Hysteresis Phenomena on Platinum and Palladium-based Diesel Oxidation Catalysts (DOCs)

Abstract

The diesel oxidation catalyst (DOC) plays a key role in diesel exhaust treatment systems. Typical noble metals used as active components are platinum (Pt) and palladium (Pd). During lightoff/lightout experiments, the catalyst reactivity during heating is in some cases different from the reactivity during cooling in the same reaction mixture. These so called hysteresis phenomena have repeatedly been reported for CO, NO, and HC conversion and are mostly attributed to noble metal oxidation and/or surface coverage effects. Hauff et al. have developed a kinetic model that is able to account for the hysteresis effects observed in NO conversion on Pt-only catalysts due to noble metal oxidation. The model was only validated for a limited range of NO concentrations and temperatures. In this follow-up, further experiments on Pt-only as well as on Pd-only catalysts will be presented for additional feed compositions. General transferability of the Pt-only model to Pd-only catalysts for NO-only is demonstrated. By addition of CO and propene to the feed, additional hysteresis phenomena are observed and will be discussed. On Pd-only, results indicate that under lean exhaust conditions, the noble metal can only be slightly reduced by CO. Interestingly, with a mix of CO and NO no reactivation is observed whereas the combination of CO, NO, and propene again shows reducing tendency. Based on these informations, a possible modeling approach will be proposed.

HC-Induced Deactivation in CO Conversion at Diesel Oxidation Catalysts

“…The solid lines represent the ignition branches and the dashed lines the extinction noble metal, which can be caused by NO or other reducing species at low temperatures would probably result in an activated catalyst at the beginning of each light-off experiment. We do not claim that noble metal oxidation does not influence the CO ignition curve, which has already been reported by Hauptmann et al and by Dubbe et al [5,7]. However, we do claim that it is unlikely that the reported effect in this work is only caused by the formation of PtO X .…”

“…The phenomenon of a constant inverse hysteresis in NO conversion can be explained by Pt oxide formation with formed NO 2 and/or O 2 at high temperatures and a reduction of the active species by NO, CO, or HC species at low temperatures [2,5,7].…”

“…It has been pointed out [2][3][4][5][6][7][8][9][10] that under typical exhaust treatment operating conditions, reversible activation/ deactivation effects can be observed. This is a fundamental challenge for the currently established LHHW-based modeling approach since, in the presence of the activation/ deactivation effects, the observed reaction rate becomes a function of the recent catalyst history.…”

“…The inverse hysteresis effect has been investigated in more detail in a number of studies [4][5][6][7], and it has been shown that for simple gas mixtures, the hysteresis behavior can be quantitatively reproduced by global [5][6][7] as well as microkinetic models [2]. Apart from the inverse NO hysteresis on platinum, further deactivation and reactivation effects caused by different precious metal oxidation states were reported for NO, CO, and propene conversion on platinum as well as on palladium by Dubbe et al [7]. They assessed the reducing potential of single pollutants and pollutant's gas mixtures, proposed a modeling approach, and simulated the effects in the case for NO for both noble metals.…”

Modeling of Conversion Hysteresis Phenomena for Pt/Pd‐based Diesel Oxidation Catalysts

“…The equilibrium constant of the NO oxidation was calculated from NIST chemistry webbook . In previous studies Dubbe et al considered additional side reactions including the formation of N 2 O , . The essential findings are listed below: Significant amounts of N 2 O (∼60 ppm) were found with a feed of 300 ppm propene on Pt‐only between 200 and 300 °C. In this region, the NO x concentration encountered a significant drop. Even if no CO was present in the feed, low amounts of CO (∼10 ppm) were found at the reactor outlet, when 500 ppm NO and 300 ppm C 3 H 6 were dosed under lean conditions. …”