Abstract
By combining scanning transmission electron microscopy, CO chemisorption, and energy dispersive X-ray spectroscopy with CO and NO oxidation light-off measurements we investigated deactivation phenomena of Pt/Al2O3, Pd/Al2O3, and Pt-Pd/Al2O3 model diesel oxidation catalysts during stepwise hydrothermal aging. Aging induces significant particle sintering that results in a decline of the catalytic activity for all catalyst formulations. While the initial aging step caused the most pronounced deactivation and sintering due to Ostwald ripening, the deactivation rates decline during further aging and the catalyst stabilizes at a low level of activity. Most importantly, we observed pronounced morphological changes for the bimetallic catalyst sample: hydrothermal aging at 750 °C causes a stepwise transformation of the Pt-Pd alloy via core-shell structures into inhomogeneous agglomerates of palladium and platinum. Our study shines a light on the aging behavior of noble metal catalysts under industrially relevant conditions and particularly underscores the highly complex transformation of bimetallic Pt-Pd diesel oxidation catalysts during hydrothermal treatment.
Highlights
Understanding the degradation mechanisms that result in catalyst deactivation is a crucial step towards an increase of catalyst durability, which allows for saving expensive noble metals
In the fresh catalyst state, the majority of particles consists of homogeneous bimetallic alloys and fresh catalyst state, the majority of particles consists of homogeneous bimetallic alloys and only in some cases core-shell-like structures with a palladium-rich shell and a platinum-rich only in some cases core-shell-like structures with a palladium-rich shell and a platinumcore were found
The pronounced sintering especially after the first hour can be attributed to Ostwald-ripening [50], as the nearly atomically dispersed palladium particles that energy dispersive X-ray spectroscopy (EDXS) uncovered in the fresh state disappear during the hydrothermal treatment
Summary
Tightening environmental legislation and more stringent emission standards continuously increase the demand for highly active and durable exhaust gas after-treatment systems. Understanding the degradation mechanisms that result in catalyst deactivation is a crucial step towards an increase of catalyst durability, which allows for saving expensive noble metals. In this context, two of the most commonly used materials in exhaust gas catalysis are platinum and palladium [1], which are exploited for diesel oxidation catalysts (DOCs) [2]. Numerous scientific studies contributed to continuous improvements of DOCs for oxidizing NO, hydrocarbons (HCs) and CO, and today DOCs are an integral part of almost every emission control system for lean-operated internal combustion engines [3,4]
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