A CHARACTERIZATION OF THE SURFACE-ACIDITY OF HFO2 BY FTIR SPECTROSCOPY OF ADSORBED SPECIES, ELECTRON-MICROSCOPY AND ADSORPTION MICROCALORIMETRY

A preparation of HfO2, derived from the hydrolysis of hafnium isopropy late, has been characterized by XRD, (HR)TEM, FTIR and adsorption micr ocalorimetry. The thermal destruction of the amorphous hafnium hydroxi de starting phase is complete at almost-equal-to 700 K, and leads to t he crystalline (monoclinic) phase of HfO2. The latter exhibits a parti cle morphology which, upon thermal treatment, evolves quickly from one of large and loose aggregates of tiny microcrystallites (microcrystal line HfO2) to one made of large single crystallites or of large polyag gregates, in which relatively small ordered microcrystals stack togeth er in a rather disordered fashion (partially sintered HfO2). The evolv ing morphology of HfO2 is monitored, on a microscopic surface scale, b y a varying IR spectrum of surface OH groups and by a varying surface Lewis acidic activity (e.g. towards CO chemisorption), due to coordina tively unsaturated Hf4+ surface centres produced upon vacuum activatio n. CO uptake, both at ambient temperature and at low temperature (almo st-equal-to 78 K), is mainly due to two families of adsorbing sites: s ites in structurally and/or coordinatively highly defective configurat ions, onto which CO adsorbs with an adsorption enthalpy of almost-equa l-to 65 kJ mol-1, and sites located in relatively extended patches of regular crystallographic planes, onto which CO adsorbs with an adsorpt ion enthalpy of almost-equal-to 50 kJ mol-1. The relative population o f the two families of adsorbing sites depends to some extent on the de gree of sintering of the material, but it is observed that, unlike oth er similar systems, the early sintering process causes a rather limite d destruction of the crystallographically/coordinatively defective con figurations. Both families of adsorbed CO species exhibit a strong dep endence of their spectral features on several parameters, among which of primary importance are the degree of surface hydration/dehydration and the surface concentration of charge withdrawing/releasing adsorbed species which, through surface inductive effects, affect the strength of the CO adsorption process.