Lattice Oxygen Extraction Research Articles

Engineering CeO2 configurations to regulate the CuOx dispersion and switch pathways of preferential CO oxidation

Constructing highly dispersed catalysts is of tremendous interest for catalysis. However, the mechanism and avenue to tune dispersive behavior and thus reactivity of supported catalysts is still facing challenges. Herein, we manipulate crystal plane-induced CeO2 configurations toward improved CuOx dispersion, and investigate the structural evolution and reaction intermediates during the CO preferential oxidation over CuOx/CeO2 catalysts. Results demonstrate that the optimal atomic arrangement appears on nanosphere-shaped CeO2 with exposed {111} and {100} facets in contrast to nanorod-shaped CeO2 exposed {100} and {110} facets and nanowire-shaped CeO2 exposed {110} facets. By combining theoretical simulations and systematic characterizations including in situ DRIFTS, in situ Raman spectra and XAFS, it reveals the optimal catalyst favors the formation of highly dispersed CuOx with surface-enriched Cu+, which is highly active site for catalysis. Furthermore, in situ Raman spectra provide direct evidence that the optimal support configuration facilitates the lattice oxygen extraction, thus promoting the carboxyl pathway.

CO oxidation by Pt2/Fe3O4: Metastable dimer and support configurations facilitate lattice oxygen extraction.

Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/Fe3O4(001) "single-atom" catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Olattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.

Rapid oxygen exchange between hematite and water vapor

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline “r-cut” (1bar{1}02) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

Ce and F Dual-Doping Strategy for High Cycle Performance of Lithium-Rich Layered Oxide Materials

Li-rich layered oxides (LrLOs) attracted much attention due to their high specific capacity. However, LrLOs have disadvantages such as fast voltage decay and poor cycle stability. In this work, we propose a dual-doping strategy based on Ce and F ions to respond to the defects of LrLOs. This work shows that Ce and F have a strong synergistic effect in LrLOs cathode materials. Dual-doping makes the structure of the cathode materials more stable, which is mainly manifested by inhibiting the extraction of lattice oxygen, reducing the migration of cations during the cycle, and reducing the corrosion of the electrolyte to the cathode materials. Thereby, the work improves the cycle performance of the cathode materials. The capacity retention rate is 84.3% for 200 cycles at 1[Formula: see text]C (versus 59.6%). The median discharge voltage is 3.2155[Formula: see text]V after 200 cycles at 1[Formula: see text]C (versus 2.9485[Formula: see text]V). The voltage decay is 0.4706[Formula: see text]V for 200 cycles at 1[Formula: see text]C (versus 0.6923[Formula: see text]V). Interestingly, our research found that Ni[Formula: see text] of the Li layer also plays an important role in the process of improving cycle performance. This work provides new ideas for solving the cycle stability of LrLOs cathode materials and suppressing voltage decay.

Temperature-Sensitive Structure Evolution of Lithium-Manganese-Rich Layered Oxides for Lithium-Ion Batteries.

Cathodes of lithium-rich layered oxides for high-energy Li-ion batteries in electrically powered vehicles are attracting considerable attention by the research community. However, current research is insufficient to account for their complex reaction mechanism and application. Here, the structural evolution of lithium-manganese-rich layered oxides at different temperatures during electrochemical cycling has been investigated thoroughly, and their structural stability has been designed. The results indicated structure conversion from the two structures into a core-shell structure with a single distorted-monoclinic LiTMO2 structure core and disordered-spinel/rock salt structure shell, along with lattice oxygen extraction and lattice densification, transition- metal migration, and aggregation on the crystal surface. The structural conversion behavior was found to be seriously temperature sensitive, accelerated with higher temperature, and can be effectively adjusted by structural design. This study clarifies the structural evolution mechanism of these lithium-rich layered oxides and opens the door to the design of similar high-energy materials with better cycle stability.

