ConspectusBecause ceria (CeO) is a key ingredient in the formulation of many catalysts, its catalytic roles have received a great amount of attention from experiment and theory. Its primary function is to enhance the oxidation activity of catalysts, which is largely governed by the low activation barrier for creating lattice O vacancies. Such an important characteristic of ceria has been exploited in CO oxidation, methane partial oxidation, volatile organic compound oxidation, and the water-gas shift (WGS) reaction and in the context of automotive applications. A great challenge of such heterogeneously catalyzed processes remains the unambiguous identification of active sites.In oxidation reactions, closing the catalytic cycle requires ceria reoxidation by gas-phase oxygen, which includes oxygen adsorption and activation. While the general mechanistic framework of such processes is accepted, only very recently has an atomic-level understanding of oxygen activation on ceria powders been achieved by combined experimental and theoretical studies using multiwavelength Raman spectroscopy and DFT.Recent studies have revealed that the adsorption and activation of gas-phase oxygen on ceria is strongly facet-dependent and involves different superoxide/peroxide species, which can now be unambiguously assigned to ceria surface sites using the combined Raman and DFT approach. Our results demonstrate that, as a result of oxygen dissociation, vacant ceria lattice sites are healed, highlighting the close relationship of surface processes with lattice oxygen dynamics, which is also of technical relevance in the context of oxygen storage-release applications.A recent DFT interpretation of Raman spectra of polycrystalline ceria enables us to take account of all (sub)surface and bulk vibrational features observed in the experimental spectra and has revealed new findings of great relevance for a mechanistic understanding of ceria-based catalysts. These include the identification of surface oxygen (Ce-O) modes and the quantification of subsurface oxygen defects. Combining these theoretical insights with Raman experiments now allows the (sub)surface oxygen dynamics of ceria and noble metal/ceria catalysts to be monitored under the reaction conditions.Applying these findings to Au/ceria catalysts provides univocal evidence for ceria support participation in heterogeneous catalysis. For room-temperature CO oxidation, Raman monitoring the (sub)surface defect dynamics clearly demonstrates the dependence of catalytic activity on the ceria reduction state. Extending the combined experimental/DFT approach to IR spectroscopy allows the elucidation of the nature of the active gold as (pseudo)single Au sites and enables us to develop a detailed mechanistic picture of the catalytic cycle. Temperature-dependent studies highlight the importance of facet-dependent defect formation energies and adsorbate stabilities (e.g., carbonates). While the latter aspects are also evidenced to play a role in the WGS reaction, the facet-dependent catalytic performance shows a correlation with the extent of gold agglomeration. Our findings are fully consistent with a redox mechanism, thus adding a new perspective to the ongoing discussion of the WGS reaction.As outlined above for ceria-based catalysts, closely combining state-of-the-art / spectroscopy and theory constitutes a powerful approach to rational catalyst design by providing essential mechanistic information based on an atomic-level understanding of reactions.
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http://dx.doi.org/10.1021/acs.accounts.1c00226 | DOI Listing |
Biochemistry
January 2025
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States.
Adv Mater
January 2025
School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, China.
Understanding the behavior of high-entropy carbides (HECs) under oxygen-containing environments is of particular importance for their promising applications in structural components, catalysis, and energy-related fields. Herein, the structural evolution of (Ta, Ti, Cr, Nb)C (HEC-1) nanoparticles (NPs) is tracked in situ during the oxidation at the atomic scale by using an open-cell environmental aberration-corrected scanning transmission electron microscope. Three key stages are clearly discerned during the oxidation of HEC-1 NPs at the atomic level below 900 °C: i) increased amorphization of HEC-1 NPs from 300 to 500 °C due to the energetically favorable formation of carbon vacancies and substitution of carbon with oxygen atoms; ii) nucleation and subsequent growth of locally ordered nanocluster intermediates within the generated amorphous oxides from 500 to 800 °C; and iii) final one-step crystallization of non-equimolar MeO and MeO (Me = metallic elements, Ta, Ti, Cr, and Nb) high-entropy oxides above 800 °C, accompanied with the reduction in atomic defects.
View Article and Find Full Text PDFNat Commun
December 2024
Department of Theory and Bio-Systems, Max Planck Institute of Colloids and Interfaces, 14476, Potsdam, Germany.
Neurodegeneration in Huntington's disease (HD) is accompanied by the aggregation of fragments of the mutant huntingtin protein, a biomarker of disease progression. A particular pathogenic role has been attributed to the aggregation-prone huntingtin exon 1 (HTTex1), generated by aberrant splicing or proteolysis, and containing the expanded polyglutamine (polyQ) segment. Unlike amyloid fibrils from Parkinson's and Alzheimer's diseases, the atomic-level structure of HTTex1 fibrils has remained unknown, limiting diagnostic and treatment efforts.
View Article and Find Full Text PDFJ Chem Inf Model
December 2024
Xuzhou College of Industrial Technology, Xuzhou 221140, Jiangsu Province, China.
The β-1,4 galactosylation catalyzed by β-1,4 galactosyltransferases (β4Gal-Ts) is not only closely associated with diverse physiological and pathological processes in humans but also widely applied in the -glycan modification of protein glycoengineering. The loop-closing process of β4Gal-Ts is an essential intermediate step intervening in the binding events of donor substrate (UDP-Gal/Mn) and acceptor substrate during its catalytic cycle, with a significant impact on the galactosylation activities. However, the molecular mechanisms in regulating loop-closing dynamics are not entirely clear.
View Article and Find Full Text PDFJ Phys Chem B
December 2024
Department of Chemistry and Biochemistry, University of Lethbridge, 4401 University Drive West, Lethbridge, Alberta, Canada T1K 3M4.
Despite the remarkable resistance of the nucleic acid phosphodiester backbone to degradation affording genetic stability, the P-O bond must be broken during DNA repair and RNA metabolism, among many other critical cellular processes. Nucleases are powerful enzymes that can enhance the uncatalyzed rate of phosphodiester bond cleavage by up to ∼10-fold. Despite the most well accepted hydrolysis mechanism involving two metals (M to activate a water nucleophile and M to stabilize the leaving group), experimental evidence suggests that some nucleases can use a single metal to facilitate the chemical step, a controversial concept in the literature.
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