Au-step atoms as active sites for CO adsorption on Au and bimetallic Au/Pd(111) surfaces

Abstract

In a combined variable temperature STM-HREELS study we could identify Au edge atoms in the first, second and higher Au layers on a Pd(111) substrate as sites of a strongly enhanced Au−CO interaction compared with CO adsorption on the respective terrace sites. CO molecules adsorbed on these Au edge atoms give rise to a C−O stretch frequency of 263 meV, indicative of linearly bound CO. Adsorption on Au terrace sites is not observed at 110 K. The increased Au−CO interaction is attributed to a modification in the electronic properties of Au edge atoms, related to their reduced coordination.The activity of supported metal catalysts is decisively influenced by various factors, such as the nature of the support material, the presence of chemical promoters, the addition of a second metal ("bimetallic catalysts") and, in particular, by the size of the chemically active particles [1,2]. For the latter case not only is the increased surface area important, but electronic modifications due to the increased fraction of low-coordination surface atoms ("defect atoms") on particles of a few nanometers in diameter may also play an important role. Significant modifications in the chemical properties caused by each of these effects have been verified in model studies, e.g. on highly stepped vicinal surfaces [3] or on bimetallic surfaces of single-crystal metal substrates [4,5]. The correct interpretation and assignment of the observed changes in the chemical properties, on a microscopic scale, however, depends critically on the detailed knowledge of the local surface structure and composition, in particular for more complex substrates, which in most studies so far has not been available.In an effort to overcome these problems we have recently started a detailed study on the local chemical properties of structurally well-defined bimetallic Au/Pd(111) surfaces, using CO as a probe molecule. By employing a combination of atomic resolution scanning tunneling microscopy (STM) as a local structural probe and high-resolution electron energy loss spectroscopy (HREELS) as an area-integrating chemical probe that is sensitive to the internal bonding properties of the adsorption complex, we can determine the local chemical properties of specific structural elements by systematically varying the surface structure. The first results, demonstrating the existence of two new weakly adsorbed CO states in a linear on-top configuration on mixed AuPd sites, were published recently [6]. It was shown that these states result from CO adsorption on an (intermixed) AuPd(111) surface alloy obtained by annealing of Au monolayer-covered Pd(111) substrates and from CO adsorption at the edges of Au monolayer islands on the same substrate, respectively. Further information on the geometry and nature of the "island edge" adsorption site, however, was not available.In this contribution we will demonstrate that CO adsorption at the Au island edges most likely takes place on the higher terrace edge of the Au islands and that the enhanced Au−CO interaction is mainly d...

Spatially Resolved Chemistry on Bimetallic Surfaces

“…A recent comparison between Pd(111) surfaces covered by a full Au monolayer with different densities of second layer Au islands on top -the morphology of these surfaces is shown in Figs. 3a and b -found a similar CO state also for second layer Au islands and no adsorption on the flat bilayer areas, and the same observations were made also on a thicker Au film [22]. These results provide clear evidence that the extra state is due to CO adsorption on top of the Au island edge atoms, and not at Pd atoms (submonolayer coverages) or Au atoms (above monolayer coverages) at the lower side of the edge.…”

“…This will be demonstrated in the following example, dealing with CO adsorption on bimetallic Au-Pd(111) surfaces (further details see in Refs. [21,22,48]). …”

Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

“…A correlation between the d-band center of transition metal and alloy surfaces and the chemisorption energy of CO and other small molecules has been established [32,34]. Enhanced CO adsorption energies at surface defect sites of bimetallic surfaces, such as atomic step edges or kinks, has been found recently, which was attributed to the reduced coordination of these metal sites [35][36][37].…”

“…The ultimate goal is to prepare surfaces with different well-defined concentrations of non-ideal defect structures. Such surfaces were prepared successfully by homo-and heteroepitaxy for some metals and bimetals and studied with respect to their adsorption properties [35,37], for metal-oxides, we are still at the beginning.…”

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

“…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.…”