Obtaining CeO2–ZrO2 mixed oxides by coprecipitation: role of preparation conditions

The preparation of CeO₂–ZrO₂ mixed oxides preparation was studied by evaluating the influence of several conditions. Coprecipitation was taken as the standard method and the effects brought about by the cerium salt precursor ((NH₄)₂Ce(NO₃)₆ or Ce(NO₃)₃), the introduction of drying and aging steps as well as pH controlling upon precipitation were analyzed. The samples were characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, infrared spectroscopy, oxygen storage capacity and surface area. The use of Ce(NO₃)₃ leads to the formation of c-CeO₂ and t-ZrO₂ mixed oxide whereas a solid solution is achieved by using (NH₄)₂Ce(NO₃)₆. It was observed that the cerium precursor is the most significant parameter of preparation procedure since it defines the crystalline phases and consequently the reducibility behavior of the CeO₂–ZrO₂ system.     

Desorption study of NO and O2 on Cu-ZSM-5

A detailed temperature-programmed desorption (TPD) study on NO and O₂ saturated Cu-ZSM-5 at different temperatures (300–723 K) has been performed. In the temperature range 373–723 K, the evolution of O₂ and NO₂ accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O₂ absorbed was very much lower than that of NO. The desorption profile of O₂ after contacting Cu-ZSM-5 with O₂ at 623 K showed a low temperature peak (369K) confirming the spontaneous ability of O₂ desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O₂ and vice versa have been performed at various temperatures (298–623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O₂ are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O₂ and other NO_x species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O₂ for the adsorption sites, therefore the adsorption of NO at 623 K on O₂ saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed     

Nanophase catalytic oxides: I. Synthesis of doped cerium oxides as oxygen storage promoters

Doped CeO₂ materials were synthesized with the aim to improve the performance of CeO₂ as oxygen storage promoter in gas catalytic reactions. The coprecipitation method was used for the synthesis of fine oxalate precursors of high homogeneity and well defined composition. The chemical and morphological properties of both the coprecipitated oxalates and the calcined oxides were examined. The influence of doping of different metal cations into the CeO₂ structure on the oxygen storage capacity in particular was investigated. Some of the doped oxides Ce_0.9M_0.1 O_{2 − δ} (M = Ca, Nd, Pb, etc.) give an increased oxygen storage capacity, 20–40% higher than the undoped. Their redox activity also remarkably increased.     

Oxidative coupling of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ tubular membrane reactors

A dense membrane tube made of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was prepared by plastic extrusion from BSCF oxide synthesized by the complexing EDTA-citrate method. The membrane tube was used in a catalytic membrane reactor for oxidative coupling of methane (OCM) to C₂ without an additional catalyst. At high methane concentration (93%), about 62% C₂ selectivity was obtained, which is higher than that achieved in a conventional reactor using the BSCF as a catalyst. The dependence of the OCM reaction on temperature and methane concentration indicates that the C₂ selectivity in the BSCF membrane reactor is limited by high ion recombination rates. If an active OCM catalyst (La-Sr/CaO) was packed in the membrane tube, C₂ selectivity and CH₄ conversion increased compared to the blank run. The highest C₂ yield in the BSCF membrane reactor in presence of the La-Sr/CaO catalyst was about 15%, similar to that in a packed-bed reactor with the same catalyst under the same conditions. However, the ratio of C₂H₄/C₂H₆ in the membrane reactor was much higher than that in the packed-bed reactor, which is an advantage of the membrane reactor.     

X-ray absorption fine structure analysis of local structure of CeO2–ZrO2 mixed oxides with the same composition ratio (Ce/Zr=1)

Three types of CeO₂–ZrO₂ (Ce:Zr=1:1 molar ratio) compounds with different oxygen storage/release capacities (OSCs) were characterized by means of the Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS). In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO₂–ZrO₂ solid solution, the OSC performance increased. Especially, the atomically homogeneous Ce_0.5Zr_0.5O₂ solid solution exhibited the highest OSC among these CeO₂–ZrO₂ samples. Additionally, the local oxygen environment around Ce and Zr was remarkably modified by enhancing the homogeneity of the CeO₂–ZrO₂ solid solution. It was postulated that the enhancement of the homogeneity of the CeO₂–ZrO₂ solid solution and the modification of the oxygen environment would be the source for the OSC improvement.     

