(755h) Systematic Identification of Active Site for Propene Combustion Using Pd and Pt Bimetallic Nanocrystal Catalysts

Emission control catalysts have gained increasing interests primarily due to the more rigorous requirement to reduce emissions of various hazardous pollutants, such as hydrocarbons and CO. Currently, most catalysts use Pt and Pd to remove the pollutants since these elements are known for their high efficiency in combusting hydrocarbons [1]. However, there still are open questions regarding the catalytic behavior of Pd and Pt since the conventional catalysts preparation always results in nonuniform metal morphology and site distribution. Thus, understanding how the structure and change of these materials relate with performance is crucial to optimize the use of these expensive metals and improve their performance at lower temperatures as needed by new engine requirements. Herein, we report a systematic study using well-defined Pd/Pt nanoparticles (NPs) to provide a fundamental understanding of hydrocarbon combustion. Propene (C₃H₆) is chosen to be the model molecule because of its large contribution to the diesel emissions [2].

Well-defined Pd and Pt nanoparticles were prepared by colloidal synthesis, with their size and shape rigorously controlled. First, the effect of metal composition was studied 2-3 nm Pd/Pt NPs with five different Pd/Pt molar ratios, including the two single metals. From light-off curves, we show that all the bimetallic catalysts are more active with or without the presence of 10 vol.% H₂O, and the equimolar Pd/Pt catalyst is the most active sample in the set. In the kinetics measurement, a surprising beneficial effect of water was observed on pure Pt and Pt-rich bimetallics, while pure Pd and Pd-rich bimetallics suffer from water poisoning. It was also observed that the improved activity in the presence of water has a positive linear relation with the Pt content.

We further investigated the effect of size on the equimolar Pd/Pt sample where the Pd/Pt ratio of 1/1 is the most active metal composition in light-off curves. Equimolar and uniform Pd/Pt NCs with sizes ranging from 2.3 to 10.2 nm were synthesized via a seed-mediated approach. These samples are proved to be random alloys by STEM-EDS Mapping and EXAFS. A clear size-dependence trend is observed from the turnover frequency on these catalysts. As the NP size becomes larger, the intrinsic activity is increased both in the absence and in the presence of 10 vol.% H₂O. Post-catalysis TEM characterization, corroborated by DFT calculations, reveal that a specific ensemble of step sites, that are active for breaking the rate-limiting C=C bond in propene combustion, was formed on larger Pd/Pt NPs. Besides, the increase in the fraction of step-site ensemble is in line with the increase in turnover frequency. Therefore, we suggest that the particular ensemble of step sites is identified as the active site for propene combustion. Reaction order measurements performed on all the materials also support this hypothesis, showing that propene adsorption becomes stronger on more active catalyst with larger nanoparticle, thus further corroborating that the presence of step sites is favorable for improved reaction rates. Taken all together, this data clarifies the active sites in Pd and Pt based materials for propene combustion. These insights help in the design of improved catalytic materials for propene combustion where the use of noble metals can be optimized.

References

1. Trinh, H. T., Imanishi, K., Tazuko, M., Hagino, H. and Takenaka, N. Journal of the Air & Waste Management Association 67 (4), 412 (2017).
2. Costagliola, M., Simio, L., Iannaccone, S. and Prati, V. Energy 111, 1162 (2013)