Model Catalysts Based on Au Clusters and Nanoparticles

Abstract

Small Au particles have been shown to exhibit interesting catalytic properties. In an attempt to parallel catalytic studies on powder supports we have undertaken a series of model studies using oxide films as support. We address the formation of Au aggregates as a function of size starting from Au atoms to clusters and islands of larger size and as a function of the support. In addition we have studied different support materials such as alumina and iron oxide and we compare ultrathin and thicker oxide films of the same material (MgO). From a comparison of charge transfer through ultrathin films with the situation encountered in thicker films, we propose the use of dopants in bulk materials to control particle shape.We include the study of carbon monoxide adsorption on Au clusters of varying size. It is demonstrated how chemical modification (hydroxylation) of oxide supports influence particle growth and properties. Finally, we report on effects to study the processes involved in particle growth by wet impregnation in order to bridge the gap to catalyst preparation under realistic conditions. On that basis one may now compare properties of supported particles prepared in ultrahigh vacuum using physical vapor deposition with those prepared by wet impregnation

Binding Behavior of Carbonmonoxide to Gold Atoms on Ag(001)

“…In contrast to the CO adsorption behaviour observed so far for single adatoms and metal islands, where CO molecules are predominantly bound on top of the Au adatoms and to the rim of Au islands [20][21][22], we find two different configurations for a single Au adatom-one on top and the other one at the side of individual Au atoms on Ag(001). Moreover, IETS reveals different low-energy vibrational energies observed between tunneling voltages of 10 and 30 meV for the two adsorption sites of CO to Au as compared to the one at 16 meV for the single CO molecule bound to Ag(001).…”

Probing the electronic properties and charge state of gold nanoparticles on ultrathin MgO versus thick doped CaO films

“…Level quantization in hexagonal potential wells with infinite walls can be described empirically with a linear relationship between eigenenergy E n and quantum number n [31]: E n = E 0 + (n/m eff * ). 1 Here, E 0 and m eff denote the onset energy and the effective electron mass of the Au 6sp band, respectively. To determine the energy position of the Au quantum well states (QWSs), we have fitted 1D cuts through the bias-dependent dI/dV maps to idealized standing-wave patterns.…”

“…We remind that the transfer electrons originate from bulk Ag(001) in the MgO case, but from Mo ions with donor character in the CaO films that are too thick for direct electron tunneling from the support. If the two charging routes indeed converge to the same final state, ultrathin oxide films could be used as model systems for bulk oxides that are difficult to explore due to their insulating nature [1,2]. In our final paragraph, we thus compile experimental evidence for the occurrence of electron transfer on MgO and CaO films, although the absolute charge state of the Au nanoislands could not be extracted from the experiment.…”

“…The charge state of oxide-supported metal nanoparticles largely governs their physical and chemical properties [1,2]. As demonstrated by experiment [3] and theory [4,5], the presence of excess electrons controls the growth morphology of the ad-metal.…”

Tunable reactivity of supported single metal atoms by impurity engineering of the MgO(001) support

“…[17][18][19][20][21][22][23] The properties of supported metal atoms and clusters are shown to be affected by properties of the substrate. 24 In particular, the presence of defects 25 and deposition of atoms onto thin oxide films supported by metallic substrates 26 alter the growth and properties of metal atoms and metal clusters, affecting their reactivity. 27 Among other properties, particular attention has been paid to the impact of the particle charge state on the chemical properties.…”