Phase Analysis and Oxygen Storage Capacity of Ceria-Lanthana-Based TWC Promoters Prepared by Sol-Gel Routes

Ceria-lanthana-based promoters of three-way catalysts are synthesized by two different sol-gel routes, involving nitrate precursors. The oxygen uptake ability of these compounds is measured by O₂ chemisorption. The specific surface area is determined by N₂ adsorption (BET). X-ray diffraction data are analyzed by Rietveld refinement, demonstrating that lanthanum forms solid solution with CeO₂; its total amount in ceria depends on the competitive formation of La-Al mixed oxides and on the synthetic method. The O₂ uptake ability is essentially determined by the La content in the ceria-lanthana solid solution, while it is independent on the surface area and on the CeO₂ particle size. The O₂ uptake ability increases with the La:Ce relative amount in the ceria-lanthana solid solution, but decreases beyond a La:Ce molar ratio greater than ?0.18. This behavior is ascribed to the stable association of vacancy-vacancy or vacancy dopant cation.     

Auto-combustion synthesis and properties of Ce0.85Gd0.15O1.925 for intermediate temperature solid oxide fuel cells electrolyte

A typical composition of the system Ce_1 − xGd_xO_2 − δ with x = 0.15 (CGO15) has been synthesized by auto-combustion method. DTA/TGA of the precursor compound indicated the completion of reaction at about 270 °C. Greater than 95% of the theoretical density has been achieved by sintering at 1300 °C for 10 h. Single phase formation in as-burnt stage has been confirmed by its powder X-ray diffraction (XRD) pattern. The structural morphology was studied employing bright field transmission electron micrograph (BFTEM) and high resolution transmission electron micrograph (HRTEM). BFTEM image indicates that particles are highly agglomerated and appear to be dispersed in amorphous matrix. Also BFTEM image reveals that the average particle size is 26 ± 5 nm. The presence of amorphous phase in as-prepared ash was also confirmed by HRTEM and selected area diffraction (SAD). The scanning electron micrograph (SEM) of the thermally etched system shows grains having an average size of 400 nm. Impedance measurements have been made in the frequency range 1 Hz to 1.3 MHz between 200 and 500 °C and the total conductivity was measured. An enhanced conductivity value is observed which may make this system suitable for application as a solid electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs).     

Investigation of the Pore Texture of SnO2-CeO2 Composite Oxides

Gas adsorption techniques have been used to study the pore texture changes occurring in composite SnO₂-CeO₂ materials of varying Sn: Ce atom ratios on calcination at temperatures up to 1273K. The data show that the uncalcined materials are largely microporous in nature, but changes in specific surface area, pore sizes and pore volume occur at an early stage in the calcination process with the formation of mesopores. However, significant changes occur at calcination temperatures>673 K at which point the mesopores are substantially reduced, and at 873 K and above the mean pore size increases greatly finally giving non-porous solids after calcination at 1273 K.     

Preparation of CeO2-ZrO2 Mixed Oxide Powders by the Coprecipitation Method for the Purification Catalysts of Automotive Emission

Influence of CeO₂ content on the specific surface area (SA), oxygen storage capacity (OSC) and crystalline structure of the powders in the CeO₂-ZrO₂ system were investigated. The change of SA value by heat-treatment is almost proportional to that of OSC. The lattice parameters of the powders had a linear relationship with the OSC value. In the CeO₂-ZrO₂ system, powders with 20 mol% CeO₂ was found to show the highest OSC value and the highest durability of SA value after calcined at high temperatures.     

On the Flocculation and Agglomeration of Ceria Dispersion

The ceria dispersions displayed a maximum in the yield stress at the isoelectric point (pI) at pH ～ 7. At pH below pI, the flocculated dispersions at solids concentration of 30 wt% and above produced a smooth and homogeneous appearance. However, at pH above pI, phase separation was observed to occur quite quickly forming a clear layer of liquid supernatant. Upon examination, relatively large discrete aggregates had formed. This behavior can be explained by the surface chemical model proposed by Nabavi et al.^{[ 1} Nabavi , M. , Spalla , O. , and Cabane , B. ( 1993 ) J. Colloid Interface Sci. , 160 : 459 – 471 . [Google Scholar] ^] According to Nabavi et al., nitrate anion, normally an indifferent electrolyte, is both adsorbed and bounded covalently to the surface of the ceria particles at pH below pI. A steric layer is formed preventing particle agglomeration. Such agglomeration was also prevented at high pH with the adsorption of pyrophosphate additive added at 1dwb% concentration. The model proposed by Nabavi et al also explained the agglomeration of ceria particles at pH > pI. Both the covalently bounded and adsorbed nitrates were displaced by the hydroxyl group forming this surface ?OH group that on the interacting particles condenses to form ?Ce?O?Ce? bridging bond and caused particle agglomeration. The yield stress-DLVO force model was obeyed by the ceria dispersion at pH below pI. A critical zeta potential of 56 mV was obtained and this gives an estimate value for its Hamaker constant in water of 82 zJ.     

