Boosting Catalysis of Pd Nanoparticles in MOFs by Pore Wall Engineering: The Roles of Electron Transfer and Adsorption Energy

Abstract

The chemical environment of metal nanoparticles (NPs) possesses significant influence on their catalytic performance yet is far from being well understood. Herein, tiny Pd NPs are encapsulated into the pore space of metal–organic frameworks (MOFs), UiO‐66‐X (X = H, OMe, NH2, 2OH, 2OH(Hf)), affording Pd@UiO‐66‐X composites. The surface microenvironment of the Pd NPs is readily modulated by pore wall engineering, via the functional group and metal substitution in the MOFs. Consequently, the catalytic activity of Pd@UiO‐66‐X follows the order of Pd@UiO‐66‐OH > Pd@UiO‐66‐2OH(Hf) > Pd@UiO‐66‐NH2 > Pd@UiO‐66‐OMe > Pd@UiO‐66‐H toward the hydrogenation of benzoic acid. It is found that the activity difference is not only ascribed to the distinct charge transfer between Pd and the MOF, but is also explained by the discriminated substrate adsorption energy of Pd@UiO‐66‐X (–OH < –2OH(Hf) < –NH2 < –OMe < –H), based on CO‐diffuse reflectance infrared Fourier transform spectra and density‐functional theory (DFT) calculations. The Pd@UiO‐66‐OH, featuring a high Pd electronic state and moderate adsorption energy, displays the highest activity. This work highlights the influence of the surface microenvironment of guest metal NPs, the catalytic activity of which is dominated by electron transfer and the adsorption energy, via the systematic substitution of metal and functional groups in host MOFs.

Dual-functional ionic porous organic framework for palladium scavenging and heterogeneous catalysis

“…52 In addition, the interactions between the Pd nanoparticles and the amine groups in the organic framework could also assist the stabilization of the nanoparticles. 53,54 As expected, the surface area of the composites is reduced to 27 m 2 g −1 . Importantly, the pore size distribution analysis of the N 2 adsorption isotherm confirms that the hierarchical porous structure of the IPOF substrate remained after loading the Pd nanoparticles (Fig.…”

Metal–Organic Frameworks Enhance Biomimetic Cascade Catalysis for Biosensing

“…[60,139] On the other hand, taking advantage of the ligand engineering of MOFs, the surface microenvironment of the encapsulated catalysts can be tuned. [140] Based on this phenomenon, reasonable combination of nanozymes and MOFs is a promising method to boost the activity of nanozymes. Another key point for the development of high-performance MOFs-based nanozymes is to sufficiently utilize the unique functionality and structure of MOFs.…”

Design of Organic/Inorganic Hybrid Catalysts for Energy and Environmental Applications

“…showed that for the hydrogenation of benzoic acid to cyclohexanecarboxylic acid, this charge interaction has a significant effect: different MOF chemistries, including UiO-66-Ome, UiO-66-NH 2 , UiO-66-3OH(Hf), and UiO-66-2OH show dramatically different activity ( Figure 14 a). 110 To understand these catalytic differences, ab initio calculations were performed to show that an increased activity for this reaction is correlated to lower charge transfer interactions from the Pd nanoparticles to the MOF structure. In addition to calculations, the changed surface state was demonstrated by DRIFTS, where a monotonic shift in CO binding energy with MOF chemistry paralleled the increase in catalytic reactivity, suggesting that a shift in electronic structure at the Pd nanoparticle surface was responsible for the catalytic activity being tunable by MOF chemistry ( Figure 14 b).…”

“…(a) Activity and selectivity of various Pd@MOF composites for selective hydrogenation of benzoic acid to cyclohexanecarboxylic acid (b) CO DRIFTS spectrum of Pd@MOF materials as a function of MOF chemistry. Reproduced with permission from ref ( 110 ). Copyright 2020 John Wiley and Sons.…”