Participation Of Lattice Oxygen Research Articles

A nine-atom rhodium-aluminum oxide cluster oxidizes five carbon monoxide molecules.

Noble metals can promote the direct participation of lattice oxygen of very stable oxide materials such as aluminum oxide, to oxidize reactant molecules, while the fundamental mechanism of noble metal catalysis is elusive. Here we report that a single atom of rhodium, a powerful noble metal catalyst, can promote the transfer of five oxygen atoms to oxidize carbon monoxide from a nine-atom rhodium–aluminum oxide cluster. This is a sharp improvement in the field of cluster science where the transfer of at most two oxygen atoms from a doped cluster is more commonly observed. Rhodium functions not only as the preferred trapping site to anchor and oxidize carbon monoxide by the oxygen atoms in direct connection with rhodium but also the primarily oxidative centre to accumulate the large amounts of electrons and the polarity of rhodium is ultimately transformed from positive to negative.

Water electrolysis on La(1-x)Sr(x)CoO(3-δ) perovskite electrocatalysts.

Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B–O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co–O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution

Rational design of efficient, stable oxygen evolution reaction (OER) catalysts is necessary for widespread adoption of electrochemical energy storage technologies. Achieving this goal requires elucidation of fundamental relationships between surface structure and reaction mechanism. Here we address this issue using ab initio computations to determine the surface structure and OER mechanism for LaNiO3, a perovksite oxide that exhibits high activity but low stability. We find a new OER mechanism in which lattice oxygen participation via reversible formation of surface oxygen vacancies is critical. We show that this mechanism has a lower reaction barrier compared to the generally proposed mechanism, leading to improved agreement with experimental data. Extending the study to La1–xSrxBO3 (B = transition metal), we demonstrate a transition to the lattice oxygen-mediated mechanism with decreasing catalyst stability. Our results suggest new approaches for next-generation catalysts design.

Greener photocatalysts: Hydroxyapatite derived from waste mussel shells for the photocatalytic degradation of a model azo dye wastewater

This paper demonstrates for the first time the feasibility of utilising waste mussel shells for the synthesis of hydroxyapatite, Ca10(PO4)6(OH)2 (denoted as HAP) to be used as a greener, renewable photocatalyst for recalcitrant wastewater remediation. HAP was synthesised from Perna canaliculus (green-lipped mussel) shells using a novel pyrolysis–wet slurry precipitation process. The physicochemical properties of the HAP were characterised using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The HAP produced was of comparable quality to commercial (Sulzer Metco) HAP. The synthesised HAP had good photocatalytic activity, whereby methylene blue (a model textile wastewater compound) and its azo dye breakdown products were degraded with an initial rate of 2.5×10−8molL−1min−1. The overall azo dye degradation was nearly 54% within 6h and 62% within 24h in an oxygen saturated feed in a batch reactor using a HAP concentration of 2.0g/L, methylene blue concentration of 5mgL−1, UV irradiation wavelength of 254nm and a stirring speed of 300rpm. The kinetics were well described by three first order reactions in series, reflecting the reaction pathway from methylene blue to azo dye intermediates, then to smaller more highly oxidised intermediates and finally degradation of the recalcitrants. The final two steps of the reaction had significantly slower rates than the initial step (rates constants of 6.2×10−3min−1, 1.2×10−3min−1 and approximately (due to limited data points) 1.6×10−4min−1 for the first, second and third step respectively), which tie in with this mechanism, however it could also indicate that the reaction is either product inhibited and/or affected by catalyst deactivation. FTIR analysis of the post-reaction HAP revealed surface PO43− group loss. Since there is good photocatalytic activity with oxygen in limited and excess supply during the photoreaction, this indicates the possibility of lattice oxygen participation in the photocatalytic reaction, which needs to be characterised more fully. However, overall, these results indicate that the HAP derived from the mussel shells is a promising greener, renewable photocatalyst for the photocatalytic degradation of wastewater components.

Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VOx/TiO2

Detailed molecular mechanisms of oxygen isotopic exchange over VOx/TiO2 catalyst following the R0, R1, and R2 mechanisms were studied using periodic DFT analysis of possible pathways by the CI-NEB method. The electronic structures of surface VOx species formed on the VOx/TiO2 model surface after interaction of molecular oxygen with fully oxidized O═V5+–O–V5+═O sites and reduced V3+–O–V3+ sites were analyzed. We found a number of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen molecules on a reduced V3+–O-V3+ site leads to the formation of a superoxide complex, followed by its transformation into a peroxide complex with low activation energy about E* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir–Hinshelwood mechanism without participation of lattice oxygen along the R0 reaction pathway. We demonstrate that adsorption of molecul...