CeO2-TiO2催化剂的表面结构及其湿式氧化活性

 采用溶胶-凝胶法和共沉淀法制备了CeO2-TiO2催化剂,利用N2吸附、X射线衍射、透射电镜、X射线光电子能谱和Zeta电位分析等手段表征了催化剂的表面结构性质. 以乙酸和苯酚为目标物,进行了湿式氧化高浓度乙酸和苯酚的活性测试,研究了CeO2-TiO2催化剂表面结构与活性之间的关系. 结果表明, Ce和Ti之间的相互作用使复合氧化物CeO2-TiO2催化剂具有小的晶粒尺寸和高的比表面积,催化剂表面化学吸附氧含量增加,且不同制备方法得到的CeO2-TiO2催化剂表面电位点不同,共沉淀法制备的催化剂更有利于乙酸和苯酚的吸附. 在湿式氧化反应中, CeO2-TiO2催化剂有较高的活性,其中共沉淀法制备的催化剂活性最高; 在230 ℃, 5 MPa条件下反应120 min后,乙酸COD去除率为79%; 在150 ℃, 4 MPa下反应120 min后,苯酚COD去除率为96%.     

Methane Decomposition over Ni/α-Al_2O_3 Promoted by La_2O_3 and CeO_2

The decomposition of methane on Ni/a-Al2O3 modified by La2O3 and CeO2 with different contents has been investigated and the ralationship between methane decomposition and removal of carbon by CO2 over these catalyst has also been studied by pulse-chromatography. The catalysts were characterized by TPR and XRD. It was shown that Ni/a-Al2O3 could be promoted by adding La2O3, and the carbon species produced over this catalyst was activated and eliminated by CO2. But CeO2 would suppress the decomposition of methane over Ni crystallite. Both La2O3 and CeO2 can inhibit aggregation of the Ni particles. Decomposition of methane over the Ni-based catalysts is structure sensitive to a certain extent.     

Preparation and Characterization of Mesoporous Ce0.5Zr0.5O2 Mixed Oxide

Mesoporous （MSU） Ce0.5Zr0.5O2 mixed oxide with a high specific surface area has been synthesized under weak acidic condition in the presence of an anionic surfactant, sodium dodecylbenzenesulfonate. The effect of the pH value on the formation of mesostructure and the thermal stability of the material has been evaluated. The products were characterized by transmission electron microscopy, powder X-ray diffraction and nitrogen adsorption-desorption measurements. The results showed that the as-prepared Ce0.5Zr0.5O2 mixed oxide possessed a specific surface area of 163.3 m^2·g^-1, which had a cubic fluorite-type structure and possessed specific surface areas of 148.4 and 62.4 m^2·g^-1 after calcination at 500 and 800 ℃ for 2 h, respectively. The material showed excellent thermal stability.     

Zeolite Catalyzed Alkylation of Biphenyl. Where Does Shape-Selective Catalysis Occur?

Liquid-phase alkylation of biphenyl (BP) was studied over large-pore zeolites. Selective formation of the least bulky products, 4,4-diisopropylbiphenyl (4,4-DIPB) occurred only in the isopropylation of BP over one- or two-dimensional zeolites, H-mordenite (HM), ZSM-12, SSZ-24, SAPO-5, SSZ-31, and CIT-5. These shape-selective catalyses are ascribed to steric restriction of transition state and to easiness of the substrates to enter into the pores. HM gave the highest selectivity among them. The dealumination of HM enhanced catalytic activity and the selectivity for 4,4-DIPB because of the decrease of coke deposition. Non-regioselective catalysis occurs on external acid sites over HM with the low SiO₂/Al₂O₃ ratio because severe coke deposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. Selective formation of 4,4-DIPB was observed at moderate temperatures such as 250°C, whereas the decrease of the selectivity of 4,4-DIPB occurred at higher temperatures as 300°C. 4,4-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250°C, while the selectivity of 4,4-DIPB decreased under low propylene pressure as 0.2 MPa. However, 4,4-DIPB was almost exclusive isomer in the encapsulated DIPB isomers inside the pores under every temperature and pressure. The decrease of the selectivity of 4,4-DIPB is due to the isomerization of 4,4-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined by the modification with cerium and other rare earth metal oxide on HM. Selectivities of 4,4-DIPB were improved over modified HM even at high temperatures because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The ethylation of BP to ethylbiphenyls (EBPs) and diethylbiphenyls (DEBPs) was non-regioselective. The ethylation of BP to EBPs was controlled kinetically. However, there was difference in reactivities of EBPs and DEBPs for their further ethylation. 4-EBP was ethylated preferentially among the isomers, although the formation of 4,4-DEBP was less selective. 4-EBP and 4,4-DEBP have the highest reactivities among EBPs and DEBPs for the ethylation to polyethylbiphenyls (PEBPs). These results show that the environments of HM pores are too loose for shape-selective formation of the least bulky isomers, 4-EBP and 4,4-DEBP, in the ethylation of BP, and that HM pores have enough space for the further ethylation of 4,4-DEBP.     

CO2 Reforming of CH4 over Ni／CeO2-ZrO2-Al203 Prepared by Hydrothermal Synthesis Method

The Ni/CeO2-ZrO2-A12O3 catalyst with different A12O3 and NiO contents were prepared by hydrothermal synthesis method. The catalytic performance for CO2 reforming of CH4 reaction, the interaction among components and the relation between Ni content and catalyst surface basicity were investigated. Results show that the interaction between NiO and A12O3 is stronger than that between NiO and CeO2-ZrO2. The addition of A12O3 can prevent the formation of large metallic Ni ensembles, increase the dispersion of Ni, and improve catalytic activity, but excess A12O3 causes the catalyst to deactivate easily. The interaction between NiO and CeO2 results in more facile reduction of surface CeO2. The existence of a small amount of metallic Ni can increase the number of basic sites. As metallic Ni may preferentially reside on the strong basic sites, increasing Ni content can weaken the catalyst basicity.