Abstract California is expected to experience great spatial/temporal variations evaporation. These variations arise from strong north‐south, east‐west gradients in rainfall and vegetation, strong interannual variability in rainfall (±30%) and strong seasonal variability in the supply and demand for moisture. We used the Breathing Earth System Simulator to evaluate the rates and sums of evaporation across California, over the 2001–2017 period. Breathing Earth System Simulator is a bottom‐up, biophysical model that couples subroutines that calculate the surface energy balance, photosynthesis, and stomatal conductance. The model is forced with high‐resolution remote sensing data (1 km).The questions we address are as follows: How much water is evaporated across the natural and managed ecosystems of California? How much does evaporation vary during the booms and busts in annual rainfall? and Is evaporation increasing with time due to a warming climate? Mean annual evaporation, averaged over the 2001–2017 period, was relatively steady (393 ± 21 mm/year) given the high interannual variation in precipitation (519 ± 140 mm/year). No significant trend in evaporation at the statewide level was detected over this time period, despite a background of a warming climate. Irrigated agricultural crops and orchards, at 1‐km scale, use less water than inferred estimates for individual fields. This leaves the potential for sharing water, a scarce resource, more equitably among competing stakeholders, for example, farms, fish, people, and ecosystems.
The reaction pathways for the dehydrogenation of ethane, propane, and butane, over Pt are analyzed using density functional theory (DFT). Pt nanoparticles are represented by a tetrahedral Pt4 cluster. The objectives of this work were to establish which step is rate limiting and which one controls the selectivity for forming alkenes as opposed to causing further dehydrogenation of adsorbed alkenes to produce precursors responsible for catalyst deactivation due to coking. Further objectives of this work are to identify the role of adsorbed hydrogen, derived from H2 fed together with the alkane, on the reaction pathway, and the role of replacing one of the four Pt atoms by a Sn atom. A comparison of Gibbs free energies shows that in all cases the rate-determining step is cleavage of a C-H bond upon alkane adsorption. The selectivity to alkene formation versus precursors to coking is dictated by the relative magnitudes of the activation energies for alkene desorption and dehydrogenation of the adsorbed alkene. The presence of an adsorbed H atom on the cluster facilitates alkene desorption relative to dehydrogenation of the adsorbed alkene. Substitution of a Sn atom in the cluster to produce a Pt3Sn cluster leads to a downward shift of the potential energy surface for the reaction and causes an increase of the activity of the catalyst as suggested by recent experiments due to the lower net activation barrier for the rate limiting step. However, the introduction of Sn does not alter the relative activation barriers for gas-phase alkene formation versus loss of hydrogen from the adsorbed alkene, the process leading to the formation of coke precursors.
Abstract Rising atmospheric CO 2 concentration ([CO 2 ]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO 2 fertilization effect. The CO 2 fertilization effect is among the most important processes for predicting the terrestrial carbon budget and future climate, yet it has been elusive to quantify. For evaluating the CO 2 fertilization effect on land photosynthesis and transpiration, we developed a technique that isolated this effect from other confounding effects, such as changes in climate, using a noisy time series of observed land-atmosphere CO 2 and water vapor exchange. Here, we evaluate the magnitude of this effect from 2000 to 2014 globally based on constraint optimization of gross primary productivity (GPP) and evapotranspiration in a canopy photosynthesis model over 104 global eddy-covariance stations. We found a consistent increase of GPP (0.138 ± 0.007% ppm −1 ; percentile per rising ppm of [CO 2 ]) and a concomitant decrease in transpiration (−0.073% ± 0.006% ppm −1 ) due to rising [CO 2 ]. Enhanced GPP from CO 2 fertilization after the baseline year 2000 is, on average, 1.2% of global GPP, 12.4 g C m −2 yr −1 or 1.8 Pg C yr −1 at the years from 2001 to 2014. Our result demonstrates that the current increase in [CO 2 ] could potentially explain the recent land CO 2 sink at the global scale.
Reducible transition metal oxides (RTMOs) comprise an important class of catalytic materials that are used for the selective oxidation and electro- and photochemical splitting of water, and as supports for metal nanoparticles. It is, therefore, highly desirable to model the properties of these materials accurately using density functional theory (DFT) in order to understand how oxide structure and performance are related and to guide the search for materials exhibiting superior performance. Unfortunately, accurate description of the structural and electronic properties of RTMOs using DFT has proven particularly challenging. The M06-L density functional, which has been shown to be broadly accurate for calculations of gas phase clusters, has recently become available to researchers carrying out calculations in the solid state, but its performance in determining the properties RTMOs has been little investigated. The aim of this work was to assess the performance of the M06-L functional for describing the structural and electronic properties of a family of RTMOs: MoO<sub>2</sub>, MoO<sub>3</sub>, and Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>. Lattice constants, band gaps, and densities of states calculated using the M06-L functional are compared to those obtained from DFT+U. We have also used the M06-L functional to determine the reaction barrier for propene activation over Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>, the rate-limiting step in the oxidation of propene to acrolein. We find that while DFT calculations carried out with the M06-L functional are roughly five times more expensive computationally than those performed with DFT+U, the results obtained using the M06-L functional provide sensible results for all properties investigated, while avoiding the necessary trade-off between accurate electronic structure and accurate thermochemistry that occurs in DFT+U.
