Phase Transformation Of γ-Al2O3 Research Articles

Optimized synthesis of highly thermal stable CeO2-ZrO2/Al2O3 composite for improved Pd-only three-way catalyst

As is well known, impregnation of precursors of Ce and Zr onto as-prepared Al2O3 powders is a commonly used approach to prepare CeO2-ZrO2/Al2O3 composite. In this work, a highly thermal stable CeO2-ZrO2/Al2O3 composite was synthesized by modifying the conventional preparation procedure, i.e., the precursor of Al2O3 thermal treated at comparatively lower temperature (200°C) was applied to conduct the impregnation process. The characterization results of the supported Pd-only three-way catalysts reveal that in the modified sample, stronger interaction between CeO2-ZrO2 and Al2O3 is achieved, which consequently gives rise to higher thermal stability of the catalyst. Upon hydrothermal aging treatment, only slight aggregation of the CeO2-ZrO2 and Al2O3 particles takes place, meanwhile phase transformation of γ-Al2O3 and phase separation of CeO2-ZrO2 component are successfully suppressed. As a consequence, Pd species supported on the modified CeO2-ZrO2/Al2O3 composite remain finely dispersed, and more oxidative PdO species are maintained owing to the intimate interaction between Pd and CeO2-ZrO2. Therefore, improved reduction capability and superior three-way catalytic performance are achieved for the modified Pd/CeO2-ZrO2/Al2O3 catalyst after aging treatment.

High-performance Mesoporous Ni/γ-Al2O3–RE (RE = La, Ce) Catalysts and Promotion Effect of Rare Earth Oxides for Carbon Dioxide Reforming of Methane

Mesoporous Ni/γ-Al2O3–RE (RE = La, Ce) catalysts through one-pot hydrolysis of nitrate salts show high activity and superior long-term stability for CO2 reforming of CH4 (CRM). Addition of RE not only significantly enhances the intrinsic activity of Ni/γ-MA but also inhibits phase transformation of γ-Al2O3 and coke deposition for CRM.

Effect of Alumina Phase Transformation on Stability of Low-Loaded Pd-Rh Catalysts for CO Oxidation

Bimetallic Pd-Rh catalysts with precious metal loading of 0.2 wt% was prepared by incipient wetness impregnation of the support (γ-Al2O3 or δ-Al2O3) with dual complex salt [Pd(NH3)4]3 [Rh(NO2)6]2. Monometallic Pd and Rh catalysts as well as its mechanical mixture were used as the reference samples. All samples were exposed for in situ prompt thermal aging procedure, and characterized by EPR spectroscopy, UV–Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The nature of the support was found to have strong effect on high temperature stability of the samples. δ-Al2O3 having non-uniform phase structure due to presence of θ-Al2O3 and α-Al2O3 traces causes the concentrating of rhodium near the interphase boundary, thus changing the mechanism of Rh3+ bulk diffusion if compare with γ-Al2O3. No noticeable anchoring effects were observed for bimetallic Pd-Rh samples neither in terms of Rh bulk diffusion nor with regard to the Pd sintering. It has been found experimentally that phase transformation of γ-Al2O3 at high temperatures does not play dramatic role for the deactivation of bimetallic Pd-Rh active species anchored to the electron-donor site of the support.

Growth and characterization of ceria thin films and Ce-dopedγ-Al2O3 nanowires using sol–gel techniques

γ-Al2O3 is a well known catalyst support. The addition of Ce toγ-Al2O3 is known to beneficially retard the phase transformation ofγ-Al2O3 toα-Al2O3 and stabilize theγ-pore structure. Inthis work, Ce-doped γ-Al2O3 nanowires have been prepared by a novel method employing an anodic aluminium oxide(AAO) template in a 0.01 M cerium nitrate solution, assisted by urea hydrolysis. Calcination at500 °C for 6 h resulted in the crystallization of the Ce-doped AlOOH gel to form Ce-dopedγ-Al2O3 nanowires.Ce3 + ions within the nanowires were present at a concentration of < 1 at.%. On the template surface, a nanocrystallineCeO2 thin film was deposited with a cubic fluorite structure and a crystallite size of 6–7 nm.Characterization of the nanowires and thin films was performed using scanning electronmicroscopy, transmission electron microscopy, electron energy loss spectroscopy, x-rayphotoelectron spectroscopy and x-ray diffraction. The nanowire formation mechanism and ureahydrolysis kinetics are discussed in terms of the pH evolution during the reaction. The Ce-dopedγ-Al2O3 nanowires are likely to find useful applications in catalysis and this novel method can beexploited further for doping alumina nanowires with other rare earth elements.

High Temperature Ceramifying Behavior of SR/Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Composites

In this work, silicone/alumina composites with 33 wt.% silicone content are prepared by compression mould technique. TGA, XRD and SEM were used to characterize the ceramifying behavior when the samples were heated from 600 to 1600°C. XRD results suggest that the ceramic residue of SR could restrain the phase transformation of γ-Al2O3 below 1100°C, indicating that the chemical bonds preserve the mechanism strength of SR/Al2O3 composites under high temperature. The flexural strength increases from 18.19 MPa to 75.31 MPa with the increasing temperature from 600 to 1600°C, owing to the sintering reaction between SiO2 and Al2O3 forming mullite phase under higher temperature. It has low liner shrinkage after firing and nearly no macrocracks or deformation under any temperature, illustrating that the SR/Al2O3 composites can be used as organic fire-retardancy materials.

Preparation and characterisation of α-Al2O3 powder from γ-Al2O3 powder using mechanical milling technique

In this presented work, α-Al2O3 is prepared from γ-Al2O3 by mechanical milling technique. Also the results obtained in studying the phase transformation of γ-Al2O3 into α-Al2O3 during intense mechanical activation by high-energy ball milling are presented. The powder samples were characterised by the commonly used bulk techniques of scanning electron microscope (SEM), differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD) analyses. The results indicated that the pure γ-Al2O3 phase showed great stability and did not transform to any other phase even after a long milling time. On the other hand, γ-Al2O3, which contained a small amount of the a-alumina phase, showed a gradual phase transformation from γ-Al2O3 to α-Al2O3 on milling. The phase transformation mechanism during milling appears to be nucleation and growth type and promoted by the α-Al2O3 seed.