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 O^2–/OH^– ions.