Rhodium has for many years been a primary component in the make up of autoexhaust catalysts because of its ability to catalyze the selective reduction of NO x to N 2 . [1,2] A historical view of this type of system is of an active, but essentially static, phase comprising particulate metal; it is from this axiom that studies of metal single crystals [2] have been accepted as models of macroscopic catalyst behavior. However, it has been established by IR [3] and XAFS [4] (X-ray absorption fine structure) spectroscopy that small rhodium particles (on alumina) undergo corrosive chemisportion to yield a mononuclear {Rh I (CO) 2 } species. In addition, the oxidation of Rh/Al 2 O 3 under an atmosphere of air and oxygen has also been demonstrated by XAFS. [5] Recently, using in situ, microreactor-based, energydispersive EXAFS (EDE) [6] and mass spectrometry [7] we have used the improved time resolution of these techniques to demonstrate that Rh on alumina is rapidly oxidized by NO. [8] Herein we utilize these procedures to probe the correlation between metal structure and catalytic performance for the reduction of NO by H 2 . Figure 1 shows the total NO conversion and N 2 O (mass 44) production as a function of reaction temperature and feedstock composition. The net conversions and selectivity of the Toromanoff, Tetrahedron 1985, 41, 5045. Spectral and analytical data for all new compounds can be found in the Supporting Information. CCDC-177450 (7 a) and CCDC-177451 (11 a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: ( 44) 1223-336-033; or deposit@ ccdc.cam.ac.uk).[21] Subjection of individual isomers cis-and trans-16 to 1.5 equiv TiCl 4 at À 78 8C 325 8C for 90 min resulted in only slight (ca. 5 ± 10 %) isomerization at the a center. Therefore, the lack of stereospecificity in 15 a 316 cannot be fully explained by rapid equilibration at the aalkyl moiety of the product.[**] This work was funded under the ™Catalysis and Chemical Processes initiative of the EPSRC∫ and through a ™long-term project∫ allocation of beamtime by the ESRF. We thank the ESPRC (MAN) and ICI (SGF) for postdoctoral funding. The technical skills of John James, Melanie Hill, Sebastian Pasternak, and Ralph Wiegel, are gratefully acknowledged as is the beamline (ID 24) stewardship of Dr. Sakura Pascarelli. Figure 1. a) NO conversion as a function of reaction temperature and active feedstock composition in the reduction of NO/He by H 2 /He over 5 wt % Rh/g-Al 2 O 3 catalysts derived from RhCl 3 ¥ 3 H 2 O: catalyst charge: 20 mg; NO ± H 2 /He 4/96; total gas flow 10 mL min À1 , GHSV ca. $ 10 4 h À1 . b) N 2 O production (mass 44) as a function of reaction temperature and active feedstock composition in the reduction of NO by H 2 over 5 wt % Rh/ g-Al 2 O 3 catalysts derived from RhCl 3 ¥ 3 H 2 O: conditions as for Figure 1 a.