Oxygen transport in zeolite Y measured by quenching of encapsulated tris(bipyridyl)ruthenium

This study deals with emission quenching of zeolite encapsulated trisbipyridyl ruthenium (II) (Ru(bpy)₃²⁺) by oxygen. Oxygen saturated solutions of Ru(bpy)₃²⁺ typically show about 70% quenching (I₀/I=3.3), where I₀ and I are the peak intensities of the emission in N₂ and O₂, respectively. However, an aqueous suspension of Ru(bpy)₃²⁺-zeolite Na–Y (Si/Al = 2.5) (abbreviated as Ru–Na–Y) showed no quenching at all. This observation motivated us to analyze how the transport of O₂ is occurring in the zeolite. Upon exposure of solid Ru–Na–Y (99% of intrazeolitic water) to N₂/O₂ dry gases, quenching in oxygen was found to be 5% (I₀/I=1.07). Partial dehydration at room temperature with loss of 33% of the water molecules from the zeolite led to 66% (I₀/I=2.96) quenching. Dehydration of Ru–Na–Y at 250 °C under vacuum overnight led to complete loss of intrazeolitic water and increased quenching to 90% (I₀/I=10.7). Nanocrystalline Ru(bpy)₃²⁺-zeolite Y upon vacuum dehydration lost 55% of the intrazeolitic water and showed 96% (I₀/I=25.3) quenching. The extent of quenching of Ru(bpy)₃²⁺ in zeolites by O₂ is by far the largest as compared to previously studied matrices, and is being attributed to confinement of O₂ in the supercages, which leads to increase in number of collisions with Ru(bpy)₃²⁺ and enhanced quenching. However, these samples showed complete lack of sensitivity (I₀/I=1) to oxygen upon exposure to water saturated gas or dissolved gas. Dealumination of zeolite framework by treatment with (NH₄)₂SiF₆ produced a framework of Si/Al = 9.5, and with SiCl₄ a framework of Si/Al > 100. With increasing dealumination, the extent of quenching by dissolved O₂ increased.     

Properties of adsorbed oxygen on Au/SiO2

Oxygen adsorption on silica-supported gold catalyst from NO₂ and O₂ exposures were investigated by temperature-programmed desorption spectroscopy under a vacuum condition. NO₂ and O₂ exposures of the surface of the catalyst at room temperature gave adsorbed oxygen in atomic state. Adsorbed oxygen penetrated beneath the gold with lower activation energy for NO₂ exposure than for O₂ exposure. Adsorbed oxygen in oxidic state which was desorbed above 600 K altered the surface properties of gold and resulted in the decrease of activation energy for oxygen to penetrate beneath the gold surface.     

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

CeO₂- and Ce_0.63Zr_0.37O₂-supported noble metal catalysts were studied. Samples were fully characterized using TEM, XRD, N₂ adsorption and H₂ 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.     

Surface reactivity of MgO oxygen vacancies

The interaction of gas-phase molecules (O₂, CO and H₂) with oxygen vacancies (F_s centers) on the MgO(1 0 0) surface was studied by cluster models ab initio wave functions. The F_s centers exhibit a high reactivity towards O₂, CO and H₂ at variance with the regular sites of the MgO surface. The reaction proceeds through the formation of radical anions, O⁻₂ and CO⁻, via the transfer of one electron trapped in the surface cavity to the empty levels of the adsorbed molecule, and can lead to the heterolytic dissociation of the H₂ molecule.     

