Lattice Oxygen Species Research Articles

Vacancy‐induced catalytic mechanism for alcohol electrooxidation on nickel‐based electrocatalyst

AbstractOwing to outstanding performances, nickel‐based electrocatalysts are commonly used in electrochemical alcohol oxidation reactions (AORs), and the active phase is usually vacancy‐rich nickel oxide/hydroxide (NiOxHy) species. However, researchers are not aware of the catalytic role of atom vacancy in AORs. Here, we study vacancy‐induced catalytic mechanisms for AORs on NiOxHy species. As to AORs on oxygen‐vacancy‐poor β‐Ni(OH)2, the only redox mediator is electrooxidation‐induced electrophilic lattice oxygen species, which can only catalyze the dehydrogenation process (e.g., the electrooxidation of primary alcohol to carboxylic acid) instead of the C−C bond cleavage. Hence, vicinal diol electrooxidation reaction involving the C−C bond cleavage is not feasible with oxygen‐vacancy‐poor β‐Ni(OH)2. Only through oxygen vacancy‐induced adsorbed oxygen‐mediated mechanism, can oxygen‐vacancy‐rich NiOxHy species catalyze the electrooxidation of vicinal diol to carboxylic acid and formic acid accompanied with the C−C bond cleavage. Crucially, we examine how vacancies and vacancy‐induced catalytic mechanisms work during AORs on NiOxHy species.

Vacancy-induced catalytic mechanism for alcohol electrooxidation on nickel-based electrocatalyst.

Owing to outstanding performances, nickel-based electrocatalysts are commonly used in electrochemical alcohol oxidation reactions (AORs), and the active phase is usually vacancy-rich nickel oxide/hydroxide (NiOx Hy ) species. However, researchers are not aware of the catalytic role of atom vacancy in AORs. Here, we study vacancy-induced catalytic mechanisms for AORs on NiOx Hy species. As to AORs on oxygen-vacancy-poor β-Ni(OH)2 , the only redox mediator is electrooxidation-induced electrophilic lattice oxygen species, which can only catalyze the dehydrogenation process (e.g., the electrooxidation of primary alcohol to carboxylic acid) instead of the C-C bond cleavage. Hence, vicinal diol electrooxidation reaction involving the C-C bond cleavage is not feasible with oxygen-vacancy-poor β-Ni(OH)2 . Only through oxygen vacancy-induced adsorbed oxygen-mediated mechanism, can oxygen-vacancy-rich NiOx Hy species catalyze the electrooxidation of vicinal diol to carboxylic acid and formic acid accompanied with the C-C bond cleavage. Crucially, we examine how vacancies and vacancy-induced catalytic mechanisms work during AORs on NiOx Hy species.

Surface engineering of ternary mixed transition metal oxides for highly efficient catalytic oxidation of low concentration VOCs

With the widespread application of waterborne painting process, effective removal technologies for low concentration volatile organic compounds (VOCs) is a major concern of the painting industries due to the low mass transfer efficiency. This work presents a unique approach for the molecular-level preparation of precious-metal-free mixed aliovalent-substituted, CeCuOx binary oxide and NiO-modified yNi/CeCuOx ternary mixed metal oxide catalysts. In addition, the excellent catalytic selectivity (no catalytic activity decreases under 5.0 vol% water vapor) of the catalysts was successfully achieved by microstructures optimization and grafting octadecyltrichlorosilane (OTS). Its catalytic activity in the oxidation of toluene at low concentrations (50 ppm) was studied. The 3Ni/CeCuOx sample exhibits the best performance, which achieve 90 % catalytic oxidation of toluene at 197 °C without using noble metals. This results from that the ternary doping and the growth of NiO increases the specific surface area, active lattice oxygen species, activates molecular oxygen species, and provides more catalytic sites, which improve the adsorption and catalysis of toluene. Furthermore, the catalytic stability of the catalysts is also significantly improved by the addition of the hydrophobic coating. Therefore, the unique catalysts have great potential in practical applications for low concentration VOCs removal of painting industries.

