Peak performance: Electrochemical experiments show that the oxygen reduction reaction (ORR) on platinum monolayers supported on various transition metals exhibits a volcano‐type behavior (see graph). Calculations reveal that bond‐breaking occurs more easily as bond‐making becomes harder, and why the Pd‐supported Pt monolayer (PtML/Pd(111)) has higher ORR activity than pure Pt(111).

This communication describes the preparation of carbon-supported truncated-octahedral Pt(3)Ni nanoparticle catalysts for the oxygen reduction reaction. Besides the composition, size, and shape controls, this work develops a new butylamine-based surface treatment approach for removing the long-alkane-chain capping agents used in the solution-phase synthesis. These Pt(3)Ni catalysts can have an area-specific activity as high as 850 muA/cm(2)(Pt) at 0.9 V, which is approximately 4 times better than the commercial Pt/C catalyst ( approximately 0.2 mA/cm(2)(Pt) at 0.9 V). The mass activity reached 0.53 A/mg(Pt) at 0.9 V, which is close to a factor of 4 increase in mass activity, the threshold value that allows fuel-cell power trains to become cost-competitive with their internal-combustion counterparts. Our results also show that the mass activities of these carbon-supported Pt(3)Ni nanoparticle catalysts strongly depend on the (111) surface fraction, which validates the results of studies based on Pt(3)Ni extended-single-crystal surfaces, suggesting that further development of catalysts with still higher mass activities is highly plausible.

We have synthesized a new class of electrocatalysts for the O2 reduction reaction, consisting of a mixed monolayer of Pt and another late transition metal (Ir, Ru, Rh, Re, or Os) deposited on a Pd(111) single crystal or on carbon-supported Pd nanoparticles. Several of these mixed monolayer electrocatalysts exhibited very high activity and increased stability of Pt against oxidation, as well as a 20-fold increase in a Pt mass-specific activity, compared with state-of-the-art all-Pt electrocatalysts. Their superior activity and stability reflect a low OH coverage on Pt, caused by the lateral repulsion between the OH adsorbed on Pt and the OH or O adsorbed on neighboring, other than Pt, late transition metal atoms. The origin of this effect was identified through a combination of experimental and theoretical methods, employing electrochemical techniques, in situ X-ray absorption spectroscopy, and periodic, self-consistent density functional theory calculations. This new class of electrocatalysts promises to alleviate some major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance. Our studies suggest a new way of synthesizing improved ORR catalysts through the modification and control of the surface reactivity of Pt-based mixed monolayers supported on transition metals other than Pt. In addition to improving the ORR catalysts, co-depositing oxophilic metals may be a promising possibility for improving a variety of other catalysts.

We explored the origin of the enhanced activity of Pd-alloy electrocatalysts for the O2 reduction reaction by correlating the electrocatalytic activity of intrinsic Pd and Pt surfaces and Pd and Pt overlayers on several substrates with their electronic properties, and established the volcano-type dependence of O2 reduction activity on the binding energy of oxygen and the d-band center of the top metal layer. Intrinsic Pd and Pt surfaces bind oxygen too firmly to allow efficient removal of the adsorbed reaction intermediates. Therefore, they do not have the highest activity and are not on the top of the volcano plot. A Pd overlayer on a Pd3Fe(111) alloy, was predicted to lie on top of the volcano plot, and thus, it appears to be the most active catalyst among investigated ones because of its moderate interaction with oxygen. The results can help in designing better electrocatalysts for fuel cells and other applications.

Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are sequence-specific posttranscriptional regulators of gene expression. Although major components of the RNA interference (RNAi) pathway have been identified, regulatory mechanisms for this pathway remain largely unknown. Here we demonstrate that the RNAi pathway can be modulated intracellularly by small molecules. We have developed a cell-based assay to monitor the activity of the RNAi pathway and find that the small-molecule enoxacin (Penetrex) enhances siRNA-mediated mRNA degradation and promotes the biogenesis of endogenous miRNAs. We show that this RNAi-enhancing activity depends on the trans-activation-responsive region RNA-binding protein. Our results provide a proofof-principle demonstration that small molecules can be used to modulate the activity of the RNAi pathway. RNAi enhancers may be useful in the development of research tools and therapeutics.

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We derived an intrinsic kinetic equation for the four-electron oxygen reduction reaction (ORR) in acidic media using free energies of activation and adsorption as the kinetic parameters. Our kinetic model consists of four essential elementary reactions: a dissociative adsorption (DA) and a reductive adsorption (RA), which yield two reaction intermediates, O and OH; a reductive transition (RT) from O to OH; and a reductive desorption (RD) of OH. Analytic expressions were found for the O and OH adsorption isotherms by solving the steady-state rate equations. For the ORR on Pt(111) in 0.1 M HClO4 solution, we analyzed the measured polarization curves, thereby deducing activation free energies that are consistent with the values from theoretical calculations. The reductive adsorption (DeltaG*0RA=0.46 eV) is not the rate-determining step (RDS) for the ORR on Pt because dissociative adsorption (DeltaG*0DA=0.26 eV) offers a more favorable pathway at high potentials. It, however, generates strongly adsorbed O. The high activation barriers for the O to OH transition (DeltaG*0RT=0.50 eV) and OH desorption (DeltaG*0RD=0.45 eV) cause a large potential loss for the desorption-limited ORR. As the OH coverage increases to a constant value with decreasing potential, the Tafel slope increases to the value determined by a symmetric electron-transfer coefficient. We discuss the role of adsorption isotherm in kinetic analysis and, via activity-and-barrier plots, illustrate why the RDS may vary with reaction conditions or may not exist. Recognizing such features of electrocatalytic reactions can facilitate reaching the long-standing goal of quantitative descriptions and predictions of electrocatalysts' activities.

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