Control of Optical Reflection in Ca2MgWO6 by Co and Mo Doping.

To develop novel inorganic red pigments without harmful elements, we focused on the band structure of Ca2(Mg, Co)WO6 and attempted to narrow its bandgap by replacing the W6+ sites in the host structure of Mo6+. Ca2Mg1-xCoxW1-yMoyO6 (0.10 ≤ x ≤ 0.30; 0.45 ≤ y ≤ 0.60) samples were synthesized by a sol-gel method using citric acids, and the crystal structure, optical properties, and color of the samples were characterized. The Ca2Mg1-xCoxW1-yMoyO6 solid solution was successfully formed, which absorbed visible light at wavelengths below 600 nm. In addition, the absorption wavelength shifted to longer wavelengths with increasing Mo6+ content. This is because a new conduction band composed of a Co3d-W5d-Mo4d hybrid orbital was formed by Mo6+ doping to reduce the bandgap energy. Thus, the color of the samples gradually changed from pale orange to dark red, with a hue angle (h°) of less than 35°. Based on the above results, the optical absorption wavelength of the Ca2Mg1-xCoxW1-yMoyO6 system can be controlled to change the color by adjusting the bandgap energy.

Color Controllable Inorganic Pigments with Ce3+ as a Color Source.

The Ba2RE1-xCexTaO6 (RE = La, Gd, Y; 0 ≤ x ≤ 1) pigments were synthesized by a conventional solid-state reaction method to develop environmentally friendly reddish inorganic pigments. The host was the double-perovskite-type Ba2RETaO6 (RE: rare-earth elements), and the color source was Ce3+. All Ba2RE1-xCexTaO6 samples were obtained in a single-phase form as solid solutions. Rietveld refinement analysis of the Ba2RETaO6 (RE = La, Ce, Gd, and Y) samples revealed that the average bond length between RE3+ and O depends on the ionic radius of RE3+, and the shorter the RE-O length, the stronger the crystal field surrounding Ce3+. A photon energy at the maximum 4f-5d absorption of Ce3+ depended on the weighted average ionic radius (rRE) at the RE3+ site (i.e., the crystal field energy around the Ce3+ ions). In response to this phenomenon, the sample color was changed in order to orange, red, pink, and violet with a decrease in the rRE value, and a hue angle (h°) was roughly linearly related to that. For validation of the tendency, we demonstrated the synthesis and characterization of Ba2La0.5-yYyCe0.5TaO6 to obtain a more reddish color. As we exactly expected, a more reddish color was obtained while maintaining a high C value. Furthermore, the h° values for y = 0.1 and 0.2 were in good agreement with our estimation. In light of the above results, by controlling the intensity of the crystal field surrounding Ce3+ and/or the concentration of Ce3+, the optical absorption wavelength and absorption intensity of the Ba2RE1-xCexTaO6 system can be changed to adjust the color.

Novel near-infrared reflective black inorganic pigment based on cerium vanadate.

Gd3+-doped cerium vanadates, Ce1-x Gd x VO4 (0 ≤ x ≤ 0.30), were synthesized as near-infrared (NIR) reflective black pigments by a conventional solid-state reaction method. Crystal structure, particle size, optical properties, and color of the samples were characterized. The Ce1-x Gd x VO4 (0 ≤ x ≤ 0.30) samples were obtained in a single-phase form and the lattice volume decreased with increasing Gd3+ concentration. Optical absorption below 630 nm was observed in all samples, which corresponded to the charge-transfer transition between Ce4f and V3d orbitals. The absorption spectrum of Ce1-x Gd x VO4 was shifted to the longer wavelength side as the Gd3+ content increased, because of the increase in the crystal field around V5+ due to the lattice shrinkage. As a result, the sample color gradually changed from dark brown to black with increasing Gd3+ content. Among the samples synthesized in this study, Ce0.80Gd0.20VO4 absorbed visible light with wavelengths shorter than 650 nm and exhibited the darkest color. Furthermore, this black pigment showed a sufficient NIR reflectance value (R = 66.3%), which was higher than those of the commercially available products (R < 50%).

High Heat Resistance of the Structural Coloration of Colloidal Arrays with Inorganic Black Additives.

