On the Surface Acid-Base Properties of Amorphous and Crystalline Mg2SiO4 as Probed by Adsorbed CO, CO2, and CD3CN

In this work, we characterize the surface sites of Mg2SiO4, a main constituent of dust grains from the interstellar medium/protoplanetary disks, by using infrared (IR) spectroscopy of adsorbed CO, CO2, and CD3CN probe molecules. With the aid of atomistic models from computer simulations, the acid-base properties of the Mg2SiO4 surface, as inferred by IR spectroscopy, are discussed as a function of each of two structures, amorphous versus crystalline, as well as that of the morphology of the particles. We found that, while the Lewis acidity of exposed cations (Mg2+) is almost independent of the amorphous or crystalline nature of the material, exposed oxygen ions on the other hand are dramatically different in terms of basicity in their crystalline and amorphous phases. Indeed, reactivity toward the CD3CN molecule only occurs in the amorphous Mg2SiO4 particles because of the strong Lewis basicity of the O2-/OH- ions.