Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Potassium nitrate catalysts supported on different oxides (CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO₂ and ZrO₂, Ce_0.5Zr_0.5O₂ presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce_0.5Zr_0.5O₂, the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.     

Graphene-supported Pd–Ru nanoparticles with superior methanol electrooxidation activity

Pd–Ru bimetallic nanoparticles dispersed on graphene nanosheets (GNS) have been obtained by a microwave-assisted polyol reduction method and investigated for methanol electrooxidation in 1 M KOH + 1 M CH₃OH at 25 °C. Structural and electrochemical characterizations of electrocatalysts are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, CO stripping voltammetry and chronoamperometry. The study shows that introduction of Ru (1–10 wt.%) into 40 wt.%Pd/GNS produces an alloy of Pd and Ru with the face centered cubic crystal structure. The electrocatalytic activity increased with increasing percentage of Ru in the Pd–Ru alloy showing maximum with 5 wt.%Ru. The electrocatalytic activity of the 40 wt.%Pd–5 wt.%Ru/GNS electrode at E = −0.10 V vs. Hg/HgO was ∼2.6 times greater than that of the base (40 wt.%Pd/GNS) electrode. Based on the methanol oxidation current, measured at 1 h during the chronoamperometry tests at E = −0.10 V vs. Hg/HgO, the active 40 wt.%Pd–5 wt.%Ru/GNS electrode exhibited ∼72% and ∼675% higher poisoning tolerance as compared to 40%Pd/GNS and 40%Pd/multiwalled carbon nanotube electrodes, respectively.     

Effect of preparation method and particle size on LaMnO3 performance in butane oxidation

LaMnO₃ with different crystal domain sizes and surface areas were prepared by citrate and sol-gel combustion methods and tested as catalysts for butane total oxidation reaction. The catalysts were characterized by N₂ physisorption, XRD and SEM. LaMnO₃ with crystal domain sizes in the range of 30–90 nm were detected by XRD characterization when high calcination temperature, at least 500 °C for sol-gel combustion method and 700 °C for citrate method, was required to prepare pure nanocrystalline phase. Although LaMnO₃ prepared by these two methods had similar crystal domain sizes, BET surface areas of samples by citrate method were significantly larger than that of samples prepared by combustion method. The difference of surface area lies in the morphology differences between the two series of samples (SEM micrographs) generated by strong sintering of samples prepared by combustion method. The catalytic activity of LaMnO₃ in butane total oxidation increased with increasing surface area being higher for materials prepared by citrate method. Thus, citrate method showed significant advantages over combustion method in preparation of perovskite catalysts.     

Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO2)

High-density monoclinic ZrO₂ was manufactured through sintering at ~1200 °C by using nanosized powders. Then, the electrical conductivity was measured at a range of high temperatures (700–900 °C) by electrical impedance spectroscopy (EIS). For the as-sintered monoclinic ZrO₂, the measured electrical conductivity was 3.2×10⁻⁵ s/cm (for 80% TD) and 4.4×10⁻⁵ s/cm (for 89% TD) at 900 °C. After aging at 900 °C for 100 h, the electrical conductivity of the monoclinic ZrO₂ of 80%-TD decreased by more than 50%. However, after reheating at 1200 °C for 1 h, approximately 80% of the conductivity was recovered compared to the value of the as-sintered monoclinic ZrO₂. The pure monoclinic crystal structure was retained despite the aging and reheating treatment. Based on microstructural observations of the aged and reheated monoclinic ZrO₂, the changes in electrical conductivity after aging and reheating were explained by the formation and recovery of micro-cracks, respectively.     

Thermal ageing of nanostructured tetragonal zirconia ceramics: Characterization of interfaces

This paper reports the preparation of nanometric powders of 3.5 mol% Y₂O₃-doped ZrO₂, with controlled microstructure, by the spray pyrolysis process, assisted by ultrasonic atomizer, at relatively low temperature. As-prepared powders were found crystalline and consisted of dense and chemically homogeneous spherical particles. Conventional sintering at 1500 °C for 2 h in air yields dense ceramics of 83 nm of average grain size. The electrical properties of electrode/electrolyte interface were determined by impedance spectroscopy measurements before, during and after thermal ageing for 2000 h at 700 °C in dry air. The effect of thermal ageing on the electrical responses of the ceramic and interfaces with platinum electrodes was investigated.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Molten salt attack on t′ yttria-stabilised zirconia by dissolution and precipitation

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na₂SO₄–30 mol% NaVO₃ on ZrO₂–8 mol% YO_1.5 (8YSZ) at 950 °C was studied by two types of experiments. Sintered compacts were exposed to 25 mg cm^?2 salt dosage for up to 96 h. In the other set of experiments, 10–35 wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25 wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950 °C, followed by slow reactions to form YVO₄ and NaYV₂O₇ at 950 °C. The unreacted Y₂O₃ and monoclinic ZrO₂ precipitate out separately during rapid cooling (～300 °C/min). Slow cooling at ～3 °C/min leads to the formation of ZrOS apart from ZrO₂ and Y₂O₃.     

