Adsorption and reaction of SO ₂ , H ₂ S, and N ₂ O on graphene/silicon(111): Successful quest for a metal-free catalyst—Theory and experiment

Graphene supported on Si(111) (short Gr/Si) is one of the very few examples of a metal-free carbon catalyst that catalyzes gas-surface reactions. Kinetics measurements indicate dissociation of SO₂ and H₂S but molecular adsorption of N₂O. In addition, spectroscopy revealed adsorbed sulfur after SO₂ and H₂S adsorption. Experiments were conducted at ultrahigh vacuum conditions, using kinetics techniques [i.e., thermal desorption spectroscopy (TDS)], spectroscopy [Auger electron spectroscopy (AES), Raman, X-ray photoelectron spectroscopy (XPS)], and imaging techniques [scanning tunneling microscopy (STM), low-energy electron diffraction]. Deviations of the gas-phase fragmentation pattern and multimass TDS pattern were observed. AES revealed adsorbed sulfur after SO₂ and H₂S adsorption. Thus, SO₂ and H₂S decompose, which contrasts with N₂O, where only the molecular pathway was present. Density functional theory (DFT) confirms experimental observations. Whereas pristine Gr/Si is nonreactive, DFT modeled grain boundary defects (GBD) (as seen by STM) are the active sites for the decomposition. GBD consist of interfacial defects and surface defects (as seen by XPS). Because carbon and silicon are inexhaustible, Gr-based metal-free catalysts would be a paradigm change. Moreover, breaking H₂S down into H₂ would allow for recycling that waste gas and synthesizing green hydrogen.