Production mechanism analysis of H2 and CO via solar thermochemical cycles based on iron oxide (Fe3O4) at high temperature

In this paper, the production mechanism analysis of H2 and CO based on iron oxide was investigated. To deeply understand the reaction mechanism of the inner working of chemistry models, detailed chemical reaction mechanisms describing H2 and CO formation were conceptually structured in a hierarchical manner with the assistance of reaction paths diagram. However, the main reaction pathways leading to syngas (H2 and CO) formation derived from the conversion of H2O, CO2 and Fe(A). The study revealed that the reactivity of H, O, HO and HO2 in the reaction medium was crucial to determine syngas yield. The species reactivity in gas-Fe(A) reaction was limited by the rates of lattice oxygen extraction and diffusion from gas species to the iron bulk surface to replenish the extracted oxygen. However, the reaction paths diagram showed the main Fe(A) bulk phase of iron was not completely transformed to the fully oxidized Fe3O4 phase because of the restricted oxygen transfer to Fe(A) possibly. The model has indicated a better control of surface temperature and operating pressure could result in high reactivity of species in the reaction medium. Moreover, the combination analysis of Fe3O4 phase transformations and produced gas led to describe syngas production over several cycles of iron oxide redox reaction.

Synthesis of nanosized Ce0.85M0.1Ru0.05O2−δ (M = Si, Fe) solid solution exhibiting high CO oxidation and water gas shift activity

Nanosized Ce0.85M0.1Ru0.05O2−δ (M=Si, Fe) has been synthesized using a low temperature sonication method and characterized using XRD, TEM, XPS and H2-TPR. The potential application of both the solid solutions has been explored as exhaust catalysts by performing CO oxidation. The addition of Si- and Fe- in Ce0.95Ru0.05O2−δ greatly enhanced the reducibility of Ce0.85M0.1Ru0.05O2−δ (M=Si, Fe), as indicated by the H2-TPR study. The oxygen storage capacity has been used to correlate surface oxygen reactivity to the CO oxidation activity. Both the compounds reversibly release lattice oxygen and exhibit excellent CO oxidation activity with 99% conversion below 200°C. A bifunctional reaction mechanism involving CO oxidation by the extraction of lattice oxygen and rejuvenation of oxide vacancy with gas feed O2 has been used to correlate experimental data. The performance of both the solid solutions has also been investigated for energy application by performing the water gas shift reaction. The present catalysts are highly active and selective towards the hydrogen production and a lack of methanation activity is an important finding of present study.

Modulated CO Oxidation Activity of M-Doped Ceria (M = Cu, Ti, Zr, and Tb): Role of the Pauling Electronegativity of M

Many properties of dopants have been investigated to explore the key factor that influenced the CO oxidation activity of M-doped ceria (CeM). Nevertheless, these reports were controversial. Herein, the Pauling electronegativity (χP) of the M was presented as a convenient guide to screen a proper dopant for ceria. Kinetics results indicated that the T10 (the temperature when CO conversion reached 10%) of CeM73 catalysts (Ce/M molar ratio was 7/3; M = Cu, Ti, Zr, and Tb) was linearly dependent on the χP of the M, which could adjust the amount of active lattice oxygen (AOL). The AOL was important to catalyst activity because lattice oxygen extraction was the rate-determining step in the overall reaction.

Effects of surface area and oxygen vacancies on ceria in CO oxidation: Differences and relationships

Many properties of ceria, such as surface area, defects, and exposed crystal faces, were found to influence the CO oxidation performance of the ceria. Nevertheless, the relations of these properties, which will facilitate the understanding of the catalytic role of ceria in CO oxidation, have not been studied. In this study, the ceria catalysts, HY and CA, were synthesized by hydrothermal method and citric acid sol–gel method, respectively, and the difference in the CO oxidation performance of them was studied using kinetics analysis. Results of temperature-programmed experiments and pulse experiment indicated the lattice oxygen extraction was the rate-determining step (RDS) in the CO oxidation on HY and CA catalysts. The RDS was influenced predominantly by the amount of the active lattice oxygen and the rate of oxygen migration on the catalyst, which were related to the surface area and oxygen vacancies in the catalyst.

