High Dispersion State Research Articles

Regeneration of Palladium Subsequent to Solid Solution and Segregation in a Perovskite Catalyst: An Intelligent Catalyst

The object of this study was to provide a function for self-regeneration of precious metals in a usage ambience without auxiliary treatment. The strategy was to control the catalytic active site of those crystalline ceramics known as perovskite-type oxides at the atomic level in order to create the new, needed function. Three series of Pd-containing perovskite catalyst systems were prepared by coprecipitation of Pd with La, Fe, and Co using the alkoxide method. It was confirmed that Pd formed a solid solution of the perovskite-type oxide. And Pd in the perovskite crystal structure exhibited an abnormal oxidation number or higher binding energy than the normal bivalence, and it was presumed to be the reason for increasing the catalytic activity. The results of dissolution analysis for the aged Pd-perovskite catalyst suggested that Pd was not only dispersed on the surface of the perovskite crystal, but was present also in the solid solution of the perovskite crystal. The formation of a solid solution in this Pd-perovskite crystal was affected by the B site elements. And Pd in LaFe0.54Co0.36Pd0.10O3 system was more durable than in LaCo0.90Pd0.10O3 or in LaFe0.90Pd0.10O3. Furthermore, the formation of Pd solid solution into these perovskite crystals also depended on atmospheres and temperatures. It appeared that a high state of dispersion was maintained as Pd repeatedly forms solid solutions in the perovskite crystal or segregates out from the crystal depending on the fluctuation of redox conditions and temperatures in automotive catalyst ambience. We named such a catalyst, wherein a precious metal regenerates itself while in operation and remains highly active without requiring any auxiliary treatment, “an intelligent catalyst”.

Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex‐shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)2C6H2(SO3Na)2 (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH3CO2)2AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.

Improvement of thermal stability of alumina by addition of zirconia

To maintain a large surface area at elevated temperatures, zirconia was added to transition alumina. The addition of a small amount of zirconia resulted in a marked suppression of phase transformation from θ- to α-alumina. After heating at 1200°C, ZrO2‐containing alumina exhibited a large surface area of 50 m2/g. UV‐VIS and XRD measurements indicated that zirconia existed in a high dispersion state after calcining at 800°C. XPS measurement also showed that zirconia existed as monolayer. Zirconia monolayers are concluded to cover the alumina surface and the interaction between them may be the cause for the suppression of phase transformation and also for the maintenance of the large surface area at elevated temperatures. The interaction remains up to 1200°C, therefore, θ phase remained at 1200°C.

Investigations of the Dispersion of Pd in H-ZSM-5

The state of Pd dispersion in Pd–H-ZSM-5 was investigated by means of infrared spectroscopy and temperature-programmed desorption spectroscopy, using CO and NO as probe molecules. Following oxidation in O2at 773 K of a freshly prepared, low-loaded sample (Pd/Al=0.048), most of the Pd is found to be present as isolated cations, viz. Pdn+(n=1–3). CO reduction at 773 K leads to the formation of Pd particles that fill the local pore space of the zeolite and are inaccessible to CO. Following CO reduction, oxidation in O2for 2 or 8 h at 773 K only partially redisperses the Pd. During treatment of a reduced sample in 5000 ppm NO for 60 min at 673 K, N2O and N2are produced in an amount sufficient to oxidize all the Pd to PdO. Characterization of the state of Pd dispersion after NO pretreatment shows that all of the Pd has been redispersed as cations. Reoxidation of the reduced sample in NO is accompanied by a decrease in the infrared band due to Brønsted acid groups in an amount nearly equivalent to 2H+/Pd. Subsequent reduction of the sample regenerates the Brønsted acid band intensity. These observations suggest that isolated Pd cations may reside as Z−H+(PdO)H+Z−in association with pairs of Al sites in the zeolite that are next-nearest neighbors. Following oxidation in O2at 773 K, a high-loaded sample of Pd–H-ZSM-5 (Pd/Al=0.48) is found to have nearly the same concentration of highly dispersed Pd cations as the low-loaded sample, with the balance of the Pd in this case being present as small particles of PdO. Treatment of a reduced, high-loaded sample of Pd–H-ZSM-5 in NO at 773 K redisperses all of the Pd in structures that are proposed to be Z−H+[(PdO)(NO)]H+Z−and Z−H+[(PdO)(NO)]. However, upon removal of the adsorbed NO by treatment in O2at 773 K, the Z−H+[(PdO)(NO)] species revert to PdO, but the Z−H+[(PdO)(NO)]H+Z species remain stable as Z−H+(PdO)H+Z−. The present results suggest that maintenance of Pd in a high state of dispersion as isolated Pdn+cations in the presence of O2at elevated temperatures requires pairs of next-nearest-neighbor Al sites in the zeolite.

