S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 3 8 H I G H L I G H T S 2 0 2 1 I

PRINCIPAL PUBLICATION AND AUTHORS

Interface dynamics of Pd CeO2 single-atom catalysts during CO oxidation, V. Muravev (a), G. Spezzati (a), Y.-Q. Su (a), A. Parastaev (a), F.-K. Chiang (b), A. Longo (c), C. Escudero (d), N. Kosinov (a), E.J.M. Hensen (a), Nat. Catal. 4, 469 (2021); https:/doi.org/10.1038/s41929-021-00621-1 (a) Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology (The Netherlands) (b) National Institute of Clean-and-Low-Carbon Energy, Shenhua NICE, Future Science and Technology City, Beijing (PCR) (c) ESRF & Instituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR UOS di Palermo (Italy) (d) ALBA Synchrotron Light Source, Barcelona (Spain)

REFERENCES

[1] M. Flytzani-Stephanopoulos & B.C. Gates, Annu. Rev. Chem. Biomol. Eng. 3, 545 (2012). [2] A.K. Datye & H. Guo, Nat. Commun. 12, 895 (2021). [3] L. DeRita et al., Nat. Mater. 18, 746 (2019). [4] G.N. Vayssilov et al., Nat. Mater. 10, 310 (2011).

lattice, while the remaining Pd atoms are atomically dispersed at the surface as the catalytically active sites for CO oxidation. Combining transient and steady-state kinetic methods with in-situ infrared and near-ambient-pressure X-ray photoelectron spectroscopy unveiled a key role of surface Pd single atoms for the low-temperature activity. In the 1PdFSP catalyst, these atomically dispersed species (Pd-Ox-Ce, Figure 118a) are stable during CO oxidation up to temperatures of at least 300°C. When Pd is impregnated on a ceria support (1PdIMP) at the same Pd loading, the initially highly dispersed Pd atoms undergo reduction and sintering under identical conditions (Figure 118a). Catalysts prepared by the one-step FSP method exhibit strong metal-support interactions, leading to stabilisation of Pd single atoms during CO oxidation, even at a high loading of 5 wt.% (5PdFSP). The unusual properties of FSP-derived catalysts were linked to enhanced reducibility of the ceria support as probed by resonant photoelectron spectroscopy (RPES) of Ce 4d-4f transitions. More evidence came from X-ray absorption near-edge structure (XANES) spectroscopy and wide-angle X-ray scattering (WAXS) investigations performed at beamline BM26. Here, the 5PdFSP catalyst was exposed to a flow of CO, and Pd K-edge XANES spectra were recorded together with WAXS patterns as a function of temperature.

These two synchrotron-based techniques were combined to simultaneously probe the oxidation state of Pd (XANES) and the reduction of ceria (WAXS). The latter is possible because WAXS is sensitive enough to monitor the unit cell changes upon reduction of Ce4+ to Ce3+. Figure 118b shows that ceria lattice expansion, indicative of bulk Ce3+

formation, is delayed compared to Pd reduction in CO, which starts at 80°C. Once sufficient metallic Pd is formed (~50% Pd0), ceria starts being reduced at ~125°C. In contrast, notable Ce3+ formation in the 5 wt.% impregnated Pd/CeO2 catalyst was observed only at ~200°C, which points to a lower mobility of oxygen in this sample. Under similar conditions, the reduction of Pd-free CeO2 starts only at ~300°C. The bulk reduction of ceria, induced by the formation of Pd metal, points to reverse oxygen spillover [4].

By following the Ce4+/Ce3+ and Pd2+/Pdδ+/0 redox dynamics, it was established that doped Pd ions in FSP- derived catalysts activate ceria lattice oxygen, leading to increased oxygen mobility at the Pd O Ce interface (Figure 119). The reverse oxygen spillover is responsible for the stability of Pd single atoms on the surface of FSP-derived catalysts. The insight that the speciation of the supported metal is strongly influenced by the redox properties of the support is relevant to other catalytic reactions that involve a metal support interface, such as methane dry reforming, the (reverse) water gas shift reaction and CO2 hydrogenation.

Fig. 119: Evolution of Pd/CeO2 SACs during CO oxidation. a) Impregnated Pd/CeO2 catalyst contains oxidised Pd single

atoms that undergo reduction and agglomeration into metallic clusters at elevated temperatures. b) Pd-doped sites, inherently

present in FSP-derived catalysts, facilitate oxygen mobility at the Pd CeO2 interface and, via reverse oxygen spillover, stabilise the

oxidised Pd single-atom moieties.