The catalytic behavior of precisely synthesized Pt–Pd bimetallic catalysts for use as diesel oxidation catalysts

Abstract

The demands of stricter diesel engine emission regulations have created challenges for current exhaust systems. With advances in low-temperature internal combustion engines and their operations, advances must also be made in vehicle exhaust catalysts. Most current diesel oxidation catalysts use heavy amounts of precious group metals (PGMs) for hydrocarbon (HC), CO, and NO oxidation. These catalysts are expensive and are most often synthesized with poor bimetallic interaction and dispersion. The goal of this work was to study the effect of aging on diesel emission abatement of Pt-Pd bimetallic nanoparticles precisely prepared with different morphologies: well dispersed core-shell versus well dispersed homogeneously alloyed versus poorly dispersed, poorly alloyed particles. Alumina and silica supports were studied. Particle morphology and dispersion were analyzed before and after hydrothermal treatments by XRD, EDX, and STEM. Reactivity as a function of aging was measured in simulated diesel engine exhaust. While carefully controlled bimetallic catalyst nanoparticle structure has a profound influence on initial or low temperature catalytic activity, the differences in behavior disappear with higher temperature aging as thermodynamic equilibrium is achieved. The metallic character of Pt-rich alumina-supported catalysts is such that behavior rather closely follows the Pt-Pd metal phase diagram. Nanoparticles disparately composed as well-dispersed core-shell (via seq-SEA), well-dispersed homogeneously alloyed (via co-SEA), and poorly dispersed, poorly alloyed (via co-DI) end up as well alloyed, large particles of almost the same size and activity. With Pd-rich systems, the oxidation of Pd also figures into the equilibrium, such that Pd-rich oxide phases appear in the high temperature forms along with alloyed metal cores. The small differences in activity after high temperature aging can be attributed to the synthesis methods, sequential SEA and co-DI which give rise, after aging, to a bimetallic surface enriched in Pd.

Effects of Hydrothermal Aging on CO and NO Oxidation Activity over Monometallic and Bimetallic Pt-Pd Catalysts

“…In this context, continuous catalyst improvement by combining different catalytically active materials are an essential aspect of environmental research and can substantially improve heterogeneous catalysts [24]. Although palladium shows only minor NO oxidation activity, its combination with platinum can serve as an effective measure to minimize deactivation phenomena for diesel oxidation catalysts [25][26][27][28][29]. Moreover, palladium shows higher thermal stability under oxidizing conditions compared to oxidized platinum, as the latter one forms volatile PtO2; hence, the usage of a Pt-Pd alloy has been reported as an efficient approach to increase the catalyst's thermal stability [30][31][32][33].…”

“…Carrillo et al [38] reported that PdO nanoparticles can trap mobile Pt species under oxidizing conditions, which is of high relevance for understanding the behavior of bimetallic Pt-Pd catalysts. Lately, a wide variety of advanced materials has been proposed for catalytic emission control under lean conditions, namely encapsulated coreshell catalysts [25,39,40] and nanostructured architectures [41,42] for oxidation reactions and microporous materials for NOx control [43][44][45][46]. Although these materials often show extremely promising catalytic activity or stability, fundamental in-depth understanding, transfer into industrial application and upscaling of preparation procedures remains challenging.…”

The Effect of Pt Particle Size on the Oxidation of CO, C3H6, and NO Over Pt/Al2O3 for Diesel Exhaust Aftertreatment

“…Even though a typical diesel exhaust contains a variety of 173 hydrocarbon components, propene (C3H6) was used as a model compound for hydrocarbon oxidation, 174 following common practice [5,10,11,23,24,46]. The propene conversion was determined based on the CO2 175 concentrations measured in the product gas, using the same analyzer as for CO oxidation, and the 176 stoichiometry of the reaction equation, forming three CO2 molecules per C3H6 molecule.…”

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

“…As a means of reducing CH 4 emissions, a major challenge for the environment, the total catalytic oxidation of CH 4 has been extensively studied [27][28][29][30][31]. The complete oxidation of CH 4 into CO 2 (a less potent greenhouse gas), particularly at low temperatures (below 500 • C), has been widely studied [32][33][34].…”