Conventional versus lattice photocatalysed reactions: Implications of the lattice oxygen participation in the liquid phase photocatalytic oxidation with nanostructured ZnO thin films on reaction products and mechanism at both 254 nm and 340 nm

This study aimed to investigate the relationship between surface morphologies of nanostructured ZnO thin films, the reaction intermediates and products produced, and the liquid phase and solid phase reaction mechanisms under both oxygen rich and oxygen limited conditions at two different wavelengths (254nm and 340nm). Four different ZnO morphologies were prepared by hydrothermal deposition on two different substrates – clean glass (CG) and direct current magnetron sputtered ZnO glass (MS). The two films grown on the MS templates were highly vertically aligned columnar structures and those grown without templates (CG) had little or no structural alignment. Methylene blue (MB) was used as the reactant, since its reaction intermediates and products are well defined and measurable, allowing predictable comparison between these catalysts.Results showed that there was a significant difference in MB degradation rates as well as reaction intermediate formation and destruction rates correlated to the morphologies and crystallinity at both UV wavelengths, with the highest reaction rates at 340nm. Reaction analysis indicates that there may be a competition between two different photocatalytic mechanisms: conventional photocatalysed radical oxidation and lattice oxygen driven oxidation. The dominant reaction mechanism present depended on the thin film morphology, crystallinity, availability of oxidant and the wavelength of the incident UV. The surface photocatalysed radical formation is thought to be predominant for more highly aligned and more crystalline morphologies, where there was plentiful oxygen and UV irradiation at 340nm. Lattice oxygen photodegradation is thought to be predominant for less aligned more amorphous morphologies and UV irradiation at 254nm. Based on these results a new MB ZnO photocatalysed oxidation mechanism is proposed.

Electrocatalysis and redox behavior of Pt2+ ion in CeO2 and Ce0.85Ti0.15O2: XPS evidence of participation of lattice oxygen for high activity

Electronic states of CeO2, Ce1 − xPtxO2 − δ, and Ce1 − x − yTiyPtxO2 − δ electrodes have been investigated by X-ray photoelectron spectroscopy as a function of applied potential for oxygen evolution and formic acid and methanol oxidation. Ionically dispersed platinum in Ce1 − xPtxO2 − δ and Ce1 − x − yTiyPtxO2 − δ is active toward these reactions compared with CeO2 alone. Higher electrocatalytic activity of Pt2+ ions in CeO2 and Ce1 − xTixO2 compared with the same amount of Pt0 in Pt/C is attributed to Pt2+ ion interaction with CeO2 and Ce1 − xTixO2 to activate the lattice oxygen of the support oxide. Utilization of this activated lattice oxygen has been demonstrated in terms of high oxygen evolution in acid medium with these catalysts. Further, ionic platinum in CeO2 and Ce1 − xTixO2 does not suffer from CO poisoning effect unlike Pt0 in Pt/C due to participation of activated lattice oxygen which oxidizes the intermediate CO to CO2. Hence, higher activity is observed toward formic acid and methanol oxidation compared with same amount of Pt metal in Pt/C.

Sonochemical Synthesis of Thermally Stable Hierarchical Ce1−xMxO2−δ (M = Pt or Pd, 0 ≤ x ≤ 0.10) Nanocrystallites: Redox Properties and Methanol Electro-Oxidation Activity

Hierarchical Ce1−xMxO2−δ (M = Pt or Pd, (0 ≤ x ≤ 0.10) nanocrystallites of ∼5 nm sizes have been synthesized by a sonication route using diethylenetriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. Ce1−xMxO2−δ (M = Pt or Pd, 0 ≤ x ≤ 0.1) crystallize in fluorite structure. Pt exists in the +4 state and Ce in the mixed valent (+3, +4) state in Ce1−xPtxO2−δ and Pd exists in the +2 state and Ce in the mixed valent (+3, +4) state Ce1−xPdxO2−δ. TEM, scanning electron microscopy (SEM), and BET surface and porosity measurements confirmed that the as-prepared compound is hierarchical structured with individual crystallites of ∼5 nm size, and even after sintering the sample at 600 °C, hierarchical structure is retained with individual crystallites of ∼7 nm size. Substitution of the Pt and Pd ion in CeO2 activated the lattice oxygen, and hydrogen spillover or a higher H/Pt ratio ∼ 8.1 and H/Pd ratio ∼ 4.2 is observed. Reversible nature of higher oxygen storage capacity or higher H/P, H/Pd ratio is due to interaction of redox couples of Pt4+/2+ (0.91 V), Pt2+/0 (1.18 V), Pd2+/0 (0.92 V), and Ce4+/3+ (1.61 V). Because of the participation of lattice oxygen, Ce0.95Pt0.05O1.95 and Ce0.95Pd0.05O1.90 have shown higher electro-oxidation of methanol compared to the same moles of Pt in 5% Pt/C.

