Trends in electrocatalytic activity of the oxygen evolution reaction (OER) are investigated on the basis of a large database of HO* and HOO* adsorption energies on oxide surfaces. The theoretical overpotential was calculated by applying standard density functional theory in combination with the computational standard hydrogen electrode (SHE) model. We showed that by the discovery of a universal scaling relation between the adsorption energies of HOO* vs HO*, it is possible to analyze the reaction free energy diagrams of all the oxides in a general way. This gave rise to an activity volcano that was the same for a wide variety of oxide catalyst materials and a universal descriptor for the oxygen evolution activity, which suggests a fundamental limitation on the maximum oxygen evolution activity of planar oxide catalysts.

Coordinatively unsaturated ferrous (CUF) sites confined in nanosized matrices are active centers in a wide range of enzyme and homogeneous catalytic reactions. Preparation of the analogous active sites at supported catalysts is of great importance in heterogeneous catalysis but remains a challenge. On the basis of surface science measurements and density functional calculations, we show that the interface confinement effect can be used to stabilize the CUF sites by taking advantage of strong adhesion between ferrous oxides and metal substrates. The interface-confined CUF sites together with the metal supports are active for dioxygen activation, producing reactive dissociated oxygen atoms. We show that the structural ensemble was highly efficient for carbon monoxide oxidation at low temperature under typical operating conditions of a proton-exchange membrane fuel cell.

Identifying the structure sensitivity of catalysts in reactions, such as Fischer−Tropsch synthesis from CO and H 2 over cobalt catalysts, is an important yet challenging issue in heterogeneous catalysis. Based on a first-principles kinetic study, we find for the first time that CO activation on hexagonal close-packed (HCP) Co not only has much higher intrinsic activity than that of face centered-cubic (FCC) Co but also prefers a different reaction route, i.e., direct dissociation with HCP Co but Hassisted dissociation on the FCC Co. The origin is identified from the formation of various denser yet favorable active sites on HCP Co not available for FCC Co, due to their distinct crystallographic structure and morphology. The great dependence of the activity on the crystallographic structure and morphology of the catalysts revealed here may open a new avenue for better, stable catalysts with maximum mass-specific reactivity.I dentifying the structure sensitivity of catalysts in chemical reactions to achieve the maximum mass-specific yet stable reactivity is one of the most challenging goals in heterogeneous catalysis. 1 A growing number of studies are conducted to understand the nature of the structure sensitivity, supported by well-defined preparation methods, in situ characterizations, surface science studies, and ab initio calculations. A prime example of this complex structure sensitivity is the Fischer− Tropsch synthesis (FTS) converting CO and H 2 from coal, natural gas, and biomass to hydrocarbon over cobalt catalysts. 2 Over the years, two notable structure sensitivities have been observed for FTS over Co catalysts, i.e., crystallographic structure and particle size. First, it is noted that Co can exist in two crystallographic structures, namely, the hexagonal closepacked (HCP) phase and the face-centered cubic (FCC) phase, and both phases are found in FTS. It has been reported by many that HCP Co has higher FTS activity than FCC Co. 3 This structure sensitivity is, however, complicated by a phase transition from HCP to FCC upon decreasing the catalyst size, 4 varying the supports and promoters, and pretreating the catalysts. 5 To our best knowledge, it remains open as to whether and why HCP Co catalysts have higher FTS activity than FCC ones, which prevents the full exploration of this structure sensitivity. Second, there are a number of reports on the effect of particle size of Co catalysts on FTS activity; namely, the turnover frequency (TOF) was constant for crystallites above a certain diameter, but when the diameter became smaller, the TOF decreased. 6 It is unclear whether the decrease of FTS activity at the smaller particle size is related to the phase transition of the Co catalyst from HCP to FCC, because of the complexity of the FTS and the absence of sufficient crystallographic structure data. Nevertheless, it is clear that one cannot increase the mass-specific activity of Co catalysts in FTS simply by decreasing the particle size.We report here a density functional theory (DFT)-base...

Progress in the field of electrocatalysis is often hampered by the difficulty in identifying the active site on an electrode surface. Herein we combine theoretical analysis and electrochemical methods to identify the active surfaces in a manganese oxide bi-functional catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). First, we electrochemically characterize the nanostructured α-Mn(2)O(3) and find that it undergoes oxidation in two potential regions: initially, between 0.5 V and 0.8 V, a potential region relevant to the ORR and, subsequently, between 0.8 V and 1.0 V, a potential region between the ORR and the OER relevant conditions. Next, we perform density function theory (DFT) calculations to understand the changes in the MnO(x) surface as a function of potential and to elucidate reaction mechanisms that lead to high activities observed in the experiments. Using DFT, we construct surface Pourbaix and free energy diagrams of three different MnO(x) surfaces and identify 1/2 ML HO* covered Mn(2)O(3) and O* covered MnO(2), as the active surfaces for the ORR and the OER, respectively. Additionally, we find that the ORR occurs through an associative mechanism and that its overpotential is highly dependent on the stabilization of intermediates through hydrogen bonds with water molecules. We also determine that OER occurs through direct recombination mechanism and that its major source of overpotential is the scaling relationship between HOO* and HO* surface intermediates. Using a previously developed Sabatier model we show that the theoretical predictions of catalytic activities match the experimentally determined onset potentials for the ORR and the OER, both qualitatively and quantitatively. Consequently, the combination of first-principles theoretical analysis and experimental methods offers an understanding of manganese oxide oxygen electrocatalysis at the atomic level, achieving fundamental insight that can potentially be used to design and develop improved electrocatalysts for the ORR and the OER and other important reactions of technological interest.