Exhaust Gases as Fluxing Agents for Palladium: Formation Enthalpies and Vapor Pressures of PdO, PdO2, Pd(CO)n (n = 1–3), Pd(NO)n (n = 1,2), Pd(OH)n (n = 1,2), HOPdNO, and 21 Other Species under Automotive Catalyst Aging Conditions

Abstract

Catalyst deactivation by precious metal sintering is a major driver of cost in automotive emission control catalysts. In order to develop strategies to mitigate sintering and maintain higher activity from lower precious metal loading, it is necessary to first develop a more complete understanding of the sintering process. In particular, it is of interest to identify the mobile intermediates that are responsible for sintering via Ostwald ripening and the extent to which the identities and concentrations of these intermediates depend upon the composition of the contacting atmosphere during catalyst use. In this work, first-principles thermodynamic calculations have been conducted on a series of small palladium-containing molecules to assess the potential of these molecules to form over palladium catalysts in the presence of automotive exhaust and serve as mobile carriers during Ostwald ripening. Among the results obtained, three stand out: (1) while exposure to reducing atmospheres in general does not enhance palladium mobility, exposure to carbon monoxide in particular leads to a multiple order of magnitude increase in palladium mobility driven primarily by the formation of Pd(CO)2; (2) under hydrothermal aging conditions, the most abundant vapor species is neither Pd nor PdOx but PdOH; and (3) combination of the latter with traces of nitric oxide enables a significant increase in palladium mobility via the formation of HOPdNO. The impact of surface adsorption on stabilization of mobile intermediates is also considered; a simple non-specific adsorption model indicates that surface stabilization enhances Pd(OH)2 formation under hydrothermal aging conditions.