Grasping periodic trend and rate-determining step for S-modified metals of metal sulfides deployable to produce (OH)-O-center dot via H2O2 cleavage

Abstract

Iron sulfides are fascinating catalytic phases because these include S-modified Fe delta+ (delta <= 2) species functioning as H2O2 activators to form (OH)-O-center dot used for oxidatively degrading aqueous contaminants (e.g., phenol). As an initial step for locating S-modified metal species (M delta+) that outperform Fe delta+ in catalytic H2O2 cleavage, hexagonal metal sulfides (MS) were synthesized using Mn, Fe, Co, Ni, and Cu to understand electric potential-assisted H2O2 scission kinetics on M delta+ species. Ni delta+ species were found to show the greatest (OH)-O-center dot productivity among all M delta+ species studied, mainly resulting from the Lewis acidic nature of Ni delta+ species adequate to expedite the liberation of (OH)-O-center dot species. This was partially evidenced by H2O2 activation/phenol degradation runs on M delta+ species, wherein initial H2O2 activation rate (-r(H2O2,0)) or initial phenol degradation rate (-r(PHENOL,0)) of Ni delta+ species was 3-9 times those of the other M delta+ species. Ni delta+ species, therefore, were located in the middle of the volcano-shaped curve plotting -r(H2O2,0) (or -r(PHENOL,0)) versus the type of M delta+. Kinetic assessment of M delta+ species under fine-tuned reaction environments also showed that regardless of varying H2O2 concentrations, M delta+ species were found to retain their -r(H2)O(2,0) values in the absence of electric potentials. Conversely, M delta+ species could enhance -r(PHENOL,0) values at larger electric potentials, where greater energies were likely exerted on M delta+ species. This indeed corroborated that (OH)-O-center dot desorption from M delta+ species was the rate-determining step to direct catalytic H2O2 scission. In addition to heterogeneous catalytic nature of Ni delta+ species in fragmenting H2O2, outstanding H2O2 scission ability provided by Ni delta+ species could also compensate for their moderate catalytic stability at pH-neutral condition.