Modeling of Diesel Oxidation Catalyst

Abstract

Optimization of hydrocarbon (HC) oxidation over a diesel oxidation catalyst (DOC) requires consideration of (i) HC gas diffusion into the catalyst layer, (ii) HC gas adsorption and desorption from catalyst sites, and (iii) kinetics of the oxidation reaction. We have already reported a brief modeling study that emphasized understanding HC storage and release from zeolitic sites within the DOC [Banno et al., SAE Tech. Pap. Ser. 2004-01-1430. In this study, more-detailed DOC modeling was attempted, using a precise gas diffusion model and experimentally determined reaction parameters. The random pore model was used for gas diffusion calculations, to simulate the bimodal nature of the catalyst structure, which has both macroporosity between catalyst particles and microporosity within the zeolite material. HC adsorption capacity, as a function of temperature and HC concentration, was measured by temperature-programmed desorption (TPD). The rate of HC desorption rate was evaluated by changing the TPD ramping rate. The rate of HC oxidation was measured using a model gas reactor. The conversion efficiency of HC, which was calculated by computer simulation with Star-CD software, reproduced the experimental measurements well, as a function of temperature, thus validating the model. The simulation methodology was used to predict two-dimensional (2D) HC conversion efficiency profiles across radial and axial positions of the monolithic catalyst during HC combustion, and it was also applied to estimate the depth profile of adsorbed HC in the catalyst layer.

Mutagenicity of biodiesel or diesel exhaust particles and the effect of engine operating conditions

“…Mutagenicity was slightly increased by the B100 exhaust (120 mg/plate) at high load mode M2 with a great increase in NO 2 concentration [ 23 ], which might be responsible for synthesizing even more genotoxic materials, nitro-PAH, under these conditions. Our results are similar to those from the different studies showing that changes depended mainly on engine load modes [ 34 , 62 , 63 ]. They suggested that high engine load increases NOx in the exhaust and NOx cannot effectively be reduced by a DOC, while high NOx levels lead to an increased formation of nPAH.…”

Mutagenicity of Diesel Engine Exhaust Is Eliminated in the Gas Phase by an Oxidation Catalyst but Only Slightly Reduced in the Particle Phase

“…The authors proposed that the DOC increases formation of direct acting mutagens under certain conditions by the reaction of NO X with PAH. 35 Therefore, the effectiveness of a DOC can depend on the engines, the fuels and the mode of operation. In fact, test cycles that simulate traffic conditions result in higher emission rates of products of incomplete combustion compared to tests at constant load mode, since at load changes the combustion is temporarily suboptimal due to an oxygen shortage.…”

“…44−46 Combustion with formation of NO X may promote nPAH production. 35 3-NBA (3-nitro-7H-benz[d,e]anthracen-7-one) was identified as a prominent compound in extracts from diesel exhaust and ambient particles which showed very strong direct mutagenic and clastogenic effects in vitro. 47 It therefore served as a positive control for direct acting mutagenicity in the present investigation.…”

Parametric study of the main parameters influencing the catalytic efficiency of a diesel oxidation catalyst: Parameters influencing the efficiency of a diesel catalyst

“…Consequently, CO tail-pipe emissions, decrease significantly with temperature. HC conversion efficiency also increases with temperature (Tanaka et al, 2005); however, the slope of conversion curves decreases at higher temperatures, indicating that there is an upper limit for this increase (Tanaka et al, 2005). HC tail-pipe emissions also decrease with temperature; however, the slope is lower than in the case of CO, indicating that temperature is more important in the case of CO oxidation than on the case of HC oxidation, as shown in Figures 1 and 2.…”

“…For constant engine-out CO missions, tail-pipe HC emissions increase with HC engine-out emissions (Tanaka, 2005), even if conversion efficiency slightly increases, because engine-out HC emissions are much higher than the increase of the corresponding conversion. For constant engine-out HC emissions, HC oxidation is very little influenced by CO, as tail-pipe HC emissions remain practically constant with engine out CO emissions.…”