10 Search Results

Acetic acid decomposition on Pt (111) in the presence of co-adsorbed water is a good model system for oxygenate decomposition on Pt (111) in aqueous phase, with application in hydrogen production and biomass conversion. Here, in this study, we present a density functional theory (DFT) theory calculation of how co-adsorbed water affects different bond cleavages of acetic acid decomposition on Pt (111). The presence of co-adsorbed water generally enhances O$$-$$H bond cleavage while inhibiting OC$$-$$O and OC$$-$$OH bond cleavage. The influence of co-adsorbed water on C$$-$$H bond cleavage varies the most and depends on the nature of the transition statemore » and how co-adsorbed water stabilizes the initial and final state. Although these trends are useful as general guidance, they are not sufficient to predict the effect on a complex reaction network such as acetic acid decomposition on Pt (111) which has several parallel reaction paths with similar energies. In the absence of co-adsorbed water, the two lowest energy pathways are decarboxylation (DCX) and decarbonylation (DCN) pathways through a common CH₂COO intermediate, in which the DCX pathway (C$$-$$H bond cleavage of CH₂COO) is more favorable than DCN pathway (OC$$-$$O bond cleavage of CH₂COO). In the presence of co-adsorbed water, the energy difference between Csingle bondH bond cleavage and OC$$-$$O bond cleavage of CH₂COO increases, suggesting an increase in the favorability of acetic acid decarboxylation (formation of carbon dioxide) over acetic acid decarbonylation (formation of carbon monoxide) on Pt (111).« less

Understanding the role of surface structure and hydroxylation in catalytic reactions on metal oxide surfaces is important for developing a mechanistic insight into the complex interface processes. Here, we investigate the reactivity of formic acid on reconstructed Fe₃O₄(001) using a combination of X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, temperature-programmed reaction spectroscopy, low energy electron diffraction, and electronic structure calculations. We find that formic acid initially dissociates at low temperatures (< 80 K) into bidentate formate and a hydroxyl up to an initial dosed coverage of two HCOOH per Fe₃O₄(001) unit cell. At higher temperatures (> 450 K), formate largelymore » decomposes along the dehydration pathway, producing CO and H₂O, with dehydrogenation to CO₂ being a minority side reaction. As a first step, water formation leads to surface oxygen extraction via the Mars-van Krevelen mechanism. Computational studies reveal formate embedded in oxygen vacancies as a key intermediate in the CO formation mechanism. CO formation proceeds via two reaction pathways with desorption that peaks at 530 K on the hydroxyl-rich surface and 560 K on the hydroxyl-deficient surface. Atomic hydrogen coadsorption experiments and ab initio calculations reveal that the presence of surface hydroxyls reduces the CO formation barrier. Furthermore, these results highlight the complex interactions between substrate and intermediate species occurring during reactions on metal oxide surfaces.« less

The stability and activity of supported single-atom catalysts (SACs) represent critical yet opposing factors limiting our ability to explore and exploit their unique properties. Here, this study demonstrates the operation of a switchable catalyst that is activated in the presence of surface intermediates and reverts back to a stable but inactive form when the reaction is completed. We employ atomically defined Rh-Fe₃O₄(001) catalysts to demonstrate how structurally stable Rh, bound in surface octahedral Fe sites, gets destabilized to form highly active Rh adatoms and small clusters. Conversion of formic acid, leading initially to surface formate and hydroxyl species, is employedmore » as a model reaction to probe the dynamics of such processes. We find that surface hydroxyl recombination to water through the Mars van Krevelen mechanism reduces Rh coordination, triggering its conversion to active Rh adatoms. Since such lattice oxygen exchange is observed in many acid-base and redox chemistries, the process can be broadly applicable to controlling the activation of the range of SACs.« less

Ni-based superalloys offer a unique combination of mechanical properties, corrosion resistance and high temperature performance. Near ambient pressure X-ray photoelectron spectroscopy was used to study in operando the initial steps of oxidation for Ni-5Cr, Ni-15Cr, Ni-30Cr and Ni-15Cr-6W at 500 °C, p(O₂)=10^-6 mbar. The comparison of oxide evolution for these alloys quantifies the outsized impact of W in promoting chromia formation. For the binary alloys an increase in chromia due to Cr-surface enrichment is followed by NiO nucleation and growth thus seeding a dual-layer structure. The addition of W (Ni-15Cr-6W) shifts the reaction pathways towards chromia thus enhancing oxide quality.more » Density functional theory calculations confirm that W atoms adjacent to Cr create highly favorable oxygen adsorption sites. The addition of W supercharges the reactivity of Cr with oxygen essentially funneling oxygen atoms into Cr sites. The experimental results are discussed in the context of surface composition, chemistry, reactant fluxes, and microstructure.« less

Abstract Despite their exceptional mechanical and corrosion properties, duplex stainless steels (DSS) have not found widespread use in high-temperature applications due to concerns over thermal aging and embrittlement at elevated operational temperatures (> 300 °C). The present study investigated the effect of thermal aging time on the electrochemical properties of lean and standard grade DSS that are exposed to a range of pressurized water reactors containing LiOH and H ₃ BO ₃ . The results indicated that the electrolyte chemistry plays a significant role in the corrosion behavior of the DSS alloys. Corrosion resistance decreased with thermal aging time for all DSSmore » alloys; however, standard grade DSS (2205 and 2209-w) alloys showed better corrosion resistance than lean grades (2003, 2101, 2101-w). The presence of dissolved oxygen in the electrolytes resulted in a significant increase in corrosion rate for the DSS alloys, but it did not affect the general trend of corrosion rates with aging time. All DSS alloys became vulnerable to pitting corrosion due to chloride addition, but the pitting resistance decreased with increasing thermal aging time. Increased boron B content resulted in degradation of corrosion resistance of the DSS alloys, while minor changes in pH did not show a significant change in corrosion resistance. Mechanical and metallurgical characterization coupled with electrochemical characterization of the DSS alloys gave a comprehensive insight into the effects of thermal aging on the electrochemical response of the DSS. Graphical abstract« less

