Carbon-templated hexaaluminates with enhanced surface area and catalytic performance

Abstract

Carbon templating was used to synthesize high-surface area LaFeAl 11O 19 hexaaluminates leading to improved catalytic performance. The oxide precursor–template composites were prepared by impregnation, complexation with citric acid, and coprecipitation routes varying the amounts of acetylene black incorporated. Calcination in static air at 1473 K led to single-phased hexaaluminates. The samples were characterized at different stages of the synthetic protocol by ICP–OES, in situ and ex situ XRD, TEM, N 2 adsorption, He pycnometry, TGA, H 2-TPR, and XPS. The templating effect of carbon originates fine oxide particles. Consequently, the specific surface area of the non-templated hexaaluminates (1–11 m 2 g −1 depending on the preparation method) increased up to a factor of 25 upon incorporation of carbon in the synthesis. The stabilization of small particles occurs despite the temperature difference between the elimination of carbon (973 K) and the attainment of the hexaaluminate phase above 1373 K. The morphology of the oxide obtained upon carbon removal determines the textural properties of the hexaaluminate particles. Our results demonstrate that the confined space synthesis is not exclusively responsible for small oxide particles. The stabilization of small crystallites on the carbon surface in the composite and breakage of the oxide layer caused by template combustion are key aspects for attaining finely dispersed particles when the carbon is macroporous. The enhanced catalytic activity and time-on-stream stability of the templated hexaaluminates was demonstrated in the direct N 2O decomposition using model and simulated feed mixtures. In this application, a quasi-linear relation between the reaction rate and the specific surface area of the samples was obtained. Characterization by H 2-TRP and XPS indicated that the nature of iron in LaFeAl 11O 19 was not altered by carbon incorporation in the synthesis.