Christos Agrafiotis

The effect of powder characteristics on washcoat quality. Part I: Alumina washcoats

Abstract The adhesion of γ-alumina washcoats on cordierite honeycombs was investigated with the aim to correlate the adhesion properties to the characteristics of the deposited powders, and, determine the conditions that can ensure integrity and endurance of the substrate/washcoat system under the severe operating conditions of an automobile catalytic system. It is shown that adhesion depends primarily on the particle size of the deposited powder. Depending on the production method, some powders have the form of large agglomerates and, if deposited as such upon the honeycombs, exhibit very poor adhesion properties. Reduction of the agglomerates size down to the order of a few (2–5) microns is necessary in order to ensure adhesion comparable to that of commercial catalysts. Particles of colloidal dimensions exhibit excellent adhesion to the support, provided that reduction of the agglomerates size that they form during calcination, takes place before deposition.

Optimization of the rheological properties of alumina slurries for ceramic processing applications Part I: Slip-casting

Aqueous powder slurries are widely employed in ceramic manufacturing processes like slip-casting and spray-drying. The aim of the present work was to identify the conditions for the preparation of stable alumina slurries with high solids content for the production of slip-cast objects with improved properties, as well as to correlate the slurry properties to the final object properties. For slurry stabilization, three commercial dispersants were compared. It was found that for each dispersant there exists an optimum concentration range within which low viscosity is achieved for a slurry of high solids content. In addition to the slurry solids content, the choice of a particular dispersant also affects the slurry viscosity and through that the casting rate. The combination of high slurry solids content and slower casting rate results in objects with higher densities both in the green and fired state.

Deposition of meso-porous γ-alumina coatings on ceramic honeycombs by sol-gel methods

Abstract The deposition of meso-porous γ-alumina coatings on multi-channeled cordierite honeycombs via sol–gel methods was investigated with the aim to correlate the deposition characteristics such as loading percentage, thickness and integrity of the coating to the support pore structure properties. Even though the mean pore size of the honeycomb supports was much higher than the size of the deposited particles, proper adjustment of sol viscosity prevented penetration of the sol into the support and led to the formation of a smooth coating of uniform, adjustable thickness. Sol viscosity was adjusted with the addition of poly-vinyl-alcohol (PVA) and with sol concentration by controlled evaporation, and fine-tuned in order to control loading percentage from 2 to 8 wt.% per impregnation, corresponding to coating thickness from 2 to approximately 10 μm respectively. The mean pore diameter of the support was found to affect the loading percentage. However, scanning electron microscopy observations have revealed that a very high loading percentage almost inevitably induces cracks on the coatings surface. The combination of sols and slurries of powders as coating media seems to be the optimum technical solution that can provide for satisfactory loading percentage per impregnation together with structural integrity of the coating.

The effect of powder characteristics on washcoat quality. Part II: Zirconia, titania washcoats — multilayered structures

Abstract Novel automotive catalyst designs based on the use of washcoat systems such as yttria-stabilized zirconia (YSZ) and titania, in combination with the traditionally used γ-alumina have been recently proposed. For the development of an integrated, robust multi-layered system, good adhesion of the inner layer on the support as well as among the various layers has to be ensured. In the present work, the adhesion of zirconia and titania washcoats on cordierite honeycombs was investigated. Irrespective of the nature of the powder used, reduction of the agglomerates’ size down to the order of few (2–5) microns is necessary in order to ensure firm adhesion of the washcoat to the support, comparable to that of commercial catalysts. In the deposition of multi-layered structures, particle size compatibility of the powders of the various washcoat layers can enhance the adhesion among them and induce better coherence of the overall washcoat.

Energy saving technologies in the European ceramic sector: a systematic review

Especially after the energy crisis of 1980s, plenty of energy efficient technologies were introduced in the European ceramic industry sector, resulting in significant energy reduction together with reduction of production time and cost. In the present review, these energy savings technologies introduced over the last two decades are presented and analysed. After a brief introduction about the current status and the typical products of the European ceramic sector, the general ceramic production flow-chart common to all ceramic industries is outlined and the energy efficient technologies introduced per sub-sector and per stage of the ceramic production process are described. Finally, an assessment of these technologies is presented and the trends for the future are discussed.

Evaluation of sol-gel methods for the synthesis of doped-ceria environmental catalysis systems. Part I: preparation of coatings

Sol-gel technology can be employed for the synthesis of multi-component nanophase environmental catalysts with enhanced catalytic activity, controlled composition and tailor-made pore structure together with their simultaneous deposition upon porous supports. The conditions for effective deposition of nano-phase doped-ceria systems onto cordierite honeycombs through sol-gel routes were investigated. For the direct casting from the sol phase, the sol rheological characteristics were adjusted and optimized so that homogeneous smooth washcoats were obtained. In this way, integrated support-catalyst assemblies suitable for high-temperature gas-solid catalysis were prepared. To overcome the inherently low loading percentage achieved per impregnation when depositing directly the sols, the synthesis of sol-gel powders and their deposition from aqueous slurries, as well as hybrid deposition methods using sol-gel powders dispersed in sols were tested as alternatives. The hybrid sol/powder systems proved to be the most attractive by combining an effective loading procedure with desirable and fine-tuned coating characteristics.

Evaluation of sol–gel methods for the synthesis of doped-ceria environmental catalysis systems: Part II. Catalytic activity and resistance to thermal aging

Abstract Nanophase calcia-doped-ceria-based washcoats with the molar composition (CeO) 0.25 (CaO) 0.02 (Al 2 O 3 ) 0.73 /(Pd, Rh), deposited on ceramic honeycombs via sol–gel methods were evaluated with respect to catalytic activity for hydrocarbons conversion and thermal aging. With the proper synthesis and deposition route, such systems were tailor-made with a meso-porous structure similar to that of commercial catalysts and with significant resistance to pore collapse and surface area loss upon thermal aging. All the prepared nanophase systems exhibited low light-off temperatures (in the range 500–550xa0K) achieved with half the amount of washcoat and half the amount of noble metals compared to currently available commercial catalysts. The performance of these systems was found to depend strongly on the deposition method (directly from the sol or from suspensions of sol–gel derived powders), on the type of precursor used for preparing the starting sols (metallo-organic or inorganic) and on the method used to insert the noble metal(s) in/on the washcoat. The performance and thermal stability of some of these systems makes them excellent candidates for three-way automotive catalysis, with a very low quantity of noble metals.