Aluminum Oxide as a Support for the Exhaust Gases Purification Catalysts
Статья (Full article),
||Aluminium Oxide: Structure, Production and Applications
Nova Science Publishers, Inc. NY, USA.2020.
||alumina, three-way catalysts, palladium, rhodium, metal-support interaction, deactivation, surface migration, agglomeration, bulk diffusion, thermal stabilization, doping with lanthanum oxide
Vedyagin Aleksey A.
Volodin Alexander M.
Department of Material Sciences and Functional Materials,
Boreskov Institute of Catalysis SB RAS
The exhaust gases purification catalysts such as three-way catalysts, used for treating the emissions from gasoline engines, and diesel oxidation catalysts, applied correspondingly, in the case of the diesel engines, are of indisputable actuality, since the requirements for automobile emission control become more and more strict every year. Aluminum oxide has been historically used as a component of such purification systems. Usually it serves as a secondary support mounted on the inner surface of the channels of the cordierite honeycomb-structured monolith. The total washcoat composition includes active metals (Pd, Pt, and Rh in varied combinations) and, depending on the type of the catalyst, can include an oxygen storage component (ceria or more complex ceria-based systems). In most cases, γ-Al2O3 is being applied due to the more developed surface area that allows distributing the active metals with the desired degree of the dispersion. The active metals in their turn, are known to interact strongly with alumina, and each metal behaves differently. For instance, palladium supported on gamma alumina in small amounts interacts with the surface electron-donor sites of the latter, thus becoming anchored and stabilized. Such catalysts retain the dispersion degree of Pd and corresponding high level of the catalytic activity even after the thermal aging procedures at temperatures as high as 1000°C. The larger palladium loadings facilitate the local oversaturation of these sites with Pd atoms, thus leading to their surface migration and agglomeration, and causing the fall in the catalytic activity. Another mechanism of the metal-support interaction (MSI) is character for the alumina-supported rhodium catalysts. Rhodium ions formed on the surface of alumina under the reaction conditions diffuse into the bulk of the support, where they initiate the formation of α-Al2O3 phase at temperatures lower than at which it should happen. Irreversible encapsulation of Rh ions within the formed corundum crystallites is the most serious problem leading to the deactivation of the rhodium-containing catalysts. The use of δ-Al2O3 support instead of γ-Al2O3 does not give a positive effect, since the mechanism of the rhodium-alumina interaction is also being changed, thus accelerating the diffusion of Rh3+ ions and affecting their localization within the bulk near the interfacial boundaries. On the other hand, alumina is rarely applied in a pure, non-modified form. For instance, the doping of γ-Al2O3 with lanthanum oxide enhances significantly the thermal stability, shifting the phase transformation towards the high temperature region. In terms of MSI, modification of alumina with La2O3 opens new diffusion routes for rhodium and changes the preferable places for its location near the La ions.