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/Al2O3 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/Al2O3 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 SnOx 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.

Automated summary

The study using FTIR spectroscopy reveals that Sn on PtSn particles has significant effects on the geometric and electronic structures of Pt sites, altering the adsorption behavior of CO. Furthermore, the reconstruction and reduction cycles of PtSn particles lead to changes in the adsorption heats of CO species, indicating the dynamic nature of the PtSn catalyst system.