Surface engineering to tailor the active sites of SrTi0.9Co0.1O3-δ perovskite for CO oxidation

We successfully engineered the surface properties of SrTiO3 perovskite for a carbon monoxide (CO) oxidation catalyst by substituting Co ions (10 at.%) into Ti-sites in SrTiO3 and subsequently applying a reductive thermal treatment. The substitution of Co ions in SrTiO3 induced charge imbalance, which was compensated by the generation of oxygen vacancies. During the subsequent reductive thermal treatment process, lattice oxygen species were released from the lattice of SrTiO3, leading to the further formation of oxygen vacancies and the segregation of Co cations on the catalyst surface. Thus, SrTi0.9Co0.1O3-δ catalyst showed superior activity compared to the pristine SrTiO3 in CO oxidation, and its catalytic activity was further enhanced after the thermal treatment. This improvement was due to the abundant surface oxygen vacancies generated by Co substitution and reductive thermal treatment. The presence of a surface Co-enriched phase increased the concentration of Co at the catalyst surface, providing more adsorption sites for CO. These findings demonstrate the effective control of the active sites (oxygen vacancies and B-site cations) in SrTiO3 using the two-step surface engineering method. This research contributes to the design of perovskite-oxide catalysts with reaction-tailored active sites by exploiting the synergistic effect of substitution and reductive thermal treatment methods.

Regulating oxygen vacancies and hydroxyl groups of α-MnO2 nanorods for enhancing post-plasma catalytic removal of toluene

Although nonthermal plasma (NTP) technology has high removal efficiency for volatile organic compounds (VOCs), it has limited carbon dioxide (CO2) selectivity, which hinders its practical application. In this study, α-MnO2 nanorods with tunable oxygen vacancies and hydroxyl groups were synthesized by two-step hydrothermal process to enhance their activity for deep oxidation of toluene. Hydrochloric acid (HCl) was used to assist in synthesis of α-MnO2 nanorods with tunable oxygen vacancies, furtherly, more hydroxyl groups were introduced to HCl-assisted synthesized α-MnO2 by K+ supplement. The results showed that the as-synthesized nanorods exhibited superior activity, improved by nearly 30% removal efficiency of toluene compared to pristine MnO2 at SIE = 339 J/L, and reaching high COx selectivity of 72% at SIE = 483 J/L, successfully promoting the deep oxidation of toluene. It was affirmed that oxygen vacancies played an important role in toluene conversion, improving the conversion of ozone (O3) and resulting in higher mobility of surface lattice oxygen species. Besides, the enhancement of deep oxidation performance was caused by the increase of hydroxyl groups concentration. In-situ DRIFTS experiments revealed that the adsorbed toluene on catalyst surface was oxidized to benzyl alcohol by surface lattice oxygen, and hydroxyl groups were also found participating in toluene adsorption. Overall, this study provides a new approach to designing catalysts for deep oxidation of VOCs.

Enhancing catalytic oxidation of toluene over Ag/Co3O4 by regulating Ag-Co interaction

Tuning the interaction between Ag and its supporters has been well accepted as a desirable way that adjusting the physicochemical properties and the catalytic performance of Ag-based catalysts. In this work, an in-situ surface reaction strategy was proposed to prepare high-performance Ag/Co3O4 catalyst by controlling the Ag-Co interaction for the purpose of efficient degradation of aromatic VOCs. The results of characterizations confirmed that the unique surface reaction strategy was conducive to anchoring Ag on the surface of Co3O4 and significantly enhanced Ag-Co interaction, as a result, Co-O bond was weakened by charge transfer between Ag and Co species, which showed great advantages in improving the reactivity of lattice oxygen species and in accelerating the adsorption of gaseous oxygen. Benefiting from this, 1%Ag/Co3O4-R synthesized by surface reaction method exhibited the best catalytic efficiency (T90 = 238 °C), excellent stability and admirable cyclic stability. In-situ DRIFTS spectroscopy investigation revealed that the loading of Ag remained unchanged reaction path of the catalytic oxidation of toluene, and benzoate was deemed a decisive stage in the oxidation course of toluene.

