Effect of the properties of hydrous zirconia prepared under various hydrothermal conditions on the isomerization activity of Pt/WO3-ZrO2

A series of hydrous zirconia samples were prepared by hydrothermal method and the effects of the properties of hydrous zirconia on the catalytic activity of Pt/WO₃-ZrO₂ in the hexane isomerization were investigated. The catalysts were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and differential thermal analysis (TDA), H₂-temperature programmed reduction (H₂-TPR) and NH₃-temperature programmed desorption (NH₃-TPD). The results showed that the hydrothermal treatment under different times and pH values led to remarkable changes in the properties (such as hydroxyl group, ordering degree and thermal stability) of hydrous zirconia. Moreover, the isomerization activity of Pt/WO₃-ZrO₂ varied distinctly with the hydrothermal treatment condition of hydrous zirconia. The correlation between the properties of hydrous zirconia and the isomerization activity of the catalyst was primarily established. It was proposed that the isomerization activity was strongly dependent on the stability and ordering degree of hydrous zirconia, while it was irrelevant to the amount of hydroxyl groups in hydrous zirconia.     

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO₃/ZrO₂, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO₄/ZrO₂) is compared with a Pt/WO₃/ZrO₂ catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO₃/ZrO₂ catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.     

Improving then-pentane hydroisomerization of Pt/SO 4 2− SiO2) catalysts through mixing with ZrO2

The performance of Pt catalysts supported on sulfated zirconia-silica with different stoichiometries is investigated in then-pentane hydroisomerization reaction. Comparatively, with respect to the Pt/SO ₄ ^2– -SiO₂ or Pt/SO ₄ ^2– -ZrO₂ catalysts, the sulfated mixed oxides show an enhancement of the catalytic activity that increases with the content of ZrO₂, reaching its maximum at values between 10 and 15 wt% zirconia. The characterization of the samples reveals that at this stoichiometry occurs the highest H2-consumption of the samples as well as the top value of strong Brónsted acid sites according to the TPD-H₂ and FTIR measurements of absorbed pyridine respectively. That is, close to these percents of zirconia content one has a compound that is homogeneously mixed and above those values the segregations of the single oxides occur as verified by X-ray diffraction characterization.     

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO₂ on the metallic properties of Pt, when supported on WO₃–ZrO₂, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO₂,^2- deposited on amorphous Zr(OH)₄ before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tungsten oxides supported on nano-size zirconia for cyclic production of syngas and hydrogen by redox operations

For cyclic production of syngas and H₂ by redox (methane reforming-water splitting) operations, samples of tungsten oxides supported on nano-size zirconia (WO₃/n-ZrO₂) were investigated at 1,223 and 1,273 K and compared with those on micron-size zirconia (WO₃/µ-ZrO₂). The reduction characteristics of WO₃/n-ZrO₂ observed in this study were consistent with those of WO₃/µ-ZrO₂ reported in the literature. Specifically, the reduction process comprised three stages, the syngas production rate decreased as WO₃ content increased, and the overall degree of reduction gradually decreased with repeated cycles. However, there were differences due to the smaller particle size, namely, WO₃/n-ZrO₂ yielded a higher syngas production rate and a lower H₂/(CO+CO₂) ratio. In addition, the hydrogen yield by water splitting was significantly lower than the amount expected based on the overall degree of WO₃ reduction. The H₂/(CO+CO₂) ratio also gradually decreased with repeated cycles. These results were mainly attributed to rapid sintering of WO₃/n-ZrO₂, which gradually began to resemble WO₃/µ-ZrO₂.     

Effect of surface finishing and thickness on the translucency of zirconia dental ceramics

The effects of different surface finishing procedures on the translucency of yttria stabilized zirconia ceramics (ZrO₂) were evaluated. ZrO₂-3mol.%Y₂O₃ (designed Zr3) or ZrO₂-5mol.%Y₂O₃(Zr5) specimens with thickness varying between 0.5 and 1.5 mm were obtained by sintering at 1500 °C for 2 h. Surface finishing of the sintered specimens was done in three distinct manners: polishing with diamond pastes, blasting with Al₂O₃ particles and blasting with Al₂O₃–SiO₂ particles, according to the preparation protocol for dental prostheses. These sample groups were characterized by X-Ray diffraction analysis, scanning electron microscopy, surface roughness and spectrophotometry. The sintered ceramics exhibited relative densities higher than 99% and were composed of 68.5% and 31.5%of tetragonal (t)-ZrO₂ and cubic(c)-ZrO₂ for composition Zr3 and 25.2% and 74.8% for composition Zr5, respectively. Furthermore, 1.5 to 1.8% of monoclinic (m)-ZrO₂, has been found in the blasted surfaces. The average grain size varied from 0.5 μm for Zr3 to 1.45 μm for the Zr5 samples. After polishing, all samples presented a surface roughness R_aof about 0.05 μm, while blasting resulted in an increased roughness ranging between 1.24 μm and 1.49 μm.Samples rich in (c)-ZrO₂ phase (Zr5) are more translucent than samples rich in (t)-ZrO₂ (Zr3) because of their larger grain size and because the cubic phase is less anisotropic than (t)-ZrO₂. Furthermore, the translucency of thinner samples is more affected by abrasive blasting because they also present the highly anisotropic monoclinic(m)-ZrO₂ phase and, therefore, the reduction of translucency is more pronounced. Parameters such as grain size, crystalline phase composition, porosity and grain boundary density are used to explain the phenomena involved in the differences of translucency of these materials.     

