More Like this

1. Differential responses of C 3 and C 4 grasses to shrub effects in a sub‐humid grassland of South America

   https://doi.org/10.1111/jvs.12715  

   Fernández, Gastón ; Altesor, Alice ; Pugnaire, ed., Francisco ( March 2019 , Journal of Vegetation Science)

   Plant–plant interactions are key processes that strongly affect the survival, growth and reproduction of individuals in plant communities. In grasslands, the micro‐environment generated under the canopy of shrubs could differentially affect co‐occurring species with different abiotic requirements. In a C₃/C₄grassland with scattered shrubs, we asked the following questions: (a) Does the aerial effect, the below‐ground effect, and the net effect of shrubs affect the vegetative and reproductive biomass, the number of tillers, the biomass allocation, and the leaf elongation rate of grasses? and (b) Do these effects differ between C₃and C₄grasses?

   Temperate sub‐humid grassland of Uruguay.

   We planted one C₃and two C₄grasses under a shrub canopy and in adjacent open sites. Half of the grasses were planted with a fabric bag to reduce root competition with the shrub. We measured leaf elongation rate, the number of tillers produced and the biomass of the grasses in every treatment. We also measured photosynthetic photon flux density (PPFD), air temperature and wind speed under shrub canopies and in adjacent open sites.

   Root biomass, aerial biomass and reproductive biomass, the number of tillers and the leaf elongation rate of the C₄grasses were negatively affected by the reduction in radiation and probably by below‐ground competition with the shrub. On the other hand, the leaf elongation rate of the C₃grasses was positively affected by the shrub canopy.PPFD, air temperature and wind speed were lower under shrubs than in adjacent open sites.

   Our results show the interplay between plant interactions and photosynthetic metabolism on the vegetative and reproductive performance of grasses. The micro‐environmental conditions generated below shrub canopies create a more appropriate site for the growth of C₃than for C₄grasses. These results show that shrubs may differentially affect co‐occurring species with different abiotic requirements.

    

   more » « less
2. Resolving the Carbon‐Climate Feedback Potential of Wetland CO 2 and CH 4 Fluxes in Alaska

   https://doi.org/10.1029/2022GB007524  

   Ma, Shuang ; Bloom, A. Anthony ; Watts, Jennifer D. ; Quetin, Gregory R. ; Donatella, Zona ; Euskirchen, Eugénie S. ; Norton, Alexander J. ; Yin, Yi ; Levine, Paul A. ; Braghiere, Renato K. ; et al ( September 2023 , Global Biogeochemical Cycles)

   Boreal‐Arctic regions are key stores of organic carbon (C) and play a major role in the greenhouse gas balance of high‐latitude ecosystems. The carbon‐climate (C‐climate) feedback potential of northern high‐latitude ecosystems remains poorly understood due to uncertainty in temperature and precipitation controls on carbon dioxide (CO₂) uptake and the decomposition of soil C into CO₂and methane (CH₄) fluxes. While CH₄fluxes account for a smaller component of the C balance, the climatic impact of CH₄outweighs CO₂(28–34 times larger global warming potential on a 100‐year scale), highlighting the need to jointly resolve the climatic sensitivities of both CO₂and CH₄. Here, we jointly constrain a terrestrial biosphere model with in situ CO₂and CH₄flux observations at seven eddy covariance sites using a data‐model integration approach to resolve the integrated environmental controls on land‐atmosphere CO₂and CH₄exchanges in Alaska. Based on the combined CO₂and CH₄flux responses to climate variables, we find that 1970‐present climate trends will induce positive C‐climate feedback at all tundra sites, and negative C‐climate feedback at the boreal and shrub fen sites. The positive C‐climate feedback at the tundra sites is predominantly driven by increased CH₄emissions while the negative C‐climate feedback at the boreal site is predominantly driven by increased CO₂uptake (80% from decreased heterotrophic respiration, and 20% from increased photosynthesis). Our study demonstrates the need for joint observational constraints on CO₂and CH₄biogeochemical processes—and their associated climatic sensitivities—for resolving the sign and magnitude of high‐latitude ecosystem C‐climate feedback in the coming decades.