Elucidating the role of Cu species in the oxidative carbonylation of methanol to dimethyl carbonate on CuY: An in situ spectroscopic and catalytic study

CuY catalysts with varied Cu content obtained by incipient-wetness-impregnation of commercial NH 4-Y zeolite with copper nitrate solution and calcination at 400 °C in air were examined by in situ FTIR investigations to characterize the Cu species and elucidate their specific role in the oxidative carbonylation of methanol. The adsorption of CO was applied to check the state of Cu(I) species whereas the acidity was characterized by pyridine adsorption. Depending on the Cu content different intensity ratios of the characteristic Cu(I)–CO bands at 2160 and 2146 cm −1 were observed which give information about the distribution of Cu(I) cations at different positions near the supercage. Lewis sites of different strengths are created by introduction of Cu resulting from Cu(I)/Cu(II) cations positioned in the super cages. MeOH adsorbs dissociatively on preferable Cu(I) Lewis sites by formation of methoxy species without additional supply of oxygen. Methoxy species and CO adsorb at the same Cu sites. Oxygenated products (monomethyl carbonate, dimethyl carbonate, CO 2) are formed during simultaneous adsorption of MeOH and CO with and without oxygen which indicates a participation of lattice oxygen of CuO x aggregates in the oxidation process and points to a Mars-van-Krevelen mechanism. Adsorbed formate species more pronounced at higher Cu loadings were additionally found. CO reacts with adsorbed methoxy species by formation of mainly monomethyl carbonate whereas the simultaneous presence of formate-like species promotes the formation of dimethyl carbonate. This implicates a specific role of adsorbed carbonate-like species for dimethyl carbonate formation. High Cu loadings were found to be beneficial because the additional formation of CuO x agglomerates in the super cages favors oxidation and oxocarbonylation reactions of methanol and enhances the formation of dimethyl carbonate which was also confirmed by the catalytic tests.

Catalytic combustion of dilute acetone over Cu-doped ceria catalysts

Cu x Ce 1− x O y ( x = 0.06, 0.13, and 0.23) were prepared by a solution combustion route using glycine as the fuel and tested for the catalytic combustion of dilute acetone in air. The structural characteristics of the catalysts were investigated by specific surface area, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR) techniques. The results reveal that the state of the CuO depends on the Cu/Ce molar ratio in the catalyst, which strongly influences the redox behavior and the catalytic activity of the sample. Among the three catalysts, Cu 0.13Ce 0.87O y was found to be the most active in the catalytic combustion of acetone. The pulse reaction of pure acetone in the absence of O 2 over the Cu 0.13Ce 0.87O y catalyst indicated the participation of lattice oxygen from the catalyst in the acetone combustion. Furthermore, the effect of the acetone concentration and the gas hourly space velocity (GHSV) on the catalytic behavior of the catalyst was examined. The stability of the Cu 0.13Ce 0.87O y catalyst for acetone combustion was also investigated and an obvious deactivation was observed under the reactant stream for 35 h at 260 °C. XRD analysis of the deactivated catalyst showed that the formation of bulk CuO was responsible for the deactivation of the catalyst.

The steady state methanol decomposition reaction over Cu/Zn and Cu/Cr catalysts: Pretreatment, operando EXAFS, and activity study

Two commercial Cu catalysts, Cu/Zn and Cu/Cr, were pretreated to produce calcined, partially reduced and completely reduced catalysts, and tested for methanol decomposition. The pretreatments resulted in different initial Cu states based on extended X-ray absorption fine structure (EXAFS) morphological analysis, but only metallic Cu was found after methanol decomposition tests. Pretreatment was found to affect catalytic activity only during the initial transient period, when atom balance analysis showed the presence of excess oxygen in the effluent. The excess oxygen is attributed to the participation of lattice oxygen in the reaction leading to CO 2 and H 2O formation. When there was no excess oxygen present in the effluent, a steady state was achieved and a similar catalytic performance was obtained regardless of catalyst pretreatment conditions.

