Stability of the Pd/Co₃O₄(111) Model Catalysts in Oxidizing and Humid Environments

Abstract

The oxidation state and stability of Pd nanoparticles supported on well-ordered Co 3 O 4 (111) films prepared on Ir(100) have been investigated in UHV and under both oxidizing and humid conditions by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near ambient X-ray photoelectron spectroscopy (NAP−XPS). After preparation, the supported Pd nanoparticles (size 4 nm) were found to be predominantly metallic. Small amounts of Pd 2+ , resulting from the electronic metal support interaction (EMSI), were found in the form of PdO and as ionic species dissolved in Co 3 O 4 (111). Annealing of the Pd/Co 3 O 4 (111) model catalyst in UHV triggers sintering of the Pd nanoparticles but leaves the oxidation states of Pd and the substrate largely unaffected. The oxidation of Pd/Co 3 O 4 (111) is coupled with dissolution of Pd 2+ species into Co 3 O 4 (111) and underlying Ir(100) resulting in a significant loss of Pd from the surface. The corresponding phenomenon occurs largely under oxidizing and, to a minor extent, under humid conditions. The reverse oxygen spillover is facilitated in the presence of Pd 2+ species dissolved in Co 3 O 4 (111) yielding Pd nanoparticles supported on CoO(111). The CoO(111) support remains stable under humid conditions but is reversibly converted to Co 3 O 4 (111) under oxidizing conditions.

A Versatile Approach to Electrochemical In Situ Ambient-Pressure X-ray Photoelectron Spectroscopy: Application to a Complex Model Catalyst

“…30 Earlier, we observed similar shifts of the Pd 3d core level in the Pd/Co 3 O 4 (111) system exposed to 1 mbar H 2 O. 29 The four components in the O 1s spectra observed under hydrated conditions are assigned to the contributions from lattice oxygen in the Co 3 O 4 (111) film (1), OH − species (2), physisorbed/liquid H 2 O (3), and vapor H 2 O (4). Note that contributions (2) and (3) partially overlap with the Pd 3p 3/2 signal which is, however, insignificant under the present conditions.…”

“…The bottom spectra in Figures a–c were obtained from a Pd/Co 3 O 4 (111) model catalyst under 30 mbar H 2 O (hydrated conditions) before contact with the liquid electrolyte. Two contributions were detected in the Pd 3d spectra, at 335.9 eV (Pd 3d 5/2 ) and 338.2 eV (Pd 3d 5/2 ), associated with metallic Pd 0 and the Pd 2+ species dispersed in the Co 3 O 4 (111) substrate, respectively. , Unfortunately, we could not identify the contribution from the partially oxidized Pd δ+ species at the Pd/Co 3 O 4 interface due to the relatively low intensity and high noise-to-signal ratio in the Pd 3d spectra. It should be noted, however, that the binding energy of the metallic Pd 0 contribution is higher with respect to the value observed in similar systems in UHV (335.1 eV) .…”

“…The sample, a disc 10 mm in diameter, was prepared by means of UHV-based surface science methods (see Supporting Information). Previously, we characterized the Pd/Co 3 O 4 (111) model catalyst with respect to its morphology and the oxidation state of Pd nanoparticles and the Co 3 O 4 (111) support. , Briefly, the Pd/Co 3 O 4 (111) model catalyst consists of two-dimensional Pd nanoparticles with an average height of 3 ML and a mean diameter of 5.8 nm (see Figure ) supported on a 6.0 nm thick ordered Co 3 O 4 (111) film. In UHV, the Pd nanoparticles are predominantly metallic.…”

Particle Size and Shape Effects in Electrochemical Environments: Pd Particles Supported on Ordered Co3O4(111) and Highly Oriented Pyrolytic Graphite

“…Based on the depth profile analysis (see Supporting Information, Figure S2), we conclude that the Pd–carbon species (labeled as Pd–C in Figure d) are primarily located on the surface of the Pd nanoparticles. With respect to the chemical nature of the minor species on Pd/Co 3 O 4 (111), we refer to our previous studies . Thus, we assign the contributions (2) and (3) at binding energies of 336.9–337.1 (Pd 3d 5/2 ) and 338.1 eV (Pd 3d 5/2 ) to a joint contribution from Pd 2+ and partially oxidized Pd δ+ aggregates (labeled as Pd 2+ /Pd δ+ ) and Pd 2+ species dispersed in the Co 3 O 4 (111) substrate (labeled as Pd sol 2+ ), respectively.…”

The Journal of Physical Chemistry C Virtual Special Issue on Metal Clusters, Nanoparticles, and the Physical Chemistry of Catalysis

“…Traditional model catalysts are simplified systems consisting of metal nanoparticles (NPs) of a controllable size and/or metal composition, often supported by a range of well-defined supports such as graphene or metal oxides. This VSI includes six experimental papers regarding the investigation of the heterogeneous catalysis and synthesis of novel heterogeneous catalysts: the selective hydrogenation of alkenes over Pd and Cu NPs, the stability of Pd NPs on Co 3 O 4 (111), the electrodeposition synthesis of size- and shape-controlled Cu 2 O NPs for CO 2 reduction, the beam deposition synthesis of binary PtCu NPs with various mixing modes for the hydrogen evolution reaction, and the solvothermal flow synthesis of immiscible and high-entropy alloy NPs . These studies illustrate the potential of nonprecious elements such as copper for catalytic application.…”