    

   more » « less
3. DFT Analysis of Methane C−H Activation and Over‐Oxidation by [Cu 2 O] 2+ and [Cu 2 O 2 ] 2+ Sites in Zeolite Mordenite: Intra‐ versus Inter‐site Over‐Oxidation

   https://doi.org/10.1002/cphc.202100580  

   Panthi, Dipak ; Adeyiga, Olajumoke ; Odoh, Samuel O. ( October 2021 , ChemPhysChem)

   Methane over‐oxidation by copper‐exchanged zeolites prevents realization of high‐yield catalytic conversion. However, there has been little description of the mechanism for methane over‐oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu₃O₃]²⁺active sites can over‐oxidize methane. However, the role of [Cu₃O₃]²⁺sites in methane‐to‐methanol conversion remains under debate. Here, we examine methane over‐oxidation by dicopper [Cu₂O]²⁺and [Cu₂O₂]²⁺sites using DFT in zeolite mordenite (MOR). For [Cu₂O₂]²⁺, we considered the μ‐(η²:η²) peroxo‐, and bis(μ‐oxo) motifs. These sites were considered in the eight‐membered (8MR) ring of MOR. μ‐(η²:η²) peroxo sites are unstable relative to the bis(μ‐oxo) motif with a small interconversion barrier. Unlike [Cu₂O]²⁺which is active for methane C−H activation, [Cu₂O₂]²⁺has a very large methane C−H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over‐oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH₃group, followed by OH and can proceed near 200 °C. Thus, for [Cu₂O]²⁺and [Cu₂O₂]²⁺species, over‐oxidation is an inter‐site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra‐site process for [Cu₃O₃]²⁺sites and the role of Brønsted acid sites.

    

   more » « less
4. “Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

   https://doi.org/10.1073/pnas.2012666118  

   Liu, Shanfu ; Udyavara, Sagar ; Zhang, Chi ; Peter, Matthias ; Lohr, Tracy L. ; Dravid, Vinayak P. ; Neurock, Matthew ; Marks, Tobin J. ( June 2021 , Proceedings of the National Academy of Sciences)

   The oxidative coupling of methane to ethylene using gaseous disulfur (2CH₄+ S₂→ C₂H₄+ 2H₂S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH₄to C₂H₄over an Fe₃O₄-derived FeS₂catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O₂(2CH₄+ O₂→ C₂H₄+ 2H₂O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH₂coupling over dimeric S–S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS₂by-product forms predominantly via CH₄oxidation, rather than from C₂products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS₂surface. The experimental rates for methane conversion are first order in both CH₄and S₂, consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD₄/CH₄kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH₄oxidative coupling with O₂. The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

    

   more » « less
5. A MOF‐based Ultra‐Strong Acetylene Nano‐trap for Highly Efficient C 2 H 2 /CO 2 Separation

   https://doi.org/10.1002/ange.202016225  

   Niu, Zheng ; Cui, Xili ; Pham, Tony ; Verma, Gaurav ; Lan, Pui Ching ; Shan, Chuan ; Xing, Huabin ; Forrest, Katherine A. ; Suepaul, Shanelle ; Space, Brian ; et al ( February 2021 , Angewandte Chemie)

   Porous materials with open metal sites have been investigated to separate various gas mixtures. However, open metal sites show the limitation in the separation of some challenging gas mixtures, such as C₂H₂/CO₂. Herein, we propose a new type of ultra‐strong C₂H₂nano‐trap based on multiple binding interactions to efficiently capture C₂H₂molecules and separate C₂H₂/CO₂mixture. The ultra‐strong acetylene nano‐trap shows a benchmarkQ_stof 79.1 kJ mol⁻¹for C₂H₂, a record high pure C₂H₂uptake of 2.54 mmol g⁻¹at 1×10⁻² bar, and the highest C₂H₂/CO₂selectivity (53.6), making it as a new benchmark material for the capture of C₂H₂and the separation of C₂H₂/CO₂. The locations of C₂H₂molecules within the MOF‐based nanotrap have been visualized by the in situ single‐crystal X‐ray diffraction studies, which also identify the multiple binding sites accountable for the strong interactions with C₂H₂.

    

   more » « less