Ensemble Effect Evidenced by CO Adsorption on the 3-Fold PdGa Surfaces

Abstract

ABSTRACT:The atomic structure and composition of a catalyst's surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa intermetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (−1− 1−1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces.

Formation of GaPd2 and GaPd intermetallic compounds on GaN(0001)

“…Annealing recovers the crystallinity of the surface. Subsequent films deposited under the same conditions on the Pd pre-covered surface are crystallographically ordered and epitaxially oriented with the Pd(111) plane parallel to the substrate surface and the Pd[100] direction parallel to the GaN [10] direction.…”

“…The covalent bonding within the compounds leads to a much higher stability against surface segregation even under reaction conditions [5]. So far, well-defined and reproducibly obtainable unsupported Ga-Pd intermetallic compounds with a rather low specific surface area and ultrathin Ga films on Pd(111) and Pd(110) surfaces have been used for basic research on physicochemical and catalytic properties of these compounds [1][2][3][4][5]10]. In this paper, we report the results of our studies on possibilities of the formation of Ga-Pd intermetallic compounds in a dispersed form by engineering the surface of the Pd/GaN(0001) adsorption system.…”

CO Adsorption on GaPd—Unravelling the Chemical Bonding in Real Space

“…Since the dangling bonds shield most of the surface (Figure c), only two potential adsorption sites out of 15 crystallographically possible are left for the CO molecule (Figure ): on‐top of the Pd 1 atom (T site) and the hollow site at the center of the triangle formed by the Ga 3 atoms (H site). Earlier studies verify this hypothesis . From the fifteen crystallographically different on‐top, bridge and hollow sites of the Pd 1 ‐terminated GaPd( ) surface, only two sites reveal an energy gain upon CO adsorption: 1) on top of the surface palladium (T), with an adsorption energy of 1.07 eV, and 2) the hollow site (H) between three surface gallium atoms, with an adsorption energy of only 0.05 eV.…”

“…Lattice planes with a high density of two‐center interactions are located perpendicular to the crystallographic direction [111], resulting in high stability of the GaPd( ) surface. The bulk‐like structure of this surface is best described by a stacking sequence of four non‐equivalent atomic layers (Pd 1 /Ga 3 /Pd 3 /Ga 1 ) along the [111] direction . In Figure , the structural model of the GaPd( ) surface is shown, where the three topmost atomic layers Pd 1 , Ga 3 and Pd 3 are highlighted.…”

The Electronic Structure and Bonding of Acetylene on PdGa(110)

“…Prinz et al 22 performed a similar study on the same PdGa surfaces, by combining scanning tunneling microscopy, temperature-programmed desorption, and ab-initio catalysis. Krajčí ( ) and (100) surfaces by DFT methods 8,25 .…”