Single-atom catalysts are often reported to have catalytic properties that surpass those of nanoparticles, while a direct comparison of sites common and different for both is lacking. Here we show ...that single atoms of Pt-group metals embedded into the surface of Fe3O4 have a greatly enhanced interaction strength with CO2 compared with the Fe3O4 surface. The strong CO2 adsorption on single Rh atoms and corresponding low activation energies lead to 2 orders of magnitude higher conversion rates of CO2 compared to Rh nanoparticles. This high activity of single atoms stems from the partially oxidic state imposed by their coordination to the support. Fe3O4-supported Rh nanoparticles follow the behavior of single atoms for CO2 interaction and reduction, which is attributed to the dominating role of partially oxidic sites at the Fe3O4–Rh interface. Thus, we show a likely common catalytic chemistry for two kinds of materials thought to be different, and we show that single atoms of Pt-group metals on Fe3O4 are especially successful materials for catalyzed reactions that depend primarily upon sites with the metal–O–Fe environment.
An asymmetric sulfa-Michael addition of alkyl thiols to enone diesters is reported. The reaction is catalyzed by a bifunctional triaryliminophosphorane-thiourea organocatalyst and provides a range of ...α-sulfaketones in high yields and enantioselectivities. Leveraging the gem-diester functional handle via a subsequent diastereotopic group discrimination generates functionalized lactones with three contiguous stereocenters.
Promoters are ubiquitous in industrial heterogeneous catalysts. The wider roles of promoters in accelerating catalysis and/or controlling selectivity are, however, not well understood. A model system ...has been developed where a heterobimetallic active site comprising an active metal (Rh) and a promoter ion (Ga) is preassembled and delivered onto a metal–organic framework (MOF) support, NU-1000. The Rh–Ga sites in NU-1000 selectively catalyze the hydrogenation of acyclic alkynes to E-alkenes. The overall stereoselectivity is complementary to the well-known Lindlar’s catalyst, which generates Z-alkenes. The role of the Ga in promoting this unusual selectivity is evidenced by the lack of semihydrogenation selectivity when Ga is absent and only Rh is present in the active site.
Polar bears (Ursus maritimus) are among those species most susceptible to the rapidly changing arctic climate, and their survival is of global concern. Despite this, little is known about polar bear ...species history. Future conservation strategies would significantly benefit from an understanding of basic evolutionary information, such as the timing and conditions of their initial divergence from brown bears (U. arctos) or their response to previous environmental change.
We used a spatially explicit phylogeographic model to estimate the dynamics of 242 brown bear and polar bear matrilines sampled throughout the last 120,000 years and across their present and past geographic ranges. Our results show that the present distribution of these matrilines was shaped by a combination of regional stability and rapid, long-distance dispersal from ice-age refugia. In addition, hybridization between polar bears and brown bears may have occurred multiple times throughout the Late Pleistocene.
The reconstructed matrilineal history of brown and polar bears has two striking features. First, it is punctuated by dramatic and discrete climate-driven dispersal events. Second, opportunistic mating between these two species as their ranges overlapped has left a strong genetic imprint. In particular, a likely genetic exchange with extinct Irish brown bears forms the origin of the modern polar bear matriline. This suggests that interspecific hybridization not only may be more common than previously considered but may be a mechanism by which species deal with marginal habitats during periods of environmental deterioration.
► Polar bears and brown bears have been hybridizing throughout the last 100,000 years ► The current polar bear matriline descends from brown bears that lived near Ireland ► This matriline most likely originated prior to or during the last ice age ► The dynamics of bear matrilines have been driven by largely by climate change
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•Mild steaming leads to partially framework-bound aluminum oxide species.•These aluminum oxide clusters are in the vicinity of Brønsted acid sites.•Site proximity leads to an increase ...of activation entropies for alkane cracking.•n-Butane cracking and D2/OH exchange reaction rates are also enhanced.
