Surface layers preparation alumina layer

Ceria and zirconia have been shown by electron microprobe spectroscopy to combine the preparation of the catalyst, forming a very thermally stable phase, and platinum group metals deposit preferentially on the alumina. The washcoat has often been applied and fired with more alumina before the platinum group metals are impregnated. Alternatively, the washcoat and metals can be applied at the same time. In either case, the conditions for the deposition can be adjusted to provide a variable surface layer of alumina and the oxides, which absotb trace poisons and protect the active metals. 

The purpose of this paper Is 1) to describe the electrochemistry of ferrl-/ferro-cyanlde and the oxidation of ascorbic at an activated glassy carbon electrode which Is prepared by polishing the surface with alumina and followed only by thorough sonlcatlon 2) to describe experimental criteria used to bench-mark the presence of an activated electrode surface and 3) to present a preliminary description of the mechanism of the activation. The latter results from a synergistic Interpretation of the chemical, electrochemical and surface spectroscopic probes of the activated surface. Although the porous layer may be Important, Its role will be considered elsewhere. 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

Metallic monoliths made of both rhodium ([HCR 1]) and FeCrAlloy (72.6% Fe, 22% Cr and 4.8% Al ([HCR 3]) carrying micro channels of 120 pm x 130 pm cross-section at various length (5 and 20 mm) were applied. The monoliths were prepared of micro structured foils by electron beam welding. After bonding, the FeCrAlloy was oxidized in air at 1 000 °C for 4 h to form an a-alumina layer, which was verified by XRD. Its thickness was determined as < 10 pm by SEM/EDX. The alumina layer was impregnated with rhodium chloride and alternatively with a nickel salt solution. The catalyst loading with nickel (30 mg) was much higher than that with rhodium (1 mg) (see Table 2.4). The amount of rhodium on the catalyst surface was determined as 3% by XPS. 

The reaction channels were coated by the sol-gel technique, revealing a 2-3 pm thick alumina layer with pore diameters ranging between 1 and 5 nm having a maximum at 4 nm. Owing to the preparation technique, no surface area data could be obtained but a surface enhancement factor of 430 m2 rrf3 was determined. Pd was introduced as active component by wet impregnation. Subsequently, the catalyst was activated at 80 °C and reduced in H2. 

The Ru/Zr02 catalysts were prepared by first annealing the FeCrAlloy and thus generating an a-alumina layer on the alloy surface (see Section 2.10.7). Then both coating with zirconia and impregnation with a ruthenium solution were performed without an intermediate calcination step. 

For example, continuous multilayered ZSM-5 films were grown on cordierite modules.[59] Similar films were generated on as-prepared and acid-treated honeycomb substrates.[60] The latter treatments led to silica-rich surface layers their composition affected the Si/Al ratios of the zeolites crystallized on the cordierite. Thin, defect-free Mid-type films were also made on porous alpha-alumina and yttria-doped zirconia substrates using tclrapropylammonium hydroxide (TPAOH) as a structure-directing agent.[61]... 

Ceria and alumina, which form the intermediate porous layer (the washcoat) between the mechanical support and the supported metals and promotors, are important components in three-way catalysts (TWC) used for car exhaust gas cleaning. Although basie studies have been published on such systems [1], the interactions which can exist between alumina and eeria and which in turn may affect the interactions between the supported metal and the washcoat [2], are not fully understood. In particular, the multiple roles attributed to ceria, like stabilization of the alumina [3], of the supported metals [4], like storage and release of oxygen [2], are veiy probably dependent on preparation methods, activation and reaction conditions. Therefore, selective characterization of each oxide in ceria-alumina can be useful for a better imderstanding of the role played by both alumina and ceria in TWC. The present work describes two "ehemical methods", the adsorption of carbon dioxide followed by IR spectroseopy and the temperature-progranuned reduction (TPR), which lead to an estimation of the surface extent of alumina and ceria respectively. 

Aluminas also play an important part in the preparation of structured catalysts. They can be present on or constitute the structured support of catalysts. Common shapes are monohthic honeycombs or open-cell foams. Aluminas can be used as washcoat, a high-surface-area layer that gives the geometrical framework a suitable morphology to support and disperse the catalytically active phase, or they can be used as primer, an intermediate layer between the geometrical support and the washcoat that acts as glue between the two layers (433). Alumina plays a central role for the preparation of efficient automotive three-way-catalysts (434). 

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