Extension of preparation methods employed with ceramic materials to carbon honeycomb monoliths

摘要

Honeycomb monolithic catalysts are being used to reduce the emissions of pollutants resulting from the combustion of fossil fuels [1]. At present, commercial catalysts include noble metals (Rh, Pt, Pd) and oxides of different elements such as V, Cr, Cu or Ti which are able to catalyze the transformation of atmospheric pollutant species into others with lower toxicity, so diminishing their concentration in the air to below allowed tolerance levels. Usually, active components are deposited over supports to obtain well dispersed phases that maintain high accessibility to the species that need to be transformed. As regards the supports, if compared with more conventional designs as those based on particle beds, monolithic structures offer several advantages such as a lower pressure drop in gas flows with high space velocity--so enhancing the catalyst's performance-together with lower weight and space requirements for filters and catalytic converters [2]. Currently, technology to make ceramic and metallic monoliths is already available. Several studies have pointed out the convenience of using carbon as raw material to make monolithic catalysts, both with and without deposited active phase [3]. Nonetheless, the plastic properties of carbon make it an inappropriate material for processing through extrusion [4]. In this sense, carbon-containing catalysts are prepared mainly as beds of powdered or granulated materials or, in the case of monoliths, by further deposition of carbon or a carbon's precursor on a ceramic monolith and not before extrusion. Although studies of honeycomb monoliths of activated carbons in the literature are available, they do not indicate how extrudibility of the carbonaceous paste is finally achieved [5]. The originality of this work lies in using a simple methodology developed for ceramics by Casa-grande and Atterberg [6] to predict the extrudibility of a carbon-based paste. As far as we know, no similar approach has been described in the bibliography devoted to the technology of carbonaceous solids.