1,735 research outputs found
An In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study of Electrochemical CO2 Reduction: Selectivity Dependence on Surface C-Bound and O-Bound Reaction Intermediates
The CO_{2} electro-reduction reaction (CORR) is a promising avenue to convert
greenhouse gases into high-value fuels and chemicals, in addition to being an
attractive method for storing intermittent renewable energy. Although
polycrystalline Cu surfaces have long known to be unique in their capabilities
of catalyzing the conversion of CO_{2} to higher-order C1 and C2 fuels, such as
hydrocarbons (CH_{4}, C_{2}H_{4} etc.) and alcohols (CH_{3}OH, C_{2}H_{5}OH),
product selectivity remains a challenge. In this study, we select three metal
catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption
spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy
(APXPS), coupled to density-functional theory (DFT) calculations, to get
insight into the reaction pathway for the CORR. We present a comprehensive
reaction mechanism for the CORR, and show that the preferential reaction
pathway can be rationalized in terms of metal-carbon (M-C) and metal-oxygen
(M-O) affinity. We show that the final products are determined by the
configuration of the initial intermediates, C-bound and O-bound, which can be
obtained from CO_{2} and (H)CO_{3}, respectively. C1 hydrocarbons are produced
via OCH_{3, ad} intermediates obtained from O-bound CO_{3, ad} and require a
catalyst with relatively high affinity for O-bound intermediates. Additionally,
C2 hydrocarbon formation is suggested to result from the C-C coupling between
C-bound CO_{ad} and (H)CO_{ad}, which requires an optimal affinity for the
C-bound species, so that (H)CO_{ad} can be further reduced without poisoning
the catalyst surface. Our findings pave the way towards a design strategy for
CORR catalysts with improved selectivity, based on this
experimental/theoretical reaction mechanisms that have been identified
Coe1 in Beta vulgaris L. Has a Tnp2-Domain DNA Transposase Gene within Putative LTRs and Other Retroelement-Like Features
We describe discovery in Beta vulgaris L. of Coe1, a DNA transposase gene within putative long terminal repeats (LTRs), and other retrotransposon-like features including both a retroviral-like hypothetical gene and an Rvt2-domain reverse transcriptase pseudogene. The central DNA transposase gene encodes, in eight exons, a predicted 160-KDa protein producing BLAST alignments with En/Spm-type transposons. Except for a stop signal, another ORF encodes a Ty1-copia-like reverse transcriptase with amino acid sequence domain YVDDIIL. Outside apparent LTRs, an 8-mer nucleotide sequence motif CACTATAA, near or within inverted repeat sequences, is hypothetical extreme termini. A genome scan of Arabidopsis thaliana found another example of a Tnp2-domain transposase gene within an apparent LTR-retrotransposon on chromosome 4
Genes Encoding Callose Synthase and Phytochrome A Are Adjacent to a MAP3Kα-Like Gene in Beta vulgaris US H20
MAP3Kα, a gene that encodes a key conserved protein kinase,
is responsible for initiating a rapid cascade of cellular events
leading to localized cell death. Hypersensitive response, as it is
termed, enables genetically resistant plants to limit microbial
invasion under the right environmental conditions. Since knowledge
of close physically linked genes is important for genome analysis
and possibly for improving disease resistance, systematic DNA
sequence analysis, gene annotation, and protein BLASTs were
performed to identify and characterize genes in close physical
proximity to a MAP3Kα-like gene in Beta vulgaris L. US H20. On the same 125 Kb BAC, callose synthase (BvCS) and phytochrome A (PhyA) genes were within 50 Kb of MAP3Kα. The close physical linkage of these genes may result from selection for coordinated responses to disease pressure. Bert, a new chromodomain-carrying gypsy-like LTR retrotransposon, resides within an intron of the BvCS gene, where it is transcribed from the opposing strand
‘Ca. Liberibacter asiaticus’ Proteins Orthologous with pSymA-Encoded Proteins of Sinorhizobium meliloti: Hypothetical Roles in Plant Host Interaction