Metal-Oxygen Hybridization of Bi/Bife(oxy)Hydroxide for Sustainable Lattice Oxygen Mechanism at High Current Density

Hydrogen energy production through the electricity-driven water electrolysis has been broadly studied to deal with growing energy demands and environment pollutions. Oxygen evolution reaction (OER) which is the half anodic reaction of water electrolysis determines overall water electrolysis due to OOH* coordination with high energy barrier. Recently, alternative reaction kinetics detouring sluggish OOH intermediate in OER pathway has been proposed as breakthrough for efficient water electrolysis. That is the strategy which directly conjugates activated lattice oxygen species to form O-O coupling instead of OOH intermediate. However, absence of facile method to realize lattice oxygen activation and structural instability during OER cycles remain as challenge, hindering practical applications of water electrolysis. In this work, metal-oxygen hybridization method has been demonstrated as not only a simple and facile strategy to activate lattice oxygen species but also sustain lattice oxygen mechanism (LOM) during OER cycles at a practical current density (> 1000 mA cm-2). Using redox potential difference between bismuth (Bi) and iron (Fe) as driving force, galvanic replacement and Kirkendall effect take place in binary metal system, resulting in heterostructure composed of amorphous BiFe(oxy)hydroxides and molecular bismuth (Bi) metal nanoparticles (BM/BiFeOxHy) with abundant oxygen non-bonding states. In 1 M KOH solution, the BM/BiFeOxHy electrocatalyst requires low overpotential of 232 and 359 mV at the current densities of 10 and 1,000 mA cm-2, respectively. Moreover, long-term catalytic stability is demonstrated up to 1,000 hours at a practically high current density of 1,000 mA cm-2 without significant degradation by virtue of the balanced hybridization of Bi/Fe-O. Electrochemical/physicochemical analysis and density functional theory (DFT) calculation reveal that the excellent OER performance and stability of BM/BiFeOxHy electrocatalyst are attributed to the optimized Fe/Bi-O hybridization and resulting heterostructure with increased oxygen non-bonding states.

Highlights on the key roles of interfaces between CeO2-based oxide and perovskite (LaMnO3/LaFeO3) in creating active oxygen species for soot oxidation

Design efficient catalyst which can activate oxygen molecules or supply active oxygen species is a key challenge for soot abatement under the much lower concentration of oxygen from gasoline vehicle exhaust. Herein, combining with the advantages of both CeO2-based oxide (CeO2–ZrO2–Y2O3–La2O3, CZ for short) and perovskite (LaMnO3 and LaFeO3) in oxygen storage/release, two hybrids catalysts, CZ-LaMnO3 (LM) and CZ-LaFeO3 (LF), with more oxygen vacancies and enhanced lattice oxygen transport at lower temperatures due to the heterointerface effects have been successfully synthesized for soot oxidation. Combining with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O2-TPD) and H2temperature-programmed reduction (H2-TPR), it was found that electrons transfer from Ce3+ to Fe3+ and an interfacial La diffusion from LM to CZ may have been occurred in LF-CZ and LM-CZ, respectively, which leads to the increased unsaturated sites and oxygen vacancies. More importantly, the obvious enhancement of active surface lattice oxygen transport at low temperatures due to interfacial interaction has also been demonstrated in LM-CZ, which highly promotes the soot oxidation. And consequently, 5LM-CZ (with 5 wt% content of LM) showed the highest soot conversion activity, whose T50 (50 % soot conversion temperature) (379 °C) was lower than 5LF-CZ (395 °C) and CZ (401°C) at tight contact mode in 1 %O2/N2 atmosphere, indicating that the activation of lattice oxygen at the interface plays an important role in soot oxidation. Furthermore, when further varying the content of LM in xLM-CZ (x = 5 wt%, 10 wt%, 15 wt%, 20 wt%), it was found that 10LM-CZ, whose content of LM reached the monolayer dispersion capacity of CZ, exhibited the most abundant active surface lattice oxygen species due to the optimized interface area. As a result, 10LM-CZ displayed superior activity even than pure LM in both loose and tight contact with/without NO, indicating that the composite catalyst has a wide application environment. This work has a certain guiding significance for the utilization of heterointerface effect among different components to prepare effective soot oxidation catalyst with multi-components.

