Performance Characterisation of a Range of Diesel Oxidation Catalysts: Effect of Pt:Pd Ratio on Light Off Behaviour and Nitrogen Species Formation 2011-24-0193

Understanding the behavior of automotive catalysts formulations under the wide range of conditions characteristic of automotive applications is key to the design of present and future emissions control systems. Platinum-based oxidation catalysts have been in use for some time to treat the exhaust of diesel-powered vehicles and have, as part of an emissions control package, successfully enabled compliance with emissions legislation. However, progressively stringent legislated limits, coupled with the need to reduce vehicle manufacturing costs, is incessantly demanding the development of new and improved catalyst formulations for the removal of pollutants in the diesel exhaust. With the introduction of low sulfur diesel fuel, and the advantageous decline in Palladium prices with respect to Platinum, bimetallic Pt:Pd-based catalysts have found an application in diesel after treatment. In this paper the findings of a study carried out on a range of lightly loaded (40 g/ft₃) diesel oxidation catalysts with varying Pt:Pd ratios are presented. The catalysts' performance characterization was measured on a state-of-the-art, commercially available, integral synthetic gas reactor and a range of exhaust analyzers was used to speciate the exhaust gas on catalyst exit. The aim of the study was to determine the effect of the Pt:Pd ratio on catalyst performance. The evaluation of the impact of varying the formulation on catalyst light-off and nitrogen species formation was of particular interest to support both catalyst modeling and exhaust system design. The study found that, at parity of exhaust flow conditions tested, the same nitrogen species are evolved in all of the catalysts, although in different concentrations. Higher Pt content distinctly favors higher conversion of NO to NO₂, which is beneficial if further treatment by LNT or SCR is implemented. Higher Pt content however, also promotes higher N₂O formation, which is undesirable. The effect on other nitrogen species of interest from an emissions control standpoint, namely NH₃ and N₂, is a more complex function of test conditions. Higher Pd content generally lowers the mixture CO catalyst light-off temperature, driven by higher metal activity towards CO conversion, which is highly desirable for the treatment of diesel exhaust gas. However, the performance of samples with higher Pd content appears to be affected by oxygen exposure. The effect on HC conversion is a more complex function of catalyst composition and reacting mixture, with CO concentration playing an important role on the overall catalyst performance.