Oxygen Storage Complete Capacity Research Articles

Defects Tune the Strong Metal-Support Interactions in Copper Supported on Defected Titanium Dioxide Catalysts for CO2 Reduction.

A highly active and stable Cu-based catalyst for CO2 to CO conversion was demonstrated by creating a strong metal-support interaction (SMSI) between Cu active sites and the TiO2-coated dendritic fibrous nano-silica (DFNS/TiO2) support. The DFNS/TiO2-Cu10 catalyst showed excellent catalytic performance with a CO productivity of 5350 mmol g-1 h-1 (i.e., 53,506 mmol gCu-1 h-1), surpassing that of almost all copper-based thermal catalysts, with 99.8% selectivity toward CO. Even after 200 h of reaction, the catalyst remained active. Moderate initial agglomeration and high dispersion of nanoparticles (NPs) due to SMSI made the catalysts stable. Electron energy loss spectroscopy confirmed the strong interactions between copper NPs and the TiO2 surface, supported by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. The H2-temperature programmed reduction (TPR) study showed α, β, and γ H2-TPR signals, further confirming the presence of SMSI between Cu and TiO2. In situ Raman and UV-vis diffuse reflectance spectroscopy studies provided insights into the role of oxygen vacancies and Ti3+ centers, which were produced by hydrogen, then consumed by CO2, and then again regenerated by hydrogen. These continuous defect generation-regeneration processes during the progress of the reaction allowed long-term high catalytic activity and stability. The in situ studies and oxygen storage complete capacity indicated the key role of oxygen vacancies during catalysis. The in situ time-resolved Fourier transform infrared study provided an understanding of the formation of various reaction intermediates and their conversion to products with reaction time. Based on these observations, we have proposed a CO2 reduction mechanism, which follows a redox pathway assisted by hydrogen.

Oxygen mobility and its relationship with the oxidative steam reforming of ethanol (OSRE)

This work focuses on the study of the relationship between oxygen mobility and catalytic activity in the oxidative steam reforming of ethanol (OSRE). The experimental strategy focused on the evaluation of redox properties using techniques such as isotopic oxygen exchange (IE 18O2), in addition to the partial oxygen storage capacity and oxygen storage complete capacity (OSC/OSCC) in mixed Ni and Co oxides. The effects of some synthesis parameters such as ultrasound and microwave assistance were also studied, as were the effects of incorporating promoters (Ce-Pr) on the oxygen mobility of the catalysts and their redox properties, establishing that the high mobility of oxygen is directly related to the performance of OSRE.

Nanoscale heterogeneity and low-temperature redox property of CeO2-ZrO2-La2O3-Y2O3 quaternary solid solution

In this paper, co-precipitation and surfactant-assisted co-precipitation were employed to synthesize CeO2-ZrO2-La2O3-Y2O3 quaternary solid solutions (CZ and CZ-N12), respectively. The redox properties, textural properties, structural properties, surface chemistry as well as the nano-structural features of the materials were investigated by hydrogen-temperature programmed reduction (H2-TPR), oxygen storage capacity complete (OSCC), powder X-ray diffraction (XRD), N2 adsorption-desorption, Raman, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) characterizations. Surfactant assisted co-precipitation shows great advantages than co-precipitation, which is presented as: CZ-N12 exhibits a low reduction temperature of 400 °C and a high OSC value of 424 μmol O2 g−1, which are about 149 °C lower and 165 μmol O2 g−1 higher than those of CZ, respectively. CZ-N12 also possesses better thermal stability than CZ, after aging at 1000 °C for 4 h, CZ-N12-a owns a high surface area of 38 m2 g−1 (18 m2 g−1 for CZ-a). In addition, XPS studies disclose that CZ-N12 exhibits Ce-enriched surface, high concentration of Ce3+ and higher amount of Oβ compared with CZ. TEM observations confirm the existence of compositional nanoscale domains in CZ-N12, which may be caused by the combustion of the surfactant. These results demonstrate that the greatly enhanced low-temperature redox activity of CZ-N12 is strongly related to the compositional and structural heterogeneity in nanoscale.