Structurally colored materials consisting of arrays of submicrometer-sized particles have drawn a great deal of attention because of their advantages, including low cost, low impact on human health as well as the environment, and resistance to fading. However, their low thermal stability is considered to be a critical issue for their practical use as colorants. Black-colored substances that can absorb the white color are added to colloidal array-type structurally colored materials to enhance their chromaticity. The poor thermal stability of commonly used black coloring additives, carbon black and Fe3O4 nanoparticles, is a main factor that reduces the heat resistance of structural coloration. Here, we demonstrate the preparation of structurally colored materials with extraordinarily high heat resistance of coloration, up to 900 °C. Several metal oxides, i.e., calcium manganese-based oxide (CCMO), chromium-iron-cobalt-nickel oxide (CFCNO), and lanthanum manganite (LMO), are synthesized and employed as black additives for structurally colored coatings prepared by the electrophoretic deposition of spherical silica particles. When CCMO is used as a black additive, the coloration heat resistance of the film is stable up to 700 °C. On the other hand, the films maintain vivid structural colors after exposure to 900 °C temperatures when CFCNO and LMO are employed as black additives. On the basis of this finding, high heat resistance of structural colors requires both heat resistance of the black additives and nonreactivity with the components of the spherical particles used for colloidal arrays.

Novel Orange Color Pigments Based on La3LiMnO7.

La3LiMn1-xTixO7 (0 ≤ x ≤ 0.05) samples were synthesized by a solid-state reaction method, and a single-phase form was observed for the samples in the range of x ≤ 0.03. Crystal structure, optical properties, and color of the La3LiMn1-xTixO7 (0 ≤ x ≤ 0.03) samples were characterized. Strong optical absorption was observed at a wavelength between 400 and 550 nm, and a shoulder absorption peak also appeared around 690 nm in all samples; orange colors were also exhibited. Among the samples synthesized, the most brilliant orange color was obtained at La3LiMn0.97Ti0.03O7. The redness (a*) and yellowness (b*) values of this pigment were higher than those of the commercially available orange pigments. Therefore, the orange color of this pigment is brighter than those of the commercial products. Since the La3LiMn0.97Ti0.03O pigment is composed of non-toxic elements, it could be a new environmentally friendly inorganic orange pigment.

Synthesis and Characterization of Ca1-x Eu x ZrO3 as Environmentally Friendly Inorganic Yellow Pigments.

Eu2+-doped calcium zirconates, Ca1-x Eu x ZrO3 (0 ≤ x ≤ 1), were synthesized as novel environmentally friendly inorganic yellow pigments by the conventional solid-state reaction method. The crystal structure, morphology, optical properties, and color were characterized. The Eu2+-doped samples strongly absorbed blue light in the wavelength range of 435-480 nm, which was caused by the 4f-5d allowed transition of Eu2+. The color of the sample gradually became brilliant yellow with increasing the Eu2+ content. Among the samples synthesized in this study, the brightest yellow color was obtained with the Ca0.7Eu0.3ZrO3 (a* = +11.5 and b* = +70.7) sample. Compared with the commercially available praseodymium yellow pigment (a* = -3.28, b* = +70.3), the yellowness value (b*) of Ca0.7Eu0.3ZrO3 was comparable and the redness value (a*) was higher. As a result, this pigment exhibited a reddish yellow color as compared with praseodymium yellow. In addition, this pigment was chemically stable. Therefore, the Ca0.7Eu0.3ZrO3 pigment has the potential to become a novel environmentally friendly inorganic yellow pigment.

Effect of [MnO6] Octahedra to the Coloring Mechanism of (Li1-x Na x )2MnO3.

