The rational design of pure and alloy metal catalysts from fundamental principles has the potential to yield catalysts of greatly improved activity and selectivity. A promising area of research ...concerns the role that near-surface alloys (NSAs) can play in endowing surfaces with novel catalytic properties. NSAs are defined as alloys wherein a solute metal is present near the surface of a host metal in concentrations different from the bulk; here we use density functional theory calculations to introduce a new class of these alloys that can yield superior catalytic behaviour for hydrogen-related reactions. Some of these NSAs bind atomic hydrogen (H) as weakly as the noble metals (Cu, Au) while, at the same time, dissociating H(2) much more easily. This unique set of properties may permit these alloys to serve as low-temperature, highly selective catalysts for pharmaceuticals production and as robust fuel-cell anodes.
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IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic ...interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) >> Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) >> Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their ...cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen ...have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag₃ clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Density functional theory calculations are presented for CHx, x=0,1,2,3, NHx, x=0,1,2, OHx, x=0,1, and SHx, x=0,1 adsorption on a range of close-packed and stepped transition-metal surfaces. We find ...that the adsorption energy of any of the molecules considered scales approximately with the adsorption energy of the central, C, N, O, or S atom, the scaling constant depending only on x. A model is proposed to understand this behavior. The scaling model is developed into a general framework for estimating the reaction energies for hydrogenation and dehydrogenation reactions.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
We estimate the activity of the oxygen reduction reaction on platinum nanoparticles of sizes of practical importance. The proposed model explicitly accounts for surface irregularities and their ...effect on the activity of neighboring sites. The model reproduces the experimentally observed trends in both the specific and mass activities for particle sizes in the range between 2 and 30 nm. The mass activity is calculated to be maximized for particles of a diameter between 2 and 4 nm. Our study demonstrates how an atomic-scale description of the surface microstructure is a key component in understanding particle size effects on the activity of catalytic nanoparticles.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Aragonite saturation state (Ωarag) in surface and subsurface waters of the global oceans was calculated from up‐to‐date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and ...total alkalinity (TA) data. Surface Ωarag in the open ocean was always supersaturated (Ω > 1), ranging between 1.1 and 4.2. It was above 2.0 (2.0–4.2) between 40°N and 40°S but decreased toward higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product (K′sp), explain the latitudinal differences in surface Ωarag. Vertically, Ωarag was highest in the surface mixed layer. Higher hydrostatic pressure, lower water temperature, and more CO2 buildup from biological activity in the absence of air‐sea gas exchange helped maintain lower Ωarag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ωarag distributions. Seasonally, surface Ωarag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open‐ocean waters of both hemispheres. Decadal changes of Ωarag in the Atlantic and Pacific Oceans showed that Ωarag in waters shallower than 100 m depth decreased by 0.10 ± 0.09 (−0.40 ± 0.37% yr−1) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.
Key Points
Climatological aragonite saturation in surface and subsurface global oceans are presented
Mechanisms controlling aragonite saturation state distributions are discussed
Subannual and decadal changes of aragonite saturation state are presented
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We address the fundamental question of which size a metallic nano-particle needs to have before its surface chemical properties can be considered to be those of a solid, rather than those of a large ...molecule. Calculations of adsorption energies for carbon monoxide and oxygen on a series of gold nanoparticles ranging from 13 to 1,415 atoms, or 0.8–3.7 nm, have been made possible by exploiting massively parallel computing on up to 32,768 cores on the Blue Gene/P computer at Argonne National Laboratory. We show that bulk surface properties are obtained for clusters larger than ca. 560 atoms (2.7 nm). Below that critical size, finite-size effects can be observed, and we show those to be related to variations in the local atomic structure augmented by quantum size effects for the smallest clusters.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A simple procedure is introduced to use periodic Density Functional Theory calculations to estimate trends in the thermodynamics of surface alloy dissolution in acidic media. With this approach, the ...dissolution potentials for solute metal atoms embedded in the surface layer of various host metals (referenced to the dissolution potential of the solute in its pure, metallic form) are calculated. Periodic trends in the calculated potentials are found to be related to trends in surface segregation energies of the various solute/host pairs. The effects of water splitting and concomitant hydroxyl adsorption on the dissolution potentials are also considered; these effects do not change the potentials for highly oxophilic solutes embedded in less active hosts, but they do decrease the dissolution potential for more inert solutes on oxophilic hosts. Finally, the dissolution of Pt “skin” layers from Pt
3X (X
=
Fe, Co, and Ni) bulk alloys is analyzed; the Pt skins are found to be stabilized compared to pure Pt.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Oceanic uptake of anthropogenic carbon dioxide (CO
2
) from the atmosphere has changed ocean biogeochemistry and threatened the health of organisms through a process known as ocean acidification ...(OA). Such large-scale changes affect ecosystem functions and can have impacts on societal uses, fisheries resources, and economies. In many large estuaries, anthropogenic CO
2
-induced acidification is enhanced by strong stratification, long water residence times, eutrophication, and a weak acid-base buffer capacity. In this article, we review how a variety of processes influence aquatic acid-base properties in estuarine waters, including coastal upwelling, river-ocean mixing, air-water gas exchange, biological production and subsequent aerobic and anaerobic respiration, calcium carbonate (CaCO
3
) dissolution, and benthic inputs. We emphasize the spatial and temporal dynamics of partial pressure of CO
2
(
p
CO
2
), pH, and calcium carbonate mineral saturation states. Examples from three large estuaries-Chesapeake Bay, the Salish Sea, and Prince William Sound-are used to illustrate how natural and anthropogenic processes and climate change may manifest differently across estuaries, as well as the biological implications of OA on coastal calcifiers.
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