A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts

Abstract

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically-dispersed Pt supported on crystalline MgO (chosen for its well-defined bonding sites for Pt) as a prototypical example, in this work, we demonstrate how systematic density functional theory calculations (for assessing all the potentially stable Pt sites) combined with automated EXAFS analysis can lead to unbiased identification of isolated, surfaceenveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging, EXAFS, and HERFD-XANES spectroscopies; the proposed Pt site are in full agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active sites than what is currently possible with conventional EXAFS analysis. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation

“…(Frećhet The Journal of Physical Chemistry Letters distances have been used previously to quantify the similarity between the experimental and simulated spectra. 13 ) Further, the structure of [100] Mg-vac /sub1 is in good agreement with the HERFD-XANES results characterizing Pt/MgO 700 , as we reported before (Figure 2h). 13 Details are presented in Figure S7.…”

“…The HERFD-XANES data were compared with FEFF-simulated XANES spectra calculated from the above-mentioned DFT-optimized geometries (Figure 2g,h). 13,26,27 The structure [100]/sub0/*O 2 is in good agreement with the HERFD-XANES data characterizing this sample (Figure 2g), with a Frećhet distance of 0.155. (Frećhet The Journal of Physical Chemistry Letters distances have been used previously to quantify the similarity between the experimental and simulated spectra.…”

“…28 The narrowness of the band implies that the species were nearly uniform in structure. 29 This band shifted to 2034 cm −1 when the sample at room temperature was exposed to 13 CO, as this ligand displaced 12 CO; the frequencies are consistent with the harmonic approximation and confirm the identification of terminally bonded CO on the platinum (Figure 3a). 30 These results are confirmed by the DFT-predicted 12 CO and 13 CO frequencies (Figure 3b, Figure S9, and Table S2).…”

“…13−15 Herein, we report results of X-ray absorption spectroscopy (XAS), atomic-resolution electron microscopy, and density functional theory (DFT) calculations that demonstrate two distinct sites for atomically dispersed platinum on MgO. In addition to a recently reported site 13 for bonding platinum in the second layer of MgOwithin the latticewe now report platinum in a more accessible site in the first MgO layer. This newly discovered site is characterized by relatively high catalytic activity for CO oxidation but low stability.…”

“…Pt/MgO 400 (the subscript is the calcination temperature in °C) was synthesized from K 2 PtCl 4 slurried with MgO powder in ethanol−water solutions, then dried, and calcined in 20% O 2 in N 2 at 400 °C (Supporting Information). 13 Low metal loadings (≈0.05 wt %) were used in an attempt to minimize the number of different bonding sites for platinum on the support. Some of the Pt/MgO 400 samples were subsequently calcined under more severe conditions, at 700 °C, giving Pt/ MgO 700 .…”