The impact of the Ru precursor on the adsorption of CO on Ru/Al2O3:: amount and reactivity of the adsorbed species

The influence of the Ru precursor (with and without chlorine) of Ru/Al2O3 catalysts is studied using CO as a probe molecule, The amounts and the reactivity of the adsorbed CO species formed on the reduced solids are studied using both mass and FTIR spectroscopy. FTIR spectra in the range of 2200-1800cm(-1) show that the introduction of 1% CO/He at 300 K on a 3.5% Ru(Cl-f)/Al2O3 solid (Cl-f: Cl free solid) mainly leads to the detection of a strong IR band at 2047 cm(-1) ascribed respectively to linear (denoted by L) CO species on the Ru particles. The total amount of CO adsorbed at 300 K is 1942 mumol/g of Ru. The evolution of the IR band of the L CO species with the increase in the adsorption temperature T, from 300 to 713 K indicates that the heat of adsorption of this adsorbed species linearly varies with its coverage 0 from 175 kJ/mol at theta = 0 to 115 kJ/mol at theta = 1. Moreover. it is shown that the L CO species is hydrogenated into CH4 at 478 K in pure hydrogen according to a mechanism with a limiting elementary step. On a 4% Ru(Cl)/Al2O3 solid the amount of adsorbed CO species at 300 K is significantly lower than on the Cl-f solid: 775 mumol/g of Ru. However. the presence of Cl (a) only slightly modifies the profile of the FTIR spectra of the adsorbed species and the heat of adsorption of the L species at high coverage and (b) has no influences on the profile of the rate of the CH4 production with the time of hydrogenation of the L species. In particular it is shown that the turnover at the maximum of the rate of CH4 production is similar on Ru(Cl-f)/Al2O3 and Ru(Cl)/Al2O3. The presence of Cl in the precursor of the Ru particles mainly decreases the number of Ru sites without strongly affecting the reactivity of the L CO species. The impact of the Ru precursor, seems mainly located on the steps involved in the preparation of the catalyst (dispersion, site blockage, reconstruction) while there are no strong electronic interactions between the residual chlorine and the adsorbed CO species. (C) 2002 Elsevier Science B.V. All rights reserved.