H2 dissociation plays crucial roles in catalytic hydrogenation reactions and hydrogen storage. Metal-nanoparticle-based heterogeneous catalysts often dissociate H2 via a homolytic pathway. The heterolytic H2 dissociation pathway was identified in several heterogeneous catalytic systems, including single-atom catalysts, metal–support interfaces, and bulk metal oxides/sulfides/nitrides/phosphides. The active site structures of these heterogeneous catalysts resemble homogeneous catalysts where the metal centers (Lewis acids) are coordinated with O/S/N/P atoms (Lewis bases). These Lewis acid–base pairs dissociate H2 molecules heterolytically into proton–hydride pairs, which favor the hydrogenation of polar functional groups in unsaturated hydrocarbons. In this review, we summarize the common structural features of heterogeneous, homogeneous, and enzyme catalysts in the heterolytic dissociation of H2. The active sites, Lewis acid–base pairs, are discussed throughout this review. The energy barriers and kinetic contributions of heterolytic and homolytic H2 dissociation pathways in heterogeneous catalytic systems are discussed. The spectroscopic evidence of the heterolytic H2 dissociation pathways is critically reviewed.

Amorphous alumina overcoats generated by atomic layer deposition (ALD) have been shown to improve the selectivity and durability of supported metal catalysts in many reactions. Several mechanisms have been proposed to explain the enhanced catalytic performance, but the accessibilities of reactants through the amorphous overcoats remain elusive, which is crucial for understanding reaction mechanisms. Here, we show that an AlO x ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (C 2 H 2 ) hydrogenation. We further demonstrate that the AlO x ALD overcoat blocks the access of C 2 H 2 (kinetic diameter of 0.33 nm), O 2 (0.35 nm), and CO (0.38 nm) but allows H 2 (0.29 nm) to access Pd surfaces. A H–D exchange experiment suggests that H 2 might dissociate heterolytically at the Pd–AlO x interface. These findings are in favor of a hydrogen spillover mechanism.

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The spatial confinement at metal−zeolite interfaces offers a powerful knob to steer the selectivity of chemical reactions on metal catalysts. However, encapsulating metal catalysts into small-pore zeolites remains a challenging task. Here, we demonstrate an inverse design of metal−zeolite interfaces, "metal-on-zeolite," constructed by area-selective atomic layer deposition. This inverse design bypasses the intrinsic synthetic issues associated with metal encapsulation, offering a potential solution for the fabrication of task-specific metal−zeolite interfaces for desired catalytic applications. Infrared spectroscopy and several probe reactions confirmed the spatial confinement effects at the inverse metal−zeolite interfaces.