Low temperature combustion (LTC) diesel engines are being developed to meet increased fuel economy demands. However, some LTC engines emit higher levels of CO and hydrocarbons and therefore diesel oxidation catalyst (DOC) efficiency will be critical. Here, CO and propylene oxidation were studied, as representative LTC exhaust components, over model bimetallic Pt-Pd/γ-Al2O3 catalysts. During CO oxidation tests, monometallic Pt suffered the most extensive inhibition which was correlated to a greater extent of dicarbonyl species formation. Pd and Pd-rich bimetallics were inhibited by carbonate formation at higher temperatures. The 1:1 and 3:1 Pt:Pd bimetallic catalysts did not form the dicarbonyl species to © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 the same extent as the monometallic Pt sample, and therefore did not suffer from the same level of inhibition. Similarly they also did not form carbonates to as large an extent as the Pdrich samples and were therefore not as inhibited from this intermediate surface species at higher temperature. The Pd-rich catalysts were relatively poor propylene oxidation catalysts;and partial oxidation product accumulation deactivated these catalysts. Byproducts observed include acetone, ethylene, acetaldehyde, acetic acid, formaldehyde and CO. For CO and propylene co-oxidation, the onset of propylene oxidation was not observed until complete CO oxidation was achieved, and the bimetallics showed higher activity. This was again related to less extensive poisoning, less dicarbonyl species formation and less overall partial oxidation product accumulation.

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Spatially resolved Fourier transform infrared spectroscopy (SpaciIR) was used to measure gas phase concentration profiles during CO and C 3 H 6 oxidation reactions over a Pt/Al 2 O 3 monolith supported catalyst. The reaction conditions were selected as representative of certain low temperature combustion (LTC) engine exhaust conditions, where in this study higher concentrations of CO, C 3 H 6 and lower concentrations of NO x were used relative to standard engine exhaust. CO and C 3 H 6 oxidation and NO X reduction reactions were examined individually and in combination via temperature programmed oxidation (TPO) experiments. Significant NO X reduction occurred right at CO and C 3 H 6 light off, and NO oxidation only occurred after the oxidation of CO and C 3 H 6. C 3 H 6 oxidation was not observed until after most of the CO oxidized, as CO was more strongly adsorbed to the active site surface at low temperature. During the TPO of CO and C 3 H 6 , the conversion versus temperature profiles did not monotonically increase; two inflections were observed where the rate of conversion change as a function of temperature slowed over a small temperature range before again accelerating with temperature. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used in order to characterize intermediates that were present on the surface at the temperatures where these steps were noted. Surface ethylene and formate species were present during the first step, with acetate and formate in the second step. The inhibition steps were therefore attributed to the partial oxidation of propylene to ethylene, and then the subsequent partial oxidation of ethylene to acetate.

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There have been ongoing research efforts focused on layering or zoning different washcoats/active metals on the catalysts constituting diesel aftertreatment systems: the diesel oxidation catalyst (DOC), the selective catalytic reduction (SCR) catalyst, the lean NOX trap (LNT), the ammonia slip catalyst (ASC), and the diesel particulate filter (DPF). This review paper aims to shed insight into the state‐of‐the‐art research on catalyst design in this area and how these catalyst designs may evolve to tackle engine emission reductions in the future. First, we discuss the motivation for zoning or layering catalysts and pioneering work on three‐way catalyst (TWC) design for reducing gasoline engine emissions; then, we focus on the catalytic systems used for diesel exhaust aftertreatment. The configuration of the aftertreatment systems for diesel engines generally consist of an oxidation catalyst for hydrocarbon (HC), CO, and NO oxidation (over the DOC), a NOX reduction catalyst (over one or combined SCR/LNT/ASC catalysts), and a particulate matter (PM) filter (using a DPF). The research to date consistently demonstrates that zoning and layering catalyst regions leads to improved performance and/or smaller system volumes required.

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