Oxygen Storage Capacity and Oxygen Mobility of Co-Mn-Mg-Al Mixed Oxides and Their Relation in the VOC Oxidation Reaction

Co-Mn-Mg-Al oxides were synthesized using auto-combustion and co-precipitation techniques. Constant ratios were maintained with (Co + Mn + Mg)/Al equal to 3.0, (Co + Mn)/Mg equal to 1.0 and Co/Mn equal to 0.5. The chemical and structural composition, redox properties, oxygen storage capacity and oxygen mobility were analyzed using X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), temperature-programmed reduction of hydrogen (H2-TPR), oxygen storage capacity (OSC), oxygen storage complete capacity (OSCC) and isotopic exchange, respectively. The catalytic behavior of the oxides was evaluated in the total oxidation of a mixture of 250 ppm toluene and 250 ppm 2-propanol. The synthesis methodology affected the crystallite size, redox properties, OSC and oxide oxygen mobility, which determined the catalytic behavior. The co-precipitation method got the most active oxide in the oxidation of the volatile organic compound (VOC) mixture because of the improved mobility of oxygen and ability to favor redox processes in the material structure.

Promoting effect of synthesis method on the property of nickel oxide doped CeO2-ZrO2 and the catalytic behaviour of Pd-only three-way catalyst

CeO2-ZrO2 doped by nickel oxide (CZN) with different method and Pd-only three-way catalysts (Pd/CZN) were prepared and characterized. The catalyst prepared by co-precipitation and supercritical drying process (Pd/CZN-s) exhibits the best catalytic activity and the widest operation working window before and after aging. Compared with other samples, CZN-s shows the formation of more homogeneous ternary solid solution with the smallest lattice parameter and crystallite size. Moreover, CZN-s maintains the high thermal and structural stability with higher surface area and larger pore volume after aging. Pd/CZN-s exhibits higher relative concentration of Ce3+ associated with the formation of more oxygen vacancies before and after aging, confirmed by X-ray photoelectron spectroscopy and Raman results. The more concentration of Ce3+ or oxygen vacancies leads to the enhancement of oxygen storage complete capacity of CZN-s. Moreover, the higher the oxygen storage capacity, the wider the working ranges of air/fuel variations.

Effect of iron doping into CeO2–ZrO2 on the properties and catalytic behaviour of Pd-only three-way catalyst for automotive emission control

Ce0.67Zr0.33O2 doped with iron oxide was prepared and the corresponding Pd-only three-way catalysts were examined and characterized. Pd/CZFe(1%) catalyst exhibits the best catalytic performance for CO, HC, NO and NO2 elimination and the widest operation window. The doping of iron oxide with 1% loading suggests the formation of more homogeneous Ce–Zr–Fe–O ternary solid solution, which seems to facilitate the reduction of Ce4+→Ce3+ or the formation of oxygen vacancy and to promote the interaction between Ce–Zr and Fe. Moreover, the Ce redox behaviour for surface reduction suggests depending not only on the formation of homogeneous Ce–Zr–Fe–O but also on the surface property of the sample. The increase in the concentration of oxygen vacancies under all atmospheres for CZFe(1%) sample also results in the enhancement of oxygen storage complete capacity.

