We describe a new class of electrocatalysts for the O 2 reduction, and H 2 and methanol oxidation reactions, consisting of a monolayer of Pt deposited on a metal or alloy carbon-supported nanoparticles. These electrocatalysts show up to a 20-fold increase in Pt mass activity compared with conventional all-Pt electrocatalysts. The origin of their increased activity was identified through a combination of experimental methods, employing electrochemical and surface science techniques, X-ray absorption spectroscopy, and density functional theory calculations. The long-term tests in fuel cells demonstrated excellent stability of the anode and good stability of the cathode electrocatalysts. We also describe the stabilization of Pt electrocatalysts against dissolution under potential cycling regimes effected by a submonolayer of Au clusters deposited on Pt surfaces. These new electrocatalysts promise to alleviate some of the major problems of existing fuel cell technology.

Oxygen reduction reaction (ORR) kinetics in acid solutions was studied by analysis of the polarization curves obtained by rotating disk electrode method for Pt(111) in HClO 4 and H 2 SO 4 solutions. The model for ORR kinetic currents assumes that the intrinsic exchange current and Tafel slope are independent of anion adsorption. The site blocking and electronic effects of adsorbed OH (in HClO 4 ) and bisulfate (in H 2 SO 4 ) were evaluated with the adsorption isotherms incorporated in the model. The best fits yielded the intrinsic Tafel slope in the range from -118 to -130 mV/dec, supporting single electron transfer in the rate-determining step with the corresponding transfer coefficients equal to 0.50 and 0.45, respectively. In addition to site blocking, a negative electronic effect on ORR kinetics was found for both OH and bisulfate with the effect of the latter being much stronger. The deviation of the apparent Tafel slope in HClO 4 from its intrinsic value can be fully accounted for by the site blocking and electronic effects of adsorbed OH ions, which vary with coverage over the mixed kinetic-diffusion controlled region. For Pt nanoparticle catalysts, the apparent Tafel slope is similar to that for Pt(111) in HClO 4 and the positive potential shift is mainly due to the increase in apparent exchange current as effective surface area increases.

We synthesized a new class of O2 electrocatalysts with a high activity and very low noble metal content. They consist of Pt monolayers deposited on the surfaces of carbon-supported nonnoble metal-noble metal core-shell nanoparticles. These core-shell nanoparticles were formed by segregating the atoms of the noble metal on to the nanoparticles' surfaces at elevated temperatures. A Pt monolayer was deposited by galvanic displacement of a Cu monolayer deposited at underpotentials. The mass activity of all the three Pt monolayer electrocatalysts investigated, viz., Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co, is more than order of magnitude higher than that of a state-of-the-art commercial Pt/C electrocatalyst. Geometric effects in the Pt monolayer and the effects of PtOH coverage, revealed by electrochemical data, X-ray diffraction, and X-ray absorption spectroscopy data, appear to be the source of the enhanced catalytic activity. Our results demonstrated that high-activity electrocatalysts can be devised that contain only a fractional amount of Pt and a very small amount of another noble metal.

Abstract. The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as −Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%. In the older samples the nitrate/CO ratio has leveled off suggesting that evaporation and formation of aerosol nitrate are in balance. In contrast, OA/CO increases with age in older samples, indicating that OA is still being formed. The amount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbC per ppm CO. At an aerosol yield of 5% and 8% for low and high yield aromatic compounds, it is estimated from surface hydrocarbon observations that only ~9% of the OA formation can be accounted for. A comparison of OA/CO in Mexico City and the eastern U.S. gives no evidence that aerosol yields are higher in a more polluted environment.

