Charge-Mediated Adsorption Behavior of CO on MgO-Supported Au Clusters

Abstract

The CO binding behavior to gold particles supported on MgO thin films has been analyzed with scanning tunneling microscopy (STM) and infrared spectroscopy (IRAS). The ad-particles accommodate excess electrons that originate either from a charge transfer through the thin oxide film or from a local interaction with electron-rich oxide defects that act as Au nucleation centers. The enhanced electron density in the Au aggregates affects both the spatial distribution and the vibrational properties of adsorbed CO species. Whereas preferential CO attachment to the chemically unsaturated and electron-rich boundary sites of the Au islands is deduced from the STM data, a continuous downshift of the CO stretching frequency with decreasing particle size is observed in IRAS. Both results are interpreted in the light of CO adsorption to negatively charged metal aggregates and used to draw general conclusions on the interplay between charge and adsorption properties of confined metal systems.

Cluster Structure and Reactions: Gaining Insights into Catalytic Processes

“…Earlier debates raised issues such as questions whether charged clusters were suitable catalyst mimics; the large number of recently reported findings that charged centers play a role in the functioning of many classes of catalytic reactions have put these concerns to rest. In fact it is becoming increasingly realized that a large variety of heterogeneous catalysts function due to the presence of defects having large local charge-density centers [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. And, in the context of our own findings, this issue was also settled in the course of comparing cluster reactions for a variety of classes of reactions with the findings of known catalysts.…”

Models for heterogeneous catalysts: studies at the atomic level

“…First attempts to image molecules at the rim where undertaken by Lin et al (2010b) using CO as a probe molecule. Using inelastic electron tunneling spectroscopy (Langan and Hansma 1975;Stipe et al 1998), as introduced by Ho and coworkers into UHV surface science, Lin et al were able to show that a low lying CO vibration frequency, in particular the frustrated rotation at about 45 meV excitation energy can only be found at the rim of an artificially shaped nano-particle similar to those in Fig.…”

Model Studies in Heterogeneous Catalysis

“…The presence of charged defects may also influence the properties of metal nanoparticles anchored to or near those defects. [64][65][66][67][68][69][70][71][72][73][74] We discuss here "color center" formation on The best studied defects in simple oxides, such as MgO, are so-called F or color centers. [64,65,75] They represent oxygen vacancies that may trap up to two electrons.…”

“…[74] The Au particles in this case accumulate on those morphological defects, that is, along the mosaic grain bound-A C H T U N G T R E N N U N G aA C H T U N G T R E N N U N G ries, and attract the charge due to their high electron affinity, as shown in the STM image (Figure 10a). [73] This may be probed through adsorption of CO, the stretching frequency of which is sensitive to the charge state of the particle. [73] A very impressive series of studies on CO stretching frequencies to free clusters of varying sizes and charge (+ , 0, and À) has been studied in Meijers group and is very useful for comparison here.…”

“…[73] This may be probed through adsorption of CO, the stretching frequency of which is sensitive to the charge state of the particle. [73] A very impressive series of studies on CO stretching frequencies to free clusters of varying sizes and charge (+ , 0, and À) has been studied in Meijers group and is very useful for comparison here. [76][77][78][79][80] In a previous publication, it was demonstrated that CO molecules adsorbed on individual Au atoms supported on MgO exhibit an enormous red shift of the stretching frequency.…”