Abstract
For
maximizing the atomic efficiency in noble metal-based catalysts,
dedicated preparation routes and high lifetime are essential. Both
aspects require an in-depth understanding of the fate of noble metal
atoms under reaction conditions. For this purpose, we used a combination
of complementary in situ/operando characterization techniques to follow the lifecycle of the Pd sites in a 0.5% Pd/5% CeO2–Al2O3
catalyst during oxygen-rich CO oxidation. Time-resolved X-ray
absorption spectroscopy showed that Pd cluster formation under reaction
conditions is important for a high CO oxidation activity. In
combination with density functional theory calculations, we concluded
that the ideal Pd cluster size amounts to about 10–30 Pd atoms. The
cluster formation and stability were affected by the applied temperature
and reaction conditions. Already short pulses of 1000 ppm CO in the
lean reaction feed were found to trigger sintering of Pd at temperatures
below 200 °C, while at higher temperatures oxidation processes
prevailed. Environmental transmission electron microscopy unraveled
redispersion at higher temperatures (400–500 °C) in oxygen atmosphere,
leading to the formation of single sites and thus the loss of activity.
However, due to the reductive nature of CO, clusters formed again upon
cooling in reaction atmosphere, thus closing the catalytic cycle.
Exploiting the gained knowledge on the lifecycle of Pd clusters, we
systematically investigated the effect of catalyst composition on the
cluster formation tendency. As uncovered by DRIFTS measurements, the Pd
to CeO2 ratio seems to be a key descriptor for Pd
agglomeration under reaction conditions. While for higher Pd loadings,
the probability of cluster formation increased, a higher CeO2 content leads to the formation of oxidized dispersed Pd species. According to our results, a Pd:CeO2 weight ratio of 1:10 for CeO2–Al2O3-supported
catalysts leads to the highest CO oxidation activity under lean
conditions independent of the applied synthesis method.
D. Gashnikova, F. Maurer, M.R. Bauer, S. Bernart, J. Jelic, M. Lützen, C.B. Maliakkal, P. Dolcet, F. Studt, C. Kübel, C.D. Damsgaard, M. Casapu, J.-D. Grunwaldt, Lifecycle of pd clusters: Following the formation and evolution of active pd clusters on ceria during co oxidation by in situ/operando characterization techniques, ACS Catalysis, 2024, 14871-14886. DOI: 10.1021/acscatal.4c02077.
