FTIR spectroscopy study of CO and NO adsorption and co-adsorption on Pt/TiO2

A Pt/TiO2 sample has been studied by adsorption and co-adsorption of CO and NO. It has been found that platinum is present in different oxidation states on the surface of the activated sample. Pt4+ ions are coordinatively saturated and form complexes neither with CO nor with NO. Pt3+ ions form, after CO adsorption, linear Pt3+–CO species stable up to 373 K and characterized by a band at 2183 cm−1. However, the Pt3+ cations are not able to coordinate NO because of the unwillingness of the latter to form electrostatic bond. Pt2+ cations form both carbonyl and nitrosyl complexes. The Pt2+–CO species are stable up to 623 K and display a band at 2134 cm−1. NO appears to be more sensitive probe for the state of the Pt2+ cations and the Pt2+–NO species are detected in the 1950–1900 cm−1 region. No evidence of Pt+ cations is found. The linear carbonyls of metallic platinum (2095–2070 cm−1) are less stable than the Pt2+–CO species. On the contrary, the most stable nitrosyls are those of Pt0 (linear species at 1850–1780 cm−1 and bent species around 1620 cm−1). It is found that CO slowly reduces cationic platinum ultimately to metal. On the contrary, NO, even at ambient temperature oxidizes metallic platinum. The results obtained are compared with the results on carbonyls formed with platinum in zeolites.

Graphical abstractAdsorption of CO on a non-reduced Pt/TiO2 sample reveals the existence of cationic (Pt3+, Pt2+) and metallic platinum. NO forms nitrosyl species with bare Pt2+ and Pt0 sites, but is not coordinated to Pt3+ ions. It appears that NO is a more sensitive probe than CO is to test the state of Pt2+ cations.Figure optionsDownload full-size imageDownload as PowerPoint slide