Differential energies of adsorption of argon and nitrogen on rutile and silica

Abstract

An adsorption microcalorimeter with two Tian–Calvet thermopiles has been used for the determination at 77.4 K of differential energies of adsorption of argon and nitrogen as a function of surface coverage on a sample of pure rutile (surface area, 22 m² g^–1) and on the rutile coated with 2.55 wt.% silica (surface area, 23 m² g^–1). With each adsorbent, the adsorption measurements were made in stages; after outgassing at 150°C, after outgassing at 250°C and after outgassing at 400°C.

All the plots of the differential energy of adsorption, Δ_aU, against surface coverage, θ, reveal energetic heterogeneity; the greatest variation of Δ_aU is given by N₂ on rutile (400°C) and the least variation of Δ_aU by argon on the silica-coated sample (150 and 250°C). The remarkably high initial adsorption energies (Δ_aU > 25 kJ mol^–1 at θ < 0.2) exhibited by N₂ on rutile (400°C) are attributed to the entry of adsorbate molecules into gaps left in the rutile surface by the removal of coordinated water molecules. The more open cationic sites on rutile (400°C) appear to be occupied over the range θ= 0.2–0.4 (Δ_aU > 20 kJ mol^–1), whilst at higher coverage the N₂—OH group interaction becomes the predominant specific contribution to the adsorption energy. Rutile (250°C) behaves in a similar manner to rutile (400°C), but cationic sites are not exposed to any significant extent on the surface of rutile (150°C) and its interaction with N₂ is therefore very similar to that given by hydroxylated silica (150 and 250°C).