Abstract. Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Aerosol Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix Factorization (PMF) of high resolution AMS spectra identified a biomass burning organic aerosol (BBOA) component, which includes several large plumes that appear to be from forest fires within the region. Here, we show that the AMS BBOA concentration at T0 correlates with fire counts in the vicinity of Mexico City and that most of the BBOA variability is captured when the FLEXPART model is used for the dispersion of fire emissions as estimated from satellite fire counts. The resulting FLEXPART fire impact factor (FIF) correlates well with the observed BBOA, acetonitrile (CH3CN), levoglucosan, and potassium, indicating that wildfires in the region surrounding Mexico City are the dominant source of BBOA at T0 during MILAGRO. The impact of distant BB sources such as the Yucatan is small during this period. All fire tracers are correlated, with BBOA and levoglucosan showing little background, acetonitrile having a well-known tropospheric background of ~100–150 pptv, and PM2.5 potassium having a background of ~160 ng m−3 (two-thirds of its average concentration), which does not appear to be related to BB sources. We define two high fire periods based on satellite fire counts and FLEXPART-predicted FIFs. We then compare these periods with a low fire period when the impact of regional fires is about a factor of 5 smaller. Fire tracers are very elevated in the high fire periods whereas tracers of urban pollution do not change between these periods. Dust is also elevated during the high BB period but this appears to be coincidental due to the drier conditions and not driven by direct dust emission from the fires. The AMS oxygenated organic aerosol (OA) factor (OOA, mostly secondary OA or SOA) does not show an increase during the fire periods or a correlation with fire counts, FLEXPART-predicted FIFs or fire tracers, indicating that it is dominated by urban and/or regional sources and not by the fires near the MCMA. A new 14C aerosol dataset is presented. Both this new and a previously published dataset of 14C analysis suggest a similar BBOA contribution as the AMS and chemical mass balance (CMB), resulting in 13% higher non-fossil carbon during the high vs. low regional fire periods. The new dataset has ~15% more fossil carbon on average than the previously published one, and possible reasons for this discrepancy are discussed. During the low regional fire period, 38% of organic carbon (OC) and 28% total carbon (TC) are from non-fossil sources, suggesting the importance of urban and regional non-fossil carbon sources other than the fires, such as food cooking and regional biogenic SOA. The ambient BBOA/ΔCH3CN ratio is much higher in the afternoon when the wildfires are most intense than during the rest of the day. Also, there are large differences in the contributions of the different OA components to the surface concentrations vs. the integrated column amounts. Both facts may explain some apparent disagreements between BB impacts estimated from afternoon aircraft flights vs. those from 24-h ground measurements. We show that by properly accounting for the non-BB sources of K, all of the BB PM estimates from MILAGRO can be reconciled. Overall, the fires from the region near the MCMA are estimated to contribute 15–23% of the OA and 7–9% of the fine PM at T0 during MILAGRO, and 2–3% of the fine PM as an annual average. The 2006 MCMA emissions inventory contains a substantially lower impact of the forest fire emissions, although a fraction of these emissions occur just outside of the MCMA inventory area.

Abstract. During the Carbonaceous Aerosols and RadiativeEffects Study (CARES), activation fraction of size-resolved aerosol particles and aerosol chemical composition were characterized at the T1 site (∼ 60 km downwind of Sacramento, California) from 10 June to 28 June 2010. The hygroscopicity of cloud condensation nuclei (CCN)-active particles (κ CCN ) with diameter from 100 to 170 nm, derived from the size-resolved activated fraction, varied from 0.10 to 0.21, with an average of 0.15, which was substantially lower than that proposed for continental sites. The low κ CCN value was due to the high organic volume fraction, averaged over 80 % at the T1 site. The derived κ CCN exhibited little diurnal variation, consistent with the relatively constant organic volume fraction observed. At any time, over 90 % of the size-selected particles with diameter between 100 and 171 nm were CCN active, suggesting most particles within this size range were aged background particles. Due to the large organic volume fraction, organic hygroscopicity (κ org ) strongly impacted particle hygroscopicity and therefore calculated CCN concentration. For the vast majority of the cases, an increase of κ org from 0.03 to 0.18, which are within the typical range, doubled the calculated CCN concentration. Organic hygroscopicity was derived from κ CCN and aerosol chemical composition, and its variations with the fraction of total organic mass spectral signal at m/z 44 (f 44 ) and O : C were compared to results from previous studies. Overall, the relationships between κ org and f 44 are quite consistent for organic aerosol (OA) observed during field studies and those formed in a smog chamber. Compared to the relationship between κ org and f 44 , the relationship between κ org and O : C exhibits more significant differences among different studies, suggesting κ org may be better parameterized using f 44 . A least squares fit yielded κ org = 2.10(±0.07) × f 44 − 0.11(±0.01) with a Pearson R 2 value of 0.71. One possible explanation for the stronger correlation between κ org and f 44 is that the m/z 44 signal (mostly contributed by the CO + 2 ion) is more closely related to organic acids, which may dominate the overall κ org due to their relatively high water solubility and hygroscopicity.

