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 isopropylate, has been characterized by XRD, (HR)TEM, MR and adsorption microcalorimetry. The thermal destruction of the amorphous hafnium hydroxide starting phase is complete at ≈700 K, and leads to the crystalline (monoclinic) phase of HfO2. The latter exhibits a particle morphology which, upon thermal treatment, evolves quickly from one of large and loose aggregates of tiny microcrystallites (microcrystalline HfO2) to one made of large single crystallites or of large polyaggregates, in which relatively small ordered microcrystals stack together in a rather disordered fashion (partially sintered HfO2). The evolving morphology of HfO2 is monitored, on a microscopic surface scale, by a varying IR spectrum of surface OH groups and by a varying surface Lewis acidic activity (e.g. towards CO chemisorption), due to coordinatively unsaturated Hf4+ surface centres produced upon vacuum activation. CO uptake, both at ambient temperature and at low temperature (≈78 K), is mainly due to two families of adsorbing sites: sites in structurally and/or coordinatively highly defective configurations, onto which CO adsorbs with an adsorption enthalpy of ≈65 kJ mol−1, and sites located in relatively extended patches of regular crystallographic planes, onto which CO adsorbs with an adsorption enthalpy of ≈50 kJ mol−1. The relative population of the two families of adsorbing sites depends to some extent on the degree of sintering of the material, but it is observed that, unlike other similar systems, the early sintering process causes a rather limited destruction of the crystallographically/coordinatively defective configurations. Both families of adsorbed CO species exhibit a strong dependence 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.