CO- and N₂-FTIR characterisation of oxidised Rh species supported on Ce_0.68Zr_0.32O₂

Abstract

The identification of a Rh-oxidised species of a Rh(0.29)/Ce0.68Zr0.32O2 catalyst that exhibits peculiar CO-O2 kinetics [I. Manuel, J. Chaubet, C. Thomas, H. Colas, N. Matthess and G. Djéga-Mariadassou, J. Catal. 2004, 224, 269] is addressed. For this purpose, various catalysts are studied by XANES, CO- and N2-FTIR, and benzene hydrogenation. The results obtained, particularly from N2-FTIR, which is, to our knowledge, reported for the first time on this kind of catalyst, suggest that Rh is mainly stabilised as electron-deficient clusters (Rhndelta+) on a Rh(0.29)/Ce0.68Zr0.32O2 catalyst after reduction at 500 degrees C under H2. The existence of these species, which may be caused by either electron perturbation induced on the metal by the reduced support or electron withdrawal from the metal clusters by an inductive effect of the neighbouring Cl anions, is also revealed through CO-FTIR experiments. In the presence of CO, however, evidence of RhI(CO)2 species is also provided.

Influence of the nature of the reducible support on CO oxidation kinetics of supported Rhδ+ catalysts: SnO2 versus Ce0.68Zr0.32O2

“…In other words, the local positive charge (d) on Rh is greater for Rh/SnO 2 . This might be assigned to the greater interaction of Rh with SnO 2 than with Ce 0.68 Zr 0.32 O 2 , which agrees well with the fact that few Rh 0 atoms were revealed for Rh/Ce 0.68 Zr 0.32 O 2 [25], whereas Rh/SnO 2 did not expose any Rh 0 atom.…”

“…In this sequence, each elementary step is far from equilibrium [14] and the catalytic sites consist of electron-deficient Rh particles (Rh d+ ) interacting with the support [25]. As also reported by other authors [27][28][29], the reaction is assumed to proceed between CO adsorbed on Rh d+ sites and oxygen species at the Rh d+ -support interface.…”

“…On Rh/Al 2 O 3 catalysts, Rice et al assigned such an absorption feature to CO bridge-bonded to Rh d+ atoms [24]. It must also be emphasised that although the reaction orders differ slightly from those obtained on Rh/Ce 0.68 Zr 0.32 O 2 (table 1), for which the presence of Rh d+ species is considered [25], they both remain positive as was the case for Rh/Ce 0.68 Zr 0.32 O 2 . The near zero order with respect to O 2 found on Rh/Ce 0.68 Zr 0.32 O 2 becomes positive on Rh/SnO 2 .…”

On the kinetics of CO oxidation by O2 over RhI(CO)2 catalytic species anchored to a zeolitic support

“…These peculiarities have been attributed to the stabilisation of oxidised rhodium species by CZ. Recent characterisation of this catalyst by means of adsorption of CO and N 2 followed by FTIR concluded to the presence of two types of Rh oxidised species, namely isolated Rh I cations and electron-deficient Rh clusters (Rh n d+ ) [4]. This study aims at shedding light on the oxidised Rh species responsible for the enhanced CO oxidation activity obtained for Rh/CZ [3].…”

“…This study aims at shedding light on the oxidised Rh species responsible for the enhanced CO oxidation activity obtained for Rh/CZ [3]. In an attempt to synthesise one of the two oxidised species identified by FTIR on Rh/CZ [4], a Rh/HY model catalyst was prepared. Although this kind of catalyst has been the subject of a large number of studies (references [5][6][7][8][9][10][11][12] and references therein), the reactivity of Rh I (CO) 2 anchored to a zeolitic support towards O 2 has hardly been investigated [5].…”