Oxidative dehydrogenation of propane on silica (SBA-15) supported vanadia catalysts: A kinetic investigation

Silica (SBA-15) supported vanadium oxide was used for a kinetic study of the oxidative dehydrogenation of propane in a fixed bed reactor. Prior to this study, spectroscopic characterization using a variety of analytic techniques such as FTIR-, Raman-, DR UV–vis- and X-ray photoelectron spectroscopy revealed the absence of bulk vanadia and a high dispersion of active surface sites for the investigated catalyst. The kinetic data evaluation was based on a first order rate law. Calorimetric measurements were used to determine the heat of adsorption of propane on the catalyst. The data indicate that the primary combustion of propane is negligible. Reaction orders of one for propane dehydrogenation and propene combustion indicate their participation in the respective rate-determining step. The zero reaction order determined for the catalyst reoxidation reveals participation of lattice oxygen in this reaction step. Higher activation energies of propane dehydrogenation as compared to propene combustion indicate the participation of the weaker allylic C–H bond of propene in the rate-determining step of the propene combustion. This results in higher propene selectivities at elevated temperatures. Kinetic parameters, including apparent and intrinsic activation energies and the equilibrium constant of propane adsorption allowed a comparison with theoretical predictions and show good agreement.

Kinetics of oxidation of methane to formaldehyde on Na4[PFeMo11O40]/SiO2

The kinetics of oxidation of CH4 to formaldehyde on the catalytic system Na4[PFeMo11O40]/SiO2 were studied, and a significant role of the redox potential of the CH4-O2 system with respect to the catalyst was shown. The density of centers participating in the reaction was determined, and dissociative competitive adsorption of methane and oxygen was established. The equation was deduced in the framework of the Langmuir-Hinshelwood theory taking into account the side conversion of formaldehyde. Possible participation of lattice oxygen in the reaction was suggested.

Manganese Oxide/Titania Materials for Removal of NO x and Elemental Mercury from Flue Gas

A novel catalyst for low temperature selective catalytic reduction (SCR) using CO as reductant, MnO x supported on titania, has been shown to be effective for both elemental mercury capture and low temperature SCR. In low temperature (200 °C) SCR trials using an industrially relevant space velocity (50 000 h −1) and oxygen concentration (2 vol %), nearly quantitative reduction of NO x was obtained using CO as the reductant. Fresh catalyst used as an adsorbent for elemental mercury from an inert atmosphere showed remarkable mercury capture capacity, as high as 17.4 mg/g at 200 °C. The catalyst effectively captured elemental mercury after use in NO x reduction. Mercury capture efficiency was not affected by the presence of water vapor. Mercury capacity was reduced in the presence of SO 2. Manganese loading and bed temperature, which influence surface oxide composition, were found to be important factors for mercury capture. X-ray photoelectron spectroscopy (XPS) results reveal that the mercury is present in its oxidized form (HgO) in spent catalyst, indicating the participation of lattice oxygen of the catalyst in the reaction. These results suggest that a single-step process integrating low temperature SCR and mercury capture from flue gas might be feasible.

CO Adsorption and Oxidation on Ceria Surfaces from DFT+U Calculations

The limiting steps of CO oxidation catalyzed by ceria via the Mars−van Krevelen reaction mechanism are identified and investigated by means of density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U). We address the adsorption of CO on the (111) and (110) surfaces, and its oxidation via participation of lattice oxygen leading to vacancy formation and CO2 desorption. CO physisorbs on the (111) ceria surface (Ebind 2 eV) yielding carbonate formation and surface reduction. The DFT+U energy of CO adsorption and oxidation is studied as a function of the parameter U. This analysis demonstrates that the values of U presently used in the literature (U > 4 eV) tend to overestimate the binding energy of CO to ceria surfaces. In contrast, the energy for the complete CO oxidation reaction is effectively independent of U and in good agreement with the available experimental data. The discuss...

