Electrocatalytic Coordination Networks
Electrocatalysis • Carbon dioxide electroreduction • Nitrogen electroreduction • polyol electrooxidation
E. Brunner,(a) X. Feng,(b) S. Kaskel,(c)
(a) Lehrstuhl für Bioanalytische Chemie, Technische Universität Dresden
(b) Molekulare Funktionsmaterialien, Technische Universität Dresden
(c) Anorganische Chemie I, Technische Universität Dresden
Porous coordination networks (PCNs) are ideal candidates for selective electrocatalytic transformations since they enable highly selective adsorptive separations of small molecules inside pores and at the same time the integration of catalytically active sites, originating from transition metals as nodes or dopants generating tunable redox functions as building blocks in electrocatalytic coordination networks. Electrocatalysis for the generation of valuable intermediates poses a huge potential for the use of low-cost electrical energy emerging from the fluctuating energy supply of renewable resources. Electrocatalytic reduction of greenhouse gases such as CO2 and conversion into valuable fuels such as methanol and methane is a promising target. Electroreduction of N2 is a visionary target to reduce energy consumption for fertilizer production. Moreover a high potential of highly selective CNs is anticipated for fine chemical intermediates, for example the selective oxidation of polyols derived from biomass.
We target the next generation of CN based materials enhancing the product selectivity in electro-catalytic applications. The whole process chain from the design and synthesis of molecular building blocks, ECN synthesis, development of required processing and advanced in situ-characterization techniques towards function demonstration in selected electrocatalytic model reactions including, CO2 and N2 electroreduction and polyol electrooxidation for the production of fine chemicals is addressed.
Large π conjugated planar monomer, such as polythiol- and amino-functionalized phthalocyanine based linkers were developed to fabricate 2D conjugated conductive PCNs and covalent-organic frameworks (COFs). Cu-phthalocyanine-based 2D PCNs with different coordinated metal complexes (PcCu-O8-M, M=Fe, Co, Ni, Cu, Zn) were synthetized and investigated toward CO2RR. PcCu-O8-Zn/CNT exhibited excellent electrocatalytic behavior with high CO selectivity (88%), turnover frequency (0.39 s-1) and long-term durability (>10 h), surpassing thus by far reported MOF-based electrocatalysts. The molar H2/CO ratio (1:7 to 4:1) can be tuned by varying metal centers and applied potential. Beside, we also developed a new strategy to construct a series of novel metal-phthalocyanine (MPc)-based pyrazine-linked 2D c-COFs (abbreviated as M COF, M=Fe, Co, Ni, Mn, Zn and Cu) for electrocatalytic NRR in acidic media. The optimized Fe COF shows high catalytic performance with NH3 yield rate of 33.6 mg h-1mg-1cat and Faradaic efficiency of 31.9 % at -0.1 V vs RHE owing to its optimized electronic/porous structure and good affinity of N2, which is comparable to current noble metal electrocatalysts.
Nanocomposites based on MOFs showing selectivity in alcohol adsorption (such as ZIF-8) and state of the art catalyst (Pt@Vulcan) were employed in electro catalytic oxidation of alcohols. The MOF functionalization significantly enhances the selectivity of the Pt@Vulcan system.
Ex situ and in situ NMR spectroscopy was applied to study the synthesized new materials as well as their behaviour in electrocatalytic reactions. Ex situ solid-state NMR is used to characterize bulk materials based on the observation of nuclei such as 1H, 13C, 15N etc. Isotope-enrichment, e.g., with 13C or 15N will be applied for selected samples of special interest, allowing to perform 2D solid-state NMR experiments such as heteronuclear correlation (HETCOR) and spin diffusion. In addition to 1D spectra, 2D solid-state NMR experiments such as 1H-13C heteronuclear correlation (HETCOR) are planned. The above-mentioned reactions will be also studied in operando under stopped flow and under continuous flow conditions. We are thus planning to develop a modified in situ setup to perform measurements also under continuous flow.
(1) H. Zhong, M. Ghorbani-Asl, K. H. Ly, J. Zhang, J. Ge, M. Wang, Z. Liao, D. Makarov, E. Zschech, E. Brunner, I. M. Weidinger, J. Zhang, A. V. Krasheninnikov, S. Kaskel, R. Dong, X. Feng, Nat. Commun. 2020, 11, 1409.
(2) J. B. Richter, C. Eßbach, I. Senkovska, S. Kaskel, E. Brunner Chem. Commun. 2019, 55, 6042.
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