Thermal Decomposition of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Adsorbate Reactions with Lattice Oxygen as Studied by Infrared Spectroscopy

The thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on high surface area TiO2 nanoparticles (Degussa P25) has been studied by transmission infrared spectroscopy. The dominant reaction channel in the low-temperature regime from 295 to 400 K is the nucleophilic attack of adsorbed DMMP by neighboring oxygen to produce Ti−OCH3 and a variety of P−Ox surface bound groups. Arrhenius studies reveal an activation energy of ∼48 kJ mol−1 for the conversion of surface bound phosphoryl (P═O) groups into P−Ox species. Above 400 K, thermally activated lattice oxygen begins to play a dominant role in driving the oxidation of surface Ti−OCH3 groups. The electrons left behind following the extraction of lattice oxygen are observed by a broad rise in the infrared absorbance as the electrons are excited from shallow traps into the conduction band. Tracking lattice oxygen removal as a function of temperature reveals an activation energy of ∼33 kJ mol−1, over the temperature range ∼400−600 K, for the high-temperature oxidation of strongly bound surface adsorbates. These measurements can be employed to provide a more complete understanding of the key role that lattice oxygen plays in the degradation of adsorbates on the surface of active nanoparticulate semiconductor oxides.

Temperature effect on electrochemical promotion of bulk lattice-oxygen extraction for hydrogen oxidation over SOFC anodes

Abstract A solid oxide fuel cell unit was constructed with yttria-stabilized zirconia (YSZ) as the solid electrolyte, Ni-YSZ as the anode and Pt as the cathode. Hydrogen oxidation was performed from 680 to 800 °C under either an open or closed circuit. The phenomena of electrochemical promotion of bulk lattice-oxygen extraction and fuel-free current were observed. The rate enhancement ratio of water formation, the Faradaic efficiency and the promotion factor all decrease with increasing temperature. The novel phenomenon of electrochemical promotion of bulk lattice-oxygen extraction, observed over the SOFC anodes without anode-side oxygen feed under generated voltage, follows the rules for conventional electrochemical promotion of catalysis, observed over electrodes with gas-phase oxygen feed under applied voltage.

FeCr gas diffusion layer with surface modification for fuel processing in direct-methane solid oxide fuel cells

A solid oxide fuel cell (SOFC) with an Ni-YSZ anode was tested both without and with porous FeCr disk as a gas diffusion layer (GDL) under direct methane feeding. The surface of GDL was modified by coating with YSZ or Ni-YSZ powders. When gold mesh was used as the current collector, the performance of the direct-methane SOFC degraded very quickly. When FeCr GDL was used as the current collector over the anode, the performance became better and stable. Surface modification of GDL increased the current density. Both in GDL and over the anode, the major methane reaction was CH 4 dissociation, which yielded C species. The C species produced in GDL can be removed via gasification by carbon dioxide that forms over the anode. The formation of CO 2 can become the major reaction in the GDL of FeCr + YSZ while that of CO is the major reaction in the GDL of either FeCr or FeCr + Ni-YSZ. The electrochemical oxidation of CO formed in GDL increases the current density. The electrochemical promotion of lattice-oxygen extraction promotes the oxidation of CO and C species over the anode.

Effect of operating conditions on the electrochemical behavior of hydrogen-fed solid oxide fuel cells

A solid oxide fuel cell (SOFC) test unit was constructed with yttria-stabilized zirconia (YSZ) as the solid electrolyte, Ni–YSZ as the anode and Pt as the cathode. The SOFC unit was tested by varying hydrogen supply time, voltage, and temperature at 700–900 °C. The phenomena of electrochemical promotion of bulk lattice-oxygen extraction and fuel-free current have been observed. The amount of lattice oxygen additionally extracted by electrochemical promotion is limited. This limitation is more pronounced as the operating temperature increases. The interstitial hydrogen species are formed as either the interstitial hydrogen atoms in the bulk of Ni electrode or the interstitial protons in the bulk of YSZ. The amount of the interstitial hydrogen species is also limited. Oxygen transport in YSZ to the YSZ–Ni interface is rate limiting for H 2O formation over the anode. These new characteristics on the electrochemical behavior can optimize the operating conditions and the designing features of hydrogen-fed SOFCs.

