Solid-state transformations, catalyst deactivation

Continuous exposure of catalysts to high temperatures may cause an alteration in its components and gradually lead to its deactivation. Thermal degradation may have an undesirable impact on both the catalyst substrate and noble metal load in various ways. Thermal degradation covers two phenomena sintering and solid-state transformation. 

Sintering of metals loaded on a support also leads to deactivation, for example, with Pt/alumina catalysts used in the reforming of hydrocarbons. The phenomenon has been modeled in mathematical terms by Elynn and Wanke [1974, 1975] and by Riickenstein and Pulvermacher [1973] and reviewed by Dadyburjor [1987]. The whole field of solid-state transformations was... 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

This contribution will address some issues which are not examined usually in meetings on catalyst deactivation. We will look at catalyst deactivation in a broader context, namely the modifications that solid catalysts (and probably also homogeneous ones ) continuously undergo while they are acting catalytically under reaction conditions. As underlined in our previous contributions, this is particularly relevant when solid-state reactions are considered, because solids have a sort of memory (1,2). Their transformations at a given moment are strongly influenced by the whole succession of conditions they have been subjected to. 

The deactivation of these several forms of catalyst (separated but supported active oxide, epitaxial oxide growth, or mixed metal oxide) can occur by the transformation of the active (catalytic) form to either of the other forms of solid. Alternately, the active form of the catalyst may involve a specific form of the metal oxide, just as the oxidation state of the metal and the irreversible oxidation or reduction of the metal (even in the same solid lattice) may cause deactivation. Our current understanding of SCR de-NOx catalysts is not detailed enough to know the nature of the active phase precisely for all forms of active catalysts. However, several researchers have analyzed their results often with a specific perspective as to the active form of the catalyst. 

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