(Li1-x Na x )2MnO3 (0 ≤ x ≤ 0.10) solid solutions were synthesized by a conventional solid-state reaction technique to investigate the relationship between the steric structure of the [MnO6] octahedra and coloration mechanisms. The color, optical properties, and crystal structure of the solid solutions were characterized. The (Li1-x Na x )2MnO3 (0 ≤ x ≤ 0.10) solid solutions absorbed the visible light at wavelengths shorter than 550 nm and around 680 nm. The former and latter optical absorption bands were attributed to the spin-allowed (4A2g → 4T1g, 4T2g) and spin-forbidden (4A2g → 2Eg, 2T1g) d-d transitions of tetravalent manganese ions, respectively. The absorption band assigned to the 4A2g → 4T2g transition shifted toward longer wavelengths with the enlargement of the average [Mn(2)O6] bond distance by doping Na+. In contrast, the latter absorption bands did not shift but the absorption intensities increased due to the distortion of the [Mn(2)O6] octahedra. Consequently, the red color purity of the sample gradually increased with the increase in the Na+ concentration. Among the (Li1-x Na x )2MnO3 (0 ≤ x ≤ 0.10) samples synthesized in this study, the highest red color purity was obtained in the (Li0.93Na0.07)2MnO3 (hue angle: h° = 39.1) sample. The results of this study provide important insights for the development of environment-friendly inorganic red pigments containing Mn4+ ions as a coloring source.

Synthesis and Color Evaluation of Ta5+-Doped Bi2O3.

Ta5+-doped Bi2O3 solid solutions, (Bi1-x Ta x )2O3+2x (0 ≤ x ≤ 0.20), were synthesized by a conventional solid-state reaction method. The crystal structure, optical properties, and color of the pigments were characterized. The (Bi1-x Ta x )2O3+2x (x = 0, 0.03, 0.04, 0.20) samples were obtained in a single-phase form, and the crystal structure depended on the Ta content; they were monoclinic α-phase (x = 0), tetragonal ß-phase (x = 0.03, 0.04), and cubic δ-phase (x = 0.20). Among them, (Bi0.97Ta0.03)2O3.06 strongly absorbed visible light at the wavelength shorter than 480 nm and exhibited the most vivid orange color. The redness value (a*) was comparable to that of commercial orange pigments, and the yellowness value (b*) was slightly larger. Although it is necessary to improve chemical stability, the (Bi0.97Ta0.03)2O3.06 pigment has potential for an environmentally friendly inorganic orange pigment.

Improvement of near-infrared (NIR) reflectivity and black color tone by doping Zn2+ into the Ca2Mn0.85Ti0.15O4 structure.

Inorganic black pigments with thermal barrier characteristics, Ca2Mn0.85-x Ti0.15Zn x O4-x (0 ≤ x ≤ 0.10), were synthesized using a conventional solid-state reaction method in order to improve the blackness without decreasing the near-infrared (NIR) reflectance of a Ca2Mn0.85Ti0.15O4 pigment, which was previously reported by our group. The composition was optimized to provide both high blackness and NIR reflection characteristics. As a result, the NIR solar reflectance value (R NIR) of Ca2Mn0.77Ti0.15Zn0.08O3.92 (R NIR = 74.6%) became larger than that of Ca2Mn0.85Ti0.15O4 (R NIR = 71.7%), and the black color tone of the former (L* = 23.2, a* = +2.81, b* = +0.83, C = 2.93) was improved in comparison with that of the latter (L* = 24.4, a* = +4.30, b* = +2.72, C = 5.09). This improvement is caused by the introduction of strain into the [MnO6] octahedra and a decrease in the manganese ion concentration. The R NIR value of the Ca2Mn0.77Ti0.15Zn0.08O3.92 pigment was also larger than those of the commercially available pigments (R NIR < 53.0%). Therefore, Ca2Mn0.77Ti0.15Zn0.08O3.92 has potential to be an inorganic black pigment for thermal shielding.

Synthesis and characterisation of Ba(Zn1-x Co x )2Si2O7 (0 ≤ x ≤ 0.50) for blue-violet inorganic pigments.

Ba(Zn1-x Co x )2Si2O7 (0 ≤ x ≤ 0.50) solid solutions were synthesized as novel blue-violet inorganic pigments by a conventional solid-state reaction method. The crystal structure, optical properties, and colour of the pigments were characterized. All the pigments were obtained in a single-phase form. The pigments strongly absorbed visible light at wavelengths from 550 to 650 nm, corresponding to the range of green to orange light. This optical absorption was caused by the d-d transition of the tetrahedrally coordinated Co2+ (4A2(F) → 4T1(P)), which was the origin of the blue-violet colour of the pigments. The most intense colour was obtained for Ba(Zn0.85Co0.15)2Si2O7, where a* = +52.2 and b* = -65.5 in the CIE (Commission Internationale de l'Éclairage) L*a*b* system. These absolute values were significantly larger than those of commercial violet pigments such as Co3(PO4)2 (a* = +33 and b* = -32) and NH4MnP2O7 (a* = +39 and b* = -21). Therefore, the Ba(Zn0.85Co0.15)2Si2O7 pigment could be a novel blue-violet inorganic pigment.