Temperature effect on electrochemical promotion of syngas cogeneration in direct-methane solid oxide fuel cells

Syngas cogeneration in direct-methane solid oxide fuel cells with Ni–yttria-stabilized zirconia (YSZ) anodes was studied with temperature varying from 700 to 900 °C. A phenomenon of electrochemical promotion of bulk lattice-oxygen extraction from the YSZ electrolyte was observed. With increasing temperature, this promotion effect increases while both the rate enhancement ratios of CO and CO 2 formations decrease. The activation energy of CO and CO 2 formation under close circuit is lower than that under open circuit. The activation energy for the lattice-oxygen extraction from the YSZ bulk is higher than that for the oxygen transport through the YSZ bulk. The process of lattice-oxygen extraction from YSZ is rate determining in direct-methane oxidation under the condition of either close circuit or open circuit. The dependence of CO formation rate on the oxygen supply rate is stronger than that of CO 2 formation rate. Electrochemical promotion of bulk lattice-oxygen extraction enhances syngas cogeneration.

Electrochemical promotion of bulk lattice-oxygen extraction for direct methane conversion to syngas in SOFCs with Ni-YSZ anodes

Abstract A phenomenon of electrochemical promotion of lattice-oxygen extraction from the bulk phase of YSZ is observed during direct methane oxidation to generate syngas at 800 °C in solid oxide fuel cells (SOFCs) with Ni-YSZ (yttria-stabilized zirconia) anodes. Markedly higher amount of oxygen than that equivalent to the measured current is extracted for anodic oxidation. This promotion effect increases as the potential (voltage) increases. A phenomenon of “fuel-free current” is observed to be a consequence of this effect. The electrochemically promoted extraction of bulk lattice oxygen is influenced by both the operating condition and the reaction type. Both rate enhancement ratios of CO and CO2 formations are linearly related to the measured current density but inversely linearly related to the voltage. As the current density increases, both methane conversion and CO selectivity increase. Electrochemical promotion of bulk lattice-oxygen extraction enhances the syngas generation.

Electrochemical promotion of bulk lattice-oxygen extraction for syngas generation over Ni-GDC anodes in direct-methane SOFCs

Gadolinia-doped ceria (GDC), a mixed ionic-electronic conductor, was used to fabricate a Ni-GDC/GDC/Pt solid oxide fuel cell (SOFC). The Ni-GDC anode has a composition of Ni:GDC = 3:5 in weight. A new phenomenon of electrochemical promotion of lattice-oxygen extraction from GDC bulk is observed during direct methane oxidation, in the absence of anode-side gas-phase oxygen, to generate syngas at 800 °C. The extraction of markedly higher amount of oxygen than that of the measured current is promoted. This effect increases as the potential (the generated voltage) increases. Both rates of CO and CO2 formations are linearly related to the measured current density but inversely linearly related to the voltage. The formation rate of CO depends on the rate of oxygen supply to the anode surface and its dependence is stronger than that of CO2. As the oxygen-supply rate increases, both methane conversion and CO selectivity increase. Electrochemical promotion of bulk lattice-oxygen extraction enhances the syngas generation.