Many metals form passive oxides which are broken or weakened by anionic-induced degradation leading to material failure and anionic diffusion is an important step of this degradation process. The diffusion of anionic species through oxides involves a combination of diffusion along grain boundaries, cracks, and channels, as well as bulk diffusion via point defects which is the focus of this study. Using density functional theory, we study bulk diffusion of Cl through O vacancies in α -Cr ₂ O ₃ as a model system for passive metal oxides. Little is known about Cl diffusion and bonding characteristics, so we benchmarkmore » our work through comparison to numerous studies on O diffusion in α -Cr ₂ O ₃ to analyze similarities and differences between the O and Cl diffusion in the passivation and degradation process of α -Cr ₂ O ₃ respectively. Unlike O diffusion, the lowest diffusion barrier for Cl is cross-plane diffusion between two (0001) planes through a vacant cation site but the much shorter in-plane diffusion path within the same coordination polyhedron has 35% higher barrier. This work provides the basis for considering the contributions of Cl bulk-diffusion in the overall diffusion kinetics of Cl through α -Cr ₂ O ₃ .« less

The ion exchange and point defect models are two prominent models describing the role of anions, such as chlorides, in the degradation of passive oxide films. Here the thermodynamic feasibility of critical steps of Cl-induced degradation of a hydroxylated α-Cr₂O₃ (0001) surface, as proposed by these two models, are studied. Both models begin with Cl substitution of surface OH and H₂O, which becomes less favorable with increasing Cl coverage. The initial stages of Cl-induced breakdown of the α-Cr₂O₃ depend on Cl coverage and the presence of O vacancy near the surface as follows: (1) neither Cl insertion (supporting the ionmore » exchange model) nor Cr vacancy formation (supporting the point defect model) is feasible at low Cl coverages except in the presence of O vacancies near the surface, where Cl insertion is thermodynamically feasible even at low coverages, (2) in the absence of O vacancies, Cr vacancy formation becomes feasible from 10/12 ML onwards whereas Cl insertion by exchange with subsurface OH only becomes feasible at full coverage. This implies that at higher coverages Cl-induced degradation first initiates through a vacancy formation mechanism, but both insertion and vacancy formation would be feasible at full coverage.« less

Lithium-ion batteries are a leading energy storage technology. One challenge with lithium-ion batteries is the reductive decomposition of electrolyte on the surface, forming a passivating, solid–electrolyte interphase (SEI). The SEI prevents further electrolyte breakdown and consumption, but if not formed properly, may also consume lithium ions and prevent lithium-ion diffusion to the anode. Fluoroethylene carbonate (FEC) is currently one of the best electrolyte additives used to form a more robust SEI on silicon anodes. In this study, we use density functional theory (DFT) to investigate the spontaneous breakdown mechanisms, energies, and charge transfers of FEC on the surface of amore » silicon anode in a lesser lithiated LiSi and a more lithiated Li₁₅Si₄ state. The reductive decomposition of FEC on LiSi and Li₁₅Si₄ to F, CO₂, and CH₂CHO is energetically most favorable on both surfaces, but F, CO, and OCH₂CHO can also be formed. The breakdown of FEC via either of the breakdown mechanisms is about 2 times more favorable on the Li₁₅Si₄ surface than on the LiSi surface. Lastly, the Bader charge transferred from the anode to the FEC breakdown products is larger when forming F, CO₂, and CH₂CHO than when forming F, CO, and OCH₂CHO and is also larger on the Li₁₅Si₄ surface than on the LiSi surface.« less

Advances in extreme ultraviolet (EUV) photolithography require the development of next-generation resists that allow high-volume nanomanufacturing with a single nanometer patterning resolution. Organotin-based photoresists have demonstrated nanopatterning with high resolution, high sensitivity, and low-line edge roughness. However, very little is known regarding the detailed reaction mechanisms that lead to radiation-induced solubility transitions. In this study, we investigate the interaction of soft X-ray radiation with organotin clusters to better understand radiation-induced chemistries associated with EUV lithography. Butyltin Keggin clusters (β-NaSn13) were used as a model organotin photoresist, and characterization was performed using ambient-pressure X-ray photoelectron spectroscopy. The changes in relative atomicmore » concentrations and associated chemical states in β-NaSn₁₃ resists were evaluated after exposure to radiation for a range of ambient conditions and photon energies. A significant reduction in the C 1s signal versus exposure time was observed, which corresponds to the radiation-induced homolytic cleavage of the butyltin bond in the β-NaSn₁₃ clusters. To improve the resist sensitivity, we evaluated the effect of oxygen partial pressure during radiation exposures. Here, we found that both photon energy and oxygen partial pressure had a strong influence on the butyl group desorption rate. These studies advance the understanding of radiation-induced processes in β-NaSn₁₃ photoresists and provide mechanistic insights for EUV photolithography.« less