Dynamic and Steady State Analysis of Low Temperature Ethane Oxidative Dehydrogenation over Chromia and Chromia-Vanadia Catalysts

Oxidative dehydrogenation of ethane to ethylene was investigated over Chromia and Cr-V mixed oxide catalysts synthesized following a complexation procedure. With an O2/C2H6 feed ratio of 0.17, Chromia exhibited a total conversion value of about 0.20 at 447°C (at a space time of 0.24 s.g/mL) with an ethylene selectivity of 0.82. Chromia catalyst was more active than Cr-V mixed oxide at temperatures as low as 200°C. Pulse-response experiments carried out with ethane pulses injected into O2-He indicated the presence of at least two different sites for the formation of CO2 and H2O over Chromia catalyst. In the dynamic experiments carried out with the Cr-V mixed oxide catalyst and by injecting O2 pulses into a gas stream containing a mixture of C2H6 and He, formation of CO rather than C2H4 was favored. Results of the dynamic runs carried out without gas phase oxygen strengthened the conclusion of lattice oxygen participation in the selective oxidation of ethane reaction through a redox mechanism.

Mechanism of ammonia oxidation over oxides studied by temporal analysis of products

A temporal analysis of products (TAP) reactor was used to study the mechanism of ammonia oxidation at high temperature (1073 K) over Fe 2O 3, Cr 2O 3, and CeO 2. The results were compared with those obtained over the industrially applied Pt 95–Rh 5 alloy. Analysis of characteristic times of the transient responses of N 2 and NO made it possible to gain insight into the sequence of their formation. A common feature of both metal oxide and noble metal catalysts is that NO is a primary product of NH 3 oxidation, whereas N 2 results mainly from secondary transformations of NO. The amount of N 2O formed over the oxides was minimal. Multipulse NH 3 experiments in the absence of gas-phase O 2 give unequivocal evidence of the participation of surface lattice oxygen of the metal oxides in the reaction of NH 3 to NO. The degree of reduction of the oxide surface determines the product distribution but does not alter the catalytic activity for NH 3 conversion. Greater reduction favors the reaction channel to N 2 over that to NO. The desired reaction follows a Mars–van Krevelen-type scheme involving the participation of lattice oxygen in the NH 3 conversion to NO and regeneration of the so-formed vacancies by gas-phase O 2 and bulk lattice oxygen. Replenishment of surface vacancies by the diffusion of bulk oxygen occurs to a significant extent over Fe 2O 3. However, when NH 3 and O 2 are fed together, dissociative adsorption of gas-phase oxygen can be considered the main mechanism for vacancy regeneration.

Production of hydrogen by steam reforming of ethanol over cobalt and nickel catalysts supported on perovskite-type oxides

Abstract The steam reforming of ethanol was performed over Co and Ni catalysts supported on perovskite-type oxides (LaAlO3, SrTiO3 and BaTiO3). Co and Ni catalysts supported on LaAlO3 and SrTiO3 showed high catalytic activities and long-term stabilities and suppressed coke formation while Co/BaTiO3 showed lower catalytic activities and poorer coking resistance. The average particle size of Co on BaTiO3 was larger than that on SrTiO3. Lattice oxygen in perovskites may play positive roles in hindering coking and Co/SrTiO3 has more frequent participation of lattice oxygen in the oxidation of intermediates or precursor of coke in comparison with Co/BaTiO3.

Extent of the participation of lattice oxygen from γ-MnO 2 in VOCs total oxidation: Influence of the VOCs nature

Investigation of the total oxidation of n-hexane and trimethylamine as well as extensive characterizations of the catalyst after tests have been coupled to identify the mechanism of oxidation involved over a very active γ-MnO 2 catalyst. This work shows that the extent of the catalyst reduction, which is a consequence of the Mars and van Krevelen redox mechanism, depends on the nature of the organic molecule. Indeed, the nature of the VOC dictates its conversion level through its ability to reduce the catalyst and accelerate the incorporation rate of gaseous oxygen.

Oxidative dehydrogenation of C 4 hydrocarbons over VMgO catalyst — kinetic investigations

The oxidative dehydrogenation (ODH) of n-butane, butenes and butadiene over VMgO catalyst containing 30 wt.% V 2O 5 was investigated. Reactivity of hydrocarbons in ODH decreases in the order 2-butenes>1-butene> n-butane⪢butadiene. Butenes are easily converted, exhibiting a 64% selectivity to butadiene while a small part 5% is isomerized. The intermediate products (butenes, butadiene and carbon monoxide) were added to the feed in order to get information on the relative importance of primary and secondary reactions. Analysis of the experimental results, derived from adding of intermediate products, led to the proposal of a reaction network comprising of 10 consecutive and parallel steps and the calculation of the reaction rate of each step. The catalyst showed significant activity in the absence gas phase oxygen, indicating the participation of lattice oxygen in the ODH of n-butane.