High temperature strength of hot pressed ZrB2–20 vol% SiC ceramics based on ZrB2 starting powders prepared by different carbo/boro-thermal reduction routes

ZrB₂–20 vol% SiC (ZS) ceramics based on ZrB₂ starting powders obtained by different boro/carbo-thermal reductions involving ZrO₂ + B₄C, ZrO₂ + B₄C + C, and ZrO₂ + B, were fully densified by hot pressing at 1900–2000 °C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600 °C. At 1600 °C, the flexural strength of the ceramics is 460 ± 31, 471 ± 32 and 345 ± 11 MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects.     

Synthesis of homoallyl alcohol from 1,4-butanediol over ZrO2 catalyst

Catalytic dehydration of 1,4-butanediol was investigated over ZrO₂ with monoclinic crystal phase at temperatures of 275–425 °C. In the reaction, 3-buten-1-ol was effectively produced together with tetrahydrofuran (THF) over ZrO₂ at 300–375 °C. 3-Buten-1-ol and THF were produced competitively, and the yield of 3-buten-1-ol increased with increasing the contact time. Formation of by-products such as 2-buten-1-ol, 2-butenal, and 1-butanol was suppressed over ZrO₂ at 325 °C.     

One-pot synthesis of monoclinic ZrO2 nanocrystals under subcritical hydrothermal conditions

This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO₂) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO₃)₂) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 °C for 24 h to obtain pure monoclinic-structured ZrO₂ nanocrystals. The crystallization temperature of the ZrO₂ phase was also greater than 200 °C. However, the products of reactions conducted at 200 °C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 °C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO₂ nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.     

Fabrication of well-crystallized mesoporous ZrO2 thin films via Pluronic P123 templated sol–gel route

Mesoporous zirconia (ZrO₂) thin films were prepared by dip-coating via Pluronic P123 templated sol–gel route. ZrOCl₂·8 H₂O was used as zirconium (Zr) precursors. Annealing of as-coated ZrO₂ thin films is important in order to stiffen the respective films and to remove the Pluronic P123. The mesoporous structure and crystallite size of ZrO₂ were characterized systematically by field-emission scanning electron microscope (FESEM), both low- and wide-angle X-ray diffraction, thermal analysis technique and Brunauer–Emmett–Teller method. At annealing temperature of 400 °C, amorphous ZrO₂ was transformed into tetragonal phase of ZrO₂ (t-ZrO₂). At 450 °C, t-ZrO₂ and monoclinic phase of ZrO₂ (m-ZrO₂) were obtained. By altering heating rate during annealing, volume fraction of t-ZrO₂ and m-ZrO₂ was changed. FESEM images showed that disordered mesostructures of ZrO₂ were formed after annealing. The surface area of mesoporous ZrO₂ obtained ranges from 54.33 to 93.39 m²/g.     

Synthesis of zirconium diboride platelets from mechanically activated ZrCl4 and B powder mixture

ZrB₂ platelets were prepared by mechanochemical processing a zirconium (IV) chloride–boron mixture with subsequent annealing from 800 °C to 1200 °C. The phases present were identified by X-ray diffraction. The size and morphology of the synthesized ZrB₂ powders were characterized by scanning electron and transmission electron microscopy. At 800 °C, ZrO₂ was detected in absence of ZrB₂. At or above 1000 °C, ZrCl₄–B converted to ZrB₂. Moreover, at 1200 °C, ZrCl₄–B completely converted to ZrB₂ without trace quantities of residual ZrO₂. The synthesized ZrB₂ consisted of platelets with a diameter of 0.1–2.1 μm and a thickness of 40–200 nm.     

Vapor-phase dehydration of 1,5-pentanediol into 4-penten-1-ol

Dehydration of 1,5-pentanediol was investigated over ZrO₂ and Yb₂O₃ catalysts at 300–450 °C. 1,5-Pentanediol was converted into 4-penten-1-ol together with tetrahydropyran over monoclinic ZrO₂ at temperatures <400 °C, and the selectivity to 4-penten-1-ol exceeded 50 mol%. Modification of ZrO₂ with Li ions increased the selectivity to 4-buten-1-ol up to 70 mol%. Yb₂O₃ also effectively worked as a catalyst in the dehydration of 1,5-pentanediol into 4-buten-1-ol at temperatures <425 °C. Especially, Yb₂O₃ with cubic structure showed higher than 75 mol% selectivity to 4-penten-1-ol.     

Self-oscillations of methane oxidation rate over Pd/Al2O3 catalysts: Role of Pd particle size

Oscillations of the methane oxidation rate were studied under methane-rich conditions on Pd/Al₂O₃ catalysts differing in Pd particle size. It was demonstrated that the temperature interval where oscillations occur narrows from 300–360 °C for the catalyst with Pd particle aggregates from 50–100 nm to 345–355 °C for the catalyst with isolated Pd particles of ~ 5 nm in size. At the same time, the period of oscillations showed ~ 6-fold increase. Structural transformations of Pd in the oscillation cycle were similar to those observed on bulk Pd used as a catalyst in the same reaction.     

