Abstract
Infrared spectroscopy is typically not used to establish the oxidation state of metal-based catalysts. In this work, we show that the baseline of spectra collected in diffuse reflectance mode of a series of Pd/Al2O3 samples of increasing Pd content varies significantly and reversibly under alternate pulses of CO or H2 and O2. Moreover, these baseline changes are proportional to the Pd content in Pd/Al2O3 samples exhibiting comparable Pd particle size. Similar measurements by X-ray absorption spectroscopy on a different 2 wt.% Pd/Al2O3 confirm that the baseline changes reflect the reversible reduction-oxidation of Pd. Hence, we demonstrate that changes in oxidation state of metal-based catalysts can be determined using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and that this behavior is part of the spectral changes that are returned by experiments under operando conditions.
Highlights
Infrared (IR) spectroscopy in its various modes—transmission, diffuse reflectance and internal reflectance—is a powerful technique among the large variety of in situ/operando methods to study materials for heterogeneous catalysis [1]
The scanning transmission electron microscopy (STEM) images collected in high-angle annular dark-field (HAADF) mode demonstrate that the Pd/Al2 O3 samples of the variable
A modulated excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiment where O2 pulses are repeatedly alternated to CO pulses on the 1 wt.% Pd/Al2 O3 of this series after reduction at 300 ◦ C is presented in Figure 2, which shows the time-resolved DRIFT spectra as well as the corresponding spectra obtained after phase sensitive detection analysis [16,17,18]
Summary
Infrared (IR) spectroscopy in its various modes—transmission, diffuse reflectance and internal reflectance—is a powerful technique among the large variety of in situ/operando methods to study materials for heterogeneous catalysis [1]. The technique strongly relies on the interaction of probe molecule reactants and products with the material of interest to probe the fundamental vibrational modes of the resulting adsorbate. Other methods, such as X-ray absorption spectroscopy (XAS) [4], are sensitive to coordination and oxidation state changes that are considered out of the capabilities of IR spectroscopy. In IR spectroscopic studies of solid catalysts, absorption level changes in IR spectra over the whole mid-IR range are often neglected [7]. Their origin may have various explanations and may depend on several effects. IR spectroscopy is used to monitor with very high time resolution (pico- to nanoseconds) electron injection into the conduction band of semiconductor materials such as those that can be exploited as supports in heterogeneous catalysts and sensors, and the subsequent slower relaxation [8]
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