Improving benzene catalytic oxidation on Ag/Co3O4 by regulating the chemical states of Co and Ag

Silver (9 wt.%) was loaded on Co3O4-nanofiber using reduction and impregnation methods, respectively. Due to the stronger electronegativity of silver, the ratios of surface Co3+/Co2+ on Ag/Co3O4 were higher than on Co3O4, which further led to more adsorbed oxygen species as a result of the charge compensation. Moreover, the introducing of silver also obviously improved the reducibility of Co3O4. Hence the Ag/Co3O4 showed better catalytic performance than Co3O4 in benzene oxidation. Compared with the Ag/Co3O4 synthesized via impregnation method, the one prepared using reduction method (named as AgCo-R) exhibited higher contents of surface Co3+ and adsorbed oxygen species, stronger reducibility, as well as more active surface lattice oxygen species. Consequently, AgCo-R showed lowest T90 value of 183°C, admirable catalytic stability, largest normalized reaction rate of 1.36 × 10−4 mol/(h·m2) (150°C), and lowest apparent activation energy (Ea) of 63.2 kJ/mol. The analyzing of in-situ DRIFTS indicated benzene molecules were successively oxidized to phenol, o-benzoquinone, small molecular intermediates, and finally to CO2 and water on the surface of AgCo-R. At last, potential reaction pathways including five detailed steps were proposed.

The influence of partial substitution of Ce with K in CeMO3 (M = Mn, Fe, Co, Ni, Cu) perovskite catalysts on soot combustion performance

A series of Ce1−xKxMO3 (M = Mn, Fe, Co, Ni, Cu, and x = 0, 0.1, 0.2, 0.3, 0.4) perovskite catalysts were synthesized by citrate sol-gel method, in which A-site element Ce was partially substituted with K. The results showed that the catalytic performance could be significantly improved by partially substituting of Ce with K in CeCoO3 catalyst for soot combustion. Ce0.8K0.2CoO3 perovskite catalyst exhibited a superior catalytic activity, and its corresponding T10, T50, T90 and SmCO2 were 326, 383, 426 °C and 98.6%, respectively. The characterization results indicated that K+ had been successfully incorporated in the perovskite crystal structure. Partially substituting of Ce with K resulted in an increase in specific surface area. It also resulted in the formation of Co4+ in the perovskite catalysts together with abundant presence of oxygen vacancies on the surface. Moreover, it was of great significance to facilitate the activation of lattice oxygen species. In situ DRIFTS result showed that NO2 produced by thermal decomposition of monodentate nitrate species played a key role in soot combustion process.

Fe-doped MnO2 as an efficient catalyst for low temperature propane oxidation

Manganese oxides were received extensive attention on account of its low cost, variable valence state and excellent redox properties. Herein, we successfully prepared various Fe-doped MnO2 catalysts to further improve the catalytic activity of MnO2 on propane. Physicochemical characterization results displayed that with the increase of Fe doping amounts, the crystalline phase of MnO2 gradually changed from α-MnO2 and β-MnO2 to Fe2O3, the particle size of catalysts decreased and the specific surface area increased. However, increasing the content of Fe could not monotonically enhance the catalytic performance of catalysts on propane. When the molar ratio of Fe/Mn was 0.25, the largest amount of Mn4+ species and lattice oxygen species were exposed on the surface of Mn8Fe2, and showed the best low temperature reducibility, which was ascribed to the synergistic effect between Mn and Fe was enhanced. It presented optimal catalytic activity for propane oxidation with temperature of propane conversion at 90% (T90) as low as 279 °C under 0.1% propane and 5% O2. This work provided a new way to design and synthesize efficient non-noble metal catalysts for the elimination of propane, which had certain application value.