Influence of La3+ additions on the phase behaviour and antibacterial properties of ZrO2–SiO2 binary oxides

The present study reports the influence of lanthanum (La³⁺) content on the phase stability and antibacterial activity of ZrO₂–SiO₂ binary oxides. Four different concentrations of La³⁺ additions in ZrO₂–SiO₂ binary oxides were synthesized using a sol–gel technique. Heat treatment of the synthesized powders resulted in the formation of t-ZrO₂ phase at 1000 °C. Heat treatment beyond 1000 °C resulted in the phase degradation of t-ZrO₂ to yield m-ZrO₂ and ZrSiO₄. Results from antibacterial tests confirmed the potential activity of La³⁺ doped ZrO₂–SiO₂ binary oxides in countering the microbial invasion.     

Structure and mechanical properties of nanofibrous ZrO2 derived from alternating field electrospun precursors

Nanofibrous zirconia (ZrO₂) meshes were prepared from precursor fibers which were synthesized using the method of free-surface, high-yield alternating field electrospinning (AFES). The weight ratio of zirconyl chloride salt to polyvinylpyrrolidone (PVP) polymer in liquid precursors was investigated for its effect on the spinnability and formation of precursor fibers as well as on the resulting fibrous ZrO₂. The precursor fiber generation measured at a rate up to 5.6 g/h was achieved with a single flat 25-mm diameter alternating current (AC) electrode, which corresponded to production of up to 1.5 g/h of fibrous ZrO₂. The calcination process involved annealing the fibers at temperatures which ranged from 600 °C to 1000 °C and produced 0.1–0.2 mm thick fibrous ZrO₂ meshes. Individual nanofibers were found to have diameters between 50 and 350 nm and either a tetragonal (t-ZrO₂) or monoclinic (t-ZrO₂) structure depending on the calcination temperature. The annealed meshes with total porosity between 98.0 ± 0.2% and 94.6 ± 0.2% showed little deformation or cracking. Tensile strength and modulus of fibrous t-ZrO₂ meshes strongly depended on porosity and varied from 0.07 ± 0.03 MPa to 1.05 ± 0.3 MPa and from 90 ± 40 MPa to 388 ± 20 MPa, respectively. The m-ZrO₂ meshes resulted similar moduli, but much lower strengths due to their brittleness. A power-law relationship between the elastic modulus and porosity of AFES-derived nanofibrous t-ZrO₂ meshes, in comparison with other porous zirconia materials, was also investigated. The results of this study have demonstrated the feasibility of free-surface AFES in sizeable production of zirconia nanofibers and highly porous nanofibrous ceramic structures.     

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.     

A study of Pt/WO3-carrier catalysts for photocatalytic purification of NO gas

The products of Pt/WO₃-carrier (e.g. ceramics, zeolite molecular sieves, activated carbon, and alumina) were fabricated by a sol-gel method, and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and Barrett-Joyne-Halenda (BJH). The XPS results suggested that Pt in the Pt/WO₃ sample existed as Pt⁰ instead of Pt^2 +. Moreover, the BET and BJH results indicated that the Pt/WO₃-zeolite molecular sieves sample shows the largest BET specific surface area, while the Pt/WO₃-Al₂O₃ sample shows the highest BJH pore volume. Furthermore, NO gas with different concentrations was used as the target pollutants to evaluate the photocatalytic activity. The photocatalytic activity of as-prepared samples was evaluated and compared by removal of NO gas under different irradiation reaction conditions, such as carriers, NO concentration, the space time and light source. The photocatalytic results showed that Pt/WO₃-zeolite molecular sieves exhibited the highest conversion efficiency (> 90% of the conversion efficiency) among all of the as-synthesized samples. What's more, the photocatalytic mechanism for NO_x removal was analyzed. Overall, this paper provides a good model for practical applications of photocatalytic purification of NO gas.     

