Heats of adsorption of linear CO species on the Pt sites of a 1.2% Pt-2.7% Sn/Al2O3 catalyst before and after reconstruction and ageing processes

The adsorption of CO in the temperature range of 300–713 K on the PtSn particles of a reduced (H 2 , 713 K) 1.2%Pt-2.7%Sn/Al 2 O 3 catalyst is studied by FTIR spectroscopy to reveal the geometric and electronic effects of Sn on the Pt sites. By comparison with a 1.2% Pt/Al 2 O 3 it is shown that Sn (a) suppresses the Pt sites forming bridged CO species due to a geometric effect and (b) displaces the IR band of a linear CO species on Pt sites at 300 K from 2066 cm −1 to 2044 cm −1 due to an electronic effect. According to the AEIR method, the change in the IR band on the PtSn particles with the increase in T a in isobaric condition (P CO = 1 kPa) provides the heats of adsorption of the linear CO species (named L1 PtSn ) at high (58 kJ/mol) and low (130 kJ/mol) coverages. By comparison with the values of the linear CO species on Pt particles (named L Pt ): 220 and 106 kJ/mol at low and high coverages, respectively, these values reveal the strong impact of the electronic effect of Sn on the heats of adsorption of CO. For T a > 463 K there is a reconstruction of the PtSn particles due to the segregation of Sn as SnO x species associated with an enrichment of the surface in Pt. On the reconstructed PtSn particles, a new IR band is observed at 2057 cm −1 after adsorption of CO at 300 K ascribed to a new L2 PtSn CO species. Its heats of adsorption are significantly higher than those of the L1 PtSn species: 165 and 65 kJ/mol at low and high coverages. Moreover, successive reconstruction/H 2 reduction at 713 K cycles leads to a sintering of the PtSn particles associated to the progressive increase in the heats of the L2 PtSn CO species until a value at low coverage: 210 kJ/mol similar to that of the L Pt species whereas at high coverage a significant difference exists between the two species (70 and 115 kJ/mol for L2 PtSn and L Pt , respectively). These data show that the impacts of Sn on the heat of adsorption of the linear CO species on the Pt sites are dependent on (a) the Pt/Sn surface ratio which changes with reconstruction and the particle size and (b) the coverage of the Pt sites. The comparison of the heats of adsorption of the L CO species on the fresh, reconstructed and aged PtSn particles with experimental and theoretical literature data reveals that they are consistent with some of them.