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Abstract: The activity of supported Ni catalysts can easily be affected by high temperatures and oxidative atmosphere. In order to study the origin of this behaviour, we performed systematic LEED and XPS investigations on a model system prepared by physical vapour deposition of Ni on a thin epitaxial alumina film on NiAl(110). Annealing experiments showed that deposited Ni is thermally stable up to ∼370K. Further annealing seems to lead first to changes in the island structure and then, at temperatures above ∼480K, to diffusion of Ni through the alumina film. Analysis of the complex oxidation behaviour revealed that there is a competition between oxidation of the Ni deposit and of the NiAl support. The latter process is probably enabled by a solid state reaction at the interface which means that Ni catalyses the further oxidation of the support. While the surface of the deposited Ni clusters is easily oxidised, the further oxidation processes of the Ni particles and the support were found to be diffusion-controlled. [Copyright &y& Elsevier]

Abstract: The adsorption of CO on well-defined Co–Pt alloy surfaces prepared by physical vapour deposition in ultra-high vacuum was studied by Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) and temperature-programmed desorption (TPD) spectroscopy. The adsorption behaviour was found to depend strongly on the composition of the surface and the first subsurface atomic layer. In the case of low Co concentrations at the surface CO adsorbs preferentially on Pt sites, however with an enthalpy of adsorption which is substantially lowered when compared to CO adsorption on Pt single crystals. TPD shifts of ∼60K were found. When Co dominates the surface composition, CO adsorbs preferentially on Co sites, however with an enthalpy of adsorption leading to a TPD shift by ∼80K to higher temperatures with respect to monometallic single crystals. This behaviour can be explained by ligand effects, i.e., by an electronic influence of the presence of a second metal in the chemical environment of a given adsorption center. The experimental findings were compared with theoretical calculations. [Copyright &y& Elsevier]

Abstract: Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO2(110) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C2H2, C2H4, H2, and hydrogen atoms. Although, C2H4 and H2 did not show any indications of reduction of oxidized gold clusters, CO, C2H2, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters. [Copyright &y& Elsevier]

Abstract: Deposition and fabrication of films of Au nanoclusters protected by alkanethiolate ligands are attempted on a TiO2(110) surface and the structures of films are observed by a scanning tunneling microscope (STM). Effects of oxygen and hydrogen-plasma etching in addition to UV irradiation on the structure and chemical composition of the films are also investigated by using STM and X-ray photoelectron spectroscopy. Alkanethiolate Au nanoclusters are produced using a modified Brust synthesis method and their LB films are dip-coated on TiO2(110). Alkanethiolate Au nanoclusters are weakly bound to the substrate and can be manipulated with an STM tip. Net-like structures of alkanethiolate Au nanoclusters are formed by a strong blast of air. Oxygen-plasma etching removes alkanethiolate ligands and simultaneously oxidizes Au clusters. At room temperature, prolonged oxygen-plasma etching causes agglomeration of Au nanoclusters. UV irradiation removes ligands partly, which makes Au nanoclusters less mobile. The net-like structure of alkanethiolate Au clusters produced by a blast of air is retained after oxygen and hydrogen-plasma etching. [Copyright &y& Elsevier]