The structure and surface acidity of zirconia-supported tungsten oxides

The structure and surface acidity of zirconia-supported tungsten oxide are studied by using BET, XRD, FT Raman, Tian–Calvet microcalorimetry as well as XPS. Three different crystal forms of ZrO₂ and their hydroxide precursors were prepared in a controllable way. It was found that the starting material and preparing conditions used have a significant effect on the structure of ZrO₂ obtained. The tungstate species on zirconium hydroxide generally promote the transformation of the hydroxide precursor into tetragonal ZrO₂, while the same species on the pre-calcined ZrO₂ has less effect during calcination. A bulk-phase WO₃ existed in the samples prepared by initially impregnating the monoclinic, tetragonal and cubic ZrO₂ or the hydroxide precursors of m- and t-ZrO₂, while no crystalline WO₃ was present in the samples prepared by initially impregnating the hydroxide precursor of c-ZrO₂. Raman results revealed that the surface tungstate(s) on three different zirconium hydroxides or their respective zirconia have similar structural features before calcination. The surface tungsten oxides and some WO₃ bulk phase are the detectable tungsten species in the final samples, the ratio of each component is strongly dependent on the preparation history and the nature of the support. Creation of very strong acidic sites on the zirconia-supported tungsten oxide is related to the crystal form of zirconia itself, the type of tungsten oxide species, and the cooperation between the surface tungsten oxide overlayer and zirconia. The strong acidic sites can be completely poisoned by the remaining sodium ion impurities, but have not been appreciably influenced by added yttrium component. The crystalline WO₃ seems of little importance in building up the strong surface acidity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2

Ni and Pt catalysts supported on α-Al₂O_3, α-Al₂O₃-ZrO₂ and ZrO₂ were studied in the dry reforming of methane to produce synthesis gas. All catalytic systems presented well activity levels with TOF (s⁻¹) values between 1 and 3, being Ni based catalysts more active than Pt based catalysts. The selectivity measured at 650 °C, expressed by the molar ratio H₂/CO reached values near to 1. Concerning stability, Pt/ZrO₂, Pt/α-Al₂O₃-ZrO₂ and Ni/α-Al₂O₃-ZrO₂ systems clearly show lower deactivation levels than Ni/ZrO₂ and Ni or Pt catalysts supported on α-Al₂O₃. The lowest deactivation levels observed in Ni and Pt supported on α-Al₂O₃-ZrO₂, compared with Ni and Pt supported on α-Al₂O₃ can be explained by an inhibition of reactions leading to carbon deposition in systems having ZrO₂. These results suggest that ZrO₂ promotes the gasification of adsorbed intermediates, which are precursors of carbon formation and responsible for the main deactivation mechanism in dry reforming reaction.     

Some crystallographic considerations on the novel orthorhombic ZrO2 stabilized with Ta doping

Some crystallographic considerations on the novel orthorhombic ZrO₂ (o-ZrO₂) stabilized at ambient conditions by Ta substitutional doping are presented. Specifically, how the Ta-doped o-ZrO₂ was observed for the first time by conventional X-ray diffractometry (XRD) is reported, after which the ab initio and non-ab initio procedures used to resolve its crystal structure are detailed. The crystal structure is then explicitly reported (i.e., space group and cell parameters, as well as atomic positions of the asymmetric unit). It is also shown that this novel Ta-doped o-ZrO₂ has a crystal structure similar, but not identical, to the high-pressure o-ZrO₂. Additionally, given that the crystal structure of the Ta-doped o-ZrO₂ is identified as being a symmetry-breaking distortion of the typical tetragonal ZrO₂ (t-ZrO₂) stabilized with the commonest dopants, a complete analysis is also made of the structural distortions relating both the Ta-doped o-ZrO₂ and the high-pressure o-ZrO₂ to the t-ZrO₂. Finally, a justification for the apparent greater immunity of the Ta-doped o-ZrO₂ to aging is provided.     

Regulation of Y2O3 addition on structure and properties of Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramic

Al₂O₃/ZrO₂(Y₂O₃) directionally solidified eutectic ceramics (DSECs) with different Y₂O₃ addition were prepared. Three polymorphs of ZrO₂: m-ZrO₂, t-ZrO₂ and c-ZrO₂ appeared and disappeared in order with the increase of Y₂O₃ addition, the effects of Y₂O₃ addition amount on the mass fraction of ZrO₂ polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m^1/2 obtained at 9 mol% and 3 mol% Y₂O₃ addition, respectively. The hardness was determined by the mass fraction of m-ZrO₂ phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.     