Catalytic liquid-phase oxidation of acetaldehyde to acetic acid over a Pt/CeO2-ZrO2-SnO2/γ-alumina catalyst.

Pt/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts were prepared by co-precipitation and wet impregnation methods for catalytic oxidation of acetaldehyde to acetic acid in water. In the present catalysts, Pt and CeO2-ZrO2-SnO2 were successfully dispersed on the γ-Al2O3 support. Dependences of platinum content and reaction time on the selective oxidation of acetaldehyde to acetic acid were investigated to optimize the reaction conditions for obtaining both high acetaldehyde conversion and highest selectivity to acetic acid. Among the catalysts, a Pt(6.4wt.%)/Ce0.68Zr0.17Sn0.15O2.0(16wt.%)/γ-Al2O3 catalyst showed the highest acetaldehyde oxidation activity. On this catalyst, acetaldehyde was completely oxidized after the reaction at 0°C for 8hr, and the selectivity to acetic acid reached to 95% and higher after the reaction for 4hr and longer.

Carbon monoxide oxidation at room temperature on Pt/CeO2-ZrO2-Bi2O3 catalysts.

A novel Pt/CeO(2)-ZrO(2)-Bi(2)O(3) catalyst was prepared to realize complete CO oxidation at room temperature or below even in the presence of moisture. Using this catalyst, a high CO oxidation activity and a high stability against moisture have been realized simultaneously.

Total oxidation of toluene on Pt/CeO2-ZrO2-Bi2O3/gamma-Al2O3 catalysts prepared in the presence of polyvinyl pyrrolidone.

Pt/CeO(2)-ZrO(2)-Bi(2)O(3)/gamma-Al(2)O(3) (Pt/CZB/Al(2)O(3)) catalysts for the catalytic combustion of toluene, which is one of the volatile organic compounds (VOCs), were prepared by the wet impregnation method in the presence of polyvinyl pyrrolidone (PVP). X-ray powder diffraction, transmission electron microscopy, and BET specific surface area measurement using N(2) adsorption have been used to characterize the catalysts. The catalytic test was conducted from room temperature in a flow of 900 ppm of toluene in air and gas hourly space velocity (GHSV) of 8000 h(-1). The catalytic activity was evaluated in terms of C(7)H(8) conversion and the gas composition after the reaction was analyzed using two gas chromatographs with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The Pt/CZB/Al(2)O(3) catalysts are specific for the total toluene oxidation and CO and any toluene-derivative compounds were not detected as by-products. The specific surface area of the catalysts was increased by the addition of PVP in the preparation process. By the optimization of the amount of platinum, complete oxidation of toluene was realized at a temperature as low as 120 degrees C on a 7 wt%Pt/16 wt%Ce(0.64)Zr(0.15)Bi(0.21)O(1.895)/gamma-Al(2)O(3) catalyst.

Advanced materials for environmental catalysts.

Recent advances in the synthesis and characterization of materials for environmental catalysts are reported in this paper. Highly advanced environmental catalysts for decomposition of volatile organic compounds and nitrogen oxides were artificially designed based on a concept usually employed in the fields of solid-state chemistry and solid-state ionics. Catalytically active materials for complete ethylene oxidation were prepared by a citrate sol-gel method as a key process to obtain CeO(2)-ZrO(2)-Bi(2)O(3) solid solutions. On the other hand, direct NO decomposition catalysts were designed and prepared focusing on the open spaces and oxide anion vacancies in the crystal lattice. Evaluation of the materials as environmental catalysts demonstrated significant advantages over the conventional ones. The design strategy, synthetic method, and structural features of these concerto catalysts are addressed.