Design of a Catalyst of Multiple Single-Atom Sites through the Synergic Effect for Chemical Transformation and Fuel Production

Abstract

Conspectus We defined the synergic effect in single-atom catalysis and demonstrated that the catalytic performance of a reaction involving two or more reactants (R1 + R2 → P) on a catalyst of multiple single-atom sites (CatM1+A1) can benefit from the coexistence of two types of single-atom sites M1α and M1β if they are responsible for activating reactants R1 and R2, respectively. The kinetics feature is that the activation barrier [Ea(CatM1+A1)] of this catalyst is lower than that of a catalyst with only one type of single-atom site [Ea(CatM1or CatA1)]. Such a catalyst can be prepared through coprecipitation/deposition, sequential precipitation/deposition, or atomic layer deposition. The synergistic effect of two types of single-atom sites on a support makes it possible to catalyze a reaction at a thermodynamically allowed low temperature. One example of such catalytic materials is Mo1+Pd1/Co3O4 consisting of two sets of single-atom sites Pd1Ox and Mo1Oy, and a support, Co3O4. While the Pd1 site activates H2 to form atomic H, the Mo1 site responds by activating C–O of C6H5OCH3 to form C6H5. The H atoms formed on the Pd1 site can diffuse on Co3O4 to couple with C6H5, leading to the formation of benzene at 110 °C. In contrast, Pd nanoparticles are not active for anisole HDO at 110 °C since Pd atoms of Pd nanoparticles preferentially chemisorb atomic H but do not activate the C–O bond of anisole at 110 °C. Another type of synergic effect in single-atom catalysis is the coupling between the role of each of the two types of single-atom sites in the catalytic cycle of a reaction. For instance, the two types of single-atom sites, Ni1Oa and Ru1Ob on CeO2 (Ce0.90Ni0.05Ru0.05O2), play a synergistic role in methane steam reforming (MSR) in which Ni1Oa is responsible for activating CH4 to form CH3, CH2O, CHO, and then CO. The oxygen vacancy on the CeO2 surface is refilled by O of H2O through the activation and dissociation of H2O by Ru1 and its adjacent oxygen vacancy (Ru1 + OVac); the synergistic role of Ni1Oa and Ru1+Ovac makes MSR occur at 500 °C, bringing the high working temperature of the solid oxide fuel cell of methane down to an intermediate temperature. A synergic effect can occur between two of the same type of single-atom sites anchored on two different supports. The two single-atom sites (M1/Zα and M1/Zβ) have different reactivities since they have different chemical and coordination environments. An intermediate of reactant A formed on a single-atom site M1 anchored on one support (M1/Zα) can spillover to another support (Zβ) to couple with the intermediate of reactant B formed on another type of single-atom site (M1/Zβ), enabling a synergistic effect of the two types of single-atom sites anchored on different supports (M1/Zα and M1/Zβ). For instance, two Pd1 sites are anchored on C3N4 and SnO2, forming Pd1/C3N4 and Pd1/SnO2, respectively. Pd1/C3N4 activates H2 to form atomic H, whereas Pd1/SnO2 responds by activating O2. We discussed future applications of this concept of materials design to the development of new heterogeneous catalysts and electrocatalysts for a great number of other reactions.