An EXAFS study of the structure of the metal–support interface in highly dispersed Rh/Al2O3 catalysts

Abstract

Four highly dispersed and fully reduced rhodium on alumina catalysts with different particle sizes in the range 6–12 Å were investigated with the EXAFS technique in order to derive information about the structure of the metal–support interface. This information can only be obtained when the signal-to-noise ratio of the experimental EXAFS data is high enough and accurate reference compounds and a modified way of data analysis are used. With the aid of phase and amplitude corrected Fourier transforms it was possible to detect a small additional signal which could be ascribed to a Rh–O bond. Since the catalysts were fully reduced and since the intensity of the small signal increased with decreasing particle size, the oxygen neighbor was assigned to be originated from the metal–support interface. From the intensity of the Rh–O bond it was estimated that, on the average, each interfacial rhodium atom is surrounded by 2–3 oxygen ions of the support. The detected Rh–O bond has a coordination distance of 2.7 Å which is about 0.6 Å larger than the first coordination distance in Rh2O3 (2.05 Å). The coordination distance of 2.7 Å can be explained by assuming an interaction between metallic rhodium (atomic radius 1.34 Å) and ionic oxygen belonging to the support (ionic radius 1.4 Å). This would possibly imply an ion-induced dipole bonding between the metal particle and the support.

Pt clusters in BaKL zeolite: Characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

“…The absence of higher Pt-Pt coordination shells, combined with the microscopic evidence of the lack of platinum outside the zeolite and the uniqueness of the structure of the intracrystalline zeolite pores, points to a nearly unique cluster size. Extremely small metal particles covered with chemisorbed hydrogen show metal-metal distances equal to bulk values [12,15]. This type of behavior is also observed in this Pt/BaKL sample.…”

“…The Pt-Pt absorber-backscatterer pair has a nonlinear phase shift and a k-dependent backscattering amplitude. Consequently, it is inferred that there are several peaks in the Fourier transform of a single Pt-Pt contribution [12,13,15].…”

“…To determine reliably the parameters characterizing the high-Z (Pt and Ba) and low-Z (O) contributions, multiple-shell fitting in k space and in r space was done, with application of k 1 and k 3 weighing [13]. Further optimization of the fit was done by applying the difference file technique and phase-and amplitude-corrected Fourier transforms [12,13].…”

“…Standard procedures were used to extract the EXAFS functions from the absorption spectra [11,12]. Experimentally determined reference data for the phase shifts and backscattering amplitudes were obtained from EXAFS measurements of platinum foil and Na2Pt(OH)6 [13].…”

Determination of metal particle size in partly reduced Ni catalysts by hydrogen/oxygen chemisorption and EXAFS

“…In the partly reduced systems four contributions are expected with R < 3.5 A: Ni-Ni of the reduced phase, and Ni-0, Ni-Ni, and Ni-Si of the hydrosihcate phase (14). Of these, the reduced Ni-Ni and the oxidic Ni-0 coordination parameters (coordination number N, coordination distance R, and Debye-Wailer factor A(r2 with respect to the appropriate reference compound) were determined using the difference file technique (15). In this technique, first an estimate for the parameters of the largest contribution (viz.…”