The CO methanation over NaY-zeolite supported Ni/Co3O4, Ni/ZrO2, Co3O4/ZrO2 and Ni/Co3O4/ZrO2 catalysts

The CO methanation was studied over zeolite NaY supported Ni, Co₃O₄, ZrO₂ catalysts. The XRD, N₂ physisorption and SEM analysis were used in order to characterize the catalysts. Catalytic activities were carried out under a feed composition of 1% CO, 50% H₂ and 49% He between the 125 °C to 375 °C. Except for the Ni/Co₃O₄/NaY catalyst, all catalysts gave high surface area because of the presence of zeolite NaY. Average pore diameter of the catalysts fell into the mesopore diameter range. The highest CO methanation activity was obtained with Ni/ZrO₂/NaY catalyst at which the CO methanation was started after 175 °C and 100% CO conversion was obtained at 275 °C using the same catalyst.     

Internal friction investigation of phase transformation in nearly stoichiometric LaMnO3+δ

Rhombohedral LaMnO_3+δ powders, prepared by two different soft chemistry routes (co-precipitation and hydrothermal synthesis), are sintered at 1400 °C for 2 h in air. Measurements of internal friction Q⁻¹(T) and shear modulus G(T), at low frequencies from −180 to 700 °C under vacuum, evidence three structural transitions of nearly stoichiometric orthorhombic LaMnO_3+δ. The first one, at 250 or 290 °C, depending on the processing followed, is associated to either a Jahn–Teller structural transition or a phase transformation from orthorhombic to pseudo-cubic. The second one at 610 or 630 °C is related to a phase transformation from pseudo-cubic or orthorhombic to rhombohedral. Below the Neel temperature, around −170 °C, a relaxation peak could be associated, for samples prepared according to both processing routes, to the motion of Weiss domains.     

Preparation and characterization of polymer-derived Zr/Si/C multiphase ceramics and microspheres with electromagnetic wave absorbing capabilities

Precursors for Zr/Si/C multiphase ceramics were synthesized by the reactions of dilithiozirconocene complex with dichlorodimethylsilane, methyltrichlorosilane and dichloromethylvinylsilane, respectively. The precursor-to-ceramic process of the precursor was investigated by TG-GC–MS and TG-FTIR analyses, confirming a complete transformation from organometallic polymers into ceramics below 800 °C. Annealing experiments of the derived ceramics at temperatures from 1000 °C to 2000 °C indicated the crystallization from ZrSiO₄, ZrO₂ to ZrC. Furthermore, micrometer-sized Zr/Si/C ceramic microspheres were successfully fabricated from the precursor at 1000 °C, showing surface morphology like wrinkled pea. According to the XRD, HRTEM and XPS analyses, such multiphase ceramic microspheres consist of ZrSiO₄, ZrO₂, and amorphous SiO_xC_y. Interestingly, the ceramic microspheres performed satisfactory electromagnetic wave absorbing capacity with the RL_max reaching −34 dB, which could be potential candidates for electromagnetic micro-devices.     

Investigation of high-temperature reactions within the ZrSiO4–Al2O3 system

Solid state reactions between ZrSiO₄ and αAl₂O₃ in powders of stoichiometric composition 3Al₂O₃·2SiO₂ were studied by X-ray diffraction and electron scanning microscopy with energy dispersive X-ray analysis (SEM + EDX). Data were obtained at temperature ranging from 1400 °C to 1600 °C for a period of time ranging from 30 min to 60 h. The results indicate that ZrSiO₄ and αAl₂O₃ react and form crystalline ZrO₂, crystalline mullite (almost 3Al₂O₃·2SiO₂ composition) and non-crystalline silicon–alumina phase (pre-mullite). At the temperature of 1600 °C the fastest stage of reaction is dissociation of ZrSiO₄. Obtained results show that dissociation of zircon is a first-order reaction. The dissolution of Al₂O₃ particles and diffusion of Al into non-crystalline phase seem to be the slowest step of the reaction.     

Effect of Al Content on the Isomerization Performance of Solid Superacid Pd–S2O82−/ZrO2 –Al2O3

The effect of Al content on the performance of the Pd–S₂O₈²⁻/ZrO₂ –Al₂O₃ solid superacid catalyst was studied using n-pentane isomerization as a probe reaction. The catalysts were also characterized by X-ray diffraction (XRD), Fourier transform Infrared (FTIR), specific surface area measurements (BET), thermogravimetry–differential thermal analysis (TG–DTA), H₂-temperature programmed reduction (TPR) and NH₃ temperature-programmed desorption (NH₃-TPD). The Pd–S₂O₈²⁻/ZrO₂ –Al₂O₃ catalyst made from Al₂O₃ mass fraction of 2.5% exhibited the best performance and its catalytic activity increased by 44.0% compared with Pd–S₂O₈²⁻/ZrO₂. The isopentane yield reached 64.3% at a temperature of 238 °C, a reaction pressure of 2.0 MPa, a space velocity of 1.0 h ^− 1 and a H₂/n-pentane molar ratio of 4.0. No obvious catalyst deactivation was observed within 100 h.