The objectives of this project have been twofold: (1) to develop a theoretical model for designing bubble-column reactors and assessing their performance and (2) to characterize the performance of a laboratory-scale bubble-column reactor. The theoretical part of the project has focused the effects of gas-liquid mass transfer; axial dispersion of gas, liquid, and catalyst; ad the interplay between the kinetics of Fischer-Tropsch synthesis and the water-gas-shift reaction. The experimental portion of the project has focused the effects of reaction conditions on product distribution and, in particular, the conditions under which gas-liquid mass transfer becomes important. Any independent set of experiments was undertaken to investigate the chemistry of carbonaceous deposits formed during Fischer-Tropsch synthesis. 8 refs.
Abstract A highly efficient solid‐base organocatalyst for the gas‐phase aldol self‐condensation of n ‐butanal to 2‐ethylhexenal was developed by grafting site‐isolated amines on tailored silica surfaces. The catalytic activity depends largely on the nature of amine species, the surface concentration of amine and silanol groups, and the spatial separation between the silanol and amine groups. In situ FTIR measurements demonstrated that the formation of nucleophilic enamines leads to the enhanced catalytic activity of secondary amine catalysts, whereas the formation of imines (stable up to 473 K) leads to the low activity observed for silica‐supported primary amines. Blocking the silanol groups on the silica support by silylation or cofeeding water into the reaction stream drastically decreased the reaction rates, demonstrating that weaker acidic silanol groups participate cooperatively with the amine groups to catalyze the condensation reaction. This work demonstrates that the spatial separation of the weakly acidic silanols and amines can be tuned by the controlled dehydration of the supporting silica and by varying the linker length of the amine organosilane precursor used to graft the amine to the support surface. A mechanism for aldol condensation was proposed and then analyzed by DFT calculations. DFT analysis of the reaction pathway suggested that the rate‐limiting step in aldol condensation is carboncarbon bond formation, which is consistent with the observed kinetics. The calculated apparent activation barrier agrees reasonably with that measured experimentally.
These datasets are supplementary to the paper "<strong>Representativeness of Eddy-Covariance Flux Footprints for Areas Surrounding AmeriFlux Sites</strong>" by Chu et al. Dataset S1. Summary of site-specific footprint metrics filename: All_site_fpt_summary.csv readme: All_site_fpt_summary-README.csv Dataset S2. All monthly footprint climatology weight maps filename: monthly_footprint_climatology_weight_map.zip the zip folder contains individual files of all monthly footprint weight maps filename: <Site-ID>_<Year>_<Month>_<DAY/NIGHT>_fpt_weight.tif readme: README.txt Dataset S3. All site-year footprint climatology overlapped with true-color satellite images. filename: site-year_footprint_climatology_realcolor_map.zip the zip folder contains individual files of footprint climatologies from all site-years filename: <Site-ID>_<Year>_<Spatial_Extent>_shrink_footprint_climatology.png readme: README.txt Dataset S4. Site-specific results and representativeness index based on the land cover type analysis. filename: All_site_land_cover_dominant_summary2.csv readme:All_site_land_cover_dominant_summary2-README.csv Dataset S5. Site-specific results and representativeness index based on the EVI analysis. filename: All_site_Landsat_EVI_fpt_comparison2.csv readme: All_site_Landsat_EVI_fpt_comparison2-README.csv Dataset S6. All available site-month EVI and time-explicit representativeness. filename: All_site_Landsat_EVI_all_cutout2.csv readme: All_site_Landsat_EVI_all_cutout2-README.csv
This is the AmeriFlux version of the carbon flux data for the site US-Snf Sherman Barn. Site Description - The Sherman Barn is pasture site on Sherman Island, that is managed by the California Department of Water Resources (DWR). A flux tower equipped to analyze energy, H2O, CO2, and CH4 fluxes was installed on June, 2018
Read moreAbstract The activity of Rh supported on SiO 2 , Al 2 O 3 , MgO, La 2 O 3 , and TiO 2 for NO reduction by CO and H 2 has been investigated in regard to automotive emission.
Read moreAbstract A series of alkali metal or alkaline earth‐exchanged NiX (X=faujasite zeolite, Si/Al=1.2) zeolites containing approximately 0.6 wt % Ni as Ni 2+ cations were examined as catalysts for propene oligomerization at 453 K and 5 bar (500 kPa). In the presence of propene, the activity of alkali metal‐exchanged zeolites (NiLiX, NiNaX, and NiKX) increased with time on stream, reached a maximum, then decreased, and finally reached steady state. In contrast, the activity of alkaline earth‐exchanged zeolites (NiMgX, NiCaX, and NiSrX) was high initially and then decreased until steady‐state activity was achieved. The primary product formed in all cases was hexene (90 %), and nonene was the only other product observed (10 %). Both the rate of propene dimerization and the ratio of branched to linear hexene isomers were determined to depend on the identity of the charge‐compensating cation, with the dimerization rate and degree of dimer branching increasing with increasing free volume in the zeolite supercages. The apparent activation energy for trimer formation was greater than that for dimer formation, which suggests that steric constraints imposed by the zeolite may inhibit the growth of larger oligomers.
Read moreResonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.
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