A novel sintering resistant and corrosion resistant Pt4ZrO2/C catalyst for high temperature PEMFCs

A novel Pt₄ZrO₂/C catalyst was prepared and compared with 20 wt.% Pt/C in terms of the sintering resistance and corrosion resistance. To evaluate their sintering resistance and corrosion resistance properties, an accelerated ageing test (AAT) was performed. The catalysts before and after AAT were characterized by cyclic voltammetry (CV), rotating disk electrode (RDE) and X-ray diffraction (XRD). After AAT, the dissolution rate of Pt and Zr in H₃PO₄ media (105 wt.%, 204 °C) was characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The electrochemical area (ECA) changes of thin film electrodes based on Pt₄ZrO₂/C and Pt/C catalysts were also evaluated using continuous CV sweep technique. All the results showed that Pt₄ZrO₂/C has higher sintering resistance and corrosion resistance than Pt/C. ‘Anchor effect’ is proposed to explain the enhanced effect of ZrO₂ in Pt₄ZrO₂/C binary catalyst compared with Pt/C that contain platinum alone.     

Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide

Tungsten oxide species form strong acid sites on ZrO₂supports and inhibit ZrO₂crystallite sintering and tetragonal to monoclinic structural transformations. W-L_IX-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WO_x–ZrO₂samples (2–21 wt.% W) oxidized at 1073 K. Maximumo-xylene isomerization turnover rates (per W atom) on WO_x–ZrO₂solids occur at WO_xsurface densities (10 W nm⁻²) that exceed the theoretical monolayer capacity of ZrO₂. Similar turnover rates are obtained on WO_x–ZrO₂samples with similar WO_xsurface densities (W nm⁻²) over a large range of oxidation temperatures (773–1223 K) and WO_xconcentrations (5–21 wt.% W). UV–visible spectra suggest an increase in WO_xdomain size with increasing surface density. High isomerization turnover rates appear to require the presence of WO_xdomains of intermediate size on ZrO₂surfaces. WO_xdomains of intermediate size appear to provide a compromise between reducibility and accessibility of WO_xcenters. These domains are necessary to delocalize a temporary charge imbalance that forms Brønsted acid sites in the presence of H₂and stabilizes carbocation intermediates. The presence of H₂duringo-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D₂/o-xylene exchange reactions indicate that H atoms from H₂are not frequently involved in the activation or desorption of xylenes. H₂is required, however, in order to reverse the occasional desorption of H atoms duringo-xylene isomerization reactions. These desorption processes lead to the destruction of Brønsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H₂. After promotion with Pt (0.3 wt.%), WO_x–ZrO₂solids catalyzen-heptane isomerization in the presence of H₂at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WO_x–ZrO₂, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.     

Controlled Hydrogenation of Acetophenone Over Pt/CeO2–MO x (M = Si, Ti, Al, and Zr) Catalysts

Vapour phase selective hydrogenation of acetophenone has been performed over a series of Pt/CeO₂–MO _x (MO _x  = SiO₂, Al₂O₃, TiO₂, and ZrO₂) catalysts. The controlled hydrogenation was carried out in the 453–533 K temperature range at normal atmospheric pressure. The ceria-based mixed oxides were prepared through a co-precipitation or deposition-precipitation route. Platinum was deposited by a wet impregnation method. The obtained catalysts were calcined at 773 K and characterized by means of X-ray diffraction, Raman spectroscopy, BET surface area, temperature programmed reduction, temperature programmed desorption, thermogravimetry, and scanning electron microscopy. XRD analyses suggest that CeO₂–SiO₂ and CeO₂–Al₂O₃ primarily consist of CeO₂ nanoparticles dispersed over the amorphous silica or alumina surface. In the case of CeO₂–TiO₂, presence of segregated nanocrystalline CeO₂ and TiO₂-anatase phase were noted. Formation of cubic Ce_0.75Zr_0.25O₂ solid solution was observed in the case of CeO₂–ZrO₂. No peaks pertaining to platinum could be detected from XRD profiles. Formation of zirconia rich tetragonal phase (Ce_0.4Zr_0.6O₂) was observed in the case of Pt/CeO₂–ZrO₂ sample. Raman measurements revealed the fluorite structure of ceria and presence of oxygen vacancies in all samples. TPR results suggest that the presence of Pt facilitates the reduction of ceria. The catalytic performance of Pt-based catalysts was found to depend strongly on the nature of the support oxide employed. Among various catalysts investigated, the Pt/CeO₂–SiO₂ catalyst exhibited better product yields.     