Effect of synthesis condition on properties of Ce 0.67Zr 0.33O 2 mixed oxides and its application in Pd-only three-way catalysts

Ce 0.67Zr 0.33O 2 (CZ) mixed oxide was prepared by coprecipitation and effect of the synthesis conditions on CZ properties were investigated in this work, as well as the catalytic performance of Pd-only three-way catalyst (TWC) supported on the prepared CZ. With the increase of aging temperature during precipitation, the crystallite size of CZ increases significantly, while the oxygen storage capacity complete (OSCC) drops and no notable effect on porosity of CZ is observed. The porosity is mainly determined by drying temperature and pH value during precipitation. Prepared under optimized conditions (precipitation at the pH of 9.5, aging at 25 °C and then drying under supercritical condition in alcohol), the CZ samples exhibit good properties. This preparation procedure leads to CZ with proper pore size distribution, larger average pore size (20.42 nm), higher pore volume (0.69 cm 3/g) and enhanced OSCC value (383.0 μmol/g). Furthermore, the Pd-only TWC supported on the CZ synthesized under the optimized conditions exhibits satisfactory catalytic activity and very low reduction temperature (53 °C).

Influence of interaction between CeO 2 and USY on the catalytic performance of CeO 2–USY catalysts for deep oxidation of 1,2-dichloroethane

Three supported CeO 2–USY catalysts were prepared by different CeO 2 loading methods and evaluated for the deep oxidation of 1,2-dichloroethane (DCE). All the catalysts were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), ammonia temperature-programmed desorption (NH 3-TPD), diffuse reflectance infrared spectra of pyridine adsorption (DRIFT), hydrogen temperature-programmed reduction (H 2-TPR), oxygen storage capacity complete (OSCC) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that a strong synergy effect occurs in the CeO 2–USY catalysts prepared by impregnation method and mechanical grinding of USY and Ce(NO 3) 3·6H 2O. Moreover, higher dispersion of CeO 2 species, better mobility of oxygen species of the catalysts as well as better catalytic activity for DCE decomposition are obtained over these two catalysts. All the CeO 2–USY catalysts show a high selectivity towards the formation of HCl. In addition, the CeO 2–USY catalyst prepared by impregnation method maintains a high conversion in the 100 h test of DCE decomposition.

Influence of H 2/O 2 redox treatments at different temperatures on Pd-CeO 2 catalyst: Structure and oxygen storage capacity

In order to investigate the metal–support interaction in PM-CeO 2 system, a Pd-doped CeO 2 catalyst was prepared by the sol–gel method and was treated in the H 2–O 2 redox atmosphere at 300, 500 and 800 °C, respectively. The Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results indicate that the redox treatment at 300 °C mainly affect the surface properties of the catalyst, while those at higher temperatures have an influence on the bulk properties. The step-scanning XRD and Raman results prove that Pd species on the surface of the sample mainly exist in the form of bulk Pd and PdO crystallites after the redox treatment at 500 °C, which are encapsulated by CeO 2 particles after the treatment at 800 °C. The CO oxidation activity, dynamic oxygen storage capacity (DOSC) and oxygen storage complete capacity (OSCC) of the catalyst vary with the treating temperature. The 300 °C-treated sample presents the highest activity for CO oxidation and oxygen storage capacity. The reduction-ratio dependent activation energy ( E a) is extremely low for the surface reduction of this sample, suggesting the participation of highly active oxygen. The active oxygen are suggested to originate from the enlarged Pd-Ce interface, which depends on the avaibility of surface Pd sites and contact between metal and support.

Preparation of Ce0.67Zr0.33O2 mixed oxides as supports of improved Pd-only three-way catalysts