Role of Lattice Oxygen in Photocatalytic Oxidation on TiO2

In the presence of low-intensity UV lights, lattice oxygen is extracted from the TiO2 surface at room temperature by adsorbed formic acid. Transient photocatalytic oxidation (PCO) and decomposition (PCD) of formic acid to CO2 and H2O were combined with interrupted reaction and temperature-programmed desorption to directly show that lattice oxygen is the oxidizing species during PCD, whereas adsorbed oxygen oxidizes organics during PCO. The rates of lattice oxygen extraction and diffusion of lattice oxygen from the TiO2 bulk to the surface to replenish the extracted oxygen were measured. The initial rate of decomposition (oxidation) of formic acid in the absence of gas-phase O2 is one-seventh the rate in 3% O2. Lattice oxygen vacancies are readily replenished by O2 in the dark at room temperature, but H2O does not re-oxidize them, even during UV exposure. Surface diffusion of formic acid to active sites does not limit photocatalytic reaction. Lattice oxygen extraction causes slow deactivation of TiO2.

Current Change Method of Reducing Gas Sensing Using ZnO Varistors

The conductivity of partially sintered doped ZnO varistor pellets was shown to increase via two mechanisms with introduction of or CO/Ar atmospheres. Reducing gas extracted chemisorbed oxygen, which in turn reduced current‐opposing potentials which had formed at grain boundaries. This mechanism was gas concentration independent. After more extended times, the conductivity showed a second increase as extraction of lattice oxygen near the grain boundaries further lowered the grain boundary Schottky barriers. During this time period, the time rates of change of current through the sensor were shown to increase with concentration (1 to 20%), when the compact sustained continuous porosity, and was heated between 300 and 500°C. This resulted from the rate of extraction of lattice oxygen being limited by the availability of gas at grain boundary neck surfaces. CO/Ar atmospheres were less effective at removing chemisorbed and lattice oxygen, and current slope sensitivity to CO concentration was not demonstrated.

Transient oxidation of volatile organic compounds on aCuO/Al 2O 3 catalyst

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to study decomposition and oxidation of methanol, ethanol, acetaldehyde, formic acid, and acetic acid on aCuO/Al 2O 3 catalyst. The volatile organic compounds (VOCs) were adsorbed on the alumina surface and diffused to the CuO to react. Lattice oxygen in CuO is active for deep oxidation, and as lattice oxygen is depleted, diffusion of lattice oxygen to the surface limits the rate of oxidation. Supported CuO dehydrogenates and oxidizes VOCs and the order of reactivity is: HCOOH > CH 3OH > CH 3COOH > C 2H 5OH > C 2H 4O. Nearly all the oxygen in CuO can be extracted by VOCs. In the absence of gas-phase O 2, extraction of lattice oxygen limits the oxidation rates of acetaldehyde and acetic acid, but gas-phase O 2 rapidly replenishes the lattice oxygen. No less-reactive, partial oxidation products were detected.

Oxidation of Volatile Organic Compounds on Al2O3, Pd/Al2O3, and PdO/Al2O3Catalysts

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to study the decomposition and oxidation of methanol, ethanol, acetaldehyde, formic acid, and acetic acid on Al2O3, Pd/Al2O3, and PdO/Al2O3catalysts. The oxidation and decomposition rates were much higher on Pd/Al2O3than on Al2O3, even though the volatile organic compounds (VOCs) were adsorbed on the Al2O3support in both cases. The VOCs surface-diffused to Pd and mostly dehydrogenated during TPD whereas they oxidized to CO2and H2O in the presence of oxygen. Partial oxidation products also apparently formed on the surface during TPO and they oxidized completely above 550 K. Above 600 K, VOCs oxidation was consistent with the Mars–van Krevelen mechanism, involving the oxidation and reduction of Pd and PdO. On PdO/Al2O3adsorbed VOCs were oxidized by lattice oxygen from PdO, but PdO was less active than Pd metal for VOC decomposition. Oxidation began at the same temperatures on PdO/Al2O3whether or not O2was present, indicating that extraction of lattice oxygen from PdO was the limiting factor initially. After lattice oxygen was removed, metallic Pd decomposed VOCs and also adsorbed O2, which was incorporated into the Pd lattice above 600 K. The reduction of PdO during TPD resulted in an autocatalytic oxidation since metallic Pd was more active than PdO. A portion of the VOCs reacted in parallel on Al2O3since these sites are active for dehydration and dehydrogenation at moderate to high temperatures.