SCIENCE/TECHNOLOGY
Optical method images catalyst surfaces By employing two techniques simulta neously, a new optical method of ex amining catalyst surfaces allows more detailed study of heterogeneous cata lytic reactions than has been previous ly possible. The method combines ellipsomicroscopy for surface imaging (EMSI) and re flection anisotropy microscopy (RAM), both based on changes in the degree of polarization of light reflected from a sur face. The method, which can be used at any pressure, was developed by re searchers Harm H. Rotermund, Guenter Haas, Ruud U. Franz, Robert M. Tromp, and Gerhard Ertl at the Fritz Haber Insti tute of the Max Planck Institute in Berlin [Science, 270, 608 (1995)]. "The new method is not restricted to high-vacuum conditions and, in effect, bridges the pressure gap that has existed in studies of catalysts at high vacuum and high pressures," Rotermund says. "Pressure gap" refers to the difficulty in extrapolating high-vacuum data to the higher pressures encountered in practical situations. There are many in stances of significant phenomenological differences between high and low pres sures. An example is the concentration patterns formed by the coupling of reac tions and surface diffusion on a catalyst. If the surface diffusion rate is different from the rate of formation of reaction products, there may be significant con centration variations on the surface. These phenomena have been studied in the past with photoemission electron microscopy at low pressures, Roter mund notes. At higher pressures, how ever, there may be significant tempera ture differences on the catalyst surface caused by variations in the local heat release characteristics. With the new method, Rotermund and his associates can image surface pat terns on the catalyst at a wide range of pressures. EMSI is based on ellipsometry, a well-established method for char acterizing surface structures with thick nesses greater than several nanometers. The research group has now further de veloped the technique to allow real-time averaging of the dynamic phenomena in the distributions of atoms adsorbed on a metal surface. Furthermore, RAM allows very sensi30
NOVEMBER 27,1995 C&EN
tive surface imaging with an optical beam reflected at near normal incidence. The optical reflectivity along two nonequivalent directions of an anisotropic surface is determined. Anisotropy in re flectivity is often changed by the pres ence of submonolayer coverage of the surface by adsorbates. For both EMSI and RAM, the reflect ed light is extinguished by appropriate settings of the compensator and the an alyzer. A uniform, featureless surface produces a dark image. Local varia tions in the surface character appear as brighter areas in the EMSI image. With RAM, regions of different reflection anisotropy are contrasted. So far, the researchers have restricted their studies with the new method to the catalytic oxidation of carbon monoxide on a Pt(110) surface. "This is a system that is very well known and is very sim ple," Rotermund says. The reaction pro ceeds through surface recombination of chemisorbed oxygen atoms and CO spe
cies formed by the adsorption of gaseous molecular oxygen and CO. Carbon diox ide produced in the reaction is immedi ately released to the gas phase. Under certain conditions the reaction rate becomes oscillatory, even chaotic, and the surface concentrations of ad sorbed Ο and CO are not uniform. The patterns indicate regions of high Ο cov erage with high reactivity and regions of high CO coverage with lower activity. With their new technique, the re searchers see the same distribution pat terns of adsorbed atoms and molecules as previously observed with the photoemission microscopy method. The new technique has a much enlarged field of view, however. Rotermund says there is no inherent upper pressure limit in applying the method. In the immediate future, Rotermund suggests his group "will probably stick to the simple reactions such as the oxi dation of hydrogen to form water." Joseph Haggin
Arizona team wins plastics recycling competition Chemical engineering students (from left) David J. Grattan, Frederick E. Bakun, and Larson C. Lindholm from the University of Arizona, Tucson, edged out three other finalists to win $1,500 in the plastics recycling competition held at the annual meeting of the American Institute of Chemical Engineers (AIChE) in Miami Beach this month. The competition, cosponsored by AIChE and the American Plastics Council, asked students to use innovative recycling technolo gies to design an economical and environmentally sound process for recycling mixed plastics recovered from municipal solid waste. The Arizona team chose a versatile dissolution process, designing separate processing units for communi ties with different recycling loads, and integrated the units to achieve maximum benefits. "We spent a lot of time making sure one unit complemented the oth er," noted Lindholm. The Arizona process allows for production of marketable polyblends—a vinyl siding, which, the team said "could be used for hog hous ing"; laminating material for large corrugated boxes; and strapping tape. Mainn Brennan
