Activity of Rhx+ species in CO oxidation and NO reduction in a CO/NO/O2 stoichiometric mixture over a Rh/CeO2-ZrO2 catalyst

The catalytic activity of a 0.30 wt% Rh/CeO2-ZrO2 catalyst (25% ZrO2 in weight) 188 m(2)g(-1) was studied in the oxidation of CO and reduction of NO in a CO/NO/O-2 stoichiometric mixture, in the temperature range of 298-773 K. The hydrogenation of benzene, a structure insensitive reaction was used for counting surface Rh(0) atoms after either thermal treatment of the catalyst in hydrogen at 773 K, or reduction by the stoichiometric CO/O-2/NO reaction mixture. The bibliography shows that the turnover rate over RhX+ is lower than that over Rh(0) by two orders of magnitude. Rhodium supported on CeO2-ZrO2 submitted to the above reducing treatments was found to be totally inactive in benzene hydrogenation but active in CO/NO/O-2 reaction. As a conclusion the present catalyst consisted exclusively of RhX+ surface species. The catalytic behaviour of this rhodium-supported catalyst was interpreted in terms of multiple active sites containing one RhX+ ion grafted in the support and able to chemisorb CO and/or NO as for a rhodium homogeneous complex. This RhX+ is associated with two adjacent oxygen vacancies (and corresponding Ce3+ cations) linked to the reduction of the support and permitting oxygen and NO chemisorption. A method was defined for counting surface RhX+ containing multiple sites by measuring the number of NO molecules adsorbed at low temperature and desorbing at the temperature of light-off of CO. A (NO/RhX+) ratio of 2 was found, in agreement with a dinitrosyl species reaction intermediate. (C) 1998 Academic Press.