Utilizing Quantitative in Situ FTIR Spectroscopy To Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al₂O₃-Supported Pt Catalysts

Abstract

Relationships between geometric structures of active metallic sites and areal rates of reaction (structure sensitivity) are extensively studied for supported metal catalysts. For CO oxidation on irreducible oxide-supported Pt catalysts, there still exists a discrepancy regarding structure sensitivity. Theoretical and single-crystal analyses suggest the CO oxidation reaction rate should be highly structure sensitive, whereas measurements on supported Pt catalysts show only minimal structure sensitivity. Here, we used quantitative in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to investigate the influence of CO oxidation reaction conditions on the fraction of well-coordinated (WC) and under-coordinated (UC) Pt active sites on a series of four α-Al2O3-supported Pt catalysts with average Pt sizes ranging from ∼1.4 to 19 nm. Pt nanoparticle surfaces were observed to restructure under CO oxidation reaction conditions, increasing the fraction of UC Pt sites. Reconstruction rendered the fraction of WC and UC sites less dependent on Pt particle size than expected from geometric models. A model, coupling the DRIFTS measurements with previous theoretical calculations, was quantitatively correlated to the measured slight structure sensitivity on the same series of catalysts. Our results bridge the gap between previous studies exploiting theory, single crystals, and supported Pt catalysts by demonstrating that WC Pt atoms are the active site for CO oxidation, but that CO-induced restructuring of Pt nanoparticle surfaces masks the inherent structure sensitivity in particle-size-dependent rate measurements.

Surface Oxidation of Supported Ni Particles and Its Impact on the Catalytic Performance during Dynamically Operated Methanation of CO2

“…This supports earlier studies in the literature [48][49][50]58] that selective blockage of the low coordinated Ni edge or corner sites leads to differed or deteriorated catalytic properties. Similar structure sensitivity due to easier oxidation of low-coordinated sites was also observed during CO oxidation on Pt catalysts [59,60]. For example, oxidation of low coordinated Pt atoms located at edge and corner sites during CO oxidation was found by XAS resulting in an irreversible CO adsorption on PtO [61].…”

High energy X-ray diffraction and IR spectroscopy of Pt/Al2O3 during CO oxidation in a novel catalytic reactor cell

“…Measurement of the sample temperature is critical in DRIFT spectroscopy, especially when kinetic information should be derived [21]. Careful determination of the sample temperature can be obtained in commercial cells using a pyrometer [21,22]. In the present cell, the position of the sample bed between the two heat cartridges [9] provides homogeneous sample temperature.…”

“…Rigorous kinetic measurements reported ca. 80 kJ/mol [22,32] suggesting that our measurements may be still suffering from mass transfer limitations. The shift of the points obtained in the catalytic reactor to lower y-values is associated again with the uncertainty in the temperature measurement (different placement of thermocouples) that caused a shift of the two activity profiles measured in the continuous linear temperature ramp.…”

Balancing Near-Field Enhancement, Absorption, and Scattering for Effective Antenna–Reactor Plasmonic Photocatalysis

“…We have previously developed detailed insights into the mechanism of thermal and photocatalytic CO oxidation on Pt NPs, allowing us to focus here on how proximal Ag antennas influence Pt photocatalysis in the context of this reaction. 31,44 The Figure S2). In this reaction, kinetic measurements under identical conditions showed that CO desorption is the rate-limiting step (RLS), because the Pt surface is poisoned by a complete monolayer of CO under reaction conditions.…”

“…We have previously developed detailed insights into the mechanism of thermal and photocatalytic CO oxidation on Pt NPs, allowing us to focus here on how proximal Ag antennas influence Pt photocatalysis in the context of this reaction. 31,44 The Photocatalysis was driven by continuous wave illumination from a Xenon lamp, which was used either in a broadband white light mode with a maximum photon flux of ~600 mW/cm 2 (spectral output shown in Figure S3), or as spectrally filtered bands with ~45 nm full-width at halfmaximum and a maximum photon flux of ~120 mW/cm 2 . We used illumination by wavelengths in the range of 350−700 nm, where direct photoexcitation of intramolecular HOMO−LUMO electronic transitions localized solely in CO is excluded and all photon absorption involves excitation of the metals.…”