Important factors of the A-site deficient Mn perovskites design affecting the CO oxidation activity

A-site deficient La0.4Sr0.4MnxTi1−xO3 (LSMT, x = 0, 0.4, 0.6, 0.8) perovskites were investigated for their structural change and effect on CO oxidation activity. The XRD, XPS, TPR and O2-TPD analysis represent the change in Mn concentration, known as an active species in oxidation, affecting the crystal structure, oxidation state of the B-site cation (Mn), number of oxygen vacancies, and the reducibility of the catalysts. The shift in the main perovskite diffraction peak in XRD spectra of the perovskites with a change in Mn content represents the formation of cationic vacancies. At the lower temperature (<200 oC), CO oxidation efficiency of LSMT perovskite catalysts was found to be almost identical. This can be attributed to their similar perovskite structure and surface area. Compared to the higher temperature (>200 oC) CO oxidation activity, the LSMT catalyst with lower Mn content of 0.4 shows relatively better performance than the higher Mn content samples, which signifies the active role of the Mn oxidation state and lattice oxygen species of the perovskite catalysts. Moreover, the LSMT4446 catalyst shows stable activity for more than 12 h with an average CO conversion of 80 %. From these results, we found that the factors of the Mn oxidation state and the type of oxygen species are crucial when designing an Mn-based perovskite catalyst for catalytic applications.

Performance and reaction mechanism of Mg-doped and Li-doped La2O3 catalysts in oxidative coupling of methane: DFT and experiment study

Lanthanum oxide (La2O3) catalyst is a promising catalytic system for oxidative coupling of methane (OCM) reaction, while low activity for methane dissociation restricts its widespread application. Low-value dopants can enhance methane conversion activity on La2O3 catalysts. This work systematically studied the mechanism and catalytic performance of OCM reaction on Mg-doped and Li-doped La2O3 catalysts using a combination of DFT and experimental methods. The results show that Mg-doped and Li-doped can significantly improve CH4 dissociation activity and C2H4 selectivity on La2O3(001) surface. The microscopic property analysis shows that the p-band center of lattice oxygen on Mg-doped and Li-doped La2O3(001) is closer to the Fermi level, compared with undoped La2O3(001), which is more conducive to the activation of methane. Furthermore, Mg-doped and Li-doped La2O3 catalysts are prepared and their OCM catalytic performance are investigated. The catalytic performance of Mg-doped La2O3 catalyst reaches its best at 750 °C, with a high CH4 conversion (above 30.9 %) and the highest C2+ hydrocarbons selectivity (about 47.5 %) and C2+ yield (about 14.7 %). However, Li-doped La2O3 shows poorer catalytic performance than undoped La2O3, primarily due to the loss of the doped Li during the OCM process. In addition, the OCM reaction cycle and the mechanism of action of oxygen species on Mg doped La2O3(001) have been studied. The results show that lattice oxygen and peroxide species are the major active oxygen species for activating methane.

Differences between atomically-dispersed and particulate Pt supported catalysts on synergistic photothermocatalytic oxidation of VOCs from cooking oil fumes

The control of volatile organic compounds (VOCs) from cooking oil fumes via synergic photothermocatalytic oxidation not only saves energy, but also helps to reduce carbon emissions. Differences between the copper oxide−ceria (CC) supported atomically-dispersed (Pt1/CC) and particulate Pt (PtNPs/CC) on photothermocatalytic heptane oxidation were investigated. The conversion efficiency at 170 oC of heptane oxidation over the Pt1/CC catalyst was 55% higher than that over the PtNPs/CC catalyst. PtNPs/CC showed a larger capacity of heptane adsorption, but limited amounts of the adsorbed oxygen and active surface lattice oxygen species were difficult to completely oxidize the over-adsorbed heptane. Heptane and its intermediates gradually accumulated at the active sites, resulting in poor catalytic activity and stability. However, Pt1/CC possessed a strong electron donating ability due to its unique coordination unsaturated sites and electron structure, which was conducive to oxygen activation. This allowed rapid conversion of heptane and intermediates, accelerating oxidation of the adsorbed heptane to CO2 and H2O. The complete oxidation of 500 ppm heptane was basically achieved at 190 oC and an optical power density of 300 mW/cm2.