Short exposure of HZSM-5 zeolites to water vapor at high temperatures (mild steaming) enhances the rates of alkane cracking and dehydrogenation as well as the rates of exchange between H2 and D2. Solid state 27Al MAS NMR results show that mild steaming leads to metastable aluminum oxide species, hypothesized to be partially framework-bound. Combining double quantum magic-angle spinning nuclear magnetic resonance experiments and kinetic analysis of cracking reactions allows us to conclude that the presence of these aluminum oxide clusters in the vicinity of Brønsted acid sites leads to an increase in the activation entropies during alkane cracking, induced by increasing steric constraints. Prolonged steaming results, in contrast, in the extraction of framework Al, which subsequently forms aggregated extra-framework Al oxide species partly blocking Brønsted acid sites and partly deposited at the outer surface of the crystallites.
Abstract
Key chemical transformations require metal and redox sites in proximity at interfaces; however, in traditional oxide-supported materials, this requirement is met only at the perimeters of ...metal nanoparticles. We report that galvanic replacement can produce inverse FeO
x
/metal nanostructures in which the concentration of oxide species adjoining metal domains is maximal. The synthesis involves reductive deposition of rhodium or platinum and oxidation of Fe
2+
from magnetite (Fe
3
O
4
). We discovered a parallel dissolution and adsorption of Fe
2+
onto the metal, yielding inverse FeO
x
-coated metal nanoparticles. This nanostructure exhibits the intrinsic activity in selective CO
2
reduction that simple metal nanoparticles have only at interfaces with the support. By enabling a simple way to control the surface functionality of metal particles, our approach is not only scalable but also enables a versatile palette for catalyst design.
The role of the specific physicochemical properties of ZrO2 phases on Ni/ZrO2 has been explored with respect to the reduction of stearic acid. Conversion on pure m‐ZrO2 is 1.3 times more active than ...on t‐ZrO2, whereas Ni/m‐ZrO2 is three times more active than Ni/t‐ZrO2. Although the hydrodeoxygenation of stearic acid can be catalyzed solely by Ni, the synergistic interaction between Ni and the ZrO2 support causes the variations in the reaction rates. Adsorption of the carboxylic acid group on an oxygen vacancy of ZrO2 and the ion of the α‐hydrogen atom with the elimination of the oxygen atom to produce a ketene is the key to enhance the overall rate. The hydrogenated intermediate 1‐octadecanol is in turn decarbonylated to heptadecane with identical rates on all catalysts. Decarbonylation of 1‐octadecanol is concluded to be limited by the competitive adsorption of reactants and intermediate. The substantially higher adsorption of propionic acid demonstrated by IR spectroscopy and the higher reactivity to O2 exchange reactions with the more active catalyst indicate that the higher concentration of active oxygen defects on m‐ZrO2 compared to t‐ZrO2 causes the higher activity of Ni/m‐ZrO2.
Finding the best: Three different Ni/ZrO2 catalysts have been tested with regard to their efficiency in the hydrogenation reaction of stearic acid. The Ni/m‐ZrO2 catalyst was found to be the best one. A higher concentration of active oxygen defects present in this combination was identified to be responsible for the superiority of this catalyst over the other systems (see scheme).
The location and stability of Brønsted acid sites catalytically active in zeolites during aqueous phase dehydration of alcohols were studied on the example of cyclohexanol. The catalytically active ...hydronium ions originate from Brønsted acid sites (BAS) of the zeolite that are formed by framework tetrahedral Si atom substitution by Al. Al K-edge extended X-ray absorption fine structure (EXAFS) and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopies in combination with density functional theory (DFT) calculations are used to determine the distribution of tetrahedral Al sites (Al T-sites) both qualitatively and quantitatively for both parent and HBEA catalysts aged in water prior to catalytic testing. The aging procedure leads to partial degradation of the zeolite framework evidenced from the decrease of material crystallinity (XRD) as well as sorption capacity (BET). With the exception of one commercial zeolite sample, which had the highest concentration of framework silanol-defects, there is no evidence of Al coordination modification after aging in water. The catalyst weight-normalized dehydration rate correlated best with the sum of strong and weak Brønsted acidic protons both able to generate the hydrated hydronium ions. All hydronium ions were equally active for the acid-catalyzed reactions in water. Zeolite aging in hot water prior to catalysis decreased the weight normalized dehydration reaction rate compared to that of the parent HBEA, which is attributed to the reduced concentration of accessible Brønsted acid sites. Sites are hypothesized to be blocked due to reprecipitation of silica dissolved during framework hydrolysis in the aging procedure.
