing the UL for CO* → COH* reduction. Trends in CO2 ER selectivity across metals predicted using scaling correlations differ signficantly from explicitly calculated values due to deviations from the linear binding energy correlations. The difference is most pronounced when the effect of explicit solvation is considered.

High Selectivity for Primary C–H Bond Cleavage of Propane σ-Complexes on the PdO(101) Surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.

Low-temperature activation of methane on the IrO2(110) surface

Low-temperature methane reactions Methane is a potential feedstock for more valuable products. The strong carbon-hydrogen bonds of methane can be activated by heterogeneous catalysts but often at temperatures that make it difficult to control reactions selectively. Liang et al. show that methane, adsorbed on the stoichiometric IrO2(110) under ultrahigh-vacuum conditions, reacts with exposed iridium atoms to break the carbon-hydrogen bonds at temperatures as low as 150 K. On heating, the surface fragments react cleanly with surface oxygen to form carbon dioxide, carbon monoxide, and water. Science, this issue p. 299 Methane undergoes highly facile C–H bond cleavage on the IrO2(110) surface at temperatures as low as 150 kelvin. Methane undergoes highly facile C–H bond cleavage on the stoichiometric IrO2(110) surface. From temperature-programmed reaction spectroscopy experiments, we found that methane molecularly adsorbed as a strongly bound σ complex on IrO2(110) and that a large fraction of the adsorbed complexes underwent C–H bond cleavage at temperatures as low as 150 kelvin (K). The initial dissociation probability of methane on IrO2(110) decreased from 80 to 20% with increasing surface temperature from 175 to 300 K. We estimate that the activation energy for methane C–H bond cleavage is 9.5 kilojoule per mole (kJ/mol) lower than the binding energy of the adsorbed precursor on IrO2(110), and equal to a value of ~28.5 kJ/mol. Low-temperature activation may avoid unwanted side reactions in the development of catalytic processes to selectively convert methane to value-added products.