Boosting the total oxidation of toluene by regulating the reactivity of lattice oxygen species and the concentration of surface adsorbed oxygen

Surface absorbed oxygen (Oads) and surface lattice oxygen (Olatt) specie are critical for accelerating oxidative removal of toluene over MnO2-based catalyst. Herein, an excellent CuMnO2 catalyst has been successfully designed and fabricated. Doping Cu2+ with strong Jahn-Taylor effect into the lattice of MnO2 not only weakens the strength of Mn-O bonds but also induces the formation of oxygen defects, which enhances the reactivity of Olatt and promotes the adsorption of gaseous oxygen molecules. As a result, CuMnO2 reaches 50 % and 90 % of toluene conversion at 212 and 241 °C respectively, showing obviously better catalytic performance compared to CeMnO2, NiMnO2, CoMnO2 and δ-MnO2. Moreover, CuMnO2 exhibits outstanding stability, good cycling stability and excellent water resistance ability. The in situ DRIFTS demonstrate that ring opening of benzoate and the dehydrogenation of benzaldehyde are key processes that affect the deep oxidation of toluene.

Low temperature catalytic elimination of dichloromethane over manganese oxides catalysts with ozone assistance

This paper synthesized MnO2 catalysts with different crystal structures including α-, β-, γ-, and δ-phase for catalytic ozonation of CH2Cl2. Among them, δ-MnO2 exhibited superior activity (100 % mineralization rate for 100 ppm of CH2Cl2 at 120 °C under O3/DCM of 12.5) as well as decent stability and potential of multi-pollutants collaborative degradation, which was attributed to its abundant oxygen vacancies and mobile lattice oxygen species. Compared with traditional catalytic oxidation, O3 assistance facilitated the contact between reactants and catalyst and suppressed the accumulation of products, thus effectively reducing energy barrier of reaction. The evolution of intermediates from transient reaction studies during catalytic ozonation and oxidation further proved O3 introduction facilitated dechlorination. This study presents an efficient crystalline-engineering MnO2 for catalytic ozonation and verifies the superiority of catalytic ozonation.

New insights into electronic structure effect of the SnMnOx nanorod catalysts for low-temperature catalytic combustion of o-dichlorobenzene.

For the catalytic combustion reaction of chlorinated volatile organic compounds (CVOCs), the electronic structure of the catalyst surface are important factors in determining the activity, selectivity, and chlorine-resistance stability. Herein, a series of SnMnOx catalysts for the catalytic combustion of CVOCs were prepared by the changing of Sn-doping way to regulate the electron valance state of Mn element, including reflux (R-SnMnOx), co-precipitation (C-SnMnOx) and impregnation (I-SnMnOx). It was discovered that the R-SnMnOx catalyst had better activity and chlorine resistance than the R-MnOx, C-SnMnOx and I-SnMnOx catalyst, and the doping ways of Sn in MnOx catalyst could regulate greatly the surface acidity, active oxygen species, the chemical state of Mn&+ species, and redox ability. Especially, the R-SnMnOx catalysts exhibit excellent water resistance, and the reasons were related to the strong interaction of Sn&+ and Mn&+, which could promote obviously the dispersion of active Mn species, form a large number of acid sites, provide the abundant lattice oxygen species, and own the excellent redox ability, which accelerate the rate of charge transfer between Sn&+ and Mn&+ (Sn4++Mn2+→Sn2++Mn4+) to produce the abundant active species and accelerate the rapid conversion of benzene and intermediates conversion.