We present in situ surface x-ray scattering observations of ordered chloride and bromide monolayers on Au(111) electrodes in aqueous solutions as an example of in situ x-ray diffraction from adsorbates of the lighter elements (Z &50). At a critical surface density both halides form incommensurate, hexagonal-close-packed monolayers that compress uniformly with increasing potential. Our structural results provide quantitative data on the residual adsorbate charge, the two-dimensional melting, and the two-dimensional compressibilities of halide monolayers.The adsorption of halogen gases on metal surfaces and of halide anions on metal electrodes in aqueous solutions has been extensively studied in the past. In a vacuum environment, chemisorbed halogens dissociate and form ordered monolayers. ' These adlayers are often incommensurate (no lateral registry) with the metal substrate and change continuously with increasing halogen coverage, not unlike the two-dimensional phases of physisorbed noble gases. However, the equilibrium properties of halide adlayers are difticult to obtain in vacuum experiments due to the strong chemical halide-metal interaction. In contrast, equilibrium is readily established at the liquidsolid interface where the chemical potential of the adsorbate can be controlled via the electrode potential. With the recent advent of modern in situ techniques the structure of these interfaces can now be determined. Studies by in situ scanning tunneling microscopy (STM) and surface x-ray scattering revealed that the strongly bound iodide ions form close-packed, ordered monolayers on gold and platinum electrodes.Much less is known about the lighter halides. Ordered bromide adlayers were found in only one preliminary STM study and ordered phases of chloride and fluoride anions at a solid-liquid interface have not been observed to our knowledge. Here we report in situ observations of ordered chloride and bromide monolayers on Au(111) by grazing incident angle x-ray diffraction. The evaluated equilibrium properties of halide monolayers will be compared to those of noble gas and metallic adlayers.G'razing incident angle x-ray diffraction measurements of the interface structure of Au(111) electrodes were carried out at beamline X22B at the National Syncrotron Light Source at a wavelength 1=1.54 A. The experiments were performed on Au(111) single crystals sputtered and annealed at 1000 K in vacuum or prepared by Game annealing.The sample was then mounted in an electrochemical cell that was filled with deaerated suprapur 0.1M HC104 solutions containing NaC1 or NaBr in the range 0.001 -1M. Subsequently the cell was deAated leaving an electrolyte film of =10 pm thickness between the Au(111) surface and the 4-pm-thick prolene (Chemplex) x-ray window. During the experiment the cell was surrounded by a N2 atmosphere to keep the electrolyte free of oxygen. The electrode potential was controlled versus a Ag/AgC1 (3M KC1) reference electrode.For further experimental details see Refs. 6 and 7. First we will describe the expe...

Abstract. Aerosol microphysics, chemical composition, and CCN properties were measured on the Department of Energy Gulfstream-1 aircraft during the Marine Stratus/Stratocumulus Experiment (MASE) conducted over the coastal waters between Point Reyes National Seashore and Monterey Bay, California, in July 2005. Aerosols measured during MASE included free tropospheric aerosols, marine boundary layer aerosols, and aerosols with high organic concentration within a thin layer above the cloud. Closure analysis was carried out for all three types of aerosols by comparing the measured CCN concentrations at ∼0.2% supersaturation to those predicted based on size distribution and chemical composition using Köhler theory. The effect of aerosol organic species on predicted CCN concentration was examined using a single hygroscopicity parameterization. For aerosols with organics volume fraction up to 70%, such as the marine boundary layer and free troposphere aerosols, CCN concentration and the corresponding first indirect aerosol effect are insensitive to the properties of organics, and can be accurately predicted with a constant hygroscopicity for all organic species. This simplification can facilitate the prediction of indirect aerosol effects using physically-based parameterizations in large scale models. However, for the aerosols within the thin layers above clouds, organics contributed up to 90% of the total aerosol volume, and an accurate knowledge of the overall organic hygroscopicity is required to accurately predict CCN concentrations. Derivations of organic properties in future closure studies, when aerosols are dominated by organic species, would help constrain the descriptions of organics and aerosol-cloud parameterizations in large scale models.