Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1.

Weijia Wang,Yuemin Wang,Alexei Nefedov,Abhinav Chandresh,Felix Studt,Dmitry I Sharapa,Stefan Heißler,Lars Heinke,Christof Wöll

doi:10.1002/anie.202000385

Abstract

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal‐organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect‐free HKUST‐1 MOF thin films, we demonstrate that Cu+/Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+/Cu2+ pairs of the intact framework, account for the high catalytic activity in low‐temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+/Cu2+. Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.

Highlights

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal-organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms
MOF-based materials hold great promise as single-site catalysts.[2]. In many cases it is unclear whether the observed catalytic activity is related to metal ions of the perfect framework, or whether the active sites are related to structural defects within the MOF
On the basis of these observations, we propose that the catalytic activity of HKUST-1 for CO oxidation results from the presence of reduced Cu+/Cu2+ dimer defects, challenging the interpretation put forward in previous work.[7]

Summary

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SURMOF to O2 at 55 K, a very weak signal at 1550 cmÀ1 (Supporting Information, Figure S4). (Figure 4 c, Supporting Information, Figure S4) as a result of the dioxygen adsorption, in line with the DFT calculations The calculated value for M06 is over 1.2 eV, while Based on DFT calculations, the latter one is assigned to CASSCF[12] and NEVPT2[13] that explicitly account for the activated, uncharged dioxygen species adsorbed to Cu2+ CUS multi-reference character calculate TS-I to 0.56 eV (see of the reduced Cu+/Cu2+ dimers (Figure 4 b). This value is in computed frequency (10 cmÀ1 red-shift in comparison to O2 fair agreement with the experimental observation of CO on pristine paddle wheel) and binding energy (0.1 eV) are in oxidation at temperatures as low as 105 K. TS-II on the other good agreement with the experimental results that show the hand, can be successfully treated with single-reference binding only occurs at low temperatures

After providing strong support for the reaction pathway

Conflict of interest