Enhanced Catalytic Performance of La-Doped CoMn2O4 Catalysts by Regulating Oxygen Species Activity for VOCs Oxidation

Tuning the oxygen species activity of transition metal oxide catalysts is essential for harnessing environmental catalytic oxidation reactions, and the mobility of lattice oxygen and activation ability of gas oxygen over CoMn2O4 were well modulated by in situ solvothermal-induced lanthanum doping for VOCs oxidation. Lanthanum species were successfully doped into the spinel lattice, and the toluene oxidation rate of Co1Mn1.9La0.1 increased by 20.74 times compared with that of the undoped Co1Mn2 catalyst at 220 °C. With lanthanum doping, the content of oxygen vacancies was remarkably increased (1.4-fold) due to oxygen atoms in unusual bonding situations. Higher mobility and reactivity of lattice oxygen species were achieved by weakening the metal–oxygen bond strength, favoring the conversion of toluene to CO2 and H2O. Furthermore, doping lanthanum into the spinel lattice also promoted the activation and replenishment of gas oxygen. Moreover, the oxidation of anhydride species, as a rate-determining step, could be significantly accelerated due to lanthanum doping. This work provided a credible comprehension of the regulation of spinel oxygen species by lanthanum doping in VOCs abatement to improve the atmospheric environment.

Oxygen activity regulation over LaNiO3 perovskites by Ti substitution for chemical looping partial oxidation of methane

The oxygen species in oxygen carriers directly influence the reaction performance in chemical looping processes. This paper describes the relationship between oxygen species and performance in chemical looping partial oxidation of methane (CLPOM) over LaNixTi1-xO3-δ (x = 0, 0.25, 0.5, 0.75 and 1) perovskites. Due to the increase of Ti substitution, the predominate reaction during CLPOM gradually changes from complete oxidation to partial oxidation. Quasi in situ XPS characterization suggests that the relative concentration of nucleophilic lattice oxygen species for partial oxidation increases with Ti substitution, while the relative concentration of electrophilic adsorbed oxygen species for complete oxidation decreases. The moderate Ti substitution for LaNi0.5Ti0.5O3-δ provides the suitable lattice oxygen activity for the methane partial oxidation. Moreover, LaNi0.5Ti0.5O3-δ exhibits high cyclic stability in redox cycles. This work demonstrates that the regulation of oxygen species distribution and activity is feasible to tailor the reaction performance of oxygen carriers in chemical looping processes.

Core-shell structure CuO-TiO2@CeO2 catalyst with multiple active sites for simultaneous removal of NO, Hg0 and toluene

The core–shell structure CuO-TiO2@CeO2 catalysts with multiple active sites synthesized via a self-assembly method was applied to study the simultaneous removal of NO, Hg0 and toluene. The results indicated that using CeO2 as sheath could remarkably enhance the catalytic performance. The CeO2 shell layer could not only be conducive to the exposure of active sites on the catalyst surface and enhance the adsorption of the toluene, NH3 and NO species, but also produce abundant oxygen vacancy defect sites through the strong interaction between CeO2 shell and CuO-TiO2 core. Coordinately NH3 on Lewis acid sites were the key intermediates for NH3-SCR reaction that takes place both through the Eley-Rideal and Langmuir-Hinshelwood pathways. Gaseous Hg0 was firstly adsorbed over multiple oxidation sites that generated via the redox cycle of Cu2+ + Ce3+ ↔ Cu+ + Ce4+, and then was oxidized to HgO by lattice oxygen species (Oβ). Meanwhile, NO could react with surface chemisorbed oxygen (Oα) to produce NO2, which could oxidize Hg0 into Hg(NO3)2. Toluene could be removed via the reaction path: toluene → benzyl alcohol → benzoate species → maleic acid species → CO2 + H2O. The interaction mechanisms between different gas components on CuO-TiO2@CeO2 were proposed.