Non-contact biomimetic mechanism for selective hydrogenation of nitroaromatics on heterogeneous metal nanocatalysts

Active site-exposed Bi2WO6@BiOCl heterostructures for photocatalytic hydrogenation of nitroaromatic compounds

Constructing heterostructured photocatalysts with highly exposed active sites proves to be an efficient strategy to improve the photocatalytic performance of bismuth-based photocatalysts. In this work, active site-exposed Bi2WO6@BiOCl (BWO@BOC) heterostructure composites based on two bismuth-based materials were fabricated by an in situ growth method for improving the photocatalytic hydrogenation of 4-aniline (4-NA) to p-phenylenediamine (PPD). BWO@BOC exhibited enhanced photoactivity for 4-NA hydrogenation compared to pure BWO and BOC. The optimal BWO@BOC composites displayed the highest conversion rate of 4-NA to PPD up to 99.3% within 12 min, with an apparent reaction rate constant of 0.414 min-1, which is 3.3 times that of pure BOC. The photoactivity enhancement is mainly ascribed to the construction of a tight Z-scheme heterostructure with improved light harvesting properties and charge carrier transport efficiency, which were revealed by optical and photoelectrochemical characterization, respectively. Furthermore, the products of the hydrogenation process were monitored by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to gain a better insight into the 4-NA hydrogenation mechanism.

Selective Reduction of Nitroarenes via Noncontact Hydrogenation

In traditional hydrogenation, where H2 and substrates with unsaturated bonds are activated on the same catalyst (contact mode), competitive hydrogenation of multiple reducible groups often occurs. We employ an unbiased H-cell for selective hydrogenation of the nitro group when multiple reducible groups are present. The setup spatially separates H2 and nitroarenes into two chambers connected by a proton-exchange membrane, thus adding barriers for a Langmuir-Hinshelwood-type mechanism that is common in thermocatalytic hydrogenation. Through a unique proton/electron transfer pathway that is specific to nitro functional group reduction to hydroxylamine, side reactions like C═C, C═O, and C≡C bond hydrogenation are fully avoided. Using Pd/C for H2 activation, and CNT for selective proton/electron transfer to -NO2 groups while being inert to C≡C, C═C, and C═O hydrogenation, the system effectively eliminates the competitive hydrogenation, achieving 100% nitro-group reduction selectivity in the hydrogenation of various nitroarenes, in sharp contrast to negligible selectivity over the same catalysts in a batch reactor under contact mode. This device enables selectivity control in hydrogenation reactions, moving beyond the traditional focus on catalyst active site engineering.

Catalytic applications of organic-inorganic hybrid porous materials

Organic-inorganic hybrid porous materials (OIHMs) inherit the unique properties from both organic and inorganic components, and the flexibility in the incorporation of functional groups renders the OIHMs an ideal platform for the construction of catalytic materials with multiple active sites. The preparation of OIHMs with precise locations of organic-inorganic components and tunable structures is one of the important topics for the catalytic application of OIHMs, but it is still very challenging. In this feature article, we describe our work related to the preparation of OIHMs via confining active sites in the nanostructure and a layer-by-layer assembly method and their applications in acid-base catalysis, catalytic hydrogenation and photocatalysis with a focus on the elucidation of the synergistic effects of different active sites and the unique properties of OIHMs in catalysis.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Microenvironment engineering of supported metal nanoparticles for chemoselective hydrogenation

Selective hydrogenation with supported metal catalysts widely used in the production of fine chemicals and pharmaceuticals often faces a trade-off between activity and selectivity, mainly due to the inability to adjust one factor of the active sites without affecting other factors. In order to solve this bottleneck problem, the modulation of the microenvironment of active sites has attracted more and more attention, inspired by the collaborative catalytic mode of enzymes. In this perspective, we aim to summarize recent advances in the regulation of the microenvironment surrounding supported metal nanoparticles (NPs) using porous materials enriched with organic functional groups. Insights on how the microenvironment induces the enrichment, oriented adsorption and activation of substrates through non-covalent interaction and thus determines the hydrogenation activity and selectivity will be particularly discussed. Finally, a brief summary will be provided, and challenges together with a perspective in microenvironment engineering will be proposed.

Atomic overlayer of permeable microporous cuprous oxide on palladium promotes hydrogenation catalysis

The interfacial sites of metal-support interface have been considered to be limited to the atomic region of metal/support perimeter, despite their high importance in catalysis. By using single-crystal surface and nanocrystal as model catalysts, we now demonstrate that the overgrowth of atomic-thick Cu2O on metal readily creates a two-dimensional (2D) microporous interface with Pd to enhance the hydrogenation catalysis. With the hydrogenation confined within the 2D Cu2O/Pd interface, the catalyst exhibits outstanding activity and selectivity in the semi-hydrogenation of alkynes. Alloying Cu(0) with Pd under the overlayer is the major contributor to the enhanced activity due to the electronic modulation to weaken the H adsorption. Moreover, the boundary or defective sites on the Cu2O overlayer can be passivated by terminal alkynes, reinforcing the chemical stability of Cu2O and thus the catalytic stability toward hydrogenation. The deep understanding allows us to extend the interfacial sites far beyond the metal/support perimeter and provide new vectors for catalyst optimization through 2D interface interaction.