Direct spectroscopic observation of the role of humidity in surface diffusion through an ionic adsorbent powder. The behavior of adsorbed pyridine on nanocrystalline MgO

The influence of water vapor on the surface diffusion behavior of pyridine adsorbed on powdered MgO surfaces has been studied by Fourier transform IR (FTIR) absorption spectroscopy. It has been found that the introduction of water vapor significantly increases the pyridine surface diffusion coefficient. FTIR spectra showed that water vapor converted Lewis acid Mg ²⁺ sites to Brønsted Mg-OH sites. These measurements also detected the change in surface bonding of pyridine to the two types of sites. The activation energy for escape of chemisorbed pyridine into a mobile precursor state is lower for pyridine bound to Mg-OH sites than for binding to Mg ²⁺ sites, causing the hydroxylated MgO to exhibit a higher diffusivity than that found for dry MgO containing no surface hydroxyl groups. This effect was confirmed by DFT calculations of the binding energy of pyridine to MgO(100) sites and to defect sites on MgO(100), where hydroxylation decreases the binding energy by ̃30 kJ mol ^-1 on each type of site.