Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce(3+) species in a ceria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce(3+) formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce(3+) was present as spectators. This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.
Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO−] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.
Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials,w hich play an important role in catalysts lowering automotive emissions,a se lectrode materials for fuel cells,and as antioxidants in biomedicine.W e demonstrated that active Ce 3+ species in ac eria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce 3+ formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is akinetically relevant step in CO oxidation, whereas afraction of Ce 3+ was present as spectators.This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redoxa nd nonredoxcatalytic mechanisms.Oxygen-storage materials play an important role in automotive three-way catalysis, [1] carbon monoxide oxidation, and water-gas shift reaction, [2,3] electrode materials for fuel cells, [4,5] antioxidants in biomedicine, [6,7] solar thermochemical and photocatalytic water and carbon dioxide splitting. [8,9] However,t he nature of active species responsible for these outstanding properties typically remains unknown. This is mainly due to the low concentration of active species and the necessity to distinguish them from inactive spectators,w hich requires direct spectroscopic identification of intermediates and quantitative comparison of their kinetic behavior to the global reaction rate. [10,11] Theb est catalysts for low-temperature CO oxidation, which are important for lowering automotive emissions,c ontain metal nanoparticles such as platinum, gold, and palladium supported on or promoted by metal oxides that demonstrate reducibility and oxygenstorage capacity (OSC), such as ceria, titania, and iron oxides. [12][13][14][15][16][17] Catalytic oxidation on such catalysts is simple only at first glance.I to ften remains unclear how oxygen molecules are activated, whether lattice oxygen is directly involved in the catalytic cycle, and if the catalytic mechanism is of redox type.Atransient infrared spectroscopy study combined with density functional theory (DFT) calculations showed that during low-temperature CO oxidation on titaniasupported gold nanoparticles oxygen activation takes place at the metal-support interface,w hereas adsorbed CO is delivered by neighboring titania. [12] Moreover,r ecent kinetic and infrared spectroscopy experiments combined with DFT calculations demonstrated that oxygen from the titania support is not directly involved in the rate-determining step of CO oxidation on the Au/TiO 2 catalyst at room temperature.Instead, weakly adsorbed water is involved in the ratedetermining step. [16] These results raise questions whether this type of associative rather than redox mechanism of lowtemperature CO oxidation can al...
Nitryl chloride (ClNO2), a precursor to highly reactive chlorine radicals and a reservoir for nitrogen dioxide (NO2), is formed from the reaction of chloride with N2O5, which has a longer atmospheric lifetime during the winter. Previous field observations, modeling, and laboratory ice flow tube results led to the hypothesis that saline snow is a source of ClNO2 following the deposition of dinitrogen pentoxide (N2O5). Due to the widespread use of road salt (primarily halite) and its deposition to the snowpack, the saline snowpack in Kalamazoo, Michigan, was investigated for the potential for direct ClNO2 production following N2O5 deposition. Vertical gas profile and snow chamber experiments were conducted during January–February 2018 with chemical ionization mass spectrometry measurements of ClNO2 and N2O5. The vertical gas profile measurements showed N2O5 and ClNO2 deposition over both bare and snow-covered ground. However, positive (upward) ClNO2 fluxes were only observed over the snow-covered ground, showing that the saline snowpack can serve as a source of ClNO2. A fraction of the ClNO2 profiles over the snow-covered ground did not exhibit gradients, indicative of a balance between ClNO2 production and loss, including through hydrolysis. Exposure of local snow to synthesized N2O5 during chamber experiments resulted in ClNO2 production that depended on the snowpack physical structure. Together, these results demonstrate a saline snowpack source of ClNO2, with expected relevance to both wintertime inland and coastal regions with snow.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.