Transport numbers and oxygen permeability of SrCe(Y)O3-based ceramics under oxidising conditions

Partial conductivities in the SrCe(Y)O_3−δ system have been studied in oxidising conditions in the temperature range 923–1273 K. Compositions with variable Y content (5 and 10 at.%), Sr deficiency (3 at.%), and with the addition of Fe₂O₃ as sintering aid (2 mol%) were analysed. A modified Faradaic efficiency method and oxygen permeation measurements were employed to appraise the oxide-ionic transport. Oxide-ion transference numbers in air lie in the range 0.19–0.80 and decrease with increasing temperature in the range 973–1223 K. Modelling of total conductivity as a function of oxygen partial pressure (p(O₂)) confirmed that protonic transport is minor under the studied conditions. SrCe_0.95Y_0.05O_3−δ exhibits greater oxide-ion conductivity than SrCe_0.9Y_0.1O_3−δ, indicative of dopant–vacancy association at high dopant contents. Conversely, oxygen permeability is slightly higher for SrCe_0.9Y_0.1O_3−δ as a result of faster surface-exchange kinetics. The oxygen flux through Fe-free membranes is dominated by the bulk in low p(O₂) gradients, when the permeate-side p(O₂) is higher than 0.03 atm, but surface exchange plays an increasing role with increasing p(O₂) gradient. Addition of Fe₂O₃ to SrCe(Y)O_3−δ lowers the sintering temperature by 100 K but results in the formation of intergranular second phases which block oxide-ionic and electronic transport, and thus oxygen permeation. The average thermal expansion coefficients (TECs) are (10.8–11.6) × 10⁻⁶ K⁻¹ in the temperature range 373–1373 K for all studied compositions.     

Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid

Catalytic wet air oxidation (CWAO) of aqueous solution of acetic acid (78 mmol L⁻¹) was carried out with pure oxygen (2 MPa) at 200 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/oxide, oxide = CeO₂, Zr_0.1Ce_0.9O₂, Zr_0.1(Ce_0.75Pr_0.25)_0.9O₂ and ZrO₂). Platinum was loaded on oxides by impregnation (5 wt%), and then the catalysts were reduced under H₂. Homogenous dispersions of 2–3 nm metal crystallites were obtained. The catalytic activity depended on the ability of the support to resist to the formation of carbonates. Ce(CO₃)OH species, determined by FT-IR and XRD, were rapidly formed during the CWAO reaction especially on mixed oxides. These carbonates were responsible to a drastic drop in catalytic performances. Amounts of carbonate species increase with the ability of the catalyst to transfer oxygen.     

Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor at high pressures

Oxygen permeation fluxes through dense disk-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm² min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO_x/γ-Al₂O₃ as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm² min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm² min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded.     

A new concept in high performance ceria–zirconia oxygen storage capacity material with Al2O3 as a diffusion barrier

CeO₂–ZrO₂ solid solution ((Ce,Zr)O₂) is an indispensable oxygen storage capacity (OSC) material for emission control in gasoline-fuelled automobiles. The high performance OSC material developed in this study is composed of Al₂O₃ as “a diffusion barrier” and (Ce,Zr)O₂ particles in intervening layers on a nanometer scale, and is abbreviated as “ACZ”. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of ACZ after durability testing in air at 1000 °C was 20 m²/g, which is higher than that of conventional CZ (2 m²/g) composed of (Ce,Zr)O₂ without Al₂O₃. After heat treatment at 1000 °C in air, the particle size of (Ce,Zr)O₂ in ACZ was about 10 nm and that without Al₂O₃ was one-half of the size in pure CZ. The OSC was roughly characterized by the total capacity (OSC-c1) and the oxygen release rate (OSC-r). In a fresh catalyst, ACZ and CZ had almost the same OSC-c1; however, the OSC-r of ACZ was twice as fast as CZ. After durability testing, the OSC-r of both ACZ and CZ were reduced significantly, but the OSC-r of ACZ was about five times as fast as CZ. While the OSC-c1 was hardly influenced by the (Ce,Zr)O₂ crystallite size and Pt particle size on the supports, the OSC-r was influenced by both of these parameters. The improvement of the OSC-r in the fresh catalyst and inhibition of the decrease in the OSC-r after durability testing were achieved by suppression of particle growth of (Ce,Zr)O₂ in ACZ by introducing Al₂O₃ as a diffusion barrier with resultant inhibition of sintering of Pt particles.     