Condensed Phase Relations in the Systems ZrO2-WO2-WO3 and HfO2-WO2-WO3

Phase relations for the systems ZrO₂–WO₂–WO₃ and HfO₂–WO₂–WO₃ from 1000° to 1700° C were determined by the quenching technique using sealed sample containers. In the system ZrO₂–WO₃, 1:2 compound, ZrW₂O₈ forms, having a cubic structure with a= 9.159 A. The ZrW₂O₈ melts incongruently at 1257°± 3°C to ZrO₂ and liquid and has a lower limit of stability at 1105°C, below which ZrO₂ and WO₃ coexist in equilibrium. One eutectic and one peritectic were established: at 1231°± 3°C and 74 mole % WO₃, and at 1257°± 3°C and 71 mole % WO₃, respectively. Along the join ZrO₂–WO₂, no compound formed. Two invariant points were determined: ZrO₂, WO₂, W, and liquid are in equilibrium at 1430°± 5°C and 76 mole % WO₂, whereas WO₂, W₁₈O₄₉, W, and liquid coexist at 1530°± 5°C and 89 mole % WO₂- Equilibrium relations in the system ZrO₂–WO₂–WO₃ were investigated at four temperatures. At 1200°C, a cubic phase with composition near W₂₀O₅₈ was found; it exists in equilibrium with ZrO₂, W₁₈O₄₉, W₂₀O₅₈, and WO₃. As the temperature increases, the liquid formed along the ZrO₂–WO₃ join extends into the ternary system, crosses the join ZrO₂–W₂₀O₅₈ at 1300°C, and crosses the join ZrO₂–W₁₈O₄₉ at 1400°C. The cubic phase can take more zirconium into its solid solution at 1300° than at 1200°C. At 1500°C, the system can no longer be treated as a simple ternary oxide system because of the presence of metallic tungsten, and equilibrium relations are presented on the basis of the system ZrO₂–W–WO₃. Phase equilibrium relations in the systems HfO₂–WO₃, HfO₂–WO₂, and HfO₂–WO₂–WO₃ in the temperature ranges studied are much like those in the corresponding zirconium system.     

Facile corrosion synthesis and sintering of disperse pure tetragonal zirconia nanoparticles

Disperse pure tetragonal zirconia (t-ZrO₂) nanoparticles smaller than 10 nm are essential for preparation of structural and functional zirconia materials, but syntheses of t-ZrO₂ nanoparticles using inorganic zirconium salts usually result in severe agglomeration. In this paper, we report a hydrothermal corrosion approach for improving the dispersity of t-ZrO₂ nanoparticles synthesized by precipitation using zirconium oxychloride without any surfactants. Disperse pure t-ZrO₂ nanoparticles with average sizes of 4.5 and 6 nm and size distributions of 2–11 and 3–12 nm were obtained by calcining precipitates at 400 °C for 2 h and 500 °C for 0.5 h followed by HCl corrosion at 120 °C for 75 h, respectively. Disperse t-ZrO₂ nanoparticles with an average size of 6 nm and a size distribution of 3–12 nm were pressed into green compacts at 500 MPa and sintered by two-step sintering (heating to 1150 °C without hold and decreasing to 1000 °C with a 10 h hold). The sintered bodies are dense pure monoclinic ZrO₂ nanocrystalline ceramic with a relative density of 99.9% and an average grain size of 110 nm.     

Solution combustion synthesis and physicochemical characterization of ZrO2-MoO3 nanocomposite oxides prepared using different fuels

A series of MoO₃-ZrO₂ nanocomposite oxides with MoO₃ content in the range of 2-20 mol% were prepared by solution combustion method. Three different fuels namely urea, glycine and hexamethylenetetramine (HMTA) were used for the preparation of MoO₃-ZrO₂ oxides. For the sake of comparison, the MoO₃-ZrO₂ composite oxides were also prepared by impregnation of zirconia with molybdenum salt precursor and subsequent heat treatment. The synthesized nanomaterials were characterized by XRD, SEM, TEM and UV-vis spectroscopic technique. XRD study indicated selective stabilization of the tetragonal phase of zirconia in the presence of MoO₃. The method of preparation was found to be crucial for the phase composition of zirconia in the composite oxide. The crystallite size and rms stain were calculated from the Fourier line shape analysis of the broadened X-ray diffraction profiles. With increase in the MoO₃ content, the crystallite size of the tetragonal zirconia phase was found to be decreased. TEM study indicated the presence of small nanoparticles with size in the 5-10 nm range. UV-vis study of the composite oxide materials revealed well dispersion of the molybdenum oxide component in the form of monomer, dimers and nanoclusters in the zirconia matrix. The nature of fuel was found to be crucial in determining the morphology and shape of the particles.