Macro‐ and Microkinetic Simulation of Diesel Oxidation Catalyst: Effect of Aging, Noble Metal Loading and Platinum Oxidation

Abstract

In automotive exhaust aftertreatment simulation, both macro-and microkinetic models are commonly used. In this contribution both models are applied for the simulation of diesel oxidation catalysts (Pt/c-Al 2 O 3 ) with different catalyst loading and degree of thermal aging. The study proves that the structure insensitive kinetics of the considered catalysts can be described with the same rate equations only by scaling the rate constants of the different reaction steps with the catalytically active Pt surface, which is accessible by CO adsorption or light-off measurements. In addition, NO oxidation is strongly influenced by a reversible, slow transformation of Pt into Pt-oxide. Catalyst aging 673give insight into the reaction network and coverage-dependent processes on the catalytic surface. For the oxidation of NO crucial information about the inhibition effect of NO can be obtained and used in the macrokinetic model. ExperimentalDOCs (Pt/c-Al 2 O 3 , 400 cpsi monolith) of a commercial catalyst supplier with three different platinum loadings have been investigated (Tab. 1). All catalysts underwent a hydrothermal pretreatment/aging in a furnace, for which the monoliths (diameter 75 mm, length 125 mm) were canned and flowed with 10 % water vapor in air. The catalysts DOC20, DOC60 and DOC120 (20, 60, 120 g ft -3 ) were pretreated for 16 h at 700°C. Two samples of DOC120 were hydrothermally aged for 16 h at 850°C (DOC120-850) and at 950°C (DOC120-950).After pretreatment/aging, catalyst slices (30 mm × 40 mm, one channel height: 1.4 mm) were taken from the center of the monolith for the kinetic measurements. The measurements were carried out in an isothermal flat bed reactor with five catalyst slices in a row [9, 10] under realistic flow conditions with synthetic exhaust gas and a space velocity of 40 000 h -1 . The measurement of the concentration profiles along the catalyst length was accomplished via lateral withdrawals after each slice. The experiment started with 5 min measurement versus the reactor end and continued for 1 min per lateral withdrawal. The synthetic exhaust gas consisted of 12 % O 2 , 7 % CO 2 , 10 % H 2 O and varying concentrations of C 3 H 6 (0 -1000 ppm), CO (0 -15 000 ppm) and NO (0 -500 ppm). Nitrogen was used for balance. The temperature was varied between 120 and 450°C.The specific platinum surface of all catalyst samples was determined by temperature programmed desorption of CO (CO-TPD). Before adsorption, the monolithic samples were reduced at 400°C with 10 % H 2 for 16 h. The temperature was set to 25°C and the catalyst was saturated with 10 % CO for 30 min followed by a temperature ramp (30 K min -1 ) to 500°C. The platinum dispersion D Pt and the specific platinum surface O Pt shown in Tab. 2 were calculated (Eqs. (1) and (2)) by the molar amount of desorbed CO, n TPD CO , and platinum deposited on the catalyst, n total Pt , assuming stoichio-metric adsorption (CO:Pt = 1:1) on the platinum surface. A detailed description of the CO-TPD procedure and experimental setup is given in [11].

Preparation of Pt/Al2O3 and PtPd/Al2O3 catalysts by supercritical deposition and their performance for oxidation of nitric oxide and propene

“…Hauff et al tested monolithic DOCs with different Pt loadings and showed that the light-off temperature for C3H6 oxidation decreased linearly with increasing Pt loading. Their results on NO oxidation indicated that there was a strong nanoparticle (NP) size effect [5,6]. In a study conducted by Boubnov et al, NO oxidation was found to be most efficient when the NP size was larger than about 5 nm, while smaller particles remained less active [4].…”

“…Among them, Pt is widely used in DOCs, and the addition of Pd was shown to increase the thermal stability and sintering resistance of the catalyst [1]. As PGMs are high cost components of ATSs, several efforts have concentrated on decreasing the amount of PGMs and increasing the stability and efficiency of PGMs by preparing DOCs with different PGM loadings, ratios, or particle sizes [3][4][5][6][7][8][9][10], and using different synthesis techniques [11][12][13][14].…”

Multiscale modeling of methane catalytic partial oxidation: From the mesopore to the full‐scale reactor operation

“…Although, depending on the local pore size and operating conditions, different dominant diffusion regimes are observed (molecular or Knudsen diffusion) . For modeling purposes, the effect of internal transport limitations on the catalyst performance can be considered implicitly in reaction kinetics (macrokinetics) or through reaction‐diffusion models in which intrinsic (transport‐free) reaction steps (microkinetics) are treated independently of transport processes …”

HC-Induced Deactivation in CO Conversion at Diesel Oxidation Catalysts

“…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 effect can be explained by a reversible oxidation of the platinum surface by NO 2 or O 2 at higher temperatures and a reduction of the platinum oxides by NO at lower temperatures. 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].…”