Papers by Author: Susana Dias

Abstract: The reactors used for Selective Catalytic Reduction (SCR) of NOx require low pressure drop structured catalyst packing. Structured packings, such as ceramic foams, are gaining increasing interest for application in low pressure drop reactors, membrane reactors and catalytic distillation units. In this work, cobalt ion exchanged mordenite (Co-HMOR)-coated cordierite-based foams produced by the replication method were evaluated for catalytic reduction of NOx with methane. The addition of 0.3 wt.% Pd to 2 wt.% Co-HMOR leads to a material that can convert 50 % NOx to N2 at 450 °C in a reaction mixture containing 2000 ppm CH4, 1000 ppm NOx, 5 % O2 and balance helium, at GHSV=17000 h-1. Although in an early stage of development, an efficient coating procedure was explored and different ways of exchange of Co and Pd cations into mordenite (Si/Al=10) were studied. Additions of 2 wt.% fumed silica enhanced adhesion of the zeolite onto the ceramic foam. Pd-exchanged Co-HMOR showed to be very sensitive to steam. A 50 % decrease in NOx conversion to N2 was observed after Pd/Co-HMOR samples were exposed at 450 °C to a reaction mixture containing 2 vol% H2O. Although further research is needed to ascertain the mechanism of this deactivation behaviour, agglomeration of Pd forming PdO particles is envisaged.

Abstract: This study focus on the synthesis of Ca-deficient apatites (CDA) by using the wet chemical precipitation method in demineralised water solutions at pH=7 and synthesis temperature of 90 °C. Upon calcination at 1000°C, the resulting HAP powders decomposed leading to the formation of small amounts of β-TCP. Surprisingly, under some conditions, minor amounts of α- TCP were also formed at 1000°C together with β-TCP. The appearance of α-TCP is favoured by the absence of mineral ions in the precipitation medium, which in turn depends on the purity of the water used. Factors affecting the formation of both β-TCP and α-TCP are discussed.

Abstract: When preparing β-tricalcium phosphate (β-TCP), it is difficult to industrialise traditional methods because of the problem of particle aggregation. In this paper, β-TCP was prepared by calcining calcium-deficient hydroxyapatite (CDHA) powders obtained by the wet precipitation method at pH=6 and 40 °C, together with additions of poly (ethylene glycol) (PEG), in order to prevent powder agglomeration. The effect of the Ca/PEG mass ratio on the particle size distribution and the morphology of the resulting powders was evaluated. For comparison purposes, one powder without PEG addition was also synthesized. All synthesized powders were characterized by means of X-ray diffraction analysis, particle size analysis and scanning electron microscopy prior and after calcination in static air at 1000 °C for 15 h. Single-phase β-TCP powders have been obtained by calcining CDHA powders in the temperature range of 850°C-1150°C. It was also found that increasing the PEG content in solution decreased the particle size distribution of the agglomerated particles and the β→α-TCP transition temperature. A plausible explanation consistent with these experimental findings is proposed.

Abstract: Open-cell ceramic foams are being considered for a variety of applications owing to their high permeability and low weight, including molten metal filters, catalytic substrates and radiant burners. In these applications, foams are exposed to high temperatures and thermal gradients and thus the materials used require resistance to severe thermal stresses during service. Unfortunately, little is known about the thermal shock behaviour of cordierite foams when subjected to sudden changes in temperature. The objective of the present study was to investigate the thermal shock behaviour of two kinds of cordierite-based ceramic foams manufactured by the replication process. Thermal shock experiments were carried out by rapidly transferring the heated samples from a resistance furnace to a quenching bath containing distilled water followed by measuring of the retained flexural strength. Unlike dense cordierite samples that showed a sudden decrease in flexural strength at quench temperature differences above 325 K, the strength retained after thermal shock by open-cell cordierite foams decreased gradually with increasing quench temperatures. This suggests a cumulative damage mechanism reflecting an increase in damage throughout the material rather than sudden failure owing to propagation of pre-existing cracks.