830
Langrnuir 1988, 4,830-836
nation for the reactivity of the encapsulated Rh surface is that these amorphous surface overlayers are sufficiently porous for the reactants to diffuse through. The T E M micrographs do not provide any information on the continuity of these films in a direction normal to the micrographs. Another possibility is that the film is discontinuous, exposing bare patches of Rh and thereby accounting for the small but measurable chemisorption uptake in the SMSI state. The reaction may proceed on these bare patches and at the metal-oxide interface. In conclusion, we have demonstrated that high-resolution transmission electron microscopy can be used to ob(22) Vannice, M. A.; Garten, R. L.J. Catal. 1979, 56,236. (23) KO.E.I.; Garten, R. L.J. Catal. 1981, 68,233.
serve the surfaces of small metal crystallites in heterogeneous catalysts. Subnanometer overlayers that are catalytically significant can be detected in edge-on views of the small metal particles. Acknowledgment. Financial support for this work from the NSF via Grant CBT-8707693 is gratefully acknowledged. ADL acknowledges receipt of a scholarship from the American Vacuum Society-NM chapter. Electron microscopy was performed at the Electron Microbeam Analysis Facility at the Department of Geology, University of New Mexico, and at the NSF national HREM facility within the Center for Solid State Science, Arizona State University (supported by Grant DMR-86-11609). Registry No. Rh, 7440-16-6; Ti02, 13463-67-7; H2, 1333-74-0.
Photohydrogenation of Acetylene in Titanium Dioxide Based Colloidal Aqueous Solutionst Zun-Sheng Cai and Robert R. Kuntz* Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211 Received January 14, 1988. I n Final Form: March 2, 1988 The photohydrogenation of acetylene on bare and modified colloidal TiOz surfaces has been studied in aqueous solutions containing a sacrificial electron donor. Yields of CzH4 and CzHe as a function of pH, loading with Mo, particle size, and light intensity were determined. The two-electron reduction to C2H, occurs on the colloidal surface by interaction with adjacent Ti(II1) centers. Mood2-,photoreduced on the colloidal particle, transfers electrons through Ti(II1) centers to the acetylene substrate, resulting in a direct four-electron reduction to CzH6 without the intermediacy of CZH4. The efficiency of the individual Mo catalytic center is quite high, but the net quantum efficiency is generally
