Control of pore structures and activity of PGM-alumina exhaust catalysts
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Pore sizes in y-alumina were controlled with additives (porogens) during formation of aluminium hydroxide. Porogens were active in the mesopore range with a sharply defined pore size distribution. Porogens that enhanced mesoporosity were diethylene glycol, glycerine, benzoic acid, ammonium tartrate and fatty acids/alcohol (stearic acid, palmitic acid and cis-oleic acid, oleyl alcohol, castor oil, olive oil and sunflower oil. These additives caused the formation of aggregates at the iso-electric point (pH 9) of aluminium hydroxide. Porogens that increased microporosity were tartaric acid, sebacic acid, lactic acid, citric acid and formic acid, 2-butanol, n-hexanol, propanol, 2-pentanol, ammonium formate and ammonium citrate. Pore sizes in the microporosity region were affected by alumina crystallite sizes, which, in turn, were determined by the inhibition effect of strong chelating agents on crystallite growth. Macroporosities were introduced by filler or spacer effects of additives, such as adipic acid, tartaric acid and fatty acids/alcohol, .as well as ammonium tartrate and ammonium carbonate. All additives produced cylindrical-shaped pores, except fatty acids which produced ink-bottle pores. Emulsified cis-oleic acid produced higher total pore, mesopore and macropore volumes as well as surface area, but smaller pore sizes than commercial alumina powder, which is being used in the automotive exhaust catalyst industry. Sintering studies indicated that ceria (11 weight per cent) only stabilised the control samples to a certain degree, but it had no effect on fatty acid-derived alumina samples. Ceria crystallites sintered preferentially compared to y-alumina, but it prevented the formation of 6-alumina during sintering at 1000 °C. Two distinctive regions were observed when Knudsen effective diffusivity (KED (D0)) (Carniglia adsorption method) and KED values were plotted as a function of tortuosity factor (TF). These were assigned to cylindrical (TF < 2.00 and KED (D0) > 0.200 cm2/s) and non-cylindrical (TF > 2.00) pores. Uniform spherical spray-dried alumina powders were prepared from cis-oleic acid - aluminium hydroxide gel (COADA) after ageing of the hydroxide, flocculation and dispersion of the gel. This active, high surface area -y-alumina was applied as wash coat · layers on monolithic cordierite supports. Factors such as purity of cis-oleic acid, milling time, peptisation (acetic acid), steam treatment and ceria amount (3 weight per cent and 11 weight per cent added or 3 weight per cent built into the structure of alumina prior to spray-drying) affected the properties of alumina spray-dried powders and wash coat layers. These also affected cracking and attrition resistance of wash coat layers. Coating of monoliths was facilitated by a charge neutralisation at the interface between monoliths and the alumina wash coat slurry at pH 3. Activity evaluation of platinum impregnated catalysts showed that inhibition of the carbon monoxide oxidation reaction occurred in the temperature range between 68 to 150 °C depending on the catalyst. Diffusion and effectiveness factors were determined for catalysts to ensure that intrinsic kinetics were measured. Effectiveness factors of unity (based on adsorption data) were obtained at 140 °C for all fresh catalysts (5 mm) comprising similar wash coat loadings. Therefore, pore diffusion was negligible for these catalysts. Experimental effectiveness factors (EEF) (based on intrinsic rate constants for lowest· wash coat loading) higher than unity were obtained for 22 weight per cent and 38 weight per cent COADA catalysts. The 58 weight per cent commercial alumina powder catalyst had a low EEF which showed high pore diffusion restrictions. Activation energies for fresh catalysts containing 11 weight per cent ceria increased in the order: commercial standard < commercial alumina powder = COADA = COADA containing 3 weight per cent ceria built in < COADA containing 3 weight per cent ceria. Specific rate for fresh monolithic catalysts (5 mm) increased with increasing platinum crystallite size, which is due to the so-called size effect. Therefore, activity of a catalyst is not confined to the high surface area of the support, but to properties of active platinum on the support material. Specific rates associated with certain platinum crystallite sizes correlated with the trends found for specific conversions and rates. This shows that the highest catalytic activity can be expected for the commercial alumina powder sample, followed by the COADA samples containing 11 weight per cent and 3 . weight per cent ceria. This was also confirmed by Tso values of 60 mm monolithic catalysts. High wash coat loading was detrimental to catalyst activity. COADA catalysts showed a reduced apparent activation energy with increasing thickness, which was consistent with measured diffusional restriction. The effect of ageing 5 mm catalysts at 980 °C was a large decrease in activity for the commercial alumina powder catalyst, while the commercial monolith increased in activity. COAD A catalysts containing 11 weight per cent ceria showed the highest activity after sintering compared to other COADA catalysts. Activation energy of the aged standard catalyst did not change significantly during sintering compared to its fresh counterpart, which indicates the high stability of this catalyst. This was also confirmed by Tso values of 60 mm catalysts. This stability was also observed for the commercial powder sample. Sintering caused severe platinum crystallite growth for aged COADA catalysts compared to fresh catalysts. Addition of 3 weight per cent and 11 weight per cent ceria resulted in a similar degree of platinum sintering for COAD A catalysts. Thus, platinum was not stabilised by addition of ceria.