Characterisation of Model Three-Way Catalysts: II. Infrared Study of the Surface Composition of Platinum–Rhodium Alumina Catalysts

In order to determine the surface composition of platinum-rhodium/alumina catalysts, successive CO and NO adsorptions were studied by FTIR spectroscopy. The objective was to develop an experimental procedure in order to reach the number of surface rhodium and platinum atoms. Thus, it was observed on Rh/Al2O3that adsorption of NO at 473 K for one night leads to a unique νNO band near 1910 cm−1corresponding to RhI-NO+species. After the same treatment under NO, the irreversible adsorption of CO at 298 K on Pt/Al2O3produces a unique νCO band near 2085 cm−1. By using monometallic solids of known dispersion, a correlation was established between the absorbance (optical density) of each of these bands and the number of surface rhodium or platinum atoms. This experimental protocol, based upon the successive adsorption of NO at 473 K and CO at 298 K, was applied to four bimetallic PtRh/alumina catalysts having a ca 3 wt% metal loading and an atomic Pt/Rh ratio between 0.4 and 3. The total numbers of surface atoms measured by this method were in rather good agreement with those obtained by hydrogen adsorption, which supports the validity of the method. Moreover, the surface composition determined from the FTIR measurements corresponded to a surface enrichment in rhodium for all the bimetallic catalysts. This point is discussed in relation to the characterisation protocol which could induce a migration of the rhodium atoms during heating under NO. The discussion includes also the possible inaccuracies involved in the method, particularly the influence of the alloying on the calibration coefficients.