One-electron reducibility of isolated copper oxide on alumina for selective NO–CO reaction

The H₂-TPR (temperature-programmed reduction) study was performed for supported copper oxide catalysts with low loading (0.5 wt% as copper). Among the various kinds of support materials (γ-Al₂O₃, TiO₂, ZrO₂, SiO₂, ZSM-5), alumina-supported copper oxide indicated a one-electron reduction behavior of Cu²⁺ into Cu⁺ ions in the presence of H₂. The reduction of the isolated Cu²⁺ species in a tetragonally distorted octahedral symmetry into the low coordinated Cu⁺ ions was identified by means of X-ray absorption spectroscopy (XANES and EXAFS). The isolated Cu⁺ ions hosted by γ-Al₂O₃ surface were prevented from further reduction into metallic Cu⁰ state under reducing condition with H₂ at 773 K. Less dispersed supported copper oxide species were easily reduced to Cu⁰ metal particles with H₂ at 573 K regardless of the kinds of support materials. It is suggested that the one-electron redox behavior of the isolated copper oxide species over γ-Al₂O₃ promotes the catalytic reduction of NO with CO in the presence of oxygen on the basis of redox-type mechanism between Cu²⁺ and Cu⁺ in atomically dispersed state.     

New insights in the understanding of the behaviour and performances of bismuth molybdate catalysts in the oxygen-assisted dehydration of 2-butanol

Pure Bi₂Mo₃O₁₂, Bi₂Mo₂O₉, Bi₂MoO₆, MoO₃ and -Sb₂O₄ and their mechanical mixtures were investigated in the oxygen-assisted dehydration of 2-butanol at atmospheric pressure and at low temperature (220 and 250°C). All catalysts were characterized before and after the catalytic reaction by BET surface area measurement, Raman spectroscopy, XRD and XPS. A strong parallelism is confirmed with the results obtained in the selective oxidation of olefins. In the frame of the remote control concept, , β and γ-bismuth molybdates are able to play a dual role: donor of spillover oxygen (Oso) with respect to MoO₃, and acceptor of Oso with respect to -Sb₂O₄. On one hand, this duality leads to mutual increase of activity when the bismuth molybdates are mixed together. In the presence of MoO₃, the phase seems to be a stronger Oso donor than β and γ, and β has a donor strength between and γ. On the other hand, when the Bi molybdates are reacted in the presence of a big quantity of spillover oxygen, like in a mixture with -Sb₂O₄, they undergo a dramatic decrease of activity. The phenomenon originates from the full oxidation of the reduced Mo species to Mo⁶⁺ induced by Oso. In parallel with other reactions involving oxygen, this confirms that the real active and selective state of molybdenum-containing oxides is that slightly reduced possessing Mo⁵⁺.     

Tetragonal structure, anionic vacancies and catalytic activity of SO4-ZrO2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO₄²⁻-ZrO₂ with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO₄²⁻-ZrO₂ catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H₂ was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work.

A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO₄²⁻-ZrO₂ is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.     

Simultaneous utilization of electro-generated O2 and H2 for H2O2 production: An upgrade of the Pd-catalytic electro-Fenton process for pollutants degradation

The Electro-Fenton (EF) process is one of the promising advanced oxidation processes (AOPs) for environmental remediation. The H₂O₂ yield of EF process largely determines its performance on organic pollutants degradation. Conventional Pd-catalytic EF process generates H₂O₂ via the combination reaction of anodic O₂ and cathodic H₂. However, the relatively expensive catalyst limits its application. Herein, a hybrid Pd/activated carbon (Pd/AC)-stainless steel mesh (SS) cathode (PACSS) was proposed, which enables more efficient H₂O₂ generation. It utilizes AC, the support of Pd catalyst, as part of cathode for H₂O₂ generation via 2-electron anodic O₂ reduction, and SS serve as a current distributor. Moreover, H₂O₂ could be catalytically decomposed upon AC to generate highly reactive ·OH, which avoids the use of Fe²⁺. Compared with conventional Pd catalyst, H₂O₂ concentration obtained by PACSS cathode is 248.2% higher, the O₂ utilization efficiency was also increased from 3.2% to 10.8%. Within 50 min, 26.3%, 72.5%, and 94.0% H₂O₂ was decomposed by Pd, AC, and Pd/AC. Fluorescence detection results implied that Pd/AC is effective upon H₂O₂ activation for OH generation. Finally, iron-free EF process enabled by PACSS cathode was examined to be effective for reactive blue 19 (RB19) degradation. After continuous running for 10 cycles (500 min), the PACSS cathode was still stable for H₂O₂ generation, H₂O₂ activation, and RB19 degradation, showing its potential application for organic pollutants degradation without increase in the running cost.     

