New frontier in transmission IR spectroscopy of molecules adsorbed on high surface area solids: Experiments below liquid nitrogen temperature

IR spectroscopy of adsorbed probe molecules is one of the most powerful characterization techniques for the investigation of surface active sites on high surface area materials like oxides and zeolites. In the last 20 years the use of specific IR cells allowing the in situ sample activation, gas dosage and sample cooling down to liquid nitrogen temperature has remarkably improved the number and the quality of the information on the surface structure with respect to the first experiments carried out at room temperature. Commercial cryostats able to reach liquid helium temperatures are available since decades, but the incompatibility of the materials used to reach and confine very low temperatures with the high temperatures usually needed to activate the surfaces of catalysts has prevented for long time the breaking down of the 77 K frontier in IR experiments of species adsorbed on active surface sites. In our group we have very recently designed, realized and tested a new experimental set-up able to perform IR experiments in the 15-300 K interval on samples previously activated under vacuum conditions (P < 10(-4) Torr, 1 Torr similar to 133.3 Pa), or in the desired atmosphere, up to 1073 K [G. Spoto, E.N. Gribov, G. Ricchiardi, A. Damin, D. Scarano, S. Bordiga, C. Lamberti, A. Zecchina, Prog. Surf. Sci. 76 (2004) 71]. The first results obtained with this innovative instrument will be reviewed and summarized in this work and compared with previous literature results on similar experiments performed at liquid nitrogen temperature. In particular, we will discuss the adsorption of CO and H-2 on MgO and H-SSZ-13 zeolite, and of H-2 on Cu+-ZSM-5 zeolite. (c) 2005 Elsevier B.V. All rights reserved.