Untitled

Abstract

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.

Rhodium Dispersion during NO/CO Conversions

“…The samples of 5 wt % Rh were prepared as described previously from RhCl 3 ·3 H 2 O and g-Al 2 O 3 (Degussa Alumina C). [4,19] Samples were pretreated in situ with a reduction-oxidation-reduction cycle.…”

“…[3] However, the mean nuclearity of the metal on Rh/Al 2 O 3 during the reduction of NO by H 2 varies with gas partial pressures and temperature. [4] Hence, an assumption of an invariant core catalyst may be invalid for highly dispersed catalysts. Indeed, combinations of CO and NO at low temperatures (< 473 K) aggressively corrode nanoparticulate rhodium to release Rh(CO) 2 centers, [5] which are mononuclear [6] and whose generation has also been monitored kinetically.…”

“…[15] This suggests that the Rh-NO site formed on g-alumina is indeed electropositive, with little amelioration by the catalyst surface. Extended X-ray absorption fine structure (EXAFS) analysis clearly showed the presence of a new Rh-O/N shell (2.22(2) atoms at 2.03 ) in addition to the reduction of Rh-Rh coordination number (mean nuclearity of [3][4]. Of the more than 20 structures tested, modeling the rhodium nitrosyl as a tetrahedral site with three O ligands and a linear Rh-N-O unit as the major site and with a minor component of the rhodium as fcc particles [16] indeed provided a close fit to the Rh XANES (XANES = X-ray absorption near-edge structure spectroscopy; Figure 3).…”

Rapid Phase Fluxionality as the Determining Factor in Activity and Selectivity of Highly Dispersed, Rh/Al2O3 in deNOx Catalysis

“…We have already shown that such processes may occur when nanoparticulate Rh is exposed to both NO [8] and O 2 . [12] Interestingly, Rh thin foils and single crystals have been reported to oxidize beyond the surface adlayer under O 2 above 500 K. [13] A consequence of these phenomena is that the phase of the supported metal at a given gas composition and temperature can be far from static and may result from a dynamic equilibrium between processes that lead to oxidation and those that reductively reform the particulate metal. While oxidation of these Rh/Al 2 O 3 catalysts by NO/He 4/96 is rapid at room temperature, re-reduction of the NO oxidized Rh sites by H 2 /He 4/96 was evident only above 400 K. [8] It is this differential that is directly responsible for the activity and selectivity of the catalyst.…”

Bestimmung der Struktur, Zusammensetzung und Dynamiken molekularer Oberflächen unter Reaktionsbedingungen bei hohen Drücken und an der Fest‐flüssig‐Phasengrenzfläche

“…In ähnlicher Weise konnte mithilfe von EXAFS-Studien, die an der Europäischen Synchrotronstrahlungsanlage in Grenoble unter Reaktionsbedingungen ausgeführt wurden, nachgewiesen werden, dass die Selektivität für die Bildung von N 2 oder N 2 O bei der Reduktion von NO 2 von der An-oder Abwesenheit oxidierter Oberflächenzentren des Rh-Katalysators abhängt. [48] An der gleichen Anlage wurden kürzlich auch EXAFS-Spektren bei Gasdrücken von bis zu 18 bar zur Untersuchung der Cobalt-katalysierten Fischer-Tropsch-Synthese aufgenommen, einer Reaktion von erheblicher industrieller Bedeutung. [49] Das weiteren wurden auch Flüssigphasenreaktionen [50] und sogar Oxidationsreaktionen in überkritischem CO 2 bei 150 bar [51] untersucht.…”