Hard x-ray photoemission spectroscopic investigation of palladium catalysts immobilized on a GaAs(001) surface

Abstract

We present studies on the structure and chemical states of a catalyst developed by immobilizing palladium on S-terminated GaAs͑001͒. Hard x-ray photoelectron spectroscopy ͑HX-PES͒ of core-level and valence band photoemission consistently indicates that the organopalladium molecules are reduced on the surface yielding Pd nanoparticles with a metallic nature. This finding is supported by high-resolution observations using scanning electron microscopy and backscattered electron image. HX-PES results also reveal that a portion of S atoms forming the S-termination is oxidized during the formation of Pd nanoparticles.

Sulfur-mediated palladium catalyst immobilized on a GaAs surface

“…This peak is therefore attributed to Pd nanoparticles on the basis of our previous SEM and HX-PES studies; it was revealed that Pd nanoparticles are generated on the surface of the catalyst prepared using the same procedure as S6 and that the Pd 3d peak of nanoparticles is located at 335.7 eV. 8 The observed shift, 0.8 eV, is comparable with that reported. [13][14][15] The formation of Pd-nanoparticles suggests that Pd(II) in Pd(OCOCH 3 ) 2 is reduced during the immobilization and heat treatment processes.…”

“…A distinct peak is observed at 2470 eV in all spectra. Since it was confirmed in previous studies 8 that the S 1s peak from the S-termination of GaAs surface is observed at 2470.1 eV, this peak is assigned to the S-termination.…”

“…As reported previously, Pd(II) (OCOCH 3 ) 2 is gradually broken down by the intense x-ray beam, and consequently a rather broad peak is yielded. 8 All the other spectra were analyzed using the same curve-fitting procedure with four Gaussian functions. The binding energies of the four peaks are, however, fixed at the same values of 335.2, 335.8, 336.2, and 337.0 eV, as determined for S6.…”

“…8 It was revealed that organopalladium molecules are reduced at the surface and that Pd nanoparticles are created. Since the presence of zero-valent Pd (Pd(0)) is essential in the Heck reaction, we concluded the Pd nanoparticles to be the origin of the catalytic activity.…”

Surface study of organopalladium molecules on S‐terminated GaAs

“…1, one possible reaction is the anion exchange by the OH group accompanying elimination of acetic acid. In this case, {Pd} is chemically immobilized on the surface, substituting the proton [6,17]. Following reduction of Pd(II) to Pd(0) is assumed [6,17] but cannot be discussed with FT-IR.…”