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

A series of palladium-substituted La 2CuO 4, corresponding to the formula La 2Cu 1 − x Pd x O 4 ( x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m 2/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C 3H 6 decreased markedly when increasing the palladium content, the activity of catalysts La 2Cu 0.9Pd 0.1O 4 and La 2Cu 0.8Pd 0.2O 4 being comparable to that of a Pt-Rh/CeO 2–Al 2O 3 catalyst for NO reduction, and higher for CO and C 3H 6 oxidation. All the La 2Cu 1 − x Pd x O 4 catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd 0 ions atomically dispersed, surrounded by Cu + and Cu 2+ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La 2Cu 1 − x Pd x O 4 structure.

Dispersion and consolidation of ultrafine BaTiO 3 powder in non-aqueous solutions

Dispersion characteristics of nanometer-sized BaTiO 3 powder in thirty-one kinds of non-aqueous solutions were evaluated by measuring the heights of cloudy layers in suspensions as a function of settling time. High dispersion state was maintained longer (100 to 500 h) in alcohols and cyclohexanone. The rheological behavior of alcohol and cyclohexanone BaTiO 3 suspensions was analyzed by the power law equation S= Kγ n , where S is the shear stress, K the consistency index, γ n is a constant. The green densities (34–54% of TD) of consolidated powder compacts were greatly influenced by the critical solid contents of colloidal solids at n = 0 which were estimated to be 17–42 vol%. The BaTiO 3 powder compacts with green densities above 57% TD, isostatically compressed after filtration, were densified to 94–96% TD after sintering at 1200°C for 2 h.

Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Re–Cu, Ir–Cu, and Pt–Cu clusters

The extended x-ray absorption fine structure (EXAFS) associated with the LIII absorption edges of rhenium, iridium, and platinum, and the K absorption edge of copper, has been used to study the structures of silica supported Re–Cu, Ir–Cu, and Pt-Cu, bimetallic clusters. The atomic ratio of copper to the other metal was approximately one in all cases. The systems were chosen to investigate the effect of varying miscibility of the components on the structural features of the bimetallic clusters. At one extreme, platinum and copper are totally miscible in the bulk. At the other extreme, rhenium and copper are completely immiscible. Iridium and copper are only slightly miscible in the bulk. In the bimetallic clusters, atoms of rhenium, iridium, or platinum are all coordinated to copper atoms about as extensively as they are to atoms of their own kind. This result is particularly interesting for the Re–Cu and Ir–Cu clusters, in view of the limited miscibility of either iridium or rhenium with copper in the bulk. The surprisingly good coordination of iridium and rhenium to copper is attributed to a very high state of dispersion of these metal clusters.

Characterization and catalytic activity of nickel-zeolite catalysts. III. Effects of parent-cations on dispersion of nickel and activity of benzene hydrogenation

A series of nickel-zeolite catalysts with alkaline earth, lanthanum and hydrogen ions as parent-cations in zeolite Y were prepared. The degrees of nickel ion reduction and the dispersion states of reduced nickel were affected by the extent of nickel-exchange and by the types of parent-cations in zeolites. Zeolite catalysts with nickel in higher dispersion state have been prepared on reduction of NiCa-Y, NiSr-Y and NiBa-Y with hydrogen. The activity of benzene hydrogenation was proportional to the dispersion of reduced nickel on zeolites.

Generation of a supported iridium catalyst of extremely high dispersion

THE preparation of supported metallic catalysts in a very high state of dispersion—that is, with a very small average metallic particle size—is of considerable current interest. Not only is the surface area per unit mass of metal thus increased, but if the metallic particles are sufficiently small, their intrinsic catalytic properties may be different from those of the massive metal. On the whole, changes of the latter sort seem to be found when the particle diameter is less than about 2 nm (refs 1, 2). The use of a metallic cluster carbonyl (MCC) as a means of preparing a dispersed metallic catalyst has been the subject of preliminary exploration3,4,5. Although it was indicated that this route could yield highly dispersed metal, it was clear3 that considerable cluster aggregation occurred during thermal decarbonylation and hydrogen reduction (at 573 K). This note reports the preparation from Ir 4 (CO)12 of a supported iridium catalyst in a very high state of dispersion.

EPR study of surface coordination effects on the stabilization of dispersed chromium (V) species at the surface of various oxides

EPR has been used to monitor the adsorption of the Cr(V)—1,2-ethylene-diol complex on various basic and acidic oxides and to study the relative stability and degree of dispersion of the obtained Cr(V) and Cr(III) species. It is shown that Cr(V) species are most stable on oxides characterized by high surface coordination numbers. After evacuation at moderate temperature, Cr(III) is usually obtained in a high dispersion state.

A STUDY OF THE STATIONARY BED OF SUPPORTED LIQUID PHASE CATALYSTS

The catalytic behavior of a porous particle containing a liquid-phase catalyst in its pores is investigated by the catalytic oxidation of ethylene (the Wacker-Hochst process). The activity of the catalyst is proved to be dependent upon the properties of the carrier materials, their sizes and the operating conditions. The experimental results are analyzed on the basis of measurement of the intrinsic kinetics and the effective diffusivity in the liquid-filled pores, and from these results the effectiveness factors of the catalysts are derived. A new technique to create a highly-dispersed state of catalyst solution inside the carrier materials and to eliminate the diffusion resistances is successfully established.