Four types of Ce0.67Zr0.33O2 (CZCP, CZH, CZHP and CZM) mixed oxides as supports were prepared by coprecipitation, hydrothermal, homogeneous precipitation and microemulsion methods, respectively. The results show that both the preparation methods and the aging procedures have significant influences on the physicochemical properties of Ce0.67Zr0.33O2 mixed oxides. All these fresh Ce0.67Zr0.33O2 samples develop mesopore structure, especially that, CZCP obtains the largest diameter (20.87nm) and volume (0.455cm3/g). The XRD results indicate that fresh CZHP and CZCP samples contain cubic and tetragonal phases, while only cubic solid solution is found in fresh CZH and CZM samples, which show better low-temperature reducibility. Moreover, higher oxygen storage capacity complete (OSCC) values and faster O2 uptake are observed in CZH and CZM samples compared with those of fresh CZCP and CZHP. After calcination at 1100°C, it is found that the OSCC values and the kinetics of the O2 uptake for CZM and CZHP samples are deteriorated due to the coexistence of cubic and tetragonal phases as well as strong sintering of samples. On the contrary, the aged CZCP and CZH samples keep satisfactory low-temperature reducibility and oxygen storage capacity. For fresh catalysts, the pore-size distribution of mixed oxides seems to be important for catalytic activities. In contrast, for aged catalysts, their activities are related to both the redox properties and the textural properties of aged mixed oxides. Therefore, the Pd-only three-way catalyst supported on the CZCP exhibits the most promising amplified amplitude of stoichiometric window.

Deterioration of the oxygen storage and release properties of CeZrO 4 by incorporation of calcium

Calcium was added to CeZrO 4 mixed oxide by wet impregnation with a Ca nitrate solution to achieve solids with nominal Ca/(Ce + Zr) atomic ratios of 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5. The solids were calcined at 873 K to decompose the Ca nitrate to CaO. The determination of oxygen storage capacity complete (OSCC) and FTIR studies of the formation of surface superoxide radicals (O − 2) were conducted to study the oxygen storage and release (OSR) properties of the solids. Both measurements indicate that the OSR properties clearly deteriorated for samples with nominal Ca/(Ce + Zr) at. ratios >0.01 (0.9 atoms of Ca nm −2). The degree of deactivation increased with increasing Ca loading; for the sample with the largest Ca loading, the OSCC was ca. 3-fold lower than that of bare CeZrO 4. The solids also were characterized by TGA, chemical analysis by TXRF, XRD, N 2 isotherms, and XPS. The results indicate that the Ca remained on the surface of the CeZrO 4 as supported CaO; the formation of large bulky crystals of CeZrCa mixed oxides by solid-state reaction between the Ca phase and the CeZrO 4 did not occur. For Ca loading below the amount required to completely cover the surface (which, experimentally, is ca. 18 atoms of Ca per nm 2 of surface), CaO existed as very well-dispersed CaO domains few atoms thick. Above this value, the surface was completely covered by CaO, and three-dimensional crystals of CaO were formed. Given these results, we propose that the main mechanism for the deterioration of OSR properties of CeZrO 4 by incorporation of CaO on the surface by impregnation is fouling: CaO physically prevents contact between the O 2 gas and the surface sites of the CeZrO 4. It also is likely that a very limited solid-state reaction occurs between the latter two phases, with a CaZrO 3-like amorphous oxide formed at the grain boundaries. The formation of this amorphous oxide also can chemically contribute to the deactivation. The relevance of these results in the understanding of the deactivation of TWC by Ca is discussed.

Thermal ageing of Pt on low-surface-area CeO2–ZrO2–La2O3 mixed oxides: Effect on the OSC performance

This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO2–ZrO2–La2O3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900°C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700°C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700°C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles.

Effect of Manganese Doping on Oxygen Storage Capacity of Ceria-Zirconia Mixed Oxides

CeO 2-ZrO 2-MnO x mixed oxide series were prepared by sol-gel method. CO pulse and CO-O 2 cycle measurements were carried out to examine the oxygen storage complete capacity (OSCC) and dynamic oxygen storage capacity (OSC) of the samples. The doping method brought about strong interactions between manganese oxide and ceria, both in the bulk and on the surface. Only a small part of Mn cations are incorporated into the ceria lattice to form solid solutions and the remaining are left on the surface as finely dispersed Mn 3O 4. The OSC behaviors of the materials are influenced by the doping amount of Mn and the solubility of Mn in the CeO 2 lattice. The OSC is more easily affected by available contents of oxygen storage components when the measurement frequency is low. Comparatively, the concentration of lattice defects, which affects the mobility of bulk oxygen, is the determining factor under high frequency.