Sinorhizobium meliloti strain 1021, a nitrogen-fixing, root-nodulating bacterial microsymbiont of alfalfa, has a 3.5 Mbp circular chromosome and two megaplasmids including 1.3 Mbp pSymA carrying nonessential ‘accessory’ genes for nitrogen fixation (nif), nodulation and host specificity (nod). A related bacterium, psyllid-vectored ‘Ca. Liberibacter asiaticus,’ is an obligate phytopathogen with a reduced genome that was previously analyzed for genes orthologous to genes on the S. meliloti circular chromosome. In general, proteins encoded by pSymA genes are more similar in sequence alignment to those encoded by S. meliloti chromosomal orthologs than to orthologous proteins encoded by genes carried on the ‘Ca. Liberibacter asiaticus’ genome. Only two ‘Ca. Liberibacter asiaticus’ proteins were identified as having orthologous proteins encoded on pSymA but not also encoded on the chromosome of S. meliloti. These two orthologous gene pairs encode a Na+/K+ antiporter (shared with intracellular pathogens of the family Bartonellacea) and a Co++, Zn++ and Cd++ cation efflux protein that is shared with the phytopathogen Agrobacterium. Another shared protein, a redox-regulated K+ efflux pump may regulate cytoplasmic pH and homeostasis. The pSymA and ‘Ca. Liberibacter asiaticus’ orthologs of the latter protein are more highly similar in amino acid alignment compared with the alignment of the pSymA-encoded protein with its S. meliloti chromosomal homolog. About 182 pSymA encoded proteins have sequence similarity (≤E-10) with ‘Ca. Liberibacter asiaticus’ proteins, often present as multiple orthologs of single ‘Ca. Liberibacter asiaticus’ proteins. These proteins are involved with amino acid uptake, cell surface structure, chaperonins, electron transport, export of bioactive molecules, cellular homeostasis, regulation of gene expression, signal transduction and synthesis of amino acids and metabolic cofactors. The presence of multiple orthologs defies mutational analysis and is consistent with the hypothesis that these proteins may be of particular importance in host/microbe interaction and their duplication likely facilitates their ongoing evolution
Assessing correlations of perovskite catalytic performance with electronic structure descriptors
Electronic structure descriptors are computationally efficient quantities
used to construct qualitative correlations for a variety of properties. In
particular, the oxygen p-band center has been used to guide material discovery
and fundamental understanding of an array of perovskite compounds for use in
catalyzing the oxygen reduction and evolution reactions. However, an assessment
of the effectiveness of the oxygen p-band center at predicting key measures of
perovskite catalytic activity has not been made, and would be highly beneficial
to guide future predictions and codify best practices. Here, we have used
Density Functional Theory at the PBE, PBEsol, PBE+U, SCAN and HSE06 levels to
assess the correlations of numerous measures of catalytic performance for a
series of technologically relevant perovskite oxides, using the bulk oxygen
p-band center as an electronic structure descriptor. We have analyzed
correlations of the calculated oxygen p-band center for all considered
functionals with the experimentally measured X-ray emission spectroscopy oxygen
p-band center and multiple measures of catalytic activity, including high
temperature oxygen reduction surface exchange rates, aqueous oxygen evolution
current densities, and binding energies of oxygen evolution intermediate
species. Our results show that the best correlations for all measures of
catalytic activity considered here are made with PBE-level calculations, with
strong observed linear correlations with the bulk oxygen p-band center (R2 =
0.81-0.87). This study shows that strong linear correlations between numerous
important measures of catalytic activity and the oxygen p-band bulk descriptor
can be obtained under a consistent computational framework, and these
correlations can serve as a guide for future experiments and simulations for
development of perovskite and related oxide catalysts
Resistance to Helium Bubble Formation in Amorphous SiOC/Crystalline Fe Nanocomposite
The management of radiation defects and insoluble He atoms represent key challenges for structural materials in existing fission reactors and advanced reactor systems. To examine how crystalline/amorphous interface, together with the amorphous constituents affects radiation tolerance and He management, we studied helium bubble formation in helium ion implanted amorphous silicon oxycarbide (SiOC) and crystalline Fe composites by transmission electron microscopy (TEM). The SiOC/Fe composites were grown via magnetron sputtering with controlled length scale on a surface oxidized Si (100) substrate. These composites were subjected to 50 keV He+ implantation with ion doses chosen to produce a 5 at% peak He concentration. TEM characterization shows no sign of helium bubbles in SiOC layers nor an indication of secondary phase formation after irradiation. Compared to pure Fe films, helium bubble density in Fe layers of SiOC/Fe composite is less and it decreases as the amorphous/crystalline SiOC/Fe interface density increases. Our findings suggest that the crystalline/amorphous interface can help to mitigate helium defect generated during implantation, and therefore enhance the resistance to helium bubble formation
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