Size‐Specific Chemistry on Bimetallic Surfaces: A Combined Experimental and Theoretical Study

Abstract

Bimetallic catalysts have long been known for their attractive catalytic properties, which are often superior to those of the components and which are generally found to depend strongly on the composition of the individual bimetallic nanoparticles making up the catalyst. [1][2][3][4] The improved catalytic performance had been explained in terms of concepts such as the electronic ligand effect or the geometric ensemble effect. [2,5] The fundamental understanding was hampered, however, by the largely unknown surface composition and, in particular, by the unknown distribution of the respective components in the surface layer. This led to a renewed interest in the chemical properties of the alloy surface where, in contrast to bimetallic nanoparticles, scanning tunneling microscopy (STM) makes it possible to directly evaluate the specific geometries available for adsorption on these surfaces, by high-resolution imaging with chemical contrast. Furthermore, density functional theory (DFT) calculations have opened up the possibility of studying the energetics of such sites. In this way, both ordered and disordered surface alloys, [6][7][8][9][10] overlayer structures, [11][12][13][14][15][16][17] steps, [18][19][20] and step modification [21] have been studied in some detail. Herein, we address the question of size effects in surface alloying and their impact on the chemical properties of the surface. We use a combined experimental and theoretical approach to quantitatively study how the surface chemistry depends on the size of islands of one metal in a matrix of another metal. The specific system we study is the adsorption of CO on well-defined bimetallic PdAu surface alloys. These surface alloys can be considered as model systems for PdAu catalysts, which have attracted considerable interest for several applications, most prominently for vinyl acetate synthesis. [22,23] The surface composition and distribution of surface atoms are determined by high-resolution STM. The interaction of CO with the respective surface is characterized by temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). For the TPD measurements, it was verified that the surface was not modified by the temperature scan. The results allow us to clearly distinguish between electronic ligand effects, geometric ensemble effects, and adsorbate-adsorbate interactions. They also demonstrate the kind of agreement achievable between state-of-the-art theoretical work (DFT calculations) and experimental data, and the quality of chemical information extractable from high-resolution STM measurements in combination with statistical evaluation and nonlocal spectroscopic measurements. During the course of this work, Goodman and co-workers published a combined TPD/IR spectroscopy study on the interaction of CO with equilibrated, MoA C H T U N G T R E N N U N G (110)-supported PdAu alloy films, whose surface composition was determined by low-energy ion scattering. [23,24] While the distribution of surface atoms could not...

Hydrogen adsorption on bimetallic PdAu(111) surface alloys: minimum adsorption ensemble, ligand and ensemble effects, and ensemble confinement

“…20,54 It should be noted that neither in this work nor in a preceding study we had any evidence for a surface reconstruction. 20 The distribution of the atoms in the surface layer was derived by quantitative evaluation of series of atomic resolution STM images for each surface concentration, which allowed us to distinguish between Pd and Au surface atoms by the so called 'chemical contrast'. 59 Representative STM images, where due to electronic effects Pd surface atoms appear bright and Au surface atoms are dark, are presented in Fig.…”

“…6), exploiting the known adsorption and desorption behavior of CO, with CO adsorbed atop on a Pd monomer desorbing at 330 K, bridge bound CO ad on Pd dimers desorbing at 380 K (Fig. 6a) and CO adsorbed on the threefold hollow sites above compact Pd trimers at 470 K. 20 For the present experiments, we first exposed the surface to CO at 140 K, and then annealed the sample to defined temperatures. This way, only specific Pd ensembles and sites remained CO ad populated.…”

From Adlayer Islands to Surface Alloy: Structural and Chemical Changes on Bimetallic PtRu/Ru(0001) Surfaces

“…In combination with adsorption/catalysis studies on these structurally well-defined surfaces, such model systems provide an ideal test case for different effects discussed as physical origin for the modified chemical and catalytic surface properties. [6][7][8][9][10][11][12][13] As part of an extensive study on the (electro-)chemical properties of bimetallic PtRu surfaces and their correlation with their structural characteristics, [6,[13][14][15][16][17][18] herein we focus on the structural modifications encountered at the transition from an overlayer system, such as a Pt monolayer or Pt monolayer island covered Ru(0001) surface, to a surface alloy, and their effect on the chemical surface properties. The structural properties of laterally equilibrated bimetallic PtRu/Ru(0001) surfaces were already studied in detail by STM.…”

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

“…23,24 This transition was also reported for Pd ensembles on PdAu and PdAg surface alloys. 3,8,10 In these alloys, the Pd ensemble size depends on the random local Pd concentration at the surface, whereas in the PdGa (111) and (−1−1−1) surfaces, size and separation are given by the crystal structure and do not allow for CO bridge site adsorption. However, the Pd ensembles found in the partially covalently bound IMC retain their adsorption properties, as adsorption sites, energy, and vibrational modes compare for CO adsorbed on the IMC and on the surface alloys, for Pd ensembles of equal size.…”