ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS

124 ESRF

investigated the nature of atomically dispersed platinum supported on ceria, the dynamic changes occurring during oxidation reactions, and the impact on the activity of such catalytic systems via complementary operando X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and in-situ infrared (IR) spectroscopy combined with DFT calculations.

In particular, due to the complexity of the diluted system, and to better appreciate subtle changes as a result of the dynamic behaviour of the Pt species under catalytic conditions, high- energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy experiments at the Pt L3-edge were performed at beamline BM16 [2]. HERFD-XANES allows for a unique increase in the resolution of Pt L3-edge spectra [3]. Together with an appropriate photon flux (maximum brilliance without beam-inducing effects on samples), this constellation offered optimal conditions for the investigation of the sensitive system. Thanks to these aspects, it was possible to follow the changes on the white line induced by the presence of adsorbates, ranging from a clear increase in intensities to fainter shifts in position, these latter not appreciable by conventional XANES.

Whereas high CO conversion was reached below 100°C for nanoparticles, in the case of single sites, there is no appreciable CO conversion below 200°C, despite a 100% dispersion of Pt atoms on the CeO2 surface. The HERFD-XANES spectra recorded at BM16 in the reaction mixture

(1000 ppm CO, 10% O2 in He, temperature range 50 400°C) showed pronounced changes in both the white line position and intensity (Figure 108). Up to 150°C, the peak intensity experiences a steep decrease (an effect not noticeable in conventional XAS), while at higher temperature, as the activity sets in, the white line undergoes a shift towards lower energies. The intermediate structural states in the spectroscopic data were interpreted and quantified using the references obtained by a multivariate curve resolution alternating least squares (MCR-ALS) algorithm [4]. Four reference states were identified and also confirmed on the basis of complementary XPS, in-situ ultrahigh- vacuum Fourier transform infrared (UHV-FTIR) studies and finite-difference-method near-edge structure (FDMNES) calculations: Pt4+, Pt2+, reduced Ptδ+ interacting with CO and a cluster- like species, PtXδ+-CO.

Thanks to these findings, also substantiated by DFT calculation, it was possible to describe in detail the relationship between the variations in the structure of Pt single sites on ceria and the corresponding effect on catalytic activity. During its formation at elevated temperature, single Pt2+ species bind to square-planar hollow sites in, e.g., {110} facets of ceria. In particular, a noble metal-induced restructuring of the ceria surface usually known for adsorbates was found. These Pt2+-CeO2 structures are very stable and explain the poor performance of Pt single sites in CO, C3H6 or CH4 oxidation. Only when the cluster- like PtXδ+-CO species becomes predominant in the HERFD-XANES spectra does the catalytic

Fig. 109: Proposed scheme for the dynamics of the active state in platinum

single sites on ceria. Figure credit: P. Dolcet, F. Maurer, M. Casapu, J.-D.

Grunwaldt (KIT).