The thermal stability of gold nanoparticles (2.5−4 nm diameter) on fully stoichiometric TiO2(110) rutile substrates in an O2 atmosphere and in air at up to ambient pressure, and for comparison under ultrahigh-vacuum (UHV) conditions, was investigated by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Substrate material and annealing parameters were chosen similar to those used for the pretreatment of realistic disperse Au/TiO2 catalysts. Under UHV conditions, the particles are rather stable even after annealing at 400 °C, and only minor changes in the particle size are detected after annealing at 500 °C. In 50 mbar O2, the particles are relatively stable at room temperature but grow significantly after annealing to 400 °C. In air, particle growth starts already at room temperature, accompanied by the formation of Au−OH species. The relevance of these results for the understanding of the pretreatment of disperse Au/TiO2 catalysts is discussed.

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.

Different types of planar Au/TiO(2) model catalysts are produced on TiO(2)(110) single-crystal substrates and thin TiO(2) films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO(2)(110) model catalysts with a stoichiometric TiO(2)(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO(2) model catalysts with thin TiO(2) films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO(2)(110) substrates on the CO oxidation activity is discussed.