Abstract
Structure/function relationships in heterogeneous catalysis play an important role in catalyst design strategies. The combination of chemisorption of suitable probe molecules alongside application of infrared spectroscopy is an established technique for providing information on the metal crystallite morphology of supported metal catalysts. Following a review of key literature on this topic, a variety of experimental arrangements that may be adopted for this task are examined. Specifically, the adsorption of CO over a 5wt% Pd/Al2O3 catalyst is investigated using transmission and diffuse reflectance sampling options and two research grade spectrometers. Although comparable spectra are obtained on all the platforms examined, differences are noted. In particular, temperature-programmed IR spectroscopy on one platform enables resolution of two features assigned to linear CO bound to the Pd particles. The relevance of this sub-division of terminal sites with respect to selective hydrogenation reactions is briefly considered.
Highlights
Infrared (IR) spectroscopy provides a platform for the observation of interactions between adsorbates and surfaces; it plays a pivotal role in the characterisation of heterogeneous catalysts [1]
The spectra obtained on warming provide an indication of the enthalpy of adsorption of CO on the Differences in absorbance values for the 303 K spectra are readily identifiable between both techniques, with the bridge bonded (μ2) CO feature more prominent in the DRIFTS set up (Fig. 2b)
As exemplified by McInroy et al in their use of vibrational spectroscopy to investigate the interaction of methanol on η-alumina [38], an adsorbate combination band at about 2600 cm−1 was relatively strong in DRIFTS spectra but considerably weaker in the corresponding transmission infrared (TIR) measurement
Summary
Infrared (IR) spectroscopy provides a platform for the observation of interactions between adsorbates and surfaces; it plays a pivotal role in the characterisation of heterogeneous catalysts [1]. FT-IR studies of CO adsorption over, for example, supported Pd catalysts has a role for defining active site distributions, which can be correlated with observed reaction trends [10,11,12]. Against this background, it is informative to consider milestones in the use of IR techniques combined with CO chemisorption to discern the active site distribution of supported metal catalysts. These studies highlight the importance of temperature-programmed infrared spectroscopy (TP-IR) In this communication, we compare the difference in spectral acquisition of adsorbed CO over a Pd/Al2O3 catalyst using TIR and DRIFTS. The relevance of the resulting morphological deductions with respect to sites effecting hydrogen supply in selective hydrogenation reactions will be briefly considered
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