As a catalyst, single-atom platinum may provide an ideal structure for platinum minimization. Herein, a single-atom catalyst of platinum supported on titanium nitride nanoparticles were successfully prepared with the aid of chlorine ligands. Unlike platinum nanoparticles, the single-atom active sites predominantly produced hydrogen peroxide in the electrochemical oxygen reduction with the highest mass activity reported so far. The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, also exhibited unique selectivity on the single-atom platinum catalyst. A lack of platinum ensemble sites changed the reaction pathway for the oxygen-reduction reaction toward a two-electron pathway and formic acid oxidation toward direct dehydrogenation, and also induced no activity for the methanol oxidation. This work demonstrates that single-atom platinum can be an efficient electrocatalyst with high mass activity and unique selectivity.

Single-atom catalysts (SACs) provide an ideal platform for reducing noble-metal usage. SACs also exhibit unusual catalytic properties due to the absence of a metal surface. The role of the support may have a significant effect on the catalytic properties, similar to that of the ligand molecules in homogeneous catalysts. Here, the support effect was demonstrated by preparing a single-atom platinum catalyst on two different supports: titanium carbide (Pt1/TiC) and titanium nitride (Pt1/TiN). The formation of single-atom Pt was confirmed by STEM, EXAFS, and in situ IR spectroscopy. Pt1/TiC showed higher activity, selectivity, and stability for electrochemical H2O2 production than Pt1/TiN. Density functional theory calculations presented that oxygen species have strong affinity into Pt1/TiN, possibly acting as surface poisoning species, and Pt1/TiC preserves oxygen–oxygen bonds more with higher selectivity toward H2O2 production. This work clearly shows that the support in SACs actively participates in the surface reaction and does not just act as anchoring sites for single atoms.

To obtain insight into the structure and surface stoichiometry of copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we perform densityfunctional theory calculations to investigate the relative stability of low-index copper oxide surfaces. By employing the technique of "ab initio atomistic thermodynamics," we identify low-energy surface structures that are most stable under realistic catalytic conditions are found to exhibit a metallic character. Three surfaces are shown to have notably lower surface free energies compared to the others considered and could be catalytically relevant; in particular, under oxygen-rich conditions, they are the Cu 2 O͑110͒ : CuO surface, which is terminated with both Cu and O surface atoms, and the Cu 2 O͑111͒-Cu CUS surface, which contains a surface ͑coordinatively unsaturated͒ Cu vacancy, while for the oxygen-lean conditions, the Cu 2 O͑111͒ surface with a surface interstitial Cu atom is found to be energetically most favorable, highlighting the importance of defects at the surface.

The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalization scheme to quantify the effect of the soft modes on the calculated properties, and confirm that the anharmonicity is an inherent feature of the Cmcm phase. These results suggest a design concept for thermal insulators and thermoelectric materials, based on displacive instabilities, and highlight the power of lattice-dynamics calculations for materials characterization.

We present density functional theory investigations of the bulk properties of cerium oxides ͑CeO 2 and Ce 2 O 3 ͒ and the three low index surfaces of CeO 2 , namely, ͑100͒, ͑110͒, and ͑111͒. For the surfaces, we consider various terminations including surface defects. Using the approach of "ab initio atomistic thermodynamics," we find that the most stable surface structure considered is the stoichiometric ͑111͒ surface under "oxygen-rich" conditions, while for a more reducing environment, the same ͑111͒ surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the ͑111͒ Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce 2 O 3 ͑0001͒ surface. This structure could represent a precursor to the phase transition of CeO 2 to Ce 2 O 3 .

As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the lowtemperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on Cu͑111͒. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the ͑p , T͒ phase diagram of O/Cu͑111͒. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late 4d transition metals ͑Ru, Rh, Pd, Ag͒, sub-surface oxygen is found to be energetically unfavorable.

Using density-functional theory within the generalized gradient approximation, we investigate the interaction between atomic oxygen and Cu͑100͒ and Cu͑110͒ surfaces. We consider the adsorption of oxygen at various on-surface and subsurface sites of Cu͑100͒ for coverages of 1/8 to 1 monolayers ͑ML͒. We find that oxygen at a coverage of 1/2 ML preferably binds to Cu͑100͒ in a missing-row surface reconstruction, while oxygen adsorption on the nonreconstructed surface is preferred at 1/4 ML coverage consistent with experimental results. For Cu͑110͒, we consider oxygen binding to both nonreconstructed and added-row reconstructions at various coverages. For coverages up to 1/2 ML coverage, the most stable configuration is predicted to be a p͑2 ϫ 1͒ missing-row structure. At higher oxygen exposures, a surface transition to a c͑6 ϫ 2͒ added strand configuration with 2/3 ML oxygen coverage occurs. Through surface Gibbs free energies, taking into account temperature and oxygen partial pressure, we construct ͑p , T͒ surface phase diagrams for O/Cu͑100͒ and O/Cu͑110͒. On both crystal faces, oxygenated surface structures are stable prior to bulk oxidation. We combine our results with equivalent ͑p , T͒ surface free energy data for the O/Cu͑111͒ surface to predict the morphology of copper nanoparticles in an oxygen environment.

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