Abstract
The metal-organic framework (MOF), NU-1000, and its metalated counterparts have found proof-of-concept application in heterogeneous catalysis and hydrogen storage among others. A vapor-phase technique, akin to atomic layer deposition (ALD), is used to selectively deposit divalent Cu ions on oxo, hydroxo-bridged hexa-zirconium(IV) nodes capped with terminal -OH and -OH2 ligands. The subsequent reaction with steam yields node-anchored, CuII-oxo, hydroxo clusters. We find that cluster installation via AIM (ALD in MOFs) is accompanied by an expansion of the MOF mesopore (channel) diameter. We investigated the behavior of the cluster-modified material, termed Cu-AIM-NU-1000, to heat treatment up to 325 °C at atmospheric pressure with a low flow of H2 into the reaction cell. The response under these conditions revealed two important results: (1) Above 200 °C, the initially installed few-metal-ion clusters reduce to neutral Cu atoms. The neutral atoms migrate from the nodes and aggregate into Cu nanoparticles. While the size of particles formed in the MOF interior is constrained by the width of mesopores (∼3 nm), the size of those formed on the exterior surface of the MOF can grow as large as ∼8 nm. (2) Reduction and release of Cu atoms from the MOFs nodes is accompanied by the dynamic structural transformation of NU-1000 as it reverts back to its original dimension following the release. These results show that while the MOF framework itself remains intact at 325 °C in an H2 atmosphere, the small, AIM-installed CuII-oxo, hydroxo clusters are stable with respect to reduction and conversion to metallic nanoparticles only up to ∼200 °C.
Highlights
Nanoparticles with well-defined shapes and structures hold important roles in many catalytic processes,1,2 for example, CO2 reduction3 and H2O2 production.4 Due to their high surface energy, nanoparticles usually agglomerate into larger particles under reaction conditions, leading to the deterioration of catalytic performance over time.5 Many different strategies have been developed to circumvent adverse sintering, such as using a catalyst support6 or coating the catalytic nanoparticles with a porous layer.7,8 With enhanced stability, the catalytic activity and the product selectivity of the resulting materials are usually maintained or even improved from adventitious support effects.9,10 controlling the size and distribution of supported and/or coated nanoparticles is non-trivial, which may change during post-deposition treatment conditions
These results show that while the metal–organic framework (MOF) framework itself remains intact at 325 ○C in an H2 atmosphere, the small, AIM-installed CuII-oxo, hydroxo clusters are stable with respect to reduction and conversion to metallic nanoparticles only up to ∼200 ○C
We investigated on NU-1000, a high-porosity Zr-MOF,31–33 that has been successfully used in multiple applications such as catalysis,23,34 gas storage and separation,33 selective adsorption in solution,35 and drug delivery
Summary
Nanoparticles with well-defined shapes and structures hold important roles in many catalytic processes, for example, CO2 reduction and H2O2 production. Due to their high surface energy, nanoparticles usually agglomerate into larger particles (sintering) under reaction conditions, leading to the deterioration of catalytic performance over time. Many different strategies have been developed to circumvent adverse sintering, such as using a catalyst support or coating the catalytic nanoparticles with a porous layer. With enhanced stability, the catalytic activity and the product selectivity of the resulting materials are usually maintained or even improved from adventitious support effects. controlling the size and distribution of supported and/or coated nanoparticles is non-trivial, which may change during post-deposition treatment conditions. Nanoparticles with well-defined shapes and structures hold important roles in many catalytic processes, for example, CO2 reduction and H2O2 production.. Nanoparticles with well-defined shapes and structures hold important roles in many catalytic processes, for example, CO2 reduction and H2O2 production.4 Due to their high surface energy, nanoparticles usually agglomerate into larger particles (sintering) under reaction conditions, leading to the deterioration of catalytic performance over time.. AIM is a selective process where the volatile metal complexes react with the nonstructural aqua and hydroxo ligands of the eight-connected, hexa-zirconium(IV)-oxy nodes of NU-1000. An in situ x-ray absorption study is performed to investigate the change in the oxidation state of Cu bonded to the MOFs nodes with temperature and acquire information about the structural changes of the Cu nanoparticles. The study provides insight into the dynamic nature of the Cu-AIM network under reducing environment at high temperature
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