A Comparison of Experimental Procedures for the Application of Infrared Spectroscopy to Probe the Surface Morphology of an Alumina-Supported Palladium Catalyst

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.

Toward High Selectivity Aniline Synthesis Catalysis at Elevated Temperatures

“…Bridge-bonded CO was held more strongly on the catalyst and was still present in the spectrum collected after the maximum desorption temperature of 450 °C. These results agree with previous CO TP-IR measurements for 5 wt % Pd/Al 2 O 3 catalysts, 29 , 40 indicating GU-1 to provide a useful reference material on which to consider morphological effects on hydrogenation activity.…”

“… 40 The significance of distinguishing between CO adsorption on corner and edge sites, adsorption sites proposed to be involved in hydrogen supply, 40 − 42 via DRIFTS has previously been considered. 29 Furthermore, and central to this study, is to note that the profile of the ambient temperature infrared spectrum shown in Figure 2 is indicative of the Pd crystallites adopting a truncated cuboctahedron structure. 40 Thus, catalytic turnover on GU-1 can be considered as occurring on the Pd surface sites, as revealed within Figure 2 .…”

“…The temperature-programmed IR spectroscopic profile for CO chemisorption over GU-1 has recently been reported over the temperature range of 28–300 °C. 29 Figure 2 extends this range to 450 °C. The room temperature spectrum depicts 4 features: a broad band at 1903 cm –1 assigned to μ 3 bridge-bonded CO on Pd(111) planes, a sharp band at 1978 cm –1 arising from μ 2 bridge-bonded CO on Pd(100) planes, and a broad feature at higher wavenumbers that can be resolved to a band at 2055 cm –1 that is associated with linear CO adsorption to edge sites and a shoulder feature at 2077 cm –1 arising from linear CO adsorption to corner sites.…”

“…As considered previously, Pd edge and corner sites are thought to play an important role in maintaining hydrogen supply. 29 With reference to Figure 2 that shows edge sites to exhibit the higher enthalpy of adsorption of the terminal sites, it is tentatively suggested that these sites are active in facilitating dissociative adsorption of dihydrogen 29 that, subsequently, participates in surface-mediated hydrogenation reactions.…”

“…The former represents a reference catalyst, where the morphology of the Pd crystallites has been previously examined by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). 29 In contrast, the lower loading sample is potentially more suited to industrial application on cost grounds; its catalytic performance and morphological attributes will be compared to the reference material. Although a reduced Pd loading has economic benefits, the lower metal loading may result in a mass-transfer limited regime.…”