Metal−Support Interaction and Redox Behavior of Pt(1 wt %)/Ce0.6Zr0.4O2

The catalyst Pt(1 wt %)/Ce(0.6)Zr(0.4)O(2) is studied by CO-temperature programmed reduction (CO-TPR), isothermal oxygen storage complete capacity (OSCC), X-ray absorption spectroscopy (XAS) at the Pt L(III) edge, and in situ X-ray diffraction (in situ XRD), with the aim of elucidating the role of supported metal in CO oxidation by ceria-based three-way catalysts (TWC). The redox behavior of Pt(1 wt %)/Ce(0.6)Zr(0.4)O(2) is compared to that of bare ceria-zirconia. OSCC of redox-aged Pt/ceria-zirconia is twice that of bare ceria-zirconia, and the maximum of CO consumption occurs at a temperature about 300 K lower than redox-aged ceria-zirconia. XAS analysis allows one to evidence the formation of a platinum-cerium alloy in redox-aged samples and the stability of the metal particles toward oxidation and sintering during high-temperature treatments. Under CO flux at 773 K, bare ceria-zirconia shows a continuous drift of diffraction peaks toward smaller Bragg angles, due to a progressive increase of Ce(III) content. Under the same treatment, the structural rearrangement of Pt-supported ceria-zirconia starts after an induction time and takes place with an abrupt change of the lattice constant. The experimental evidence points to the role of supported Pt in modifying the redox properties of ceria-zirconia with respect to the bare support. It is proposed that the much faster bulk reduction observed by in situ XRD for redox-aged Pt/ceria-zirconia can be attributed to an easier release of reacted CO(2), producing a more effective turnover of reactants at the catalyst surface.

Ceria-Zirconia Nanostructured Materials for Catalytic Applications: Textural Characteristics and Redox Properties

Highly dispersed solid solutions of ceria-zirconia with nominal composition Ce0.6Zr0.4O2 were prepared by sol-gel technique and the influence of the gel aging time (1 day–3 weeks) on the morphological and textural properties of the materials was investigated. The samples were calcined at 923 K for 8 h, then characterized by XRD, BET surface area and pore size measurements by gas adsorption and by mercury porosimetry. DTA, TEM and SEM analyses were also carried out. The aging of the gel seems to have a remarkable influence on the morphology of the materials, whose surface area and porosity increase by increasing aging time. The phase composition of the solid solutions of ceria-zirconia, as determined by XRD analysis, seems to be independent from the synthetic procedure; in any case a cubic phase (space group Fm-3m) was identified as the main component. XRD and TEM characterizations show the presence of highly dispersed crystallites, with diameters in the range 60–80 A. The redox properties and hydrogen chemisorption capacity have been studied by TPR/TPD/TPO experiments. Oxygen storage capacity measurements were carried out by a pulse technique, to determine the overall reducibility of the solid, OSCC (Oxygen storage capacity complete). OBC (Oxygen buffering capacity) measurement were also performed to determine the ability of the ceria-zirconia oxides to attenuate fast oscillations (0.1 Hz) of oxygen partial pressure. The correlation between textural, structural and redox properties of the oxides is discussed.

Towards the comprehension of oxygen storage processes on model three-way catalysts

CeO 2- and Ce 0.63Zr 0.37O 2-supported noble metal catalysts were studied. Samples were fully characterized using TEM, XRD, N 2 adsorption and H 2 chemisorption. The oxygen storage process was investigated focusing on the evolution as a function of temperature of both the oxygen storage capacity (OSC) and the oxygen storage complete capacity (OSCC). Aging effect on OSC was also examined in details in the case of Rh catalysts. Finally, the major role of oxygen diffusion, partly influenced by the metal/support interface quality, was confirmed.