Pd/Co₃O₄(111) Interface Formation

Abstract

The formation of the metal−oxide interface in the Pd/Co 3 O 4 (111) model catalyst was investigated by means of density functional theory (DFT), synchrotron radiation photoelectron spectroscopy (SRPES), and scanning tunneling microscopy (STM). The electronic metal−support interaction results in a substantial charge transfer at the interface yielding atomically dispersed Pd 2+ species and partially oxidized Pd δ+ aggregates coupled with a partial reduction of Co 3 O 4 (111). Atomically dispersed Pd 2+ species at the fcc site on the Co 3 O 4 (111) surface were found to be the most energetically favorable configuration. In comparison to the dispersed Pd 2+ species, the formation of Pd dimers, trimers, and tetramers was found to be less favorable. The analysis of the Bader charges revealed a substantial net positive charge on Pd atoms in dimers, trimers, and tetramers which is consistent with the formation of partially oxidized Pd δ+ aggregates detected by SRPES. The analysis of the charge distribution in Co 3 O 4 (111) revealed a partial reduction of Co 3+ to Co 2+ cations in the first and second Co layers. According to DFT, Pd δ+ aggregates are prone to oxidation to PdO in the presence of O 2 and H 2 O. The partially oxidized Pd δ+ and Pd 4 O x aggregates form 1 to 2 monolayer thick clusters which serve as nuclei for the growth of metallic Pd 0 nanoparticles. At high Pd coverage, Pd nanoparticles coalesce resulting in the growth of two-dimensional islands that densely cover the Co 3 O 4 (111) substrate.

Constructing a Pd–Co Interface to Tailor a d-Band Center for Highly Efficient Hydroconversion of Furfural over Cobalt Oxide-Supported Pd Catalysts

“…The surface element chemical valence of Pd/Co 3 O 4 - x was explored by XPS spectroscopy to disclose the metal–support interaction, and the deconvoluted XPS spectra are depicted in Figure . Pd 3d 5/2 binding energy peaks centered at 335.1, 336.8, and 338.0 eV in the Pd 3d XPS spectrum of Pd/Co 3 O 4 -0 were separately assigned to Pd 0 , Pd 2+ , and Pd 4+ , in accordance with the values reported in the literature. , Lee et al and Kastenmeier et al attributed the presence of Pd 2+ species in Pd/Co 3 O 4 to the electron transfer from Pd to ionic cobalt via −Pd–O–Co– bonds on account of the metal–support electronic interaction, which was also confirmed by the charge distribution via DFT calculations. − Schuschke et al associated the Pd 3d 5/2 binding energy peak at 338.1 eV with palladium ions dissolved in Co 3 O 4 . Similarly, ionic palladium species in Pd x Ce 1– x O 2−δ solid solution exhibited a Pd 3d 5/2 binding energy in the range of 337.5–338.2 eV .…”

Atomistic picture of electronic metal support interaction and the role of water

“…Typically, we quantify the oxidation state of Co 3 O 4 (111) based on the changes in the Co 3+ /Co 2+ concentration ratios obtained by the analysis of the valence band spectra. 52 The Co 3+ /Co 2+ concentration ratio was monitored using two different photon energies yielding surface (60 eV) and sub-surface (115 eV) information. Prior to Pt deposition (see circled dots in Fig.…”