Epitaxial thin film model catalysts

Fig. 15.3 Preparation of epitaxial thin film model catalysts, (a) Electron micrograph of a Pt-AljOj model catalyst with a mean particle size of 5 nm the insets show the corresponding electron diffraction pattern and the (200) weak-beam dark-field image of a pyramidal Pt nanocrystal (b) an atomically resolved TEM micrograph of a slightly rectangular Pt particle. A structural model of a pyramidal Pt particle is presented in (c). To illustrate the epitaxial growth the NaCl substrate is also included...

Preparation and Nanoparticle Structure of Epitaxial Thin Film Model Catalysts... 

Apparently, epitaxial thin-film model catalysts provide a well-defined initial state for a systematic study of microstructural changes and structure-activity correlations. Model catalysts were prepared for various noble metal-oxide combinations, including Pt, Rh, Ir, Pd, Re supported by Al Oj, SiO, TiO, CeO, VO, Ga Oj, etc. The number density of the metal particles (island density particles per cm ) and their size can be controlled via the NaCl(OOl) substrate temperature during evaporation and the amount of metal deposited (as measured by a quartz microbalance), respectively (Pig. 15.4). 

Fig. 17.3 The preparation of epitaxial thin-film Ga O, oxide model catalysts, (a) Electron micrograph of a Ga O thin film prepared on NaCl(OOl) by the evaporation of Ga O in 10 Pa O at 623 K. The insets show the corresponding SAED pattern, (b) Electron micrograph of a Ga O thin film prepared on NaCl(OOl) in the absence of at 623 K and its corresponding SAED pattern (inset), (c) Ga2p XPS spectra obtained in the as-grown state and after different annealing treatments [26].

An ideal study of support effects requires model catalysts with metal particles that are identical in size and shape (so that only the support oxide varies). This is difficult to achieve for impregnated catalysts, but identical metal particles can be prepared via epitaxial model catalysts [36]. Well-faceted Rh nanocrystals were grown on a 100-cm area NaCl(OOl) thin film at 598 K. One half of a Rh/NaCl sample was covered with Al Oj, and the other half with TiO. The preparation of Rh particles for both Al Oj- and TiO -supported model catalysts in a single step prevents any differences in particle size, shape, and surface structure which could occur if the samples were prepared in separate experiments. Three model catalysts were prepared, with a mean Rh particle size of 7.8, 13.3, and 16.7 mn (the films were finally removed from the NaCl substrate by flotation in water). Activation was performed by O /H treatments, with the structural changes followed by TEM (Fig. 15.6). Oxidation was carried out in 1 bar O at 723 K prodncing an epitaxially grown rhodium oxide shell on a Rh core (cf Fig. 15.5e), whereas the hydrogen reduction temperature was varied. 

Rupprechter G, Hayek K, Rendon L, Jose-Yacaman M (1995) Epitaxially grown model catalyst particles of platinum, rhodium, iridium, palladium and rhenium studied by electron microscopy. Thin Solid Films 260 148... 

Fig. 17.2 The preparation of epitaxially grown thin film oxide model catalysts suitable for structural, spectroscopic and catalytic characterization. A schematic of the recirculated microreactor setup is shown on the right [22, 28]...

Peden CHF, Herman GS, Ismagilov E, Kay BD, Henderson MA, Kim YJ, Chambers SA (1999) Model catalyst studies with single crystals and epitaxial thin oxide films. Cat Today 51 513-519 Peterson KA, Xantheas SS, Dixon DA, Dnnning THJ (1998) Predicting the proton affinities of H2O and NHj. J Phys Chem A 102 2449-2454... 

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