A spillover approach to oxidation catalysis over copper and manganese mixed oxides

The activity of a hopcalite-type catalyst for H₂ and CO oxidation is compared with that of single-phase oxides CuO, Mn₂O₃ and CuMn₂O₄ (spinel) and a mixture of 1:1 CuO and Mn₂O₃ in order to elucidate the effects that are responsible for the high catalytic activity of the former. The reaction rates over the hopcalite catalyst calcined at 550°C (a mixture of CuO, Mn₂O₃ and CuMn₂O₄) are very close to those of the physical mixture of CuO and Mn₂O₃, being much greater than the rates over the single-phase oxides. CuO, Mn₂O₃ and CuMn₂O₄ show a kinetic compensation behavior both in H₂ and CO oxidation. By taking into account the activation energies and the reducibility measured by TPR it was concluded that the oxidation reactions over CuO follow a redox mechanism using lattice oxygen, while over Mn₂O₃ the mechanism is associative involving adsorbed oxygen species. Based on the TPR and kinetic results, the synergy between copper and manganese oxides in hopcalite and in the CuO–Mn₂O₃ mixture is assigned to a spillover effect.     

Oxygen partial pressure-dependent behavior of various catalysts for the total oxidation of VOCs using cycled system of adsorption and oxygen plasma

In this work, comprehensive investigation was done on the oxygen partial pressure-dependent behavior of the various catalysts using a flow-type plasma-driven catalyst (PDC) reactor. These data provide a useful guideline for the optimization of the cycled system using adsorption and the O₂ plasma-driven catalysis of adsorbed volatile organic compounds (VOCs). The potentials of the tested catalysts for the cycled system were evaluated based on the enhancement factor and the adsorption capability. All the tested materials (TiO₂, γ-Al₂O₃, zeolites) exhibited positive enhancement factor, while negative values with the dielectric-barrier discharge (DBD) plasma alone. TiO₂ catalysts showed the highest enhancement factor of about 100 regardless of the type of metal catalysts and their supporting amount. Based on the experimental findings in this study and the literature information, a plausible mechanism of plasma-driven catalysis of VOCs was suggested.     

F2, H2O, and O2 etching rates of diamond and the effects of F2, HF and H2O on the molecular O2 etching of (110) diamond

Oxidation kinetics of natural (110) diamond by oxygen and water were investigated using in situ Fizeau interferometry. Apparent activation energies of 53 and 26 kcal mol⁻¹ were obtained for the etching of (110) type Ia diamond by O₂ and H₂O respectively. The etch rate was found to follow second-order kinetics with respect to O₂ pressure in the pressure range 0.04–10 Torr. For water over the vapour pressure range 0.1–2 Torr, the reaction has a reaction order near unity. The diamond (110) surface was impervious to etching by molecular fluorine at all temperatures up to 1300 °C. Fluorine, hydrogen fluoride and water were found to inhibit the molecular oxygen etching of diamond. Below 900 °C, oxidation is inhibited by the addition of F₂ and HF presumably by blocking reactive sites on the diamond surface through formation of C---F bonds. Above 900 °C, the fluorine is thought to desorb from the diamond (110) surface, rendering the surface susceptible to further oxidation. Addition of water below 800 °C was found to retard etching by molecular oxygen. This is attributed to the formation of C---OH bonds, analogous to C---F.