•We compare major global transportation models with considerable technology details to 2050.•There are significant differences in the base-year data and key parameters for future projections.•Main ...GHG mitigation differs for economics IA models vs transport-only expert models.•Significant EV gaps between policy targets and model trajectories to 2°C target.•We offer suggestions on access to data, model improvements, and future comparisons.
This paper focuses on comparing the frameworks and projections from four global transportation models with considerable technology details. We analyze and compare the modeling frameworks, underlying data, assumptions, intermediate parameters, and projections to identify the sources of divergence or consistency, as well as key knowledge gaps. We find that there are significant differences in the base-year data and key parameters for future projections, especially for developing countries. These include passenger and freight activity, mode shares, vehicle ownership rates, and energy consumption by mode, particularly for shipping, aviation and trucking. This may be due in part to a lack of previous efforts to do such consistency-checking and “bench-marking.” We find that the four models differ in terms of the relative roles of various mitigation strategies to achieve a 2°C/450ppm target: the economics-based integrated assessment models favor the use of low carbon fuels as the primary mitigation option followed by efficiency improvements, whereas transport-only and expert-based models favor efficiency improvements of vehicles followed by mode shifts. We offer recommendations for future modeling improvements focusing on (1) reducing data gaps; (2) translating the findings from this study into relevant policy implications such as gaps of current policy goals, additional policy targets needed, regional vs. global reductions; (3) modeling strata of demographic groups to improve understanding of vehicle ownership levels, travel behavior, and urban vs. rural considerations; and (4) conducting coordinated efforts in aligning historical data, and comparing input assumptions and results of policy analysis and modeling insights.
Pseudomonas aeruginosa encodes eight members of the Rid protein superfamily. PA5339, a member of the RidA subfamily, is required for full growth and motility of P. aeruginosa. Our understanding of ...RidA integration into the metabolic network of P. aeruginosa is at an early stage, with analyses largely guided by the well-established RidA paradigm in Salmonella enterica. A P. aeruginosa strain lacking RidA has a growth and motility defect in a minimal glucose medium, both of which are exacerbated by exogenous serine. All described
mutant phenotypes are rescued by supplementation with isoleucine, indicating the primary generator of the reactive metabolite 2-aminoacrylate (2AA) in
mutants is a threonine/serine dehydratase. However, the critical (i.e., phenotype determining) targets of 2AA leading to growth and motility defects in P. aeruginosa remained undefined. This study was initiated to probe the effects of 2AA stress on the metabolic network of P. aeruginosa by defining the target(s) of 2AA that contribute to physiological defects of a
mutant. Suppressor mutations that restored growth to a P. aeruginosa
mutant were isolated, including an allele of
(encoding cysteine desulfurase). Damage to IscS was identified as a significant cause of growth defects of P. aeruginosa during enamine stress. A suppressing allele encoded an IscS variant that was less sensitive to damage by 2AA, resulting in a novel mechanism of phenotypic suppression of a
mutant.
2-aminoacrylate (2AA) is a reactive metabolite formed as an intermediate in various enzymatic reactions. In the absence of RidA, this metabolite can persist
where it attacks and inactivates specific PLP-dependent enzymes, causing metabolic defects and organism-specific phenotypes. This work identifies the cysteine desulfurase IscS as the critical target of 2AA in Pseudomonas aeruginosa. A single substitution in IscS decreased sensitivity to 2AA and suppressed growth phenotypes of a
mutant. Here, we provide the first report of suppression of a
mutant phenotype by altering the sensitivity of a target enzyme to 2AA.
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